WO2005060653A2 - Inductive probe having a looped sensing element or a terminated transmission line sensing element and methods and system for using the same - Google Patents

Inductive probe having a looped sensing element or a terminated transmission line sensing element and methods and system for using the same Download PDF

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Publication number
WO2005060653A2
WO2005060653A2 PCT/US2004/042161 US2004042161W WO2005060653A2 WO 2005060653 A2 WO2005060653 A2 WO 2005060653A2 US 2004042161 W US2004042161 W US 2004042161W WO 2005060653 A2 WO2005060653 A2 WO 2005060653A2
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WIPO (PCT)
Prior art keywords
signal
medium
probe
change
radio frequency
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PCT/US2004/042161
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French (fr)
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WO2005060653A3 (en
Inventor
Mehrdad Mehdizadeh
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E.I. Dupont De Nemours And Company
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Publication of WO2005060653A2 publication Critical patent/WO2005060653A2/en
Publication of WO2005060653A3 publication Critical patent/WO2005060653A3/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/221—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance by investigating the dielectric properties
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/24—Earth materials
    • G01N33/246—Earth materials for water content

Definitions

  • the present invention relates to an inductive probe apparatus having a looped sensing element or a terminated transmission line sensing element and to methods and a system for using the same, including: methods for detecting the presence of a material causing a change in the dielectric characteristics of a medium in which the material is contained; a method sensing of moisture in a granular medium and a system employing the same; and a method for detecting the interface between materials.
  • prior art capacitive devices utilize at least two metallic electrodes, which are placed into contact with the medium. Current flow through the medium is measured and provides an indication of moisture content of the medium.
  • the electrodes must have a relatively large surface area to achieve sufficient contact with the medium to be measured.
  • the electrodes also must be separated at a distance so that the medium, which is often coarse and granular, can be positioned between the electrodes. Due to these limitations the physical size of a capacitive probe is often quite large. Such a probe is not usable, for example, in a small plant pot, such as a so- called seed pot, found in experimental greenhouses.
  • prior art devices Due to the relatively low AC frequencies used the prior art devices are also subject to electrical currents produced by ionic conductivity of soils that are acidic, basic, or saline.
  • the prior art capacitive devices must therefore be calibrated for these types of conditions to permit accurate measurement of the moisture content.
  • the metallic electrodes of prior art capacitive devices are subject to detrimental corrosion or oxidation effects since they are required to contact the soil. To prevent corrosion the probe electrodes are typically made of relatively expensive metals and thus suffer a cost disadvantage.
  • An apparatus capable of accurate moisture measurement, in either a continuous or an intermittent mode of operation while being insensitive to ionic conductivity would facilitate precise moisture measurement of the soil and would thus provide an important tool for facilitating improved moisture control useful for automating both commercial and experimental greenhouses.
  • the present invention is directed to an apparatus for detecting the presence of a material that causes a change in the dielectric 5 characteristic of the medium in which the material is contained.
  • the apparatus comprises a source of a radio frequency signal at a predetermined amplitude, an inductive probe placeable into contact with the medium, and a receiver for receiving the radio frequency signal transmitted through the probe.
  • a detection network is associated with the0 receiver for determining a change in a property of the signal arriving at the receiver.
  • a transmission line couples the first end of the sensing element of the probe to the source and the second end of the sensing element of the probe to the receiver.
  • the transmission line may be a coaxial structures with the inner conductor being surrounded by the dielectric material and the shielding conductor.
  • the transmission line may be a layered structure comprising an inner conductor surrounded by a dielectric material and first and second shielding conductor layers.
  • the sensing element iso configured as a looped sensing element having a first and a second end.
  • the sensing element is configured as a monopole sensing element.
  • the probe is implemented using a terminated transmission line that itself comprises an5 inner conductor surrounded by a dielectric material and a shielding conductor. An exposed portion of the inner conductor defines the monopole sensing element.
  • Both the looped sensing element and the monopole sensing element may be constructed as a rigid element suitable to be insertableo into the soil.
  • either sensing element may be flexible so that it may be placed in contact with the soil by positioning it along or attaching it to the interior wall of the container in which the soil medium is placed.
  • the source, receiver and detection network and the sensing element structure itself may be constructed of materials and circuits that are readily available at low cost, such as those used in wireless consumer electronic devices. In use, upon placement of the probe into contact with the medium, a property of the signal transmitted from the source to the receiver is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe.
  • the detection network generates a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver.
  • the changed property of the signal may be either: the attenuation of the signal at a predetermined frequency; the attenuation of the signal amplitude at each of a plurality of predetermined frequencies; or, the change in the propagation velocity of the signal through the probe.
  • the present invention facilitates accurate moisture determination in a granular medium, such as soil, in a compact, easy to use, and inexpensive device. By applying radio frequency electromagnetic waves to the medium to measure dielectric loss without making direct electrical (i.e., ohmic) contact with the medium the present invention is sensitive to water content in the medium, but is not sensitive to the ionic conductivity of the medium.
  • the apparatus may be made quite small.
  • the sensing element of either embodiment of the apparatus of the present invention is sized such that it may be either inserted into or placed in contact with the soil in the smallest of seeding pots or seeding trays. Ionic currents are not induced in the soil since the ions in the soil are too large to respond to the frequencies of operation.
  • the present invention is directed to a method for detecting the presence of a material in a medium by detecting a change in a property of an ultrahigh frequency radio frequency signal passed through the medium.
  • the method comprises the steps of: placing an inductive probe into contact with the medium; passing a radio frequency signal through the probe; detecting a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe; and generating a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal.
  • the sensing element of either embodiment of the apparatus of the present invention may be used.
  • the radio frequency signal has a predetermined frequency and the change in the property of the signal detected is an attenuation in signal amplitude.
  • the radio frequency signal has a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency.
  • the radio frequency signal (at the receiver) has a maximum amplitude at a predetermined one of the frequencies, the change in the attenuation of the signal as a function of frequency being detected by determining a change in the frequency at which the maximum amplitude occurs.
  • the probe may be inserted into the medium or placed against a container having electrically non-conducting walls in which the medium is disposed.
  • the present invention is directed to an automated irrigation system and method for controlling the moisture content in a medium within a container.
  • the system and method automatically controls the operation of a valve connectible to a source of water supply. Assertion of the valve supplies water to the medium.
  • the system includes at least one inductive probe having the sensing element of either embodiment of the apparatus of the present invention.
  • the probe is operative to detect a change in dielectric characteristic of the medium due to the amount of moisture present in the vicinity of the probe and a detection network associated with the probe for generating a signal representative of the amount of moisture in the medium.
  • a comparator compares the signal representative of the amount of moisture in the medium with a reference signal representative of a desired moisture content and generates a control signal in accordance with the comparison.
  • the valve may be controlled to the supply water in accordance with any one of at least three predetermined water control protocols.
  • the present invention is directed to method for detecting an interface between materials.
  • a sensing element in accordance with either embodiment of the apparatus of the present invention is excited by a radio frequency signal at a predetermined amplitude and progressively inserted into the volume of materials.
  • the position of the sensing element within the volume is tracked and the attenuation of the radio frequency signal is monitored as a function of insertion distance into the volume to detect first distance ranges having a substantial rate of change of attenuation and second distance ranges having substantially no change of attenuation.
  • the location of the interface between strata within the volume is detected by noting a transition from a second distance range to a first distance.
  • Figure 1 is a side elevational view, partly diagrammatic and partly in section, of an apparatus in accordance with a first embodiment of the present invention that includes a probe in the form of a looped sensing element for detecting the presence of a material in a medium disposed within a container in accordance with method aspects of the present invention
  • Figure 2 is a perspective view of one form of the first embodiment of the apparatus of the present invention in which the looped sensing element is implemented as a coaxial structure with portions broken away for clarity of illustration
  • Figure 3 is a perspective view of an alternate form of the first embodiment of the apparatus of the present invention in which the looped sensing element is implemented as a layered structure disposed in a single plane with portions broken away for clarity of illustration
  • Figure 4 is a perspective view of another alternate form of the first embodiment of the apparatus of the present invention in which the loop
  • FIG. 1 shown is one form of a first embodiment of an apparatus 10 in accordance with of the present invention for detecting the presence of a material in a medium M.
  • the medium M is shown disposed within a container or soil pot P.
  • the container P may have planar or curved boundary walls.
  • the apparatus 10 includes an inductive probe in the form of a looped sensing element 20 (generically illustrated as a single turn of conductor) and associated electronics network 25.
  • the looped sensing element 20 has a first end 22 and a second end 24.
  • the electronics network 25 comprises a radio frequency signal source 26 and a receiver 28.
  • the first end 22 of the looped sensing element 20 is coupled to the signal source 26 by a first transmission line 30.
  • the second end 24 of the looped sensing element 20 is coupled to the receiver 28 by a second transmission line 32.
  • a detection network 40 is associated with the receiver 28 for determining a change in a property of the signal arriving at the receiver 28.
  • the detection network 40 may be implemented as either an amplitude detection circuit ( Figure 6), a resonant frequency detection circuit (Figure 7), or a phase shift detection circuit 48 ( Figure 8). Each form of detection network 40 is more fully described herein.
  • Optional capacitors 50 and 52 aid in increasing the sensitivity of the apparatus 10 by matching the impedance of the source 26 to transmission line 30 and matching the impedance of transmission line 32 to the receiver 28. In the operating position illustrated in Figure 1 the probe 20 is inserted into or placed in contact with the medium M within the container P.
  • a property of the signal transmitted from the source 26 to the receiver 28 is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material (e.g., water) present in the medium M (e.g., soil) in the vicinity of the probe.
  • the detection network 40 generates at its output 42 a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver.
  • the signal representative of the amount of material in the medium may be transmitted to an indicating device 44, which may take the form of a liquid crystal display or a light emitting diode display.
  • the looped sensing element 20 may be implemented in a coaxial structural form ( Figure 2) or layered structural forms ( Figures 3 or 4).
  • Figure 2 shows a perspective view of the inductive probe having a looped sensing element 20 exhibiting a coaxial structure.
  • the inductive probe is formed from a length of coaxial transmission line having a center conductor 124, a dielectric layer 126 and an outer shielding conductor 128.
  • the looped sensing element 20 is formed by removing the outer shielding conductor 128 from a central portion of the length of the transmission line 122 thereby to expose a region 130 of the center conductor 124 surrounded by the dielectric layer 126.
  • the remaining shielded portions 132, 134 define the respective first and second transmission lines 30, 32 that couple the respective ends 22, 24 of the looped sensing element 20 to the source 26 and receiver 28 ( Figure 1).
  • the shielded portions 132, 134 are arranged in parallel and attached together, as by soldering the outer shields.
  • mechanical rigidity can be achieved by mounting the looped sensing element 20 of the probe on a planar rigid substrate (shown in dot-dash lines at reference character 136).
  • the rigid substrate may alternatively be curved (as shown in dot-dash lines at reference character 136') to facilitate placement of the looped sensing element 20 of the probe into contact with the medium within the curved boundary wall of the container P.
  • the electronics network 25 (comprising the source 26 and the receiver 28) may be mounted on the substrate 136, 136' with the looped sensing element 20 thereby to form an integrated structure.
  • a source of electrical energy such as a battery, may be included within the integrated structure. When used in a greenhouse environment having bright lighting a solar cell may supplement the battery.
  • the entire structure may be enclosed or encapsulated in a suitable dielectric material, as suggested at reference character 138.
  • the coaxial line may be made of a commercially available product commonly called "semi-rigid coax".
  • the outer shielding conductor 128 is made of a flexible grade solid copper and the center conductor 124 is made of either solid copper or silver- plated steel alloy.
  • the dielectric layer 126 is made of solid PTFE, which is known to have very low dielectric loss.
  • the usage of such materials for and application like soil moisture sensing has the advantage of durability in a naturally corrosive wet soil environment.
  • attached shielded portions provide a robust mechanism to space the electronics network 25 from the corrosive environment in which the sensing element is placed while transmitting the radio frequency signals faithfully between the probe and the electronics network 25.
  • the lengths of the remaining shielded portions 132, 134 are preferably equal and are determined by the mechanical requirements of the apparatus.
  • Figures 3 and 4 show respective perspective views of alternative forms of the first embodiment of the apparatus of the invention in which the looped sensing element of the inductive probe 20 has a layered structure.
  • the probe in each Figure employs a structure known in the art as "stripline".
  • the transmission lines 30, 32 are implemented in a laminar structure comprising (from top to bottom as viewed in Figure 3): a first conducting shield layer 140; a first dielectric layer 142; a layer 143 including both first and second stripe conductors 144, 146; a second dielectric layer 148; and a second conducting shield layer 150.
  • the various layers may be in the form of parallel planes (as illustrated) or concentric curved (e.g., cylindrical) surfaces.
  • An additional electromagnetic shielding conductor 152 is disposed between the stripe conductors 144, 146 to isolate the conductor 144 included in the transmission line 30 from the conductor 146 included in the transmission line 32.
  • This conductor 152 should be connected to the shield layers 140, 150.
  • the inner signal stripe conductors 144, 146 may be spaced from each other a sufficient lateral distance to isolate the conductors 144, 146.
  • the first and second stripe conductors 144, 146 and the dielectric layers 142, 148 all extend past an edge of the shield layers 140, 150.
  • the extending portions of the conductors 144, 146 are joined between the ends 22, 24 to form the looped sensing element 20.
  • the stripline structure is preferably symmetrical about the layer 143 containing the inner signal stripe conductors 144, 146.
  • the lengths of the remaining shielded portions 132, 134 are preferably equal and are determined by the mechanical requirements of the apparatus.
  • the stripline structure may be either flexible or rigid.
  • a flexible structure is useful for placement in contact with the soil along the wall of a curved wall soil container P.
  • the flexible structure may be attached to wall of the soil container P using a suitable adhesive. If the container has electrically non-conducting walls the looped sensing element may be attached to the outside of the container P and still be in sufficient electromagnetic contact with the medium to effect the desired measurement.
  • a rigid structure is useful for insertion into soil in a container.
  • a rigid structure may be achieved by mounting the looped sensing element of the probe on a rigid substrate 136, 136' as discussed in connection with Figure 2.
  • a rigid structure may be achieved by forming the dielectric layers 142, 148 from a glass fiber-filled polymer material or from a ceramic material, such as a low temperature co-fired ceramic. These materials (which may be planar) are sufficiently stiff to obviate the need for a separate rigid substrate.
  • the source 26 and the receiver 28 may be mounted on the substrate with the looped sensing element 20 thereby to form an integrated structure. Surface mount technology is preferably used. If desired the entire structure may be enclosed or encapsulated in a suitable dielectric material 138.
  • Figure 4 illustrates a stacked layer stripline structure in which the first and second signal conductors 144, 146 are disposed on respective separate layers 143A, 143B separated by electromagnetic shielding layer 154 (functionally equivalent to the conductor 152 in the embodiment of Figure 3). Again some portions of the Figure are broken-away for clarity.
  • a dielectric layer 156A, 156B is respectively interposed between the ground conductor layer 154 and each signal layer 143A, 143B.
  • Dielectric layers 142, 148 are respectively disposed above and below the first separate layers 143A, 143B.
  • Outer shielding conductor layers 140, 150 are respectively disposed above and below the dielectric layers 142, 148.
  • the stacked layer stripline structure shown in Figure 4 comprises, in order from top to bottom: a first outer shielded conductor layer 140; a first dielectric layer 142; a layer 143A including a first inner signal conductor 144; a second dielectric layer 156A; a ground conductor layer 154; a third dielectric layer 156B; a layer 143B including a second inner signal conductor 146; a fourth dielectric layer 148; and a second outer shielded conductor layer 150.
  • the electromagnetic shielding layer 154 is electrically connected to the shield layers 140, 150.
  • the first and second stripe conductors 144, 146 and the dielectric layers 142, 156A, 156B, and 148 all extend past an edge of the shield layers 140, 150.
  • the extending portions of the conductors 144, 146 are joined between the ends 22, 24 to form a vertically oriented looped sensing element 20.
  • the first transmission line 30 connected to the first end 22 of the looped sensing element 20 is formed from the combination of the stripe conductor 144, surrounding dielectric layers 142 and 156A, shielding layer 140 and ground conductor layer 154.
  • the second transmission line 32 connected to the second end 24 of the looped sensing element 20 is formed from the combination of the stripe conductor 146, surrounding dielectric layers 148 and 156B, shielding layer 150 and ground conductor layer 154.
  • the layers are planar in form, although they may be implemented as concentric curved surfaces.
  • the structure may be flexible or rigid, enclosed in a suitable dielectric material 138 and integrated with the electronics network 25 (not shown in Figure 4), all as described in connection with Figures 2 and 3.
  • Figure 5 is an electrical equivalent circuit of the apparatus of Figure 1 , whether implemented using the looped sensing element 20 configured as in Figures 2, 3 or 4, by which the principles of operation of the present invention may be understood.
  • the transmission line sections 30, 32 are connected to the looped sensing element 20.
  • an inductive loop carrying an AC current produces an AC magnetic field in its vicinity.
  • Magnetic fields are not known to interact with dielectric media (e.g., soil) and materials (e.g., water).
  • an inductive loop also produces stray electric fields that may, at a "high” frequency, interact with such dielectric media.
  • the interaction of such stray electric fields with the dielectric media is the principled underpinning of the operation of the present invention.
  • An excitation frequency is suitably "high" when the length dimension (from end 22 to end 24) of the looped sensing element 20 is at least an appreciable fraction [such as from about two percent (2%) to about ten (10%) percent] of the operating wavelength.
  • a "high" frequency greater than one megahertz (1 MHz), particularly greater than one hundred megahertz (100 MHz), and more particularly, greater than five hundred megahertz (500 MHz), and specifically in the eight hundred to nine hundred megahertz range (800 to 900 MHz, the present operating range of wireless consumer electronics) has been found useful.
  • the practical upper limit of the frequency is determined by the state of the art of source and receiver electronics. Stated alternatively, it has been found that to achieve sufficient sensitivity to the dielectric characteristic of the material in the medium, the length of the loop-sensing element 20 should be at least a minimum length as dictated by the operating frequency.
  • Capacitor C typically has a small value (typically less than one picofarad) and the parallel resistance R has a typically high value (greater than tens of megohms). Therefore at low frequencies of operation the reactive impedance of this parallel combination is very large and can be ignored. At high frequencies, however, the total impedance of this capacitance/resistance combination becomes significant. Therefore the changes in the dielectric characteristics become easier to measure using the apparatus disclosed in this invention.
  • the combination of capacitors 50 and 52 with the transmission line portions 30, 32 and with the total impedance L, C, R of the looped sensing element 20 cooperate to form a resonant circuit having a broad resonance curve.
  • capacitors 50, 52 and the lengths of the transmission line 30, 32 and the length of looped sensing element 20 are chosen in accordance with the frequency of the radio frequency signal from source 26. Typical values of these lengths are given in the example below. From the foregoing it should thus be understood that in the present invention the electric field (E field), rather than the magnetic field (H field), interacts with the medium in the immediate vicinity of the looped sensing element 20. The change in dielectric characteristics caused by the presence of the material in the medium in the immediate vicinity of the looped sensing element 20 causes a change in one or more properties of the radio frequency signal passing through the looped sensing element 20.
  • E field electric field
  • H field magnetic field
  • Properties of the radio frequency signal passing through the looped sensing element 20 that may be affected by the presence of the material in the medium in the immediate vicinity of the looped sensing element 20 include: attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at a single predetermined frequency; attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs; or the change in the propagation velocity through the looped sensing element as manifested as a shift in phase of the radio frequency signal at the receiver.
  • Figure 6 is a schematic diagram of a network for detecting the magnitude of attenuation in signal amplitude of a radio frequency signal.
  • the source 26 (shown schematically as implemented by an oscillator chip) generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium.
  • the dielectric characteristics of the medium M attenuate the amplitude of the signal as received by the receiver 28 (shown schematically as implemented by a receiver chip).
  • the detection circuit 40 associated with the receiver 28 detects a change in this property of the signal in accordance with the change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe.
  • the detection circuit 40 (shown as implemented by a detector chip that produces an encoded binary output) produces an output signal 42 that is representative of the amplitude of the received signal and thus representative of the quantity of material in the medium. As shown one bit of the encoded output signal drives the indicating device 44 (implemented as a light emitting diode element). The output 42 may serve as an input to a control system.
  • Figure 7 is a functional block diagram of a network for detecting the attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs.
  • the source 26 may be implemented as a sweeping source that generates a radio frequency signal at a predetermined amplitude at each of a plurality of frequencies across a predetermined range of frequencies.
  • a voltage-controlled oscillator driven by a sawtooth waveform generator is illustrated as the source 26.
  • the source 26 may generate a signal having predetermined amplitude at each of a plurality of predetermined discrete frequencies. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium.
  • the property of the signal that changes in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is the attenuation of the signal through the probe at each frequency.
  • FIG. 8 is a functional block diagram of a network for detecting the change in the propagation velocity of the radio frequency signal through the looped sensing element 20 when material is present in the medium. This change in the propagation velocity is manifested as a shift in phase of the radio frequency signal at the receiver 28.
  • the source 26 In operation, the source 26 generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium. The propagation velocity of the signal through the probe 20 changes in accordance with the amount of material present in the medium M.
  • a reference signal carried over a transmission line 55 provides a phase reference signal directly from the source 26 to the receiver 28.
  • the detection network 40 compares the phase of the signal received from the probe 20 with the phase of the reference signal received from transmission line 55 to generate a signal representative of the amount of material in the medium.
  • EXAMPLE Example 1 A probe having a looped sensing element was constructed as shown in Figure 2.
  • a semi-rigid coaxial line stock was used, having a copper shield 128 with an outer diameter of 2.29 mm (0.09 inches), a PTFE dielectric insulation 126 with an outer diameter of 1.6 mm (0.063 inches), a silver-plated steel center conductor 124 with an outer diameter of 0.42mm (0.018").
  • the coaxial line sections 132, 134 were each 48mm (1.85") in length.
  • the probe was arranged in use in accordance with Figure 1 with capacitors 50 and 52 being three picofarads (3pF) each.
  • a Hewlett Packard model HP8553 network analyzer was used as the source 26 and receiver/detection network 28/40. The optimum frequency for this arrangement was found to be eight hundred forty four megahertz (844MHz).
  • the attenuation between the input and output of the sensor was as follows. Attenuation when probe was not inserted in any medium: -1.91 dB. When inserted in dry soil the attenuation was - 1.94 dB. When the sensor was inserted in soil saturated with water the attenuation was -4.15 dB. The attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at the single predetermined frequency (844MHz) increased by 2.21 dB from dry soil to water saturated soil.
  • Example 2 In this example the probe described in Example 1 was used. Integrated circuit components were used as the source 26 and receiver 28/detection network 40.
  • the source 26 of radio frequency energy was a model #MAX2620 integrated oscillator manufactured by Maxim Integrated Products, Sunnyvale, CA.
  • the detector 28 was a model #AD8361 Detector made by Analog Devices Inc., Norwood, MA.
  • the voltage output signal 42 from the detector 40 was measured using a recording laboratory voltmeter.
  • the oscillator of the source 26 was adjusted to generate a signal at 844 MHz.
  • the probe was placed in a pot containing soil saturated with water. A small flow of dry air was passed continuously through the soil to promote evaporation of the water.
  • the test apparatus was operated for three days to allow the soil to dry.
  • the voltage signal 42 from the detector 40 was recorded.
  • Figure 9 shows a plot of the attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at the single predetermined frequency (844MHz) versus elapsed time.
  • the voltage output 42 is about 0.45 volts.
  • the voltage 42 gradually increased to 0.9 volts when the soil completely dried.
  • FIG. 10 shown is one form of a second embodiment of an apparatus 10' in accordance with the present invention for detecting the presence of a material in a medium M.
  • the medium M is again shown disposed within the container or soil pot P.
  • the utility of this embodiment of the apparatus of the present invention may extend to any situation involving sensing a material present in a medium wherein the material has a dielectric characteristic different from that of the medium.
  • such other uses could include the sensing of moisture content of stored grain, emulsified gas in oil, moisture in wood, or liquid within another liquid or slurry.
  • the apparatus 10' includes an inductive probe in the form of a monopole sensing element 220 and an associated electronics network 225.
  • the electronics network 225 includes a radio frequency signal source 226 and a receiver 228.
  • a directional coupler 230 couples the source 226 to the monopole sensing element 220 and the sensing element to the receiver 228.
  • a detection network 232 is associated with the receiver 228 for determining a change in a property of the signal arriving at the receiver 228.
  • the detection network 232 may be implemented as either an amplitude detection circuit (Figure 14), a resonant frequency detection circuit (Figure 15), or a phase shift detection circuit ( Figure 16). Each form of detection network 232 is more fully described herein.
  • a transition network 234 to be described aids in increasing the sensitivity of the apparatus 107
  • the monopole sensing element 220 having an end 222 is implemented using a terminated transmission line 238 comprising an inner conductor 240 surrounded by a dielectric material 242 and a shielding conductor 244.
  • An exposed portion of the inner conductor 240 defines the monopole sensing element 220.
  • the exposed portion of the inner conductor may be open-circuited (as illustrated in solid lines) or may be short-circuited to the shielding conductor 244 (as illustrated in dotted lines). If the open-circuit implementation is used the transition network 234 includes one or more capacitor(s) 250 and one or more inductor(s)
  • inductor(s) 254 may be omitted from the transition network 234.
  • the transition network 234 matches the impedance of the directional coupler 230 with the transmission line 238.
  • the transmission line 238 physically spaces the electronics network 225 from the corrosive environment in which the sensing element is placed, while transmitting the radio frequency signals faithfully between the probe and the electronics. In the operating position illustrated in Figure 10 the probe 220 is inserted into or placed in contact with the medium M within the container P.
  • a property of the signal transmitted from the source 226 to the receiver 28 is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material (e.g., water) present in the medium M (e.g., soil) in the vicinity of the probe.
  • the detection network 232 generates at its output 236 a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver.
  • the signal representative of the amount of material in the medium may be transmitted to an indicating device 237, which may take the form of a liquid crystal display or a light emitting diode display.
  • the apparatus 10' using the monopole sensing element 220 may be implemented in coaxial structural form (Figure 11) or in a layered structural form ( Figure 12).
  • FIG. 11 the transmission line 238 is coaxial in structure with the inner conductor 240 concentrically surrou nded by the dielectric material 242 and the shielding conductor 244.
  • the open-circuit implementation is shown in solid lines in Figure 11 while the short circuit implementation is illustrated in dotted lines.
  • a portion of the shielding conductor 244 is removed adjacent the end of the transmission line to expose a length of the inner conductor 240 so that the shielding conductor then surrounds part of the length of the inner conductor 240.
  • the exposed length of the inner conductor 240 may be surrounded by the dielectric material 242.
  • the dielectric material 242 may also be removed leaving a bare the length of the inner conductor 240.
  • the monopole sensing element may be formed in an additive manner by surrounding a conductor over part or all of its length with a dielectric material.
  • the transmission line 238 may be constructed from a material that imparts sufficient mechanical strength to permit the monopole sensing element 220 to be insertable directly into the medium. Additional mechanical rigidity can be achieved by mounting the monopole sensing element of the probe on a planar rigid substrate (shown in dot-dash lines at reference character 266).
  • the rigid substrate may alternatively be curved (as shown in dot-dash lines at reference character 266') to facilitate placement of the monopole sensing element 220 of the probe into contact with the medium M within the curved boundary wall of the container P.
  • the source 226, the receiver 228, the directional coupler 230 and the transition network 234 may be mounted on the substrate 266, 266' with the monopole sensing element 220 thereby to form an integrated structure.
  • a source of electrical energy such as a battery, may be included within the integrated structure. When used in a greenhouse environment having bright lighting, a solar cell may supplement the battery.
  • the entire structure may be enclosed or encapsulated in a suitable dielectric material, as suggested at reference character 268.
  • the coaxial line may be made of a commercially available product commonly called "semi-rigid coax".
  • the outer shielding conductor 244 is made of a flexible grade solid copper and the center conductor 240 is made of either solid copper or silver-plated steel alloy.
  • the dielectric 242 is made of solid PTFE, which is known to have very low dielectric loss. The usage of such materials for an application like soil moisture sensing has the advantage of durability in a naturally corrosive wet soil environment.
  • the layered form of the second embodiment of the apparatus is illustrated in Figure 12. In this form of the probe 220 the transmission line 238 employs a laminar structure known in the art as "stripline".
  • the transmission line 238 comprises (from top to bottom as viewed in Figure 12): a first conducting shield layer 244A; a first dielectric layer 242A; a layer 243 including a stripe conductor 240; a second dielectric layer 242B; and a second conducting shield layer 244B.
  • the various layers may be in the form of parallel planes (as illustrated) or concentric curved surfaces.
  • the stripe conductor 240 and the dielectric layers 242A, 242B extend past an edge of the shield layers 244A, 244B to define the monopole sensing element 220.
  • the open-circuit implementation is shown in solid lines while the short-circuit implementation is illustrated in dotted lines.
  • the stripline structure may be either flexible or rigid.
  • a flexible structure is useful for placement in contact with the soil along the wall of a curved soil container P.
  • the flexible structure may be attached to wall of the soil container using a suitable adhesive. If the container has electrically non-conducting walls the monopole sensing element may be attached to the outside of the container P and still be in sufficient electromagnetic contact with the medium M to effect the desired measurement.
  • a rigid structure is useful for insertion into soil in a container.
  • a rigid structure may be achieved by mounting the monopole sensing element of the probe on a rigid substrate 266, 266' as discussed in connection with Figure 11.
  • a rigid structure may be achieved by forming the dielectric layers 242A, 242B from a glass fiber-filled polymer material or from a ceramic material, such as a low temperature co-fired ceramic.
  • FIG. 13 is an electrical equivalent circuit of the apparatus of Figure 10, whether implemented using the monopole sensing element 220 configured as in Figure 11 or Figure 12, by which the principles of operation of the present invention may be understood. It is conventionally known that an inductive monopole carrying an AC current produces an AC magnetic field in its vicinity.
  • Magnetic fields are not known to interact with dielectric media (e.g., soil) and materials (e.g., water).
  • dielectric media e.g., soil
  • materials e.g., water
  • an inductive monopole also produces stray electric fields that may, at a "high” frequency, interact with such dielectric media.
  • the interaction of such stray electric fields with the dielectric media is the principled underpinning of the operation of the present invention.
  • An excitation frequency is suitably "high” when the length dimension of the monopole sensing element is an appreciable fraction of the operating wavelength [such as from about two percent (2%) to about ten (10%) percent] of the operating wavelength.
  • a "high" frequency greater than one megahertz (1 MHz), particularly greater than one hundred megahertz (100 MHz), and more particularly, greater than five hundred megahertz (500 MHz), and specifically in the eight hundred to nine hundred megahertz range (800 to 900 MHz, the present operating range of wireless consumer electronics) has been found useful.
  • the practical upper limit of the frequency is determined by the state of the art of source and receiver electronics. Stated alternatively, it has been found that to achieve sufficient sensitivity to the dielectric characteristic of the material in the medium, the length of the monopole sensing element 20 should be at least a minimum length as dictated by the operating frequency.
  • Capacitor C typically has a small value (typically less than one picofarad) and the parallel resistance R has a typically high value (greater than tens of megohms). Therefore at low frequencies of operation the reactive impedance of this parallel combination is very large and can be ignored. At high frequencies, however, the total impedance of this capacitance/resistance combination becomes significant. Therefore the changes in the dielectric characteristics become easier to measure using the apparatus disclosed in this invention.
  • capacitor(s) 250 and inductor(s) 254 and the length of the transmission line 238 and the length of monopole sensing element 220 are chosen in accordance with the frequency of the radio frequency signal from source 226. Typical values of these parameters are given in the example below. From the foregoing it should thus be understood that in the present invention the electric field (E field), rather than the magnetic field (H field), interacts with the medium in the immediate vicinity of the monopole sensing element 220.
  • the change in dielectric characteristic caused by the presence of the material in the medium in the immediate vicinity of the monopole sensing element 220 causes a change in one or more properties of the radio frequency signal passing through the monopole sensing element 220. Properties of the radio frequency signal passing through the monopole sensing element 220 (whether implemented as shown in either
  • Figure 11 or Figure 12 that may be affected by the presence of the material in the medium in the immediate vicinity of the monopole sensing element 220 include: attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at a single predetermined frequency; attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs; or the change in the propagation velocity through the monopole sensing element as manifested as a shift in phase of the radio frequency signal at the receiver.
  • Figure 14 is a schematic diagram of a network (generally similar to that shown in Figure 6) for detecting the magnitude of attenuation in signal amplitude of a radio frequency signal.
  • the source 226 (shown schematically as implemented by an oscillator chip) generates a radio frequency signal having predetermined amplitude at a single predetermined frequency.
  • This signal is coupled by the directional coupler 230 to the monopole sensing element 220 of the probe while the probe is in contact with the medium.
  • the reflected signal from the monopole sensing element is coupled by the directional coupler 230 to the receiver 228.
  • the dielectric characteristics of the medium M attenuate the amplitude of the signal as received by the receiver 228 (shown schematically as implemented by a receiver chip).
  • the detection circuit 232 associated with the receiver 228 detects a change in this property of the signal in accordance with the change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe.
  • the detection circuit 232 (shown as implemented by a detector chip that produces an encoded binary output) produces an output signal 36 that is representative of the amplitude of the received signal and thus representative of the quantity of material in the medium. As shown one bit of the encoded output signal drives the indicating device 237
  • Figure 15 is a functional block diagram of a network for detecting the attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs.
  • the source 226 may be implemented as a sweeping source that generates a radio frequency signal at a predetermined amplitude at each of a plurality of frequencies across a predetermined range of frequencies.
  • a voltage controlled oscillator driven by a sawtooth waveform generator is illustrated as the source 226.
  • the source 226 may generate a signal having predetermined amplitude at each of a plurality of predetermined discrete frequencies. This signal is coupled into the monopole sensing element 220 of the probe while the probe is in contact with the medium.
  • the property of the signal that changes in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is the attenuation of the signal through the probe at each frequency. This change in property is manifested as a change in the frequency at which the maximum amplitude occurs at the receiver.
  • the detector circuit 232 detects the attenuation at each frequency generated by the source.
  • the detection circuit determines the frequency value at which the maximum amplitude is received. This value is compared to a stored calibration value previously obtained when the medium contains no material of interest. The magnitude of the change in frequency at which the maximum amplitude is received determines the amount of material in the medium.
  • Figure 16 is a functional block diagram of a network for detecting the change in the propagation velocity of the radio frequency signal through the monopole sensing element 220 when material is present in the medium. This change in the propagation velocity is manifested as a shift in phase of the radio frequency signal at the receiver 228.
  • the source 226 generates a radio frequency signal having predetermined amplitude at a single predetermined frequency.
  • This signal is coupled into the monopole sensing element 220 of the probe while the probe is in contact with the medium.
  • the propagation velocity of the signal through the probe 220 changes in accordance with the amount of material present in the medium M.
  • a reference signal carried over a transmission line 255 provides a phase reference signal directly from the source 226 to the receiver 228.
  • the detection network 232 compares the phase of the signal received from the probe 220 with the phase of the reference signal received from transmission line 255 to generate a signal representative of the amount of material in the medium.
  • EXAMPLE Example 3 A probe having a monopole sensing element was constructed using the open circuit configuration as shown in Figure 11.
  • a semi-rigid coaxial line stock was used, having a copper shield 244 with an outer diameter of 2.29 mm (0.09 inches), a PTFE dielectric insulation 242 with an outer diameter of 1.6 mm (0.063 inches), a silver-plated steel center conductor 240 with an outer diameter of 0.42 mm (0.018").
  • the coaxial line section 238 was 48 mm (1.85") in length.
  • the probe was arranged in use in accordance with Figure 10 with the capacitor 250 being about two picofarads (2 pF) and the inductor 254 being about three nanohenries (3 nH).
  • a Hewlett Packard model HP8553 network analyzer was used as the source 226 and receiver 228/detection network 232.
  • the optimum frequency for this arrangement was found to be eight hundred forty four Megahertz (844MHz).
  • the resonant frequency when the monopole sensing element was inserted in soil saturated with water was approximately four hundred five Megahertz (405 MHz).
  • the container was placed on a scale and allowed to air dry. As the soil dried the resonant frequency increased to a final value of approximately 430 MHz four hundred thirty Megahertz (430 MHz). This represented a moisture loss of approximately two hundred seventy grams (270 g).
  • either embodiment of the apparatus in accordance with the present invention i. e., apparatus 10, Figure 1 , having a looped sensing element 20 or apparatus 10' Figure 10, having a monopole sensing element 220
  • apparatus 10, Figure 1 having a looped sensing element 20
  • apparatus 10' Figure 10 having a monopole sensing element 220
  • an inductive probe (20, 220) is placed into contact with the medium; a radio frequency signal is passed through the probe; and a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is detected; and a signal representative of the quantity of material in the medium is generated in accordance with the change in the property of the signal.
  • the probe 20, 220 may be inserted into the medium M or, if the medium M is disposed within a container having electrically non- conducting walls, the probe 20, 220 may be placed against the exterior of the container.
  • the radio frequency signal may have a predetermined frequency and the change in the property of the signal detected is the attenuation in signal amplitude ( Figure 6 and Figure 14).
  • the radio frequency signal may have a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency ( Figure 7 and Figure 15).
  • the radio frequency signal at the receiver has a maximum amplitude at a predetermined one of the frequencies, and the change in the attenuation of the signal as a function of frequency is detected by determining a change in the frequency at which the maximum amplitude occurs.
  • the radio frequency signal sweeps across the predetermined range of frequencies.
  • the radio frequency signal has a predetermined propagation velocity through the probe, and the change in the property of the signal is a change in the propagation velocity. ( Figure 7 and Figure 16).
  • the change in the propagation velocity of the signal is detected by determining a shift in phase of the radio frequency signal.
  • FIG 18 A system aspect of the present invention is illustrated in Figure 18.
  • an apparatus 10 having an inductive probe with a looped sensing element 20 ( Figure 1 ) or an apparatus 10' having an inductive probe with a monopole sensing element 220 ( Figure 10) may be utilized in an automated irrigation system 100 for controlling the moisture content in a medium within one or more container(s) by controlling at least one solenoid actuated valve V disposed between the container(s) P and a water supply source W.
  • a communal system wherein one valve services all of the containers P is illustrated.
  • the system 100 may be configured with a dedicated valve V associated with each container.
  • Each container P is provided with an inductive probe 20/220 operative to detect a change in dielectric characteristic of the medium M due to the amount of moisture present in the vicinity of the probe.
  • the detection network 40/232 associated with each probe 20/220 generates a signal 42/236 representative of the amount of moisture in the medium, as described.
  • a valve controller 102 for controlling the operation of the valve V to supply water to a medium includes a comparator 104 for comparing the signal 42/236 from the detection network 40/232 representative of the amount of moisture in the medium M in which a given probe 20/220 is disposed with a reference signal 106 representative of a desired moisture content.
  • a valve control signal 108 is generated in accordance with the comparison and is used to control the operation of the valve V.
  • valve control signal 108 is applied a solenoid timer network 110.
  • the solenoid timer network 110 is operative to assert the valve using any one of several watering protocols.
  • the duty cycle of each protocol is illustrated in Figure 19A, 19B, 19C.
  • Figure 19A illustrates one watering protocol.
  • the solenoid timer network 110 asserts the valve V to enable water to flow from the water source W to the container(s) P for that period of time necessary for the moisture content of the medium in the container(s) to exceed the reference threshold.
  • a second watering protocol is illustrated in Figure 19B.
  • FIG. 19C illustrates a third watering protocol. This protocol is similar to that of Figure 19B. However, instead of one continuous time interval the valve V is asserted for an intermittent series of time intervals.
  • Example 4 The same sensing element as in Examples 1 and 2 was used to control soil moisture in a full cycle of a plant life from seed to fully-grown plant.
  • the output voltage 42/236 was used as an input in a comparator circuit 104 to assert a solenoid valve V for water flow when the output voltage 42/236 was above a given reference voltage 106 signifying dry soil. As a result, the soil moisture was controlled without human intervention for the period of three (3) weeks.
  • an apparatus 10 having an inductive probe with a looped sensing element 20 ( Figure 1) or an apparatus 10' having an inductive probe with a monopole sensing element 220 ( Figure 10) may be utilized to detect an interface between a first material Mi and a second material M 2 .
  • an electronics module 25/225 (shown in Figures 1 , 10, respectively) be associated with the appropriate sensing apparatus for the method under discussion.
  • the combination of the sensing apparatus 10/10' and the electronics module 25/225 defines a useful system for detecting an interface defined between a first material and a second material disposed in a stratified manner in a volume of materials.
  • a first distance range "a" is defined in which the attenuation increases at a substantial rate. This is graphically illustrated in Region I of the plot of Figure 21. The attenuation increases until the sensing element 20/220 is fully immersed in material Mi, at which time the attenuation reaches level Ai . As long as the sensing element 20/220 is within material M- ⁇ further insertion results in no further change in attenuation. As illustrated in Region II of Figure 21 this serves to define a second distance range "b" in which the attenuation has substantially no change.

Abstract

An apparatus (10, 10’) for detecting the presence of a material that causes a change in the dielectric characteristic of a medium (M). The sensing element of the probe may be a looped (20) or a monopole sensing element (220). With the probe contacting the medium (M) a property of a signal transmitted through the probe is changed in accordance with a change in dielectric characteristic of the medium (M) due to the quantity of the material present in the vicinity of the probe. The changed property of the signal may be either: the attenuation of the signal at a predetermined frequency; the attenuation of the signal amplitude at each of a plurality of predetermined frequencies; or the changes in the propagation velocity of the signal through the probe. The probe (20, 220) may be used to determine moisture in soil, for example, as part of an automated irrigation system and method for controlling the moisture content in a medium within a container (P). The probe (20, 200) may also be used to detect an interface between materials.

Description

INDUCTIVE PROBE HAVING A LOOPED SENSING ELEMENT OR A TERMINATED TRANSMISSION LINE SENSING ELEMENT AND METHODS AND SYSTEM FOR USING THE SAME BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an inductive probe apparatus having a looped sensing element or a terminated transmission line sensing element and to methods and a system for using the same, including: methods for detecting the presence of a material causing a change in the dielectric characteristics of a medium in which the material is contained; a method sensing of moisture in a granular medium and a system employing the same; and a method for detecting the interface between materials. Claim to Priority This application claims the priority of the following provisional applications: Inductive Probe Having A Looped Sensing Element, S.N. 60/531 ,031 , (CL-2469), filed December 18, 2003 in the name of Mehrdad Mehdizadeh and assigned to the assignee of the present invention; Terminated Transmission Line Probe, S.N. 60/531 ,034,(CL-2470), filed December 18, 2003 in the name of Mehrdad Mehdizadeh and assigned to the assignee of the present invention. Method of Detecting A Material In A Medium, S.N. 60/530,684, (CL- 2492), December 18, 2003 in the name of Mehrdad Mehdizadeh and assigned to the assignee of the present invention; and Irrigation System Using An Inductive Probe, S.N. 60/530,683, (CL-
2506), filed December 18, 2003 in the name of Mehrdad Mehdizadeh and assigned to the assignee of the present invention; and System For Detecting An Interface Between First And Second Strata Of Materials, S.N. 60/608,984, (CL-2783), filed September 10, 2004 in the name of Mehrdad Mehdizadeh and assigned to the assignee of the present invention. Description of Related Art Both commercial and experimental greenhouses, such as those involved in the research and development of crop protection chemicals, are in need of an apparatus to measure accurately moisture of the soil in which experimental plants are grown. Prior art devices typically utilize capacitive sensing and operate in a frequency range of up to tens of kilohertz. These prior art capacitive devices utilize at least two metallic electrodes, which are placed into contact with the medium. Current flow through the medium is measured and provides an indication of moisture content of the medium. There are several disadvantages with prior art capacitive devices. The electrodes must have a relatively large surface area to achieve sufficient contact with the medium to be measured. The electrodes also must be separated at a distance so that the medium, which is often coarse and granular, can be positioned between the electrodes. Due to these limitations the physical size of a capacitive probe is often quite large. Such a probe is not usable, for example, in a small plant pot, such as a so- called seed pot, found in experimental greenhouses. Due to the relatively low AC frequencies used the prior art devices are also subject to electrical currents produced by ionic conductivity of soils that are acidic, basic, or saline. The prior art capacitive devices must therefore be calibrated for these types of conditions to permit accurate measurement of the moisture content. The metallic electrodes of prior art capacitive devices are subject to detrimental corrosion or oxidation effects since they are required to contact the soil. To prevent corrosion the probe electrodes are typically made of relatively expensive metals and thus suffer a cost disadvantage. An apparatus capable of accurate moisture measurement, in either a continuous or an intermittent mode of operation while being insensitive to ionic conductivity would facilitate precise moisture measurement of the soil and would thus provide an important tool for facilitating improved moisture control useful for automating both commercial and experimental greenhouses. Methods using such apparatus for detecting the presence of moisture or the interface between strata of materials are also believed advantageous. SUMMARY OF THE INVENTION In a first aspect the present invention is directed to an apparatus for detecting the presence of a material that causes a change in the dielectric 5 characteristic of the medium in which the material is contained. The apparatus comprises a source of a radio frequency signal at a predetermined amplitude, an inductive probe placeable into contact with the medium, and a receiver for receiving the radio frequency signal transmitted through the probe. A detection network is associated with the0 receiver for determining a change in a property of the signal arriving at the receiver. A transmission line couples the first end of the sensing element of the probe to the source and the second end of the sensing element of the probe to the receiver. The transmission line may be a coaxial structures with the inner conductor being surrounded by the dielectric material and the shielding conductor. Alternatively, the transmission line may be a layered structure comprising an inner conductor surrounded by a dielectric material and first and second shielding conductor layers. In one embodiment of the apparatus the sensing element iso configured as a looped sensing element having a first and a second end. In a second embodiment of the apparatus the sensing element is configured as a monopole sensing element. In the case of the monopole sensing element the probe is implemented using a terminated transmission line that itself comprises an5 inner conductor surrounded by a dielectric material and a shielding conductor. An exposed portion of the inner conductor defines the monopole sensing element. - Both the looped sensing element and the monopole sensing element may be constructed as a rigid element suitable to be insertableo into the soil. Alternatively either sensing element may be flexible so that it may be placed in contact with the soil by positioning it along or attaching it to the interior wall of the container in which the soil medium is placed. The source, receiver and detection network and the sensing element structure itself may be constructed of materials and circuits that are readily available at low cost, such as those used in wireless consumer electronic devices. In use, upon placement of the probe into contact with the medium, a property of the signal transmitted from the source to the receiver is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe. The detection network generates a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver. The changed property of the signal may be either: the attenuation of the signal at a predetermined frequency; the attenuation of the signal amplitude at each of a plurality of predetermined frequencies; or, the change in the propagation velocity of the signal through the probe. The present invention facilitates accurate moisture determination in a granular medium, such as soil, in a compact, easy to use, and inexpensive device. By applying radio frequency electromagnetic waves to the medium to measure dielectric loss without making direct electrical (i.e., ohmic) contact with the medium the present invention is sensitive to water content in the medium, but is not sensitive to the ionic conductivity of the medium. If the operating frequency of the present invention is on the * order of hundreds of megahertz the apparatus may be made quite small. The sensing element of either embodiment of the apparatus of the present invention is sized such that it may be either inserted into or placed in contact with the soil in the smallest of seeding pots or seeding trays. Ionic currents are not induced in the soil since the ions in the soil are too large to respond to the frequencies of operation. ( In another aspect the present invention is directed to a method for detecting the presence of a material in a medium by detecting a change in a property of an ultrahigh frequency radio frequency signal passed through the medium. The method comprises the steps of: placing an inductive probe into contact with the medium; passing a radio frequency signal through the probe; detecting a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe; and generating a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal. The sensing element of either embodiment of the apparatus of the present invention may be used. In one embodiment of the method the radio frequency signal has a predetermined frequency and the change in the property of the signal detected is an attenuation in signal amplitude. In another embodiment the radio frequency signal has a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency. In yet another embodiment the radio frequency signal (at the receiver) has a maximum amplitude at a predetermined one of the frequencies, the change in the attenuation of the signal as a function of frequency being detected by determining a change in the frequency at which the maximum amplitude occurs. The probe may be inserted into the medium or placed against a container having electrically non-conducting walls in which the medium is disposed. In still another aspect the present invention is directed to an automated irrigation system and method for controlling the moisture content in a medium within a container. The system and method automatically controls the operation of a valve connectible to a source of water supply. Assertion of the valve supplies water to the medium. The system includes at least one inductive probe having the sensing element of either embodiment of the apparatus of the present invention. The probe is operative to detect a change in dielectric characteristic of the medium due to the amount of moisture present in the vicinity of the probe and a detection network associated with the probe for generating a signal representative of the amount of moisture in the medium. A comparator compares the signal representative of the amount of moisture in the medium with a reference signal representative of a desired moisture content and generates a control signal in accordance with the comparison. The valve may be controlled to the supply water in accordance with any one of at least three predetermined water control protocols. In yet another aspect the present invention is directed to method for detecting an interface between materials. In this aspect of the invention a sensing element in accordance with either embodiment of the apparatus of the present invention is excited by a radio frequency signal at a predetermined amplitude and progressively inserted into the volume of materials. The position of the sensing element within the volume is tracked and the attenuation of the radio frequency signal is monitored as a function of insertion distance into the volume to detect first distance ranges having a substantial rate of change of attenuation and second distance ranges having substantially no change of attenuation. The location of the interface between strata within the volume is detected by noting a transition from a second distance range to a first distance. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description taken in connection with the accompanying drawings, which form a part of this application and in which: Figure 1 is a side elevational view, partly diagrammatic and partly in section, of an apparatus in accordance with a first embodiment of the present invention that includes a probe in the form of a looped sensing element for detecting the presence of a material in a medium disposed within a container in accordance with method aspects of the present invention; Figure 2 is a perspective view of one form of the first embodiment of the apparatus of the present invention in which the looped sensing element is implemented as a coaxial structure with portions broken away for clarity of illustration; Figure 3 is a perspective view of an alternate form of the first embodiment of the apparatus of the present invention in which the looped sensing element is implemented as a layered structure disposed in a single plane with portions broken away for clarity of illustration; Figure 4 is a perspective view of another alternate form of the first embodiment of the apparatus of the present invention in which the looped sensing element is implemented as a planar structure disposed in parallel planes with portions broken away for clarity of illustration; Figure 5 is an electrical equivalent circuit useful in understanding the operation of the apparatus of Figure 1 ; Figure 6 is a schematic diagram of a circuit for detecting the magnitude of attenuation of a signal that is useful with any form of the first embodiment embodiment of the invention shown in Figures 2, 3, or 4; Figure 7 is a functional block diagram of a circuit for detecting the shift in resonant frequency of a signal that is useful with any form of the first embodiment of the invention shown in Figures 2, 3, or 4; Figure 8 is a functional block diagram of a circuit for detecting the phase shift of a signal that is useful with any form of the first embodiment of the invention shown in Figures 2, 3, or 4; Figure 9 is a plot of the detector voltage output versus time using the apparatus in accordance with Figure 1 to measure water-saturated soil as it dries as described in Example 2; Figure 10 is a side elevational view, partly diagrammatic and partly in section, of a second embodiment of an apparatus in accordance with the present invention that includes a probe having a monopole sensing element for detecting the presence of a material in a medium with method aspects of the present invention; Figure 11 is a perspective view of one form of the second embodiment of the apparatus of the present invention in which the monopole sensing element is implemented as a coaxial structure with portions broken away for clarity of illustration; Figure 12 is a perspective view of an alternate form of the second embodiment of the apparatus of the present invention in which the monopole sensing element is implemented as a layered structure layered structure with portions broken away for clarity of illustration; Figure 13 is an electrical equivalent circuit useful in understanding the operation of the apparatus of Figure 10; Figure 14 is a schematic diagram of a circuit for detecting the magnitude of attenuation of a signal that is useful with either form of the second embodiment of the invention shown in Figures 11 or 12; Figure 15 is a functional block diagram of a circuit for detecting the shift in resonant frequency of a signal that is useful with either form of the second embodiment of the invention shown in Figures 11 or 12; Figure 16 is a functional block of a circuit for detecting the phase shift of a signal that is useful with either form of the second embodiment of the invention shown in Figures 11 or 12; and Figure 17 is a plot of resonant frequency versus soil moisture loss, using the apparatus of Figure 10 measuring water-saturated soil as it dried; Figure 18 is a functional block diagram of an automated irrigation system in accordance with a system aspect of the present invention; Figures 19A, 19B, 19C are plots showing various watering protocols able to be implemented with the system of Figure 18; Figures 20A, 20B are schematic views showing a generic representation of the sensing element of an apparatus having either a looped sensing element (Figure 1) or a monopole sensing element (Figure 10) in use in accordance with another method aspect of the present invention to detect an interface between first and second materials Mi, M2 respectively, disposed in a stratified manner in a volume of materials, where the sensing apparatus is inserted progressively into the volume; and Figure 21 is a plot showing the attenuation of a radio frequency signal passing though the apparatus as a function of insertion distance. DETAILED DESCRIPTION OF THE INVENTION Throughout the following detailed description similar reference characters refer to similar elements in all figures of the drawings. With reference to Figure 1 shown is one form of a first embodiment of an apparatus 10 in accordance with of the present invention for detecting the presence of a material in a medium M. The medium M is shown disposed within a container or soil pot P. The container P may have planar or curved boundary walls. Although the invention will be discussed in the context of the sensing of moisture (material) in soil (medium), it should be understood that the utility of the present invention may extend to any situation involving sensing a material present in a medium wherein the material has a dielectric characteristic different from that of the medium. For example, such other uses could include the sensing of moisture content of stored grain, emulsified gas in oil, moisture in wood, or liquid within another liquid or slurry. The apparatus 10 includes an inductive probe in the form of a looped sensing element 20 (generically illustrated as a single turn of conductor) and associated electronics network 25. The looped sensing element 20 has a first end 22 and a second end 24. The electronics network 25 comprises a radio frequency signal source 26 and a receiver 28. The first end 22 of the looped sensing element 20 is coupled to the signal source 26 by a first transmission line 30. The second end 24 of the looped sensing element 20 is coupled to the receiver 28 by a second transmission line 32. If the source 26 and/or receiver 28 are located remotely from the looped sensing element 20, respective transmission lines 34, 36 may be provided. A detection network 40 is associated with the receiver 28 for determining a change in a property of the signal arriving at the receiver 28. The detection network 40 may be implemented as either an amplitude detection circuit (Figure 6), a resonant frequency detection circuit (Figure 7), or a phase shift detection circuit 48 (Figure 8). Each form of detection network 40 is more fully described herein. Optional capacitors 50 and 52 aid in increasing the sensitivity of the apparatus 10 by matching the impedance of the source 26 to transmission line 30 and matching the impedance of transmission line 32 to the receiver 28. In the operating position illustrated in Figure 1 the probe 20 is inserted into or placed in contact with the medium M within the container P. A property of the signal transmitted from the source 26 to the receiver 28 is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material (e.g., water) present in the medium M (e.g., soil) in the vicinity of the probe. The detection network 40 generates at its output 42 a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver. The signal representative of the amount of material in the medium may be transmitted to an indicating device 44, which may take the form of a liquid crystal display or a light emitting diode display. The looped sensing element 20 may be implemented in a coaxial structural form (Figure 2) or layered structural forms (Figures 3 or 4). Figure 2 shows a perspective view of the inductive probe having a looped sensing element 20 exhibiting a coaxial structure. The inductive probe is formed from a length of coaxial transmission line having a center conductor 124, a dielectric layer 126 and an outer shielding conductor 128. It should be understood that the portion of Figure 2 identified by the bracketed numeral 122 is broken-away solely for considerations of clarity to illustrate the construction of the inductive probe. The looped sensing element 20 is formed by removing the outer shielding conductor 128 from a central portion of the length of the transmission line 122 thereby to expose a region 130 of the center conductor 124 surrounded by the dielectric layer 126. The remaining shielded portions 132, 134 define the respective first and second transmission lines 30, 32 that couple the respective ends 22, 24 of the looped sensing element 20 to the source 26 and receiver 28 (Figure 1). To provide mechanical rigidity the shielded portions 132, 134 are arranged in parallel and attached together, as by soldering the outer shields. Alternatively, mechanical rigidity can be achieved by mounting the looped sensing element 20 of the probe on a planar rigid substrate (shown in dot-dash lines at reference character 136). The rigid substrate may alternatively be curved (as shown in dot-dash lines at reference character 136') to facilitate placement of the looped sensing element 20 of the probe into contact with the medium within the curved boundary wall of the container P. Further, the electronics network 25 (comprising the source 26 and the receiver 28) may be mounted on the substrate 136, 136' with the looped sensing element 20 thereby to form an integrated structure. A source of electrical energy, such as a battery, may be included within the integrated structure. When used in a greenhouse environment having bright lighting a solar cell may supplement the battery. The entire structure may be enclosed or encapsulated in a suitable dielectric material, as suggested at reference character 138. In the coaxial embodiment of Figure 2 the coaxial line may be made of a commercially available product commonly called "semi-rigid coax". The outer shielding conductor 128 is made of a flexible grade solid copper and the center conductor 124 is made of either solid copper or silver- plated steel alloy. The dielectric layer 126 is made of solid PTFE, which is known to have very low dielectric loss. The usage of such materials for and application like soil moisture sensing has the advantage of durability in a naturally corrosive wet soil environment. Furthermore, attached shielded portions provide a robust mechanism to space the electronics network 25 from the corrosive environment in which the sensing element is placed while transmitting the radio frequency signals faithfully between the probe and the electronics network 25. The lengths of the remaining shielded portions 132, 134 are preferably equal and are determined by the mechanical requirements of the apparatus. Figures 3 and 4 show respective perspective views of alternative forms of the first embodiment of the apparatus of the invention in which the looped sensing element of the inductive probe 20 has a layered structure. The probe in each Figure employs a structure known in the art as "stripline". Referring first to Figure 3 the transmission lines 30, 32 (again with some portions broken-away for clarity) are implemented in a laminar structure comprising (from top to bottom as viewed in Figure 3): a first conducting shield layer 140; a first dielectric layer 142; a layer 143 including both first and second stripe conductors 144, 146; a second dielectric layer 148; and a second conducting shield layer 150. The various layers may be in the form of parallel planes (as illustrated) or concentric curved (e.g., cylindrical) surfaces. An additional electromagnetic shielding conductor 152 is disposed between the stripe conductors 144, 146 to isolate the conductor 144 included in the transmission line 30 from the conductor 146 included in the transmission line 32. This conductor 152 should be connected to the shield layers 140, 150. Alternatively, the inner signal stripe conductors 144, 146 may be spaced from each other a sufficient lateral distance to isolate the conductors 144, 146. The first and second stripe conductors 144, 146 and the dielectric layers 142, 148 all extend past an edge of the shield layers 140, 150. The extending portions of the conductors 144, 146 are joined between the ends 22, 24 to form the looped sensing element 20. As shown in Figure 3 the stripline structure is preferably symmetrical about the layer 143 containing the inner signal stripe conductors 144, 146. The lengths of the remaining shielded portions 132, 134 are preferably equal and are determined by the mechanical requirements of the apparatus. The stripline structure may be either flexible or rigid. A flexible structure is useful for placement in contact with the soil along the wall of a curved wall soil container P. The flexible structure may be attached to wall of the soil container P using a suitable adhesive. If the container has electrically non-conducting walls the looped sensing element may be attached to the outside of the container P and still be in sufficient electromagnetic contact with the medium to effect the desired measurement. A rigid structure is useful for insertion into soil in a container. A rigid structure may be achieved by mounting the looped sensing element of the probe on a rigid substrate 136, 136' as discussed in connection with Figure 2. Alternatively, a rigid structure may be achieved by forming the dielectric layers 142, 148 from a glass fiber-filled polymer material or from a ceramic material, such as a low temperature co-fired ceramic. These materials (which may be planar) are sufficiently stiff to obviate the need for a separate rigid substrate. Further, the source 26 and the receiver 28 may be mounted on the substrate with the looped sensing element 20 thereby to form an integrated structure. Surface mount technology is preferably used. If desired the entire structure may be enclosed or encapsulated in a suitable dielectric material 138. Figure 4 illustrates a stacked layer stripline structure in which the first and second signal conductors 144, 146 are disposed on respective separate layers 143A, 143B separated by electromagnetic shielding layer 154 (functionally equivalent to the conductor 152 in the embodiment of Figure 3). Again some portions of the Figure are broken-away for clarity. A dielectric layer 156A, 156B is respectively interposed between the ground conductor layer 154 and each signal layer 143A, 143B. Dielectric layers 142, 148 are respectively disposed above and below the first separate layers 143A, 143B. Outer shielding conductor layers 140, 150 are respectively disposed above and below the dielectric layers 142, 148. Thus, the stacked layer stripline structure shown in Figure 4 comprises, in order from top to bottom: a first outer shielded conductor layer 140; a first dielectric layer 142; a layer 143A including a first inner signal conductor 144; a second dielectric layer 156A; a ground conductor layer 154; a third dielectric layer 156B; a layer 143B including a second inner signal conductor 146; a fourth dielectric layer 148; and a second outer shielded conductor layer 150. The electromagnetic shielding layer 154 is electrically connected to the shield layers 140, 150. The first and second stripe conductors 144, 146 and the dielectric layers 142, 156A, 156B, and 148 all extend past an edge of the shield layers 140, 150. The extending portions of the conductors 144, 146 are joined between the ends 22, 24 to form a vertically oriented looped sensing element 20. The first transmission line 30 connected to the first end 22 of the looped sensing element 20 is formed from the combination of the stripe conductor 144, surrounding dielectric layers 142 and 156A, shielding layer 140 and ground conductor layer 154. The second transmission line 32 connected to the second end 24 of the looped sensing element 20 is formed from the combination of the stripe conductor 146, surrounding dielectric layers 148 and 156B, shielding layer 150 and ground conductor layer 154. Preferably the layers are planar in form, although they may be implemented as concentric curved surfaces. The structure may be flexible or rigid, enclosed in a suitable dielectric material 138 and integrated with the electronics network 25 (not shown in Figure 4), all as described in connection with Figures 2 and 3. Figure 5 is an electrical equivalent circuit of the apparatus of Figure 1 , whether implemented using the looped sensing element 20 configured as in Figures 2, 3 or 4, by which the principles of operation of the present invention may be understood. The transmission line sections 30, 32 are connected to the looped sensing element 20. It is conventionally known that an inductive loop carrying an AC current produces an AC magnetic field in its vicinity. Magnetic fields are not known to interact with dielectric media (e.g., soil) and materials (e.g., water). However, an inductive loop also produces stray electric fields that may, at a "high" frequency, interact with such dielectric media. The interaction of such stray electric fields with the dielectric media is the principled underpinning of the operation of the present invention. An excitation frequency is suitably "high" when the length dimension (from end 22 to end 24) of the looped sensing element 20 is at least an appreciable fraction [such as from about two percent (2%) to about ten (10%) percent] of the operating wavelength. A "high" frequency greater than one megahertz (1 MHz), particularly greater than one hundred megahertz (100 MHz), and more particularly, greater than five hundred megahertz (500 MHz), and specifically in the eight hundred to nine hundred megahertz range (800 to 900 MHz, the present operating range of wireless consumer electronics) has been found useful. The practical upper limit of the frequency is determined by the state of the art of source and receiver electronics. Stated alternatively, it has been found that to achieve sufficient sensitivity to the dielectric characteristic of the material in the medium, the length of the loop-sensing element 20 should be at least a minimum length as dictated by the operating frequency. At a typical frequency in the eight hundred to nine hundred megahertz range (800 to 900 MHz) a loop length of about two (2) to four (4) centimeters has been found sufficient. This stray electric field can be represented in the equivalent circuit of Figure 5 as a stray capacitance, represented by capacitor C. The leakage current due to the dielectric characteristic of the medium M (in this case the dielectric loss) is represented as a leakage resistance R in parallel with the capacitor C. The inherent inductance of the conductors is indicated by the reference character L. In Figure 5 both the leakage resistance R and the stray capacitance C are shown as variable elements since they change in accordance with the amount of the material in the medium M. Capacitor C typically has a small value (typically less than one picofarad) and the parallel resistance R has a typically high value (greater than tens of megohms). Therefore at low frequencies of operation the reactive impedance of this parallel combination is very large and can be ignored. At high frequencies, however, the total impedance of this capacitance/resistance combination becomes significant. Therefore the changes in the dielectric characteristics become easier to measure using the apparatus disclosed in this invention. The combination of capacitors 50 and 52 with the transmission line portions 30, 32 and with the total impedance L, C, R of the looped sensing element 20 cooperate to form a resonant circuit having a broad resonance curve. The values of capacitors 50, 52 and the lengths of the transmission line 30, 32 and the length of looped sensing element 20 are chosen in accordance with the frequency of the radio frequency signal from source 26. Typical values of these lengths are given in the example below. From the foregoing it should thus be understood that in the present invention the electric field (E field), rather than the magnetic field (H field), interacts with the medium in the immediate vicinity of the looped sensing element 20. The change in dielectric characteristics caused by the presence of the material in the medium in the immediate vicinity of the looped sensing element 20 causes a change in one or more properties of the radio frequency signal passing through the looped sensing element 20. Properties of the radio frequency signal passing through the looped sensing element 20 (whether implemented as shown in either Figures 2, 3 or 4) that may be affected by the presence of the material in the medium in the immediate vicinity of the looped sensing element 20 include: attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at a single predetermined frequency; attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs; or the change in the propagation velocity through the looped sensing element as manifested as a shift in phase of the radio frequency signal at the receiver. Figure 6 is a schematic diagram of a network for detecting the magnitude of attenuation in signal amplitude of a radio frequency signal. In operation, the source 26 (shown schematically as implemented by an oscillator chip) generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium. The dielectric characteristics of the medium M attenuate the amplitude of the signal as received by the receiver 28 (shown schematically as implemented by a receiver chip). The detection circuit 40 associated with the receiver 28 detects a change in this property of the signal in accordance with the change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe. The detection circuit 40 (shown as implemented by a detector chip that produces an encoded binary output) produces an output signal 42 that is representative of the amplitude of the received signal and thus representative of the quantity of material in the medium. As shown one bit of the encoded output signal drives the indicating device 44 (implemented as a light emitting diode element). The output 42 may serve as an input to a control system. Figure 7 is a functional block diagram of a network for detecting the attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs. In operation, as shown, the source 26 may be implemented as a sweeping source that generates a radio frequency signal at a predetermined amplitude at each of a plurality of frequencies across a predetermined range of frequencies. A voltage-controlled oscillator driven by a sawtooth waveform generator is illustrated as the source 26. Alternatively, the source 26 may generate a signal having predetermined amplitude at each of a plurality of predetermined discrete frequencies. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium. The property of the signal that changes in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is the attenuation of the signal through the probe at each frequency. This change in property is manifested as a change in the frequency at which the maximum amplitude occurs at the receiver. The detector circuit 40 detects the attenuation at each frequency generated by the source. The detection circuit determines the frequency value at which the maximum amplitude is received. This value is compared to a stored calibration value previously obtained when the medium contains no material of interest. The magnitude of the change in frequency values determines the amount of material in the medium. Figure 8 is a functional block diagram of a network for detecting the change in the propagation velocity of the radio frequency signal through the looped sensing element 20 when material is present in the medium. This change in the propagation velocity is manifested as a shift in phase of the radio frequency signal at the receiver 28. In operation, the source 26 generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is transmitted through the looped sensing element 20 of the probe while the probe is in contact with the medium. The propagation velocity of the signal through the probe 20 changes in accordance with the amount of material present in the medium M. A reference signal carried over a transmission line 55 provides a phase reference signal directly from the source 26 to the receiver 28. The detection network 40 compares the phase of the signal received from the probe 20 with the phase of the reference signal received from transmission line 55 to generate a signal representative of the amount of material in the medium. EXAMPLE Example 1 : A probe having a looped sensing element was constructed as shown in Figure 2. A semi-rigid coaxial line stock was used, having a copper shield 128 with an outer diameter of 2.29 mm (0.09 inches), a PTFE dielectric insulation 126 with an outer diameter of 1.6 mm (0.063 inches), a silver-plated steel center conductor 124 with an outer diameter of 0.42mm (0.018"). The coaxial line sections 132, 134 were each 48mm (1.85") in length. The probe was arranged in use in accordance with Figure 1 with capacitors 50 and 52 being three picofarads (3pF) each. A Hewlett Packard model HP8553 network analyzer was used as the source 26 and receiver/detection network 28/40. The optimum frequency for this arrangement was found to be eight hundred forty four megahertz (844MHz). The attenuation between the input and output of the sensor was as follows. Attenuation when probe was not inserted in any medium: -1.91 dB. When inserted in dry soil the attenuation was - 1.94 dB. When the sensor was inserted in soil saturated with water the attenuation was -4.15 dB. The attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at the single predetermined frequency (844MHz) increased by 2.21 dB from dry soil to water saturated soil. Example 2: In this example the probe described in Example 1 was used. Integrated circuit components were used as the source 26 and receiver 28/detection network 40. Referring to Figure 6, the source 26 of radio frequency energy was a model #MAX2620 integrated oscillator manufactured by Maxim Integrated Products, Sunnyvale, CA. The detector 28 was a model #AD8361 Detector made by Analog Devices Inc., Norwood, MA. The voltage output signal 42 from the detector 40 was measured using a recording laboratory voltmeter. The oscillator of the source 26 was adjusted to generate a signal at 844 MHz. The probe was placed in a pot containing soil saturated with water. A small flow of dry air was passed continuously through the soil to promote evaporation of the water. The test apparatus was operated for three days to allow the soil to dry. The voltage signal 42 from the detector 40 was recorded. Figure 9 shows a plot of the attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at the single predetermined frequency (844MHz) versus elapsed time. As seen from Figure 9, at the beginning, when the soil is saturated with water the voltage output 42 is about 0.45 volts. As the soil dried over the period of three (3) days, the voltage 42 gradually increased to 0.9 volts when the soil completely dried.
-o-O-o- With reference to Figure 10 shown is one form of a second embodiment of an apparatus 10' in accordance with the present invention for detecting the presence of a material in a medium M. The medium M is again shown disposed within the container or soil pot P. It should be again understood that although the invention will be d iscussed in the context of the sensing of moisture (material) in soil (medium), the utility of this embodiment of the apparatus of the present invention may extend to any situation involving sensing a material present in a medium wherein the material has a dielectric characteristic different from that of the medium. For example, such other uses could include the sensing of moisture content of stored grain, emulsified gas in oil, moisture in wood, or liquid within another liquid or slurry. The apparatus 10' includes an inductive probe in the form of a monopole sensing element 220 and an associated electronics network 225. The electronics network 225 includes a radio frequency signal source 226 and a receiver 228. A directional coupler 230 couples the source 226 to the monopole sensing element 220 and the sensing element to the receiver 228. A detection network 232 is associated with the receiver 228 for determining a change in a property of the signal arriving at the receiver 228. The detection network 232 may be implemented as either an amplitude detection circuit (Figure 14), a resonant frequency detection circuit (Figure 15), or a phase shift detection circuit (Figure 16). Each form of detection network 232 is more fully described herein. A transition network 234 to be described aids in increasing the sensitivity of the apparatus 107 The monopole sensing element 220 having an end 222 is implemented using a terminated transmission line 238 comprising an inner conductor 240 surrounded by a dielectric material 242 and a shielding conductor 244. An exposed portion of the inner conductor 240 defines the monopole sensing element 220. The exposed portion of the inner conductor may be open-circuited (as illustrated in solid lines) or may be short-circuited to the shielding conductor 244 (as illustrated in dotted lines). If the open-circuit implementation is used the transition network 234 includes one or more capacitor(s) 250 and one or more inductor(s)
254 connected in parallel between the directional coupler 230 and the terminated transmission line 238. Alternatively, if the short-circuit implementation is used inductor(s) 254 may be omitted from the transition network 234. The transition network 234 matches the impedance of the directional coupler 230 with the transmission line 238. The transmission line 238 physically spaces the electronics network 225 from the corrosive environment in which the sensing element is placed, while transmitting the radio frequency signals faithfully between the probe and the electronics. In the operating position illustrated in Figure 10 the probe 220 is inserted into or placed in contact with the medium M within the container P. A property of the signal transmitted from the source 226 to the receiver 28 is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material (e.g., water) present in the medium M (e.g., soil) in the vicinity of the probe. The detection network 232 generates at its output 236 a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver. The signal representative of the amount of material in the medium may be transmitted to an indicating device 237, which may take the form of a liquid crystal display or a light emitting diode display. The apparatus 10' using the monopole sensing element 220 may be implemented in coaxial structural form (Figure 11) or in a layered structural form (Figure 12). As with Figures 2, 3 and 4, portions of Figures 11 and 12 are omitted for clarity of illustration. In the form shown in Figure 11 the transmission line 238 is coaxial in structure with the inner conductor 240 concentrically surrou nded by the dielectric material 242 and the shielding conductor 244. The open-circuit implementation is shown in solid lines in Figure 11 while the short circuit implementation is illustrated in dotted lines. A portion of the shielding conductor 244 is removed adjacent the end of the transmission line to expose a length of the inner conductor 240 so that the shielding conductor then surrounds part of the length of the inner conductor 240. To provide mechanical and chemical protection the exposed length of the inner conductor 240 may be surrounded by the dielectric material 242. Alternatively, the dielectric material 242 may also be removed leaving a bare the length of the inner conductor 240. The monopole sensing element may be formed in an additive manner by surrounding a conductor over part or all of its length with a dielectric material. The transmission line 238 may be constructed from a material that imparts sufficient mechanical strength to permit the monopole sensing element 220 to be insertable directly into the medium. Additional mechanical rigidity can be achieved by mounting the monopole sensing element of the probe on a planar rigid substrate (shown in dot-dash lines at reference character 266). The rigid substrate may alternatively be curved (as shown in dot-dash lines at reference character 266') to facilitate placement of the monopole sensing element 220 of the probe into contact with the medium M within the curved boundary wall of the container P. Further, the source 226, the receiver 228, the directional coupler 230 and the transition network 234 may be mounted on the substrate 266, 266' with the monopole sensing element 220 thereby to form an integrated structure. A source of electrical energy, such as a battery, may be included within the integrated structure. When used in a greenhouse environment having bright lighting, a solar cell may supplement the battery. The entire structure may be enclosed or encapsulated in a suitable dielectric material, as suggested at reference character 268. In the coaxial form of the second embodiment of the apparatus shown in Figure 11 the coaxial line may be made of a commercially available product commonly called "semi-rigid coax". The outer shielding conductor 244 is made of a flexible grade solid copper and the center conductor 240 is made of either solid copper or silver-plated steel alloy. The dielectric 242 is made of solid PTFE, which is known to have very low dielectric loss. The usage of such materials for an application like soil moisture sensing has the advantage of durability in a naturally corrosive wet soil environment. The layered form of the second embodiment of the apparatus is illustrated in Figure 12. In this form of the probe 220 the transmission line 238 employs a laminar structure known in the art as "stripline". In this form the transmission line 238 comprises (from top to bottom as viewed in Figure 12): a first conducting shield layer 244A; a first dielectric layer 242A; a layer 243 including a stripe conductor 240; a second dielectric layer 242B; and a second conducting shield layer 244B. The various layers may be in the form of parallel planes (as illustrated) or concentric curved surfaces. The stripe conductor 240 and the dielectric layers 242A, 242B extend past an edge of the shield layers 244A, 244B to define the monopole sensing element 220. Again, as in Figure 11 , the open-circuit implementation is shown in solid lines while the short-circuit implementation is illustrated in dotted lines. The stripline structure may be either flexible or rigid. A flexible structure is useful for placement in contact with the soil along the wall of a curved soil container P. The flexible structure may be attached to wall of the soil container using a suitable adhesive. If the container has electrically non-conducting walls the monopole sensing element may be attached to the outside of the container P and still be in sufficient electromagnetic contact with the medium M to effect the desired measurement. A rigid structure is useful for insertion into soil in a container. A rigid structure may be achieved by mounting the monopole sensing element of the probe on a rigid substrate 266, 266' as discussed in connection with Figure 11. Alternatively, a rigid structure may be achieved by forming the dielectric layers 242A, 242B from a glass fiber-filled polymer material or from a ceramic material, such as a low temperature co-fired ceramic. These materials (which may be planar) are sufficiently stiff to obviate the need for a separate rigid substrate. Again, the source 226, the receiver 228, the directional coupler 230 and the transition network 234 may be mounted on the substrate 266, 266' with the monopole sensing element 220 thereby to form an integrated structure. Surface mount technology is preferably used. Again, if desired, the entire structure may be enclosed or encapsulated in a suitable dielectric material 268. Figure 13 is an electrical equivalent circuit of the apparatus of Figure 10, whether implemented using the monopole sensing element 220 configured as in Figure 11 or Figure 12, by which the principles of operation of the present invention may be understood. It is conventionally known that an inductive monopole carrying an AC current produces an AC magnetic field in its vicinity. Magnetic fields are not known to interact with dielectric media (e.g., soil) and materials (e.g., water). However, an inductive monopole also produces stray electric fields that may, at a "high" frequency, interact with such dielectric media. The interaction of such stray electric fields with the dielectric media is the principled underpinning of the operation of the present invention. An excitation frequency is suitably "high" when the length dimension of the monopole sensing element is an appreciable fraction of the operating wavelength [such as from about two percent (2%) to about ten (10%) percent] of the operating wavelength. A "high" frequency greater than one megahertz (1 MHz), particularly greater than one hundred megahertz (100 MHz), and more particularly, greater than five hundred megahertz (500 MHz), and specifically in the eight hundred to nine hundred megahertz range (800 to 900 MHz, the present operating range of wireless consumer electronics) has been found useful. The practical upper limit of the frequency is determined by the state of the art of source and receiver electronics. Stated alternatively, it has been found that to achieve sufficient sensitivity to the dielectric characteristic of the material in the medium, the length of the monopole sensing element 20 should be at least a minimum length as dictated by the operating frequency. At a typical frequency in the eight hundred to nine hundred megahertz range (800 to 900 MHz) a monopole length of about two (2) to four (4) centimeters has been found sufficient. This stray electric field can be represented in the equivalent circuit of Figure 13 as a stray capacitance, represented by capacitor C. The leakage current due to the dielectric characteristic of the medium M (in this case the dielectric loss) is represented as a leakage resistance R in parallel with the capacitor C. The inherent inductance of the conductors is indicated by the reference character L. In Figure 13 both the leakage resistance R and the stray capacitance C are shown as variable elements since they change in accordance with the amount of the material in the medium M. Capacitor C typically has a small value (typically less than one picofarad) and the parallel resistance R has a typically high value (greater than tens of megohms). Therefore at low frequencies of operation the reactive impedance of this parallel combination is very large and can be ignored. At high frequencies, however, the total impedance of this capacitance/resistance combination becomes significant. Therefore the changes in the dielectric characteristics become easier to measure using the apparatus disclosed in this invention. The combination of the capacitor(s) 250, inductor(s) 254 and the transmission line 238 with the total impedance of the capacitor C and the leakage resistance R of the monopole sensing element 220, cooperate to form a resonant circuit having a broad resonance curve. The values of capacitor(s) 250 and inductor(s) 254 and the length of the transmission line 238 and the length of monopole sensing element 220 are chosen in accordance with the frequency of the radio frequency signal from source 226. Typical values of these parameters are given in the example below. From the foregoing it should thus be understood that in the present invention the electric field (E field), rather than the magnetic field (H field), interacts with the medium in the immediate vicinity of the monopole sensing element 220. The change in dielectric characteristic caused by the presence of the material in the medium in the immediate vicinity of the monopole sensing element 220 causes a change in one or more properties of the radio frequency signal passing through the monopole sensing element 220. Properties of the radio frequency signal passing through the monopole sensing element 220 (whether implemented as shown in either
Figure 11 or Figure 12) that may be affected by the presence of the material in the medium in the immediate vicinity of the monopole sensing element 220 include: attenuation in signal amplitude (at the receiver) of a radio frequency signal having a predetermined amplitude at a single predetermined frequency; attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs; or the change in the propagation velocity through the monopole sensing element as manifested as a shift in phase of the radio frequency signal at the receiver. Figure 14 is a schematic diagram of a network (generally similar to that shown in Figure 6) for detecting the magnitude of attenuation in signal amplitude of a radio frequency signal. In operation, the source 226 (shown schematically as implemented by an oscillator chip) generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is coupled by the directional coupler 230 to the monopole sensing element 220 of the probe while the probe is in contact with the medium. The reflected signal from the monopole sensing element is coupled by the directional coupler 230 to the receiver 228. The dielectric characteristics of the medium M attenuate the amplitude of the signal as received by the receiver 228 (shown schematically as implemented by a receiver chip). The detection circuit 232 associated with the receiver 228 detects a change in this property of the signal in accordance with the change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe. The detection circuit 232 (shown as implemented by a detector chip that produces an encoded binary output) produces an output signal 36 that is representative of the amplitude of the received signal and thus representative of the quantity of material in the medium. As shown one bit of the encoded output signal drives the indicating device 237
(implemented as a light emitting diode element). The output 236 may serve as an input to a control system, as will be developed in accordance with a system aspect of the present invention (Figures 18, 19A, 19B and 19C). Figure 15 is a functional block diagram of a network for detecting the attenuation of the amplitude (at the receiver) of a radio frequency signal as a function of frequency over a plurality of predetermined frequencies as manifested as a change in the frequency at which the maximum amplitude occurs. In operation, as shown, the source 226 may be implemented as a sweeping source that generates a radio frequency signal at a predetermined amplitude at each of a plurality of frequencies across a predetermined range of frequencies. A voltage controlled oscillator driven by a sawtooth waveform generator is illustrated as the source 226. Alternatively, the source 226 may generate a signal having predetermined amplitude at each of a plurality of predetermined discrete frequencies. This signal is coupled into the monopole sensing element 220 of the probe while the probe is in contact with the medium. The property of the signal that changes in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is the attenuation of the signal through the probe at each frequency. This change in property is manifested as a change in the frequency at which the maximum amplitude occurs at the receiver. The detector circuit 232 detects the attenuation at each frequency generated by the source. The detection circuit determines the frequency value at which the maximum amplitude is received. This value is compared to a stored calibration value previously obtained when the medium contains no material of interest. The magnitude of the change in frequency at which the maximum amplitude is received determines the amount of material in the medium. Figure 16 is a functional block diagram of a network for detecting the change in the propagation velocity of the radio frequency signal through the monopole sensing element 220 when material is present in the medium. This change in the propagation velocity is manifested as a shift in phase of the radio frequency signal at the receiver 228. In operation, the source 226 generates a radio frequency signal having predetermined amplitude at a single predetermined frequency. This signal is coupled into the monopole sensing element 220 of the probe while the probe is in contact with the medium. The propagation velocity of the signal through the probe 220 changes in accordance with the amount of material present in the medium M. A reference signal carried over a transmission line 255 provides a phase reference signal directly from the source 226 to the receiver 228. The detection network 232 compares the phase of the signal received from the probe 220 with the phase of the reference signal received from transmission line 255 to generate a signal representative of the amount of material in the medium.
EXAMPLE Example 3: A probe having a monopole sensing element was constructed using the open circuit configuration as shown in Figure 11. A semi-rigid coaxial line stock was used, having a copper shield 244 with an outer diameter of 2.29 mm (0.09 inches), a PTFE dielectric insulation 242 with an outer diameter of 1.6 mm (0.063 inches), a silver-plated steel center conductor 240 with an outer diameter of 0.42 mm (0.018"). The coaxial line section 238 was 48 mm (1.85") in length. The probe was arranged in use in accordance with Figure 10 with the capacitor 250 being about two picofarads (2 pF) and the inductor 254 being about three nanohenries (3 nH). A Hewlett Packard model HP8553 network analyzer was used as the source 226 and receiver 228/detection network 232. The optimum frequency for this arrangement was found to be eight hundred forty four Megahertz (844MHz). As seen from Figure 17 the resonant frequency when the monopole sensing element was inserted in soil saturated with water was approximately four hundred five Megahertz (405 MHz). The container was placed on a scale and allowed to air dry. As the soil dried the resonant frequency increased to a final value of approximately 430 MHz four hundred thirty Megahertz (430 MHz). This represented a moisture loss of approximately two hundred seventy grams (270 g).
-o-O-o-
As should be appreciated from the foregoing discussion and Examples, either embodiment of the apparatus in accordance with the present invention (i. e., apparatus 10, Figure 1 , having a looped sensing element 20 or apparatus 10' Figure 10, having a monopole sensing element 220) may be used to implement a method aspect of the present invention to detect the presence, in a medium, of a material that causes a change in dielectric characteristic of the medium. In accordance with the method aspect of the present invention: an inductive probe (20, 220) is placed into contact with the medium; a radio frequency signal is passed through the probe; and a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe is detected; and a signal representative of the quantity of material in the medium is generated in accordance with the change in the property of the signal. The probe 20, 220 may be inserted into the medium M or, if the medium M is disposed within a container having electrically non- conducting walls, the probe 20, 220 may be placed against the exterior of the container. The radio frequency signal may have a predetermined frequency and the change in the property of the signal detected is the attenuation in signal amplitude (Figure 6 and Figure 14). Alternatively, the radio frequency signal may have a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency (Figure 7 and Figure 15). In one case the radio frequency signal at the receiver has a maximum amplitude at a predetermined one of the frequencies, and the change in the attenuation of the signal as a function of frequency is detected by determining a change in the frequency at which the maximum amplitude occurs. In another case the radio frequency signal sweeps across the predetermined range of frequencies. As yet another alternative the radio frequency signal has a predetermined propagation velocity through the probe, and the change in the property of the signal is a change in the propagation velocity. (Figure 7 and Figure 16). The change in the propagation velocity of the signal is detected by determining a shift in phase of the radio frequency signal.
-o-O-o-
A system aspect of the present invention is illustrated in Figure 18. As shown in Figure 18 either an apparatus 10 having an inductive probe with a looped sensing element 20 (Figure 1 ) or an apparatus 10' having an inductive probe with a monopole sensing element 220 (Figure 10) may be utilized in an automated irrigation system 100 for controlling the moisture content in a medium within one or more container(s) by controlling at least one solenoid actuated valve V disposed between the container(s) P and a water supply source W. A communal system wherein one valve services all of the containers P is illustrated. If desired the system 100 may be configured with a dedicated valve V associated with each container. Each container P is provided with an inductive probe 20/220 operative to detect a change in dielectric characteristic of the medium M due to the amount of moisture present in the vicinity of the probe. The detection network 40/232 associated with each probe 20/220 generates a signal 42/236 representative of the amount of moisture in the medium, as described. A valve controller 102 for controlling the operation of the valve V to supply water to a medium includes a comparator 104 for comparing the signal 42/236 from the detection network 40/232 representative of the amount of moisture in the medium M in which a given probe 20/220 is disposed with a reference signal 106 representative of a desired moisture content. A valve control signal 108 is generated in accordance with the comparison and is used to control the operation of the valve V. In the implementation illustrated the valve control signal 108 is applied a solenoid timer network 110. The solenoid timer network 110 is operative to assert the valve using any one of several watering protocols. The duty cycle of each protocol is illustrated in Figure 19A, 19B, 19C. Figure 19A illustrates one watering protocol. In this case the solenoid timer network 110 asserts the valve V to enable water to flow from the water source W to the container(s) P for that period of time necessary for the moisture content of the medium in the container(s) to exceed the reference threshold. A second watering protocol is illustrated in Figure 19B. Here, when the signal 42/236 representative of the quantity of moisture in the medium is below a predetermined threshold 106 the valve V is asserted by the solenoid timer network 110 to enable water to flow from the water source to the medium for a predetermined continuous time interval. The predetermined time interval is pre-set and no feedback monitoring of the medium in the container(s) is performed. Figure 19C illustrates a third watering protocol. This protocol is similar to that of Figure 19B. However, instead of one continuous time interval the valve V is asserted for an intermittent series of time intervals. Example 4: The same sensing element as in Examples 1 and 2 was used to control soil moisture in a full cycle of a plant life from seed to fully-grown plant. The output voltage 42/236 was used as an input in a comparator circuit 104 to assert a solenoid valve V for water flow when the output voltage 42/236 was above a given reference voltage 106 signifying dry soil. As a result, the soil moisture was controlled without human intervention for the period of three (3) weeks.
-o-O-o-
As seen from the generic representation of Figures 20A and 20B, either an apparatus 10 having an inductive probe with a looped sensing element 20 (Figure 1) or an apparatus 10' having an inductive probe with a monopole sensing element 220 (Figure 10) may be utilized to detect an interface between a first material Mi and a second material M2. Of course, in order to practice this method aspect of the present invention it is necessary that an electronics module 25/225 (shown in Figures 1 , 10, respectively) be associated with the appropriate sensing apparatus for the method under discussion. The combination of the sensing apparatus 10/10' and the electronics module 25/225 defines a useful system for detecting an interface defined between a first material and a second material disposed in a stratified manner in a volume of materials. As the sensing element 20/220 is progressively inserted into the material Mi (Figure 20A) a first distance range "a" is defined in which the attenuation increases at a substantial rate. This is graphically illustrated in Region I of the plot of Figure 21. The attenuation increases until the sensing element 20/220 is fully immersed in material Mi, at which time the attenuation reaches level Ai . As long as the sensing element 20/220 is within material M-\ further insertion results in no further change in attenuation. As illustrated in Region II of Figure 21 this serves to define a second distance range "b" in which the attenuation has substantially no change. When the sensing element 20/220 passes into the material M2
(Figure 20B) the change in attenuation resumes, thus defining another distance range "a" (Region III of Figure 21). Assuming the loss factor L2 in the material M2 is greater than the loss factor Li in the material M-i, attenuation increases to reach the level A2 when the sensing element 20/220 is fully immersed in material M2. From that point on further insertion of the sensing element 20/220 produces no further increase in attenuation (i.e., another distance range "b"). The attenuation is monitored as a function of insertion distance to detect first and second distance ranges "a" and "b". An interface between materials is denoted by a transition from a second distance range "b" to a first distance "a".
-o-O-o- Those skilled in the art, having the benefit of the teachings of the present invention may impart numerous modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention, as defined by the appended claims.

Claims

CLAIMS What Is Claimed Is: 1. Apparatus for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the apparatus comprising: a source of a radio frequency signal; an inductive probe placeable into contact with the medium, the probe being in the form of a looped sensing element having a first and a second end; a receiver for receiving the radio frequency signal transmitted through the probe, a detection network associated with the receiver for determining a change in a property of the signal arriving at the receiver; and a transmission line for coupling the first end of the probe to the source and the second end of the probe to the receiver, such that, in use, upon placement of the probe into contact with the medium a property of the signal transmitted from the source to the receiver is changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe, the detection network generating a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver.
2. The apparatus of claim 1 wherein the source is operative to generate the radio frequency signal having a predetermined amplitude at a single predetermined frequency, and wherein the change in the property of the signal detected by the detection network is an attenuation in signal amplitude.
3. The apparatus of claim 1 wherein the source is operative to generate the radio frequency signal having a predetermined amplitude at each of a plurality of predetermined frequencies, the radio frequency signal at the receiver having a maximum amplitude at a predetermined one of the frequencies, and wherein the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency, and 5 wherein the change in the attenuation of the signal as a function of frequency is manifested as a change in the frequency at which the maximum amplitude occurs.
4. The apparatus of claim 1 whereino the source is operative to generate the radio frequency signal having a predetermined propagation velocity through the probe, and wherein the change in the property of the signal detected by the detection network is a change in the propagation velocity, and wherein the change in the propagation velocity of the signal through thes probe is manifested as a shift in phase of the radio frequency signal at the receiver.
5. The apparatus of claim 1 wherein the transmission line comprises an inner conductor surrounded by a dielectric material and a coaxial shielding conductor, an exposed portion of the inner conductoro formed by removal of a portion of the shielding conductor defining the looped sensing element.
6. The apparatus of claim 1 wherein the transmission line comprises an inner conductor surrounded by a dielectric material and two shielding conductor layers, the looped sensing element of the probe being5 formed by extending the inner conductor beyond the two shielding conductor layers.
7. The apparatus of claim 1 , wherein the transmission line comprises: a signal layer comprising first and second inner signal conductors, 0 the first and second inner signal conductors being electrically connected to form the looped sensing element of the probe, a first and second dielectric layer surrounding the signal layer, and a first and second outer shielding conductor layer, the first and second inner signal conductors extending beyond the shielding conductor layers.
8. The apparatus of claim 7 wherein the signal layer further
5 comprises a ground conductor disposed between the first and second inner signal conductors, the ground conductor being electrically connected to the first and second outer shielding conductor layers.
9. The apparatus of claim 1 wherein the transmission line comprises a layered structure comprising, in order:o a first outer shielded conductor layer, a first dielectric layer, a layer including a first inner signal conductor, a second dielectric layer, a ground conductor layer,s a third dielectric layer, a layer including a second inner signal conductor, a fourth dielectric layer, and a second outer shielded conductor layer, the first and second inner signal conductors extending beyond theo shielding conductor layers, the first and second inner signal conductors being electrically connected to form the looped sensing element of the probe.
10. The apparatus of claim 1 wherein the radio frequency is greater than one megahertz.5
11. The apparatus of claim 1 wherein the radio frequency is greater than one hundred megahertz.
12. The apparatus of claim 1 wherein the radio frequency is greater than eight hundred megahertz.
13. The apparatus of claim 1 wherein the length of the looped 0 sensing element of the probe is at least ten percent of the wavelength of the radio frequency signal.
14. The apparatus of claim 1 wherein the material is water.
15. The apparatus of claim 1 further comprising a rigid substrate on which the source, the looped sensing element of the probe and the receiver are mounted thereby to form an integrated structure.
16. The apparatus of claim 15 further comprising a source of electrical energy being included within the integrated structure.
17. The apparatus of claim 1 further comprising a rigid substrate on which the looped sensing element of the probe is mounted.
18. The apparatus of claim 17 wherein the medium is disposed within a container having a curved wall, and wherein the rigid substrate is curved in conformance with the curved wall thereby to facilitate placement of the looped sensing element of the probe into contact with the medium within the container.
19. The apparatus of claim 18 further comprising an adhesive material on the rigid substrate to hold the same to the curved wall of the container.
20. The apparatus of claim 1 wherein the medium is disposed within a container having a curved wall, and further comprising a flexible substrate on which the looped sensing element of the probe is mounted, the flexible substrate being able to be shaped into conformance with the curved wall, thereby to facilitate placement of the looped sensing element of the probe into contact with the medium within the container.
21. The apparatus of claim 20 further comprising an adhesive material on the flexible substrate to hold the same to the curved wall of the container.
22. Apparatus for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the apparatus comprising: a source of a radio frequency signal at a predetermined amplitude and a predetermined frequency; an inductive probe placeable into contact with the medium, the probe being in the form of a looped sensing element having a first and a second end; a receiver for receiving the radio frequency signal transmitted through the probe, the receiver including an amplitude detection network therein; and a transmission line for coupling the first end of the looped sensing 5 element to the source and the second end of the looped sensing element to the receiver, such that, in use, upon placement of the probe into contact with the medium a signal transmitted from the source to the receiver is attenuated due to dielectric loss in accordance with the quantity of the material0 present in the medium in the vicinity of the probe, the detection network detecting the magnitude of the attenuation of the radio frequency signal arriving at the receiver and generating a signal in accordance therewith representative of the amount of material in the medium.s
23. Apparatus for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the apparatus comprising: a sweeping source of a radio frequency signal at a predetermined amplitude at each of a plurality of frequencies across a predetermined o range of frequencies, an inductive probe placeable into contact with the medium, the probe being in the form of a looped sensing element having a first and a second end; a receiver for receiving the radio frequency signal transmitted5 through the probe, the receiver including a resonant frequency detection network therein, and a transmission line having a distributed inductance and a distributed capacitance for coupling the first end of the looped sensing element to the source and the second end of the looped sensing element to the receiver,o at least one capacitor connected to the transmission line, the capacitor being cooperable with the source, the transmission line, the inductive probe to form a resonant circuit having a predetermined baseline resonant frequency, such that, in use, upon placement of the probe into contact with the medium the sweeping signal transmitted from the source to the receiver is attenuated as a function of frequency due to dielectric loss in accordance with the quantity of the material present in the medium in the vicinity of the probe, the detection network detecting the resonant frequency of the radio frequency signal arriving at the receiver and generating a signal in accordance therewith representative of the amount of material in the medium.
24. Apparatus for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the apparatus comprising: an inductive probe placeable into contact with the medium, the probe being in the form of a loop having a first and a second end; a source of a radio frequency signal, the radio frequency signal having a predetermined propagation velocity through the loop portion of the probe, a receiver for receiving the radio frequency signal transmitted through the probe, the receiver including a phase detection network therein, and a transmission line for coupling the first end of the probe to the source and the second end of the probe to the receiver, such that, in use, upon placement of the probe into contact with the medium the propagation velocity of the signal through the loop portion of the probe changes in accordance with the quantity of the material present in the medium in the vicinity of the probe, the phase detection network detecting the magnitude of change in propagation velocity of the radio frequency signal through the probe and generating a signal in accordance therewith representative of the amount of material in the medium.
25. Apparatus for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the apparatus comprising: a source of a radio frequency signal at a predetermined amplitude; an inductive probe placeable into contact with the medium, the probe being in the form of a monopole sensing element; a receiver for receiving the radio frequency signal transmitted through the probe; a detection network associated with the receiver for determining a change in a property of the signal arriving at the receiver; and a directional coupler for coupling the source to the probe and for coupling the probe to the receiver, so that, in use, upon placement of the probe in contact with the medium a signal coupled from the source to the probe is reflected by the sensing element and coupled to the receiver, a property of the signal coupled from the source to the receiver being changed in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe, the detection network generating a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal arriving at the receiver.
26. The apparatus of claim 25 wherein the source is operative to generate the radio frequency signal having a predetermined amplitude at a single predetermined frequency, and wherein the change in the property of the signal detected by the detection network is an attenuation in signal amplitude.
27. The apparatus of claim 25 wherein the source is operative to generate the radio frequency signal having a predetermined amplitude at each of a plurality of predetermined frequencies, the radio frequency signal at the receiver having a maximum amplitude at a predetermined one of the frequencies, and wherein the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of frequency, and wherein the change in the attenuation of the signal as a function of frequency is manifested as a change in the frequency at which the maximum amplitude occurs.
28. The apparatus of claim 25 wherein the source is operative to generate the radio frequency signal having a predetermined propagation velocity through the probe, and wherein the change in the property of the signal detected by the detection network is a change in the propagation velocity, and wherein the change in the propagation velocity of the signal through the probe is manifested as a shift in phase of the radio frequency signal at the receiver.
29. The apparatus of claim 25 wherein the probe is implemented using a terminated transmission line itself comprising an inner conductor surrounded by a dielectric material and a shielding conductor, an exposed portion of the inner conductor defining the monopole sensing element.
30. The apparatus of claim 29 wherein the transmission line is a coaxial structure with the inner conductor being surrounded by the dielectric material and the shielding conductor.
31. The apparatus of claim 29 wherein the transmission line is a layered structure, comprising an inner conductor surrounded by a dielectric material and first and second shielding conductor layers, the monopole sensing element being formed by extending the inner conductor beyond the extent of the two shielding conductor layers.
32. The sensing apparatus of claim 29 wherein the exposed inner conductor is surrounded by the dielectric material.
33. The sensing apparatus of claim 29 wherein the exposed inner conductor is bare.
34. The apparatus of claim 25 wherein the radio frequency is greater than one megahertz.
35. The apparatus of claim 34 wherein the radio frequency is greater than one hundred megahertz.
36. The apparatus of claim 35 wherein the radio frequency is greater than eight hundred megahertz.
37. The apparatus of claim 25 wherein the length of the monopole sensing element of the probe is at least ten percent of the wavelength of the radio frequency signal.
38. The apparatus of claim 25 wherein the material is water.
39. The apparatus of claim 25 wherein the medium is soil.
40. The apparatus of claim 25 wherein the source, the probe, the coupler and the receiver are mounted on a rigid substrate to form an integrated structure.
41. A method for detecting the presence of a material in a medium, the material causing a change in dielectric characteristic of the medium, the method comprising the steps: placing an inductive probe into contact with the medium; passing a radio frequency signal through the probe; detecting a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the material present in the vicinity of the probe; and generating a signal representative of the quantity of material in the medium in accordance with the change in the property of the signal.
42. The method of claim 41 wherein the probe is inserted into the medium.
43. The method of claim 41 wherein the medium is disposed within a container having electrically non-conducting walls, and wherein the probe is placed against the container.
44. The method of claim 41 wherein the radio frequency signal has a predetermined frequency and the change in the property of the signal detected is an attenuation in signal amplitude.
45. The method of claim 41 wherein the radio frequency signal has a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of
5 frequency.
46. The method of claim 45 wherein the radio frequency signal at the receiver has a maximum amplitude at a predetermined one of the frequencies, and wherein the change in the attenuation of the signal as a function ofo frequency is detected by determining a change in the frequency at which the maximum amplitude occurs.
47. The method of claim 45 wherein the radio frequency signal sweeps across the predetermined range of frequencies.
48. The method of claim 41 wherein the radio frequency signal hass a predetermined propagation velocity through the probe, and wherein the change in the property of the signal is a change in the propagation velocity.
49. The method of claim 48 wherein the change in the propagation velocity of the signal is detected by determining a shift in phase of the0 radio frequency signal.
50. An automated irrigation system for controlling the moisture content in a medium within a container comprising: a valve connectible to a source of water, an inductive probe operative to detect a change in dielectric5 characteristic of the medium due to the amount of moisture present in the vicinity of the probe; a detection network associated with the probe for generating a signal representative of the amount of moisture in the medium; and a valve controller for controlling the operation of the valve to supplyo water to a medium, the valve controller including a comparator for comparing the signal from the detection network representative of the amount of moisture in the medium with a reference signal representative of a desired moisture content and for generating the valve control signal in accordance with the comparison.
51. The irrigation system of claim 50 wherein the valve is asserted in response to the valve control signal to enable water to flow from the
5 water source to the medium when the signal representative of the quantity of moisture in the medium is below a predetermined threshold until the signal representative of the quantity of moisture exceeds the threshold.
52. The irrigation system of claim 50 wherein the valve is asserted in response to the valve control signal to enable water to flow from theo water source to the medium for a continuous time interval when the signal representative of the quantity of moisture in the medium is below a predetermined threshold.
53. The irrigation system of claim 50 wherein the valve is asserted in response to the valve control signal to enable water to flow from thes water source to the medium for an intermittent series of time intervals when the signal representative of the quantity of moisture in the medium is below a predetermined threshold.
54. An automated method for controlling the moisture content in a medium within a container comprising the steps of:0 a) detecting the moisture content in the medium by detecting the change in dielectric characteristic of the medium caused by the presence of moisture in the medium by the steps of: placing an inductive probe into contact with the medium; passing a radio frequency signal through the probe;5 detecting a change in a property of the signal in accordance with a change in dielectric characteristic of the medium due to the quantity of the moisture present in the vicinity of the probe; and generating a signal representative of the quantity of moisture in the medium in accordance with the change in the property of the signal;o b) comparing the signal representative of the quantity of moisture in the medium a reference signal representative of a desired moisture content; and c) generating an irrigation valve control signal in accordance with the comparison.
55. The method of claim 54 wherein valve is asserted in response to the valve control signal to enable water to flow from the water source to
5 the medium when the signal representative of the quantity of moisture in the medium is below a predetermined threshold until the signal representative of the quantity of moisture exceeds the threshold.
56. The method of claim 54 wherein valve is asserted in response to the valve control signal to enable water to flow from the water source too the medium for a continuous time interval when the signal representative of the quantity of moisture in the medium is below a predetermined threshold.
57. The method of claim 54 wherein valve is asserted in response to the valve control signal to enable water to flow from the water source tos the medium for an intermittent series of time intervals when the signal representative of the quantity of moisture in the medium is below a predetermined threshold.
58. The method of claim 54 wherein the probe is inserted into the medium. 0
59. The method of claim 54 wherein the medium is disposed within a container having electrically non-conducting walls, and wherein the probe is placed against the container.
60. The method of claim 54 wherein the radio frequency signal has a predetermined frequency and the change in the property of the signal5 detected is an attenuation in signal amplitude.
61. The method of claim 54 wherein the radio frequency signal has a predetermined amplitude at each of a plurality of predetermined frequencies and the change in the property of the signal detected by the detection network is the attenuation of the signal as a function of o frequency.
62. The method of claim 61 wherein the radio frequency signal at the receiver has a maximum amplitude at a predetermined one of the frequencies, and wherein the change in the attenuation of the signal as a function of 5 frequency is detected by determining a change in the frequency at which the maximum amplitude occurs.
63. The method of claim 61 wherein the radio frequency signal sweeps across the predetermined range of frequencies.
64. The method of claim 54 wherein the radio frequency signal haso a predetermined propagation velocity through the probe, and wherein the change in the property of the signal is a change in the propagation velocity.
65. The method of claim 64 wherein the change in the propagation velocity of the signal is detected by determining a shift in phase of thes radio frequency signal.
66. A method of detecting an interface between first and second materials having different dielectric loss factors disposed in a stratified manner in a volume of materials using a sensing apparatus having a length of transmission line including an inner conductor surrounded by a 0 dielectric material and at least one shielding conductor, a single sublength of the inner conductor being exposed along the length of the transmission line, the method comprising the steps of: exciting the sensing apparatus by a radio frequency signal at5 a predetermined amplitude, inserting the excited sensing apparatus into the volume of materials, tracking the insertion distance of the sensing apparatus within the volume, o monitoring the attenuation of the radio frequency signal as a function of the insertion distance into the volume to detect first distance ranges "a" having a substantial rate of change of attenuation and second distance ranges "b" having substantially no change of attenuation; and detecting an interface in the strata by noting a transition from a second distance range "b" to a first distance "a".
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