CA1196706A - Fiber optic sensor - Google Patents
Fiber optic sensorInfo
- Publication number
- CA1196706A CA1196706A CA000422005A CA422005A CA1196706A CA 1196706 A CA1196706 A CA 1196706A CA 000422005 A CA000422005 A CA 000422005A CA 422005 A CA422005 A CA 422005A CA 1196706 A CA1196706 A CA 1196706A
- Authority
- CA
- Canada
- Prior art keywords
- sensor
- resonant
- wire
- signal
- central station
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/10—Measuring force or stress, in general by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings
- G01L1/103—Measuring force or stress, in general by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings optical excitation or measuring of vibrations
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/08—Measuring force or stress, in general by the use of counterbalancing forces
Abstract
Abstract of the disclosure An instrumentation system for use in measuring and processing industrial process variables, such as flow, pressure, or temperature, includes a resonant element sensor whose reso-nant frequency varies in accordance with changes in the desired process variable communicating through an optical fiber link to a distant control room. The sensor is acti-vated into resonant physical motion by light energy from a source in the control room, while the motion of the wire is sensed optically and retransmitted to the control room to produce an output signal whose frequency is equal to that of the resonating element. A feedback network maintains the sensor in resonance by synchronizing the delivery of light energy to the motion of the resonant element. The powering and sensing aspect may be performed by individual fiber optic cables or alternatively this function may be combined by utilizing a single fiber optic strand.
Description
9~6 103.020 Fiber optic sensor This invention relates to improvements in industrial process measurement apparatus capable of developing a signal that corresponds to the magnitude of a measurable physical para-meter. More particularly, this invention relates to such apparatus employing resonant element sensors with fiber optic means to excite the resonant element and sense the resonant frequency.
Instrumentation systems for use in measuring industrial process variables such as flow, pressure, temperature, and liquid level typically employ a sensing element located in a field location adjacent the process which responds directly to the process variable. The output signal of the sensing element is transmitted to a distant central station, e.g., a control room, for further signal conditioning and processing.
In the majority of present industrial applications 7 an elec-trical measurement signal is produced at the sensor, and a two-wire transmission line provides the interconnection necessary to power the sensor and receive the measurement signal.
One class of measurement instrument for developing such a measurement signal that has been known for many years employs resonant elements as the primary sensing device. More re-cently an accurate, practical family of instruments of this general type has been devised and successfully marketed by The Foxboro Company as its 800-Series resonant wire sensors.
~hile these devices represent a significant advance as evi-denced by the high degree of commercial success which they have obtained, they do possess certain limitations, particu-larly when operating in severel highly electrically noisy process environments.
:~9~
Thus, room for improvement exists in the design and construction of industrial measurement instruments, especially in their accur-acy while operatina within troublesome process environments, bv eliminating or minimizing undesired electri~al eff~cts.
The present inv~ntion provides a significant departure from those industrial measurement instruments cf the past by providing an optical link between a resonant sensing element adjacent the pro-cess and a distant central station containing signal conditioning electronics. Energy necessary to activate the sensing element and induce mechanical vibration is thus supplied optically.
More particularly, the invention provides an instrumentation system for developing at a central station a measurement signal representing the value of a process condition of the type inclu-ding a sensor that produces an output signal that varies in accor-dance with changes in said condition, said sensor being at a field location remotely positioned from said central station adjacent the process, light source means within said central station adapted to produce optical energy, said system fur-ther including means for transferring said optical energy between said sensor and said central station characterized in that: a single optical fiber is used both for supplying optical energy to activate said sensor and for transmitting said output signal of said sensor to said central station for producing said measurement signal.
This preferably involves -the use of wavelength multiplexing onto the single fiber -to provide the function oE powering the sensor and de-tec-tin~ its ou-tput signal.
In a preferred embodiment to be described in detail below, a differential pressure measurement instrument of the resonant-wire type is used as the sensor. Pulsed optical energy activates the resonant-wire sensor, whose tension and hence resonant fre-quency varies in accordance with the pressure to be measured, ~g~7~
while an information-bearing signal representative of the pres-sure measurement is sent back along the fiber. The transmitted pulsed optical energy is photovoltaically converted into corres-ponding pulses of electric current which induce the wire, in the presence of a magnetic field, to vibrate at its resonant frequency.
Oscillatory movement of the wire is sensed by reflecting trans-mi-tted steady-s-tate light, which illuminates the moving wire, back into the fiber, thereby modulating the intensity o~ the steady-state light a-t a frequency that corresponds to the resonant frequency of the wire. To maintain the wire in resonance and thus minimize the amount of power required to drive the wire, a feedback network couples this composite reflected light signal to the supply of pulses to provide synchronization at the reson-ant frequency.
Other aspects and advantages of the present invention will become more evident after a review of the following detailed description taken in context with the accompanying drawings illustrating the principles of the inveniion~
FIG. 1 is a schematic diagram in block format of a field-located differential pressure measurement device communicating with signal processing elements within a control room constructed in accor-dance with a preferred embodiment of the invention; and FIG. 2 is a schematic diagram showing the optical communications network employing a single fiber for transmitting power to and sensing the outpu-t of -the pressure measurement device of FIG. 1.
As used throughout this written descrip-tion and in the appended claims, the term "resonan-t element" is to be construed broadly.
That is, it is intended to encompass not only vibrating wires or strings but also any characteristic structure that, when subjected to an external stimulus such as a pressure or force, will vibrate at a frequency which corresponds -to the applied stimulus.
. . , -3a-Turning now to FIG. 1, there ls shown schematically a measuring instrument 10 employing a resonant element sensor 12 arranged to measure -the magnitude of an unknown force (or :"
;','`
pressure). The instrument is located in a process field 13 and is coupled by a pair of optical fibers 14, 15 to a central control room 16 having signal generating and processing equipment located there-in. Although shown schematically as two distinct fiber optic cables, it will be appreciated that for typical process installations where the distance between field instruments and the control room is about one mile, these two Eibers may be jacketed in a single cable with appropriate cladding to propaga-te the light.
The left-hand portion of Figure 1 shows in block diagram for-mat the mechanical components of the resonant element sensor 12, namely a wire 20 tautly positioned within the gap 21 of a magnetic assembly.
This assembly consists of a permanent magnet and suitable pole pieces (collectively indicated by numeral 22) arranged to produce an intense magnetic field perpendicular to the longitudinal axis of the wire.
Although the operation of resonant element sensors is well understood by those of skill in the art, the following discussion repre-sents a brief summary. The wire 20 is anchored at one end to a section of the instrument body indicated by numeral 2~, while the other end is operatively coupled to a diaphragm 26 which alters the tension on the wire in response to an applied force. The exact arrangement o~ components is not important for an understanding of the principles of the present inven-tion, but reference should be made to United States Patent No. ~,165,651, if a detailed explanation of the pressure measuring instrument is required.
The wire is formed of electrically conductive material prefcrably with a polished refLec-tive surface, and is electrically insulated from the instrument body by a bushing 23. When an alternating electric current is caused to flow through the wire, it is induced to vibrate at its _ ~
~9~7~6 resonant frequency which in turn is a function of the applied pressure. For purposes of illustration, it is assumed that the magnetic field is directed through the wire orthogonally to the plane of the drawing sheet, and thus the wire dis-placement follows the profile given by the dashed lines. A
vibrating cycle is defined as a single excursion of the wire from its at rest or central null position to the left-most displacement back through the null position to its right-most displacement and back to the null position.
As shown the fiber 14 extends through a hole in the magnet assembly 22 to a position proximate the expected maximum deflection of the wire 20. This configuration permits the wire to be irradiated with light while a portion, depending on the instantaneous distance of the wire from the fiber, is reflected back into the fibeI for transmission to the control room 16.
In operation, the electro-optical circuitry within the con-trol room 16 provides the system drive energy through a regulated d-c power supply 30 that delivers a voltage input to a light emitting diode (LED) 32 and a feedback network 50 which in turn powers a second LED 33. The LED 32 provides, in conjuction with a pair of microlenses 34, 35 and a beam splitter 40, steady-state light into the fiber 14 for trans-mission to the wire 20. The use of microlenses at optical interFaces throughout the system to enhance optical energy transfer is well understood by those of skill in the art and such lenses are commercially available ~rom Nippon Sheet Glass Company.
As mentioned, motion of the wire 20 results in a modulated light signal being reflected back to the control room 16 over the same optical fiber 14 where it is received at a photo-diode 42 located at the return output 40A of the beam split-ter 40. The electrical feedback network 50 coupled between the photodiode 42 and the LED 33 provides through a microlens 36 light energy for the optical fiber 15 to activate motion of the wire 20. For this embodiment being described, a transformation of light energy into mechanical motion occurs at the field mounted end of the fiber 15 by a photodiode 62 whose electrical output is applied across the primary winding 64 of a transformer 66. The secondary winding 65 is directly connected to the wire 20.
It will be appreciated that this overall arrangement, al-though involvina a mixture of electrical, mechanical and optical components, defines a closed loop oscillator. ~ore-over, as is well known by those of skill in the art, the sys~em can be designed utilizing appropriate gain and phase shift selection to self-start from the electrical noise present or even from slight mechanical vibrations i nduced within the resonant-wire sensor 12 such that the loop will be at resonance within a few operatino cycles.
Considering in more detail the operation of the system, and assuming that the wire 20 has begun vibrating, an a-c elec-trical signal will be developed at the photodiode 42 whose frequency is equal to that of the vibrating wire. This a-c signal is then applied to the feedback network 50. Th'is network consists of a low-level a-c amplifier 52 to amplify the signal from the photodiode 42, a phase shift network 54 to compensate for phase differences within the closed loop to sustain oscillation, a pulse shaper 56, and a power amp-lifier 58. The output of the amplifier 58 becomes the drive voltage for the LED 33 which is thereby caused to emit a series of pulses of light. These light pulses, transmitted via the optical fiber 15 to the pho todiode 62, produce ~after i7~3~
suitable impedance matching by the transformer 66) corresponding current pulses through the wire that are precisely synchronized with the motion of the wire to produce m~;ml~m deflection (and hence a m~ir~l~ amplitude resonant signal) with each successive pulse. Thus the output of the pulse shaper 56 represents the resonant frequency of vibration and hence the pressure measure-ment This frequency signal may be read out directly at an output terminal 70 or alternatively supplied to a frequency to d-c converter ~0 -to produce a d-c control signal proportional to the pressure measurement.
In similar fashion changes to the resonant frequency of vibration caused by changes in pressure exerted on the di~yhragm 26 are detected and automatically adjusted for within the closed loop to produce a new output signal representative of the change in the process parameters. The design details of an appropriate amplifier circuit described above are well within the knowledge of a skilled artisan.
In certain applications it may be desirous to provide a single optical fiber for communication between the process field and the control room. For these purposes, the arrangement of Figure 2 may be particularly advantageous which focuses primarily on the optical energy transfer of the present invention.
For simplicity, details of the electronic drive and feedback circuitry have been omitted, suffice it to say their operation will be similar to that already pre-sented in detail above. More particularly, beam splitter 40' functions identi-cally to beam splitter 40 shown in Figure 1. ~lere a pair of LED sources 100, 200 of discernibly different wavelength (~ 2) are wavelength multiplexed at a dichroic beam splitter 300. The source 100 produces a pulse train of light at a frequency within the operative range (e.g., 1700-3000 llz) of the resonant sensor 10 while the source 200 provides a steady-state beam of light. These two wave-lengths are transported from the control room 16 over :~ l 9~6 a single optical fiber 40Q to a field-located dichroic beam splitter 500 which passes substantially all of ~1 to the photodiode 62 for powering the sensor 10 while blocking ~2 In turn, effectively all of the steady-state light (~2) is reflected by the beam splitter 500 so as to illuminate the wire ~0, with essentially none of ~1 being directed along this path.
The return signal reflected from the wire 20 is as before the steady-state beam (A2) modulated in ~ntensity by an alter-nating signal corresponding to the motion of the wire. This signal is then detected at a photodiode 600 and fed back through a suitable network 700 to close the loop with the LED
source 100 thereby setting the pulse train frequency at the resonant frequency of the wire.
It may also be possible to utilize a single optical communi-cation fiber to both power the sensor and detect its output without employing multiple sources and dichroic beam split-ters. In such an arrangement, a pulsed beam of light is transmitted to the field and split in two paths, one to drive the wire, the other to illuminate the moving wire on a peri-odic basis. Although the waveforms of the reflected signal would be somewhat complicated due to the chopped nature of the incident light, the intensity of light reflected from the resonant wire would still be proportional to the distance between the wire and the adjacent optical fiber, with less light being reflected when the wire is furthest from the fiber and vice-versa. The returned illumination combined with the transmitted light produces a composite waveform representing the total illumination in a given instant of time within the optical fiber, i.e., a pulsed signal with a periodic alternating signal thereon. ~ith suitable adjust-ments in the electronic design, a ccmpatible oscillator ~36~
could be built such that at resonance the transmitted light pulses would be synchronized with the motion o~ the wire.
Such source synchronization is arrived at by the feedback arrangement previously discussed in detail above.
Thus numerous advantages of the present invention have been set forth in detail above. An instrumentation system employ-ing a resonant element sensor has been demonstrated that operates by converting light energy into resonant physical motion, while transmitting measurement data in terms of fre-quency through optical sensing means. Py eliminating elec-trical transmission between control room and field locations over copper wire conductors, problems associated with elec-tromagnetic interferences as in past such systems have been alleviated. Installation of the present optical network within process plants may be simplified by eliminating the need for separate optical fiber conductors for powering and sensing by effectively providing two-way communication over a single optical fiber. Additionally, the feedback technique of the present invention besides sustaining oscillations also allows the largest amplitude of vibration for the lowest possible power input. This arrangement thus is particularly suitable to permit the use of low power LED sources for com-municating over the distances involved while still maintain-ing an effective signal to noise ratio.
Although a preferred embodiment of the invention has been described in detail above, this is solely for the purpose of illustration and is not intended to be limiting. Numerous modifications will become apparent to those of skill in the art. For example, the invention has been described through-out as operating with resonant element sensors that are activated by electro-magnetic energy and hence a conversion from light energy to electrical energy has been shown. It ~6~
will be understood that other techniques could be devised for applying the supplied light energy to the sensor element to effect resonant physical motion without departing from the scope of the invention as defined in the accompanying claims.
Instrumentation systems for use in measuring industrial process variables such as flow, pressure, temperature, and liquid level typically employ a sensing element located in a field location adjacent the process which responds directly to the process variable. The output signal of the sensing element is transmitted to a distant central station, e.g., a control room, for further signal conditioning and processing.
In the majority of present industrial applications 7 an elec-trical measurement signal is produced at the sensor, and a two-wire transmission line provides the interconnection necessary to power the sensor and receive the measurement signal.
One class of measurement instrument for developing such a measurement signal that has been known for many years employs resonant elements as the primary sensing device. More re-cently an accurate, practical family of instruments of this general type has been devised and successfully marketed by The Foxboro Company as its 800-Series resonant wire sensors.
~hile these devices represent a significant advance as evi-denced by the high degree of commercial success which they have obtained, they do possess certain limitations, particu-larly when operating in severel highly electrically noisy process environments.
:~9~
Thus, room for improvement exists in the design and construction of industrial measurement instruments, especially in their accur-acy while operatina within troublesome process environments, bv eliminating or minimizing undesired electri~al eff~cts.
The present inv~ntion provides a significant departure from those industrial measurement instruments cf the past by providing an optical link between a resonant sensing element adjacent the pro-cess and a distant central station containing signal conditioning electronics. Energy necessary to activate the sensing element and induce mechanical vibration is thus supplied optically.
More particularly, the invention provides an instrumentation system for developing at a central station a measurement signal representing the value of a process condition of the type inclu-ding a sensor that produces an output signal that varies in accor-dance with changes in said condition, said sensor being at a field location remotely positioned from said central station adjacent the process, light source means within said central station adapted to produce optical energy, said system fur-ther including means for transferring said optical energy between said sensor and said central station characterized in that: a single optical fiber is used both for supplying optical energy to activate said sensor and for transmitting said output signal of said sensor to said central station for producing said measurement signal.
This preferably involves -the use of wavelength multiplexing onto the single fiber -to provide the function oE powering the sensor and de-tec-tin~ its ou-tput signal.
In a preferred embodiment to be described in detail below, a differential pressure measurement instrument of the resonant-wire type is used as the sensor. Pulsed optical energy activates the resonant-wire sensor, whose tension and hence resonant fre-quency varies in accordance with the pressure to be measured, ~g~7~
while an information-bearing signal representative of the pres-sure measurement is sent back along the fiber. The transmitted pulsed optical energy is photovoltaically converted into corres-ponding pulses of electric current which induce the wire, in the presence of a magnetic field, to vibrate at its resonant frequency.
Oscillatory movement of the wire is sensed by reflecting trans-mi-tted steady-s-tate light, which illuminates the moving wire, back into the fiber, thereby modulating the intensity o~ the steady-state light a-t a frequency that corresponds to the resonant frequency of the wire. To maintain the wire in resonance and thus minimize the amount of power required to drive the wire, a feedback network couples this composite reflected light signal to the supply of pulses to provide synchronization at the reson-ant frequency.
Other aspects and advantages of the present invention will become more evident after a review of the following detailed description taken in context with the accompanying drawings illustrating the principles of the inveniion~
FIG. 1 is a schematic diagram in block format of a field-located differential pressure measurement device communicating with signal processing elements within a control room constructed in accor-dance with a preferred embodiment of the invention; and FIG. 2 is a schematic diagram showing the optical communications network employing a single fiber for transmitting power to and sensing the outpu-t of -the pressure measurement device of FIG. 1.
As used throughout this written descrip-tion and in the appended claims, the term "resonan-t element" is to be construed broadly.
That is, it is intended to encompass not only vibrating wires or strings but also any characteristic structure that, when subjected to an external stimulus such as a pressure or force, will vibrate at a frequency which corresponds -to the applied stimulus.
. . , -3a-Turning now to FIG. 1, there ls shown schematically a measuring instrument 10 employing a resonant element sensor 12 arranged to measure -the magnitude of an unknown force (or :"
;','`
pressure). The instrument is located in a process field 13 and is coupled by a pair of optical fibers 14, 15 to a central control room 16 having signal generating and processing equipment located there-in. Although shown schematically as two distinct fiber optic cables, it will be appreciated that for typical process installations where the distance between field instruments and the control room is about one mile, these two Eibers may be jacketed in a single cable with appropriate cladding to propaga-te the light.
The left-hand portion of Figure 1 shows in block diagram for-mat the mechanical components of the resonant element sensor 12, namely a wire 20 tautly positioned within the gap 21 of a magnetic assembly.
This assembly consists of a permanent magnet and suitable pole pieces (collectively indicated by numeral 22) arranged to produce an intense magnetic field perpendicular to the longitudinal axis of the wire.
Although the operation of resonant element sensors is well understood by those of skill in the art, the following discussion repre-sents a brief summary. The wire 20 is anchored at one end to a section of the instrument body indicated by numeral 2~, while the other end is operatively coupled to a diaphragm 26 which alters the tension on the wire in response to an applied force. The exact arrangement o~ components is not important for an understanding of the principles of the present inven-tion, but reference should be made to United States Patent No. ~,165,651, if a detailed explanation of the pressure measuring instrument is required.
The wire is formed of electrically conductive material prefcrably with a polished refLec-tive surface, and is electrically insulated from the instrument body by a bushing 23. When an alternating electric current is caused to flow through the wire, it is induced to vibrate at its _ ~
~9~7~6 resonant frequency which in turn is a function of the applied pressure. For purposes of illustration, it is assumed that the magnetic field is directed through the wire orthogonally to the plane of the drawing sheet, and thus the wire dis-placement follows the profile given by the dashed lines. A
vibrating cycle is defined as a single excursion of the wire from its at rest or central null position to the left-most displacement back through the null position to its right-most displacement and back to the null position.
As shown the fiber 14 extends through a hole in the magnet assembly 22 to a position proximate the expected maximum deflection of the wire 20. This configuration permits the wire to be irradiated with light while a portion, depending on the instantaneous distance of the wire from the fiber, is reflected back into the fibeI for transmission to the control room 16.
In operation, the electro-optical circuitry within the con-trol room 16 provides the system drive energy through a regulated d-c power supply 30 that delivers a voltage input to a light emitting diode (LED) 32 and a feedback network 50 which in turn powers a second LED 33. The LED 32 provides, in conjuction with a pair of microlenses 34, 35 and a beam splitter 40, steady-state light into the fiber 14 for trans-mission to the wire 20. The use of microlenses at optical interFaces throughout the system to enhance optical energy transfer is well understood by those of skill in the art and such lenses are commercially available ~rom Nippon Sheet Glass Company.
As mentioned, motion of the wire 20 results in a modulated light signal being reflected back to the control room 16 over the same optical fiber 14 where it is received at a photo-diode 42 located at the return output 40A of the beam split-ter 40. The electrical feedback network 50 coupled between the photodiode 42 and the LED 33 provides through a microlens 36 light energy for the optical fiber 15 to activate motion of the wire 20. For this embodiment being described, a transformation of light energy into mechanical motion occurs at the field mounted end of the fiber 15 by a photodiode 62 whose electrical output is applied across the primary winding 64 of a transformer 66. The secondary winding 65 is directly connected to the wire 20.
It will be appreciated that this overall arrangement, al-though involvina a mixture of electrical, mechanical and optical components, defines a closed loop oscillator. ~ore-over, as is well known by those of skill in the art, the sys~em can be designed utilizing appropriate gain and phase shift selection to self-start from the electrical noise present or even from slight mechanical vibrations i nduced within the resonant-wire sensor 12 such that the loop will be at resonance within a few operatino cycles.
Considering in more detail the operation of the system, and assuming that the wire 20 has begun vibrating, an a-c elec-trical signal will be developed at the photodiode 42 whose frequency is equal to that of the vibrating wire. This a-c signal is then applied to the feedback network 50. Th'is network consists of a low-level a-c amplifier 52 to amplify the signal from the photodiode 42, a phase shift network 54 to compensate for phase differences within the closed loop to sustain oscillation, a pulse shaper 56, and a power amp-lifier 58. The output of the amplifier 58 becomes the drive voltage for the LED 33 which is thereby caused to emit a series of pulses of light. These light pulses, transmitted via the optical fiber 15 to the pho todiode 62, produce ~after i7~3~
suitable impedance matching by the transformer 66) corresponding current pulses through the wire that are precisely synchronized with the motion of the wire to produce m~;ml~m deflection (and hence a m~ir~l~ amplitude resonant signal) with each successive pulse. Thus the output of the pulse shaper 56 represents the resonant frequency of vibration and hence the pressure measure-ment This frequency signal may be read out directly at an output terminal 70 or alternatively supplied to a frequency to d-c converter ~0 -to produce a d-c control signal proportional to the pressure measurement.
In similar fashion changes to the resonant frequency of vibration caused by changes in pressure exerted on the di~yhragm 26 are detected and automatically adjusted for within the closed loop to produce a new output signal representative of the change in the process parameters. The design details of an appropriate amplifier circuit described above are well within the knowledge of a skilled artisan.
In certain applications it may be desirous to provide a single optical fiber for communication between the process field and the control room. For these purposes, the arrangement of Figure 2 may be particularly advantageous which focuses primarily on the optical energy transfer of the present invention.
For simplicity, details of the electronic drive and feedback circuitry have been omitted, suffice it to say their operation will be similar to that already pre-sented in detail above. More particularly, beam splitter 40' functions identi-cally to beam splitter 40 shown in Figure 1. ~lere a pair of LED sources 100, 200 of discernibly different wavelength (~ 2) are wavelength multiplexed at a dichroic beam splitter 300. The source 100 produces a pulse train of light at a frequency within the operative range (e.g., 1700-3000 llz) of the resonant sensor 10 while the source 200 provides a steady-state beam of light. These two wave-lengths are transported from the control room 16 over :~ l 9~6 a single optical fiber 40Q to a field-located dichroic beam splitter 500 which passes substantially all of ~1 to the photodiode 62 for powering the sensor 10 while blocking ~2 In turn, effectively all of the steady-state light (~2) is reflected by the beam splitter 500 so as to illuminate the wire ~0, with essentially none of ~1 being directed along this path.
The return signal reflected from the wire 20 is as before the steady-state beam (A2) modulated in ~ntensity by an alter-nating signal corresponding to the motion of the wire. This signal is then detected at a photodiode 600 and fed back through a suitable network 700 to close the loop with the LED
source 100 thereby setting the pulse train frequency at the resonant frequency of the wire.
It may also be possible to utilize a single optical communi-cation fiber to both power the sensor and detect its output without employing multiple sources and dichroic beam split-ters. In such an arrangement, a pulsed beam of light is transmitted to the field and split in two paths, one to drive the wire, the other to illuminate the moving wire on a peri-odic basis. Although the waveforms of the reflected signal would be somewhat complicated due to the chopped nature of the incident light, the intensity of light reflected from the resonant wire would still be proportional to the distance between the wire and the adjacent optical fiber, with less light being reflected when the wire is furthest from the fiber and vice-versa. The returned illumination combined with the transmitted light produces a composite waveform representing the total illumination in a given instant of time within the optical fiber, i.e., a pulsed signal with a periodic alternating signal thereon. ~ith suitable adjust-ments in the electronic design, a ccmpatible oscillator ~36~
could be built such that at resonance the transmitted light pulses would be synchronized with the motion o~ the wire.
Such source synchronization is arrived at by the feedback arrangement previously discussed in detail above.
Thus numerous advantages of the present invention have been set forth in detail above. An instrumentation system employ-ing a resonant element sensor has been demonstrated that operates by converting light energy into resonant physical motion, while transmitting measurement data in terms of fre-quency through optical sensing means. Py eliminating elec-trical transmission between control room and field locations over copper wire conductors, problems associated with elec-tromagnetic interferences as in past such systems have been alleviated. Installation of the present optical network within process plants may be simplified by eliminating the need for separate optical fiber conductors for powering and sensing by effectively providing two-way communication over a single optical fiber. Additionally, the feedback technique of the present invention besides sustaining oscillations also allows the largest amplitude of vibration for the lowest possible power input. This arrangement thus is particularly suitable to permit the use of low power LED sources for com-municating over the distances involved while still maintain-ing an effective signal to noise ratio.
Although a preferred embodiment of the invention has been described in detail above, this is solely for the purpose of illustration and is not intended to be limiting. Numerous modifications will become apparent to those of skill in the art. For example, the invention has been described through-out as operating with resonant element sensors that are activated by electro-magnetic energy and hence a conversion from light energy to electrical energy has been shown. It ~6~
will be understood that other techniques could be devised for applying the supplied light energy to the sensor element to effect resonant physical motion without departing from the scope of the invention as defined in the accompanying claims.
Claims (4)
1. An instrumentation system for developing at a central station a measurement signal representing the value of a process condition of the type including a sensor that produ-ces an output signal that varies in accordance with changes in said condition, said sensor being at a field location re-motely positioned from said central station adjacent the process, light source means within said central station adapted to produce optical energy, said system further in-cluding means for transferring said optical energy between said sensor and said central station characterized in that:
a single optical fiber is used both for supplying optical energy to activate said sensor and for transmitting said out-put signal of said sensor to said central station for produ-cing said measurement signal.
a single optical fiber is used both for supplying optical energy to activate said sensor and for transmitting said out-put signal of said sensor to said central station for produ-cing said measurement signal.
2. The apparatus of claim 1 wherein said sensor is a reso-nant element whose resonant frequency of vibration is respon-sive to the value of said condition and including means coupled to said optical fiber for optically detecting the frequency of said resonant vibration.
3. The apparatus of claim 1 wherein said light source means comprises a single light source that produces a pulse train of light.
4. The apparatus of claim 3 including means to divert a portion of said pulse train of light along said single opti-cal fiber for detecting motion of said resonant element by reflecting light energy from said element back along said single optical fiber, the intensity of which is a measure of the position of said resonant element, and means to detect said reflected energy to derive a signal to cause said single light source to pulse on and off at the resonant frequency of said resonant element.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US350,687 | 1982-02-22 | ||
| US06/350,687 US4521684A (en) | 1982-02-22 | 1982-02-22 | Optical measurement system with light-driven vibrating sensor element |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1196706A true CA1196706A (en) | 1985-11-12 |
Family
ID=23377776
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000422005A Expired CA1196706A (en) | 1982-02-22 | 1983-02-21 | Fiber optic sensor |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4521684A (en) |
| EP (1) | EP0090167A3 (en) |
| JP (1) | JPS58155320A (en) |
| AU (1) | AU551797B2 (en) |
| CA (1) | CA1196706A (en) |
Families Citing this family (44)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2138137B (en) * | 1983-03-07 | 1987-01-14 | Brown Boveri Kent Ltd | Improvements in sensing apparatus |
| GB2144547B (en) * | 1983-08-04 | 1986-10-01 | Gen Electric Plc | A strain sensor |
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| SE422111B (en) * | 1980-06-23 | 1982-02-15 | Asea Ab | FIBEROPTIC COUPLET METDON |
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| US4356396A (en) * | 1980-12-17 | 1982-10-26 | Siemens Corporation | Fiber optical measuring device with compensating properties |
| US4379226A (en) * | 1981-02-02 | 1983-04-05 | Siemens Corporation | Method and sensor device for measuring a physical parameter utilizing an oscillatory, light modulation element |
-
1982
- 1982-02-22 US US06/350,687 patent/US4521684A/en not_active Expired - Fee Related
-
1983
- 1983-02-17 EP EP83101517A patent/EP0090167A3/en not_active Withdrawn
- 1983-02-17 AU AU11604/83A patent/AU551797B2/en not_active Ceased
- 1983-02-21 CA CA000422005A patent/CA1196706A/en not_active Expired
- 1983-02-21 JP JP58027505A patent/JPS58155320A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS58155320A (en) | 1983-09-16 |
| US4521684A (en) | 1985-06-04 |
| AU1160483A (en) | 1983-09-01 |
| AU551797B2 (en) | 1986-05-08 |
| EP0090167A3 (en) | 1984-08-29 |
| EP0090167A2 (en) | 1983-10-05 |
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