CA1277001C

CA1277001C - Proximity sensing device

Info

Publication number: CA1277001C
Application number: CA000528512A
Authority: CA
Inventors: Jeffrey David Edwards
Original assignee: Intellect Electronics Ltd
Current assignee: Intellect Electronics Ltd
Priority date: 1986-01-30
Filing date: 1987-01-29
Publication date: 1990-11-27
Anticipated expiration: 2007-11-27
Also published as: EP0256004A1; JPS63502540A; US5063306A; EP0256004A4; KR870007609A; WO1987004851A1; MY102421A

Abstract

ABSTRACT

A proximity sensing device comprising a sensor including a planar capacitor (1) having a central region (1a) and an outer region (1b) separated by a dielectric area, differentiator means (2,3) associated with said capacitor to form a spiked output pulse from a drive input pulse having at least one stepped edge and comparitor means (10) to repeatedly compare said spiked output pulses with a reference datum indicative of no proximity body being located adjacent to or touching said sensor to determine if such a proximity body is so located. The ratio of the perimeters of the central region and dielectric area of the planar capacitor is approximately 2:1 and its capacitance is comparable to that of a human finger.

Description

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PRO~IMITY SENSING DEVICE

The present invention relates to proximity sensing devices, and in particular capacitive sensing devices.
Prior art devices used to detect the proximity of a person or object, have as their principle o~ operation, the 5utili~ation of a change in capacitance o~ the sensing component altering the frequency of an oscillator, and the detection of this change in frequency as indicative of the presence of a person or object.
Such frequency dependent devices have had limited 10application particularly in situations where compact size is required and where environmental influences and objects, not being the desired subjects for detection, interfere with the frequency detection and ~enerate undesired and spurious results.
15This is found to be the case for capacitive sensing devices such as switches or keys that have been used in keyboa~ds and the like where there is a plastic or glass overlay. The presence of the overlay, tinting, moisture, dust, and finger grease have been known to affect the 20frequency of oscillation resultin~ in inaccurate detection.
Another prior art device known to the applicant is described in French Patent Application No. 7346455 (publication No. 2271538) entitled "Improvements in Capaciti~e Approach Detectors".
25The detector described therein only operates upon the approach of a conductive surface which is connected to earth~
The present inventio~ will operate whenever any capacitive body is proximate to the proximity sensin~ device the subject of the invention, and there is no necessity for 30the proximate body to be connected to earth. Accordinyly, the present invention has more applications than ~he detector described in the aforesaid French Patent.
Furthermore, the detector described in the French Patent is only able to detect two states, namely, the touching or not 35of the detector by the earthed conductive surface. The ,-1~770~

proximity sensing device of the present invention on the other hand, can detect capacitive bodies that are a diskance away from the detector, and in envirol~ents as above described such as through various thicknesses such as glass or plastic The present invention is also able to automa-tically correct for thicknesses of any barrier present between it and the proximate body whereas the detector described in the French Patent will not work as well behind a barrier unless there are provided extra plates such as those numbered 23 and 27 or there is additional circuitry for earthing such plates.
Furthermore, the French detector requires a shield which is not present in the subject invention. Accordingly, the subject invention is less expensive than the French detector.
In an effort to try and overcome the aforementioned disadvantages, the present invention provides a specially adapted capacitive sensor and permits the detection of a change in a differentiated signal caused by the proximity of a person, (or other capacitive object), and particularly, the accurate detection of a person's finger through glass or plastic.
In the case of a keyboard, where a number of sensors are re~uired, the sensors for the differentiated signal and any change thereto, which minimises interference from the scanning circuitry.
It has been found that the detection of a differentiated signal and any changes thereto rather than detection of a frequency change, provided advantages in that such previous environmental disturbances from cover plates, dust moisture etc., have a minimal or no effect upon the differentiated signal.
One such advantage is that, as the subjeGt invention is detecting changes in differentiated signal, the said change is proportional to the capacitance of the proximate body and also proportional to the distance away of the pro~imate body.
The specially adapted capacitive sensor comprises, a planar capacitor having an outside area and a central plate.
The planar capacitor is pref~rably etched directly on a ~27~

prin-ted circuit board, and this contribu-tes to the space an~
cost savings of the device.
It has been found that the size of the central plate affects the required dielectric distance between the ouksi~e area and central plate for the proper functioning of the capacitor as part of the proximity device. For a one ~inger size central plate a ratio of 1/3 between the cen-tral plate size and the dielectric distance is preferable. For a two finger size central plate, a ratio of 2/3 is preferable.
The layout of the planar capacitor can also be determined alternatively, and preferably, by the ratio between the outer perimeter of the dielectric between the outer area and central plate, and the perimeter of the central plate. A preferable ratio of about 2.1 has been determined for proper functioning.
Preferably connected to the back of the central plate of the planar capacitor, are a transistor, preferably bipolar, and a bias resistor for the transistor in a configuration such that the circuit behaves as an electronic differentiator of the drive signal. The connection to the back of the central plate allows a user's finger to approach the sensor without obstruction, and also the configuration is preferred as it minimises electrical interference whilst being as close as possible to the plate for a usable amount of signal. The gain is set for maximum output without introducing finger noise.
(It has been found that finger noise manifests itself as a 50 hertz oscillation).
The drive signal, generated externally, is applied preferably to the outside area of a planar capacitor, and picked up by the central plate. The drive signal needs to be a pulse with a sharp leading edge and preferably is a square wave pulse. It has been found that a CMOS driver circuit provides a suitable quality pulse.
As the circuit is behaving as an electronic differentiator, the square wave input results in a corresponding output of a sharp spike, at the Gollector of the transistor.

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The detection of a change in the amplitude of the spike is the means by which the presence of a user is determined.
The presence of ~ user's finger through glass or plastic over a planar capacitor interferes with the pickup of the drive pulse by the central plate. It is considered that the finger acts as a third plate for the capacitor, absorbing some of the drive pulse and thereby reducing the amplitude of the spikes coming from the transistor output. ~ finger on glass or plastic should preferbaly have a capacitance comparable to the capacitance of the planar capacitor and it has been fo~nd that where the value of capacitance is about the same a change of approximately 25% in the amplitude of the spike is observed.
The detection of the change in the differentiated signal (spike signal) is done by comparison of the differentiated signal, preferably after amplification, with a reference signal in a comparator circuit.
The comparator circuit is part of a spike processing circuit which comprises, in general an operational amplifier/filter circuit connected to a microprocessor. The micropocessor generates the square wave drive pulse and controls the delivery of such pulses to the planar capacitor sensor.
The comparator reference signal is initially obtained from the first square wave pulse differentiation when there is no finger present on the sensor.
The reference is constantly being monitored and re-established by the microprocessor. As a result, a ~urther advantage of the subject invention is that the invention automatically corrects for the thickness of any barrier between it and a proximate body. Logic in the microprocessor reads and checks the differentiated peak level to decide that it is within the expected range.
The spike processing circuitry serves the further purposes of amplifying the spikes and filtering out background noise.

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In a keyboard configuration, comprising a number of capacitative proximity sensors, a multiplexer controlled by the microprocessor is provided to switch to each sensor in turn and thereby scan the whole keyboard matrix.
As the d.c. level of each transistor is slightly different (due to manufacturing characteristics and resistor biasing), when each transistor level is switched through th~
multiplexer, a series of edges are produced which generate spikes that can swamp the spikes that need to be detected.
To overcome this, a window is provided by the spike processing circuitry under microprocessor control.
The window is a logical signal such as from a buffer or latch chip, driven by the microprocessor, which prevents the receipt of signal from a sensor until the multiplexer has switched to that sensor. In ~his way, the undesired spikes associated with switching are not processed.
In a preferred mode of operation, the drive pulse is not activated until after switching has occurxed and the window is opened. The whole process has been found to take a period of approximately 50 micro seconds per key which is amply sufficient for detection.
The microprocessor performs the further function of key character or character string output and key debouncing which is the filtering out of unwanted threshold oscillations associated with the approach of a finger to a sensor key.
FrGm the above, it can be seen that a keyboard ~planar capacitor sensors, differentiators, and multiplexer) and a microprocessor and support circuitry (spike processing circuitry comprisin~ amplifiers/filters) are necessary for the operation of the proximity device as a keyboard in applications such as a consumer access keyboard.
Conveniently the keyboard can be a separate part of the remaining device, being joined by a cable to the microprocessor circuitry and thereby increase the possible appllications of the device.
One preferred embodiment of the invention will be further described with reference to the figures as follows:

~2~7(~1 Figure 1 la~out sketch of a printed planar capacitor (shaded areas ar0 copper on a printed circuit board).
Figure 2 side view of a differentiator sensor comprising a planar capacitor, bipolar transistor and resistor.
Figure 3a single sensor circuit (the transistor and resistor of the differentiator of figure 2 are shown externally for clarity).
Figure 3b equivalent circuit for -the circuit of figure 3a.
Figure 4 key~oard sensor circuit (4 sensors) showing connection ~o multiplexer circuitry.
Figure 5 graphical representation of the square wave pulse in relation to the time se~uence of the window and clock. (All produced by microprocessor).
Figure 6 spike processing circuitry.
Referring to figures 1, 2, 3a and 3b, a planar capacitor 1 etched on a printed circuit board, has a bipolar transistor 2 and a bias resistor 3 connected from behind to the central plate la.
A square wave pulse received at the outside area lb is picked up by the central plate and differentiated by the circuit to produce a spike signal output at the collector o~
the transistor 4.
Referring to figure 4, an analog multiplexer 5, is shown connected to a keyboard array 6 consisting of 4 sensors. The multiplexer is controlled by a microprocessor (not shown) to switch to each key and thereby scan the ke~board continuously.
A counter 7 driver by a clock pulse from a microprocessor (not shown) is used to regulate the scan. The microprocessor is also used for ke~ character output and key debouncing as described earlier.
Referring next to figures 4, 5 and 6, a square wave drive signal is transmitted to a planar capacitor sensor from the microprocessor, only a~ter the multiplexer has switched to that particular sensor. The s~ike processing circuit shown in figure 6 performs its function under software control from the microprocessor.

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The microprocessor software opens a window 8, which is a buffer or latch chip, which allows signal to pass into the processing circuitry.
The graphical representation of this process is shown in 5figure 5, and the leading edge of the drive pulse (emphasized for clarity in figure 5) provides the detected spike signal.
The spike signal is further amplified by the amplifier 9 in the top branch of the figure 6 circuit as shown, and i~
cc~mpared with a ref~rence signal from the lower branch of the 10ci.rcuit at the junction 10 where an operational amplifier acts as the comparator. The lower branch of the circuit figure 6 develops the reference signal from the microprocessor which utilizes the signal on switching on of the device as previously described.

Claims

1. A proximity sensing device comprising:
a sensor including a planar capacitor having a central region and an outer region, the capacitance value of said planar capacitor being substantially unaffected by the presence of a proximity body adjacent to said sensor, differentiator means comprising a differentiator circuit, said planar capacitor forming part of said differentiator circuit, said circuit forming a spiked output pulse having an amplitude, said output pulse being formed in response to a drive input pulse having at least one stepped edge, and comparator means in circuit connection with said differentiator means for receiving said spiked output pulse and repeatedly comparing the amplitude of said spiked output pulse with a reference datum, the amplitude of said spiked output pulse being influenced by said proximity body located adjacent said sensor, said comparator means providing a proximity sensing output signal when the amplitude of said spiked output pulse is reduced as compared with said reference datum, said proximity sensing output signal thereby providing an indication of whether said proximity body is located adjacent to said sensor.

2. A proximity sensing device as claimed in claim 1, wherein the comparator means is comprised of an amplifier/filter circuit in circuit connection with a microprocessor.

3. A proximity sensing device as claimed in claim 1, wherein said planar capacitor is etched onto a printed circuit board.

4. A proximity sensing device as claimed in claim 1, wherein the differentiator circuit includes a transistor and a resistor biasing said transistor in circuit connection with one region of said planar capacitor.

5. A proximity sensing device as claimed in claim 4, wherein said transistor and said resistor are mounted on the back of a printed circuit board, the front side of the circuit board having said planar capacitor etched thereon.

6. A proximity sensing device as claimed in claim 1, wherein the size of said central region of the planar capacitor determines a dielectric distance or area between said central region and said outer region and the capacitance of said planar capacitor is similar to that of a human finger.

7. A proximity sensing device as claimed in claim 6, wherein the ratio of the perimeter of a dielectric area between said central region and said outer region and the perimeter of the said central region is approximately 2:1.

8. A proximity sensing device as claimed in claim 6, wherein the said central region and said outer region of said planar capacitor are a central and an outside area substantially in the same plane.

9. A capacitive proximity sensing device comprising:
a planar capacitor, the capacitance value of which being substantially unaffected by the presence of a proximity body adjacent the planar capacitor, drive signal means in circuit connection with said planar capacitor for providing a plurality of drive signal pulses to said planar capacitor, a differentiator means including said planar capacitor in circuit connection with other circuit elements, said differentiator means providing respective spiked output pulses in response to said plurality of drive signal pulses, each said spiked output pulse having an amplitude, and comparator means in circuit connection with said differentiator means for receiving said spiked ouput pulses, said comparator means comparing the amplitude of a first output spiked pulse referenced as indicating the absence of a proximate body with the amplitude of a subsequent spiked output pulse, the comparator means providing a proximity output signal when a reduction in the subsequent pulse amplitude as compared to the referenced pulse amplitude is determined, the proximity output signal providing an indication of said proximity body located adjacent said planar capacitor.

10. A sensing device comprising:
a plurality of proximity sensors such as in a keyboard, each sensor comprising:
a planar capacitor, the capacitance value of which being substantially unaffected by the presence of a proximity body adjacent the planar capacitor;
a transistor and a resistor in circuit connection with said planar capacitor to form a differentiator;
said sensing device further comprising:
drive signal means for providing a plurality of drive signals to said proximity sensors;
switch means operably connected to each of said sensors for selectively connecting a comparator means with one of said sensors, one at a time, such that each sensor is connected to the comparator before a respective drive signal pulse is applied to the sensor; and said comparator means being in circuit connection with said proximity sensors for receiving a respective spiked output signal, the respective output signal being in response to a respective drive signal being applied to a respective sensor, and for repeatedly comparing the amplitude of each said respective spiked output signal with a reference datum, the amplitude of each said respective spiked output signal being reduced by said proximity body located adjacent said respective sensor, said comparator means providing a signal indictive of the proximity of said body to said respective sensor when the amplitude of said respective spiked output signal is reduced compared with said reference datum.

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