AU602652B2 - Proximity sensing device - Google Patents

Description

rI 41 AU-A 1-55101/86 LJIPh~ 1'

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PCT WORLHNT E LP RT "6 N TION T Y INTERNATIONAL APPLICATION B HE 2 NDER THE PATENT COOPERATION TREATY (PCT) i (51) International Patent Classification 4 (11) International Publication Number: WO 87/ 04851 H01G 1/015, G01B 7/08 Al HO1G 7/14 (43) International Publication Date: 13 August 1987 (13.08.87) (21) International Application Number: PCT/AUS6/00043 (81) Designated States: AT (European patent), AU, BE (European patent), CH (European patent), DE (Euro- (22) International Filing Date: 20 February 1986 (20.02.86) pean patent), FR (European patent), GB (European patent), IT (European patent), JP, LU (European patent), NL (European patent), SE (European patent), (31) Priority Application Number: PH 4377 US.

(32) Priority Date: 30 January 1986 (30.01.86) Published (33) Priority Country: AU With international search report.

With amended claims.

(71) Applicant (for all designated States except US): INTEL- LECT ELECTRONICS LTD. [AU/AU]; Level 2, 39 Hay Street, Subiaco, W.A. 6008 (AU).

(72) Inventor; and Inventor/Applicant (for US only) EDWARDS, Jeffrey, David [AU/AU1; 48A Alston Avenue, Como, W.A. J. EP 1 6152 E P (74) Agent: EDWD. WATERS SONS; 50 Queen Street, Melbourne, VIC 3000 2 This document contains the 4 OT dmients made undr 5 OF Section 49 and is correct for prin tin.D (54) Title: PROXIMITY SENSING DEVICE ij 1k BC549 SIMPLtFIED VIEW OF 5IN .SENSOQ..

(57) Abstract A proximity sensing device comprising a sensor including a planar capacitor having a central region (la) and an outer region (lb) separated by a dielectric area, differentiator means 3) associated with said capacitor to form a spiked output pulse from a drive input pulse having at least one stepped edge and comparator 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.

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WO 87/04851 PCT/AU86/00043 PROXIMITY 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 of operation, the utilization of a change in capacitance of 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 application 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 generate undesired and spurious results.

This is found to be the case for capacitive sensing devices such as switches or keys that have been used in keyboards 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 frequency of oscillation resulting in inaccurate detection.

In an effort to try and overcome the aforementioned disadvantages, the present invention provides a specially adapted capacitive sensor and 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 persons finger through glass or plastic on a capacitive sensor according to the invention.

In the case of a keyboard, where a number of sensors are required, the present invention further provides a method of scanning 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 WO 87/04851 PCT/AU86/00043 2 such previous environmental disturbances from cover plates, dust moisture etc., have a minimal or no effect upon the differentiated signal.

The specially adapted capacitive sensor comprises, a planar capacitor having an outside area and a central plate. The planar capacitor is preferably etched directly on a printed circuit board, and this contributes to the space and cost savings of the device.

It has been found that the size of the central plate affects the required dielectric distance between the outside area and central plate for the proper functioning of the capacitor as part of the proximity device. For a one finger size central plate a ratio of 1/3 between the central plate size and the dielectric distance is preferable. For a two finger size central plate, a ratico 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 a electronic differentiator of the drive signal. The connection to the back of the central plate allows a users 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 by up the central plate. The drive signal needs to __111~ WO 87/04851 PCT/A U86/00043 3 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 collector of the transistor.

The detectioh of a change in the amplitude of the spike is the means by which the presence of a user is determined.

The presence of a users 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.

A finger on glass or plastic should preferably have a capacitance comparable to the capacitance of the planar capacitor and it has been found that where the value of capacitance is about the same a change of approximately 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 microprocessor 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. Logic in the L F WO 87/04851 PCT/AU86/00043 4 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 amplifing the spikes and filtering out 1 5 background noise.

1 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 the 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 micropressor, which prevents the receipt of signal from a sensor until the multiplexer has switched to that sensor. In this way, the undersired spikes associated with switching are not processed.

In a preferred mode of operation, the drive pulse is not activated until after switching has occured 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.

From the above, it can be seen that a keyboard (planar capacitor sensors, differentiators, and multiplexer) and a microprocessor and support circuitry (spike processing i S WO 87 /04851 PCT/A 86/00043 5 /f circuitry comprising 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 applications of the device.

One preferred embodiment of the invention will be further described with reference to the figures as follows: Figure 1 layout sketch of a printed planar capacitor (shaded areas are 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 differentator of figure 2 are shown externally for clarity).

Figure 3b equivalent circuit for the circuit of figure 3a.

Figure 4 keyboard sensor circuit (4 sensors) showing connection to multiplexer circuitry.

Figure 5 graphical representation of the square wave pulse in relation to the time sequence 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 pointed 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 Ib is picked up by the central plate and differentiated by the circuit to produce a spike signal output at the collector of the transistor 4.

Referring to figure 4, an analog multiplexer 5, is shown connected to a keyboard array 6 consisting of 4 WO 87/04851 PCT/AU86/00043 6 sensors. The multiplexer is controlled 'y a microprocessor (not shown) to switch to each key and thereby scan the keyboard 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 key 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 after the multiplexer has switched to that particular sensor. The spike processing circuit shown in figure 6 performs its function under software control from the microprocessor.

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 figure 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 is compared with a reference signal from the lower branch of the circuit at the junction 10 where an operational amplifier acts as the c. nparator. 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.

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Claims (11)

1. A proximity sensin 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 cc-nparator means in circuit connection with said differentiator means for receiving said spiked output pulse and repeatedly comparing the amplitude of said spiked output *ago 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 0 microprocessor.

3. A proximity sensing device as claimed in claim 1 or 2, wherein said planar capacitor is etched onto a printed circuit board. -i -8-

4. A proximity sensing device as claimed in claim 1, 2 or 3, wherein the differentiator circuit includes a transistor and a resistor biasing said transistor in circuit connection with one region of said planar capacitor.

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 any one of claims 1 to 5, 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 j 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. agog

8. A proximity sensing device as claimed in claim 6 or 7, wherein the said central region and said outer region of said planar capacitor are a central and an outside area 0 90, substantially in the same plane.

9. A capacitive proximity sensing device comprising 0 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, each drive signal pulse having at least one stepped edge, F -9- 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 output pulses, said comparator means comparing the amplitude of a first output spiked pulse referenced as indicating the absence of a proximate body with une 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 I providing an indication of said proximity body located adjacent said planar capacitor. e 0o

10. A sensing device comprising OO •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, each drive signal having at least one stepped edge; 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 nf n -n 10 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 indicative 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.

11. A device as claimed in claim 1, 9 or substantially as herein described with reference to the accompanying drawings. DATED THIS 15TH DAY OF JANUARY, 1990 INTELLECT ELECTRONICS LTD. WATERMAR PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN, VICTORIA 3122 *AUSTRALIA LJD:RCTS:JC (14.4) I eeoc L Lr