GB2119931A

GB2119931A - Proximity detection devices

Info

Publication number: GB2119931A
Application number: GB08213296A
Authority: GB
Inventors: Graham Michael Stephens
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-05-07
Filing date: 1982-05-07
Publication date: 1983-11-23
Also published as: GB2119931B

Abstract

A proximity detection device comprises an oscillator adapted to produce a square wave oscillation, capacitive proximity sensor means whereby the self-capacitive proximity effect of an article or body to be detected modifies the rise and/or fall time of the square wave, and means for detecting the change between the two waveforms. Preferably means is provided for squaring up the modified waveform, which is then similar to the original square wave but delayed in time, and means is provided for detecting the delay between the two square waveforms. The detection device is readily adapted as a portable alarm the circuitry of which is shown in Figure 12. <IMAGE>

Description

SPECIFICATION Detection devices This invention relates to detection devices and has for its object the provision of a detection device for warning of the presence of articles or bodies (both animate and inanimate) by their self-capacitive proximity effect.

The common techniques for detecting articles or bodies (animate or inaminate) bytheirself- capacitive proximity effect usually involve methods whereby this effect modifies a capacitive potention divider (as indicated by Figure 1 of the accompanying drawings) or alters the frequency (see Figure 2) or amplitude (see Figure 3) of a high frequency oscillat9r. In general, the higher the frequency, the greater the effect. This effect depends upon the self-capacitance of the body (an apparent capacity to ground) and its distance from the sensing point.

The circuitry usually used in Figures 1,2 and 3 suffers from at least some of the following disadvantages: 1) The operation characteristics, e.g., sensitivity and susceptibility to false triggering, are often power supply dependent, and may vary according to fluctuations or variations in the supplied voltage.

This could come from the mains voltage, ageing batteries or the differences in power consumption between quiescent and "active" states.

2) High frequency oscillators often require specialised or precision components e.g. inductors, high stability capacitors etc., which are usually comparatively large and expensive.

3) Typicai high frequency oscillators are often prone to false or erratic operation, especially when in an electrically noisy environment (fluorescent lights, electric motors, radios, etc.).

4) High frequency oscillators often generate interference in their own right unless adequate "screening" precautions are taken, and this often reduces the sensitivity of the detection circuit.

Thus a further object of the invention is to overcome these disadvantages.

According to the present invention, a detection device comprises a high frequency oscillator adapted to produce a square wave oscillation, means whereby the self-capacitive proximity effect of an article or body to be detected is to modify the rise and/or fall time of the square wave, and means for detecting the change between the two waveforms.

The basis of the invention is illustrated by Figure 4 in which the left-hand side shows the basic circuit elements and the right-hand side shows the two waveforms. The nominal midpoints of the waveforms are separated by time 't'. It should be noted that neither the duty cycle nor the frequency of the square wave is critical to observe this effect, as time 't' is a function of the time constant formed by Rand the body's capacitance. (The square wave duty cycle or its frequency should not be such that time 't' approaches that of the positive-or negative-going pulse of the waveform as the resultant waveform at 'b' should be allowed to have a similar amplitude to the signal at 'a').

Conveniently, means is provided for squaring up the modified waveform, which is then similar to the original square wave but delayed in time, and means is provided for detecting the delay between the two square waveforms. The basis of this is illustrated in Figure 5.

The delay between the two waveforms may be detected in several different ways using, for exam ple, shift registers, counters, flip-flops, monostables etc., but some of these techniques require additional circuitry to operate reliably - especially when using edge-triggered devices.

However, a much simpler circuit, which also allows a differential mode of operation, is shown in Figure 6. The VRI/CI combination is used to remove any residual "spikes" caused by the inverter's propagation delay. The various waveforms of Figure 6 are shown in Figures 7. The gate function may be OR, NOR, AND, NAND, EXOR or EXNOR, Depending upon the type of gate function used, shorting the sense points to either + or - could render the gate "inactive". (This is also true for detectors using shift registers etc). However this may be overcome by ensuring that the sense points are joined to the gate via capacitors, thus blocking any D.C. component. If the EXOR or EXNOR gate function is used, then the inverter, and therefore its propagation delay, may be eliminated, as shown in Figure 8, although the choice of gate function depends upon the application.

If an integrated circuit is used for the gate function then this has some advantages in that the input characteristics (switching thresholds, stray capacitance etc.) are usually better matched, and such integrated circuits usually contain four independent gate functions. For economy, the unused gates may be used to generate the square wave, and to select the polarity of the output pulses - assuming that the gate function can be made into an inverter. (If the aforementioned circuits are used as remote sensors, then the output polarity should be chosen for "fail-safe" operation in the event of power loss at or disconnection of the detector).

The detector portion may be improved in some applications by the addition of resistors (R) between the sense points and the gate input. These resistors can protect the gate inputs from accidentally-applied high voltages and, in conjunction with the gate's inherent input capacitance, form a low pass filterl spike remover which may reduce even further any susceptibility to external interference. To define this filter characteristic more accurately, capacitors (C) may be added at these points if necessary, as shown in Figure 9, and if high voltages are a possibility, then additional precautions should be taken.

For differential use, one of the resistors may be a fixed value, R, and the other resistor variable (VRI) to coverthe required range determined by the application. Any change in capacitive effect at the sensing input e.g. the addition of a body at one of the inputs, or the removal of a body from the other, could be detected. A VRI may be used to balance out the quiescent state, as shown in Figure 10.

For single ended applications, the resistors in the non-sensing gate input may be repiaced with one resistor which nominally equals the value of the other input's resistors, as shown in Figure 11.

The square wave oscillator frequency may be very low, and for many applications it may typically be below 100 Hz. In this way, any generated interference is negligible. The frequency may also vary both in the long and short term, as shown in FigureS. The waveform amplitude - and hence the power supply may vary within reasonable limits (typically set by the type of gate), the components used are neither critical nor specialised, and the gate functions are readily available in integrated circuit form. If CMOS gates are used with a battery power supply, then battery life may well approach its "shelf" life.

The arrangement of Figure 11 forms the basis of a portable alarm, the circuitry of which is shown in Figure 12 and the controls of which are shown in Figure 13 on a casing having a loudspeaker grille.

When the sensing wire is touched, or a metallic object (such as a door handle or latch) to which the sensing wire is attached is touched, the alarm is triggered, even if the potential intruder - or other being to be detected - is wearing rbber-soled shoes, or gloves. An optional delay of about ten seconds is incorporated to allow the owner of the alarm to disable it, and this delay can also serve to confuse the potential intruder as to what action set off the alarm and so scare the potential intruder away. The alarm may be set to sound continuously or for about ten seconds after which time it will automatically cancel and reset itself, so if it has not disturbed the owner then it may well have served its purpose by scaring away a potential intruder, instead of sounding an alarm after an intruder has gained entry.

It should be noted that this particular application as described in the preceding paragraph is not limited to use as a burglar alarm, but may also be used to protect children, the infirm, or pets from dangerous or forbidden areas or articles by giving a warning of approach.

Claims

1. A detection device comprising an oscillator or adapted to produce a square wave oscillation, means whereby the self-capacitive proximity effect of an article or body to be detected is to modify the rise and/or fall time of the square wave, and means for detecting the change between the two waveforms.

2. A detection device as in Claim 1, wherein means is provided for squaring up the modified waveform, which is then similar to the original square wave but delayed in time, and means is provided for detecting the delay between the two square waveforms.

3. A detection device as in Claim 1, and substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

4. A detection device as in Claim 1, and substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

5. A detection device as in Claim 1, and substantially as hereinbefore described with reference to Figures 6 and 7 of the accompanying drawings.

6. A detection device as in Claim 1, and substantially as hereinbefore described with reference to Figures 8 to 11 of the accompanying drawings.

7. A detection device as in Claim 1, and substantially as hereinbefore described with reference to Figures 12 and 13 of the accompanying drawings.