GB2205950A

GB2205950A - Capacitive proximity sensor

Info

Publication number: GB2205950A
Application number: GB08709828A
Authority: GB
Inventors: Paul Coleman; Tom Bach
Original assignee: Coleman Electronic Designs Ltd
Current assignee: Coleman Electronic Designs Ltd
Priority date: 1987-04-24
Filing date: 1987-04-24
Publication date: 1988-12-21
Also published as: WO1988008595A1; AU1625188A; GB8709828D0

Abstract

A loop (1) or strip (2) of conducting foil is attached to an object to be protected. The strip or loop is connected to one end of a high inductance coil (3), the other end being grounded. Another coil (5) of high inductance is wound co-axially of coil (3). Coil (5) is connected via a diode (6) to an oscillator (7) operating at about 80 KHz. The oscillator output is coupled to a means which produces a signal which is frequency modulated (FM) by proximity of e.g. an intruder to the protected object. Intruder induced frequency variations tend to fall within a fixed frequency band, and thus can be distinguished from variations induced by other causes. The intruder induced variations are detected by using a band-pass frequency discriminating amplifier (10) and a comparator (11) which triggers (14) an alarm (13). The high inductance coils and use of FM reduce interference.

Description

CAPACITANCE PROXIMITY SENSOR This invention relates to a capacitance proximity sensor, an embodiment of the sensor comprises an alarm system capable of the protection of precisely defined areas such as table displays or pictures, without any obvious visible indications of the employment of said system.

Current devices suffer from one or more of the following disadvantages and limitations; 1) They only offer protection for metal objects, or the objects have to be covered or filled with metal foil.

2) The consumption of electric current is wastefully high resulting in the frequent need of battery replacement in those devices which are battery operated.

3) Protection is only achieved very close to the surface of a defined area, typically within one or two inches.

4) There is no provision for any variation of sensitivity.

5) The electronics may be subject to drift and consequent false alarms.

6) The device requires a long time to set up and also requires a long stabilisation period before being ready for use.

The present invention aims to substantially improve on at least one or more of the above limitations.

According to an embodiment of the present invention, there is provided a configuration of conducting foil strip attached to the selected boundary of the protected area, said configuration of conducting foil strip, which forms a capacitance to earth, is connected to an electronic circuit contained in a suitable enclosure. Said electronic means comprises an inductive coil which together with said configuration of conducting foil strip controls the frequency of an oscillator operating at a typical frequency around 80kHz. The presence of an intruder, or intruders, or hand or other member of said intruder or intruders changes the capacitance of the configuration of conducting foil strip and thus alters the oscillator frequency.Following various means to eliminate interference either to or from the outside environment, said means to be subsequently described, a standard charge pump circuit achieves a voltage output dependant on the frequency of the aforementioned oscillator. There follows means to amplify this change of voltage output, enabling minute changes to be detected. A subsequent window comparator, whose thresholds can be set by the user, determines the voltage upper and lower limits required to trigger the alarm.

There follows a description by way of specific embodiment of the present invention, reference being made to the accompanying drawings, in which; Fig 1 is a schematic diagram of one embodiment of the system.

Fig 2 is a schematic block diagram of the circuitry employed.

Fig 3 is a schematic circuit diagram of the high Q oscillator.

Fig 4 is a schematic circuit diagram of the reset mechanism.

Fig 5 is a schematic diagram of the protection area provided by a single loop of the conducting foil.

Fig 6 is a schematic diagram of the protection area provided by a double strip of conducting foil.

Fig 7 is a schematic diagram of the protection area provided by a triple strip of conducting foil.

In one embodiment the system is intended to protect a table display. A loop or strip 1 of conducting foil is attached to the underside of the display table 2 around the perimeter, thus defining the area to be protected. Said loop or strip 1 is connected to one end of a high inductance coil 3, typically around 30-lOOmH, the other end of the coil being connected to earth 4. Another high inductance coil 5 is wound coaxially of the coil 3 and is connected through a diode 6 to a first stage transistor oscillator 7 operating around 80kHz. The diode 6 allows the oscillator to swing above and below the rail voltage permitting the full Q of the circuit to be realised.

This first stage oscillator 7 consumes little power and oscillates in a sine-wave mode; both these features advantageously minimise interference to the outside environment. The utilisation of high inductance coils has several advantages; a) It permits the use of lower frequencies (typically around 80kHz) minimising interference to outside radio reception including long-wave in particular.

b) It permits the use of low loop capacity needing minimal areas of conducting foil instead of large metal plates as with some systems. This in turn means that tiny changes in capacitance due to an intruders hand up to five feet away produce a proportionately larger change which is detectable in this present invention.

c) It allows the limiter 8 and charge pump 9 to operate on very small currents contributing to the overall very low current consumption of the present invention. (Typically around 50-100 micro-amps).

In addition the use of two coils results in a considerable reduction in false alarms due to the loop picking up the effects of nearby electrical disturbances such as are produced by the turning on or off of fluorescent lights or other spark inducing devices. The exact cause of this benefit is not known.

The output from the first oscillator 7 is fed into a limiter 8 incorporating some feed back which also acts as the second stage of the oscillator. The subsequent output is no longer amplitude modulated, only frequency modulated by the intruder. This also benefits outside interference suppression in addition to that obtained by the coil 3, 5 as heretofore described.

The oscillator output is fed into a standard charge pump 9. Said apparatus achieves the aforementioned varying voltage output dependant on the varying oscillator frequency caused by the movement of the intruder or intruders, or hand or other member of said intruder or intruders.

Said varying voltage is amplified by a band-pass frequency discriminating amplifier 10. For changes above about O.1Hz but below about 15 Hz the gain is around 250. For changes above about 15 Hz or below about 0.1 Hz down to DC, the gain is around unity.

Intruder induced variations are generally above about 0.1 Hz and below about 15 Hz, while temperature or other unwanted electronic component changes will be typically below 0.1 Hz or above 15 Hz, and their undesirable effects eliminated.

The output of the amplifier is fed into a window comparator 11 which has two levels, the lower one is set permanently around 0.4 colts, and the upper one is set by a potentiometer 12. If the earth lead 4 or loop 1 are not connected or become disconnected the output of the amplifier goes below 0.4 volts and the alarm 13 is triggered by means of a triggering device 14. This prevents any tampering with the system. A timing mechanism 15 resets the alarm after a suitable period typically about fifteen seconds. Normally the amplifier output lies above 0.4 Volts. The approach of an intruder or intruders causes the voltage output to rise until it reaches the level set by the potentiometer 12, when the alarm 13 is triggered.

This potentiometer 12 is thus a sensitivity control.

By placing it between the positive and earth rails, which are regulated, stabilised sensitivity settings are achieved virtually independantly of temperature or power supply variations.

Low settings will protect only near the foil strip and not in the centre of the loop as in 16. Higher settings add protection towards the centre of the loop as in 17 and 18.

The relatively low frequency response of the system (typically around 0.1 Hz) would necessitate a long wait after turning on (typically 20 minutes or more) before said system was sensitive and read for use, were it not for the use of an auto-reset mechanism 19 also shown in Fig 4. When the unit is turned on power is supplied from the positive rail to the series connected capacitor 40, 41 via a switch 42. For about one second the gate 20 is on, thus setting the output of amplifier 10 close to the input of said same amplifier. This forms a partial coarse reset. Another gate having a large series resistor (typically 1 MegOhm) is also turned on for about ten seconds. This forms a fine reset helping the high gain amplifier to reach its final voltage, allowing the circuit to become quickly stabilised and ready for the detection of intruders. During this reset period the alarm is held off. The alarm sounder 13 may be typically a loud, highly efficient piezo-electric buzzer or siren or other acoustic apparatus. Alternatively outputs and sockets are provided for connection to other standard alarm indicators.

In another embodiment of the invention, (see Fig 6) two single parallel foil strips are used 22, 23, and are connected to respective ends of the first coil 3.

This eliminates the need for an earth lead and creates a protection barrier 24 having applications within door-frames 25 where access is to be restricted or monitored.

In a further embodiment of the invention (see Fig 7), three concentric foil strips are used 26, 27, 28, and connected to the first coil 3 which is now centre tapped; one strip being connected to each end of the coil 3 and one strip being connected to the centre tap. This creates a narrower protection barrier 29, and also eliminates the need for an earth lead.

In a further embodiment other configurations of the conducting foil strip may be used to define other secure regions to be protected by the invention.

In a further embodiment the system may be used to trigger additional apparatus either by wire or by telemetric means employing radio, or infra-red, or other such means, for the purpose of security or other monitoring of the proximity or movement of animate or inanimate object or objects, whether their motion be due to means internal or external to the object or objects.

Although reference has been made to foil strips and concentric arrangements of loops, the conductive foil may be of any suitable shape and configuration, and need not be in the form of closed loops.

While embodiments of the present invention have been disclosed hereinbefore, it will be appreciated that the invention is not limited thereto, but may be otherwise embodied within the scope of the following claims:

Claims

CLAIMS 1) A capacitance proximity sensor wherein electrically conductive foil is connected to electronic means capable of sensing changes in capacitance.
2) A sensor according to claim 1 wherein said electronic means comprises an oscillator having a high inductance coil arrangement coupled to the foil, the frequency of the oscillator being varied by the proximity of an object to the foil.
3) A sensor according to claim 2 wherein the inductance of the coil arrangement is in the range 30 to lOOmH.
4) A sensor according to claim 2 or 3, wherein the coil arrangement comprises a pair of coaxial coils, one connected to the foil, the other connected to the oscillator.
5) A sensor according to claim 1, 2 or 3 or 4, wherein the oscillator output is fed into a means to produce a signal that is frequency modulated by the proximity of said object.
6) A sensor according to claim 5, wherein the output of the frequency modulated signal producing means is fed into means to produce an output of varying voltage dependant on the frequency variations.
7) A sensor according to claim 6 wherein the output of the varying voltage producing means is amplified by a bandpass frequency discriminating amplifier to eliminate voltage variations other than those due to the proximity of said object or objects.
8) A sensor according to claim 7 wherein the output from the band pass frequency discriminating amplifier is fed into a comparator.
9) A sensor according to claim 8 wherein the output of the comparator triggers an alarm device.
10) A sensor according to claim 8 or 9 wherein the comparator is a window comparator with upper and lower thresholds.
11) A sensor according to claim 10 wherein the upper threshold is variable.
12) A sensor according to claim 10 or 11 wherein the lower threshold is set to detect faulty connection between the said electronic means and said foil, and/or earth connection.
13) A sensor according to any preceding claim wherein an auto-reset mechanism is incorporated to achieve rapid setting up and stabilisation of the sensor.
14) A sensor according to claim 2 or any of claims 3 to 13 when dependant on claim 2 wherein a diode is connected in series with the coil to allow oscillations above and below the rail voltage resulting in a high Q factor.
15) A sensor according to clam 2 or any of claims 3 to 14 when dependent on claim 2 wherein said coil has a primary and secondary winding, both of high inductance, to minimise electrical interference external to the said configuration and said electronic means.
16) A sensor according to claim 9 wherein said alarm device is a piezo-electric sounder.
17) A sensor according to claim 9 or 16 wherein a time delay is incorporated to switch the alarm off after a predetermined period of sounding.
18) A capacitance proximity sensor substantially as hereinbefore described with reference to the accompanying drawings.
19) A system for use with an electrically conductive member for detecting changes in capacitance with respect to the member, the system comprising an oscillator including an inductive arrangement for coupling to the coil, the inductive arrangement having a high inductance.
20) A system according to claim 19, wherein the inductive arrangement comprises a pair of magnetically coupled coils.
21) A system according to claim 20, wherein the coils are co-axial.
22) A system according to claim 19, 20 or 21 wherein the electrically conductive member comprises a plurality of conductive foils connected to the inductive arrangement.