GB2197462A - Sensor output change detection circuit - Google Patents

Abstract

A circuit for detecting fast changes in the voltage output of a sensor 2 includes a voltage controlled oscillator (10) connected to the sensor, which produces a signal which is frequency modulated when the sensor output fluctuates and a processor 14 which produces an alarm signal when the rate of change of frequency exceeds a predetermined threshold rate. The processor may operate digitally and may operate with a feedback circuit 12 for stabilizing the output of oscillator 10. The circuit may be used with a pyroelectric detector 4 in a passive infra-red sensor as shown or in sensors for detecting the presence of chemicals or temperature in which the detector includes a ChemFET or thermopile respectively. <IMAGE>

Description

SPECIFICATION Sensors The present invention relates to sensors of the type including a detector which products a voltage output the level of which represents the amount of the quantity to be sensed. The voltage signal output also typically includes a DC offset. Such sensors include heat sensors where the detector is a thermopile, sensors for various chemicals where the detector is a ChemFET, and intruder sensors where the detector is an infra-red sensitive ceramic pyroelectric device. The invention more specifically relates to the electrical circuitry used in such sensors for detecting fluctuations in the sensed quantity.

For the purposes of explaining the invention a passive infra-red sensor will be referred to but it will be appreciated that the ideas also relate to other types of sensor. In the case of a passive infra-red sensor the sensed quantity is changes in incident infra-red radiation which may be due to the presence of an intruder.

The circuitry must convert this information into an alarm signal.

In an existing type of passive infra-red sensor an optical arrangement directs infra-red radiation from a plurality of distinct zones onto an element of an infra-red sensitive detector.

As is well known, such sensors work on the principle that as an intruder passes in and out of the zones defined by the optical arrangement, the infra-red radiation received by the detector will fluctuate with a low frequency above a substantially constant amount representing the radiation received from the surroundings. In order to detect this low frequency oscillation, which is generally within the range 0.3 to 10Hz, the voltage signal which represents the level of radiation received by the detector is fed through a band limited amplifier which amplifies the AC component of the voltage signal output from the detector within the required frequency range.

Any DC offset, which may be introduced into the output of the detector because of the biasing of the FET which acts as a buffer amplifier for the pyrodevice, must be removed by the amplifier. The output of the amplifier is then compared with a threshold and if the amount of energy within the monitored frequency range exceeds the threshold, an alarm signal is generated. Because of the low frequencies to which the band limited amplifier must be tuned, large values of capacitance are required in the amplifer stages requiring the use of bulky electrolytic capacitors. Such capacitors cannot be provided within integrated circuits. Therefore, the electrical circuitry provided for use with existing passive infra-red sensors cannot be produced on an integrated circuit and must be assembled on a printed circuit board.This leads to longer and more expensive assembly than would be possible if the required circuitry were reduced to one or more integrated circuit chips.

The present invention is directed towards solving the technical problem of providing circuitry for use in any sensor of the defined type which circuitry can be manufactured by integrated circuit techniques.

The present invention accordingly provides an electrical circuit for use in a sensor, comprising a detector adapted to produce a voltage output having a magnitude related at least in part to the amount of a quantity to be sensed, a voltage controlled oscillator having a control input connected to the output of the detector, a processing circuit connected to the output of the voltage controlled oscillator for sensing any rate of change in the frequency output of the oscillator in excess of a predetermined rate in order to initiate an alarm.

By connecting the output of the detector to a voltage controlled oscillator (VCO) the low frequency fluctuations in the amplitude of the sensed quantity, for example infra-red radiation received by the detector, are converted into a frequency modulated output from the voltage controlled oscillator. Preferably the processing circuit comprises means for converting the varying frequency signal to a sequence of digital samples each representing the instantaneous frequency of the input signal and a microprocessor for processing the digital samples.

All the components of this electrical circuitry can readily be integrated since the processing now required is essentially a timing measurement rather than a measurement of the amplitude of a low frequency signal.

Preferably the processing circuit produces a control signal representing the difference between the output frequency of the voltage controlled oscillator and a predetermined frequency, which control signal is fed back via a long time constant feedback circuit to the control input of the oscillator. Such a feedback arrangement maintains the fundamental frequency output of the VCO at a constant value despite slow changes in the ambient infra-red radiation received by the element from the surroundings, or changes in DC conditions caused by changes in leaking current, temperature changes etc. Feedback may also be applied to one input of a frequency subtraction device which has another input connected to the output of the oscillator so that only the difference frequency is fed to the processing circuit.

Some embodiments of electrical circuitry for use in passive infra-red sensors will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of a first embodiment of electrical circuitry for use in a passive infra-red sensor; Figure 2 is a block diagram of a second embodiment; and Figure 3 is a block diagram of a third embodiment.

A pyroelectric infra-red detector 2 includes an element 4 which is sensitive to received infra-red electromagnetic radiation. The element 4 is typically connected to a buffer such as an FET 6 with the output taken from the source terminal of the FET. The drain terminal of the FET is connected to a power line 8 via a resistor. Such pyroelectric infra-red detectors are commerically available, for example, as manufactured by Mullard (e.g. RPY 97). The output from the FET buffer is a voltage signal the magnitude of which represents the changes in level of infra-red radiation impinging on the element 4. The biasing arrangement for the FET may also introduce a DC offset into the output.

When the detector is mounted in a passive infra-red sensor which includes an optical arrangement that defines a number of discrete zones, the output from the detector 2 will include the DC offset and a low frequency component which occurs when an intruder is crossing the zones defined by the optical arrangement. It is the purpose of the present circuitry to detect the existence of any low frequency component in the detector output and thereby sense the presence of an intruder.

In the embodiment of the circuit of the present invention illustrated in Fig. 1, the output of the detector 2 is connected to a control input of a VCO 10. If the input from the detector is substantially only the DC offset then the output from the VCO 10 will be a signal at a substantially constant frequency. However, where there is an amplitude variation in the input from the detector, the output signal from the VCO 10 will be frequency modulated at a frequency corresponding to the frequency of the amplitude variation, that is the frequency at which an intruder is crossing the zones defined by the optical arrangement of the infra-red sensor.

The output of the VCO 10 is fed to a processor 14, which carries out a digital filtering operation. Firstly, the procesor includes means for converting the input signal to a sequence of digital samples each representing the instantaneous frequency of the input signal. This means may effectively repeatedly measure the period of the input signal and output a digital sample representing that period. The digital samples received over a preceding time interval are stored so that they can be processed in a suitably programmed microprocessor within the processor 14. The microprocessor program is designed to identify any rapid change in the frequency output from the VCO which is due to the presence of an intruder.

For example the processor may measure the frequency of the VCO output at successive intervals and differentiate this frequency signal to produce an alarm signal on output line 16 if the frequency differential in time exceeds a predetermined threshold. The frequency signal can also be fed out on line 18 to a feedback circuit 12. This frequency signal is converted into a control voltage which represents the difference between a fixed frequency and the measured frequency. The feedback circuit also includes a time delay and smoothing circuit to remove brief fluctuations in the frequency signal and only pass on long term variations to the VCO in order to stabilize the output from the VCO at a substantially constant frequency, which is independent of the slow variations in background infra-red radiation and variations in the DC offset of the detector.Since such frequency variations occur over relatively long periods and therefore, provided the time constant of the feedback circuit 12 is sufficiently long, this stabilization will not mask the more rapid frequency modulation of the VCO output due to the amplitude variations of the detector input.

The above described circuit operates as follows.

In the steady state when there is no intruder present, the only variation in the input to the VCO will be due to slow variations in the ambient level of infra-red radiation and to variations in the DC offset. Typically these may cause the voltage level of the control input of the VCO 10 to vary between 900 and 1100 mV. Without the feedback, this would result in a variation in the VCO output between, say, 100 Hz and 10K Hz. However, the feedback circuit 12 is used to stabilize the output of the VCO 10 to, say, 1 kHz. This is achieved by arranging that the input from the feedback circuit is added to the voltage received from the detector 2. Therefore, if the processor 14 senses an increase in the frequency output the feedback circuit 12 will generate a negative voltage to be added to the output from the detector to reduce the frequency of the VCO output.

When an intruder is present the amplitude of the detector output will vary with a frequency in the range of 0.3 to 10 Hz. The amplitude variation however will be slight, typically 0.1 mV. In the example given this will result in the output from the filter 14 deviating either side of the stabilized 1 kHz frequency by plus and minus 10 Hz. Since this frequency change will be relatively rapid (taking place within 0.05 to 2 seconds) relative to the time constant of the feedback circuit 12 it will be effectively ignored by this feedback circuit.

The processor measures the rate of change of frequency and outputs an alarm signal when the rate of change exceeds a predetermined threshold, for example 5Hz/sec.

The embodiment of Fig. 2 is similar to the embodiment of Fig. 1 except that the output of the feedback circuit 12 is fed to one input of a frequency subtraction device 19, which receives at its other input the output from the VCO 10. The feedback circuit 12 outputs a signal with a frequency representing a base level produced by the VCO in response to the DC offset plus any variations due to slow temperature changes, fluctuations in the leakage current etc. The output of the frequency subtraction device 19 therefore represents only instantaneous frequency variations and in the absence of an intruder there should be no output to the processor. Any output from the device 19 may represent either a variation in the DC offset which will be cancelled out by the feedback circuit or an intruder.The distinction is made as described with reference to the embodiment of Fig. 1 by measuring the rate of change of frequency.

An alternative embodiment of the processing circuit for a dual element detector is illustrated in Fig. 3. In this case, there are two elements 2A, 2B in the detector. The output from each element is fed to a respective VCO 10A,10B. The outputs from these oscillators are then fed to a mixer 20, the output of which is connected to a processing circuit 22.

The processing circuit 22 includes a filter for producing the difference frequency between the two inputs from the VCOs 10A, 10B. The element 2A is connected so as to receive radiation from the zones defined by the optical arrangement, and the detector 2B is positioned so as to receive the infra-red background radiation but not to be affected by the passage of an intruder across the zones. This can be achieved by placing the detector 2B in the sensor behind a plain infra-red transmitting window whereas the element 2A is positioned relative to the optical arrangement so that it receives radiation only from the zones.In this case the frequency input from the VCO 10B will vary in response to the variation in background infra-red radiation whereas the frequency from the oscillator 10A will vary in response to that radiation and also in response to the presence of an intruder. The difference frequency generated by the mixer 20 will therefore be only the frequency variation due to the intruder. In this case the processing circuit will need to monitor the change in frequency output from the mixer to produce an alarm output when the frequency changes at greater than a predetermined threshold rate.

The whole or parts of the described electrical circuitry can readily be implemented on integrated circuit chips. In the circuit of Fig. 1, the VCO, digital frequency sampling means, and feedback circuit could be integrated into a single chip with the processing carried out by a further chip or by a suitably programmed microprocessor. In this manner existing pyroelectronic detectors could be connected to the intregrated circuit elements to produce a readily assembled sensor. Different detectors and different alarm processing circuits could readily be used to produce a range of sensor suitable for different purposes. Such sensors would clearly also have appropriate optical arrangements.

Another possibility is to integrate onto a signle chip an infra-red detector element together with an associated oscillator. The VCO could be connected directly to the infra-red sensitive element 4 instead of the conventional FET buffer 6.

It will readily be appreciated that the described circuitry can be adapted for use with other detectors which produce a voltage output.

Claims (12)

1. An electrical circuit for use in a sensor comprising a detector adapted to produce a voltage output having a magnitude related at least in part to the amount of a quantity be sensed, a voltage controlled oscillator having a control input connected to the output of the detector, a processing circuit connected to the output of the voltage controlled oscillator for sensing any rate of change in the frequency output of the oscillator in excess of a predetermined range in order to initiate an alarm.

2. An electrical circuit according to claim 1, wherein the processing circuit comprises means for converting its input signal to a sequence of digital samples each representing the instantaneous frequency of the input signal, and a microprocessor for processing the digital samples.

3. An electrical circuit according to claim 1 or 2, further comprising means for stablizing the output of the voltage controlled oscillator to compensate for slow variations in the voltage output from the detector independent of the quantity to be sensed.

4. An electrical circuit according to claim 3, wherein said stabilizing means comprises feedback means for producing a control signal representing the difference between the output frequency of the voltage controlled oscillator and a predetermined frequency.

5. An electrical circuit according to claim 3, further comprising a frequency subtraction device having one input connected to the output of the voltage controlled oscillator and the other input to a control output from the processing circuit, said control output from the processing circuit carrying a signal having a predetermined frequency controlled to match the output from the voltage controlled oscillator in the absence of the quantity to be sensed so as to compensate for slow variations in this output.

6. An integrated circuit comprising an element sensitive to a quantity to be sensed, means for converting the output of said element into a voltage signal, and a voltage controlled oscillator for producing an output signal having a frequency related to said voltage signal.

7. An integrated circuit according to claim 6, wherein said converting means is coupled to the element so as to transmit only changes in the quantity to be sensed.

8. An integrated circuit according to claim 7, wherein the element is a pyroelectric detector.

9. An integrated circuit according to claim 6, wherein the element and converting means comprises a ChemFET.

10. An integrated circuit according to claim 6, wherein the element and converting means comprises a thermopile.

11. An integrated circuit according to any one of claims 6 to 10, further comprising a processing circuit connected to the output of the voltage controlled oscillator for sensing any rate of change in the frequency output of the oscillator in excess of a predetermined range in order to initiate an alarm.

12. A sensor substantially as herein described with reference to the accompanying drawings.