GB2085626A - Movement detector - Google Patents

Abstract

A movement detector has a receiver TD2 receiving reflected ultrasonic radiation from a transmitter TD1 and generating an A.C. signal which is amplified by a slow response A.G.C. amplifier 5 and a range-control amplifier 7. The resulting signal is demodulated and compared with a threshold at 8, 9 to yield a detected motion pulse to a line 10 each time the threshold is exceeded. Each pulse is converted into a constant width pulse by a pulse generator 11 and a succession of these pulses is applied through a charging circuit 12 which is adjustable to vary the sensitivity, to a memory capacitor C4 with a fixed discharge path R6. When the charge on C4 exceeds a predetermined value, a trigger circuit 14 gives an output alarm signal. <IMAGE>

Description

SPECIFICATION Signal variation detector This invention relates to detectors for detecting variations in signals and for assessing the significance of the detected signal variations.

According to the present invention, there is provided a detector for detecting variations in an input signal, comprising means for continuously comparing the signal with a threshold value and means for comparing the rate of occurrence of excesses of the signal above the threshold value with a present value for such rate.

Advantageously, the rate comparing means include means for generating a standard pulse for each said occurrence and means for assessing the rate at which the said pulses are generated.

Conveniently, the pulses may be applied through a charging circuit to charge a capacitor having a discharge path. When the charge on the capacitor, as indicated for example by its voltage, exceeds a predetermined value, a switch circuit is operated to change its state.

By varying the relative time constants of the charging cicuit and discharge path, the sensitivity of the assessing means may be varied.

Where the signal is modulated onto a carrier, the variation detector preferably includes an automatic-gain-control amplifier having a response time which is slow compared with the signal variations to be detected, for adjusting the steady or mean value of the carrier amplitude advantageously to a standard or predetermined value, prior to demodulation and application to the means for comparing the signal with the threshold value.

The invention may be applied to movement detectors, for example of the kind having a source of energy radiation (e.g. ultrasonic) and a receiver including a transducer for generating a signal corresponding to the received radiation.

The application of the invention to an ultrasonic movement detection system will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of the movement detector; Fig. 2 is a circuit diagram of the movement detector; and Fig. 3 shows schematically the signals at various parts of the circuit.

The ultrasonic movement detector shown in Fig. 1, comprises a transmitting ultrasonic transducerTD1 which is supplied with electrical energy by an oscillator 1 tuned to the natural frequency of the transducer (about 40 kHz) through an output stage 2.

A second ultrasonic transducerTD2 matched to the frequency of the transducer TD1 receives ultrasonic radiation from the environment into which the transducer TD1 is transmitting. The output from the transducer TD2 is fed into a highgain pre-amplifier 3, the output of which is passed through a high-pass filter 4 into a variable gain amplifier 5. The output from the amplifier 5 is passed, through a d.c.-level restorer 6 to a variable gain amplifier 7. The control voltage for determining the gain of the amplifier 5 is derived from the output of the d.c. restorer 6.

The output of the variable gain amplifier 7 is d.c.-restored and detected or demodulated in a d.c. restorer and detector 8. The output of the detector 8 is applied to a switching circuit 9 and each time the output of the detector exceeds the predetermined value, the circuit 9 is switched on to give an output to a line 10.

The output signal given to the line 10 causes a "motion-detected" photodiode D8 to emit light.

The line 10 is connected to a constant width pulse generator 11 which generates a pulse of constant width at the end of each signal received from the line 10.

The pulses of constant width are applied through a variable charging circuit 12 to a memory device 1 3 with a fixed decay rate. The output of the memory 13 is monitored by a schmitt trigger circuit 14; when the threshold level of the latter has been exceeded, it emits an alarm signal to an output 1 5.

Turning now to Fig. 2, it will be seen that the circuits are fed from a 1 2 volt supply, the positive terminal being connected to line 16 and the negative to earth at 1 7.

The oscillator 1 is conveniently in the form of a multi-vibrator and the transducers TD1 and Tod, are of the capacitive type having a very narrow frequency response range.

Fig. 3a shows diagrammatically the wave form at point a in Fig. 2 after amplification by the preamplifier 3, the a.g.c. amplifier 5 and the variable gain amplifier 7. This wave form corresponds to an amplitude-modulated 40 kHz signal. The variation in the amplitude of this signal is caused by variations in intensity of the radiation picked up by the transducer TD2 as a result of environmental changes such as movement of objects or air currents within the range of the transducer.

The gain of the variable gain amplifier 5 is determined by a control voltage built up on a capacitor C12 discharging slowly a resistor R2, and applied through a field-effect junction-type transistor Tr6 in series with a capacitor C5 in the base circuit of transistor Er5 of the a.g.c. amplifier.

Correspondingly, the gain of the variable gain amplifier 7 is adjusted by a variable resistorVr3 in the base circuit of that amplifier.

The signal is detected by the diode D7 to give a waveform corresponding to Fig. 3b and this signal is applied to capacitor Cis and resistor R30 to turn on an off the switching circuit transistor Tor,0. In adjusting the resistor Vr3, the operator can set the range up to which the detector is operative by adjusting Vr3 until the photodiode D8 indicates that the circuit is just picking up movements at the desired range.

The constant width pulse generator 11 includes a first integrated circuit IC2a which produces square wave pulses at its output terminal 3 (Fig.

3d). These pulses are differentiated by capacitor C3 and resistance R4 and the positive-going pulses are clipped by diode D1 (Fig. 3e). The second integrated circuit IC2b and diode D2 combine to give a positive constant width pulse corresponding to the end of each pulse delivered to line 10.

These pulses are applied through a charging circuit comprising a fixed resistor R5 and a variable resistor Vr2 to a memory capacitor C4. Adjustment of the resistor Vr2 adjusts the sensitivity of the circuit. With increasing frequency of pulses, the voltage on the capacitor C4 increases. When this exceeds the threshold value of the schmitt trigger, the latter gives an output signal to a switching circuit incorporating a transistorTr, to change the state of the latter to give an alarm signal.

Typical values of significant components are shown in Table 1.

Table 1 C3 1 nF C4 100 nF C8 10,uF C9 10nF C12 100 nF C,O 220 nF C13 22 nF TR6=J175 C,4 220 nF C15 100 of C16 10nF C17 1 ,uF C19 100 nF R4 39K R8 1K R6 10M R16 22K R17 10K VR2=10K R19 4.7K VR3=4.7K R20 1M R28 4.7K TABLE 1 TABLE 1 continued R29 220 R30 390K As indicated in Figure 2, the circuits require only three connections to external equipment. The circuits and transducers TD1 and TD2 can be mounted within a small casing to form a sensing head and can be connected to the external equipment by a standard jack and socket connection, the body, ring and tip of the jack being for example connected respectively to the lines 17, 1 5 and 16. The jack may be mounted vertically and form the only support for the casing, so that the orientation of the transducers can be easily adjusted simply by rotating the casing about the axis of the jack. This mounting may be directly onto the casing of the external equipment (including a battery, audible alarm with or without delay, etc) or a plurality of sensing heads can be connected to a common external equipment casing by a three-core lead. Thus, the whole installation can be portable and quickly set up wherever required.

Claims (7)

1. A detector for detecting variations in an input signal, comprising means for continuously comparing the signal with a threshold value and means for comparing the rate of occurrence of excesses of the signal above the threshold value with a preset value for such rate.

2. A detector according to claim 1, wherein the rate comparing means include means for generating a standard pulse for each said occurrence and means for assessing the rate at which the said pulses are generated.

3. A detector according to claim 1 or 2, wherein the pulses are applied through a charging circuit to charge a capacitor having a discharge path.

4. A detector according to any of the preceding claims, wherein the signal is modulated onto a carrier and the variation detector includes an automatic-gain-control amplifier having a response time which is slow compared with the signal variations to be detected, for adjusting the steady or mean value of the carrier amplitude to a standard or predetermined value, prior to demodulation and application to the means for comparing the signal with the threshold value.

5. A movement detector, comprising a source of radiation energy and a receiver including a transducer for generating a signal corresponding to the received radiation, wherein the receiver includes a detector according to any of the preceding claims for detecting variations in the said signal.

6. A movement detector according to claim 5, wherein the radiation energy is ultrasonic.

7. A movement detector substantially as hereinbefore described with reference to the accompanying drawings.