GB1602441A

GB1602441A - Detector

Info

Publication number: GB1602441A
Application number: GB1267077A
Authority: GB
Original assignee: RCA SECURITY SYSTEMS Ltd
Current assignee: RCA SECURITY SYSTEMS Ltd
Priority date: 1978-05-03
Filing date: 1978-05-03
Publication date: 1981-11-11

Description

(54) DETECTOR (71) We, RCA SECURITY SYSTEMS LIMITED, a British Company, of Lincoln Way, Windmill Road, Sunbury-on-Thames, Middlesex, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a detector, for example for detecting the presence of an object to be protected or detecting the presence of an intruder approaching such an object, and aims to give reasonably precise information without undue tendency to false alarms.

According to the present invention there is provided a detector analyzer circuit comprising means to receive bursts of oscillations separated by quiet periods and to pass the received bursts to a transmission gate, means to define successive window periods at the burst rate by appropriate intermittent opening of the transmission gate, a Schmitt trigger device coupled to the output of the gate to deliver square pulses when bursts are being received during the window periods, a "missing pulse" detector coupled to the Schmitt trigger device to deliver a signal when no square pulse is received therefore during a given period, and means to give a sustained warning on said signal being delivered from said missing pulse detector.

A cooperating vibrator may consist of a small fairly directional loud speaker, and then the receiver could be a microphone positioned possibly beside it; the vibrations will be mechanical vibrations, and preferably at a frequency above the audible range so as not to cause a disturbance. They might be at 25 KHZ and they might be transmitted at an intensity of 100 DB which is sufficient to get a good signal from an object some feet away, but is not disturbing because the frequency is above the audible range. The vibrator and receiver could be constituted by the same device.

The delay period will be calculated or set by trial and error, to be equal to the time for vibrations to reach the object from the transmitter and be reflected to the receiver, so that if reflected vibrations are detected during the "window" period, then it is established that there is an object present at that range from the transmitter. The oscillator could of course be controlled to generate coded or modulated signals to which the receiver is sensitive, so that a false signal will not be received from ambient noise. Alternatively the receiver could have an amplifier set so that the ambient noise is not sufficient to produce a detectable signal in the "window" period, and the received signal can be substantially greater than the ambient noise, because of the possibility of transmitting at high power as discussed above.

The delay can be adjusted to suit the range of any particular object from the transmitter, and also the quiet period can be adjusted to give sufficient time for any reflection to be received before the next burst or vibrations is transmitted. The length of the "window" period will conveniently correspond with the length of the burst of vibrations.

The circuit can be arranged to give an alarm signal if no reflected signal is received during the "window" period.

It will be appreciated that the system is suitable for detecting that an object is present at a certain range from the transmitter, for example a valuable object on display, and also that the operation does not depend upon movement of the object, as would be the case with a Doppler type of detector. Alternatively the detector could be used to detect a person or other object coming between the transmitter and the object, and thus preventing a reflection from being received during the "window" period. The system might even be set to receive a reflection from a wall which reflection would be lost if someone appeared between the transmitter and the wall, or between the wall and the receiver.

Other uses of such a detector system will become apparant.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the drawing accompanying the Complete Specification, of which the single figure is a block diagram of a detector together with a set of signal characteristics showing the signals appearing at different places in the diagram.

A transmitter or vibrator 'T' is driven by an oscillator I which is it self controlled by an astable circuit II to drive the transmitter during short bursts spaced by quiet periods as shown in characteristic 'A'. In the example being described, the bursts last for 3 milliseonds, and the quiet periods for 40 milliseconds. The resulting vibrations from the transmitter are reflected from the object to a receiver 'R', the output of which is amplifier by an amplifier 'V' and supplied to a transmission gate VI.The characteristic'B' shows the transmission from the transmitter, and the characteristic 'C' shows the direct signal received by the receiver from the transmitter without having travelled to the object, a further reflection of lower amplitude showing the vibrations reflected from the object, and one further set of reflections from the wall 'W' behind the object before the next transmitted burst is received directly.

The astable circuit II drives a monostable circuit III from the trailing edge of the burst, and that in turn drives a monostable circuit IV after a settable time delay as indicated in the characteristic 'D'. The monostable circuit IV gives a positive output for a pre-set period corresponding to the length of the burst, starting with the end of the delay from the monostable circuit III, and during that output from the monostable circuit IV a "window" period is provided, as shown in the characteristic 'E' by opening of a transmission gate VI which receives the signal from the amplifier V. Characteristic 'F' shows that that "window" period is set only to receive reflections from an object at the appropriate range from the transmitter. Those signals are amplified in an amplifier VII as shown as 'G' and square in a Schmitt Circuit VIII as shown at characteristic 'H'.The output of the Schmitt trigger circuit is applied to a missing pulse detector IX which is arranged to produce a positive output as long as it receives a signal from the Schmitt circuit in the complete cycle, but to drop in output if 80 milliseconds have passed without receipt of a reflected signal in the "window" period.

That is shown at 'H' 'I' & 'J'. the drop in signal triggers a monostable circuit X if no pulse is received, and that provides a 2 seconds pulse for operating an alarm relay.

Once the relay has sounded it provides a continuing signal to the monostable Xso that the warning is continued.

The output from the amplifier V is also applied through an a.c. to d.c. converter XI to a second Schmitt circuit XII, and thence to the relay. The circuit is normally set so that the background noise is not sufficient to trigger the relay, but if someone trying to defeat the system introduces a random signal to the receiver of sufficient amplitude, that can also be arranged to trigger the alarm relay.

Thus provided no such jamming noise is received no alarm is given as long as the object is in position and is not obscured either from the transmitter or the receiver.

Both a loudspeaker/transmitter and a microphone/receiver have been referred to above, and they are conveniently mounted side by side. However it is also possible to have a single electro-magnetic vibrator acting both as transmitter and receiver with an appropriate electrical circuit for separating the drive signals and the receiver signals.

WHAT WE CLAIM IS: 1. A detector analyzer circuit comprising means to receive bursts of oscillations separated by quiet periods and to pass the received bursts to a transmission gate, means to define successive window periods at the burst rate by appropriate intermittent opening of the transmission gate, a Schmitt trigger device coupled to the output of the gate to deliver square pulses when bursts are being received during the window periods, a "missing pulse" detector coupled to the Schmitt trigger device to deliver a signal when no square pulse is received therefore during a given period, and means to give a sustained warning on said signal being delivered from said missing pulse detector.

2. A detector analyzer circuit as claimed in Claim 1 and a pulsed oscillation transmitter cooperative therewith, in an installation arranged such that the window periods correspond to a reflected wave from the transmitter via an object or a wall.

3. A detector analyzer circuit as claimed in Claim 2 wherein the window periods correspond in length to the burst periods.

4. A detector analyzer circuit as claimed in Claim 3 comprising means to pulse the oscillator of the transmitter from the output of an astable circuit, and means to control the transmission gate in the analyzer from the same output via a circuit of a given delay.

5. A detector analyzer as claimed in Claim 4 wherein the delay circuit comprises a first monostable driven from the astable circuit at the time of the trailing edge of the burst of oscillations, a second monostable

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. which reflection would be lost if someone appeared between the transmitter and the wall, or between the wall and the receiver. Other uses of such a detector system will become apparant. The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the drawing accompanying the Complete Specification, of which the single figure is a block diagram of a detector together with a set of signal characteristics showing the signals appearing at different places in the diagram. A transmitter or vibrator 'T' is driven by an oscillator I which is it self controlled by an astable circuit II to drive the transmitter during short bursts spaced by quiet periods as shown in characteristic 'A'. In the example being described, the bursts last for 3 milliseonds, and the quiet periods for 40 milliseconds. The resulting vibrations from the transmitter are reflected from the object to a receiver 'R', the output of which is amplifier by an amplifier 'V' and supplied to a transmission gate VI.The characteristic'B' shows the transmission from the transmitter, and the characteristic 'C' shows the direct signal received by the receiver from the transmitter without having travelled to the object, a further reflection of lower amplitude showing the vibrations reflected from the object, and one further set of reflections from the wall 'W' behind the object before the next transmitted burst is received directly. The astable circuit II drives a monostable circuit III from the trailing edge of the burst, and that in turn drives a monostable circuit IV after a settable time delay as indicated in the characteristic 'D'. The monostable circuit IV gives a positive output for a pre-set period corresponding to the length of the burst, starting with the end of the delay from the monostable circuit III, and during that output from the monostable circuit IV a "window" period is provided, as shown in the characteristic 'E' by opening of a transmission gate VI which receives the signal from the amplifier V. Characteristic 'F' shows that that "window" period is set only to receive reflections from an object at the appropriate range from the transmitter. Those signals are amplified in an amplifier VII as shown as 'G' and square in a Schmitt Circuit VIII as shown at characteristic 'H'.The output of the Schmitt trigger circuit is applied to a missing pulse detector IX which is arranged to produce a positive output as long as it receives a signal from the Schmitt circuit in the complete cycle, but to drop in output if 80 milliseconds have passed without receipt of a reflected signal in the "window" period. That is shown at 'H' 'I' & 'J'. the drop in signal triggers a monostable circuit X if no pulse is received, and that provides a 2 seconds pulse for operating an alarm relay. Once the relay has sounded it provides a continuing signal to the monostable Xso that the warning is continued. The output from the amplifier V is also applied through an a.c. to d.c. converter XI to a second Schmitt circuit XII, and thence to the relay. The circuit is normally set so that the background noise is not sufficient to trigger the relay, but if someone trying to defeat the system introduces a random signal to the receiver of sufficient amplitude, that can also be arranged to trigger the alarm relay. Thus provided no such jamming noise is received no alarm is given as long as the object is in position and is not obscured either from the transmitter or the receiver. Both a loudspeaker/transmitter and a microphone/receiver have been referred to above, and they are conveniently mounted side by side. However it is also possible to have a single electro-magnetic vibrator acting both as transmitter and receiver with an appropriate electrical circuit for separating the drive signals and the receiver signals. WHAT WE CLAIM IS:

1. A detector analyzer circuit comprising means to receive bursts of oscillations separated by quiet periods and to pass the received bursts to a transmission gate, means to define successive window periods at the burst rate by appropriate intermittent opening of the transmission gate, a Schmitt trigger device coupled to the output of the gate to deliver square pulses when bursts are being received during the window periods, a "missing pulse" detector coupled to the Schmitt trigger device to deliver a signal when no square pulse is received therefore during a given period, and means to give a sustained warning on said signal being delivered from said missing pulse detector.

initiated by the end of a period set by the first monostable, and a coupling between said second monostable and a control input of said transmission gate.

6. A dectector analyzer as claimed in any of Claims 2-5 in which the transmitter comprises a loudspeaker for radiating acoustic waves.

7. A detector analyzer as claimed in any preceding claim in which the receiver comprises a microphone.

8. A detector analyzer as claimed in Claim 6 in which the vibrator is arranged to operate at a frequency above the audible range.

9. A detector analyzer as claimed in any of the Claims 2-8 in which an or the oscillator is arranged to generate coded of modulated signals to which the receiver is sensitive.

10. A detector analyzer is claimed in any of the preceding claims, in which the receiver is set so that the ambient noise is not sufficient to produce a detectable signal in the "window" period.

11. A detector analyzer as claimed in Claim 10 comprising means to detect noise above ambient or background noise, thereon to activate said sustained warning giving means.

12. A detector analyzer as claimed in Claim 11 comprising a second Schmitt trigger circuit in a path parallel with that containing the transmission gate and the first Schmitt trigger circuit.

13. A transmitter and detector analyzer substantially as described with reference to the drawing.