GB2234068A

GB2234068A - Infra-sonic detectors for use in alarm systems

Info

Publication number: GB2234068A
Application number: GB8905654A
Authority: GB
Inventors: * Mistry Chand
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-03-13
Filing date: 1989-03-13
Publication date: 1991-01-23
Also published as: GB8905654D0

Abstract

The use of infra-sonic detectors, comprising of a electromechanical transducer (t) and a differential method of detection is described for use in an alarm system. It is suggested that the invention could be used to produce commercially viable alarm systems which overcome some of the problems of false triggering of the cruder forms of infra-sonic detectors available at present. The output of the transducer (t) is amplified in an amplifier (a) and then divided between a reference filter (f') and a filter (f'') which is adjustable for frequency band and sensitivity selection. A differentiator (d) separates the useful signal from noise and applies the signal to a processor (p). <IMAGE>

Description

Infar-sonic detectors for use i alarm systems.

Introduction The security/alarm industry is for ever looking for methods of providing security at low cost and high reliability.

The conventional loop wiring, the ultrasonic, the passive infrared and radar types of detectors are well used and have dominated the industry. However, the use of infra-sonics have been relatively ignored due to the unreliability of the sensors which are currently available.

It is the intention, within this illustrated embodiment of the invention, to demonstrate that a principle of infra-sonics can be employed to produce commercially viable alarm systems. The main merits of these alarm systems will be the low cost of manufacture, high immunity to false triggering and the large volume/area which can be protected by such alarms.

The infra-sonic principle The human ear is an excellent detector for detecting the variations in the air pressure (ie sound). The range is nominally from lOHz to 20KHz.

Higher frequencies are used by the ultrasonic type detectors which suffer from excessive attenuation by normal materials around the home/office and tend to only respond to a straight line of sight.

Lower frequencies are generally referred to as infra-sonic or sub-sonic frequencies. These have the advantage of lower attenuation by conventional fabrics and can be used within security applications.

An in depth research has revealed, that the infra-sonic range of frequencies can be sub-divided into discreet frequency bands which correspond directly to the type of activity which produces such infra-sonic disturbances.

This illustrative embodiment (refer to figl and fig2) of the invention considers firstly taking an electromechnical transducer (t) which responds to the required frequency band and then amplifier (a') to amplify the signal. Slight filtering and conditioning of the signal is provided at this stage. The signal from a low impedance source is now split into two and passed through two unique filters (f' and f''). Filter (f') is the reference filter which provides the noise reference, consisting of ac hum, rfi, emi, temperature compensation and other forms of noise signals. Filter (f") produces characteristics which provide the selectivity and sensitivity (adjusted by the operator) of the required frequency band. It fundamentally also provides, superimposed on to the signal, the noise which is attributed by filter (f'). A differentiator (d) performs the signal processing which destinguishes the common mode noise and produces the usable signal. Finally, signal processing (p) which includes possible inhibiting circuits to prevent self-triggering can be employed to construct the alarm system. Examples of such applications could include the operation of relays, provide entry/exit delays trigger further sensors and/or communication systems etc.

Claims

What is Claimed

1) A device which provides a unique combination of the mechanical arrangements of the transducer- (eg electric microphone) and the differential method of detecting infra-sonic signals.

2) A device according to claim 1) which provide a method of selecting narrow frequency bands, at correct sensitivity within the infra-sonic band width, when in a noisy environment to prevent false signaling.

3) A device according to claims 1) and 2) which provide a good reliable method of controlling the sensitivity of the alarm and hence the detection of an intruder within a secure area.

4) A device which is easily interfaced with any subsequent signal processing and/or conditioning to provide commercially viable and workable intruder alarm systems.