GB1045923A - Improvements in or relating to moving object detection systems - Google Patents

Abstract

1,045,923. Detecting objects by ultrasonic vibrations. DECCA Ltd. May 19, 1965 [Feb. 20, 1964], No. 7200/64. Heading H4D. [Also in Division G4] In a self-contained, battery-operated system for detecting moving objects, e.g. an intruder, an electrical signal indicative of the presence of a moving object is derived from a circuit responsive to the difference of frequency or phase between a transmitted ultrasonic signal and the signal received after reflection from the object, and is transmitted to a remote point to operate an audible or visual alarm. In the embodiment of Fig. 2 (not shown), cylindrical piezo-electric transmitting and receiving transducers 11, 12, respectively, are mounted one above the other, with their axes vertical, in a cylindrical housing 40 which also constitutes a baffle between them. A detection region is defined by the overlap of the transmission and reception beams, which are of relatively narrow beam width in the vertical plane (Fig. 3, not shown). A 70 kc/s. continuous wave oscillator 10 (Fig. 1) drives transducer 11, and the phase difference between the transmitted and received ultrasonic signals is detected by a detector 14 which may be of the type disclosed in Specification 1,004,615. Low frequency variations in the phase difference, due to movement of an object in the detection region, are detected by circuits 15, 16, 17, and passed to a logic circuit 18 which discriminates against short-duration variations which would give rise to false alarms. An alarm signal is transmitted to the remote point by a radio transmitter 19 whose aerial 20 may form part of the cylindrical housing 40. Operation of the system may be inhibited, for servicing or setting up the apparatus, by switching off oscillator 10 by means of a time switch or by means of a coded radio signal transmitted from the remote point and received by a receiver 22. The beam width of transducers 11 and 12 may be restricted in the horizontal plane when the apparatus is used near a large solid object, e.g. a wall.