GB1004615A - Improvements in or relating to moving object detection systems - Google Patents

Abstract

1,004,615. Intruder alarm systems. DECCA Ltd. July 25, 1963 [April 30, 1962], No 16364/62. Heading G4N. [Also in Division H4] A moving object detection system comprises a transmitter 11 arranged to radiate continuous or modulated ultrasonic signals, a receiver 12 arranged to receive echoes of such signals, a phase-sensitive detector 14 arranged to give an output representative of the phase of the received signals compared with the phase of the radiated signals and means for indicating any variations in the output of the phase-sensitive detector due to moving objects. Substantially the whole subject-matter of this Specification is contained within Specification 1,004,614, but whereas the latter Specification describes and claims the arrangement of the transmitting and receiving transducers, this Specification describes and claims the phase-sensitive detector as shown in Fig. 2. The signal from the amplifier 13 is fed through a capacitor 15 to the collector of a transistor 16 and a continuous wave reference signal, in fixed phase relationship with the ultrasonic frequency output of the transducer 11 from the oscillator 10, is applied to the base of this transistor. The reference signal is arranged to switch the transistor 16 on and off for alternate half-cycles, and thus the output across the collector-to-emitter circuit depends on the phase relationship between the applied signals from the amplifier 13 and the reference signal. This output is passed through a low-pass filter consisting of series resistors 18, 19 and a shunt capacitor 20 and is fed through a capacitor 21 to a low-frequency amplifier 22 (Fig. 1). In a pulse modulated system it is necessary to generate a reference signal for controlling the phase-sensitive detector representative of the phase of the ultrasonic oscillations in the radiated pulses, the amplitude of which must be sufficient to switch the transistor 16. Further if the received signals drive the transistor into a limiting condition rectangular output pulses are formed which will in affect be pulse width and polarity modulated with the modulation representing the required phase. The indicating means comprise a filter 24 for removing the high frequency components to provide a lowfrequency signal which actuates a visual or audible alarm 27. The output from the filter depends on all the various fixed objects and any change in this output indicates the presence of a moving object. The indicator may be made responsive only to changes in the output of filter 24, or in some cases it may indicate the total output of the filter, which if it had a magnitude different from normal would indicate the presence of an abnormal fixed object. The phase-sensitive detector may comprise a half-wave or full wave rectifier which is switohable on and off at the reference signal frequency.