Patent GB-1062727-A (GB)
Title: Object detecting acoustic system
IPC: G01S7/529
Filed: 1965-07-05
Abstract: 1,062,727. Ultrasonic vehicle-detection systems. GENERAL SIGNAL CORPORATION. July 5, 1965 [July 22, 1964], No. 28418/65. Heading H4D. In an object - detecting system, the sensitivity of receiving apparatus is increased on reception of a detector signal from the object to prevent plural manifestations of a single object due to fluctuation of the strength of the received signal. In the ultrasonic vehicle-detecting system of Fig. 3, pulses from one output of a multivibrator 40 trigger an oscillator 42 to produce ultrasonic acoustic pulses directed towards a roadway from a transmitting transducer 44, and stretched pulses from the other output of multivibrator 40 are fed directly to a coincidence gate 57 and through a delay circuit 61 to a coincidence gate 58. Echo pulses reflected from the roadway or a vehicle on the roadway are received by a transducer 50 whose output is fed to a pair of twostage tuned amplifiers 51, 53 and 52, 54, and the demodulated outputs of the pair of amplifiers provide a second input signal for gates 57 and 58, respectively. Transducers 44 and 50 are situated a predetermined distance above the roadway such that the travel-time of a roadway-reflected pulse between transmitter 44 and receiver 50 causes production of output pulses from demodulators 55 and 56 simultaneously with the output pulse from delay circuit 61. If no vehicle is present in the detection zone, pulses thus occur at both inputs of gate 58 simultaneously and a " reset " pulse is applied to a bistable circuit 62. In the " reset " state of circuit 62, a relay 64 is de-energized and a negative feedback cathode resistor 125 in amplifier stage 54 is shorted out by relay contacts 66, whereby the gain of stage 54 is made greater than that of stage 53. If a vehicle is present in the detection zone the echo pulse is received earlier than a roadway echo, and the output pulses of demodulators 55, 56 are coincident in time with the undelayed stretched pulse fed from multivibrator 40 to gate 57. Gate 57 then applies a " set " pulse to circuit 62, causing energization of relay 64. Relay contacts 66 change over, thereby shorting out a cathode resistor 123 in stage 53 instead of resistor 125. The relative gains of stages 53 and 54 are thus inverted, increasing the sensitivity of the system to vehiclereflected acoustic pulses and eliminating relay " chatter " due to variations in the strength of the echo signal (e.g. due to air turbulence or the different reflectivity of different parts of a vehicle). Indication of the presence of a vehicle in the detection zone is given by operation of relay 64, which closes further contacts 65. In an alternative embodiment (Fig. 2, not shown) a combined transmitting/receiving transducer (14) is pulsed from an oscillator (12) gated by a multivibrator (10), and received echo signals are fed to a two-stage amplifier (16). The output of amplifier (16) is passed, after further amplification, to circuits (18, 19), which provide amplitude discrimination between received signals reflected from vehicles and those reflected from other objects (e.g. people). The output of circuit (19) is passed through an amplifier (20) to a relay control system (21). The presence of a vehicle in the detection zone causes control system (21) to de-energize a relay (22) whose contacts (23) then short out a cathode resistor (106) in amplifier (16) to increase the amplifier gain. Both amplifiers (16 and 20) are gated off during the transmission of an acoustic pulse by a control signal from multivibrator (10) whose operating period is made equal to the traveltime of a roadway-reflected acoustic pulse, whereby relay control system (21) is operated only by vehicle-reflected pulses. Relay control system (21) may cause relay (22) to remain deenergized as long as pulses are received at a repetition rate above a predetermined minimum.