Patent GB-1227593-A (GB)
IPC: G01S1/72
Filed: 1968-04-05
Abstract: 1,227,593. Sonic object-detector; transistor pulse circuits. GENERAL SIGNAL CORP. 5 April, 1968 [7 April, 1967], No. 16456/68. Headings H3T and H4D. In a sonic pulse-echo system for detecting vehicles the emitted pulse is given a predetermined duration and a vehicle presence indication is provided only if an echo signal having a duration correlative with the emitted pulse duration is detected within a selected interval. This prevents false indications due to spurious signals. In the Fig. 4 embodiment of a road vehicle detector the emitted sonic pulse is of predetermined repetition frequency, carrier frequency and duration, as determined by units 1, 4, 200, respectively. A tuned amplifier 7 passes echo signals to recognition circuitry 33, 34, 35 which actuates a vehicle presence indicator only if two echo pulses of duration exceeding a minimum are received in two successive system cycle (repetition periods), each such pulse occurring within a time gate embracing expected arrival times of vehicle echoes. Detection memory unit 9 increases the gain of amplifier 7 when a vehicle is detected, thereby causing the system to "lock on" and preventing "stutter" due to vehicle echo fluctuations. Vehicle indication is cancelled (by timer 31) if there is no output from the recognition circuitry in an interval embracing several cycles. Amplifier 7 gain may be increased during each time gate period so that the system does not respond to "ringing" of transducer 5 following pulse emission. Instead of the pulse duration and two-pulse requirements of the recognition circuitry being used together the duration requirement may be used on its own. Fig. 2 illustrates a known circuit providing an output if two echo pulses, each occurring within a given time gate, are received in two successive cycles. Coincidence detector 23 comprising SCS 120 receives the echo pulses at its anode gate and the time gate pulses at its anode, and coincidence output pulses charge capacitor 131 via diode 130. Capacitor 131 tends to discharge via resistor 134 with a time constant such that the capacitor voltage will reach a value sufficient to switch transistor 141 and so trigger SCS 150 only if the two-pulse requirement is met. Any subsequent lack of coincidence output pulses removes the shunting effect of transistor 160 across capacitor 161 and enables unijunction timer 28 to supply a pulse to SCS 150 cathode for switching SCS 150 off after a time embracing several system cycles, thereby cancelling vehicle indication. Fig. 5B shows a biased diode circuit which prevents high amplitude pulses applied to transducer 5 by transmitter 4 from reaching receiver 7, while allowing low amplitude echo pulses to pass unattenuated from transducer 5 to receiver 7. Fig. 8A details an arrangement whose general function is similar to the Fig. 2 circuit. Coincidence detector 34, output stage 9 and unijunction timer 31 correspond to circuits 23, 27, 28 of Fig. 2. A coincidence output pulse at A drives capacitor 260 negatively with its trailing edge and the time for recovery of the capacitor voltage to the clamping level determined by diode 261 is greater than one system cycle so that a positive-going voltage step at B is provided by the leading edge of the next coincidence pulse providing said next pulse is in the next system cycle. This step is detected by network 263, 264, 265 and triggers stage 9, indicating that the two pulse requirement is met. The triggering of stage 9 switches on transistor 292 in shunt with an emitter feedback resistor in receiver amplifier 7 to increase the amplifier gain. The pulse duration recognition circuit (Fig. 7B, not shown), comprises echo amplifier (239), diode clamp (242) and detector (243, 244). Positive voltage is applied to resistor (248) during the time gate period. The detected echo signal removes the shunting effect of transistor (246) across capacitor (247), which will charge to a level sufficient to switch transistor (249) only if the signal has a required minimum duration. Time gate generator details are also given (Fig. 6A, not shown).