Patent GB-1513173-A (GB)
Title: Detector apparatus
IPC: G01V3/10
Filed: 1975-04-29
Abstract: 1513173 Road-vehicle detectors CANOGA CONTROLS CORP 29 April 1975 [29 April 1974] 17834/75 Heading G4Q [Also in Division G1] An inductive loop vehicle detector system of the type described in Specification 1,448,967 wherein the loop frequency is monitored by counting the number of clock pulses which occur within a predetermined number of loop frequency cycles and comparing the count with a reference number, is modified to operate with a number of independent detector loops in sequence. Basic system. Fig. 1.-A loop oscillator 50 is coupled in turn to each of the four detector loops under the control of the logic network 54 which includes three sets of input switches 70. The first set of switches allows the user to select a sensitivity appropriate to the particular loop to be used: low sensitivity for a single small loop, or large sentivity for a large loop or multiple loops connected to a single input channel. The second set of switches control system sensitivity according to whether or not bicycles are to be detected, and determines whether the predetermined number of loop cycles is 1024 or 512. The third set of switches determines whether the system responds to vehicle presence or merely provides one output pulse if a vehicle arrives and remains in the loop. A power monitor 60 restores the system to an initial state if the power supply is interrupted. Counting circuits, Fig. 5.-Clock pulses from oscillator 106 are normally counted by the D counter 104, of which the final stages D17, D18 are used to select each of the four channels in turn. Cycles from the loops are fed through gates 218 and 220 to the L counter 100. At the beginning of each counting period, after D00 D01 and D04 all reach 1 states the output of gate 200 disables gate 232 and stops the D counter; the next change in the loop signal sets latch 164 and causes the L counter to be re-set and start counting loop cycles which are fed in through gates 218 and 220. The pulse at L02 is used to re-start the D counter which continues to count until stopped by a pulse at L10, i.e. after 1024 cycles of the loop frequency, when the contents of the D counter are transferred to the S register (Fig. 8). A NOT ABORT flip-flop 160 and its associated circuitry operate if (a) at the start of a period measurement the loop oscillator fails to oscillate. In this case an RC network 270 provides a pulse to make the D counter resume counting, (b) if the loop oscillator signal stops prior to transferring the contents of the D counter to the S register, the network 270 again generates a substitute for the loop signal (c) if the signal L10 is not produced within the measurement period the flipflop 160 is re-set to indicate a faulty period measurement. Logic circuits, Fig. 8.-The H register 120 comprises four sections (one for each detector loop) each storing two 16 bit words. One word is a reference number which fed into the subtract unit 350 for comparison with the current count which has been transferred into the S register 112 from the D counter, and the resulting difference is fed to the compare circuit 380. If the difference is negative a call is registered via flip-flop 382. If the difference is positive and less than a threshold value TLQ, the reference number stored in the H register is reduced by 1. If the difference is positive and greater than TLQ, an INITIALISE process is carried out which results in the reference number in the H register being replaced by S-TLQ (where S is the current number stored in the S register). The second word stored in the H register is an age count which is incremented by 1 in each cycle if the mster call is set and is set to zero by the INITIALISE process. The effect of the circuit is that if a car stalls over a detector loop its signal will fade out in time and the loop will then be able to respond to other vehicles which may enter; when the stalled car moves, the INITIALISE process permits a rapid response to a subsequent vehicle. When operating in the PULSE mode instead of the presence detecting mode an additional compare means (Fig. 11, not shown) responds to a particular age count corresponding for example to 2 seconds to force an INITIALISE process so that the system is then responsive to a new arrival. In a modification, Fig. 15 (not shown), each detector loop is coupled to its own oscillator which is pulsed on by the channel selection system. A loop may monitor a single or a plurality of traffic lanes.