Patent GB-1497917-A (GB)
Title: Vehicle identification system
IPC: G08G1/09
Filed: 1975-09-26
Abstract: 1497917 Radio frequency interrogation and identification system for vehicles PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 Sept 1975 [1 Oct 1974] 39526/75 Heading H4L In a vehicle identification system comprising an interrogation unit 1 for the wireless transmission of an interrogation signal to a transponder on the vehicle to be identified and a receiver 10 for detecting an identification signal transmitted by the transponder, the receiver 10 comprising a carrier detector 22 and switching means for resetting the receiver 10 to a state in which it is ready to receive a further identification signal if, during reception of an identification signal the carrier is absent or interference present for a given small number of periods of the carrier. The invention is applicable to a system of the type in which traffic light priority is given to certain vehicles such as buses and fire engines in response to interrogation and identification signals transmitted by a wireless. The interrogation unit 1 comprises a 100 kHz carrier generator 4 the output of which is keyed at 6 by means of a logic circuit 9 and coupled by an amplifier 7 and hybrid circuit 2 to a loop aerial 3 fitted in the road surface. When a vehicle fitted with a transponder, e.g. of the type described in Specification 1,451,832 comes within the range of the aerial 3, it responds with an identification signal composed of a start code which is the same for all transponders, a recognition code with parity bit and a stop code which is also the same for all transponders. These codes which may comprise a total of 32 bits are transmitted by frequency-shift keying at frequencies of 90 and 100 kHz. If the length of the aerial loop is 2 metres, the maximum vehicle speed for correct performance is 116 km./hour. The identification signal is picked up by aerial 3 and fed to an amplifier-limiter 4 follower by a trigger 12, which provides a rectangular signal whose variations coincide with the zero crossings of the carrier, demodulated at 13 under control of a clock signal from 8 and fed for decoding to a shift register 14 and a parity check 15. As soon as the start code has been completely written into shift register 14, the start code detector 16 enables the parity checker 15. Thereafter. the bits of the recognition code are written into register 14 followed by the stop code. The stop code detector 17 and parity checker 15 supplies signals to AND gate 19 which enables a further AND gate 20 so that the contents of the register 14 are entered into a buffer 21 in series or parallel. In the presence of a noise signal, the receiver 10 may not respond accurately. The limiter 11 is thus arranged to be swamped by large amplitude noise signals so as to suppress the carrier for several periods. The trigger 12 thus provides a constant output during this time which is fed to the carrier detector 22 which is arranged to deliver a signal following the failure of at least two successive zero-crossings of the carrier. This signal is fed to resetting inputs of shift register 14, parity checker 15 and logic circuit 9 so that the receiver 10 is prepared for the reception of a further identification signal from unit 1. Fig. 2 (not shown) shows an arrangement of D flips-flops and logic gates suitable for use as the carrier detector 22 and arranged to produce a resetting signal after the absence of a predetermined number of zero crossings.