System for detecting the passage of a mobile including a passive responder

A system for detecting the passage of a mobile at a predetermined point during a guided displacement thereof along a track, comprising an interrogation assembly and a passive responder, one of which is carried by the mobile and the other of which is carried by the track, wherein

• the interrogation assembly includes:
• a low frequency transmitter and a medium frequency transmitter both designed to operate continuously, said transmitters having respective antennas arranged to transmit towards a zone having a predetermined location relative to the respective antennas which is selected for the responder to pass through it during displacement of the mobile, and
• a unit responsive to the characteristics of the responder when the latter is in said zone,
• said interrogation assembly being connectable to an electrical power supply;
• wherein
• the responder comprises a circuit for receiving a medium frequency signal from said medium frequency transmitter and a circuit for receiving a low frequency signal from said low frequency transmitter, arranged to control the medium frequency circuit, said low frequency circuit and said medium frequency circuit comprising respective tuned radiating circuits and being operatively associated in such a manner that said low frequency signal, when induced in the low frequency circuit by the low frequency transmitter of the interrogation assembly short-circuits the medium frequency circuit at the low frequency, and wherein
• said interrogation assembly is arranged to be responsive to disturbances of the medium frequency transmitter caused by the short-circuiting of the medium frequency tuned radiating circuit of the responder.