Patent ID: 6137411
Filing Date: 2000-10-24
Classification: G01V,G08B

Abstract:
An article surveillance system for detecting the presence of articles in a detection zone, comprising at least one transmitter means and at least one receiver means for transmitting and receiving, respectively, electromagnetic radio-frequency signals in a detection zone, or in a proximity thereof, each article being provided with a sensor operating as a transponder for transmitting an electromagnetic radio-frequency reply signal to the receiver means when receiving a signal from the transmitter means,characterized bya coil arrangement with driving means for generating a low-frequency magnetic modulation field in the detection zone, anda controller (14) operatively connected to the transmitter means, the receiver means and the coil arrangement,wherein each sensor is arranged to transmit a reply signal, the amplitude of which is modulated by the magnetic modulation field,wherein the receiver means is arranged to receive and to demodulate the amplitude-modulated reply signal, andwherein the controller is arranged to supply modulating signals (i.sub.mod) to the coil arrangement, receive demodulated signals (i.sub.demod) from the receiver means and use the demodulated signals (i.sub.demod) when determining the position of the sensor in relation to the detection zone.