Demodulation technique for rip detector signals

Abstract

Synchronous demodulation using a lock in amplifier technique improves substantially the signal to noise ratio in a conveyor belt rip detector. The lock in amplifier technique uses an analog multiplier to form a product of a received AC transmitter signal, i.e. that received by the rip detector receiver from an antenna carried by the conveyor belt, and an AC reference signal, which has the same frequency as the transmitted AC transmitter signal, i.e. that transmitted by the rip detector transmitter. A shift in the phase of the AC transmitter signal as it is coupled from the transmitter to the receiver requires a corresponding shift in the phase of the AC reference signal so that the latter is tuned to the same phase as that of the received AC transmitter signal. The invention lends itself to computer control and self diagnosis of faults of the overall rip detector system. To effect such fault detection not only is there produced an AC transmitter signal and an AC reference signal, but there also is produced a test signal representative of proper operation of the transmitter, and this parameter is checked to learn whether a rip has occurred causing an antenna not to be sensed at an expected location on the belt or a fault has occurred in the rip detector equipment.