Patent ID: 8314713

Claim:
A method for monitoring the operation of a metal detection system that comprises a balanced coil system with a transmitter coil that is connected to a transmitter unit adapted to provide transmitter signals having one of a fixed transmitter frequency and a variable transmitter frequency and with a first receiver coil and a second receiver coil that are adapted to provide output signals to a receiver unit that are adapted to compensate one another in an event that the metal detection system is in balance, said method comprising: providing a carrier signal with the transmitter frequency and a monitoring signal with a monitoring frequency to a modulation unit that suppresses the carrier signal and provides a modulated monitoring signal; supplying the modulated monitoring signal to a monitoring coil that is inductively coupled with at least one of the receiver coils; demodulating output signals of the receiver coils in a demodulation unit that provides a demodulated monitoring signal to a signal processor; and comparing the demodulated monitoring signal in at least one of phase and amplitude with a reference and providing an alarm if a measured deviation exceeds a given threshold value.