Metal detector with nulling of imbalance

摘要

A metal detector includes a receive coil and a transmit coil connected in an inductive bridge. To overcome imbalances in the bridge due for instance to misalignment of the coils or the presence of mineralization in the medium which is being examined, the metal detector automatically produces a nulling (bucking) signal to cancel out the effects of any unwanted receive coil signals detected during calibration. This nulling signal is a nulling current both in terms of level and phase, and its level and phase are determined during a calibration process prior to actual metal detection. By inclusion in the metal detector of a microprocessor (microcontroller) operating at a much higher frequency than the variations in the magnetic field used to detect metal, the nulling signal generation is performed with a high degree of time resolution, resulting a precision metal detector which adaptively ignores any unwanted signals.