Automatic compensation of primary field coupling for a frequency-domain electromagnetic induction sensor

Residual mutual coupling between the transmitter and receiver coil(s) is a well-known problem in frequency-domain (FD) electromagnetic induction (EMI) sensors. In cases where a ratio of the measured signal to primary field coupling signal is sufficiently high, careful geometrical arrangements of coils, such as those used in EMI gradiometers, are normally sufficient for suppressing the primary field coupling. However, in applications requiring very high sensitivity, additional active means of sensor compensation are needed, since the measured object response may become commensurable with the primary field response and its thermally-induced fluctuations. In this paper, we investigate a technique for closed-loop automatic compensation of residual primary field coupling and its thermal drift, based on the use of an active sensor balancing. The technique is experimentally validated in a laboratory environment using a prototype of FD EMI sensor with magnetic cavity type design. Preliminary results indicate that direct coupling voltage can be effectively controlled, providing larger dynamic range and increased accuracy in measurements of EMI responses of small metallic objects.