The design of a proximity inductive sensor

In this paper we discuss the advantages of flat coil inductive proximity sensors. The coil copper helix may be realized by application of a photoengraving technique to two side-printed boards by a very cheap process. The low thickness allows one to get the coil closer to the target. This fact allows one to attain a higher sensitivity. The coil's low intrinsic capacitance and inductance allow one to work at high frequencies (up to some megahertz). As a consequence the sensor's time constant may decrease to some microseconds. A discussion of the parameters that influence the sensor's performance is reported. Special attention has been given to the coil's shape, the target and the electronic revealing circuit required for promptly functioning and precise devices. The complete analysis of the influences of these parameters on the sensor's performance gives useful suggestions that allow one to obtain a fast response, high sensitivity and high intrinsic insensitivity versus temperature. This is obtained by adopting flat coils, bimetallic (magnetic-non-magnetic) targets and an electrical bridge or frequency-shifting circuit. This provides a substantial improvement in performance with respect to previous solutions.