Eddy current sensing using planar coils

Abstract

The eddy current technique is a method of determining by the proximity of a conducting target to a coil energized by an alternating current by measuring the electrical losses in the coil caused by the presence of the target. The coil is normally wound on a bobbin with a ferrite core to increase the magnetic field strength at the target, however reductions in size and manufacturing cost can be achieved by using a planar coil etched on a printed circuit board. The aim of this project was to deign a planar coil with a performance comparable to an existing conventional coil. To reduce the number of cut-and-try evaluations, computer simulations were used to determine the properties of various planar coil geometries, both with and without a ferrite core. Before embarking on the design process, the simulation was verified by comparison with physical measurements made on existing conventional and planar coils.These simulations provided useful information for the optimal design of a planar coil. The selected design, based on a four-layed printed circuit board and without a ferrite core, had an improved performance (in terms of sensitivity to target position) over the existing conventional coil. The performance of this coil was verified by physical measurements on a prototype, and was found to closely match the simulation results. This project was conducted in collaboration with Methode Electronics Malta Ltd.