Mutual Impedance of Planar Eddy-Current Driver–pickup Spiral Coils

A framework for the analysis of planar driver–pickup coil arrays is presented by examining the response of a pair of planar circular spiral coils. Closed-form expressions are presented for the (i) mutual inductance of the coils in air, (ii) change in mutual impedance for the coils above a conductive linear ferromagnetic half-space, and (iii) change in mutual impedance for the coils in the thin-skin regime due to a long slot with uniform depth and width. The model predictions are compared with experimental measurements over the frequency range 100 Hz to 10 MHz using both Al-alloy and steel test specimens. When used within their known range of validity, the model predictions are in good agreement with experiment. The results demonstrate the flexibility and sensitivity of planar driver–pickup coils in eddy-current nondestructive evaluation.