Transient response of a driver-pickup coil probe in transient eddy current testing

• First correct and complete analytical description of a transient pickup signal.
• Excellent experimental agreement; infamous feedback challenge addressed.
• Method pertains to inductive coupling with ferromagnetic and conducting structures.
• EM field solutions incorporated into circuit equations for exact driver response.
• Arbitrary excitation waveforms, including sinusoidal and step, may be considered.

Novel solutions that correctly incorporate all electromagnetic interactions arising in inductively coupled circuits are presented for the case of a coaxial driver and pickup coil probe encircling a long ferromagnetic conducting rod. The differential circuit equations are formulated in terms of the rod׳s impulse response using convolution theory, and solved by Fourier transform. The solutions presented here are the first to account for feedback between a ferromagnetic conductor and the driver and pickup coils, providing correct voltage response of the coils. Experimental results, obtained for the case of square wave excitation, are in excellent agreement with the analytical equations.