Accurate Thickness Measurement Using Eddy Current System Based on Novel Transformer Model

Eddy current (EC) testing has been widely regarded as a popular tool for thickness measurement due to the advantages of noncontact, low cost, and efficiency. Phase signature is believed to change linearly as a function of specimen thickness using the conventional transformer model, which, however, does not take into account the effect of EC diffusion and reflection. In this work, a novel transformer model is developed, and an accurate method for thickness measurement is proposed subsequently. Firstly, true skin depth is formulated with a simple but accurate function, and an equivalent thickness is introduced to characterize ECs considering the difference between the standard and true skin depth. Secondly, the use of the thickness of equivalent ECs develops the novel transformer model, and a novel method is figured out to improve the accuracy of thickness measurement. Basically, the difference between the true and standard skin depth is closely related to excitation frequency, liftoff distance and the electromagnetic properties of a specimen. Therefore, a correction parameter is needed and it should be redetermined for different cases. Finally, an EC system was built, and the experiments were carried out to evaluate the novel method. The results show that it outputs more accurate specimen thickness, especially for low conductive materials.