Abstract
Abstract Electromagnetic-acoustic transducers (EMATs) work on transduction principles which allow them to operate with a clearance (liftoff) between them and a conducting specimen. They have the potential for on-line ultrasonic measurements of rapidly moving materials. Liftoff causes changes in the effective inductance and resistance of the EMAT. Consequently, it causes a phase shift in the output voltage of a receiving EMAT. This can cause errors when the EMAT is used for velocity measurements. In this paper, we develop a model for the effect of liftoff. The model gives good agreement with measured liftoff-induced arrival time changes. The model can be extended to the case of an EMAT used with a voltage stepup transformer. The maximum signal is obtained for EMATs operating at resonance; however, the maximum sensitivity to liftoff also occurs then. Thus, a tradeoff must be made between optimum signal and suppressing liftoff artifacts. Our model, and experimental results, can be used to make these tradeoffs.
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