Lift-off performance of electromagnetic acoustic transducers (EMATs) for surface acoustic wave generation

Electromagnetic acoustic transducers (EMATs) have a behaviour that depends on the distance between the transducer and the surface of the material under test, namely the lift-off. Rayleigh waves generated by EMATs suffer from waveform distortion as the lift-off is increased. This paper describes this distortion numerically and experimentally, focusing on the spatial distribution of the induced currents and hence the ultrasound pulses. This is verified experimentally using an EMAT consisting of a magnet and a single 1.5 mm wide linear coil, showing a decrease in peak frequency for the wideband Rayleigh wave of the order of 100 kHz/mm with lift-off. The behaviour when using EMATs in an array configuration (equivalent to a meander-coil, where the current through neighbouring coils is in opposite directions) is then described. Coil spacing is shown to affect the lift-off behaviour, with the spatial model predicting well the frequency behaviour. The lift-off behaviours of physically spaced coils and those which are pulsed with time delays to give an effective separation are shown to be equivalent for all but the smallest separations; unless very small separations are required to give a high frequency signal, the effective spatial current distribution spread for a single coil counteracts the benefits of reducing the dipole effect. Good performance with lift-off variation is hard to achieve at high frequency, and lift-off must be considered if a particular frequency of operation is required.