Improvements in EMAT Sensitivity
Electromagnetic-acoustic transducers (EMATs) are generally regarded to have a poor sensitivity compared to piezoelectric transducers. However, EMATs are contactless transducers and it is expected that they outperform piezoelectric transducers on applications where couplant cannot be used; there are unfavourable coating layers; or a certain lift-off from the sample under inspection is required.
EMATs are commonly operated at hundreds of volts. This tends to make the electronics bulky and unsafe in explosive environments; therefore, there is a need to explore how to increase the sensitivity of EMATs.
The magnetic fields of permanent magnets and coils that compose the EMATs are studied so as to understand the optimal configurations and dimensions to build an EMAT optimised for sensitivity.
Regardless of any improvements that could be obtained in the design of EMATs, the level of the signals expected will still be very low if low voltages are used to drive EMATs. In these cases, signals are commonly found below the noise threshold; hence, special electronics and signal processing are needed to retrieve the signals.
Electronics must be carefully designed, keeping noise and interference to a minimum while procuring a very high gain – usually around 100dB. Complex modulation techniques are used to maximise the energy injected into the EMATs in the least time possible; for example, in pulse-compression techniques.
The different modulations; associated electronics; and signal processing are addressed in order to maximise the efficiency of the acquisition system.