Non-contact thickness gauging of aluminium strip using EMAT technology

Accurate measurement of aluminium thickness is an important aspect of quality control for rolled aluminium products. This paper presents the findings of trials to measure the thickness of aluminium strip using a send-receive, radially polarised broadband Electromagnetic Acoustic Transducer (EMAT). Two broadband EMAT systems have been used, with centre frequencies of approximately 5 MHz and frequency content up to 12 and 20 MHz, respectively. The resultant ultrasonic waveforms have been processed using Fast Fourier Transform (FFT) analysis. Strip samples of an aluminium alloy in the thickness range between 0.28 and 2.8 mm have been measured in static tests to a resolution of 1 μm using this non-contact approach at stand-offs of up to 2 mm. Thickness measurement tests was also carried out successfully on a moving strip at speeds up to 2 m/min, to detect the gauge variation over the length of the strip.