Inspection of drinks cans using non-contact electromagnetic acoustic transducers

Electromagnetic acoustic transducers (EMATs) have been used to transmit ultrasonic signals across liquid-filled aluminium beverage containers. The work has demonstrated a non-contact method for both determining the location of liquid boundaries, and detecting the presence of foreign bodies using fan-beam tomographic imaging. Pulse compression signal processing, followed by acoustic mode extraction, has been used in a new technique, whereby unwanted guided waves that travelled around the container wall can be identified and removed from the image reconstruction process. Such waves would normally complicate the identification of the signals require for image formation that propagated through the liquid. The result is a method that has been used to provide images of simulated foreign bodies, by scanning EMATs around outer surface of aluminium drinks cans. The approach has also demonstrated the successful measurement of fill level.