Imaging using air-coupled polymer-membrane capacitive ultrasonic arrays

Polymer-membrane capacitive ultrasonic linear and 2-D arrays have been fabricated for use in air-coupled imaging. By using arrays as receivers, there is a possibility of much faster imaging as the need for physically moving the receiver to scan a sample can be replaced by electronic multiplexing. In order to utilise this, a through-thickness air-coupled image of a composite plate has been made using a 2-D array as a receiver and a comparatively large planar source in air. This was made possible by the use of a chirp drive signal and cross-correlation on the measured waveform. Larger 2-D arrays with an increased number of elements have been simulated using a small scanned single receiver, and excellent imaging potential demonstrated. In addition two array receivers have been used in conjunction with two methods of post-processing, SAFT and ellipse crossing, to locate objects accurately.