• Lines, David
• Nicolson, Ewan
• Macleod, Charles N.

Abstract

The introduction of the ultrasonic phased array has inspired many advanced ultrasonic imaging techniques, through either physical beamforming or by the post-processing of Full Matrix Capture (FMC) datasets. However, post-processing methods have struggled to meet the demand for fast, lightweight imaging solutions for Non-Destructive testing (NDT). Impractical hardware and software requirements have led to the limited deployment of such techniques.<br/>Despite offering improvements in both flaw detection and characterisation, a fundamental drawback of post-processing algorithms is the increase in algorithmic complexity relative to beamformed inspection methods. TFM requires extensive focusing, and struggles to meet real-time requirements unless often expensive hardware acceleration is available. Furthermore, the volume of data required to be transferred through the imaging system is orders of magnitude larger than conventional beamforming techniques. Furthermore, the use of single-element excitation during FMC acquisition often requires high-excitation voltages to maximise acoustic energy transfer during inspection, to achieve a reliable reflection from flaws.<br/>The use of coded excitation has shown to limit the requirement for high-voltage excitation, by altering the transmission process of the phased array. Waveforms are generated using corresponding pairs of Golay codes of length 2N, which are fired successively – doubling the number of transmission events. This has shown improved Signal-to-Noise Ratio (SNR) of post-processing algorithms in attenuative materials, and can maintain SNR at low excitation voltages.<br/>Single-bit quantisation of FMC data has been shown to greatly reduce data transfer rates and image processing times, in particular for Phase Coherence Imaging (PCI). This has allowed reduced computational hardware requirements for data processing, with a typical 12-bit Analog-to-Digital Converter (ADC) being replaced by a comparator working as a single-bit ADC. <br/>By combining single-bit quantisation and coded excitation, an ultra-lightweight imaging system can be constructed with the potential to drastically reduce hardware and software requirements for post-acquisition phased array imaging. The capability of this concept has been tested using a 5 MHz array to image tip diffraction of an EDM notch in a thick carbon-steel block. By performing the required post-acquisition correlation between Golay pairs with single-bit precision, higher-order precision data has been shown to be effectively reconstructed and used to produce PCI images. Comparative studies have shown no reduction in SNR of PCI images for low-voltage excitation relative to regular high-voltage imaging processes, using a 32-cycle Golay code waveform.

Topics

• impedance spectroscopy
• Carbon
• phase
• steel
• ultrasonic