Alternative coded excitation for multi-channel low-power ultrasonics

Coded excitation is a well-researched signal processing technique that employs phase modulations to improve signal quality in acquired A-scan ultrasonic timetraces. By utilising phase modulations based on sequences with favourable time-compression properties, total excitation energy can be increased. This directly improves Signal-to-Noise Ratio (SNR) without necessitating an increase in peak output power or reducing range resolution. Most prior research into this topic has focussed on understanding how typical ultrasonic inspection systems can be improved by incorporating coded excitations. This work instead highlights the benefits of an ultrasonic inspection system that is designed especially for coded excitations. Traditional systems ensure a high SNR by using high-voltage excitations, isolation circuits and pre-amplifiers before received signal digitisation. Instead, in this work we propose the use of low-voltage transmissions amplitudes which are within the operating range of the receiver components such that receiver isolation is not required. This removes the limitation on applicable sequence lengths for coded excitation measurements. Therefore, ever increasing sequence lengths can be utilised to improve the measurement SNR without increasing signal dead-zone. Further, this opens the possibility of using different types of sequence which can achieve quasi-ideal compression from a single transmission event, and facilitates the use of quasi-orthogonality for suppressing crosstalk during simultaneous operation of multi-channel systems. Through physical experiments, we show how this low-power coded approach can be utilised to match the performance (45dB SNR) of conventional high voltage systems (200V transmission amplitude with 15dB receiver amplification) whilst using only +/-2V transmission signals (40dB reduction in peak excitation power). The ability to remove inefficient high-voltage transmission hardware by adopting this technique can open the door for equally capable but much more cost effective and compact ultrasonic inspection hardware, making it ideal for applications such as permanently installed systems for Structural Health Monitoring.