Coded excitation for low-SNR systems and EMATs

Abstract

This thesis addresses two main subjects: the development of efficient electromagnetic-acoustic transducers (EMATs) and the synthesis of coded sequences for pulse-echo mode. EMATs are desirable because no mechanical contact with the sample is required; however, EMATs are inherently inefficient. To improve their performance, a ferromagnetic core surrounded by permanent magnets whose like poles face the core is proposed as the bias magnetic field source; this configuration can outperform a single magnet by an order of magnitude. Coil configurations that result in linear and radial polarisations of the ultrasonic waves are also compared; the linear polarisation was found to yield higher mode purity and penetration depths. Furthermore, the optimal impedance of the EMAT coil is discussed.

Although improvements in EMAT sensitivity of up to 20 dB were achieved, this is not enough to obtain adequate signal-to-noise ratios (SNR) (> 30 dB) without averaging over a long period of time. Therefore, efficient encoding techniques to achieve a greater SNR over shorter periods of time were investigated. The maximum length of conventional coded sequences and hence the maximum SNR increase is limited by the location of the closest reflectors. In this thesis, coded sequences that have receive intervals are introduced so that their overall length and therefore the SNR increase is not affected by the location of the reflectors; the proposed sequences can produce a given SNR increase an order of magnitude faster than averaging. These sequences are then used to drive EMATs using less than 0.5 W and a repetition rate of 10 Hz; this rate can be perceived by a human inspector as quasi-real-time. Moreover, a set of pseudo-orthogonal sequences that have common receive intervals can simultaneously be transmitted through several transducers. This makes it possible to increase the number of active elements in an array, which combined with synthetic focusing, can increase the resolution and contrast of the resulting image without affecting the frame rate.

Binary quantisation is also investigated in this thesis. It can be used with low-SNR systems resulting in minimal loss of information while reducing the data throughput and the complexity of the electronics, especially in arrays with many elements. The theory behind binary quantisation is reviewed and the maximum input SNR range that does not cause unacceptable distortions is investigated.

Finally, the first low-power, pulse-echo EMAT phased array system is proposed. This array can perform similarly to conventional piezoelectric arrays mounted on a wedge to excite angled shear wave in the sample. Racetrack coils are used as the array elements laid out in an overlapping pattern that minimises inter-element crosstalk and results in an array element width and pitch equivalent to 1 and 2/3 of the wavelength respectively. Coded sequences that have receive intervals are used to reduce the drive power to just a few watts (24 Vpp). The performance of an 8-channel, 1-MHz prototype in the detection of surface cracks which have a length of less than 1 mm is reported.