Coded Excitation for Pulse-Echo Systems

Underwater quantitative thickness mapping through marine growth for corrosion measurement using shear wave EMAT with high lift-off performance

Underwater inspection is important to ensure the safety, integrity and functionality of underwater structures. Although numerous conventional methods have been adopted for underwater inspection, successful application of most methods relies on the surface condition of the object, which, however, is typically covered by marine growth. Consequently, routine inspection requires thorough cleaning of marine growth, which is time-consuming and costly. Hence a method which can inspect objects without the need for extensive surface cleaning is necessary. Two methods have the potential to achieve this: pulse eddy current (PEC) and electromagnetic acoustic transducer (EMAT). PEC attains a significant lift-off distance, enabling inspection through marine growth. However, it suffers from high sensitivity to environmental conditions and low inspection accuracy due to 'relative' property which means its results are interpreted by comparing received signals to reference values. In contrast to PEC, EMAT provides 'absolute' measurements, ensuring precise results in the inspection. But it is limited by a small lift-off distance (<2∼3 mm), rendering it unsuitable for underwater applications with marine growth. Therefore, if the lift-off distance can be enhanced to a specific value, this method may offer a superior solution for underwater inspections. In this paper, a quantitative measurement method is proposed through employing a shear wave EMAT with high lift-off performance. A repelling configuration of magnets is introduced to achieve a significantly improved maximum effective lift-off distance of up to 5 mm in both air and seawater conditions with only 400 Vpp applied. This EMAT is then demonstrated to measure thickness through marine growth, showing excellent underwater performance in quantitative thickness mapping for corrosion inspection.

A New Design to Rayleigh Wave EMAT Based on Spatial Pulse Compression

The main disadvantage of the electromagnetic acoustic transducer (EMAT) is low energy-conversion efficiency and low signal-to-noise ratio (SNR). This problem can be improved by pulse compression technology in the time domain. In this paper, a new coil structure with unequal spacing was proposed for a Rayleigh wave EMAT (RW-EMAT) to replace the conventional meander line coil with equal spacing, which allows the signal to be compressed in the spatial domain. Linear and nonlinear wavelength modulations were analyzed to design the unequal spacing coil. Based on this, the performance of the new coil structure was analyzed by the autocorrelation function. Finite element simulation and experiments proved the feasibility of the spatial pulse compression coil. The experimental results show that the received signal amplitude is increased by 2.3~2.6 times, the signal with a width of 20 μs could be compressed into a δ-like pulse of less than 0.25 μs and the SNR is increased by 7.1-10.1 dB. These indicate that the proposed new RW-EMAT can effectively enhance the strength, time resolution and SNR of the received signal.

Simultaneous transmission and reception on all elements of an array: binary code excitation

Pulse-echo arrays are used in radar, sonar, seismic, medical and non-destructive evaluation. There is a trend to produce arrays with an ever-increasing number of elements. This trend presents two major challenges: (i) often the size of the elements is reduced resulting in a lower signal-to-noise ratio (SNR) and (ii) the time required to record all of the signals that correspond to every transmit-receive path increases. Coded sequences with good autocorrelation properties can increase the SNR while orthogonal sets can be used to simultaneously acquire all of the signals that correspond to every transmit-receive path. However, a central problem of conventional coded sequences is that they cannot achieve good autocorrelation and orthogonality properties simultaneously due to their length being limited by the location of the closest reflectors. In this paper, a solution to this problem is presented by using coded sequences that have receive intervals. The proposed approach can be more than one order of magnitude faster than conventional methods. In addition, binary excitation and quantization can be employed, which reduces the data throughput by roughly an order of magnitude and allows for higher sampling rates. While this concept is generally applicable to any field, a 16-element system was built to experimentally demonstrate this principle for the first time using a conventional medical ultrasound probe.

EMAT phased array: A feasibility study of surface crack detection

Electromagnetic-acoustic transducers (EMATs) consist of a magnet and a coil. They are advantageous in some non-destructive evaluation (NDE) applications because no direct contact with the specimen is needed to send and receive ultrasonic waves. However, EMATs commonly require excitation peak powers greater than 1kW and therefore the driving electronics and the EMAT coils have to be bulky. This has hindered the development of EMAT phased arrays with characteristics similar to those of conventional piezoelectric phased arrays. Phased arrays are widely used in NDE because they offer superior defect characterization in comparison to single-element transducers. In this paper, we report a series of novel techniques and design elements that make it possible to construct an EMAT phased array that performs similarly to conventional piezoelectric arrays used in NDE. One of the key enabling features is the use of coded excitation to reduce the excitation peak power to less than 4.8W (24 Vpp and 200mA) so that racetrack coils with dimensions 3.2×18mm² can be employed. Moreover, these racetrack coils are laid out along their shortest dimension so that 1/3 of their area is overlapped. This helps to reduce the crosstalk between the coils, i.e., the array elements, to less than -15dB. We show that an 8-element EMAT phased array operating at a central frequency of 1MHz can be used to detect defects which have a width and a depth of 0.2 and 0.8mm respectively and are located on the surface opposite to the array.