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.

High accurate flow measurement is important for process monitoring. The clamp-on ultrasonic flow meter is a type of non-invasive and non-intrusive flow measurement technique. However, the traditional used transit-time difference method in the clamp-on ultrasonic flow meters cannot achieve accurate measurement under small flow rates because the measurement accuracy is limited by the equipment’s sampling rate and system error. This article proposes a novel flow measurement method by using signal voltage amplitude difference (VAD). The transit-time difference information is converted into voltage amplitude difference information, therefore the measurement error due to sampling rate at small flow conditions can be eliminated and the measurement accuracy is improved. The analytical solutions of ultrasonic vibrations are analyzed and a new “M” shaped ultrasonic transducer design is introduced. The new transducer can generate and receive downstream and upstream ultrasonic signals simultaneously. In this scenario, the VAD method can be used for flow measurement. In addition, a variety of influencing factors are analyzed to further improve the measurement accuracy of the proposed method. Experimental results indicate that the combined uncertainty of the VAD method exhibits a 62.5% reduction compared to that of the transit-time difference method.

Rayleigh wave electromagnetic acoustic transducer (RW-EMAT) has been widely used in nondestructive testing. It is suitable for metal surface crack detection due to its energy concentration within a small depth. However, the EMAT has low energy conversion efficiency and poor signal quality. The spatial resolution is degraded after applying a coil with several turns and an excitation signal with a few cycles. To enhance the spatial resolution of RW-EMAT, a new technique based on temporal-spatial pulse compression (TSPC) is proposed. The transmitter with a linear wavelength modulated coil acts as a spatial matched filter. It spatially compresses the temporal excitation signal with corresponding linear frequency modulation. Then, a novel ultrashort ultrasonic field for Rayleigh waves is generated. The finite element model and experimental results indicate that the TSPC technique compresses the ultrasonic field into a spike distribution. Compared with the conventional approach, the spatial signal width of RW-EMAT is reduced by about 98%. For the adjacent defects which cannot be identified by the conventional approach, the TSPC method can identify them clearly and the calculation error of defect spacing is less than 2%. This work significantly enhances the spatial resolution of RW-EMAT.