Research On The System And Method Of Electromagnetic Acoustic Measureing Thickness

In the field of industry manufacturing, it is necessary to measure the thickness of parts of metal pipe to ensure these materials quality. Conventional ultrasonic detection applications are based on piezoelectric sensors, which need the coupling medium between the sample and sensor. This technology of thickness detection difficultly achieves high-speed automatic measurement. The technology of Electromagnetic acoustic (EMA) measuring thickness attracts highly attention from researchers, it can detect the steel pipe with high-speed and contactlessness, and need no coupling medium, the mechanical structure of EMAT is relatively simple. With the rapid development of computer, digital and detection technology, the study on the electromagnetic acoustic wall thickness inspection system has become the inevitable trend of the detection technology.First of all, the paper discusses the theoretical basis of Electromagnetic Acoustic phenomenon and designs the transducer of EMAT. The shear wave is selected as testing wave. The direction of magnetic field, the coil geometry and the frequency of the electromagnetic field determine the wave type of transducer. Then we select the permanent magnet and the track coil to stimulate the shear wave. At the same time, Transmission-line transformer was applied to the impedance matching of the circuit.In order to improve the electro-acoustic conversion efficiency, the drive circuit and high voltage switch circuit produces a narrow pulse of high voltage to excite electromagnetic ultrasonic probe in the prompting part of hardware circuit. The system design two level of high-frequency ultrasound receiving circuit, which carries on de-nosing, amplification, filtering to the echo signal, and forms thickness pulse signal with the pulse-width extraction circuit. In the paper, the FPGA and DSP is designed to realize high-speed digital signal acquisition and processing, it can change the frequency of controlling signal and high-speed counter with software programming. The wavelet threshold denoising algorithm is programmed with Combined Language on the DSP to process echo signal, improve SNR and resolution. In the end, It is necessary to set up visualizatical detect interface for real-time detecting and reflecting the information of steel pipe's wall-thickness.The preliminary experimental results show that the EMAT have the feature of need-no-couplant, EMAT measuring wall thickness technology can be used the online detection of the pipeline.