Research Of Electromagnetic Ultrasonic Signal Acquisition System

Due to EMAT(Electromagnetic Acoustic Transducer, EMAT) has many advantages, such as non-contact, no coupling agent, good reproducibility, wide application range, high detection precision etc. It is widely used in railway tracks, pipes, sheet metal and other non-destructive testing. However, the signals received by EMAT are small in amplitude, easy to be interfered by noise and high in frequency, so it is inconvenient for the acquisition of electromagnetic ultrasonic signal. So, the study of electromagnetic ultrasonic signal acquisition system in this paper has important theoretical significance and practical value.Based on the study of working mechanism of EMAT, this paper gives an analysis on the characteristics of electromagnetic ultrasonic signals received by EMAT, determines the overall design scheme of the electromagnetic ultrasonic signal acquisition system and modular design. The system consists of EMAT probe, signal conditioning circuit, data acquisition card, the personal computer and LABVIEW.The design of signal conditioning circuit is the key part of this paper, which is mainly composed of five modules: preamplifier circuit, band-pass filter, third stage amplification, equal phase modulation and linear power supplies. Under the strong noise background, the preamplifier circuit can realize the broadband and low noise amplification for weak signal with three different gain 20 dB, 30 dB and 40 dB. Band-pass filter circuit can filter out the noise with high value of Q to greatly improve the SNR of the received signal, the center frequency f0 and Q also can be adjusted continuously, so the applicability of the circuit is expanded. Third stage amplifier circuit eliminates the influence of the load variation on the filter circuit, and make up for the inadequacy of preamplifier magnification. The equal phase modulation circuit can realize a phase delay within 0~90° for the input signals whose frequency range from 100 kHz to 2MHz, so it reduces the error caused by phase shift and improves the accuracy of defect detection. The design of DC linear power supply can eliminates the noise caused by the switching power supply.This paper mainly discusses the simulation analysis, circuit structures and experimental verification of the preamplifier, band-pass filtering and the phase-shift circuit. Then the overall signal condition circuit structures and the experimental verification were completed, and the experimental results meet the design requirements.