| EMAT is a new non-destructive testing in the use of ultrasound technology in recent years. Compared with a conventional piezoelectric ultrasonic nondestructive testing technology,EMAT can do detection process without contact with the workpiece, without the use of a coupling agent, can adapt to the non-contact detection, high-speed online testing, high temperature testing and other special testing requirements. The metal plate thickness metal is the basic parameters of raw materials, oil pipelines, chemical storage tanks and other industrial materials. Measurement of the thickness of the sheet metal processing, chemical thinning of the pipe-line thickness measurement is directly related to the quality and safety of products. Application of electromagnetic ultrasonic technology to measure the thickness of the metal plate can well meet the inspection requirements. But its low conversion efficiency, the echo signal is weak, to the subsequent processing and signal detection process makes it difficult. How to improve the efficiency of acoustic emission stages to make the acoustic emission intensity, echo attenuation slow is a prerequisite for the use of sound waves to detect and echo subsequent processing. Therefore, the study of electromagnetic ultrasonic acoustic emission technology has practical significance.Starting from the basic principle, this paper describes the working principle of electromagnetic ultrasonic conducted, the mathematical model of electromagnetic ultrasonic were reasoning. Provide theoretical support for the follow-up study. Technical rationale for the application of electromagnetic ultrasonic thickness measurements were carried out instructions. Transducer structures were described in detail and the design optimization. This paper designed a small, battery-operated high-frequency, high-voltage dedicated to the electromagnetic ultrasonic thickness excitation circuit. This circuit uses STM32 chip and chip DDS to achieve a precise adjustable pulse frequency values. And build a digital gating circuit to select the number of pulses train. Overall structure is simple, low cost. Through the experiment, research the main factors affecting the acoustic emission about he frequency of the excitation voltage, excitation voltage amplitude, pulse number string, impedance matching coil parameters, the lift-off distance. Obtain parametric curves about these parameters and acoustic wave amplitude. The main factors affecting the amount of echo were explored and experimental verification. Describe the application of multiple echoes cumulative difference method for thickness measurement methods. Compare their advantages with poor single echo thickness measurement method through data analysis.Experimental study on an aluminum plate, the results show: Design of pulse excitation frequency transmitter circuit precisely adjustable, the output voltage range of 0 to 200 V. Electromagnetic ultrasonic thickness system can meet requirements. The frequency of the excitation voltage, excitation voltage amplitude, pulse number string, coil parameters and shape, impedance matching, lift-off value and other parameters have a greater impact on the electromagnetic emission of ultrasonic acoustic waves. Can do experiment to find the most suitable for the detection of parameters. Linear excitation coil excitation than helical coil has better performance on multiple echoes. The number of echo which can be used to measure the thickness is up to 50 or even higher. The cumulative difference between the use of multiple echo method can improve the detection accuracy, reduce errors caused due to insufficient sampling frequency. At 25 MHz sampling frequency, the application of echo difference ten times the cumulative accuracy have close precision with application method of a single echo difference method 10 sets of measurements averaged. |
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