The Research On Dynamic Impedance Matching System Of EMAT

This paper investigates the problem of ultrasonic energy degradation because of changing input impedance of electromagnetic ultrasonic transducer(EMAT)that is used for measuring the thickness of continuous casting billet in high temperature environment due to circumstances.In a high temperature environment,the input impedance of EMAT will change with temperature,load,parasitic parameters and lift-off change.When the output impedance of driving power supply does not match the input impedance of EMAT,excitation signal will reflect along the transmission path,causing standing wave.As a consequence,the efficiency of the transducer is degraded and the energy of ultrasonic is depreciated.Thus,the power demand of ultrasonic testing cannot be met,the accuracy of thickness detection of continuous casting billet is affected.This paper studies the impedance characteristics of EMAT.We design a dynamic impedance matching system based on feedback control of the ultrasonic energy(envelop amplitude).This system can improve the efficiency of EMAT by adjusting the matching speed and accuracy,which has great value in industry usage.First,we calculate the impedance of inflection type coil of EMAT using finite-element analysis under normal temperature,and provide the distributive capacitance.Such information is used to determine the matching capacity under a certain frequency,which enables the transducer to work under static impedance matching.On the other hand,we use impedance analyzer MICROTEST 6379 to measure the impedance change of matching coil under different lift-off,temperature and testing conditions.We compared several impedance matching schemes,and adopt the ?-impedance matching network to build the dynamic model between ultrasonic energy collected by Pico Scope 5444 A and the parameters of impedance matching network.Then,we build a testing platform with the ultrasonic excitation power supply to conduct envelope analysis of the ultrasonic singles collected by Pico Scope.The main amplitude is used as the input to the dynamic model,a modified Hill Climbing Algorithm is used to iterate new solution,which is propagated to MCU to change the parameters of the matching network in real-time.The optimal solution is searched to match the impedance of driver output and transduce input impedance.The envelope analysis and Hill Climbing Algorithm to the ultrasonic signal is simulated in matlab.At the same time,the impedance matching network and the equivalent electromagnetic acoustic coupling circuit is simulated to study the impact of the end-point echo waveform of the matching network.Finally,we evaluate the dynamic impedance matching in the testing platform,the results are close to the simulation and good performance is achieved.