| The electrical impedance matching of piezoelectric transducer is important in the application of power ultrasound. As the rapid economic development, the application of power ultrasound become wider and wider. But traditional static impedance matching can not meet the impedance change at work. Automatic impedance matching that widely used at present is just for the ultrasound power supply with changed output frequency, and it also has the problem of only focusing on resonance but ignoring impedance transformation. The output frequency of ultrasound power supply is fixed in the ultrasound therapy device. This paper proposed a new automatic impedance matching method for such condition based on the basic principle and requirements of electrical impedance matching, achieved the resonance and impedance transformation by on-line detection of matched transducer impedance and accordingly adjusting the matching network.This paper established the overall design scheme for the system after analyzing the impedance characteristics of piezoelectric transducer.The impedance detection and matching network adjust were achieved by the vector voltage/current method and step motor respectively. And the implementation of hardware and software parts were also presented. The hardware circuit consists of C8051F015, voltage/current transformer, active band-pass filter circuit,RMS conversion circuit,an external counting circuit and stepping motor. The structure and function of every circuit unit were introduced and the error source and improvement method were also presented. In the software design, the design plan and function are given and all programs were completed by C language. In the experimental part, every circuit unit was tested by standard signal generator, and the results show that the system has high precision and can be used in the on-line detection of ultrasound transducer impedance.
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