| Among the acoustics branches, the ultrasonic technique is fastest developed and widest applied, In recent years, with the development of the electronic technology and the material science, the high-power ultrasonic technics such as ultrasonic cleaning, ultrasonic melding, ultrasonic crushing, ultrasonic atomization, ultrasonic emulsification, ultrasonic mashing, etc, are getting wider and wider application. This in reverse has promoted the study on the principle and application of ultrasonic technique. In the study on high-power ultrasonic apparatus, the power adjusting technique and the frequency tracing are two important subjects for research. Usually, the power adjusting is made either by manual work or through a complicated circuit system, and the latter often brings difficulties to the systematic debugging and designing. However, automatic power adjusting can be realized by using the piezoelectric transducer's characteristic that its equivalent resistance reduces with increase in external load when working at the parallel resonance frequency f_p. Under this condition, the transducer's output will increase with the increase in load as long as the driving voltage is kept constant. To make this function active, it is necessary to realize the automatic frequency tracing around the f_p.Based on the method combining theory with experiment, the behavior of the piezoelectric transducer around its f_p and auto-frequency tracing are studied in this paper. The behavior of the piezoelectric transducer around its f_p is discussed and summarized by using the equivalent-circuit method, and the result is testified by experiment. Through comparing various frequency-tracing methods at present, the phase-locked loop(PLL) method is selected for research because it can provide a high power factor to the system. The scheme tracing the series resonance frequency f_S is modified. Concerned with the poor sampling signals around transducer's f_p, a band-pass filter is used to modify the sample's waveform. The influence of the filter on the tracing frequency is analyzed, and a practical circuit is given. It may be a perfect scheme that PLL frequency tracing combines with the auto-power-adjusting feature around the transducer's f_p. It can realize auto-power adjusting for the system without the cost of increasing the circuit complexity. It is meaningful for simplifying ultrasonic apparatusand increasing the capacity to adapt the load.Following conclusions are drawed from this work:(1) The ultrasonic transducer has a good auto-power-adjusting feature around itsfp. The experiment shows that this feature is even better if the tranducer's voltage and current are in phase.(2) The auto-power-adjusting feature of the transducer exists not exact at/p, but in a range around the^,. The method for determinating the range is given.(3)Although a higher driving voltage is needed for the transducer working around thefp, the lower Q value actually makes the transducer bear a relative low voltage. Also, the matching inductor's bearing voltage is reduced (but the output transformer of the generator needs to bear a higher voltage). On the other hand, the poor sampling current waveform aroud the^ may bring certain difficulties to frequency tracing .(4) When tracing the fs with PLL system, an unlock circuit can be added to expand the frequency-tracing range and to strengthen system's ability to adapt the imitative load.(5) A band-pass filter can be used to help realize the parallel resonance frequency tracing.(6) When tracing the fp with a PLL system, the actual working frequency can be determined by either the imductance of the matching inductor or the central frequency of the band-pass filter.(7) The band-pass filter will shift the tracing frequency unless its central frequency is equal to thejp. Therefore, the auto-adjusting of the band-pass filter' central frequency is important to the^ tracing.(8) The fp tracing is successfully made on two parallel transducers with the PLL system.
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