An improved frequency tracking strategy in ultrasonic transducer

Abstract

The realization of automatic frequency tracking is of great significance in ultrasonic transducer due to its wide application area in nowadays industrial manufacture. However, excessive overshoot current, serious overheating and long response time remain the urgent issue. To tackle these existing ultrasonic frequency tracking problem, this paper proposes a hybrid technique that integrates fuzzy control theory with PID control on the system. The technique involves coarse adjustment by fuzzy control and accurate adjustment by PID. Then, direct digital synthesizer (DDS) is used to generate required PWM wave with high accuracy. Initial driving frequency of the system and its corresponding phase difference of the feedback voltage and current can both be acquired during sweep frequency test before the tracking process, which corresponds to the parallel resonant point where the current is the minimum. This phase difference is also regarded as the target tracking value. During the tracking process, fuzzy control will be adopted when phase difference value error between feedback value and target value is greater than 10%, or PID will be used. Current difference AI and change rate AI/Af are used as two input values while driving frequency is the output value in fuzzy control when coarse adjustment is needed. Incremental PID arithmetic is used when accurate adjustment is needed. The result shows that the technique can reduce overshoot current of the ultrasonic transducer by approximately 8%, shorten the time for achieving stabilization of the system by 12%. In conclusion, this novel hybrid technique can quickly and stably track the frequency of parallel resonant point of the ultrasonic transducer system, meanwhile keep the whole system operating more efficiency and stable.