Parallel resonant frequency tracking based on the static capacitance online measuring for a piezoelectric transducer

Piezoelectric transducers working series resonant frequency (SRF) may suffer damage due to the over-heat of the piezoelectric device caused by the large current. It is especially severe when the load value is large and changes fast. On the other hand, it is proved that a transducer enjoys a load-based self-adjustable output power when operating under its parallel resonant frequency (PRF). To take advantage of such feature and avoid the damage at SRF, a method of PRF tracking was proposed based on the online measurement of static capacitance. First, the relationship between the output power and load was derived based on the parallel equivalent circuit. Then, a target impedance value was defined and tracked as the flag of PRF. It shared the same changing rules as that of total equivalent impedance with respect to working frequency. The variable-step frequency sweeping method was improved to find PRF in less than 1 s and the error is less than 1 Hz. The proposed method monitored the static equivalent capacitance in real time since the capacitor always changes irregularly with the working condition. Also, the self-unlock mechanism was added to eliminate local optimum. The experimental results based on the proposed methods agreed with the theoretical analysis, which indicated the potential of the proposed method in practice.