A Novel High-Speed Resonant Frequency Tracking Method Using Transient Characteristics in a Piezoelectric Transducer

Abstract

When driving the piezoelectric transducer (PT: piezo transducer), which is a key device, it is important for the ultrasonic system (using ultrasonic waves of 20 kHz or higher) to operate at a resonant frequency that can maximize the conversion of mechanical energy (vibration) from electrical energy. The resonant frequency of the PT changes during the actual operation according to the load fluctuations and environmental conditions. Therefore, to maintain a stable output in an ultrasonic system, it is essential to track the resonant frequency in a short time. In particular, fast resonant frequency tracking (RFT: resonant frequency tracking) is an important factor in the medical ultrasonic system, i.e., the system applied in this thesis. The reason is that in the case of a medical ultrasonic system, heat-induced skin necrosis, etc., may cause the procedure to be completed within a short period of time. Therefore, tracking the RFT time for maximum power transfer is an important factor; in this thesis, we propose a new high-speed RFT method. The proposed method finds the whole system resonance frequency by using the transient phenomenon (underdamped response characteristic) that appears in an impedance system, such as an ultrasonic generator, and uses this to derive the mechanical resonance frequency of the PT. To increase the accuracy of the proposed method, parameter fluctuations of the pressure of the PT, the equivalent circuit impedance analysis of the PT, and a MATLAB simulation were performed. Through this, the correlation between the resonance frequency of the ultrasonic system, including the LC filter with nonlinear characteristics and the mechanical resonance frequency of the PT, was analyzed. Based on the analyzed results, a method for tracking the mechanical resonance frequency that can transfer the maximum output to the PT is proposed in this thesis. Experiments show that using the proposed high-speed RFT method, the ultrasonic system can track the mechanical resonance frequency of the PT with high accuracy in a short time.

Review of the design of power ultrasonic generator for piezoelectric transducer

“…Usually, the pure PLL phase control tracking method is only used to track the local resonant frequency, but not suitable for large frequency range sweep tracking [15] , [174] , [179] . Other methods (including the maximum current method [182] , maximum power method [183] , self-sensing model method [184] , composite tracking method [106] , etc.) can usually perform both global sweep frequency tracking and local resonance frequency tracking.…”

Advances in high-performance MEMS pressure sensors: design, fabrication, and packaging

“…This design was responsible for maintaining the sensor’s resonant frequency at a predetermined value, which enables accurate pressure measurements 97 . Various techniques, including the frequency locking technique 98 , 99 , phase-locked loop (PLL) technique 100 – 104 , adaptive control technique and digital control technique 105 , 106 , and proportional-integral-derivative (PID) control technique 74 , 77 , 94 , 107 , have been developed to drive mechanical motion, control feedback, and track frequency changes. For the circuit design described above, establishing the equivalent circuit model of an RPS provides an important foundation for sensor characterization and closed-loop circuit design.…”

“… a Schematic of the closed-loop circuit with automatic gain control 100 . b Schematic view of the oscillator and the frequency response of the loop gain 101 .…”

“…9c, d, and e . The operating mechanism of a PLL is based on comparing the phase of the RPS output signal with that of a reference signal and adjusting the operating conditions accordingly 100 – 104 . In general, a PLL circuit consists of a phase detector that compares the phase of the PRS output with the reference signal, a VCO generating a frequency signal based on the phase difference, a low-pass filter that reduces the signal gain of noise, a high-frequency component, a loop filter shaping the control signal to the VCO based on the filtered output, and a feedback loop connecting the output of the VCO to the sensor to adjust its resonant frequency.…”

A Frequency-Dependent Dynamic Electric–Mechanical Network for Thin-Wafer Piezoelectric Transducers Polarized in the Thickness Direction: Physical Model and Experimental Confirmation

“…Theoretical and experimental studies reported by Piquette showed that, under the excitation of a sinusoidal voltage signal, the dynamical state on the surface of a piezoelectric transducer went through a transient transition process from a static state to a steady state sinusoidal vibration [ 52 , 53 ]. Moon et al found that the resonant frequency of the piezoelectric transducer changed with the load’s fluctuations and environmental conditions and proposed a method to track the resonant frequency using the transducer’s transient characteristics [ 54 ]. Wang et al investigated the relationship between eigenfrequency and the early strength development of cement mortar by using a piezoelectric transducer [ 55 ].…”