Dynamic resonant frequency control system of ultrasonic transducer for non-sinusoidal waveform excitation

Abstract This study verifies the effectiveness of a dynamic resonant frequency control system for transducers. This system enables the resonant frequency of the transducer to match the driving frequency. In general, the resonant frequency of an ultrasonic transducer is fixed based on its design and material properties. Therefore, it is difficult to actively control the frequency when driving the transducer. However, for high-power piezoelectric actuators, it is important to control the ratio of the fundamental and higher-order resonant frequency of the longitudinal vibration precisely at 1:2. A high-power and high mechanical quality factor (high-Q) ultrasonic transducer requires precise control of its resonant frequency. However, the resonant frequency may shift due to changes in boundary conditions or non-linear phenomena in piezoelectric vibration while driving the ultrasonic transducer. To maintain the resonant frequency ratio of the ultrasonic transducer at 1:2, we propose to dynamically control the resonant frequency ratio constant. In this study, we made two main proposals to our dynamic resonant frequency control system. One is the stepped structure of the transducer, and the other is the completely automatic control. In the stepped structure, a Langevin transducer was designed to have a resonant frequency ratio of almost 1:2 for the first and third longitudinal mode in the initial condition. Additionally, this structure could achieve control of only one of two resonant frequencies of the transducer. For the utterly automatic control system, piezoelectric elements were introduced for controlling the resonant frequency ratio precisely. For this propose, switching the electrical boundary conditions of these piezoelectric elements was carried out by MOSFETs connected to the ultrasonic transducer and control its optimum duty ratio automatically by our feedback system. This system realized dynamic control of the resonant frequency. As a result, the resonant frequency of the transducer matched the driving frequency in the frequency band from 23.23 kHz to 23.93 kHz. It was also confirmed that the shape of the excited non-sinusoidal waveform could be controlled by using resonant frequency control.