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In this study, a resonant-type smooth impact drive mechanism (R-SIDM) actuator fabricated with lead-free piezoelectric materials was investigated. There is high demand for compact high-power actuators, and ultrasonic motors are a promising way to meet these requirements because of their high power density. The R-SIDM actuator is a type of linear ultrasonic motor that is driven with a quasi-saw-shaped displacement excited by a combination of two resonant sinusoidal waves. The advantage of the R-SIDM actuator is its low heat generation compared with that of conventional SIDM actuators because of its resonant drive operation. A previously proposed R-SIDM used a transducer composed of lead zirconate titanate (PZT), which has excellent piezoelectric properties but is harmful to the environment. Therefore, in the presently proposed R-SIDM actuator, PZT was replaced by the lead-free piezoelectric ceramic (K,Na,Li)NbO 3 (KNLN). The actuator was used to rotate a bearing rotor to measure the driving performance, and a driving speed of 53 mm/s with no load and a maximum thrust force of 69 mN were obtained.

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.

This paper proposes a wireguide-driving piezoelectric actuator for narrow space driving such as in endoscopic devices. Conventional wireguide-driving actuators use torsional or bending vibration. We examined a resonant-type smooth impact drive mechanism (R-SIDM) utilizing longitudinal vibration as the driving principle. The longitudinal vibration is resistant to outside disturbances. Different to conventional smooth impact drive mechanism (SIDM) actuators, R-SIDM actuators use the resonant effect. The quasi-saw-shaped displacement is excited by combining two resonant vibrations with a frequency ratio of 1:2. This driving principle allows lower input voltage compared to that of the conventional SIDM. As the driving source, a step-shaped Langevin transducer was fabricated. With this step-shaped design, the longitudinal resonant frequency ratio f 3 / f 1 could be adjusted to 2. At the tip of this transducer, an aluminum wireguide was attached to excite a quasi-saw-shaped vibration at the end tip. The length of the wireguide was designed to match the resonant frequencies of the transducer. It was confirmed that a bearing rotor (diameter: 10 mm) could be rotated. As we expected, a quasi-saw-shaped vibration was excited, and we could measure a non-loaded driving speed of 176 mm/s and a maximum load of 0.47 N for a 100 V p – p input voltage and a 6.0 N preload.