東京大学大学院 新領域創成科学研究科 人間環境学専攻 人間支援デバイス分野

東京大学大学院 新領域創成科学研究科 人間環境学専攻 人間支援デバイス分野

Introduction of Research

Miniaturized R-SIDM actuator / Resonant frequency control

As one of promising miniaturized piezoelectric actuator, we are studying on the SIDM (Smooth Impact Drive Actuator). The driving principle is based on asymmetric piezoelectric motion, namely slow expansion and quick returning motion. Due to inertia of the slider, slip-stick phenomena is induced, which results in unidirectional frictional motion of the slider. In this study, we proposed to realize this asymmetric motion by combining two resonant vibration motions to reduce the input voltage. For this purpose, the resonant frequency ratio must be controlled to be 1:2. As a result of low-voltage operation, temperature increase is eliminated. To realize this RSIDM (Resonant type SIDM) operation, the resonant frequency ration should be controlled precisely. During operation, the temperature increase and the nonlinear piezoelectric vibration affect this ratio. To overcoming this problem, we proposed the resonant frequency control system was developed. In general, the piezoelectric material is utilized for driving the transducer; however, in this study, the resonant frequency controlling piezoelectric parts were introduced. By changing the electrical boundary condition for these parts, the resonant frequency of the stator transducer can be adjusted during actuator operation.



Hydrothermal Method

Piezoelectric materials are promising for miniaturized actuator or sensors due to its simple mechanism for energy conversion. For developing these elemental technologies, large thickness of the piezoelectric films is strongly required. Compared to sol-gel method, sputtering, CVD, the hydrothermal method has various unique advantages. An important feature is its low temperature deposition, 150 degree C which is more than 450 degree C lower than the conventional methods. In addition, the hydrothermal method enables to deposit the film on the three dimensional substrate because the process is carried out based on the chemical reaction in solution. Until now, by using the hydrothermal method we realized the micro piezoelectric actuator, ferroelectric memories and so on. Recently, we succeeded in increasing the thickness larger than 10 micro meters by developing the ultrasonic assist hydrothermal method. By irradiation ultrasonic power during the hydrothermal process, the larger thickness and smooth surface profile were confirmed. As target materials, PZT and environmental friendly material such as (KNbO3) type piezoelectric materials are studying in this project.



Evaluation of Nonlinear Piezoelectric Vibration

For developing the high-power ultrasonic devices, such as medical ultrasonic knives and ultrasonic motors, nonlinear piezoelectric vibration must be taken into consideration. When the piezoelectric material is excited with large amplitude, complicated phenomena appear, such as the increase in heat generation, the change in resonance frequency, the vibration speed / current jump phenomenon (Figure 1).
In this research, focusing on the influence of higher order elasticity, nonlinear modeling and quantitative evaluation for piezoelectric high power characteristics were conducted. In the nonlinear piezoelectric equivalent circuit model (Fig. 2b), the parameters of the single-plate piezoelectric vibrator (Fig. 2a) are examined and nonlinear mechanical stiffness was introduced. As a next step, for considering a distributed parameter model (Fig. 3), transfer matrix method was improved by putting nonlinear vibration parameters. These modeling enabled to study on the influence of heat generation and the practical ultrasonic transducer such as Langevin transducer. Our evaluation method is quite important for the material development process, for example finding the suitable piezoelectric materials for high power ultrasonic devices. Now, we are trying to find such materials in environmental friendly materials.



Self-sensing of Piezoelectric Manipulator

The technology of miniature manipulators is highly required for surgical robots and other electrical devices. To estimate the holding state or the softness of the gripped object, one approach is to attach a sensor directly to a manipulator. However, miniaturization becomes more challenging in this case. Our research suggests that piezoelectric materials are used as both actuators and sensors in a small piezoelectric manipulator. The grip is opened and shut by amplifying the displacement of a piezoelectric stack through a flexure hinge. When an object is gripped, the change in mechanical boundary condition results in a deviation of electrical frequency characteristics which is utilized to estimate the mechanical impedance, softness, and viscosity of the object. In addition to the driving voltage of the gripper, a small AC sensing voltage is applied for measuring electrical characteristics changes. Therefore, a single piezoelectric material can provide both gripping and sensing functions. We are working on designing of a manipulator and measuring a mechanical impedance of a soft object.