Efficient Modulation of Friction in Ultrasonic Motors Using Functional FluidsEfficient Modulation of Friction in Ultrasonic Motors Using Functional Fluids Efficient Modulation of Friction in Ultrasonic Motors Using Functional Fluids 機能性流体を用いた超音波モータにおける摩擦制御に関する研究
Ultrasonic motors (USMs) have been extensively studied for three decades, due to their attractive features, such as high torque in low speed, simple structure, and precise positioning capability. However, since USMs are driven by the friction force between the rotor and the stator in most cases, they inherently possess the friction loss and the wear of contact materials, which cause their low efficiency and short life and limit their applications to some niche areas. Reducing the friction loss to enhance the efficiency and prolong motor life is, therefore, a significant issue for broadening the application areas of USMs. To resolve these problems in conventional USMs, in this thesis, we propose to efficiently modulate the friction force in USMs using functional fluids, i.e. lubricants and giant electrorheological (GER) fluids. The former can dynamically change the friction coefficient as it relates to fluid viscosity, sliding speed, and load, while the rheological characteristics of the latter can be varied by applying electric field. In this thesis, we firstly introduce the concept of friction modulation using lubricant and proposed lubrication mechanisms in USMs. Numerical simulation is conducted by an equivalent circuit model to hybrid transducer-type ultrasonic motors (HTUSMs), which shows that the motor characteristics, i.e. motor efficiency, no-load speed, and maximum torque, are more desirable at high static preloads in lubricated condition than in dry condition. Then we experimentally clarify the improvement of the motor transduction efficiency using lubricant in standing-wave type USMs. With lubrication, the motor performance is desirable at high static preloads, which indicates that high pressure is required to keep sufficient friction force if lubricant is applied. The maximum efficiency is enhanced from 28% in dry condition to 68% in lubricated condition. In addition, significant enhancement in motor output torque is observed. The maximum torque as high as 1.01 Nm is obtained in a 25-mm-diameter HTUSM, which is 2.6 times higher than that in dry condition. In order to understanding the lubrication mechanisms in USMs, the transient variation in lubricant film thickness is measured at an oscillating frequency >50 kHz by newly proposed stroboscopic optical interferometry, which is the first measurement of film thickness at such a high frequency. This thesis further describes the tribological performance of engineering ceramics as friction materials in lubricated USMs. Mechanical fracture is found to be the main wear mechanisms of the tested ceramics in lubricated USMs. ZrO2 showed the mildest wear of all the tested ceramics, indicating that the ceramics possessing high fracture toughness are desirable for lubricated USMs. The lubricating effect in traveling wave ultrasonic motors is also experimentally studied. Unlike the situation in standing wave ultrasonic motors, lubricant significantly lowered the mechanical characteristics of motor, including motor efficiency, no-load speed, and maximum torque. This phenomenon is attributed to that the quality factor of the stator is largely reduced due to the presence of lubricant, resulting in high vibration loss and poor motor performance. A bending vibrator with higher vibration energy might be the solution for lubricated traveling-wave type USMs.The last part of this thesis discusses a novel way to modulate the friction in USMs using GER fluids. A non-contact rotary motor using a piezoelectric torsional vibrator and the GER fluid is developed. Ideal motor performance is obtained under 2 kV/mm electric field strength with 30% duty cycle, and 1.04 mN m torque at the rotational speed of up to 6.98 rad/s is achieved, offering force at least two orders of magnitude larger than that of conventional non-contact USMs.