Grasping Force Control of Robot Hand with Fingers using Ultrasonic Motors

利用超声电机（USM）作为机械手，可以实现无线圈结构简单、响应速度快、低速转矩大、静止自锁等优点，从而实现高质量的机械手。然而，关于利用超声波电机的自锁来控制转矩的研究很少。本研究提出了一种用PWM（脉宽调制）控制机械手用三指旋转圆柱体时的抓握力。在ON-OFF型PWM中，即使占空比很小，也能获得较大的自锁摩擦。但是，PWM占空比与静转矩的关系呈现非线性，存在较大的磁滞。结果表明，在行波型超声电机中引入驻波可以减小自锁摩擦。在这种情况下，当驻波在PWM的关断期诱导到超声电机上时，PWM的占空比与超声电机的静转矩几乎成线性关系。结果表明，超声电机的驱动特性取决于PWM的开关频率。在开关频率fs=500Hz的情况下，观察到驻波降低了USM的驱动力。在这种情况下，USM在抑制驻波强度后恢复线性转矩特性。此外，本研究还提出了一种稳定旋转圆柱体机械手驱动特性的方法。三根手指作为超声波电机的振动型。超声电机的驱动频率通过其振动幅值的反馈跟随共振频率。

Utilizing ultrasonic motor (USM) for the robot hands, the high quality robot hands will be realized since the ultrasonic motor has a simple structure without coils, a high response, large torque at low speed, and a self-locking at rest. Little research, however, has been reported regarding the torque control by use of the self-locking of the ultrasonic motor. This study proposes a grasping force control by PWM (pulse width modulation) when the robot hand rotates a cylindrical body by three fingers.In the case of ON-OFF type PWM, the large self-locking friction can be obtained even at the small duty ratio. However, the relation between the duty ratio of PWM and the static torque indicates nonlinear and a large hysteresis occurs. It was confirmed that the introduction of a standing wave to a traveling wave-type ultrasonic motor diminishes the self-locking friction. In this case the relation between the duty ratio of PWM and the static torque of ultrasonic motor becomes almost linear when the standing wave is induced to the USM in the OFF period of PWM.It is confirmed that the driving characteristic of the ultrasonic motor depends on the switching frequency of the PWM. At the case of the switching frequency of fs=500Hz it was observed that the standing wave reduced the driving force of the USM. In this case, the USM regains the linear torque characteristic after suppressing the strength of the standing wave. In addition, this study proposes the method to stabilize the driving characteristic of a robot hand that rotates a cylindrical body. Three fingers work as the vibration-type of ultrasonic motors. The driving frequency of the ultrasonic motors follows the resonance frequency by the feedback of its vibration amplitude.