Grasping Force Control of Robot Hand with Fingers using Ultrasonic Motors

超声波电机具有结构简单、无线圈、高响应、低速大扭矩、静止自锁等特点，利用超声波电机可以实现高质量的机械手。然而，关于利用超声波电机的自锁进行扭矩控制的研究报道很少。提出了一种机械手三指旋转圆柱体时的脉宽调制抓持力控制方法，在开关式脉宽调制的情况下，即使在较小的占空比下也能获得较大的自锁摩擦力。但是，脉宽调制的占空比与静态转矩之间的关系呈现非线性，存在较大的滞环现象。在行波型超声波电机中引入驻波可以减小自锁摩擦。在这种情况下，在脉宽调制关断期间，当驻波被感应到超声波电机时，脉宽调制的占空比与超声波电机的静态扭矩几乎成线性关系，证实了超声波电机的驱动特性取决于脉宽调制的开关频率。在开关频率为飞秒=500赫兹的情况下，观察到驻波降低了超声电机的驱动力。在这种情况下，超声电机在抑制驻波强度后恢复了线性扭矩特性。此外，本研究还提出了稳定旋转圆柱体机器人手驱动特性的方法。三个手指作为振动型超声波电机工作。超声波电机的驱动频率通过其振动幅度的反馈跟随谐振频率。

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.