Three-DOF angular positioning control using a multi-DOF ultrasonic motor in the pre-loaded condition: application to the auditory tele-existence robot "TeleHead"

A multi degree of freedom (multi-DOF) ultrasonic motor is anticipated as an actuator in the next generation, because it can drive three-DOF motion with high torque and low noise that can not be perceived by the human auditory system. Using a multi-DOF ultrasonic motor as an actuator that constitutes the mechanism of the neck of the auditory tele-existence robot "TeleHead" is a good example of its applicability. However getting enough torque requires a suitable pre-loading method, and properly controlling three-DOF motion requires a suitable measurement method of the three-DOF angular position of the rotor and a control algorithm that takes into account the high-DOF motion and complex physical characteristics of the motor. This paper proposes both a hardware system for pre-loading and measuring 3-DOF angular position and a software system for controlling 3-DOF motion of the motor from the view point of application to TeleHead (see video). The proposed pre-loading method can generate enough pre-loading force and reduce resistance torque when the dummy head of "TeleHead" is in the inclined state. The proposed measuring method for three-DOF motion achieves a wide range of angular position of the motion of the dummy head in each rotating direction, especially in the yaw direction. The proposed control algorithm adopts a serial switching method with a very high frequency, and does not need any dynamics model of the physical characteristics of the motor for control signal calculation. Experimental results show the high performance of the proposed system when a 1-kg load is attached to the rotor of the multi-DOF ultrasonic motor.