Basic position/force control of a single-axis arm designed with an ultrasonic motor

Recently, many studies on assist robots have been conducted in which the development of a unique system is required to support elderly people, physically handicapped people, and/or care givers. One typical system is called the assist suit, which has been used on a trial basis. The assist suit is a mechatronics device which can assist physical human actions. However, the current assist suit has a few problems with respect to cost, size, weight, endurance, and so on. This article describes a fundamental study concerning a compact assist device. The assist device supports one spot on the body, such as a knee, an elbow, or a shoulder. First, a simple single-axis arm was designed using an ultrasonic motor which can generate a high torque from a low velocity range. Then a servo-system, a torque control system, and a passive torque control system were designed and their characteristics were evaluated. In addition, the stability of the servo-system was analyzed. Here, the passive torque control includes a stiffness control and a compliance control.