In industrial robots, each joint is driven through a reduction gear unit such as a harmonic drive gear. The elasticity of the drive systems cannot be ignored when the high-speed and precise motion control is required for robots. Due to the elasticity of the drive systems, vibratory behaviors are caused, and the control performances of robot systems deteriorate. In this study, we develop a new actuator that consists of an AC servomotor, a harmonic drive gear and an electrorheological (ER) damper. The ER damper can change the damping coefficient by applying an electric field. Since the gap between electrodes of the ER damper is 80 /spl mu/m only the voltage of 160 V is needed for the electric field of 2 kV/mm. The ER damper consumes little electric power. The actuator is installed on a one-link robot arm. The mathematical model of the robot arm is derived, and the eigenvalues of the robot system are analyzed. Experiments were carried out, and the effectiveness of the actuator is confirmed.

The authors propose a joint torque sensing technique making use of the existing structural elasticity of robots. The technique provides joint torque sensing without reducing the stiffness of the robot or changing the mechanical structure of the joints. The elasticity of the flexsplines of the harmonic drives is utilized to measure the joint torque. The flexsplines are flexible thin cups, made from steel, in the harmonic drives that are driven by the wave generators. A finite-element analysis of the flexsplines shows that a special configuration of strain gauges, mounted on the flexspline, has to be employed to eliminate errors in sensor information due to rotation of the wave generator. Characteristics of the torque sensor are examined experimentally. The linearity and the dynamic response are almost the same as those of a conventional sensing technique. For a one-link robot arm, both theoretical and experimental investigations support the validity of the sensing technique.>

When positive feedback of joint torques is applied to the motion control of robot arms, the nonlinear dynamic couplings between links are canceled and a servo system with high stiffness is obtained. The problems which arise due to joint flexibility in robot motion control based on joint torque positive feedback are discussed. The effects of joint flexibility are investigated by numerical and experimental studies on a one-link arm with a harmonic drive. It is shown that the joint flexibility destabilizes the control system with joint torque positive feedback. A low-pass filter is used in the torque feedback loop to improve the oscillation characteristics of the control scheme.>

A joint torque sensing technique for robots with harmonic drives is proposed. Elasticity of harmonic drives is utilized to measure the joint torque. The technique provides joint torque sensing without reducing the stiffness of the robot and changing the mechanical structure of the joints. A finite-element analysis of flexsplines, which are flexible thin cups made from steel in harmonic drives, is performed. The analysis shows that a special configuration of strain gauges on the flexspline has to be employed to eliminate errors in sensor information due to rotations of wave generators. Characteristics of the torque sensor are examined experimentally. The linearity and the dynamic response are almost the same as those of a conventional sensing technique. Both theoretical and experimental investigations support the validity of the proposed sensing technique.>