The authors have proposed a torque sensor which utilizes elasticity of harmonic drives. The characteristics of joint torque control were experimentally examined using this torque sensor. Three types of torque control laws were implemented with a one-link arm to find a control method which provides excellent friction reduction and dynamic response in joint torque. The experimental results of the joint torque control showed that the torque sensor was very useful for compensating for the nonlinear friction. The torque control was effective in improving the accuracy of motion control at low velocity and suppressed the vibration caused by the joint flexibility.>

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.>