We have proposed practical torque sensing which utilizes a flexible part of a harmonic drive gear. The sensing technique provides joint torque sensing without reducing stiffness of the robot and changing the mechanical structure of the joints. The characteristics of the torque sensing have been studied under an immovable condition. The dependence of characteristics on the rotational velocity have not been discussed. We describe the characteristics under rotational conditions of the harmonic drive. The experimental results show that the accuracy of the torque sensing under high velocity rotation is 2% of the gear torque capacity.

Torque sensing built into harmonic drives has advantages in its size and stiffness comparing to the torque sensors added to the robot joints. A relatively high fluctuation signal of a built-in torque sensor is a drawback in its practical applications. However, we present a simple and effective method to compensate the fluctuation signal and thus improve an overall accuracy of the harmonic drive built-in torque sensing. The method does not require precise positioning of the strain gauges, does not require online-estimations of the fluctuation signal, and does not increase the number of applied strain gauges. It requires initial tuning of the gains. For the signals of the separate strain gauges, two techniques to define the gains are presented: one is based on a mathematical model of the fluctuation signal, and the other employs a heuristic approach. Practical results show the effectiveness and usefulness of the proposed method.