Precision Control of Robots with Harmonic Drives

Aimed at achieving ultra-high precision control performance for high-end applications of robots equipped with harmonic drives, an adaptive joint torque controller developed previously is extended to all the joints of seven degrees of freedom (DOF) robot manipulator using four different types of harmonic drives. The developed adaptive joint torque controller uses additional sensing including the joint and motor positions and the joint torque, and adaptively compensates the large friction associated with harmonic drives, while incorporating the dynamics of flexspline. With guaranteed L₂/L_infin stability and asymptotic stability, the adaptive joint torque controller allows any motion controller to be employed. Experimental results using the developed adaptive joint torque controller incorporating a motion controller based on the virtual decomposition control demonstrate precision trajectory tracking control of a 7-DOF robot at both moderate and ultra-low speeds. The precision control with encoder-resolution accuracy at an ultra-low joint speed of 0.001 (rad/s) characterizes the effectiveness of the friction compensation.