An identification procedure for rate-dependency of friction in robotic joints with limited motion ranges

This paper proposes a procedure for identifying rate-dependent friction of robotic manipulators of which the motion is limited due to the configuration or the environment. The procedure is characterized by the following three features: (i) the rate dependency is represented by line sections connecting sampled velocity-force pairs, (ii) the robot is position-controlled to track desired trajectories that are some cycles of sinusoidal motion with different frequencies, and (iii) each velocity-force pair is sampled from one cycle of the motion with subtracting the effects of the gravity and the inertia. The procedure was validated with a six-axis industrial robotic manipulator YASKAWA MOTOMAN-HP3J, of which the joints are equipped with harmonic-drive transmissions of the reduction ratios of 81.5-224. The experimental results show that the identification is achieved with a sufficient accuracy with the 20 degrees of motion of each joint. In addition, the results were utilized for friction compensation, successfully reducing the effect of the friction by 60-80%.