Method for Estimating Combined Controller, Joint and Link Stiffnesses of an Industrial Robot

In this paper a new combined local/global approach for estimating the combined stiffnesses of joints in anthropomorphic robots is presented. The stiffness of each joint is a combination of several effects: i) stiffness of the links, ii) stiffness of joint bearings and gears and iii) stiffness of the position control loops given by the individual axis controller gains in the controller software. Experimental results are presented for an ABB IRB6600 industrial robot using measurements from a FARO Xi laser tracker and an ATI Omega160 force/torque sensor. The results show that there is a significant variation in stiffness among the individual joints of the robot and that the stiffnesses of the main axes (1-3) are significantly higher than the stiffnesses of the wrist axes (4-6). The measured forces and estimated stiffness are used in SimulationX modeling software to validate the proposed method, where the model includes both the kinematics and joint stiffness. The results presented in this paper are valid in the home position of the robot, but the method can be modified and used for any robot position. The method requires much less experimental data compared to a global approach.