Torque control based direct teaching for industrial robot considering temperature-load effects on joint friction

Abstract

Purpose The purpose of this paper is to design a practical direct teaching method for the industrial robot with large friction resistance and gravity torque but without expensive force/torque sensor, where the gravity torque is just a function of joints position, whereas the friction is closely associated with joint velocity, temperature and load. Design/methodology/approach In the teaching method, the output torque of joint motor is controlled through current to compensate gravity torque completely and friction resistance incompletely. Three variables closely associated with friction are investigated separately by experiment and theoretical analysis, and then a comprehensive friction model which is used to calculate the required compensated friction torque is proposed. Finally, a SIASUN 7 degrees of freedom robot was used to verify the model and the method. Findings Experimental results demonstrated that the teaching method enables an operator to teach the robot in joint space by applying small force and torque on either end-effector or its body. The friction investigation suggests that the velocity and temperature have a strong nonlinear influence on viscous friction, whereas load torque significantly influences the Coulomb friction linearly and causes a slight Stribeck effect. Originality/value The main contribution includes the following: a practical joint space direct teaching method for a common industrial robot is developed, and a friction model capturing velocity, temperature and load for robot joints equipped with commercialized motors and harmonic drives is proposed.