Experimental assessment and feedforward control of backlash and stiction in industrial serial robots for low-speed operations

Abstract

Serial robots have been successful in numerous applications; however, their limited motion accuracy hinders the potential for high precision robotic machining. Among the many sources of error in robot movement, non-linearities at robot joints, namely backlash and stiction/friction, are yet to be thoroughly investigated and addressed. When a robot joint reverses, backlash-induced excitation switches direction and stiction induces localized errors that occur over a short duration. Together, backlash and stiction from joint reversal lead to significant positional errors. This paper closes the research gap by proposing feedforward backlash and stiction compensation with a focus on suppressing errors during joint reversals. The controllers are designed using experimental studies of robot joints and the nominal tool centre point (TCP) trajectories. Based on the robot kinematics, the strategy interpolates the nominal trajectories to ensure accurate identification of joint reversals along the path, and therefore provides optimal error compensation with minimum latency. The performance of the controller is evaluated in single axis tests for suppressing backlash in static conditions and in multi-axis path tests for suppressing backlash and stiction in continuous motions. The experimental results show that the feedforward compensation greatly improves the robot accuracy during joint reversal, reducing errors by more than 70%.