Modeling and Compensation of Backlash and Harmonic Drive-Induced Errors in Robotic Manipulators

Harmonic drives, used widely in robot transmission systems, can induce significant periodic, joint-dependent position errors. Further, backlash in transmission systems, caused by wear or improper assembly, can considerably limit the overall repeatability, and therefore accuracy, of robotic manipulators. To measure the kinematic errors induced by both the harmonic drive and backlash, a laser tracker system, accurate to 10 μm at 20 m, is used to measure the end-effector position of a FANUC 200i LR Mate as its first joint is actuated randomly through ±130° (i.e., the range visible by the laser tracker). A joint-dependent model is then derived to account for the error seen in the measurements. Using a maximum likelihood estimator, the joint-dependent model coefficients and the amount of backlash are simultaneously identified. After backlash compensation is implemented, the maximum residual calculated between the nominal predicted position and the measured position of the end-effector, 0.2969 mm, is reduced by approximately 68%, to 0.0947 mm and the mean is reduced by 58% from 0.0631 to 0.0264 mm, after the error is modeled and compensation is implemented.