Passive Friction Compensation Using a Nonlinear Disturbance Observer for Flexible Joint Robots with Joint Torque Measurements

The friction and ripple effects from motor and drive cause a major problem for the robot position accuracy, especially for robots with high gear ratio and for high-speed applications. In this paper we introduce a simple, effective, and practical method to compensate for joint friction of flexible joint robots with joint torque sensing, which is based on a nonlinear disturbance observer. This friction observer can increase the performance of the controlled robot system both in terms of the position accuracy and the dynamic behavior. The friction observer needs no friction model and its output corresponds to the low-pass filtered friction torque. Due to the link torque feedback the friction observer can compensate for both friction moment and external moment effects acting on the link. So it can be used not only for position control but also for interaction control, e.g., torque control or impedance control which have low control bandwidth and therefore are sensitive to ripple effects from motor and drive. In addition, its parameter design and parameter optimization are independent of the controller design so that it can be used for friction compensation in conjunction with different controllers designed for flexible joint robots. Furthermore, a passivity analysis is done for this observer-based friction compensation in consideration of Coulomb, viscose and Stribeck friction effects, which is independent of the regulation controller. In combining this friction observer with the state feedback controller \cite{Albu-Schaeffer2}, global asymptotic stability of the controlled system can be shown by using Lyapunov based convergence analysis. Experimental results with robots of the German Aerospace Center (DLR) validate the practical efficiency of the approach.

Position Tracking of a Pneumatic Actuator Using Backstepping-Sliding Mode Control With Adaptive Friction Observer

In this paper, an enhanced sliding mode control (SMC)-based controller known as backstepping-sliding mode control (B-SMC) with an adaptive friction observer is proposed for position tracking control of pneumatic actuators. The adverse effect of friction encountered in the pneumatic actuators which play a major role in pneumatic servo system is highlighted and a solution to rectify this is proposed using an adaptive LuGre-based friction observer for friction compensation. The B-SMC is experimentally applied, for the first time, on a double-acting single-rod industrial pneumatic cylinder. Comprehensive derivation as well as stability proof of the controller is presented. The performance of B-SMC with and without the friction observer is experimentally investigated. From the results, it is clearly observed that the B-SMC with the adaptive friction observer performs better (i.e. reduces the tracking as well steady-state errors of up to 30%) than the one without friction observer. Thus, the adaptive friction observer of B-SMC is identified as the key element that improved the control performance by compensating the adverse effect of friction during the position tracking tasks.