Robust compliant motion control of robot manipulators with nonlinear friction using time delay estimation

Abstract

This thesis deals with robust compliant motion control of robot manipulators with nonlinear friction. Robust compliant motion is that controlled robots not only follow desired trajectories exactly in free space, but also make compliant contact with the environment in constrained space. Friction compensation is important in robot control because the friction torque accounts for more than 60\% of the motor torque, and hard nonlinearities due to Coulomb friction and stiction severely degrade control performances as they account for nearly 30\% of the industrial robot motor torque. A simple, robust compliant motion control technique is presented for robot manipulators with nonlinear friction. The nonlinear terms in robot dynamics are classified into two categories from the time-delay estimation (TDE) viewpoint: soft nonlinearities (gravity, viscous friction, and Coriolis and centrifugal torques, disturbances, and interaction torques) and hard nonlinearities (due to Coulomb friction, stiction, and inertia force uncertainties). TDE is used to cancel soft nonlinearities, and ideal velocity feedback is used to suppress the effect of hard nonlinearities. The proposed control structure is transparent to designers; it consists of three elements that have clear meaning: a soft nonlinearity canceling element, a hard nonlinearity suppressing element, and a target dynamics injecting element. The proposed control turns out simple in form and easy to tune; specifying desired dynamics for robots is all that is necessary for a user to do. The same control structure can be used for both free space motion and constrained motion of robot manipulators. The robustness of proposed method has been confirmed through experimental comparisons with other controllers such as natural admittance control and internal force-based impedance control using a two-degree-of-freedom (2-DOF) SCARA-type industrial robot. The proposed control enables us to analyze and compensate nonlinear...