Nonlinear contact control for space station dexterous arms

In October 1996, a research and development task was initiated at JPL to develop and demonstrate nonlinear contact control schemes for the dexterous robotic arms planned for the Space Station. This paper reports on the progress made to-date in this task. Specifically, the paper introduces a new class of contact controllers comprised of a nonlinear gain in cascade with a linear fixed-gain PI force controller and PD compliance controller. The nonlinear gains used are simple hyperbolic functions of force error and contact force, respectively. The stability of the closed-loop systems incorporating nonlinear PI and PD controllers are investigated using the Popov stability criterion. Experimental results are presented to demonstrate the efficacy of the nonlinear force and compliance control schemes for a dexterous 7-DOF arm. These results highlight the advantages of the proposed nonlinear contact controllers compared to conventional fixed-gain controllers