On Force Control for Robot Manipulators in Contact with Uneven Planar Surfaces

This paper studies a robot manipulator in contact with an uneven planar surface. The robot contact-task is to exert a desired force on normal direction to the surface while a desired reference is tracked along the surface. The uneven characteristic of the surface is modeled with ramp disturbances in the position, on normal direction to the surface. The magnitude of the ramp is proportional to the speed of the trajectory along the surface. Also, uncertainty in the stiffness is analyzed. Considering delays and filters commonly found in practical applications, a force control system based on admittance is analyzed. Disturbance rejection in terms of steady-state error is considered, and a tuning method for disturbance attenuation is developed. Furthermore, the relative stability of the system is studied giving a metric of how much uncertainty in the stiffness the control system can handle. The analysis and methods are verified by simulations.