Robust position/force control of robot manipulators during contact tasks

In this paper, a new approach employing smooth robust controllers is proposed for simultaneous position and force control of uncertain robots in contact with uncertain environments. By employing Lyapunov´s direct method, the authors have shown that the closed-loop system is globally and asymptotically stable, furthermore, the closed-loop system is robust to the uncertain system parameters, as well as the frictional forces at both the internal joints and on the contact surface. The employed controller is insensitive to the variation of the stiffness of the environment. Thus, it is more practical in the sense that there is no need to switch between controllers for robots to perform contact tasks in dynamically changing environments. Unlike other commonly used robust controllers, the proposed one can simultaneously guarantee both asymptotic stability of the closed-loop system and smooth control input. A numerical example of a two-link robot with its end-effector moving along a stiffness changing surface is presented for illustration.