Hybrid position/force sliding mode control of a class of robotic manipulators

This paper deals with the hybrid position/force control of a class of robotic manipulators. To perform the control scheme design, it is necessary to characterize the dynamical model of the force sensor which is mounted at the end-effector of the robot. The objective is to perform reliable contact force measurements by estimating all the forces which are generated at the level of the tip which is directly connected to the sensor. A dynamical model of the sensor motion is formulated and identified, by considering also the kinematics of the robot. The proposed hybrid control scheme includes position and force controllers based on first and second order sliding modes. These kind of controllers guarantee suitable robustness properties to perform a satisfactory trajectory tracking, also allowing one to make the robot move in an environment with unknown obstacles by using the possibility of touching the obstacles as a way to pass them by. Experimental tests are performed on a COMAU SMART3-S2 anthropomorphic rigid robot manipulator with an ATI Gamma force sensor by comparing four different position/force control schemes.

I. Introduction

Recent advances in robotics include the capability of planning a suitable trajectory in order to drive the robot from an initial configuration to a pre-determined goal point, or to follow, when possible, a pre-specified trajectory even in unknown environments [1]. Various methods can be adopted to accomplish this task. These methods are mainly classified in relation to the capability of the sensors which are employed to map the environment and the obstacles near the robot. When distance sensors and cameras are considered, the trajectory can be planned without colliding with the obstacles (i.e. no force measurements are required), see [2]–[4].