TRACKING AND FORCE CONTROL FOR A CLASS OF ROBOTIC MANIPULATORS

The increasing utilization of robotic manipulators in industrial tasks, such as assembly, forming or shaping of surfaces, as well as the handling of hazardous materials depends greatly on available hybrid force and position control schemes. Since the robot as well as its environment are often subject to parameter uncertainties which can not be neglected, it is necessary to design controllers which are robust with respect to these uncertainties. In addition, the dynamics of the robot are nonlinear requiring consideration of nonlinear control concepts. Another aspect to be taken into account is the relative stiffness of the robot, the force sensor and the manipulated surface. That is, the behavior of the system normal to the surface is relatively stiff while that tangential to the surface is relatively free. Separation of the controller for these two directions is therefore indicated. We propose a controller design which accounts for this point of view and demonstrate its efficacy with respect to robustness and accuracy of position and force tracking by means of numerical simulations. The design is based on the control concept of [CORLESS, LEITMANN 1989]. The example, considered is a Manutecr3 robot with three degrees of freedom. In addition, we account for the dynamics of the actuator which also possesses three degrees of freedom. The considered parameter uncertainties are friction moments in the links, friction between the end effector and the manipulated object, as well as nonlinear dynamics which are difficult to characterize.