Control of a 2-DOF manipulator with a flexible forearm

In this paper we present experimental results on the position and vibration control of the end-effector of a 2-DOF parallelogram manipulator with a flexible forearm. A dynamic model of the manipulator is first obtained. Control strategies are implemented to control the manipulator. The first control strategy uses the computed torque method based on a reduced-order dynamic model of the manipulator which is obtained by assuming that all the links are rigid. This method is referred to as the reduced-order computed torque (ROCT) method. Experimental results demonstrate that such a strategy is not good for vibration control of the end-effector of the manipulator. The second control strategy is a state feedback control law designed based on a local linearization of the nonlinear dynamic model. Experimental results show that this control strategy achieves good vibration control of the end-effector of the manipulator. However, since the strategy is based on local linearization, it is valid only in a neighbourhood of the operating point. A hybrid controller that uses the ROCT method for the initial large movement of the manipulator is then implemented. Based on a switching rule, the controller is switched to the state feedback controller based on the linearized model when the manipulator is sufficiently close to the equilibrium state. Experimental results are reported and the successful performance of the controller in dampening out end-point vibrations is demonstrated.