Effects of geometric imperfections to the control of redundantly actuated parallel manipulators

The model-based control of robotic manipulators relies on an exact model of the manipulator. Redundantly actuated PKM posses the ability to exhibit internal prestress that does not affect its environment. This allows for a purposeful distribution of control forces, taking into account secondary tasks, such optimal force distribution, active stiffness, and backlash avoiding control. In the presence of kinematic uncertainties this feature can become a serious problem since then the control forces may be annihilated or even some of the intentional prestress components may interfere with the environment. The effect of such kinematic uncertainties and the application of standard model-based control schemes is analyzed in this paper. It is shown that, in the presence of model uncertainties, it leads to parasitic perturbation forces that can not be compensated by the controls. An amended version of the augmented PD and computed torque control scheme is proposed that removes the parasitic feedback forces.