Approaches of Stiffness Control of Parallel Manipulators with Actuation Redundancy (original) (raw)

To execute high-value added operations, such as fine motion control and force control in assembly, deburring and many other manufacture tasks, one needs to design control strategies which would provide accurate and stable motion. One of the natural solutions of those tasks is to incorporate into the control strategy inputs whose number is larger than the needed minimum. If a manipulator possesses degrees of mobility that outnumber the dimensions of the task space, the redundancy thus obtained is called "kinematic redundancy". On the other hand, if the number of the actuators is larger than that of the degrees of mobility, the resulting redundancy is called "actuation redundancy" . While the kinematic redundancy is characteristic for serial manipulators, parallel manipulators are designed involving actuation redundancy. One can use kinematic redundancy control to realize gross motion, dexterity, obstacle avoidance etc. However, actuation redundancy control can be employed to enhance the load handling capacity of the manipulator, to minimize actuator torque and to obey the actuator constraints -[2]. Furthermore, actuation redundancy can also be used to reduce the force singularities of a parallel manipulator. Yet, the antagonistic actuation of redundant manipulators allows for the increase of their stiffness. Note that the internal forces balance each other in a closed structure and do not execute effective work, but generate stiffness. One can use this property to design control strategies, most generally known as stiffness control by redundant actuation . The strategies are part of impedance control schemes where one can directly follow the ratio "force/displacement", and not each quantity separately. These considerations imply the necessity to describe the desired dynamic relationship between the end-effector force and position. The present study aims at the review of known schemes of stiffness control by mean of redundant actuation. The aim of the study is to design and experiment approaches of specifying a desired stiffness whit is appropriate for the manipulator control. The present work proposes a stiffness control scheme of a serialparallel manipulator with actuation redundancy.