Based on its simple structure, base-fixed actuators, high payload capacity, high accuracy, and high mechanical rigidity, the 3-RlowbarRR mechanism is a valuable planar parallel manipulator. However, the 3-RlowbarRR mechanism is known to have singular loci within its workspace affecting its use. In this paper, singularity avoidance of the 3-RlowbarRR mechanism using kinematic redundancy is presented. First, singularity analysis of the proposed 3-RlowbarPlowbarRR mechanism is described and a simple and effective redundancy resolution algorithm based on local optimization suitable for real-time control is developed. Here, the cost function in the optimization is designed to avoid the most problematic singularity configurations, where the end-effector can be locally moved even though all actuated joints are locked. Results from simulation show that the resultant 3-RlowbarPlowbarRR mechanism can be used to avoid singularities associated with the 3-RlowbarRR mechanism, and enlarges the usable workspace.

A reference force-tracking impedance control system is proposed, consisting of a conventional impedance controller in the inner-loop and a trajectory modifying controller in the outer-loop for force-tracking. The design of the outer-loop is presented and the stability of the two-loop control system is analyzed. A computationally efficient control algorithm for the inner-loop is suggested. Simulation results are presented. The controller is able to achieve excellent position and force-tracking with unknown environment stiffness and the presence of a burr on the tracking surface.>

An approach is presented for the design of simple robust independent joint controllers which result in linear decoupled joint motions. In this approach each joint is treated as a simple inertial systems plus a disturbance torque representing all the unmodeled dynamics. By means of a very simple procedure, the disturbance is instantly estimated and rejected, thus allowing simple PD control to be used. The controllers are inherently robust. Experimental evaluations of the controller on a PUMA 560 are are performed, and the experimental results are presented.>