Path planning for assembly mode changes for the 3-RRR using global workspace roadmaps

Path planning for assembly-mode changes in the joint space for parallel manipulators is a challenging task due to parallel singularities. The process is made even more difficult by multiple direct kinematic solutions that these manipulators generally exhibit, especially for manipulators with a high number of degrees of freedom. This paper proposes a path planning scheme to tackle this problem; a hierarchical path planning method that uses a global workspace roadmap. It is a very powerful tool which allows us to efficiently determine path feasibility in joint space and to find the existence of possible assembly-mode changes. However in our previous works, it has only been shown to be viable on 2-DOF manipulators. With enhancements to the algorithm, the proposed path planning process is applied to the 3-RRR, a 3-degree of freedom parallel manipulator which exhibits up to 6 direct kinematics solutions in its joint space. Through numerical experiments, we show that the global workspace roadmap allows us to check path validity of a proposed assembly-mode change in 3-degrees of freedom that is viable in both time and memory constraints.