Computation of singularity free region and design optimisation of a four-degrees-of-freedom hybrid arm

This paper presents a method for the dimensional design of a spatial hybrid manipulator, namely, a four-degrees-of-freedom robotic arm. The design aims at finding the link parameters of the arm in such a manner that it possesses a singularity-free region that is bigger than a specified volume, in the shape of a cylinder. It is also required that the static performance of the manipulator, over this region, is better than a specified nominal level, quantified in terms of the actuator torques required to carry a fixed payload. The design problem is posed as a bi-objective optimisation problem, and solved using a GA-based optimiser code, namely, NSGA-II. The method is demonstrated via a numerical example. The Pareto-optimal solutions are obtained in a reasonable time, and the design achieves the required targets, thus demonstrating its efficacy.