Singularity-free workspace is a very important criterion for the design of manipulators, especially for parallel manipulators which are well known for their limited workspace and complex singularities. This paper studies geometric parameters and dexterity measures that affect the size of a singularity-free joint space and proposes methods for the development of 6-DOF Stewart–Gough parallel manipulators that have better singularity-free joint space. With a local dexterity measure as the objective function, a systematic method is employed to search for the design with a maximal singularity-free joint space. The related workspaces are also investigated. It is shown that the workspace is not proportional to the size of the joint space and that manipulators with a larger singularity-free workspace usually have relatively poor dexterity.

Isotropic manipulators are generally considered as designs with optimum dexterity. Currently, many 6-DOF (degrees-of-freedom) isotropic parallel manipulators can be developed by numerical or analytical methods. At an isotropic configuration, a manipulator is equidistant from its neighboring singular points. The distance, however, can be very small, so an isotropic design might have relatively smaller singularity-free workspace. This paper presents methods to develop traditional 6-DOF parallel manipulators with better dexterity and larger singularity-free workspace. Some fully symmetric nontraditional designs are then proposed. The evaluation of kinematic properties shows that the fully symmetric designs have very good global dexterity, better rotatability, and relatively larger singularity-free workspace. The manipulators are suitable for some special tasks requiring higher precision, better rotatability, or larger workspace.