This paper introduces a class of reconfigurable parallel robots consisting of a fixed base and a moving platform connected by serial chains having RRPS (Revolute-Revolute-Prismatic-Spherical) topology. Only the prismatic joint is actuated and the first revolute joint in the chain can be locked or released online. The introduction of these lockable joints allow the prismatic actuators to maneuver to approximate 6-DoF motions for the moving platform. An algorithm for generating these maneuvers is first described. Then, a motion planner, based on the generation of a Probabilistic RoadMap (PRM) whose nodes are connected using the described maneuvers, is presented. The generated trajectories avoid singularities and possible collisions between legs.

The static analysis of a parallel wrist, known as the 3-universal-prismatic-universal (UPU) wrist, is presented, and the 3-UPU wrist singularity conditions are interpreted from a statics point of view. An equation (singularity equation) that enables finding all the 3-UPU wrist singular configurations is written in explicit form. Moreover, the use of the Rodrigues parameters to parameterize the 3-UPU wrist platform orientation allows the singularity equation to become a fourth-degree polynomial equation in three unknowns. Finally, a numerical example is presented.

Among the new architectures for parallel wrists, recently presented in the literature, one of the simplest is the architecture of the 3-revolute-spherical-revolute (RSR) wrist. The kinematics of the 3-RSR wrist has not been presented yet, in the literature. This paper presents the kinematic analysis of the 3-RSR wrist. In particular, the demonstration that the 3-RSR wrist makes the end-effector perform finite spherical motion is reconsidered from a different point of view. The new demonstration directly provides the constraint singularities of the 3-RSR wrist. Moreover, the position analysis of the 3-RSR wrist is reduced to the solution of problems already solved in the literature. Finally, the mobility analysis of the 3-RSR wrist is addressed, and all its singularity conditions are found in analytical form and geometrically interpreted.