Design of a Non-Holonomic Spherical Wrist

The article presents a prototypal design of a spherical parallel machine that can be used for the orientation of parts or devices: the peculiarity of this wrist is that, by exploiting non-holonomic constraints, it is able to bring the end-effector in any 3 d.o.f.'s orientation within its working space by only using 2 actuators. It is shown that, despite the higher complexity of the mechanical design, such architecture features good kinematic performances with respect to similar holonomic machines. Moreover, the use of 4 position sensors allows to obtain just one solution for the direct orientation problem, characterized by simple mathematical expressions. The kinematic performances of the wrist make it a suitable choice for the functional design of machines to be used for static tasks or point to point motions.