Design and performance analysis of an (nS)-2SPU underactuated wrist

The paper presents the concept of an underactuated spherical wrist with parallel kinematics architecture and the use of non-holonomic constraints: two linear actuators are sufficient to drive the 3 d.o.f.'s of the spherical motion since the instantaneous kinematics is limited to span a space of dimension 2 by the presence of a (non-holonomic) roller-sphere contact. The use of 4 position sensors allows to obtain just one solution for the direct orientation problem, with very simple mathematical passages. After having developed the direct and inverse orientation and velocity kinematics, the potential performances of the concept are investigated with reference to a virtual prototype and are compared with the corresponding performances of the original S-3SPU concept it is derived from.