ET: A Wrist Mechanism without Singular Positions

Abstract

A robot that responds to sensed changes in its environment needs well-conditioned kinematics to maintain adequate dynamic performance and to avoid practical difficulties with flexible cables and actuator limits. We show that it is possi ble to design a wrist mechanism with a large angular work space without any singularities. Although the singularities exist, they are allocated to a single wrist position outside the mechanical limits. The kinematics of the mechanism show that, although the degeneracy cone is larger than for the other wrist mechanisms, it is also largely outside the me chanical limits. Some extensions of the established concept of singularity cones are required. We introduce the concept of "least favorable reorientation " to define the usable workspace of the wrist. Dynamic performance specifications for the wrist are framed in terms of desired single actuator responses under given worst-case situations. Lightweight linear hydrau lic actuators have been used; however, the design can be rearranged to use precision electric drives. The wrist will be used in a new research robot for shearing sheep.