The Design of a Chain of Spherical Stephenson Mechanisms for a Gearless Pitch-Roll Wrist

Although bevel-gear wrists are widely used in industrial manipulators due to their simple kinematics and low manufacturing cost, their gear trains function under rolling and sliding, the latter bringing about noise and vibration. Sliding is inherent to the straight teeth of the bevel gears of these trains. Moreover, un-avoidable backlash introduces unmodeled dynamics, which mars robot performance. To alleviate these drawbacks, a gearless pitch-roll wrist is currently under design for low backlash and high stiffness. The wrist consists of spherical cam-rollers and spherical Stephenson linkages; two roller-carrying disks drive a combination of cams and Stephenson mechanisms rotating as a differential mechanism. In this paper, the design of the chain of spherical Stephenson mechanisms (SSMs) is introduced. The problem of the dimensional synthesis is addressed and interference avoidance is discussed. An embodiment of the concept is also included.