| Many a robotic pitch-roll wrist uses a bevel-gear differential train to drive the gripper. The innovative design of pitch-roll wrists using spherical cam-roller pairs is currently underway at McGill University's Centre for Intelligent Machines, with the aim of overcoming the drawbacks of bevel-gear trains. This innovative design relies on Speed-o-Cam, a new concept of speed-reduction mechanisms based on cams and pure-rolling contact, intended to replace gears and harmonic drives in applications where backlash, friction, and flexibility cannot be tolerated. The new mechanism consists mainly of a spherical conjugate cam subassembly and two roller-carrying disks.;The optimum design of the pitch-roll mechanism based on cam-roller pairs is reported here. The optimization is intended to simplify the subassembly of spherical conjugate cams of the old design by means of a layout of two pairs of spherical mechanisms of the Stephenson type and two conjugate cams mounted on distinct shafts. We focus on the optimum design of both the spherical cam-roller mechanism and the spherical Stephenson mechanism.;We start with a study of cam curvature, with special focus on its machinability. Drawing from experience, we introduce the hypothesis that high curvature changes of a cam profile are at the source of the concentration of machining errors. As a consequence, the machining accuracy of the concave regions in a cam profile is substantially lower than that of its convex regions. To produce a more accurate cam we developed the geometric condition that guarantees a fully convex spherical cam profile. |
