A slip model for the spherical actuation of the atlas motion platform

The Atlas platform represents a novel six degree-of-freedom motion platform architecture. Orienting is decoupled from positioning, and unlimited rotations are possible about every axis. The decoupling is accomplished by fixing a three degree-of-freedom spherical orienting device, called the Atlas sphere, on a gantry with three orthogonal linear axes. The key to the design is three omni-directional wheels in an equilateral arrangement, which impart angular displacement to a sphere, providing rotational actuation. The free-spinning castor rollers provide virtually friction-free motion parallel to each omni-wheel rotation axis creating the potential for unconstrained angular motion. Since the sphere directly contacts the omni-wheels, there are no joints or links interfering with its motion, allowing full 360° motion about all axes. However, the kinematic constraints are non-holonomic. This paper explores the slip at the interface between each omni-wheel and the Atlas sphere. A kinematic slip model is presented, introducing the slip ratio, which is the ratio of the kth omni