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-wheel's transverse velocity component, S⊥k, which is perpendicular to the free-spinning castor wheel axis, and the tangential velocity component, Stank, which is perpendicular to the omni-wheel driving axis, parallel to the tangential velocity vector, Vk. The long-term goal is to incorporate the slip model into a control law for position level control of the sphere. Two illustrative examples are given.

Transactions of the Canadian Society for Mechanical Engineering
Department of Mechanical and Aerospace Engineering

Holland, J.B. (J. B.), Hayes, M.J.D, & Langlois, R.G. (2005). A slip model for the spherical actuation of the atlas motion platform. In Transactions of the Canadian Society for Mechanical Engineering (Vol. 29, pp. 711–720).