Abstract

A method for designing ball wheel mechanisms for vehicles with omnidirectional motion capability is presented in this paper. A ball wheel mechanism holds a spherical tire while allowing it to rotate with two degrees of freedom such that it may roll in an arbitrary direction on the floor. This paper concerns a particular class of ball wheel mechanisms which hold a spherical tire in a special arrangement of rollers. Due to point contacts between these rollers and the tire, and between the tire and the ground, specific considerations are required to achieve large payload capacity and traction as well as high position control accuracy. First, we review the kinematic conditions for theoretical functionality of the class of ball wheels and describe a proof-of-concept prototype vehicle. Starting from the generalized ball wheel mechanism, we then identify a subset of designs that contains the optimal ball wheel mechanisms for most applications. Within this subset, the design is optimized by parameterizing the design space. By analyzing the phenomena relating the design parameters to three performance indices — payload capacity, acceleration and repeatability — a surface can found in the performance space that represents the maximum ball wheel performance for all permutations of the design parameters. This surface will be used by vehicle designers to evaluate ball wheel performance in comparison with other omnidirectional drive mechanisms. A simple lookup table can then be used to find the design parameter values which provide desired performance.

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