Development of the ball wheel mechanism for a robust omnidirectional mobile platform

This dissertation presents work on the design of a new ball wheel drive mechanism for a robust omnidirectional wheeled mobile platform. This platform is designed for use in the highway maintenance and construction area, which is generally an unstructured and congested environment. The proposed ball wheel mechanism can move in all directions on the plane, instantaneously and isotropically. Through the course of this project, three devices were developed: a bench-top proof of concept, a fully-mobile, operator-controlled demonstration platform, and a linear test-bed for controls design. Novel features of the final design include: active regulation of traction drive contact pressure and an optimally controlled reconfigurable drive system, based on redundant actuators.; The development of the ball wheel mechanism for more sophisticated applications requires velocity tracking of the spherical tire, which has three rotational degrees of freedom. The tracking scheme must also be robust and suitable non-ideal environments. To address this issue a three-dimensional, non-contacting angular velocity sensor has been developed based on magnetometry. The proposed scheme tracks the rotation of a magnet embedded at the center of the sphere. A direct inverse-kinematics solution, based on natural invariants of rigid-body rotation, is used to extract the instantaneous angular velocity vector of the sphere from the orientation data. Results from experimental verification demonstrate that the scheme can accurately track the speed and rotation-axis orientation of a spinning magnet. The scheme can also track transient motion such as step changes in velocity and precession of the rotation axis.