Kinematic modeling and simulation of active-caster robotic drive with a ball transmission (ACROBAT-S)

In this paper, a new type of an active-caster with a single ball transmission (ACROBAT-S) is proposed. The proposed ball transmission is a novel mechanism which realizes combining of two motor powers together to rotate the single ball in a 2Dway and simultaneously distributing the combined power to a wheel shaft and a steering shaft of an active-caster in an appropriate ratio. The transmission design enables to remove a sensor for detecting the wheel orientation for coordinated control. Also, it would be possible to build an omnidirectional robot with three active casters to be controlled its 3D motion by three motors with no redundancy. The new concept of ACROBAT-S gives many advantages compared with the original ACROBAT since the number of balls is reduced from two to one, therefore the number of friction drive between a ball and rollers or a ball to a ball is also reduced from five to three. These features contribute not only to simplify the mechanism design, but also to enhance its performance since slippages and energy losses would be reduced. To verify the design concept, we derive and analyze a kinematic model of a single ball transmission mechanism. Furthermore the motions of ACROBAT-S are verified by Solid Works 3D simulator. From the results, it is confirmed that the proposed single ball transmission is applicable for an omnidirectional wheel mechanism to realize omnidirectional motions.