Analysis and Prototyping of the Active-caster Robotic Drive with a Ball Transmission

In this paper, the active-caster robotic drive with a ball transmission (ACROBAT-S) is presented. The proposed mechanism includes a unique drive train with a ball-roller traction system to drive the active-caster omnidirectional wheel system. A large steel ball is driven by two rollers, which act as a power composition function. The third roller takes the combined power out from the ball and decomposes it to drive a wheel axis and a steering axis of the active-caster to create a caster motion. The proposed ball transmission contributes to simplify a control system since it needs no angle sensor and complicated calculations for cooperative motor control. To realize this concept, we analyze the kinematics and statics of the proposed mechanism with a ball transmission. The results of the analysis provide a mechanical condition for a prototype design, such as relationship among a gear ratio, a wheel radius and a caster offset, together with pre load conditions to avoid a slip between a ball and rollers. To verify the kinematic analysis and mechanical condition, simulation studies are performed. Based on the analysis and the simulation, we design and build a wheel mechanism with two step motors. The step motors are controlled by feed forward control in which velocity commands along x-axis and y-axis are sent to motor drivers. From the results the prototype design and experiments, it is confirmed that the proposed active-caster mechanism, ACROBAT-S, realized expected omnidirectional motions with no angle sensor and no complicated control.