Traction analysis for Active-caster omnidirectional Robotic drive with a Ball Transmission (ACROBAT)

This paper presents a traction analysis for the Active-Caster omnidirectional RObotic drive with a BAll Transmission (ACROBAT). The original Active-Caster omnidirectional RObotic drive (ACRO) allows one to design a holonomic and omnidirectional mobile robot without using so-called omni-wheels such as Mechanum wheel or Universal wheel, etc. The ACRO equips with two motors to generate 2DOF active motion by coordinating two motors based on an angle sensor to detect the wheel orientation. ACROBAT has been developed to remove the needs for an absolute encoder for detecting the wheel orientation and complexities on the coordinated motor control for a robot with ACRO system. ACROBAT needs ball transmission to compose propelling power from two motors and distribute it to wheel axis and steering axis mechanically in an appropriate ratio. To realize this functionality, a drive torque has to be transmitted certainly by traction drive with no slip via the contact point between rollers to a ball and a ball to a ball. Therefore, transmitted torque and a required load to the contact point has to be analyzed to design and maintain a reliability of the mechanism. For the purpose, a traction force analysis is performed in this paper. To transmit motor rated torque to drive the wheel and the steering of the mechanism, the maximum traction force at each contact point is derived from a static model of the mechanism. To guaranty the transmittable traction force, contact loads applied to the components such as rollers or balls are derived as well. To satisfy the required contact loads, the individual load control mechanism has been designed in the first prototype. To meet the required contact load, the loading mechanism is modified from the first one. By the modification, the second prototype is able to be driven by two motors to realize 2DOF motion of the wheel mechanism which shows a holonomic and omnidirectional motion without angle sensor or a high coordinated motor control system.