Omnidirectional control of a four-wheel drive mobile base for wheelchairs

This paper presents a new type of omnidirectional mobile base with four-wheel drive (4WD) mechanism for improving traction on the slippery surface and enhancing mobility on rough terrain of omnidirectional electric wheelchairs. The mobile base includes a pair of normal wheels on the rear side of the base and a pair of omni-wheels on the front side. The normal wheel in rear and the omni-wheel in front, mounted on the same side of the base, are interconnected by a power transmission to rotate in unison with a wheel motor. The omni-wheels allow the front end of the mobile base to roll freely from side to side. Additionally, the third motor is installed to turn a chair about a vertical axis on the center of the mobile base. For omnidirectional control of the 4WD mobile base, two wheel motors are coordinated to translate the center of the chair in an arbitrary direction while the chair orientation is controlled by the third motor separately. Thus a wheelchair with proposed mobile base can move in any direction without changing the chair orientation and turn in a place. The 4WD mechanism guarantees the traction force even on irregular surfaces and the step climbing ability is enhanced since all wheels are large in diameter and no passive caster is used. Moreover three motors provide 3DOF motion of the chair with no redundancy. The configuration with the minimum number of motors contributes not only to cost saving but also to high reliability of the mechanism. In the paper, motion analysis of the 4WD mechanism and the omnidirectional control method are presented followed by kinematic simulations for verification of the proposed 4WD omnidirectional wheelchair.