An omnidirectional 4WD mobile platform for wheelchair applications

In this paper, a new type of omnidirectional mobile platform with four-wheel drive (4VM) mechanism is presented. The mobile platform includes a pair of normal wheels on the rear side of the platform 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 platform, are interconnected by a chain or a belt transmission to rotate in unison with a drive motor, i.e. a synchro-drive transmission. The omni-wheel allows the front end of the mobile platform to roll freely from side to side while it provides traction force in the heading direction. The third motor is installed to turn a chair about a vertical axis about the center of the platform. The two drive motors are coordinated by the omnidirectional control to translate the center of the chair in an arbitrary direction while chair orientation is controlled by the third motor individually. Thus a wheelchair with proposed 4WD mobile platform can move in any direction without changing the chair orientation and turn in a place, namely holonomic. The 4WD mechanism provides certain traction force even on the irregular surface and step climbing capability is enhanced since all wheels are actively driven and no passive caster is used. The non-redundant configuration with three motors contributes not only to a cost saving but also the high reliability of the mechanism. In the paper, kinematics and statics of the 4VM mechanism are analyzed and an omnidirectional control method is developed. The onmidirectional motions of the proposed 4VM drive system are tested by the computer simulations.