Omnidirectional and Holonomic Mobile Platform with Four-Wheel-Drive Mechanism for Wheelchairs

Abstract

This paper presents a new type of omnidirectional and holonomic mobile platform with a four-wheel-drive (4WD) mechanism for improving traction of electric wheelchairs on slippery surfaces and enhancing mobility on rough terrain. The 4WD mechanism includes a pair of normal wheels on the back and a pair of omniwheels on the front. The normal wheel in back and the omniwheel in front, on the same side of the drive mechanism, are connected by a power transmission to rotate in unison with a common motor. Omniwheels enable the front of the mechanism to roll freely from side to side. A third motor turns the chair about a vertical axis at the center of the mobile platform. One goal of this project is to apply the 4WD mechanism to a holonomic omnidirectional mobile base for wheelchairs to enhance both maneuverability and mobility in single wheelchair design. The 4WD mechanism guarantees traction on irregular surfaces and enhances step climbing over that of standard wheelchairs because all wheels have a large diameter and no passive casters are used. For omnidirectional control of the 4WD mobile base, two wheel motors are coordinated to move the center of the chair in an arbitrary direction while chair orientation is controlled separately by the third motor. The three motors thus provide nonredundant 3DOF chair movement. A wheelchair with our proposed mobile base moves in all directions without changing chair orientation and turns in place, i.e., holonomic. The configuration minimizing number of motors cuts costs and ensures a high reliable mechanism. We analyze the kinematics of planar motion and statics on the wheel step of the synchronized 4WD, then discuss the development of omnidirectional 4WD control. A series of experiments using a small robotic vehicle verifies kinematic and static models and the feasibility of the 4WD omnidirectional system proposed.