Step climbing capability of a 4WD omnidirectional wheelchair

This paper presents analysis and experiments of a 4WD omnidirectional wheelchair for climbing a high step. For enhancing the mobility and maneuverability of standard wheelchairs, a new type of omnidirectional mobile platform with four-wheel drive (4WD) mechanism is introduced. The mobile platform equips four wheels, two omni-wheels in front and two normal tires in rear. The normal wheel and the omni-wheel, mounted on the same side of the base, are interconnected by belt transmissions to rotate in unison with a drive motor, i.e. a synchro-drive transmission. To rotate a chair at the center of the mobile base about vertical axis, the third motor is installed on the platform. The prototype wheelchair with the proposed 4WD system is capable of moving in any direction and climbing over a high single step. The conventional 4WD system provides enhanced step climb capability. However the maximum step height is restricted by the rear wheels. Namely, front wheels can climb up higher step than the rear wheels although the both wheels are identical in diameter. To clarify the difference between front and rear wheels in the step climb capability, the statics of the 4WD system is analyzed. It is suggested that the reaction of a drive torque cause the front-rear differences and its influence depends on a wheelbase and a vehicle weight distribution ratio between front and rear wheel axes. For varying a vehicle weight distribution ratio, we develop a chair tilting mechanism for the 4WD wheelchair with a linear drive mechanism controlled by a stability algorithm using an inclinometer. To verify the analysis results and availability of the chair tilting mechanism, step climb capabilities of a 4WD wheelchair prototype is tested in experiments. In the experiments, both front and rear wheels of the prototype wheelchair with a human can successfully climb a 90 mm step while the rear wheels fail when the chair tilting system is disabled.