A novel vehicle design for holonomic omnidirectional wheelchairs is presented in this paper. The new design allows the vehicle to change the shape of its footprint as well as to continuously vary the gear ratio of the drive mechanism. The variable footprint mechanism consists of a pair of beams intersecting at a pivotal point in the middle. Two pairs of ball wheels at the diagonal positions of the vehicle chassis are mounted, respectively, on the two beams. The angle between the two beams varies actively so that the ratio of the wheel base to the tread may change. Four independent servomotors driving the four ball wheels allow the vehicle to move in an arbitrary direction from an arbitrary configuration as well as to change the angle between the two beams and thereby change the footprint configuration. The objective of controlling the beam angle is threefold. One is to augment the static stability of the vehicle by varying the footprint so that the mass centroid of the vehicle may be kept within the footprint at all times. The second is to reduce the width of the vehicle when going through a narrow doorway. The third is to apparently change the transmission ratio relating the vehicle speed to individual actuator speeds. Required vehicle tasks are coordinated by the hierarchical control architecture. First the concept of the varying footprint mechanism is described briefly, then three control algorithms for varying the footprint are presented followed by the hierarchical control architecture for coordinating the different tasks. A proof of concept prototype wheelchair is designed, built, and tested.

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