Figure 4

Drive mode change function. (a) Rear drive mode. When all five wheels are in contact with the ground, the wheelchair can perform a pivot turn motion around the center of the large wheels. The large wheels contact on the ground and are rotated since the markers of the large wheels move gradually. (b) Front drive mode. When the large wheels are hovered above the ground and the steering shafts of the front casters are fixed, the wheelchair can perform the same motion around the center of the front casters, avoiding the nonholonomic constraint of the large wheels. The large wheels float from the ground since the markers of the large wheels don't move.