Study on an Add-on Type Electric Wheelchair Using Active Caster with the Differential Mechanism

Abstract

This study developed an electric-powered manual wheelchair with a differential-drive active caster mounted at the rear of the wheelchair. The active caster has two axes: a caster wheel axis and a steering axis, and the coordinated control of these axes enables the wheel-mounted part to move in all directions. Conventional active casters have separate motors for the steering and wheel axes, and only the motor on the wheel axis operates when the wheelchair moves straight ahead, resulting in a low drive efficiency. Therefore, we developed a differential-drive-type active caster that uses a differential-drive mechanism. The differential-drive-type active caster uses the sum of the rotations of the two motors to move the steering axis, and the difference to move the wheel axis. Therefore, the two motors always operate, thus the drive efficiency is higher than that of a conventional type. However, we found that the two motors interfere with each other owing to their structure, making it difficult to stabilize the steering axis when speed increases. A control method was developed to solve this problem. Through verification experiments, we demonstrated the ability of the wheelchair to move at speeds over 6 km/h, the legal speed limit in Japan, the improved stability of the steering axis, and the accuracy of its omnidirectional movement. Additionally, to reduce the wobble that occurs when the electric-powered manual wheelchair moves straight ahead, we introduced a yaw-rate feedback to the joystick input system. The feedback was then verified in a lane-change experiment, confirming the wobble reduction.