In this paper, we propose semi-automatic step climbing system for a wheelchair with an active-caster drive unit. The wheelchair is composed by a manual wheelchair with an add-on mechanism, which configuration has the active-caster, the linear actuator, and link frames, attached on the back of a manual wheelchair. The active-caster is used for a propelling device of a wheelchair. The add-on mechanism is connected to a wheelchair frame with links which configuration is varied by a linear actuator. In the previous works, we confirmed that the wheelchair prototype could climb over a step using the add-on mechanism by manual operation. However, it was quite difficult for normal persons to achieve the proposed step climbing strategy since it includes skillful operations such as cooperative control of the drive wheel and the linear actuator. In this paper, we propose the semi-automatic control system to realize the step climbing motion for normal users. The wheelchair starts the step climbing motion by a trigger given by a user and sequentially climbs over a step. The control system consists of the step position detecting system and the cooperative control of the linear actuator and the drive wheel. By the experiments using a prototype, we confirm that the wheelchair can pass over a step by the semi-automatic control system.
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