In this paper, we studied about a power assist control method to climb stairs with carry carts with planetary wheels, which have three pairs of a rotational arm and a free wheel on the end of the arm. In the proposed control, when drag force on handle become larger than threshold, the planetary wheels rotate by 120 degrees to climb a single step. The rotating speed varies proportional to the drag force and the proportional gain varies according to phases of contact between stairs and the wheels. Experiments are performed and the maximum drag force is reduced about to 50%. From comparison with two dimensional types of wheels, it is found that smaller wheels require more complex control to realize stable climbing on stairs.
