In this paper, we expand the human-friendly control concept which has been restricted only to the robot controls into general assistive controls. For a demonstration of this generalized human-friendly control, we adopt a power-assisted wheelchair and apply a novel control to it. We investigate some requirements for this generalized human-friendly control and suggest solutions to it. Our solution approach consists of two parts: one is on the observation problem, the other is on the control problem. We pick up two problems which happen in power-assisted wheelchair control by gravity. The first is the tendency of falling backward that is related to the observation problem, and the other is the difficulty in propulsion on a hill that is related to the control problem. We propose solutions to each problem, generalize them, and try to establish the human-friendly control.