“…As far as we understand, however, most previous works (including [10][11][12]) based on the relative motion equations in the Hill frame have used purely mathematical approaches in synthesis of their control laws, and the physical properties of the relative motion have been overlooked. As Slotine and Li have mentioned in their works [14,15], some physical properties, such as energy conservation, may hold significant potential in analysis and control design for multiinput nonlinear mechanical systems.…”