I. Introduction
In the past decades, quadrotor UAVs have become popular due to their advantages of maneuverability [1] and the vertical take-off and landing (VTOL) capability [2], [3]. These features make the quadrotors ideal choices for cargo delivery by air [4]. Among the basic ways of the quadrotor transportation [5], cable-suspended load transportation has received increasing attention recently. However, the control of the quadrotor transportation systems is a much more challenging task compared with single quadrotor control problems due to the dual underactuated property, strong nonlinearity and complex dynamic coupling [6], hence it is of great necessity to design efficient control schemes for mission safety.