Different from the quadrotor suspended payload system with fixed length cable, the system with variable length cable can load and unload cargoes more efficiently under many conditions. This paper presents a method of modeling, control, and trajectory planning for a quadrotor suspended payload system with variable length cable. In order to effectively reduce the swing of the slung payload under the quadrotor, a coupled dynamic model and an integral backstepping control scheme for the system are designed, and a basic trajectory planning is carried out. The presented coupled dynamic model is composed of two parts, one is the dynamic equation of the quadrotor with payload pull, and the other is the dynamic model of the payload pendulum angles with variable length cable related with quadrotor flight acceleration. Meanwhile, the interaction forces between them are calculated. Based on the coupled dynamic model of the system, a cascade control scheme with trajectory planning is designed. The attitude and position control loop of the system adopt the integral backstepping algorithm to solve the reliable flight of quadrotor with slung payload. At the same time, the flight trajectories are generated by trajectory planning as the reference input of the control system, which can further reduce the payload swing. Simulation results show that the proposed scheme can realize stable and reliable flight of the quadrotor and limit the pendulum angles within the desired range to reduce the payload swing.

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