A path following controller based on an uncertainty and disturbance estimator (UDE) for a quadrotor with a cable suspended payload is proposed. The quadrotor and the payload are subject to unknown wind disturbances. The controller resembles a cascade architecture. For the outer loop, a UDE-based translational control law is proposed. The controller asymptotically stabilizes the quadrotor along a given path and estimates the lumped disturbances with a low pass filter. For the inner loop, an attitude tracking controller is used to control the direction of the lift vector so that the actual lift force can asymptotically follow the reference force generated by the translational controller. The stability of the system with the translational controller and the attitude tracking controller has been shown to be asymptotically stable using the reduction theorem. With the help of the reduction theorem, the design of the translational and the attitude control can be decoupled, providing the flexibility of implementing different attitude controllers without redoing the stability analysis. As shown in the simulation, the control law can stabilize the quadrotor on the desired path under different wind disturbances.

This paper presents a novel guidance and control design for a parcel tethered to a drone in its delivery mission. The proposed proportional navigation based guidance, integrated with custom built control, enable us to achieve autonomous payload landing. The novelty of the proposed method lies on two aspects: 1) the guidance law allows for soft landing; while 2) the path following control ensures the swing-free payload transportation that sets the solid foundation for landing. The development is verified by extensive simulations and further demonstrated by flight experiments.

The dynamics and control of a quadrotor with a cable suspended payload is studied in this paper. The payload whose tether point deviates from the center of mass of the quadrotor is called an off-centered suspended payload. By using the matrix form of Kane's method, the equations of motion are obtained. The system is decomposed into rotational and translational channels. A nonlinear controller is developed to enable stabilization of the quadrotor by using partial linearization of the system. Simulations of a set point regulation are presented to verify the effectiveness of the dynamics modeling control design.

Cooperative localization is a promising approach to achieving safe quadrotor formation flight through precise positioning via low-cost interdrone sensors. This paper develops an observability-aware control principle tailored to quadrotor formation flight with range-only interdrone measurements. The control principle is based on a novel approximation of the local observability gramian (LOG), which we name the short-term local observability gramian (STLOG). The validity of STLOG is established by proving its link to directional estimation precision in nonlinear systems. We propose the observability predictive controller (OPC), a receding-horizon controller that generates optimal inputs to enhance information gain in weakly observable state directions by maximizing the minimum eigenvalue of the STLOG. This reduces the risk of estimator divergence due to the unbounded growth of uncertainty in weakly observed state components. Monte Carlo simulations and flight experiments are conducted with quadrotors in a GNSS-denied ferrying mission, showing that the OPC improves positioning confidence and estimator robustness.