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Modeling and Control for UAV with Off-center Slung Load

Zongyang Lv, Yanmei Jia, Yongqing Liu, Alan F. Lynch, Qing Zhao, Yuhu Wu

TL;DR

This work addresses the control of UAVs carrying off-center slung loads by formulating a dynamic model from the suspension-point frame, revealing that the payload swing is driven by the suspension-point acceleration and that coupling appears in the attitude dynamics. A cascade control architecture is developed: an outer-loop load-velocity controller, a middle-loop swing-angle controller, and an inner-loop attitude controller, coupled via a decoupler and mixer to deliver actuator commands. The authors prove local exponential stability of the closed-loop system using Lyapunov methods and validate the approach with simulations and real-flight experiments, demonstrating improved attitude tracking, anti-swing performance, and robust load-velocity tracking under offset conditions. The results indicate practical benefits for UOSL operations, enabling more reliable payload transport without requiring CoM-centric modeling or external positioning systems.

Abstract

Unmanned aerial vehicle (UAV) with slung load system is a classic air transportation system. In practical applications, the suspension point of the slung load does not always align with the center of mass (CoM) of the UAV due to mission requirements or mechanical interference. This offset creates coupling in the system's nonlinear dynamics which leads to a complicated motion control problem. In existing research, modeling of the system are performed about the UAV's CoM. In this work we use the point of suspension instead. Based on the new model, a cascade control strategy is developed. In the middle-loop controller, the acceleration of the suspension point is used to regulate the swing angle of the slung load without the need for considering the coupling between the slung load and the UAV. Using the off-center reference frame, an inner-loop controller is designed to track the UAV's attitude without the need of simplification on the coupling effects. We prove local exponential stability of the closed-loop using Lyapunov approach. Finally, simulations and experiments are conducted to validate the proposed control system.

Modeling and Control for UAV with Off-center Slung Load

TL;DR

This work addresses the control of UAVs carrying off-center slung loads by formulating a dynamic model from the suspension-point frame, revealing that the payload swing is driven by the suspension-point acceleration and that coupling appears in the attitude dynamics. A cascade control architecture is developed: an outer-loop load-velocity controller, a middle-loop swing-angle controller, and an inner-loop attitude controller, coupled via a decoupler and mixer to deliver actuator commands. The authors prove local exponential stability of the closed-loop system using Lyapunov methods and validate the approach with simulations and real-flight experiments, demonstrating improved attitude tracking, anti-swing performance, and robust load-velocity tracking under offset conditions. The results indicate practical benefits for UOSL operations, enabling more reliable payload transport without requiring CoM-centric modeling or external positioning systems.

Abstract

Unmanned aerial vehicle (UAV) with slung load system is a classic air transportation system. In practical applications, the suspension point of the slung load does not always align with the center of mass (CoM) of the UAV due to mission requirements or mechanical interference. This offset creates coupling in the system's nonlinear dynamics which leads to a complicated motion control problem. In existing research, modeling of the system are performed about the UAV's CoM. In this work we use the point of suspension instead. Based on the new model, a cascade control strategy is developed. In the middle-loop controller, the acceleration of the suspension point is used to regulate the swing angle of the slung load without the need for considering the coupling between the slung load and the UAV. Using the off-center reference frame, an inner-loop controller is designed to track the UAV's attitude without the need of simplification on the coupling effects. We prove local exponential stability of the closed-loop using Lyapunov approach. Finally, simulations and experiments are conducted to validate the proposed control system.
Paper Structure (14 sections, 3 theorems, 50 equations, 7 figures, 2 tables)

This paper contains 14 sections, 3 theorems, 50 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Dynamic Modeling From Off-center Perspective
  3. Control Design
  4. Mixer
  5. Cascade control system
  6. Inner-loop Attitude Controller
  7. Middle-loop Swing Angle Controller
  8. Decoupler
  9. Outer-loop Load Velocity Controller
  10. Simulation and Experimental Results
  11. Simulation
  12. Ground experiment
  13. Real flight experiment
  14. Conclusions and Future Work

Key Result

Theorem 1

For the attitude model of the quadrotor defined in (model_eta), if the torque $\bm{\tau_\eta}$ is set as (taueta), the zero equilibria of the attitude tracking errors $\bm{e_\eta}$ and $\bm{e_{p_\eta}}$ are exponentially stable.

Figures (7)

  • Figure 1: The quadrotor UAV with an off-center slung load.
  • Figure 2: The schematic of the control strategy.
  • Figure 3: Experimental platform.
  • Figure 4: Simulation results of UAV attitude tracking
  • Figure 5: Ground test experiment.
  • ...and 2 more figures

Theorems & Definitions (5)

  • Remark 1
  • Theorem 1
  • Theorem 2
  • Theorem 3
  • Remark 2