Abstract
The application of rotorcraft to autonomous load carrying and transport is a new frontier for Unmanned Aerial Vehicles (UAVs). This task requires that hovering vehicles remain stable and balanced in flight as payload mass is added to the vehicle. If payload is not loaded centered or the vehicle properly trimmed for offset loads, the robot will experience bias forces that must be rejected. In this paper, we explore the effect of dynamic load disturbances introduced by instantaneously increased payload mass and how those affect helicopters and quadrotors under Proportional-Integral-Derivative flight control. We determine stability bounds within which the changing mass-inertia parameters of the system due to the acquired object will not destabilize these aircraft with this standard flight controller. Additionally, we demonstrate experimentally the stability behavior of a helicopter undergoing a range of instantaneous step payload changes.
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Notes
Rotor thrust is taken as constant, exactly canceling helicopter weight.
Note that all rotations are considered to occur around the unloaded CoG of the helicopter; offset mass effects are accounted for in the load bias torque.
The R-50 control structure does not use a pure integral term—its integral action is derived from position. The value of k i used is inferred.
Compared with helicopters of the same mass, quadrotors have significantly smaller rotors with faster blade tip speeds, making the Froude Numbers very different. Furthermore, unlike small platforms, larger quadrotors more strongly exhibit flapping, complicating the comparison.
In practice all rotorcraft exhibit flapping, however small.
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Acknowledgements
The authors would like to thank Joe Acosta of Build Right Fly Right Hobbies, Wallingford CT, USA, Greg Brown and Joseph Belter for their support of this work. This work was supported in part by the Office of Naval Research grant N000141010737.
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A preliminary version of this work was presented at International Conference on Robotics and Automation 2011 (Pounds et al. 2011).
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Pounds, P.E.I., Bersak, D.R. & Dollar, A.M. Stability of small-scale UAV helicopters and quadrotors with added payload mass under PID control. Auton Robot 33, 129–142 (2012). https://doi.org/10.1007/s10514-012-9280-5
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DOI: https://doi.org/10.1007/s10514-012-9280-5