Control system for fixed-wing unmanned aerial vehicles: automatic tuning, gain scheduling, and turbulence mitigation

Poksawat, P 2018, Control system for fixed-wing unmanned aerial vehicles: automatic tuning, gain scheduling, and turbulence mitigation, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Control system for fixed-wing unmanned aerial vehicles: automatic tuning, gain scheduling, and turbulence mitigation
Author(s) Poksawat, P
Year 2018
Abstract Over the past decade, there has been a rapid growth in the deployment of small unmanned aerial vehicles (UAV). This type of aircraft has become a feasible solution to many applications such as surveying complex terrain, remote sensing, filming and many military purposes. Due to the nature of fixed-wing UAVs' nonlinear flight dynamics, a robust control system is required in order to achieve accurately controllable flights. However, this desirable flight property presents significant design challenges from a control systems perspective.

There are various problems with the conventional model-based method of controller design for UAVs. Firstly, the physical and aerodynamic parameters can be extremely difficult and time consuming to determine, especially for miniature UAVs with very small parameters. Secondly, the actuator and sensor dynamics are often ignored. Lastly, some nonlinear parameters may vary depending on operating points. For these reasons, the identified parameters for the physical model may not be ideal for flight controller design.

As a major contribution to the field, a novel automatic tuning approach was proposed, one that ensures the desired closed-loop performance can be achieved, while also being simple and straightforward to implement. The main idea is to identify the open loop frequency response through closed-loop relay feedback experiment, to be used in the controller parameter selection.

Futhermore, due to existing nonlinearities, a gain scheduled controller was adopted to allow the aircraft to compensate for the change in airspeed. The gain scheduled controller was designed based on several linear models identified with the autotuner.

Additionally, it is commonly known that the main challenges for small UAVs are predominantly associated with maintaining steady and controllable operations in turbulence. This type of UAV typically operates at low altitude, which is characterised as having high level of turbulence intensity. Instability in the attitude system can easily lead to deviation from the commanded trajectory or crashes as a consequence of turbulence-induced perturbation.

Therefore, leading to another contribution of this research, which is the formulation, modelling and implementation of the disturbance feedforward controller for perturbation mitigation. This work was performed on top of the autotuned system to improve flight stability in turbulence. The feedforward controller was experimentally validated and compared with several disturbance rejection controllers in a wind tunnel, which has more challenging turbulence spectrums than what UAVs typically experience in atmospheric turbulence. Finally, an outdoor flight test was conducted to show how the system reacts to atmospheric gusts.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Control Systems, Robotics and Automation
Flight Dynamics
Keyword(s) control system
unmanned aerial vehicle
automatic tuning
gain scheduling
turbulence mitigation
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Created: Tue, 18 Sep 2018, 15:37:18 EST by Keely Chapman
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