Model-based Flight Control for a VTOL Aircraft with Independently Tilting Rotors

Master thesis (2019)

Authors

J.M. de Jong Mechanical Engineering

Contributors

T. Keviczky Team Tamas Keviczky - Mechanical, Maritime and Materials Engineering (mentor)
C.C. de Visser (graduation committee member)
J. Alonso-Mora Learning & Autonomous Control - Mechanical, Maritime and Materials Engineering (graduation committee member)
Faculty
Mechanical Engineering, Mechanical Engineering
Copyright: © 2019 Joost de Jong
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Published Date

15-02-2019

Language

English

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Faculty

Mechanical Engineering

Abstract

This thesis reports on the research and design of a real-time Time-Varying Model Predictive Control (TVMPC) scheme to stabilize a tilt-rotor aircraft with four independently tilting rotors. The aircraft design stems from a prototype system constructed by the drone technology start-up Avy.
First, a motivation is presented for implementation of model-based flight control techniques. Consequently, TVMPC is introduced as a middle ground between Adaptive Model Predictive Control (MPC) and fully nonlinear MPC, capturing model variation over the control horizon while retaining the computational efficiency of quadratic optimization algorithms. In a simulation of a simple nonlinear system, TVMPC outperforms fixed-model and adaptive MPC using only a small set of extra models.
Secondly, the tilt-rotor system model is developed. Models for rotor torque and thrust responses and stepper motor movement are introduced and calibrated using experiments, leading to a full nonlinear model of 21 states, 8 control inputs and 8 box constraints. Further envisioned experimental work and its preparations such as aerodynamic testing and tracked constrained flight are briefly mentioned.
The implementation of TVMPC is tested in simulation. Five scenarios compare the effectiveness of time-varying control versus adaptive and fixed-model MPC, with TVMPC not outperforming fixed-model control in horizontal vehicle orientations, but improving on oscillatory behavior shown by adaptive control. TVMPC outperforms the other two controllers in a past 90 degree pitch scenario (partial backflip).
In parallel, the setup of Robot Operating System (ROS) in conjunction with a Hardwarein-the-Loop (HIL) experiment is presented to test tilt-rotor control in real-time. In closed loop, the system was stabilized for limited degrees of freedom using a combination of PID controllers, validating the framework and providing more opportunities for research.
It is concluded that TVMPC improves on the predictions of adaptive MPC, but is prone to oscillatory behavior. Only in highly nonlinear situations, fixed-model MPC is outperformed. For real-time, embedded results, more research still needs to be performed.