Flight Control of a Modular Multirotor System for Flight of Rigid Objects


Student thesis: Doctoral Thesis

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Award date1 Sep 2020


Micro aerial vehicles (MAVs), or drones, have been remarkably popular in a broad spectrum of civil and military applications. However, commercial off-the-shelf drones in the market are mostly designed for a single function. These drones are designed to carry a particular payload with flights controller devised for respective platforms.

In this thesis, a single multirotor system that can carry various payloads for different requirements is proposed. This is achieved by adopting a modular design. The proposed system consists of multiple propelling modules, a control module, and a rigid payload. All flight modules are affixed to a payload to form a flying vehicle. These flight modules act as the system's thrust generation part while the payload becomes a functional component and an airframe. The modular design produces a highly versatile platform as it is reconfigurable by the addition or removal of propelling modules, adjustment of the modules' arrangement, or replacement of payloads. The flight strategy for the proposed modular multirotor system is carried out as follows.

First, all thrusts from the propelling modules of the modular system are assumed to align. To facilitate the flight control, we first propose an IMU-based parameter estimation strategy for rapid computation of the system's configuration to deal with different module and payload attachments. An adaptive geometric controller is then developed to stabilize the system for further refinement of uncertain system parameters. The parameter estimation and adaptive control strategies are applied in real flights. In these experiments, various system's configurations with different dummy payloads and numbers of propelling modules were explored. The experimental results reveal that stable flights of the modular system are attained thanks to the proposed estimation method and adaptive control.

Second, the constraint that all propelling thrusts must be aligned is relaxed. To this end, the research focus is on the development of flight controller for underactated quadrotors with unaligned thrusts. The position and heading angle of these systems can still serve as references for the trajectory generation and tracking. The corresponding nominal states and control inputs exist and can be numerically computed. To stabilize the systems with unaligned thrusts, a novel flight controller is developed with considering the planned trajectories. The flight controller has been tested in numerical simulations and real experiments concerning quadrotors with unaligned thrusts. This controller can be potentially applied to the proposed modular system.

Overall, the proposed modular concept allows the transformation of flight modules to be used for different purposes in a versatile fashion. Scientific merits of the work include but not limited to (i) implementation of modular robot concept in aerial robots; (ii) development of robust self-calibrating, adaptive flight control strategies for a reconfigurable multirotor system with aligned thrusts; and (iii) a potential flight control scheme for underactuated quadrotors with unaligned thrusts.