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Title:  Guidance and control of swarms of spacecraft 
Author(s):  Morgan, Daniel James 
Director of Research:  Chung, SoonJo 
Doctoral Committee Chair(s):  Chung, SoonJo 
Doctoral Committee Member(s):  Conway, Bruce A.; Hutchinson, Seth A.; Voulgaris, Petros G.; Hadaegh, Fred Y 
Department / Program:  Aerospace Engineering 
Discipline:  Aerospace Engineering 
Degree Granting Institution:  University of Illinois at UrbanaChampaign 
Degree:  Ph.D. 
Genre:  Dissertation 
Subject(s):  swarms of spacecraft
sequential convex programming model predictive control 
Abstract:  There has been considerable interest in formation flying spacecraft due to their potential to perform certain tasks at a cheaper cost than monolithic spacecraft. Formation flying enables the use of smaller, cheaper spacecraft that distribute the risk of the mission. Recently, the ideas of formation flying have been extended to spacecraft swarms made up of hundreds to thousands of 100gramclass spacecraft known as femtosatellites. The large number of spacecraft and limited capabilities of each individual spacecraft present a significant challenge in guidance, navigation, and control. This dissertation deals with the guidance and control algorithms required to enable the flight of spacecraft swarms. The algorithms developed in this dissertation are focused on achieving two main goals: swarm keeping and swarm reconfiguration. The objectives of swarm keeping are to maintain bounded relative distances between spacecraft, prevent collisions between spacecraft, and minimize the propellant used by each spacecraft. Swarm reconfiguration requires the transfer of the swarm to a specific shape. Like with swarm keeping, minimizing the propellant used and preventing collisions are the main objectives. Additionally, the algorithms required for swarm keeping and swarm reconfiguration should be decentralized with respect to communication and computation so that they can be implemented on femtosats, which have limited hardware capabilities. The algorithms developed in this dissertation are concerned with swarms located in low Earth orbit. In these orbits, Earth oblateness and atmospheric drag have a significant effect on the relative motion of the swarm. The complicated dynamic environment of low Earth orbits further complicates the swarmkeeping and swarmreconfiguration problems. To better develop and test these algorithms, a nonlinear, relative dynamic model with $J_2$ and drag perturbations is developed. This model is used throughout this dissertation to validate the algorithms using computer simulations. The swarmkeeping problem can be solved by placing the spacecraft on J2invariant relative orbits, which prevent collisions and minimize the drift of the swarm over hundreds of orbits using a single burn. These orbits are achieved by energy matching the spacecraft to the reference orbit. Additionally, these conditions can be repeatedly applied to minimize the drift of the swarm when atmospheric drag has a large effect (orbits with an altitude under 500 km). The swarm reconfiguration is achieved using two steps: trajectory optimization and assignment. The trajectory optimization problem can be written as a nonlinear, optimal control problem. This optimal control problem is discretized, decoupled, and convexified so that the individual femtosats can efficiently solve the optimization. Sequential convex programming is used to generate the control sequences and trajectories required to safely and efficiently transfer a spacecraft from one position to another. The sequence of trajectories is shown to converge to a KarushKuhnTucker point of the nonconvex problem. In the case where many of the spacecraft are interchangeable, a variableswarm, distributed auction algorithm is used to determine the assignment of spacecraft to target positions. This auction algorithm requires only local communication and all of the bidding parameters are stored locally. The assignment generated using this auction algorithm is shown to be near optimal and to converge in a finite number of bids. Additionally, the bidding process is used to modify the number of targets used in the assignment so that the reconfiguration can be achieved even when there is a disconnected communication network or a significant loss of agents. Once the assignment is achieved, the trajectory optimization can be run using the terminal positions determined by the auction algorithm. To implement these algorithms in real time a model predictive control formulation is used. Model predictive control uses a finite horizon to apply the most uptodate control sequence while simultaneously calculating a new assignment and trajectory based on updated state information. Using a finite horizon allows collisions to only be considered between spacecraft that are near each other at the current time. This relaxes the alltoall communication assumption so that only neighboring agents need to communicate. Experimental validation is done using the formation flying testbed. The swarmreconfiguration algorithms are tested using multiple quadrotors. Experiments have been performed using sequential convex programming for offline trajectory planning, model predictive control and sequential convex programming for realtime trajectory generation, and the variableswarm, distributed auction algorithm for optimal assignment. These experiments show that the swarmreconfiguration algorithms can be implemented in real time using actual hardware. In general, this dissertation presents guidance and control algorithms that maintain and reconfigure swarms of spacecraft while maintaining the shape of the swarm, preventing collisions between the spacecraft, and minimizing the amount of propellant used. 
Issue Date:  20150420 
Type:  Thesis 
URI:  http://hdl.handle.net/2142/78367 
Rights Information:  Copyright 2015 Daniel Morgan 
Date Available in IDEALS:  20150722 
Date Deposited:  May 2015 
This item appears in the following Collection(s)

Dissertations and Theses  Aerospace Engineering

Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois