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There are benefits to the use of internal actuators for rotational maneuvers of small-scale underwater vehicles. Internal actuators are protected from the outside environment by the external pressure hull and will not disturb the surrounding environment during inspection tasks. Additionally, internal actuators do not rely on the relative fluid motion to exert control moments, therefore they are useful at low speed and in hover. This paper describes the design, fabrication and testing of one such autonomously controlled, internally actuated underwater vehicle. The Internally Actuated, Modular Bodied, Untethered Submersible (IAMBUS) can be used to validate non-linear control strategies using internal actuators. Vehicle attitude control is provided by three orthogonally mounted reaction wheels. The housing is a spherical glass pressure vessel, which contains all of the components, such as actuators, ballast system, power supply, on-board computer and inertial sensor. Since the housing is spherically symmetric, the hydrodynamics of IAMBUS are uncoupled (e.g. a roll maneuver does not impact pitch or yaw). This hull shape enables IAMBUS to be used as a spacecraft attitude dynamics and control simulator with full rotational freedom. / Master of Science

Pontryagin's principle is applied to several significant problems associated with optimal large angle spacecraft rotational maneuvers. Both rigid and flexible body dynamical models for these vehicles are considered. Three relaxation/analytic continuation methods are developed for iteratively solving the two-point-boundary value problem which results in the treatment of these problems. The solutions obtained are required to rigorously satisfy the necessary conditions derived from Pontryagin's principle. These methods include: (1) boundary condition relaxation processes; (2) differential equation relaxation processes; and (3) hybrid relaxation processes, combining (1) and (2) above. In the literature these relaxation processes are closely related to a number of methods for solving nonlinear equations, known as Davidenko's method, imbedding, and homotopy chain methods. For rigid vehicles a general nonsingular optimal maneuver formulation is obtained, treating all kinematic and dynamical nonlinearities, for general orientation and angular velocity boundary conditions. For flexible vehicles restricted to single axis maneuvers and anti-symmetric elastic deflection modes, a general optimal maneuver formulation is obtained; treating all kinematic, dynamical, and first order structural nonlinearities. In the case of general motion for a flexible vehicle a general formulation is provided, though a solution is not obtained; due to a previously unidentified and as yet unresolved computational difficulty associated symmetry in the dynamical model for the spacecraft. / Ph. D.

Polynomial feedback controls are developed for large angle, nonlinear spacecraft attitude maneuvers. Scalar and two-state systems are presented as simple examples to demonstrate the method, and several systems of state variables to parameterize spacecraft motion are considered. Both external and internal control torques are treated; in the latter, attention is restricted to momentum transfer maneuvers that permit several order reductions. Several stability theorems with their application to polynomial feedback systems are discussed. / Ph. D.

Attitude control laws that use control moment gyros (CMGs) and momentum wheels are derived with nonlinear techniques. The control laws command the CMGs to provide rapid angular acceleration and the momentum wheels to reject tracking and initial condition errors. Numerical simulations of derived control laws are compared. A trend analysis is performed to examine the benefits of the derived controllers. We describe the design of a CMG built using commercial off-the-shelf (COTS) equipment. A mixed attitude control strategy is implemented on the spacecraft simulator at Virginia Tech. / Master of Science

Among the various challenges involved in the development of CubeSats lies the attitude determination and control of the satellite. The importance of a properly functioning attitude determination and control system (ADCS) on any satellite is vital to the satisfaction of its mission objectives. Due to this importance, three-axis attitude control simulators are commonly used to test and validate spacecraft attitude control systems before flight. However, these systems are generally too large to successfully test the attitude control systems on-board CubeSat-class satellites. Due to their low cost and rapid development time, CubeSats have become an increasingly popular platform used in the study of space science and engineering research. As an increasing number of universities and industries take part in this new approach to small-satellite development, the demand to properly test, verify, and validate their attitude control systems will continue to increase. An approach to CubeSat attitude determination and control simulation is in development at the Virginia Tech Space Systems Simulation Laboratory. The final test setup will consist of an air bearing platform placed inside a square Helmholtz cage. The Helmholtz cage will provide an adjustable magnetic field to simulate that of a low earth orbit (LEO), and the spherical air bearing will simulate the frictionless environment of space. In conjunction, the two simulators will provide an inexpensive and adjustable system for testing any current, and future, CubeSat ADCS prior to flight. Using commercial off the shelf (COTS) components, the Virginia Tech CubeSat Attitude Control Simulator (CSACS), which is a low cost, lightweight air bearing testing platform, will be coupled with a 1.5-m-long square Helmholtz cage design in order to provide a simulated LEO environment for CubeSat ADCS validation. / Master of Science / The attitude determination and control subsystem is a vital component of a spacecraft. This subsystem provides the pointing accuracy and stabilization which allows a spacecraft to successfully perform its mission objectives. The cost and size of spacecraft are dependent on their specific applications; where some may fit in the palm of your hand, others may be the size of a school bus. However, no matter the size, all spacecraft contain some form of onboard attitude determination and control. This leads us to the introduction of a miniaturized class of spacecraft known as CubeSats. Their modular 10×10×10 cm cube structural design allows for both low cost and rapid development time, making CubeSats widely used for space science and engineering research in university settings. While CubeSats provide a low cost alternative to perform local, real-time measurements in orbit, it is still very important to validate the attitude determination and control subsystem before flight to minimize any risk of failure in orbit. Thus, the contents of this thesis will focus on the development, design, and testing of two separate spacecraft attitude determination and control simulation systems used to create an on-orbit environment in a laboratory setting in order to properly validate university-built CubeSats prior to flight.

Thesis (MScEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: With CubeSats becoming popular as a cheap alternative to larger satellites, the need for advanced miniature attitude determination and control systems (ADCS) arises to meet the pointing requirements of satellite operations such as earth imaging and orbit maintenance. This thesis describes the design of a complete ADCS for use on CubeSats. A previously designed CubeSat on-board-computer, CubeComputer, and ne sun and nadir sensor, CubeSense, is incorporated in the design. The remaining requirements with regard to sensors and actuators were met by CubeControl, an additional module, the design, manufacturing and testing of which are described. CubeControl can implement magnetic control with the use of a magnetometer and three magnetorquers. It is also capable of driving three reaction wheels for accurate active 3-axis stabilization. / AFRIKAANSE OPSOMMING: Met CubeSats wat gewild raak as 'n goedkoop alternatief tot groter satelliete ontstaan die behoefte vir gevorderde miniatuur ori entasiebepaling en -beheerstelsels wat satelliet operasies soos aardwaarneming en wentelbaan korreksies moontlik maak. Hierdie tesis beskryf die ontwerp van 'n volledige ori entasiebepaling en -beheerstelsel vir CubeSats. 'n Voorheen ontwikkelde CubeSat aanboordrekenaar, CubeComputer, en 'n fyn sonsensor en nadirsensor, CubeSense, is ingesluit in die ontwerp. Die orige benodighede met verband tot sensors en aktueerders word vervul deur CubeControl, 'n addisionele module waarvan die ontwerp, vervaardiging en toetsing beskyf word. CubeControl kan magnetiese beheer implementeer deur gebruik te maak van 'n magnetometer en drie magneetstange. Dit kan ook drie reaksiewiele aandryf vir akkurate aktiewe 3-as stabilisering.

The design of an attitude control system for a flexible spacecraft using magnetic actuation is considered. The nonlinear, linear, and modal equations of motion are developed for a general flexible body. Magnetic control is shown to be instantaneously underactuated, and is only controllable in the time-varying sense. A PD-like control scheme is proposed to address the attitude control problem for the linear system. Control gain limitations are shown to exist for the purely magnetic control. A hybrid control scheme is also proposed that relaxes these restrictions by adding a minimum control effort from an alternate three-axis actuation system. Floquet and passivity theory are used to obtain gain selection criteria that ensure a stable closed-loop system, which would aid in the design of a hybrid controller for a flexible spacecraft. The ability of the linearized system to predict the stability of the corresponding nonlinear system is also investigated.

The design of an attitude control system for a flexible spacecraft using magnetic actuation is considered. The nonlinear, linear, and modal equations of motion are developed for a general flexible body. Magnetic control is shown to be instantaneously underactuated, and is only controllable in the time-varying sense. A PD-like control scheme is proposed to address the attitude control problem for the linear system. Control gain limitations are shown to exist for the purely magnetic control. A hybrid control scheme is also proposed that relaxes these restrictions by adding a minimum control effort from an alternate three-axis actuation system. Floquet and passivity theory are used to obtain gain selection criteria that ensure a stable closed-loop system, which would aid in the design of a hybrid controller for a flexible spacecraft. The ability of the linearized system to predict the stability of the corresponding nonlinear system is also investigated.