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Integrated Optimal and Robust Control of Spacecraft in Proximity OperationsPan, Hejia 09 December 2011 (has links)
With the rapid growth of space activities and advancement of aerospace science and technology, many autonomous space missions have been proliferating in recent decades. Control of spacecraft in proximity operations is of great importance to accomplish these missions. The research in this dissertation aims to provide a precise, efficient, optimal, and robust controller to ensure successful spacecraft proximity operations. This is a challenging control task since the problem involves highly nonlinear dynamics including translational motion, rotational motion, and flexible structure deformation and vibration. In addition, uncertainties in the system modeling parameters and disturbances make the precise control more difficult. Four control design approaches are integrated to solve this challenging problem. The first approach is to consider the spacecraft rigid body translational and rotational dynamics together with the flexible motion in one unified optimal control framework so that the overall system performance and constraints can be addressed in one optimization process. The second approach is to formulate the robust control objectives into the optimal control cost function and prove the equivalency between the robust stabilization problem and the transformed optimal control problem. The third approach is to employ the è-D technique, a novel optimal control method that is based on a perturbation solution to the Hamilton-Jacobi-Bellman equation, to solve the nonlinear optimal control problem obtained from the indirect robust control formulation. The resultant optimal control law can be obtained in closedorm, and thus facilitates the onboard implementation. The integration of these three approaches is called the integrated indirect robust control scheme. The fourth approach is to use the inverse optimal adaptive control method combined with the indirect robust control scheme to alleviate the conservativeness of the indirect robust control scheme by using online parameter estimation such that adaptive, robust, and optimal properties can all be achieved. To show the effectiveness of the proposed control approaches, six degree-offreedom spacecraft proximity operation simulation is conducted and demonstrates satisfying performance under various uncertainties and disturbances.
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Modeling and Analysis of a Thermospheric Density Measurement System Based on Torque EstimationAceto, Christopher James 12 July 2023 (has links)
This thesis models and analyzes an in-situ method for measuring the density of the thermosphere at low Earth orbit (LEO) altitudes in real time. As satellites orbit in the thermosphere, the sparse yet present air perturbs their orbits via the drag force. The drag force is poorly characterized and has a significant effect at LEO altitudes relative to other forces, making this perturbation force one of the greatest uncertainties in LEO orbit propagation. A steadily increasing number of satellites orbit at LEO altitudes, so for safety, it is critical to accurately track these satellites to avoid collisions. Therefore, better knowledge of the drag force is required. The drag force depends directly on the air mass density in the thermosphere, and current knowledge of the thermospheric density is limited. Models exist to describe the variations in density over time, but due to the many unpredictable factors which affect the thermosphere, the best of these models are only accurate to within 10%. Also, currently available techniques to measure the thermospheric density can only return time-averaged measurements, which causes inaccuracies in orbit propagation due to local density variations. Some planned in-situ density measurement missions rely on measuring acceleration caused by the drag force, but this requires a highly accurate accelerometer to be able to separate the drag force from other stronger forces acting on a satellite. The Satellite Producing Aerodynamic Torque to Understand LEO Atmosphere (SPATULA) concept was introduced as an alternative method, which infers density based on measurements of the drag torque. In the rotational regime, drag produces the strongest torque at LEO altitudes by far, making it possible to acquire accurate density measurements with inexpensive, commercially available sensors and actuators on a SPATULA spacecraft. This thesis expands upon a preliminary study of the SPATULA concept. A SPATULA spacecraft's dynamics are modeled in three dimensions, and a novel method is introduced for modeling the dependence of external torques on the geometry and attitude of the spacecraft. In addition to the dynamics model, discrete-time algorithms for guidance, system state filtering, attitude control, and density estimation are developed for the six degrees of freedom case. The MathWorks tools MATLAB and Simulink are used to simulate the physics and system models. The simulations are used to evaluate the performance of the SPATULA system's density measurements and compare them to conventional methods. It is found that the accuracy and bandwidth of the SPATULA system have a significant dependence on the assumed accuracy of the torque models in the system's filter. When the bandwidth is set to avoid significant phase shift errors, the SPATULA system can produce real-time measurements of density accurate over a minimum time scale of about 60 seconds, and the density error has a standard deviation of about 2 x 10^-14 kg/m^3. This accuracy is about 6 times better than the best thermospheric models, and it is also better than reported accuracies of most other density measurement methods. If bandwidth is sacrificed, the density error standard deviation can be decreased by a factor of 4. This introduces additional error due to phase shift delays, but these can be corrected with signal processing techniques. With the higher accuracy, the SPATULA system loses its real-time ability, but the data it produces would still provide excellent insight for improving thermospheric models. With high accuracy and low cost, the SPATULA concept is a promising path to pursue toward improving thermospheric density knowledge. / Master of Science / This thesis models and analyzes a method for measuring the density of the upper atmosphere in real time directly onboard a satellite. As low Earth orbit (LEO) satellites orbit at low altitudes, the sparse yet present atmosphere changes their orbits via the drag force. The drag force is poorly characterized and has a significant effect at LEO altitudes relative to other forces, making this perturbation force one of the greatest uncertainties in LEO orbit prediction. A steadily increasing number of satellites orbit at LEO altitudes, so for safety, it is critical to accurately track and predict the orbits of these satellites to avoid collisions. Therefore, better knowledge of the drag force is required. The drag force depends directly on air density, and current knowledge of the upper atmospheric density is limited. Models exist to describe the variations in density over time, but due to the many unpredictable factors which affect the atmosphere, the best of these models are only accurate to within 10%. Also, currently available techniques to measure the upper atmospheric density can only return time-averaged measurements, which causes inaccuracies in orbit prediction due to local density variations. Some planned density measurement missions rely on measuring acceleration caused by the drag force, but this requires a highly accurate accelerometer to be able to separate the drag force from other stronger forces acting on a satellite. The Satellite Producing Aerodynamic Torque to Understand LEO Atmosphere (SPATULA) concept was introduced as an alternative method, which infers density based on measurements of the drag torque. Drag produces the strongest torque at LEO altitudes by far, making it possible to acquire accurate density measurements with inexpensive, commercially available parts on a SPATULA spacecraft. This thesis expands upon a preliminary study of the SPATULA concept. A SPATULA spacecraft's motion and rotation is modeled in three dimensions, and a novel method is introduced for modeling the dependence of external torques on the geometry and orientation of the spacecraft. In addition to the dynamics model, algorithms that could be implemented on a satellite's computer are developed for determining the best orientation, estimating the state of the system, controlling the orientation, and estimating density. The MathWorks tools MATLAB and Simulink are used to simulate the physics and system models. The simulations are used to evaluate the performance of the SPATULA system's density measurements and compare them to conventional methods. It is found that the accuracy and bandwidth of the SPATULA system have a significant dependence on the assumed accuracy of the torque models used by the system. When a high bandwidth is used to avoid problems associated with low bandwidth, the SPATULA system can produce real-time measurements of density accurate over a minimum time scale of about 60 seconds, and the density error has a standard deviation of about 2 x 10^-14 kg/m^2. This accuracy is about 6 times better than the best upper atmospheric models, and it is also better than reported accuracies of most other density measurement methods. If bandwidth is sacrificed, the density error standard deviation can be decreased by a factor of 4. This introduces additional error due to delayed measurements of quickly varying components of the density, but these can be corrected with signal processing techniques. With the higher accuracy, the SPATULA system loses its real-time ability, but the data it produces would still provide excellent insight for improving atmospheric density models. With high accuracy and low cost, the SPATULA concept is a promising path to pursue toward improving density knowledge.
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Optimal Attitude Control Management For A CubesatDevelle, Michael James 01 January 2011 (has links)
CubeSats have become popular among universities, research organizations, and government agencies due to their low cost, small size, and light weight. Their standardized configurations further reduce the development time and ensure more frequent launch opportunities. Early cubesat missions focused on hardware validation and simple communication missions, with little requirement for pointing accuracy. Most of these used magnetic torque rods or coils for attitude stabilization. However, the intrinsic problems associated with magnetic torque systems, such as the lack of three-axis control and low pointing accuracy, make them unsuitable for more advanced missions such as detailed imaging and on-orbit inspection. Threeaxis control in a cubesat can be achieved by combining magnetic torque coils with other devices such as thrusters, but the lifetime is limited by the fuel source onboard. To maximize the mission lifetime, a fast attitude control management algorithm that could optimally manage the usage of the magnetic and thruster torques is desirable. Therefore, a recently developed method, the BSpline-augmented virtual motion camouflage, is presented in this defense to solve the problem. This approach provides results which are very close to those obtained through other popular nonlinear constrained optimal control methods with a significantly reduced computational time. Simulation results are presented to validate the capabilities of the method in this application
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Fuzzy Attitude Control of a Magnetically Actuated CubeSatWalker, Alex R. January 2013 (has links)
No description available.
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Low drag aerodynamic attitude control for high-speed missiles using transpirationZbierajewski, Kathryn Ann 09 September 2010 (has links)
No description available.
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Attitude determination of a spinning spacecraft through application of detected and identified star transits to the estimation of spacecraft model parameters /Walsh, Thomas Michael January 1974 (has links)
No description available.
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Optimal large angle spacecraft rotational maneuversTurner, James D. January 1980 (has links)
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.
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Mixed Control Moment Gyro and Momentum Wheel Attitude Control StrategiesSkelton, Claude Eugene II 20 January 2004 (has links)
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
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Advancements in the Design and Development of CubeSat Attitude Determination and Control Testing at the Virginia Tech Space Systems Simulation LaboratoryWolosik, Anthony Thomas 07 September 2018 (has links)
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.
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An Autonomous Underwater Vehicle for Validating Internal Actuator Control StrategiesSchultz, Christopher R. 13 July 2006 (has links)
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
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