Space station stabilization and control study Final engineering report

Simulation of stabilization and control for spinning, manned space station to provide artificial gravity station environmen

Recovering Linear Controllability of an Underactuated Spacecraft by Exploiting Solar Radiation Pressure

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140657/1/1.G001446.pd

Attitude motion planning for a spin stabilised disk sail

While solar sails are capable of providing continuous low thrust propulsion the size and flexibility of the sail structure poses difficulties to their attitude control. Rapid slewing of the sail can cause excitation of structural modes, resulting in flexing and oscillation of the sail film and a subsequent loss of performance and decrease in controllability. Disk shaped solar sails are particularly flexible as they have no supporting structure and so these spacecraft must be spun around their major axis to stiffen the sail membrane via the centrifugal force. In addition to stiffening the structure this spin stabilisation also provides gyroscopic stiffness to disturbances, aiding the spacecraft in maintaining its desired attitude. A method is applied which generates smooth reference motions between arbitrary orientations for a spin-stabilised disk sail. The method minimises the sum square of the body rates of the spacecraft, therefore ensuring that the generated attitude slews are slow and smooth, while the spin stabilisation provides gyroscopic stiffness to disturbances. An application of Pontryagin’s maximum principle yields an optimal Hamiltonian which is completely solvable in closed form. The resulting analytical expressions are a function of several free parameters enabling parametric optimisation to be used to provide reference motions which match prescribed boundary conditions on the initial and final configurations. The generated reference motions are utilised in the repointing of a 70m radius spin-stabilised disk solar sail in a heliocentric orbit, with the aim of assessing the feasibility of the motion planning method in terms of the control torques required to track the motions

Advances in Underactuated Spacecraft Control

This dissertation addresses the control of a spacecraft which either becomes underactuated due to onboard failures or is made underactuated by design. Successfully controlling an underactuated spacecraft can extend spacecraft operational life in orbit and improve the robustness of space missions. The novel contributions of the dissertation include the following. Firstly, switching feedback controllers are developed for the attitude control of an underactuated spacecraft equipped with two pairs of thrusters, or two reaction wheels (RWs), or two control moment gyros (CMGs). The problem is challenging; e.g., even in the zero total angular momentum case, no smooth or even continuous time-invariant feedback law for stabilizing a desired orientation exists. The method exploits the separation of the system into inner-loop base variables and outer-loop fiber variables. The base variables track periodic reference trajectories, the amplitude of which is governed by parameters that are adjusted to induce an appropriate change in the fiber variables towards the desired pointing configuration. Secondly, nonlinear Model Predictive Control (MPC) is applied to the attitude dynamics of an underactuated spacecraft with two RWs and zero angular momentum. MPC has the remarkable ability to generate control laws that are discontinuous in the state. By utilizing nonlinear MPC, the obstruction to stabilizability is overcome and attitude maneuvers can be performed while enforcing constraints. Thirdly, an unconventional pathway is discussed for recovering the linear controllability of an underactuated spacecraft with two RWs by accounting for the effects of solar radiation pressure (SRP) in the spacecraft attitude model. Necessary and sufficient conditions for recovering linear controllability are given, and with linear controllability restored, conventional controllers can be designed for underactuated spacecraft. Lastly, two sets of coupled translational and rotational equations of motion for a spacecraft in a central gravity field are derived. The spacecraft is assumed to have only internal attitude actuators and the equations of motion are relative with respect to an equilibrium orbit. Under reasonable assumptions on the spacecraft configuration and equilibrium orbit, the coupled dynamics are small-time locally controllable (STLC), which opens a path to utilizing conventional control techniques to move translationally in space by employing attitude control only.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133430/1/cdpete_1.pd

Retrospective Cost-based Adaptive Spacecraft Attitude Control.

Fixed gain attitude control laws are sensitive to modeling errors and actuator nonlinearities. Adaptive control can solve many of these challenges. We present a retrospective cost-based adaptive spacecraft attitude controller designed using the system's impulse response as modeling information. The performance metric is based on rotation matrices and thus, the controller does not suffer from singularities or discontinuities present in vector attitude representations. We demonstrate robustness to inertia and actuator scaling as well as actuator misalignment and nonlinearities, unknown disturbances, sensor noise and bias for thrusters and reaction wheels through numerical simulations. We implement an averaged Markov parameter and decentralized control to address the problem of the singular input matrix of magnetic torquers. For control moment gyros, we develop a hybrid linearization and impulse response-based Markov parameter and present new guidelines to evaluate the feasibility of desired rest-to-rest maneuvers. Finally, we address the problem of angular velocity-free attitude control of a flexible spacecraft with noncollocated sensors and actuators. We present a new approach to controlling harmonic nonminimum-phase systems using the step and impulse response of the linearized system. We demonstrate robustness to model uncertainty through system analysis and numerical simulations.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111607/1/gecruz_1.pd

Underactuated Spacecraft Switching Law for Two Reaction Wheels and Constant Angular Momentum

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140660/1/1.g001680.pd

Single axis pointing for underactuated spacecraft with a residual angular momentum

The problem of aiming a generic body-fixed axis along an inertially fixed direction is dealt with for an underactuated spacecraft in the presence of a non-zero residual angular momentum, when only two reaction wheels can exchange angular momentum with the spacecraft platform. An analytical condition for the feasibility of the desired pointing is derived first, together with a closed-form solution for the corresponding attitude with zero platform angular rate. A nonlinear controller is then developed in the framework of singular perturbation theory, enforcing a two-timescale response to the system. Convergence to the desired attitude, when the pointing direction falls within admissible limits, is then proved for rest-to-rest maneuvers and randomly generated initial tumbling conditions for a configuration representative of a small-size satellite

UNSCENTED GUIDANCE FOR POINT-TO-POINT REACTION WHEEL MANEUVERS

Attitude control system failures are often mission ending even when the mission payload remains operational. In this dissertation, the concept of unscented guidance is applied to reorient a reaction wheel satellite in the absence of feedback from star trackers or an inertial measurement unit (IMU). It is shown that an open-loop maneuver, properly designed using optimal control theory, can be used to achieve terminal attitude errors that are comparable with closed-loop control in the presence of uncertainty in the satellite inertia tensor. Typically, coarse closed-loop control is used to achieve < 1 degree pointing accuracy before more accurate pointing is done using fine guidance sensors to close the loop for science acquisition. It is shown that reaction wheel maneuvers designed using unscented guidance can also achieve sub-degree pointing accuracy of the spacecraft, making control hand-off to a functioning fine pointing control mode possible. The approach presented here enables large angle attitude control to be recovered so that mission operations may be continued despite IMU or star tracker failures.DoD Space, Chantilly, VA 20151Civilian, Department of the NavyApproved for public release. Distribution is unlimited