Timothy Sands

Nonredundant Single-Gimbaled Control Moment Gyroscopes

Abstract : Two objectives dominate consideration of control moment gyroscopes for spacecraft maneuvers: high torque (equivalently momentum) and singularity-free operations. This paper adds to the significant body of research toward these two goals using a minimal three-control-moment-gyroscope array to provide significant singularity-free momentum performance increase spherically (in all directions) by modification of control-moment-gyroscope skew angles, compared with the ubiquitous pyramid geometry skewed at 54.73 deg. Spherical 1H (one control moment gyroscope s worth momentum) singularity-free momentum is established with bidirectional 1H and 2H in the third direction in a baseline configuration. Next, momentum space reshaping is shown via mixed skew angles permitting orientation of maximum singularity-free angular momentum into the desired direction of maneuver (yaw in this study). Finally, a decoupled gimbal angle calculation technique is shown to avoid loss of attitude control associated with singular matrix inversion. This technique permits 3H (maximal) yaw maneuvers without loss of attitude control despite passing through singularity. These claims are demonstrated analytically, then heuristically, and finally validated experimentally.

Acquisition, tracking, and pointing technology development for bifocal relay mirror spacecraft

The purpose of the research is to develop acquisition, tracking, and pointing technologies for the Bifocal Relay Mirror Spacecraft and verify these technologies with the experimental test-bed. Because of the stringent accuracy requirement of the laser beam and the agile maneuverability requirement, significant research is needed to develop acquisition, tracking, and pointing technologies for the Bifocal Relay Mirror Spacecraft. In this paper, development of the Bifocal Relay Mirror Spacecraft experimental test-bed is presented in detail. The current operational results are also presented including precision attitude control of the spacecraft for fine tracking and pointing.

2H Singularity-Free Momentum Generation with Non-Redundant Single Gimbaled Control Moment Gyroscopes

Two objectives dominate consideration of control moment gyroscopes (CMGs) for spacecraft maneuvers: high torque (equivalently momentum) and singularity-free operations. This paper adds to the significant body of research towards these two goals utilizing a minimal 3-CMG array to provide 646% singularity-free momentum performance increase spherically, compared to the ubiquitous pyramid arrangement skewed at 54.73deg. Spherical 1H (1 CMGs-worth momentum) singularity free momentum is established with birectional 1H and 2H in the third direction in a baseline configuration. Lastly, momentum space reshaping is shown via mixed skew angles. These claims are demonstrated analytically, then heuristically, and finally validated experimentally

Control moment gyroscope singularity reduction via decoupled control

Two objectives dominate consideration of control moment gyroscopes (CMGs) for spacecraft maneuvers: High torque (or equivalently momentum) and singularity-free operations. Utilizing a 3/4 CMG skewed-pyramid in the optimal singularity-free configuration, this paper develops a decoupled control strategy to reduce the remaining singular conditions. Analysis is provided to justify the argument with promising simulations followed by experimental verification using a free-floating satellite simulator.

Nonlinear-Adaptive Mathematical System Identification

By reversing paradigms that normally utilize mathematical models as the basis for nonlinear adaptive controllers, this article describes using the controller to serve as a novel computational approach for mathematical system identification. System identification usually begins with the dynamics, and then seeks to parameterize the mathematical model in an optimization relationship that produces estimates of the parameters that minimize a designated cost function. The proposed methodology uses a DC motor with a minimum-phase mathematical model controlled by a self-tuning regulator without model pole cancelation. The normal system identification process is briefly articulated by parameterizing the system for least squares estimation that includes an allowance for exponential forgetting to deal with time-varying plants. Next, towards the proposed approach, the Diophantine equation is derived for an indirect self-tuner where feedforward and feedback controls are both parameterized in terms of the motor’s math model. As the controller seeks to nullify tracking errors, the assumed plant parameters are adapted and quickly converge on the correct parameters of the motor’s math model. Next, a more challenging non-minimum phase system is investigated, and the earlier implemented technique is modified utilizing a direct self-tuner with an increased pole excess. The nominal method experiences control chattering (an undesirable characteristic that could potentially damage the motor during testing), while the increased pole excess eliminates the control chattering, yet maintains effective mathematical system identification. This novel approach permits algorithms normally used for control to instead be used effectively for mathematical system identification.

Simulation of spacecraft damage tolerance and adaptive controls

The nature of adaptive controls, or controls for unpredictable systems, lends itself naturally to the concept of damage tolerant controls in high performing systems, such as aircraft and spacecraft. Recent technical demonstrations of damage tolerant aircraft prove the concept of adaptive controls in an operational environment. Research covered by this paper expands on the topic by discussing the application of adaptive controls to spacecraft and theory behind simulating damage tolerant control implementation. Simulation is then used to demonstrate the stability of adaptive controls when experiencing sudden mass loss and rapid changes in inertia.

Physics-Based Automated Control of Spacecraft

Spacecraft control suffers from inter-axis coupling regardless of control methodology due to the physics that dominate their motion. Feedback control is used to robustly reject disturbances, but is complicated by this coupling. As pointing requirements have become more stringent to accomplish military missions in space, decoupling dynamic disturbance torques is an attractive solution provided by the physics-based control design methodology. Promising approaches include elimination of virtual-zero references, manipulated input decoupling, sensor replacement and disturbance input decoupling. This paper compares the performance of the physics-based control to control methods found in the literature typically including cascaded control topology and neglecting factors such as back-emf.

Improved Hamiltonian Adaptive Control of spacecraft

Spacecraft control is complicated by on-orbit inertia uncertainties. Considerable initial, on-orbit check-out time is required for identification of accurate system models enabling fine pointing. Smart, plug-n-play control algorithms should formulate smart control signals regardless of inertia. Adaptive control techniques provide such promise. Spacecraft control has been proposed to be adapted in the inertial frame based on estimated inertia to minimize tracking error. Due to unwieldy computations, later researchers suggested adapting the control in the body frame. This paper derives this later suggested approach using the recommended 9-parameter regression model for 3-axis spacecraft rotational maneuvers. Additionally, a new 6-parameter regression model is shown to be equivalent. A new, further-reduced 3-parameter regression model is demonstrated to yield similar performance. A new improved, simplified adaptive feedforward technique is developed and shown to provide superior performance. Following promising simulations, experimental verification is performed on a free-floating three-axis spacecraft simulator actuated by non-redundant, single-gimbaled control moment gyroscopes.