Global steering of single gimballed control moment gyroscopes using a directed search

Abstract

A guided depth-first search that manages null motion about torque-producing trajectories calculated with a singularity-robust inverse is proposed as a practical feedforward steering law that can globally avoid (or minimize the impact of) singular states in minimally redundant systems of single gimballed control moment gyroscopes. Cost and heuristic functions are defined to guide the search procedure in improving gimbal trajectories. On- orbit implementation of the steering law is proposed as an extension to momentum management algorithms. A set of simulation examples is presented, illustrating the search performance for a minimally redundant, pyramid- mounted array. Sensitivities o£ feedforward gimbal trajectories are examined in the presence of unmodeled disturbances, and techniques are proposed for avoiding excessive divergence. HE next generations of manned and unmanned space- craft will require enhanced control algorithms in order to efficiently achieve their proposed mission objectives. Reg- ularly coping with uncertainties in the orbital environment, dynamically changing spacecraft configurations (i.e., via docking and buildup), nonlinear actuator properties, and the need to tolerate potential hardware failures will mandate develop- ment of control strategies considerably beyond the available state-of-the-art. Because of the priority placed on minimizing cost, weight, and consumable requirements, future spacecraft will not always be able to rely on highly complex, multiply- redundant actuator systems (as is often now the case), but must employ more flexible and intelligent schemes that effi- ciently exploit all available onboard control capability. This study has examined methods of applying heuristic search techniques to perform adaptable inverse kinematics and ac- tuator management for spacecraft attitude control. In partic- ular, single gimballed control moment gyros (SGCMGs) were chosen as torque actuators. Although many existing al- gorithms may suffice for steering double gimballed CMG (DGCMG) arrays, such as envisioned for the NASA Space Station, no powerful techniques have been developed to man- age minimally redundant arrays of SGCMGs. These devices, however, are ideally suited as momentum-exchange effectors for a wide class of future spacecraft because of their large torque output and momentum capacity. They offer significant cost, power, weight, and reliability advantages over DGCMGs and could provide attitude control for a wide variety of future spacecraft, as torque requirements in many proposed mod- erate-sized vehicles may surpass the capability of available reaction wheels. Because of the complicated nonlinear map- ping between the input (gimbal) space and output (momen- tum) space, however, effective steering laws that reliably avoid problematic singular states have not been developed for min- imally redundant SGCMG systems, discouraging their appli- cation in many situations. To fully exploit the capability of SGCMGs, intelligent steering laws must be developed that address the system nonlinearities and avoid singular states over global trajectories.