Combined attitude control and energy storage for small satellites using variable speed control moment gyroscopes.

This work presents the first known energy storage and attitude control subsystem (ESACS) for small satellites, proving this technology to be viable, applicable to more complex, demanding space missions, and laden with substantial benefits, such as agile slewing, robust singularity avoidance, increased lifetime, mass savings, and favourable peak power density. In capturing the key features of this novel system, it investigates the design sizing, feasibility, mission utility, experimentation, and performance benefits for using variable-speed control moment gyroscopes (VSCMGs) to store and drain energy while controlling satellite orientation. First, a novel optimal ESACS sizing algorithm is developed for a practical, miniature spotlight synthetic aperture RADAR (SAR) space mission. When given a set of small satellite agility and energy storage requirements, the design is cast as a constrained nonlinear programming problem using a performance index constructed from subsystem design margins including the attitude torque, peak power, energy capacity, and subsystem mass margins and solved using a reduced-order, gradient-based solver software code. The resulting method permits an efficient, structured approach to designing an optimally sized ESACS while enabling comparison of new technology performance to an existing system in order to identify the advantages and disadvantages of such new technology. The process shown generates point designs which are then compared via a design scoring process. Then, realistic usable energy capacity is studied, yielding a more practical system capable of meeting the desired requirements albeit with reduced mass savings benefits from theoretical levels. This factor, although presented in the early 1970s, is often overlooked in the literature. Next, a novel ESACS gimbal steering law is derived to permit independent gimbal and wheel control of VSCMGs with continued singularity avoidance, a situation that allows direct incorporation of an ESACS into an existing small satellite energy storage (ES) subsystem. This law rejects the disturbances generated during independent ES wheel control which can be significant if the power is stored and drained rapidly, demanding high wheel deceleration/acceleration. Meanwhile, the separation of control renders simultaneous control law singularity avoidance through coordinated wheel torquing and gimballing impossible, thus a conventional CMC gimbal singularity avoidance steering algorithm was also added to this new law. As it permits directly interfacing this small satellite ESACS into a conventional satellite, this novel, composite gimbal steering law is more immediately practical than pre-existing simultaneous steering laws. Finally, a low-cost prototype using current off-the-shelf technology was effectively employed in the first known hardware-in-the-loop, three-axis, experimental demonstration of an ESACS. This is also the first known demonstration of VSCMGs for a small spacecraft scale ESACS. Key test results taken at moderate, less efficient wheel speeds show that a small ESACS can yield the depth-of-discharge and round-trip energy efficiencies near anticipated theoretical values while imparting attitude change, but has limitations in energy density due to the use of COTS motors. It is also shown that a resistive load can be powered from the flywheel system for a set time period (depending upon the resistive load), but drains energy much faster than a magnetically-levitated system would due to motor bearing friction. Nevertheless, these developments open the door for further practical advancement of these concepts and future employment on a small satellite.