Design and Analysis of a Practical Heliogyro Blade Control System for Deployment and Flight

The heliogyro solar sail consists of high aspect ratio blades rotating about the central hub of the spacecraft. This sail concept requires less support structure than a traditional solar sail since the centrifugal forces help to deploy and maintain a rigid blade shape. The heliogyro maneuvers by actuating the blades at their root with sinusoidal pitch profiles, producing spin-averaged forces and moments. The lightweight blade material has little inherent damping, requiring active control to attenuate blade vibration caused by maneuvering. However, due to the small root pitch control torques required, on the order of 2 µNm, compared to the large friction torques associated with a root pitch actuator, a major impediment to advancing the Technology Readiness Level of the heliogyro is designing a root control system that takes friction into account and can still add damping to the blade. Therefore, implementation of a physically realizable root pitch control system for a heliogyro blade that is robust to modeling errors in the blade dynamics, specifically the amount of inherent blade damping at higher frequency modes, is critical to advancing the heliogyro towards a flight demonstration mission and is the purpose of this research. The three main research questions focus on making contributions to the heliogyro control design, understanding of the uncertainties in the blade dynamics and a scalable and physically realizable testbed for the controller. The approach begins with classical and impedance control techniques to develop a physically realizable root sensor/actuator/control system, suited for space, that can provide the small root pitch torques as well as add damping to the blade’s structural modes. The root pitch controller ameliorates the friction torques in the root actuator with a high gain inner feedback control loop and adds damping to the larger magnitude low frequency torsion modes with an additional outer feedback loop of the differential twist rate at a proximal point on the blade. The robustness of this controller, called the proximal blade twist feedback control (PBTFC) design, depends on the inherent material damping of the blade. In other words, the non-collocated nature of the control design requires some nonzero level of inherent damping at the high frequency modes to actively damp the larger magnitude low frequency modes, making it critical to obtain a confident lower bound on the blade’s torsional damping at high frequencies. Therefore, the next approach uses modal testing on small-scale heliogyro blades hanging in a high-vacuum chamber, the closest analog possible for centrifugally loaded blades in space, to characterize the inherent damping. Improvements to the measurements and a new modal damping algorithm, dubbed the incremental and reduced frequency response function method, allow separate flap and twist damping ratios to be calculated for the first three modes of these lightly damped blades to within ±2% error. The unprecedented precision of these results debunks the assumption of a linear viscous torsional blade damping model and allows for the formulation of a new modal damping model based on the experimentally determined values. By redesigning the PBTF controller around the updated single blade heliogyro model, the required low frequency damping ratio (ζ>0.1582) and settling time (τ<720 seconds) necessary for a heliogyro technology demonstration mission are achieved. Further uncertainty in the blade dynamics will be addressed with adaptive system identification in the control loop. Lastly, validation of these models and the resulting simulated control performance will be sought using experimental testbeds at CU and NASA facilities. This research provides a comprehensive study of the performance bounds and tradeoffs of physically realizable heliogyro blade root pitch controllers, thereby increasing the heliogyro’s Technology Readiness Level and guiding future mission designers on their selection of blade control.