Design of Attitude Controller for Alticube+
Design of LQR Controller for Alticube+ and Evaluation of the Pointing Performance Based on Reaction Wheel Jitter and Flexible Structure Interactions
J.Y. Shioda (TU Delft - Aerospace Engineering)
J. Guo – Mentor (TU Delft - Space Systems Egineering)
E. van Kampen – Mentor (TU Delft - Control & Simulation)
A. Cervone – Graduation committee member (TU Delft - Astrodynamics & Space Missions)
J. Bouwmeester – Graduation committee member (TU Delft - Space Systems Egineering)
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Abstract
Interest in oceanic climate change and to better understand the oceanic dynamics, a key interest in oceanic topography pushes for cheaper and smaller EO satellites which can achieve similar resolution and measurement accuracy. Alticube+ lies at the forefront of a new frontier, aggregated system of CubeSats connected by booms to fulfil the scientific objective of providing accurate ocean height measurements on par with large monolithic satellites. This configuration introduces strong coupling between attitude dynamics, flexible structural modes, and reaction wheel jitter, posing a challenge for attitude controller design.
This research aims to design a centralised LQR attitude controller that enables effective utilisation of reactions wheel to satisfy scientific motivated pointing control and knowledge requirements. The second objective is to research how reaction wheel jitter interacts with the large aggregated structure to degrade the pointing control, affecting the measurement accuracy. To achieve this, a comprehensive simulation framework was developed in MATLAB/Simulink, capable of modelling both rigid-body and flexible spacecraft dynamics within a unified environment. A centralised Linear Quadratic Regulator (LQR) combined with a Kalman filter was designed to address multi-axis attitude regulation and pointing knowledge requirements under realistic actuator and sensor constraints.
The flexible spacecraft model was formulated using Kane’s equations and a lumped-parameter representation of the dominant structural modes. Reaction wheel jitter was modelled via static imbalance effects, enabling realistic excitation of flexible modes. The simulation framework was verified through analytical comparisons for the rigid-body case and validated for the flexible model by comparison with a finite element model, including modal frequency alignment and structural response under controlled torque excitation.
Simulation results demonstrate that the centralised LQR controller achieves stable attitude convergence within the allocated \(450\,\mathrm{s}\) operation window for the majority of initial conditions. Monte Carlo analysis shows that approximately \(80\%\) of the simulated cases satisfy the absolute pointing error requirement of \(0.2\,\mathrm{deg}\), with performance primarily limited by reaction wheel saturation. Flexible dynamics introduce oscillations, particularly in the pitch axis, where reaction wheel jitter and inertia uncertainty result in pointing errors up to \(0.3\,\mathrm{deg}\) under worst-case conditions. Despite this, internal antenna misalignment and pointing knowledge errors remain within mission requirements for nominal operating conditions.
The results indicate that centralised LQR-based control remains a viable and effective solution for flexible assembled CubeSat systems such as Alticube+, provided that actuator saturation, structural flexibility, and uncertainty effects are explicitly accounted for during design. This work provides insight into the achievable pointing performance envelope of Alticube+'s architecture.