RBOT [RWA Bias Optimization Tool (wherein "RWA" signifies "Reaction Wheel Assembly")] is a computer program designed for computing angular momentum biases for reaction wheels used for providing spacecraft pointing in various directions as required for scientific observations. RBOT is currently deployed to support the Cassini mission to prevent operation of reaction wheels at unsafely high speeds while minimizing time in undesirable low-speed range, where elasto-hydrodynamic lubrication films in bearings become ineffective, leading to premature bearing failure. The problem is formulated as a constrained optimization problem in which maximum wheel speed limit is a hard constraint and a cost functional that increases as speed decreases below a low-speed threshold.
The optimization problem is solved using a parametric search routine known as the Nelder-Mead simplex algorithm. To increase computational efficiency for extended operation involving large quantity of data, the algorithm is designed to (1) use large time increments during intervals when spacecraft attitudes or rates of rotation are nearly stationary, (2) use sinusoidal-approximation sampling to model repeated long periods of Earth-point rolling maneuvers to reduce computational loads, and (3) utilize an efficient equation to obtain wheel-rate profiles as functions of initial wheel biases based on conservation of angular momentum (in an inertial frame) using pre-computed terms.
This work was done by Clifford Lee and Allan Lee of Caltech for NASA's Jet Propulsion Laboratory. NPO-42011
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Optimization of Angular-Momentum Biases of Reaction Wheels
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Overview
The document discusses the optimization of angular-momentum biases of reaction wheels used in the Cassini/Huygens spacecraft, which was launched on October 15, 1997, to explore Saturn, its rings, and its moons. Reaction wheels are crucial for precise and stable attitude control of the spacecraft, enabling it to point its science instruments accurately. The performance of these wheels is vital, as degradation can significantly impact the quality of scientific data collected. Loss of more than one reaction wheel could severely compromise the mission's scientific return.
The document outlines the challenges faced by the Cassini attitude control team, particularly the need to minimize the time the reaction wheels operate at low spin rates (±300 rpm). Operating at low speeds can lead to premature failure of the bearings due to insufficient lubrication, which is critical for protecting the mechanical components. The team recognized the importance of optimizing the reaction wheel biasing rates to ensure the long-term health of the wheels and to avoid operational issues, such as unexpected drag torque spikes that had previously occurred.
To address these challenges, the Cassini Attitude and Articulation Control (AACS) team formulated a constrained optimization problem. The goal was to minimize the time the reaction wheels spent in the low-rpm region while satisfying various operational constraints. The optimization process involved using the Nelder-Mead simplex algorithm, a well-established method for finding optimal solutions. The team implemented several computational enhancements to improve efficiency, including detecting periods of low activity in the spacecraft to perform calculations and using sinusoidal approximation sampling for modeling maneuvers.
The document highlights the development of the Reaction Wheel Bias Optimization Tool (RBOT), which was created to assist the attitude control team in selecting optimal biasing rates. This tool has been routinely used since 2002, allowing for systematic management of the reaction wheels and contributing to mission success.
Overall, the document emphasizes the innovative approaches taken by the Cassini team to ensure the effective operation of reaction wheels, thereby safeguarding the mission's scientific objectives and enhancing the longevity of the spacecraft's systems.
