Doctoral Degrees (Electrical and Electronic Engineering)

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by Subject "Aerodynamic measurements"

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    Estimation of external disturbance torques on CubeSats to enhance full state attitude estimation accuracy and mission success
    (Stellenbosch : Stellenbosch University, 2023-03) Habila, Asia Saeed Kajo; Steyn, Herman; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: CubeSats are becoming increasingly popular, and the number of launches is growing rapidly. This is due to their low manufacturing costs and ease of launch. CubeSats were initially utilized for educational purposes; however, over time, they began to be used for more complex missions, such as astronomy and remote sensing applications. For these complex missions to be successful, precise control is required, which is the role of the attitude determination and control subsystem (ADCS). ADCS, on the other hand, has challenges achieving precise control due to the CubeSat's small moment of inertia, which makes it susceptible to disturbance torques. This thesis focuses on the accurate estimation and development of models for disturbance torques that considerably improve the full-state estimate accuracy and, as a result, the CubeSats' control and, eventually, preserve the mission. As measuring the disturbance torque that affects an object is as simple as measuring its angular acceleration, the gyroscope is chosen as the main attitude sensor in this thesis to estimate attitude disturbance torques. However, the gyroscope was found to have shortcomings due to noise and errors that affect its output and performance, resulting in an inaccuracy of measurement. As a consequence, two noise drift estimation algorithms are developed, tested, and evaluated in order to determine which is best to use to boost the accuracy of disturbance torque estimation. The general models for attitude disturbance torques, along with the estimators, are developed, tested, and evaluated. The outcomes from the models are used to assist in the eventual development of the estimators. For the estimators, deterministic and nonlinear recursive estimation methods are developed, tested, and evaluated through simulations in order to determine which are best to be used in the thesis to estimate the attitude disturbance torques. The deterministic algorithms that are utilized are QUEST and Linear Least Square, while the recursive ones are Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF). In addition, a suggested method (algorithm) for estimating attitude disturbance is proposed, followed by simulations for assessment. Furthermore, an identification technique is proposed for determining which attitude disturbance torques are acting on the satellite body, and their magnitude is also identified and evaluated through simulations. A design and practical implementation of a disturbance torque estimator on the ground is developed. An air bearing platform is used to represent the satellite in terms of potential space disturbances and to evaluate the effectiveness of the disturbance torque estimator. Firstly, the mass properties of the air bearing are obtained using two different methods to ensure the air bearing system dynamics is correct, and then the estimator is developed and evaluated. The EKF was found to be the most suitable choice that can be utilized in this research to estimate the disturbance torque. It gives a more accurate estimate compared to QUEST and Weighted Least Square (WLS), is more precise, and requires less execution time compared to UKF. Accordingly, a bank of EKFs is employed to independently estimate each of the disturbance torques and used in combination with the identification function to accurately estimate the total disturbance torque in orbit.