Adaptive Fault Compensation And Identification For Satellite Attitude Control Systems

Actuator faults, such as loss of control failures and uncertain actuation signs, may cause deterioration of satellite attitude control accuracy, and even lead to system instability resulting in the loss of satellites. Actuator faults are uncertain in occurring time instants, patterns and values, which can bring significant parametric and structural uncertainties into systems and make the control design more difficult. To enhance reliability and safety for satellites, development of attitude control systems with fault compensation and identification capacities has received considerable attention and become an advanced research hotspot in aerospace engineering. Adaptive control, for its desired capability of dealing with system parametric and environmental uncertainties, has high potentials for effective fault compensation and identification of satellite attitude control systems. There are some important problems to be solved, such as fault identification that has self-stabilization ability and compensation of loss of control failures and actuation sign uncertainties.This thesis studies new adaptive compensation and identification approaches for dealing with actuator faults of satellite attitude control systems. Adaptive compensation schemes are developed to compensate system uncertainties caused by loss of control failures or actuation sign uncertainties, which can ensure desired system stability and asymptotic tracking properties. Based on the adaptive compensation scheme proposed for compensating loss of control failure, an adaptive identification scheme that has selfstabilization property is further developed to accurately identify failed actuators. This thesis studies the adaptive compensation and identification problems for different satellite attitude control dynamics, and establishes a systemic theoretical framework, which are presented as follows:(1) Two adaptive failure compensation schemes are developed for rigid satellites with known and unknown inertia parameters, respectively, to compensate loss of control failures. System uncertainties, especially control gain uncertainties, caused by failures and inertia parameters are parameterized. When inertia parameters are known, a new positive definite control gain matrix is constructed, which can be absorbed by an appropriate choosing of Lyapunov function. For the case of known inertia parameters, a positive definite control gain matrix is constructed, which helps to form an appropriate Lyapunov function. For the case of unknown inertia parameters, a control gain matrix with positive leading principal minors is constructed. An SDU factorization is used to parameterize such a control gain matrix, which ensures the nonsingularity of the gain matrix estimate. Based on such new control gain matrix formulation and parametrization, system uncertainties can be appropriately estimated, and two adaptive failure compensation schemes are developed which ensure desired system performance, despite the presence of uncertain actuator failures.(2) An adaptive failure compensation scheme is developed for flexible satellites with known and unknown inertia parameters to compensate loss of control failures. The uncertainties of dynamics, flexibility, failures and control gain matrix are parameterized. Then, an adaptive failure compensation scheme is developed to ensure system stability and asymptotic tracking properties, in which, an approximation function for sign function is employed to avoid system chattering caused by the discontinuousness of control signal.(3) An adaptive failure identification scheme, which has self-stabilization ability for ensuring stable identification, is developed for microsatellites with uncertain loss of control failures and unknown inertia parameters. An adaptive failure compensation scheme is first developed that ensures system stability and asymptotic tracking properties. Then, multiple adaptive estimators are designed for all possible failure patterns, each of which is designed for one pattern, and multiple cost functions are also calculated by using the estimation errors. The failure identification is implemented by comparing such cost functions and identifying the failure pattern corresponding to the minimum one. Finally, a special reference motion is specified by using a signal independence condition, for ensuring desired failure identification. When some actuators have failed and both identification and control are applied, the controller, while stabilizing the system, makes the microsatellite engage in the special motion so that the minimum cost function matches the actual failure pattern, and the failed actuators are thus identified.(4) An adaptive compensation scheme is developed for rigid satellites with unknown inertia parameters to compensate actuation sign uncertainties. For each possible actuation sign pattern, an adaptive estimator is designed, and a control signal is generated to guarantee that the estimator states asymptotically track the reference signal when the satellite operates under the failure pattern and this control signal. Then, multiple cost functions are formed by using all possible estimation errors, and the control signal corresponding to the minimum cost function is selected as the actual control signal. The desired system performance is ensured by a new error transformation technique, involving the system tracking error, estimator tracking error and estimation error.The developed adaptive fault compensation and identification schemes have been applied to some specific satellite models. Simulation results show that the proposed adaptive fault compensation schemes can ensure desired system stability and asymptotic tracking properties despite the presence of loss of control failures or uncertain actuation signs, and the proposed failure identification scheme has selfstabilization property and can accurately find out the failed actuators that are loss of control.