Fault tolerant control allocation in systems with fixed magnitude discrete controls
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The promise and potential of controllers that can reconfigure themselves in the case of control effector failures and uncertainties, and yet guarantee stability and provide satisfactory performance, has led to fault tolerant control being an active area of research. This thesis addresses this issue with the design of two fault tolerant nonlinear Structured Adaptive Model Inversion control schemes for systems with fixed magnitude discrete controls. Both methods can be used for proportional as well as discrete controls. However, discrete controls constitute a different class of problems than proportional controls as they can take only binary values, unlike proportional controls which can take many values. Two nonlinear control laws based on Structured Adaptive Model Inversion are developed to tackle the problem of control failure in the presence of plant and operating environment uncertainties. For the case of redundant actuators, these control laws can provide a unique solution. Stability proofs for both methods are derived and are presented in this thesis. Fault Tolerant Structured Adaptive Model Inversion that has already been developed for proportional controls is extended here to discrete controls using pulse width modulation. A second approach developed in this thesis is Fault Tolerant Control Allocation. Discrete control allocation coupled with adaptive control has not been addressed in the literature to date, so Fault Tolerant Control Allocation for discrete controls is integrated with SAMI to produce a system which not only handles discrete control failures, but also accounts for uncertainties in the plant and in the operating environment. Fault tolerant performance of both controllers is evaluated with non real-time nonlinear simulation for a complete Mars entry trajectory tracking scenario, using various combinations of control effector failures. If a fault is detected in the control effectors, the fault tolerant control schemes reconfigure the controls and minimize the impact of control failures or damage on trajectory tracking. The controller tracks the desired trajectory from entry interface to parachute deployment, and has an adaptation mechanism that reduces tracking errors in the presence of uncertainties in environment properties such as atmospheric density, and in vehicle properties such as aerodynamic coefficients and inertia. Results presented in the thesis demonstrate that both control schemes are capable of tracking pre-defined trajectories in the presence of control failures, and uncertainties in system and operating environment parameters, but with different levels of control effort.
Marwaha, Monika (2008). Fault tolerant control allocation in systems with fixed magnitude discrete controls. Master's thesis, Texas A&M University. Available electronically from
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