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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 . |
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