Application Research of MRAC in Fault-Tolerant Flight Controller
1. Introduction
High safety is the primary condition of aircraft system operating. Evaluating the safety of the flight control system
is the most important part of achieving safe flight. When the unforeseeable dynamic characteristics changes, such as
structure damage of the aircraft occurring, control surface failure, or the position of center of gravity shifting, the
conventional controller cannot adjust control parameters according to the control error, being likely to lead to
disastrous consequences. The fault-tolerance flight control system can reconstruct the control law according to
control deviation, and still maintain the satisfying performance, at least possess the ability of safety returning, which
will greatly improve the safety level of aircraft system [1-2].
Fault-tolerant control can be divided into passive fault-tolerant control and active fault-tolerant control according
to its realization methods. Passive fault-tolerant control is a kind of high robustness scheme, using fixed controller to
ensure the low sensitivity to system fault. The attention of constructing active fault-tolerant control is to redesign a
new controller according to expected control characteristics after fault taking place, in order to stabilize the system
newly. It is clear that the latter design has more developing place and application potential.
Based on whether depending on fault information provided by Failure Detection and Identification (FDI) system,
active fault-tolerant control is divided as direct reconfiguration control and indirect reconfiguration control [3].
Because of the accurate fault information provided by FDI system, it is reasonable to require the reconfiguration
to complete faster [4], and the flying qualities recover better. Some reconfiguration method could rapidly switch
corresponding control law designed beforehand offline when fault occurs.
On 29 April 2013, a Boeing 747-400 which operated by National Airlines between the British military base Camp
Bastion in Afghanistan and Al Maktoum Airport in Dubai, crashed moments after taking off from Bagram, killing all
seven people on board. On 2 June 2013, investigators from the Ministry of Transport and Civil Aviation of
Afghanistan confirmed the load shift hypothesis; three armored vehicles and two mine-sweeping vehicles came loose
and rolled backwards onto the rear bulkhead, damaging the aircraft and pushing the center of gravity outside its rear
limit. Consequently, the aircraft became uncontrollable, pitched up sharply and stalled, and crashed moments later.
In this failure condition, the direct reconstruction control would not work if it was chosen; but the indirect
reconstruction control which doesn't rely on FDI has the potential of taking the control, thereforeˈ it is the
faulttolerant control's developing direction in the future.
Model Reference Adaptive Control (MRAC) is kind of control method which follows a desired response from a
reference model. It has the advantages of simple structure, reconfiguration fast and stable. The general idea behind
Model Reference Adaptive Control (MRAC) is to create a closed loop controller with parameters that can be updated
to change the response of the system. The control parameters are updated based on this error [6]. The goal for the
parameters is to converge to ideal values that cause the plant response to match the response of the reference model.
A state feedback multivariable MRAC scheme is developed for such a linear model, with adaptive compensation
of the uncertain dynamics offset as well as system parametric uncertainties in [7], and NASA GTM were used in
simulation study and the result was verified. Multivariable MRAC is also used in unsymmetrical.
In the control system design, linear design method based on linear time-invariant systems is more mature.
However, the stability, security and the performance of design results will have some uncertainty because of the
nonlinear characteristic of the controlled object and changes in flight environment.
To solve the problems above, this paper presents a fault-tolerant flight controller containing adaptive control
modules derived theoretically. For a certain type of aircraft, longitudinal overload fault-tolerant flight controller is
designed to evaluate the effectiveness of major failure reconfiguration when a serious backward shifting of the center
of gravity cannot be monitored by FDI. The reasonableness of results has also been assessed.
