Robust fault-tolerant control for flexible spacecraft against partial actuator failures
Ran Zhang
Jianzhong Qiao
Tao Li
Lei Guo
In this paper, we present a robust faulttolerant control scheme to achieve attitude control of flexible spacecraft with disturbances and actuator failures. It is shown that the control algorithms are not only attenuate exogenous bounded disturbances with attenuation level, but also able to tolerate partial loss of actuator effectiveness. The proposed controller design is simple and can guarantee the faulty closed-loop system to be quadratically stable with a prescribed upper bound of the cost function. The design algorithms are obtained by combining free weighting matrices method with linear matrix inequality technique. The effectiveness of the proposed design method is demonstrated in a spacecraft attitude control system subject to loss of actuator effectiveness. High precision attitude control has been a difficult and important problem for flexible spacecraft in commu-
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nication, navigation, remote sensing, and other
spacerelated missions. It is because modern spacecraft often
employ large, deployed and light damping structures
(such as solar paddles and antenna reflectors) to provide
sufficient power supply and reduce launch costs [16].
During the control of the rigid body attitude, actuators
play an important role of linking control commands to
physical actions [7,8]. Normally, the actuators should
execute commands demanded by the controller
faithfully and completely. In this condition, the actuators
need to be 100 % effective. However, when a fault
occurs in the actuator, the handicapped actuator may
not complete the control command fully. Naturally, the
control channel effectiveness (or lack of it) becomes an
appropriate measure of the severity of the actuator fault
[9]. In an spacecraft, actuator faults may cause
discrepancies between the desired and the actual movements
of these control surfaces due to incorrect supply
pressure in the hydraulic lines, change in hydraulic
compliance, and line leakage [10]. Any of these problems
can prevent the primary control surfaces such as
elevators, ailerons, or rudder from moving to the
positions demanded by the controller [9]. On the other
hand, the complex space structure may lead to the
decreased rigidity and low-frequency elastic modes.
However, elastic vibration of the flexible appendages
may cause degradation of the performance of attitude
control [7,11]. Thus, the desired control scheme should
tolerate partial loss of actuator effectiveness and be
robust enough to overcome various disturbances from
structural vibrations of the flexible appendages.
Due to the increasing demands for high
reliability and survivability of the complex control systems,
the fault-tolerant control (FTC) has attracted extensive
interests and attention [1221]. FTC can be divided
into passive FTC [12,13] and active ones [14,15]. An
active FTC uses the diagnosis results provided by the
fault detection and diagnosis to actively adjust the
control efforts, thus is potentially capable of dealing with
a larger number of faults [14,15]. Compared with the
active FTC, the passive one has the advantage of not
requiring the exact actuator fault information; thus, it is
simple to implement. The passive FTC can also ensure
system stability and desired performance after the
actuator fault occurs and before the fault detection and
diagnosis phase finishes [12,13].
Motivated by the preceding discussion, in this paper,
a passive FTC scheme for flexible spacecraft with
disturbances and partial loss of actuator effectiveness is
studied. First, the partial loss of actuator effectiveness
problem is transformed into uncertain parameters
problem. Second, the fault tolerant control is designed by
combining H control technique and robust control
method. The proposed control algorithms are not only
attenuate disturbances from structural vibrations of the
flexible appendages with H attenuation level, but also
able to robust to partial loss of actuator effectiveness.
Meanwhile, the resultant FT controller may be simply
designed and can guarantee the faulty closed-loop
system to be quadratically stable with a prescribed upper
bound of the cost function. Finally, a numerical
example is shown to demonstrate the good performance of
our method.
The rest of this paper is organized as follows.
The single-axis model of flexible spacecraft model and
partial loss of actuator effectiveness are described in
Sect. 2. The passive FT controller is designed and
analyzed in Sect. 3. Numerical simulations on different
control effectiveness factor situations are presented in
Sect. 4 to demonstrate the performance of the
proposed control method. Finally, we conclude the paper
in Sect. 5.
Notation: Throughout this paper, Rn denotes the
ndimensional Euclidean space; the space of
squareintegrable vector functions over [0, ) is denoted by
l2[0, ); the superscripts and 1 stand for
matrix transposition and matrix inverse, respectively;
P > ( 0) means that P is real symmetric and posit (...truncated)