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Aeroelastic flutter non linear control
Professor Mario Di Bernardo
Faculty of engineering, Naples
Masters degree in control engineering
Giovanni Pugliese Carratelli M58/30
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"Heavier than air flying machines are impossible"
Lord Kelvin
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Summary
The purpose of this document is to investigate some non linear control strategies for a 2 degree of freedom
(DOF) wing section subject to aero elastic flutter. In the beginning of the document it will be shown what aero
elastic flutter is with some examples. A mathematical model is then shown, the BACT model, with system some
analysis. After the system analysis results, two control strategies are developed and results and simulations are
shown.
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Contents
Summary 5
1 Model and system analysis 9
1.1 What is aereoelastic fluttering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.2 The NASA BACT model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.1 BACT MODEL with two control surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3 State space representation and system open loop analysis . . . . . . . . . . . . . . . . . . . . . . 16
1.3.1 State space representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.2 Open loop analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 Input Output Feedback linearization 25
2.1 Pitch FBL control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.1.2 Hidden dynamics analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.1.2.1 Stability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.2 Plunge FBL control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.2.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.2.2 Hidden dynamics analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2.2.1 Lyapunov stability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.2.2.2 Bifurcation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3 MRAC Minimum Controller Synthesis control for flutter 43
3.1 MCS algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1 MCS extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.1.1.1 MCS-LQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.1.1.2 MCSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.1.1.3 EMCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.1.1.4 NEMCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.1.1.5 LQ-NEMCSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2 MCS control synthesis and simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2.0.6 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.0.7 MCS controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.2.0.8 MCSI controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.0.9 EMCSI controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.0.10 NEMCSI controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.0.11 LQ-NEMCSI controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.2.0.12 Gain-locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.2.0.13 Velocity variation rejection and comparison . . . . . . . . . . . . . . . . . . . . . 63
3.2.0.14 Hybrid parameter variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
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3.2.0.15 Chaos recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Bibliography 72
List of figures 73
List of tables 79
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Chapter 1
Model and system analysis
It is possible to fly without motors, but not without knowledge and skill. This I conceive to be fortunate, for man, by
reason of his greater intellect, can more reasonably hope to equal birds in knowledge than to equal nature in perfection
of her machinery.
Wilbur Wright, Letter to the Western society of engineers, 1900
1.1 What is aereoelastic fluttering
Aeroelasticity is the interaction of structural, inertial and aerodynamic forces. It occurs to systems subject to an
airstream (or more generally in a fluid stream), for example to airfoils or even bigger structures such as bridges
or buildings. Aeroelasticity is under certain conditions characterised by what is called flutter. Aeroelastic flutter
is an oscillatory aeroelastic instability characterized by the loss of elastic recall and low damping due to the
presence of aerodynamic loads. In the aerospace industry this is a very well known problem as it happens to
occur to airplanes wings.
The conditions under which flutter can be observed are various and depend mainly on: the speed at which the
structure is moving in the fluid, the elastic recall to which the foil is subject to (as the foil is a structure it has
an elastic behaviour) and the angle between the fluid and the foil (also known as angle of attackAoA). Indeed,in the case of an airplane, wing structural deformation leads to higher aerodynamic forces making flutter a
self-feeding mechanism that may lead to catastrophe, moving so from an equilibrium point to a Limit Cycle
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Model and system analysis
Oscillation (LCO) or to chaos. If the damping is not adequate, the imbalance between energy input and the
structural dissipation will result in very large oscillations or unconstrained increase of amplitude.
A simple example can show qualitatively what happens: consider first a foil in a steady airstream. Such foil in
a fluid stream is subject to lift, air resistanceand a pitchmoment as can be seen in Fig.1.1.
Figure 1.1: Lift (L), resistance (R) and pitch moment (P) of a foil in a steady airstream
Lift, resistance and pitch moment depend, among other factors, on the square of the airstream velocity, the
exposed surface to the airstream and the AoA, .
L= Cl()12
U2CxA
R= Cr()12
U2CxA
P=Cm()12
U2CxA
FunctionsCl,Crand Cmare non linear functions ofand are different for every foil. Some typical behaviours
are depicted in Fig.1.2.
Figure 1.2: Typical Cl,Cm and Cr behaviour
A simple model to show how LCO occur on such system is to consider a 2nd order torsional ODE as follows:
Consider now a foil, that is immerged in an airstream and is subject also to an elastic recall K, for example a
wing section like as Fig.1.3.
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1.1. What is aereoelastic fluttering
Figure 1.3: Foil subject to elastic recall K
A simple model to show how LCO occur on such system is to consider a 2nd order torsional ODE as follows:
I+c+K() = M(,) (1.1)
M(,) is the pitch moment, c( ) is the damping term and K() is the torsional elastic recall. Assuming
that M is only function of, possible for low velocities, the model can be recast to the following form:
I+c+ (K()M()) = 0 (1.2)
When hit by an airstream at a critical speed U0 the foil will be subject to a pitch moment increase, that will
lead to higher values of the AoA. Pitch angle will increase ifK()M()0, the elastic
recall will be greater than pitching moment and bring the foil towards = 0. As the system damping is low,
which is typical for structures, overshooting from = 0 will occur. This will result in a negative AoA and an
increasing pitch moment in opposite direction as shown in Fig.1.4
Figure 1.4: Foil subject to elastic recall K in opposite direction after overshoot
After overshooting, very much as what happens for positive AoA, the structures recall is lower than pitching
moment and so there will be a negative increase of AoA. Since the airstream is steady, pitching moment will grow
until it is greater than the elastic recall for negative values of, increasing the AoA until K()M()> 0.
The major elastic recall over the pith moment will bring the system towards = 0, and an overshoot will occur
again for positive values of the AoA. A a self feeding mechanism will so begin, this is called flutter.
This example shows how flutter is in this simple dynamical system a LCO. More complex models are of course
possible with more than one degree of freedom where chaotic behaviour is possible.
1.1.1 Examples
Flutter is not only limited to the aerospace industry, it can indeed occur to other structures in a fluid, as for
instance chimneys and bridges, or even simply sign posts. Although the focus of this report is towards airplane
(or gliders) wing active fluttering control, in the past many other ways to avoid the phenomena have been
successfully applied in aeronautics (i.e. passive or structural fluttering control). The main strategy along this
line is to dimension the structure in such a way that the energy introduced in the system is well damped in
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Model and system analysis
normal use (mass distribution is the main parameter on which to work).
Some examples of what flutter can lead to are reported in the following. As for the aerospace industry an
example of wing flutter was shown in a test flight by NASA in 1966. The test flight pilots brings the Piper
PA-38 to speed that causes flutter and slows down just before any structural failure.
Figure 1.5: The tail of the Piper during an LCO
Some accidents have happened due to fluttering, the first example is the Tackoma bridge in 1940. The bridge
was so that it would oscillate at its natural mode when subject to wind at approximately 67mph. When this
occurred the bridge structure failed as shown by these impressive images.
In recent times some studies have qualitatively related how flutter and dry Coulomb friction are a close
phenomena as shown in [2]. In this paper a simple 2 Degree of Freedom (DoF) mechanical arm is built with
two elastic hinges on which a load is applicable. The arm is shown in Fig., and known as Ziegler Column.
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1.1. What is aereoelastic fluttering
Figure 1.6: The Ziegler Column 2 DoF arm
This is a two-degree-of-freedom structure made up of two rigid bars, internally jointed with an elasticrotational spring, externally fixed at one end through another elastic rotational spring, while at the other end
subject to a tangential follower load Pcoaxial to the rod to which it is applied as schematically show in 1.7.
One side of the arm is fixed to a and on the other a wheel is mounted so to create a follower force P with a
movable metal plate that allows friction between the wheel and the plate.
Figure 1.7: Schematic representation of the 2 DoF mechanical arm. vp is the plate velocity
It is shown that the structure becomes dynamically unstable at a certain load level , so that it evidences
flutter and, at higher load, divergence instability. In Fig.1.8it is quite clear how the system moves towards a
LCO both from the model predictions and the experimental data.
Figure 1.8: Experimental data and model prediction for the Ziegler Column
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Model and system analysis
1.2 The NASA BACT model
In the past many approaches have been used to model and actively control flutter. The major modeling results
are by Theodorsen who developed the classical unsteady aerodynamic theory which accounts for the aerodynamic
damping at different conditions, and showed how flutter depends on it. After Theodorsen, Mukhopadhyay and
Gangsaas created 20th and 50th order models,and strategies for order reduction. They used state feedback asthe control method, and implemented estimators to describe unmeasured states. They employed proportional
gain feedback methods developed from root locus plots. A quasi-steady aerodynamic model coupled with a two
degree-of-freedom structural model was used to develop several types of feedback. The development directly
feeds one of four variables to the control surface through a proportional gain.
Aeroelastic systems typically contain nonlinearities which are either neglected or simplified to a linear form
for analysis. Nonlinearities which occur in aeroelastic systems include control saturation, free play, hysteresis,
piece-wise linear, and continuous nonlinearities.
Later on it was shown that a poor agreement between theory and experiment in flutter is most likely due to
nonlinear structural elastic models. So detailed examination of many types of nonlinearities that may affect
aeroelastic systems is presented in various articles. Tang and Dowell introduced a free play nonlinearity in thetorsional stiffness and examined the nonlinear aeroelastic response. For various initial condition they show that
LCO is dependent upon free stream velocity, initial pitch condition, magnitude of the free play nonlinearity and
initial conditions.
One the major developments was given by NASA in 1997, by building the Benchmark Active Control Technology
(BACT). It is a two degree of freedom model where the pitching movement and the plunging one, are respectively
restrained by a pair of springs attached to the elastic axis(EA)of the airfoil. A prototype was also built as shown
in Fig.1.9where one or two trailing-edge control surfaces are used to control the air flow, thereby providing
maneuverability to suppress instability. The BACT model is accurate for airfoils at low velocity and has been
confirmed by wind tunnel experiments.
Figure 1.9: Configuration of the nonlinear 2-D prototypical aeroelastic wing
In this report the BACT model will be shown, thus with respect to the example in equation 1.2,a second
degree of freedom known asplungeis taken into account. Plunge is introduced so to consider also flapping of the
considered wing section. The model is a simple representation of an aeroelastic system for low speed, where all
non linear terms from experimental data are taken into account as shown in [9]. Hence the equations governing
the motion of the aereoelastic system are:
m mxb
mxb I
h
+
ch 0
0 c
h
+
Kh 0
0 K()
h
=
L
M
(1.3)
whereh is the plunge motion and is the pitch or AoA. In equation1.3,m is the mass of the considered section
of the wing, and I
is the mass moment of inertia about the elastic axis. The position of the elastic axis with
respect to the center of mass of the considered wing section can be varied and is referred as x. Constants chandcare linear damping constants of the system. Khis the spring constant for the plunge motion and K()
is the non linear stiffness of the pitch motion. In this report the non linear stiffness K() is assumed to be a
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1.2. The NASA BACT model
4th order polynomial function:
K() =4
i=0
Kii =K0+K1+K2
2 +K33 +K4
4 (1.4)
Figure 1.10: Wing cross-section representation
M and L, in case of a single control surface as in Fig. 1.10, are respectively the input moment and the
quasi-steady aerodynamic lift and are modeled as [3]:
L= U2bcl(+h
U+ (
12
a)bU
) +U2bcl (1.5)
M=U2b2cm(+h
U + (
12
a)bU
) +U2b2cm (1.6)
where is air density, Uis the air stream velocity, is the angle between the foil and the trailing edge control
surface. cl and cm are the lift and momentum coefficients for the AoA and cl and cm are respectively the
lift and moment coefficients for the control surface. a is the distance between mid-chord1 and the elastic axis
(EA) as shown in Fig.1.11.
Figure 1.11: Wing cross-section schematic representation showinga and mid-chord b
After substituting the quasi-stead forces from equations1.5 and1.6into equation1.3: m mxb
mxb I
h
+
ch+Ubcl U b
2cl(12 a)
U b2cm cU b3cm(
12
a)
h
+ (1.7)
Kh U2bcl0 U2b2cm+K()
h
= bcl
b2cm
U2
1Chord is the imaginary line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an
airfoil
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Model and system analysis
It is important to note that the only source of non linearity is given by the stiffness and an extension for higher
velocity of the model is possible by considering the pith moment as a quadric function of.
1.2.1 BACT MODEL with two control surfaces
A extension of the model presented in equation1.7 can be derived for when two control surfaces are availableon the trailing edge as depicted in Fig. 1.12The model can be derived considering the following lift and pitch:
Figure 1.12: Two trailing edge control surfaces
L= U2bcl(+h
U+ (
12
a)bU
) +U2bcl2 2+U2bcl2 2 (1.8)
M=U2b2cm(+h
U+ (
12
a)bU
) +U2b2cm1 1+U2b2cm22 (1.9)
That substituted in equation1.3leads to: m mxb
mxb I
h
+
ch+Ubcl U b
2cl(12 a)
Ubcm cU b3cm(
12 a)
h
+ (1.10)
+
Kh U2bcl0 U2cmK()
h
=
bcl1 bcl2b2cm1 b
2cm2
1
2
U2
1.3 State space representation and system open loop analysis
1.3.1 State space representation
It is convenient for the analysis that follows, and for control design to have a state space formulation (SS) ofthe system in equations1.7and1.10. For this purpose the following state variable vector is used:
x=
x1
x2
x3
x4
=
h
h
, x 4 (1.11)
and is the control input.
The SS formulation expresses the system in the following affine form:
x= f,x(x) +g(x)
where = U2 and it is to note the equations are dependant on the airstream velocity and the elastic axis
location x. The notation with the two subscripts and x emphasizes the system dependance on the two
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1.3. State space representation and system open loop analysis
1.3.2 Open loop analysis
A first simulation of an open loop response of the system is carried out considering the air flow velocity U= 15 ms
and a = 0.4 with the following initial conditions = 0.1[rad], h= 0.1[m] and = h= 0 as depicted in Fig.
1.14a.
6 4 2 0 2 4 6 8150
100
50
0
50
100
150
(x2) [deg]
x4
[deg/s]
(a)AoA Phase portrait with = 0.1[rad], h= 0.1[m] and
= h= 0,U= 15ms
,a= 0.4
0.015 0.01 0.005 0 0.005 0.010.2
0.15
0.1
0.05
0
0.05
0.1
0.15
(x2) [deg]
x4
[deg/s]
(b) Plunge phase portrait with = 0.1[rad], h = 0.1[m]
and = h= 0,U = 15ms
,a= 0.4
The system clearly shows a LCOs due to the structural non linearity in equation 1.4and due to the low
damping. The suppression of the LCO and of a possible chaotic behaviour will be in the following chapters, one
of the main goals for the controller.
Other simulation are for different initial conditions. It is interesting to notice that the system other than LCO
behaviour also shows a chaotic behaviour 2 for some initial conditions. Indeed for low values ofh and h, the
system exhibits chaos as shown in the following figures.
0.1 0.05 0 0.05 0.1 0.15
0.6
0.4
0.2
0
0.2
0.4
x2
x4
(a)AoA chaotic Phase portrait with = 0.1[rad], h = 0[m]
and = h= 0,U= 25ms a= 0.4
0.05 0.04 0.03 0.02 0.01 0 0.01 0.02 0.03 0.04
0.4
0.2
0
0.2
0.4
0.6
X1
X3
(b)Plunge chaotic phase portrait with = 0.1[rad], h[m]
and = h= 0,U = 25ms a= 0.4
The behaviour shown in Fig.1.14aand1.14bgiven by a chaotic vector field for the values of parameters
indicated, and it will be shown in chapter 3 how a MCS controller can recover from chaos.
Setting the initial conditions so to have as solution to the system an LCO behaviour, and because of its
dependance on the parameters as shown in equation 1.12, it is possible to do a bifurcation analysis . Before
showing the results of the bifurcation analysis we qualitatively show on a 3d plot what happens for different
values ofUand a as depicted in the following figures because it gives a quick glance of how the LCO grow with
respect to velocity:2notice that chaos is possible only for systems of order n >3, this is a consequence of the Poincar-Bendixson theorem
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Model and system analysis
15
20
25
30
35
40
15
10
5
0
5
10
15
20
25
800
600
400
200
0
200
400
600
U[m/s]x2= [degrees]
x4
=
[degrees/s]
Figure 1.14: Open loop responses on AoA for the system with = 0.1[rad], h = 0.1[m] and = h = 0 and U = 15, 17, 20, 19, 40,
a= 0.4
15
20
25
30
35
40
0.1
0.08
0.06
0.04
0.02
0
0.02
0.04
1.5
1
0.5
0
0.5
1
1.5
U[m/s]x1= h[m]
x3
=
h[m/s]
Figure 1.15: Open loop responses on plunge for the system with = 0.1[rad], h = 0.1[m] and = h= 0 and U= 15, 17, 20, 19, 40,
a= 0.4
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1.3. State space representation and system open loop analysis
Similarly as what has been done for U, is done for various values ofa at a settled velocity ofU= 25.
0.80.7
0.60.5 0.4
0.30.2
0.1
0.15
0.10.05
0
0.05
0.1
0.15
0.2
0.25
0.3
8
6
4
2
0
2
4
6
a
x1
x3
Figure 1.16: Open loop responses on AoA for the system with = 0.1[rad],h = 0.1[m] and = h= 0 anda = 0.1,0.3,0.4,0.6
0.80.7
0.60.5
0.40.3
0.20.1
0.15
0.1
0.05
0
0.05
0.1
0.15
0.2
0.25
8
6
4
2
0
2
4
6
a
x1
x3
Figure 1.17: Open loop responses on AoA for the system with = 0.1[rad],h = 0.1[m] and = h= 0 anda = 0.1,0.3,0.4,0.6
As stated earlier a bifurcation analysis is performed for the LCOs of the system. The bifurcation for the
LCOs is a Hopfbifurcation starting for a= 0.9; no bifurcations for airstream velocity are found. The result
of the continuation calculations is a family of LCOs shown in the following figures.
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Model and system analysis
0.1 0.05 0 0.05 0.1 0.15 0.2
5
4
3
2
1
0
1
2
3
4
x2
x4
(a)Family of LCOs for AoA, U= 25ms
0.2 0.1 0 0.1 0.2 0.3
8
6
4
2
0
2
4
6
x2
x4
(b)Bigger family of LCOs for AoA, U= 25 ms
20 15 10 5 0 5
x 103
0.3
0.2
0.1
0
0.1
0.2
0.3
x1
x3
(c)Family of LCOs for plunge, U= 25 ms
0.06 0.05 0.04 0.03 0.02 0.01 0 0.01 0.02 0.03 0.04
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
X1
X3
(d)Bigger family of LCOs for plunge, U= 25ms
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1.3. State space representation and system open loop analysis
8
64
20
24
60.2
0.15
0.1
0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
a
x4
x2
(a)3d visualization of a family of LCOs for AoA, U = 25 ms
0.2 0.15 0.1 0.05 0 0.05
0.1 0.15 0.21
0.5
0
0.5
1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
a
x1
x3
(b) 3d visualization of a family of LCOs for plunge, U =
25ms
The analysis shown for the system is only numerical and qualitative. It is although possible even if highlycomplex, to show the behaviour of the system use a Lyapunov approach. Being the system mechanical, as a
Lyapunov function it is possible to use the energy of the system; being the system of the 4th order the candidate
Lyapunov function is the following:
V =12
((x3, x4)M(x3, x4)T +K1,1x
21+x1x2K1,2) +
x20
K2,2()d >0
Where M is the mass matrix, Ki,j are the elements of the elastic matrix as in equation1.7. The calculations of
the time derivative can be carried out, but from what has been shown in the previous figures it is not possible
to show the V
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Chapter 2
Input Output Feedback linearization
In this chapter a non-linear controller based on Feedback linearization (FBL) is developed for the systems
introduced in equation1.12. The main idea of this control technique is to build a non-linear controller so to
cancel the non-linear dynamic terms of the system. After the linearizing law is built a linear controller on asecond control loop is synthesized with classic methods as in [10] or, for instance, an optimal LQ controller can
be developed.
The control objective is to build a possibly global FBL controller, either with partial FBL or total FBL, to
suppress the LCO shown in the previous chapter. Indeed a FBL presents with respect to linearization the
advantage of being global.
There are some downsides in FBL, the main one is that it is not always possible and trivial to fully linearize a
system, indeed sometimes only partial FBL is possible and in this case hidden dynamics are to be investigated
for the stability of the entire system. The second downside, is that the linearizing control law may (and often
does) use parameters that are not always known; if the system parameters change the FBL law may not yield
the mismatch between the assigned parameter in the law and the actual value of the parameter thus may leadthe system to instability. The last downside concerns structural robustness, indeed non modeled dynamics can
affect the systems performance.
Input output FBL (IO FBL) strategy is derived by using partial linearization and thus analysing hidden
dynamics for the system with one control surface. Partial FBL will be applied first to the AoA control and then
to the plunge dynamics.
2.1 Pitch FBL control
To derive the control law with IO FBL it is necessary to define an output function y = h(x). In the case of
pitch control the most reasonable variable to considered as output is the AoA since it is also is the easiest tomeasure. So another equation is to be added to the system defined in equation 1.12:
y= h(x) = x2
In order to accomplish partial FBL the first step is to calculate1 the relative degreerof the system. The relative
degree of the system is calculated as in [1]:
Lg(h) = h
xg(x) = 0
Lf(h) = h
xf(x) = x4
Lg(Lf(h)) = g4= 0
1notice the following calculations the Lie derivative Lf(h) is calculated because it will be used as both for finding the relative
degreer , and for the state transformation as shown in [5]
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Input Output Feedback linearization
Showing that the relative degree of the system is r = 2 allowing to linearize two equations leaving the other
nr as hidden dynamics. In order to accomplish partial FBL a state transformation is carried out as follows:
xz
z1= h(x) = x2
z2= Lf(h) = x4
z3= x1
z4= g3x4+g4x3 (2.1)
The criterion in choosing the state transforation is shown in [5]. It is to note that z3and z4could have been
chosen differently but a different choice would not have had the benefit of having Lg(zi) = 0, 3 i 4(the
n rhidden dynamics equations ) thus not allowing any effect of the input towards the variables z3and z4and
simplifying the hidden dynamics stability analysis. The picked choice for the SS transformation guarantees the
transformation to be invertible.
The new SS representation is the following, again in affine form:
z= f,x(z) +g(z)
where:
f,x(z) =
z2
(k4+q(z1))z1+ (c3g3g4
c2)z2k3z3 c3g4
z41g4
(z4+g3z2)
((g3k4g4k2)+g3q(z1)g4p(z1))z1+ (c3g23
g4+c4g3c1g3c2g4)z2+ (g3k3g4k1)z3+ (
c1g3g4
c1)z4
,
g(z) =
0
g4
0
0
Partial FBL can be achieved by selecting a control law where the non linear dynamics are compensated 2. Thus
by choosing:
=(k4+q(z1))z1+ (c3
g3g4
c2)z2k3z3 c3g4
z4+v
g4
the closed loop system is defined as in equation 2.2:
z1
z2
z3
z4
=
z2
v1g4
(g3z2+z4)
((g3k4g4k2)+g3q(z1)g4p(z1))z1+ (c3g23
g4+c4g3c1g3c2g4)z2+ (g3k3g4k1)z3+ (
c1g3g4
c1)z4
(2.2)
Some considerations are to be made on the closed loop system in 2.2. The first is that a linear controller v is
to be built for stabilizing the dynamics ofz1 and z2; secondly the dynamics ofz3 and z4 are to be carefully
analysed for stability because there is no direct control on them and more importantly, if not linear, they can
be subject to bifurcations.
The linear controller v is first build as an LQR controller with a feed forward action, later on a derivative action
will be used as well moving so to a PD controller. The LQR controller is build using a full information scheme,
this is reasonable even if it is not possible to measure all the state vector[ z1 z2]T, indeed it is still realistic tomeasure AoA (z1) and estimating the pitch angular velocity (z2)is not difficult e.g. using a Kalman filter.
2this in the aerospace industry is know as linear dynamic inversion
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2.1. Pitch FBL control
The LQ controller is built by solving the LQ problem in the MATLAB environment, using the Brysons rule to
define the weight matrixes R and Q. The resulting gain values for the controller are the following:
kz1= 70.7107, kz2= 13.8355
The controller scheme design in the Simulink/Matlab environment is shown depicted in the following image:
Figure 2.1: SS graph for AoA partial FBL with = 0.1[rad], h= 0.01[m] and = h= 0
In Fig.2.1it possible to see the LQ controller3, and the subsystem consisting of the SS transformation and the
compensator for the non-linear dynamics. It should be clear that no transformation is applied to the reference
signal; since the FBL is partial and the transformation -and thus only variables z1and z2are of interest- consists
in no more than a simple exchange of position. The full SIMULINK Block diagram is plotted in fig.
To Workspace6
pfblalpha
To Workspace5
pfblhp
To Workspace4
pfblh
To Workspace3
t
To Workspace2
betaFBLalpha
To Workspace1
pfblalphap
To Workspace
fblalphatrack
Signal Builder
Signal 1
Scope5
Scope4 Scope3
Scope2
Scope1
Radiansto Degrees3
R2D
Radiansto Degrees2
R2D
Radians
to Degrees1
R2D
Radians
to Degrees
R2D
Plant
beta x
Ground
Filtro riferimento
1/(s/25+1)
FBL Controller + Transformation
reference
z_1,z_2
Fsb
Beta
Clock
Figure 2.2: FBL control scheme for AoA
2.1.1 Simulations
Some simulations for the system are reported in the following figures both for and h. The parameters used
areU= 15 ms
, a=0.4.
3not the feedforward action which is outside of this block.
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Input Output Feedback linearization
0 1 2 3 4 5 6 7 8 95
0
5
10
15
20
25
30
35
40
z1=x2=[deg]
z2=x4=d/dt[
deg/s]
Figure 2.3: Phase Plane of the controlled AoA dynamics with the use of a LQR controller
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9
Time [seconds]
z1=x2=[
deg]
Reference
Figure 2.4: Step response of the controlled AoA dynamics with the use of a LQR controller
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2.1. Pitch FBL control
0 1 2 3 4 5 6 7 8 9 105
0
5
10
15
20
25
30
35
40
Time [seconds]
z2=x4=d/dt[
degree/s]
0 1 2 3 4 5 6 7 8 9 1014
12
10
8
6
4
2
0
2
4
Time [seconds]
[degrees]
Figure 2.5: Time response and control input for = z1 = x2 and = z2 = x4
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Input Output Feedback linearization
The control goal, which is to suppress LCO and possibly permit tracking of a reference signal, is achieved
on the interested variables. Before discussing the stability of the hidden dynamics it is interesting to see a
qualitative behaviour of the variables [z3 z4]. The following figure shows a phase plane of the plunge and its
velocity. Clearly being the relative degree r= 2 there is no direct control over it; and also having chosen the
SS transformation as shown2.1 allows to have no control input in the hidden dynamics equations.
0.03 0.025 0.02 0.015 0.01 0.005 0 0.005 0.010.3
0.25
0.2
0.15
0.1
0.05
0
0.05
0.1
0.15
0.2
z3=x
1= h[m]
z4=x3=d/dth[m/s]
Figure 2.6: SS graph for the plunge after partial FBL on the AoA. It is clear that the variable is indirectly influenced
Another tracking examples is shown in Fig. 2.7, after appropriate filtering of the reference. In this case the
plunge phase plane also is of interest because it can be seen that not only it is qualitatively stable but it shows
a stable focus as depicted in the following figure and as will be shown further:
0 1 2 3 4 5 6 7 8 9 100
2
4
6
8
10
Time [seconds]
[degree]
0 1 2 3 4 5 6 7 8 9 1040
20
0
20
40
d/dt[degrees/sec]
reference
Figure 2.7: Tracking graph and reference signal with null initial conditions
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2.1. Pitch FBL control
0 1 2 3 4 5 6 7 8 9 100.03
0.02
0.01
0
0.01
Time [seconds]
h[m]
0 1 2 3 4 5 6 7 8 9 100.3
0.2
0.1
0
0.1
0.2
Time [seconds]
d/dth[m/s]
Figure 2.8: Plunge position and speed with null initial conditions when the AoA subject to reference signal as in Fig. 2.7
FBL control is highly model based and it is interesting to show a simple example of what happens when
there is uncertainty on the value of one or parameters. In the following figures a FBL law is built by considering
for the air stream velocity value atUFBL different from the real parameterUof the system. Indeed a 1g positive
acceleration is introduced at = 5.9[s]; the following figure show the time response.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9
Time [seconds]
Reference
Figure 2.9: Tracking error for give by building FBL control law on different parameters (UFBL, aFBL) from the systems one
(U,a)
A constant tracking error and an increase of settling time with respect to Fig. 2.7are present that the LQ
controller is not capable of rejecting. Some more information can be obtained by looking at the plot of :
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Input Output Feedback linearization
0 1 2 3 4 5 6 7 8 9 105
0
5
10
15
20
25
30
35
40
Time [seconds]
d/dt
Figure 2.10: dynamics when subject to non rejected parameter variations
Indeed when using FBL a robust linear controller is essential in order to handle unexpected parameter
variation that are not handled by the non linear dynamic compensating law. As stated earlier, for this purpose
a derivative action is introduced in the controller so to allow to reject some parameter variations. After the
introduction of a PD controller the resulting step response allows a null tracking error as can be seen in fig.t
would have been possible also to introduce a LQI scheme or a PI linear controller, but in this occasion a
derivative action is preferred.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9
Time[seconds]
[
deg]
Reference
FBL with uncertain parameters tracking
Figure 2.11: Null tracking error for subject to parameter variation with a PD controller linear controller
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2.1. Pitch FBL control
2.1.2 Hidden dynamics analysis
The hidden dynamics are to be investigated; in order to accomplish the stability analysis the zero dynamics of
the system are investigated, by setting [z1 z2]T = 0 thus leading to the following system:
z3z4
=
0 1,22,1 2,2
z3
z4
(2.3)
where some constants of system2.2 have been renamed as follows:
1,2= 1g4
, 2,1= (g3k3g4k1), 2,2= c1g3
g4c1
2.1.2.1 Stability analysis
As qualitatively shown in Fig2.6there is a stable focus in [z3 z4] = (0, 0) that can be easily found solving thesystem2.3with null solution. The eigenvalues calculated using as parameters the same as previous simulations
a=0.4U= 15m/s. The eigenvalues are: [1.3122+17.1258j], [1.312217.1258i] showing so a stable focus,
in accordance with what has been qualitatively shown in previous Fig .2.6. It is important to notice that the
zero dynamics system is linear, but still parameter dependant; it is necessary so to evaluate the position of the
eigenvalues for different values ofa and of the velocity Uas illustrated in Fig. 2.1.2.1
0
20
40
6080
100
1 0.9
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
8
6
4
2
0
2
4
6
8
10
12
U[ms]
Eigenvalues
Figure 2.12: Plot of the eigenvalues with respect to velocity U and a. Color pattern towards red indicates increasing values for the
eigenvalues
Fig.2.1.2.1 shows the how the real part of the eigenvalues is smaller than zero for all velocity values a
a < 0.55 and greater than zero for a >0.55
Another possibility to investigate the systems stability is to use the Lyapunov equation. This technique is not
followed since the use of the stability analysis using eigenvalues is a more synthetic way to for linear systems.
Secondly being the system parameter dependant the use of eigenvalues well shows how the stability varies with
respect to U and a; indeed using the Lyapunov equation would have requested to analyse a parameter varying
matrix, solution of the equation: ATP+ P A 0.
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Input Output Feedback linearization
2.2 Plunge FBL control
The FBL control law for plunge is built in a similar way as it was done for the AoA partial FBL control. At
first an output function is selected:
y= h(x) = x1
After this the relative degree r of the system is calculated:
Lg(h) = h
xg(x) = 0
Lf(h) = h
xf(x) = x3
Lg(Lf(h)) = g3= 0
The relative degree of the system is r = 2 thus allowing a partial FBL on the plunge dynamics. Sincer < n
there are 2 hidden dynamics to be investigated.
Following the calculation ofr, a SS transformation is introduced as following, again, the criterion shown by [5]
is used:xz
z1= h(x) = x1
z2= Lf(h) = x3
z3= x2
z4= g3x4+g4x3 (2.4)
Fortunately also in this case it is possible, and easy, to find a SS transformation where Lgzi = 0, 2 i 4
making orthogonal the variables not interested by the FBL law to the control input.
The the transformation introduced in2.4allows a recast of the systems to the affine form of equation2.2where:
f,x(z) =
z2
k1z1(c1g4+c2g4g3
)z2)(k2+p(z3))z3 c2g3
z41g3
(g4z2+z4)
(g4k1k3g3)z1+ (c1+c2g24
g3c3g3c4g4)z2+ (g4(k3+p(z3))g3(k4+q(z3)))z3+ (
g4g3
c3c4)z4
,
g(z) =
0
g3
0
0
By selecting an appropriate control law plunge FBL can be achieved. Thus chosen as follows:
=+k1z1+ (c1g4+c2
g4g3
)z2) + (k2+p(z3))z3+ c2g3 z4+v
g3
The resulting system equations after the compensator law has been selected as following:
z1
z2
z3
z4
=
z2
v1g3
(g4z2+z4)
(g4k1k3g3)z1+ (c1+c2g24
g3c3g3c4g4)z2+ (g4(k3+p(z3)g3(k4+q(z3))))z3+ (
g4g3
c3c4)z4
(2.5)
It is now essential to build a linear controller v on an outer control loop so to stabilize the plunge dynamics.
Also in this case,a s what has happened for the IO FBL of the AoA an LQ controller is developed. By solvingthe LQ problem and choosing the weight matrixes using the Brysons rule the following gains are obtained:
kz1=70.7107, kz2= 13.8355
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2.2. Plunge FBL control
In the case of the plunge dynamics no derivative or integral action is used, but it is clear that it can easily bee
implemented.
2.2.1 Simulations
With the linear controllerv deployed some simulations can be shown before analysing the zero dynamics of thesystem. In the following figures some significant simulations are shown:
0.02 0 0.02 0.04 0.06 0.08 0.10.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
h [m]
d/dt
h[m/s]
Figure 2.13: Phase plane for a closed loop response for partial FBL on the plunge dynamics, with the following initial conditions
h= 0.1 and = = h= 0
0 1 2 3 4 5 6 7 8 9 100.02
0
0.02
0.04
0.06
0.08
0.1
Time [seconds]
h[m]
Figure 2.14: Time response for partial FBL on the plunge dynamics, with the following initial conditions h = 0.1 and = = h= 0
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Input Output Feedback linearization
0 1 2 3 4 5 6 7 8 9 100.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
Time [s]
d/dth[m/s]
Figure 2.15: Time response for partial FBL on the plunge dynamics, with the following initial conditionsh = 0.1 and = = h= 0
Clearly the controller stabilises the system for the plunge dynamics for which there is no reason to have
tracking but just a regulation to zero. When dealing with partial FBL a qualitative check to the dynamics with
no control is often interesting as show in the following phase plane figure:
5 0 5 10 15 20300
200
100
0
100
200
300
[degrees]
d/dt[
degrees/sec]
Figure 2.16: Phase plane for and for a closed loop response for partial FBL on the plunge dynamics, with the following initial
conditions h= 0.1 and = = h= 0
2.2.2 Hidden dynamics analysis
To analyse the systems zero dynamics the variables z1
and z2
are set to zero, obtaining so the following
equations: z3z4
=
1,2z4
(g4(k2+p(z3))g3(k4+q(z3)))z3+2,2z4
(2.6)
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2.2. Plunge FBL control
Where1,2 and2,2 rename some constants from system2.2as follows:
1,2= 1g3
, 2,2= g4g3
c2c4
It is clear that the zero dynamics of the system are a family of non-linear equations depending on parameters
Uanda.
2.2.2.1 Lyapunov stability analysis
To analyse the stability of the zero dynamics system equations 2.6 a Lyapunov theory approach is used. The
first step to simplify the process of finding a Lyapunov function is to recast the system to 2 ndorder ODE. This
can be achieved as follows:z3z4
=
1,2z4
((g4mxb
d g3
md)K(z3) +(g4k2g3k4))z3+2,2z4
=
1,2z419.94z354.57z23+ 2593.6z
3316916.5z
43+ 34088.5z
53+2,2z4
And thus we can recast the hidden dynamics form as if it is a non linear mass-spring-damper system:
z2,2z(z) = 0, (z) = 4.51z+ 9.86z2 5.87z3 + 3.82z4 7.71z5
We now search for a candidate Lyapunov function. A possibility is to use the energy of the system, which is
the sum of kinetic and potential energy. Thus the candidate Lyapunov function V(z) is selected as follows:
V(z) =12
z2 + 0
() =12
z2 + 2.25z2 3.28z3 + 1.46z4 0.774z5 + 1.28z6
(a) (b)
Figure 2.17: Lyapunov function for the plunge Zero dynamics. It is clearly continuous, differentiable, null in (0 , 0), positive definite
and radially unbounded
Now if we calculate V(z) and use the input-output form of the zero dynamics we obtain:
z(z+ (z)) = 2,2z2
Thus the system is locally asymptotically stable because 2,2< 0 as can bee seen from Fig.2.18.
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Input Output Feedback linearization
Figure 2.18: Derivative of the Lyapunov function of Fig.2.18.
A possibility to show that the system is globally stable, is to use the La Salle invariant set theorem and so
use the so called sector conditionsas shown in [1]. The sector conditions, given a 2ndorder ODE in the form
of y+d(y) +ky = 0, are the following:
d(y)y >0, x >0
k(y)y >0, x >0
d(0) =k(0) = 0
if the conditions are full filled then the system is globally stable because being the biggest invariant set for whichV= 0 the equilibrium point in (0, 0). In this case as can be seen in the following figures the condition are full
filled thus the zero dynamics are globally stable.
A second possibility to show that the system is globally stable is to use the Babashin Krasovskii theo-
rem;indeed from what can be seen in earlier figures, the condition on radial unboundedness is respects, and
having V(z) = 2,2z2
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2.2. Plunge FBL control
0 100 200 300 400 500 600 700 800
0.08
0.06
0.04
0.02
0
0.02
0.04
0.06
0.08
0.1
z3
Figure 2.19: Bifurcation diagram with respect to and with a = 0.4
0 100 200 300 400 500 600 700 8000.08
0.06
0.04
0.02
0
0.02
0.04
0.06
0.08
z3
Figure 2.20: Bifurcation diagram with respect to and with a = 0.5
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Input Output Feedback linearization
0 100 200 300 400 500 600 700 8000.04
0.03
0.02
0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
z3
Figure 2.21: Bifurcation diagram with respect to and with a= 0.6
0 100 200 300 400 500 600 700 8000.03
0.02
0.01
0
0.01
0.02
0.03
0.04
0.05
z3
Figure 2.22: Bifurcation diagram with respect to and with a= 0.63
It is important to notice that Fig.2.19,2.20,2.21,2.22show a pitchfork bifurcation but it is not a symmetric
bifurcation. Indeed this sort of bifurcation is also know as an imperfect bifurcationorperturbated bifurcationas
showed in [13] and [4]. In These kind of bifurcation the normal form of the bifurcation, that in the case of a
standard pitchfork is x= x x3, becomes x= x +x2 x3, therefor adding a quadratic term. Although
these bifurcations are perturbated it is still possible to show a critical velocityc after which the uncontrolled
plunge dynamics equilibrium is unstable.
Bifurcation diagrams are show with respect to a, varying as shown in the Fig.2.23and2.24:
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2.2. Plunge FBL control
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 00.08
0.06
0.04
0.02
0
0.02
0.04
0.06
0.08
a
z3
Figure 2.23: Bifurcation diagram with respect to a and with = 115
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0.1
0.05
0
0.05
0.1
0.15
a
z3
Figure 2.24: Bifurcation diagram with respect to a and with = 115
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Chapter 3
MRAC Minimum Controller Synthesis
control for flutter
In this chapter an MRAC adaptive controller will be applied to the plant defined in chapter 1 using the Minimum
Controller Synthesis algorithm (MCS)1. We will show briefly how the MCS algorithm works before showing the
results, and the extensions of the MCS and simulations.
3.1 MCS algorithm
The MCS strategy is based upon an extension of Landaus MRAC approach [6] as shown in [11]. The MCS
control strategy aims to track asymptotically a reference model as defined in 3.3 trough a gain adaptation
law. One of the most important characteristics of the control strategy is that there is no assumption on the
knowledge of the plant parameters. The only assumption that is held is that the plant structure is known and
fully controllable in a canonical form like follows:
x=Ax(t) +Bu(t) (3.1)
wherex(t) Rn,A(t) nn,u(t) and vector B n1 are:
A=
0 1 0 0
0 0 1 0...
... ...
. . . ...
a1 a2 a3 an
B=
0
0...
b
(3.2)
Given the plant as in3.1,it is necessary to define a reference model as follows2:
xm= Amxm(t) +Bmr(t) (3.3)
where
Am=
0 1 0 0
0 0 1 0...
... ...
. . . ...
am1 am2 am3 amn
Bm=
0
0...
1
(3.4)
The control law used in the MCS strategy is the following:
uMCS(t) = K(t)x(t) +KR(t)r(t) (3.5)
1from now on the MCS algorithm will be referred to as if it a control strategy with a slight name abuse2note that the reference model is supposed to be stable, as so, the matrix Am is a Hurwitz matrix
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MRAC Minimum Controller Synthesis control for flutter
where:
K(t) = t0
ye()xT()d+ ye(t)x
T(t) KR(t) = t0
ye()r()d+ ye(t)r(t) (3.6)
K(0) = 0, KR(0) = 0, xe(t) = xm(t)x(t)In the equation3.6 the terms ye and is defined as:
ye(t) = Cexe(t), Ce=
0 0 0 . . . 1
P
and P is the solution of the Lyapunov equation
P Am+ATmP=Q P >0, P=PT
With the control law defined in3.5the error system shown in figure 3.1, is asymptotically stable, as proven
in [1]
Figure 3.1: MCS scheme
The MCS controller can be applied to non linear plants, as it has been shown in [12] that MCS control strategy
rejects non linear disturbances3 and vanishing disturbances towards the input direction.
3.1.1 MCS extensions
Some extension of the MCS control strategy have been proposed in literature. Part of the extensions focus on
conjugating optimal control with the MCS control strategy, while others aim to modify the control law so obtain
specific proprieties of the controlled system.
3.1.1.1 MCS-LQ
In this extension of the MCS strategy the reference model is an optimally controlled linear model. The reference
model, that in applications is typically derived from the non linear model of the plant, is controlled with an
optimal LQ controller so to reach desired proprieties of optimality. This scheme, show in figure3.2 allows to
increase the LQ robustness because the MCS strategy compensates the mismatch between plant and the LQ
optimal trajectory.
3note that in this propriety well fits with the use of the MCS strategy that is used in this report. Indeed in aeronautics often
not only parameters are unknown but non linear effects cannot be correctly modeled due to order reduction and other factors
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3.1. MCS algorithm
Figure 3.2: LQMCS scheme
3.1.1.2 MCSI
This extension of the MCS strategy modifies the control law by inserting an integral action (MCSI). This
extension in necessary in those cases in which it is mandatory to have a null tracking error. The MCS control
law is modified as in equation3.7:
uMCSI(t) = K(t)x(t) +KR(t)r(t) +KI(t)xI(t) (3.7)
In equation3.7the terms KI(t) and xI(t) are defined:
xI(t) =
(ry) KI(t) = t0
ye()xI()d+ ye(t)xI(t), KI(0) = 0 (3.8)
The stability of the MCSI strategy has been proven in [1].
3.1.1.3 EMCS
EMCS stands for Extended MCS and it is a modified control of the standard MCS law so to reject generic
disturbance.
The MCS law presented in3.5 is modified by adding a commuting control action:
uEMCS(t) = K(t)x(t) +KR(t)r(t) +Nsgn(ye) (3.9)
where the value ofNmust respect the following propriety as shown in [1]:
N >1
b
max{|d|} (3.10)
It is important to observe that in control law3.9it is necessary to have a measurement, or at least an estimation,
of the disturbance d. The main advantage of using the control law in3.9 is so have a more robust controller.
This can be simply understood by recalling the main results of commuting controllers such a Sliding controllers.
3.1.1.4 NEMCS
Based upon3.9, and based upon the fact the value Nis set by a measurement of the disturbance amplitude,
the NEMCS introduces a gain varying law for the commuting action.
uNEMCS(t) = K(t)x(t) +KR(t)r(t) +Kn(t)sgn(ye) (3.11)
In equation3.11Kn(t) is defined as follows:
Kn(t) = t0
ye(t) (3.12)
the proof of stability for this controller, with some interesting experimental results, is given in [7]
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3.1.1.5 LQ-NEMCSI
The LQ-NEMCSI an extension of the LQ-MCS and NEMCS control strategy where the control laws are modified
so to have integral action and good rejection capabilities following so an optimal trajectory from the linear model.
The control law is defined as follows:
uNEMCSI(t) = K(t)x(t) +KR(t)r(t) +Kn(t)sgn(ye) +KI(t)xI(t) (3.13)
It has been show in [7] to be asymptotically stable.
3.2 MCS control synthesis and simulations
In this section MCS and some extensions of MCS control strategy are deployed to control the plant defined in
1.7. The main goal is the suppression of LCO and tracking of a given reference signal.
The system defined in 1.12is defined with x 4 and with and does not result in the affine canonical
form. This poses a first problem in applying the MCS strategy. Indeed to control such a system a possible
solution is to add a second control surface and defining two MCS controllers each one working on a separateaffine system and rejecting the effects of the remaining part of the system. Another possible solution, marly
qualitative, is to use only on control input for allow tracking on AoA and show that the plunge dynamics is
stable. In this report two different controllers are deployed one for AoA and one for plunge dynamics.
As a first step we can rewrite the system defined in 1.12f and g functions in so to separate clearly what is seen
as a disturbance from the controller:
f,x(x) =
x3
x4
k1x1(k2 2.82mxbd )x2c1x3c2x4+d3(x, t)
k3x1(k4+ 2.82md )x2c3x3c4x4+d4(x, t)
, g(x) =
0
0
g3
g4
(3.14)
whered3(x, t) and d4(x, t) as the following:
d3(x, t) = p(x2)2.82mxb
d , d4(x, t) =
2.82md
+q(x2)
The following step is to define a reference model for both the plunge and AoA dynamics. This is accomplished
by selectingAm andBm as the following matrixes:
Am=
0 1
1000 75
, Bm=
0
1
(3.15)
The step response of this model is shown in Fig.3.3and has no overshooting and a settling time of approximately0.3[seconds].
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3.2. MCS control synthesis and simulations
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
Time [s]
Amplitude[deg]
[m]
Reference
Position
Figure 3.3: Reference model step response
Under the assumption of a single control surface and thus controlling only the AoA dynamics 4 the closed
loop equations can be given for a MCS controller as in 3.16:
x1
x2
x3
x4
=
x3
x4
k1x1(k2 2.82mxb
d )x2c1x3c2x4+d3(x, t) +g3MCS
k3
x1
(k4
+ 2.82md
)x2
c3
x3
c4
x4+d
4(x, t) +g
4MCS
(3.16)
Similarly by changing the control law for it is possible to write the closed loop equations for the various MCS
extensions.
If using two control surfaces by recalling system defined in1.10it is possible, with some manipulation, to redefine
in a SS representation the closed loop equations as in equation 3.17:
x1
x2
x3
x4
=
x3
x4
k1x1(k2 2.82mxbd )x2c1x3c2x4+d3(x, t) +g131MCS + g232MCSk3x1(k4+ 2.82md )x2c3x3c4x4+d4(x, t) +g141MCS+ g242MCS
(3.17)
As stated previously since the system in not in canonical form two different controllers are deployed one forcontrolling the plunge dynamics and one for controlling the AoA as can be seen in Fig.??thus the two control
surface model is used. Clearly the is no necessity for the plunge dynamics to have tacking capabilities and the
plunge controller has no feed forward, and the control surface used is smaller than the one used for the AoA
dynamics thus it is just a stabilising controller. Also, in a realistic application there is no real need to have
plunge control that introduces a cost growth, due to the introduction of a seconds control surface; indeed in
the aerospace industry the plunge dynamics typically is not controlled but some action are taken so to reduce
or cancel undesired behaviours. From now on only simulations for the AoA are shown, but the reader should
keep in mind that all the simulation have on the plunge dynamical a gently tuned stabilising controller. Only
one simulation is displayed in fig.3.4awith the relative feedback gains n fig.3.4b.
4it is no difficult to imagine that this is the main goal for control, plunge can be controlled with structural stiffness or wight
distribution
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Riferimento filtrato
Filtered_rif
Radiansto Degrees3
R2D
Radiansto Degrees2
R2D
Radiansto Degrees1
R2D
Radiansto Degrees
R2D
Plant
beta1
beta2
xMCS h
rif
x
beta1
MCSalpha
rif
x
Beta1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.02
Time [seconds]
h[m]
(a)Controlled plunge dynamics with MCS
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.930
20
10
0
10
20
30
Time [seconds]
Feedbackgains
(b) Feedback gains for controlled plunge dynamics
3.2.0.6 Simulations
Some simulations are shown using the equations defined in the previous sections. At the beginning of this
chapter it is stated that no assumption is made on the numerical values of the plant parameters; this on one
side means that the MCS controller works as an identifier other than as a controller - as can be seen in [1], and
on the other side means that in order to allow a correct identification of the plant parameters a transient is
necessary so to allow the gains to adapt. In this report this phase will be shown by using long simulations but,as stated in [7], a more industrial approach would be to use as parameters from linear models of the system as
a rough first estimation. This, that might seems a minor aspect for the use of the MCS control, turns out to
be a limitation. Indeed, as will be shown further, if no knowledge of the plant parameters is assumed the gain
adaptation law will start with a peak that will be reflected on the control input. This can result in unexpected
behaviours on the real plant that can lead to instability, LCO, or chaos because of un modelled dynamics. Due
to these reasons the reference signal for the AoA dynamics is periodic square wave at a frequency of 0.2[Hz].
A single period of the reference signal is shown in Fig3.4. If necessary a first order filter is used so to slow
down the reference signal with a bandwidth of approximately 10 rads
. If other reference signals are used it will
be clearly stated.
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3.2. MCS control synthesis and simulations
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Time [s]
Amplitude[deg]
[m]
Reference
Figure 3.4: Reference signal period
Simulations are shown on wide time range so to allow the gains to settle and there are performed on the
basic MCS, some extension, and some possible advanced use cases and problems are also addressed.
The first simulations are conducted under the assumption of a two control surfaces, only the AoA dynamics,
since a simply stabilising controller is on the plunge dynamics, as stated previously.
3.2.0.7 MCS controller
An MCS controller, as show in Fig. ??, is so deployed using the following parameters = 15000, = 10
and
the results are show in the following figures:
Ce
Beta1
1
StateSpace
x = Ax+Buy = Cx+Du
Radiansto DegreesR2D
MatrixMultiply
MatrixMultiply
ye KN(t)*sgn(ye)
r
ye
Ki*int
r
ye
Kr
ye
x
K0
K*u
x2
rif
1
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MRAC Minimum Controller Synthesis control for flutter
0 50 100 1502
0
2
4
6
8
10
12
Time [s]
Angleofattack[
deg]
Reference
Angle of attack
Figure 3.5: Reference signal and output for a MCS controller on the AoA dynamics
0 50 100 150100
80
60
40
20
0
20
Time [s]
[deg]
Figure 3.6: Control signal for a MCS controller on the AoA dynamics
In Fig.3.5it can be seen how the controller adapts the gains after a transient. It is also important to notice
that there happens to be no null tracking error because of the lack of an integral action
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0 50 100 1508
6
4
2
0
2
4
6
8
Time [s]
Error[deg]
Error
Figure 3.7: Tracking error for a MCS controller on the AoA dynamics
In Fig.3.7 the tracking error is shown; during the transient in which the gains are adapting the error is
consistent but gradually decreases as the controller adapts the gain. The tracking error does not go to zero
specially in the rise phase of the signal, this problem will be solved, at least in stationary conditions by the use
of a MCSI controller.
When using an adaptive control scheme, like the MCS, one of the most important things to check is the values
of the gains. This is crucial in the use of a MCS control scheme since, as for instance in the case of saturations
o persistent disturbances, the gain can assume very high values leading so the system to instability. In this caseso the feed forward and feedback gains are show in Fig.3.8and Fig3.9.
0 50 100 1503.5
3
2.5
2
1.5
1
0.5
0
Time [s]
KR
Figure 3.8: Feed Forward gains for the single MCS controller on the AoA dynamics
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0 50 100 1505
4
3
2
1
0
1
2
Time [s]
K
Figure 3.9: Feed back gains for the single MCS controller on the AoA dynamics
In Fig.3.9it is important to notice two details: the first regards the initial peak of the gains this often in
practical applications has to be saturated since can cause big initial control actions, the second aspect regards
the gains that do not settle at a fixed value. What really happens is that the gains do settle at a fixed value but
over a much bigger period of time as illustrated in Fig.3.10. The reason for this behaviour is to be researched int
the persistent disturbances the controller is faced to contrast due not only to the uncontrolled plunge dynamics
but also to the non linearities. As will be shown later other control actions will allow to reduce the settling time
for the gains, and their values.
0 500 1000 1500 2000 2500 3000 3500 4000 4500 500010
5
0
5
10
15
Time [s]
K
Figure 3.10: Feed back gains for the single MCS controller on the AoA dynamics
Before showing the results obtained with a MCSI controller a closer look to the control signal and relative
feedback gain is of interest so to show what happens on a short period of time. Indeed the gain grows briefly
for the single maneuver but the reassest on a lower value. The reason for such a behaviour is because the
MCS control action does not only have a integral law for the gain adaptation but also a faster proportional law
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3.2. MCS control synthesis and simulations
weighed by the parameter .
77 78 79 80 81 82 83 84
35
30
25
20
15
10
5
0
5
10
Time [s]
[deg]
76 77 78 79 80 81 82 83 84 85
2
1.5
1
0.5
0
0.5
1
Time [s]
K
Figure 3.11: Control signal and relative Feedback gain for a single maneuver
3.2.0.8 MCSI controller
An implementation of an MCSI increases the performance of the closed loop system as show in the following
figures.
0 10 20 30 40 50 60 70 80 90 1001
0
1
2
3
4
5
6
7
8
9
Time [s]
(
x2
)[deg]
Figure 3.12: Reference signal and output for a single MCSI controller on the AoA dynamics
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0 10 20 30 40 50 60 70 80 90 1008
6
4
2
0
2
4
6
8
Time [s]
Trackingerror[deg]
Figure 3.13: Tracking error for a single MCSI controller on the AoA dynamics
Differently from what happens in Fig.3.13the tracking error is null specially after the transients for the rise
time of the reference signal.
0 10 20 30 40 50 60 70 80 90 100120
100
80
60
40
20
0
20
40
Time [s]
[deg]
Figure 3.14: Control signal for a single MCSI controller on the AoA dynamics
In Fig. 3.14the control signal is shown; the major difference with the Fig.3.6results in smaller peak values
of the control signal but a closer look shows in Fig.3.18a, also a smoother control action. The feedback and feed
forward gains for the MCSI control settle in a much shorter time with respect to what happens in the MCS
controller (a good assessment can be seen from Fig. 3.10after more or less 3000[s] ). This is due to the integral
action show in Fig.3.17that helps a faster stabilisation. The gains for feed forward, feedback and integral action
can be seen respectively in Fig.3.16,3.15and Fig.3.17.
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3.2. MCS control synthesis and simulations
5 10 15 20 25 30 35 40 45 50 55
5
4
3
2
1
0
Time [s]
K
Figure 3.15: Feed back gain for the single MCSI controller on the AoA dynamics
0 10 20 30 40 50 60 70 80 90 1002.5
2
1.5
1
0.5
0
Time [s]
KR
Figure 3.16: Feed Forward gains for the single MCSI controller on the AoA dynamics
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10 20 30 40 50 60 70 80 90
0.4
0.2
0
0.2
0.4
0.6
0.8
Time [s]
KI
Figure 3.17: Integral gain KIfor the single MCSI controller on the AoA dynamics
Before showing other controller implementations a comparison is helpful to show the increase of performance
between the MCS ad MCSI control strategy as can be seen in the following figures:
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3.2. MCS control synthesis and simulations
68.5 69 69.5 70 70.5 71 71.5 72
0
1
2
3
4
5
6
7
8
Time [s]
(
x2
)[deg]
MCSI
MCS
Reference
71 71.2 71.4 71.6 71.8 72 72.2 72.4 72.6
7
7.2
7.4
7.6
7.8
8
8.2
8.4
8.6
8.8
9
Time [s]
(
x2
)[deg]
MCSI
MCS
Reference
69 70 71 72 73 74 75
6
4
2
0
2
4
6
Time [s]
TrakingerrorsMCSVSMCSI[deg]
MCS
MCSI
73.5 74 74.5 75 75.5 76 76.5 77 77.5 78 78.5
25
20
15
10
5
0
5
10
15
20
Time [s]
Controlsignals[deg]
MCSI
MCS
Figure 3.18: MCS vs MCSI controller on AoA dynamics
3.2.0.9 EMCSI controller
An EMCSI controller is implemented in this section and a first comparison result is shown with respect to the
MCSI controller in the first seconds of simulation.
The dimensioning of the parameter Nhas been carried out with in a qualitative manner as follows. In equation
3.14,all values of variables x1and x3 have been substituted with what is a reasonable physical value for them.
The functiond4has been evaluated a 1.5[rad] - which is a high value for the angle of attack). Then by following
the rules showed in 3.10 Nresults approximately 15000. Thus by leaving the parameters and the same
as what has been done with the MCS and MCSI simulations from Fig. 3.19, it is possible to see that time to
settle good performance is much lower with respect to the MCSI. The reason for such behaviour is to seek in
the commuting action that introduced by the EMCSI controller, that as stated before make the system more
robust.
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
1
2
3
4
5
6
7
8
Time [s]
(
x2
)[deg]
MCSI
Reference
EMCSI
Figure 3.19: Reference signal and output for a single EMCSI controller on the AoA dynamics
Similarly for what has been done for the MCS and MCSI controller the gains are plotted so to show that
the not go grow indefinitely.
0 50 100 1500.05
0.04
0.03
0.02
0.01
0
0.01
Time [s]
K
Figure 3.20: EMCSI Feedback gains for AoA dynamics with one control surface
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3.2. MCS control synthesis and simulations
0 50 100 1500.06
0.05
0.04
0.03
0.02
0.01
0
0.01
Time [s]
KR
0 50 100 1500.5
0.4
0.3
0.2
0.1
0
0.1
0.2
0.3
0.4
0.5
Time [s]
Kn
sgn(y
e)
Figure 3.21: EMCSI gains for AoA dynamics with one control surface
The down side of this control scheme is to be searched in the commuting action introduced on the actuator
and show in Fig.3.22.
32 34 36 38 40 42 44
30
20
10
0
10
20
30
Time [s]
[deg]
Figure 3.22: Control signal for a single EMCSI controller on the AoA dynamics
A final simulation with respect to the MCSI controller is shown in Fig.3.23
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78.9 79 79.1 79.2 79.3 79.4 79.5 79.6 79.70.01
0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
Time [s]
(
x2
)[deg]
Reference
EMCSIMCSI
Figure 3.23: Controller tracking comparison between MCSI and EMCSI controller on the AoA dynamics after a longer simulation
3.2.0.10 NEMCSI controller
For this controller only major results are shown as a comparison with the EMCS and the values of the gains.
In Fig.3.24it is possible to see after the gain have reached a stable value a performance increase.
101.5 102 102.5 103 103.5 104 104.5
0
1
2
3
4
5
6
7
8
Time [s]
(
x2
)[deg]
NEMCSI
ReferenceEMCSI
101.2 101.3 101.4 101.5 101.6 101.7 101.8 101.9 1027.5
8
8.5
9
Time [s]
(
x2
)[deg]
NEMCSI
ReferenceEMCSI
Figure 3.24: Output performance comparison between NEMCSI and EMCSI
The gains are reported, with exception for KNreported in Fig.3.26,are reported in Fig. 3.25.
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3.2. MCS control synthesis and simulations
10 20 30 40 50 60 70 80
1.5
1
0.5
0
0.5
Feedback Gain K
Feedback Gain K
Feedforward gains KR
Figure 3.25: NEMCSI controller gains on the AoA dynamics after a gain assessment, with exception ofKNreported in Fig.3.26
10 20 30 40 50 60 70
6
4
2
0
2
4
6
x 103
Time [s]
KN
Figure 3.26: NEMCSI controller KNgain on the AoA dynamics after a gain assessment
3.2.0.11 LQ-NEMCSI controller
This extension of the MCS strategy uses an LQ controlled linear model as a reference model for the plant
and using the control laws seen for he NEMCI. An LQ model is o synthesised by solving the LQ problem on
the reference model as in equation 3.3; the selection of the weight matrixes is done using the Bryson rule.
The simulation results obtained, and showed in comparison to the other MCS extensions, are show on in the
following figures.
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100 100.5 101 101.5 102 102.5 103 103.5 104 104.5 1051
0
1
2
3
4
5
6
7
8
9
Time [s]
(
x2
)[deg]
Reference
LQNEMCSI
NEMCSIEMCSI
Figure 3.27: LQ-NEMCSI controller performance
10 20 30 40 50 60 70 80 90
4
3
2
1
0
1
2
3
Time [s]
Gains
Feedforward gain
Feedback gain
Feedback gain
10 20 30 40 50 60 70 80
10
8
6
4
2
0
2
4
6
8
10
Integral gain KI(t)
Figure 3.28: Gain plot for LQ-NEMCSI
3.2.0.12 Gain-locking
It is important to show that the MCS control strategy is not free of problems. One of the major ones is to be
searched in the infinite value that the controller gains can reach. A very simple complication of the system allows
to show how this can severely affect the performance of the controller. A saturation can be introduced to model
the actuator limitation for high values of the angles, the behaviour is shown in the following figure. Saturation
on the control input is introduced and as can be seen in Fig.3.29athe values of the gains grows indefinitely.
This happens because the controller reads a null tracking error that cannot compensate fully because of the
saturation o the actuators; the result of this a fast growth of the gains. To solve this issue, that is also known
as gain drifting, a gain lockingcriterion has to be introduced. As for shown example a possible solution is to
lock the gains after that the tracking error is below 1% and 1[s] has passed after the assigned settling time.
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3.2. MCS control synthesis and simulations
340 345 350 355 360 365 370 37520
15
10
5
0
5
10
15
20
Time [s]
[deg]
340 345 350 355 360 365 370 3750
1
2
3
4
5
6
7
8
Time [s]
(
x2
)[deg]
Reference
0 50 100 150 200 250 300 350 400 450120
100
80
60
40
20
0
20
40
Time [s]
Gains
Feedback Gain
Feedback Gain
Feedforward gain
340 345 350 355 360 365 370 375 380
100
50
0
50
Time [s]
Gains
Feed back gain
Feed back gain
Feed forward gain
3.2.0.13 Velocity variation rejection and comparison
One of the main characteristics of the MCS control scheme is its capacity to face parameter variations if the
variation of the parameter is slow than the gain adaptation law. Two interesting case are here shown, the first
shows a 2gdeceleration of the air stream and sudden re-acceleration in a steady state, the second case will show
only a 2gdeceleration during a transient of a manoeuver.
The first velocity profile that is used is the shown in fig .3.29.
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MRAC Minimum Controller Synthesis control for flutter
120 122 124 126 128 130 132 134 136 138 140
5
10
15
20
25
Time [seconds]
U[m/s]
Aistream Velocity
Figure 3.29: Airstream velocity variation
The results of the simulation carried on all the controllers are shown in the following figure
128.5 129 129.5 130 130.5 131
0.2
0.15
0.1
0.05
0
0.05
0.1
0.15
0.2
Reference
MCSI
NEMCSI
MCS
LQNEMCSI
Figure 3.30: Output for the various controllers under Fig.3.29airstream parameter variation
The feedback gains are displayed as well for the MCS controller and the LQ-NEMCSI.
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126 128 130 132 134 136 138 140
5
4
3
2
1
0
1
2
3
4
5
Time [seconds]
FeedbakgainsMCS
Figure 3.31: Feedback gain for the MCS controller under Fig.3.29airstream parameter variation
128.5 129 129.5 130 130.5 131 131.5 132
0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time [seconds]
FeebbackGainsLQ
NEMCSI
Figure 3.32: Feedback gain for the LQ-NEMCSI controller under Fig.3.29airstream parameter variation
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MRAC Minimum Controller Synthesis control for flutter
127 128 129 130 131 132 133 134
4
3
2
1
0
1
2
3
x 104
Time [seconds]
NEMCSIKN
(t)
Figure 3.33: Kn(t) gain for the LQ-NEMCSI controller under Fig.3.29airstream parameter variation
As can be seen from Fig.3.31,3.32and3.33, while the MCS controller can only compensate the variation
of airstream speed with a Feedback action in the LQ-NEMCSI the variation is compensated main by the
discontinuous actionKN(t)sgn(ye) and the integral action (here not shown).
The seconds parameter variation wave form used is displayed in fig.3.34.
0 50 100 150
6
8
10
12
14
16
18
20
22
24
26
Time [seconds]
Airstreamv
elocity[m/s]
Aistream Velocity U
Figure 3.34: Airstream velocity variation
The output for the MCS and LQ-NEMCSI controller is shown in Fig .3.35
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126 127 128 129 130 131 132 133 134
2
1
0
1
2
3
4
5
6
7
8
Time [seconds]
Reference
MCS
LQNEMCSI
Figure 3.35: Output for the MCS and LQ-NEMCSI controllers under Fig.3.34airstream parameter variation
3.2.0.14 Hybrid parameter variation
A simulation that is worth to be displayed is one with a hybrid system. Although this of no physical interest
in this type of system (U or a vary with continuity), it interesting to show how the controller reacts to such
variation. To achieve a hybrid system a sudden variation (a 20 ms
velocity decrease) of U in a null time is
introduced, att = 126.5[s] as shown in fig.3.36
0 20 40 60 80 100 120 140 160 180 200
5
10
15
20
25
Time [s]
U
airstreamv
elocity
Figure 3.36: Airstream velocity "hybrid" variation
The output of the system is shown in3.37. The controller does take some time to handle the variation but
the over all behaviour is good, especially considering the tracking error is below 1[degree] approximately. The
gains are also plotted for such variation. A fine tuning allows better performances.
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126 128 130 132 134 136 138 140 142 144
0
1
2
3
4
5
6
7
8
Time [seconds]
[
degrees]
Reference
Figure 3.37: Feedback gains for the chaos recovery simulation
120 125 130 135 140
0.1
0.08
0.06
0.04
0.02
0
0.02
0.04
0.06
0.08
0.1
Time [s]
KN
(t)
(a) KN(t) gain variation for the closed loop system under
going parameter variation in fig.3.36
120 125 130 135 140 145 150
1.5
1
0.5
0
0.5
Time [seconds]
Feedbackgains
K
(b)Feedback K gain variation for the closed loop system
under going parameter variation in fig.3.36
120 125 130 135 140 145
60
40
20
0
20
40
60
Time [seconds]
IntegralgainKI
(c)Integral gain variation for the closed loop system un-
der going parameter variation in fig.3.36
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3.2. MCS control synthesis and simulations
3.2.0.15 Chaos recovery
As mentioned in chapter 1 we proposed to show how the MCS strategy - or an extension - can control and
recover from