DEVELOPMENT OF FUZZY LOGIC LQR CONTROL INTEGRATION FOR AERIAL REFUELING AUTOPILOT

Proceeding of the 12-th ASAT Conference, 29-31 May 2007 GUD-04 1
Military Technical College
Kobry El-Kobba
Cairo, Egypt
12-th International Conference
on
Aerospace Sciences &
Aviation Technology
DEVELOPMENT OF FUZZY LOGIC LQR CONTROL
INTEGRATION FOR AERIAL REFUELING AUTOPILOT
Ahmed Momtaz*, Han Chao**
ABSTRACT
This paper investigates the performance on the aircraft control system during air
refuel purposes of the Linear Quadratic Regulator (LQR) control alone, and the
integration between fuzzy control and LQR. LQR is modern linear control that is
suitable for multivariable state feedback and is known to yield good performance
for linear systems or for nonlinear systems where the nonlinear aspects are
presented. The fuzzy control is known to have the ability to deal with
nonlinearities without having to use advanced mathematics. The LQR integrated
fuzzy control (LQRIFC) simultaneously makes use of the good performance of
the LQR in the region close to switching curve, and the effectiveness of the fuzzy
control in region away from switching curve. A new analysis of the fuzzy system
behavior presented helps to make possible precise integration of LQR features
into the fuzzy control. The LQRIFC is verified by simulation to suppress the
uncertainty instability more effectively than the LQR besides minimizing the time
of the mission proposed.
KEY WORDS
Fuzzy control, LQR, Aerial refueling, Autopilot
NOMENCLATURE
(LQR) linear quadratic regulator, (FLC) fuzzy logic controller, (COA) central of
area, (FOS) fuzzy net output signal
*PHD Candidate, School of astronautics, Beihang University, Beijing, China
**Professor, School of astronautics, Beihang University, Beijing, China

1. INTRODUCTION
The objective of the present work is to minimize the net performance index (PI) of
the aircraft control system during aerial refueling process by using fuzzy logic
LQR control integration instead of using LQR alone. Effectiveness of this method
is mostly appeared when the system is highly nonlinear and exposed to
surrounding uncertainty. Using of such integration permit stability and robustness
of the system specially when applying different optimization methods for fuzzy
logic parameters tuning.
2. AIRCRAFT MODELING
One of the key phases in aircraft design is the knowledge acquisition about the
aerodynamic data that can be used in all the computer oriented design
approaches, using this data the designer can build up a simulated nonlinear
model for this aircraft. For our research we had used flight aerodynamic data
given by (Brumbaugh, 1991) and (Stevens & Lewis, 1992) [1]. The employed
flight model is a stability axis model, with a state vector [α q δe αf ] for pitch rate
control and [β φ p r δa δr] for lateral control.
3. OPTIMAL FLIGHT CONTROL (LQR)
The modern control system design is based directly on the state variable model,
which contains more information about the system. Another central concept is
the expression of performance specifications in terms of a mathematically
precise performance criterion that then yields matrix equations for the control
gains. The classical successive loop closure approach means that the control
gains are selected individually. In contrast, solving matrix equations in modern
control allows all the control gains to be computed simultaneously so that all the
loops are closed at the same time with stable closed loop poles (as an algorithm
condition). This could be achieved by selecting the control input to minimize
a quadratic cost or performance index (PI) of the type
)(tu
dteVeuRuxQxJ TTT ~~
2
1
)~~~~(
2
1
0
++= ∫
∞
(1)
Where Q and R are symmetric positive semi-definite weighting matrices,
x~ , u~ , e~ are state, control, steady state error respectively

4. APPLYING FUZZY INFERENCE SYSTEM FOR CONTROL
The fuzzy inference technique can be very useful in control engineering. A
standard control system would utilize a numerical input and produce numerical
output and so should a fuzzy controller. The knowledge base contains the set of
inference rules chosen to achieve the control objectives and the parameters of
the fuzzy systems used to define the data manipulation in the fuzzification,
inference engine, and defuzzification processes as shown in Fig. 1. Input to the
fuzzification process is the measured or estimated variable that appears in the
antecedent part of the if-then rule. This input variable has associated linguistic
values to describe it. Each linguistic value is defined by a membership function,
parameterized by data from the knowledge base .In the inference engine the
decision–making logic is conducted, inferring control laws from the input
variables through fuzzy implication. The final step is the defuzzification process
where a crisp control command is determined based on the inferred fuzzy control
law.
4.1 Structure of Fuzzy Rules
A fuzzy rule is the basic unit for capturing knowledge in many fuzzy systems. A
fuzzy rule has two components: an if-part (also referred to as the antecedent)
and a then-rule (also referred to as the consequent):
IF <antecedent> THEN <consequent>
The antecedent describes a condition, and the consequent describes a
conclusion that can be drawn when the condition holds.
The structure of a fuzzy rule is identical to that of a conventional rule in artificial
intelligence. The main difference lies in the content of the rule antecedent. The
antecedent of the fuzzy describes an elastic condition (a condition that can be
satisfied to a degree) while the antecedent of a conventional rule describes a
rigid condition (a condition that is either satisfied or dissatisfied)[2].
4.2 Rule Derivation
Motivated by the need of a systematic method to generate and modify fuzzy rule-
bases, much research is being conducted on developing learning approaches.
This technique begins with the self-organizing controllers which consist of two
levels of fuzzy rule bases. The first rule base is the standard control fuzzy rule

base. The second level contains a fuzzy rule base consisting of meta-rules,
which attempt to assess the performance of the close loop control system and
subsequently used to modify the standard rule base. Learning approaches based
on evaluation theory, such as genetic algorithms[3], have a promising potential
towards the derivation of fuzzy rule bases as done in this work. Fig. 2 shows one
sort of the speed controllers that have been used utilizing both control signals
(error and error rate) according to the line of sight distance between the aircraft
and the tanker, tuning the normalized and the denormalized factors along
different operating points simulating the gain scheduling technique for the LQR
control. The result of applying COA defuzzification to a fuzzy conclusion can be
expressed by the formula:
A i
i
A i
i
( )y y
y
( )y
iμ
μ
×∑
=
∑
(2)
where Aμ is the membership of area A.
In the same manner nonlinear PID fuzzy logic controller was proposed to be
integrated with LQR controller (inner loop) for both pitch and lateral control
purposes as an outer loop control. Fig. 3 shows the different output control gains
after performing the (COA) defuzzification process for Mamdani type 49 rule
base PID nonlinear fuzzy logic controller (7X7 membership functions). The
following rules describe the logic behavior of the PID nonlinear controller as an
example, in the same way it possible to deduce the rule-base for the speed
controller.
1. if (e is NB) and (ec is NB) then (kp is PB)(Ki is NB)(Kd is PS)(1)
: : : : : : : : : :
49. if (e is PB) and (ec is PB) then (kp is NB)(Ki is PB)(Kd is PB)(1)
5. THE LQR INTEGRATED FUZZY CONTROL (LQRIFC)
The LQR integrated fuzzy control utilizes both advantages from the LQR
controller and fuzzy logic controller as LQR controller can easily satisfies the
flying qualities and pilot rating requirements and fuzzy control can cope with the
nonlinearity of the system introducing a smart way to modifying the output gains
according to the actual performance blending the dynamic response that
generating better performance than using LQR alone. Fig. 4 describes the LQR
integrated fuzzy control (LQRIFC) for pitch controller. The system dynamics is
evaluated by LQR design process at each trimmed point for pitch control, in the

same manner it is possible to introduce the same procedure for lateral-directional
control Fig. 5. Eventually the LQR closed loop system poles can easily satisfy the
flying qualities specifications such as the damping ratio and the natural
frequencies which is the main requirement in designing closed loop aircraft
control system [4].
The states and output of the plant plus the compensator for both pitch and lateral
motions are
[ ]T
Fepitch qx εαδα= , [ ]T
Fpitch qy εα=
[ ]T
wralateral xrpx εδδφβ= , [ ]T
rlateral epey φε=
GrBuAxx ++=•
(3)
FrCxy += (4)
HxZ = (5)
Kyu −= (6)
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
−
−
−=
0002958.570
010000.10
002.2000
00
~~~
00
~~~
eqqqq
eq
BAA
BAA
A
δα
αδααα
, , ,
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
=
0
0
2.20
0
0
B
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
=
1
0
0
0
0
G
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
=
0
0
0
F
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
=
10000
0002958.570
02958.57000
C , [ ]0002958.570=H , [ ]lq KKKKyu α−=−=
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
−
−
−
−
=
00000010
01002958.57000
002.2000000
0002.200000
00
~~~~~~
00
~~~~~~
00
~~~~~~
00
~~~~~~
rrarrrprrr
rpaprppppp
rarp
rarp
BBAAAA
BBAAAA
BBAAAA
BBAAAA
A δδϕβ
δδϕβ
ϕδϕδϕϕϕϕβϕ
βδβδββϕβββ
, ,
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
=
00
00
2.200
02.20
00
00
00
00
B
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
=
01
00
00
00
00
00
00
00
G
Hxx
r
z
w
=⎥
⎦
⎤
⎢
⎣
⎡
−
=⎥
⎦
⎤
⎢
⎣
⎡
=
01002958.57000
00000010ϕ

]
c
r
FrCx
e
p
e
y +=
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎣
⎡∈
=
ϕ
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
−
−
=
00000010
00000100
00002958.57000
10000000
C
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
=
01
00
10
00
F
The control input may be expressed as[ T
ra uuu = Kyu −=
with ⎥
⎦
⎤
⎢
⎣
⎡
=
000
0
2
431
k
kkk
K
6. APPLICATION METHODOLOGY FOR AERIAL REFUELING INTEGRATED
CONTROL SYSTEM
The refueling procedure requires the tanker to fly straight and level at a constant
velocity. The receiving aircraft then closes in and moves to a standard position
behind and below the tanker boom. The two aircraft continue to fly in this
formation while the boom operator in the tanker's tail uses a joystick to move the
boom into position just above the receiver of the plane to be refueled. The
operator then extends the telescoping component of the main boom until the
nozzle is inserted into the receiver. This can be done automatically using some
kind of autopilot consists of three controllers one for pitch control, the second for
lateral control and the third for controlling the aircraft speed. Both controllers
proposed for pitch and lateral control will apply the LQR fuzzy control integration
and the other speed controller uses only fuzzy logic controller 14 rule-base
Mamdani type. The input references for both pitch and lateral controllers will be
fed by the tanker location on the head up display inside the cockpit. The input
reference for the speed controllers will be fed by the line of sight distance
between the aircraft and the tanker. For simulation purposes it is required to map
the tanker position from the earth fixed frame of reference to the aircraft wing
coordinates and this can be done by the proposed transformation as shown in
Fig. 6. Implementing the whole integrated aircraft control system on MATLAB/
SIMULINK thus calculating the actual trajectories and relative distance between
the aircraft and the tanker as shown in Fig. 7. Practically the relative distances
can be calculated by the Head Up display or GPS information that mounted on
both tanker and the aircraft connected to each other through UHF channels .
The following equations explain the transformation procedure starting from the
tanker position with respect to fixed frame of reference to wing coordinate of the
receiving aircraft with initializing small perturbed step (0.0001) to avoid the
singularities during simulation calculation.
X = Tanker X component relative to the fixed frame of reference

Y = Tanker Y component relative to the fixed frame of reference
Z = Tanker Z component relative to the fixed frame of reference
XF = Fighter X component relative to the fixed frame of reference
YF = Fighter Y component relative to the fixed frame of reference
ZF = Fighter Z component relative to the fixed frame of reference
XBO = Tanker X component relative to aircraft body axes at the fixed frame of
reference
YBO = Tanker Y component relative to aircraft body axes at the fixed frame of
reference
ZBO = Tanker Z component relative to aircraft body axes at the fixed frame of
reference
XB = Tanker X component relative to aircraft body axes
YB = Tanker Y component relative to aircraft body axes
ZB = Tanker Z component relative to aircraft body axes
θ , Ф, ψ = Rotation around X,Y, Z axes
A = cos θ cos ψ
B = (sin Ф sin θ cos ψ – cos Ф sin ψ)
C = (cos Ф sin θ cos ψ+ sin Ф sin ψ)
D = cos θ sin ψ
E = (sin Ф sin θ sin ψ + cos Ф cos ψ)
F = (cos Ф sin θ sin ψ – sin Ф cos ψ)
G = – sin θ
M = sin Ф cos θ
N = cos Ф cos θ
)])(())([(
)])(())([(
BDEANACGMABGCDFA
BDEAZAXGMABGXDYA
Z BO
−−−−−
−−−−−
= (7)
)(
)]()[(
BDEA
CDFAZXDYA
Y BO
BO
−
−−−
= (8)
A
CZBYX
X BOBO
BO
)( −−
= (9)
)])(())([(
)])(())([(
BDEANACGMABGCDFA
BDEAAZGXMABGDXAY
Z FFFF
SHIFT
−−−−−
−−−−−
= (10)
)(
)]()[(
BDEA
CDFAZDXAY
Y SHIFTFF
SHIFT
−
−−−
= (11)
A
CZBYX
X SHIFTSHIFTF
SHIFT
)( −−
= (12)
XB = XBO – X shift (13)
YB = YBO – Y shift (14)
ZB = ZBO – Z shift (15)
Fig. 8 shows all the control efforts of the control surfaces and the aircraft engine
power during the whole mission starting from the start point as in Fig. 7. until to

the point of contact to the tanker which is the most essential interval in the
mission. The alignment terminal control secures the position of the receiving
aircraft beneath the tanker of about 30 (ft) and the steady flight speed is equal
the same as the tanker flight speed. The figures show also the relative speed and
distances between the receiving aircraft and the tanker.
7. CONCLUSION
It is clear that designing aircraft controllers using the integration of both fuzzy
logic and LQR has a great benefit in tuning the output performance of the aircraft
particularly in aerial refueling systems. As the designing such autopilot requires
high precision in controlling the receiving aircraft to a certain lower point under
the flying tankers. The control strategy used will be divided into 3 phases, first
phase is the stability system augmentation stabilizing the receiving aircraft at
initial steady state flight. The second phase is the tracking phase controlling the
receiving aircraft from the starting point to the 1000 (ft) behind the tanker at the
desired lower point which is straight and lower from the tanker with 30 (ft) at this
phase it is clear that the receiving aircraft speed profile maintained at almost
constant speed as long as the receiving aircraft is away from the tanker then
decelerating with different acceleration rates till starting the next phase, actually
the smart fuzzy logic speed controller provide such profile according to the rule
bases that was previously discussed. The third phase will switch the control to
alignment controller which aligns the receiving aircraft with the center line of the
tanker with steady level flight decelerating the speed to reach the tanker speed at
the point of contact. Mean while the fuel boom will be extended down manually
by using joystick until connecting the receiving valve in the upper surface of the
receiving aircraft starting the refueling process. As shown in the previous figures
the control strategy satisfies all the requirements stated for aerial refueling
process such as flying qualities and the minimum estimated time (356 seconds)
for initial relative distances of (38000 ft) with application of a DRYDEN gust
model as a vertical gust . The previous figures show the effectiveness of using
such integration to cope with the applied disturbance more than using LQR
alone.
REFERENCES
[1] Stevens, B. L., Lewis, F.L., & Al-Sunni, F. (1991). Aircraft flight controls
design using output feedback. Journal of Guidance, Control, and Dynamics.
[2] Sugeno M. and Yasukawa T.,”A fuzzy-logic-based approach to qualitative
modeling,” IEEE Trans. Fuzzy Syst., vol. 1, pp. 7-31, 1993.
[3] Mendel J. M. and Mouzouris G. C.,”Designing fuzzy logic systems,” IEEE
Trans.Circuits Syst., vol. 44, pp. 885-895, Nov. 1997.
[4] Flying Qualities of Piloted Airplanes, MIL-F-8785C, 5 Nov. 1980.

[5] M.L. Fravolini, A. Ficola, M. R. Napolitano, G. Campa, M. G.
Perhinschi ”Development of Modeling and Control Tools for Aerial Refueling for
UAVs” Perugia University, 06100 Perugia, Ital,2000
Fig. 1. The structure of a Fuzzy Logic Control System
Fig. 2. Mamdani type fuzzy logic speed controller
Fig. 3. PID fuzzy logic controller output gains

Fig. 4. LQR integrated fuzzy control (LQRIFC) for pitch controller
Fig. 5. LQR integrated fuzzy control (LQRIFC) for lateral-directional controller
Fig. 6. Tanker position transformation to aircraft wing axes

Fig. 7. Actual trajectories for tanker and receiving aircraft

Fig. 8. F-16 nonlinear model performance

DEVELOPMENT OF FUZZY LOGIC LQR CONTROL INTEGRATION FOR AERIAL REFUELING AUTOPILOT

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