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3-1
Smart Icing Systems Review, June 19-20, 2001
Aircraft Autopilot Studies
Petros Voulgaris
Vikrant Sharma
University of Illinois
3-2
Smart Icing Systems Review, June 19-20, 2001
Objectives
Investigate the A/P & FCS
behavior under icing conditions.
Develop methods to enhance the
A/P behavior :
- Envelope Protection - Adaptation of the current A/P
- Robust redesign
3-3
Smart Icing Systems Review, June 19-20, 2001
Overview of the Talk
• Autopilot Modes
• Autopilot structures
• A few simulations
• Current and Future work
3-4
Smart Icing Systems Review, June 19-20, 2001
Autopilots
Longitudinal Modes
– Pitch Attitude Hold (PAH)– Altitude Hold (ALH)
Lateral Modes
– Roll Attitude Hold (RAH)– Heading Hold (HH)
3-5
Smart Icing Systems Review, June 19-20, 2001
Block Diagram for PAH
Ki/s
K
Kq
ActuatorDynamics&CompDelay
A/C Dynamics
q
PAH
integrator
refref
--++
++
++--
ee
3-6
Smart Icing Systems Review, June 19-20, 2001
Block Diagram for ALH
Ki/s
Kh
Kq
Actuator Dynamics&Comp
Delay
A/C Dynamics
q
Hr
integratorH
dKs
s
K
Washout filter
++
--
++
++--
++--
ALHALH
ee
3-7
Smart Icing Systems Review, June 19-20, 2001
Block Diagram for RAH
A/C
Turn CoordinationLoop gains
ref
K
Ki/s
ActuatorDynamics
&Comp Delay
a
r
r
RAH
+-
+
++
3-8
Smart Icing Systems Review, June 19-20, 2001
Heading Hold
A/CDyn.
Turn CoordinationLoop gains
K
Ki/s
ActuatorDynamics
&Comp Delay
a
r
r
K
ref
HH
+
++
+
-+-
3-9
Smart Icing Systems Review, June 19-20, 2001
A/P Performance
• Local designs exhibit good performance and stability margin properties
• Gains are scheduled on A/C speed
• Overall A/P performs well over the operational envelope of Twin Otter for clean conditions
3-10
Smart Icing Systems Review, June 19-20, 2001
Simulation results : Case 1c
• Aircraft State : Initially trimmed at V = 76 m/s and H = 2300 m • No icing• Autopilots engaged : Altitude Hold till T = 370 s and then Pitch
Hold is engaged.
• Maneuver made : pitch up by 11.5 degrees at T = 370 s and the pitch back at T = 420 s.
0 50 100 150 200 250 300 350 400 450 50040
45
50
55
60
65
70
75
80
Velocity response
Time (s)
Vel
oci
ty (
m/s
)
0 50 100 150 200 250 300 350 400 450 5000
1
2
3
4
5
6
7
8
9
10
Angle of Attack Vs Time
Time (s)
Ang
le o
f A
ttac
k (d
eg)
3-11
Smart Icing Systems Review, June 19-20, 2001
Case 1c : Pitch Up
• Case 1 continued :
0 50 100 150 200 250 300 350 400 450 5002250
2300
2350
2400
2450
2500
2550
2600 Height Vs Time
Time (s)
Hei
ght (
m)
0 50 100 150 200 250 300 350 400 450 500-2
0
2
4
6
8
10
12
14Pitch Angle vs Time
Time (s)
Pitc
h A
ngle
(de
g)
0 50 100 150 200 250 300 350 400 450 500-10
-8
-6
-4
-2
0
2 Elevator Deflection Vs Time
Time (s)
Ele
vato
r D
ef (
deg)
3-12
Smart Icing Systems Review, June 19-20, 2001
Case 1i
• Aircraft State : Initially trimmed at V = 76 m/s and H = 2300 m.• Icing : Gets fully iced in 100 seconds Starting at T = 0.• Autopilots engaged : Altitude Hold engaged till T = 370 s and then
Pitch Hold is engaged.• Maneuver made : Pitch up to 17 degrees at T = 370 s and the
pitch back at T = 420 s to its trim condition at T = 370 s.
0 50 100 150 200 250 300 350 400 450 50030
35
40
45
50
55
60
65
70
75
80Velocity Vs Time
Time (s)
Vel
oci
ty (
m/s
)
0 50 100 150 200 250 300 350 400 450 5000
5
10
15
20
25Angle of attack Vs Time
Time (s)
Ang
le o
f A
ttac
k (d
eg)
3-13
Smart Icing Systems Review, June 19-20, 2001
Case 1i : Pitch Up
0 50 100 150 200 250 300 350 400 450 5002220
2240
2260
2280
2300
2320
2340
2360
2380Height Vs Time
Time (s)
He
igh
t (m
)
0 50 100 150 200 250 300 350 400 450 500-5
0
5
10
15
20
25Pitch angle vs Time
Time (s)
Pitc
h a
ng
le (
de
g))
0 50 100 150 200 250 300 350 400 450 500-20
-15
-10
-5
0
5Elevator deflection vs Time
Time (s)
Ele
vato
r d
efle
ctio
n (
de
g))
3-14
Smart Icing Systems Review, June 19-20, 2001
Comparison
Case 1c Case 1i
350 360 370 380 390 400 410 420 430 440-5
0
5
10
15
20
Blow up of the pitch response
Time (s)
Pitc
h a
ng
le (
de
gre
es)
350 360 370 380 390 400 410 420 430 440-20
-15
-10
-5
0
5Blow up of the elevator response
Time (s)
Ele
vato
r d
ef
(de
g)
350 360 370 380 390 400 410 420 430 440-5
0
5
10
15
20
Blow up of the pitch response
Time (s)
Pitc
h a
ng
le (
de
g)
350 360 370 380 390 400 410 420 430 440-20
-15
-10
-5
0
5
Blow up of the elevator response
Time (s)
Ele
vato
r d
ef
(de
g)
3-15
Smart Icing Systems Review, June 19-20, 2001
Case 2c
• Aircraft State : Aircraft initially trimmed at V = 76 m/s and H=2300m • No icing• Autopilots engaged : Altitude Hold engaged till T = 370 s and then
Pitch Hold is engaged. • Maneuver made : Pitch up by 2 degrees at T = 370 s and the pitch
back to the initial trim at T = 420 s.
0 50 100 150 200 250 300 350 400 450 50069
70
71
72
73
74
75
76
77
78Velocity vs Time
Time (s)
Vel
oci
ty (
m/s
)
0 50 100 150 200 250 300 350 400 450 5000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8Angle of Attack vs Time
Time (s)
Ang
le o
f at
tack
(de
gre
es)
3-16
Smart Icing Systems Review, June 19-20, 2001
Case 2c : Pitch Up
0 50 100 150 200 250 300 350 400 450 5002280
2300
2320
2340
2360
2380
2400Height vs Time
Time (s)
Hei
ght
(m)
0 50 100 150 200 250 300 350 400 450 500-0.5
0
0.5
1
1.5
2
2.5
3
3.5Pitch angle vs Time
Time (s)
Pitc
h an
gle
(deg
rees
)
0 50 100 150 200 250 300 350 400 450 500-5
-4
-3
-2
-1
0
1
2
3
4
5Elevator deflection vs Time
Time (s)
Ele
vato
r de
flect
ion
(deg
)
3-17
Smart Icing Systems Review, June 19-20, 2001
Case 2i
• Aircraft State : Initially trimmed at V = 76 m/s and H = 2300 m.• Icing : The aircraft is allowed to get fully iced in 100 seconds.• Autopilots Engaged : Altitude Hold engaged till T = 370 s and then
Pitch Hold is engaged.• Maneuver made : Pitch up by 2 degrees at T = 370 s and then pitch
back to the trim state just before pitching up at T = 420 s.
0 50 100 150 200 250 300 350 400 450 50045
50
55
60
65
70
75
80 Velocity vs Time
Time (s)
Vel
oci
ty (
m/s
)
0 50 100 150 200 250 300 350 400 450 5000
1
2
3
4
5
6
7
8
9 Angle of Attack vs Time
Time (s)
Ang
le o
f at
tack
(de
gre
es)
3-18
Smart Icing Systems Review, June 19-20, 2001
Case 2i : Pitch Up
0 50 100 150 200 250 300 350 400 450 5002280
2290
2300
2310
2320
2330
2340
2350Height vs Time
Time (s)
Hei
ght
(m)
0 50 100 150 200 250 300 350 400 450 5000
2
4
6
8
10
12
14Pitch angle vs Time
Time (s)
Pitc
h an
gle
(deg
rees
)
0 50 100 150 200 250 300 350 400 450 500-10
-8
-6
-4
-2
0
2Elevator deflection vs Time
Time (s)
Ele
vato
r de
flect
ion
(deg
)
3-19
Smart Icing Systems Review, June 19-20, 2001
Comparison
Case 2c Case 2i
350 360 370 380 390 400 410 420 430 440-0.5
0
0.5
1
1.5
2
2.5
3
3.5
Blown up pitch angle response
Time (s)
Pitc
h an
gle
(deg
rees
)
350 360 370 380 390 400 410 420 430 4404
5
6
7
8
9
10
11
12
13Blow up of the pitch angle response
Time (s)
Pitc
h an
gle
(deg
rees
)
3-20
Smart Icing Systems Review, June 19-20, 2001
Case 3c
• Aircraft State : Initially trimmed at V = 60 m/s and H = 2300 m.• No icing• Autopilots Engaged : Altitude Hold engaged throughout and RAH
is engaged after T = 370 s.• Maneuver made : Roll by 10 degrees at T = 370 s and then roll
back at T = 420 s.
0 100 200 300 400 500 60058
59
60
61
62
63
64 Velocity vs Time
Time (s)
Vel
oci
ty (
m/s
)
0 100 200 300 400 500 6002.4
2.6
2.8
3
3.2
3.4
3.6
3.8
4Angle of Attack vs Time
Time (s)
Ang
le o
f A
ttac
k (d
egre
es)
3-21
Smart Icing Systems Review, June 19-20, 2001
Case 3c : Roll
0 100 200 300 400 500 6002295
2296
2297
2298
2299
2300
2301
2302
2303
2304 Height vs Time
Time (s)
He
igh
t (m
)
0 100 200 300 400 500 600-2
0
2
4
6
8
10
12 Roll angle vs Time
Time (s)
Ro
ll a
ng
le (
de
gre
es)
0 100 200 300 400 500 600-10
0
10
20
30
40
50
60
70
80
90 Yaw Angle vs Time
Time (s)
Ya
w a
ng
le (
de
gre
es)
0 100 200 300 400 500 6001.5
2
2.5
3
3.5
4
4.5
5 Pitch Angle vs Time
Time (s)
Pitc
h A
ng
le (
de
gre
es)
3-22
Smart Icing Systems Review, June 19-20, 2001
Case 3c : Control deflections
0 100 200 300 400 500 600-2.6
-2.4
-2.2
-2
-1.8
-1.6
-1.4
-1.2Elevator deflection vs Time
Time(s)
Ele
vato
r de
flect
ion
(deg
)
0 100 200 300 400 500 600-1.5
-1
-0.5
0
0.5
1
1.5Rudder deflection vs Time
Time (s)
Rud
der
defle
ctio
n (d
eg)
0 100 200 300 400 500 600-6
-4
-2
0
2
4
6Aileron Deflection vs Time
Time (s)
Aile
ron
defle
ctio
n (d
eg)
3-23
Smart Icing Systems Review, June 19-20, 2001
Case 3i
• Aircraft State : Initially trimmed at V = 60 m/s and H = 2300 m.• Icing : Aircraft gets fully iced in the first 300 s.• Autopilots Engaged : Altitude Hold engaged throughout and RAH
is engaged after T = 370 s.• Maneuver made : Roll by 10 degrees at T = 370 s and then roll
back at T = 420 s.
0 100 200 300 400 500 60025
30
35
40
45
50
55
60
65 Velocity vs Time
Time (s)
Vel
oci
ty (
m/s
)
0 100 200 300 400 500 6000
5
10
15
20
25
30 Angle of attack vs Time
Time (s)
Ang
le o
f A
ttac
k (d
egre
es)
3-24
Smart Icing Systems Review, June 19-20, 2001
Case 3i : Roll
0 100 200 300 400 500 600600
800
1000
1200
1400
1600
1800
2000
2200
2400 Height vs Time
Time (s)
He
igh
t (m
)
0 100 200 300 400 500 6002
4
6
8
10
12
14
16
18
20
22 Pitch angle vs Time
Time (s)
Pitc
h a
ng
le (
de
gre
es)
0 100 200 300 400 500 600-40
-20
0
20
40
60
80
100
120
140Yaw angle vs Time
Time (s)
Ya
w a
ng
le (
de
gre
es)
0 100 200 300 400 500 600-6
-4
-2
0
2
4
6
8
10
12
14Roll angle vs Time
Time (s)
Ro
ll a
ng
le (
de
gre
es)
3-25
Smart Icing Systems Review, June 19-20, 2001
Case 3i : Control deflections
0 100 200 300 400 500 600-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2 Elevator deflection vs Time
Time (s)
Ele
vato
r d
efle
ctio
n (
de
g)
0 100 200 300 400 500 600-1.5
-1
-0.5
0
0.5
1 Rudder deflection vs Time
Time (s)
Ru
dd
er
de
flect
ion
(d
eg
)
0 100 200 300 400 500 600-3
-2
-1
0
1
2
3 Aileron deflection vs Time
Time (s)
Aile
ron
de
flect
ion
(d
eg
)
3-26
Smart Icing Systems Review, June 19-20, 2001
Comparison
Case 3c Case 3i
350 400 450 500 550-2
0
2
4
6
8
10
12Blowup of the Roll response for the clean case
Time (s)
Ro
ll a
ng
le (
de
gre
es)
350 400 450 500 550-6
-4
-2
0
2
4
6
8
10
12
14Blowup of the roll response for the Iced case
Time (s)
Ro
ll a
ng
le (
de
gre
es)
3-27
Smart Icing Systems Review, June 19-20, 2001
Some Conclusions
• Icing can cause saturation of control surfaces
• Icing can cause severe degradation in A/P performance
• Altitude cannot be held with elevator only
• There is a need to adapt overall A/P structure
3-28
Smart Icing Systems Review, June 19-20, 2001
Adaptation
• Three levels
- Level 1 : Envelope Protection
- Level 2 : Adapt current FCS gains
- Level 3 : Augment with new FCS design
3-29
Smart Icing Systems Review, June 19-20, 2001
Level 1 : Pilot Command Module Adaptation
A/C Dynamics
IcingCharacterization
A/P & SASK=K(V)
EnvelopeProtection
ModulePilot
Control
Inputs
Ref.
Comm.Inputs
Aircraft Icing Parameters
Sensor
Meas.
ice
3-30
Smart Icing Systems Review, June 19-20, 2001
Level 2 : Pilot Command Module & A/P Adaptation
A/C Dynamics
IcingCharacterization
A/P & SASK=K(V, )
EnvelopeProtection
ModulePilot
Ref.
Comm.Inputs
Aircraft Icing Parameters
Control
Inputs
Sensor
Meas.
ice
ice
3-31
Smart Icing Systems Review, June 19-20, 2001
Level 3 : Augment with New A/P Design
A/C Dynamics
IcingCharacterization
A/P & SASK=K(V, )
EnvelopeProtection
ModulePilot
Ref.Comm.
Inputs
Aircraft Icing Parameters
Control
Inputs
NewA/P
Design SensorMeas.
++
ice
ice
3-32
Smart Icing Systems Review, June 19-20, 2001
Envelope protection module
• Account for peak transient values• Use of robust control methods
Want for all t
What is the maximum allowable ?
Ytyp )(
)(tr
A/C&
A/P
ypr
Pilot inputs Variables to be limited
3-33
Smart Icing Systems Review, June 19-20, 2001
Our Approach to EP
• A Fact
for all t iff for all t
where : L1 norm
• A simple bound on pilot stick commands
Ytyp )(Y
sGtr 1
),()(
1),( sG
G(s, )r yp
Pilot inputs Variables to be limited
A/C & A/P linearized dynamics
3-34
Smart Icing Systems Review, June 19-20, 2001
Concepts for new design
• Use robust control methods
• G(s,) depends on A/P• Can find limits of A/P performance :
want for
What is maximum allowable Overall possible A/P’s ?
• Leads to guidelines for new A/P design
A/C&
A/P
yp
Variables to be limited
r
Pilot inputs
Ytyp )( Wtw )()(tr
}
G(s,)
w Disturbances
3-35
Smart Icing Systems Review, June 19-20, 2001
Current and future work
Use a nonlinear model for Cm, Cl
and Cd curves.
Performance characterization.
Adaptation : Three levels
Incorporate throttle control