Flight Test for PIOs on the Advanced Technologies Testing Aircraft System (ATTAS)
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Transcript of Flight Test for PIOs on the Advanced Technologies Testing Aircraft System (ATTAS)
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Slide 1ACGSC Meeting, October 16, 2008
Flight Test for PIOs on the Advanced Technologies Testing Aircraft System (ATTAS)
Oliver Brieger, Group Leader, Flight Test Manching, German Aerospace Center (DLR)
Matt Turner, Senior Lecturer, Dept. of Engineering, University of Leicester
102 ACGSC Meeting
16 October 2008, Niagara Falls, NY
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ACGSC Meeting, October 16, 2008
Slide 2
Abstract
Summary of SAIFE (Saturation Alleviation In-Flight Experiment) flight
test campaigns:
SAIFE I – July 2006
SAIFE II – September 2007
Study of real-world effects and implications of theoretically sound anti-
windup compensators for PIO reduction
Are such tools useful (do they reduce susceptability to PIOs)?
Do they function as envisaged in-flight?
Are they transparent?
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ACGSC Meeting, October 16, 2008
Slide 3
Content
PIO Phenomena and Research Motivation
Possible Compensation Schemes and Anti-Windup Theory
SAIFE I+II – Saturation Alleviation In-Flight Experiment and Flight Test Results
Conclusions and Outlook
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ACGSC Meeting, October 16, 2008
Slide 4
PIO Phenomena and
Research Motivation
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ACGSC Meeting, October 16, 2008
Slide 5
PIO (Pilot Involved Oscillations)
sustained or uncontrollable oscillations resulting from the efforts of the pilot to control the aircraft
CAT I: effective aircraft dynamics and pilot behavior are considered to be essentially linear and time stationary PIO development is
associated with high open-loop system gain and excessive phase lags in the effective vehicle dynamics
CAT II: quasi-linear Pilot-Vehicle System oscillations with control surface rate and/or position-limiting as the only explicitly non-linear elements
(introduces amplitude dependant lag)
CAT III: essentially non-linear Pilot-Vehicle System oscillations with transitions
PIO Classification
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ACGSC Meeting, October 16, 2008
Slide 6
Example of CAT II PIO Event Due to Rate Saturation
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ACGSC Meeting, October 16, 2008
Slide 7
Current State of the Art
CAT I PIO's reasonably well understood
Several prediction methods addressing CAT II PIO phenomena available (e.g. OLOP, time-domain Neal & Smith, Hess)
Limited flight test data on CAT II PIO events
Current real-time prevention methods lack systematic design criteria and are tuned empirically
Aim
Progress understanding of CAT II PIO
Implement online algorithms for PIO prevention
Test resulting schemes in flight (DLR ATTAS test bed)
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ACGSC Meeting, October 16, 2008
Slide 8
Possible Compensation Schemes
and Anti-Windup Theory
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Slide 9
Magnitude and Rate Saturation Problems
Saturation (rate or magnitude) introduces a troublesome nonlinearity into the systemParticularly dominant for large/fast control signals
Two types of system behaviour:1. Small signal: actuator behaves essentially linearly2. Large signal: actuator behaves nonlinearly
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ACGSC Meeting, October 16, 2008
Slide 10
Methods for the Suppression of CAT II PIOThree basic methods of tackling rate/magnitude saturation:
Re-design controller which accounts for saturation problems a priori (requires complete re-design: expensive, time consuming)Introduce extra compensator which becomes active only during periods of saturation (anti-windup compensation)Restrict magnitude/rate of command signals (can limit small signal performance)
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ACGSC Meeting, October 16, 2008
Slide 11
Anti-Windup Design Philosophy
Two stage design procedure:
Design nominal (linear) controller ignoring saturation constraints
Design anti-windup compensator purely to treat saturation problems
Design goal of anti-windup compensator:
RECOVERY OF UNSATURATED BEHAVIOUR
- Anti-windup compensator only activated upon saturation
- Nominal behaviour undisturbed unless saturation encountered
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ACGSC Meeting, October 16, 2008
Slide 12
Notes on Anti-Windup
Most anti-windup techniques in use today are based on ad hoc design methods
Fragile theoretical basis
No guarantees of stability/performance
Poorly understood tuning rules
Mainly aimed at magnitude (rather than rate) limits
Type II PIO problems invite more advanced anti-windup solutions
Systematic
Stability/performance guarantees
Ability to treat large, complex systems
Ability to tackle rate-saturation
Anti-windup: difficult nonlinear control problem
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ACGSC Meeting, October 16, 2008
Slide 13
Rate-Limit AW Design Approach
Two figures mathematically equivalent
Lower figure simplifies the design task
Decoupling of linear from nonlinear design
Anti-windup aims to:
Stabilise nonlinear loop
Ensure disturbance filter output is small as possible
Mathematically must minimise nonlinear operator:
dlinp ydT ~: dlinp ydT ~:
ydT linp~:
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ACGSC Meeting, October 16, 2008
Slide 14
Features of anti-windup compensator (I)
Anti-windup compensators designed using nonlinear/optimal control techniques to ensure:
Rigorous stability guarantees given
Deviation from nominal performance is minimised
Target for AW compensator:
Lyapunov stability
L2 gain “Reduced” sector condition.
Dictates size of local
stability region
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Slide 15
Features of anti-windup compensator (II)
Anti-windup compensators obtained from solution of LMIs or Riccati
equations
Trade-off between stability region size and performance (also
demonstrated in ground tests)
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Slide 16
SAIFE I – Saturation
Alleviation In-Flight
Experiment I
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Slide 17
ATTAS – Advanced Technologies Testing Aircraft System
Highly modified VFW 614 aircraft
System manipula-tion possible in 5 DOF
Allows testing of new control law concepts in a real world environment
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ACGSC Meeting, October 16, 2008
Slide 18
From Design to Flight Test
• Linear based AW-design (linear model extracted from full non-linear 6 DOF model)
• Desk top offline non-linear simulation
• Conversion of Simulink models into C-source code via RTW
• Ground based manned simulation
• Identical S/W loading in experimental CLAWS on aircraft
• Flight Test
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ACGSC Meeting, October 16, 2008
Slide 19
Purpose of the Flight Tests
Proof of concept
Motivate industry acceptance
Basic understanding of modern AW-compensation methods
Application in practice
PIO-alleviation properties
Can a theoretically sound technique deliver real world performance improvement?
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ACGSC Meeting, October 16, 2008
Slide 20
Scrutinized Test Conditions (I)
(1) 10000 ft, Ma 0.3 / (2) 10000 ft, Ma 0.4 /(5) 20000 ft, Ma 0.4 / (6) 20000 ft, Ma 0.5 /(8) pattern altitude, 135 kEAS
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Slide 21
Scrutinized Test Conditions (II)
Focus on roll axis due to structural constraints in the pitch axis and high roll agility of ATTAS aircraft
Rate limits artificially degraded to 50% and 60% (inherent ATTAS limits for approach and landing) of nominal values
Allow comparison between AW and no AW scenarios
Dedicated AW designs for individual flight conditions
For enhanced robustness
(potential to use single AW-compensator across envelope)
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ACGSC Meeting, October 16, 2008
Slide 22
Anti-windup compensator structure for SAIFE I
Lateral/directional design
Full-order designs tested:
one per flight condition
Multivariable
AW compensator
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Slide 23
Up & Away Test Techniques and Test Philosophy (I)
A complete HQ evaluation must include an evaluation of the full range of gain, bandwidth, and compensation that pilots bring to a task
An ordered build-up approach must be used to ensure that hazards are approached in a safe manner
- Phase 1: Low Bandwidth Testing
- Phase 2: High Bandwidth Testing
- Phase 3: Operational Testing
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ACGSC Meeting, October 16, 2008
Slide 24
Up & Away Test Techniques and Test Philosophy (II)
Phase 1 - Low Bandwidth Testing:
semi-closed loop and closed loop bank angle capture tasks, to enable pilot to become familiar with aircraft dynamics, also referred to as warm-up’ testing
Phase 2 - High Bandwidth Testing :
Employs HQDT (Handling Qualities During Tracking) test technique
Tasks are conducted at safe up-and-away flight conditions
Pilot comments are supported by PIO ratings
Currently the only method that allows for systematic, high bandwidth PIO resistance testing
Referred to as Handling Qualities Stress Testing (HQST) - serves as a handling qualities ‘safe gate’
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ACGSC Meeting, October 16, 2008
Slide 25
BuildUp
The HQDT Piloting Technique
Evaluation pilot is required to track ‘precision aim’ point as aggressively and as assiduously as possible, striving to correct even the smallest tracking error as rapidly as possible
- increases pilot bandwidth and minimizes lead/lag compensation
- emulates pilot control strategy in a high stress situation
Sti
ck A
mp
litu
de
Stick Frequency
Step 1Step 2
Step
3
Step 1 - Low bandwidth, non-aggressive, small amplitude
Step 2 - ‘tighten-up’ to small amplitude, high frequency tracking inputs
Step 3 - Increase input amplitude at high frequency (up to ‘bang-bang’ control)
Pilot was required to capture wings level roll attitude from an initial bank angle offset applying HQDT
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ACGSC Meeting, October 16, 2008
Slide 26
Up & Away Test Techniques and Test Philosophy (III)
Birdy Target Tracking Task
requires pilot to closely track generic birdy (aircraft symbol) projected into MHDD with aircraft water line symbol
Evaluation pilots were tasked to provide Handling Qualities Ratings (HQRs) to quantify system performance during gross
0 10 20 30 40 50 60 70-35
-30
-25
-20
-15
-10
-5
0
5
10
15
(
deg)
time (sec)
desired
adequate
birdy
WL-symbol
desired
adequate
desired
adequate
birdy
WL-symbol
Offset Approaches
- 2 different AW compensator designs tested for approach/landing
- From an initial 200 m lateral offset to the nominal approach path focus was placed on the centerline capture task
Phase 3 – Operational Testing:
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Slide 27
Video Footage
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ACGSC Meeting, October 16, 2008
Slide 28
SAIFE I Flight Test Results
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ACGSC Meeting, October 16, 2008
Slide 29
Exemplary Up & Away Results (FC 6: 20 kft, Ma 0.5)
Comparison of PIO ratings with/without AW (FC6)
0
1
2
3
4
5
6
P1, capture P1, birdy P2, capture P2, birdy
Bank Angle Capture / Birdy Tracking Task
PIO
rat
ing
without AW
with AW
Comparison of HQRs with/without AW (FC6)
0
1
2
3
4
5
6
7
P1, gross P1, fine P2, gross P2, fine
Birdy Tracking Task
HQ
R without AW
with AW
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ACGSC Meeting, October 16, 2008
Slide 30
Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (I)[deg] with no/ with AW compensation
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Slide 31
Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (II)Stick Input [deg] with no/ with AW compensation
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Slide 32
Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (III)Control signals u and ur with no/ with AW compensation
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ACGSC Meeting, October 16, 2008
Slide 33
SAIFE I Critique
Anti-windup compensators successfully tested
Clear improvement in aircraft-pilot system's tendency to induce
PIOs.
Accompanying improvements in basic handling qualities at certain
test conditions
Some room for improvement
Robustness not assessed
Complexity of AW compensators very high (equal states to that of
aircraft)
Link between abstract AW design criteria and performance not
completely understood
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ACGSC Meeting, October 16, 2008
Slide 34
SAIFE II Aims
To build on results of SAIFE I
To design more practical low order compensators (1 or 2 states)
while preserving the theoretical rigour of the original designs
To demonstrate robust compensators which work off flight
condition
To establish clearer links between design criteria and actual in-
flight performance of compensators (e.g. OLOP criterion, L2 gain
etc.)
Comparison of different compensators
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ACGSC Meeting, October 16, 2008
Slide 35
Anti-windup modifications (I)
Focus purely on roll axis – aileron main source of saturation
Only aileron rate-saturation
considered: SISO AW compensator
Pilot model used in AW design
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ACGSC Meeting, October 16, 2008
Slide 36
Anti-windup modifications (II)
Low-order compensators designed using LMIs
Filters chosen by designer Optimal static gains
Filters based on existing full order anti-windup designs and fine-tuned using frequency domain tools and nonlinear simulation.
Gain matrix obtained as solution of either QFT-inspired classical design or LMI-based absolute stability optimisation
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ACGSC Meeting, October 16, 2008
Slide 37
Scrutinized Test Conditions for SAIFE II
6 AW compensatorsdesigned at FC6
Tests at FC6 and “best”compensators retested at FC2
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Slide 38
SAIFE II Flight Test Results
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ACGSC Meeting, October 16, 2008
Slide 39
Exemplary Up & Away Results (FC 6: 20 kft, Ma 0.5)
Basic results:Low-order compensators
deliver similar PIOR/HQR improvements
to full order compensators
...and robustly!
PIOR HQDT HQR fine tracking HQR gross acquisition PIOR Birdy
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ACGSC Meeting, October 16, 2008
Slide 40
Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (I)[deg] with no/ with AW compensation
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Slide 41
Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (II)[deg] with no/ with AW compensation
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Slide 42
SAIFE I/II CompensatorComparison
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Slide 43
limiter inactivetr
ansi
tion
pha
se
Normalized Frequency /onset
Ma
gn
itud
e
[dB
]P
ha
se
[de
g]
onset
1.86
Bode plot of the rate limiter describing function
Open Loop Onset Point (OLOP)
2
arccos4)(
onsetjonset ejN
Describing function of the fullyactivated rate limiter:
Phase I Phase IIIPhase II
Op
en
Lo
op
Ma
gn
itud
e [
dB
]
Open Loop Phase [deg]
Op
en
Lo
op
Ma
gn
itud
e [
dB
]
Nichols chart
typical A/C frequency response
onset
6 dB
3 dB
1 dB
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Slide 44
The OLOP Criterion for Conventional Rate Limiters
Application
Pilot model (pure gain model)
Rate limit activation frequency onset
OLOP-Parameter: Magnitude and phase of theopen-loop frequency responseat onset in the Nichols chart
OLOP boundary
Cat II PIO-prone
Cat II PIO-free
Open Loop Phase [deg]
Op
en
Lo
op
Ma
gn
itud
e [
dB
]
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Slide 45
SAIFE I/II Compensator comparison
“Best” compensators
AWC1, AWC9
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Slide 46
Conclusions and Future Work
Proof of concept demonstrated
Flight tests clearly showed an improvement in PIO suppression and handling qualities due to enhanced predictability of system dynamics
At certain flight conditions pilot workload was reduced
Future Challenges:
Investigate design rules further
Synthesis with fault detection algorithms
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ACGSC Meeting, October 16, 2008
Slide 47
Questions?