flight test for pios on the advanced technologies testing aircraft system (attas)

ACGSC 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


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?


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


Slide 4

PIO Phenomena and

Research Motivation


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


Slide 6

Example of CAT II PIO Event Due to Rate Saturation


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)


Slide 8

Possible Compensation Schemes

and Anti-Windup Theory


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


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)


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


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


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~:


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


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)


Slide 16

SAIFE I – Saturation

Alleviation In-Flight

Experiment I


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


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


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?


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


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)


Slide 22

Anti-windup compensator structure for SAIFE I

Lateral/directional design

Full-order designs tested:

one per flight condition

Multivariable

AW compensator


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


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’


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


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:


Slide 27

Video Footage


Slide 28

SAIFE I Flight Test Results


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


Slide 30

Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (I)[deg] with no/ with AW compensation


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


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


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


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


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


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


Slide 37

Scrutinized Test Conditions for SAIFE II

6 AW compensatorsdesigned at FC6

Tests at FC6 and “best”compensators retested at FC2


Slide 38

SAIFE II Flight Test Results


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


Slide 40

Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (I)[deg] with no/ with AW compensation


Slide 41

Exemplary Time Histories for Birdy Tracking Task(FC 6: 20 kft, Ma 0.5) (II)[deg] with no/ with AW compensation


Slide 42

SAIFE I/II CompensatorComparison


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


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

]


Slide 45

SAIFE I/II Compensator comparison

“Best” compensators

AWC1, AWC9


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


Slide 47

Questions?

flight test for pios on the advanced technologies testing aircraft system (attas)

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