Flight Test for PIOs on the Advanced Technologies Testing Aircraft System (ATTAS)

1. Flight Test for PIOs on the Advanced Technologies Testing Aircraft System (ATTAS) 102 ACGSC Meeting16 October 2008, Niagara Falls, NY Oliver Brieger, Group Leader, Flight Test Manching, German Aerospace Center (DLR) Matt Turner, Senior Lecturer, Dept. of Engineering, University of Leicester

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?

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

4. PIO Phenomena and Research Motivation

5. PIO (Pilot Involved Oscillations) sustained or uncontrollable oscillations resulting from the efforts of the pilot to control the aircraft PIO Classification 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

6. Example of CAT II PIO Event Due to Rate Saturation

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)

8. Possible Compensation Schemesand Anti-Windup Theory

9. Magnitude and Rate Saturation Problems Saturation (rate or magnitude) introduces a troublesome nonlinearity into the system Particularly dominant for large/fast control signals • Two types of system behaviour: • Small signal: actuator behaves essentially linearly • Large signal: actuator behaves nonlinearly

10. Methods for the Suppression of CAT II PIO Three 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)

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

12. Notes on Anti-Windup Anti-windup: difficult nonlinear control problem • 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

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: ~ T : d y  p lin

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: L2 gain “Reduced” sector condition. Dictates size of local stability region Lyapunov stability

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)

16. SAIFE I –SaturationAlleviationIn-FlightExperiment I

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

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

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?

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

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)

22. Anti-windup compensator structure for SAIFE I Lateral/directional design Full-order designs tested: one per flight condition Multivariable AW compensator

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

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’

25. Stick Amplitude Step 3 Step 2 Step 1 Stick Frequency 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 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) Build Up Pilot was required to capture wings level roll attitude from an initial bank angle offset applying HQDT

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 Phase 3 – Operational Testing: • 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

27. Video Footage

28. SAIFE I Flight Test Results

29. Exemplary Up & Away Results (FC 6: 20 kft, Ma 0.5)

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

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

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

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

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

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

36. Anti-windup modifications (II) Low-order compensators designed using LMIs Optimal static gains Filters chosen by designer 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

37. Scrutinized Test Conditions for SAIFE II Tests at FC6 and “best” compensators retested at FC2 6 AW compensators designed at FC6

38. SAIFE II Flight Test Results

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

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

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

42. SAIFE I/II CompensatorComparison

43. 1 dB 3 dB 6 dB Open Loop Onset Point (OLOP) Describing function of the fullyactivated rate limiter: Phase II Phase III Phase I typical A/C frequency response Magnitude [dB] limiter inactive wonset Open Loop Magnitude [dB] Open Loop Magnitude [dB] Phase [deg] transition phase wonset 1.86 Normalized Frequency w/wonset Bode plot of the rate limiter describing function Open Loop Phase [deg] Nichols chart

44. Cat II PIO-prone Cat II PIO-free The OLOP Criterion for Conventional Rate Limiters Application • Pilot model (pure gain model) • Rate limit activation frequency wonset • OLOP-Parameter: Magnitude and phase of theopen-loop frequency responseat wonset in the Nichols chart Open Loop Magnitude [dB] Open Loop Phase [deg] OLOP boundary

45. SAIFE I/II Compensator comparison “Best” compensators AWC1, AWC9

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

47. Questions?