Flight Control Law Design: An Industry Perspective

1
Flight Control Law DesignAn Industry Perspective
2003 European Control Conference

• Gary J. Balas
• balas_at_aem.umn.edu
• Aerospace Engineering and Mechanics
• University of Minnesota
• Minneapolis, MN 55105 USA
• September 4, 2003

2
Presentation Overview

• Survey of the control techniques being used by
  industry in Brazil, Europe, Russia and the United
  States of America to design flight control laws
  for fixed-wing aircraft.

3
Outline

• 100 years of Controlled Flight
• Introduction
• Background
• Countries

• Brazil
• Europe
• France
• Germany
• Italy
• Sweden
• United Kingdom

• Israel
• Russia
• United States of America
• Boeing
• Honeywell
• Lockheed Martin

• Summary

4
Wright Brothers December 17, 1903

• First to develop a fully aerodynamic control
  system coupled to a powered aeroplane which was
  both flyable and maneuverable.

5
United Kingdom

• Most Influential Predecessor of the Wrights
• Sir George Cayley in 1799 at Brompton, near
  Scarborough in Yorkshire sketched a
  conventional configuration of an aeroplane.
• Indeed, in 1909 Wilbur Wright himself paid Cayley
  the following tribute
• "About 100 years ago, an Englishman, Sir
  George Cayley, carried the science of flight to a
  point which it had never reached before and which
  it scarcely reached again during the last
  century.
• The History of Flight from Around the World,
  United Kingdom, Eur. Ing. Dr. J.A.D. Ackroyd

6
Cambridge University, England

• Lord Kelvin, President, Royal Society,
  undergraduate at Cambridge University, Senior
  Wrangler

7
Cambridge University, England

• Lord Kelvin, President, Royal Society,
  undergraduate at Cambridge University, Senior
  Wrangler

Heavier-than-air flying machines are impossible
(1895) "I have not the smallest molecule of
faith in aerial navigation other than
ballooning...I would not care to be a member of
the Aeronautical Society," (1896)
8
Current/New Commercial Aircraft
9
Current/New Military Aircraft
Saab Gripen
10
Current/New Military Aircraft
Sukhoi SU-37
F/A-18 E/F
11
Uninhabited Aerial Vehicles (UAVs)
12
Uninhabited Aerial Vehicles (UAVs)
Dragon Drone
Pioneer
FPASS
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Uninhabited Aerial Vehicles (UAVs)

• US Department of Defense (DoD) has 20 UAVs in
  service or under conceptual development
• DoD will have invested over 10 Billion in UAVs
  by 2007.
• DoD UAV systems will grow to 300 by the year
  2010.
• 32 Nations are developing more than 250 models of
  UAVs.
• Over 60 small and Micro UAV programs are under
  way through out the world.
• DoD Unmanned Aerial Vehicle Roadmap 2002-2027

14
Background

• Flight control design research is a very active
  area
• In 2002 alone, 490 flight control design related
  papers were published.
• Majority new theory or applying theory to
  aircraft simulations.
• Basis from which the aircraft industry draws its
  new ideas.
• Working Group 23 of Advisory Group for Aerospace
  Research and development (AGARD now RTO) noted in
  1996
• Skill required to design an advanced flight
  control system is not easily transferred and very
  little material exists in the open literature to
  be used as a reference handbook by designers.
• RTO recommended better documentation of existing
  flight control system development process,
  lessons learned and best practices.

15
Background

• Survey of the current practices and control
  design approaches used by the aircraft industry.
• Brazil , Europe (France, Germany, Italy, Sweden,
  United Kingdom), Israel, Russia, United States
  of America
• Caveats
• Limited information on techniques used in
  industry.
• Some companies consider the control architecture,
  algorithms etc. to be IP.
• Companies that publish more are better
  represented in this talk.
• Almost all the references cited were published in
  English.

16
Brazil

• Embraer ERJ-170 aircraft
• Flight control design philosophy
• Docile and benign as possible flying qualities
  behavior (behaviour)
• Digital fly-by-wire (FBW) control system would
  allow complex flight controllers, cost
  constraints and accelerated time schedule led to
  selection of standard classical flight control
  system.
• In-flight simulation using Veridian Variable
  Stability Learjet.

17
Europe
18
Europe

• Long tradition of aeronautical research.
• Close links between universities and industry
• To increase cooperation, the Association of
  European Research Establishments in Aeronautics
  (EREA) found in 1994.
• CIRA (Italy), DERA (Great Britain), DLR
  (Germany), FFA (Sweden), INTA (Spain), NLR
  (Dutch) and ONERA (France, 1999).
• Group for Aeronautical Research and Technology in
  Europe (GARTEUR)
• GARTEUR Flight Mechanics Action Group 08
  (1994-1997)
• Robust control design methods
• GARTEUR Flight Mechanics Action Group 11
  (1999-2002)
• New Analysis Techniques for Clearance of Flight
  Control Laws
• Many European aircraft industries are
  multi-national and parts of the same aircraft
  flight control laws are designed in more than one
  country.

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France
Concorde
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France

• Mirage 2000, Rafale C
• Airbus
• Major partner in European consortium of French,
  German, Spain and U.K. companies
• A300/A310, A320, A330/A340, A380 under design.
• Developer of commercial fly-by-wire (FBW) system.
• A320 was first commercial aircraft to enter
  service with a FBW flight control system (1988).
• A340 2nd generation FBW certified in 1992.
• All Airbus flight control surfaces are
  electronically controlled and hydraulically
  activated.
• UAVs

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France Airbus A320

• A320 Flight Control Laws
• Improve the natural flying qualities,
  particularly the stability, control and flight
  envelope protection.
• Longitudinal Control
• Load factor demands
• Classical proportional plus integral control
• Lateral-directional Control
• Roll rate, sideslip and bank angle commands
• Classical proportional plus integral control with
  a gain matrix for stability and roll
  rate/sideslip decoupling.

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France Airbus A340

• A340 Flight Control Laws
• Reproduced architecture/principles for A320
• Increased size and flexibility, required addition
  of structural mode control to reduce structural
  mode vibration.

• Structural Mode Suppression Controller
• Manual flight controller/Autopilot modified to
  eliminate interaction.
• Turbulence damping function added to attenuate
  fuselage response.
• Sufficient bandwidth separation between two
  controllers minimized interaction.
• Accelerometers were added to sense vibration.

23
France Airbus A380

• A380 Flight Control Laws
• High capacity, long range
• Flexibility increases interaction between control
  laws and structural dynamic modes.
• Aeroservo-elastic coupling traditionally solved
  by filtering/decoupling, would require reducing
  control bandwidth.

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France Airbus A380

• Airbus A380 Approach
• Airframe flexibility taken directly into account
  when designing the flight control laws.
• Integrated flight control laws to achieve desired
  handling qualities and flexible mode damping
  requirements leading to extended control
  bandwidth.
• Flight tested on A340 using A380 models
• Robust to fuel, payload, etc. variations
• Same concept applied to autopilot and manual
  control laws.

25
Germany

• Commercial Aviation
• Airbus
• DLR experimental aircraft (ATTAS) Advanced
  Technologies Testing Aircraft System.
• Military Aviation
• Eurofighter
• X-31 (US/Germany program)

26
Germany

• X-31A Post stall experimental aircraft
  (US/Germany program)

• First X program with Intl partner
• Enhanced Fighter Maneuverability
• EFM using thrust vectoring
• Goal tactical advantage of EFM in post stall up
  to 70 deg AoA
• Rockwell and MBB
• X-31A Flight Control Laws
• Pilot cmd (p,q,r)
• Sensed feedback (p,q,r,?,?)
• Actuation cmd (?SF ,?DF, ?C, ?R,?,?)
• K - LTI controller
• Optimal LQ digital regulator
• Scheduled with ?, M, h
• Nonlinear feedforward blocks

27
Germany

• VECTOR X-31A program

• Vectoring Extremely short-takeoff-and-landing
  Control Tailless Operation Research (VECTOR)
• Boeing, US Navy, German BWB and EADS

28
Germany

• Flight control system for A380
• Size and flexibility
• EU funded REAL (Robust and Efficient Autoland
  control Law design).
• Industry and research institutions from France,
  Germany and the Netherlands
• Benchmark was DLR ATTAS aircraft.

• DLR REAL flight control design approach
• Multi-Objective Parameter Synthesis (MOPS).
• Robustness addressed via multi-model,
  optimization, Monte-Carlo analysis.

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Germany - REAL

• Stability/command augmentation, tracking,
  guidance
• Inner loops designed using dynamic inversion.
• Total Energy Control System (TECS)
• Lateral tracking uses classical PI control with
  tuning parameters
• Tuning based on multi-criteria/multi-model
  parameter opt using MOPS.

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Germany - REAL

• DLR autoland controller flight tested in 2000
• DLR autoland control performed well.
• Nonlinear ATTAS simulations
• Technical University of Deflt (Netherlands)
  developed a ?-controller to replace the dynamic
  inversion inner-loop controller.
• ONERA (France) developed a fixed-order H?
  controller.

31
Germany - IRIS-T

• Infra-Red Imagining System-Tail (IRIS-T) Missile
• Thrust-vectored control, next generation
  short-range missile.
• Being developed with Greece, Italy, Norway and
  Sweden.
• Extreme maneuverability.
• ?-synthesis robust control technique used to
  design lateral and roll controllers.
• Scheduled on dynamic pressure
• Successful flight test in May 2000.

32
Italy

• Eurofighter
• Germany, United Kingdom, Italy, Spain.
• Quad redudant, full authority DFCS.
• Alenia responsible for basic autopilot
• Longitudinal axis controls attitude or pitch
  angle, lateral axis controls bank angle and
  heading.
• Autopilot designed using classical control tools.
• Control structure defined, Nicholas/Bode plots,
  linear time responses.
• Large amplitude, nonlinear closed-loop
  simulations.
• Modified control structure with nonlinear
  elements.
• Mode logic increased nonlinear elements to
  satisfy mode schedules.

33
Sweden

• Saab/BAE JAS 39 Gripen
• Contract awarded in 1982
• Lightweight, multi-role combat aircraft
• All moving, delta canard configuration
• Hungary (2003) orders 14, 232 ordered

34
Sweden

• Pilot-induced Oscillation (PIO)
• First test aircraft crashed after 6th flight
  (1989), first operational aircraft crashed in
  1993.
• Partial cause PIO related to control surface
  servo rate limits.
• Reduced phase margin or extra delay in feedback
  loop.

35
Sweden

• Controller modification based on PIOs
• Feedback phase compensation based on anti-windup
  methods.
• Increased or advanced phase around nonlinearity.
• Low pass filters used to eliminate biases and
  high frequency roll off issues
• Flight tested and verified.

• Phase compensation technique used in place of
  rate limiters in Gripen production flight control
  system.

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United Kingdom

• Commercial
• Airbus

• Military
• Harrier
• Saab/BAE JAS 39 Gripen
• Eurofighter
• Lockheed Martin/BAE F-35 JSF

37
UK - Jaguar and EAP Programs

• FBW Jaguar program (1980s) precursor to VAAC

• Prove principles of active control technology,
  establish design and flight clearance techniques
  for DFCS.
• First UK aeroplane equipped with full authority
  DFCS.

38
UK - Jaguar and EAP Programs

• Experimental Aircraft Programme (EAP, 1983-1995)
• Follow on from Jaguar programme.
• Control design process
• Linear low frequency PI scheduled as function
  control
• Nonlinear trim distribution, nonlinear control
  power, nonlinear variations of stability
• Linear high frequency avoid structural coupling
• Lessons learned separate regulator and command
  path designs.
• FBW Jaguar and EAP shaped Eurofighter flight
  controller

39
United Kingdom

• Vectored thrust Aircraft Advanced Control (VAAC)
  program
• Inception in 1984.
• Handling, control and display requirements for
  future short takeoff/vertical landing (STOVL)
  aircraft.
• Experimental FBW VAAC Harrier.
• Development and testing of advanced aircraft
  flight control algorithms.
• Longitudinal axis, integrated management of
  thrust vectoring and aerodynamic forces for
  decoupled control.

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UK - VAAC Control Strategies

• Classical control, loop-at-a-time
• Frequency shaping, gain-scheduling, significant
  nonlinearities linearized with inverse functions,
  iterative design.
• Anti-windup scheme and control allocation.
• Nonlinear static inverse
• Nonlinear inverse of the aircraft to determine
  control effectors to trim. aircraft at a given
  maneuvering state.
• Constrainted design process used to define unique
  solution to non-linear inverse problem (trim
  map).
• Nonlinear inverse feed-forward combined with low
  gain, classical feedback design for stability.

41
UK - VAAC Control Strategies

• Nonlinear Dynamic Inversion (NDI)
• Nonlinear dynamic model of aircraft used to
  invert nonlinearities and a classical PI
  controller designed to track desired pitch rate
  command.
• Pilot commands filtered prior to input to NDI
  controller.

42
UK - VAAC Control Strategies

• H? loop shaping
• Multivariable linear controllers at 4 points
  hover-to-forward flight.
• Inner-loop pitch rate feedback used to reduce
  effect of pitch moment due to thrust changes.
• Outer-loop 3-input/3-output, H? loop shaping to
  control normal and forward acceleration and
  incidence.
• Weight selection similar to classical
  loop-shaping.
• Four linear point designs gain-scheduled
  throughout flight envelope.
• Controller implemented in observer form.
• Interpolated controller gains and interpolated
  controller outputs.
• H? loop shaping techniques also used to
  synthesize an integrated longitudinal/lateral
  flight and propulsions control system for VAAC.

43
UK - VAAC Control Strategies

• Linear, parameter-varying (LPV) controller
• System dynamics written as LTI models whose
  state-space coefficients are a function of
  scheduling variable(s).
• LPV H? loop shaping uses LPV model of nonlinear
  aircraft dynamics to directly synthesize a
  scheduled LPV controller.
• Successfully implemented at tested between 1995
  and 1998.

44
Israel

• Light, multi-mission fighter Lavi
• Initial flight test 31 Dec 86, program
  terminated 30 Aug 87
• Flight control laws
• Classical technique with optimal control methods
  used in preliminary design process.
• Lessons learned Relationship between control
  design parameters and flying qualities.
• UAVs

45
Russia
46
Russia
47
Russia
48
Russia

• Extensive history of military/commercial aircraft
  development.
• English language literature on Russian industry
  flight control design techniques is limited.
• Sukhoi 37 FBW flight controller
• Quad redundant DFCS
• Design requirements
• Good handling qualities.
• Optimal trimming.
• Reconfigurable under flight control system
  failures to maximize control moments and trim
  configuration.
• Adaptive controller designed to eliminate small
  amplitude self-induced oscillations due to
  actuator nonlinearities.

49
Russia - SU-37 Aircraft

• Canards and thrust vectoring (TV loop not
  shown.).
• Longitudinal controller synthesized with
  classical control methods.

50
United States of America (USA)

• Commerical
• Boeing (McDonnell Douglas)
• B-717, B-737, B-747, B-757, B-767, B-777
• Honeywell
• Military
• Lockheed Martin (General Dynamics)
• F-16, F-22, F-35 (JSF)
• Northrup Grumann
• F-14, F-20, B-2
• Boeing (McDonnell Douglas, North American
  Rockwell)
• B-52, B-1B, C-17 C-40A, F/A-18, KC-10
• Honeywell

51
USA
B-747
52
USA
B-747
53
USA - 1947
B-747
54
USA Flight Control Specifications

• Multivariable Control
• Multiple sensors, disturbances, objectives,
  surfaces (TV) with coupling.
• Military specifications focus on SISO,
  loop-at-a-time analysis
• Multivariable Control Design Guidelines (1996)
• Honeywell Research Labs, Lockheed Ft. Worth,
  Lockheed Skunk Works
• Eigenstructure assignment, dynamic inversion,
  ?-synthesis
• F-177, YF-22 and MCT/F-16
• Report provides a reference point for the US Air
  Force to evaluate the design of future flight
  control systems.

55
Boeing

• The Boeing Company
• Largest manufacturer of aircraft in the world.
• Merger of Boeing, Rockwell International and
  McDonnell Douglas
• Variety of approaches to flight control design
• Multivariable Flight Control
• First application in 1978 as part of a NASA
  research program.
• Since 1980s, multivariable control as been
  applied to a number of aircraft
• Multivariable control with classical
  frequency-domain interpretations.
• Guidelines to transform design requirements into
  math
• Training of control engineers
• User-friendly control software programs.

56
Boeing

• Multivariable Control Design
• LQR/LQG based
• Performance and robustness
• Direct tradeoff between command response, control
  activity, disturbance rejection and loop
  bandwidth
• Key Selection of variables to regulate and
  controls to perform regulation
• Integral regulators augmented, zero
  steady-state errors to constant inputs

57
Boeing

• Integral LQR/LQG design procedure
• Select controls and regulated outputs controls
  regulated variables.
• Check singular values and transmission zeros.
• Attach integrators and set target zeros.
• Select Q and R matrices for LQR problem.
• Q and R selected based on command loop crossover
  frequencies.
• Q ( R ) diagonal, qi (ri) adjusts bandwidth of
  command loop for yi (ui).
• Check loop at input to integrator and actuator.
• Note that the control loop crossover frequency is
  limited by the actuator capabilities and
  structural mode coupling.
• Verify that the phase margins at higher
  frequencies are sufficient.
• Feed-forward gains adjusted for flying qualities.

58
Boeing

• Applied to B-767, JSF (Boeing), UCAV, JDAM MMT
  and ACTIVE F-15.

59
Boeing X-36

• X-36 Prototype fighter aircraft
• Stealth and agility prior to UCAV
• 28 scale, remotely piloted
• Reconfigurable flight control laws

• Reconfigurable flight control laws
• Dynamic inversion (DI)
• Explicit model following framework
• On-line neural network (NN) to adaptively
  regulate inversion error
• Uncertainties
• Failures
• Damage
• Desired dynamics and control mixer same for DI
  and adaptive NN

B-747
60
Boeing X-36

• Reconfigurable flight control laws
• NN able to stabilize vehicle following failures
  and damage.
• On-line NN adaptively canceled inversion error.
• NN/DI controller provide improved HQ when
  failures occurred.

B-747
61
Honeywell

• Honeywell Research Center (Honeywell Labs)
• Significant contribution to areas of robust
  control and dynamic inversion.
• Approach to NDI is MACH (Multi-Application
  Control).
• MACH is a modular, nonlinear multivariable design
  approach.
• Blends classical control design with inversion.
• Outer-loop controllers Proportional or PI.
• Inner-loop controllers Dynamic Inversion .

62
Honeywell MACH

• Outer loops multivariable signal shaping
  provides position command and rate tracking.
• Inner loops NDI to normalize vehicle dynamics to
  be integrators.
• NDI requires on-board model (OBAC).
• Estimate derivatives of aircraft states and
  control variables (CV).

B-747
63
Honeywell MACH

• Five components of MACH
• 1. Select controlled variables (CV) for
  performance/robustness.
• 2. Outer-loops convert pilot/guidance commands
  into CV commands.
• Nonlinear elements such as limiters.
• 3. Desired dynamics define dynamic behaviour the
  CVs should follow while tracking their commands.
• 4. NDI attempts to solve the equations of motion
  for actuator commands to satisfy CV desired rate
  of change.
• Control allocation logic may be necessary.
• 5. OBAC computes nonlinear vehicle dynamics for
  inversion.
• Least-squares used to approximate aerodynamic
  data.

B-747
64
Honeywell MACH

• Initial controller for Lockheed Martin JSF
  vehicle (1995-97).
• Successfully flight tested on X-38 V132.

B-747
65
Lockheed Martin

• Leading producer of military aircraft
• F/A-22 Raptor
• Air superiority.
• Stealth and agility.
• Sustained supersonic cruise.
• YF-22 (initial demonstrator) flight control
  designed using eigenstructure assignment.
• Series of pitch oscillations 13m above ground
  lead to aircraft impacted on runway (1992).
• PIO due to control surface rate/position
  saturation, time delays.

• JSF F-35

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Lockheed Martin - F/A-22

• JSF F-35

• F/A-22 flight control law redesign
• Excellent flying qualities and lessons learned.
• Classical control combined with eigenstructure
  assignment.
• Removal of pitch integrator key to redesign.
• Addition of first order pitch stick command
  pre-filter needed to recover flying qualities
  requirements.
• Classical tradeoff between pitch attitude and
  flight path angle bandwidth.
• Redesigned flight control law successfully
  achieved Level I handling qualities for all
  closed-loop tracking tasks.

67
Lockheed Martin - JSF

• JSF F-35

• F-35 Joint Strike Fighter (JSF).
• Conventional takeoff/landing (CTOL/AF).
• Aircraft carrier landing (CV/Navy).
• Short-takeoff/vertical landing (STOVL/Marines).
• All variants will fly same set of flight control
  laws.

• JSF Flight control law design
• Direct mapping of flying qualities to control
  laws.
• Nonlinear dynamic inversion control design.

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JSF Flight Control Laws

• Controller structure decouples flying qualities
  from a/c dynamics.
• Regulator/Commands implement desired.
• Effector blender optimally allocates desired
  acceleration commands.
• On-board model.
• Control effectiveness matrix.
• Estimated acceleration for dynamic inversion.

B-747
69
JSF- Mission X
70
Summary

• Use of multivariable control techniques to
  design the flight control laws for new aircraft
  is standard.
• Dynamic inversion is the most widely applied
  multivariable control design technique in the
  aircraft industry.
• Dramatic change from 15 years ago when almost all
  flight control laws for aircraft were designed
  using classical control techniques.

71
Acknowledgments

• Rick Hyde Martin Hanel
• Kevin Wise Ralph Paul
• Dale Enns Dominique Briére
• Chris Fielding Frank Thielecke
• Dagfin Gangass Greg Walker
• George Papageorgiou Prof. Alexander Efremov
• Krister Fersan David Bodden
• Prof. Fred Culick
• This work was funded in part by the NASA Langley
  Cooperative Agreement NCC-1-337, Dr. Celeste
  Belcastro Program Manager, Dr. Christine
  Belcastro, Technical Monitor.