flight control system design and test for unmanned rotorcraft · flight control system design and...

Flight Control System Design and Test forUnmanned Rotorcraft

Flight Control and Cockpit Integration BranchArmy/NASA Rotorcraft Division

NASA Ames Research CenterMoffett Field, CA

Chad R. Frost

IEEE Santa Clara Valley

Control Systems Society Technical Meeting

September 21, 2000

UAV Control Design

Overview

• Background• Design Tools• Design Methods• UAV programs• Example

UAV Control Design

Background• A UAV is an uninhabited, reusable aircraft that is

controlled:– Remotely,– Autonomously by pre-programmed on-board equipment,

– Or a combination of both methods

• Currently >241 UAV systems developed by 31 countriesare operational or in test

• Numerous missions, current and proposed:– Military

– Civilian– Space

Source: Bob Keith, NATO UAV C2 Workshop 1999 (Unclassified).

UAV Control Design

Background

UAV Control Design

Background

• Many vehicle configurations, but rotary-wingedvehicles form a significant and growing portion

• Hover-capable UAVs offer unique capabilities, butcome with unique challenges

UAV Control Design

Background

• Significant industrial and military expertise exists in fixed-wing UAV development.

• Initial work on rotary-wing UAVs did not exploit thecapabilities of the configuration:– Lack of familiarity with rotorcraft issues

– Inability to foresee problem areas

• NASA involvement in rotorcraft UAV developmentsought to take performance to a new level.

UAV Control Design

Background

• Ames is NASA rotorcraft center:– Army / NASA Rotorcraft Division

• NASA: Aerospace Directorate

• Army: Aviation & Missile RD&E Center

– Flight Control and Cockpit Integration Branch

• Expertise in rotorcraft:– Flight control– Modeling– Simulation

• Design tools developed in-house

UAV Control Design

Design Tools

• CIFER®

– Comprehensive Identification from FrequencyResponses

• CONDUIT– CONtrol Designer’s Unified InTerface

• RIPTIDE– Real-time Interactive Prototype Technology

Integration/Development Environment

UAV Control Design

Design Tools

• CIFER®

– Extraction of mathematicaldescription of vehicle dynamicsfrom test data

– “Inverse” of simulation

– Robust software, widely used inaerospace industry

UAV Control Design

Design Tools

• CONDUIT– Evaluation and analysis of any

modeled system• Linear model from CIFER• Non-linear simulation code

– Control system design• Simulink or SystemBuild block-

diagram modeling

– Control system optimization• User-selected specifications

• Multi-variable, multi-objective FSQP

UAV Control Design

UAV Control Design

Design Tools

• RIPTIDE– Real-time simulation

environment– Can use models from

CIFER, CONDUIT, orstand-alone code

– Can use control systemdesigns fromCONDUIT

– Hardware-in-the-loopcapability

UAV Control Design

Design Tools

RIPTIDESHAREDMEMORY

A/C Model

Controller

Auto Code

Cockpit Displays

Out-the-window Displays

Pilot Inceptors

MatlabSIMULINK

MathModel

Development

CockpitDisplay

Development

CONDUIT

Elements of RIPTIDE real-time simulation environment

UAV Control Design

Design Methods

Design

Simulation

Flight Test

Development

Specifications

Flight VehicleDevelopment Cycle

UAV Control Design

Design Methods

• Typical sequence of control system development:– Collect data from vehicle

– Extract linear math model using CIFER

– Design control system using CONDUIT

– Optimize control system gains using CONDUIT

– Shakedown tests in RIPTIDE

– Fly control system on vehicle• If modeling done correctly, vehicle response should match

model predictions.

UAV Control Design

UAV Programs

• VTUAV• LADF• R-50/R-MAX• BURRO

Ames participation in:

UAV Control Design

Example: BURRO

• USMC demonstration program• Broad-area

Unmanned Responsive Re-supply Operations

• UAV to pick up loads frommoving ship, deliverautonomously to inland troops

UAV Control Design

Introduction

• Kaman Aerospace K-MAX– In production– Designed for load-lifting– 6,000 lb vehicle– 6,000 lb slung load capacity– Synchropter configuration– Servo-flap rotor

UAV Control Design

Introduction

• Army/NASA CRDA with Kaman to supportFCS development

• Three integrated tools– CIFER® : System identification– CONDUIT: Control system modeling, analysis,

optimization– RIPTIDE: Desktop real-time simulation

UAV Control Design

IntroductionDesign

Simulation

Flight Test

Analysis

ManualLittle or no optimizationStop when "good enough"

Not real timeNot "pilot in the loop"

ManualSubjectiveGuesswork

SlowExpensiveRisky Design

Simulation

Flight Test

Analysis

CONDUITMulti-objective optimization

CONDUIT (NRT)RIPTIDE (RT, piloted)

CIFERAutomatedAccurateObjective

Fewer hours requiredHigher initial confidence

Traditional

State-of-the-Art

UAV Control Design

Introduction

• Scope:– Hover / low-speed– Unloaded– Ground operator control

UAV Control Design

Aircraft Modeling

• Start with pilotedfrequency sweeps ofunaugmented K-MAX

• 8-DOF (rigid-body + 2rotor states) linearstate-space modelidentified from flightdata using CIFER®

Real Axis

Imag

Axi

s

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Unstable roll

Unstable pitch

Pitch

Lateral-directional

Heave

Eigenvalues, hover

UAV Control Design

Aircraft Modeling

• Verified in time domainusing CIFER®

Ref: Jason Colbourne, et al., American HelicopterSociety Forum, May 2000.

-10

1

Lateral Control Deflection

Flight data

Math model

-20

020

Roll Rate Response

-20

020

Pitch Rate Response

Time (Sec)0 1 2 3 4 5 6 7

UAV Control Design

Aircraft Modeling

• Sensor dynamics– Equivalent delays estimated

from manufacturer specs (25ms)– 2nd-order Padé approximations

• Actuator dynamics– Identified from bench-test

frequency sweeps– 2nd-order systems

(ω=20 r/s, ζ=.5)

– Rate- and position-limiting

UAV Control Design

Aircraft Modeling

• Aircraft, actuator and sensor modelsimplemented in Simulink block-diagram

1

Aircraft response

Sensor models

Linearized 8DOFAircraft dynamics

Flight controllaws Actuator models

1

Control input

UAV Control Design

Control Law Development

• Inner Loops– Attitude Command / Attitude Hold

• PID controller

– Heading Command• PD controller

– Altitude Rate Command• PD controller

• Outer Loops– Translational Rate Command

• PI controller (or position feedback)

• Modeled in Simulink

3

Attitude Rate

2

Attitude

1

TranslationalRate

Sensor models

PIDAttitude

controller

PITranslational

Ratecontroller

Actuator input

Velocity

Attitude

Attitude rate

Linearized 8DOFAircraft dynamics

Actuator model

1

Control input

UAV Control Design


AttitudeCommand

Mode

VelocityCommand

Mode

1

Lateral_Servo_Command [stu]

.05

trim sensitivity (right)

.05

trim sensitivity (left)

1

stick sensitivityft/sec per stu

1

stick sensitivitydeg per stu

trigger

in out

discrete latchHolds input value

when FCC_State = Active (1)(Velocity hold)

trigger

in out

discrete latchHolds input value

when FCC_State = Active (1)(Attitude hold)

1/57.3

deg > rad

dpp_aKp

Proportional gainstu/rad

dpp_aKp_v

Proportional gainrad/(ft/sec)

dpp_aKi_v

Integrator gain

dpp_aKi

Integrator gain

s

1

Integratorlimited windup

.523 rad(30 deg)

s

1

Integratorlimited windup

(50%)

dpp_aTp

Feedback time constantrad/(rad/sec)

m

FCC_State

Trim +

Trim -

Direct

Total Control

Control Mixer

Attitude limit+/ .523 rad

(30 deg)

6

Command_Mode [ 1= Velocity 2 = Attitude ]

5

sensors

4

Trim_Right

3

Trim_Left

2

Stick Input[stu]

1

FCC_State

Roll angle

Vd

Roll rate

• Complete Simulink model:– 22 inputs– 32 outputs– 331 states (continuous and discrete)

– 27 tunable gains (“design parameters”)

• Includes nonlinearelements:

– Limited integrators

– Authority limits

– Mode switching

Lateral controller shown

UAV Control Design

Control Law Development• CONDUIT Optimization Engine:

– Multi-objective optimization using FSQP

– Adjusts design parameters (system gains)to meet requirements of specifications

– Specifications represented graphically, 3regions based on level of performance

• Categorizes specifications:– Hard

• must be met

– Soft• should be met, without violating Hard

specs

– Objectives• minimized after all specs satisfied

GM [db] P

M [d

eg]

Gain/Phase Margins

0 5 10 15 200

20

40

60

80

Level 1

Level 2

Level 3

UAV Control Design


• Specification selection– Stability

– Performance and “Handling Qualities”– Objectives

• Rationale– Airframe originally designed as a manned vehicle– Safety pilot on board demonstrator vehicle

– Ground operator control will be VFR / simple tasks

UAV Control Design


• Stability Specifications (Hard constraints)

GM [db]

PM

[deg

]

(rigid-body freq. range)Gain/Phase Margins

0 5 10 15 200

20

40

60

80

Real Axis

Eigenvalue Location

-1 -0.5 0 0.5 1

UAV Control Design


• Performance Specifications (Soft constraints)

d_theta [deg]

Change in 1 Second [deg]Pitch Attitude

0 1 2 3 4Time (sec)

Normalized Attitude Hold(Disturbance Rejection)

0 5 10 15 20

-1

-0.5

0

0.5

1

Actuator Rate Saturation

Act

uato

r P

ositi

on S

atur

atio

n

Actuator Saturation

0 0.5 10

0.2

0.4

0.6

0.8

1

UAV Control Design


• Handling Qualities Specifications (Soft constraints)

Eq.

Ris

e Ti

me

(Tx-

dot, T

y-do

t) [s

ec] Translational Rate Rise Time

1

2

3

4

5

6

7

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEs;UCE=1;Fully AttBandwidth & Time Delay (roll)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Dam

ping

Rat

io (

Zet

a)

Attitude HoldDamping Ratio

0

0.2

0.4

0.6

0.8

1

heave mode, invThdot, [rad/sec]

time

dela

y, ta

u_hd

ot, s

ec

Hover/LowSpeedHeave Response

0 0.5 10

0.1

0.2

0.3

0.4

0.5

UAV Control Design


• Objective Specifications– Spec selection reduces actuator sizing,

component fatigue, and noise sensitivity

Actuator RMS

Actuator RMS

0 0.5 1

Crossover Frequency [rad/sec]

(linear scale)Crossover Freq.

0 5 10 15 20

UAV Control Design


• Control system gains tunedusing CONDUIT– Initial tuning:

• 27 parameters• 33 specifications

– All specifications satisfied(Level 1)

– RMS actuator position andcrossover frequencyminimized

Real Axis

Eigenvalues (All)EigLcG1:

-1 -0.5 0 0.5 1-0.1

-0.05

0

0.05

0.1

GM [db]

PM

[deg

]

(rigid-body freq. range)StbMgG1: Gain/Phase Margins

H

0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80


(linear scale)CrsLnG1:Crossover Freq.

0 5 10 15 201

1.2

1.4

1.6

1.8

2


time

dela

y, ta

u_hd

ot, s

ec

shipboard landingFrqHeH2:Heave response

0 0.5 10

0.1

0.2

0.3

0.4

0.5


Act

uato

r P

ositi

on S

atur

atio

n

Actuator SaturationSatAcG1:

0 0.5 10

0.2

0.4

0.6

0.8

1


Act

uato

r P

ositi

on S

atur

atio

n

Actuator SaturationSatAcG1:

0 0.5 10

0.2

0.4

0.6

0.8

1

COL

LON (ACAH) LAT (ACAH)

PED

LON

COL, LON LAT, PED

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEs;UCE=1;Fully AttBnwPiH2:BW & T.D. (pitch)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEs;UCE=1;Fully AttBnwRoH2:BW & T.D. (roll)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEs (Yaw)BnwYaH2:BW & T.D.

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Actuator RMS

RmsAcG1:Actuator RMS

0 0.5 10

0.2

0.4

0.6

0.8

1

Time (sec)

HldNmH1:Normalized Attitude Hold

0 5 10 15 20

-1

-0.5

0

0.5

1

Dam

ping

Rat

io (

Zet

a)Attitude Hold

OvsPiH1:Damping Ratio

0 0.5 1

0

0.2

0.4

0.6

0.8

1

d_theta [deg]

Change in 1 secRisPiV1:Pitch Attitude

0 1 2 3 40

0.2

0.4

0.6

0.8

1

d_phi [deg]

Change in 1 Second [deg]RisRoV1:Roll Attitude

0 2 4 6 80

0.2

0.4

0.6

0.8

1

d_psi [deg]

Change in 1 Second [deg]RisYaV1:Yaw Attitude

0 1 2 3 4 50

0.2

0.4

0.6

0.8

1

LON

PITCHROLL

YAW

PITCH

ROLL YAW

COL

LAT

PED

COL

LAT

PED

UAV Control Design


• Conditionally stable lat & lon

• Model predicts stable, well-damped responses

0 6

1Roll response

Time (sec)

Atti

tude

(de

g)

-40

-20

0

20

Gai

n (d

B)

Lateral Stability Margins (Initial):PM = 46.8 deg. (ωc = 3.75 rad/sec)

GM = 9.7 dB, (ω180 = 11.23 rad/sec)

1 10-400

-300

-200

-100

0

Frequency (rad/sec)

Pha

se (

deg)

UAV Control Design

Flight Test

• First flight test with CONDUIT-tuned gains• Aircraft responses did not agree with model

(lon and lat)

0 5 10 15 20 25 30 35 40 45-40

-30

-20

-10

0

10

20

30

40

Time (sec)

Rol

l Atti

tude

(de

gree

s)

Roll attitude command

Roll attitude

UAV Control Design

Flight Test

• Looking for source of discrepancy:– Lon and lat doublets flown closed-loop– CIFER® used to extract frequency responses– Actual sensor and actuator dynamics identified

• Equivalent time delay greater than originallyestimated

UAV Control Design

Flight Test

ComponentEstimated

Delay (ms)Actual Delay

( m s )

Actuators 5 0 107Sensors 2 5 5 3

Computer 2 0 6 0Filters 0 7 0

TOTAL 9 5 2 9 0

UAV Control Design

Flight Test

• Updated Simulink model with identified delays• Added delay results in highly constrained

system

1 10-400

-300

-200

-100

0

Frequency (rad/sec)

Pha

se (

degr

ees)

K-MAX BURRO

XV-15 Tilt Rotor

Narrowed range of stability

UAV Control Design

Flight Test

• Added lead filter to lon& lat attitude feedback

• FCS gains re-tunedwith CONDUIT

• CONDUIT successfullytraded off phasemargin for gain margin

-60

-40

-20

0

20

Gai

n (d

B)

1 10-400

-300

-200

-100

0

Frequency (rad/sec)

Pha

se (

degr

ees)

acceptable range of crossover frequency

CONDUIT-tuned result

UAV Control Design

Flight Test

• CONDUIT tuning results

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80


Act

uato

r P

ositi

on S

atur

atio

n

Actuator Saturation

0 0.5 10

0.2

0.4

0.6

0.8

1


Act

uato

r P

ositi

on S

atur

atio

n

Actuator Saturation

0 0.5 10

0.2

0.4

0.6

0.8

1

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Bandwidth & Time Delay (pitch)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Bandwidth & Time Delay (roll)

0 1 2 3 4 50

0.1

0.2

0.3

0.4


LON LAT

PITCH ROLL

After CONDUIT tuning

Baseline gains

UAV Control Design

Flight Test

CONDUIT results:

– Level 2 (8 specs)

– Reduced bandwidth

Real Axis

Eigenvalue Location

1 0.5 0 0.5 1GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80

GM [db]

PM

[deg

]


0 5 10 15 200

20

40

60

80


(linear scale)Crossover Freq.

0 5 10 15 20


Act

uato

r P

ositi

on S

atur

atio

n

Actuator Saturation

0 0.5 10

0.2

0.4

0.6

0.8

1


Act

uato

r P

ositi

on S

atur

atio

n

Actuator Saturation

0 0.5 10

0.2

0.4

0.6

0.8

1

Bandwidth [rad/sec] P

hase

del

ay [s

ec]

Other MTEs;UCE=1;Fully AttBandwidth & Time Delay (pitch)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEs;UCE=1;Fully AttBandwidth & Time Delay (roll)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Bandwidth [rad/sec]

Pha

se d

elay

[sec

]

Other MTEsBandwidth & Time Delay (yaw)

0 1 2 3 4 50

0.1

0.2

0.3

0.4

Actuator RMS

Actuator RMS

0 0.5 1

Time (sec)

Normalized Attitude Hold(Disturbance Rejection)

0 5 10 15 20

-1

-0.5

0

0.5

1

Dam

ping

Rat

io (

Zet

a)

Attitude HoldDamping Ratio

0

0.2

0.4

0.6

0.8

1

d_theta [deg]

Change in 1 Second [deg]Pitch Attitude

0 1 2 3 4d_phi [deg]

Change in 1 Second [deg]Roll Attitude

0 2 4 6 8d_psi [deg]

Change in 1 Second [deg]Yaw Attitude

0 1 2 3 4 5

Eq.

Ris

e Ti

me

(Tx-

dot, T

y-do

t) [s

ec] Translational Rate Rise Time

1

2

3

4

5

6

7


time

dela

y, ta

u_hd

ot, s

ec

Hover/LowSpeedHeave Response

0 0.5 10

0.1

0.2

0.3

0.4

0.5

COL


PEDLON LAT

PITCH

ROLL

LAT

LON COL

LON

LAT

PED

COL

LON

LAT

PED

PITCH,ROLL

YAW

PITCH ROLL YAW

LON (TRC)LAT (TRC)

UAV Control Design

Flight Test

• Roll response muchimproved; modelresponses agree wellwith flight results

0 5 10 15 20 25 30 35 40 45-60

-40

-20

0

20

40

60

Time (sec)R

oll A

ttitu

de (

degr

ess)

Roll attitude command

Roll attitude

UAV Control Design

Flight Test• BURRO successfully demonstrated to USMC nine

months after start of development

UAV Control Design

Conclusions

•• Design space is very limitedDesign space is very limited– Aircraft dynamics

– Control system hardware– CONDUIT was able to extract the best achievable

performance within design limitations

•• High frequency dynamics were key driver of closed-High frequency dynamics were key driver of closed-loop performanceloop performance– CIFER was useful in identifying system elements

•• Advanced design tools allowed rapid developmentAdvanced design tools allowed rapid developmentof a successful UAVof a successful UAV– 9 month time span– Recovery from added delay

UAV Control Design

Current and Future Work

• Build 1 of K-MAX BURRO UAVsuccessfully demonstrated toUSMC

• Build 2 now in development

• 2000-lb loaded hover– 10-DOF EOM and CIFER ident

complete– FCS design complete

• 5000-lb case in development

• Envelope expansion to 70 KTASin progress

UAV Control Design

Questions?

For additional information:

http://caffeine.arc.nasa.gov

http://uavinfo.homepage.com

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