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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
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UAV Control Design
Overview
• Background• Design Tools• Design Methods• UAV programs• Example
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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).
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UAV Control Design
Background
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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
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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.
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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
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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
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UAV Control Design
Design Tools
• CIFER®
– Extraction of mathematicaldescription of vehicle dynamicsfrom test data
– “Inverse” of simulation
– Robust software, widely used inaerospace industry
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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
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UAV Control Design
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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
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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
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UAV Control Design
Design Methods
Design
Simulation
Flight Test
Development
Specifications
Flight VehicleDevelopment Cycle
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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.
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UAV Control Design
UAV Programs
• VTUAV• LADF• R-50/R-MAX• BURRO
Ames participation in:
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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
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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
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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
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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
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UAV Control Design
Introduction
• Scope:– Hover / low-speed– Unloaded– Ground operator control
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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
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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
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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
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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
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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
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UAV Control Design
Control Law Development
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
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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
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UAV Control Design
Control Law Development
• 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
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UAV Control Design
Control Law Development
• 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
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UAV Control Design
Control Law Development
• 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
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UAV Control Design
Control Law Development
• 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
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UAV Control Design
Control Law Development
• 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
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UAV Control Design
Control Law Development
• 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
]
(rigid-body freq. range)StbMgG1: Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)StbMgG1: Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)StbMgG1: Gain/Phase Margins
0 5 10 15 200
20
40
60
80
Crossover Frequency [rad/sec]
(linear scale)CrsLnG1:Crossover Freq.
0 5 10 15 201
1.2
1.4
1.6
1.8
2
heave mode, invThdot, [rad/sec]
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
Actuator Rate Saturation
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
Actuator Rate Saturation
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
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UAV Control Design
Control Law Development
• 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)
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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
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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
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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
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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
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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
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UAV Control Design
Flight Test
• CONDUIT tuning results
GM [db]
PM
[deg
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
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
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
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 (ACAH) LAT (ACAH)
LON LAT
PITCH ROLL
After CONDUIT tuning
Baseline gains
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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
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
GM [db]
PM
[deg
]
(rigid-body freq. range)Gain/Phase Margins
0 5 10 15 200
20
40
60
80
Crossover Frequency [rad/sec]
(linear scale)Crossover Freq.
0 5 10 15 20
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
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
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
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
COL
LON (ACAH) LAT (ACAH)
PEDLON LAT
PITCH
ROLL
LAT
LON COL
LON
LAT
PED
COL
LON
LAT
PED
PITCH,ROLL
YAW
PITCH ROLL YAW
LON (TRC)LAT (TRC)
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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
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UAV Control Design
Flight Test• BURRO successfully demonstrated to USMC nine
months after start of development
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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
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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
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UAV Control Design
Questions?
For additional information:
http://caffeine.arc.nasa.gov
http://uavinfo.homepage.com