BELL V-280 APPLICATION OF JOINT INPUT-OUTPUT …

© 2020 Bell Textron Inc.

BELL V-280 APPLICATION OF JOINT INPUT-OUTPUT METHODOLOGY FOR HOVER MODEL IDENTIFICATION

VFS Southwest Chapter MeetingVirtual

November 10th, 2020 Distribution Statement A: Approved for public release; distribution is unlimited.

Paper Authors© 2020 Bell Textron Inc.

2

Mrs. Caitlin S. BerriganV-280 Control Law Engineer

BellFort Worth, Texas, USA

Dr. Mark J. S. LopezAerospace Engineer

DEVCOM Aviation & Missile CenterMoffett Field, CA, USA

Mr. Paul RuckelV-280 IPT Control Law Manager

BellFort Worth, Texas, USA

Dr. J.V.R. PrasadProfessor and Associate Director of VLRCOE

School of Aerospace EngineeringGeorgia Institute of Technology, Atlanta, GA, USA

System Identification Background and Motivation

System Identification (SID) is an important part of control law development• Reduce control law development risks • Reduce costs associated with in-flight optimization • Improve correlation of flight dynamics models with flight test data


3

Non- Linear

Linear

DESKTOP SIMULATION Simulated Hardware SIMULATION GROUND / FLIGHT

TEST

SILPILOT / HARDWARE

IN-THE-LOOP SIMULATION

Autocode

REQUIREMENTS

PICTURE of SIM CAB

AC

AGC FCC

Background: System Identification (Direct Method)© 2020 Bell Textron Inc.

Sys ID

0 10 20 30 40 50 60 70

Time (sec)

-150

-100

-50

0

50

100

p (d

eg/s

ec)

Flight Testing

0 10 20 30 40 50 60 70-200

-100

0

100

200

lat

[PW

M]

-80-60-40-200

Mag

nitu

de(D

B)

p/a3 (J 63 )

-450-360-270-180

Phas

e (D

eg)

0.20.40.60.81

Coh

eren

ce

Effective Bare-airframe Frequency Response/ latp δ

ID Model

Flight Data

0

-40

-80

Mag

nitu

de

[dB]

-180

-360

Phas

e [d

eg]

1

0.6

0.2

Coh

eren

ce

Frequency [rad/s]1 10 100

Time [s]

δ lat

[PW

M]

p[d

eg/s

]

P: Bare-airframe

F: Feed-forward

Stick Inceptor Sweep

Bare-airframe Outputs

Σ

H: Feedback

C: Mixer

…

4

Collect Closed-Loop Flight Data

Effective Bare-airframe: �𝑷𝑷 = 𝑷𝑷 � 𝑪𝑪

𝛿𝛿𝐴𝐴𝛿𝛿𝑙𝑙𝑙𝑙𝑙𝑙𝛿𝛿𝑆𝑆 𝑝𝑝

Direct method works well for effective bare-airframe 𝑝𝑝/𝛿𝛿𝑙𝑙𝑙𝑙𝑙𝑙 identification

V-280 Valor

Background: Joint Input-Output Motivation

Modern advanced aerospace vehicle configurations require a more sophisticated SID approach (Berger 2019)

• Off-axis inputs are correlated with primary inputs‒ Mechanical mixer‒ Highly-coupled redundant control effectors

• Joint Input-Output (JIO) method can be used to identify control effectiveness of each effector


5

F-16 VISTA CCDC AvMC TDD Octocopter V-280

Knapp 2018 Lopez 2019

Objective of this work is to develop and initially demonstrate JIO methodology in V-280 System Integration Lab (SIL), a controlled environment and then proceed into flight test

Methodology – Highly Correlated Inputs

• Joint Input-Output methodology proposed by Akaike 1967 to mitigate measurement noise correlation when identifying systems with feedback

• Used recently by:‒ Gennaretti 2017 and Hersey 2017 for ID of rotorcraft

inflow models‒ Knapp 2018 for VISTA F-16 ID‒ Berger 2019 for Learjet LJ-25D‒ Lopez 2019 for UAS‒ Berrigan 2020 for V-280 SIL


6

Visualization of FreewakeSweep (Hersey 2017)

Learjet LJ-25D

Berger 2019

Prior JIO work has proven JIO correctly identifies systems with highly correlated inputs.Focus here is on JIO application to V-280, manned-size rotorcraft, high fidelity simulation.

Vert

ical

Dis

tanc

e

Lateral Distance

Methodology – Joint Input-Output

• Bare-airframe control effector inputs 𝛿𝛿𝐴𝐴 and outputs treated as outputs to set of uncorrelated (or partially-correlated) reference inputs 𝑟𝑟𝑟𝑟𝑟𝑟


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P: Bare-airframe

F: Feed-forward

Stick Inceptor Inputs


Σ Σ

H: Feedback

C: Mixer

EffectorSweep Inputs

…

……

𝛿𝛿𝐴𝐴𝑖𝑖𝑖𝑖

𝛿𝛿𝐴𝐴𝛿𝛿𝑀𝑀𝛿𝛿𝑆𝑆 𝑦𝑦

Block Diagram with Redundant Effectors:


• Bare-airframe control effector inputs 𝛿𝛿𝐴𝐴 and outputs treated as outputs to set of uncorrelated (or partially-correlated) reference inputs 𝑟𝑟𝑟𝑟𝑟𝑟


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Joint Input-Output Method:

P: Bare-airframe

F: Feed-forward



Σ Σ

H: Feedback

C: Mixer

EffectorSweep InputsBlock Diagram with Redundant Effectors:

…

……

𝛿𝛿𝐴𝐴𝑖𝑖𝑖𝑖

𝛿𝛿𝐴𝐴𝛿𝛿𝑀𝑀𝛿𝛿𝑆𝑆 𝑦𝑦

Reference to EffectorReference to Output

Effector to Output(Bare-airframe)

𝑦𝑦𝛿𝛿𝐴𝐴

=𝑦𝑦𝑟𝑟𝑟𝑟𝑟𝑟

𝛿𝛿𝐴𝐴𝑟𝑟𝑟𝑟𝑟𝑟

−1

RedundantEffectors


JIO Takeaways:• JIO works because Direct Method outputs can be fully correlated

‒ JIO is a post-processing extension of the Direct Method

• Single-Input-Single-Output (SISO) case: JIO can be thought of as type of “chain-rule” expansion

• References chosen as sweep signal (𝒓𝒓𝒓𝒓𝒓𝒓 = 𝜹𝜹𝑺𝑺 or 𝒓𝒓𝒓𝒓𝒓𝒓 = 𝜹𝜹𝑨𝑨𝒊𝒊𝒊𝒊) for any given sweep

• JIO available natively in CIFER® (system ID tool developed by CCDC AvMC)


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Joint Input-Output Method:Reference to EffectorReference to Output

Effector to Output(Bare-airframe)

𝑦𝑦𝛿𝛿𝐴𝐴

=𝑦𝑦𝑟𝑟𝑟𝑟𝑟𝑟

𝛿𝛿𝐴𝐴𝑟𝑟𝑟𝑟𝑟𝑟

−1

Bell V-280: Next Generation Tiltrotor Technology© 2020 Bell Textron Inc.

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VTOL MODE

CRUISE MODE

In 36 Months of Flight Test• >300 kts Airspeed• >180 hours of flight time• >336 operating hours• Low speed agility• High speed maneuverability

• Fixed Engine• RPM Variability • Straight Wing

• 2 Pilots/ 2 Crew Chiefs• Fly-by-wire• Sidestick Controls

• V-tail• Tail Gear w/Active Steering

V-280 control system is well harmonized and redundant control effectors are used for large portions of the flight envelope

Basic Tiltrotor Controls

PitchSymmetric Ruddervator

YawDifferential Ruddervator &

Diff Coll

RollAileron

Power Lever Throttle

CRUISE

Automatic blendingfrom VTOL to Cruise

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YawDifferential Cyclic

Power Lever Symmetric Collective

PitchSymmetric Cyclic

VTOL

RollDifferential Collective &

Lat Cyclic

V-280: Hover Coupled Effectors© 2020 Bell Textron Inc.

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HoveringLeft / Right Collective

and Lateral Cyclic are symmetric

Hovering Tilt-Rotor – Collective and Lateral Cyclic are symmetric


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Pilot Commands Right Roll which Results in Both DCP and Lateral Cyclic

Hovering – Right Piloted Input

Differential Collective Pitch (DCP) &

Lat Cyclic to effect control


14

RollDifferential Collective

Pitch (DCP) &Lat Cyclic to effect

control

Pilot Commands Right Roll which Results in Both DCP and Lateral Cyclic

Right Roll

Flight Test Execution: Frequency Sweeps in Hover

Performed in 3 different ways1. Manual piloted sweep at the stick inceptor


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PF


Bare-Airframe Outputs

Σ Σ

H

C


𝛿𝛿𝑀𝑀𝛿𝛿𝑆𝑆 𝑝𝑝

𝐷𝐷𝐷𝐷𝑃𝑃𝑖𝑖𝑖𝑖

𝐷𝐷𝐷𝐷𝑃𝑃

𝐿𝐿𝐿𝐿𝐿𝐿 𝐷𝐷𝑦𝑦𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑖𝑖𝑖𝑖

𝐿𝐿𝐿𝐿𝐿𝐿 𝐷𝐷𝑦𝑦𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶

Swee

p [N

D]

DC

P [d

eg]

lat c

yclic

[deg

]

Time [s]

p [d

eg/s

]

Lateral Inceptor STIM Sweep

Time

Swee

pD

CP

LatC

yclic

p


Performed in 3 different ways1. Manual piloted sweep at the stick inceptor2. Automated (STIM) sweep at the inceptor


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PF



Σ Σ

H

C








Swee

p [N

D]

DC

P [d

eg]

lat c

yclic

[deg

]

Time [s]

p [d

eg/s

]

Time

Swee

pD

CP

LatC

yclic

p


Performed in 3 different ways1. Manual piloted sweep at the stick inceptor2. Automated (STIM) sweep at the inceptor


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PF



Σ Σ

H

C








Swee

p [N

D]

DC

P [d

eg]

lat c

yclic

[deg

]

Time [s]

p [d

eg/s

]

Time

Swee

pD

CP

LatC

yclic

p

Fully correlated DCP and Lat Cyclic requires JIO to separate control effectors


Performed in 3 different ways1. Manual piloted sweep at the stick inceptor2. Automated (STIM) sweep at the inceptor3. Automated (STIM) sweep at the effector


18

PF



Σ Σ

H

C

EffectorSweep Inputs𝐷𝐷𝐷𝐷𝑃𝑃𝑖𝑖𝑖𝑖





DCP Effector STIM Sweep

Swee

p [N

D]

DC

P [d

eg]

lat c

yclic

[deg

]

Time [s]

p [d

eg/s

]

Time

Swee

pD

CP

LatC

yclic

p


Performed in 3 different ways1. Manual piloted sweep at the stick inceptor2. Automated (STIM) sweep at the inceptor3. Automated (STIM) sweep at the effector


19

PF



Σ Σ

H

C

EffectorSweep Inputs𝐷𝐷𝐷𝐷𝑃𝑃𝑖𝑖𝑖𝑖





DCP Effector STIM Sweep

Swee

p [N

D]

DC

P [d

eg]

lat c

yclic

[deg

]

Time [s]

p [d

eg/s

]

Time

Swee

pD

CP

LatC

yclic

p

Fully correlated DCP and Lat Cyclic still occurs due to feedback. Requires JIO to separate control effectors

Cross Control Coherence© 2020 Bell Textron Inc.

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Lateral Stick SweepQuantitative

Coherence is > 0.5, MIMO Conditioning will not work

All forms of sweep quantitatively and qualitatively show high correlation between the two effectors

(lat cyclic) / DCP

Frequency [rad/sec]0

0.5

1

Coh

eren

ce

lat sweep DCP sweep

Frequency

Coh

eren

ce

DC

PD

CP

LatC

yclic

LatC

yclic

Time

TimeDCP Sweep

Frequency Response Identification© 2020 Bell Textron Inc.

21

JIO methodology correctly allocated the frequency response for both effectors, the SISO method over predicted the allocation for lateral cyclic

Roll Rate Response


.5

1

Coh

eren

cePh

ase

[deg

]M

agni

tude

[dB

]

p/dcp (JIO) p/lat (JIO) p/lat (SISO) p/dcp (SISO)FrequencyCoh

eren

cePh

ase

Mag

nitu

de

p/DCP (JIO)

p/lat cyclic (SISO)

p/lat cyclic (JIO)

p/DCP (SISO)

Flight Test and SIL Frequency Response Comparison© 2020 Bell Textron Inc.

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.5

1

Coh

eren

cePh

ase

[deg

]M

agni

tude

[dB

]

p/DCP (SIL) p/DCP (FT)


.5

1

Coh

eren

cePh

ase

[deg

]M

agni

tude

[dB

]

p/lat (SIL) p/lat (FT)

DCP to Roll Rate Lateral Cyclic to Roll Rate

Transfer Function Identification

Hovering vehicle, low-order transfer function from McRuer 1973:

Interested only in control derivatives. At high-freq (s→∞):


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𝑝𝑝 𝑠𝑠𝛿𝛿 𝑠𝑠

=𝐿𝐿𝛿𝛿 𝑠𝑠2+ −𝑌𝑌𝑣𝑣+

𝑌𝑌𝛿𝛿𝐿𝐿𝛿𝛿

𝐿𝐿𝑣𝑣 𝑠𝑠

𝑠𝑠3+ −𝑌𝑌𝑣𝑣−𝐿𝐿𝑝𝑝 𝑠𝑠2+𝑌𝑌𝑣𝑣𝐿𝐿𝑝𝑝𝑠𝑠−𝑔𝑔𝐿𝐿𝑣𝑣𝑟𝑟−𝜏𝜏𝑠𝑠

𝑝𝑝 𝑠𝑠𝛿𝛿 𝑠𝑠

→𝐿𝐿𝛿𝛿 𝑠𝑠2

𝑠𝑠3𝑟𝑟−𝜏𝜏𝑠𝑠 =

𝐿𝐿𝛿𝛿𝑠𝑠𝑟𝑟−𝜏𝜏𝑠𝑠

Identified transfer function accurately captures high-frequency responses

Roll Rate Response (p / DCP)

Mag

nitu

de [d

B]

Frequency [rad/sec]

Phas

e [d

eg]

Flight Test ModelFrequency

Mag

nitu

dePh

ase

Comparison of Roll Control Effectiveness Flight Test vs. SIL© 2020 Bell Textron Inc.

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Identified roll control effectiveness has been correctly identified using JIO method

Con

trol

Effe

ctiv

enes

s D

CP

Flight Test SIL

Con

trol

Effe

ctiv

enes

s la

t cyc

lic

Flight Test SIL

Con

trol E

ffect

iven

ess 𝐿𝐿 𝛿𝛿

Con

trol E

ffect

iven

ess 𝐿𝐿 𝛿𝛿

Identified Roll Control Effectiveness DCP

Identified Roll Control Effectiveness Lat Cyclic

Concluding Remarks

1. JIO methodology accurately extracted frequency responses from correlated effectors. JIO is an extension of Direct

Method for highly-correlated inputs. Control derivatives can be directly

identified from frequency responses using high frequency, low order transfer function approximations.

2. V-280 SIL was validated against flight test data and accurately captures control effectiveness

3. To minimize flight test time, only one of the correlated effectors needs to be individually swept per axis if the inceptor is also swept.


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Acknowledgments

• This joint project success is due to efforts and support of many contributors

• Authors would like to thank:‒ V-280 team‒ CCDC Aviation & Missile Center‒ Pilots from Bell


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