A Way to Mitigate Force-Fight Oscillation Based on Pressure andPosition Compensation for Fly-by-Wire Flight Control Systems*

Ying XUE1),2)† and Zhen Qiang YAO2)

1)Shanghai Aircraft Design and Research Institute, Shanghai 200210, China2)School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

This paper focuses on force-fight oscillation mitigation in fly-by-wire (FBW) flight control systems (FCSs) for civilaircraft. The theory of “force fight” was first introduced after analyzing its impact on active-active actuators system. Thearchitecture of one typical FBW FCS and the layout of an electronic hydraulic servo actuator (EHSA) were described. Aforce-fight mitigation algorithm applying pressure and position compensation feedback was then presented. To validatealgorithm performance, an associated actuator-surface model was created to simulate force fight. Finally, the algorithmwas tested using worst-case scenarios by calculating the system latency and tolerance, and verified using special ‘‘IronBird’’ tests. The results show an obvious decrease in the difference delta press (DDP) when comparing with/without mit-igation, and thereby meeting force fight limitation requirements.

Key Words: Force Fight, Actuators, Fly-By-Wire, Flight Control System, Mitigation, Compensation

1. Introduction

1.1. BackgroundOscillatory failure case (OFC) refers to abnormal oscilla-

tion of aircraft control surfaces due to component malfunc-tion in actuator servo-loops. This oscillation, of unknownmagnitude and frequency, can be propagated downstreamof the control loop to the control surface, leading to structuralloads produced by the airplane.

According to the standard requirements of FAR/CS/CCAR 25, structural load will cover the following items:25.301, 303, 305, 307, 333, 471, 561, 571, 601, 603, 605,607, 609, 613, 691, 521, 623, 625 and 629. These certifica-tion requirements are generally classified into four groups:dynamic loads, flutter loads, fatigue loads on actuator andcontrol surfaces, and static load.1,2) The main source of theseloads is OFC. To analyze OFC, the following effects of dif-ference levels are studied:

Level 1—Airplane controllability: Oscillation makes itdifficult for pilots to control the airplane during continuedflight and landing.

Level 2—Excessive limit load: Oscillation results in ex-cessive loads in the overall airplane, including wings, fuse-lage, and empennage. The magnitude and frequency of theoscillations should be kept to within an allowable limitation.Specifically, if the control surfaces are at saturation, then thechannels should be shutdown within three to five OFCcycles.

Level 3—Global structural fatigue: Due to oscillation,loads are act upon the airplane structure, producing unac-cepted structural fatigue and damage. The magnitude and fre-

quency of oscillation should be defined as the number of os-cillation during one flight.

Level 4—Low-cycle local fatigue (Force fight): Each ac-tuator’s motion of one surface is out of sync because of theOFC, thus producing load. This may result in unaccepted fa-tigue damage on the control surface.

This study mainly focuses on Level 4. Force fight is dis-cussed in the following section.

Flight control surface actuation systems with two or moreactuators coupled to a single flight control surface typicallyimplement one of two operational configurations: active-standby and active-active.3) In the active-standby mode,which was used by Airbus, one actuator is actively poweredwhile the other is in standby; no redundancy management isneeded. In active-active mode, which was used by Boeing,all of the actuators are simultaneously powered. This allowseach individual actuator to be sized relatively smaller thanthe one used for active-standby mode. However, this maybring the potential resultant force fight between each activeactuator,4) as shown in Fig. 1.

The voter can compare the difference between each chan-nel and calculate the average output, and each actuator iscontrolled using the same command. However, due to the er-ror accumulated during signal processing, manufacturingand installation of the valves and actuators, the displacementof each actuator is usually a different coupling, with largetorsion stiffness in the control shaft. Thus, force fight occursbetween the actuators, easily causing fatigue and structuralfailure.5)

In previous literature, mainly two approaches were pro-posed to mitigate the impact of force fight:

One is to eliminate the command difference by optimizingthe actuator command. The opposing position signal, syn-chronism position reference, backlash and friction were ana-lyzed as the main source of force fight. A corresponding way

© 2020 The Japan Society for Aeronautical and Space Sciences+Received 28 November 2018; final revision received 4 August 2019;accepted for publication 5 August 2019.†Corresponding author, 53539499@qq.com

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1

to optimize system performance was provided to reduce thedifference.6,7) Signal rigging to eliminate the command dif-ference by decreasing and compensating the installation con-stant error was also proposed by Airbus.8)

In the other approach, the addition a mitigation augmenta-tion command was used to compensate the command differ-ence, and this was studied in this paper. A way to equalizeforce using a pure integral part was propose by Wang etal.4) The integral of each actuator’s pressure differentialwas used to compensate the displacement control signal;while the command compensated is calculated using a spe-cial PID. Wang et al.9) presented the mid-value-selectionmethod to balance force fight. It simulates the difference con-tributors using the simulation tool, Symbolic Analysis Labo-ratory (SAL).9) A plurality of force sensors were used byKirkland and Mukilteo,10) of Boeing Company, to feedbackthe force fight value. A flight computer calculated the differ-ence in actual actuator rates and summed up the differencewith a computed difference in actuator force. Thus, positioncommands that equalize the actuator forces on a control sur-face were generated.10) Cochoy et al., from Hamburg Univer-sity, proposed a new control mode: active/no-load mode be-yond with active-active and active-standby mode, and studiedthe force fight of a mixture of actuators using an electro-mechanical actuator (EMA) and a electronic hydraulic servo-actuator (EHSA), and analyzed the control concept basedon the mathematical models.11,12)

Based on the above-mentioned references, the researchconcluded fell into three categories:

a. Active-standby mode, which was used by Airbus andthe series, did not refer the force fight issue.

b. Active-active mode, which was used by Boeing Com-pany,10) was achieved using EHSA. The force fight objectwas similar to the one discussed in this article.

c. Active-active mode, which was achieved using mixedactuator types such as hydraulic actuator (HA), electro-hydrostatic actuator (EHA) and EMA, and the object was dif-ferent from the one discussed in this article.

Different from the proposal of Boeing and others, the nov-elty of the method proposed in this article is: The mitigationaugmentation command is calculated using pressure and po-sition feedback built into the EHSA without additional hard-ware support, such as additional force sensors. It is more ap-plicable to electronic hydraulic servo-actuators (EHSAs).1.2. System description

To explain the mitigation method proposed, the flight con-

trol system structure is described below. The flight controlsystem structure of one type of civil aircraft is illustrated inFig. 2.

As can be seen in Fig. 2, there are three flight control com-puters (FCCs) and four actuator control electronics (ACEs)that provide flight control signals for calculation and redun-dancy management. In addition, each surface actuator is con-trolled by a remote electrical unit (REU) to receive digitalsignals from the FCCs and ACEs. A pilot uses a side stickor pedal to transfer operation into electrical signals and sendthem to the ACEs. Finally, the electrical power of the FCCs,ACEs and REUs comes from four power conditioning mod-els (PCMs).

There are two modes of operation: Normal mode (NM)and Direct mode (DM). In NM, the FCS provides full systemfunctions, closed-loop flight control, system monitoring,crew annunciation and maintenance support. This mode runswhen a sufficient sensor set is available and at least one FCCis valid. If all FCCs fail, the FCS will be degraded to DM. InDM, the FCS provides a basic mechanic link between thestick and control surfaces with body rate damping via manualairplane operation. Only basic system monitoring is offered.This mode presents a simple and deterministic control pathfrom stick input to control surfaces.

2. Force Fight Mitigation Algorithm

Generally, in the force fight mitigation algorithm, the po-sition and pressure sensors feedback the pressure signal,thereby removing the force fight condition from the surface.One obvious solution is to eliminate the pressure differentialdelta press (DP) from neighboring actuators by driving theDP in both actuators to an average value. A force fight mit-igation algorithm that uses only DP as feedback is shown inFig. 3 for two actuators on one surface. The difference DP(DDP) between each actuator and the average pressure oftwo actuators is used to generate an equalization signal foradditional command input to each actuator.

Com

Mon

Comlane

Monlane

FCC1Comlane

Monlane

FCC2Comlane

Monlane

FCC3

ACE1

Com

Mo n

ACE4

Com

Mon

ACE2

Com

Mo n

ACE3

Left outboardelevator actuator

Left inboardelevator actuator

Right outboardelevator actuator

Right inboardelevator actuator

COMMON

REU

COMMON

COMMON

REUCOM

MON

REUREU

PCM1

PCM2

PCM3

PCM4

Upper rudder actuator

Lower inboard rudder actuator

Mid outboard rudder actuator

COMMON

REU

COMMON

COMMON

REU

REU

Left outboardaileron actuator

Left inboardaileron actuator

Right outboardaileron actuator

Right inboardaileron actuator

COMMON

REU

COMMON

COMMON

REU

COMMON

REUREU

Fig. 2. Flight control system structure.

Actuator 1 force

Actuator 2 force

Hinge axis

Tensional strain

Control surface

Fig. 1. Illustration of an active-active actuator.

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The actuator studied in this paper is the EHSA, see Fig. 4.An electronic hydraulic servo-valve (EHSV) is controlled bythe FCC and ACE to drive the piston displacement. Pistonposition is fed back via an embedded linear variable differen-tial transformer (LVDT) in the main ram of the actuator.

The piezo-resistive pressure sensor of the actuator is usedto measure the extended and retracted chamber pressures ofthe hydraulic actuator, and to transmit the data back to theACE. DDP is used in the FCC for force fight mitigation be-tween two adjacent actuators on a given primary surface andfor oscillatory malfunction detection.

The algorithm of the force fight mitigation that activelycompensates force fight on the surface is shown in Fig. 5.The essential goal of the pressure feedback algorithm is tobalance the load between the actuators that are engaged.

The compensating command is implemented with a com-bination of position and pressure differentials between eachactuator and average value. The force fight command is cal-culated based on the difference between the pressure differ-ential across the piston head of a single actuator and the aver-age pressure differential across all piston heads in theactuators engaged on the surface, as referred to inEqs. (1)–(7).

��Pinave ¼�Pin þ�Pout

2��Pin ð1Þ

Xinddp ¼Z

ð��Pinavek1Þdt þ��Pinavek2 ð2Þ

��Poutave ¼�Pin þ�Pout

2��Pout ð3Þ

Xoutddp ¼Z

ð��Poutavek1Þdt þ��Poutavek2 ð4Þ

XPdiff ¼XPin �XPout

2ð5Þ

XPincompen ¼ Xinddp �XPdiff ð6ÞXPoutcompen ¼ Xoutddp �XPdiff ð7Þ

where,�Pin and�Pout are the DP signals of the inboard andoutboard actuators. ��Pinave and��Poutave are the pressuredifferentials between actuator pressure and average pressureof the inboard and outboard actuators. Xinddp and Xoutddp arethe mitigation contributors of the inboard and outboard posi-tion commands converted by DDP. XPdiff is the position dif-ferential between the inboard and outboard actuator ram po-sitions. XPincompen and XPoutcompen are the combinedmitigation compensation commands of the inboard and out-board actuator position command. The integral path calcu-lates the steady-state force fight mitigation, and the scaledgains k1 (inch/sec/psi) are used to adjust the commandsfor all engaged actuators to that of the average commandfrom the engaged actuators. The proportional path k2

(inch/psi) is the compensation for high-rate pressure changesduring actuator movement.

The compensating command is performed on the actuatorpiston ram position based on the current ram position, so asto reduce the algorithm dependency on pressure feedback.Ram position is always sent to the engaged actuators, evenif only one actuator is engaged.

In the force fight mitigation logic, the mitigation commandis calculated for each actuator separately. If some data on thecorresponding logic input are wrong, the actuator will beswitched to standby mode and part of the mitigation calcula-tion based on mitigation command and pressure values is ob-tained as zero. Another actuator is still in active mode if validdata are received, and the mitigation calculation for that ac-tuator will continue.

ACE

ACE

Inboard actuator

Outboard actuator

+-P

Retraction press P1

Extension press P2

Retraction press P1

Extension press P2

+-P

P= PIB- POB

Surface

FCC

Command

∆

∆

∆ ∆ ∆∆

Fig. 3. Force fight illumination.

Actuator cylinderLVDT

EHSV

Port pressure

sensor

Compensator

Damp valve

Relief valve

Piston

Fig. 4. EHSA layout.

PinavePin

Pout

Pave

Poutave

Xinddp

Xoutddp

XPin

Xout

XPdiff

XPincompen

XPoutcompen

Fig. 5. Force fight mitigation algorithm.

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3. Actuator Simulation Model

To examine the performance of the algorithm proposed,the dynamic characteristics of the EHSA simulation modeare created using the following mathematics mode. The fol-lowing equations are referenced in other literature.13,14)

The quantity flow rate Q (inch3/sec) of the EHSV valve isexpressed as Eq. (8):

Q ¼ cdwXv

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

�Ps � ApPmanvp �

Xv

jXvjPl

!vuut ð8Þ

where cd is the coefficient of the EHSV orifice; w (inch) is theEHSV slot width; Xv (inch) is the EHSV valve spool stroketo control the hydraulic pressure; µ (lb/inch3) is the density ofthe hydraulic oil; Ps is the hydraulic supply pressure (i.e., thetypical supply pressure is 3000 psi);Ap (inch2) is the actuatorpiston area; Pman (psi/inch3/sec) is the ratio of pressure decaybetween Q and pressure based on actuator internal leakagespecifications; vp (inch/sec) is the feedback of actuator pistonmovement speed; ApPmanvp is the supply pressure decay; andPl is the external load from aerodynamic hinge moment. Pistonmovement positionXp (inch) is expressed as Eqs. (9) and (10):

vp ¼Q

Apð9Þ

Xp ¼Z

vpdt ð10Þ

where, vp is the output of actuator piston movement speed and itis also used as the feedback to calculate the pressure decay inEq. (8).

The actuator output force Fp is expressed as Eqs. (11)–(13):

Pe ¼Ps þ Pl

2ð11Þ

Pr ¼Ps � Pl

2ð12Þ

Fp ¼ ðPe � PrÞAp ð13Þ

where, Pe and Pr are the pressures of extend and retract ram,respectively.

When two actuators are mounted on a surface, the surfaceposition is balanced by the force from the two actuators andexternal load. The basic actuator-surface model is illustratedin the Fig. 6. The actuators and surface are regarded as aspring-damp system, and the dual actuators are linked byother springs.15)

To simulate the surface load, the following equivalents areused to build the model.16) One actuator-surface load modelis defined as Eqs. (14) and (15):

Fp ¼ ApðP1 � P2Þ

¼ Md2ðXhÞdt2

¼ cdðXp �XhÞ

dtþKsðXp �XhÞ

ð14Þ

Th ¼ FpLarm ð15Þ

where, M is the surface mass; c is the surface damping coef-ficient; Ks is the surface tensional stiffness; Xh (inch) is thesurface hinge stroke; Th is the hinge moment of the actuator;and Larm is the arm of force between the actuator piston andactuator hinge.

Two actuators-surface load models are created, as shownbelow, Eqs. (16)–(18):

Thsur ¼ ðThin þ ThoutÞ � jðXhin �XhoutÞjKt ð16Þ

Xhsur ¼Xhin þXhout

2� jFpin � Fpoutj=Ks ð17Þ

Thsur ¼ kXhsur ð18Þ

where, Thsur is the surface hinge moment; Xhsur is the surfacemovement; Thin and Thout are the inboard and outboard actua-tor piston rod hinge moments, respectively, calculated usingTh;Xhin andXhout are the inboard and outboard surface hingemovements, respectively, calculated using Xh; Kt is the sur-face torsional stiffness; and k is the scale factor between thesurface stroke and hinge moment.

4. Force Fight Simulation

In order to analyze the efficiency of the force fight mitiga-tion algorithm, worst case scenarios are used to simulate itslimitation. System latency as the main contributor of forcefight was analyzed to determine the worst case.4.1. System latency

In the force fight mitigation algorithm proposed, pressureis used as feedback data. An equalization signal for inputtingeach actuator command is generated using the pressure dif-ferential between one actuator and the average pressure oftwo (or three) actuators.

The transmission delay between the change in actuatorchamber pressure and the system reaction led to difficulty us-ing this approach. After sensing pressure change, the pres-sure sensor processes and sends the data to a REU. Basedon the pressure data received, the controller effectively re-moves the steady-state force fight. However fast surfacemovements and large chamber pressure changes result in aforce fight risk; in this case, the system latency needs to beanalyzed to find the worst asynchronous condition of two

Ks

C

M

KT

Thin

Th surface

Inboard actuator

Outboard actuator

Xp Xh surfcaeFp

Xhin

XhoutThout

Outboard hinge

Inboard hinge

Fig. 6. Dual actuator-surface simulation model.

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4©2020 JSASS

actuators. The system latency allocation is expressed inFig. 7.

The worst pressure differential Pdiff is expressed asEqs. (19)–(22):

tdelay ¼X

tsens þ tACE þ tFCC þ ttrans ð19ÞPdiff ¼ tdelayQ ð20Þ

tACE ¼ 1

fACEð21Þ

tFCC ¼ 1

fFCCð22Þ

where, tACE and tFCC are the maximum delay time of ACEand FCC, respectively, depending on the sample frequencyof FCC fFCC and ACE fACE . ttrans is the maximum delaytime of data transmission in the data bus. tsens is the delaytime of the pressure sensor phase difference. Q is the EHSVpressure flow rate. The accumulated delay time is calculatedas 43.5 msec in the worst case.

Another latency case is the asynchronous clock in theACE. The clock in the ACE has a tolerance �10%, allowingeach ACE to run with a slightly different execution rate. Be-cause of large pressure gradients and these time delays, thesystem cannot react until the chamber pressure changes atleast 480 psi. Such a large delay results in an inadequate sys-tem response to completely equalize the pressure differential.

4.2. Force fight simulationForce fight is simulated for several severe cases to validate

the force fight mitigation algorithm. In all of the cases simu-lated, the full command stroke is chosen. Two test cases areselected to simulate the worst-case conditions for force fight,which are: two actuator pressure signal transmissions are notsynchronized, and two actuator command phases are incon-sistent because of different ACE processing frequencies. Thetest case descriptions are given in Table 1.

The test results are shown in Figs. 8–10, where the blueline of the first figure is the surface deflection position, theblue line of the second figure represents the DDP in two ac-

Filtering

REUActuator

ACE FCC

Aircraft Surface

Demodulation

AD

IO processing

Data bus

Data bus

Packet validation

ACE

IOC

Software process

IOC

Packet validation

ACE

Pilot input

ttrans

ttrans

tsens

Pressure sensor

Dat

a bu

s

Dat

a bu

s

ttrans

tsens

REUActuator Pressure sensor

tACEtFCC

tACE

tACE

Fig. 7. System latency allocation.

Table 1. Test case descriptions.

No. Test case Rational Passing criteria

1One pressure sensor feedback signal delay is43.5msec to another, with the highestdeflection rate of 25 deg/sec

The transmission delay of pressure data betweentwo actuators.

The DDP is lower than 500 psi, which is thelimitation of surface strength.

2One ACE output command frequency is480Hz and other is 480Hz �10%

The command sample clock in the ACE has atolerance that may cause the ACEs run atdifferent execution rates.

Sur fa

ce d

efle

ctio

n (°

)D

DP

(psi

)Fo

rce

fight

miti

gatio

n al

gorit

hm c

omm

and

(inch

)

Time (sec)

Time (sec)

Time (sec)

Fig. 8. Simulation results for case 1.

Surfa

cede

flect

ion

(°)

DD

P(p

si)

Forc

efig

htm

itiga

tion

algo

rithm

com

man

d(in

ch)

Time (sec)

Time (sec)

Time (sec)

Fig. 9. Simulation results for case 2 (480Hz +10%).

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tuators, and the blue and green lines of the third figure areforce fight mitigation commands calculated using the mitiga-tion algorithm.

For test case 1 shown in Fig. 8, the maximum DDP offorce fight is approximately 150 psi (1034 kPa) and lowerthan the 500 psi (3447 kPa) required by the surface structurefatigue limitation.

For scenarios of test case 2 in Figs. 9 and 10, one ACE runswith a higher/lower frequency than the other ACE. The DDPin the maximum value did not increase significantly.

5. Verification Test

5.1. Iron Bird test platformSince the algorithm is performed using FCC software, in

addition to validation by simulation, it needs to be verifiedusing actual FCS hardware. Thus, the Iron Bird test is carriedout.

The Iron Bird test platform is used to verify the FCS func-tions and performance where FCS hardware configurationsare the same as real aircraft (including FCCs, ACEs, REUsand actuators). Additionally, the platform provides simula-tion and recording devices (AOA/inertia/air speed/landinggear/flap, slat external signals, aircraft mode and signal sim-ulation system, data recording devices, etc.) to support test-ing. The architecture of the Iron Bird test platform is shownin Fig. 11.

5.2. Force fight mitigation testTo verify force fight mitigation capability, high-rate sur-

face deflection tests are conducted to check the DDP value.The test command simulation system injects the high-ratesurface deflection command to drive the actuators, and thedata recorder measures the DDP value from FCC output data.

In order to compare the effect between the case with forcefight mitigation and the one without it, an additional test casewithout the mitigation function is designed to express theoriginal DDP. The mitigation algorithm is the one functionof flight control software in the FCCs; if the FCCs are inac-tive, the direct mode would start and the force fight mitiga-tion function is not activated.

The surface is driven by the fastest rate to achieve the max-imum force fight with and without external load, as describedin Table 2. The test results are shown in Figs. 12 and 13.

It can be seen that the original DDP reaches 400 psi for thecase where the mitigation function is inactive. In contrast, inthe cases where the actuators are driven by the highest deflec-tion rate with external load, or in a static position, DDP is on-ly 190 psi at the most. This indicates that the mitigation proc-ess results in an obvious decrease in force fight.

Display

Flight scene

Inertia/AOA/CAS system

Aircraft six degree of freedom

Aerodynamic model

Pilot

Engine model

Wind model

REU and actuator

FCC ACE

Landing gear

Aircraft model

Flight control system

Landinggear

model

Flapslat

model

Fig. 11. Iron Bird test rig.

Table 2. Test case descriptions.

No. Test case Passing criteria

1Force fight mitigation functioninactive

Dynamic force fight: Triangle command with 20�/sec rate, three cycles, external load(max. hinge moment)

None

2 Dynamic force fight: Triangle command with 20�/sec rate, three cycles, no external load

DDP <500 psi3 Force fight mitigation function

active

Dynamic force fight: Triangle command with 20�/sec rate, three cycles, external load(max. hinge moment)

4Static force fight: Position command to 0�¼¹10�¼¹20�¼¹30�¼¹20�¼¹10�¼0�,stay for 4 sec, external load (max. hinge moment)

Fig. 12. DDP test without mitigation.

Surfa

cede

flect

ion

(°)

DD

P(p

si)

Forc

efig

h tm

itig a

tion

alg o

rithm

com

man

d( in

ch)

Time (sec)

Time (sec)

Time (sec)

Fig. 10. Simulation results for case 2 (480Hz ¹10%).

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6. Conclusion

A force fight mitigation algorithm has been studied forforce fight OFC events in FBW flight control systems of civilaircraft. After introducing the principle of OFC (includingforce fight), we explored regular OFC defined in CCAR25and illustrated the theory of force fight.

We then proposed a force fight mitigation algorithm tomitigate DDP between the actuators. The algorithm exhibitshighly satisfactory results in terms of robustness and detec-tion. The DDP mitigation results for three worst-case scenar-ios were also analyzed. Further investigations are necessaryto monitor force fight OFC Level 4 for closed-loop actuators.

Finally, the mitigation algorithm was tested using a dedi-cated Iron Bird test bench with actual FCS hardware; the aimbeing to assess algorithm performance. The test resultsshowed obvious mitigation of DDP enabled by using the al-gorithm.

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Seiya UenoAssociate Editor

Fig. 13. DDP test with mitigation.

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