Recent Advances in Aerospace Hydraulics, November 24-25, 1998, Toulouse, France

POWER CONTROL UNITS WITH SECONDARY CONTROLLEDHYDRAULIC MOTORS – A NEW CONCEPT FOR APPLICATION IN

AIRCRAFT HIGH LIFT SYSTEMS

BIEDERMANN OlafTechnical University Hamburg-HarburgSection Aircraft Systems Engineering

Nesspriel 5, D-21129 Hamburg, GermanyPhone +49 (0)40 74315 210Fax +49 (0)40 74315 270

Email biedermann@tu-harburg.de

GEERLING GerhardMannesmann Rexroth Brueninghaus Hydromatik GmbH

An den Kelterwiesen 14, D-72160 Horb, GermanyPhone +49 (0)7451 92 1338

Fax +49 (0)7451 1796Email gerhard.geerling@bru-hyd.com

ABSTRACT

Today’s high lift systems of civil transport aircraft are drivenby Power Control Units using valve controlled constantdisplacement hydraulic motors. This concept leads to com-plex valve blocks, attended by high power losses to realisediscrete speed control, positioning and pressure maintainingfunctionality. The concept of secondary controlled hydraulicmotors with variable displacement offers reduction in flowconsumption without pressure losses and decreases thecomplexity of the valve block design. Instead of controllingflow of the hydraulic motor with valves, torque is adjusted tothe load by varying displacement. An electronic controlcircuit allows flexible digital control concepts e.g. loadindependent speed control, pressure maintaining function-ality, smooth start-up sequences and continuous positioningof the mechanical transmission system.This paper introduces the concept of today’s Power ControlUnits, the principle and mathematical model of secondarycontrolled hydraulic motors and the cascade control loopstructure. A new hydraulic concept for Power Control Unitsusing secondary controlled hydraulic motors is presented.Theoretical, simulated and experimental results show typicaloperation sequences under load and a comparison of powerrequirement to conventional systems.

KEYWORDS

Secondary controlled hydraulic motor, variable displacementhydraulic motor, aircraft high lift system, secondary flightcontrols, power drive units, Power Control Unit (PCU)

INTRODUCTION

The power requirements of future large civil transportaircraft open an attractive field for the application of secon-dary controlled hydraulic or so called variable displacement

hydraulic motors (VDHM). Especially during landingapproach, the operation of power drive units of high liftsystems, so called Power Control Units (PCU), heavily loadsthe hydraulic power supply (Ivantysynova et al, 1995).Figure 1 shows a typical load profile for a civil transportaircraft hydraulic system. The consumption during theapproach phase is one decisive design case for today’saircraft hydraulic systems sizing. In this flight phase, largehydraulic consumers (flaps/slats, landing gear) have to beoperated while the available hydraulic power pump flowreaches its minimum due to idle condition of the engines.

Figure 1. Typical hydraulic load profile

A new concept of PCU design using secondary controlledhydraulic motors with variable displacement reduces theflow demand from the hydraulic system during take-off andlanding (Geerling, 1997). Compared to conventional PCU, aflow reduction of about 50% is expected.This technology is under investigation and development atthe Section Aircraft Systems Engineering at the TechnicalUniversity of Hamburg-Harburg in cooperation withLiebherr Aerospace Lindenberg. For that, a test set-up andcomprehensive simulation models were established todevelop the new concept.

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I POWER CONTROL UNIT

Today’s high lift systems of civil transport aircraft are drivenby PCU using valve controlled constant displacementhydraulic motors (CDHM). Figure 2 shows a typical high lifttransmission system with a conventional PCU of the leadingedge (slats). The same principle is not shown but also appliedfor the trailing edge (flaps). For reliability aspects the PCUhas two independent hydraulic motor/valve group assem-blies. A speed summing differential gear (DG) connects bothhydraulic motors to the transmission shaft. In case of a singlehydraulic system failure the slats resp. flaps are operatedwith half speed. The feedback position pick-up unit (FPPU)indicates the position of the transmission system. If thedesired flap position is reached, the whole transmissionsystem is set by applying pressure-off brakes (POB).

Figure 2. Conventional high lift system with CDHM-drivenPCU

Figure 3 illustrates a detailed example of the hydraulicscheme of one CDHM with valve block as it is applied in theAirbus A340. Different hydro-mechanic control functions arerealised. Direction and two discrete rates of speed are con-trolled by the main control valve and a pilot flow limitingrestrictor. A pressure maintaining function, using a pilotpressure maintaining valve is included to give priority toprimary flight controls, reducing flow consumption of themotor if the system pressure drops. The flaps are positionedby depressurising the POB with the brake solenoid valve(Brake SV). The functions are thus reached by switchingseveral discrete solenoid valves (SV).The implemented control functions require a complex valveblock design. This kind of speed control, by varying thehydraulic resistance, leads to pressure losses up to 80 %.

Figure 3. Hydraulic concept of a valve controlled CDHM

II SECONDARY CONTROLLEDHYDRAULIC MOTORS

The principle of secondary controlled hydraulic units or socalled variable displacement hydraulic motors (VDHM)allows conversion of hydraulic to mechanical power withoutpressure losses. It has been successfully applied in a varietyof industrial fields since the early eighties. The use in air-craft’s hydraulic systems requires reliability and safety underextreme environmental conditions and life time demands.

2.1 Design and FunctionFigure 4 shows the principle and a cross section of an axialpiston motor. The motor torque is regulated by the angle ofthe swash plate, changing the motor displacement. It ispositioned by a swash plate actuator (SPA) which is con-trolled by an electro-hydraulic servovalve (EHSV).

Figure 4. Design of an axial piston motor (MannesmannRexroth Brueninghaus Hydromatik, type A10VSO)

Swash Plate Actuator

Electro-hydraulic Servovalve

Swash Plate

+ -

The design described allows a very flexible application ofVDHMs in aircraft hydraulic architecture (Biedermann et al,1998). Depending on the swash plate angle and the loadtorque at the output shaft the unit works either as a pump oras a motor in 4-quadrant mode. This kind of hydraulic motorallows control of torque, power, speed and position at theoutput shaft.

2.2 Model of the Hydraulic MotorThis section presents a non-linear mathematical model of thehydraulic unit. Figure 5 shows a scheme of a VDHM withconstant pressure supply.The secondary controlled hydraulic motor can be character-ised by two equations, namely for flow and motor torquedepending on the variable displacement VM. The motortorque MM at constant differential pressure supply is

( )π2

RSMM

ppVM −⋅= (1)

with the displacement VM being proportional to the swashplate actuator stroke xP. This leads to

( )π2

RS

max,P

max,MPM

ppxV

xM−

⋅= . (2)

The motor flow QM for a constant speed ω is given by

ωπωπ ⋅⋅⋅=⋅⋅= 22max,P

max,MPMM x

VxVQ . (3)

The hydraulic input power of a VDHM Phyd,in at constantdifferential pressure supply

( ) MRSin,hyd QppP ⋅−= (4)

is only reduced by the hydro-mechanical efficiency=ηhm andvolumetric efficiency=ηvol. Hence, the mechanical powerPmech,out at the output shaft is calculated by

tin,hydvolhmin,hydLout,mech PPMP ηηηω ⋅=⋅⋅=⋅= (5)

with the load torque ML at the motor output shaft resp. at thePCU output shaft, considering aerodynamic loads andmechanical losses of the transmission system.The power equation (5) shows, that power loss of a VDHMonly depends from the volumetric and hydro-mechanicalefficiencies. VDHM are used to work with an overall, totalefficiency=ηt up to 90% at the operation point.

Figure 5. Scheme of a VDHM

A non-linear mathematical model of the hydraulic motor isused to simulate dynamic behaviour and for controllerdesign. The mechanical system of the VDHM is described bythe equation of momentum

LFM MMMJ −−=⋅ω� . (6)

The friction term MF considers Coulomb and viscous frictionas well as the stiction moment:

0e)sign()sign( ωω

ωωω−

⋅+⋅+= SticCoulF MdMM .(7)

Equations (2), (6) and (7) lead to

( )

0e)sign()sign(-

2

ωω

ωω

πωω

−⋅−

−−−⋅=⋅+⋅

SticCoul

LRS

max,P

max,MP

MM

MppxV

xdJ �(8)

a non-linear, time-invariant ordinary first order differentialequation with the actuating input xP. The system dynamicdepends on the difference between motor torque MM (xP) andload torque ML. Simplified the swash plate actuator is repre-sented by an integral behaviour

P

SVP A

Qx =� (9)

and the flow through the EHSV by a proportional term

SVSVSV iKQ = , (10)

with a linear approximation for the servovalve gain KSV.

III NEW CONCEPT FOR AIRCRAFTAPPLICATION

The use of VDHM in PCU application leads to new hydrau-lic interface and controller concepts. Therefore, a test set-upand simulation model was established at the Section AircraftSystems Engineering of the Technical University of Ham-burg-Harburg to examine the new concept in practicaloperation.

3.1 Hydraulic ConceptFigure 6 shows a new possible configuration for a VDHM-driven PCU in 4-quadrant mode. The hydraulic unit isseparated from the pressure supply by an isolation or socalled enable valve during non-operational time. In combi-nation with the Brake SV it ensures adequate protectionagainst failure cases which might lead to uncommanded flapsmovement or runaway. In pump mode, i.e. for ‘aiding load’cases, the VDHM inlet and suction port is connected to thesystem return pressure line and pump flow is swept via apressure relief valve to the return pressure line.

3.2 Controller DesignThe VDHM concept with continuous control of motor torqueallows a flexible and application specific integration ofdifferent modes as speed control, start-up and positioningsequences and pressure maintaining function into the closed

loop high lift positioning circuit. Compared to the conven-tional CDHM in Figure 3 the functions of the main controlvalve, pilot flow limiting restrictor and pilot pressure main-taining valve can be realised by the digital controller usingsignals of motor shaft speed ω, swash plate position xP andsystem pressure pS.

Figure 6. Hydraulic concept of PCU with VDHM in 4-quadrant operation

Figure 7 illustrates a principle controller structure. Swashplate stroke, speed and transmission position are controlledin a cascade control loop design. Each VDHM is associatedto one controller. Depending on the difference betweentransmission position ϕFPPU and desired position ϕin a speedtrajectory ωin is defined. The pressure maintaining function(PMT) affects speed limiting if the system pressure pS dropsunder a certain limit.

Figure 7. Block diagram of the cascade control loop design

3.3 Experimental ResultsThe test set-up consists of an Airbus A310 DG driven by twosecondary controlled axial piston VDHM as shown in Figure4. Loads at the output shaft of the DG are simulated by aservovalve controlled constant displacement motor. Thecascade control concept was realised as digital controller andexecuted on a personal computer (Geerling, 1997).Figure 8 demonstrates a comparison of simulated and meas-ured results for swash plate angle resp. displacement andspeed during a full flap extension assuming an Airbus A340load profile. For simulation the non-linear model mentionedbefore was used.The start-up sequence releases the POB with a simultaneousinput speed ramp being applied. The shaft speed ω showsload independent behaviour. The displacement indicated bythe piston stroke xP adjusts to the changing load. At max.load conditions just 60% of max. displacement is needed.The difference between required and max. displacementdirectly shows the power reduction between constant (max.)displacement and variable displacement hydraulic motors.When the desired flap position is approached, a shut downsequence is initiated and the POB is set.

Figure 8. Comparison of experiment and simulation withVDHM (flap extension against load)

Figure 9. Comparison of power need between CDHM andVDHM

0 5 10 15 20

0

0.5

1

Experimentx P

/xP

,max

, ω/ω

max

ω/ωmax

xP/x

P,max

0 5 10 15 20

0

0.5

1

Simulation

Time [s]

x P/x

P,m

ax, ω

/ωm

ax

Start−up sequence

max. load

Setting of POB

0 5 10 15 20

0.2

0.4

0.6

0.8

1

Time [s]

P/P

max

Phyd,CDHM

Phyd,VDHM

Pmech,out

Simulation of a flap extension operation

Power loss atcontrol valve

Power loss due tomotor efficiency

3.4 Hydraulic Power ConsumptionFigure 9 shows the theoretical, simulated power consumptionof a conventional CDHM-driven and the new VDHM-drivenPCU concept. A comparison for a typical flap extensionbased on Airbus A340 data is made. The given load profile atthe PCU output shaft considers aerodynamic loads as well asmechanical losses of the transmission system. This loadprofile multiplied with the actuated speed leads to themechanical output power Pmech,out at the PCU output shaft.The power consumption of the VDHM-driven PCU Phyd,VDHMadapts to the changing output power Pmech,out and compen-sates losses due to the total efficiency ηt of the motor. Theflow controlled CDHM-driven PCU has a constant powerconsumption Phyd,CDHM assuming steady speed. The powerdifference between Phyd,CDHM and Phyd,VDHM is caused bypressure losses in the main control valve and the flow limit-ing restrictor.The comparison shows a possible power reduction between53% and 80% applying VDHM instead of CDHM in theA340 PCU.

CONCLUSION

Introducing variable displacement to power drive units offersa high potential for hydraulic system power reduction.At the Section Aircraft Systems Engineering of the TechnicalUniversity of Hamburg-Harburg a first step to investigatethis technique has been applied for the PCU of high liftsystems. Different hydraulic concepts and controller designshave been investigated. Feasibility, practicability and reli-ability have been proved by experimental, numerical andanalytical results.This paper has presented theoretical and experimental resultson a new concept for application in aircraft high lift systems.The comparison to today’s conventional system verifiespower reduction between 53% and 80%. Moreover, valveblock complexity is decreased. A digital controller allowsflexible transfer of hydro-mechanical control functions andoffers all kinds of speed, position, torque or power controlfor future concepts.The consequent use of hydraulic motors with variable dis-placement in aircraft’s hydraulic system architecture coulddecrease system power requirements. The principle is trans-ferable to any other consumers with rotary power drive units,e.g. Trimmable Horizontal Stabiliser Actuator.

REFERENCES

Biedermann O., Engelhardt J. and Geerling G. (1998), MoreEfficient Fluid Power Systems Using VariableDisplacement Hydraulic Motors, Proceedings of the21th Congress of the International Council of theAeronautical Sciences ICAS '98, Melbourne,Australia, 1998

Geerling G. (1997), Secondary Controlled Variable Dis-placement Motors in Aircraft Power Drive Units,Proceedings of the 5th Scandinavian InternationalConference on Fluid Power SICEP '97, Ed.1 , pp 167-179, Linköping, Sweden, 1997

Ivantysynova M., Kunze O. and Berg H. (1995), EnergySaving Hydraulic Systems in Aircraft - a Way to SaveFuel, Proceedings of the 4th ScandinavianInternational Conference on Fluid Power SICEP '95,Tampere, Finland, 1995

NOTATIONS

SymbolsA Surface areaJ InertiaK GainM TorqueP PowerQ FlowV Displacementd Viscous friction numberi Currentp Pressurex Actuator strokeω Revolving shaft speedϕ Transmission positionη Efficiency

Indices and AbbreviationsCDHM Constant displacement hydraulic motorCoul CoulombDG Differential gearEHSV Electro-hydraulic servovalveF FrictionFPPU Feedback position pick-up unithm hydro-mechanicalhyd hydraulicin InputL LoadM Hydraulic motormech mechanicalout OutputP PistonPCU Power Control UnitPMT Pressure maintainingPOB Pressure-off brakeR ReturnS SupplySPA Swash plate actuatorStic StictionSV Servovalve, Solenoid valvet totalVDHM Variable displacement hydraulic motorvol volumetric

ACKNOWLEDGEMENT

The authors thank the Liebherr Aerospace LindenbergGmbH for promoting and supporting the research project‘Development and Investigation of new PCU concepts’ andthe Daimler-Benz Aerospace GmbH for providing typicalhigh lift system loads.