More Efficient Fluid Power Systems Using Variable Displacement Hydraulic Motors

O. Biedermann, J. Engelhardt, G. GeerlingTechnical University Hamburg-Harburg, Section Aircraft Systems Engineering

Nesspriel 5, D-21129 Hamburg, Germany

Abstract

The approach and landing phase is dimensioning fortoday’s aircraft fluid power systems. In this flight phase,large hydraulic consumers (flaps/slats, landing gear) haveto be operated while the available hydraulic powerreaches it’s minimum due to the reduced engine speed.

During most of the flight the installed resources exceedthe hydraulic power requirements by far; resulting in alow overall-efficiency.

This paper presents an approach to increase theefficiency of today’s fluid power system by usingvariable displacement hydraulic motors (VDMHs). Twoapplications will be introduced: the VDMH drivenslat/flap power control unit (PCU) and the bi-directionalhydraulic-electrical power conversion unit (HEPCU).

The variable displacement PCU reduces the design loadsfor the fluid power system during take-off and landing.Compared to conventional PCUs used today, a flowreduction of about 50% is expected using the VDHMtechnique.

The HEPCU transfers hydraulic into electrical power andvice versa depending on the current load and flightsituation (“hydraulic-electrical power management”).The main benefit from this approach is down sizing theprimary power sources (engine driven pumps andgenerators), a significant increase in reliability and ahigher efficiency for both, the hydraulic and the electricalpower generation system.

Symbols

A Surface areaC CapacityE Bulk modulusJ InertiaK GainM TorqueP PowerQ FlowV Displacement, VolumeW Work, Energyd Viscous friction numberi Current

p Pressurex Actuator strokeω Revolving shaft speedη Efficiency

Indices

CDHM Constant displacement hydraulic motorF FrictionLE LeakageL LoadM Hydraulic motorP PistonR ReturnS SupplySV ServovalveVDHM Variable displacement hydraulic motorfl Fluidhm hydro-mechanicalhyd hydraulicin Inputmech mechanicalout Outputt totalvol volumetric

Abbreviations

AC Alternating currentCDHM Constant displacement hydraulic motorCSMG Constant speed motor generatorDG Differential gearEDP Engine driven pumpEHSV Electrohydraulic servovalveHEPCU Bi-directional hydraulic-electrical power

conversion unitIDG Integrated drive generatorIGBT Insulated gate bipolar transistorPCU Power control unitPOB Pressure-off brakeSOV Solenoid valveT/O Take offTUHH Technical University Hamburg-HarburgVDHM Variable displacement hydraulic motorVSCF Variable speed – constant frequency

2

Introduction

Since the late 1930s hydraulic systems are used in allkind of aircraft. The importance of these systems hasgrown significantly since then and the hydraulic powerdemands have consequently increased greatly.

Modern commercial aircraft are equipped with three orfour independent hydraulic systems. Hydraulic power isneeded for a large variety of functions including primaryand secondary flight controls, brake systems, dooractuation, landing gear systems and nose wheel steering.

In flight, engine driven pumps (EDPs) supply thepressure system. For ground and emergency operationadditional electric motor driven pumps are provided. Theuniversally used pump type is the variable displacementpump designed to guarantee a constant system pressure.

FIGURE 1 shows a typical load profile for an aircrafthydraulic system. Dimensioning is the approach phasewhen large consumers (slats/flaps, landing gear) have tobe operated while the available hydraulic power providedby the EDPs reaches it’s minimum due to reduced enginespeed.

FIGURE 1 - Typical hydraulic load profile for acommercial aircraft

During cruise phase hydraulic power is only needed foroperating the primary flight controls and for coveringinternal leakage. This purpose requires only a rathersmall amount of hydraulic power. Concerning a longrange flight the installed power resources exceed thepower demands by far for more than 90% of the missiontime.

This low overall efficiency can be increased through theconsequent use of variable displacement hydraulicmotors (VDHMs). Therefore, two approaches concerningthe integration of VDHMs in aircraft fluid power systemwill be presented in this paper.

• Replacing conventional constant displacement motor(CDHM) concepts through VDHM techniques yields

in a reduction of consumer demands, especiallyduring the critical flight phases, i.e. take off,approach and landing. One particular application, theuse of VDHMs for the slap/flap power control unit(PCU) will be described in detail.

• The concept for a bi-directional hydraulic electricalpower conversion unit (HEPCU) will be introduced.HEPCU provides additional hydraulic power “ondemand” during the critical flight phases on the onehand and, on the other hand, HEPCU uses the powerresources during cruise for generating electricalpower.

Both applications are developed at the Section AircraftSystems Engineering, Technical University Hamburg-Harburg (TUHH). Test rigs (FIG. 6) and comprehensivesimulation models were established to examine the twoconcepts in practical operation.(1)

Variable Displacement Hydraulic Motors (VDHMs)

The principle of displacement controlled hydraulic unitsenables power control without pressure losses. It issuccessfully applied in a variety of industrial fields sincethe early eighties. The use in aircraft’s hydraulic systemsrequires high level reliability and safety under extremeenvironmental conditions and life time demands.

Although variable displacement hydraulic units arecommonly referred to as “motors” they are also able towork as a pump. The term “motor” was established sincethe first units working with this principle were used ashydraulic motors.

Today’s aircraft’s hydraulic power controls uses valvecontrolled constant displacement hydraulic motors(CDHM) at constant pressure supply. The speed controlof these motors is realised by varying the hydraulicresistance of the control valve. This kind of velocitycontrol leads to high pressure losses up to 80 %.

The required hydraulic power is provided by variabledisplacement pumps driven directly by the engine gearbox or by AC motors. These pumps are not able to workas a motor due to design limitations.

Design and Function

FIGURE 2 shows the design of a axial piston motor. Themotor torque is regulated by the angle of the swash platechanging the motor displacement. It is positioned by aswash plate actuator. The flow to the cylinder is usuallycontrolled by an electro-hydraulic servovalve (EHSV).

0

50

100

150

200

250

300

350

400

Flow

[l/m

in]

Flight phase

Available flow

Requiredflow

Ground Taxi T/O Climb Cruise Desc. Appr. Land.

extensive gap betweenavailable and required power

Primary Flight Controls (+ Internal Leakages)

GearDoorsSlatsFlaps

GearDoorsSlatsFlaps

design case forhydraulic system

3

The design described allows a very flexible applicationof VDHMs. Depending on the swash plate angle and theload torque at the output shaft, the unit works either as apump or as a motor. This kind of hydraulic motor allowsto control torque, power, speed and position at the outputshaft. Moreover, it is possible to realise pressure andflow control of the hydraulic power supply.(3)

FIGURE 2 - Design of a axial piston motor (Mannes-mann-Rexroth, type A10VSO)

Model of the Hydraulic Motor

This section presents a linear mathematical model of thehydraulic unit that captures its main dynamics. FIGURE3 shows a scheme of a VDHM at constant pressuresupply.

When working as a hydraulic motor in a speed controlloop, the controlled outputs are the swash plate actuatorstroke xP and the revolving speed at the output shaft ω.The actuating input signal of the EHSV is the current iSV.

FIGURE 3 – Scheme of a VDHM

Compared to CDHMs, attended by control valve inducedpower losses, the hydraulic input power of a VDHMPhyd,in at constant differential pressure supply

( ) MRSin,hyd QppP ⋅−= (1)

is only reduced by the hydro-mechanical efficiency=ηhmand volumetric efficiency=ηvol. The mechanical powerPmech,out at the output shaft is calculated by

tin,hydvolhmin,hydLout,mech PPMP ηηηω ==⋅= . (2)

The power balance (2) describes the power loss of aVDHM. Usually, the volumetric and hydro-mechanicalefficiency is very high. Generally, VDHMs are used towork with an overall efficiency of 90 % at the operationpoint.

For controller design purposes a mathematical model ofthe hydraulic motor is needed. The mechanical system ofthe VDHM is described by the equation of momentum

LFM MMMJ −−=ω� . (3)

Neglecting the stiction moment, the friction term MF isreduced to the viscous part

ωMF dM = . (4)

The motor torque MM is calculated by

( )π2

RSMM

ppVM −= , (5)

with a linear dependence between the displacement VMand the actuator stroke xP

( )PMxP

RS

max,P

max,MM xKxpp

xV

M =−=π2

. (6)

Equations (3) to (6) lead to a first order differentialequation

LPMxM MxKdJ −=+ ωω� , (7)

a linear, time-invariant ordinary differential equationwith the actuating input xP. The transient behaviour ofthe revolving speed ω depends on the difference betweenmotor torque MM (xP) and load torque ML.

Simplified the swash plate actuator is represented by anintegral behaviour

P

SVP A

Qx =� (8)

and the flow of the EHSV stands for a proportional term

Swash Plate Actuator

Electro-hydraulic Servovalve

Swash Plate

+ -

EHSV

pS pR

iSV

VDHM

Swash Plate Actuator

ML, ω

xP

QMQP

4

SVSVSV iKQ = . (9)

When working as a hydraulic pump the controlled outputis the supply pressure pS. The derivative of the pressureis calculated by

( )LELPhyd

S QQQC

p −−= 1� , (10)

with the hydraulic capacity Chyd which is a function ofthe system volume Vhyd and the hydraulic fluid’s bulkmodulus E

EV

C hydhyd = . (11)

The load flow QL stands for the consumed system flowand the leakage flow QLE is represented by

SLELE pKQ = . (12)

The pump flow QP for a constant speed is given by

PQxPmax,P

max,M xKxx

VQ =⋅⋅⋅= ϖπ2 (13)

in which the actuating input xP can be derived fromequations (8) and (9). The speed ω depends on thedriving unit (engine, AC motor), respectively. Equations(10) to (12) lead to a first order differential equation

LPQxSLEShyd QxKpKpC −=+� . (14)

Controller Design

FIGURE 4 - Simple block diagram of a speed controlloop (a) and a pressure control loop (b)

The use of VDHMs in aircraft environment requiresrobust and discrete-time controllers. The reduced linearplant of the open loop, presented by equations (7), (8)and (9), completed with the speed control system isshown in FIGURE 4 (a). The pressure control loop basedon equation (10) and (11) is shown in FIGURE 4 (b).

The digital controller calculates the actuating signal ofthe servovalve iSV using motor shaft speed ω or systempressure pS.

Moreover, the swash plate position xP can be controlledindependently. This might be needed to stabilise thecontrol system because of several kinds of disturbancese.g. leakage of the swash plate actuating cylinder leadingto undesired movement of the swash plate. Besides themotor torque can be set adjusting the swash plateactuator e.g. for starting sequences.

Aircraft Applications

Bi-directional hydraulic-electrical power conversionunit (HEPCU)

Modern civil aircraft’s hydraulic and electrical powergeneration systems are largely separated. The transferoptions between both systems are limited to ground andemergency operations. The conventional transfer unitsused today (electrical driven motor pumps, hydraulicdriven emergency generators) are working exclusivelymono-directional.

FIGURE 5 - Bi-directional hydraulic-electricalpower conversion unit (HEPCU)

A far stronger coupling of the hydraulic and electricpower system can be achieved by using a bi-directionalhydraulic-electrical power conversion unit (HEPCU).Combining a variable displacement hydraulic motor(VDHM), an electrical synchronous machine and highlyintegrated power electronics, the HEPCU is able to work

-

-MF

ML

ω

dM

�ωxP

KA

SV

PiSV

1JMMω in Digital

Controller-

KMx

-xP

-

KA

SV

PiSV

1ChydQ

QLE

QL

KLE

DigitalController-

KQxpSpin

(a)

(b)

pressure line

Solenoidvalve

Variable displacementhydraulic motor (VDHM)

reservoir

suctionline

returnline

Power electronics(VSCF Converter)

Synchronous machine(motor/generator)

Rectifier IGBTs Filter

AC Bus

Hydraulic powergeneration system

Elelectrical powergeneration system

Pel

- Controller- Monitoring

Servo valve

Pel

Pilot

HEPCU

5

either as a pump or as a generator (compare FIGURE 5).With this functionality, HEPCUs can replace today’s ACmotor pumps and hydraulic driven emergencygenerators.

When working in pump mode, the electrical motor drivesthe VDHM converting electrical into hydraulic power. Inthis mode the hydraulic unit works in a pressure controlloop supplying the 3000 psi hydraulic system.

FIGURE 6 - HEPCU test rig at the Section AircraftSystems Engineering at TUHH

In generator mode, the hydraulic unit works as a speedcontrolled motor driving the synchronous generator. Thegenerator produces a wild-frequency voltage dependingon the motor speed. This voltage is rectified and suppliedto a VSCF converter (VSCF = variable speed constantfrequency). The converter, build up of highly integratedIGBTs (insulated gate bipolar transistors), produces aconstant frequency voltage which fits the strictrequirements of today’s electrical power generationsystems (115 VAC, 400 Hz).

In both modes, the HEPCU is able to work as anemergency or ground power source, similar to today’sAC motor pumps or emergency generators. The HEPCUcombines both functions in a single unit.

FIGURE 7 - Bi-directional power transfer

In addition, the HEPCU is able to work in parallel toprimary power sources (engine driven generators andpumps). This functionality is used for a hydraulic-electrical power management concept:

Typical hydraulic and electrical load profiles show thatthe maximum loads in both systems do not occursimultaneously (FIG. 7). There is an extensive hydraulicpower consumption during take-off, approach andlanding when large hydraulic consumers (gear,flaps/slats) are operated. During cruise, only a rathersmall amount of hydraulic power is needed for theprimary flight controls, whereas the electrical powerconsumption reaches its maximum (operation of galleysand passenger entertainment systems).

With the HEPCU working as a "demand pump" or as a"demand generator", a power transfer between thesystems depending on flight phase and load situationbecomes possible. During cruise hydraulic power istransferred into electrical power, whereas during take offand landing electrical power is converted into hydraulicpower.

This flexible transfer option reduces the designrequirements for both systems because the maximumloads can partly be supplied from the other system,respectively. As a result, down-sizing the installed powersources (engine pumps and engine generators) is themain benefit of this approach.

FIGURE 8 - Increased power availability for theAC power generation system

In addition to this weight and cost benefit, the use ofHEPCUs increases the flexibility in power distributionbetween the hydraulic and electrical system. This resultsin a higher reliability and better power availability forboth systems (FIG. 8).

0

20

40

60

80

100

Electrical Load Profile

Elec

tric

al P

ower

[kV

A] Grd. Taxi T/O Climb Cruise Desc. Appr. Land.

Flight phase

0

100

200

300

400

Hydraulic Load Profile

Flow

[l/m

in]

Flight phase

Demand Generator�

��Climb��Cruise��Descent

Available flow

Demand Pump�

��Take-Off

��Approach

��LandingAvailable electrical power

Requiredflow

Total load

Grd. Taxi T/O Climb Cruise Desc. Appr. Land.

10-15 10-10 10-5 10 00

100

200

300

AC P

ower

[kVA

]

Probability P for a power < x

configuration with HEPCUs (reduced primary power)conventional configuration

max. total load

max. "technical load"

Example:

Higher availibility for safety relevant loads

Reduction of IDG powerfrom 4x75 kVA to 4x50 kVA

weight

increased efficiency

Replacing AC motor pumps andemergency generator by HEPCUs (rated power: 3x15 kW)

6

Power Control Unit (PCU)

Today’s high lift systems of civil transport aircraft aredriven by power control units (PCU) using valvecontrolled fixed displacement hydraulic motors. FIGURE9 shows a typical trailing edge (flaps) of a conventionalhigh lift transmission system with PCU. Because ofreliability reasons the PCU is driven by two independenthydraulic actuating circuits. The speed of both hydraulicmotors is summed by a differential gear (DG). In the caseof a single hydraulic system failure the high lift systemcan be operated with half speed. The position of thewhole transmission system is set by throwing pressure-off brakes (POB). Using VDHM driven PCUs enablessmooth start-up and positioning sequences. Moreover itallows steady position control of the high lift system.(1),(2)

FIGURE 9 - Conventional high lift transmission (flap-)system with VDHM driven PCU

The power requirements of future large civil transportaircraft open an attractive field for the application ofVDHMs. Especially the PCU-operation during landingapproach is one decisive design case of hydraulic powersupply (compare FIGURE 1). FIGURE 10 demonstratesthe theoretical, simulated power transients of the speedcontrolled conventional CDHM and the VDHM conceptfor a given load profile at PCU output of a full flapextension operation. The power loss between themechanical output power and the curve of the VDHMhydraulic input power is explained by the losses due tototal efficiency ηt of the unit whereas the difference

between CDHM and VDHM can be found in valveinduced pressure losses.

FIGURE 10 - Comparison of power transients between CDHM and VDHM

The comparison of power peaks and hydraulic workshows the VDHM concept’s advantage. The power peakat maximum load demand is reduced to 47% (FIG. 11).The overall hydraulic work needed is decreased to 32%.

FIGURE 11 - Comparison of power peaks andhydraulic work between CDHM andVDHM

0

0.2

0.4

0.6

0.8

1

Comparison of power peaks and work for a full extension operation

P/P

max

, W

/Wm

ax

Pmech,out

Phyd,VDHM

Whyd,CDHM

Whyd,VDHM

Wmech,out

Phyd,CDHM

EHSV

pS pR

iSV

VDHM

POB POB

2.HydraulicSystem

DG : Differential GearEHSV : Elektrohydraulic ServovalvePCU : Power Control UnitPOB : Pressure-Off BrakeSOV : Solenoid ValveVDHM : Variable Displacement Hydraulic Motor

DG

1.HydraulicSystem

PCU

SOV

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

P/P

max

Time (s)

Comparison of power transients for a full extension operation

Phyd,CDHM

Phyd,VDHM

Pmech,out

7

Fluid power system architecture using VDHMtechniques

Combining the two approaches, VDHM driven slat/flapPCU and HEPCU, in an alternative fluid power systemarchitecture may lead to a significant change of thetypical hydraulic load profile on the consumer and on thepower generation side (FIG. 12).

FIGURE 12 - Hydraulic load profile of an aircrafthydraulic system using VDHMtechniques

On the consumer side the hydraulic power demands foroperating the slat/flap systems during take off andlanding decreases by approximately 50% compared to anconventional configuration. This yields in a directreduction of the hydraulic design load.

With the HEPCU transferring hydraulic into electricalpower during cruise, the gap between power demandsand power resources decreases. The efficiency of thehydraulic power generation system increases.

On the power generation side the HEPCU providesadditional power “on demand” during take off, approachand landing. It depends on the respective hydraulic andelectrical system architecture and consumer profiles howmuch power can be transferred between the two systems.For a typical long range aircraft configurationapproximately 20% of the rated hydraulic and electricalpower is available for a bi-directional transfer.

By reducing the design load and by providing additionalboost power down sizing of the primary power sources(engine driven pumps) becomes possible. For a typicallong range aircraft the rated power of the engine drivenpumps can be reduced by 30...40%. Besides, thehydraulic installations (pipes, filters, valves, etc.) can be

downsized as well. Both results in weight and costreduction.

Apart from the advantages for the fluid power system theapplication of VDHMs may also influence other aircraftsystems:

The VDHM driven slat/flap PCU is the basis for avariable camber wing. This concept allows to choose anoptimum lift coefficient depending on the current flightphase.

The HEPCU influences the electrical power generationsystem similar to the hydraulic system. The enginedriven generators can be downsized because the designloads during cruise are covered from the hydraulicsystem (electrical power on demand). Besides, thereliability of the electrical system is increased due tomore flexible power distribution options.

Summing all effects the application of VDHMs can saveup to 20% weight in the hydraulic and electrical powergeneration systems of a typical long range aircraft.

Conclusion

The consequent use of variable displacement motors(VDHM) on the consumer and on the power generationside can help to increase the efficiency of aircraft’shydraulic systems. Even the efficiency of other systems(namely the electrical power generation system) can beinfluenced in a positive way.

Especially, the application of VDHMs in future largeaircraft is a very interesting issue. Compared to existingconfigurations the hydraulic power requirements in theseaircraft will increase enormously.

With conventional hydraulic system architectures seriousdesign problems are expected for these future largeaircraft. These problem can be avoided by using energysaving VDHM-based applications, i.e. the hydrauli-electrical power conversion unit (HEPCU) and theVDHM driven power control unit.

0

50

100

150

200

250

300

Flow

[l/m

in]

Flight phase

Requiredflow

Ground Taxi T/O Climb Cruise Desc. Appr. Land.

additional power provided by HEPCU

reduction of PCUpower demandhydraulic power

transferred into electrical power by HEPCU

new main pumpcharateristic

8

References

(1) GEERLING, G.: Secondary Controlled VariableDisplacement Motors in Aircraft Power DriveUnits. 5th Scandinavian International Conferenceon Fluid Power SICEP '97. Ed. 1. Linköping,Sweden 1997, pp. 167-179.

(2) IVANTYSYNOVA, M.; KUNZE, O.; BERG, H.:Energy Saving Hydraulic Systems in Aircraft - aWay to Save Fuel. 4th Scandinavian InternationalConference on Fluid Power SICEP '95. Tampere,Finland 1995.

(3) MURRENHOFF, H.: Regelung von verstellbarenVerdrängereinheiten am Konstant-Drucknetz.PhD Thesis, Rheinisch-Westfälische TechnischeHochschule Aachen, Germany 1983.