Prototyping platform - Design and implementation of FPGA-based LQ control of active magnetic be

Intheprototypingplatform,weusetworadialbearingsandoneaxial bearing,

thatis,vepairsofelectromagnetstocontrolmagneticsuspensionoftherotor.

Thetechnical detailsanddimensionsofthebearingsandtherotorusedin the

studiedprototypecanbefoundinAppendixB.1. Thetotalrotormassis46.2kg.

Using Eq. (2.17) and the dimensions of the bearings, the maximal static

force loadcapacity ofthe radialbearing canbe estimated as

f max ≈ 2 kN

^(at

thenominalair-gap). Alternatively,themoreaccurateestimateofthemaximal

bearing load capacity (which still does not resort to the complex numerical

solutions)maytakeintoaccountthemagnetizationoftheiron(Schweitzeretal.,

2003). Equation(2.17) denesthe loadcapacityof the studied radialbearing

intheweakestdirection. Itassumesthatthemaximalradialforceoccursatthe

current

i sat

^that^roughly^can^be^determined^from^(2.5),^as

i sat ≈ 2l 0 B sat

µ 0 N ≈ 8.5 A,

^(2.22)

wherethemagneticair-gaplength

l 0

^is^estimated^for^the^rotor^remainingⁱⁿ^the

centralposition(Appendix B.1). InEq.(2.22),weassumesucha

B sat

^that^we

stillcanneglectthe eect ofironfor thecoilcurrent

i < i sat

⁽

i max > i sat

^). ^In

this work, themaximal current

i max = 10 A

^; ^it ^is ^the ^same^for ^all ^used ^types

of bearings. In the thrustbearing, the

i max

^is ^smaller ^than^the

i sat ≈ 12.7 A

Therefore, it is safe to estimate the load capacity of the axial bearing from

Eq.(2.8)as

f max ≈ 2.13 kN

^. ^In^order^to^tackle^the^power^lossesⁱⁿ^the^ampliers,

thereducedbiascurrentwaspre-selected

i bias = 0.25 · i max = 2.5 A.

^This^choice

aects thecurrentstiness

k i

^and^an ^open-loop^gain,^which ^becomes ^low^at ^a

zeroload. Additionaly,thepossiblerateofchangeofforcedecreases,compared

withthenon-reduced

i bias

^. ^This^can^be^observe^by^examining^Fig.^3.13.

Atthebeginningofthischapter,theAMBcontrolsystemasawholeis

pre-sentedusingthemechatronicsVenndiagramandgeneralassociationswith

dif-ferentengineeringelds. Thephysicalcomponentsoftheparticularprototyping

platformandtheirphysical associationscanbepresentedusingtheconceptual

staticmodel (accordingto Goma, 2000),whichoersamoredetailed physical

insighttothestudied system(Fig. 2.13andFig.2.14).

2.5.1 Control electronics

Themaincomponentsofthecontrolelectronics,apartfromtheprocessingunits,

oftheprototypingcontrolsystemwerethefollowing:

1. Thecustom-builtpowercircuitboard, withtheIGBTSemitop

R2SK30 GAR 123 wasused. Thedigital switching signalsfrom the control unit

weredeliveredthroughtheopticallyisolatedgatedrivers.

2. Forthe current sensors weutilized ten closed-loop (compensated)

Hall-eect LEMtransducers(LA25-NP).

3. Thepositionsensors,weusedthreesingle-channelDT3701U1-A-C3,two

dierential(two-channel)DT3703U3-A-C3andonesingle-channelCMSS

68eddycurrentdisplacementsensorsfromMICRO-EPSILON(usedwith

A M B C o n t r o l S y s t e m

Figure 2.13: Conceptual static model for AMB system, where each number

refersto thenumberofparticularentities

P C

Figure2.14: MainassociationsinthestaticmodelforAMBsystemaredepicted;

thePC Consoleis asaproperty ofconnectionbetweentheCustomHardware

&Softwareblockand the PC; theSimulinkControl Panel isasa property of

connectionbetweenthedSPACEand thePC.

respectively. ThesensorsusedfortheradialAMBshadaluminumsleeves

attachedonthesteelshaftasthemeasurementsurface,andthesensorfor

theaxialAMBhadsteelasthemeasurementsurface. Thearrangementof

thesensorsintheprototype,computingtherotordisplacementsinx and

y axesandthecalibrationofthesensorsaresummarizedinAppendixA.3.

4. The measuredsignalsare split in thesignalsplitters, and then theyare

sampledbytwosetsofADCs,connectedtotwocontrolplatforms:

FPGA-based, and a modular electronic control unit from dSPACE (dSPACE

platform). The ADC board that is a part of the dSPACE platform is

DS2001,and theADCboardthat isapartof theFPGA-basedplatform

iscustom-built.

The selected, more detailed characteristics of the control electronics can be

foundinAppendix B.2.

2.5.2 Processing units

Whenbuilding thecontrol electronicsfor thestudied system, wewere looking

forasolution, which would onthe onehand providethepowerfulandexible

FPGA-basedcontrolplatform,andontheotherhandsecurethemeanstoeasily

test and modify newalgorithms. With this in mind, wecombinedaMemec's

developmentboardcontainingaXilinx'sVirtex-II ProFPGAwithadSPACE

platformthatistraditionallyusedincontroldevelopment. Insuchanarranged

prototyping platform, the control algorithms canbe developed in a graphical

Simulinkenvironment,thenautomaticallycompiledintothePowerPCprocessor

(in dSPACE) and tested in the system in real-time. The FPGA provides at

leastaPWM forthegatedrivers,ormoreifit usesitsown ADC board. The

Memec'sdevelopmentboardcomprisestheVirtex-IIProFPGA,whichcontains

30816logiccells (13696slices) and twoPowerPC 405(32-bit implementation)

embeddedprocessorblocks.

ThedSPACEInc. oersmodular,commercialsolutionsforelectroniccontrol

unit softwaredevelopment;it isusedfor rapidcontrol prototypingin a

graph-icalSimulinkenvironment. Inparticular, we used theDS1005-09board, from

dSPACE, that contains itsown PowerPC TM

processor, capable of working in

realtime. TheDS4003DigitalI/OBoard(96TTLI/Os)servedasaninterface

between two control platforms. The selected FPGA contains two embedded

32-bit microprocessorcores, and it enables exible partitioning of the control

software into the programmable hardware and microprocessor program. The

development of the control for the FPGA was carried out in VHDL, and the

control software developmentforthedSPACE platformwascarriedoutin the

Simulinkenvironment.

Apartfromthealreadylistedcomponents,acontrolelectronicsfeaturedsuch

componentsas: hostPC,custom-builtpowerdistributionboard,anda

custom-built interface board containing a Spartan TM

-II FPGA interface between the

Virtex-II FPGAandDS4003boardfromdSPACE.

The components of the control electronics and the Memec's FPGA board

DC -li nk ca pa cit an ce

S p a r t a n - I I V i r t e x - I I P r o

R S - 2 3 2 A T X P o w e r s u p p l y

E t h e r n e t

Sp lit te rs fo r m ea su re d sig na ls an d AD C fo r a xi al p os iti on

Cu sto m A DC B oa rd A m p l i f i e r s o f p o s i t i o n s e n s o r s

S y s t e m A C E

Figure 2.15: Custom-built components of the control electronics and the

Memec'sFPGAboardaremountedinasinglebox.

In document Design and implementation of FPGA-based LQ control of active magnetic bearings (sivua 49-53)