support.Thereisnothrustbearing,butthetworadialmagneticbearingsprovideadditionalaxial restoringforcesthroughreluctancecenteringeffects.Thestaticpredictedloadcapacityofeachradial bearingis113kg(250Ib).Intypicalapplications, aradialbearingsystemisdesignedwithaload capacityofthreetimestherotorweight.Inthiscasethebearingsaregrosslyoversizedandalmostan orderofmagnitudestrongerthanrequired.Faulttolerantbearingdesignswillmostlikelybeoversized, butnotthismuch.Oversizedbearingseliminateapotentialsourceofnon-linearity duringfaulttolerance testing.AsshowninFigure2,eachbearinghas8coils(includingfourredundantcoils),whichare individually controlledbytheirowndedicated PWMtransconductance poweramplifiers. Forthis configuration, adecouplerisrequiredforeachbearing(seeMeeker,1996).Thebearingneartheair turbine(in-board)ispoweredbytwo-stateamplifiersandtheoutboardbearingispoweredbythree-state amplifiers.Teneddycurrentdisplacement probesinthreedifferentplanesmonitorthemotionoftherotor. Thereareseveralwaystosimulatecircuitfaultsinthisfacility.Therearemechanical switchesand voltagecontrolledswitchesinthecircuitsbothinfrontoftheeachamplifierandeachcoil.The mechanical switchescanbecontrolledmanuallyfromthecontrolpanel(Figure3).Thevoltage-controlled switchescanbeopenedandclosedwitha0and5voltsignal,respectively. Circuitfaultscanalsobe simulatedbycontrollingthecommandsignalsfromthemagneticcontrollertothePWMamplifiers. SystemDynamics Forproportional-integral-derivative (PID)feedback controlthefollowing relation isrepresented the decentralized dynamics ofthesuspension system: meqJt:+K,.Ka_:+(KiKp-K,),+K,K,Ixdt=O (1) whereKI,istheproportional controlgain,Ktistheintegralcontrolgain,Kaisthederivative controlgain, and,m_,jistheequivalent lumpedmassoftherotor. Forcentralized controlforcefromeachradialbearing, thefollowing equation represents thecentralized dynamics ofthefault-tolerant magnetic suspension system F,.2(k,X.,q+.,-:)+(k,+k ).l.o+(q = - "' " (2)4 ControlSystem Thecontrolsystemusesaproportional-integral-derivative (PID)controlalgorithm, whichisimplemented withtheMATLAB/Simulink software. Areal-time ANSICcodewasgenerated, compiled, and downloaded toadSPACE controlsystem(Figure4),whichisanintegrated software andelectronic controlunitcombination (MATLAB software anddsl003/dsl004 alpha-combo, multiprocessor board).It foundthatfundamental shortcoming existsintheblockdiagram basedcontroller asappliedtotheeventdrivenfault-tolerant controlapplication. Thenumber ofblocksusedintheoverallcontrolsystem drastically increased thelooptime(orsampletime).Toovercome thisdeficiency, acustomized block wasusedtoincorporate afault-tolerant controlalgorithm writteninANSICwiththecontrolhardware. It reduced thelooptimebyabout45%,reducing alotofSimulink blockstoimplement afault-tolerant controlalgorithm. Unlikeothermagnetic suspension testrigsatGRC,thefault-tolerant testrighadseveresensornoise, whichcausedanintolerable operating noise.Tocancelsensornoise,amovingaverage (MA)filterwas implemented tosmooth outfiverunning points.SinceaMAfilterisaFiniteImpulse Response filter, whichcannotbeimplemented inrealtimefashion(thenexttwofuturevaluesareunknown). Tomake thisnon-causal filtercausal,wehadtoshifttheindexbackbytwo(thatis,twosampletimes). Figure5showsacontroller cockpitwindowthatallowsmodification ofmagnetic bearingparameters such asbias,stiffness, damping, andintegral. Alsoitincludes afeatureforvariousfault-tolerant situations for multiple coilandamplifier failures. Ithasaswitchforadecentralized PIDcontroller orcentralized modal controller. Atcriticalmodes, acentralized modalcontroller wassufficient tosuppress thevibration 128