FaultTolerant Magnetic Bearing forTurbomachinery Benjamin Choi*andAndrewProvenza* Abstract NASAGlennResearch Center(GRC)hasdeveloped aFault-Tolerant Magnetic Bearing Suspension rig toenhance thebearingsystemsafety.Itsuccessfully demonstrated thatusingonlytwoactivepolesout ofeightredundant polesfromeachradialbearing(thatis,simply12outof16polesdead)levitated the rotorandspunitwithoutlosingstability anddesired position uptothemaximum allowable speedof 20,000rpm. Inthispaper,itisdemonstrated thatasfarasthesummation offorcevectorsoftheattracting polesand rotorweightiszero,afault-tolerant magnetic bearingsystemmaintained therotoratthedesiredposition without losingstability evenatthemaximum rotorspeed.Aproportional-integral-derivative (PID) controller generated autonomous corrective actionswithnooperator's inputforthefaultsituations withoutlosingloadcapacity intermsofrotorposition. Thispaperalsodealswithacentralized modal controller tobettercontrolthedynamic behavior oversystemmodes. Introduction Magnetic suspension isnowacompetitive technology foraerospace applications suchasgasturbine engines andenergystorageflywheels. Despitetheirmanybenefits, activemagnetic suspension systems maysustainfailures inanyoneofmanycomplex components: displacement probes, DSPcontrol boards,poweramplifiers, communication buses,wiring,etc.Afailureinanyoneofthesecouldmeana complete failureoftheentiresystem. Because ofthis,concerns aboutoverallsystemreliability have arisen.Forsomeapplications, doubleoreventripleredundancy inmechanical systems isrequired. Redundancy inatypicalmagnetic systemmayrequireextraduplicate components powered upand readytogowithavotingmechanism inplacetochose(seeField,etal.,1998).Thiscanworkwith amplifiers, probes,DSPboards,andperhaps evenwirebundles, butnotnecessarily forelectromagnetic coils. Maslen, etal.(1998)showed thatsymmetry ofpolesinamagnetic bearing isnotrequired forcontrol. Appropriate coilcurrents canbedetermined toprescribe anarbitrary forceformanycombinations of faultsincoils,amplifiers, orconnectors. Na,etal.(2000)developed optimized solutions forfault scenarios foran8-poleheteropolar magnetic bearing. Theyfoundthatan8-polebearingwithuptofive failedcoilscanmaintain controloverarotorprovided therewassufficient position stiffness anddamping. Chen(2000)hasalsoshownthatamagnetic bearingcanoperatewithoutallofitscoilsfunctioning. Inthispaper,afail-safe controller forafault-tolerant magnetic suspension testrigwasdemonstrated. Without changing theinitialcontrolgainparameters, thecontroller successfully enabled arotortospinup tothemaximum allowable speedof20,000rpmwithonlytwocontrolled polesoutofeightfromeach radialbearing. Fault-Tolerant Maqnetic Bearing Rig TheNASAGlennResearch Centerhasafacilitytostudyafaulttolerance inmagnetic suspension systems. Thetestrigandassociated hardware areshowninFigure1.Therotoris68.6cm(27")long, weighs10.4kg(23Ib),andisdrivenbyanairturbine.Theairturbinecanspinupto60,000rpm,butthe rotorcanonlybespunupto20,000rpmduetothestresslimitations intheHyperco 50rotorlaminations. Therotorisattached totheairturbinewithahelicalflexiblecoupling, whichprovides someaxialrotor "NASAGlennResearch Center,Cleveland, OH Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001 127