Undernormaloperation,thepositionsensorwillprovidefeedbacktotheelectronics tominimizethe powerdissipationintheactuatorcoils.Thesensorpositionwillbecontinuously checkedafterthe actuatorissentthe60mApull-incurrentlevel.Oncethesensorreadsathresholdindicatingtheshutter isclosed,theelectronicswillautonomously dropthecurrenttotheholdlevelof3mA.Thedelayfromthe timethesensorreachesthethresholdtothedropincurrentis1.0E-03sec.Whentheshutterisgiventhe signaltoopen,thesensorverifiestheopenpositionandreturnsthissignalinadatapacketpriorto completely poweringdowntheshuttermechanism. Unique Operation Thereareadditional uniquecharacteristics ofthismechanism. Asstatedabove,theshutter isdesigned tofailopenifthereisaflexure breakage. However, theshutterisalsodesigned nevertocloseagainif thisfailureshouldoccur.Theinternal forcesoftheactuators (discussed intheProblems section) are largeandtherotorisinaninherently unstable position. Therefore, without acontinuous flexure under tension, therotorispulledawayfromthefailuresideandlargefrictional loadsaregenerated either between therotorandthestatorintheactuator, orbetween themirrorarmandthehardstopupright. Thisfictionisenough toprevent theactuator fromclosing. Finally, thechoiceinflexuresuspension system combined withthebearing surfaces asdesigned cause theinternal forcestoapproach zerowhentheshutter ispowered offandthemirrorisreturning tothe openposition. Theflexureallowsustoavoidconventional ball-bearing designs thataredifficult for cryogenic usageandalwayshavesomelevelofinternal friction. Inaddition, thefatigueproperties of BeCuincrease dramatically atliquidHeliumtemperatures. Wehaveperformed alifetestonanETUand havedriventhemechanism togreater than450,000 cycles,22.5timestheratedlife. Thebushings ontheshaftaredesigned tohave38.1_(0.0015 in)radialclearance withtheholeinthe endcaps.Careful alignment andtolerancing isperformed tocenterthebushing withinthisholeas precisely aspossible andlimitthecontact between thetwoparts.However, duetooperation under1G andphysical limitations intheprecision ofthemachining, thesepartsdotouch.Astheshutteris actuated, theinternal loadsoftheactuators pulltheshaftoffaxisandthesebearing surfaces rub.Under thisloadedcondition, therearefrictional lossesattheseinterfaces duetotheoffaxisnormalloadinthe bearing interface. However, withpoweroff,thisloadgoestozeroasdoesthefrictionforce.Therefore, thefrictional forcefighting theopening ofthemirrorisminimized. Thiseffectwillberealized toaneven greater extentinthezero-Genvironment ofspace. Fvn(;:tion01 Testinq Theshutterhasbeensuccessfully testedcoldmanytimes.Thesetestsincluded coldfunctional, position sensorrepeatability, lifeoperation fortheETU,infantmortality fortheflight,andelectronics boardcheckout.Theperformance duringeachofthesetestswasrepeatable andconsistent. Inaddition, lifetests werecompleted withnoapparent degradation inperformance. Oneadditional testwasinadewarwithawindow totakeopticalmetrology ontheshutter mirror. Usinga theodolite, wewereabletomeasure themirroranglerelative toastarting position. Thiswasourfirst opportunity tocharacterize theposition sensorcold.Thewindow prevented usfrombeingableto achieve temperatures colderthan28K,buttheoperation oftheshutter attemperatures below10Khad beenpreviously verified. Inaddition, theshutter isactuated withacurrentdrivesystem thatcompensates foranyvariance inactuator coilresistance. Theperformance ofboththeactuator andthesensorprovedtoberepeatable. Mirrorangleswere measured atseveral different current inputs. Ateachofthesecurrentlevels,thevoltage outputfromthe sensorelectronics wasmeasured aswell.Theresultsareincluded hereasFigures 8and9.The response oftheposition sensor issecond orderwithregardtothemirrorangle(Figure 8).Figure9 indicates thattheactuator needsapproximately 42mAbeforeovercoming thetorsional preload ofthe flexure. Above42mA,themirrorwillrotateaccording totheplotuntilthecurrent reaches 53mA.Atthis time,theattractive forceoftheactuator tabsoverwhelms therestoring forceoftheflexureandcauses the 6