ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Figure 3.  Scan Mirror As sembly  Componen t Layout 

The unique quality of the SMA was its ability to prov ide stable and continuous scans for the duration of  the missi on. The stability of the SMA w as in sim plicity of construct ion. All the m oving part s, i.e. the mirror,   motor rotor, a nd re solve r rotor, sh are a comm on shaft supp orted by two ball be aring s forming one of the   two main sub assembli es. By using a co mmon shaft conf iguration the SMA was also a ble to eliminate any   errors in po sition due to ba cklash as wel l as red uce single point fail ure a reas. Th e scan mirror is attache d  to the shaft throu gh the u se of flexure mounts joi ned to the mirror’s structural  ribs. Early in the desig n  stepp er moto rs were co nsidered but we re ruled o ut due to con cerns in lifetime. 

Mirror Ac tuation is  provided by a limited rotation, brushless , samarium cobalt permanent magnet torque  motor with re dund ant wind ings. A limited rotation mo tor was use d to achieve zero rip ple torque. Torq ue  for the moto r ranged from 0.085 N-m/A mp at the null  posit ion to 0 at ±50 d egrees. Positio nal feedba ck is  provide d by a 16-speed brushless resolver with r edundant win dings with an accuracy  of ±20 arcse conds  over 22.5 de grees.   

The other subassem bly was the hou sing, which supported the ba ll bearing s and contain ed the stators of   the motor an d resolver. T wo sepa rable race b all bearings supp orted the rotor on either side of the sca n  mirro r, inboard closest  to the m otor and  outbo ard farthest from th e moto r. Th e bearing housing for the   SMA was made fro m titanium to mat ch the therma l expan sion of the bea rings. Bea ring prelo ad was  achieved th ough th e use of a di aphragm spring on the outboard side and adj usted with shims. To  redu ce  the amount of  frictional torq ue the axial p reload wa s kept low.   

During op eration, images were acquire d while the mirro r sca nned and while  the SMA held the mirro r at  fixed position . Norm al ope ration involve d scan speeds ranging from 0.14 to 0.2 8 degree/second follo wed  by a return to  starting position at 6 degrees/second1. The cont rol sy stem diag ram is shown in Figure 4. As  can be seen the SMA op erated with a close d loop system usi ng software  compo nent mo dels and filters to   modulate the control sig nal for maximum  stability.  

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Figure 4. SMA Con trol Sy stem Diag ram 

Testing   In ord er to decrease the ti me ne eded for lifetime te sts to fit within  the p rogram sche dule, two parall el tests  were run on two me chanisms. It was determi ned t hat a single accele rated t est would not be accu rate  enou gh to d etermine th e SMA’s beh avior while in  operat ion. On e test was a n accel erated test and  the  other wa s a test duplicating on-orbit scan ning. These tests were pe rform ed in vacuum and p rimarily  measured two sig natures; averag e mo tor current to determi ne any chan ge in mech anical chara cteristics,  and freq uency contest to d etermin e if any bearing d egradatio n occu rred. 

Another te st con ducted on  the SMA was tensile te sting of the joints betwe en the mirror an d its flexure   mounts. During this testin g seve ral catastro phic failure s occurre d resulting in  the loss of several mirro rs.  These fail ures were due to incorre ctly estimated  valu es of the  mirror te nsile strength. First numbers used   for ten sile strength of Sint ered Silicon Carbide  227.5E3 kPa did not take into accou nt the wide  varia bility  of the materi al from 90E3  kPa to 345 E3 kPa. Afte r an in-h ouse analysi s, which in corpora ted margi ns,  62E3 kP a wa s cho sen to repre sent the tensile strength of the mirror. Thi s new numbe r then resulted in a  design chan ge of how the mirro r mou nts we re atta ched to sp read t he load ove r a greate r area.               SMA On-Orbit Performance   The pe rform ance of the SMA on orbi t was mo st often determi ned prim arily by measuri ng the kno wn  positio n of a star with the expecte d position of the SMA. Unfortun ately, this method wo uld b e accurate to  within a few t enths of a degree du e to the nature of t he optics. In  addition, for t he LORAAS missi on, any   errors that were le ss then 0.1 deg ree would b e unnoticeable a nyway due to la rger errors from  the attitude   determi nation of the ARGOS spac ecraft. ARGOS horizon sensors prov ide spacecraft‘s attitude with  fluctuation up to 0.2 deg ree with one sigma error. As for m echanism state of health the  down-lin ked  telemetry incl uded resolve r errors a s well as torque  motor curre nt. Durin g the entirety of the missio n  errors o ccurred that would  have comp romised  the in strument d ata beyond th at due the spacecraft attitude   control system.  4
In addition  to the sca nning mirror me chani sm, the nature  of the  optics a nd detectors used  for the E UV  and FUV n ecessitate havin g two other m echanisms used by LORAA S, the DCDA and the D DM.  

Dust Cov er Door Assem bly 

As with all o ptics, contam ination fr om dirt and hydroca rbons is an issue that deserve s spe cial attention.   The sensitivity of VUV opti cs to contami nation i s greater tha n for optics in other wavelengths especially  when the con tamination a rises from thin layers of oi ls or othe r hydroca rbons. A mirro r with a hydro carbon  layer me asuri ng on th e ord er a fe w ten s of ang strom s thick can have its thro ughput re duced by as mu ch  as half in the  reflected V UV. This sen sitivity to contamination d rives the nee d for the LO RAAS one time   activation du st cover d oor assem bly, sho wn in Fig ure 5. Unli ke most instru ment doo rs the LO RAAS  DCDA i s unique in  that it u ses a guillotin e type of operation rath er than a swin g-away or clam shell type of  operation. Th e driver for t he guillotine operation wa s to pre serve envelope co nstrai nts imp osed by the   rest of the spacecraft. The DCDA is also unique in t hat it uses no l ubrication even though it is a slidi ng  mech anism.  The lack of lubricants was due to the se nsitivity of the sca n mirror to  contami nation. For this  particula r optic, a stray dro plet of lubricant would cause mo re da mage than a  dust parti cle. Also unli ke  other door mechanisms, t he DCDA is not mou nted over t he front aperture of the in strum ent instea d it is  mounted after the sunshade an d there fore ha d ad ditional con straints imp osed on it by th e sunshad e  mounting  scheme a nd structure. Th e use of the  guillotine-type  door requi red an extensiv e amo unt of   analysi s to be  condu cted to  ensu re sati sfactory pe rformance of the DCDA whil e on earth an d in orbit. The  result of this analysi s was to determine  operation al tempe ratures,  lifetimes, re sistances due to fri ction,  and op eratin g margin s for t he actu ation springs and d oor latch. 

Figure 5.  DCDA Comp onent Lay out 

The DCDA consi st of an Aluminum 60 61-T6 plate that slides downward from the area between the USA  and the sunshade. Two of  the outer edges  of the door are confined within  aluminum tracks that constrai n  its motion d ownward alo ng the USA. A  wou nd metal  strip called the ne gator spring as is shown i n Figu re  6 drive s the motion of the  door. During laun ch and storage the door is held in place by a l ock arm a s  shown in Figure 6. The lo ck arm i s rotated out the door-latching hole by a Star sys Re search Corporation  paraffin  actuator with red undant heate rs. DCDA status is monito red by  three sets of  hermetically sealed  microswitche s. One mi croswitch is loca ted in the la tching system f or lock arm rotation statu s. The se cond  and third set of micro switches, a prim ary and b ackup unit is lo cated at the door limit s of travel and   confirms that the door i s either fully clo sed or fully ope n.   5
Lock Arm  Dust Cover  Door  Housing  Microswit ch Sun Shade  Microsw itch  Housing  Paraffin  Actuator 

Figure 7.  DCDA La tchin g System Co mponets 

In desig ning the DCDA an alysis was done to aid in  sizing the n egator spring and latchi ng system. The   analysi s shown in Figu re 7 wa s used t o dete rmine  the ne gator spring  size. Sh own along the curved li nes  are sprin g margin s at different value s of coeffici ents of friction b etwee n alumi num on alum inum. Thre e  curve d lines are plotted fo r sprin gs that exerted differ ent value s of force  on th e door at its closed position.  The d esign margi n of 2 50% is pl otted  as th e thick horizontal  line in the l ower portion of the g raph. Th e  high m argin helps to en sure succe ssful openi ng of th e DCDA by p roviding  enough force  to ov ercome  any  highe r than n ormal fri ctional force s. The highe r fricti onal force s may occur from sticking o r mild galling   from u sing similar m etals i n sliding contact. To help minimize the shock from openin g, the spring  margin  was not raise d high er tha n what i s listed. Any sprin g which exe rts more th an 2. 2 N of fo rce u pon the door  at closed po sition op erates a 25 0% margi n or ab ove. This an alysis h elped to reduce th e size of the   negato r spring by 10% from what wa s detailed in ea rlier de signs.   

Figure 7.  DCDA Do or Slide Spring Margins 

Another an alysis wa s done to determin e how the ne gator sprin g margi n woul d be affected by temperature  change. T his analysi s is shown in Fi gure 8. T he m argin was determin ed to be suffi cient to  operate the   6
DCDA up to  the 67° C testing limit. Howeve r as the temperature increa ses the margin d ecreases as  shown and is reduced to 12 0% at the testing limit. 

Figure 8.  DCDA Op erati onal Temper ature M argins 

The DCDA Latchin g syste m is de signed so that the re exist s suffi cient fri ctiona l force b etween the lockarm an d the door (exerte d by the negator sp ring) to prevent the d oor from o pening prematu rely duri ng  launch vibrations. T he negator spring and the lat ching syste m were also an alyzed. Thi s analysis was to  determi ne if t here would be sufficient f orce exe rted by the p araffin actu ator t o release the  doo r. On e  analysi s dete rmine d the a mount of force exe rted o n the lock arm by the paraffi n actuato r at different  rotation angle s. Anothe r analysis as sh own in Figu re 9, determi ned the am ount of ope rating margi n from  the actu ator above that require d to release th e door. The m argins are above  1000% of that  need ed to  open  the door. Thi s data  could have been  used to reduce th e size of th e latching  system for wei ght  reduction. Howeve r, the paraffin a ctuator u sed at  the time was the al ready the small est availabl e.  Fortun ately for thi s particular m echani sm, hig h margins are desirable  in that a failure  of th is me chanism  would result in a compl ete missi on failu re.   

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Figure 9.  DCDA Do or Latching Sy stem Margins 

In hinds ight the high marg ins benefited the LORAAS program unexpec tedly as a result from a s light flaw  in the wiri ng of the paraffi n actuato r. Normal o peration of a pa raffin actuato r require s a conti nuou s flow of  curre nt in th e prim ary he ater with th e redu ndant heater avail able but not e nergi zed. In an ide ntical  appli cation fo r anoth er NRL instrume nt both the p rimary and redundant h eaters were wired in parallel. It  was decided  that rewirin g the instrum ent to prev ent the powe r applied fro m going ove r the spe cified  values would be too costl y and inconv enient. A soft ware solution  was found which involve d modul ating  the duty cycl e of power a pplied. Thi s resulte d in th e pulsin g of po wer with a 5 0% duty cycle on the orde r  of one se cond per cycle so that on average the po wer rem ains within sp ecifie d limits. If one of the heaters  were to burn out, a n ew comm and could be  uploaded to power the remaining he ater co ntinuou sly.  Grou nd testin g was d one t o confirm thi s method woul d wo rk. Beca use of the hig h margin s for the latching   system, the l oss of h eat that may o ccur d uring pulsed operation should not cause failure in that t he  actuator woul d still be working well below its specif ied limits.       Detector Do or Mecha nism 

The vast ma jority of VUV detectors h ave photo cathode films d eposited ont o their active  surfa ces to  increa se sen sitivity in a given passba nd. The LORA AS detectors use d cesium iodide, CsI. Unfortunately,   CsI is hygroscopi c and breaks do wn o n the ord er of hours when exposed to moi sture. L ong-term stora ge  of Cs I, on the order of years , is best performed by s ealing it under high vacuum as  compared to a rough  vacuum  or a dry atmo sphere3. The DDM , sho wn in Fi gure 10, was used to  maintain the  high  vacu um o n  the detecto r. Ordin arily a tran sparent window wo uld be used to seal off the high vacu um interior of the  detecto r from the ambie nt atmosph ere. However,  the lack of  a material that is tran sparent at the   wavele ngths of interest pre vents the use  of a window. A detector do or me chani sm was d esigned and built   to provide the  nece ssary hi gh-va cuum hermeti c seal durin g inst rument con struction, betwee n calib ration s  as well as d uring gro und storag e, tran sport, and laun ch.   

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Figure 10.  Tw o Detector Door Me chanisms with Mounted De tector s 

DDM F eature s  The majo rity of detector doors de signs ope rate on ly once from  close d to open and the n have to be  manually reset. The LORASS DDM  is unique in that it can open and close wit hout needing to be reset.  The DDM’s have a  design lifetime of over 1 00 open and close cycl es with out seve re degradation i n  perfo rman ce. This lifetim e requi rement follows from  the number of o pen/close cycles predi cted fo r  calibration p urposes of the  detecto r and the instrum ent while o n the grou nd rather than fo r flight use.  Designing the DDM to the aforem entioned lifetim e requi rements wa s co mplicate d by the adde d  requi rements that the DDM be a ble to withsta nd a high temp erature b akeout of 200° C. It is sta ndard  practice in F UV an d EUV  instrume ntation to b akeou t comp onents used in  the optical  system due to th e  extreme sen sitivity to contamination.   

Another uni que q uality of  the DDM  is its extremel y low m ass. The DDM can provide th e ne cessary   sealing fo rces and lifetime  in a packag e that weighs slightly less than 1  kg. Prese ntly all DDM’s u sed at  NRL h ave p rovided  the unpo wered high va cuum  seal for many months at  a time with no l oss of  perfo rman ce after tens of cycle s. Unli ke other d etector doo r mechani sms, whi ch are custo m desig ned f or  a particular detector, the LORAAS DDM  is adaptable fo r use with other detectors . This adaptability is due  to the detector hou sing being an ind epend ent stru cture from the DDM support frame. Th e detecto r is  mounted to t he DDM by clamping a flange, which is par t of the d etector housing, to the DDM flange. As  long a s the d etector housing po ssesse s a flange d face it sho uld be com patible with the DDM  barri ng any   kind of me chanical inte rference fro m another portio n of the dete ctor. Figu re 10 shows  two DDMs  with two  different mo dels of d etecto r mou nted. T he cl amp-mounting schem e employe d by the LORAA S DDM also  allows th e detector to be  rotated at any orientatio n with res pect to the DDM. S ometimes this is necess ary  as it i s with the L ORAAS instrument s when the detecto r/DDM a ssembly  might go th rough seve ral  iteration s during cali bratio n to determine the best o rientation. 

DDM De sign and Kinem atics  The DDM i s built aro und a 4-Ba r toggl e linka ge system to achi eve the necessary to rque multiplicatio n.  Weig ht wa s minimized by  prop erly mat ching the lin kage to u se the small est input torqu e possible while  still providing the necessary force to  seal the detector. Sealing the detector requires a force of 136  kilograms to comp ress, by  30%, a 75-durometer Vito n-A o-ring with a 1.8 -mm (0.07”) cross section. In   addition, th e torque on the door a rm exe rted by the seal is not enough to  back-drive the  moto r against its  detent to rque. Prope r matching the linkage components hel ps give the  DDM its ability t o provide  the  vacuum seal while unpowered.   9
The DDM  linkage consists of the follo wing components:  the DDM Door, the d oor arm, the tog gle ba r, and  the cranks haft. A s pur gear with a 3: 1 reduc tion is  connec ted to the cranks haft and is driven by a  Rapidsyn st epper m otor providing 0.073  N-m of torqu e at 22 VDC.  The output o f the motor is transfe rred  to a Globe 4-Spage plan etary gearbox with a 319:1 es timated mi nimum Torqu e ratio and 7 60.6:1 Speed   ratio. The driv e system rota tes the cran kshaft whi ch is conne cted to  the toggle bar. The toggle bar rotate s  the do or arm and closes the door. The compon ents of  the li nkage and drive system a re shown in  Figu re  11.   

Stepper  Stepper M otor  Planetary  Drive  Gearing  Toggle Bar Limit Switch  Actuator  Dete ctor Flange  ClampBellville  Springs Crankshaf t  Drive Gear Cranks haft Door Arm  Door 

Figure 11.  DDM Comp onent Lay out 

The DDM Li nkage was designed so that the fo rce imparted by  the d oor in creases as the door move s  closer to the clampi ng surface. As the door closes, the linka ge moves to its toggle po sition whe re the  angle bet ween the cran kshaft and th e toggle app roaches zero , as sho wn in Figure 12.  This toggle  positio n occurs just befo re the door is close d and thi s is where the maximu m clamping  force  is achieved   with the mini mum amo unt of input torq ue. The closing force p roduced by the  linka ge is adjuste d and   regul ated th rough  the use of two custom Bellville wash ers lo cated bet ween the door an d door arm. The   Bellville washers have a small range of deflection (about 0.5 mm) in whi ch the force exerted by them   remai ns relati vely con stant. This con stant force helps to en sure the prope r amount of prelo ad is exerted   despite any linka ge relaxat ion or the rmal distortio ns, and p rovide s margi n for an y toleran ces in the DDM   system. If desire d, the pre load can be adjusted by changi ng the Bellville st ack cha racteristics. However,   any ch ange i n preload m ust be d one so that ba ck-driving does not occur. Regulation of the  DDM i s also  accompli shed through the use of two mechanical stops. One stop limits the  door travel  to 95 degree s and  the other stop preve nts the toggle fro m going ove r cente r. Stopping the togg le from movi ng over the   cente r preve nts any re sistance du ring door openi ng from the Bellville washers and o -ring. Minimizin g the  resistance during op ening was anoth er requireme nt driving the DDM desig n.   

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Crankshaf t Toggle Bar  Door Arm  Door XY 

Figure 12.  DDM Kinema tic Diagram 

To minimi ze resistance fro m friction i n the lin kage,  extra-preci sion needl e bea rings from To rrington were   used in all li nkage joint s. These ne edle bea ring were mo dified b y removing t he cup which allowed for  cleaning and lubricant insertion. The  lubricant used for the bea rings as well as for the drive system wa s  Demn um 20 0-L-280 Perflouro polyethe r. The cran kshaft gear u sed impre gnated Teflon for lubrication.  Out-ga ssing  and th e requi reme nt to survive the b akeout  procedure drove lubricant selection. Surviving   the ba keout pro cedure inf luenced th e design of the  DDM st ructure as far as how th e com pone nts were  attac hed. 

Bake -outs mostly occur d uring the d etector inte gration peri od. Th ese b ake-outs ca n be low l evel were the   detecto r is brought to 10 0°C for a mi nimum of one h our and they can be at hig h level whe re temperatures  can reach up to 200°C. Rather than increase the c ost of the DD M by using comp onents that ca n  withsta nd th ese tem peratures it was de cided that t he DDM should be m ade easy to di sassemble. Duri ng  a high level b akeout the do or arm is fa stened to the fr ame an d mai ntains the seal while it is d isconn ected  from the linkage. The e ntire linkage and drive system ca n then  be discon nected from th e one -piece  frame. For lo w-level b akeout the detector and DDM  syst em rem ain as a unit with a tempe rature co ntrol  channel pla ced on the driv e system to e nsure that it does n ot go ab ove 100° C. 

DDM An alysis  Extensive an alysis was condu cted on the DDM d esign to ensure satisfact ory pe rforma nce during  operation, a chieve a savin g in wei ght a nd meet its lif etime re quire ment s. Th e analysi s performed on th e  DDM determined the bearing ra dial lo ads at e ach joint, cra nkshaft stre sses durin g op eration, toggle  torque, a nd st ructu ral stiffness of the fra me and lin kage co mpon ents. When possible the se analyse s were   verified with data from di rect mea surement as i s shown in Figu re 14. Th e bottom line sh ows the doo r  force  as it rel ates to positi on. As was describ ed earlier, the a pplie d force in creases as the door clo ses  until the Bellville washer deflection poin t is reached. At this point force remains constant unti l the door is  closed. The door fo rce margi n is de termine d when the mea surem ents of  the actual door fo rce is  combi ned with the cal culations fo r the a vailable doo r arm force which i s the top line. The m argin i s the  differen ce between the top and bottom  lines an d is represented by the mi ddle line. Th e force  exerte d by  the doo r was measured b y using a l oad cell lo cated in the pla ce that the dete ctor wo uld o ccupy. A strain  gaug e mou nted on th e tog gle is used to  mea sure DDM perfo rmance when a detector i s in place, such as  in instrume nt level enviro nmental test. This m apping of the strain experie nced by the toggle  as the door  closes was the result of the analysi s during de sign verified by direct  measu rement on a finishe d unit.  11
Figure 14.  DDM Forc e Margin 

The analy sis that was con ducte d duri ng design allo wed seve ral ch ange s to take  place to pro duce a final  model with a lowe r weight and le ss compone nts th an ea rlier estimated. Th e origin al 38-mm diamet er  gearbox wa s repla ced wit h one 31.75 mm in diame ter. T he motor wa s also repla ced from  the original   Globe brushl ess moto r to a Ra pidsyn stepper moto r. Simplificatio n wa s achiev ed by changing both  the  crankshaft and door a rm from 3 piece a ssemblie s to singl e part s.   

DDM Co st  Due to the l ong design history of the DDM a ny detail ed costs h ave be en lo st as personnel have come and  gone f rom th e proje ct. Th e be st estimates fo r the total non recurring costs are between  $350,00 0 an d  $500,0 00. M ost of these costs are due to the extensive amo unt of analysi s condu cted in addition to that  incurred by DDM  design.  The recurrin g costs for each unit is somewhe re around $55,0 00 per unit. Al l  testing was done at NRL, which greatly redu ces the cost. The cost for testi ng wa s between $3,0 00- $5,000 p er DDM. All cost s are ap proxim ate and in 19 93 dolla rs. Te n flight units were mad e so a total co st  would be just  under a milli on dolla rs. Since the de sign has be en so su ccessful it has been u sed on two  other missio ns and will most likely be used on many future missi ons. Thi s will hel p offset the i nitial  nonrecurring costs an d ma ke the de sign even more v aluabl e. 

In addition to  the LORAA S missio ns, a modified fo rm of the en ginee ring d esign u nit wa s succe ssfully  flown o n NRL’ s Joint Astrop hysical Plasma dynamic Expe riment so unding ro cket missi on. Th e  modificatio ns for this DDM was the reduction of th e openin g time from 30 minute s to about 3 0 seconds by  changing the motor/drive system to a motor with higher to rque and le ss reduction in the gears. The se  modificatio ns were ne eded to accomm odate the al ready short am ount of time available to t ake data. Th e  cost of this modificatio n was on the o rder of $ 30,000 for t he new parts and  analysi s to v erify the DDM  would still provide the necessary seal. Also an identi cal form of t he DDM as that flown on LORAAS was  used successfully in the NRL Hi gh Resolution Iono spheri c Thermospheric Spectrograph in strume nt. 

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