ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
A visual examination showed the following results:  - MoS 2 on “Cronidur 30” (upper row left): Massive  removal of the lubrication layer.  - Ag on “Cronidur 30” (upper row right): Undistorted Ag covered surfaces kept    unimpaired.  - Pb on “Cronidur 30” (lower row left): Formation of a lead salt layer.  - WS 2 on “Cronidur 30” (lower row middle): No visible degradations.  - Uncoated witness sample of “Cronidur 30” (lower row right): No visible degradations. 

For this reason, the MoS 2 coating was not an option fo r the gimbal lubrication. 

To gather information about galvanic corrosion, t he test was performed on samples with distorted  coatings. The upper end of the rollers kept uncoat ed to simulate a distortion of the coating. 

Figure 3.  Salt Fog Test Results of Samples With Distorted Coatings  In Figure 3, the test specimens are shown after t he performance of the test. Two samples of each kind  are shown in the picture: 2x MoS 2, 2x Ag, 2x Pb, 2x WS 2, and 2x uncoated samples of “Cronidur 30”  (from left to right). 

Surprisingly the silver-coated samples showed indicati ons of severe galvanic corrosion in combination  with the “Cronidur 30”. 

XRF examinations were performed by ESTL to verify this result. One of the results was that the XRF of  the red colored areas of the silver coated samples s uggested a slightly higher c oncentration of iron. This  result confirmed that galvanic co rrosion has occurred. For this reason, the silver coating was not an  option for the gimbal lubrication. 

The XRF examination of the lead coated sample showed that still enough lead was present to provide  lubrication. As lead salt itself is soft and w ill be cracked up and embedded between the remaining lead no  negative impact on the lubrication effect is to be expected. 

Besides the environmental loads on the tribological sy stem, there are demands on the friction coefficient  and the load capability to be satisfied. For both remaining candidates, WS 2 and lead literature describes  sufficiently low friction coefficients for the intended oper ation profile as well for operation in air as in  vacuum. Further investigations on the WS 2 showed that the coating is not  appropriate at the high Hertzian  pressures that occur in the gimbal bearings in the light of the bearing life time requirements. 

Result and Outlook 

The result of this investigation and the test campaign was that only t he sputtered lead coating showed the  potential to act as lubricant for the gim bal bearings. This selection is based on:  140
• The proven compatibility of the lead coating with t he environmental loads, as concerns our specific  application.  • The known and demonstrated excellent performance of a lead coating under high mechanical surface  pressures and under oscillatory motion [2].  • The vast heritage of the standard sputtering lead coating process by ESTL (European Space  Tribology Laboratory, UK).  • The lead’s compatibility to oxygen. If oxidation should occur a lead-oxide layer is formed which  encapsulates the underlying lead. Th is lead oxide itself has lubr icating properties and acts as  lubricant itself. Moreover the thickness of the oxide layer (about 20nm) is small compared to the  coating thickness. 

Currently the qualification of the coating (performed by ESTL) and manuf acturing of the bearings for the  first 4 gimbal models (performed by FAG) is under  way and representative friction measurements are  planned beginning of August 2004. 

These friction measurements will include investigations on the impact of oxygen, life time behaviour of the  coating, and overload testing of the bearings. 

Conclusion 

Based on investigations of a variety of lubrication al ternatives showed that for the specific application on  the gimbal requiring survival under  high Hertzian contact stresses and high resistance to environmental  impacts it was determined that lead lubrication performs best on the “Cronidur 30” substrate. 

In contrast to the expectations that  • Silver coatings which is widely used on conv entional bearing steels on space application would  perform in an excellent manner, and that  •  “Cronidur 30” material is fully resistant to corrosion  It turned out that  • Silver coating on a “Cronidur 30” substrate leads to severe galvanic corrosion problems  • Severe corrosion effects can occur under certain circumstances even though “Cronidur 30” steel has  excellent corrosion resistance co mpared to other bearing steels. 

References 

1.  Trojahn, W. et al. "Progress in Bearing Perf ormance of Advanced Nitrogen Alloyed Stainless Steel,  Cronidur 30."  Presented at the 5th International ASTM Sym posium  on Bearing Steels, New Orleans,  Louisiana, USA - Nov. 19-21, 1996  2.   N.N. "Space Tribology Handbook, 3rd Edition."  AEA Technology plc - ESTL Cheshire, UK, 2002 

Rem ark 

Ariane 5 is an ESA program, managed by CNES, pr ime contractor of VINCI is SNECMA.  141
The Integration of a  Load Limiter to  an Orbiter Over-Center Mechanism 

Adam Gilmo re* and Tim Rupp* 

Abstract 

This p aper summari zes the desi gn process used to relieve the p redicted hi gh loads on a Space Sh uttle  Orbite r mechanism prior to the STS-1 12 flight. The overloa ding of the mechani sm was due t o a dynami c  respon se between the orbiter a nd payload that was specific to this paylo ad’s mass an d attachment   scheme.  A solution wa s devised  by adding a com ponent  that prevented overl oad of the  mechanism. In   addition, the introdu ction of  the new com pone nt neithe r interfered with the norma l operatio n nor req uired  extra-vehi cular activity from a cre wmember. By utiliz ing rapid p rototyping tech nology, engin eers we re able  to verify clearances and feasibility while preparing to build  the flight hardware. This desi gn solution was  successfully flown o n STS-112 an d STS-113.  Introduc tion 

The Sh uttle Remote  Manipulator Syste m (SRMS) (also known a s the Shuttle Robotic Arm ) is located and  stowed on the port sill of t he payload bay for orbiter l aunch and landing (Figure 1) . The SRM S is used to  deploy satellit es into orbit or move  and attach segments to the  International  Spa ce Station. Th e SRMS  is  supporte d by four Manip ulator Positio ning Mechani sms (MPM s), which rotate the SRMS inboard to   enabl e closure of the  payl oad bay d oors (stowed configuratio n) and outboard to allow clearance for the   removal of pa yloads fro m the orbite r (deployed config uration ). 

Shoulder MPM Retention Pedestal MPMsSRMS XO911 MPM Stowed Deployed 

Figure 1.  SRMS and X o911 MPM Location 

                                                  * NASA J ohns on Spac e Center, Hous ton, TX 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  143
Problem Des cription 

Prior to the  launch of  the S 1 Truss (STS -112,  October 2002), unusually hig h loads were predi cted i n the  Xo911 MPM due to the S1  Truss m ass and uni que attachme nt sch eme u sing a dual keel O rbiter interfa ce.  As a result of the Solid Rocket Boo ster ignitio n overpressu re and dynami c response between th e S1   Truss and the Orbite r, a low frequ ency stru ctural mo de wa s excite d in the SRMS. This  mode, in turn, was  predi cted to cause a defle ction i n the X o911 MPM th at wo uld ove rload and fail  an inte rnal MPM spli ned  shaft (Fi gure 2). Beca use the MP M is an over-cen ter me chanism, launch lo ads drive the  mech anism  further ove r center, thu s incre asing the splined shaft load s.     While the re were many t echnical ch allenge s associated with thi s probl em, schedul e wa s also imp ortant.  Implementin g a solution t hat woul d en sure a safe f light wa s of the utmost importa nce; howeve r, the  Orbite r was alrea dy at th e launch pad  and the S1 Truss could not be install ed until this probl em wa s  addresse d.  Bell Crank Splined ShaftOver-Ce nter  Measureme nt Point Increasing OverCenter Rotation due  to Inerti al Loa dsSRMS StowedCritical Inertial Load Direction SRMS Deployed

Figure 2.  Vie w Looking Aft with Lo ad Applied to Xo911 MPM in Sto wed Position 

Solution Des cription 

The solutio n involved devi sing a m eans of limiting the  deflectio n in the MPM, thu s preventin g overloa d in  the splined shaft. There were several constraints that  had to be addressed in designing this solution:  no  perm anent modificatio ns were to be pe rforme d on existi ng orbiter h ardware, interferen ce with t he functio n  of the mecha nism was p rohibited, it had to be assem bled on the l aunch pad, a nd the solutio n had to be   designed and  installed within a ten day sche dule. 

The de sign of a two-pa rt assembly, n amed the Splined Sh aft Load Limiter (SSLL), satisfied all the   aforem ention ed re quirem ents. The a ssembly, whic h san dwiched an existin g MPM com pone nt, was  locate d usin g existing MPM features a nd mechani cally fastened together. The  primary loca ting feature s  on the MPM were lighte ning po ckets on the Bell Crank (Figure 3 ). Matchi ng protrusions we re cre ated on  144
the SSLL to provide p roper alignm ent. These p rotrusions also provided a shear rea ction point on ea ch  half of the SSLL. The featu res that a ctually limit MP M rotation, thus relieving splined shaft load s, were the  lower legs th at are shown belo w. The lower legs contact the  MPM pe destal (Figure 4 ) when the  loads in  the spli ned shaft approa ch its op erational limit. As t he mechan ism sta rts to  stru cturally overloa d the  splined shaft, the legs cont act the MPM Pedestal creating an al ternate load path.  

Figure 3.  SSLL (Le ft) and  MPM Bell Crank (Right) 

 Lightening  PocketMating   Protrusion Lower LegsFasteners Stowed Deployed Partia lly DeployedMPM Pedestal SSLL Lower Le gs Figure 4.  Sequential MP M Deplo yment referen cing Figure 2 Detail Vie w 

During the d esign process, kin ematic modelin g and  rapid p rototy ping were used to optimi ze the de sign  and streamlin e the i nstallation. To  ensure that inte rferences were  eliminated, a  kinematic mo del was built   to examine th e MPM throu gh its full range of motio n. The re sults of the model are sh own in Figure 4 with  clearances verified in the stowed thro ugh deploye d position. Rap id prototypin g wa s use d to cre ate full  145
scale, hig h-fidelity plasti c part s that were used in fit checks on  other Orbiters, to ve rify the ea se of  installatio n, and to create installatio n pro cedures. The critical feature s on the p rototypes were  manufa ctured  within 0.25 mm (0.01 0 in) of their theo retical val ues. The informa tion obtaine d throug h the   use of rapi d prototype s provided valua ble informatio n that wa s incorpo rated i nto the final d esign of the  SSLL. The S SLL was succe ssfully flo wn on  STS-1 12 and subsequently, on  STS-113  due to paylo ad  similarities . 

Figure 5.  Isometric Vie w of Model (l eft) and SSL L Installed for STS-112 (right) 

Lesso ns Lea rned 

stallation procedures for compl ex me chani sms a re often difficult  to dete rmine  in a timely m anne r with  or this design process, existing featu res on a n MPM  comp onent were u sed to locate th e new ha rdware.   Conclu sion 

his design process provides a go od model for m aintaining a critical sche dule and mini mizing d esign  In comm on soli d modeling p ractices. In this ca se, rapid prototypin g was used t o create  full-scale pl astic  parts th at not only helped create inst allation pro cedures, b ut also verified asse mbly clea rances. The MP M  was ope rated throu gh its full range of motion with a prototyp e of the SSLL install ed; this allo wed  engin eers to che ck for inte rferences while techni cians rehea rsed in stallation p rocedures. 

F In desig ning the first version of the SSLL, engin eers overl ooked a subtle drawing revision o n this  particula r feature. The u se of rapid p rototypes provided immedi ate feedba ck to corre ct the S SLL de sign  with minimal cost and no sche dule imp act.  T risks. The use of a high fidelity kine matic mod el and a rapid  proto typing process contri buted  to a d esign  that increased the load ca rrying capability of the ov er-center m echani sm wit h no impact to the launch   sched ule. At the time this pape r was written, the SSL L had succe ssfully  flown o n two se parate mission s  (STS-1 12 an d STS-113 ).  146
numbe r of sub-increment s betwe en detent po sition s. This i s achieved by simult aneo usly ex citing two o r  more  stato r phases to generate  an electromag netic fi eld at a desired angle. Th e angle of the generated   field is co ntrolled by the re lative amplitudes of the ex citation si gnals appli ed to each win ding phase. Th e  vector sum of  the excitation sign als di ctate the effective angle of the resultant m agneti c field.  

The ex citatio n appli ed to each wi nding is a quantized si nusoid. Each quantu m level of th e sin usoid  corre sponds to a uniqu e excitation state for the motor.  The si ze of a  single mi crostep is determi ned by the  numbe r of excitation state s per inhe rent step of the motor. In an ideal sy stem,  the step increments are  uniform a nd the micro step size is eq ual to the moto r step angl e divided by the nu mber of state s per step. 

System Desi gn 

Figure 2.   NPP solar ar ray driv e with ultra  quiet sli p ring assembl y Figure 1.  Low distu rbance Deep I mpact  antenna pointing gimbal The de sign o f the micro steppin g syste m starts with accurate p erforma nce estimates to de termine key  requi rements for moto r, mech anism and d rive el ectronics. The  necessa ry perform ance estimate s are  made by constructing a high-fidelity co mputer simulation mo del. This mod el includes re presentation s for  the motor, motor d river and m echanical syste m. The m otor driver generates q uanti zed si nusoidal  excitation signals, th e mot or produces torque  in respon se to  the excitation, and the  struct ural model  respon ds to the appli ed torque. The  result from thi s simulatio n is a time domain e stimat e for to rque  disturban ces resulting from  the input motion profile. 

In many application s the ultimate torq ue distu rbance requireme nts are de rived from rate  and pointing   errors  impart ed to the spac ecraft. To determine thes e errors it is nec essary to trans late the dis turbanc e  torque b ack to the spa cecraft through th e transfe r function of the attitude control system. As such, furthe r  processing of  the simul ation data is required to o btain freque ncy d omain data. G enerally, the time domai n  data obtain ed throug h sim ulation is converted to the freque ncy d omain thro ugh a fast Fou rier tran sform.  The outp ut freque ncy d omain data i s passe d thro ugh the transfer fu nction to determi ne the re sultan t  pointing a nd rate errors. 

Motor Desig n  The step per motors used in microste pping appli cations can be of  either perm anent mag net or hybrid2  design. Ho wever, in ea ch ca se it is important  to spe cify ke y motor parameters to i mprove the                                                     2 Hybri d moto rs use a  single perman ent magnet oriented axially to magneti ze end cap s with  small teeth   machi ned al ong the circu mferen ce. Th ey are refe rred to as hyb rids a s they combi ne bot h perm anent   magnet an d variabl e relu ctance tech nology in the de sign.  148
perfo rman ce of the mi crosteppin g system. The key characteristics of the m otor that affect m icrostepping  perfo rman ce are the ba ck EMF profile a nd detent torque. 

The ideal b ack EMF profile  for a micro steppin g syste m is sinu soidal with low h armonic distortion. Having   a sinu soidal back EMF profile insu res accurate incremental moti on with qu antized si nusoidal excitation .  With pe rman ent magnet motors the q uality of sinu soidal profile is prim arily cont rolled by the width of the  individual ma gnet pole s on the rotor. Fo r hybrid stepp er moto rs, th e back EMF profile is controlled by the   tooth geomet ry of both rotor and stator.  

Minimizi ng detent torque i s also criti cal for extrem e low torq ue di sturban ce application s. Appl ied torqu es  to stepp er motors result in some finite deflection of the motor shaft resulting fro m the effective com pliance  of the energi zed moto r. This is al so true for motors  used in mi crostep ping system s. Whe n the motor is  subject to  the sinusoidal  detent torque durin g operation, the output is pertu rbed and velo city jitter re sults.  These sm all variation s in velocity and a ssociated a cceleration s produce torque di sturb ance. 

In low distu rbance ap plications, the d etent torqu e of the motor is minimize d. This do es redu ce the   unpo wered h olding torque  of the mechanical syste m but this is the comp romise re quired i f disturba nce  torque i s critical. The dete nt in the stepper moto r is controlled through the stat or geom etry. Like a typical   servo m otor, the stato r slots are skewed  to minimize magneti c detent. 

Motor Driv er Design 

The fun ction of the motor driver in a mi crostepping system is to g enerate qua ntized si nusoidal excitation  signals.  The typical motor used in micro steppi ng ap plication s is a t wo phase mo tor req uiring a bipola r  two pha se driver.  Three p hase motors can al so be used in mi crostepping a pplication s but th e electroni c  control is sli ghtly more co mplex.  The basi c archite cture of a micro steppi ng mot or drive r is illustrate d in  Figure 3.  Phase CounterSine/Cosine Look-up TableSine Register Cosine RegisterControl LogicA/D Converter Output AmplifierCurrent Controller A/D Converter Output AmplifierCurrent ControllerMotor Phase A Motor Phase BStep Enable Direction Bus PowerBASI C MICRO STEPP ING MOTOR DRIVER Step Frequenc yGate Enable Sine Data Cosine Data Moto r Power

Figure 3.  Block diagram  of basic microste pping motor driv er 

As the fo cus of this pa per is on th e ove rall ele ctromechanical sy stem, the detail s of the electronic design  are not di scussed. Howe ver, the sele ction bet ween voltage m ode an d cu rrent mod e operatio n are   relevant an d are p resented.  149
basic motion profile required for th e mission. Durin g this te st, time dom ain d ata wa s collected at vari ous  points while t he mechani sm was m oving at different spe eds. 

In gen eral, th e me asure d disturban ce torques did correlate to  analytical p rediction s however there were  some n otable  exception s. Some sig nificant disturban ce was o bserved at  the first resona nt freque ncy of   the sy stem. The a nalytica l model  did not predict thi s well but thi s may be attrib uted to th e relatively low  order mo del used for the structure. Th e suspicion was that parasitic m echanical n oise was excitin g the  structural resonan ce, however, this remains un confirmed. Seve ral test s were perfo rmed  in an attempt to   avoid exciting  the structu re but so me re sidual distu rbance rem ained. While this disturban ce wa s outsi de  the spe cification at that particula r freq uency, furthe r analysi s at the spa cecraft level determin ed that the  resultant poin ting and rate errors we re within establi shed bud gets. 

Figure 5.  Deep Impac t antenn a gimbal  installed in instrumented  hexap od.Figure 4.  Typical disturbance torqu e  results from Deep Impac t  tests. 

Conclu sions   From  this de velopment experie nce it h as been demonstrated th at microste pping system s are a viable  option fo r low torque disturban ce app lication s. It h as also bee n lea rned that nume rous asp ects of  electrome chanical d esign must be considered fo r proper system  operation. S pecifically, the lesso ns  learn ed in the  work to date are summa rized belo w.  • Accu rate system modelin g is criti cal for dete rminin g desi gn re quirements for the motor, moto r  driver a nd me chanism.  • Low harmoni c distortio n of the motor back EMF is importa nt fo r obtai ning uniform mi cro-step   increments .  • Minimizi ng m otor detent is requi red to achieve low di sturbance level s.  • Well controll ed p erformance is be st achieved using ave rage curre nt control for the m otor  excitation.  • Mech anism should b e designed for hi gh stiffnes s, ze ro backla sh and smo oth op eration.  • Motion p rofile needs to b e constructed in a m anner to limit maxim um instantan eous changes in  step fre quen cy.  • Operation of the micro stepping syste m need s to accomm odate t he inh erent limitations of the  technology.  • Test fixture should b e suffi ciently stiff to preclude amp lification of structural mod es.  • Para sitic me chani cal noi se can be signif icant when extremely low disturban ce torques and forces  are requi red. 

152
SARA®21 - A New  Rotary  Actuator for Space Applications 

Olivier Secheresse *, Laurent Cadiergues ** and Julien Rabin + 

Abstract  SNECMA MOTEURS and CNES have recently qualif ied a new rotary actuator called SARA®21, intended  for fine orientation of space mechanisms. This SARA®21 is a very small, light and compact actuator  designed around a new super-flat CSD 20-160 harmonic dr ive. Its optimized and robust design allows for  great performance such as sustaining high loads and rotating into position with an extreme precision of  0.00625°. All characteristics, from  lifetime functional behavior to strength against mission environments  like launch vibrations, were demonstrated with marg ins during the 2003 qualification test program. The  investigation described in this paper confirmed the good behavior of the SARA®21 components. 

Introduction  For many years, SNECMA MOTE URS has developed Solar Array Drive Mechanisms for spacecraft, and  more than 40 satellites and space vehicles are now equipped with SNECMA SADM products. This past  and present experience has borne fr uit to a new product called SARA®21. In association with CNES who  co-funded the program, SNECMA MOTEURS has des igned, developed and qualified a highly efficient  rotary actuator for space mechanisms. 

 Y  X  Z

Figure 1.  SARA®21 rotary  actuator  

It can be used for one or multiple-axis rotating mechani sms, such as for antenna accurate orientation,  solar array deployment, or other specific applicati ons. Its design has been highly optimized to minimize  mass and increase compactness, mainly with the us e of the new CSD 20-160 super flat Harmonic Drive  component. 

                                                  * SNECMA Motors, D.M.S. – D.P. E.S., Villaroche Nord, France   ** CNES, Toulouse, France  + ACSIENCE, Paris , Franc e 

Proceedings of the 37th Aerospace Mechanisms Symposium, Johnson Space Center, May 19-21, 2004  153