ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Deplo yment Process, Mechan ization, and Testing   for the Mars Exploration Rovers 

Ted Iskend erian∗ 

Abstract 

NASA’s Mar Exploration Rover (MER) robotic prospecto rs were produced in an environment of unusually  challengin g sche dule, vol ume, an d mass restri ction s. The technical challen ges pushed the system’ s  design to wards extensive  integratio n of functio n, which resulte d in complex  system engineering issue s.  One example  of the  system’s integrat ed complexity can be foun d in the deployment p rocess for th e rover.   Part of this proce ss, rover “stand up”, is o utlined in th is paper. Parti cular attention is given to the Rover Lift   Mech anism’s (RLM) rol e and its de sign.  Analysi s methods are presented and compa red to test results. It   is sh own that  becau se prudent de sign principles were followed, a robu st me chani sm wa s created that  minimized th e duration of integratio n and test, and enabl ed re covery without pertu rbing related syste ms  when reasonably foreseea ble pro blems did occu r. Ex ample s of avoidabl e, unne cessary difficulty are also   presented. 

Introduc tion 

The highly i ntegrate d, collabo ratively engi neered mechani sms of th e 2003 Ma rs Ex ploratio n Rover  represent a signifi cant achievement for the team that produ ced this spa cecraft in record ti me. Two of  these spacecraft were cre ated as successors to th e accl aimed Mars Pathfinder “Soj ourner” rover that  lande d on Ma rs on July 4, 1997. The id entical rove rs lande d on Ma rs on January 3rd and 24th, 2004. 

Relative to most spacec raft, each Rover and the landing system that accom panies it feature a uni quely   high b ulk de nsity. The roughly recta ngular pri sm of the rove r fits snugly with in the stowe d tetrah edron  config uratio n of the la nder. A complex deployment process results from thi s dense packagi ng. A total of   24 devices and mi ssion -critical d eploym ents must correc tly func tion jus t to prepare the ME R for driving off  its ba se petal landi ng pl atform. The design of several of the se me chanical system s depen ded on  successfully impleme nting the con cept for sta ndup to  which the ro ver wa s com mitted at an early date.  Figure 1 sh ows an overall view of the MER before sta ndup.  Rover +X Lander +YWheels &  Wheel  Restraints (6) Forward Ro cker ( 2) Rocker De ployment A ctuator (2)  Aft Rocker ( 2) Bogi e Differential (1) ForwardRear Cable Cutt er #3

Figure 1.  The sto wed MER in its pre-s tandu p con figuration                                                         ∗ Jet Propul sion Lab orato ry, Pasadena, CA 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  197
RLM Fun ctional Require ment & Stan dup Sequen ce  The nom enclature of the RLM a nd its relation ship to the rover is sh own in Figure 3. Thi s simplified  comp uter mo del sh ows the  rover’ s configuratio n just before the standup sequence is initiate d.  

 Rear surface support  conta ct to rov er’s “belly ”  RLA   flexure-pivot U-joint  RLA with lea d screwFront roller suppo rt  contac t with rover s ocket  Link Arm 

 RLM Rocker-bri dge latch & pivot WEB (Rover body  structure)  Wheel hold-down  device s 864 mmBogie pivot immobili zed b y  4 locked d own rea r wheels  (released afte r stand up)  Figure 3   (Above) the  RLM an d its nome nclatur e, s hown in s towed and deplo yed  configur ations. (Belo w) The rov er before pyrotechnic se paration dev ices releas e,  showing the RLM  nested within the thick ness of the base  petal. For  clarity, only   two of six wheels are sh own.  199
Some wheels are not sho wn for clarit y  Separation n uts rele ase — sp rings push off 5mm    RLM be gins lifting  

 42º rotation RLA nut trave ls abo ut 244 mm until RL M stop s enga ge.     Configuration at ma ximum deplo yment 

Front bo gie & wheel d eploy ment.         RLM lo wers, ro cker b ridge latches eng age. RLM co ntinues to  lower rove r until wheel contacts ba se petal, then conti nues to  retract and di sengage  Figure 4   Sequenc e of deplo yments during sta ndup  

It was al so desirable  to accompli sh the conve rsion between  kinematic config uration s in require ment s 2  and 3 witho ut reso rting to pyrotechni c devices, as we  anticipated t esting the en ginee ring u nit many times  while i n a compre ssed schedul e. Safety pre cautio ns and other issue s peculia r to pyrote chnic devices  posed a formi dable threat to perfo rming many tests in  rapid succe ssion. 

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Figure 5  Rocker-bri dge latch  pivot betwee n front  and re ar rockers  Aft contac t point to RLM Socket receives  RLM’ s front  conta ct rolle r  Free body  diagram of the  rover and its interface s to the  su spension s ystem and RLM 

  1,250 N Figure 6  RLA force requirement v s. time  for a nominal, flat lander  attitude (le ft) and 20° pitc h (right) 

One g ratifyin g indication f rom ou r analysis was that the maximu m force  required from th e RLA wo uld n ot  be sig nificantly different as a functio n of rover/lan der tilt attitude; although th e slope of the force cu rve  would change, no one lo ad ca se was clea rly more  difficult than  anothe r. We  calculated f orces at the  actuato r and joints ove r the whole envelope of p ossible d eploy ment attitude s (ME R is require d to b e  deploya ble at any attitude up to 20º fro m horizontal ).  

Testing, Fail ure & Recov ery  Our te sting sche dule d emande d that p arts mu st be fabricated b efore a chievin g com plete confiden ce in   the de sign. But CAE provi ded esse ntial advan ce warn ing of problems when co ntinuing analy sis predicted  that the rover’s stability wa s threat ened at certain tilt angle s within  the specifi cation envelope . The RLM  depe nds on a bal ance of force s due to gravity to hold it in fi rm contact with th e rove r, and it was fea red  that there wa s insuffi cient margi n to pre vent the front conta ct roller from pulling out of its socket at lande r  tilt attitudes g reater than 2 0°. Anticipati ng that this j oint wo uld ne ed refin ement as a result of testing, the   critical inte rface shape was designed as an easily machined and repla ced titan ium-all oy insert. The t wo  plots in Fi gure 7 contrast the estimat ed cont act force on th e RLM’ s roll er between t he un stable  config uratio n on the  left wi th the reme died config uration on the right, that wa s designed lon g befo re the  first test could be executed. This  instability was confi rmed in a (c arefully controll ed!) test. The  repla cement part was rea dy to be i nstalled imme diately, and the  test p rogram contin ued with little delay.  Figure 8 illust rates th e socket and repla ceable in sert.  203
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 Time (sec) @25 deg gravity & 16 deg socket-22 -7  7 22 36 50 65 79 93108122137151165 Force - Z (newton)  0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 Time (sec) @ 25 deg gravity & 3 deg socket-47-42-38-34-29-25-21-17-12-8-415Force - Z (newton)  Negative values are sta ble:  roller in  comp ression  Figure 7  Contact force on the R LA’s fron t contact roller v s. deploy ment tim e. Not e the  pronoun ced, monotonic transition thr ough ze ro fo r the uns table joint confi guration o n  the left. This  indicates th at th e roller has lost com pressiv e contac t with its socke t 

       Figure 8  Front roller to socke t inte rface and th e replace able socke t insert 

Lesso n 3  A modeling effort that continue s con currently  with hardware assembly an d test can p rove  benefi cial, especi ally if un certai nties can be foreseen and m odular com ponen ts are utili zed  which can be easily modified or re placed as ne w inform ation is m ade available.  

Late in the te st pro gram, it was fo und th at the real system st raine d so mu ch in E arth gravity loads that the   maximum tra vel of the RL A had to be extended by approximatel y 12 mm to cause ad equa te rotation at  the rocke r-bridge latch to approximatel y 42°. When modelin g wit h ADAMS we  had attempted to quantify  the extra depl oyment made  nece ssary b y complian ce and backla sh, but the rea l system wa s too compl ex  for an a ccurate estimate. The RLA’s le ad screw was designed to be lon g enough to p rovide ade quate   margi n to allow this extensi on, although i t was ne cessary to abandon the hardst op’s adjustability feature  and de sign a new h ardstop with smalle r volume to accompli sh the stroke in crease. 

At about this same time, we noted anot her p roblem: a cra ck had open ed up at  the root of the hardsto p  that was ma chined into the  Torlon nut. T his occu rred due to an oversi ght in the stre ss analysi s; no stre ss  concentration s had been  considered in that lo cation . The reme dies fo r the  cracked  nut and to p rovide   increased range of travel were ac complis hed in the same redes ign; there was  room at both ex tended and  retracted positions to mo unt new steel p arts (adding about 0.1 kg) that shu nted the stop l oads aroun d the  nut altogethe r. The bro ken Torlo n nut is shown in Fig ure 9. Figu re 10 illustrates the origin al extended sto p  design and the final  versi on that used  all the availa ble screw length. The  final  design of th e nut housing  and ret raction stop is sho wn in Figure 11 .   204
Lesso n 4   This reaffirm s the pre vious lesson: m odularit y in de sign p roved to be benefi cial. Sm all, key   compone nts could be qui ckly u nbolte d and ine xpensive re placement desi gns inco rporated in  a short tim e, rathe r than having to m odify an e xpensive, com plex m achining with integ ral  feature s. Try to foresee the  need for e xtended capability and de sign for it. 

Lesso n 5  When significant un certaint ies exist in th e development of a d esign, try to  provi de room  and  feature s that  perm it different ap proaches to acco mplishing th e sam e task. There was  adeq uate room at both en ds of t ravel to modify the hardstops so that the o riginal ap proach  was disca rded and ne w ha rdware fabri cated at a late date. 

The RLA wa s specified to  work relia bly at loads up to 3,685 N (8 30 lb f). This would allow t esting of the  compl ete rover/ suspension system in Earth gravity without the assistance of off-loading devices. We  considered this an imp ortant requi rement, as it wo uld su pport the difficult lo gistics of rap idly testing  deployme nt in the many perm utation s of attitude and enviro nment. It was also de sirable to avoid offloadin g devi ces to prevent  possible damage to  the delicate rover appe ndages. Ho wever, dynamomete r  tests on the a ctuator m ade it plain that it would n ot reliably exert the force req uired to test the lift proce ss  in Earth g ravity. Instead, we found th at, even at room  temperature it would f requently stall at l oads as low  as 2,31 3 N (5 20 lb f). Its capability was, of cour se, worse at –60°C, the lo west test temperature.  

The force  at whi ch the RLA stalled was inconsistent from o ne test to a nother. In fa ct, one unit  successfully (albeit slo wly) lifted a rover in cold th ermal-va cuum t est, only to fail anothe r test at ambie nt  temperature  in a supposedly identical RLM. We al so noted th at sta ll gen erally occurred  at a motor speed  of 26.0 rad/s (248 rpm) ±5% instea d of nearly ze ro speed as we  expected. F ollowing the lo gic indi cated in  Figure 12, it wa s sho wn to high confid ence that th e poor and erratic pe rform ance was due to variable   friction of the  Torlo n nut on the le ad screw. The m otor/gea rhead put out con sistent torque t hat indi cated  reasonable g earhead effici ency. 

Calculate gearhead efficiency Is gearhead efficiency acceptable?Calculate dry lead screw range of friction µCalculate greased lead screw friction µ Characterize failure-critical friction and torque valuesQuantify functional ma rgin: • At 28v, ambien t by friction µ and torque margin • At –60 ºC by speedyes no Investigate motorgearhead problemsInput available data: • Unpowered motor detent torque • Powered motor on dyno speed/torque/current • Motor minimum static ripple torque • Stall torque of RLA assembly

Figure 12  Diagno stic logic for RL A performa nce   206
We inve stigat ed several remedie s. The f irst solution we attempted was to dry fil m lubri cate th e lead screw  with a thin fil m of sputtere d-on MoS 2. This p rocess made the fri ction pro blem worse! Next, we removed   the lub ricant film and p olished the  scre w with  lapping compound. The T orlon nuts h ad a noticea bly poo r  finish o n their threa ds, so we tri ed polishing tho se too. Polishi ng improve d the RLA’s pe rformance b riefly,  but the g ains woul d not persist through all qu alificati on te sts; up on subsequ ent running-in, the level and  variability of friction at the nut increa sed again. 

With p erformance too poor fo r Ea rth gravity op eration and sche dule almost exh austed, we  finally  experim ented  with g rease lubrication  on the screw. Im mediately aft er a failure to  lift the rove r in ambi ent  conditions, we injected Braycote® 60 0 grea se into t he cavity just in front of th e nut. As hoped, the rove r  deploye d on the next try without the  slighte st trouble. Persua ded to favor grease lubrication by this  dram atic imp roveme nt, now we h ad to prove that a blast of dirt woul d not jam the exposed lead screw!  The brush wipers, mentio ned ea rlier, were mad e of electri cally cond uctive brass-fibe rs to facilitate  grounding across th e inte rface between  the steel l ead screw to th e u-joint that housed the T orlon plastic  nut. They were intended to remove  all but a  fine l ayer of du st that might cling to the screw,  but no w we  need ed to show th at a h eavy load of mix ed di rt would not pose significant ri sk to RLM operation. To m ake  matters worse, we noted t hat some brush fibers came loose in some te sts (Figure 13). It then becam e  necessa ry to sho w that the se fibers th emselve s could not jam the mech anism.  

To prove that  the RL M co uld tolerate stray bru sh fibers , we repeated a set of RLA dynamometer tes ts  with a g reased lead screw. Several b rush seal s we re chopped up to prepa re la rge a mounts of b rass  fibers, a nd th ese fib ers we re ap plied aggressively to  the lead screw befo re the RLA was activated. To ou r  relief, no am ount of effort to intentionall y jam fibers of any length  into the nut/scre w interfa ce succe eded  in ma king a signifi cant (>5%) in crease in current re quire d to lift any loa d up to the maxim um test loa d.  Photos of the  lead scre w and the Torl on nut covere d with bru sh fibers are shown in Figure 14 . 

Figure 13  Photo sho wing brass br ush fibers that were pulled  out of th eir brush wiper assembly  

   Figure 14   Left, the gre ased lead s crew in the d ynamomete r cov ered with brush fiber s. Right, the  Torlon nut a fter th e tes t. Performan ce was unaffected de spite the trap ped fi bers   207
Finally, many system-level  tests we re perfo rmed ov er the whol e standup sequence. The engin eering  model of th e rover/ suspension/RLM p ackage was tested for the RLM’s ability to tolerate a h eavy load of  dirt. We form ulated a mix of quart z particle s of variou s sizes and basalt di rt and applie d it to the full length   of the RLM’ s lead screw in its p re-lift co nfiguratio n. This materi al application was inte nded to simulate a  worst case caking of dirt onto the leadscrew resulti ng from ai rbag impact and rollin g. When the RLM   achieved m aximum lift, a coating of fi ne “M ars Dust Simulant” was applie d to the lead screw i n order t o  simulate the effect of airb orne p articles that might  stick to the lead screw when it’s expo sed in the   extended p osition. Thi s measure add resse s a contingen cy pl an that anticipates the g round team i s  workin g on a stand up or system ano maly for multiple sol s. 

Table 1 summariz es the material mixt ure that was  applied prior to lift. Figure 15 s hows a picture of this   mixtu re and the appearance of the lead screw with this contaminant before liftin g. The lead screw had  residual Braycote 60 0 gre ase on it from a prior test. The material wa s forced  onto and in betwe en th e  thread s of th e scre w from  all side s through the e ntire length of t he scre w. Th us, both the total quantity  applie d an d the level of penetratio n of the mate rial into the th read de pths was felt to b e conservative  relative to wh at could b e naturally achie ved durin g impact an d rolli ng on Ma rs.  Table 1  Hard, sh arp dirt contamination mixtu re applied to  the RL A’s le adscr ew 

Element Volume  Fractio n  Nomin al 420 -1000 mi cron crushed qu artz 1/3  MIL-E 5007 crushed q uartz 1/3  ISIL (bas alt) sand, ≤ 5mm particl es 1/6  Athena “Ma rs Dust Simula nt” 1/6 

Figure 1 6 shows that  a significa nt am ount of m aterial is g athere d in the extensio n brush wipe rs of the  RLM a nd that  a small a mount doe s escape pa st the brushes to th e Torl on nut. In addition to the materi al  entrain ed by the brushe s, a significa nt fractio n of th e materi al is b rushed  off th e lea d scre w and falls to  the grou nd during o peration.  

No signifi cant perfo rmance degradatio n wa s seen as a result of di rt testing, a s measure d by m otor curre nt  necessa ry to raise the rov er. Table 2 summari zes current and sp eed test data  from  system tests an d the  safety margi ns that co rrespond to those result s. 

       Figure 15  The mixture of quar tz and sand appli ed to th e RL A’s leadsc rew for th e dirt toleranc e test  208
Figure 16  Views of the exte nsion br ush wiper a fter th e dirt te st and the  Torlon nut with an accum ulation of dir t 

Table 2  Final MER 1 and MER 2 RLM/RL A mar gins as mea sured in s ystem tes ts. 

Test Test   Temp  ºC Test   Voltage Measured   stall   current   [mA] Predicted   stall   Velocity,  mm/ s Measured   current   [mA] Measured   Velocity,  mm/ s  current dr aw velocity current dr aw velocity  MER 1 STT -55 32.7 785 0.2 350 0.52 2.2 2.6 3.7 3.5  MER 2 STT2 -57 28.5 810 0.2 350 0.44 2.3 2.2 4.3 3.4 Demonstrated M argin,   Earth Weight Predicted Margin, Mars   Weight 

The pe rform ance m argins noted he re are rob ust, eve n in Earth g ravity where wi de margin is not req uired.  It can be se en that the ke y to obtaining con sistent per forman ce was to control friction at the nut/scre w  interface of the RLA, and  grea se was the appro priate mean s to achi eve this co ntrol. Th is affirms a  motto—to qu ote Dou g Packard, an este emed, ve teran engin eer at JPL: “G rease is Good ”! 

Lesso n 6  We respon ded to earl y indication s that the self-l ubricating To rlon® nut pre sented high a nd  erratic fri ction with m easure s such a s polishi ng a nd dry film  lubricat ion. Unfortunatel y, the  best solutio n we fo und could h ave b een im plem ented m uch soone r if we had n ot been  dogm atic abo ut not flying a grea sed, “dust m agnet ” lead screw. As it turned out, the grea sed  screw was hi ghly tole rant of large q uantities of dirt, esp ecially with its brush wi per-shield s. If  resou rces permit, it is wise to pre pare and te st a ra nge of d esign solutions when signifi cant  uncertaintie s exist in all ca ndidate soluti ons. 

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torque f rom 1350 N-m to  3300 N-m. This requi rement wa s to be acco mplished witho ut a signifi cant  attendant in crease in ma ss or size. The high torque requi reme nt was a result of the increa se in the ma ss  of the rove r and lan der package from 3 30 kg for t he Pathfinder mi ssion to 4 41 kg for th e ME R prog ram.  The finalized requi rements for the MER LPA’s we re as follows: 

Output Torqu e:    3300 N-m  Speed:      0.0153 RPM Minimum  Mass:      6.5 kg maxim um  Size:     App'x. 150-m m dia. X 250-mm long  Operating Te mperature:    -60 to +55º C  Survival Temperatu re:   -120 ºC  Powe r Consumption:   50 watts M aximum  Un-po wered Brake Hol ding Torq ue: > 228 0 N-m 

Figure 1.  LPA First Articl e 

Actuator De sign 

As mention ed, the use of  a Harmoni c Gear drive was not d eemed practi cal due to the ex cessive m ass  penalty. Aero flex had previously de signed, tested an d delivere d to JPL a low mass actu ator, capa ble of  deliverin g approximately 2 N-m of to rque at a total m ass of 42 g, for u se in th e Muse s program. Many of  the de sign details as well as the overa ll philo sophy of that a ctuator design were brought to  bear on the  LPA tas k.  

This a pproa ch re sulted in a desi gn that  consisted of an ele ctronically commuta ted BLDC mo tor driving a  7-sta ge gear red uction  system. The  motor shaft was fitted with  a brake to provide th e required u n212
powere d hold ing torqu e. The gea r reduction was divi ded into 3 se parate h ousings so that each sta ge may   be ma ss optimized a s much as possible. Gear g eometry wa s selected b ased on tran smitte d torqu e and  runni ng spee d to allow a n optimum bal ance betwee n the numb er of teeth un der lo ad, the  numbe r of  planet s shari ng the loa d and the ove rall mass of the  unit. Gear te eth we re of a  size 64 DP at the moto r  input sid e to an 8 DP si ze at the output.  

The a ctuato r has an ove rall redu ction ratio of app roximately 3200 00:1 and i s capable of d elivering well i n  exce ss of th e requi red 3300 N-m of torque.  Howeve r final torque output  is limite d to 3300  N-m  by  utilization of a current limi t to saf eguard the actuator as well as the hardware dow nstream i n the torque  path. 

Figure 2.  Ac tuator Sec tional Vie w 

Gear Design  

All of the  gearing  utilized  in th e LPA  was designed an d optimi zed using conventional  CAD a nd FEA  softwa re. In  each i nstance, maximu m utilization  of volume and ma ss was pa ramount to ensu re  compli ance with the requireme nts. Ea ch comp onent was analy zed to allo w maximum safe red uction in  mass without comp romising the req uired design margins.  Particul ar attentio n was required for sele ction of  beari ngs, bea ring mo unting  and bea ring pre-load sele ction. 

As a result o f the larg e overall redu ction ratio, gea r teeth at the  input si de were required to withsta nd  considerably more l oad cycles than t he output sta ge gearing. Thi s would require that the inp ut side gears  be de signed to have fairly mode rate contact and b ending stresse s whereas the output sectio n gea rs were  prima rily affected by the b endin g load (f atigue) stresses.  

As the design prog ressed , it becam e apparent that t he size of th e input side gears wo uld ultimately be  govern ed by  the availabl e sizes of th e rolling el ements. The required dime nsions for th ese b earings  resulted i n tooth be nding  loads and contact st resses that wo uld easily meet th e design loads. The effect  of these si ze accomm odati ons were a n et incre ase of 70 g in the mass of the act uator.  

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