behin d the n ut. We sele cted a nut made of Torl on® re sin 42 03 for its hi gh strength and fri ctional   properties, which we had cha racterized and  found  to be a dequate in limite d testing  for other MER  device s.     The RLM was required to  have integ rated ha rd stops to preve nt the appli cation of high  force to th e  suspension system’s ele ments when th ey meet their end of travel. The unce rtainty in requi red st roke  length wa s reco gnized ea rly in the design pro cess, so the se sto ps were d esigned into the  RLA’s lea d  screw and m ade adju stable. The stop  “dogs” we re cu stom machined i nto adj ustable  steel nuts on each  RLA and timed to  engage with  an appropriate  overlap with  corre spondi ng dogs on the T orlon nut. Nook  Industri es ma chined th eir thread form in to the n uts so that they could be adju sted by screwing back and  forth on the leadscrew. One half-dog setscrew on each nut secured its fin al positio n and reacted t he  torque o n each stop throug h she ar force.   

An early dev elopme nt test  was designed to gain confidence in the  leadscre w’s ability to tolerate dirt. We   purcha sed the clo sest commercially a vailable item  we could find that rese mbled ou r flight desi gn: a  single-start, rolled-th read acme screw with Polyethy lene (PET) n ut. We applie d load s up to  1,156 N (2 60  lbf), whi ch represented the high est lo ad antici pated in Mars’ s gravity. Results were en couraging: torque   requi red incre ased from 10 % to 30%, with sma ller increases n oted at the highe st load.  

Most of th e early pa rt mod eling a nd analysis was done in Solid Works® an d associated analy sis softwa re.  However, du e to limitations in So lidWorks at that tim e, the whole spacecraft da tabase wa s maintaine d on  Comp utervi sion® CA DDS5. Whe n the engin eer was satisf ied wit h the ge neral  layout of the  assembly, it  was trans ferred by STEP or Parasolid  file to the sys tem to be int egrated with the s pacecraft. This  transl ation prese nted an opportu nity for erro r, an d we did discove r a significant  erro r in the 4-bar pivot  locatio ns when the rover attitude at sp ecific point s along the lift  trajecto ry did not match betwe en the   CADDS 5  model and the SolidWorks model.  The  discrepa ncy was discov ered and remedied  by a   thorou gh co mpari son of the CAD mod els. We had made ea rly plans to cre ate an Interfa ce Drawing to  control swept volume s and critical i nterface s, but the co mplexity of the de sign problem a nd the limited   time available  forced us to simply rely on  the CADDS 5 databa se as the Interfa ce control tool. 

Lesso n 1  Complex me chanisms comprising man y parts with mu ltiple motion trajectories are muc h  easier to m anage i n CAD system s that sh ow so lid form  in real tim e, that allow q uick  assignm ent o f motion constraints, and al low real tim e movement of the system .  

Lesso n 2  Errors due  to poor com munication crept i nto the lo cation of the  pivots in the  4-ba r link system .  The e rrors re sulted i n frustration a nd wasted tim e when the  two models g ave incom patible   trajecto ries for the WEB. This e xperience n ot only affirm s the common wi sdom  that  communicati on am ong te am members is im portant,  but espe cially points o ut that usi ng two  indep ende nt databa ses for desig n wo rk requi res caref ul planni ng to  safegu ard consistency of   data. Much better still, use only one dat abase. 

Loads Analy sis  The lo ads in the RLM were e stimated with CAE sof tware , and the effec t of joint fric tion was  estimated.   Figure 5 sho ws the fo rces acting o n the interfa ce betwee n the WEB and the  RLM du ring the upward lift  process. Two  tools were  used to eval uate force s: Dynamic Designer®, software that i s integrated  with   SolidWo rks, and A DAMS ®. Several  variabl e parameters were  evaluate d: rover/la nder attitude with   respe ct to g ravity, different joint locatio ns, and lin k length s. The  effect of in creased friction at any on e of  the system’ s many pivoting joints could  be studie d this way a s a mean s of qua ntifying functional ma rgins.  CAE sim ulation p rovide d insight and  understan ding of system b ehavior, helpi ng to d esign the sy stem   stand up te st plan. An  example of an alysis outp ut for the li near actuator’s force  output  is sho wn in Figure  6. Gary O rtiz of JPL pe rformed the A DAMS modelin g and d ocumented the system’s fo rces and ki nemati c  behavio r. 

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