Developm ent of Mars Explorati on Rover Lander Petal Actuators 

Boz Sharif* and Ed Jo scelyn* 

Abstract 

The twi n robotic mi ssions “Spirit” and  “Oppo rtunity” were launched in the  sum mer of 200 3 as part of the   MER (Mars Exploration  Rover) proj ect for a ren dezvou s with Mars in  early 200 4. The se ide ntical twin  rovers were desi gned to prospect the surfa ce of Mars for evide nce of past g eologi cal a ctivity related to  presence of liquid water.  The mi ssion was mo deled after the  successful “Pathfinde r” missi on that  delivere d the “Sojourne r” rover to the surfa ce of Ma rs in 1997. The ne w rov ers are m uch large r and  heavier than  the “Sojou rner” an d are equip ped with a large array of scientific in strument s as well a s  comm unication gea r for di rect tra nsmission of data back to Earth . The landing  vehicles h ave also g rown  in size to a ccommod ate the new rove r configuration.  After the Lan der m akes it s hard l anding prote cted by  a co coon of  air ba gs, it is the job of any one of  the 3 Lan der Petal Act uators (LPA’s) to right th e  spacecraft, if necessary, and open the re maining pet als to allow deployment of the rover onto the surf ace  of the red pla net.  

This p aper describ es the chall enge s in developin g, building and t esting the new LPA’s in suppo rt of the   MER proj ect. 

Summar y 

One of the challen ges to Aeroflex wa s to desi gn th e Land er Pet al Actuator t o develop 3, 300 N-m of   torque within  the requi red mass bud get of 6.5 kilogr ams. The earlier “Pathfin der” LPA’ s we re ba sed on  the use of a size 40, 16 0:1 red uction ratio, Harm onic Drive gea r set a s the out put section of  the actuato r.  The a ctuato r was limited t o an o utput torque of 1580 N-m as a result of the “Ratchet” limit of t he Harm onic  Drive ge ar set. These com pone nts were  mass optimized to yield a total mass of 5.7 kg for the “Pathfinde r”  LPA. If a si milar approach was take n for the MER a ctuato rs, the requi red size 6 5 Harmonic g ear set   would weig h 20.9 kg. Cl early, a new ap proach was required.  

Each petal n eeds to rotate 110 deg rees in 20 minut es or le ss giving a minimum spe ed re quire ment o f  0.0153 RPM.  The allo wable power was a maximum of 50 watts per actuato r. Whe n the a ctuators were   delivere d to the program, the final  power with brake wa s only 10 watts per act uator. This wa s  accompli shed throu gh the use of plan etary ge ar reduction system s that yield mu ch greater effi ciency at all   operating  temperature s (approximat ely 83% p er stage at  the cold operatin g temp eratu re). Th e usua l  penalty for this app roach is size fo r a given red uction ratio and stiffn ess. 

Another chall enge  was the  testing  of such a large system over th e requi red environme ntal temperatures  (-60 to +5 5ºC) as well a s dynamic te sting of the bra kes at a cold t emperature o f –120º C. No off-the-shelf  equipm ent was avail able for this ta sk. A test app aratus wa s developed from  scrat ch to p erform  the  requi red testi ng whil e safe guarding the missi on-critical har dware. This te st fixtu re will be d escrib ed late r. 

The ME R LP A req uirements were  established based on the  mass, size and th e perfo rmance of the LPA  units on the previou s Pathfi nder mi ssion. These earlier units, d eveloped  by JPL, were approxi mately 15 0  mm in diam eter, had a mass of 5.7 kg a nd we re ca pable of a su stained output torque of 135 0 N-m  and a  peak torq ue of 1580 N-m as a result of the “Ratch et” limit of the gea r reduction unit emp loyed. This  missi on successfully land ed and de ployed the Sojourne r rove r onto the surfa ce of Mars in 1997. The   MER mission  strives to leverag e the su ccessful he ritage of the Pathfi nder mi ssion to the extent possibl e.  However, a s it is usually  the ca se when utili zing a herita ge d esign for a new mi ssion,  some mino r  “twea ks” are necessa ry. In the case  of the MER LPA’s, the mino r tweak wa s an increa se in the output                                               * Aeroflex Mo tion Control Products, Hauppauge, N Y 

Proceedi ngs of the  37th Aerospa ce Mechani sms Sympo sium, John son Space Center, May 19-21, 2004  211