The Challenges of Designing the Rocker-Bogie Suspension  for the Mars Exploration Rover 

Brian D. Harri ngton* and Chris Vo orhees* 

   Abstract 

Over the pa st decade, the  rocke r-bogie suspen sion design ha s become a pro ven mobility appli cation   known for its superior vehi cle sta bility and ob stacle-climbing capa bility [1,2]. Fo llowing seve ral techn ology  and research rover imple mentation s, the sy stem was successfu lly flown a s part of M ars Pathfinder’ s  Sojourner ro ver [3]. When the Mars E xploration Rover (MER) Project wa s first propo sed, the use of a  rocke r-bogi e suspension wa s the obvi ous choice due to its exte nsive h eritage. The challenge p osed by  MER was to design a light weight ro cker-bogi e suspension that would pe rmit th e mobility to stow within   the limited  space availa ble and deploy i nto a configu ration that the rover could  then safely use to egre ss  from the lander and expl ore the Martian surface. This  paper will describe ho w the MER rocker-bogie  suspension subsy stem wa s abl e to me et these conflicting de sign requiremen ts whil e hig hlighting the  variety of deployment and l atch me chanisms employe d in the desi gn.   

Introduc tion 

The prim ary role of the ME R suspension subs ystem is to provide the rover with a mobility system that has  the ki nemati c rang e to permit th e rover to safel y traverse 20 cm obstacles and allow the  wheel  assembli es to rotate fo r rover “arc-turn” a nd “t urn-in-pla ce“ ma neuvers. In addition to t hese ge neral   traversability requi rements,  there we re several re quirements u nique to the particula r issue s of the MER  vehicle. Sp ecifically, the su spension was req uired to 1.) Stow in  an extremely small sp ace a nd deploy th e  mobility into a sta nce tha t woul d provide the  rove r with 45 degree stability and 2.) Ab sorb a large  percenta ge of  the impact lo ads the rover would experience duri ng lande r egress and surfa ce traverse. 

The rocker-b ogie suspension is a mechanism  that, al ong with a differential, ena bles a six-wh eeled  vehi cle  to passively keep all six wheels in contact with a surface even whe n driving on seve rely u neven te rrain  (see Figu re 1).  There a re two key ad vantage s to th is feature. The first adva ntage is that  the wheel s’  pressure on the gro und will be equilib rated.  This is extremely important in soft terrai n where excessive   ground pressure can resul t in the vehi cle sinking into the d riving surface. The seco nd advant age is that  while climbin g over ha rd, uneven te rrain, all six whe els will n ominally remain in  conta ct with the su rface   and un der lo ad, helping to  propel the v ehicle over th e terrai n. MER take s adva ntage of this config uratio n  by integrating  each wheel with a drive a ctuator, maxi mizing the ve hicle’ s motive  force capa bility.    

Another key feature of the  suspen sion t hat has n ot been emp hasized in p revious technol ogy and flight  appli cation s is the ability to absorb sig nificant d riving load s. In the past, rocke r-bogie suspensions have   been used on rovers wh ere the loads gene rated du ring dr iving have bee n relatively low. T herefo re, the  suspension served p rimarily as a “rigid ” kinem atic lin k betwee n the rover body a nd the wheel s. Ho weve r,  the MER rover has the  challeng e of egressin g from a lan der poi sed o n airbags and surface featu res, a  maneuve r that coul d require the vehi cle to dro p from a sig nificant height a bove the su rface. Instrum ents  that had b een stowed during the la nding phase of the mission  will be de ployed  durin g driving and were  not de signed to withsta nd large loads in thei r science-g athering configu ration. A compelling design  requi rement was to therefore create a “soft” suspension to limit the accele rations exp erienced by the   payload du ring driving. Ho wever, on e of the more challengin g design issue s to address was how soft to   make the suspensi on. A suspensi on that  was too soft will result in large deflections where the rover body   or its scie nce appe ndages may conta ct Martian  surface features in an uncontro lled m anner. Therefore,                                                     * Jet Propul sion Lab orato ry, California In stitute of Te chnology, Pasad ena, 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 

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