rocker-bogie lunar rover.pdf

Acta Astronautica 64 (2009) 925934/locate/actaastroDesignofahighperformancesuspensionforlunarroverbasedonevolutionBaichao Chena, Rongben Wanga,YangJiab, Lie Guoa,LuYangaaIntelligent Vehicle Group, Traffic college, JiLin University, ChinabChina Academy of Space Technology, Beijing, ChinaReceived 7 March 2008; received in revised form 17 October 2008; accepted 4 November 2008Available online 21 December 2008AbstractIn this paper, we propose a new suspension for lunar rover called obverse and reverse four-linkage suspension (ORF-Lsuspension for short). Its two components are designed based on the evolutions of bogie and rocker. Firstly, we analyze thecharacter of bogie and research the approach to improve its performance. Based on that research, an evolved mechanism ofbogie is proposed, named obverse four-linkage. It has better capacity than bogie. In addition an evolved mechanism of rockeris also proposed, named reverse four-linkage. The bogie, rocker and their evolved mechanisms can compose four availablesuspensions including the interested ORF-L suspension. Because ORF-L suspension is composed of two evolved mechanisms, ithas the highest performance. In order to check that, the performance comparison between ORF-L suspension and rocker-bogiesuspension are carried out based on simulation. Finally, a prototype rover with ORF-L suspension is designed and manufactured.It shows excellent performance as expected.Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.Keywords: Suspension; Four-linkage; Lunar rover; Exploration vehicle1. IntroductionWheeled locomotive system can move in variouskinds of soils with high efficiency. And not only are itsimpact load, energy consumption and abrasion smaller,but also its configuration is simpler than other types oflocomotive systems, for example tracked locomotivesystem. Therefore wheeled locomotive system is usedbroadly in planet exploration. However, it is weak intrafficability, in order to enhance it, all kinds of sus-pensions are developed. For example, rocker-bogieCorresponding author.E-mail address: (B. Chen).0094-5765/$-see front matter Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.actaastro.2008.11.009suspension is used in Sojourner, Rocky7, MER, FIDO,etc by NASA 1,2; parallel architecture of the bo-gies and spring suspended fork suspension is used inSHRIMP by Swiss federal institute of technology, etc.3; pentad grade assist suspension is used in Mcro5by ISAS Japan 4,5; RCL Concept series and CRABare also used in prototype vehicles for mars by ESAand ASL 6,7, and so on. Even though all suspensionsare designed to perform well in rough terrain, eachdesign has its advantages and drawbacks. For example,the SHRIMP with parallel architecture of the bogiesand spring suspended fork suspension has excellentclimbing capacity, but its platform stationarity is not sowell as a result of rear wheel being fixed on platformdirectly. Therefore it is still necessary to develop a new926 B. Chen et al. / Acta Astronautica 64 (2009) 925934suspension to enhance the performance of wheeledlocomotion system completely.Because Sojourner and MER had worked success-fully on Mars and all their suspensions are rocker-bogiesuspension, rocker-bogie suspension is very worth re-searching. Therefore, this paper starts with the analysisof rocker and bogie, and then proposes a new suspen-sion mechanism based on the evolutions of rocker andbogie.2. Obverse four-linkage mechanism design2.1. The character of bogieBogie is a component of rocker-bogie suspension.The bogie of Mar rover Sojourner is shown in Fig. 1.Because it is a curved bar, its stability is not strong andit is easy to overturn on uneven road.The pivot point of bogie and the center points ofwheel 1 and wheel 2 make the angles afii9826 and afii9825, and forma triangle (see Fig. 1). Through analyzing the stabilityoftriangle,weknowthatthebiggertheangle afii9825 themoreeasily the triangle turns clockwise; the bigger angle afii9826the more easily the triangle turns counter-clockwise.Additionally, when one wheel is uplifted relative to theother one on uneven road, the bigger the angles afii9825 and afii9826the more the variation of wheel load is and the biggerthe probability of bogie turnover is.Inordertoenhancethestabilityofbogieanddecreasethe variation of wheel load, it is necessary to adjust thebogie to make the angles afii9825 and afii9826 less or even zero asthe bogie of Rocky7 8 shown in Fig. 2. Rocky7 is a re-formative type of Sojourner by NASA and its bogie is astraightbar.Butthatadjustmentalsodecreasestheclear-ance between bogie and ground, which leads the bogielikely to be blocked with obstacles. Although to shortenthe length of bogie as Rocky7 can decrease the proba-bility blocked with obstacles, but which also decreasesthe stability. Thus, for bogie, stability, wheel loadFig. 1. Sojourner and its rocker-bogie suspension.invariance and ground clearance restrain each other. Inorder to make a tradeoff, a proper afii9825 or afii9826 ought to beexistent.The climbing of bogie can be regarded that thetorque which is loaded in bogie to help wheel climb(active torque) overcomes the torque which is loadedin bogie to prevent wheel from climbing (negativetorque). For bogie, no matter how to adjust the shape,length and location of pivot, the climbing capacity isstill weak. That is because the torque for wheel 1 toclimb and that for wheel 2 to climb are restrained eachother and those adjustments cannot break this restraint.In other words, the weak climbing capacity is inher-ent for bogie and cannot be enhanced by optimizing.Thus we must design a new mechanism to break thatrestraint.2.2. Approach to enhance climbing capacity of bogieThrough Section 2.1, we know that the bogie stabilityand the wheel load invariance of Rocky7 are better thanthose of Sojourner. Therefore the research starts withthe bogie of Rocky7.ThecorrelativeparametersaregiveninFig.3(a)whenwheel 1 contacts obstacle. The forces and torques ofwheel 1 giving bogie are F1and Tf1. The forces andtorques of wheel 2 giving bogie are F2and Tf2.Thedistances between the pivot point of bogie and the cen-ters of wheel 1 and wheel 2 are, respectively, L1andL2. The active torque for climbing is T1,clockwise.Thenegative torque for climbing is T2, counter-clockwise.The arms of F1and F2are Lb1and Lb2. The anglesbetween bogie and the arms of F1and F2are, respec-tively, afii98281and afii98282. The angle between horizon and F1is afii98261. The angle between horizon and F2is afii98262.Theangles between bogie and horizon are, respectively, afii98251and afii98252.When wheel 1 contacting obstacle, as a rule0180(afii98291+ afii98351) when the two load linkages turn counter-clockwise as the result of wheel 3 climbing up obstacle.According to Eqs. (5) and (6), afii9835= f (afii9829) is an increasingfunction. Based on above analysis, it is not difficult todeduce afii98352afii98351. Therefore the rover with reverse four-linkage mechanism has better cab stationarity than therover with rocker when wheel 3 climbs up obstacle.With the same way, it is not difficult to draw the sameconclusion when wheel 3 goes down into crater.4. Obverse and reverse four-linkage suspensiondesignThroughSections2and3,weknowthatobversefour-linkage mechanism (OF-L) has the better performancesthan bogie and reverse four-linkage mechanism (RF-L)has also the better performances than rocker. It is rea-sonable to confirm that the suspension composed ofOF-L and RF-L is high-performance (this conclusion isalsovalidatedbysimulationinSection6).Withanavail-able combination of OF-L and RF-L being achieved,930 B. Chen et al. / Acta Astronautica 64 (2009) 925934a new high performance suspension is born, named ob-verse and reverse four-linkage suspension (ORF-L sus-pension), shown in Fig. 8. ORF-L suspension includessix linkages and seven pivot points. The installation anduse methods of ORF-L suspension are the same as thoseof rocker-bogie suspension.ORF-L suspension inherits the advantages of OF-Land RF-L, thus it has higher performances than bogie-rocker suspension in climbing obstacle, cab stationarity,wheel load invariance and suspension stability.5. Combination mechanisms analysisOF-L, RF-L, bogie and rocker can compose fouravailable suspensions, named, respectively, ORF-L sus-pension, OF-L-rocker suspension, bogie-RF-L suspen-sion and bogie-rocker suspension, shown in Fig. 9.Fig. 8. Rover model with ORF-L suspension.Fig. 9. The combination mechanisms: (a) ORF-L; (b) OF-L-rocker; (c) Bogie-RF-L; (d) Bogie-rocker.Table 1Performance comparisons of the four suspensionsPerformance Climbing obstacle Cab stationarity Mechanism stability Wheel load invariance Weight and complexityORF-L A A B B COF-L-rocker B C B B BBogie-RF-L B B C C BBogie-rocker C C C C ANote: excellent A; good B; fair C.ORF-L suspension and OF-L-rocker suspension areall novel suspensions without being used. The bogie-RF-L suspension is similar to the suspension of RCLConcept-C rover by ESA, and the bogie-rocker suspen-sion is used in Sojourner, MER and other prototyperovers by NASA.We think that the performances of suspension can bedecided by the performances of its component mecha-nisms more or less, and based on that, the relative per-formances of above four suspensions can be inferred.The result is shown in Table 1.6. Simulation validation6.1. Simulation modelAimingattheORF-Lsuspensionroverandthebogie-rocker suspension rover, some simulation comparisonworks are carried out based on ADAMS. In simulation,no control is set and all wheel velocities are a constantvalue.According to the design requirements of Chineselunar rover, the outline size and mass of the ORF-Lsuspension rover model (ORF-L rover for short) aredetermined. For a fair comparison, the bogie-rockersuspension rover model (Rocker-Bogie rover for short)is designed with the same size and mass of ORF-Lrover.The same parameters are as followed: outline sizeis 1.5m1.0m0.8m; rover mass is 200kg; wheelmass is 3.5kg; centroid height is 500mm; diameter andB. Chen et al. / Acta Astronautica 64 (2009) 925934 931Fig. 10. The structures and sizes of two rover models: (a) ORF-L rover; (b) Rocker-Bogie rover.Fig. 11. The course of the two rovers crossing the different height obstacles.Fig. 12. Comparison of friction coefficients of two rovers.widthofwheelare300and200mm;wheeltrackisallthesame, and the wheelbase between front-wheel and rear-wheel is 1200mm. In the course of simulation, gravityacceleration is set at 9.8m/s2and wheel velocity is at0.3rad/s. The structures and sizes of ORF-L rover andRocker-Bogie rover are shown in Fig. 10.932 B. Chen et al. / Acta Astronautica 64 (2009) 925934Fig. 13. Comparison of invariance coefficients of two rovers.Fig. 14. Comparison of cab pitch angle of two rovers.6.2. Simulation and comparisonToincreasethefrictioncoefficientbetweenwheelandground until Rocker-Bogie rover and ORF-L rover allcan climb over the 250mm-high obstacle, and then tomeasure some interesting parameters. Fig. 11 shows thecourse of the two rovers crossing the different heightobstacles (the arrows in Fig. 11 are the forces of groundgiving wheels).6.2.1. Comparison of friction coefficientThe friction condition needed in climbing can re-flects the climb capacity of rover. The smaller the fric-tion coefficient the stronger the climbing capacity is.Fig. 12, respectively, shows the friction coefficients offront wheels, middle wheels and rear wheels of the tworovers during crossing the 250mm-high obstacle. Dur-ing the time of zone A, the front wheels of ORF-L rovercontact, climb up and detach from obstacle. With theB. Chen et al. / Acta Astronautica 64 (2009) 925934 933same state, zone B, C and D, E, F corresponds, respec-tively, to middle, rear wheels of ORF-L rover and front,middle, rear wheels of Rocker-Bogie rover. Obviously,the friction coefficients leap and come to peak at themoments of wheels contacting obstacle, therefore thosemoments are the most difficult for wheel to climb. Ac-cording to Fig. 12 the friction coefficients of ORF-Lrover are all less than 0.7, but for Rocker-Bogie roverthe maximum friction coefficient is close to 1.0. There-fore the climb capacity of ORF-L rover is stronger thanthat of Rocker-Bogie rover, which is the same as theconclusion of the analysis in Sections 4 and . Comparison of invariance coefficient of wheelloadThe invariance coefficient of wheel load is also animportant performance index for rover. It reflects thevariation degree of wheel load in rough terrain, andthe smaller the value the less the wheel sinkage is andthe more the wheel tractive power is. The invariancecoefficient is defined as followed:Invariancecoefficient=current wheelload initialwheelloadinitialwheelloadFig. 13, respectively, shows the load invariance coef-ficients of front wheels, middle wheels and rear wheelsof the two rovers during crossing the 250mm-high ob-stacle. Obviously, the coefficients of ORF-L rover aresmaller than those of Rocker-Bogie rover.6.2.3. Comparison of cab pitch angleCab stationarity can be evaluated through measuringthe pitch angle of cab, and the smaller the angle thebetter the cab stationarity is. From Fig. 14, it is obviousthat the pitch angle of ORF-L rover cab is nearly sym-metric and the maximum pitch angle of ORF-L rovercab is smaller than that of Rocker-Bogie rover cab.6.3. Simulation conclusionsIn simulation, the ORF-L rover performs more ex-cellent performances than Rocker-Bogie rover in climb-ing obstacle, wheel load invariance and cab stationarity.That verifies the conclusions of the theory analysis inSections 35. Meanwhile, we find there are also somethe same trends in the performance curves of two mod-els. We think that those improvements and similaritiesjust reflect the effect of evolving.Fig. 15. The prototype lunar rover with ORF-L suspension.7. ExperimentsA prototype lunar rover with ORF-L suspension isdesigned, shown in Fig. 15. Because the key technolo-gies of ORF-L suspension in manufacture and instal-lation are the same as that of rocker-bogie suspension,ORF-L suspension has good practical value.Some interesting tests are carried out on lunar sim-ulation ground to check the practical performances ofrocker-bogie suspension. In test, the rover passes overvarious typical blocks and craters facilely, and not onlyis its cab relatively stationary but also the loads in sixwheels is relatively homogeneous. The test states arepartially shown in Fig. 16.8. ConclusionIn this article we use an effective method to de-sign the suspension of lunar rover, which is evolution.Based on that, we design a high-performance suspen-sion, called ORF-L suspension and evaluate the relativeperformances of four different evolutionary suspen-sions. Because ORF-L suspension has the all-roundexcellent performance, it can make rover interestingto cross higher obstacles rather than to avoid and godown steeper slope instead of abandoning. Its goodcab stationarity is in favor of the on-board instrumentsto work. Its good wheel load invariance can decreasewheel sinkage and produce more tractive power. Itskey technologies in manufacture and installation arethe same as those of bogie-rocker suspension, whichmakes it has high working reliability.934 B. Chen et al. / Acta Astronautica 64 (2009) 925934Fig. 16. The test to climb block and cross crater.Sofarwehavenotfoundthesimilarsuspensionstruc-ture with ORF-L suspension all over the world. Chinahas determined to carry an exploring vehicle to lunar in2012 and as a forecast, the new suspension and the evo-lution thought will provide a valuable technical supportto it.AcknowledgmentThis work was supported by China Natural ScienceFoundation, subject No. 50675086. China Academy ofSpace Technology provides a lot of help in capital andtechnology.References1 D.B. Bickler, A new family of JPL planetary surface vehicles,Missions, Technologies, and Design of Planetary MobileVehicles, Toulouse, France, 1992, pp. 301306.2 B. Harrington, C. Voorhees, The cha