Improving skid-steering on a 6x6 all-terrain vehicle: A preliminary experimental study

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Mitchell Howard
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In order to increse steering efficiency, it is possile to modify the reprtition of norml contct forces on the wheels.

1 Improving skid-steering on 6x6 ll-terrin vehicle: A preliminry experimentl study J.-C. Furoux LMI/IFMA/UBP P. Vslin2 LIMOS/UBP Clermont-Ferrnd, Frnce Astrct 6x6 ll-terrin vehicles without steering system must use gret power when skid-steering. In order to increse steering efficiency, it is possile to modify the reprtition of norml contct forces on the wheels. This pper descries preliminry experimentl study on the Kokoon vehicle, n llrod 6x6 electric wheelchir. Contct forces of the wheels on the ground re mesured vi six component force-plte. Reprtition of the norml forces on the six wheels is chnged vi suspension modifiction. The first results show tht steering cn e sustntilly improved only y minor djustments on suspension. Keywords: ll-terrin vehicle (ATV), 6x6, skid-steering, force-plte, Kokoon. I. Introduction This pper intends to present preliminry experimentl results in order to evlute the efficiency of skid-steering with wheeled vehicle. Skid is phenomenon tht ppers with every type of ground vehicle when the dynmic forces pplied on the vehicle exceed the cpilities of the vehicle-ground interfce []. Skid my e due to longitudinl inertil forces when ccelerting-rking or to lterl inertil forces when steering t high speed nd low rdius. It my lso e due to the design of the vehicle. Skid lwys ppers with trcked vehicles during turns (even if some of them hve front steering trcks [2]) ecuse the long contct surfce of the trck with the ground cnnot e steered without lterl slipping, which induces rection torque. On the contrry, wheels ensure reduced contct surfce on plnr ground: punctul contct with toric tires such s motorike tires; liner contct with cylindricl tires, such s those used for crs. In relity, ecuse of tire deformtion, the contct point or contct line ecomes contct ptch nd moderte steering torque ppers. However, wheels hve excellent ilities for turning round one dimetrl xis while keeping ground contct. For trcks s well s wheels, grip depends strongly on the vlues nd distriution of the norml forces [3]. The lrge mjority of wheeled vehicles hve steering wheels. It my e the front wheels (in clssicl cr); the rer wheels (in power lift truck or lwn mower [4, 5]); ll the wheels (on certin types of moile roots nd sport crs [6]); two front nd two rer wheels out of six [7] or E-mil: furoux@ifm.fr 2 E-mil: Philippe.Vslin@isim.fr G. Dourre IFMA four front wheels out of eight [8] (on militry wheeled rmoured vehicles or truck mounted crnes). The steering mechnism is complex one, prticulrly when there re more thn two steering wheels. Constrints pper such s respecting the Ackermnn steering geometry (lso known s Jentud geometry in Europe) to minimize skid during turns t low speed nd eing comptile with independent suspension mechnisms. Another drwck of frmes with steering wheels is tht they generlly do not llow to rotte the vehicle on itself without longitudinl displcement. With two steering wheels, this would men steering ngles round 9, which is complex to design nd dngerous t high speeds. For this reson, mny ll-terrin vehicles still rely on fixed wheels with possily suspension system ut no steering mechnism. Such vehicles hve roust nd relile ehviour on rough terrin. They must turn with skid-steering nd ehve similrly to trcked vehicles [9]. Most of them hve 4x4 trnsmission, such s the Pioneer3-AT root [], nd some hve 6x6 one, such s multi-purpose mphiin vehicles []. The purpose of this work is to evlute the energy consumption of the steering process in 6x6 configurtion nd to propose solutions to enhnce steering on such types of vehicles with non-directionl wheels. II. Description of the Kokoon vehicle This pper will use the exmple of the Kokoon vehicle, n ll-rod 6x6 electric wheelchir designed y the students of the French Institute for Advnced Mechnics (IFMA) since 999 [2] (Fig. ). The vehicle hs two electric motors of 33W ech nd is cple to go t 8 km/h on 2 % slope. Two led-cid Fig.. Overview of the Kokoon vehicle developed t IFMA since 999.

tteries ensure 4h utonomy. Ech motor is directly connected to the three wheels on sme side y trnsmission mechnism using five elts (Fig. 2 nd 3-).

Kokoon ws designed to clim esily over 5 cm ostcles. Mérignc, Frnce), rigidly olted into wooden ox uried in the ground so tht the top plte is pproximtely t the level of the ground (Fig.4 nd 5-).

three two-component strin guge force trnsducers (Fig. 4), which re lso firmly fixed on the se plte [6, 7].

top plte in the reference frme linked to the force-plte (Fig. 5-).

CAD model of the vehicle frme. The first tests showed excellent climing ilities ut some difficulties to steer.

2 tteries ensure 4h utonomy. Ech motor is directly connected to the three wheels on sme side y trnsmission mechnism using five elts (Fig. 2 nd 3-). The wheels hve independent suspensions with hydropneumtic djustle dmpers. The vehicle is 75 cm long nd 3 cm wide. The wheelse is 47 cm nd the trck width 93 cm. Wheels hve 4 cm dimeter. Kokoon ws designed to clim esily over 5 cm ostcles. Mérignc, Frnce), rigidly olted into wooden ox uried in the ground so tht the top plte is pproximtely t the level of the ground (Fig.4 nd 5-). The six-component force-plte used in this study (Dimensions: 6 x 8 cm - Mesurement rnges: Fx = N, Fy = 9 N, Fz = N - Resolution: N) is composed of rigid composite top plte (cron/luminium) fixed on three two-component strin guge force trnsducers (Fig. 4), which re lso firmly fixed on the se plte [6, 7]. Ech of these trnsducers mesures one component of the resultnt force in the plne of the top plte ( shering component: Fx, Fy) nd the other in the direction perpendiculr to the top plte ( compressive component: Fz). The signls produced y the six force trnsducers re multiplied y the thirty-six coefficients of the clirtion mtrix for clculting the six components (Fx, Fy, Fz, Mx, My, Mz) of the wrench pplied on the top plte in the reference frme linked to the force-plte (Fig. 5-). In norml using, the wrench components llow to compute the horizontl coordintes (, ) of the point of force ppliction on the force-plte, which is usully clled centre of pressure (COP). Fig. 2. CAD model of the vehicle frme. The first tests showed excellent climing ilities ut some difficulties to steer. Even with smll tires (tire tred width: 7 cm), the vehicle cnnot steer on itself on high dherence grounds such s mcdm. Steering is still possile with gret turning rdii nd non null longitudinl speed. Initilly designed for disled people, this vehicle is lso n interesting reserch pltform ecuse it hs modulr design nd cn e reconfigured esily [3]. This property will llow us to esily modify suspensions (Fig. 3) while keeping unchnged ll the other prmeters, only y djusting the loction of some criticl joints. Force Wooden Ground trnsducers ox level Z T Top plte Bse plte T2 T3 Fig. 4. Setting of the force-plte in the ground (). The force-plte including the three trnsducers (). The signls of the force-plte trnsducers re simultneously smpled t Hz y 6-it A/D conversion crd (AT-MIO-6, Ntionl Instruments) slotted into PC, where experimentl dt is recorded using n cquisition softwre (LView 5., Ntionl Instruments). The cquisition PC nd signl conditioner re rought close to the force pltform (Fig. 5-). Further postprocessing is performed in Open Office spredsheet. Fig. 3. Belt trnsmission (). Adjustle swing rm suspensions (). Z III. Experimentl settings The steering process of vehicle is complex phenomenon tht my e etter understood vi n experimentl preliminry pproch [4, 5]. For this 6x6 vehicle, it ws decided to mesure experimentlly the contct forces of the wheels on the ground. Fig. 5. Integrtion of the wooden ox into the ground (). The entire experimentl system (). A. The six-component force-plte B. Centre of mss During its displcement, the wheels of the Kokoon vehicle roll on the top plte of six-component force-plte (TSR, Mss reprtition on the wheels hs gret influence on the ground contct. It is determined independently y three

z hs O x S vs θs Prmeter Vlue Fs vs 8 mm hs std 45 mm ls B θ l2 Fig. 6. Computing the centre of mss on the CAD model (). Mesuring the weight reprtition with six scles ().

Modifying the suspensions This pper focuses on improving steering efficiency of multi-xle vehicles such s Kokoon. One solution my e to modify mss reprtition.

In this work, we choose to modify the centrl suspension reltively to the front nd rer suspensions.

For simplicity resons, it ws chosen to chnge the hs length from hs std = 45 mm to the miniml possile length hs mod = 45 mm.

3 z hs O x S vs θs Prmeter Vlue Fs vs 8 mm hs std 45 mm ls B θ l2 Fig. 6. Computing the centre of mss on the CAD model (). Mesuring the weight reprtition with six scles (). Middle xle Rer xle 7 83 Totl 367 G (mm) TABLE I. Mss reprtition on the xles nd position of the centre of mss. C. Modifying the suspensions This pper focuses on improving steering efficiency of multi-xle vehicles such s Kokoon. One solution my e to modify mss reprtition. This cn e done y physiclly dding mss or moving existing components nd pylod in the frme. Another simpler solution is to modify suspension chrcteristics. In this work, we choose to modify the centrl suspension reltively to the front nd rer suspensions. Our purpose is to increse the verticl lod on the centrl xle, which is equivlent to unlod the front nd rer xles. The suspension mechnism is represented in Fig. 7 with its min dimensions. For simplicity resons, it ws chosen to chnge the hs length from hs std = 45 mm to the miniml possile length hs mod = 45 mm. This ws otined y trnslting forwrd the top-ttchment points of the front nd rer dmpers in their T-slot. Fw 45 mm ls 2-28 mm l2 7 mm l 35 mm e 5 mm e l different methods nd results re coherent. The first method uses the CAD model of Kokoon (Fig. 6-). Ech component is given uniform density (pproximte ssumption) nd the Solid Edge CAD softwre cn evlute the volume nd clculte the weight of the component. The whole ssemly, including severl hundreds of prts, reches totl weight of out 367 kg without externl composite pnels. The second method consists in putting delictely the vehicle on six scles with winch (Fig. 6-). The results re summrized in Tle I nd re very close to those otined with the first method. The longitudinl position G of the centre of mss G is lso given reltively to the middle xle. G is locted 33 mm ehind the middle xle without driver nd only 78 mm ehind with 83 kg driver, for totl weight of kg. The lod is higher on the rer xle nd secondly on the middle xle, even if the driver mss contriutes to re-centre point G. The third method relies on the force-plte to mesure sttic lods on ech wheel. The results re lso perfectly coherent nd will e commented lter (Fig. -). Front xle Without driver 66 With 83kg driver 8 C hs mod S2 Fig. 7. The suspension mechnism with its dimensions. By chnging hs of mm, the stiffness of the middle suspension is reinforced nd the initil verticl position of the centrl wheel is chnged. Most of ll, the centrl xle is strongly overloded, which is visile on Fig. 8-. Fig. 8. Stndrd suspension configurtion (). Modified configurtion (). Tle II summrizes the chnges in weight reprtition. Front nd rer xles re off-loded of 26% nd 35% respectively while middle xle ers 58% more. Stndrd configurtion Modified configurtion Front xle 8 8 (-26%) Middle xle (+58%) Rer xle 83 2 (-35%) Totl TABLE II. Chnging mss reprtition (including 83kg driver). IV. Experimentl results A. Reference 6x6 vehicle with stndrd suspension The experimentl setting cn e seen on Fig. 9. Lines showing the desired trjectories re drwn on the ground with flour. Three types of trjectories re considered in this work: stright line (which is equivlent to turn with infinite rdius); turn with 6m rdius; turn with 3 m rdius. Severl experiments were performed in the im of following s much s possile the desired trjectories. Results re summrized in Fig..

The cquisition frequency is /th of second, which is enough for low speed vehicle such s Kokoon. Test durtion vries ccording to the the trjectory: rolling stright forwrd tkes generlly no more thn.

4 The cquisition frequency is /th of second, which is enough for low speed vehicle such s Kokoon. Test durtion vries ccording to the the trjectory: rolling stright forwrd tkes generlly no more thn.5 s (Fig. -) while turning is slower nd requires round 2.5 s (Fig --c) ecuse steering requires much power from the electric motors nd this power must e djusted in rel time y the driver on ech side of the vehicle. Z R = 3m R = 6m Fig. 9. Experimentl setting with reference frme nd trjectories. Fig. - shows the rection forces pplied to the vehicle when rolling stright forwrd. Becuse the se wheel of the vehicle (47 cm) is smller thn the forceplte width long the rolling direction (6 cm), it occurs tht there re sometimes one nd sometimes two wheels t the sme time on the force-plte. This explins the shpe of the curves. The time xis cn e divided into five phses:. First, only wheel pplies efforts on the pltform; 2. Then wheel 2 clims on the pltform (left trnsprent re) nd the verticl component of rection force increses suddenly; 3. After tht, wheel goes out of the pltform nd only wheel 2 remins on it; 4. Then, it is up to wheel 3 to go on the pltform (right trnsprent re) nd nother pek on ppers; 5. Finlly, wheel 2 leves the pltform nd only wheel 3 remins on it. Tle III is otined y verging the,, vlues on the single wheel intervls nd gives n order of mgnitude of the rection components, thus eliminting the smll vritions in the signl due to electricl perturtions nd vehicle virtions on smll grvels. Concerning norml force, it cn e seen tht wheel 3 (93 N) ers more thn wheel 2 (848 N) which ers more thn wheel (635 N). These results include the driver weight nd confirm the previous clcultions of the centre of mss. If grip coefficient is supposed equivlent on ech wheel, this mens tht the rer nd centrl wheels re le to pply higher propulsion force nd to undergo higher lterl force. When the trjectory vries (Fig. - nd -c), the overll shpe of the norml force does not chnge lot: the first pek is identicl; the second pek chnges little, proly ecuse of trnsient phenomen. The propulsion force hs n originl shpe: it seems tht only the centrl wheel pplies propulsion force. This could e proly justified y insufficient tension in the front nd rer trnsmission elts. It should e corrected very soon ecuse the centrl elts cnnot trnsmit the complete torque so only one third of the potentil propulsion force is currently used y the vehicle. Another interesting point is out the evolution of with the steering rdius R: when R decreses, must increse to mke the vehicle rotte. Along stright line, does not need to e very high to generte vehicle movement. But during turn with R = 6 m (respectively 3 m), reches 454 N (resp. 553 N). This is clerly felt y the driver, who needs to increse the power for smll turns. The lterl force hs negligile vlue when rolling stright forwrd. This vlue increses prticulrly on front nd rer wheels when the turning rdius R decreses. For instnce, for the 3 m turn, hs vlue of -32 N (resp. 532 N) on wheel (resp. wheel 3). The signs re opposite, which is norml. The solute vlues of re not symmetricl, proly ecuse they re proportionl to. Another possile explntion would e tht the instnt rottion centre of the vehicle is not exctly on the the centrl xle xis (trjectory error). This is proly the cse for the 3 m turn, where is non null on the centrl wheel (-46 N). B. Modified 6x6 vehicle On the modified vehicle, test durtion during turn tkes only 2 s (Fig. --c) insted of 2.5 s (Fig. --c) ecuse steering nd power control re much esier for the vehicle nd the driver respectively. These improvements were clerly experienced y the driver during tests. For the stright line trjectory (Fig. -), the norml force hs strnge shpe with pek during Phse 3. This might e cused y forwrd lening of the driver or y vehicle tilting on smll ostcle. On Fig. ---c, it cn e noticed tht the curve does not chnge lot with the turning rdius R. For propulsion force, the curves still show tht only the centrl wheel pplies propulsion force. However, the modified suspension seems to hve decresed the propulsion force. This mens tht smller force genertes the sme movement. For 6 m turn, decreses from 454 N to 282 N, which mens 38% gin. The driver needs to inject fewer energy in the electric motors. Moreover, s the overll turning time is shorter thn with clssicl suspension, it cn e concluded tht the glol turning efficiency is definitely improved. The lterl force hs more or less the sme shpe in Fig. s in Fig.. For 6 m turn whith the modified vehicle, there is 28% decrese of on the front xle nd 36% decrese on the rer xle. This mens fewer energy dissipted during skid-steering.

5 W heels +2 Rection forces (N) Stndrd suspension Stright line W heels 2+3 W heel W heel 2 Rection forces (N) Modifiedsuspension Stright line W heel 3 - -,5,5 Rection forces (N) Stndrd suspension,5,5 Rection forces (N) Modified suspension - -,5,5 2 Rection forces (N) Stndrd suspension,5 Rection forces (N) Modified suspension,5 c c - -,5,5 2 2,5 Fig.. Forces on the right wheels for the stndrd suspension. Stndrd suspensions Stright line Wheel (Front) R (N) R (N) Wheel 2 (Middle) R (N) R (N) ,5,5 Fig.. Forces on the right wheels for the modified suspension. Wheel 3 (Rer) R (N) R (N) Modified suspensions Wheel (Front) R (N) R (N) Stright line Wheel 2 (Middle) R (N) R (N) Wheel 3 (Rer) R (N) R (N) TABLE III. Averge forces on the right wheels for stndrd suspensions. TABLE IV. Averge forces on the right wheels for modified suspensions. Another experimentl result is represented in Fig. 2: the trce of the centre of pressure on the force-plte for different trjectories of the modified vehicle. Phses, 3 nd 5 re represented y curve segment going upwrd. Phses 2 nd 4 (where two wheels re present t the sme time on the pltform) re represented y sudden inflexion of the curve downwrd. It is interesting to see tht the centre of pressure follows qusi-perfect rcs of circles during phses This confirms tht trjectory ws correctly followed (see rrows on Fig. 2). mss reprtition. This phenomenon cn e prtilly explined y existing results. In [8], Hlconruy shows tht the lterl force is function of the slip ngle s. The eginning of the curve is liner (Fig. 3-) up to limit ngle sl, which is of the order of for typicl cr tire. Aove this threshold, there is trnsition zone nd the tire strts to slip on the ground. It hs lso een proved tht the lterl force depends on the norml force. An increment on genertes n increment on. This phenomenon is exctly wht ws noticed during our experiments. The slip ngle s is the ngle etween the medin plne of the tire nd rolling direction. There is direct reltion etween s, trjectory nd vehicle geometry. C. Some elements of physicl explntion These preliminry results re quite encourging nd tend to suggest tht turning efficiency is highly sensitive to

6 3 reprtition on the wheels without chnging neither mss nor pylod reprtition in the vehicle. This is currently done mnully. In future version, the suspension djustment could e performed utomticlly during turns y dedicted mechnism. Further work will focus on extended experimentl results including mesurements on ll the wheels of the vehicle nd n nlyticl model of the involved phenomen. 2 Acknowledgement 6 (cm) 5 Going stright 4 Turn R=6m Turn R=3m (cm) Fig. 2. Trce of the centre of pressure for the modified vehicle nd for three types of trjectories. References If the centre of rottion of the vehicle is supposed to e locted on the middle xle, slip ngles s nd s2 pper on the front nd rer xles of the skid-steering vehicle (Fig. 3-). Lrger wheelse nd smller turning rdius generte higher slip ngles. Consequently, lterl forces increse nd the vehicle needs more energy to turn. Force-pltform Lterl force R R Z > R Z2 > R Z3 R Z s V V2 V3 s3 R Z2 R Z3 Slip ngle s sl The Kokoon prototype ws designed with finncil support of ANVAR (French Ntionl Agency for Development of Reserch) nd MICHELIN compny. The other sponsors nd people involved in Kokoon development re given on the Kokoon Project we pge: Circulr trjectory of rdius R Fig. 3. The lterl force depends on slip ngle nd norml force (). The slip ngles depend on trjectory nd vehicle geometry (). VI. Conclusion This preliminry experimentl work proved tht steering efficiency of the 6x6 ll-terrin Kokoon vehicle cn e sustntilly improved only y minor djustments on the vehicle suspension. An importnt modifiction in weight reprtition ws otined y cm djustment on dmper fixtures. This minor djustment llowed to reduce the propulsion forces of out 2-3 % nd lso decresed the lterl forces in the sme proportion. The driver lso reported tht he felt sustntil improvement of steering cpcities during tests with the modified vehicle. This method could e generlized to mny types of multi-xle vehicles in order to improve their steering performnce. The sence of ny steering system on vehicle is gurntee of roustness nd control simplicity ut hs the drwck of high power consumption during steering phses. With such solution, one cn imgine n dpttive suspension cple of modifying norml force [] Kececi E.F., nd To G. Adptive vehicle skid control. Mechtronics, 6(5):29-3, June 26. [2] Wtne K., Kitno M., nd Fugishim A. Hndling nd stility performnce of four-trck steering vehicles. Journl of Terrmechnics, 32(6):285-32, Novemer 995. [3] Mokhimr O., nd Ae M. How the four wheels should shre forces in n optimum coopertive chssis control. Control Engineering Prctice, 4(3):295-34, Mrch 26. [4] Besselink B. C. Development of vehicle to study the trctive performnce of integrted steering-drive systems. Journl of Terrmechnics, 4(4):87-98, Octoer 24. [5] Besselink B. C. Computer controlled steering system for vehicles hving two independently driven wheels. Computers nd Electronics in Agriculture, 39(3):29-226, August 23. [6] Shoichi S., oshimi F. nd utk T. Steering pprtus for vehicle hving steerle front nd rer wheels. Ptent numer US46435, , Hond Motor Co Ltd (Jpn). [7] FNSS Corp., PARS 6x6 nd 8x8 Wheeled Armoured Vehicles, [8] Ptri Corp., The Armoured Modulr Vehicle, [9] Mclurin B. A skid steering model with trck pd flexiility. Journl of Terrmechnics, In Press, Corrected Proof, Aville online 9 June 26. [] Roosoft Corp. The Pioneer 3 ATRV. [] Osis LLC Corp. The Mx 6x6 ATV. [2] Furoux J.C., Chrlt S., nd Limenitkis M. Tem design process for 6x6 ll-rod wheelchir. In Proc. Interntionl Engineering nd Product Design Eduction Conference, pp , IEPDE'24, Delft, The Netherlnds, Septemer 2nd - 3rd, 24. [3] Furoux J.C., Chrlt S., nd Limenitkis M. Conception d'un véhicule tout-terrin 6x6 pour les personnes à moilité réduite. In Proc. 3ème conférence Hndicp 24, June 7th - 8th, 24, Pris Expo, Porte de Versilles / Frnce, pp [4] Itoh H., Oid A. nd mzki M. Mesurement of forces cting on 4WD-4WS trctor tires during stedy-stte circulr turning in rice field. Journl of Terrmechnics, 32(5): , Septemer 995. [5] Foster J.R., Ayers P.D., Lomrdi-Przyylowicz A.M. nd Simmons K. Initil effects of light rmored vehicle use on grsslnd vegettion t Fort Lewis, Wshington. Journl of Environmentl Mngement, In Press, Corrected Proof, Aville online 23 Mrch 26. [6] Couétrd. Cpteur de forces à deux voies et ppliction notmment à l mesure d un torseur de forces. INPI, Brevet N (Frnce), 993. [7] Couétrd. Crctéristion et étlonnge de dynmomètres à six composntes pour torseur ssocié à un système de forces. PHD thesis report, Bordeux University,. [8] Hlconruy T. Les liisons u sol. 995, 2p., E.T.A.I.

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