Control of AWD System for Vehicle Performance and Safety

Transcription

1 Control of AWD Syste for Vehicle Perforance and Safety Hojin Jung, Seibu Choi KAIS, Departent of Mechanical Engineering, 344 Daejeon, Korea Abstract. AWD (All-Wheel Drive) syste transfers drive force to all wheels so that it can help vehicle escape low u surface or clib hill ore conveniently. Recently, AWD syste for on road vehicle has becoe popular to iprove vehicle driving perforance. However, there has not been enough research of applying AWD syste for vehicle stability especially for lateral oveent. Copared with ESC (Electronic Stability Control), AWD syste does not cause any inconveniences to the driver because it controls vehicle only by distributing front and rear drive torque, without using brake. By allowing slipping/locking of wet clutch inside the transfer case, AWD syste can distribute different aount of torque between front and rear ale. his paper introduces odelling of AWD syste and suggests the control of AWD syste based on peak slip ratio and slip angle at which tyre saturates. Carsi based vehicle siulation results of AWD controller is presented. Introduction Copared with original WD vehicle which drives front wheels or rear wheels only, AWD syste has a roll to distribute engine drive to torque to both front ale and rear ale. Usually AWD syste was equipped with offroad vehicle that runs on rough road. However, with the growing of engine power, AWD syste has becoe being equipped in high perforance sedan. Hardware configuration which is dealt with in this paper differs fro center differential 4WD and selective 4WD because it distributes soe portion of drive torque of ain drive shaft which is directly linked to transission output shaft to sub drive shaft by controlling wet clutch inside the transfer case. Actuator type for control varies fro ball rap control type by electroagnetic force to hydraulic piston control type by otor pup. By utiliing advantages of active control syste, AWD syste can be used for both of longitudinal and lateral driving safety. In this regard it is iportant to find out the present vehicle state as well as road state. For eaple, vehicle clibs high slope hill, escapes fro low u surface and has lateral oveent like double lane change or circular otion with high velocity. And research about odel predictive control should be done including these various driving situations. hen it is epected that vehicle perforances like driver s riding quality and steady state cornering are iproved. his paper suggests the control ethod of AWD syste of wet clutch with hydraulic piston actuator type. Vehicle siulation environent was set based on siulink built transfer case syste coupled with carsi software. Especially, possibility of lateral stability iproveent by AWD syste without intervention of ESC is discussed in detail as well as longitudinal perforance. Syste odelling. Driveline dynaic odel States of AWD vehicle change depending on the aount of torque which is delivered front or rear shaft. AWD syste which is dealt in this paper can control the aount of torque which is transfered to front drive shaft in transient state, by using ulti plate wet clutch. hus siulation description should be available of variable distriburtion of front shaft torque depending on clutch engageent force especially between clutch slipping state and clutch lock up state. Driveline odel fro transission output shaft to front and rear differential gear can be represened as follows, atheatical odel is also as follows : Figure. Scheatic diagra of transfer case syste. t c r Jt t c c, f F () () c f / if Jc c (3) he Authors, published by EDP Sciences. his is an open access article distributed under the ters of the Creative Coons Attribution License 4. (

2 k b f f c f f c f k b r r t r r t r (4) Where t is a transission output torque, c is a clutch engageent torque, f and r are front and rear shaft torque each. J t is a ain drive shaft inertia, J c is front drive shaft inertia, F c is clutch engageent force given by hydraulic actuator. ωis a clutch slip angular velocity. Spring and daper syste was added at each end of drive shaft to represent transfer torque given by distortion of shaft. ransitted clutch torque is epressed as a function f (F c, ω slip ) and detailed epression is introduced in section.... Wet clutch odel Clutch is generally divided into dry clutch and wet clutch, depending on the kind of aterial and whether the oil is included or not. Dry clutch can be burned if slip duration prolonged. Wet clutch copensate the disadvantage of dry clutch, by absorbing the heat which is generated in friction plate. However, wet clutch is short of aiu allowable torque which friction plate endures. By overlapping ultiple plates, aiu allowable torque can be enhanced. ().. Generalied Mawell Slip (GMS) friction odel It is iportant to have a feasibility of friction odel for the syste which includes engageent force between two rolling plates to guarantee the validity of siulation. GMS odel [,], which is well known for friction odelling, was opted for odelling transfer case syste. When clutch is fully lock up, differential equation of clutch transfer torque is as dmi k i p, t (6) dt Where k i (p, t ) is the function of clutch engageent force and transission output torque. k i can be apped by coparing carsi internal 4WD odel front shaft torque and developed transfer case syste front shaft torque. When clutch is slipping, differential equation of clutch transfer torque is as dm i M i sgn C i dt s, p (7) Where s(ω, p) is a stribeck equation about torque, which is epressed as Vs p s, psgnms Mc Mse (8) Wet clutch should consider viscous effect by the oil fil between disc plates. Especially for wet clutch, drag torque takes quite soe part of transitted torque so that this should be odelled carefully to iprove the accuracy of transient response [4]. Drag torque of wet clutch is originated fro the shear stress between plates and can be odelled by fluid dynaic analysis [4-6]. hen, viscous friction torque equation is obtained as M vis r 3 o r.94 h dr (9) r h. Re Where r i is an clutch inner diaeter, r o is an effective diaeter due to the centrifugal force, which is usually less than clutch outer radius. Re h is a Reynolds nuber Autoatic ransission Fluid (AF), which is written as rh Re h () Where μ is a viscous coefficient of AF, ρ is a density of AF and his a clearance length of clutch plate. Finally, clutch transfer torque when slipping is as c Mi Mvis () o iprove the accuracy of wet clutch engageent siulation, clutch eperiental friction coefficient data which will be anufactured for vehicle ipleentation was used..3 Lock up state representation by applying DOF (Degree Of Freedo) reduction odel GMS odel which is introduced in section. atches well in noral driving siulation. However, cople or severe driving situation like rapid acceleration in low surface can cause odel instability like chattering response of clutch transfer torque. Equation should be replaced when lock up state so that it was described by designing DOF reduction odel. When lock up state, angular accelerations at both transission output shaft and clutch driven shaft are sae. hen, dynaic odel of driveline is as () t t f r t c t c J J (3) k b (4) f f c f f c f k b () r r t r r t r.4 State transition between clutch slipping and lock up It is iportant to decide the current state whether clutch is slipping or locked up to iprove the accuracy of AWD syste siulation [3]. GMS odel distinguish current state only by the liit of allowable static torque. However, for the case of siulation which include full vehicle odel. Above condition was not enough to deterine state transition accurately when clutch is engaged while input torque is rapidly increasing. hus two conditions of static torque and clutch slip were used siultaneously to enhance the accuracy of state transition. c Ms (6). rad / s (7) slip

3 3 Controller design 3.. Vehicle state estiation For the AWD syste controller based on tyre odel, current vehicle states should be estiated. Generally, the lowest wheel angular velocity value aong 4 angular velocity sensors can be assued to vehicle longitudinal velocity in WD. However, when clutch is engaged, vehicle longitudinal velocity of AWD cannot be assued only with wheel sensors because all wheels becoe driven wheels. hus vehicle longitudinal velocity should be estiated. In addition, vehicle lateral velocity should be estiated to know wheel side slip angle. By utiliing vehicle longitudinal dynaics, lateral dynaics, yaw dynaics and wheel dynaics, these vehicle states can be estiated by etended Kalan filter [7,8] using planar bicycle odel shown in figure. he equation of otion for the bicycle odel shown in figure can be categoried three dynaics. Longitudinal dynaics is: sin v Ff cos f Fyf f Fr (8) aircd Av gsin vyr Where v is the longitudinal velocity, is the vehicle ass, F f is the front longitudinal ale force, F yf is the front lateral ale force, δ f is the tire steer angle, ρ air is the air density, C d is the drag coefficient, v y is the lateral velocity, ris the yaw rate and θ is hill angle. Lateral dynaics is: cos v y Ff sin f Fyf f Fyr vr (9) Where F yr is the rear lateral ale force. Yaw dynaics is: r lf Ff sin f Fyf cos f lr Fyr () I Where I is the vehicle yaw oent of inertia, l f is the distance fro the center of ass to the front ale and l r is the distance fro the center of ass to the rear ale. Another dynaics which force ters are related is wheel dynaics which is epressed as d b RF e, i RF e r () Iw Where ω is the wheel angular velocity, I w is the wheel inertia, d is the wheel drive torque, b is the wheel brake torque. In this paper, d is assued to be known and does not consider braking situation. R e is the wheel effective radius, F r is the tire rolling resistance. F,i is the each tire longitudinal force whose subscript i includes fl (front left), fr (front right), rl (rear left), rr (rear right) which is written throughout the paper. Figure. Bicycle odel. Using dynaics which are entioned above, Kalan filter can be obtained. States of Kalan filter is defined as t v, vy, r,, ω F () Where ω fl, fr, rl, rr (3) F Ffl, Ffr, Frl, Frr, Fyf, F yr (4) ire longitudinal forces which are epressed in this paper are related as: Ff Ffl Ffr, Fr Frl Frr he easureents signals fro vehicle are as y t a, ay, r, fl, fr, rl, rr () Where a and a y are related as: a Ff cos f Fyf sin f Fr aircd Av gsin (6) ay Ff sin f Fyf cosf Fyr (7) he above states and easureents are epressed as nonlinear function f t, u t and h t, u t as t f t, u t w t (8), yt h t u t vt (9) Estiated states are usually denoted as ˆ. hen, the algoriths of discrete tie etended kalan filter can be epressed as follows []: Pk t k ˆk ˆ k f tk, u d (3) Pk tk d (3) F P P F LL tk k k k k k k k h t K P H H P H R (3) ˆk ˆk K k y k ˆ k, u (33) P k I K k H k P k (34) 3

4 Where F and H are the lineariation of equation (9) and (3) around ˆ k at each tie step. P is the error covariance and L is the sensor noise disturbance atri. Wheel slip ratio and side slip angle are obtained fro the estiated states. Longitudinal slip ratio at each wheel in acceleration is given by: Rei vˆ i (3) Rei Also side slip angle of front ale and rear ale which will be used in the AWD logic developent are: lf r f f (36) vˆ lr r r (37) vˆ Where β is side slip angle which is defined as v y / v. 3. AWD driving logic developent Certainly, AWD driving condition can be decided based on saturation point of tyre longitudinal and cornering stiffness. AWD syste has a tendency to stabilie the vehicle both of longitudinal and lateral oveents. here any approaches to develop vehicle control logic based on tyre stiffness. However, there has not been saturation point detection logic considering vehicle weight shifting and slip siultaneously. o solve this proble tyre lateral force function of two variables about vertical force and slip angle is presented here., i,i F C C (38) F Fy Cy, Cy, F (39) n, Where F,n is noinal vertical force of noral load. ΔF is the difference between shifted vertical force and noinal vertical force. Saturation point of slip ratio and slip angle can be calculated by differentiating equation (39) and (4). C, sat (4) C, Cy, sat F (4) Cy, F n, Above saturation point is used in this paper to deterine the engageent of wet clutch. 4 Siulation o validadte the AWD syste driving siulation by ipeenting developed transfer case plant odel, carsi software was used. As shown in figure, siulink based transfer case odel was coupled in carsi. Ecept for transfer case part carsi internal odel was used. Siulation coupled with coercial sofware not only can verify the versatility of developed odel but also help to easiliy grasp of change of vehicle state depending on various driving siulation. Vehicle driveline is based on front ship rear wheel drive (FR) type. Figure 3. ransfer case siulink odel coupled with carsi. 4. Accelerating in split u situation For the validation of longitudinal driving perforance of AWD syste, straight line with split u siulation was conducted. Left wheel is driven on the low u with friction coefficient of. and right wheel is driven on the high u with friction coefficient of.8. While driving split u with 3k/h, rapid full acceleration was conducted between 8-4 seconds to siulate the situation of high slip ratio. Based on the slip calculation logic and trigger condition which was entioned in section 3. and 3.3, AWD syste was activated. o eclude the influence of gear shifting, only st and nd gear shifting was allowed. Longitudinal velocity [/s] Slip ratio v v e ie [s] (a). w rl (awd) w rl (rwd) ie [s] (c) orque [N] Longitudinal velocity [/s] v (awd) v (rwd) ie [s] (b) f r ie [s] (d) Figure 4. Siulation results of accelerating in split u situation. (a) Coparison of longitudinal velocity between real value and estiated value (b) Coparison of longitudinal velocity between awd and rwd (c) Coparison of slip ratio between awd and rwd (d) front and rear shaft torque response in awd Figure 4(a) shows the results of estiated awd longitudinal velocity. Although there appeared discrepancy between real value and estiated value, it can be used for slip ratio calculation. And figure 4(b) shows the results of longitudinal velocity between awd and rwd. Certainly, awd showed a better perforance to escape the split u road. Slip ratio reduction in awd copared to rwd can be verified in figure 4(c). Figure 4(d) shows the torque response of awd by the control logic. 4

5 4. Accelerating in steady state cornering situation For the validation of lateral stability of AWD syste, feet radius circle lane siulation was conducted. While driving high u of friction coefficient with 9k/h, rapid full acceleration was conducted between - seconds to siulate the situation of oversteering. hen, AWD syste was activated based on sae process which was entioned in section 4.. Maiu transission gear was set to th. Side slip angle Side slip angle difference [rad] beta(awd) beta(rwd) -. 3 ie [s] (a).. -. af-ar(awd) af-ar(rwd) -. 3 ie [s] (c) orque [N] Yaw rate [rad/s] - r(awd) r(rwd) - 3 ie [s] (b) 8 f r ie [s] (d) Figure. Siulation results of accelerating in steady state cornering. (a) Coparison of side slip angle between awd and rwd (b) Coparison of yaw rate between awd and rwd (c) Coparison of front and rear slip difference between awd and rwd (d) front and rear shaft torque response in awd Figure (a) and (b) shows the results of side slip angle and yaw rate, which are principal indices of lateral stability, between awd and rwd. Certainly, rwd showed high side slip angle and yaw rate when tyre saturates. Figure (c) shows the results of front and rear ale side slip angle difference. Actually, awd showed slight side slip angle difference, which eans vehicle oves siilar to neutral steer. However, rwd showed an over-steering response when tyre force saturates. Figure (d) shows the torque response of awd by the control logic. It is noteworthy that sall aount of torque distribution prevented over-steering response. Conclusion AWD control logic developent for vehicle stability was conducted in this paper. For controller validation, transfer case odeling ethod was introduced and reliable siulation environent was built first, by ipleenting siulink transfer case as an eternal odel in carsi software. Noticable reuslt of siulation is that upper level controller based on tyre force function of wheel slip ratio and slip angle helped tyre not to fall into saturation region, without intervention of ESC syste. In addition, sall aount of torque transfer in subdrive shaft stabilie the vehicle lateral oveent efficiently. However, eplanation of soe principal vehicle states estiator is oitted by assuing they are easurable in this paper. hus it should be furthur studied for practical application. Also, real tie tyre stiffness and u estiation based logic will iprove the awd control logic for various driving scenario. Acknowledgeent his work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea governent(msip) (No. -868). his research was supported by the MSIP(Ministry of Science, IC and Future Planning), Korea, under the C- IRC(Convergence Inforation echnology Research Center) (IIP--H86--) supervised by the IIP(Institute for Inforation & counications echnology Prootion) References. Agusian Partogi Opusunggu, Paul Sas, Hendrik Van Brussel, Modeling and siulation of the engageent dynaics of a wet clutch syste subjected to degradation: An application to condition onitoring and prognostics, Mechatronics (3). Vincent Lapaert, Farid Al-Bender, Jan Swevers, A Generalied Mawell-Slip Friction Model appropriate for Control Purposes, IEEE Physics and Control Proceedings. International Conference (3) 3. Jiwon J. Oh, Seibu B. Choi, Jinsung Ki, Driveline odeling and estiation of individual clutch torque during gear shifts for dual clutch transission, Mechatronics (4) 4. Hirou HASHIMOO, Sanae WADA and Yoshiaki MURAYAMA, An Influence of Centrifugal Force on the Perforance of hero-hydrodynaic urbulent hrust Bearings, Bulletin of JSME, 7, No. 8 (984). Yiqing Yuan, Eysion A. Liu, Jaes Hill, Qian Zou, An Iproved Hydrodynaic Model for Open Wet ransission Clutches, ASME Journal of Fluids Engineering (7) 6. Youhei akagi, Yasunori Okano, Masatoshi Miyayaga and Nobuyuki Katayaa, Nuerical and Physical Eperients on Drag orque in a Wet Clutch, Japanese Society of ribologists, pp () 7. Laura R. Ray, Nonlinear state and tire force estiation for advanced vehicle control, IEEE RANSACIONS ON CONROL SYSEMS ECHNOLOGY, 3, NO. (99) 8. Mooryong Choi, Jiwon J. Oh and Seibu Choi, "Linearied Recursive Least Square Methods for Realtie Identification of ire-road Friction Coefficient", IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, 8, (3)