Pag000217 EVS25 Shnzhn, China, Nov 5-9, 2010 Rsarch on Braking Stability of Elctro-mchanical Hybri Braking Systm in Elctric Vhicls nzhu Ji 1, Mi Tian 2 School of transportation scinc an nginring, Bihang Univrsity No.37 Xu Yuan Roa, Hai Dian Zon Bijing, China jfz@buaa.u.cn Abstract or th lctro-mchanical hybri braking systm, which is compos of lctric brak an gnral friction brak, th mols of lctric braking forc, total braking forc an th utilization ahsion cofficint for front an rar axls wr stablish bas on th analysis of braking torqu istribution. Th variation rlationship btwn lctric braking forc an friction braking forc in iffrnt braking intnsity was calculat an analyz with th paralll-hybriiz braking control stratgy. Taking USA urban riving circl UDDS as an xampl, th utilization ahsion cofficint of front an rar axls was calculat at iffrnt braking intnsity for a crtain Elctric Vhicls (EVs), an th braking stability was also analyz for front-whl riv EVs. Th calculation rsults inicat that th utilization ahsion cofficint of front axl is always gratr than that of rar axl, which mans th front axl always lock aha of th rar axl, thus th braking stability mts th rquirmnt. Th calculation rsults also hav crtain instructiv significanc on th anti-lock braking systm (ABS) an lctric brak-forc istribution (EBD) of EVs. Kywors: lctric vhicls, lctric brak, lctro-mchanical hybri brak, braking stability, utilization ahsion cofficint. 1 Introuction Rgnrativ braking tchnology for lctric vhicls, which also nam lctric brak tchnology, is an ffctiv tchnical masur to improv th riving rang by convrting kintic nrgy into lctric nrgy which thn was posit into accumulator [1]. Howvr, th maximum charging currnt an charging powr of th onboar accumulator in lctric vhicls ar usually limit, thus th braking ability of lctric braking systm is also rstrict, which can t satisfy th havy braking intnsity. Hnc, lctric vhicls with lctric braking systm shoul also EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 1
Pag000218 hav th mchanical friction braking systm. Namly, th lctro-mchanical hybri braking systm shoul b aopt to mt th n in iffrnt braking intnsitis. Th currnt rsarch subjcts on lctric brak mainly focus on th aspcts such as th ncssity an fficincy of braking nrgy rcycl [2], th control stratgy of lctric braking [3], th control tchniqu of riving lctric motor an powr convrtr [4], an th coorination of th lctro-mchanical hybri braking systm [5]. Howvr, th papr on braking stability of lctro-mchanical hybri braking systm for lctric vhicls is rarly rport. Taking som typ of EVs as th rsarch objct, th braking stability of lctro-mchanical braking systm was stui in orr to harmoniz th lctric braking with th mchanical friction braking, thus th braking stability was guarant. Th rsarch rsults can stablish thortical basis for rsarch on ABS an EBD of EVs, which hav crtain nginring application valus. 2 Braking torqu istribution principl of lctro-mchanical hybri braking systm In orr to achiv both th nrgy rcycl an braking stability, a crtain braking torqu istribution principls must b satisfi for lctro-mchanical hybri braking systm [6]. Th principls wr us to istribut th rgnrativ braking torqu an th frictional braking torqu, which wr shown in ig.1. igur 1: th curvs of braking torqu of rgnrativ braking systm in HEV sction1: rgnrativ braking torqu in harmonious; sction2: rgnrativ braking torqu quivalnt to convntional ful vhicl Whn rlasing th acclrator pal an th EV sliing, th braking intnsity is rlativly wak (quivalnt to th ngin braking of convntional ful vhicl), hr, th rgnrativ braking (lctric braking only) coul sufficintly mt th mans of braking prformanc; Th rivr stp on th brak pal whil mor braking intnsity is rquir. If th forc a to th pal is lss, th braking forc ffcting on riving whls is only th rgnrativ braking forc which incrasing in irct proportion to pal forc; onc th pal forc xcs a crtain valu an th braking intnsity coul not b satisfi by rgnrativ braking only, th braking torqu gnrat by mchanical friction on both front an rar whls wr n. With th procing of braking, vhicl slows own an rgnrativ braking torqu graually climbs up an rachs to th maximum at th point which th vhicl sp has ruc to th zon of motor constant torqu, whil th friction braking torqu clins corrsponingly. Whn EV is to stop, th rgnrativ braking torqu promptly fall to zro whil th rquir friction braking torqu rapily riss. 3 Braking forc mathmatical EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 2
Pag000219 mol in lctro-mchanical hybri braking systm 3.1 Elctric braking forc mathmatical mol Accoring to th analysis mntion abov, th calculation formula of lctric braking forc is as follows: n n, T T 9549P n n, T n (1) Th lctric braking forc convrt to riving whls,, is calculat as follows: T i i g 0 (2) rt whr, n is th rat rvolution of th motor, r/min; T is th rat torqu, Nm; P is th rat powr, kw; i g an i 0 ar th riving ratio of transmission an main rucing gar, rspctivly; r is th rolling raius of whls, m; T is th riving fficincy. As th EVs garing own, th rlationship btwn vhicl sp an motor sp is as follows: rn u 0.377 (3) i i In th formula, u is th vhicl sp, km/h; whil n is motor sp, r/min. rom formula (1) ~ (3), w can gt th calculation formula of lctric braking forc of riving whls: n n, n n, g 0 Ti gi r T 0 3600P u T (4) It can b sn that th lctric braking forc is rlat to vhicl sp, an it crass graually with th incrasing of vhicl sp. Th braking forc of lctro-mchanical hybri braking systm must b rgulat accoring to th variation of vhicl sp in orr to guarant th braking stability. 3.2 Total braking forc mathmatical mol It was suppos that th ratio of braking forc of front an rar axl arrstrs is fix in convntional mchanical friction braking systm, an th partition cofficint of braking forc is, thn th rlationship btwn braking forc of front an rar arrstrs, b1 b2, is as follows [7] : 1 (5) b 2 b 1 It was takn into account that th front whls ar riving whls. Thn th total braking forc of rar whls is b2, an total braking forc of front whls is as follows:, b1 b1 (6) 3.3 Utilization ahsion cofficint of front/rar axls Utilization ahsion cofficint rprsnts th using conition of groun ahsion cofficint as th EVs braking, That is to say, it is th groun ahsion cofficint that thr ar no whls lock at crtain gr of braking intnsity. It was fin as follows: b 1 b2 f, r (7) z 1 z 2 Whr, b1 b2 rprsnts groun braking forc prouc by front axl an rar axl rspctivly whn braking intnsity is z; z1 z2 rprsnts th normal groun-raction which th groun applis to th front axl an rar axl rspctivly whil th braking intnsity quals to z; f r shows th utilization ahsion cofficint of front an rar axl rspctivly with th braking intnsity z. Th utilization ahsion cofficint calculation mol can b foun combining with th calculation formulas of groun braking forc an normal groun-raction in rfrncs [7], about th rar whls ar going to b lock or both front EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 3
Pag000220 an rar whls ar going to b lock unr braking intnsity z. Th mol is scrib as follows: [ z (1 ) / G] L f L zh L(1 )( z / G) r L zh 1 2 g g (8) In th formula, f r ar th utilization ahsion cofficint of front axl an rar axl, rspctivly; L is th axl bas, m; L 1 is th istanc btwn th cntroi an th front axl, m; L 2 is th istanc from th cntroi to th rar axl, m; G is th whol gravity of vhicl; h g is th hight of th cntroi, m; z is th braking intnsity. Th utilization ahsion cofficint of front an rar axls, f r, ar rlat to vhicl sp sinc th lctric braking forc is rlat to th vhicl sp. Th ahsion utilization curv of riving whls coul go byon th limit of ECE rgulation with th incras of th vhicl sp. It coul la to iffrncs btwn th braking stability of lctro-mchanical hybri braking systm an that of th traitional vhicls. 4 Braking stability of lctro- mchanical hybri braking systm 4.1 Braking forc of lctro-mchanical braking systm In orr to nsur th braking stability of lctric vhicls, it is not only to b proprly istribut for th lctric an friction braking torqu, but also to b rasonably controll on th braking torqu of th front an rar axl. Thortically, thr ar thr kins of control stratgis for th braking torqu of front an rar axl: ial front an rar axl braking torqu control stratgy, optimal nrgy fback control stratgy, an paralll-hybriiz braking control stratgy [6]. Th paralll-hybriiz braking control stratgy was aopt in this rsarch, which mainly bcaus th braking controllr for whol vhicl is not n unr this control stratgy. A motor controllr only ns to b a to control th lctric braking forc in accoranc with braking intnsity an vhicl sp. In aition, th friction braking systm is th sam as that in traitional ful vhicls. Taking a crtain typ of lctric passngr car as th rsarch objct, a mathmatical mol of lctro-mchanical hybri braking systm was stablish bas on th braking torqu allocation principl an braking forc calculation mol mntion abov, with th input paramtr as vhicl sp, brak pal isplacmnt, battry SOC an transmission gar. Th lctric braking forc an friction braking forc coul b calculat an ajust timly to satisfy th braking stability of EVS. Th main tchnical paramtrs of this EVS ar as follows: th total mass (full loa) is 1615kg; th istanc from cntroi to front axl is L 1 =1060mm; th istanc from cntroi to rar axl is L 2 =1590mm; th hight of cntroi is h g =495mm; th whl raius is r=0.287mm; th transmission gar ratios from 1 to 5 location ar rspctivly 3.181,1.842,1.250,0.864,0.707; th main rucr ratio is i 0 =4.266; th riving fficincy is T =0.92. Th AC asynchronous motor is us in th EVS. Th rat powr an sp rspctivly ar 20 kw an 3600rpm. Th motor charactristic curvs tst wr show in ig.2. It can b sn that th charactristics of gnrating an motoring stat is similar basically. In rgnrativ braking conition, th motor brak at rat powr as th sp is highr than rat sp; an th motor brak at rat torqu as th sp is lowr than rat sp. Rgnrativ braking will b isabl an lctric braking forc will cras rapily to zro if motor sp is quit low. EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 4
Pag000221 ig.2: th motor tsting curvs ig.3 show th braking forc istribution curv of th front an rar axls, an th total braking forc is born both by lctric braking forc an friction braking forc unr a fix proportion. Thr is only lctric braking forc acting on th whol vhicl whn th z0.1; thr ar both lctric braking forc an friction braking forc bring to bar at a fix ratio whil 0.1<z0.7; th z> 0.7 is th stat of mrgncy braking, an thr is only friction braking forc in this situation. braking stability of vhicls, firstly, it must mak sur that th instancs that only th rar whls lock or rar whls lock aha of th front whls o not happn; Sconly, it shoul also b avoi that only th front whls lock or th front an rar whls lock simultanity. Th ial instanc is that non of th whls lock whn vhicls braking. In orr to analyz th stability of lctro-mchanical braking systm, th utilization ahsion cofficint was chosn to rprsnt braking conitions of front an rar axl in this stuy. Th simulation on th braking stability of EVs was calculat at th conition of th USA urban riving circl UDDS, in which th maximum vlocity is 91.25km/h, whil th avrag vlocity is 31.51km/h. Th calculation rsult was shown in ig.4. (a) Constant torqu braking ig.3: braking forc istribution curv of an EVs 4.2 Stability of lctro-mchanical braking systm Braking stability is th capacity that vhicls o not viat, sislip, or los th string ability whn braking; it is closly rlat to th istribution an rgulation of th braking forc of th front an rar arrstr. In orr to nsur th (b) Constant powr braking EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 5
Pag000222 ig.4: th rlation btwn utilization ahsion cofficint an braking intnsity ig.4 illustrat that th curv of utilization ahsion cofficint of front axl f is always abov of that of rar axl r no mattr what th motor is in constant torqu braking ara or constant powr braking ara at th conition of lctro-mchanical hybri braking. That is to say, th front axl is always lock aha of th rar axl. It inicats that th braking stability of EVs mt th rquirmnts. But th utilization ahsion cofficint curv of th front axl has an intrsction with that rgulat by ECE rgulation whn th EVs brak at th top gar, which accounts for that th front axl coul b lock in th top gar. Bsis, th istanc btwn th curvs of f an = (z+0.07)/0.85, in ECE rgulation, is growing along with th incrasing of lctric braking forc (low gar), so os th r curv. Th braking stability is affct isavantagously. 5 Conclusion Elctric braking forc is rlat to th vhicl sp, an its braking capacity was limit, which can t mt rquirmnts of havy braking intnsity. Thus th lctro-mchanical hybri braking systm shoul b aopt in th EVs with rgnrativ braking systm. Th braking stability is influnc by lctric braking forc which only acts on riving whls an changs th istribution portion of braking forc of front an rar axls of convntional friction braking systm. It can b achiv to mt th rquirmnts of braking stability that th front axl is lock bfor th rar axl or th two axls ar lock simultanity by allocating lctric braking torqu an friction braking torqu rasonably, choosing propr braking forc control stratgy of front an rar axls. Howvr, th utilization ahsion cofficint curv of front axl analyz in this papr was abov th curv rgulat by ECE rgulation, which might la to an arly lock of th front axl. It can b nsur that th utilization ahsion cofficint curv of front axl b always unr th ahsion cofficint curv rgulat by ECE rgulation at any sp by optimizing or ajusting rlvant paramtrs (such as cntroi position, lctric braking forc controll by powr convrtr, tc.), which will improv on th utilization ratio of lctric braking forc. Rfrnc [1] Knji, Morita. Automotiv powr sourc in 21st cntury, JSAE Rviw, ISSN 0389-4304,24(2002), 3-7. [2] A. M. Walkr, M. U. Lamprth an S. Wilkins. On riction Braking Dman with Rgnrativ Braking, 20th Annual Brak Colloquium An Exhibition, Oct. 2002, 2002-01-2581. [3]Mhra Ehsani,Yimin Gao an Sbastin Gav. Charactrization of Elctric Motor Drivs for Traction Applications,Th 29th Annual Confrnc of th IEEE, 2003, 891-896. [4]Hongwi Gao, Yimin Gao an Mhra Ehsani,A Nural Ntwork Bas SRM Driv Control Stratgy for Rgnrativ Braking in EV an HEV, Elctric Machins an Drivs Confrnc(IEMDC 2001), Jun.2001, 571-575. [5] Zhang Jianlong, Yin Chnlian, Zhang Jianwu, tc.. Rsarch on Braking Control Systm for Hybri Elctric Vhicl with Elctro-mchanical Hybri Brak, Journal of Systm Simulation, ISSN 1004-731X,21(2009), 5169-5178. [6]Zhao Guozhu. Rsarch on Braking Stability of Rgnrativ Braking Systm in EV an HEV. Nanjing Univrsity of Aronautics an Astronautics, 2006. [7]Yu Zhishng, Automotiv Thory, ISBN 7111020766,Bijing, China Machin Prss, 2008. EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 6
Pag000223 Authors nzhu Ji Rciv Ph.D. gr from Bihang Univrsity. Th major rsarch subjcts ar vhicl nginring, spcially th ky tchnology of EVs, an ngin thrmal managmnt. Mi Tian Grauat from Bijing Institut of Tchnology. Th major is vhicl opration nginring, th main rsarch subjct is ky tchnology of EVs. EVS25 Worl Battry,Hybri an ul Cll ElctricVhicl Symposium 7