A fast actuator for an anti-lock braking system

Transcription

1 74 Philips ech. Rev. 36, 74-84, 1976, No."3 A fas acuaor for an ani-lock braking sysem D. R. Skoyles I is very imporan for road safey ha cars should no skid when he brakes are applied suddenly. There is a danger of his on we or icy roads, where he wheels can easily become locked. To preven hisfrom happening various ani-lock braking sysems have been developed. This aricle describes an elecro-hydraulic brake-pressure conrol wih a number of new feaures. I was iniially developed for esing elecronic circuis for ani-lock sysems and he design has given such good resuls in rials ha i has been aken as he saring poinfor hefurher developmen of ani-lock braking sysems. Inroducion When he brakes of a vehicle are applied a cerain amoun of 'slip' mus occur beween he wheels and he road. The word 'slip' is used here as an indicaion ha he braked wheels roae more slowly han hey would if hey rolled freely [*1. When he brakes are firs applied, he wheels coninue o roll over he road, bu he rae of roaion decreases because of he elasic defermaion in he par of he yre ha forms he conac pach on he road surface. As he brake pressure increases, he speed of he wheels relaive o he road decreases and evenually he wheel will lock. The degree of slip as he wheel approaches lock is a complicaed funcion of he characerisics of he brake, he yre and he road surface. Oher facors such as he speed and deceleraion of he vehicle also come ino play. When he wheels of a vehicle are locked, he sopping disance generally becomes larger, and if he fron wheels are locked, seering becomes impossible. Mos drivers are fully aware of his frighening loss of conrol and do no aemp o apply he brakes fully if he road surface appears o be slippery. Even experienced professional drivers are unable o brake a he maximum value as hey canno allow for he difference in characerisics from wheel o wheel. In he ineress of safey i is clear ha wheel lock should be avoided. Some way mus herefore be found of prevening he brakes from being applied oo hard. If his can be done auomaically, a driver would be able o brake fully in an emergency, while full seering conrol and normal handling will be mainained. Many pracical soluions o his problem have been proposed: a few of hem are now in use on a limied scale. This aricle describes a hydraulic pressure con- D. R. Skoy/es, M./.E.E., is wih Mullard Research Laboraories, Redhill, Surrey, England. rol or acuaor for an ani-lock sysem; he device was devised and consruced for esing elecronic conrol modules for such a sysem in a vehicle. A he ime here was no exising acuaor wih a fas enough response for he purpose. The acuaor was iniially developed for privae cars, bu i may well be suiable for oher vehicles. Many feaures of his acuaor are used in more recen ani-lock sysems under developmen. Before we begin he acual descripions of he design and consrucion ofhe acuaor, we should firs consider he behaviour of a skidding vehicle, yre characerisics and he arrangemen of ani-lock sysems. Consrucion and use of ani-lock sysems There are hree fundamenal pars o any ani-lock sysem: a sensor ha measures he wheel speed (or some relaed speed), a processor uni (ypically elecronie or fluidic) and a mechanical acuaor ha can briefly reduce he brake pressure and hen resore i again as long as here is pressure in he maser cylinder of he braking sysem. The wheel sensor may measure he speed of a single wheel or of a number of wheels. The oupu quaniy is a signal whose frequency or volage is proporional o he wheel speed. This signal is applied o he processing uni, where i is used o predic an imminen wheellock. If necessary, he oupu signal from he processor is made o operae a solenoid valve or oher acuaor o reduce he brake pressure. More han one such acuaor may be used. In mos ani-lock sysems he brake force falls while he solenoid (or oher device) is energized, and increases when he solenoid is swiched off (afer he danger of a wheel lock has passed). Since he passive sae of he pressure regulaor corresponds o normal

2 Philips ech. Rev. 36, No. 3 ANTI-LOCK BRAKING SYSTEMS 75 braking, a cerain measure of fail-safe behaviour is auomaically buil ino he sysem. Ideally he combinaion of sensor conrol circui and acuaor would be fied o each wheel separaely, allowing each yre o have he opimum grip on he road. This would give he driver maximum braking and seering conrol, bu i is cosly. In many exising sysems a single acuaor is used o conrol one axle or one pair of wheels. In such cases he conrol mus be applied o he rear wheels, as he following argumen shows. Alhough an ani-lock sysem will in general reduce he sopping disance, is mos imporan funcion is o preven loss of conrol of he vehicle. A firs sigh i would appear ha i is more imporan o preven direcion. If he driver now keeps he fron wheels poining in a direcion parallel o AB, he vehicle coninues o move wih is long axis parallel o AB unil i comes o a sop. This moion is very unsable, however, and requires grea skill on he par of he driver. Ifhe driver does no aler he posiion ofhe fron wheels, hen small random deviaions will cause he vehicle o move in a direcion oher han AB, e.g. a righ angles o OIFI. The cenre of graviy coninues o move along AB, i.e. a righ angles o OIPI. The vehicle herefore describes a circle, cenred on 0, and sars o roae in a clockwise direcion a an angular velociy direcly proporional o he velociy in he forward direcion and inversely proporional o he radius OIPI. Afer a shor ime he cenre of graviy of he vehicle has moved o P2; he deviaion of he fron wih respec o AB has become larger and he cenre of he roaion has moved o 02. Since 02P2 < OIPI he angular velociy ofhe roaion has increased. In his way he vehicle goes via posiion 3 o posiion 4, where i makes an angle of 90 wih he original direcion. Here he fron is saionary for an insan B Ps P4+=~~== _-"<--.M... P -_"L.k,~ 3 F,...-_....._.-._..-_.._-.-_. OT A Fig.!. Illusraing he behaviour of a vehicle if wheels become locked during braking. he fron wheels from locking, since hese do he seering. (This would also appear o give a shorer sopping disance, since mos of he braking force is provided by he fron wheels owing o weigh ransfer.) This is no so, however: a moving vehicle wih a single pair of wheels locked will evenually ravel in he original direcion bu wih he rolling pair of wheels railing [11. If only one pair of wheels is o be conrolled, i mus herefore be he rear wheels. The behaviour of a vehicle when wheels are locked during braking will be discussed wih he aid of fig. 1. The vehicle is ravelling along he line AB. I is assumed ha only he rear wheels lock, a he insan when he cenre of graviy of he vehicle is a he poin PI. If he long axis of he vehicle now devi~es slighly from AB, he fron of he vehicle sars o move. in he 'direcion deermined by he posiion of he fron wheels, while he cenre of graviy of he vehicle coninues o move in he direcion AB and he locked rear wheels can skid equally in any and he vehicle roaes abou he cenre of he fron axle. In posiion 5 he rear wheels move in fron and he cenre of he roaion is a 05, while he radius OsPs now becomes greaer and he angular velociy decreases. In posiion 6 he vehicle is almos in sable moion again, parallel o AB, bu wih he locked rear wheels in fron. A number of second-order effecs have been negleced in his qualiaive descripion [21. If ani-lock conrol is applied o he rear wheels only, he fron wheels will lock and skid when he vehicle is srongly braked under adverse condiions. If he long axis of he vehicle now [*1 This is he echnical definiion of slip, which we shall use in his aricle. A sliding movemen of a car wih locked wheels will be called 'skid'. [11 H. Darwin and C. V. Buron, Side-slip in moor cars, Engineering, Sep G. Jones, The skidding behaviour of moor vehicles, Proc. Auo. Div. Insn. Mech. Engrs 1962/63, No. 1. [21 Fig. I and he associaed explanaion are mainly aken from: J. Bradley and S. A. Wood, Facors affecing he moion of a four-wheeled vehicle when some of is wheels are locked, Proc. Insn. Auo. Engrs 25, 59-62, 1930/31..

3 76 D. R. SKOYLES Philips ech. Rev. 36, No. 3 deviaes slighly from he direcion of ravel, he fricional forces beween he fron yres and he road ill he direcon of moion wil! produce a orque ha will end o swing he vehicle furher round. However, he ransverse forces on he rolling wheels (he rear wheels here) will se up a much larger orque, ending o resore he long axis of he vehicle o he direcion of ravel. The yre-road inerface Since rubber is elasic, he yre and he read have a cerain flexibiliy, and his has an influence on he fricion beween yre and road [3l. Fig. 2 shows, grealy exaggeraed, he deformaion of he rubber where he wheelconacs he road during braking. As a poin on he yre periphery moves ino he conac pach is effecive disance from he cenre of he wheel decreases and i is a he same ime subjeced o a shear force which disors i. The disorion is a funcion of boh he shear (rearding) force and he weigh acing on he wheel. For a given weigh, here is an insan when he disorion wih increased braking reaches a maximum value a which i is oo grea o be mainained by he fricional drag of he road surface and a his poin sliding of he rubber occurs. Breakaway and consequenly sliding begin firs a he rear edge of he conac pach where he disorion is greaes and he verical conac force lowes. As he rearding force increases, he read disorion increases and he break-away spreads owards he cenre of he pach, where he verical conac force is greaes. Evenually, as he brake pressure increases furher, he shear force ineviably reaches a value which causes sliding even a he cenre [4l. Before his poin is reached he rearding force available is always increasing bu once he sliding area has grown sufficienly o include he cenre of he original conac pach, he force from he gripping rubber drops rapidly as sliding sars, lowering he fricion force from he saic o he sliding value. This decreases he effeciveness of he fron par of he conac pach. The acion is cumulaive: increase of sliding area resuls in sill lower resoring forces and he wheel roaion rapidly degeneraes o he locked condiion. This process is illusraed in fig. 3 where vehicle rearding force and braking pressure are ploed agains ime. The laws of fricion impose upper limis on he maximum reardaion forces available a he yre-road inerface. For a ypical yre on a good, dry level surface a vehicle reardaion of abou 0.8 g is possible. (1 g is he acceleraion due o graviy.) This figure is independen of he yre conac area and of he mass of he vehicle. Fig. 4 shows how he angular reardaion ë achieved on various surfaces depends on he slip [5l. The slip S is' defined by: Fig. 2. Disorion of a yre by he drag of he road when a wheel is braked. C represens he conac pach and he arrow represens he direcion of moion of he vehicle. The dashed lines represen radial planes hrough he cenre of he wheel; he solid lines show schemaically how hese planes are disored when he wheel is braked. Par of he disorion lies in he side wall of he yre and par in he read. The disorion is greaes near he rear of he conac pach. - Fig.3. Variaions of he fricional force beween yre and road surface for increasing brake pressure. The fricional force Fr and he brake pressure Pb are ploed in arbirary unis. A R here is sill complee grip of he road surface; C indicaes he poin where he break-away from he road surface has expanded o include he cenre of he conac pach. The wheel hen locks and Fr decreases o he consan value ha applies for he sliding yre.. where Bw is he acual angular velociy of he wheel and Bv is he angular velociy he wheel would have if rolling freely. Alhough oher measuremens could give resuls somewha differen from hose of fig. 4, i is in general rue ha if wheel slip could be held beween 12 and 15%, a rearding force would be available which (on mos surfaces) would be higher han he rearding force available from a locked wheel..

4 Philips ech. Rev. 36, No. 3 ANTI-LOCK BRAKING SYSTEMS 77 The reardaion-slip characerisics show ha even ligh braking (low reardaion) is accompanied by a cerain amoun of wheel slip. This does no necessarily imply sliding as such, bu each par of he rubber read is disored as i forms a par of he conac pach during braking and each new disorion occurs a he expense of he angular velociy of he wheel. This produces a roaion which is slower han ha of a freely rolling wheel. The higher he peak reardaion raio (= peak reardaion divided by reardaion a S = 100%, see fig. 4), he greaer he increase in sopping power given by a good ani-lock sysem. On we slippery surfaces he peak reardaion raio of he slip characerisic (fig. 4) depends on he abiliy of he yre read o break hrough or squeeze ou he moisure film beween rubber and road. Since his is more difficul a high speeds, he peak of he slip characerisic has a value which decreases wih speed. I is also very dependen on yreread design and surface exure. A worn yre wih a read of less han 1-2 mm has inadequae drainage [6]. and he braking force on a smooh we surface is herefore less han ha wih a full read. A rough road surface allows improved drainage and he difference beween a worn and an unworn read is less marked. Experimens have shown ha read is unimporan on normal dry surfaces, where even bald yres give excellen performance. A good, dry road surface offers higher road adhesion and higher braking orques are possible wihou he danger ofwheellock. Fig. 5 shows some slip characerisics for various road surfaces. The rearding orque given by he road-yre drag is ploed (insead of he reardaion) as a funcion of he slip S. When he high-slip region of he orque-slip curve is nearly horizonal, he rearding force of he locked wheel is no grealy differen from he peak braking orque near 15% slip. In his siuaion he reducion in brake pressure ha he ani-lock device mus provide o allow he wheel o recover o he sable side of he curve mus in fac be less han in he case of a seeper slip characerisic. The way in which he ani-lock device reduces he brake pressure will be discussed in he nex secion. Gravel surfaces and loose snow behave anomalously (see figs. 4 and 5) because he maerial piles up in fron of he wheels. T,- Fig.5. Braking orque Tr available on various surfaces as funcion of he slip S. 1 good surface, 2 gravel (he hump is due o piling up). If T; is he braking orque a which he wheel sared o lock on a good surface, a small redneion I5T o Tr will reurn he wheel o a safe braking condiion. The same orque reducion would be inadequae for a more slippery surface, as can be seen from curve 3. ij I I % 75 -s Fig. 4. Reardaion-slip characerisics of various road surfaces, showing ha he peak reardaion is mosly aained for a slip of beween 10 and 15 %. 1 dry asphal, 2 we asphal (hin waer film), 3 we asphal (hick waer film), 4 fresh snow, 5 packed snow,6ice. 5 6 Principles of ani-lock sysems Before discussing he principles on which an anilock sysem can be designed, i is imporan o recognize he surprising fac ha one essenial piece of informaion - he rue road speed of he vehicle - is no readily available. As we sawearlier, he peripheral speed of he wheel is only equal o he road speed if here is zero slip. As soon as slip appears, he wo speeds differ. The wheel speed can easily be measured by a ransducer a he wheel. To deermine he slip, however, he acual speed relaive o he road mus be known. Apar from such obvious mehods of measuremen as [3] G. Temple, The dynamics of he pneumaic yre, Endeavour 15, , [4] K. N. Chandler, Theoreical sudies in braking, Par I: Effecs of longiudinal slip for a single wheel, Proc. Auo. Div. Insn. Mech. Engrs 1960/61, No. 4.. [5] P. Müller and A. Czinczel, FISITA 14h Congress, London 1972, p. 3/92. [6] G. C. Saughon, The effec of read paern deph on skidding resisance, Repor of he Transpor and Road Research Laboraory LR 323, 1970.

5 78 D. R. SKOYLES Philips ech. Rev. 36, No. 3 a fifh wheel (no accepable for privae cars), or more complex mehods such as Doppler radar (oo expensive), here is no simple way of doing his. The roadspeed informaion necessary for ani-lock conrol - or some approximaion o i - mus herefore be exraced in anoher way. An esimae of he minimum road speed a any given insan can be derived from he wheel speed a he momen he brake is applied and an assumed maximum aainable deceleraion of he vehicle. This can be convenienly obained elecronically by using a volage proporional o he wheel speed and a circui in which he maximum deceleraion is simulaed. This circui gives a reference volage proporional o he esimaed road speed. The volage corresponding o he insananeous wheel speed is now compared o he above reference volage. If i is less han 85 % of his value his means ha he slip of he wheel is excessive. The bes possible braking would be obained if he brake pressure could be always held a he value coinciding wih he maximum rearding force available a each insan from he road (see figs. 4 and 5). If he wheel is held a his 'sae of maximum adhesion' he maximum laeralor seering force is in general obained [71. In pracice he opimum brake pressure canno be used because.regulaing he pressure o his (changing) opimum value requires he rue slip of he wheel o be known accuraely a every insan. I migh appear ha a good compromise would be o regulae he brake pressure a a level lying a a more sable poin jus below he peak adhesion region of he reardaion-slip curve (fig. 4). However, his also urns ou o be impracicable because of he variaion in road-surface fricion, and here are also problems of long-erm sabiliy. In pracice he pressure is herefore 'modulaed' abou he opimum, i.e. cycled abou he poin of maximum adhesion, high slip values causing a reducion of mean pressure. This allows for varying surfaces and changes of weigh beween he wheels. Since lower slip values arise repeaedly, a fairly simple mehod can be used for supplying he elecronic conrol module wih regularly updaed compued informaion abou he wheel slip: his informaion is essenial for deermining he correc insans for he relaxaion and re-applicaion of he brake pressure. For a level-force-slip characerisic (fig. 5, curve 1), reducion o below he locking pressure may appear o provide a braking force lower han ha of a locked wheel bu his is no necessarily so. The pressure may be modulaed abou a value well below he opimum pressure, wihou any reducion in he average vehiclerearding force. Wheel ineria plays an imporan par here. The following saemen may help o clarify his: if he vehicle is moving fas wih all wheels locked owing o excessive brake pressure and he brake pressure is insananeously reduced o zero, full 'braking' is mainained unil he wheels have been acceleraed (by he road) o abou 85 % of he vehicle speed (i.e, abou 15 % slip). The rearding f~rce afer zero brake pressure is produced here no by brake orque bu by wheel ineria. The sum of he orques acing on he wheel is zero: Tr + Tb + ë; = 0, where Tr is he road orque, Tb is he brake orque, 1 is he momen of ineria of he wheel and ë; is he angular acceleraion. When he brakes are applied, he sored energy of he vehicle is reduced bu so is he sored energy of he wheels as hey decrease in speed. When he brakes are released he acceleraion of he wheels by he road is associaed wih an inerial reacion orque Q 9 IJ Fig. 6. a) The angular velociy Ö of he wheel, ploed as a funcion of ime. ev represens he vehicle road speed expressed in erms of he angular velociy of a (hypoheical) freely roaing wheel. For < o he speed is consan; a = o he brakes are applied. The deceleraion ij~ of he vehicle, which is given by he slope of he curve ev, hen changes from (jv = 0 o ëv = -K, where K is "assumed consan. The maximum value of K in pracice is found o be abou 0.8 g. When he brakes are applied here is always some slip. The curve ë; shows how he wheel speed would vary in he ideal case, i.e. braking such ha he slip remains beween 10 and 15% (see he peak reardaion in fig. 4). b) Variaions of he rue wheel speed Ow. The upper dashed line is he angen o he curve iw a a poin where he slope ijw corresponds o a deceleraion of 1.5 g. This line is he velociy reference. The lower line is drawn parallel o he upper line and below i such ha he disance S' represens a 5 % slip wih respec o he velociy references. This implies ha a he ime "l he deceleraion ofhe wheel is well in excess ofhe reference a 1.5 g; a his poin he solenoid valve is energized. c) The variaion of he brake pressure Pb wih ime, as conrolled by he energizaion of he solenoid valve. (The shape of his curve is discussed laer in his aricle, wih he consrucion of he valve.) A Ol he solenoid valve is de-energized. '02' 1 02 ec. are subsequen cycles of operaion.

6 Philipsech, Rev. 36, No. 3 ANTI-LOCK BRAKING SYSTEMS 79 lij',. which has he same sense as Tb. The energy required o build up his orque is exraced direcly from he kineic energy of he vehicle. Many ani-lock conrol devices make use of he wheel ineria effec; i is a feaure which grealy faciliaes he realizaion of pracical sysems. When he conrol circui deecs a poenial wheel lock, a solenoid valve is opened o relieve he brake pressure. Afer a cerain ineviable delay, he grip of he brake on he wheel will have decreased sufficienly for he drag force of he road o re-accelerae he wheel: he slip herefore sars o decrease again. When he slip has been sufficienly reduced, he brake pressure can be increased again by closing he solenoid valve. The whole process - deecion of excessive slip, opening of solenoid valve and closing i - is now repeaed as shown schemaically infig. 6a, band c. The brake pressure and he slip are hus coninually cycled abou he poin of maximum adhesion (fig. 4). The firs energizaion of he solenoid valve akes place jus afer he wheel has been deceleraing a a rae greaer han ha corresponding o a vehicle deceleraion of 1.5 g (well above he maximum vehicle deceleraion aainable). Acual swich-on occurs when \/" 9, (Vó(')~ Vc-Vö () RC Fig. 7. a) Elemenary circui for he comparison of he wheelspeed sensor volage wih an arificial reference corresponding o a wheel deceleraion of 1.5 g. The oupu volage Vó from he wheel sensor (a volage generaor) is proporional o he wheel speed, VB = k{jw. The capacior is charged o his volage via he diode bu he charge leaks away a a rae deermined by CR. Rand C are chosen such ha he charging rae corresponds o a deceleraion of 1.5 g. b) Variaions of he volage Vó from he wheel sensor. As soon as he wheel deceleraion is lower han 1.5 g he volage Vc across C is equal o Vó, buifhe deceleraion exceeds 1.5 g (a ime '), Vó falls below Vc. A sensing circui gives a signal if he volage Vs becomes negaive, i.e. if VB falls below 0.95 Vc (he line VO.05).This signal is used o energize he.. ani-lock solenoid (a he ime el)' The slip is hen abou 5 % 'greaer han i was when he reference was firs exceeded. The energizaion is removed again a he ime Ol when VB again becomes larger han VO.05. he wheel slip amouns o more han 5% relaive o 'he reference speed corresponding o a deceleraion of 1.5 g (hence a ime el in fig. 6b). This is no he ideal insan for he sar of he energizaion, bu he acual slip will be greaer han 5 % since vehicle deceleraion will always be far lower han 1.5g, and he vehicle speed will always be above he compued reference. The slope of he ideal curve {Jw in fig. 6a corresponds o a wheel-speed conrol such ha here is always 15 % of slip, i.e. such ha {Jw remains coninuously 15 % below he curve ev. I migh appear o be beer o wai unil he wheel speed had dropped o he value corresponding o 15% slip (maximum adhesion) before energizing he solenoid o relieve he brake pressure, bu in pracice i is more advanageous o energize as soon as possible in he case of rapid brake applicaion (slow applicaion allows he desired slip value of 15 % o be reached). In addiion, i is more convenien o operae wih a single slip seing for he wheel sensor (he apping in fig. 7a, see below). When he slip of he wheel begins o decrease (re-acceleraion o he vehicle speed) he brake pressure has o be re-inroduced sufficienly slowly o enable he wheel o reach he sable side of he curve o he lef of he peak in fig. 4). A repeiive pressure-cycling acio'n is hen possible. If he sable side of he curve were' no reached, he conrol circui would no ge enough informaion o assess he slip and he wheel would lock. Fig. 7a gives a highly simplified example of a circui in which he arificial reference volage corresponding o a vehicle deceleraion of 1.5 g is compared wih he volage from he wheel-speed sensor. This volage Vil, which is always proporional o he wheel speed Ow, is applied o he capacior C via a diode. A resisance R is conneced across he capacior, giving a consan RC corresponding o a deceleraion of 1.5 g (RC is usually greaer han he cycle ime fe2 - el): ( ) RC Vo RC V = Vo e- / CR R:I Vo 1- - = Vo - -. The volage across R canno herefore fall faser han V = vsmc, see he line Vc in fig. 7b. a volage Vil = kow is For a given wheel speed Bw, generaed. The capacior is charged o his volage, which herefore appears across R. On sudden braking, he wheel velociy can fall faser han he fixed rae V = Vo/RC, which is made o correspond o a vehicle deceleraion of abou 1.5 g. Whenever his happens we know ha ë w has a value larger han ha corresponding o 1.5 g. The solenoid valve is energized if he volage V ö falls below he volage a a apping on R corresponding o 0.95 R (a he ime el' if he curves Viland VO.95 inersec). This will occur a a slip value 5% in excess of he reference line. When he speed of he wheel has increased again such ha Vö > VO.95 (a ime Ol)' he solenoid valve is de-energized, and he [7J K. E. Holmes and R. D. Sone, Tyre forces as funcions of cornering and slip on we road surfaces, Repor of he Transpor and Road Research Laboraory LR 254, 1969.

7 80 D. R. SKOYLES Philips ech. Rev. 36, No. 3 whole cycle is repeaed again if required. The circuis used in pracice are more complex o allow for a consan slope a all wheel speeds and also o accommodae all kinds of delays and second-order effecs. An acuaor for an ani-lock sysem The acuaor used has he form shown in jig. 8. I consiss of a solenoid valve, a variable resricor and a pump. The pump is driven direcly by he wheel. The acuaor was designed wih he seven following aims in view: I should be a small compac uni which can be housed wihin he wheels (no vulnerable pipes or signal wires across he vehicle, poenial sources of failure). The acuaor should no affec he normal operaion of he brakes. The insan a which he brakes are reapplied should depend, on he decrease in slip during an ani-lock period and also on ani-lock behaviour during previous cycles. The brake pressure should no be allowed o fall so far ha under-braking resuls when he wheel is freed from lock. Fail-safe arrangemens should be included in case of circui or wiring failures or oher fauls. There should be a single solenoid valve for brakepressure conrol. The conrol sysem should be compleely independen of he driver. Incipien wheel lock causes he conrol circui o open he solenoid valves and hence reduce he brake pressure. When he wheel has recovered o a safe condiion (lef-of-peak in fig. 4) he solenoid is closed and he fluid ha has flowed from he brake cylinder B is reurned o he pressurized maser cylinder M. If his were no done he drivers' foo would coninue o move owards he floor in an effor o mainain he pressure in a 'leaking' sysem. A he same ime he pressure in he brake cylinder increases again. To obain he maximum braking his should happen as quickly as possible; he rae of pressure build-up should no be affeced by. facors oher han he behaviour of he wheel. The variable resricor On a good road surface he wheel can recover speed rapidly because of he high fricional drag of he road on he yre. For his reason a higher rae of pressure recovery is required on a good surface han on a poor surface. This is arranged by. designing he acuaor in such a way ha he pressure-recovery rae is a funcion of he oal ime for which he solenoid has been energized. This is done by he variable resricor V (fig. 8). The longer he solenoid is opened (poorer surface) he more he cylinder D wih he plunger h is driven o he righ and he slower he build-up of pressure in he brake afer he solenoid has closed again. On a good road surface he wheel recovers is speed very rapidly afer a high slip: he solenoid is only energized for a shor ime and he plunger hardly Fig. 8. The ani-lock braking sysem. The acual braking sysem consiss of he maser cylinder M and he brake cylinder B ha operaes he brake of a wheel. The main pars are a solenoid valve S which relieves he brake pressure when he wheel is abou o lock, a variable annular resricor Vwih plunger Ji, which deermines he rae of pressure build-up in he brake afer he solenoid valve has closed again, and a pump P which reurns divered brake fluid o he maser cylinder. The pump can be locaed in he wheel and driven by i. F is a fail-safe conrol valve which closes in he even offauly operaion of S. R is a reservoir for he divered brake fluid; his brake fluid displaces he pison D, so ha he quaniy of fluid in R deermines he amoun of closure ofhe resricor V (see also fig. 9). his a second plunger mouned on D, which modifies he rae of brake-pressure recovery. Fig. 9. Pressure in he brake cylinder decreasing as a funcion of ime, and is dependence on he solenoid energizaion ime. e represens he insan a which he solenoid is energized. There is firs a sligh delay -,; before he pressure begins o drop (a few milliseconds). If he energizaion coninues for a long ime he pressure can drop o pmin. A ypical energizaion lasing for a ime (ra - e) allows a build-up of pressure a he rae indicaed by he angle r afer he solenoid has closed. A shorer energizaion resuls in a more rapid pressure build-up. Longer energizaions are accompanied by a slower increase unil e is exceeded. A his ime he reservoir R (fig. 8) is full and pressure would be resored, in he absence of pump P, even if he solenoid remained energized. If, however, he pump was operaing, all fluid would be reurned o he maser cylinder and no pressure build-up would be possible unil he solenoid was de-energized.

8 Philips ech. Rev. 36, No. 3 ANTI-LOCK BRAKING SYSTEMS 81 peneraes he opening, so ha he brake pressure can build up sufficienly rapidly o keep up wih he rapid recovery of wheel speed. This is illusraed infig. 9. The variable resricor canno be designed o provide he opimum rae of pressure rise for all kinds of road surfaces, because is resricive effec depends on facors oher han he posiion of D (fig. 8), which are deermined by he amoun of brake fluid in he reservoir R; he pressure rae also depends on he viscosiy, for example. However, he variable resricor allows high raes of pressure recovery for good surfaces wihou compromising he performance on a poor surface. Also, because he closure of he resricor depends on he posiion of D in he reservoir, he conrol becomes self-correcive, i.e. if he wheel under ani-lock conrol on a poor surface suddenly encouners a good surface, he resricor is rapidly wihdrawn since he fluid is quickly pumped from he reservoir and here is no furher replenishmen via he solenoid. The resricor adaps rapidly o he new condiions and allows a higher rae of pressure recovery. Conrol of he brake pressure in he manner described relies on he fac ha he slip decreases o a lower value han he opimum (15 %, see fig. 4). Any sysem in which he vehicle speed is no acually measured or compued has o rely on his. If for any reason he slip does no decrease from he original high value o a value in he sable region, he conrol sysem will measure oo high a value of slip. However, under hese circumsances he variable resricor exends he conrol logic in he sense ha each solenoid energizaion ensures a succesively lower rae of pressure build-up (more and more closure of he resricor) unil he slip does become sufficienly small. Alhough in his mehod of slip reducion he slip emporarily exceeds he opimum value, his is preferable o a complee lock of he wheel. The variable rae of pressure build-up is very imporan and eases he ask of he elecronic conrol uni. The resricor plunger 11 is compleely wihdrawn for normal braking. When fluid has been divered from he brake via he solenoid valve o he reservoir (R, fig. 8), he brake pressure exercises pressure on he fluid in he reservoir via he plunger. When he reservoir is nearly empy, he oule o he pump is parially blocked by he plunger Jz. A his poin he plunger 11 akes up a posiion such ha when ani-lock conrol has ceased he brake fluid can flow ino he brake cylinder a he maximum rae for good surfaces. The rae a which his happens increases as he reservoir finally empies, despie he damping acion of he plunger J2. The maximum ime for he wheel o regain speed during an ani-lock cycle is in he region of 0.1 s; his is shor compared wih he ime aken for 11 o reach he open posiion when ani-lock conrol has ceased. The rae of increase of he brake pressure afer an ani-lock energizaion of he solenoid valve is conrolled mainly by he resricor bu j also depends on he viscosiy of he brake fluid, he pressure in he maser cylinder, he brake pressure, he capaciy of he brake cylinder, he compression of he brake linings, he spring pressure of he brake calipers, he cross-secion of he plunger and is radial clearance (referred o above qualiaively as he closure). The flow rae of he brake fluid hrough he resricor is given by ndg3 q = 121]1 /::"p, where i is he volume flow in m3/s, d he diameer of he resricor in m, g he annular gap in m,!1p he pressure difference in Nfm2, 1 he lengh of he resricor in m and 1] he viscosiy in Ns/m2 The formula shows ha he flow rae is very sensiive o changes in he clearance of he resricor plunger. Very close olerances are herefore required. The viscosiy of brake fluid a 20 DCis en imes he viscosiy a 100 DC and a enh of he viscosiy a -20 DC. Changes in viscosiy can be compensaed by using maerials wih differen expansion coefficiens for he resricor housing and plunger; his gives an annular clearance which varies linearly wih emperaure. The change in flow rae is hen proporional o he square of he emperaure bu he variaion 'in viscosiy wih emperaure is logarihmic. The compensaion can herefore never be perfec and furher compensaion may be necessary for opimum operaion a exreme emperaures. The pump The pump (P in fig. 8), driven direcly by he wheel, mus have he following characerisics: a) Even a low road speeds i mus be able o reurn all he fluid o he maser cylinder, i.e. i mus be able o rapidly empy he reservoir when he solenoid valve is closed. b) A high speeds and low brake pressures he pumping efficiency should fall o a low value. The firs feaure ensures ha he ani-lock device operaes well even a low speeds. The second feaure ensures ha he wheels have sufficien ime o regain speed even afer locking a high speed and low brake pressure. This feaure is forunaely presen in mos simple pumps. In a ypical case a simple plunger pump may be used, he plunger being pushed agains a shallow cam (which revolves wih he wheel of he vehicle) by he brake fluid in he reservoir (R in fig. 8). A low brake pressures he pump plunger canno move rapidly because he passage beween he reservoir and plunger acs as a resricor. Also, a high speeds he pump plunger will be unable o follow he cam compleely. The rae of pumping Q is herefore conrolled by he resrieion o he pump inle. This is' shown in fig.j). The effec of he pumping acion is o reduce he amoun of fluid in he reservoir R and hence give a displacemen o he lef of he plunger Jl of he variable resricor V. The rae of flow of'fluid from maser cylinder M o he brake cylinder B herefore increases

9 82 D. R. SKOYLES Philips ech. Rev. 36, No. 3 as pumping coninues and his enables he sysem o adap o a good surface afer having been on a poor surface. High vehicle speed would heoreically give scavenging (empying) of R a oo high a rae (he scavenging rae is proporionalo he speed) bu he resricive effec of JI and he connecion beween R and P (fig. 8) is sufficien o limi his o a suiable value wihou he necessiy for any addiional impedance. d - ê(rad/s) Fig. 10. Illusraing he pumping rae Q as a funcion of wheel speed e, a various posiions of he plunger J2 (fig. 8). Curves J, 2, 3 are for high and medium brake pressures, showing he pumping raes Q aainable when he plunger Jz permis full flow. The line Qm represens he maximum pumping rae when he plunger does limi flow. Curves 4 and 5 represen he raes for low brake pressures. The solenoid valve When no energized he solenoid valve is closed. A pressures up o abou knfm 2 (3000 p.s.i., 200 am) i compleely prevens he flow of fluid. When energized i mus release fluid a a rae which permis a brake-orque decay rae of abou 2100 Nm/s and, equally imporan, when de-energized i mus inerrup and sop he flow of fluid very rapidly. The operaing curren for he solenoid has o be provided by he car baery and in pracice his means ha he maximum curren is abou 10 A. The armaure pull-in ime should be below 7 ms a his curren and any reducion of his ime improves he braking performance. The ani-lock sysem described here uses a solenoid wih a flow conrol which allows an iniial rapid drop of fluid pressure corresponding o a decrease in brake orque of 180 Nm, followed by a linear fall rae corresponding o Nmfs (mached o he brakes ofhe es vehicle). If he iniial drop in pressure alone deermined he 'modulaion deph' (he relaive decrease in. brake pressure, see fig. 6c) his would give rise o ± 17 % of modulaion on a dry surface (coefficien of fricion p, R:! 1) and ± 100% modulaion on a slippery surface (p, ÄJ 0.15).,!he large modulaion deph is necessary on slippery surfaces for he wheel o recover ~ is speed wih a reasonable acceleraion (abou 4 g). The maximum heoreical acceleraion is abou 7 g bu only abou 4 g is usually available on a slippery surface, because here is always some residual brake pressure due o seal resilience. The fixed iniial drop in pressure a el in fig. 6c provides an effeciveand simple way. of obaining he smaller pressure modulaion required for good road surfaces. I would be difficul o conrol a fas pressure drop accuraely by elecronic mehods, on accoun of he delay ha would occur beween he signal from he wheel sensor and he reacion from he acuaor. The linear drop in pressure (el o Ol in fig. 6c) is inroduced parly o reduce he effec of ransducer and circui delays bu mainly o reduce he urn-off deláy of he solenoid iself. The operaion is as follows. In he much simplified diagram of he solenoid valve shown in fig. 11 i can be seen ha he pison G around he armaure conains a number of orifices H. The pressure drop across an orifice is proporional o he flow, and his 'flow' force ends o close he valve a E, while armaure curren I will end o open i. A consan flow resuls, bu his can be alered by change in 1. Since he valve a E is always parially closed, if I drops sharply o zero, he flow is quickly sopped. The acion described above comes from he ineracion of various forces. Wih he valve opened here is an equilibrium of forces if paaa - (PI - P2)A2 - Fs + FA = 0, where Fs represens he sum of he spring forces and FA he magneic force exered on he armaure; he oher quaniies are shown in fig. 11. Neglecing he small erms paaa and Fs, we find ha he armaure force FA required is The force exered on he armaure in a coil is proporional o he square of he curren I, i.e. FA = KI/2. The pressure difference beween he wo sides of he pison G is proporional o he rae of flow of brake fluid hrough he orifice H: PI-P2 = K2q. I is his flow ha provides he linear drop in pressure referred o above. The equilibrium equaion for he forces is.hen showing ha he flow rae q is conrolled only by he curren, since all he oher facors are consans.

10 / Philips ech. Rev. 36, No: 3 ANTI-LOCK BRAKING SYSTEMS 83 P (via R) P2 PI G A 51 W Fig. 11. The solenoid valve (S in fig. 8. Fluid eners from he brake cylinder B via valve F). W solenoid windings, A mild-seel armaure. E main flow orifice. G concenric pison conaining orifice H. CO-ring leak proecion. Sl, S2 and S3 compression springs. Pl is he pressure o he righ of pison G, P2 he pressure o he lef of G, Pa he pressure in he exi line o he reservoir R and pump P. The effecive area of G is A2, and he area of he exi line is A3. 5 I -î-b Fig. 12. Fail-safe flow-conrol valve (F in fig. 8). Brake fluid eners a B from he brake and leaves a S for he solenoid. During he firs momens of braking before ani-lock funcioning of he solenoid can begin any flow hrough he fail-safe valve mus be due o fauly opening of he solenoid valve. The pressure drop produced across A by he flow drives he pison o he lef, blocking furher fluid flow. If, however, here is no faul, A mus be cleared from he bore before he expiry ofa cerain ime which depends on he brake pressure (see ex and fig. 13). Al crosssecion of pison rod. A2 area of pison, g widh of annular gap, d diameer of A, I lengh of channel C wih pison in equilibruim posiion (zero brake pressure), deermined by spring Ss, Fig. 13. Brake pressure ps as a funcion of ime for hard braking H and ligh braking L. The shaded areas are equal. Their magniude deermines he ime wihin which he wheel has deceleraed so far ha he solenoid valve is energized (he wors case is when he road is so slippery ha he brake pressure only has o overcome he ineria of he wheel). The solenoid valve should herefore be esed wihin his ime for correc operaion, and any remedial acion aken. If no faul is found he fail-safe valve mus be opened before 2 (for hard braking) or : (for ligh braking).. H Safey feaures As menioned earlier i is imperaive ha he anilock equipmen should no increase he chance of he brakes failing in normal operaion. Two fail-safe proecions are herefore included: he firs allows for he condiion where here is a permanen leak hrough he solenoid valve, and he second akes care of he dangerous siuaion which would arise if he solenoid for some reason became permanenly energized. The undesired consequences of a leak are easily avoided by including an O-ring seal in he sealing face of he armaure of he solenoid valve (C in fig. 11). A ligh spring S2 biases he armaure o he lef' so ha he O-ring makes good conac wih he face. If here is a leak hrough he seal E he pressure inside he space sealed off by he O-ring is low and he brake pressure acing on he back of he armaure presses he armaure o he lef wih a force ha exceeds he force available from he solenoid. This of course inhibis any ani-lock conrol in he even of emergency braking bu i does allow normal braking in spie of solenoid leakage. When here is no leak, he armaure is free o move o he righ when he solenoid is energized. The second and mos imporan proeeion is provided by a hydraulic flow-conrol valve, illusraed' schemaically infig. 12 (F in fig. 8). I would be no use relying on elecrical proeeion (swiching off he power), as his would no be effecive in he case of some ypes of shor-circui or in he even of a fauly seal or a 'sicking' armaure in.he solenoid valve. The hydraulic conrol valve of fig. 12 deecs fluid flow due o fauly operaion or failure of he solenoid valve. Only under one special se of circumsances can flow due o his cause be disinguished from flow due o normal ani-lock operaion. This siuaion arises a he onse of braking, before i is possible even heoreically for he ani-lock mechanism o have been called ino use. Wheel ineria plays an essenial role here, since a cerain brake pressure will always be required o reduce wheel speed even if he wheel is no in conac wih he ground. This means ha a cerain value of he pressure-ime inegral (fig. 13) mus have been reached before he ani-lock mechanism can be called ino operaion. During his sage of inerial braking, herefore, any flow of fluid hrough he solenoid valve mus be due o a faul. This flow is used o drive he pison A (fig. 12) o he lef in order o sop any furher flow. If he sysem has no' failed, he assembly mus bé driven o he righ before he pressure-ime inegral has exceeded a given value in order ha A may be cleared from he bore so ha he solenoid can perform is ani-lock funcion if required. The pison is driven o'

11 84 ANTI-LOCK BRAKING SYSTEMS Philips ech. Rev. 36, No. 3 he righ when he brake pressure acing on A overcomes he spring force. When here is zero leakage a he solenoid valve, he ne force on he pison A driving i o he righ is PbA1 - Fs. This is approximaely equal o PbA1 if he 'spring pressure Fs is low. Movemen of A is opposed by he viscous flow q of fluid hrough he annular channel C: _ :n;g3d I:!. q - 121]x p, where d is he diameer of he pison, g he gap widh, and x is he remaining lengh of he channel (he iniial equilibrium lengh, when here is no brake pressure, is I). The pressure difference!lp across he channel is equal o he excess hydraulic pressure se up in he fluid o he righ ofhe pison by he force acing on he lefhand face, i.e. Since he spring force Fs «PbA1 and Al «A2:!lp The velociy of he pison is hen RJ PbA1/A2.. q q :n;g3d PbA1 :n;1/2g3pba1 x = A2- Al RJ A2 = 121]x A22 = 61]x A23/2 ' where dis expressed in erms of A2. The ime -c for A o clear he bore C, i.e. for he fail-safe valve o be pu ou of acion, is hus given by he equaion from which i follows ha: o :n;1/2g3al.t - f xdx = 6A23/2 J pbd!, 11]0 f T _ 31]A 2 3/2[2 Pb d!- 1/2 3A. o :n; g 1 The value of he inegral mus be less han ha corresponding o he ime for he wheel o be braked o he chosen slip crierion (;;;. 10 % in our example) under adverse condiions. This value depends on he momen ofineria ofhe wheel and is given by Kbl!lÖ, where!lo is he permied velociy change corresponding o he chosen slip (> 10%) and Kb is a consan relaing he braking orque o he braking pressure. Provided he value of he inegral saisfies he requiremens, he fail-safe valve is cleared well before he ani-lock acion could be called on o ake place. The ani-lock acuaor described here, originally buil for esing he elecronic circuis of ani-lock sysems, has worked well in field rials. Laer versions have been consruced wih flow conrol via edge orifices insead of annular channels o give a more viscosiy-independen behaviour. Some of hese acuaors, incorporaing many of he feaures described here, are now under furher developmen elsewhere. Summary. Descripion of an elecro-hydraulic brake-pressure conrol, developed for esing elecronic circuis for ani-lock braking sysems for road vehicles. Afer an inrodueion o he problem and a shor descripion of he conac beween he yre and he road surface on braking, a survey is given of he basic principles of ani-lock sysems. Such a sysem consiss of hree main pars: a sensor for he angular velociy of he wheel, a processing uni, and an acuaor ha, if required, can reduce he brake pressure and increase i again (his has since been modified and improved). The acuaor includes an elecronically conrolled solenoid valve, which reduces he brake pressure if wheellock is imminen by causing brake fluid o flow ou of he brake sysem, a pump ha reurns he fluid o he maser cylinder, and a variable resricor, which ensures ha he rae a which he brake pressure increases again depends on he ype of road surface. Various fail-safe arrangemens are included o give rouble-free operaion: if a leak occurs in he solenoid valve, or even if he valve says open, normal braking is reained. Volume 36, 1976, No. 3 pages Published 1s Sepember 1976