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brake11 OTHER ABS ACTUATORS.pdf

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BRAKE11 OTHER ABS ACTUATORS
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1. Describehowthesolenoidcontrolledpowersteeringfluidcontrols brakepressureintherearwheelABSsystem. 2. Describethefunctionofthemechanicalvalveinthe3-position solenoidandmechanicalvalveactuator. 3. Describethethreecontrolpositionsofthe3-positionsolenoidin maintainingABSoperation. Section 11 OTHER ABS ACTUATORS Lesson ObjectivesSection 11 154 LEXUS Technical Training ToyotausesseveraltypesofABSactuators,eachdiffersinhowthe modulationofpressureisaccomplished.Thefunctionofsensorsand ECUcontrolalreadydiscussedinSection8,donotdiffer. The3-positionsolenoidvalveusesa3-positionvalve,electricalcoiland checkvalve.Ascurrentflowsthroughthesolenoidwindings,itcreates amagneticfieldaroundthe3-positionvalvecausingittomovetoward thecenterofthecoilcompressingthereturnspring.Currentfromthe ABSECUisswitchedinthreesteps;0amps,2ampsand5ampsin ordertocontrolthestrengthofthemagneticforceinthecoil. Therearefour3-positionsolenoidvalvesintheABSactuatordescribed here;thoseforthefrontwheelscontroltheleftandrightwheels independently,whilethosefortherearwheelscontrolboththeleftand rightwheelssimultaneously.Thesystemisthereforeknownasa three-channelsystem. 3-Position Solenoid Actuator The 3-position solenoid valves for the front wheels control the left and right wheels independently, those for the rear wheels control both left and right wheels simultaneously. Other Actuator Designs 3-Position Solenoid TypeActuator Types DuringnormalbrakingABSisnotactivatedandthe3-positionvalveis pusheddownbyareturnspring.Thesolenoidinlet,portA“,remains openwhiletheoutlettothereservoir,portB“remainsclosed. Whenthebrakepedalisdepressed,brakefluidpassesfromportA“to portC“,tothediscbrakecylinder.Brakefluidispreventedfrom flowingintothepumpbytheNo.1CheckValvelocatedinthepump circuit. Whenthebrakepedalisreleased,thebrakefluidreturnsfromthedisc brakecylindertothebrakemastercylinderthroughportC“toport A“andtheNo.3CheckValveinthe3-positionsolenoidvalve. Normal Braking With zero amps applied to the solenoid, port A is open, pressure is applied to the brake cylinder. Operation During Normal Braking (ABS Not Activated)Section 11 156 LEXUS Technical Training WhentheECUdeterminesthatawheelisabouttolockup,itswitches totheholdingmodetostoptheincreaseinhydraulicpressure.Asthe pressureinsidethediscbrakecylinderisreducedorincreased,andthe speedsensorindicatesthatthespeedisatthetargetlevel,theECU suppliesa2amperesignaltothesolenoidcoiltoholdthepressurein thediscbrakecylinderatthatlevel. Whenthecurrentsuppliedtothesolenoidcoilisreducedfrom5 amperes(inthepressurereductionmode)to2amperes(inholding mode),themagneticforcegeneratedinthesolenoidcoilalsodecreases. The3-positionsolenoidvalvethenmovesdowntothemiddleposition bytheforceofthereturnspring,closingportB“. WiththeECUholdingPortAclosed,andpedalpressureclosingcheck valves#1&#3,brakecaliperpressureholdssteady,andcannotbe increased. “Holding” Mode Two amps applied to the solenoid, Port A and Port B are closed, pressure remains constant. “Holding” ModeActuator Types Whenthewheelsareabouttolock,theECUsuppliesa5amperesignal tothesolenoidtocloseportA“andopenportB“inthe3-position solenoid.Asaresult,brakefluidfromthediscbrakecylinderpasses throughportC“toportB“inthe3-positionsolenoidandflowstothe reservoir.Atthesametime,acomputersignalcausesthepumpto operate.Fluidstoredinthereservoirispumpedtothemastercylinder. Theforcedreturnofpressurefromthebrakecalipercircuittothe mastercylinderforcesthepedalupslightly.Thiscausesthedriverto feeltheABSsystemoperation. “Pressure Reduction” Mode Five amps applied to solenoid, port A dosed, port B open, allowing pressure from wheel cylinder to flow to the reservoir. “Pressure Reduction” ModeSection 11 158 LEXUS Technical Training Whenthepressureinthediscbrakecylinderneedstobeincreasedto applymorebrakingforce,theECUstopssendingcurrenttothe solenoidcoil.ThisopensportA“ofthe3-positionvalveandclosesport B“.ThisallowsthefluidinthemastercylindertopassfromportC“ inthethree-positionsolenoidvalvetothediscbrakecylinder.The hydraulicpressureincreaserateiscontrolledbytherepetitionofthe pressureincreaseandholdingmodes. Brakecaliperpressurewillincreaseaslongasthedrivercontinuesto applypedalpressure. “Pressure Increase” Mode Zero amps applied to solenoid, Port A open allowing pressure application at wheel cylinder. “Pressure Increase” ModeActuator Types Thisactuatorisfundamentallythesameasthe3-positionsolenoidtype justdiscussed.Itconsistsofthree,3-positionsolenoidvalves,a mechanicalvalve,pumpandreservoir.Thesolenoidthatcontrols pressuretotherightrearwheelsalsousesamechanicalvalvethat controlspressuretotheleftrearwheel.Thisactuatorwasfirst introducedonthe1994CamryproducedbyToyotaMotor Manufacturing(TMM)inGeorgetown,Kentuckyandlaterinthe Avalon. 3-Position Solenoid and Mechanical Valve Actuator The solenoid which controls pressure to the rear wheels also uses a mechanical valve which controls pressure to the left rear wheel. 3-Position Solenoid and Mechanical ValveSection 11 160 LEXUS Technical Training Themechanicalvalveconsistsoftwosetsofcylindersandpistonsand aplungertolinktheirmovement.PistonAmonitorsthepressurefrom themastercylinderonitsleftsideandmonitorspressuretotheright rearbrakecircuitfromthe3-positionsolenoidvalveonitsrightside. PistonAmovesbasedondifferencesinpressuresincepistonsurface areasareequal. AnymovementofpistonAistracedbypistonBthroughtheplunger.In additionaspringloadedvalveopensandclosesthemastercylinder passagetotheleftrearbrake.PistonBisspringloadedtotheleft whichleavesthevalveunseatedandthefluidpathopen. Mechanical Valve This valve controls pressure to the left rear brake eliminating the need for another solenoid. Thepressureincreasemodeisalsothenormalbrakingpositionfor brakeoperationwithoutABS,asshownintheillustrationabove. Pressurefromthemastercylinderisequaltothepressurecoming throughthe3-positionsolenoidtotherightrearbrakecylinder.Piston AdoesnotmoveandisheldtotheleftbythespringactingonpistonB. Pressureincreasecomesfromthemastercylinderthroughthebrake pedalpressure. Mechanical Valve Construction Pressure Increase ModeActuator Types Whenthe3-positionsolenoidgoesintotheholdposition,itblocks hydraulicfluidandpressureinthecircuitbetweenthesolenoid,the rightrearbrakecylinderandtherightsideofpistonA.Withpressure intherightrearbrakeheld,pressurefromthemastercylinder increases,causingpistonAtomoveright.Thepistonforcestheplunger tomovepistonBtotheright,blockingfluidfromtheleftrearbrake cylinder.Thisactionmirrorsthepressureholdontherightrear cylinder. Pressure Hold Mode The Mechanical Valve causes pressure at the left rear brake to mirror the pressure to the right rear brake. Pressure Hold ModeSection 11 162 LEXUS Technical Training The3-positionsolenoidgoesintothereductionpositionventing hydraulicfluidandpressureinthecircuitbetweenthesolenoid,the rightrearbrakecylinderandtherightsideofpistonA.Pressurefrom themastercylinderincreasescausingpistonAtomoverightforcing theplungertomovepistonBtotherightcompressingthespring.The movementofpistonBincreasestheareavolumeintheleftrearbrake cylinderhydrauliccircuit.Thisactionmirrorsthepressurereductionin therightrearcylinder. Pressure Reduction Mode Pressure reduction is accomplished by the movement of piston B creating an increase in volume to the circuit to the left rear brake. Pressure Reduction ModeActuator Types Thissystemwasanoptionon’90-’94Trucks,4RunnersandT100’s.It representsthegreatestdeparturefromthesystemswehavestudiedto thispoint.Thefollowingsummaryhighlightstheuniquefeaturesof thissystem: w Controlstherearwheelsonly. w Single2-positionsolenoidintheactuator. w Usesadecelerationsensorandasinglespeedsensor. w Powersteeringpressureisusedtocontrolbrakepressure. Rear Wheel ABS Diagram The ECU uses input from the single speed sensor and deceleration sensor to control rear wheel brake pressure. Asingle2-positionsolenoidcontrolsthehydraulicpressurefromthe powersteeringtocontrolbrakepressure.Thissystemusesonlyone speedsensor,mountedontopoftheringgearonthedifferentialcase. Forthisreason,adeceleration(G“)sensorisnecessaryfortheECUto sensethedecelerationratetohelpdeterminethesliprateoftherear wheels. Power Steering Pressure Controls Brake PressureSection 11 164 LEXUS Technical Training Theactuatormodulatespowersteeringfluidpressuretocontrolbrake hydraulicpressurewhentherearwheelsslipwhilethebrakesare applied.ModulationisaccomplishedbytheECUcontrolled two-positionsolenoidbasedoninputfromthedecelerationsensorand speedsensor. Theactuatorismadeupofseveralcomponentseachhavingaspecific function: w Solenoid valveiscontrolledbytheABSECUandmodulatesthe powersteeringpressureonthepressurereductionpiston. w Pressure regulator valveregulatesthepowersteeringpressure inrelationtobrakepressure.Brakepressureactingontheright sideofthevalvecompressingthespringwouldraisepowersteering pressure.(LocatedintheActuator) w Relief valverelievesthepowersteeringpressureifitgetstoohigh intheactuator. w By-pass pistonopensandclosestheby-passvalveaccordingto powersteeringpressure.Itconstantlymonitorspowersteering pressuretokeeptheby-passvalveclosed.Ifthepowersteering pressureislost,theby-passvalveopenstoallowbrakepressureto by-passactuatorcontrol. w Pressure reduction pistonopensandclosesthecutandby-pass valvetodirectthebrakefluid.Changesthebrakepressuretothe rearwheelsbyincreasingordecreasingthevolumeinitsbore. w Cut valve and By-pass valvecontrolsthepathofbrakefluid dependingonmodeofoperation. Actuator Hydraulic Circuit The actuator modulates power steering fluid pressure to control brake hydraulic pressure when the rear wheels slip.Actuator Types Controlofthebrakefluidpressureactingontherearbrakecylindersis carriedoutinthreemodes: w pressureholding. w pressurereduction. w pressureincrease. Therear-wheelanti-lockbrakesystemisnotactivatedduringnormal braking.Inthismodethepowersteeringfluidpressureactson chambersC“andD“pushingboththeby-passvalveandcutvalve towardtheright.Thiscausesthecutvalvetoopenandthenormalport ontheleftsideoftheby-passvalvetoopen. Whenthebrakepedalisdepressed,themastercylinderfluidpressure rises.Thebrakefluidpassesfromthecutvalvetothenormalportin theby-passvalve,andissenttotherearbrakewheelcylinders. Normal Braking Power steering fluid pressure acts on chambers “C” and “D” pushing both the by-pass valve and cut valve toward the right. Operation During Normal BrakingSection 11 166 LEXUS Technical Training Thissystemdoesnothaveaspecificholdingmode:instead,rearbrake cylinderpressureholdmodeismaintainedbytheECUquicklypulsing thesolenoidvalveONandOFFbetweenpressureincreaseand pressurereductionmodes.TheECUmaintainsthepressuretotherear wheelcylinderswithinanarrowrangeasitcontinuestomonitorwheel speed. Holding Mode The holding mode is maintained by the ECU quickly pulsing the solenoid valve ON and OFF between pressure increase and reduction to maintain the brake pressure to the rear brakes. Holding ModeActuator Types Whentherearwheelbeginstolock-up,theECUenergizesthesolenoid coil,generatingamagneticforce.Theplungermovesupwardandport A“closeswhileportB“opens.Asaresult,thepowersteeringfluid actingonchamberD“returnstothepowersteeringreservoir.This causesthepressurereductionpistontomovetotheleft,firstclosing thecutvalve,thencausingthebrakefluidpressure,actingontherear brakewheelcylinders,toaccumulateinchamberE“. Asaresult,thepressurelevelinsidetherearbrakewheelcylinders decreasestopreventwheellocking. Pressure Reduction Mode The solenoid plunger moves upward and port “A” closes while port “B” opens. As a result, the power steering fluid acting on chamber “D” returns to the reservoir. Pressure Reduction ModeSection 11 168 LEXUS Technical Training Whenthefluidpressureintherearbrakewheelcylindersneedstobe increasedtoapplymorebrakingforce,theECUchangestheratioin whichthesolenoidvalveisturnedonandoff.Inthepressureincrease mode,thebrakefluidpressureintherearwheelbrakecylindersis increasedwhilethesolenoidvalveisswitchedonandoffrepeatedly.By extendingtheamountoftimethesolenoidisswitchedoffintheon/off cycle,theamountoftimeportA“isopenandportB“isclosedis extendedandthiscausesthepressureinchamberE“torisegradually. Thecutvalveremainsclosedduringtheoperation.Thepressure reductionpistonismovedgraduallytotherightandincreasesthe brakefluidpressureactingontherearbrakewheelcylinders. Pressure Increase Mode By extending the amount of time the solenoid is switched off in the on/off cycle, the amount of time port “A” is open and port “B” is closed is extended and this causes the pressure in chamber “E” to rise gradually. Pressure Increase ModeActuator Types Intheeventpowersteeringfluidpressureisinsufficient,theBy-pass PistonandPressureReductionPistonmovetotheleftbybrake hydraulicpressure.ThiscausestheCutValveandtheNormalPortof theBypassValvetoclose.WiththeNormalPortclosed,theBy-pass Portisopenallowingthemastercylindertoapplypressuretothe wheelcylinder. IntheeventofamalfunctioninthesignalsystemtotheABSECU,the solenoidrelayisshutOFF.ThespringloadedSolenoidValveallows powersteeringpressuretomovethePressureReductionpistonand By-passPistontotherightopeningtheCutValve.Thebrakesystem operatesasanormalbrakesystemwithoutABS. Fail-safe Mode Allows brake fluid to by-pass the actuator control in the event of power steering pressure loss. Fail-safe ModeSection 11 170 LEXUS Tec
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