BRAKING CONTROL
`
`15.11
`
`command
`
`Bulle
`
`
`Decoy_§
`command
`Bullecommand
`Decay
`Build
`Build
`FIGURE15.8 Typical antilock brake cycling.
`
`1>==a
`
`Wheel speed
`sensor inputs
`
`valves
`
`To lamps
`Stop lamp
`switch inputs~"|
`
`Systen
`monitors
`
`To electric
`motor
`
`
`
`Valt,
`reg,
`
`FIGURE 15.9 Electronic control unit block diagram.
`
`decreasing microcontroller costs have made microcontroller-based electronic control units
`the norm rather than the exception. Although the control units can be either engine compart-
`ment-mounted or passenger compartment-mounted, reduced wiring costs favors the former.
`Also, for enhancedreliability, electronic control units may be either attached to or integrated
`with the hydraulic modulator.
`
`15.3.3 Safety Considerations
`
`Standard automotive brake systems have been developed and refined over the years to be
`highly reliable and safe. Because of its ability to decrease pressure in brakes, an antilock sys-
`tem must be designedusing a disciplined methodology and mustbe rigorously tested prior to
`release for production.
`
`351
`
`
`
`command
`
`Decoy
`
`
`Wheelspeed
`
`Brakepressure
`Battery —
` To solenoid
` Qutput
`circuitry
`Inputcircuitry
`
`
`351
`
`

`

`15.12
`
`CONTROLSYSTEMS
`
`Failure mode and effects analyses
`Failure Mode and Effects Analyses/Fault Tree Analyses.
`(or fault tree analyses) are essential to the proper design of antilock systems. Both system-
`level and subsystem-level analyses need to be performed and fault effects and detection
`assumptions must be tested. No single failure can result in an unsafe condition and,if a fault
`is undetectable in thefield, that fault in conjunction with any other fault must not result in an
`unsafe condition. Because of the complexity of the electronic control units, simulation tech-
`niques are used to test those fault effects in which benchorfield testing is impractical.
`
`Common Design Techniques to Improve Safety. One of the most commontechniques used
`to improvesafety in antilock systemsis to include extensive built-in-test within the electronic
`control unit. Typically, all inputs to the electronic control unit and outputs to the other com-
`ponents of the antilock system are tested for proper signals and loads, respectively, andall
`functions internal to the electronic control unit are extensively tested.
`In addition, redundant processing is commonly used to insure the proper internal working
`of a microcontroller. This may take the form either of identical microcontrollers or of a main
`and a watchdog microcontroller that can inhibit operation.
`In order to ensure inhibition of faulty antilock operation, antilock systems employa relay
`function to remove actuation powerfrom the outputactuators; this function may take the form
`ofa discrete relay or it may be a transistorized circuit. This relay function is a key element of
`the design sinceit affords a secondary methodin which to inhibit energization ofvalves or the
`motor/pump and,therefore, a secondlevel of safety relative to improper antilock operation.
`Figure 15.9 is a typical electronic control unit block diagram.? Inputs are filtered and
`buffered prior to being presented to the microcontrollers for processing. Likewise, the micro-
`controller outputs are buffered/amplified and filtered prior to exiting the electronic control
`unit. In the diagram shown, the main microcontroller is responsible for the majority of pro-
`cessing and control of the outputs; the watchdog microcontroller, as its name implies, is
`responsible for monitoring for proper operation and inhibiting antilock if faults are indicated.
`A characteristic of modern antilock electronic control units is bidirectional communication
`between functional blocks;this is a result of the high level of built-in-test designed into the
`contro] units. For example, the output circuitry may be commandedtotest the solenoid valves
`for proper current draw and conveythetest results to the microcontrollers; similarly, the input
`circuitry may be commanded to perform tests on the sensors and other antilock components
`external to the electronic control unit, and convey the test results to the microcontrollers.
`
`15.3.4 Antilock Control Logic Fundamentals
`
`Due to the complexity of antilock braking and the requirements ofstability and steerability
`as well as good stopping distance, the brake control algorithm is more easily represented as a
`state-space diagram than asa classical proportional-integrative-derivative control scheme.
`A simplified state diagram for a single-channel antilock system is shown in Fig. 15.10. In this
`diagram, a vehicle not braking or decelerating would be in the NORMAL BRAKINGstate. If
`antilock action is warranted,it is because the brake pressure on a given channel has caused the
`wheelto begin to lock;the first action would be to decrease the brake pressure (DECAYstate)
`in an effort to permit the locking wheel to reaccelerate. Fine control of the brake pressureis
`indicated by the states labeled HOLD OR BUILD/DECAY and SLOW BUILDand course
`control is indicated by the FAST BUILD state. (The course control is typically used during
`rapidly changing road surface conditions such as ice-to-asphalt transitions.) During the
`antilock cycle the state will change, as needed, to attain the type of brake pressure and result-
`ing wheel speed activity as shownin Fig. 15.8. Once the need for antilock action has ended,the
`END ANTILOCKstate is entered, the pump motoris de-energized, the valves are de-ener-
`gized, and the system can return to the original NORMAL BRAKINGstate.*
`Howthis state-space approach is integrated into a typical microcontroller flowchartis
`shown in Fig. 15.11. After RESET and INITIALIZATION,a microcontroller enters into a
`
`352
`
`352
`
`

`

`BRAKING CONTROL
`
`15.13
`
`Normat
`braking
`
`
`
`FIGURE 15.10 Simplified single-channel state diagram.
`
`aI
`
`
`nitialize
`
`
`
`
`
`
`
`State/
`space
`analysis
`iValve act.
`Elec. motor
`act,
`
`Simplified antilock flowchart.
`
`353
`
`Wheel speeds,
`vehicle speed
`calculations
`:
`System and
`controller
`checks
`
`Allow antilock
`
`
`
`
`
`
`
`Analyze
`
`system
`for rules
`
`
`
`
`Inhibit
`
`Channel
`treatment
`for
`antilock
`shutdown
`
`t
`FIGURE15.11
`
`353
`
`

`

`15.14
`
`CONTROLSYSTEMS
`
`MAINloop that includes extensive system and electronic control unit checks as well as cal-
`culations of wheel speeds, prediction of vehicle speed, analysis of conditions warranting
`antilock action/state-space control law, and valve and motor/pumpactuations.
`Calculation of wheel speeds consists of scaling the wheel speed sensor inputs to a more
`usable form and possibly filtering noise due to axle deflection, brake squeal, other electrical
`systems, etc. A consideration is that the bandwidth of wheel acceleration and deceleration is
`large—50 g’s may beattainable.
`The vehicle velocity predictionis critical to many control schemes because wheel velocity
`relative to vehicle velocity, as well as wheelslip, may be used asa factor in determining appro-
`priate valve action. Vehicle velocity prediction becomesdifficult once the wheels begin to lock
`because the sensors will no longer be reliable indicators of vehicle speed. The methods used
`to predict vehicle velocity once the wheels have begun to lock consist of a set of rules that
`have been developed by antilock manufacturers through years of experience to ensure a pre-
`diction that has a high degree of accuracy to true vehicle speed.
`The antilock system checks typically consist of sensor and valve/motor continuity tests and
`system voltage rangetests. In addition, the checks normally include tests internal to the elec-
`tronic control unit, such as inter-microcontroller communication.
`Once it is determined that conditions are such that antilock action can be safely invokedif
`warranted, the wheel speed conditions are analyzed to establish the appropriate state for that
`channel. Primary indicators for most antilock control schemes are wheelslip and wheel decelera-
`tion. Anotherfactor consideredis the effect on vehicle stability if a particular state iscommanded.
`Actuation of the valves or electric motor actuators is a direct result of the decisions made
`in the analysis/state-space logic. Other than actuators requiring pulse-width modulation
`drives, the actuators normally will remain in the commandedstate until the microcontroller
`loops back through the code (usually a few milliseconds).
`
`15.3.5 Antilock System Testing
`
`Antilock vehicle testing has evolved over the years to include the following most commontests:
`
`* Straight-line stopping
`¢ Braking in a turn
`¢ Split coefficient stopping with associated stability criteria
`* Transitional road surface testing including checkerboard and low/high and high/low coeffi-
`cient surfaces
`
`* Lane change maneuver
`
`All of these tests may be performed ona variety of surfaces, at a variety of speeds, and with
`lightly loaded and heavily loaded vehicles.
`
`15.4 FUTURE VEHICLE BRAKING SYSTEMS
`
`A trend that will impact braking systems is the industry’s desire to reduce vehicle wiring
`through the use of multiplexing techniques. As increasing numbers of vehicles are outfitted
`with antilock,this trend is expected to result in an increased numberof antilock systems com-
`municating with other vehicle systems through a multiplex link. In addition to the wheel
`speed/vehicle velocity information available from the antilock system, the antilock electronic
`control unit could benefit from this technology by being able to receive engine, transmission,
`steering angle, and other subsystem information.
`
`354
`
`354
`
`

`

`GLOSSARY
`
`BRAKING CONTROL
`
`15.15
`
`Another trend in advanced electronically controlled braking systems is vehicle dynamics
`control during nonbraking maneuvers, as well as during braking. This is accomplished through
`use of the traction control actuators normally integrated in antilock hydraulic modulators, the
`addition of sensors to more accurately determine the dynamicstate of the vehicle, and com-
`munication links with the drivetrain electronic controllers. Vehicle dynamic control holds the
`promise of safer vehicle operation through improvedstability in all maneuvers.
`The vehicle brake systems engineering community also is investigating the addition of
`radar to individual vehicles. This addition could lead to semiautomatic or automatic braking
`in. emergency situations as the brake system anticipates the potential problem and aids the
`operator in safely applying the vehicle brakes in time to avoid a collision. This conceptalso
`lends itself to automatic braking in nonemergency situations to maintain safe distances
`between vehicles at high speeds.
`Continuing interest in electric vehicles and the need for regenerative braking in these vehi-
`cles likely will significantly impact future braking systems. It is expected that the regenerative
`braking function will not be sufficient to provide adequate braking deceleration underall
`conditions and to provide operators the comfort and safety obtainable with conventionalfric-
`tion brake systems augmented by antilock systems. It is expected that a more complex elec-
`tronic control system will be used in conjunction with electric vehicles to afford optimum
`powerregeneration without sacrificing braking stopping distance,stability, or steerability.
`These trends point to a continued useoffriction brake systems through the end of the cen-
`tury and a significant expansionofthe role of electronics in these systems.
`
`Antilock (or ABS or Antiskid) A system designed to prevent wheel lock during overbraking.
`
`Antilock hydraulic modulator A hydraulic brake pressure modulation actuator used in
`antilock systems.
`
`Booster A brake pedal force amplifier, typically vacuum or hydraulically powered.
`
`Boostercrack point
`
`+The brake pedal/push rod travelpoint initiating booster force amplification.
`
`Booster runout A condition in which the brake booster can no longer provide the gain
`required due to high input forces and the input force/output force slope becomesless positive.
`
`Brake caliper A part of a disc brake that contains the brake pads and the brake cylinder.
`
`Braking force A force tending to stop a moving vehicle. Usually applied to the force result-
`ing from brake torque being applied to a wheel of a moving vehicle.
`
`Braking maneuver Any vehicle braking action intended to decelerate a moving vehicle,
`including partial as well as full stops.
`
`Diagonal split brake system A brake system configuration in which a front brake andits
`opposing rear brake are included in the same brake channel. This technique is used to allow
`braking on one front wheel in the case of catastrophicfailure of the other brake channel.
`
`Disk brake A type of brake characterized by force being applied to both sides of a rotor,
`thereby creating braking torque.
`
`Drum brake A type of brake characterized by brake force being applied to the inner surface
`of a drum, thereby creating braking torque.
`
`Dynamic load transfer The characteristic of weight shift during deceleration that places
`more weight on the front wheels and reduces weight on the rear wheels.
`
`355
`
`355
`
`

`

`15.16
`
`CONTROL SYSTEMS
`
`Electric motor/pump The typical hydraulic power source used in antilock systems; an elec-
`tric motor driving a hydraulic pump.
`
`Lateral force Force perpendicularto the direction oftravel.
`
`Longitudinal slip Relative slip between the wheels and the road surface in the direction of
`travel.
`
`Master cylinder A two-chambered hydraulic cylinder operated by the driver through actu-
`ation of the brake pedal.
`
`Proportioning valve A hydraulic valve designed to reduce pressure to the rear brakesrela-
`tive to the front brakes once a crack point is reached. The valve may befixed or load-sensing.
`
`Regenerative braking A type of braking usedin electric vehicles in which the drive motoris
`used as a generator during braking, and it serves as the load to brake the vehicle. This tech-
`nique is used to reclaim a portion of the energy expended during vehicle motion.
`
`Vertical split brake system A brake system configuration in which both front brakes are on
`one channel and both rear brakes are on the other channel.
`
`Wheel slip The difference between tangential wheel speed and road speed.A rollingtire
`with no braking torque on it exhibits 0 percentslip; a nonrotating tire on a moving vehicle
`exhibits 100 percentslip.
`
`REFERENCES
`
`1. JY. Wong, Theory of Ground Vehicles, John Wiley & Sons, New York, 1978.
`2. T.D. Gillespie, Fundamentals of Vehicle Dynamics, Society of Automotive Engineers, Inc., Warrendale,
`Pa., 1992.
`
`3. J. L. Cage, “The Bendix ABS underhood electronic control unit,” Automotive Technology Interna-
`tional '90, Sterling Publications International Limited, London, 1990.
`J. L. Cage, and J. T. Hargenrader, “Safe electronic modules for antilock applications,” Automotive
`Technology International ’91, Sterling Publications International Limited, London,1991.
`
`4.
`
`ABOUTTHE AUTHOR
`
`Jerry L. Cage is the managerof electrical/electronics engineering, AlliedSignal Braking Sys-
`tems. He has over 20 years experience in the design and development of automotive and
`aerospace control systems and harsh environmentelectronics. He has been awarded five U.S.
`patents in electronics and electrohydraulics.
`
`356
`
`356
`
`

`

`
`CHAPTER16
`
`TRACTION CONTROL
`
`
`
`Armin Czinczel
`Development Engineer
`Robert Bosch GmbH
`
`
`
`16.1 INTRODUCTION
`
`Traction control systems designed to prevent the drive wheels from spinning in response to
`application of excess throttle have been on the market since 1987. Vehicles with powerful
`engines are particularly susceptible to drive-wheel slip under acceleration from standstill
`and/or on low-traction road surfaces. The results include attenuated steering response on
`front-wheel-drive (FWD) vehicles, diminished vehicle stability on rear-wheel-drive (RWD)
`cars, and loss of effective accelerative force.
`Large mutual discrepancies in left- and right-side traction levels engender early drive-
`wheelslip on slick surfaces. Under these conditions, the effective accelerative forces at both
`drive wheels are limited to a level corresponding to the adhesion available at the low-traction
`side. The traction control system inhibits wheelspin, allowing the wheel on the high-traction
`surface to apply maximum accelerative force to the road.
`
`16.1.1 Optimizing Stability (Steering Control)
`
`The essential requirement for systems designed to optimize vehicle stability (with RWD) and
`steering control (with FWD)is to maintain adequate lateral traction. The most basic arrange-
`ments achieve this end by controlling engine torque alone. Both drive wheels transmit the same
`level of motive force, dosed in accordance with the adhesion available at the low-traction wheel
`and thereby providing particularly large lateral-traction reserves at the wheel with the greater
`adhesion. Whenthetraction levels are roughly equal at both drive wheels, the system enhances
`vehicle stability (steering control) while providing a certain increase in available effective accel-
`erative force beyond that available on an uncontrolled vehicle with slipping wheels.
`
`16.1.2 Optimizing Traction
`
`The optimization of traction becomesa top priority when the motive force must be transmit-
`ted to surfaces on which the adhesion varies substantially between sides.
`
`16.1
`
`357
`
`357
`
`

`

`16.2
`
`CONTROLSYSTEMS
`
`The typical passenger car features a differential unit at the drive axle; this unit allows vir-
`tually loss-free differences in wheel speed (for instance, in corners) in combination with uni-
`form torque distribution to the drive wheels. This layout generally provides favorable
`dynamic vehicle response, as the equa! distribution of drive torque inhibits vehicle yaw. How-
`ever, a difference in the force-transmission potentials at the drive wheels can combine with
`demands for maximum traction to exposebasic liabilities in the design principle.
`Figure 16.1 illustrates the system dynamics of drive shaft, differential, and drive wheels on
`road surfaces affording differing levels of traction with adhesion coefficients My, [;, (high
`wheel, low wheel). The torque emanating from the driveshaftis distributed equally between
`the drive wheels. The low wheel responds to inadequate adhesion potential by spinning dur-
`ing brief wheel acceleration. The accelerative force transmitted through the high wheel then
`corresponds to the sum of the accelerative force at the low wheelplusits inertia Og [QJ]/R.
`Once the low wheel reachesits terminal speed, the accelerative force available at both wheels
`is limited to the maximum at the low wheel.
`
`
`
`FIGURE16.1 Braking interventionto limit differentialslip.
`
`The only way to increase the accelerative force at the high wheel is to prevent the low
`wheelfrom spinning. Thefirst option, application of the wheel brake,is illustrated in Fig. 16.1.
`The application of braking force F, at the low wheel preventsit from spinning. This makes the
`additional accelerative force F,* (the product of Fg multiplied by theratio of effective brak-
`ing radius to wheelradius) available at the high wheel.
`A second option for maximum exploitation of traction potential is represented by the
`application of fixed, variable, or controlled differential-slip limitation mechanisms. These pro-
`vide fixed coupling to ensure equal slippage rates at the drive wheels, thereby allowing them
`to develop maximum accelerative force.
`During cornering at high rates of lateral acceleration, lateral variations in drive-wheel load
`occur, again producing a difference in acceleration potential. Brakes and limited-slip differ-
`ential arrangements can also be applied to assist in ensuring maximum traction under these
`conditions.
`
`16.1.3 Optimizing Stability and Traction
`
`Traction-control systems incorporating engine-torque control and supplementary braking
`intervention (or controlled differentials) can be applied simultaneously to ensure consistent
`
`358
`
`358
`
`

`

`TRACTION CONTROL
`
`16.3
`
`vehicle stability (steering control) and optimal acceleration within the limits imposed by
`physical constraints. Engine-torque control is the preferred method on road surfaces afford-
`ing uniform adhesion, while application of braking force (or differential control) provides
`optimal acceleration at both drive wheels for dealing with surfaces displaying lateral varia-
`tions in traction.
`
`16.2 FORCES AFFECTING WHEEL TRACTION:
`FUNDAMENTAL CONCEPTS
`
`The dynamic forces that define the tires’ braking response on straights and during cornering
`are already familiar from the technical literature. The transmission of accelerative force in
`straight-line operation and in curves is subject to the same qualitative principles that apply
`during braking. Theslip ratio which applies for braking.
`
`is replaced bythe ratio
`
`_ Vr-OpR
`hg =a
`
`Na ord
`
`Ogr— Ve
`F
`
`with Orr 2 Ve
`
`Acceleration slip rates can range all the way from 0 to the very high numbers used to
`describe the conditions that can occur when the drive wheels spin freely during attempts to
`accelerate from rest.
`Figures 16.2 to 16.4 show acceleration and side-force coefficients as a function of the accel-
`eration slip. Figure 16.2 applies for acceleration during straight-line operation. The demand
`for reserves in lateral adhesion is fairly diminutive under these conditions (including, for
`instance, compensation for side winds); thus, traction remains the salient factor.
`
`
`
`EaEeeGlaeice
`
`4
`4 Ld :
`3.0
`Acceleration slip 2.
`
`FIGURE 16.2 Adhesion coefficient for acceleration u, as a function of
`accelerationslip A,.
`
`359
`
`a5
`
`Dry road
`Loose sand, gravel
`(off-road)
`(ot
`a
`
`Heavy snow cover _.—
`ne eeeee
`
`_
`
`=:
`
`oO
`;
`8
`o
`
`tc
`§
`§
`
`2 8S
`
`359
`
`

`

`Dry road surface
`10)
`
`a@=4* a, m
`
`.
`
`Te
`
`16.4
`
`CONTROL SYSTEMS
`
`
`
`AccelerationandsideforcecoefficientL,,Us
`
`0.2
`
`“sl
`
`gs 4?
`
`0
`
`0.2
`
`L
`1!
`0.6
`0.4
`Acceleration slip ha
`
`pn qareo:
`0.8
`1.0
`
`FIGURE 16.3 Acceleration and lateral traction coefficients at differ-
`ent slip angles a.
`
`o =i ao
`
`o 3
`
`
`
`
`
`Accelerationandsideforcecoefficientj1a,Us
`
`Glare ice
`
`
`
`Acceleration slip A,
`
`FIGURE 16.4 Acceleration and side-force coefficients at different slip
`angles ct.
`
`On dry road surfaces, maximum accelerative force is available at slip rates of 10 to 30 per-
`cent, with traction enhancements of 5 to 10 percent possible relative to spinning drive wheels.
`Onglare ice, maximum traction is achieved at extremely diminutive acceleration slip lev-
`els (2 to 5 percent). On loose sand and gravel and in deep snow (especially in combination
`with snow chains), the coefficient of acceleration force increases continually along with the
`slip rate, with the respective maximaonly being reached somewhere beyond 60 percent. Thus,
`the slip rates of 2 to 20 percent found within the ASR’s operating range will not provide ade-
`quate traction underall operating conditions.
`For this reason, all known ASRsystems incorporate slip-threshold switches or ASR deac-
`tivation switches, which allow the vehicle operator to either reset the ASR slip-control thresh-
`old to substantially higherlevels, or to switch the system off entirely should the needarise.
`Figures 16.3 and 16.4 apply to acceleration during cornering; under these conditions the
`drive wheels are subject to various degreesof lateral force as a function of the vehicle’s rate
`of lateral acceleration. Increasing acceleration slip (and increasing accelerative forces) cause
`a dropin the lateral forces, which then respondtostill higher slip rates by collapsing to small
`residual levels.
`
`360
`
`360
`
`

`

`TRACTION CONTROL
`
`16.5
`
`Figure 16.3 represents the response pattern on a dry road surface. The curvestarts at a rate
`of accelerationslip of zero. Initially, the side-force coefficient displays a moderate downward
`trend. However, continuing increases in the coefficient of acceleration force induce a sub-
`stantial fall in the side-force coefficient. The figure shows that the accelerative force must be
`limitedto a fraction of its ultimate potentialif sufficient lateral forces are to be maintained.
`On glare ice (Fig. 16.4), the extremely limited friction potential means that vehicle stabil-
`ity under acceleration remains available only at relatively small slip angles (ca. <2°). Rela-
`tively diminutive slip angles (<0.05°) will be sufficient to induce a radical drop in the
`side-force coefficient. This makesit clear that an extremely precise and sensitive slip control
`is required on glare ice (and other low-friction road surfaces). The traction-control system
`must thus exhibit a high degree of monitoring accuracy, while signal processing and actuation
`of the final-control elements must be rapid and precise.
`
`16.3 CONTROLLED VARIABLES
`
`The four wheel speeds used for the ABS supply the following closed-loop control parameters
`for the ASR traction control system: the acceleration slip from the lateral variation in the
`rotation speeds of the driven and nondriven wheels, and the angular acceleration of the
`driven wheels.
`The following secondary control parameters are also calculated: vehicle velocity and accel-
`eration based on the speeds of the nondriven wheels, and curve recognition, derived from
`comparisonsof the speeds of the nondriven wheels.
`The target value for accelerationslip is defined as the meanrotational velocity of the non-
`driven wheels plus a specified speed difference known asthe slip threshold setpoint. The main
`goal of regulating accelerationslip can thus be divided into two subsidiary objectives: closed-
`loop control of acceleration slip to maintain slip rates at the specified levels with maximum
`precision, and calculation of optimalslip setpoints for different operating conditions and their
`implementation as control objectives.
`Depending on the final-control strategy being used, various contro] concepts can be
`employed to meet the first objective. With throttle-valve control, a setpoint calculated from a
`numberofsignals is adopted for regulation as soon as the closed-loop control enters opera-
`tion. The subsequent control process basically correspondsto that of a PI controller. When the
`brakes are used, arrangements are necessary to compensate for the nonlinear pressure-vol-
`ume curve which governs the response in the brake calipers. The first stage of the closed-loop
`control program thus employs a sensing pulse corresponding to a relatively large volume; this
`compensates for compliance in the brake caliper. In the next stage, the system responds to
`positive deviations from the setpoint with graduated pressure increases; the rate of increase
`correspondsto the degree of divergence. A subsequent drop below the control setpoint initi-
`ates a pressure-relief stage (sequence of defined pressure-relief and holding phases). This
`impulse series, in which the length of the pressure-relief phases increases continually,is fol-
`lowed by termination of braking intervention.
`Ignition and fuel-injection intervention essentially conform to the D controller closed-loop
`control concept. The difficulty associated with determining satisfactory setpoint values results
`from the fact that optimal acceleration and lateral forces cannot be achieved simultaneously.
`The ASR control algorithm must therefore meet varying operator demandsforlineartraction
`and lateral adhesion by using priority-control strategies and adaptive response patterns
`High vehicle speeds are accompanied by lower operator requirements for traction, espe-
`cially with low coefficients of adhesion. At the same time, reductions in vehicle stability and
`steering response are not acceptable. The control strategy is thus designed to provide pro-
`gressively lower slip threshold setpoints as the vehicle speed increases, with priority being
`shifted from linear traction to lateral adhesion.
`
`361
`
`361
`
`

`

`16.6
`
`CONTROL SYSTEMS
`
`The vehicle’s acceleration rate and the regulated level of engine output provide the basis
`for reliable conclusions regarding the coefficient of friction. Thus, another importantstrategy
`takes into accountthe coefficientof friction at the road surface. The slip threshold setpointis
`raised in response to higherfriction coefficients. This ensures that an ASR system designed
`for optimum performance on low-friction surfaces will not intervene prematurely on high-
`traction surfaces.
`Yet another important controlstrategy is based on the cornering detection mentioned pre-
`viously. This system employs the difference in the wheel speeds of the nondriven wheels as a
`basis for reductions in the slip setpoint to enhance stability in curves. This speed differential
`can be used to calculate the vehicle’s rate of lateral acceleration. A large discrepancy indicates
`a high rate of lateral acceleration, meaning that a high coefficient of friction may also be
`assumed.In this case, the slip setpoint should not be reduced, but rather increased.
`
`16.4 CONTROL MODES
`
`16.4.1 Modulation of Engine Torque
`
`Various control intervention procedures can be employed, either singularly or in combina-
`tion, to regulate engine torque:
`
`¢ Throttle-valve control with the assistance of the electronic performance control or an auto-
`matic throttle-valve actuator (ADS)
`e Adjustment of the ignition-advance angle
`¢ Selective ignition cutout, combined with suppression of fuel injection
`¢ Fuel injection suppression alone
`
`Slippage at the drive wheels generally occurs in response to an excess of torque relative to
`the coefficient of friction available at the road surface. Controlled reduction of engine torqueis
`thusa logical step. It is always the most suitable method in cases wherevirtually identical adhe-
`sion is present at both drive wheels. At the same time, the response times for the individual
`engine controls must be considered if adequate vehicle stability is to be ensured (see Fig. 16.5).
`
`
`ASRwith
`
`=== Throttle valve
`—— Throttle valve
`andignition/ injection
`same Throttle valve
`and wheel brake
`
`
`
`km/h
`
`
`
`Controldeviation
`
`
`
`
`Time ——»
`
`FIGURE 16.5 Deviation of the controlled variable during first control
`cycle with different actuators.
`
`362
`
`362
`
`

`

`TRACTION CONTROL
`
`16.7
`
`If control is restricted to the throttle-valve position alone, the throttle valve’s response
`time, response delays within the intaketract, inertial forces in the engine, and drivetrain com-
`pliance will all result in palpable wheel slippage continuing for a relatively long period of
`time. Throttle regulation alone cannot ensure adequate vehicle stability on rear-wheel-drive
`vehicles. This qualification is particularly applicable to vehicles with a high power-to-weight
`ratio. On FWD and 4WDvehicles, throttle-valve control alone can be sufficiently effective if
`response delays are minimized.
`Arrangements combining throttle-valve regulation with interruptions in the fuel injection
`produce substantial reductions in the amplitude and duration of wheelslip. Thus, this concept
`can be used to guarantee good vehicle stability regardless of which axles are driven.
`In principle, it is also possible to design an ASR system based solely on regulation of the
`ignition and injection systems. This concept employs a system with sequential fuel injection.It
`alternately cuts out individual cylinders, while the ignition is also adjusted for the duration of
`the control process. Although this concept can be employed to ensure adequate vehicle sta-
`bility regardless of drive configuration, certain sacrifices in comfort are unavoidable, espe-
`cially during operation on ice and during the warm-up phase.
`
`16.4.2 Brake Torque Control
`
`The brakesat the drive wheels are capable of converting large amounts of kinetic energy into
`heat, at least for limited periods of time. In addition, the response times can be held extremely
`short, makingit possible to limit slippage increases to very low levels.
`ASRsystems relying exclusively on braking intervention appear suitable for regulating
`spin at the drive wheels.
`Traction enhancements during starts and under acceleration on road surfaces affording
`varying levels of adhesionat the left and right sides are especially significant with this system.
`The ASR hydraulic unit used to generate the braking forces employs components which
`are already present for the ABS. Cost considerations makeit important that ASR hydraulic
`systems require an absolute minimum in additional components beyond those already avail-
`able for the ABS.
`The hydraulic concepts can be classified in two categories, according to whether stored
`hydraulic energy is employed or not. A dual-strategy system including rapid braking interven-
`tion with stored hydraulic energy is always to be recommended where the engine torque con-
`trol is based entirely on throttle valve adjustments with their relatively long response times.
`Figures 16.6 and 16.7 illustrate two examples of ASR braking intervention using stored
`energy.
`
`In this ASR system, designed for RWD vehicles in
`Brake Torque Control with Stored Energy.
`the upperprice range, engine torque