BRAKING CONTROL
`
`15.1 1
`
`Speed aca5.I.ct.-
`Wheel
`pressure
`Broke
`
`1au.aa
`
`cnmund
`
`at...“
`
`:ccey
`
`De:n9___
`
`DEL‘u)‘—
`
`com-ond
`Cm!chlld
`
`DD.an
`FIGURE 15.8 Typical antilock brake cycling.
`
`Huilu
`
`Itulld
`
`Blind
`
`To scienold
`Valves
`
`To {amps
`
`To Electric
`motor
`
`%
`E.
`‘_l'_’
`
`3 U
`
`+4
`
`a
`l;
`
`i
`
`5‘
`L
`+’
`
`U
`.2
`U
`
`+J
`3
`1:b—t
`[1
`
`—1—
`
`Volt.
`reg.
`
`
`:l
`
`
`Wheel speed
`sensor Inputs
`
`Stop lamp
`switch Inputs
`
`System
`monitors
`
`Battery
`
`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-monmed or passenger compartment-mounted, reduced wiring costs favors the former.
`Also, for enhanced reliability, 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 designed using a disciplined methodology and must be rigorously tested prior to
`release for production.
`
`351
`
`351
`
`

`

`15.12
`
`CONTROL SYSTEMS
`
`Failure mode and effects analyses
`Failure Mode and EfieersAnolyses/Foolr 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 the field, 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 bench or field testing is impractical.
`
`Common Design Techniques to Improve Safety. One of the most common techniques used
`to improve safety in antilock systems is 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, and all
`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 nucrocontroller. 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 employ a relay
`function to remove actuation power from the output actuators;this function may take the form
`of a discrete relay or it may be a transistorized circuit. This relay function is a key element of
`the design since it affords a secondary method in which to inhibit energization of valves or the
`motorlputnp and, therefore, a second level 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 nucrocontrollers for processing. Likewise, the micro-
`controller outputs are bufferedi‘arnplified 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
`control units For example, the output circuitry may be commanded to test the solenoid valves
`for proper current draw and convey the test 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 of stability and steerability
`as well as good stopping distance, the brake control algorithm is more easily represented as a
`state—space diagram than as a classical proportionaI—integrative-derivative control scheme.
`A simplified state diagram for a single—channel anti10ck system is shown in Fig. 15.10. In this
`diagram, 3 vehicle not braking or decelerating would be in the NORMAL BRAKING state If
`anti10ck action is warranted, it is because the brake pressure on a given channel has caused the
`wheel to begin to Iock;the first action would be to decrease the brake pressure (DECAY state)
`in an effort to permit the locking wheel to reaocelerate. Fine control of the brake pressure is
`indicated by the states labeled HOLD OR BUILDIDECAY and SLOW BUILD and 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 shown in Fig. 15.8. Once the need for antilock action has ended, the
`END ANTILOCK state is entered, the pump motor is de—energized, the valves are de—ener—
`gized, and the system can return to the original NORMAL BRAKING state.‘1
`How this state-space approach is integrated into a typical microcontroller flowchart is
`shown in Fig. 15.11. After RESET and INITIALIZATION, a microcontroller enters into a
`
`352
`
`352
`
`

`

`BRAKING CONTROL
`
`1 5. 1 3
`
`HOW
`or
`
`Normal
`braking
`
`lold/olec,
`
`FIGURE 15.10 Simplified single—channel state diagram.
`
`?I
`
`
`nitialize
`
`
`
`
`
`Wheeispeeds
`vehicle speed
`cakculutions
`
`
`
`
`i S
`
`ystem and
`controuer
`checks
`
`ALiow untilock
`
`
`
`
`
`
`
`
`ntilock
`worrun‘te
`
`
`
`
`
`
`Inhibit
`
`
`Channel
`Analyze
`
`treatment
`system
`For rules
`For
`
`
`
`antilock
`shutdown
`
`Yes
`
`LS
`
`tate/
`space
`analysis
`__T_Valve oc‘t.
`Elec. motor
`Qc‘t.
`
`
`
`
`i
`
`i
`
`FIGURE 15.11 Simplified antilock flowchart.
`
`353
`
`353
`
`

`

`15.14
`
`CONTROL SYSTEMS
`
`MAIN loop 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 actionlstatenspace control law, and valve and motori‘pump actuations.
`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 be attainable.
`The vehicle velocity prediction is critical to many control schemes because wheel velocity
`relative to vehicle velocity, as well as wheel slip, may be used as a factor in determining appro—
`priate valve action.Vehicle velocity prediction becomes difficult 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 valvel'motor continuity tests and
`system voltage range tests 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 invoked if
`warranted, the wheel speed conditions are analyzed to establish the appropriate state for that
`channel. Primary indicators for most antilock control schemes are wheel slip and wheel decelera-
`tion. Another factor considered is the effect on vehicle stability if a particular state is commanded.
`Actuation of the valves or electric motor actuators is a direct result of the decisions made
`
`in the analysisfstate—space logic. Other than actuators requiring pulse~width modulation
`drives, the actuators normally will remain in the commanded state 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 common tests:
`
`- Straight—line stopping
`- Braking in a turn
`- Split coefficient stopping with associated stability criteria
`
`- Transitional road surface testing including checkerboard and lowlhigh and highjlow coeffi-
`cient surfaces
`
`I Lane change maneuver
`
`All of these tests may be performed on a 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 number of antilock systems com-
`municating with other vehicle systems through a multiplex link. In addition to the wheel
`speedl'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
`
`

`

`_GLOSS_ARY
`
`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 dynamic state of the vehicle, and com—
`munication links with the drivetrain electronic controllers. Vehicle dynamic control holds the
`promise of safer vehicle operation through improved stability 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 CollisionThis concept also
`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 under all
`conditions and to provide operators the comfort and safety obtainable with conventional fric-
`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
`power regeneration without sacrificing braking stopping distance, stability, or steerability.
`These trends point to a continued use of friction brake systems through the end of the cen—
`tury and a significant expansion of the role of electronics in these systems.
`
`Antilock (or ABS or Antiskid) A system designed to prevent wheel lock during overbraking.
`
`Anljlock hydraulic modulator A hydraulic brake pressure modulation actuator used in
`antilock systems.
`
`Booster A brake pedal force amplifier, typically vacuum or hydraulically powered.
`
`Booster crack point The brake pedalipush rod travel point initiating booster force amplification.
`
`Booster runoot A condition in which the brake booster can no longer provide the gain
`required due to high input forces and the input forcer’output force slope becomes less 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 and its
`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 catastrophic failure 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
`
`

`

`1 5.1 6
`
`CONTROL SYSTEMS
`
`Electric motorlpump The typical hydraulic power source used in antilock systems; an elec-
`tric motor driving a hydraulic pump.
`
`Lateral force Force perpendicular to the direction of travel.
`
`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 brakes rela—
`tive to the front brakes once a crack point is reached. The valve may be fixed or load—sensing.
`
`Regenerative braking A type of braking used in electric vehicles in which the drive motor is
`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 rolling tire
`with no braking torque on it exhibits 0 percent slip; a nonrotating tire on a moving vehicle
`exhibits 100 percent slip.
`
`REFEFENCES
`
`J. Y. Wang, Theory of Ground Vehicles, John Wiley & Sons, New York, 1978.
`1.
`2. T. D. GilleSpie, Fundamnmls ofVehicl‘e Dynamics, Society of Automotive Engineers, Inc..Wan:endale,
`Pa, 1992.
`
`3.
`
`4.
`
`.T. L. Cage, “The Bendix ABS underhood electronic control unit,” Automotive Technoiogy forem—
`tionol ’90, Sterling Publications International Limited, London, 1990.
`J. L. Cage, and J. T. Hargenradcr, “Safe eiectronic modules for antilock applications,” Automotive
`Technology International ‘91, Sterling Publications International Limited, London, 1991.
`
`ABOUT THE AUTHOR
`
`Jerry L. Cage is the manager of electricallelectronics engineering, AlliedSignal Braking Sys—
`terns. He has Over 20 years experience in the design and development of automotive and
`aerospace control systems and harsh environment electronics He has been awarded five US.
`patents in electronics and eiectrohydraulics
`
`356
`
`356
`
`

`

`
`
`CHAPTER 16
`
`
`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—
`wheel slip 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. When the traction 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 becomes a 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
`
`CONTROL SYSTEMS
`
`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 equal 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 expose basic 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 pH, 11L (high
`wheel, low wheel). The torque emanating from the driveshaft is 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
`corresyonds to the sum of the accelerative force at the low wheel plus its inertia (9R [9}1R.
`Once the low wheel reaches its terminal speed, the accelerative force available at both wheels
`is limited to the maximum at the low wheel.
`
`
`
`FIGURE 16.] Braking intervention to limit differential slip.
`
`The only way to increase the aocelerative force at the high wheel is to prevent the low
`wheel from spinning. The first option, application of the wheel brake, is illustrated in Fig. 16.1.
`The application of braking force F}; at the low wheel prevents it from spinning.'Ihis makes the
`additional accelerative force Fat {the product of F3 multiplied by the ratio of effective brak—
`ing radius to wheel radius) 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 variatioris 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
`
`1 6.3
`
`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 AFFEC'HNG WHEEL TRACTION:
`
`EUNDAMENTAL CONcEars
`
`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. The slip ratio which applies for braking.
`
`is replaced by the ratio
`
`_ VF— GRR
`1.5 —
`VF
`
`AA =M with GRr 2 V;
`V;
`
`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.
`
`
`
`-—--—._.__...—__
`Dry road
`
`Loosesand,gravei
`_.—_.—
`{off—mad)
`_u_ __ _ _
`,1
`
`Heavy snowooverfl-’
`
`coefficient14A
`Accelerationforce
`
`
`Acceleration Slip 1,.
`
`FIGURE 16.2 Adhesion coefficient for acceleration uh as a function of
`acceleration slip 7L“.
`
`359
`
`359
`
`

`

`16.4
`
`CONTROL SYSTEMS
`
`
`Dry road Surface
`1.0'
`
`40
`
`
`
`u
`
`‘5 ....
`
`‘4.“
`
`
`
`Accelerationandsideforcecoefficientup"us
`
`0-2
`
`"'-.
`
`t1=4°
`
`0
`
`0.2
`
`l
`I
`0.6
`0,4
`Acceleration slip AA
`
`l
`0.8
`
`' “ "1 ---- --
`1.0
`
`FIGURE 16.3 Acceleration and lateral traction coefficients at differ-
`ent slip angles oz.
`
`Glare ice
`
`
`
`Accelerationandsideforcecoefficientus,#5
`
`
`
`FIGURE 16.4 Acceleration and side—force coefficients at different slip
`angles or.
`
`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 poseible relative to spinning drive wheels
`On glare 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 maxjma only 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 under all operating conditions.
`For this reason, all known ASR systems 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 higher levels, or to switch the system off entirely should the need arise.
`Figures 16.3 and 16.4 apply to acceleration during cornering; under these conditions the
`drive wheels are subject to various degrees of lateral force as a function of the vehicle‘s rate
`of lateral acceleration. Increasing acceleration slip (and increasing accelerative forces) cause
`a drop in the lateral forces, which then respond to still higher slip rates by collapsing to small
`residual levels
`
`360
`
`360
`
`

`

`TRACTION CONTROL
`
`1 8.5
`
`Figure 163 represents the response pattern on a dry road surfaceT'he curve starts at a rate
`of acceleration slip 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
`limited to a fraction of its ultimate potential if 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. 32°). Rela-
`tively diminutive slip angles (30.05“) will be sufficient to induce a radical drop in the
`side—force coefficient. This makes it 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 VARJAQLES
`
`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 acccln
`eration based on the speeds of the nondriven wheels, and curve recognition, derived from
`comparisons of the speeds of the nondriven wheels.
`The target value for acceleration slip is defined as the mean rotational velocity of the non-
`driven wheels plus a specified speed difference known as the slip threshold sctpoint. The main
`goal of regulating acceleration slip 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 optimal slip setpoints for different operating conditions and their
`implementation as control objectives.
`Depending on the final—control strategy being used, various control concepts can be
`employed to meet the first objective. With throttle-valve control, a setpoint calculated from a
`number of signals is adopted for regulation as soon as the closed-loop control enters opera-
`tion. The subsequent control process basically corresponds to 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 caiipers. The first stage of the closed-loop
`control program thus employs a sensing pulse correspOndin g 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
`corresponds to 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 demands for linear traction
`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 important strategy
`takes into account the coefficient of friction at the road surface. The slip threshold setpoint is
`raised in response to higher friction 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 control strategy 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)
`- 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 torque is
`thus a logical step. It is always the most suitable method in cases where virtually 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).
`
`ASH with
`
`km/ h
`
`
`
`
`x \ x\ \
`
`\\
`
`\ \ ‘ ~ ~
`
`.‘N\L
`
`ms
`
`- - - Throttle valve
`— Throttle valve A
`and ignition/ injection
`- - - - - Throttle valve
`and wheel brake
`
`.. ~
`
`’
`
`T
`53
`33
`a'C
`'9'Eo
`O
`
`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 intake tract, 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 4WD vehicles, 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 wheel slip. 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 brakes at 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, making it possible to limit slippage increases to very low levels.
`ASR systems 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 adhesion at 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 make it 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