Anti-lock Braking System

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Automatic stability control plus traction & Anti-lock braking system Abstract Automatic stability control + traction (ASC+T) are the technology that revolutionized the world of automobile handling and control. asc+t assist the motorist to have a better control over the vehicle it was realized that there is a need for a better handling system to assist the driver in the dire situations, where he could steer through the obstacle even while braking at the limit which would not be possible with the conventional type of braking system. The antilock braking system (ABS) was brought into the market which met the requisites, the further advancements over the abs is the ASC+T which has control over the engine output for better control. ABS and ASC+T will be dealt separately ABS will be dealt with firstly as it becomes very necessary, first to understand ABS to get the concepts behind ASC+T. Like most materials, the rubber on a tyre has the greatest friction (traction) on pavement when it isn’t sliding. When the tyre is rolling, the contact patch on the ground is stationary when a tyre is skidding, the contact patch is sliding. To achieve maximum braking (or acceleration or cornering) force, it’s necessary to keep the contact patch from sliding. Due to a number of force interactions when the tyre rubber ant the tread flexes, the maximum traction force occurs when there is 10-20% slippage. This turns out to be really useful. If there was no slippage at all before loss of traction, it would be almost impossible for the system to predict impending loss until after it actually occurred. (Learning a feel for this slippage is an extremely useful skill for the driver too, but that’s another subject). By adjusting (reducing) the braking force at a wheel that’s about to lock up, maximum traction can be maintained.

K.V.S.KIRAN (PIRMEC) N.SARATH CHANDRA (MRRITS) SVITS (JKC) E-mail: [email protected] Department of Mechanical Engineering

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Antilock braking systems

Introduction Abs is an abbreviation for antilock braking system it was designed to help the driver maintains some steering ability and avoid skidding while braking Abs was introduced in the mid 1980s and has become “standard” equipment on the majority of vehicles sold in Canada .abs in cars and most multi-purpose vehicles (mpv’s) works on all four wheels .This promotes directional stability and allows steering while maximizing braking. The abs in most pick-up trucks works only on the rear wheels, which promotes directional stability only. However, there are some available with abs on all four wheels. Control under heavy breaking ABS allows you to maintain control of the vehicle. Since four-wheel ABS prevents all wheels from skidding, it allows you to steer the vehicle and still maintain braking.

Where is it located? The abs is a three channel system .that means that the abs hydraulic unit has a separate control valve for each front wheel, and a third control valve that is shared by both back wheels. Because of that, when one rear wheel starts to loose traction, braking has to be reduced to both rear wheels. Example of a bosch breaking system 1. Vehicle stabilization. 2. ESP sensors. 3. Brake booster and master cylinder. 4. Disc brake front axle. 5. Disc brake rear axle. 6. Brake assist (here: integrated in ESP). 7. Communication with engine management.

Page 3 of 12 Abs uses wheel speed sensors to determine if one or more wheels are trying to lock up during braking. if a wheel tries to lock up, a series of hydraulic valves limit or reduce the braking on that wheel. This prevents skidding and allows you to maintain steering control there is a toothed ring that spins at each wheel next to a magnetic hall affect sensor. As the wheel turns, the sensor send out a pulse to the abs controller as each tooth passes by it. By measuring the frequency of the pulses coming in, the controller can determine how fast the wheel is turning. Note that it can’t tell which direction the wheel is turning; the pulses are the same either way. it assumes the wheel is rolling forward. By comparing the speed difference of each wheel, it can detect when one or more wheels are slowing down faster than the car, indicating an Impending loss of traction. The abs controller then commands the abs hydraulic unit to release the pressure on that wheel’s brake. It then reapplies brake pressure as soon as it senses that the wheel has sped back up. This happens rapidly over and over (about eight times a second) so that there is a perceived pulsing or buzzing sensation. By adjusting the braking this way, this wheel’s tyre is held right at its maximum traction limit. The abs control firmware takes into account not just the difference in speed between each wheel, but also a maximum deceleration rate (incase the system misses and a wheel does actually lockup), as well as compensation for cornering (the outside wheels in a corner need to spin faster than the inside wheels. So, it checks to see if the difference between the inside and outside at the front is similar to the difference between the inside and outside at the rear) another important compensation that abs performs is for “split-mu” surfaces. an example of this is when the two wheels on one side of the car are on the road (lots of traction), and the wheels on the other side are off the road(less traction).if the system just adjusted to each wheel’s maximum braking force (which is what some less advanced abs systems do), the tyres on the side of the car on the pavement would apply a force that would make the car tend to spin out. The split-mu detection algorithm will reduce the over all braking force just enough to prevent this from happening. 2.4 Why do we need this? Abs has three significant advantages: it reduces the need for driver skill during panic situations. It can separately control braking thresholds. Most important: it allows the driver to steer while braking at the limit. Since the abs will not allow the tyre to stop rotating, you can brake and steer at the same time. Braking and steering ability of the vehicle is limited by the amount of traction the tyre can generate. Situations where, abs would give assistance, a tyre for all practical purposes has a fixed amount of traction in any directions (accelerating, braking and cornering).when steering and braking at the same time, this traction has to be shared between the two functions. When braking in a straight line, traction of all the the tyres can be used for braking. When cornering at the limit, the tyre has no available traction for braking. Between these two

Page 4 of 12 extremes, the traction can be shared. ABS automatically adjusts the brakes for the traction that’s left over after the cornering force. If you demand steering while braking, the 100% of traction that the tire can generate will be divided between both tasks. For example, if you require 50% for steering then there is 50% of available traction left for braking. If you require 10% for steering then there is 90% left for braking. Be aware that 100% traction on a dry road is a great deal more traction than 100% traction on ice! Therefore, your vehicle is unable to steer and brake as well on a slippery surface as it can on a dry road. Road surfaces and ABS Road hazards that will cause ABS to function unexpectedly are gravel, sand, ice, snow, mud, railway tracks, potholes, manhole covers and even road markings when it is raining. The ABS cannot makeup for road conditions or bad judgment. It is still the driver’s responsibility to drive at reasonable speeds for weather and traffic conditions. Always leave a margin of safety. 2.5 Instructions while driving with abs We need to take some necessary tips for operation of vehicle possessing abs Do keep your foot on the brake. Do allow enough distance to stop. Do practice driving with abs. Do consult the vehicle’s owner’s manual. Do know the difference between four wheel and rear wheel abs. Don’t drive an abs equipped vehicle more aggressively. Don’t pump the brakes. Don’t forget to steer. HOW DO I USE ABS Apply steady and constant pressure- don’t take your foot off the brake pedal until the vehicle is stopped and don’t pump the brake. 6 Disadvantages of abs Abs has a couple of disadvantages: in deep snow or gravel it’s actually better for the wheels to lock up. On completely glare ice, locked wheels will often stop a car faster because even though the sliding friction is less than non-sliding friction, it is applied 100%.

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Small video clip on ABS…………..

3. Automatic stability control + traction. ASC+T is the improvement over the ABS what does it do that makes it stand out from ABS. Automatic Stability Control + Traction (ASC + T) is a suspension control system that reduces engine output, until the vehicle can move or acceleration can take place, without the wheels spinning.

3.1 Introduction. Intelligent regulation systems prevent the vehicle from swerving when accelerating on a loose surface and thus ensure that directional stability is reliably maintained. If a wheel threatens to spin, the wheel brake is applied or else the system intervenes to regulate the engine management accordingly. Only so much throttle is accepted for acceleration so as not to impair lateral stability, even if the accelerator is fully depressed. A similarly integrated engine drag torque regulation system stops the drive wheels from blocking if the driver suddenly removes his/her foot from the accelerator. The system can be switched off if, for example, over steering in bends is required for a sporty driving style. Intelligent regulation systems prevent the vehicle from swerving when accelerating on a loose surface, insuring that directional stability is reliably maintained. •

• •

If a wheel threatens to spin, the wheel brake is applied or else the system intervenes to regulate the engine management accordingly. Only so much throttle is applied to insure lateral stability, even if the accelerator is fully depressed. An integrated engine drag torque regulation system stops the drive wheels from blocking if the driver suddenly removes his foot from the accelerator. The system can be switched off if you desire.

3.2 Benefits: Optimum safety on slippery road surfaces such as ice, snow, loose gravel etc., Even sudden patches of slippery road surface are immediately registered and the system reacts accordingly. No blocking of wheels occurs, even when the accelerator is suddenly released on a slipper surface or there is a shift change to a lower gear.

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3.3 The Basics The stability control system constantly monitors the vehicle’s wheel speed, steering angle, lateral acceleration and yaw rate while you are driving. If the system determines that the vehicle is beginning to slide or spin, it can brake individual, wheels and adjust the throttle in milliseconds, to correct the situation and keep the vehicle headed in the direction it’s being steered. That is something even a professional rare car driver can’t do. Stability control works in conjunction with a vehicle’s antilock brake system, traction control system, and engine management controller without any driver intervention. Car manufacturers will typically rename the systems with their own proprietary designations to make them stand out. For example, Audi and Mercedes-Benz call their systems Electronic Stability Control, Cadillac and Buick use the name Stabilitrak; the systems for BMW and Jaguar are called Dynamic Stability Control; Ford products use Advance Trac: and Volvo calls its system Dynamic Stability Traction Control. So sometimes it can be hard to know what a particular system does If in doubt, ask the salesperson if a certain feature is indeed stability control and includes steering and yaw sensors. 3.4 Function in detail: As ASC+T prevents the drive wheels from spinning, it is a logical extension of the Anti-lock Brake System (ABS) To ensure optimum driving stability with enhanced traction under all driving conditions, ASC+T acts on three factors as a function of the specific driving situation. Drive power control: Whenever there is a substantial power surplus with slip on the drive wheels, engine torque is cut back by changing the position of the throttle butterfly or the parameter responsible for the power provided. If this is not sufficient to quickly reduce wheel spin, drive power is cut back briefly by changing the ignition angle and canceling out the ignition. Brake force control: A further function serves to automatically apply the brakes on a drive wheel about to spin. Since each drive wheel can be braked individually, this system has the same effect as a limited-slip differential. The result is optimum traction comparable with the efficiency of a limited-slip differential with locking action up to almost 100%. This limited slip-effect is generated only at speeds up to 40 km/h. At speeds of more than 80 km/h and 100 km/h, there is no further intervention in the brakes. Instead, control action is limited to a modification of

Page 7 of 12 the throttle butterfly position in accordance with the driving situation by means of Digital Motor Electronics (DME)/Digital Diesel Electronics (DDE) Engine drags torque control (MSR): The MSR modification is performed by ASC, as when the engine begins to brake the car (transition from drive to coasting), the drag torque is so large that the driven wheels could lock at low road-surface friction coefficients. This vehicle reaction may particularly arise at high engine speeds in low gears and when the road-surface friction coefficient is low (e.g. ice, snow and loose gravel). If the driven wheel speed is too small compared with the non-driven wheels, the command to increase engine torque is issued to the engine management control unit by the ASC control Unit. The engine management control unit then inhibits the coasting function and, at the same time, energizes the idle speed charger actuator, which is opened as a result. This measure convert’s drag torque into a positive engine torque. The engine management control unit informs the ASC control unit that the engine torque has been increased. During MSR control, neither cylinder fade-out nor ignition timing is modified. MSR control is possible only at speeds greater than 20 km/h. This is to prevent it from beginning its control routine when a car negotiates a narrow bend at a low speed, thus producing an undesirable coasting effect. An applied handbrake is identified by a logic system stored in the ASC control unit. The handbrake-light warning switch is not used for identification purposes. An applied handbrake is identified if a wheel spin is less than 5 km/h for longer than 200 ms and engine speed greater than 1 200 rpm. Automatic stability control is an extension of ABS, but there are few differences worth discussing. ABS has to control the braking force at all four wheels. ASC+T have to control the power delivery of the engine and the way the rear differential distributes torque between the two back wheels. 3.5 What lies within? These systems generally consist of an electronic control unit (ECU), a hydraulic control unit (HCU), an electrically driven hydraulic pump and a set of sensors. These sensors typically are: a steering wheel angle sensor, a wheel speed sensor for each wheel, a yaw rate sensor and a lateral acceleration sensor. These sensors provide information about the driving state of the vehicle. The ESP system then activates the brake on one or more wheels as appropriate to help prevent a skid and more generally, to improve the feedback to the driver and make the handling more linear. In a typical even in which ESP comes into play, the driver decides that the vehicle is slightly too fast and wide in a corner, and so applies more steering lock. Tyres,

Page 8 of 12 which are already heavily, loaded laterally, supply a little more lateral force, but not enough for the driver, so he adds more steering lock. The ESP monitors the steering wheel angle and the vehicle’s yaw velocity, and when the error between the two exceeds pre-programmed limits, it intervenes, and brakes the inner rear wheel (typically). This tends to cause the rear axle to slide outwards, hence pointing the nose of the car tighter into the corner, which is what the driver wanted to do. The system may also reduce the speed of the vehicle by adjusting the throttle. 4. Traction control: Traction control and Vehicle Stability Control systems, on current production vehicles, are typically (but not necessarily) electro-hydraulic systems designed to prevent loss of control when excessive throttle or steering is applied by the driver. The intervention can consist of any, or all of the following. 1. 2. 3. 4.

Retard or suppress the spark to one or more cylinders. Reduce fuel supply to one or more cylinders. Brake one or more wheels. Close the throttle, if the vehicle is fitted with drive by wire throttle. The brake actuator, and the wheel speed sensors, is the same as that used for Anti lock braking system. Traction Control is usually considered as a performance enhancement, allowing maximum traction under acceleration without wheel spin. It is particularly advantageous to 4X4 vehicles when driven off road on a loose surface. Conversely Vehicle Stability Control is considered a safety feature preventing operation of a vehicle at the edge of the safety envelope.

5 GETTING TECHNICAL: There are two ways in which a vehicle typically loses control when cornering: under steer or over steer. Understeer occurs when the front wheels of a vehicle lose traction in a turn. It feels as though the car is going straight rather than in the direction of the turn. Turning the steering wheel more doesn’t help. Backing off the throttle will eventually bring the vehicle back into line. Over steer occurs when the rear wheels lose traction and the rear of the vehicle begins to swing out, potentially causing a vehicle to spin or slide sideways. This motion can sometimes be corrected by steering into the direction of the slide. Stability control helps prevent these situations by taking control of the vehicle before or as the driver loses control. It rapidly applies brake force on one or more wheels, depending on the behavior of the vehicle. In an under steer situation (below right), the vehicle is making a turn to the right, but begins to under steer. Stability control initially increases brake force on the right rear wheel. This causes a rotation in the vehicle; helping the front tire gain grip and helping the driver maintain control in the bend. The car’s first traction problem comes from the differential

Page 9 of 12 The differential is a gear unit that couples the rear wheels to the single driveshaft coming from the transmission. It has to transmit power from the transmission, while allowing the rear wheels to spin at different speeds. The wheels need to turn t different speeds because when the car goes around a corner, the outside wheels have a longer path to travel than the inner ones do. If both back wheels were directly coupled to each other (forcing them to both turn at the same speed), then they would ‘fight’ any effort of the driver steering around a corner. A conventional differential is a set of gears that couples twisting forces of three devices (in a car’s case, the transmission and two wheels). Torque must be distributed to all three. Under normal conditions, both wheels are coupled together through the ground. In this situation, torque can be transmitted from the transmission to both wheels. If the coupling of the two wheels through the ground is disrupted (as when a wheel is spinning), then the differential can’t usefully distribute the torque. All of the power is being applied to the spinning wheel, and not to the wheel that still has traction. An electronic traction control system can help prevent a wheel from spinning by applying that wheel’s brake. This not only maximizes the traction at that wheel, but more importantly it enables the differential to apply power to the other wheel, which probably has more traction. The car’s second traction problem is when the torque from the engine exceeds the total traction available at both wheels. An electronic traction control system can deal with this problem by reducing the engine output. The ASC + T system intervenes in two stages: Threshold of adhesion, it starts to rapid pulse the brake to that wheel (just like ABS). When the second rear wheel nears the limit of adhesion, engine power is reduced. The first stage (single wheel braking) actually improves vehicle performance. The second stage (engine reduction) doesn’t improve performance available, but it adjusts output so that all that is available is fully utilized. Small video clip on traction control…………………

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6. Technologies: The following chart shows the safety features found on four-wheel anti-lock brake systems(ABS),traction control and automatic stability control + traction and handling systems characteristics Prevents wheel lock-up under many road conditions. Allows driver to maintain control when brakes are fully applied. Sensors detect impending wheel lock-up. Pumps the brakes like a driver would, only much faster and more effectively. Engages when the driver stumps on brake pedal. Prevents unwanted wheel spin in low traction situations. Adjusts vehicle acceleration when driving in low traction situations, such as rain or snow. Helps drivers accelerate safely Detects a vehicle’s position in relation to steering input with use of sensors Monitors & compares a vehicle’s movement with the direction a driver is steering .

Four Wheel ABS

Traction control

Automatic stability + traction Control systems

X

X

X

X

X

X

X

X

X

X X

X

X

X

X

X X X

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7. Equipped Vehicle List Specific to Make and Model: There are auto companies which have got the same technology under their own trade name as can be seen from the table below. TABLE -2 Make/model Acura Jaguar Chevrolet Ford

Model/equipment made available VSA (Vehicle DSC (Dynamic stability control) AHS (Active Handling System) IVD (Interactive vehicle dynamics)” Vehicle tract”

BMW Lexus Lincoln Mercedes Benz

DSC (Dynamic stability control) VSC (Vehicle stability control) Advanced trac ESP (Electronic stabilization program)

Oldsmobile Pontiac Porsche

PCS (precision control system) Stabilitrack3 PSM (Porsche stability management)

Toyota Volvo

VSC (Vehicle Stability Control) DSTC (Dynamic Stability and Traction Control System)

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8. Conclusion. With such sophisticated technology which has made driving simpler than ever before, there is an immense demand for these systems becoming more and more common in the vehicles to come in future, though some auto enthusiasts don’t prefer to drive with the ASC+T on as it takes away some of the driving skill from the driver, but this is true only in the case of serious auto enthusiasts. But overall the system works towards the safety of the driver, and it has got praises from world wide market. So the technology just goes on growing and working toward more and more safety.

References: Websites • www.autoeducation.com “BRAKES” • www.a-car.com “ NEW TECHNOLOGY” • www.transportcanada.com. “What you should know about anti lock braking system” • www.318ti.org/notebook/asc/” an article by Brian Brown” • www.freescale.com “electronic stability control” • Popular mechanics. • Auto India.