כתבה והסבר על TCS ESP .......

White-Night · ב-18 למרץ, 2004

TRACTION CONTROL SYSTEMS

ON ROAD CARS

ADVANCED VEHICLE SYSTEMS 241MEC

31.01.2003

D. Missirys

J. Pienaar

S. Pawsey

R. Talvitie

SUMMARY

Traction control systems are one of the most important developments in the active safety of road cars in the past 20 years. There are two types of system on the market today: normal traction control and stability control.

Both systems were created and developed by Bosch GmbH and Mercedes-Benz AG in Germany, and both systems made their debut on Mercedes-Benz cars: traction control in 1981 and stability control in 1995.

Traction control acts only on the driven wheels of the car and stops any wheel spin by cutting engine revs or by applying the brakes.

Stability control on the other hand monitors all the wheels of the vehicle and corrects both oversteer and understeer slides.

Both systems are very effective safety devices in slippery conditions and in unexpected road situations. However, both systems are also not infallible: carrying too much speed into a corner can still cause an accident.

CONTENTS

INTRODUCTION

HISTORY

TRACTION DATA

TRACTION CONTROL

STABILITY CONTROL

CONCLUSION

INTRODUCTION

Some of the biggest advances in automotive technology in the past 25 years have come in the area of safety. Spurred by improvements in microprocessor speed, miniaturisation, and software development, the automobile continues to evolve. Traction control systems are one of these safety developments that have reached the automobile during this period.

Traction control systems are the most important development in the active safety of road cars since the popularisation of ABS brakes and form part of a series of three braking technology developments that began appearing on vehicles in the mid-eighties. In chronological order these three developments are: ABS, traction control and stability control. The difference between traction control and stability control is that normal traction control systems only limit the slipping of the driven wheels whereas stability control systems control the slipping of all the four wheels.

HISTORY

Both traction control and stability control systems were created and developed by Bosch GmbH and Mercedes-Benz AG in Germany and the systems fitted to most vehicles are made by Bosch. The system was invented a surprisingly long time ago in 1959 by Dr Fritz Nallinger, the then Chief Engineer at Daimler-Benz who patented a control device designed to prevent the spinning of the driven wheels by controlling the engine, transmission and brakes. However, the device remained just a theory due to the lack of sensors and control systems capable of performing with the split-second speed required of a traction control system.

Bosch test track http://www.bosch.com

In 1978 Bosch launched the ABS (Antiblockiersystem) brakes which included the wheel rotation sensor technology required in traction control systems. This paved the way for the development of traction control systems, or ASR (Antischlupfregelung) as it was named by Bosch and Mercedes-Benz. The system was launched in 1981 on the W126 S-Class Mercedes-Benz and intervened in both the braking system and the engine to regulate the slipping of the rear wheels. In 1986 the system was launched by Bosch for use by other car manufacturers.

1981 Mercedes-Benz S-Class W126 http://www.mercedes-benz.com

Following computer simulations, the first trials were started in 1987 with vehicles equipped with a Transverse Slip Control System which controlled the skidding movements of the vehicle by selective intervention in the chassis, engine and transmission. After the success of these tests the development of production version began in 1992 with more than 40 engineers from Mercedes-Benz and Bosch being involved in the project. The system, now named ESP (Electronic Stability Program) was launched to the press on a frozen lake in Sweden, in March 1994 and production began in the spring of 1995 on the W140 Mercedes-Benz CL600 Coupé. The system was also made available to other manufacturers.

1995 Mercedes-Benz CL600 W140 http://www.mercedes-benz.de

TRACTION DATA

In order to achieve the maximum acceleration possible, the slip between the tyre and the road has to be decreased to a point where the tire is just beginning to slip against the road.

The graph above shows that this optimum slip value is around 10% for a dry road surface and 5% for a wet road surface. It is most important that this level of slip has to be carefully kept at these levels, or close to them, since grip decreases dramatically above them. This makes for a very fine balance between too much power and too little acceleration.

The difficulty of maintaining the correct levels of slip during varying conditions makes experienced rally drivers go for a slip level of 10%-20%, thus using almost 90% of the available traction. This way, they are close to decreasing acceleration too much, but also closer to retaining 100% of the traction. The less experienced drivers, not having as precise throttle control and timing as the more experienced ones do, tend to spin their wheels more (20%-30% slip) than letting the car accelerate poorly due to insufficient throttle input.

This shows that good drivers still stay cautious about their throttle modulation, in case the slightly spinning wheels suddenly grip the road surface and stop spinning completely. This, however, might decrease the total acceleration of the vehicle, but will definitely give them nearly 100% traction figures. The above driver habit has to do with the response time of a human being, which is nearly 1/10th of a second. Also the fastest throttle response time is around that range, making for a total driver-car lag of 0.2 seconds. The advent of traction control with 1/1000th of a second response time significantly helps keeping wheels near, but never over, the critical slip levels, for varying conditions.

TRACTION CONTROL

Traction control works at exactly the opposite end of the scale from ABS dealing with acceleration rather than deceleration. In other words, traction control regulates wheel-spin during acceleration to ensure maximum contact between the road surface and the tyres, even under less than ideal road conditions.

Traction control systems are design to control wheel-spin by monitoring the speed of all four wheels and controlling the engine power. ABS sensors are used to monitor the speed of all four wheels every 1000th of a second. The information is sent to the Electronic Control Unit (ECU) to adjust the amount of power to the wheels through engine speed and selective braking. These systems are extremely sensitive comparing right hand driven and undriven wheels separately to the left hand driven and undriven wheels. This allows the system to utilise different strategies for when the vehicle is driving in a straight line or for when the vehicle is cornering.

There are two methods of controlling the engine power. For fuel injected engines, the system controls power by means of a fuel cut and for a carburettor engine power is controlled by means of a spark cut.

This pictures shows a BMW M Coupé going around a corner at high speed with the traction control turned off.

In this picture the traction control is turned on and although the car approached the corner at the same speed the traction control system slowed the car down to safely go through the corner without any wheel-spin.

http://www.racelogic.co.uk

Traction control systems makes the biggest difference when driving on a wet and slippery roads.

STABILITY CONTROL

Also known as,

- Electronic Stability Program (ESP)

- Dynamic Stability Control (DSC)

Stability control is the latest in a series of braking-related developments that began appearing in production vehicles in the 1980s. The first versions of this new technology appeared in the top of the range vehicles produced by manufacturers Mercedes and BMW, namely in the S-Class and the 7 Series. The reason for this German advance in technology is because the Bosch group that first designed the system is one of the largest Original Equipment Manufacturers and is based in Stuttgart. For many years the Bosch group has been a world leader in safety system development, and by the time of the release of their stability control system they already had their ABS brakes and their ASR traction control systems on the market and in production.

The Stability Control system came out of the natural progression of these safety orientated systems. The Bosch definition of Stability Control is as follows,

ESP = (ABS + TCS)^2

ESP combines both the Antilock Braking System with the traction Control system and multiplies the advantages of both these tried and tested systems: by improving transverse dynamics along with longitudinal dynamics ESP achieves the ultimate objective stability in all directions.

http://www.bosch.com

Following the success of, and the ground work covered by the ABS and traction control systems the first two building blocks were in place onto which the Stability Control system could be built.

Stability control incorporates everything ABS and traction control can achieve, however, the major difference is the inclusion of a Yaw-Sensing feature, this is also known as a rotational speed sensor. Yaw can be described as "the movement of an object turning on its vertical axis." The yaw sensor enables the electronic control unit of the system to sense when the vehicle begins to side skid and will then work to correct the situation. Components of the Bosch Electronic Stability Program http://www.bosch.com

Traction Control and ABS only function in the longitudinal direction of the vehicle, it is the ability of the Stability Control system to work also in the lateral directions that makes it a far more powerful safety feature.

The Bosch ESP system works initially from the yaw sensor sensing the amount of roll the vehicle is experiencing, Bosch claim that this is sampled at a rate of twenty five times per second.

The sensor is able to determine how far off-axis the car is tilting, this information is then fed to the system control unit which will then compare the data with what it is receiving from the other sensors that make up the stability control system, and also what the traction control system is sending.

Information is fed to the control unit from the steering mechanism, giving the current steering angle, and the accelerator position, the demand being made on the engine.

It is the combination of the steering position, throttle position and yaw sensor that make up the basic stability control system, the other sensor will already have been installed for the traction control and ABS systems.

If the information fed to the control unit is outside the safe boundaries it will act to correct the situation. This is done by the application of appropriate breaking forces applied to wheels that, under the circumstances, have the most grip. It is here that the Stability Control will rely upon the traction control system as, as has previously been discussed, it has already been monitoring the grip levels. The traction control will then rely upon the ABS to ensure that best use is made of the available grip and so the chain is completed

Understeer Oversteer http://www.mercedes-benz.com

Stability Control is particularly successful when driving conditions are poor, ice on the road, rain or snow. It has the ability to do most of the thinking for the driver enabling safer progress while driving. Depending on the particular driving situation, the system may activate an individual wheel brake or any combination of the four, as well as control the throttle, until the vehicle is once again stable.

ESP not engaged ESP engaged http://www.edmunds.com

The pictures above show the stability control in action. The vehicle in the left hand set of pictures shows the effects of entering a corner carrying too much velocity. The vehicle will, depending on the suspension set-up, understeer or oversteer towards the outer radius resulting in a collision. Pay particular attention to the amount of body roll that the left centre picture shows the vehicle is experiencing, this is what the stability control system will initially sense and act to control. The same test has been repeated in the right hand set of pictures; however, in this series the stability control system is active. The system senses the increased amount of yaw angle, as seen in the first test, but then reduces engine power output - cutting the throttle and will brake the wheels with the highest level of grip.

This process will continue until the system senses that the vehicle has returned to a safe situation where the yaw angle is within tolerance.

In conclusion it is important to understand that the Stability Control system is not a stand alone system. In effect it piggy-backs the existing systems and could not function without them, hence the equation quoted by Bosch in their definition.

The stability control system masquerades under many other names as dictated by the different vehicle manufacturer, below is a list of the most popular systems available to the public market.

Audi: Electronic Stability Program (ESP).

BMW: Dynamic Stability Control (DSC).

DaimlerChrysler: Electronic Stability Program (ESP).

Ford Motor Company: Advance Trac.

General Motors: Active Handling System (Corvette), Precision Control System (Oldsmobile), Stabilitrak (Pontiac, Buick, Cadillac).

Jaguar: Dynamic Stability Management (DSM).

Lexus: Vehicle Skid Control (VSC)

Porsche: Porsche Stability Management (PSM).

Volkswagen: Electronic Stability Program (ESP).

Volvo: Dynamic Stability Traction Control (DTSC).

Stability Control is the successful operation of three safety systems, Traction Control, ABS and the Stability Control system itself. It requires its own sensors for function, a throttle position sensor, steering position sensor and fundamentally the yaw sensor.

Bosch are not the sole manufacturer of these safety systems, three other manufacturers also build systems for passenger cars they are TRW, Delphi and Continental-Tevis.

Currently the Stability Control system is seen as something of a luxury by car manufacturers. Many only offer the system as standard on the highest level models, and only few will offer it as an option on lower level models if at all. It is strange considering that even a cheap car has the same chance of a loss of control through driver error or unseen/inclement driving conditions.

The future is stability control offered throughout the product range of vehicle manufacturers. This will happen when equipment and component costs fall far enough to make it feasible.

Stability Control is an investment in safety for not only the consumer but also the manufacturer, safety is what reputations are made and broken on.

"ESP helps drivers maintain control of their vehicles in critical situations and thus avoid serious accidents"

Bosch 2003

DISCUSSION

Like most other new automotive inventions both traction control and stability control made their debut in top of the range luxury cars. There is little practical reason for this but customers of luxury cars may be more willing to pay for new technology in their cars than normal buyers. Also, since new technology can be very expensive, they have a smaller effect on the price of an already expensive car.

CONCLUSION

Traction control and stability control systems are the most important development in the safety of road cars since ABS brakes. Both these German inventions offer excellent additional safety in slippery or otherwise unexpected road conditions. However, neither system is infallible: driving too fast into a corner can still cause an accident. The systems act as aides rather than safety nets.

SOURCES

http://www.mercedes-benz.com

http://www.bosch.com

http://www.racelogic.co.uk

http://www.edmunds.com

http://www.jaguar.com

http://www.lexus.com

http://www.porsche.com