Electronic Stability Program

CONTENTS




 INTRODUCTION
 WHAT IS ESC
 IMPORTANCE
 COMPONENTS
 WORKING
 ADVANTAGES
 APPLICATION
 SUMMARY

















Electronic Stability Program (ESP)

Electronic Stability Program (ESP) is an interactive high-tech safety system that significantly improves the stability of a vehicle in all driving situations .When starting off, When driving it self and when braking and so increases the driver’s chances of avoiding a potential accident. ESP helps keep the driver in control of the vehicle even in critical situations.
ESP is based on already familiar components; the anti-lock brake system (ABS) and traction control (TCS) and also include electronic brake power distribution (EBD) and engine drag torque control (EDC).
ESP is an active safety system which improves vehicle stability in all driving conditions. It operates by actuating the brakes individually on one or more wheels on the front or rear axle. ESP stabilizes the vehicle when cornering, braking ,or during non-driven coasting to keep it on the road and in the desired line.
ESP is a registered trademark of the Robert Bosch GmbH and used originally for Mercedes-Benz.
ESP compares the driver's intended direction in steering and braking inputs, to the vehicle's response, via lateral acceleration, rotation (yaw) and individual wheel speeds. ESP then brakes individual front or rear wheels and/or reduces excess engine power as needed to help correct understeer (plowing) or overseer (fishtailing). ESP also integrates all-speed traction control, which senses drive-wheel slip under acceleration and individually brakes the slipping wheel or wheels, and/or reduces excess engine power, until control is regained. ESP cannot override a car's physical limits. If a driver pushes the possibilities of the car's chassis and ESP too far, ESP cannot prevent an accident.
Stability control equipment is now generally known as electronic stability control or ESC, a category recognized by the Society of Automotive Engineers. Electronic stability control combines anti-lock brakes, traction control and yaw control (yaw is spin around a vertical axis). To grasp how it works, think of steering a canoe. If you want the canoe to turn or rotate to the right, you plant the paddle in the water on the right to provide a braking moment on the right side. The canoe pivots or rotates to the right. ESC fundamentally does the same to assist the driver.
The electronic stability program (ESP) is a further enhancement to the anti-lock braking system (ABS) and traction control system (TCS). The ESP is designed to detect a difference between the driver's control inputs and the actual response of the vehicle. When differences are detected, the system intervenes by providing braking forces to the appropriate wheels to correct the path of the vehicle. This automatic reaction is engineered for improved vehicle stability, particularly during severe cornering and on low-friction road surfaces, by helping to reduce over-steering and under-steering.

To implement ESP functionality, additional sensors must be added to the ABS system. A steering wheel angle sensor is used to detect driver input with a yaw rate sensor and a low-G sensor that measure the vehicle response. Some ESP systems include a connection to the powertrain controller of the vehicle to enable reductions in engine torque when required.

As soon as skidding becomes imminent, the ESP Electronic Stability Program prevents it immediately. ESP continually monitors where the driver is steering and where the vehicle is actually going. When instability threatens, ESP selectively can brake each wheel individually, and intervene in the engine-management system. ESP stabilizes the vehicle and makes it more controllable in critical situations.
Rapid intervention: briefly applied braking pressure keeps the car on track
One of the strengths of the Electronic Stability Program is the speed with which it works: the sensing of oversteer and understeer, and the automatic braking intervention, are all completed within fractions of a second. For example if the rear of the car starts to swing out when taking a corner too fast, the ESP microcomputer first of all reduces engine power, thus increasing the lateral forces at the rear wheels. If this is not enough to eliminate the skidding tendency, the system also applies the brakes to the outer front wheel. The braking counteracts the critical rotational movement and restores stability. The simultaneous reduction in speed has further benefits for safety





Fig 1. Under steering drive
When ESP corrects the vehicle , it is not a one-off event which is completed after a brief application of the brakes. The stabilization is an ongoing process which is continuously adapted to take account of situational changes in the dynamics of the vehicle, until the risk of skidding is eliminated. This adaptive control requires the sensors and actuators in the Electronic Stability Program to react and adapt with extreme speed. The system has to cope not only with fast lane changes or patches of black ice, it must also function whatever the carload or tyre tread depth.
ESP was developed and tested with the aid of the most advanced techniques available, which systematically evaluated all potential malfunctions. Using these techniques, every conceivable system error was analyzed and methods were developed to eliminate the risk of malfunction. Amongst other things, the individual ESP components carry out self-checking routines at regular intervals. For example the vitally important yaw sensor is checked each time it supplies information, at intervals of just 20 milliseconds. The switch disables ESP's capability to reduce the engine torque. It also reduces the ESP intervention threshold to about 20%

How do I know ESP is working?
ESP monitors the vehicle's response to the driver's steering and braking inputs to detect oversteer or understeer. If sensors detect that a skidding condition is developing, ESP brakes individual front or rear wheels and/or reduces excess power as needed to help keep the vehicle going in the direction the driver is steering.
ESP could be realized especially through the remarkable progress in modern microelectronics. Sensors constantly record the driver and vehicle behavior and send their data to an electronic control unit. It compares the current driving condition with an appropriate nominal condition for the respective situation, and thus detects impending swerving within fractions of a second. If the car deviates from the calculated “ideal line”, ESP intervenes according to a special logic, and helps to keep the vehicle on track through accurately proportioned brake impulses at the front and rear axle as well as a reduction in engine torque. Thus, the system helps to correct driving errors and swerving movements, that are caused by slipperiness, wetness, gravel or other adverse road conditions. The stabilization takes place permanently – within the physical limits – and adjusts to the vehicle movements caused by the respective situation.
The triangle in the center of the speedometer flashes when ESP intervenes; either with ESP switched on or off. It's a reminder to adjust your speed to the prevailing road conditions, usually by reducing it. If instead one "steps on it", with ESP ON, the engine power may be reduced to prevent a potentially critical situation.
Electronic stability program
The standard-fitted ESP system selectively applies braking forces to the front and rear wheels in such a way as to reduce the risk of skids and slides and help the driver maintain control in critical situations. The system extends the technology of the anti-lock braking and acceleration skid control systems with a range of additional sensors which are used principally to detect yaw motion.
The ESP computer continuously compares the actual behaviour of the vehicle with the computed ideal values. The moment the car deviates from the direction intended by the driver, specially developed control logic causes the system to intervene with split-second speed to bring the car back on track. It does this in two ways:
1)Precisely controlled braking at one or more wheels 2)Reducing engine power.
ESP in this way helps to stabilize the vehicle in critical situations.

Active safety systems help prevent accidents]

1) Antilock braking system ABS
2) Traction control system TCS

Antilock Braking System (ABS)

Laying the groundwork for stability control, in the mid-80s Bosch brought the antilock braking system (ABS) to market through Mercedes and BMW. As most consumers probably know by now, ABS has become a standard feature on many new cars. It works by sensing and preventing wheel lock-up, thereby improving the vehicle's traction and enhancing steerability during hard braking.
1) Prevents the wheels from locking and thus allows avoiding obstacles.
2) The vehicle remains under control even while braking on one-sided slippery road.
3) The stopping distance is usually shortened compared to locked wheels

Traction Control System (TCS)
1) Fast interventions in engine management and brakes prevent the driven wheels from spinning
2) Safe drive off is possible even on one-sided slippery road
3) TCS prevents the vehicle from skidding when accelerating too much in a turn.

What does ESP do?
ESP actively enhances vehicle stability (staying in lane and in direction)

1) Through interventions in the braking system or the engine management.
2) To prevent critical situations (i.e. skidding), that might lead
to an accident
3) To minimize the risk of side crashes.

What is so special about ESP?
ESP watches out:-
1) Surveys the vehicle’s behavior (longitudinal and lateral dynamics)
2) Watches the driver’s commands (Steering angle, brake pressure, engine torque)
3) Is continuously active in the background.

ESP knows:

Recognizes critical situations –in many cases before the driver does
Considers the possible ways of intervening:
Wheel-individual brake pressure application Intervention in the
engine management.
Frequent cause for accidents:
The driver loses control of his vehicle. I.e. through
1) Speeding
2) Misinterpretation of the course or the road condition
3) Sudden swerving.
25% of all accidents involving severe personal injury are caused by skidding.
(Source: GDV – General Association of German Insurance Companies)

60% of all accidents with fatal injuries are caused by side crashs. These side crashs are mainly caused by skidding because of excessive speed, driving errors or excessive steering movements Source: GDV – General Association of German Insurance Companies)

What are the components of ESP?
The Bosch ESP components:

1) Hydraulic modulator with attached ECU
2) Wheel-speed sensors
3) Steering-angle sensor
4) Yaw-rate and lateral acceleration sensor
5) Communication with engine management.
First generation: complex hydraulic system develops braking pressure
The central control unit of the first-generation ESP system comprised two microprocessors, each with 48 kilobytes of memory, and the hydraulic system consisted of a pressurising pump, a charge piston and a central hydraulic unit. The pressurising pump was required for fast and reliable development of braking pressure under all temperature conditions. The hydraulic unit distributed the pressure individually to the wheels

WORKING

The heart of ESP is a yaw velocity sensor which resembles the ones used in aircraft and space vehicles. Like a compass, it constantly monitors the exact altitude of the car and registers every incipient spin. Other sensors report how high the current brake pressure is, what the position of the steering wheel is, how great the lateral acceleration is, what the speed is and how big the difference in wheel speed is. Whenever handling becomes instable, the necessary commands are executed and the vehicle is brought under control in a fraction of a second.
The ESP continuously compares the actual driving condition conveyed by the sensor with the driver’s intention. The software recognizes where the driver wants to steer via the anti-lock brake system sensors from the speeds of the four wheels and by the steering movement. This is recorded by another sensor mounted on the steering column. If there is now a difference between the driver’s intent determined in a vehicle model inside the computer and the current driving condition, which cannot be compensated without increased effort by the driver, ESP becomes active—upto 150 times per second. If there is a tendency to understeer, ESP brakes the wheel on the inside of curve as a priority and brings the car back on to the desired course. If the vehicle is oversteering toward the edge of the road, ESP brakes the outside of the car generating a moment opposed to the yawing moment that results in a driving condition that is easy to correct.

ESP works according the principle of an “observer”: Sensors acquire driver and vehicle behavior, send their data to a powerful microcomputer, that is loaded with a mathematical model. Thus, the actual state of the vehicle is compared with a nominal state appropriate for the respective situation, and impending swerving is detected.

From a physical standpoint, that swerving is nothing else than the turning of the vehicle about its own vertical axis. The faster that turning takes place, the greater the swerving movement and accident risk. Only: For the reliable measurement of the rotational speed, a complicated system was required, which then only existed in aerospace technology: a so-called turning or yaw rate sensor, which was too susceptible to breakdown and too expensive for automotive applications
however. Therefore, the experts developed a comparable measuring element for the stability program, which consists of a small hollow cylinder made of steel. Quartzes excite it to defined vibrations,that shift through the rotational movement of the vehicle. To compensate for that shift, an electrical potential was needed, whose value is the measuring signal for the rotational speed of the car.
In addition to the rotational speed, the ESP computer processes additional sensor information through the respective desire of the driver and the actual behavior of the vehicle:

The steering angle sensor measures the turn-in of the steering wheel and thus acquires where the driver wants to go.

The wheel speed sensors register the speed of the vehicle and the tendency of the wheels to loose adhesion with the road surface.

The lateral acceleration sensor detects when the vehicle is drifting off laterally.

The rotational speed sensor is the heart of the electronic stability program. It measures the rotational movement of the vehicle. If it deviates from the calculated “ideal line”, impending swerving is detected.

The admission pressure sensor detects the brake pressure input by the driver. In addition, the ESP ECU is connected with the engine and automatic transmission per CAN data bus (Controller Area Network), so that it also receives the current data about engine torque, gas pedal position and gear ratio at any time.

Constant Control: ESP Is Ready in Any Driving Situation

While driving, the ESP computer constantly compares the actual vehicle behavior with the programmed nominal values. If the vehicle deviates from the safe “ideal line”, the system intervenes with lightning speed according to a specially developed logic, and can bring the vehicle back onto the right course in two ways: Through exactly measured out brake impulses at the front or rear axle, and through reduction of the engine torque. Here, within the limits of physics, ESP corrects driving errors as well as swerving movements caused by ice, wetness, gravel or other adverse road surface conditions, where the driver

Normally hardly has a chance anymore to keep his vehicle on track through steering or braking maneuvers. Therefore, the system – compared with traction control – is always ready: While braking, during acceleration or when coasting.

As soon as skidding becomes imminent, the ESP Electronic Stability Program prevents it immediately. ESP continually monitors where the driver is steering and where the vehicle is actually going. When instability threatens, ESP selectively can brake each wheel individually, and intervene in the engine-management system. ESP stabilizes the vehicle and makes it more controllable in critical situations.
Advantages of ESP
1) Improves moving-off and acceleration capabilities by increasing traction; especially useful on road surfaces with different levels of grip and when cornering.
2) Improves active dynamic safety, since only a wheel which is not spinning can provide optimum traction with no loss of lateral stability.
3) Automatically adapts the engine torque to suit the ability of the wheels to transmit this to the road when the driver applies too much throttle.
4) Reduces the danger of traction loss under all road conditions by automatically stabilizing the vehicle during braking, acceleration and in spins.
5) Significantly improves the directional stability of the vehicle when cornering-up to the limit range.





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CONCLUSION
. Numerous international studies have confirmed the effectiveness of ESC in helping the driver maintain control of the car, help save lives and reduce the severity of crashes. In the fall of 2004 in the U.S., the National Highway and Traffic Safety Administration confirmed the international studies, releasing results of a field study in the U.S. of ESC effectiveness. NHTSA concluded that ESC reduces crashes by 35%. The prestigious Insurance Institute for Highway Safety later issued their own study that concluded the widespread application of ESC could save 7,000 lives a year. That makes ESC the greatest safety equipment development since seat belts, according to some experts. Other manufacturers use electronic stability control systems under different marketing names:
ESP reduces the danger of spinning in curves, in avoidance maneuvers or during braking, and supports the driver in better controlling critical situations. “The current analysis of the accident statistics shows, that ESP makes an important contribution to accident prevention, and is therefore as significant for traffic safety as ABS, seat belt and airbag,”
Practice shows that vehicle dynamic control systems like ESP are capable of making skidding avoidable or at least increase control. With their widespread introduction a substantial decrease in the number of serious accidents could be expected.
RESIKO-Survey of GDV – General Association of German Insurance Companies)

REFERENCES



• ‘Robert Bosch GmbH-systems and products for Automobile Manufacturer.
• Vehicle stability enhancement systems-TRAXXAR
• ESP-Electronic stability program by Bosch.
• www.autoweb.com.au
• www.answers.com