1. INTRODUCTION
The best protection against crashes is to avoid them completely, but sometimes, even good drivers encounter circumstances that are out of their control. Fortunately, a revolutionary active safety technology, Electronic Stability Control (ESC), serves as a vehicle's sixth sense by anticipating crashes before they occur and automatically intervening to prevent them. ESC provides enhanced stability and control in all driving situations, significantly reducing the risk of a crash, especially on... twisting, country roads or highway ramps. ESC helps to prevent rollovers, skids, spins, and can keep your vehicle on the road.
ECS IN ACTION
When avoiding an obstacle, negotiating a sharp curve, or while driving in bad weather, ESC is there to help. ESC is especially useful in protecting you during oversteering, understeering and in potential rollover situations.
ADVANTAGEOUS OR NOT?
With the invention of the Electronic Stability Program ESP®, manufacturers make an important contribution to the improvement of driving safety. The unique system can defuse accident-prone situations by the detection of delayed or inappropriate reactions of the driver and targeted correction of his steering or braking errors – all the way to the limit area, but always within the laws of driving physics.
Based on sensor signals and simulations, the electronics detect dangerous moments before the driver can even respond. Therefore, ESP® can intervene extremely quickly when the situation gets serious. Thus, the system complements the practice oriented safety concept of Manufacturers in an important point: accident avoidance.
2. HOW ESC WORKS
Electronic Stability Control (ESC) is a stability enhancement system designed to electronically detect and assist the driver in critical driving situations. ESC compares a driver's intended course with the vehicle's actual movement. When instability is detected, ESC automatically applies brakes to individual wheels and can also reduce engine torque to help keep you on track.
ESC is your safety co-pilot, especially during cornering and avoidance maneuvers. The best part is that the system works automatically, so you can concentrate on driving. The ESC system relies on information from:
Wheel speed sensors
Steering wheel angle sensor
Yaw rate and lateral acceleration sensors
Master cylinder pressure sensor
Antilock Braking System (ABS) – Controls break pressure to help prevent wheel lock-up during braking, so you can steer and maneuver around obstacles during braking.
Traction Control System (TCS) – Applies brakes at drive wheels and reduces engine torque to help reduce wheel spin during acceleration. It works across a full range of speeds, whether you're accelerating after a stop or passing on the highway.
Electronic Stability Control (ESC) – Applies brakes to individual wheels and reduces engine torque to help correct oversteer and understeer. It helps you maintain control in all weather conditions, helping to prevent skids, spins and rollovers.
UNDERSTEER
It's late at night and you are on your way back from a weekend trip. You decide to take a new route home. Unfamiliar with the off-ramp, you miscalculate the sharpness of the curve and the front of your vehicle begins to slide to the outside of the turn – a situation known as understeer. ESC begins to work in a split-second.
As the vehicle begins the turn in an understeering situation, the front-end of the vehicle slides out of the lane and out of the driver's control as the wheels lose traction.
ESC automatically applies the inside rear brake to help you achieve your desired turn. Depending on vehicle speed, the system may also reduce engine torque.
OVERSTEER
On an apple-picking outing, you and your family explore the scenic back roads of the country. The rain holds off until you are ready to leave. Everyone piles into the vehicle and you begin to navigate your way back home. During the trip, you encounter an unexpected sharp turn. You jerk the wheel, and the rear-end of the vehicle begins to slip or "fishtail" – a situation known as oversteer. Luckily, ESC is there to help you stay on track.
As the vehicle begins to oversteer, the rear wheels lose traction and you could start to spin out of control.
However, ESC automatically applies the outside front brake to correct the problem and help to prevent a dangerous situation before it happens.
ROLLOVER
You are driving at dusk, the road is wet, you have poor visibility and an animal suddenly runs out in front of you. You slam on the brakes and quickly swerve left then right to avoid it.
In similar circumstances, some vehicles may become unstable and enter into a "tripped rollover" by leaving the road. However, your ESC equipped vehicle stays on the road, keeping you and your family safe.
ESC also helps to prevent vehicles from going sideways or skidding, another condition that contributes to an "on-road rollover."
3.INCURSION
In principle, this event involves the driver performing a double lane change to avoid an obstacle while driving at a high rate of speed. Specifically, the event is designed around a hidden drive way out of which another vehicle backs out in to the path of the driver. The event timing was design to ensure that steering is necessary to avoid colliding with the incurring vehicle. At the same time, on coming traffic was used as a motivator to the second part of the double lane change manure.
To ensure that driver do not slow down in anticipation of a critical event they encounter a similar situation before reaching the actual event nothing of interest takes place when passing a similar house containing various stationary vehicles arranged in a similar configuration .
The posted limit is 65 mph and the road profile used for this scenario is identical to the profile used in the curve departure scenario.
4. TECHNICAL OVERVIEW
The idea behind Electronic Stability Control is simple. It is an active safety system that helps you prevent skidding that can occur in all kinds of weather and on all kinds of roads; in situations where even the best of drivers might struggle to keep their vehicles on the road.
Sensing Safety The system has sensors that compare a vehicle's behavior in relation to steering wheel position. When ESC detects a discrepancy between a driver's intended path and the vehicle's actual path, it intervenes to help bring the movement of the vehicle back in line with your intentions.
Stability The stabilizing effect is based on calculations performed by a microcomputer that evaluates signals from the ESC sensors. The steering-angle sensor registers the steering-wheel position, and wheel speed sensors mounted at each wheel provide the wheel speeds. The data from these sensors enables the system's microcomputer to recognize what maneuvers you intend to perform.
The Heart of the Matter The yaw rate sensor is at the heart of the ESC system. It registers the vehicle's rotation around its vertical axis. A highly sensitive lateral-acceleration sensor reacts to the centrifugal forces that are generated when cornering.
Step 1 The sensors provide a microcomputer known as the Electronic Control Unit (ECU) with information it needs to evaluate the vehicle's behavior at a given instant.
Step 2 The microcomputer uses this information to continuously compare the vehicles actual and the desired movement, and intervenes immediately if the vehicle shows a tendency to leave the intended path of travel.
Step 3 Using programmed set points, which are precisely tailored to the particular vehicle and specific logic operations, the microcomputer, calculates the steps which need to be taken within a fraction of a second.
Step 4 ESC then transmits the appropriate commands to the braking system, which applies a precisely defined brake pressure at appropriate wheels.
Step 5 ESC can also reduce the engine torque when understeering or if wheel spin is detected during acceleration.
5. DEFINATIONS
Electronic Stability Control is a closed-loop stability-control system that relies on proven ABS (Antilock Brake System) and TCS (Traction Control System) components. It incorporates sensors for determining vehicle parameters as well as an ECU (Electronic Control Unit) to modulate braking and traction forces.
Sensors – ESC utilizes the following sensors to deliver the safest driving experience possible:
Wheel speed sensors
Steering wheel angle sensors
Yaw rate and lateral acceleration sensors
Master cylinder pressure sensor
Steering Wheel Angle Sensor – measures the steering wheel angle and steering input rate.
Wheel Speed Sensors – deliver electric pulses from which the ECU (Electronic Control Unit) computes the speed of the wheels. The vehicle speed is derived from the rotational speeds of all wheels by means of a special computational algorithm.
Yaws Rate Sensor – is a gyroscopic sensor that monitors the vehicle's rotation around its vertical axis.
Lateral Acceleration Sensor – measures the acceleration of the vehicle in the direction of the vehicle's lateral axis (side to side motion).
Electronic Control Unit (ECU) – the microcomputer that processes and interprets information from each sensor and then generates the necessary activation commands to control brake pressure and engine torque.
Wheel-speed sensors
The electronic control unit (ECU) uses the signals from the individual wheel-speed sensors to compare the speeds of the wheels. Two different operating principles are used: passive (inductive) and active (Hall) speed sensors. Active sensors can also recognize the direction of wheel rotation.
Hydraulic modulator with attached ECU
Through commands from the ECU, the hydraulic modulator regulates the pressure in the individual brake cylinders by means of solenoid valves. The hydraulic unit is located in the brake system between the master cylinder and the brake cylinders. The ECU controls all electrical and electronic tasks as well as controls the functions of the system.
Steering-wheel angle sensor
The combination of signals from the wheel-speed sensors and steering-wheel angle sensor provides the input for ESC to interpret the intended course of the vehicle. The steering-wheel angle sensor has a working range of 720° with a tolerance of ± 5°.
ESC communicates with engine management systems
Involved engine management system components include:
- Engine ECU
- Accelerator-pedal sensor
- Throttle-valve actuator
- Fuel injectors
- Ignition module
Yaw-rate and acceleration sensor
A yaw-rate sensor detects the vehicle's rotational movements around its vertical axis.
The principle for measuring rate of yaw is based on how forces (known as Coriolis forces) act on a mass moving forward in a rotating system (this movement is termed "translation"). If there are oscillating mass elements in this system, this oscillatory action is affected once the system starts to rotate. The manipulated variable needed to return the oscillatory motion produces a voltage which is translated into a measurement of the yaw rate.
6. PRINCIPLE OF ESP OPERATION
7. SAFETY
Mercedes Passenger Cars Get Into Fewer Accidents
‧Accident rate dropped by 15 percent especially due to standard ESP
‧Share in driving accidents with serious consequences declining greatly since 2000
‧Stability program makes important contribution to accident prevention
The number of Mercedes passenger cars with road traffic accidents has declined sharply since the series production/standard employment of the Electronic Stability Program ESP.
This is shown by a representative random sample analysis of the accident data* of the Federal Statistics Office, which Mercedes-Benz published today. Accordingly, the accident rate of Mercedes Models first licensed in 2000/2001 compared with 1999/2000 has dropped by 15 percent, and thus dropped by 4 percentage points more than the average of other automobile manufacturers. Mercedes-Benz analyzed a total of over 1.5 million traffic accidents. As the first automobile brand, Mercedes-Benz put the Electronic Stability Program ESP into all of it's passenger car models, starting in the summer of 1999, as standard equipment, which 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," says Dr. Hans-Joachim Schöpf, head of passenger car development at Mercedes-Benz.
Reduction of Casualty rate by Extent of Injury for Toyota in Japan
Toyota has named its program as Vehicle Stability Control
8. HOW EFFECTIVE IS IT?
The results are extraordinary, in that the effectiveness of ESP is large and consistent, and just on an isolated accident type or event it is therefore essential, that the method and the possible bias are challenged. There are a few possible explanations to the results, given that ESP was effective in reducing the accidents. Firstly, if rear end impacts was an accident type, where ESP drivers showed their aggressiveness and risk compensation more, than in all other accident types, the results could show the same profile as in this study. The fact that both ESP cars and non-ESP cars show the same profile in being target and bullet vehicles in rear is an indication of that this assumption does not show in reality. Secondly, same results could be present if ESP cars were exposed to driving on low friction surface. Such factor is very complicated to analyze with induced exposure. It is, given the way the data was assembled, not a plausible explanation. Still, the study should be repeated also in Sweden, also to study the long term effects of ESP. In the meantime, the likely explanation to the results that ESP in fact is effective. The method used in the present study, does not allow an analysis on the actual function of the system, and in what sequence of driving it has its potential.
Wheather ESP works as an intelligent system to warn the driver about low friction, or if it has a direct function in the driver-vehicle loop in critical manoeuvres, either in controlling stability and/or reduce speed, was not possible to study. It was not possible to study to what extent the effectiveness varied over accident types or events. Further studies should try to analyze this in-depth. It should also be analyzed, if the severity of impacts is reduced by ESP. A recent development of ESP that would most probably increase the effectiveness is the link between the ESP being active and the passive safety system of the car being alerted and start to act. In Mercedes-Benz S-Class update 2002, both the ESP and the brake assist system will trigger a reversible seat belt pretension, to bring the occupant in a favorable position before a possible crash. Also other functions of the vehicle are also triggered. In this sense, the ESP is used as a device for increasing the crash protection of the car. The effectiveness of ESP is promising enough, to stimulate automotive industry and consumers to implement ESP in new cars as quickly as possible. Therefore, consumers should be advised to choose cars with ESP, especially in countries with wet and icy road conditions. While the effectiveness on dry roads was not significant, accidents on both wet as well as roads covered with snow or ice were significantly reduced.
9. CONCLUSION
- ESC is found to reduce accidents with personal injuries.
- The effectiveness on all accidents, except rear-end impacts on dry road surface, was 1.1% to 43.1%, with best estimate 22.1%
- The effectiveness on roads with lower friction was substantially higher, 7.8-55.2% and 12.1-64.3% (best estimates 31.8 and 38.2%), for wet roads and roads covered with ice or snow, respectively.
- The results broken down to vehicle types were consistent with the above results, in some cases though not statistically significant.
A new database, EACS (the European Accident Causation Survey) was used for the first time to estimate the potential opportunity for an active safety system based on the number of accidents in which it is judged that the presence of the system would have influenced the outcome. The EACS data suggest that ESC could have a probable or definite influence in about 67% of fatal accidents involving loss of vehicle control and 42% of the corresponding injury accidents. However, some cautions about potential effectiveness estimates produced in this way have been raised. In particular, data consistency between teams in different countries and lack of national or international representivity in the EACS sample were identified as particular concerns.
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