Electronic stability control
Electronic stability control, also referred to as electronic stability program or dynamic stability control, is a computerized technology that improves a vehicle's stability by detecting and reducing loss of traction. When ESC detects loss of steering control, it automatically applies the brakes to help steer the vehicle where the driver intends to go. Braking is automatically applied to wheels individually, such as the outer front wheel to counter oversteer, or the inner rear wheel to counter understeer. Some ESC systems also reduce engine power until control is regained. ESC does not improve a vehicle's cornering performance; instead, it helps reduce the chance of the driver losing control of the vehicle on a slippery road.
According to the U.S. National Highway Traffic Safety Administration and the Insurance Institute for Highway Safety in 2004 and 2006, one-third of fatal accidents could be prevented by the use of this technology. In Europe the electronic stability program had saved an estimated 15,000 lives as of 2020. ESC became mandatory in new cars in Canada, the US, and the European Union in 2011, 2012, and 2014, respectively. Worldwide, 82 percent of all new passenger cars feature the anti-skid system.
History
In 1983, a four-wheel electronic "Anti-Skid Control" system was introduced on the Toyota Crown. In 1987, Mercedes-Benz, BMW and Toyota introduced their first traction control systems. Traction control works by applying individual wheel braking and throttle to maintain traction under acceleration, but unlike ESC, it is not designed to aid in steering.In 1990, Mitsubishi released the Diamante in Japan. Developed to help the driver maintain the intended line through a corner; an onboard computer monitored several vehicle operating parameters through various sensors. When too much throttle had been used when taking a curve, engine output and braking were automatically regulated to ensure the proper line through a curve and to provide the proper amount of traction under various road surface conditions. While conventional traction control systems at the time featured only a slip control function, Mitsubishi's TCL system had an active safety function, which improved course tracing performance by automatically adjusting the traction force, thereby restraining the development of excessive lateral acceleration while turning. Although not a 'proper' modern stability control system, trace control monitors steering angle, throttle position and individual wheel speeds, although there is no yaw input. The TCL system's standard wheel slip control function enabled better traction on slippery surfaces or during cornering. In addition to the system's individual effect, it also worked together with the Diamante's electronically controlled suspension and four-wheel steering to improve total handling and performance.
BMW, working with Bosch and Continental, developed a system to reduce engine torque to prevent loss of control and applied it to most of the BMW model line for 1992, excluding the E30 and E36. This system could be ordered with the winter package, which came with a limited-slip differential, heated seats, and heated mirrors. From 1987 to 1992, Mercedes-Benz and Bosch co-developed a system called Elektronisches Stabilitätsprogramm to control lateral slippage.
Introduction, second generation
In 1995, three automobile manufacturers introduced ESC systems. Mercedes-Benz, supplied by Bosch, was the first to implement ESP with their Mercedes-Benz S 600 Coupé. Toyota's Vehicle Stability Control system appeared on the Toyota Crown Majesta in 1995.General Motors worked with Delphi Automotive and introduced its version of ESC, called "StabiliTrak", in 1996 for the 1997 model year on select Cadillac models. StabiliTrak was made standard equipment on all GM SUVs and vans sold in the U.S. and Canada by 2007, except for certain commercial and fleet vehicles. While the StabiliTrak name is used on most General Motors vehicles for the U.S. market, "Electronic Stability Control" is used for GM's overseas brands, such as Opel, Holden and Saab, except in the cases of Saab's 9-7X and 9-4X.
The same year, Cadillac introduced an integrated vehicle handling and software control system called the Integrated Chassis Control System, on the Cadillac Eldorado. It involves an omnibus computer integration of engine, traction control, Stabilitrak electronic stability control, steering, and adaptive continuously variable road sensing suspension, with the intent of improving responsiveness to driver input, performance, and overall safety, similar to Toyota/Lexus Vehicle Dynamics Integrated Management.
In 1997, Audi introduced the first series production ESP for all-wheel drive vehicles. In 1998, Volvo Cars began to offer their version of ESC called Dynamic Stability and Traction Control on the new Volvo S80. Meanwhile, others investigated and developed their own systems.
During a moose test, Swedish journalist Robert Collin of Teknikens Värld rolled a Mercedes A-Class at 78 km/h in October 1997. Because Mercedes Benz promoted a reputation for safety, they recalled and retrofitted 130,000 A-Class cars with firmer suspension and sportier tyres; all newly produced A- class featured ESC as standard along with the upgraded suspension and wheels. This produced a significant reduction in crashes, and the number of vehicles with ESC rose. The availability of ESC in small cars like the A-Class ignited a market trend; thus, ESC became available for all models.
Ford's version of ESC, called AdvanceTrac, was launched in the year 2000. Ford later added Roll Stability Control to AdvanceTrac which was first introduced in the Volvo XC90 in 2003. It has been implemented in many Ford vehicles since.
Ford and Toyota announced that all their North American vehicles would be equipped with ESC standard by the end of 2009. However, as of November 2010, Ford still sold models in North America without ESC. General Motors had made a similar announcement for the end of 2010.
Third generation and after
In 2003 in Sweden the purchase rate on new cars with ESC was 15%. The Swedish road safety administration issued a strong ESC recommendation and in September 2004, 16 months later, the purchase rate was 58%. A stronger ESC recommendation was then given and in December 2004, the purchase rate on new cars had reached 69% and by 2008 it had grown to 96%. ESC advocates around the world are promoting increased ESC use through legislation and public awareness campaigns and by 2012, most new vehicles should be equipped with ESC.Legislation
In 2009, the European Union decided to make ESC mandatory. Since November 1, 2011, EU type approval is only granted to models equipped with ESC. Since November 1, 2014, ESC has been required on all newly registered cars in the EU.The NHTSA required all new passenger vehicles sold in the US to be equipped with ESC as of the 2012 model year, and estimated it will prevent 5,300–9,600 annual fatalities.
Concept and operation
During normal driving, ESC continuously monitors steering and vehicle direction. It compares the driver's intended direction to the vehicle's actual direction.Normal operation
ESC intervenes only when it detects a probable loss of steering control, such as when the vehicle is not going where the driver is steering. This may happen, for example, when skidding during emergency evasive swerves, understeer or oversteer during poorly judged turns on slippery roads, or hydroplaning. During high-performance driving, ESC can intervene when unwanted, because steering input may not always be indicative of the intended direction of travel. ESC estimates the direction of the skid, and then applies the brakes to individual wheels asymmetrically in order to create torque about the vehicle's vertical axis, opposing the skid and bringing the vehicle back in line with the driver's commanded direction. Additionally, the system may reduce engine power or operate the transmission to slow the vehicle down.ESC can function on any surface, from dry pavement to frozen lakes. It reacts to and corrects skidding much faster and more effectively than the typical human driver, often before the driver is even aware of any imminent loss of control. This has led to some concern that ESC could allow drivers to become overconfident in their vehicle's handling and/or their own driving skills. For this reason, ESC systems typically alert the driver when they intervene, so that the driver is aware that the vehicle's handling limits have been reached. Most activate a dashboard indicator light and/or alert tone; some intentionally allow the vehicle's corrected course to deviate very slightly from the driver-commanded direction, even if it is possible to more precisely match it.
All ESC manufacturers emphasize that the system is not a performance enhancement nor a replacement for safe driving practices, but rather a safety technology to assist the driver in recovering from dangerous situations. ESC does not increase traction, so it does not enable faster cornering. More generally, ESC works within the limits of the vehicle's handling and available traction between the tyres and road. A reckless maneuver can still exceed these limits, resulting in loss of control. For example, during hydroplaning, the wheels that ESC would use to correct a skid may lose contact with the road surface, reducing its effectiveness.
Due to the fact that stability control can be incompatible with high-performance driving, many vehicles have an override control which allows the system to be partially or fully deactivated. In simple systems, a single button may disable all features, while more complicated setups may have a multi-position switch or may never be fully disengaged.
Off-road use
ESC systems—due to their ability to enhance vehicle stability and braking—often work to improve traction in off-road situations, in addition to their on-road duties. The effectiveness of traction control systems can vary significantly, due to the significant number of external and internal factors involved at any given time, as well as the programming and testing performed by the manufacturer.At a rudimentary level, off-road traction varies from typical operational characteristics of on-road traction, depending on the terrain encountered. In an open differential setup, power transfer takes the path of least resistance. In slippery conditions, this means when one wheel loses traction, power will counter-productively be fed to that axle instead of the one with higher grip. ESCs focus on braking wheels that are spinning at a rate drastically different from the opposing axle. While on-road application often supplements rapidly intermittent wheel braking with a reduction of power in loss-of-traction situations, off-road use will typically require consistent power delivery to retain vehicle momentum while the vehicle's braking system applies intermittent braking force over a longer duration to the slipping wheel until excessive wheel-spin is no longer detected.
In intermediate level ESC systems, ABS will be disabled, or the computer will actively lock the wheels when brakes are applied. In these systems, or in vehicles without ABS, the performance in emergency braking in slippery conditions is greatly improved as grip state can change extremely rapidly and unpredictably off-road when coupled with inertia. When the brakes are applied and wheels are locked, the tyres do not have to contend with the wheel rolling and braking repeatedly. Grip provided by the tyres is constant and as such can make full use of traction wherever it is available. This effect is enhanced where more aggressive tread patterns are present as the large tread lugs dig into the imperfections on the surface or below the substrate, as well as dragging dirt in front of the tyre to increase the rolling resistance even further.
Many newer vehicles designed for off-road duties from the factory, are equipped with Hill Descent Control systems to minimise the risk of such runaway events occurring with novice drivers and provide a more consistent and safe descent than either no ABS, or on-road orientated ABS. These systems aim to keep a fixed speed while descending, applying strategic braking or acceleration at the correct moments to ensure wheels all rotate at the same rate while applying full locking braking when required.
In some vehicles, ESC systems automatically detect whether to operate in off- or on-road mode, depending on the engagement of the 4WD system. Mitsubishi's unique Super-Select 4WD system, operates in on-road mode in 2WD as well as 4WD High-range with the centre differential unlocked. However, it automatically activates off-road traction control and disables ABS braking when shifted into 4WD High-range with centre differential locked, or 4WD Low-range with centre differential locked. Most modern vehicles with fully electronically controlled 4WD systems such as various Land Rovers and Range Rovers, also automatically switch to an off-road-orientated mode of stability and traction control once low range, or certain terrain modes are manually selected.