Electronic stability control
Electronic stability control (ESC) is a computerized technology that improves the safety of a vehicle's stability by detecting and minimizing skids. When ESC detects loss of steering control, ESC automatically applies the brakes to help "steer" the vehicle where the driver intends to go. Braking is automatically applied to individual wheels, 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. Electronic stability control does not improve a vehicle's cornering performance; rather it helps to minimize a loss of control. The NHTSA estimates 5,300-9,600 traffic fatalities could be avoided if all passenger vehicles were equipped with the feature. According to the IIHS one-third of fatal accidents could be prevented by the technology.
In 1987, the earliest innovators of ESC, Mercedes-Benz, BMW and Toyota introduced their first traction control systems. Traction control works by applying individual wheel braking and throttle to keep traction while accelerating but unlike ESC it is not designed to aid in steering.
In 1990, Mitsubishi released the Diamante (Sigma) in Japan. It featured a new electronically controlled active trace & traction control system (the first integration of these two systems in the world) that Mitsubishi developed. Simply named TCL in 1990, the system has since evolved into Mitsubishi's modern Active Skid and Traction Control (ASTC) system or ESC. 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 has been used when taking a curve, engine output and braking are 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 newly developed TCL system had a preventive (active) safety function which improved the course tracing performance by automatically adjusting the traction force (called "trace control") thereby restraining the development of excessive lateral acceleration while turning. Although not a ‘true’ 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 enables better traction on slippery surfaces or during cornering. In addition to the TCL system's individual effect, it also works together with Diamante's electronic controlled suspension and four-wheel steering that Mitsubishi had equipped to improve total handling and performance.
BMW, working with Robert Bosch GmbH and Continental Automotive Systems, developed a system to reduce engine torque to prevent loss of control and applied it to the entire BMW model line for 1992. From 1987 to 1992, Mercedes-Benz and Robert Bosch GmbH co-developed a system called Elektronisches Stabilitätsprogramm (Ger. "Electronic Stability Programme" trademarked as ESP) a lateral slippage control system, the electronic stability control (ESC).
In 1995, automobile manufacturers introduced electronic stability control systems. Mercedes-Benz supplied by Bosch was the earliest with their W140 S-Class model. That same year BMW, supplied by Bosch and ITT Automotive (later Continental) and Volvo began offering ESC on some of their models while Toyota's own Vehicle Stability Control system (also in 2004, a preventive system called VDIM) appeared on the Crown Majesta. Meanwhile others investigated and developed their own systems.
During a moose test (swerving to avoid an obstacle) which became famous in Germany as "the Elk test" the Swedish journalist Robert Collin of Teknikens Värld (World of Technology) during October 1997 and the number of vehicles with ESC rose. Today virtually all premium brands have made ESC standard on all vehicles, and the number of models with ESC continues to increase. Ford and Toyota have announced that all their North American vehicles will be equipped with ESC standard by the end of 2009 (Toyota SUVs standard in 2004, Toyota has yet to fit the Scion tC and Yaris). General Motors had made a similar announcement for the end of 2010. The NHTSA requires all passenger vehicles to be equipped with ESC by 2011 and estimates it will prevent 5,300-9,600 annual fatalities once all passenger vehicles are equipped with the system.
During normal driving, ESC works in the background, continuously monitoring steering and vehicle direction. ESC compares the driver's intended direction (by measuring steering angle) to the vehicle's actual direction (by measuring lateral acceleration, vehicle rotation (yaw), and individual road wheel speeds).
ESC only intervenes when it detects loss of steering control, i.e. 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. ESC calculates (since all states are not readily available, this calculation is actually an "estimation" according to the control terminology) 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 work 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. In fact, this 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 inform the driver when they intervene, so that the driver knows that the vehicle's handling limits have been approached. 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.
Indeed, 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 (although it can facilitate better-controlled cornering). More generally, ESC works within inherent limits of the vehicle's handling and available traction between the tires and road. A reckless maneuver can still exceed these limits, resulting in loss of control. For example, in a severe hydroplaning scenario, the wheel(s) that ESC would use to correct a skid may not even initially be in contact with the road, reducing its effectiveness.
In July 2004, on the Crown Majesta, Toyota offered a Vehicle Dynamics Integrated Management (VDIM) system that incorporated formerly independent systems including ESC and worked not only after the skid was detected but also worked to prevent the skid from occurring in the first place. Using electric variable gear ratio steering power steering this more advanced systems could also alter steering gear ratios and steering torque levels to assist the driver in evasive maneuvers.
Numerous studies around the world confirm that ESC is highly effective in helping the driver maintain control of the car and saving lives and reducing 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. The National Highway Traffic Safety Administration in United States concluded that ESC reduces crashes by 35%. Additionally, Sport utility vehicles (SUVs) with stability control are involved in 67% fewer accidents than SUVs without the system. The United States Insurance Institute for Highway Safety (IIHS) issued its own study in June 2006 showing that up to 10,000 fatal US crashes could be avoided annually if all vehicles were equipped with ESC The IIHS study concluded that ESC reduces the likelihood of all fatal crashes by 43%, fatal single-vehicle crashes by 56%, and fatal single-vehicle rollovers by 77-80%.
ESC is described as the most important advance in auto safety since the seat belt by many experts including Nicole Nason, Administrator of the NHTSA, Jim Guest and David Champion of Consumers Union Max Mosley of the Fédération Internationale de l'Automobile (FIA), E-Safety Aware, Csaba Csere, editor of Car and Driver, and Jim Gill, long time ESC proponent of Continental Automotive Systems. The European New Car Assessment Program (EuroNCAP) "strongly recommends" that people buy cars fitted with stability control.
The IIHS requires that a vehicle must have ESC as an available option in order for it to qualify for their Top Safety Pick award for occupant protection and accident avoidance.
Components and design
ESC incorporates yaw rate control into the anti-lock braking system (ABS). Yaw is rotation around the vertical axis; i.e. spinning left or right. Anti-lock brakes enable ESC to brake individual wheels. Many ESC systems also incorporate a traction control system (TCS or ASR), 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. However, ESC achieves a different purpose than ABS or Traction Control.
The ESC system uses several sensors to determine what the driver wants (input). Other sensors indicate the actual state of the vehicle (response). The control algorithm compares driver input to vehicle response and decides, when necessary, to apply brakes and/or reduce throttle by the amounts calculated through the state space (set of equations used to model the dynamics of the vehicle). The ESC controller can also receive data from and issue commands to other controllers on the vehicle such as an all wheel drive system or an active suspension system to improve vehicle stability and controllability.
The sensors used for ESC have to send data at all times in order to detect possible defects as soon as possible. They have to be resistant to possible forms of interference (rain, holes in the road, etc.). The most important sensors are:
- Steering wheel angle sensor: determines the driver's intended rotation; i.e. where the driver wants to steer. This kind of sensor is often based on AMR-elements.
- Yaw rate sensor : measures the rotation rate of the car; i.e. how much the car is actually turning. The data from the yaw sensor is compared with the data from the steering wheel angle sensor to determine regulating action.
- Lateral acceleration sensor: often based on the Hall effect. Measures the lateral acceleration of the vehicle.
- Wheel speed sensor : measures the wheel speed.
Other sensors can include:
- Longitudinal acceleration sensor: similar to the lateral acceleration sensor in design but can offer additional information about road pitch and also provide another source of vehicle acceleration and speed.
- Roll rate sensor: similar to the yaw rate sensor in design but improves the fidelity of the controller's vehicle model and correct for errors when estimating vehicle behavior from the other sensors alone.
ESC uses a hydraulic modulator to assure that each wheel receives the correct brake force. A similar modulator is used in ABS. ABS needs to reduce pressure during braking, only. ESC additionally needs to increase pressure in certain situations and an active vacuum brake booster unit may be utilized in addition to the hydraulic pump to meet these demanding pressure gradients.
The heart of the ESC system is the Electronic Control Unit (ECU). The various control techniques are embedded in it. Often, the same ECU is used for diverse systems at the same time (ABS, Traction control system, climate control, etc.). The input signals are sent through the input-circuit to the digital controller. The desired vehicle state is determined based upon the steering wheel angle, its gradient and the wheel speed. Simultaneously, the yaw sensor measures the actual state. The controller computes the needed brake or acceleration force for each wheel and directs via the driver circuits the valves of the hydraulic modulator. Via a CAN interface the ECU is connected with other systems (ABS, etc.) in order to avoid giving contradictory commands.
Many ESC systems have an "off" override switch so the driver can disable ESC, which may be desirable when badly stuck in mud or snow, or driving on a beach, or if using a smaller-sized spare tire which would interfere with the sensors. Some systems also offer an additional mode with raised thresholds so that a driver can utilize the limits of adhesion with less electronic intervention. However, ESC defaults to "On" when the ignition is re-started. Some ESC systems that lack an "off switch", such as on many recent Toyota and Lexus vehicles, can be temporarily disabled through an undocumented series of brake pedal and handbrake operations. Furthermore, unplugging a wheel speed sensor is another method of disabling most ESC systems.
Availability and cost
ESC is built on top of an anti-lock brake (ABS) system, and all ESC-equipped vehicles are fitted with traction control. The ESC components include a yaw rate sensor, a lateral acceleration sensor, a steering wheel sensor, and an upgraded integrated control unit. According to National Highway Traffic Safety Administration research, ABS in 2005 cost an estimated US$368; ESC cost a further US$111. The retail price of ESC varies; as a stand-alone option it retails for as little as $250 USD. However, ESC is rarely offered as a sole option, and is generally not available for aftermarket installation. Instead, it is frequently bundled it with other features or more expensive trims, so the cost of a package that includes ESC could be several thousand dollars. Nonetheless, ESC is considered highly cost-effective and it might pay for itself in reduced insurance premiums.
Availability of ESC in passenger vehicles varies between manufacturers and countries. In 2007, ESC was available in roughly 50% of new North American models compared to about 75% in Sweden. However, consumer awareness affects buying patterns so that roughly 45% of vehicles sold in North America and the UK are purchased with ESC, contrasting with 78-96% in other European countries such as Germany, Denmark, and Sweden. While few vehicles had ESC prior to 2004, increased awareness will increase the number of vehicles with ESC on the used car market.
ESC is available on cars, SUVs and pickup trucks from all major auto makers. Luxury cars, sports cars, SUVs, and crossovers are usually equipped with ESC. Midsize cars are also gradually catching on, though the 2008 model years of the, Nissan Altima and Ford Fusion only offered ESC on their V6 engine-equipped cars. While ESC includes traction control, there are vehicles such as the 2008 Chevrolet Malibu LS and 2008 Mazda6 that have traction control but not ESC. ESC is rare among subcompact cars as of 2008. The 2009 Toyota Corolla in the United States (but not Canada) has stability control as a $250 option on all trims below that of the XRS which has it as standard. In Canada, for the 2010 Mazda3, ESC is as an option on the midrange GS trim as part of the moonroof package, and is standard on the top-of-the-line GT version. The 2009 Ford Focus has ESC as an option for the S and SE models, and standard on the SEL and SES models.
ESC is also available on some motor homes. Elaborate ESC and ESP systems (including Roll Stability Control (RSC) are available for many commercial vehicles, including transport trucks, trailers, and buses from manufacturers such as Bendix Corporation, WABCO, Daimler, Scania AB, and Prevost.
In the US, the Insurance Institute for Highway Safety (IIHS) website shows availability of ESC in individual US models and the National Highway Traffic Safety Administration (NHTSA website) lists US models with ESC.
The market for ESC is growing quickly, especially in European countries such as Sweden, Denmark, and Germany. For example, 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.
Just as ESC is founded on the Anti-lock braking system (ABS), ESC is the foundation for new advances such as roll stability control (RSC), RSC applies brakes, reduces throttle, induces understeer, and/or slows down the vehicle.
The computing power of ESC facilitates the networking of active and passive safety systems, addressing other causes of crashes. For example, sensors may detect when a vehicle is following too closely and slow down the vehicle, straighten up seat backs, and tighten seat belts, avoiding and/or preparing for a crash.
While Sweden used public awareness campaigns to promote ESC use, others implemented or proposed legislation.
The Province of Quebec was the first jurisdiction to implement an ESC law, making it compulsory for carriers of dangerous goods (without data recorders) in 2005.
The United States was next, requiring ESC for all passenger vehicles under 10,000 pounds (4536 kg), phasing in the regulation starting with 55% of 2009 models (effective 1 September 2008), 75% of 2010 models, 95% of 2011 models, and all 2012 models.
Canada will require all new passenger vehicles to have ESC from 1 September 2011.
In Australia, following the lead of the State of Victoria, the Australian Federal Government announced on 23 June 2009 that ESC would be compulsory from 1 November 2011 for all new passenger vehicles sold in Australia, and for all new vehicles from November 2013.
The European Parliament has also called for the accelerated introduction of ESC. The European Commission has confirmed a proposal for the mandatory introduction of ESC on all new cars and commercial vehicle models sold in the EU from 2012, with all new cars being equipped by 2014.
The United Nations Economic Commission for Europe has passed a Global Technical Regulation to harmonize ESC standards.
Electronic stability control (ESC) is the generic term recognised by the European Automobile Manufacturers Association (ACEA), the North American Society of Automotive Engineers (SAE), the Japan Automobile Manufacturers Association, and other worldwide authorities. However, vehicle manufacturers may use a variety of different trade names for ESC:
- Acura: Vehicle Stability Assist (VSA)
- Alfa Romeo: Vehicle Dynamic Control (VDC)
- Audi: ESP - Electronic Stability Programme
- Bentley: ESP - Electronic Stability Programme
- Bugatti: ESP - Electronic Stability Programme
- Buick: StabiliTrak
- BMW: Dynamic Stability Control (DSC) (including Dynamic Traction Control)
- Cadillac: StabiliTrak & Active Front Steering (AFS)
- Chery Automobile: Electronic Stability Programme
- Chevrolet: StabiliTrak; Active Handling (Corvette only)
- Chrysler: Electronic Stability Programme (ESP)
- Citroën: Electronic Stability Programme (ESP)
- Dodge: Electronic Stability Programme (ESP)
- Daimler: Electronic Stability Programme (ESP)
- Fiat: Electronic Stability Programme (ESP) and Vehicle Dynamic Control (VDC)
- Ferrari: Controllo Stabilità (CST)
- Ford: AdvanceTrac with Roll Stability Control (RSC) and Interactive Vehicle Dynamics (IVD) and Electronic Stability Programme (ESP); Dynamic Stability Control (DSC) (Australia only)
- General Motors: StabiliTrak
- Honda: Vehicle Stability Assist (VSA)
- Holden: Electronic Stability Programme (ESP)
- Hyundai: Electronic Stability Programme (ESP), Electronic Stability Control (ESC), and Vehicle Stability Assist (VSA)
- Infiniti: Vehicle Dynamic Control (VDC)
- Jaguar: Dynamic Stability Control (DSC)
- Jeep: Electronic Stability Program (ESP)
- Kia: Electronic Stability Control (ESC), Electronic Stability Programme (ESP)
- Lamborghini: ESP - Electronic Stability Programme
- Land Rover: Dynamic Stability Control (DSC)
- Lexus: Vehicle Dynamics Integrated Management (VDIM) with Vehicle Stability Control (VSC)
- Lincoln: AdvanceTrac
- Maserati: Maserati Stability Programme (MSP)
- Mazda: Dynamic Stability Control (DSC) (Including Dynamic Traction Control)
- Mercedes-Benz (co-inventor): Electronic Stability Programme (ESP)
- Mercury: AdvanceTrac
- MINI: Dynamic Stability Control
- Mitsubishi: Active Skid and Traction Control MULTIMODE and Active Stability Control (ASC)
- Nissan: Vehicle Dynamic Control (VDC)
- Oldsmobile: Precision Control System (PCS)
- Opel: Electronic Stability Programme (ESP)
- Peugeot: Electronic Stability Programme (ESP)
- Pontiac: StabiliTrak
- Porsche: Porsche Stability Management (PSM)
- Renault: Electronic Stability Programme (ESP)
- Rover Group: Dynamic Stability Control (DSC)
- Saab: Electronic Stability Programme
- Saturn: StabiliTrak
- Scania: ESP - Electronic Stability Programme
- SEAT: ESP - Electronic Stability Programme
- Škoda: ESP - Electronic Stability Programme
- Smart: Electronic Stability Programme (ESP)
- Subaru: Vehicle Dynamics Control (VDC)
- Suzuki: Electronic Stability Programme (ESP)
- Toyota: Vehicle Dynamics Integrated Management (VDIM) with Vehicle Stability Control (VSC)
- Vauxhall: Electronic Stability Programme (ESP)
- Volvo: Dynamic Stability and Traction Control (DSTC)
- Volkswagen: ESP - Electronic Stability Programme
ESC system manufacturers include:
- Robert Bosch GmbH
- Aisin Advics
- Bendix Corporation
- Continental Automotive Systems
- ITT Automotive, since 1998 part of Continental AG
- Mando Corporation
- Nissin Kogyo
- Teves, now part of Continental AG
- Bosch ESC Information
- ChooseESC! a combined initiative from the European Commission, eSafetyAware, and Euro NCAP
- E-Safety list of ESC Media Articles from eSafety Support
- NHTSA on ESC including US Regulation and list of US vehicles with ESC
- Canadian Association of Road Safety Professionals on ESC (PDF)
- Transport Canada on ESC
- Australia (Victoria) on ESC
- NHTSA announces mandatory ESC all light US vehicles by 2012
- Proposed Canadian ESC Law
- European ESC Mandate for Trucks and Coaches
- European ESC Campaign
- Canadian ESC Campaign
- Swedish ESC Study
- Australian (Monash) ESC Study
- Pacific Institute ESC Study
- Paine (2005) review of ESC research and regulations
- UK ESC Video by What Car
- Four Little Words: "Does it have ESC?"
- ABC News ESC Video