Electronic Stability Control
Electronic stability control (ESC) is a technology that improves the safety of a vehicle's handling, by detecting and preventing skids and slides, helping the driver maintain control of the vehicle. This technology is applied through a computerized system.
ESC simplifies steering for the driver. ESC uses the vehicle's braking system as a tool for "steering" the vehicle back on track. Braking is automatically applied to individual wheels, such as the inner rear wheel to counter understeer, or the outer front wheel to counter oversteer. Some ESC systems also intervene by reducing engine power or accelerating the driven wheels.
Operation
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 wheel speeds). If the vehicle is not going where the driver is steering, ESC then brakes individual front or rear wheels and/or reduces excess engine power as needed to help correct understeer (plowing) or oversteer (fishtailing).
ESC incorporates yaw angle control into anti-lock brakes. Yaw is rotation around the vertical axis; i.e. spinning left or right. Anti-lock brakes enable ESC to brake individual wheels. ESC may also incorporate 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.
ESC cannot override a car's physical limits or increase traction. If a driver pushes the vehicle's traction beyond its limits, ESC cannot prevent a crash. It is a tool to help the driver maintain control using available traction.
Components and Design
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 (25 times per second) and decides, when necessary, to apply brakes and/or reduce throttle.
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 sensor : measures the yaw angle (rotation) 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.
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.
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, 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.
Most ESC systems use an indicator light on the dash to tell the driver when the system is active (i.e. has detected and corrected skidding).
Many ESC systems have an "off" switch so the driver can disable ESC, for example, when stuck in mud or snow. However, ESC defaults to "On" when the ignition is re-started.
Effectiveness
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. NHTSA concluded that ESC reduces crashes by 35%. Additionally, Sport utility vehicles (SUV's) with stability control are involved in 67 percent fewer accidents than SUVs without the system. The Insurance Institute for Highway Safety (IIHS) issued its own study results in 2004 concluding that universal use of ESC could save 7,000 lives a year in the United States. In June 2006, the IIHS released a second study showing that up to 10,000 fatal US crashes could be avoided annually if all vehicles were equipped with ESC. The 2006 study concluded that ESC reduces the likelihood of all fatal crashes by 43 percent, fatal single-vehicle crashes by 56 percent, and fatal single-vehicle rollovers by 77-80 percent. 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[10], and Bill Kozyra, CEO of Continental Automotive Systems[6]. The European New Car Assessment Program (EuroNCAP) "strongly recommends" that people buy cars fitted with stability control. On November 21, 2006 the IIHS announced that 13 of the 2007 vehicles had earned its TOP SAFETY PICK rating which is based primarily on crashworthiness; however, a major new requirement for this top rating is that the vehicle must be equipped with ESC (which helps avoid crashes). The NHTSA issued a final research report in July 2007, stating, "ESC reduced all fatal crashes by 14 percent for passenger cars and 28 percent for LTVs (Light Trucks and Vans). Only the reduction in LTVs is statistically significant." Although that data was not statistically significant for passenger cars generally, it was found to be statistically significant that "police-reported crash involvements decreased by 8 percent in passenger cars and 10 percent in LTVs.".
Cost
ESC is built on top of an anti-lock brake (ABS) system. 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 costs an estimated US$368 (in 2005) and ESC costs an additional US$111. The retail price of ESC varies; as a stand-alone option, it retails for as little as $450. Unfortunately, many auto makers bundle ESC with other features so the cost of a package that includes ESC could be several thousands of dollars.
ESC is highly cost-effective and it might pay for itself in reduced insurance premiums.
Availability
ESC is not generally available for after-market installation. The only way consumers can get ESC is to buy a vehicle equipped by the manufacturer with standard or optional ESC.
Availability of ESC in passenger vehicles varies between manufacturers and countries. In 2007, ESC was available in roughly 50% of new North American models, whereas that figure is 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 78-93% in Nordic 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, SUV's, and pickup trucks from all major auto makers. 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[19], WABCO [20], Daimler Chrysler, Scania AB, and Prevost.
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 National Roads and Motorists' Association NRMA shows availability of ESC in Australian models. CHOOSE ESC! shows availability of ESC in European countries.
Laws
While Sweden used public awareness campaigns to promote ESC use, others implemented or proposed legislation. Quebec was the first jurisidiction to implement an ESC law, making it mandatory for carriers of dangerous goods (without data recorders) in 2005. The United States was next, mandating ESC for all passenger vehicles under 10,000 pounds (4536 kg), phasing in the regulation starting with 55% of 2009 models, 75% of 2010 models, 95% of 2011 models, and 100% of 2012 models. Australia and Canada will likely be next, requiring 100% of passenger vehicles to have ESC on September 1, 2011, matching the U.S. The United Nations Economic Commission for Europe is working on a Global Technical Regulation to harmonize standards for ESC.
Future
The market for ESC is growing at a very robust rate, especially in European countries such as Sweden and Germany. For example, in Sweden, in 2003, 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 2007 it had grown to 93%. 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) that works in the vertical plane much like ESC works in the horizontal plane. When RSC detects impending rollover (usually on transport trucks or SUV's, 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.
