Intelligent Safety

Mechanical Engineering, Dec 2007 by Brown, Alan S

Newly mandated electronic stability control takes over braking to prevent spinouts-and promises smarter, safer vehicles in the future.

traffic accidents occur with mind-numbing regularity. According to the U.S. Department of Transportation, more than 10 million vehicles were involved in 6.2 million accidents in 2005. Of that, 1.8 million collisions injured nearly 2.7 million people. Another 39,189 crashes left 43,443 dead.

It sounds like wholesale carnage. Yet the number of fatalities and injuries per mile has fallen by half over the past 20 years.

The reason is that passenger vehicles protect their occupants better. Active safety systems, such as traction control and antilock brakes, provide more control during emergency maneuvers. Passive safety systems, such as seat belts, air bags, and energy-absorbing crumple zones, lessen the severity of injuries in a crash. According to the Department of Transportation, seat belts saved 16,000 lives and air bags 3,000 lives in 2005.

This past April, the U.S. Department of Transportation announced that it would go beyond active and passive safety systems to mandate the first use of a truly intelligent safety system. The new standard requires automakers to equip all vehicles with electronic stability control, which automatically brakes individual wheels during skids, by Sept. 1, 2011.

The agency estimates that electronic stability control will save between 5,300 and 9,600 lives and prevent as many as 238,000 injuries each year.

ESC is more than a safety breakthrough. It opens the door to entirely new types of intelligent safety systems that use sensors and computers to anticipate and respond to threats-independendy of the driver.

The Department of Transportation safety estimates are based on experience. The number of ESC-enabled cars on the road has been growing steadily since Germany's Robert Bosch GmbH and Daimler AG introduced the technology in 1995. Today, most European cars and about one-third of U.S. vehicles use ESC. U.S. automakers make it standard on nearly all sport utility vehicles and vans, and plan to increase the number of cars with ESC well in advance of the 2011 deadline.

This has given researchers plenty of data to analyze. In 2004, the National Highway and Traffic Safety Administration looked at 1997-2002 crash data from the first cars with ESC. It found that the system reduced single-vehicle crashes by 35 percent in passenger cars and by a remarkable 67 percent in SUVs. It also reduced fatalities by similar percentages.

In 2006, the Insurance Institute for Highway Safety concluded that electronic stability control could prevent nearly one-third of all fatal crashes and reduce rollovers by as much as 80 percent. Automakers apparendy knew this well before the study because ESC comes as standard equipment on most topheavy SUVs.

A 2006 study by the University of Michigan's Transportation Research Institute found that electronic stability control reduced non-fatal, loss-of-control crashes by 53 percent for SUVs and 40 percent for passenger cars. On wet, snowy, or icy roads, those percentages climb to 88 percent for SUVs and 75 percent for cars.

"Electronic stability control is probably the most significant automotive safety technology since the seat belt," said John Woodrooffe, who heads the institute's safety analysis division.

Over and Under

ESC helps maintain control of a vehicle by keeping it headed in the direction the driver wants it to go.

Spinning out, or oversteering, occurs when a car turns too quickly. Imagine, for example, that an object falls off the back of a truck. The driver swerves sharply to the left to avoid it and then tries to straighten the car. Turning the front wheels back to the right orients the car in the right direction, but the momentum from the turn keeps the rear of the car sliding to the left. The car fishtails, starts to spin, and can go off the road.

Drivers can maintain control by working the brakes and countersteering, momentarily turning away from their intended direction. Even a driver who learns how to do this may fail to execute during the few seconds that a crisis lasts.

ESC works in the background, constantly comparing the direction of the vehicle's front wheels-its intended direction-with its actual direction. It can tell when the car's direction changes too quickly, and apply the brakes selectively to individual wheels (some ESC systems also reduce engine torque) to keep the vehicle from fishtailing and spinning out.

The system also works when drivers understeer. This often happens when they misjudge a curve. They enter too fast, and then try to execute a sharp turn at high speeds. Electronic stability control senses that the vehicle's direction is not changing fast enough for the steering wheel position, and when the front of the car starts to drift, it applies brakes selectively to keep the vehicle on the road.

Electronic stability control builds on two earlier advances, antilock brakes and traction control, according to Phil Headley, chief engineer for advanced technology in Continental AG's Continental automotive systems division, a major ESC supplier. "It has been an evolution," Headley said.