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Intelligent Safety PUBLIC ACCESS

Newly Mandated Electronic Stability Control Takes Over Braking to Prevent Spinouts-and Promises Smarter. Safer Vehicles in the Future.

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Associate Editor

Mechanical Engineering 129(12), 35-38 (Dec 01, 2007) (4 pages) doi:10.1115/1.2007-DEC-3

The US 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 September 1, 2011. According to a senior staff member, electronic stability control is probably the most significant automotive safety technology since the seat belt. Electronic stability control combines sophisticated sensors and high-octane computing to take intelligent brake control to an entirely new level. Ford Motor Co. takes Electronic steering control (ESC) one step further with roll stability control, which senses when a van or SUV begins to tilt during a turn or emergency manoeuvre. It automatically takes countermeasures to prevent the vehicle from rolling over. Code-making organizations are currently developing broadcast and message standards for such systems, but it will take many vehicles with communications capacity to make them effective.

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 us. 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 energyabsorbing 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 us. 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-independently 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 Daintier AG introduced the technology in 1995. Today, most European cars and about one-third of US. vehicles use ESC. US. 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 apparently 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 gathers information about wheel speed, steering wheel direction, lateral acceleration, and yaw rate. It compares the data with a computer algorithm to determine if the vehicle has begun to skid.

Grahic Jump LocationElectronic stability control gathers information about wheel speed, steering wheel direction, lateral acceleration, and yaw rate. It compares the data with a computer algorithm to determine if the vehicle has begun to skid.

"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.

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 ;ight 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.

Antilock brakes, he noted, can only reduce, not increase, brake pressure. They generally use induction or magnetic sensors to monitor the speed of each wheel as it rotates. When the driver brakes and the system senses some wheels moving slower than the others, it releases brake pressure on the slow wheels to keep them from locking. This results in faster, more accurate braking.

Traction control keeps the car from losing traction when the driver applies too much throttle or steering. Both traction control and antilock brakes measure wheel spin. But whereas antilock brakes release pressure on wheels that are slowing down, traction control increases brake pressure on wheels that are rotating too fast.

"This system adds more valves and more logic to antilock brakes," Headley s;lid. "It can brake the drive wheels and use the engine controller to reduce torque. The most important difference is that traction control can apply the brakes without the driver touching them."

Electronic stability control combines sophisticated sensors and high-octane computing to take intelligent brake control to an entirely new level.

A typical ESC system starts with some of the same basic elements as antilock brakes and traction control. These elements include wheel speed sensors and a hydraulic modulator unit that senses and controls brake pressure for each individual wheel. ESC takes over operation of the hydraulic modulator when engaged.

ESC uses three types of sensors not fo~nd on other active safety systems. The first measures the angle of the steering wheel to determine where the driver wants to go. One variation uses an LED to shine a light through a pelforated disc on the steering column that turns with the wheel, but it takes a few moments of driving to fully enable the system. A second variant uses a calibrated microprocessor that retains the position of the steering wheel in memory, even if the car battery has been removed.

The second critical ESC sensor is the microelectromechanical accelerometer, which measures lateral acceleration. The accelerometers usually use a cantilever or comb that deforms during acceleration or deceleration (much like the antenna on a car whipping back and forth). The deformation changes the cantilever's electrical properties in proportion to the degree of acceleration. Micro accelerometers have been used on vehicles to activate air bags since the rnid-1990s.

The yaw rate sensor, which measures the degree of rotation around a vehicle's vertical axis, was new when Bosch and Mercedes introduced it in 1995. At its heart is a MEMS gyroscope that takes advantage of the tendency of vibrating objects to keep vibrating in the same plane. When the gyro rotates out of that plane, it creates a bending strain that electronics sense and transmit to the ESC computer.

The sensors work together, measuring, calcuiating, and comparing to determine when the cbntrol system should intervene. ESC monitors the yaw rate sensor and accelerometers to gauge the position of the car and how fast it is changing. It compares that information with the direction of the steering wheel. It must be finely tuned, so it can tell the difference between a driver shooting around a slower car on a two-lane country road and a driver who loses control to avoid an unexpected obstacle.

If using a computer to make judgment calls sounds complicated, that's because it is. ESC cannot afford to make a mistake, to start braking when a car is trying to pass a truck on a highway at 75 miles per hour. ESC has to make the right call every single time or it puts the driver in peril.

That means creating a foolproof algorithm, the set of mathematical routines that dictate how the system will behave under every conceivable condition. ESC algorithms collect data from every sensor and compare the data with a model of vehicle behavior between 50 and 150 times per second. As it does this, it looks for inconsistencies, cases where the wheels point in one direction but the vehicle is not following properly.

There is no one-size-fits-all algorithm to do this. According to Scott Dahl, director of marketing and product planning for Bosch's chassis division, "We start out with an algorithm template, but we have to tune it for each vehicle. Originally, those instructions had to fit on microcontrollers with 256 kilobytes of memory. Now they can go up to 4 megabytes and have more than 400 variables."

Algorithms, Dahl said, start with modeling the vehicle and its dynamics. Some initial variables are easy to find, such as wheelbase, track width, height, engine torque, center of gravity, and brake pressure. Others, such as body and side slip angles, are estimates.

"When you do a fi'ee body diagram on a sheet of paper, it's easy to create these things," Dahl said. "The algorithm's based on equations, and we draw on our experience for where to set the original variables in these equations.

"That's a good starting point, but then you have to put the vehicle on the bench and do the testing to characterize its performance. We've developed a standardized set of tests that include things like straight line stops, slippery stops, lane changes, and double lane changes. Then we analyze the data and bring it into the algorithm. Then we go out and do it again. We do the lane change again, and usually wind up tweaking one of the more than 400 parameters you can calibrate and test."

Bosch said the tests last for two winter test cycles. They try to anticipate all types of dr-iving conditions. An ESC test includes performance on loose snow, hard-packed snow, slush, and snow over ice.

Not only do technicians tune each car differently, but some require massive changes. "Bosch's first ESC systems were developed for rear-wheel-drive sedans," Dahl said. "When we did our first pickup truck, we had to take into account four-wheel-drive as well as the modes in which the axles are locked together.

"We had to understand the driveline. If it was in fourwheel- locked mode and you tried to apply brakes to the front, you'd also slow down the rear axle. For intervention to be effective, we had to decide whether we should decouple the transfer case or keep it coupled and then how much torque to apply."

Tuning also varies with brand character, according to Bosch's chassis marketing manager, Paul Mercurio. "If you have a luxury car, like a Mercedes S Class, it's typically driven by older drivers and ESC has to intervene quickly," Mercurio said. "If the car is very sporty, like a Corvette, you have to allow a lot of body slip angle because drivers like to push them. In fact, the Corvette's ESC has an 'off'button so they can do spinouts and burnouts, though the system must always default to the 'on' mode."

ESC developers like Bosch and Continental always tune to manufacturer specifications. That poses a problem no one has overcome yet: how to handle aftermarket modifications. What happens if a consumer decides to jack up a car's suspension or install oversize tires? The change in center of gravity and tire size and track can easily throw off the ESC system.

Electronic stability control detects when a vehicle begins to skid, then applies brakes to individual wheels to keep the vehicle safely on its desired course.

Grahic Jump LocationElectronic stability control detects when a vehicle begins to skid, then applies brakes to individual wheels to keep the vehicle safely on its desired course.

Despite challenges, intelligent safety systems are here to stay. "It's not just about brakes," Dahl said. "The first systems controlled brakes and engine torque, but then we started to ask what else we could do to impact safety, comfort, and convenience."

Ice and snow pose a difficult test for electronic stability control systems. Bosch pioneered microscale yaw rate sensors for the system. Microelectromechanical accelerometers detect lateral movement.

Grahic Jump LocationIce and snow pose a difficult test for electronic stability control systems. Bosch pioneered microscale yaw rate sensors for the system. Microelectromechanical accelerometers detect lateral movement.

Brake disc wiping is an example. "There are systems in production now that interface with windshield wiper systems:' Dahl said. "If the wiper is on at a certain rate, they pulse the brakes every so often to wipe water fi:om them so they stop better in the rain." Another system senses when a car is stopped on a hill and will not let it roll backward when it starts.

Dahl also points to side air bag activation. "IfESC detects the vehicle is sliding sideways at high velocity, it sends a signal to the side air bag controller," he said. "A vehicle contacting a pole is the most deadly accident for an occupant. Ordinarily, a pressure sensor will feel the impact, but wait for confirmation from the accelerometer before deploying the air bag. If the system detects sliding, we ready the air bag so that it reacts immediately to the pressure sensor. That saves 5 to 9 milliseconds."

Ford Motor Co. takes ESC one step further with roll stability control, which senses when a van or SUV begins to tilt during a turn or emergency maneuver. It automatically takes countermeasures to prevent the vehicle from rolling over. The technology debuted on the 2003 Volvo XC90 and 2004 Lincoln SUVs.

Active or adaptive cruise control is the system most likely to make the next big splash . It combines many ESC elements with radar or lidar sensors and more automatic features.

Active cruise control recognizes if another car pulls into the lane or if an obstacle, such as a large animal, suddenly appears. It immediately takes a number of actions. Bosch's system, for example, prefills the brake system to reduce the distance between brake pads and wheel rotor for faster stopping. It also prepares the panic brake assist, a system now shipping with some Bosch ESC systems. It recognizes panic situations and rapidly builds pressure on the brake faster than the driver can apply it. "Even if the driver applies only 30 bar pressure, we'll boost the pressure ifhe does it at a fast rate," said ssDahl.

What if the driver is fiddling with the radio or falling asleep as the distance continues to close? Active cruise control applies the brakes in a series of quick, short jabs.

"We looked at buzzers, vibrating seats, and lots of other warnings," Dahl recalled. "They don't necessarily focus the driver's attention on the road. Instead, he or she rIlight look around the cabin for the source of the buzzing noise. But our studies showed nearly everyone knows to look straight ahead when they suddenly decelerate."

If the driver still doesn't take action and the distance continues to close, the system sends a signal to ready the air bags so they deploy without confirmation from the acceleration sensors. The system also readies the car for a crash. It automatically decelerates the vehicle, closes the windows and sunroof, tightens seat belts, and adjusts seats so passengers won't submarine under their seat belts.

Ultimately, the system will apply brakes to reduce the chance of secondary collisions. This concept still needs work, Dahl said. The problem: "How do you confirm that you have not slowed the vehicle down in the left lane with a truck coming at it?" he said.

Continental's Headley is optinlistic about such near-term capabilities as blind spot detection using cameras mounted in mirrors and lane departure warnings.

Ultimately, vehicles nuy link with one another through telematics, in-car wireless systems that create dynamic networks to warn drivers of safety hazards while helping to speed traffic. "Such systems could warn drivers of a stalled car or slippery road conditions," he said. "It could identify and help emergency vehicles get to accidents."

Code-making organizations are currently developing broadcast and message standards for such systems, but it will take a large number of vehicles with conm1Unications capacity to make them effective.

Yet those advances, or ones like them, are certainly coming. The proven ability of intelligent system.s has already saved lives, and it promises to save even more of them in the future. One day, receiving an assist from our car's on-board computer may feel as natural as snapping on our seat belts.

Copyright © 2007 by ASME
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