By A/an S. Brown, Associate Editor Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/129/12/35/6356619/me-2007-dec3.pdf by guest on 24 September 2021

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

rafflc accidents occur vent as many as 238,000 injuries each year. with mind-numbing ESC is more than a safety breakthrough. It opens regularity. According the door to entirely new types of intelligent safety to the us. Department systems that use sensors and computers to anticipate ! of Transportation, more and respond to threats-independently of the driver. I.' than 10 million vehicles The Department of Transportation safety estimates were involved in 6.2 mil- are based on experience. The number of ESC-en­ lion accidents in 2005. Of abled cars on the road has been growing steadily that, 1.8 million collisions since Germany's Robert Bosch GmbH and Daintier injured nearly 2.7 million AG introduced the technology in 1995. Today, m.ost people. Another 39,189 crashes left 43,443 dead. European cars and about one-third of US. vehicles It sounds like wholesale carnage. Yet the number use ESC. US. automakers make it standard on nearly of fatalities and injuries per mile has fallen by half all sport utility vehicles and vans, and plan to increase over the past 20 years. the number of cars with ESC well in advance of the The reason is that passenger vehicles protect their 2011 deadline. occupants better. Active safety systems, such as trac­ This has given researchers plenty of data to analyze. tion control and antilock brakes, provide more In 2004, the National Highway and Traffic Safety control during emergency maneuvers. Passive safety Administration looked at 1997-2002 crash data from systems, such as seat belts, air bags, and energy­ the first cars with ESC. It found that the system. re­ absorbing crumple zones, lessen the severity of in­ duced single-vehicle crashes by 35 percent in pas­ juries in a crash. According to the Department of . senger cars and by a remarkable 67 percent in SUVs. Transportation, seat belts saved 16,000 lives and air It also reduced fatalities by similar percentages. bags 3,000 lives in 2005. In 2006, the Insurance Institute for Highway Safe­ This past April, the us. Department of Trans­ ty concluded that electronic stability control could portation announced that it would go beyond active prevent nearly one-third of all fatal crashes and re­ and passive safety systems to mandate the first use of duce rollovers by as much as 80 percent. Automakers a truly intelligent safety system. The new standard apparently knew this well before the study because requires automakers to equip all vehicles with elec­ ESC comes.·as standard equipment on most top­ tronic stability control, which automatically brakes heavy SUVs. individual wheels during skids, by Sept. 1,2011. A 2006 study by the University of Michigan's The agency estimates that electronic stability con­ Transportation Research Institute found that elec­ trol will save between 5,300 and 9,600 lives and pre- tronic stability control reduced non-fatal, loss-of-

mechanical engineering December 2007 35 Electronic Stability Program ESp® Electronic stability control senses that the vehicle's direc­ Components of the Electronic Stability Program ESp® from Bosch: 1 ESP-Hydraulic unit with integrated ECU tion is not changing fast enough for the steering wheel po­ 2 Wheel speed sensors 3 Steering angle sensor sition, and when the front of the car starts to drift, it ap­ 4 Yaw rate sensor with integrated plies brakes selectively to keep the vehicle on the road. acceleration sensor 5 Engine-management Electronic stability control builds on two earlier ad­ ECU for comrryunication vances, antilock brakes and traction control, according to Phil Headley, chief engineer for advanced technol­ ogy in Continental AG's Continental automotive sys- tems division, a major ESC supplier. "It has been an evolution," Headley said. . Antilock brakes, he noted, can only reduce, not in- crease, brake pressure. They generally use induction or

magnetic sensors to monitor the speed of each wheel as it Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/129/12/35/6356619/me-2007-dec3.pdf by guest on 24 September 2021 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. Electronic stability control gathers information about wheel speed, steering wheel direction, lateral acceleration, and yaw rate. It compares the data Traction control keeps the car from losing traction when with a computer algorithm to determine if the vehicle has begun to skid. the driver applies too much throttle or steering. Both trac­ tion control and antilock brakes measure wheel spin. But whereas antilock brakes release pressure on wheels that are control crashes by 53 percent for SUVs and 40 percent for slowing down, traction control increases brake pressure on passenger cars. On wet, snowy, or icy roads, those percent­ wheels that are rotating too fast. ages climb to 88 percent for SUVs and 75 percent for cars. "This system adds more valves and more logic to antilock "Electronic stability control is probably the most signif­ brakes," Headley s;lid. "It can brake the drive wheels and icant automotive safety technology since the seat bel~," use the engine controller to reduce torque. The most im­ said John Woodrooffe, who heads the institute's safety portant difference is that traction control can apply the analysis division. brakes without the driver touching them." Electronic stability control combines sophisticated sensors and high-octane computing to take intelligent brake con­ Over and Under trol to an entirely new level. ESC helps maintain control of a vehicle by keeping it A typical ESC system starts with some of the same basic headed in the direction the driver wants it to go. elements as antilock brakes and traction control. These ele­ Spinning out, or oversteering, occurs when a car turns ments include wheel speed sensors and a hydraulic modu­ too quickly. Imagine, for example, that an object falls off the . lator unit that senses and controls brake pressure for each back of a truck. The driver swerves sharply to the left to individual wheel. ESC takes over operation of the hydraulic avoid it and then tries to straighten the car. Turning the modulator when engaged. front wheels back to the right orients the car in the ;ight ESC uses three types of sensors not fo~nd on other active direction, but the momentum from the turn keeps the rear safety systems. The first measures the angle of the steering of the car sliding to the left. The car fishtails, starts to spin, wheel to determine where the driver wants to go. One and can go off the road. variation uses an LED to shine a light through a pelforated Drivers can maintain control by working the brakes and disc on the steering column that turns with the wheel, but countersteering, momentarily turning away from their in­ it takes a few moments of driving to fully enable the sys­ tended direction. Even a driver who learns how to do this tem. A second variant uses a calibrated microprocessor that may fail to execute during the few seconds that a crisis lasts. retains the position of the steering wheel in memory, even ESC works in the background, constantly comparing the if the car battery has been removed. direction of the vehicle's fi-ont wheels-its intended direc­ The second critical ESC sensor is the microelectro­ tion-with its actual direction. It can tell when the car's di­ mechanical accelerometer, which measures lateral accelera­ rection changes too quickly, and apply the brakes selective­ tion. The accelerometers usually use a cantilever or comb ly to individual wheels (some ESC systems also reduce that deforms during acceleration or deceleration (much engine torque) to keep the vehicle from fishtailing and like the antenna on a car whipping back and forth). The spinning out. deformation changes the cantilever's electrical properties in The system also works when drivers understeer. This of­ proportion to the degree of acceleration. Micro accelerom­ ten happens when they misjudge a curve. They enter too eters have been used on vehicles to activate air bags since fast, and then try to execute a sharp turn at high speeds. the rnid-1990s.

36 December 2007 mechanical engineering The yaw rate sensor, which measures the degree of rota­ you can calibrate and test." tion around a vehicle's vertical axis, was new when Bosch Bosch said the tests last for two winter test cycles. They and Mercedes introduced it in 1995. At its heart is a MEMS try to anticipate all types of dr-iving conditions. An ESC test gyroscope that takes advantage of the tendency of vibrating includes performance on loose snow, hard-packed snow, objects to keep vibrating in the same plane. When the gyro slush, and snow over ice. rotates out of that plane, it creates a bending strain that Not only do technicians tune each car differently, but electronics sense and transmit to the ESC computer. some require massive changes. "Bosch's first ESC systems The sensors work together, measuring, calcuiating, and were developed for rear-wheel-drive sedans," Dahl said. comparing to determine when the cbntrol system should "When we did our first pickup truck, we had to take into intervene. ESC monitors the yaw rate sensor and ac­ account four-wheel-drive as well as the modes in which celerometers to gauge the position of the car and how fast the axles are locked together. f"i" Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/129/12/35/6356619/me-2007-dec3.pdf by guest on 24 September 2021 it is changing. It compares that information with the direc­ "We had to understand the driveline. If it was in four­ eJ tion of the steering wheel. It must be finely tuned, so it can wheel-locked mode and you tried to apply brakes to the tell the difference between a driver shooting around a front, you'd also slow down the rear axle. For intervention ~~.l slower car on a two-lane country road and a driver who to be effective, we had to decide whether we should de­ ~ loses control to avoid an unexpected obstacle. couple the transfer case or keep it coupled and then how ~ much torque to apply." ~~ ~j Tuning also varies with brand character, according to Fao/proof Math Bosch's chassis marketing manager, Paul Mercurio. "If you If using a computer to make judgment calls sounds com­ have a luxury car, like a Mercedes S Class, it's typically plicated, that's because it is. ESC cannot afford to make a driven by older drivers and ESC has to intervene quickly," mistake, to start braking when a car is trying to pass a truck Mercurio said. "I£the car is very sporty, like a Corvette, you on a highway at 75 miles per hour. ESC has to make the have to allow a lot of body slip angle because drivers like to right call every single time or it puts the driver in peril. push them. In fact, the Corvette's ESC has an 'off'button so That means creating a foolproof algorithm, the set of they can do spinouts and burnouts, though the system must mathematical routines that dictate how the system will be­ always default to the 'on' mode." have under every conceivable condition. ESC algorithms ESC developers like Bosch and Continental always tune collect data from every sensor and compare the data with to manufacturer specifications. That poses a problem no a model of vehicle behavior between 50 and 150 times per one has overcome yet: how to handle aftermarket modifi­ second. As it does this, it looks for inconsistencies, cases cations. What happens if a consumer decides to jack up a where the wheels point in one direction but the vehicle is car's suspension or install oversize tires? The change in cen­ not following properly. ter of gravity and tire size and track can easily throw off the There is no one-size-fits-all algorithm to do this. Accord­ ESC system. ing to Scott Dahl, director of marketing and product plan­ ning for Bosch's chassis division, "We start out with an al­ gorithm 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, cen­ ter of gravity, and brake pressure. Others, such as body and side slip angles, are estimates. D Vehicle ap'proa'cfies "When you do a fi'ee body diagram on a sheet of paper, an obstacle El Vehicle goes off course, it's easy to create these things," Dahl said. "The algorithm's enters oncoming traffic based on equations, and we draw on our experience for lalle and (triv~r.loses 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 pel{ormance. We've developed a standardized set of tests Electronic stability control detects when a vehicle begins to skid, then applies that include things like straight line stops, slippery stops, brakes to individual wheels to keep the vehicle safely on its desired course. lane changes, and double lane changes. Then we analyze the data and bring it into the algorithm. Then we go out Despite challenges, intelligent safety systems are here to and do it again. We do the lane change again, and usually stay. "It's not just about brakes," Dahl said. "The first sys­ wind up tweaking one of the more than 400 parameters tems controlled brakes and engine torque, but then we

mechanical engineering December 2007 37 Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/129/12/35/6356619/me-2007-dec3.pdf by guest on 24 September 2021

Ice and snow pose a difficult test for electronic stability control systems. faster than the driver can apply it. "Even if the driver ap­ Bosch pioneered microscale yaw rate sensors for the system. Microelectro­ mechanical accelerometers detect lateral movement. plies only 30 bar pressure, we'll boost the pressure ifhe does it at a fast rate," said Dahl. What if the driver is fiddling with the radio or falling started to ask what else we could do to impact safety, com­ asleep as the distance continues to close? Active cruise con­ fort, and convenience." trol applies the brakes in a series of quick, short jabs. Brake disc wiping is an example. "There are systems in "We looked at buzzers, vibrating seats, and lots of other production now that interface with windshield wiper sys­ warnings," Dahl recalled. "They don't necessarily focus the tems:' Dahl said. "If the wiper is on at a certain rate, they driver's attention on the road. Instead, he or she rIlight look pulse the brakes every so often to wipe water fi:om them so around the cabin for the source of the buzzing noise. But they stop better in the rain." Another system senses when a our studies showed nearly everyone knows to look straight car is stopped on a hill and will not let it roll backward ahead when they suddenly decelerate." when it starts. If the driver still doesn't take action and the distance con­ Dahl also points to side air bag activation. "IfESC detects tinues to close, the system sends a signal to ready the air the vehicle is sliding sideways at high velocity, it sends a sig­ bags so they deploy without confirmation from the accel­ nal to the side air bag controller," he said. "A vehicle con­ eration sensors. The system also readies the car for a crash. tacting a pole is the most deadly accident for an occupant. It automatically decelerates the vehicle, closes the windows Ordinarily, a pressure sensor will feel the impact, but wait and sunroof, tightens seat belts, and adjusts seats so passen­ for confirmation from the accelerometer before deploying gers won't submarine under their seat belts. the air bag. If the system detects sliding, we ready the air Ultimately, the system will apply brakes to reduce the bag so that it reacts immediately to the pressure sensor. chance of secondary collisions. This concept still needs That saves 5 to 9 milliseconds." 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. The Next Stop Continental's Headley is optinlistic about such near-term Ford Motor Co. takes ESC one step further with roll sta­ capabilities as blind spot detection using cameras mounted bility control, which senses when a van or SUV begins to in mirrors and lane departure warnings. tilt during a turn or emergency maneuver. It automatically Ultimately, vehicles nuy link with one another through takes countermeasures to prevent the vehicle from rolling telematics, in-car wireless systems that create dynamic net­ over. The technology debuted on the 2003 Volvo XC90 works to warn drivers of safety hazards while helping to and 2004 Lincoln SUVs. speed traffic. "Such systems could warn drivers of a stalled Active or adaptive cruise control is the sys tem most car or slippery road conditions," he said. "It could identify likely to make the next big splash . It combines many and help emergency vehicles get to accidents." ESC elements with radar or lidar sensors and more auto­ Code-making organizations are currently developing matic features. broadcast and message standards for such systems, but it will Active cruise control recognizes if another car pulls into take a large number of vehicles with conm1Unications ca­ the lane or if an obstacle, such as a large animal, suddenly pacity to make them effective. appears. It immediately takes a number of actions. Bosch's Yet those advances, or ones like them, are certainly com­ system, for example, prefills the brake system to reduce the ing. The proven ability of intelligent system.s has already distance between brake pads and wheel rotor for faster saved lives, and it promises to save even more of them stopping. It also prepares the panic brake assist, a system in the future. One day, receiving an assist from our car's now shipping with some Bosch ESC systems. It recognizes on-board computer may feel as natural as snapping on our panic situations and rapidly builds pressure on the brake seat belts .•

38 December 2007 mechanical engineering