It is All About µ

Engineers have found a way to keep vehicles stable while driving on slippery surfaces. Now they need to cut the technology's cost, so they convince the public to buy it.

Charles J. Murray -- Design News, May 15, 2005

In many cases, the fatal scenario is simple: skid. trip. roll. crash. Within seconds, the deadly chain of events is irreversible, and another life is lost on the highways.

Now, however, that may be changing. After more than a decade of experience with electronic stability control systems (ESC), automotive researchers are comprehending the value of this little used technology. Stop the skid with ESC, they're saying, and you can head off the other events in the gruesome chain.

"People are realizing now that if you can make the driver better, you can reduce crashes," says Rich Golitko, marketing director for electronic stability control at Bosch Automotive.

Indeed, the realization has arrived in the form of an avalanche of technical studies. In January, the Insurance Institute for Highway Safety published findings declaring that ECS reduced fatal single-vehicle crashes in one study by an astounding 56 percent. That study followed on the heels of a September paper from the National Highway Traffic Safety Administration (NHTSA) describing a potential 35 percent reduction in single-vehicle crashes if ECS were used across the board in all vehicles. Similar studies from the University of Iowa and the Swedish National Road Commission showed 34 percent and 38 percent control improvements on wet and snowy roads, respectively.

In the wake of such findings, automakers and electronics suppliers have begun gearing up for a huge wave of demand for ESC and for a possible government mandate of the technology. Although ESC now has a paltry 11 percent market penetration in the U.S., GM announced on January 30 that its version of the technology, known as Stabilitrak, will be standard on all its cars and trucks by 2010. Engineers at Bosch estimate that about one-third of the nation's 17 million vehicles built annually will have stability control by 2008. The reason, they say, is public demand for ECS' life-saving measures.

"When you consider that there are between 14,000 and 17,000 single-vehicle fatalities every year, and we can reduce those fatalities by 50 percent, you realize we're dealing with a huge number of lives that could be saved," Golitko says.

SUVs experience a greater reduction in fatal crashes due to ESC because they have a higher center of gravity, and because an SUV that yaws, or skids, can sometimes strike a curb or other object, causing it to "trip." The tripping action, combined with the high CG, cause an SUV to have an inordinately high number of roll-related crashes.

How ESC Works

To make that vision happen, however, automakers, tier-one suppliers, and component vendors need to gear up for a sharp increase in demand. Experts say that such an increase is feasible, mainly because ESC is basically an electronic overlay that sits atop a vehicle's antilock braking system (ABS).

The technology's primary components include a lateral accelerometer, gyroscopic yaw-rate sensor, and steering angle sensor, as well as electronics module and wiring harness. The system works by measuring yaw and lateral acceleration, then comparing that to the driver's desired path, as indicated by the steering angle sensor. If a microcontroller determines that the difference between the desired path and actual path is too great, the system activates one or more of the wheel brakes via the antilock braking system.

"It may take a few milliseconds, but the controller looks at the deviation from the target, and then responds with sufficient force to put the vehicle back in line," notes Scott Dahl, marketing director for chassis system management at Bosch Automotive. By selectively applying brake force at one wheel, the system can apply a moment to counteract undesired yaw, Dahl says.

Most vehicle control systems use microcontrollers that sample data about 50 times per second, Dahl says. The moment they "see" a yaw that's outside the target threshold, they send a command signal to the ABS system, which takes anywhere from 80 to 500 msec to react, depending upon the magnitude of braking force that's required.

To fully comprehend the nature of what's happening, most stability control systems also employ vehicle data, such as vehicle mass, wheel mass, center of gravity, drag coefficient, engine torque, pitch moment of inertia, roll moment of inertia, and more.

A stability control system uses that data to determine whether additional measures, such as pulling back on the throttle or braking another wheel, are necessary.

"The system 'knows' what every vehicle is capable of," Dahl says. "Obviously, a Corvette has higher thresholds than an Expedition or another large SUV."

ESC also uses the sensor data to derive a rough estimate of the coefficient of friction (µ) on the driving surface. Knowing µ, as well as the vehicle's performance characteristics, helps ESC determine how much intervention is required and when the time has come for action. Bosch then tests the systems on all kinds of surfaces, including ice, snow, dry asphalt, wet asphalt, gravel, sand, bumps, and rocks.

"It's all about µ," Dahl says.

ESC employs a combination of sensor data and a stored database of vehicle information to make its intervention decisions.

But Costs Must Come Down . . .

For widespread adoption to take place as planned, however, tier-one suppliers, such as Bosch, Siemens VDO Automotive and Delphi Corp., need to minimize costs, so that low-end vehicles can afford the technology. Many such entry-level vehicles can't currently afford to integrate ABS, let alone the extra $300-$700 that's required for ESC.

To meet the need, electronics vendors are working on lower-cost sensors, as well as on electronic integration methods that would help reduce factory installation costs. Analog Devices Inc., for example, has developed silicon-based yaw-rate sensors, that could ultimately cost substantially less than the quartz-based, piezoelectric tuning fork "gyros" now used in most ESC systems.

Unlike the quartz gyros, which use the frequency of the tuning fork to measure yaw, the silicon sensors employ a "vibrating mass," which produces a voltage signal that's proportional to the vehicle's angular rate of change. By employing silicon instead of quartz, Analog Devices engineers believe they can bring sensor costs down from their current levels of $20 to $30 apiece, to less than $10.

"Today, silicon and quartz are on a par in terms of price," notes David Krakauer, product marketing manager for gyro sensors at Analog Devices. "But in three to four years, as the technology matures, silicon will definitely be cheaper."

Ultimately, such cost reductions could become critical, experts say, because the number of extra ESC sensors per vehicle (which can be as few as two today) could be expanded to include three axes of accelerometer-based sensing, two gyroscopes, and a roll sensor.

Analog Devices engineers also argue that silicon provides a more robust solution. That's important, they say, when a vehicle crosses gravelly roads and kicks up dust and rocks that could trick the sensor.

"If you're driving across a bumpy road and a rock hits the underside of your car, you don't want the sensor to see that shock and think the car is spinning out of control," Krakauer says.

Other vendors, including Silicon Sensing, a joint venture of BAE Systems and Sumitomo Precision Products, based in Hyogo, Japan, have joined in the development of silicon yaw-rate sensors, employing slightly different technologies. Freescale Semiconductor is also considering entry into the silicon yaw rate sensor market.

Device integration could also be a key way to cut costs, engineers say. Analog Devices, among others, is working on integrating lateral accelerometers onto a single die with yaw-rate sensors. Such efforts go hand in hand with attempts to integrate ESC electronics into ABS systems under the hood, instead of in their current location in the passenger compartment. Doing so would eliminate the need for costly wiring harnesses that must pass through firewalls between the passenger and engine compartments. Some automakers are also said to be looking at the possibility of leaving the ESC electronics in the passenger compartment, and instead cutting costs by integrating the ESC module with the airbag module.

"It's better to put more sensors in a common spot, rather than trying to create a new location within the vehicle," Krakauer says.

Stability control systems combine lateral acceleration sensors with existing ABS hydraulics to keep yawing forces under control.

… And The Public Must Buy Into It

Automotive suppliers believe that if they can enable across-the-board employment of ESC, fatalities will decrease, especially in SUVs and trucks. In those vehicles, where rollover fatalities are inordinately high, ESC could help by preventing the skid that often precipitates the rollover. Experts say that such skidding vehicles sometimes strike bumps, or gravel surfaces, that cause vehicles to trip and roll. And while such accidents are unusual, rollovers ultimately account for 33 percent of automotive fatalities in the U.S., Bosch engineers say.

"Stability control is especially effective at preventing tripped rollovers, because it keeps the vehicle from going sideways and leaving the road," says Golitko of Bosch.

For the technology to achieve widespread adoption, however, suppliers say they must first convince the public that stability control is a lifesaver. That will be a more difficult task in the U.S., where the current rate of adoption is 11 percent, than in Germany, where it's 64 percent, or across Europe as a whole, where it's now 36 percent.

"Our job is to show people that we can reduce single-vehicle crashes by more than 40 percent and reduce fatalities by more than half," Golitko concludes. "Potentially, we can save more than 7,000 lives a year with this technology."

Reach Senior Editor Chuck Murray at charles.murray@reedbusiness.com.

It's All About µ
Coefficients of friction for various surfaces.
Dry asphalt	0.8 - 0.9
Gravel	0.3 - 0.4
Ice	0.10 - 0.15
Snow	0.25 - 0.35
Wet ceramic tile	0.13 - 0.15
Banana peel	0.10 - 0.11
Water over ice	0.07 - 0.08

Web Resources

To view an animation of Bosch's ESC technology, go to http://rbi.ims.ca/4391-556