At first glance, the scenario seems simple: A car fishtails on a patch of ice; on-board sensors “feel” it skidding; a controller applies the brake at the correct wheel; fishtailing stops.
Simple, yes, but the technology behind it is incredibly powerful. Not since the advent of the seat belt has an automotive technology shown such promise for saving lives. Not air bags. Not antilock brakes (ABS). Not traction control, tire pressure monitoring, all-wheel drive or telematics.
“Very few safety technologies show this kind of large effect in reducing crash deaths,” writes the Insurance Institute for Highway Safety (IIHS).
That's why the National Highway Traffic Safety Administration (NHTSA) has mandated across-the-board availability of electronic stability control (ESC) on passenger cars and small trucks by the 2012 model year. Studies conducted by the IIHS and NHTSA have shown ESC could save from 5,000 to 10,000 lives per year in the U.S. alone. Save for seat belts, no other technology comes close.
“In stability control, we see more life-saving potential than in antilock brakes or airbags,” says David Champion, director of testing at Consumer Union's automotive test facility. “It has huge potential.”
Halting the Deadly Chain
Ironically, it wasn't always so. Electronic stability control was introduced quietly in the mid-1990s, with little fanfare. Journalists eased the news into print rather obliquely, usually describing it as an add-on to antilock braking systems. Few experts foresaw its life-saving capabilities.
Starting in 2004, however, scientific studies began to emerge, declaring ESC could reduce single-vehicle crashes at amazing rates. One IIHS study found it reduced fatal single-vehicle crashes by an astonishing 56 percent. Another, conducted by NHTSA, described a 35 percent fatality reduction. Similarly, 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.
ESC is able to achieve such miraculous numbers through the innovative use of hardware and software. During everyday driving, its software algorithms work with the vehicle's sensors and microcontrollers to compare the intent of the driver to real-world vehicle behavior. The system recognizes driver intent by monitoring steering angle and throttle sensors, as well as through the application of the brakes. It makes similar use of sensors— wheel speed sensor, lateral accelerometer and yaw-rate sensor — to build a software model of the car's performance.
“By looking at yaw rate, lateral acceleration and wheel speed, we know what the car is actually doing,” says Scott Dahl, director of marketing and product strategy for Bosch Automotive. “And if there's a delta between that and the driver's intent, then we have to bring the two back together.”
Indeed, the main intent of the system is to see if the vehicle is yawing and, if so, to bring it back into line. That's critical because yaw — the clockwise or counterclockwise rotation of the vehicle about an axis perpendicular to the ground — is the culprit in an extraordinary number of accidents. Yaw's fatal scenario — skid, trip, roll and crash — is a deadly chain of events which, once started, is nearly irreversible through ordinary driver action.
Stability control stops the deadly chain, however, before it has time to gain momentum. It accomplishes that by applying the brake at a selected wheel to produce a moment that counteracts the yaw.
To be sure, the multitude of commercial ESC systems offered by automakers use a complex array of software algorithms, enabling their systems to understand what's happening in the outside world. Bosch, for example, uses its software vehicle to infer the coefficient of friction on the surface beneath the car. Doing so, it “knows” whether it's traveling on ice or gravel or dry asphalt and it prepares itself accordingly.
“We have a very complex vehicle model that considers the coefficient of friction, the vehicle weight, the car's wheelbase and just about every other physical parameter you can think of,” Dahl says.
Who Will Pay?
Given the impressive evolution of such systems and the results of so many safety studies, NHTSA decided late last year to mandate the technology. The U.S. mandate — which starts in 2008 and moves to 100 percent compliance by 2012 — is expected to have tremendous implications for consumers and engineers alike. Approximately 17 million vehicles sold annually in the U.S. in 2012 will incorporate the technology. For consumers, that translates to fewer fatal crashes. For engineers, it means millions more electrohydraulic modules, lateral accelerometers, gyroscopic sensors, steering angle sensors and microcontroller chipsets.
It also means more cost. Every vehicle not currently equipped with ESC faces the certain addition of an “inertial module,” which incorporates the yaw rate sensor and lateral accelerometer. The inertial module, typically mounted in the passenger compartment under the center console, is costly because of the yaw rate sensors. Yaw rate sensors have dropped dramatically in cost over the years — from more than $50 apiece in 2000 to about half that today — but they're still terribly expensive to an industry that worries about extra pennies. The yaw rate devices, which include MEMS-type and tuning-fork-type sensors, are offered by several companies, including Analog Devices, Bosch and Systron Donner.
A new electrohydraulic module will also be necessary for all vehicles moving up to ESC. Although the new modules are similar to those of ABS systems, additions are necessary to enable them to do stability control tasks. Prime among those additions is the need for a system that builds pressure without the need for the driver to apply the brakes. Because ABS electrohydraulic modules can't do that, the new breed of modules must incorporate extra valves and an upgraded hydraulic pump. In addition, the associated electronics of the unit must be upgraded to include a larger read-only memory (ROM), as well as means for more effective thermal dissipation and more powerful microcontrollers.
Such costs can mount up quickly. Microcontrollers, which typically run anywhere from $3 to $5 for ABS, jump to $4 to $7 in ESC. The reason: Most ESC controllers require twice as much on-chip flash memory (from a high of 512 KB to 1 MB); twice as much RAM (from a high of 24 KB to 48 KB), and about 50 percent more speed (from about 48 MHz to 70 MHz). Suppliers for such devices include Texas Instruments and Freescale Semiconductor, among others.
Moreover, the cost curve grows steeper for those low-end vehicles that don't currently offer ABS. For those vehicles, it's not merely a matter of upgrading the electrohydraulic ABS unit. They must add the inertial module, electrohydraulic ESC unit, steering angle sensors, wheel speed sensors and microcontrollers.
As a result, the extra cost is absorbed with a great deal more difficulty in entry-level vehicles than in luxury models. According to NHTSA estimates, the average extra cost for a passenger vehicle will be $90.30 and for a light truck, $29.20. Luxury vehicles, which mostly require upgrades to existing equipment, will be less than the average. Entry level vehicles, which require first-time incorporation of electrohydraulic modules and sensors, will rack up about $479 in additional costs, NHTSA says. Total cost to implement all vehicles is expected to reach $985.2 million.
Automakers aren't yet saying whether the additional costs will be paid by consumers, OEMs, or a combination of the two.
“This is a mandate, so customers will expect the additional capabilities to come, more or less, for free,” says Matthias Poppel, automotive marketing manager for Texas Instruments. “The question now is whether carmakers should 'feature-reduce' to come up with a stability control system that is cost-optimized. Given the fact that U.S. carmakers are under such extreme pressure right now to gain back market share, we expect to see some cost reduction.”
No matter who pays, however, ESC is coming. Already, it's being offered by automakers under dozens of different trade names — from Vehicle Stability Assist (Acura) to Dynamic Stability Control (BMW) to StabiliTrak (GM) and many others. GM, in particular, has announced across-the-board availability of the feature by 2010, about two years ahead of the mandate.
Moreover, automakers are laying plans to build atop stability control to create new and better kinds of safety systems. Ford Motor Co., for example, now offers Roll Stability Control, which turns the yaw sensors on a 90 degree angle and employs them to sense motion about the roll axis. Such systems are expected to see use in SUVs and other vehicles with a high center of gravity.
Beyond that, engineers are planning to use ESC as a foundation for more advanced safety products, including collision avoidance.
“The hydraulic system for stability control builds up pressure on its own and that sets the stage for all kinds of other features,” says Dan Mailot, chief engineer for advanced control systems for TRW in North America. “With that kind of system, you can now look at collision avoidance, collision mitigation, radar-based autonomous cruise control and all sorts of haptic feedback systems.”
For reasons such as those, experts expect stability control to reach sales levels of about 35 million units per year in the next five years. The U.S., which now has ESC penetration of about 35 to 40 percent, will ultimately jump to 100 percent during that timeframe. Japan and Europe, which currently hover around 50 to 60 percent penetration, are expected to move more gradually to across-the-board usage.
“There are a lot of stepping stones you can build upon to do things with vehicles that you couldn't do previously,” Mailot says. “Stability control gives us one of those stepping stones.”
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