"Why It's Good"

Aerospace Technology For The Ground

By Carey Russ

One of the latest innovations in automotive technology is the stability control system. Mercedes-Benz was first with its "Electronic Stability Program" (ESP). Cadillac followed with "Stabilitrak". Lexus has "Vehicle Skid Control" (VSC), and BMW calls its system "Dynamic Stability Control" (DSC). It's called the "Active Handling System" (AHS) in the fifth-generation Corvette. "Porsche Stability Management" (PSM) can be found in the newest 911 Carrera 4. Jaguar calls its system "Automatic Stability Control" (ASC), and Ford is working on "Interactive Vehicle Dynamics" (IVD). Volvo has just introduced the "Dynamic Stability and Traction Control" (DSTC) in its premium S80 sedan. And more applications are on the way.

If you think the alphabet soup in the preceding paragraph sounds like the latest high-tech wizardry from NASA or the military, congratulations. You're almost right. Stability control may be among the newest safety and performance-enhancing technologies for automobiles, but, in concept and some hardware, it is indeed a spinoff from the aerospace world. Its use there goes back well over 30 years.

Aerospace use of stability control is, not surprisingly, very different from automotive use in detail, but not purpose. For various reasons, many high-performance military aircraft are aerodynamically unstable and require rapid, continuous control adjustment in order to fly safely and controllably. Adjustments need to be made faster than humanly possible, so the flight control computer is actually in charge of the aircraft. The pilot points the airplane in the desired direction, but the computer really makes all necessary detail adjustments.

Swords to plowshares. It should be no surprise that when the microchip-sized yaw sensors and accelerometers that were used in military avionic systems became available for civilian use their application to automotive systems could be seen. Street-legal automobiles are not inherently unstable, except under certain circumstances such as in poor weather and road conditions and at the limits of cornering abilities. Stability control systems keep the car more stable under such conditions by tying together many of the electronic control systems already found in modern automobiles, including engine management, traction control, and antilock braking systems, with data from a steering wheel angle sensor, a yaw sensor, and lateral accelerometers. The yaw sensor detects sideways motion; the accelerometer measures the rate of change in motion.

Basically, a stability-control system works as follows: the intended direction of the car is determined by comparing inputs from the steering wheel angle sensor, yaw sensor, lateral accelerometer, and individual wheel speeds. (Wheel speed sensors are part of the ABS and traction control systems. ) If the computer determines that the car's actual path is different from its intended path, action is taken. Here's where it goes beyond human abilities. If the car is not responding quickly enough to steering input, a condition known as understeer or "push", the system applies the inside front or rear brake. In oversteering (or "loose") conditions, where the rear of the car steps out of line, the outside front brake is applied. Engine power may also be reduced, as in most traction control systems. If partial low-traction conditions such as a patch of ice are encountered in straight-line travel, partial braking and/or power reduction may be applied to keep the car travelling in a straight line.

Like the control computer of an F16 fighter jet, the stability control system can react to sensor inputs and activate control systems far faster and more precisely than any human being. Imagine trying to co-ordinate four brake pedals...good luck.

I've had first-hand experience with the Cadillac Stabilitrak system on a mixture of dry pavement, soapy water, and dirt at Phoenix International Raceway, the Lexus VSC system on the GS400 and LS 400 on the tight, hilly, twisting "Streets of Willow" road course at Willow Springs International Raceway, and, more recently, the Volvo DSTC system on ice and snow in Quebec. How well do these systems work? Wonderfully, within their limits. Remember, you can break traffic laws, but you can't break the laws of physics. The most important stability control system is still the one in the driver's head.

At each demonstration, members of the press were encouraged at some point to attempt to lose control of the car. Each system successfully brought the car under control, but these tests were done in areas where there was nothing to hit - just in case. On the racetracks and on a closed-to-the-public ice-covered hilly road on the Volvo test in Quebec that was better suited to snowmobiles than cars, we were encouraged to test the systems but not hit anything. Trying the same maneuvers with and without the stability system was an eye-opener. A car that was practically uncontrollable on dirt, wet pavement, or ice without the system was well-behaved and predictable with it. High- speed cornering on a racetrack felt much more secure, and the car was faster through the corners. I live where ice is a rarity, and am not at all practiced in driving on it. Driving a DSTC-equipped Volvo S80 on the ice and snow-covered Quebec hill was not terrifying at all. I could sometimes feel the traction control, ABS, and DSTC systems work, but never felt in danger of leaving the road. High-quality snow tires on the cars certainly helped by allowing any sort of traction at all (never underestimate the importance of proper and properly-inflated tires!), but modern electronics helped control considerably.

During the Quebec ice test, one journalist did manage to collect a snow bank with an S80, fortunately without damage to himself or the car. His comment, "There's a problem with the system.. It doesn't work when the wheels are pointed straight and the car is going sideways," was most instructive. None of the current stability control systems can do anything in that circumstance - traction and control are already lost, and anyone in the car is just along for the ride. Everyone else had successfully negotiated that section of road. Modern technology can expand the envelope of control, but it can't repeal the laws of physics. Common sense and caution are still the best safety factors. The most important stability control system is still the one in the driver's head.