Body Control as Easy as A-B-C

PAPENBURG, Germany - It's not that it hasn't been tried - and done - before. Engineers have crafted schemes to regulate undesirable vehicle-body motion practically since the days of the Tin Lizzie. Only now, though, do the engineers have high-speed electronic minions to do their bidding.

Governing body motion requires, in a larger sense, governing suspension movement. The weight of the vehicle reacts to changes in momentum; those changes occur due to acceleration and deceleration forces and cornering forces, or because of irregularities in the road surface. As the vehicle's weight shifts, the suspension attempts to counteract those movements. How well the suspension does its job is based on a variety of factors - and a lot of compromise.

Yes, one could design the suspension so that it barely moves, regardless of the forces acting upon it. But that vehicle would have a terrible ride. Think, for example, of a Formula One race car: rock-hard suspension means the body seldom pitches or rolls, but the race car doesn't have to deal with bumps of any significance. Take the race car out on the road and the ride would pulverize your internal organs.

So there's the idea behind "active" suspension. A suspension that is compliant enough to give a good ride, but then "reacts" to road disturbances or drastic forces like hard acceleration or braking.

How to achieve that ideal happens to be the problem. Nissan Motor Co. Ltd. tried with the first-generation Q45 luxury car. It worked - sort of. And PSA Peugeot Citroen has for years done a credible job with various elegantly engineered but only partially successful systems.

Now DaimlerChrysler AG's Mercedes-Benz rings in with its attempt at active suspension, and as one might expect from an automaker with Mercedes' reputation for painstaking development, the company's engineers didn't start yesterday on the so-called Active Body Control (ABC) system.

Frank Knothe, head of S/SL/SLK-Class in Mercedes passenger-car development, says the ABC system has been under development for 20 years. "The sheer length of the development process is testimony to the difficulty of the task," he explains.

The heart of the ABC system is a suspension strut at each corner that incorporates a hydraulically adjusted "plunger," along with conventional "passive" dampers and coil springs. There, of course, is the expected maze of sensors: 13 in all, one riding each strut, one at each axle to detect the level of the car, six sensors to detect localized vertical and transverse acceleration and a yaw sensor. Two microprocessors soak up all the required inputs and issue forth a new suspension-adjustment command every 10 milliseconds. Then there's the separate 290-psi hydraulic system that provides the force necessary to act on each corner's strut plunge.

Taking body position readings from the body sensors, ABC forces the plunger to act upon the strut's coil springs, counteracting various body movements and effectively adapting the spring and damping rates. In some cases, the plunger preloads the springs, in others it limits the amount of rebound. The amount of pressure the plunger exerts on the coil springs, and the duration, is what keeps suspension and body movement in check.

Mercedes claims several advantages for the new CL coupe, which with this month's Job One becomes the first Mercedes car to be fitted with ABC. Arno Rohringer, head of spring/shock absorber development, says body roll during cornering is reduced by 68% and by 75% if the driver has selected ABC's "sport" mode. Braking dive and acceleration "squat" is all but eliminated. Body stabilizing time after a rapid swerve is cut by 30%. Ride height also can be continually adjusted, and at 140 mph (225 km/h) the ride height is lowered by almost a half-inch (10 mm).

Mr.Rohringer says the advent of high-speed electronics has made ABC reactive enough to answer Mercedes'high goals.

And engineers avoided another prior active-suspension drawback - excessive parasitic power requirement - by designing ABC to act only on body movement frequencies up to 5 Hz. For higher-frequency wheel motions, the passive dampers and springs handle the action. Mr. Rohringer explains that ABC's maximum power draw is about 4.5 hp; on average, the engine loses just 0.8 hp to run the system. The effect on fuel economy, he assures, is negligible.

Although the system sees its initial fitment on the high-cost CL, Mercedes hopes it can volume-produce ABC for other vehicle lines (i.e. sport/utilities) so that more DC products can benefit from what Mr. Knothe calls "the latest example of the 'mechatronic' trend."