No slip-ups with ESP control system

By T. Uncle

IMAGINE what it would be like to have F1 driver Michael Schumacher take over the wheel of your car just as you've lost control on a slippery corner. If you can, then you will begin to have some idea of the function of a remarkable new control system developed by Mercedes-Benz.

The system -- dubbed ESP, for Electronic Stability Program -- allows the car's electronics, largely independently of the driver, to play an active part in vehicle control.

Most drivers will know the feeling of going into an unplanned slide on a slippery road, either the rear of the vehicle swings around in "oversteer" or the front end wants to continue straight ahead in "understeer". In either situation the car is not doing what the driver wants or, usually, is capable of correcting.

The sophisticated Mercedes system takes care of that. ESP actually steps in and returns the vehicle to its intended course through clever manipulation of the braking system. Electronic messages sent to each wheel apply the brakes in a specific way, so directional control is resumed.

How does this work? To help grasp the concept, it might be helpful to have familiarity with the effects of a faulty brake system, where the vehicle lurches to one side as one brake "grabs". ESP uses the same principles, except that it works in the opposite direction, canceling impending diversions from the chosen path through the subtle application of brake pressure. The clever thing about the system is that it's able to brake each wheel individually and selectively, obviously something even the most skilled driver cannot do.

The ESP system is overseen by sensors placed around the vehicle that are able to detect whether it is heading in the desired direction. The sensors detect such things as the "yaw" angle, the angle of the vehicle relative to the direction of travel, lateral acceleration (or sliding), and steering angle.

ESP was developed using much existing technology. The antilock braking system now widely adopted by most carmakers, which modulates brake pressure so a locked wheel is able to continue rotating, thus improving braking traction, was a starting point.

The ability to control the brakes on specific wheels, except that in this case the brakes were to be applied rather than momentarily disengaged, was what was needed to develop an effective stability control.

The system needed to know exactly the right amount of braking pressure that should be applied to the correct wheel in given circumstances. Here, the massive computer power available to today's design engineer played a significant part.

Of course, no system is able to control every possible control situation a driver may have to contend with, but there is no question ESP will pave the way to fewer driver-related accidents as it becomes more widely adopted.

And that may not be far away. If you consider stability control systems like ESP are probably still a long way off for the lower end of the market, think how familiar antilock braking is today, and the fact that it, too, was once confined to the luxury class. Economies of scale have enabled a lot of high technology to filter down through all levels of the new-car market.

Undoubtedly the most active car safety system yet developed, ESP takes us a step closer to the "intelligent" motor vehicle.

Other carmakers are using "active" braking to improve control, too. BMW's Automatic Stability Control, Plus Traction (ASC+T), system controls power-induced rear-end slides by applying braking pressure to individual rear-wheel brakes to virtually eliminate wheel spin. BMW started offering ASC+T on its top-of-the-line models, but is now making it standard in six-cylinder versions of its smaller 3-Series range, as well as all 5-Series models.