Advanced Vehicle Stability Systems

Huge strides have been made in improving vehicle dynamics during the past several decades, and these changes are destined to continue as we head into the 21st century. The next significant enhanced-stability control advance has just become a production reality. Designed to prevent dangerous oversteer or understeer conditions, this new strain of anti-spin/anti-plow driver assist integrates the main elements of existing ABS and traction control with a more powerful electronic brain and the latest iteration of decision-making software. While the new system can't override the basic laws of physics, it does promise a safer driving experience over virtually every type of road surface, and in any weather condition.

A number of automakers plan to introduce stability-control systems during the next several years, but this spring Mercedes-Benz and BMW became the first to market them to the public. (Mercedes' initial fitment is on '95 Euro-spec S-Class models, with select U.S. counterparts receiving it for '96; BMW has made a slightly less-sophisticated variation standard this year on V-12 7- and 8-Series cars worldwide.) Although both firms called upon Robert Bosch GmbH to provide the fundamental technology, Delco and ITT Automotive also are developing similar systems. While differing in certain details, the basic function of each stability-control system is the same: to ensure that a car's path of travel conforms as closely as possible to the driver's intent.

We first encountered one of these high-tech guardians this spring in an S600 Coupe fitted with Mercedes-Benz' new Electronic Stability Program (ESP) at the firm's cold-weather test facility in Arjeplog, Sweden, 60 miles south of the Arctic Circle. M-B research indicates that nearly 90 percent of all accidents are caused by human error, with about half of those calamities attributed to an incorrect steering response, and another 10 percent due to improper braking. The firm believes that ESP will materially impact these rather sobering figures.

Key elements in the ESP system include a battery of sensors, an upgraded computer control module, and a sophisticated mathematical algorithm that determines the appropriate vehicle response for a given situation. Individual wheel speeds, steering angle, lateral acceleration, throttle position, and brake pressure are monitored, and input is processed by a pair of computers in the ECM that have a combined capacity of 48 kilobytes, about twice as much memory as a conventional ABS/traction-control setup requires.

Unique to the ESP package is a yaw sensor that directly measures the vehicle's rotation around its vertical axis and calculates the extent of understeer or oversteer. A bit of hardware borrowed from the aerospace industry, the sensor is a compact unit capable of withstanding the rigors of an automotive installation. So far, all other stability systems (including the one Bosch codeveloped with BMW) rely soley on an algorithm that computes the rotation differential between wheels to establish these yaw-angle variations.

When a car begins to deviate from the intended path of travel, ESP swings into action, comparing the real versus the ideal arc being traced by the wheels and intervening as needed to bring the vehicle back onto the desired line. The system's response is blindingly quick, on the order of 40 milliseconds. If understeer is detected, ESP applies primary braking force to the opposite-side rear wheel. (In a lefthand corner, for example, a push to the outside right would be counteracted by main braking to the inside rear wheel.) Conversely, in the case of an oversteer condition, the main supplemental braking force is applied to the outside front wheel.