New Level of Control

BOXBERG, Germany — Stability management systems are rapidly gaining acceptance as consumers discover the technology that instantaneously can sense loss of control and apply the brakes to one or more wheels to keep a vehicle going straight.

The world's top vehicle markets are embracing the technology, known widely as ESP (for Electronic Stability Program).

In Germany, one out of two new passenger vehicles already comes equipped with ESP, and in France it's one in five, according to Robert Bosch GmbH, one of the top suppliers of the technology. In North America, where rollover concerns prevail, ESP appeared on 5% of new passenger cars last year and penetration is expected to surge to 12% by 2005, Bosch reports.

ESP is about to get even better. Since its volume introduction in 1995, ESP has relied solely on braking functions and sensors that detect yaw motion of the vehicle.

Today, ZF Lenksysteme (a joint venture between Bosch and steering specialist ZF Friedrichshafen AG) takes the technology one step further by integrating steering input into the stability control equation.

The all-new '04 BMW 5-Series is the first production vehicle to feature the optional technology, known as Active Steering.

The system uses a planetary gear with two input shafts and one output shaft — one linked to the steering column, the other to an electric motor to generate extra steering angle, if necessary.

The device allows a driver-independent steering input to the front axle without having to disconnect the primary mechanical linkage between the steering wheel and front axle, says Colin Carter, sales director of ZF Lenksysteme.

Software processes the necessary sensor signals, monitors the whole steering system and controls an ancillary electric motor. The motor and the overriding drive allow a steering impulse to be transmitted to the front axle independently of the driver, Carter says.

Active Steering receives inputs of vehicle speed and steering angle, then compares the actual vehicle motion to that desired by the driver. The system adjusts to enhance stability, even for very small deviations from the desired motion, which the driver may not even notice, BMW AG says.

In potentially dangerous driving situations, the electronics of Active Steering can use the data from its own system sensors to alter the steering to keep a vehicle under control.

If the driver applies the brakes on an uneven surface with uneven traction, one side may be slippery while the other offers good traction. This can cause the vehicle to pull to one side. Active steering recognizes this tendency and corrects it by steering against the pull, BMW says. The driver doesn't even know anything out of the ordinary is happening.

Active Steering is amazingly fast. It processes the necessary information about 100 times per second. When the electric motor is idle, the conventional direct steering connection between the steering wheel and the road wheels is restored, Carter says.

Active Steering has benefits in normal driving conditions, too. At low speeds, the system eases steering ratio for simpler parking; at high speeds, Active Steering “firms” the steering ratio to impart optimum control and better handling.

The technology proved remarkable during a test drive of two BMWs (one with, one without Active Steering) here at Bosch's Boxberg proving grounds near Frankfurt.

One of the nice features of Active Steering is that it does its job quietly and undetected. Antilock brakes, on the other hand, can startle the driver with the characteristic clicking and pulsating that occurs when the system is activated.

Other braking suppliers appear to be headed down the same technological path, including Continental Teves Inc., TRW Automotive Inc. and Delphi Corp.

Active steering will be optional on the BMW 525i, 530i and 545i and will be part of the Sport Package on those models. The feature will be standard on the 545i 6-speed. Expect pricing information sometime in September.