Nearly Steer-by-Wire

Columns From: 11/1/2003 Automotive Design & Production

What Is It?Active Steer is as close to steer-by-wire as you can get in production.

What Is It?
Active Steer is as close to steer-by-wire as you can get in production. Built by ZF Lenksysteme, a joint venture of Robert Bosch GmbH and ZF Freidrichshaffen AG, it allows driver-independent steering input without disconnecting the mechanical linkage between the steering wheel and front axle. At its heart is a standard rack-and-pinion steering system to which is added a planetary gearset and an electric motor. Active Steer debuts as an option on the 2004 BMW 5 Series sedan.

How Does It Work?
A planetary gear with two input and one output shaft is built into the steering column; one of the input shafts is connected to the steering wheel. An electric motor drives the other input shaft via a worm gear that wraps around the planetary gearset. Based on the situation, the electric motor can increase or decrease the effective steering angle, or allow a direct connection between the steering and road wheels. As you might expect, software and sensors play a key role in Active Steering’s abilities. There are sensors for steering wheel, pinion and motor angle on the steering system itself. These tie into the vehicle’s yaw rate and lateral acceleration sensors, road wheel speed sensors, and add Active Steering’s functionality to the vehicle stability system.

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The heart of ZF Lenksysteme’s Active Steering unit is the motor-driven planetary gearset.

What Does It Do?
At parking speeds, Active Steering takes less than two turns of the wheel to go from lock to lock. In the medium speed range, it is either direct or adds a slight amount of steering angle. At higher speeds, the steering becomes more indirect for greater stability. In a severe lane-change maneuver, for example, the system can add up to 2.5°º of steering angle in order to stabilize the vehicle. As a result, the driver and vehicle stability system are freed from having to “catch” the tail. This not only helps driver concentration, it expands the performance envelope of the vehicle stability system. On BMW’s 5 Series, the standard mechanically variable steering system has a mean ratio of 14.1:1. The Active Steer version, on the other hand, has a ratio that varies from 10:1 to 18:1, depending on the situation. Should it fail, the electric motor and worm gear are locked in place, and the planetary gearset rotates as a unit. The driver then has a fixed ratio steering system with a direct connection between the steering wheel and the road wheels; something a true drive-by-wire system could never offer.

What’s The Downside?
Unless given a back-to-back demonstration of Active Steering’s abilities, it’s possible some drivers will never notice the steering’s role in the vehicle’s greater agility and comfort. Unless they place a premium on parking lot maneuverability, they’ll be hard-pressed to tell the system is working. But that invisibility also is a plus because it gives the driver “authentic” steering feel with many of the benefits of steer-by-wire. Of course, this technology comes at a price. The unit is part of a $3,300 Sport Package on the 2004 5-Series. Expect to see the system on other–including non-BMW–vehicles in the near future.