BMW Opts for Hybrid Steering SystemBMW Opts for Hybrid Steering System

You've read in Design News about the weight, complexity, and potential power management advantages of electronically controlled steer-by-wire (see DN 10.07.02, p.84). Why then, when they redesigned their flagship 5 Series performance sedan for 2004, did BMW engineers decide to go instead with a hybrid of traditional hydraulics augmented by an electrically driven system to vary steering ratio (movement of the steering wheel relative to the turning of the front wheels)? The answer: Feel and handling.

The company says BMW drivers are a particular lot: They want a car that provides a feel of the road and is optimized for handling and performance. According to Philip Koehn, the lead engineer for what the company terms Active Steering for the hybrid steering system, steer-by-wire systems cannot currently provide natural, direct-feedback cues that BMW drivers expect (see sidebar).

The Active Steering system uses a Papst (St. Georgen, Germany, www.papst.de) brushless dc motor to rotate the cage (ring gear) containing the planet gears a planetary gearbox at the bottom of the steering column. The motor effectively changes the steering ratio by increasing or decreasing the amount of gearbox output rotation relative to changes in the steering-wheel angle input. The resulting ratio depends on the speed of the car. For instance, a low 10:1 ratio gives high maneuverability for parking in tight spaces, with less than two turns of the steering wheel, lock to lock. To track straight at highway speeds, where even a small steering input can produce a quick sideways acceleration, the electric drive boosts the ratio to 20:1. Cars without Active Steering typically have ratios halfway between (specifically 14.1:1 for the BMW).

By using a steering rack with varying tooth spacings at the outer extremes from the neutral, straight ahead position, it is possible to change the steering ratio on conventional hydraulic power steering on the order of 10-15% to aid parking. But the driver still inputs more hand motion, with possible steering wheel "shuffling."

While the Active Steering changes steering ratio when its motor rotates the gearbox ring gear, the effect is to change the front axle steering angle during a turn as well. BMW engineers took advantage of this characteristic to introduce limited steering inputs in emergency maneuvers, such as a series of sharp turns to avoid an obstacle. The Active Steering controller applies up to 2.5 degrees of front wheel angle countersteering to avoid a skid without the car's Dynamic Stability Control initiating selective wheel braking with its sometimes disconcerting noise and deceleration. "The closed-loop Active Steering controller compares the driver's steering wheel angle input with the vehicle's output (lateral acceleration and yaw rate). The amount of countersteer provided depends on the difference between the driver's input and vehicle reaction," Koehn says.

Run silent

Because the electric motor was an addition to the steering system, the engineers wanted it to produce as little sound as possible to avoid adding to cabin noise. Koehn notes they selected Papst, rather than traditional automotive suppliers, to furnish the brushless dc motor based on its history of producing quiet, robust fans both for computers and cooling automotive electronics and seats. As to motor design specifics to achieve quiet operation, all Koehn says is, "We paid a lot of attention to that issue and it is know-how we must protect."

The motor produces up to 400W output, but "90% of the time, only 20-40W is needed," adds Koehn. He also cites other advantages of the brushless motor including fewer parts to be "maintenance free," good power density, and inherent safety characteristics. For the latter he adds, "The solenoids can only follow the direction of the electromagnetic field generated by the controller. If the field is stationary, that is a failure due to a short circuit; the motor will not move." The system then reverts to conventional power steering operation until repaired.