by Jim Kerr
Steering has come a long way from the days of the horse and buggy. Steering boxes with their large worm and sector gears have been replaced on many vehicles by more compact and precise rack and pinion steering units. Power steering is found on all but a few of the lowest price economy vehicles. Variable effort steering systems that vary the force required to turn the steering wheel are now found on many vehicles. Electric steering systems are even beginning to replace the hydraulic pumps, hoses and steering gears on some vehicles. Now a new steering innovation has been introduced by BMW, and it is so simple and works so well, the question comes to mind: “Why didn’t we think of this before?” BMW calls it Active Front Steering.
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The concept of Active Front Steering is based around a planetary gear set, such as found in most automatic transmissions. A planetary gear set is made up of three main components: the sun gear in the center, a set of planet gears (several gears in a unit that rotate around the sun gear, and a ring gear around the outside that has internal teeth meshing with the planet gears. Any one of the three components can be the drive input and any can be the output, as long as one or more of the components is held. This provides a variety of gear ratios in forward and reverse. In the BMW steering system, a small planetary gear set is located between the steering wheel and the conventional steering rack. BMW calls this unit a superimposing gear.
On the BMW Active Steering, the sun gear and planet gears are the input and output. The ring gear is held from rotating by a computer-controlled electric motor meshed to the outside of the ring gear. If the ring gear is held stationary by the electric motor, the gear ratio of the gear set is fixed. However, the computer can operate the electric motor to turn the ring gear at the same time the driver turns the steering wheel, providing a variable steering ratio. The effect of the system operation is amazing. In a parking situation, the computer varies the ratio so that the steering wheel needs less than two turns to move the wheels lock to lock. As vehicle speeds increase, the steering ratio increases, so it takes more turns of the steering wheel to move the wheels and increases vehicle stability.
Now the magic begins. Besides providing variable steering ratios, the computer is linked with the vehicle stability control system to aid in directional stability of the vehicle. As the vehicle is travelling down the highway, road surfaces and wind gusts can affect the vehicle directional stability. The car may wander a little or dart to one side, as many who have met a tractor-trailer unit on a windy day have experienced. Sensors on the car detect this sudden unintentional movement and the computer will stabilize the car by moving the Active Steering electric motor and steering gear. The driver doesn’t turn the steering wheel at all!
If the driver experiences a skid or slide because of poor road conditions, the Active Steering will react to information from the yaw rate sensors to modify the steering angle of the front wheels to stabilize the vehicle. This occurs much faster than the driver can react. If the Active Steering angle is not enough, then the Stability Control system intervenes to help as well.
Safety is one of the prime objectives of this system. Steering angle sensors on the steering column sense the direction the driver wants to go and the system only intervenes if the car is beyond stable limits. If an error or problem occurs in the electronics, the computer shuts down the operation of the electric motor, locking the ring gear of the planetary gear set and making it fixed ratio steering. Finally, if there is a problem inside the planetary gear unit (an unlikely scenario), there is a second shaft that runs all the way through from the steering wheel shaft to the steering rack so that conventional steering is available.
On the road, most drivers wouldn’t feel the system operation, although because I was trying to, I could feel it very slightly when the electric motor was operating. The car just seems very stable at all speeds. Parking was also a snap, because of the short lock to lock turns of the steering wheel. The most noticeable change was when I hopped back into another vehicle without Active Steering. The steering seemed so unresponsive, which demonstrates improvements in steering performance the Active Steering system provides.