Active front steering (AFS) was originally developed by BMW in 2003 to make front wheels turn a certain number of degrees based on vehicle speed. This prevents oversteering and understeering. A typical AFS system uses sensors to detect steering angle and activate an electric motor that turns the front wheels via a planetary gear assembly. Most systems employ the ZF Lenksysteme approach, which varies the steering ratio to provide more precise control at lower speeds and reduced effort at higher speeds. The AFS adapts the steering response based on signals about vehicle dynamics.

1. Active front steering
Basic system
Active front steering (AFS) is technology designed to make the front wheels turn a certain
number of degrees in accordance with the speed of the vehicle. It was originally developed
by Bavarian Motor Works (BMW) in 2003 and the ZF Lenksysteme method used is pretty
much the same used in the AFS of other cars. The slower the speed, the greater number of
degrees the wheels are turned per degree of movement of the steering wheel; more front
wheel turning is required than at higher speeds. This prevents over and under-steering, as in
parking situations or high speed highway driving, when the former involves more turning of
the wheels and the latter does not. One stark example is locking the steering wheel after
parking. It should take less than half a turn. In normal vehicles, it can take more than two
turns of the steering wheel to lock, as opposed to AFS, where fewer than two turns is needed.
Sensors located in the steering column and detecting steering angle recognize where the
driver wants to go and activate the AFS. If the electronics shuts down, the planetary gear in
the differential controlled by the AFS is locked, and fixed ratio steering takes over. In the
event of a planetary gear problem conventional steering then takes over, as there is a second
shaft running from the steering rack running from the to the planetary gear assembly.
A typical AFS looks like the following:
[1]
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2. with a detailed configuration being:
[2]
1. Main gear
2. Servotronic valve
3. AFS actuator including the synchronous motor
4. Upper position gear system
5. AFS electronic control system with the AFS Electronic Control Unit (ECU)
6. Motor angle sensor
7. Electromagnetic locking unit
8. Pinion angle sensor
9. Steering pump
10. Oil reservoir with filter
11. Hoses
Other: Respective electrical connections of the ECU and the required software modules
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3. The typical motor and electromagnetic locking units is:
and the AFS actuator:
[3]
Active front steering and driveline dynamics functions
Two methods exist for steering adjustment, the ZF Lenksysteme approach and the Ackerman
method. With the ZF Lenksysteme the variable steering ratio (VSR) is the ratio between the
steering wheel angle and the average road wheel angle and this is changed in accordance
with the driving environment, as a function of such factors as velocity. The VSR also depends
upon the pinion gear angles, or the rack displacement, it being less at higher speeds than
lower ones. This means more precision for smaller steering angles and reduced steering
effort at larger steering angles [4]. This system has steering lead function (SLF) that adapts
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4. the steering response to signals about the vehicle situation, such as wheel angular velocity
that determines the desired SLF. The whole system has a feedback system, where the
driver's actions help control AFS actuator motion and the system response is fed back to the
driver.
The Ackerman method adjusts the steering angle by computing the difference between a
reference yaw rate (movement around the vehicle's vertical axis) and actual yaw rate.
Steering ability depends upon vehicle mass, road conditions, and velocity, among other
factors, so better control is achieved by controlling the yaw rate [5].
Most systems use the ZF Lenksysteme.
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References (Subject is indicated by URL – accessed 9 July 2011)
[1] Willy Klier, Gerd Reimann and Wolfgang Reinelt, Concept and Functionality of the Active
Front Steering System, ZF Lenksysteme GmbH, Schwäbisch Gmünd, Germany, No. 2004-21-
0073, 2003 SAE International (2003), pp. 1-3
[2] Ibid.
[3] Ibid.
[4] Ibid., p. 3
[5] http://resources.metapress.com/pdf-preview.axd?code=22113q1143813182&size=largest
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