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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.

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
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 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. 

Improving active front steering performance 

Steering is dependent upon suspension integrity, so it is only to be expected that AFS and suspension control is being integrated [6]. Shock absorbers, leaf springs, and McPherson Struts can affect steering geometry, as an inspection of the following shows:

Typical front wheel suspension assembly – Left front wheel suspension of a Saab Quantum IV with double wishbones, showing kingpin axis, wheel hub, disc brake, steering arm and tie rod end [7]

For example, weaken the spring, and there will be sag, thus altering the sideways angles of the vehicle, in turn altering the yaw rate. Note here, also, that tire pressure and stiffness can affect height, as well, resulting in similar steering control issues. There are, of course, tie rod end, bushing, and bearing wear. This is not to mention motor mounts, tire condition, differential (in the case of rear wheel-driven systems), and condition of the transmission torque obviously affects steering, as when one wheel contacts a slippery surface, thus not gaining as much traction as the other, hence not having as much torque. Thus the vehicle will have a tendency to turn in the direction of the wheel having more torque, thus affecting steering control. Adaptive feedback is being investigated to transmit the torque status of wheels to the AFS [8].

Steering is not an isolated action, as we have just seen. The ability to control a vehicle also depends upon other components of the vehicle other than the front end, and for this reason there is ongoing the development and deployment in the integration of active steering into the chassis control system to increase active safety.

Whenever there is a loss of steering, the brakes need to be applied. In electronic steering control (ESC), this is what happens, each wheel being braked individually so as to control the vehicle until it is brought to a stop. Central to the ESC is the electronic control unit (ECU), which often manages ABS, traction control, and even climate control inside the vehicle. Steering wheel angle and gradient, as well as wheel speed are input to the ECU, along with the yaw sensor data. From this is computed the force needed to slow down or speed up the wheel and direct a hydraulic modulator to make the changes. 

Anti-skid and traction control systems have been available since the late 1980s, with engine output and braking being regulated in conformity with throttle control. The early systems, such as the trace control system developed by Mitsubishi in 1990, monitored steering angle, throttle position, and individual wheel speeds and worked in conjunction with electronically controlled suspension and four wheeled steering. Later developments by other companies incorporated the monitoring of yaw and a system to regulate engine torque. Sensors track yaw, lateral acceleration, and wheel speed, with other being longitudinal acceleration and roll rate (collective measurement of other sensors). The anti-lock brake system (ABS) enables the ESC to brake each wheel by itself. 

In some integrated systems, variable steering ratios are taken into account, but there also are systems where, the computer is linked with the vehicle stability control system to, as the name implies, manage vehicle stability. Built into these systems are sensors to detect yaw change due to sliding or skidding, perhaps, and the AFS will change the steering angle of the front wheels to achieve vehicle stability. All of this happens almost instantaneously, far beyond the capacity of the driver to react. A stability control system intervenes to assist in bringing the car under control if the steering angle is not sufficient [9]. Other integrated systems incorporate the variation in cornering stiffness and breaking torque for controlling vehicle stability, especially under difficult road surface conditions. Several chassis stability schemes that integrate AFS are being presented in research papers.

For some 20 years anti-lock brake systems (ABS), acceleration slip regulation (ASR), and vehicle stability control (VSC) have existed. More recently direct yaw control (DYC), a further development of individual braking of wheels, has come on line. For electric vehicles differential driving is the basis of the DYC; most recently it uses the drive-by-wire.

Dynamic stability control (DSC) systems have been developed that are driveline and brake-based, as well as a combination of these two. Both forward speed and lateral acceleration are factors influencing how DSC systems are built. The AFS controller improves steering in the lower to middle range speeds, while the DSC controller takes over in managing vehicle stability in extreme situations, such as sliding. Research is being conducting in how to have these two systems operate together, where one does not interfere with the other.

For hybrid electric vehicles, research is being done to optimize energy consumption and chassis control, so as to “accurately identify critical issues affecting driving such as time for a system to respond, actual vehicle movement from uncontrolled to controlled state, and hesitation in torque. Event-based rules combine with driveline dynamics to control the engine start and stop function. [10]. 

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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
Resources (Subject is indicated by URL – accessed 9 July 2011)
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