output = driver • e + AFS • (1-e) [1]
Active Front Steer (AFS) is a control system for altering road wheel steer angle by feeding a second steering input (in addition to driver steering wheel input) through a planetary gear set. The system is intended to allow faster steering at low vehicle speeds for convenience, gradually switching to slower steering at high vehicle speeds for stability. In addition, AFS is intended to serve a Yaw Control function, correcting steering input in extreme handling situations (split-mu braking, for example) for improved vehicle stability.
You model AFS using the rack_afs steering gear template. This template can be used both with and without an external controls system.
In the shared_chassis_database, steering_gear_rack_afs.xml is an example subsystem file using this template.
Learn about:
Modeling AFS without External Control System
An option is available to model AFS without the use of a supplemental control system. This allows quick studies of vehicle behavior with speed-dependent rack rates. Using this method, you specify a VARIABLE that establishes a target rack position. One possible technique to create this VARIABLE is to use the CustomAdmText portion of your steering gear subsystem file.
For example, Speed and Steering Wheel Angle might be used to set the target rack position based on a target speed-dependent rack rate. The AFS planet gear carrier motion necessary to achieve target rack position is then imposed with a MOTION statement. (See AFS Steer Input Superposition Mechanism for more information on AFS gear sets).
The more realistic way to model AFS is with the supplemental control system interface. In this case, the external controller is responsible for establishing a target rack position, and the controller attempts to maintain this target by applying a torque through the AFS gear set.
AFS Steer with Control System - Input Superposition Mechanism
| Note: To model AFS steering with a Control System you must activate the AFS Worm Torque control output in your controls subsystem. |
AFS is modeled in ADAMS/Chassis by adding a three-way COUPLER to the steering system between intermediate shaft and torsion bar. The COUPLER incorporates driver input from the intermediate shaft and rotational input from an external controller. These two inputs are summed within the coupler, and then fed as output to steering input shaft/torsion bar.
An example of how such a three-way coupler might work is illustrated in Figure 1. In this modified planetary gearset, driver input is modeled as the red gear, while AFS input is modeled as the green gear. The two inputs are combined through the purple planetary gears, which rotate both about their own axis, and about the axis of the green AFS input gear. The end result is rotation of the blue output gear. This output gear serves as the input to the torsion bar of the steering gear input shaft. In the ADAMS/Chassis implementation, the purple planetary gears are not included as their own PART. Rather, the relative rotations of the system inputs and output are modeled in the three-way COUPLER. It is left to you to supply the gear set relative rotation relationships. This allows you to model AFS gearset architectures other than the type shown in Figure 1.
Rotation relationships for the Figure 1 gearset are given by:
where:
e= N2N35/N32N5 [2]
driver - Angular velocity of steering
input from driver steering wheel (shown in red)
AFS - Angular velocity of steering
input from planet gear carrier (shown in green)
output - Angular velocity of output
gear (shown in blue), which in turn serves as input to the pinion on the
rack-and-pinion steering gear.
N2 - Number of teeth in driver input gear (shown in red)
N32 - Number of teeth in driver-input planet gear (shown in pink on left)
N35 - Number of teeth in output planet gear (shown in pink on right)
N5 - Number of teeth in output gear (shown in blue on right)
Figure 1. Example of AFS Steer Input Superposition Mechanism
Not shown in Figure 1 is that the green AFS input gear is actually a worm gear, driven by a worm. The relative rotation rate between the worm and worm gear can be easily modeled with a two-way COUPLER, but it is more difficult to model the self-locking friction characteristic of a worm gear. This cannot be modeled directly, as friction in ADAMS/Chassis is intended for individual JOINTs, not COUPLERs. In addition, the friction characteristics vary depending on the direction of torque flow. To model this, a torque flow function VARIABLE was created as the product of worm torque and worm rotational direction. The sign (+/-) of this function determines whether the driving or driven friction moment is applied to the worm. It is left to the FRICTION statement to actually apply the friction torque, but the JOINT load that determines friction moment magnitude varies as a function of the torque flow direction. This JOINT load is artificially induced with an axially-oriented SFORCE statement.
Figure 2. Worm Gear Friction Modeling Method
The example file steering_gear_rack_afs.xml contains all necessary data for AFS. Compared with the standard rack gear template, the AFS template has additional parts for the AFS mechanism, as well as additional parameters.
AFS Part List
The following parts comprise the AFS mechanism.
| Name | Comment |
| AFS_Input | Active Front Steer Driver Input Gear |
| AFS_Carrier | Active Front Steer Spur Gear Carrier |
| AFS_Worm | Active Front Steer Worm (drives AFS_Carrier) |
| AFS_Worm_Shaft | Auxiliary Worm part, allowing "Torque Sensor" between two parts |
For the AFS Worm Torque control output, + adds more left steer.