Traction Control System

Critical driving situations can occur not only while braking, but also whenever strong longitudinal forces are transferred at the contact area between the tire and the ground. This is because the transferable lateral forces are reduced as excessive longitudinal slip is introduced. Critical situations can also occur when starting off and accelerating, particularly on a slippery road surface, on hills, and when cornering. These kinds of situations can make the driver react incorrectly and the vehicle to become unstable.

Traction Control System (TC) detects if one or more wheels are about to spin and regulates engine torque to keep wheel slip ratio at desired values, just like Anti-Lock Braking System (ABS) does while braking. As a result, the vehicle maintains its controllability.

Figure 1.

In modern vehicles the throttle pedal is connected to a sensor, at which the throttle position is measured. The vehicle’s computer sends the measured signal to the throttle actuator controlling the engine intake. This set up is called drive by wire and it is necessary for TC function. TC utilizes wheel speeds sensors and an accelerometer to calculate longitudinal slip and then regulates driver’s throttle reference to prevent wheels from spinning.

When you build a car/light truck model using the Assembly Wizard, you can select to include a Traction Control System. The TC in MotionView system collects information like wheel speeds, vehicle longitudinal acceleration, and Driver throttle demand from the MotionView model as input to the TC Twin Activate Model imported into MotionView as a Functional Mock-up Unit (FMU). The TC Twin Activate Model estimates the wheel slips and outputs the modulate throttle to the MotionView powertrain system. The powertrain system then outputs the equivalent torque to drive the vehicle based on the modulated throttle. The Traction Controller Twin Activate model (.scm file) is included in the MDL Library for viewing and editing (…\hwdesktop\hw\mdl\mdllib\Common\FMU_Library\TC).

The figure below shows the schematic relationship between the TC Twin Activate Model and the MotionView powertrain model.

Figure 2.

TC Modules in Twin Activate

The complete TC block is shown below:

Figure 3.

TC Throttle Controller

The TC throttle controller is responsible for creating the control signal for the throttle modulator using slip error and a PI controller with anti-windup on the integral term.

Figure 4.
Anti-windup is responsible for the saturation of the signal between 0 and the driver throttle demand, so that the vehicle will not accelerate faster than desired at any moment. The PI with anti-windup controller is shown below:

Figure 5.
Longitudinal slip is calculated by the wheels rotational velocity and the vehicle's longitudinal velocity as:
  • : longitudinal slip
  • : wheels rotational velocity
  • : vehicle’s longitudinal velocity
  • : tires radius
  • : a small number to avoid undefined value when = 0

Figure 6.
The vehicle’s longitudinal velocity used in slip calculation refers to an estimation that is performed using signals from wheel speed sensors and the vehicle’s longitudinal acceleration measured by an accelerometer.

Figure 7.
Each sampling instantly computes three auxiliary signals:
  • Average wheel speed of the four tires
  • Average wheel speed of the two non-driven tires
  • Longitudinal Acceleration

As seen in the block diagram below a switch is also available to identify the non-driven wheels:

Figure 8.
Estimation algorithm behavior changes according to the status of the vehicle which can be represented by four values:
  • Vehicle's velocity is very low
  • Vehicle is accelerating
  • Vehicle has constant velocity or decelerating softly
  • Vehicle is decelerating

Status is computed based on the previous step status and some threshold values alongside with some hysteresis in order to keep the algorithm stable.

Figure 9.
Velocity estimation based on the current status of the vehicle is:
Finally, the TC electronic control unit is also responsible to trigger TC. This happens after a significant amount of wheel slip occurs. Here, the electronic control unit works as a switch specifying if the throttle is controlled by the TC module.

Figure 10.
Also, a manual switch is introduced so that the user/driver can activate or deactivate TC.

Figure 11.

Create a Vehicle Model with Traction Control System (TC)

Follow the steps below to create a full vehicle model with TC.
  1. Load the MBD-Vehicle Dynamics Tools preference file (File > Load > Preference File > MBD-Vehicle Dynamics Tools)

    Figure 12.
  2. From the Model tab, select the Set Wizard Path and pick Car/Small Truck.
  3. From the Model tab, select the Assembly Wizard.

    Figure 13.
  4. On Page 8 of the dialog, select Traction Controller (TC).

    Figure 14.
Completing the Assembly Wizard’s selections will lead to a full vehicle model with Altair Driver and a Traction Control System.

Figure 15.

Figure 16.
By selecting FWD or RWD vehicle Traction, the controller is defined automatically to the desired setting. If you switch the powertrain, then the proper value for traction control needs to be set inside the traction control Forms.
Figure 17.