Electric Power Assisted Steering System

A power assisted steering system that helps driver steer the vehicle by adding controlled energy to the steering system, making it easier for drivers to perform a turn or a maneuver.

In the car industry two types of power assisted steering systems can be found:
  • A Hydraulic Power Assisted Steering System (HPAS) which uses a pump providing pressure in a fluid through a hydraulic system connected with cylinders to the rack-pinion assembly, reducing driver’s effort.
  • An Electric Power Assisted Steering System (EPAS) which uses a DC electric motor providing a controlled assist torque to the steering system.

Electric Power Assisted Steering Systems have become more popular during the years for several reasons. Firstly, an EPAS system can be easily “tailored” to its individual needs by having different behavior for different vehicle velocities. Most EPAS systems in the market tend to provide higher assist when vehicle is moving at slow speed and reducing it for faster vehicle’s speed. Also, unlike a HPAS that continuously drives a hydraulic pump to maintain system pressure, the efficiency advantage of an EPAS system is that it powers the EPAS motor only when necessary. This results in reduced vehicle fuel consumption compared to the same vehicle with an HPAS. Finally, an EPAS system consists of less parts and needs less maintenance making it more appealing as a power assisted steering system.

There are four forms of EPAS systems based on the position of the assist motor. They are the column assist type (C-EPS), the pinion assist type (P-EPS), the direct drive type (D-EPS) and the rack assist type (R-EPS). The C-EPS type has a power assist unit, torque sensor, and controller all connected to the steering column. In the P-EPS system, the power assist unit is connected to the steering gear's pinion shaft. This type of system works well in small cars. The D-EPS system has low inertia and friction because the steering gear and assist unit are a single unit. The R-EPS type has the assist unit connected to the steering gear. R-EPS systems can be used on mid- to full-sized vehicles due to their relatively low inertia from high reduction gear ratios.

An Electric Power Assisted Steering System of a double pinion configuration is available in the Assembly Wizard plug-in for a Rack-pin style steering system in the Car/Small truck library and a Pitman steering system in the Heavy Truck library.

Working with MotionView/MotionSolve Models

The EPAS electronic control unit calculates the desired assist torque as a function of driver’s torque, measured by a torque sensor placed on steering column, and vehicle’s speed.

The assist motor rotates a steering gear applying the desired force and reducing steering effort.

Figure 1.
As described, three systems need to be added to the existing vehicle model for EPAS implementation:
  • Electric Motor
  • Electronic Control Unit
  • Torque Sensor

Electric Motor

A simple electric DC motor is governed by two main dynamic equations:

Figure 2.

where and are inertial and damping constants of the motor, and are inductance and resistance of motor armature winding, is the armature current, and are t he back EMF and torque constants of the motor, and are the electromagnetic torque created by the motor and the counter balance torque acting on motor shaft due to tire forces.

A complete block diagram model would look like the one below. Here, the main problem is how to calculate the reaction torque on pinion due to tires lateral force.

Figure 3.
For this reason, a split model strategy is used with motors electrical dynamics being modeled in Twin Activate and motors pinion being part of MotionView/MotionSolve model and coupled with rack system.
  • The Twin Activate motor part uses motor’s angular velocity to calculate the back EMF voltage while with current value feedback the desired electrical torque is exported.

    Figure 4.
  • The MotionView motor part contains a body to represent motor’s inertial properties and a rack and pinion coupler with the rack body to interact with the system.

    Figure 5.
The Electric Motor’s parameters can be accessed and changed:
Parameter Model Default Value Unit
Inductance () Twin Activate 9.06e-5
Resistance () Twin Activate 0.0035
Torque Coef. () Twin Activate 0.05285 -
Back EMF Coef. () Twin Activate 0.05285 -
Inertia () MotionView 1000

The MotionView diagram and the FMU used in MotionView model can be accessed from this location within the installation: <install_dir>\hwdesktop\hw\mdl\mdllib\Common\FMU_Library\EPAS\FMU_source

Torque Sensor

Measuring driver’s input torque is performed via a torsion bar connecting the Gear Input Shaft and Pinion (or corresponding bodies in other steering systems). In this model, a torque sensor is modeled as a beam element and driver’s input torque is the torque output of the beam element. Its stiffness and damping characteristics can be modified via the beam’s properties.

Figure 6.
Torsion bar stiffness and damping characteristics can be modified via MotionView model beam's properties.
Parameter Default Value Unit
Young's Modulus (E) 21000
Shear Modulus (G) 75000
Outer Diameter (OD) 7
Inner Diameter (ID) 0
Damping Coef (CRatio) 0.01 -

Boost Curve

Boost curve logic is used in order to set the desired assist torque for the electric motor. As shown in the diagrams below the assist torque is zero for , increasing linearly until value which is reached when value is also decreasing linearly with the vehicle’s longitudinal velocity in order to maintain vehicle’s stability and controllability. Driver’s input torque and vehicle’s velocity are the plant output variables from vehicle’s model in MotionView.

Figure 7.
The parameters of boost curve can be also changed via the MotionView model.
Parameter Default Value Unit
Driver’s min torque () 1
Driver’s max torque () 8
Motor max torque () 20
Vehicle critical velocity () 100

Create a Full Vehicle Model with EPAS System

The MBD-Vehicle Dynamics Tools must be loaded as a preference file.

The Update Model utility is only available when the profile MBD-Vehicle Dynamics Tools is loaded. Follow the steps below to load this profile and access the utility:

  1. From the Menu bar, select File > Load > Preference File.

    Figure 8.
  2. Choose the MBD-Vehicle Dynamics Tools profile and click Load.

    Figure 9.
  3. Click Model > Assembly Wizard.

    Figure 10.
  4. In the Model Type window, select the following:
    1. Select Full vehicle with driver.
    2. In driveline configuration select Front wheel drive.
    3. In primary systems in order to add a power assisted steering system Rackpin steering must be selected as steering linkages.

      Figure 11.
  5. In steering subsystems EPAS (Rackpin) will be available as a Steering boost method.

    Figure 12.

    Other options for the model systems are left to your discretion.

    Completing the Assembly Wizard’s selections will lead to a full vehicle model with Altair Driver and an Electric Power Assisted Steering System.

    Figure 13.

    Steering maneuver simulation such as Sinusoidal Steering or Step Steer events can help you understand the behavior of an EPAS system. In the following diagrams these two events are presented for the displayed event settings and the results are compared with those of a vehicle with no steering boost system, performing the exact same maneuvers.

    Figure 14.