POWER STEERING SYSTEM DETAILS


  1. FUNCTION OF MAIN COMPONENTS


    1. The main components of the EPS system are as follows:

      Component Function
      Steering Column Assembly Power Steering ECU Assembly Actuates the power steering motor assembly mounted on the steering column assembly to provide power assist, based on the signals received from various sensors and ECUs.
      Power Steering Motor Assembly Generates assist torque in accordance with a signal received from the power steering ECU assembly.
      Power Steering Motor Assembly Rotation Angle Sensor Outputs the rotation angle of the power steering motor assembly to the power steering ECU assembly.
      Electric Power Steering Column Sub-assembly Torque Sensor Detects the amount of twist of the torsion bar. Based on the torque that is applied to the torsion bar, the sensor creates an electrical signal, and outputs this signal to the power steering ECU assembly.
      Reduction Mechanism Reduces the speed of the power steering motor assembly through the use of a worm gear and a wheel gear and transmits it to the column shaft.
      Brake Booster with Master Cylinder Assembly Skid Control ECU

      • Outputs the vehicle speed signal to the power steering ECU assembly.

      • Requests steering torque assist during steering cooperative control.
      Air Conditioning Amplifier Assembly Receives a signal from the power steering ECU assembly to limit electrical use.
      Combination Meter Assembly EPS Warning Light

      • Illuminates to alert the driver when the power steering ECU assembly detects a malfunction in the EPS system.

      • If the power steering ECU assembly detects low battery voltage, it will illuminate the EPS warning light.
      Buzzer Sounds to warn the driver of a malfunction in the system.
      ECM Outputs the engine speed signal to the power steering ECU assembly.

  2. SYSTEM CONTROL


    1. The EPS system has the following controls:

      Control Outline
      Basic Control Calculates the assist current from the steering torque value and the vehicle speed, and actuates the power steering motor assembly.
      Inertia Compensation Control Ensures the starting movement of the power steering motor assembly when the driver starts to turn the steering wheel.
      Recovery Control During the short interval between the time the driver fully turns the steering wheel and when the wheels try to recover, this control assists the recovery force.
      Damper Control Regulates the amount of assist when the driver turns the steering wheel while driving at high speeds, thus damping the changes in the yaw rate of the vehicle body.
      System Overheat Protection Control Estimates the power steering motor assembly temperature based on the amperage and the length of time the amperage has been applied for. If the temperature exceeds the standard, it limits the amperage to prevent the power steering motor assembly from overheating.
      Electric Load Control Prevents a reduction of assist torque if the battery voltage decreases.
      Cooperative Control Brake Control System

      The power steering ECU assembly performs cooperative control with the skid control ECU in order to control the steering assist torque in accordance with information received from the skid control ECU. This facilitates the steering operation of the driver, thus realizing a high level of vehicle stability. For details, Click here.

    2. Electric Load Control


      1. When the power steering ECU assembly detects a battery voltage decrease, it transmits the electric load control signal to the air conditioning amplifier assembly in order to limit the electrical usage.

      2. The air conditioning amplifier assembly limits operation of rear window defogger and mirror heater until the power steering ECU assembly releases the limitation demand.

  3. CONSTRUCTION


    1. Steering Column Assembly


      1. The steering column assembly consists of an electric power steering column sub-assembly, power steering motor assembly and power steering ECU assembly.

      2. The electrical power steering column sub-assembly includes a torque sensor and reduction mechanism.

        A01HRX5E01
        Text in Illustration
        *1 Steering Column Assembly *2 Power Steering ECU Assembly
        *3 Power Steering Motor Assembly *4 Torque Sensor
        *5 Reduction Mechanism - -
        *a A-A Cross Section *b B-B Cross Section

      3. The torque sensor consists of a sleeve, stub shaft and 2 detection coils.

      4. The sleeve is mounted on the input shaft, and the stub shaft is mounted on the output shaft. The input shaft and the output shaft are joined by the torsion bar. The detection coils are placed on the outside of the sleeve to complete an excitation circuit without making contact.

      5. When the steering wheel is turned, the twist that is created in the torsion bar creates a relative displacement between the sleeve and the stub shaft. The torque sensor outputs this change as an electrical signal to the power steering ECU.

        A01HS8ZE01
        Text in Illustration
        *1 Torque Sensor *2 Stub Shaft
        *3 Sleeve *4 Detection Coil
        *5 Input Shaft *6 Torsion Bar
        *7 Output Shaft - -

      6. A low inertia, low noise, and high power output brushless type power steering motor assembly is used.

      7. The power steering motor assembly consists of a rotor, stator, motor shaft and rotation angle sensor.

      8. The rotation angle sensor consists of a highly reliable and durable resolver sensor. The rotation angle sensor detects the rotation angle of the motor and outputs it to the power steering ECU assembly. As a result, it ensures efficient EPS control.

      9. The torque that is generated by the power steering motor assembly is transmitted via the joint to the reduction mechanism.

      10. The reduction mechanism reduces the speed of the power steering motor assembly via the worm gear and the wheel gear, and transmits it to the column shaft.

      11. The wheel gear is made of a high strength, low friction, and low wear plastic material, to realize low noise and a lightweight construction.

      12. A worm gear supported by ball bearings is used. Also, a leaf spring is provided to ensure optimal gear engagement at all times.

        A01HRUJE03
        Text in Illustration
        *1 Spring *2 Ball Bearing
        *3 Worm Gear *4 Rotation Angle Sensor
        *5 Rotor *6 Column Shaft
        *7 Wheel Gear *8 Motor Shaft
        *9 Stator - -
        *a This illustration is an example only. - -

  4. OPERATION


    1. Power Steering Torque and Steering Direction Detection


      1. The torque detection circuit built in the power steering ECU assembly calculates the torque sensor 1 output (TRQ1) and the torque sensor 2 output (TRQ2) in accordance with the signals from the 2 detection coils of the torque sensor.

      2. The power steering ECU assembly detects the steering torque and the steering direction according to TRQ1 and TRQ2.

        A01HS4DE02

      3. When the driver does not turn the steering wheel, TRQ1 and TRQ2 output a specified voltage (approximately 2.5 V). As long as the specified voltage is output, the power steering ECU assembly determines that steering torque is not being generated.

      4. When the driver turns the steering wheel to the right or left, TRQ1 and TRQ2 changes as shown in the graph below. Based on the changes, the power steering ECU assembly determines the steering torque and steering direction input by the driver.

        A01HRZHE01

  5. FAIL-SAFE


    1. If the power steering ECU assembly detects a malfunction in the EPS system, the power steering ECU assembly illuminates the EPS warning light and sounds the buzzer in the combination meter assembly to inform the driver.

    2. If a system malfunction is detected, the power steering ECU assembly changes control mode to fail-safe mode. For details, refer to the Repair Manual.

  6. DIAGNOSIS


    1. The power steering ECU assembly will also store Diagnostic Trouble Codes (DTCs). The DTCs can be accessed through the use of the Techstream. For details, refer to the Repair Manual.