POWER STEERING SYSTEM


  1. FUNCTION OF MAIN COMPONENTS


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

      Component Function
      Steering Column 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.
      Rotation Angle Sensor Outputs the rotation angle of the power steering motor to the power steering ECU assembly.
      Power Steering Motor Generates assist torque in accordance with a signal received from the power steering ECU assembly.
      Reduction Mechanism Reduces the speed of the power steering motor through the use of a worm gear and a wheel gear and transmits it to the column shaft.
      Power Steering ECU Assembly
      • Actuates the power steering motor mounted on the steering column assembly to provide power assist, based on the signals received from various sensors and ECUs.

      • Supplies the required voltage to the power steering motor by boosting the voltage of the auxiliary battery from 12 V to 30-35 V as required by the state of the vehicle.

      Skid Control ECU Outputs the vehicle speed signal to the power steering ECU assembly.
      Combination Meter Assembly EPS Warning Light Illuminates to alert the driver when the power steering ECU assembly detects a malfunction in the EPS system.
      Buzzer Sounds to warn the driver of a malfunction in the system.
      Master Warning Light* Illuminates when the warning message is displayed in the multi-information display.
      Multi-information Display* Shows a warning message to alert the driver, when the power steering ECU assembly detects a malfunction in the EPS system.
      ECM Sends an engine revolution signal via CAN communication to the power steering computer assembly.

      Tech Tips

      *: Models with TFT LCD type multi-information display

  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.
      Inertia Compensation Control Ensures the starting movement of the power steering motor when the driver starts to turn the steering wheel.
      Recovery Control During the short interval between the times the driver fully turns the steering wheel and 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 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 from overheating.
      Assist Function Continuation Control Controls each assist control so that it will be continued if the ignition is turned off while driving. An assist function continuation control period will be determined based on the vehicle speed when the ignition was turned off.
  3. CONSTRUCTION


    1. Steering Column Assembly


      1. The steering column assembly includes a torque sensor, power steering motor and reduction mechanism.

        B0082LTE02
        Text in Illustration (LHD Models (Except Models for Korea):)
        *1 Steering Column Assembly *2 Torque Sensor
        *3 Reduction Mechanism *4 Power Steering Motor
        *a A-A Cross Section *b B-B Cross Section
        B0082JJE03
        Text in Illustration (LHD Models (Models for Korea):)
        *1 Steering Column Assembly *2 Torque Sensor
        *3 Reduction Mechanism *4 Power Steering Motor
        *a A-A Cross Section *b B-B Cross Section
        B0082JIE02
        Text in Illustration (RHD Models:)
        *1 Steering Column Assembly *2 Torque Sensor
        *3 Reduction Mechanism *4 Power Steering Motor
        *a A-A Cross Section *b B-B Cross Section
      2. The power steering torque sensor is built into the steering column assembly. A multipole magnet is mounted to the input shaft, and a yoke is mounted to the output shaft. The input and output shafts are joined by a torsion bar.

      3. The power steering torque sensor contains 2 Hall ICs which face opposite to each other. The system detects the steering direction in accordance with the direction of the magnetic flux that passes between the Hall ICs. Furthermore, the system detects steering torque in accordance with the amount of change in the magnetic flux density based on the relative displacement of the multipole magnet and the yoke. The power steering ECU assembly monitors the torque sensor signals output by the 2 Hall ICs to detect malfunctions.

        B0082IYE03
        Text in Illustration
        *1 Input Shaft *2 Torsion Bar
        *3 Output Shaft *4 Hall IC 2
        *5 Hall IC 1 *6 Multipole Magnet
        *7 Yoke - -
        *a Power Steering Torque Sensor - -
      4. A low inertia, low noise, and high power output brushless type power steering motor is used.

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

      6. 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.

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

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

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

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

        B0082MIE03
        Text in Illustration
        *1 Leaf Spring *2 Ball Bearing
        *3 Worm Gear *4 Rotation Angle Sensor
        *5 Rotor *6 Column Shaft
        *7 Wheel Gear *8 Motor Shaft
        *9 Stator - -
  4. OPERATION


    1. When the steering wheel is not turned


      1. If the vehicle is driven straight and the driver does not turn the steering wheel, the yoke is centered between the N and S poles of the multipole magnet. Thus, no magnetic flux passes between the Hall ICs. In this case, the Hall ICs output a specified voltage to the power steering ECU assembly, to indicate that no turning torque is being applied to the steering wheel. Therefore, current is not applied to the motor.

        B0082LEE13
        Text in Illustration
        *1 Yoke Tab *2 Multipole Magnet
        *3 Hall IC 2 *4 Hall IC 1
    2. When the steering wheel is turned right or left


      1. When the driver turns the steering wheel to the right or left, the twist created in the torsion bar creates a relative displacement between the multipole magnet and yoke.

      2. At this time, the magnetic flux from the N to S poles of the multipole magnet passes between the Hall ICs. The system detects the direction the steering wheel is being turned in accordance with the direction of the magnetic flux that passes between the Hall ICs. Hall IC1 and Hall IC2 are installed facing opposite to each other. As a result, the output characteristics of the 2 Hall ICs are constantly opposite each other. The system monitors the different outputs of these Hall ICs in order to detect malfunctions.

      3. The magnetic flux density becomes higher as the Hall ICs get closer to the center of each respective pole. Each Hall IC converts these magnetic flux fluctuations into voltage fluctuations in order to transmit the rotational torque of the steering wheel to the power steering ECU assembly.

        B0082LME16
        Text in Illustration
        *1 Yoke Tab *2 Multipole Magnet
        *3 Hall IC 2 *4 Hall IC 1
        *a When the steering wheel is turned right *b When the steering wheel is turned left
        B0082MO Direction of the Magnetic Flux - -
    3. Power Steering Torque Sensor Output Characteristics


      1. When the driver does not turn the steering wheel, the power steering torque sensor outputs a specified voltage (2.5 V) to the power steering ECU assembly. As long as the specified voltage is output, the power steering ECU assembly determines that no turning torque is being applied to the steering wheel.

      2. When the driver turns the steering wheel to the right or left, the voltage output from the power steering torque sensor to the power steering ECU assembly changes. Based on the changes, the power steering ECU assembly determines the steering torque and steering direction input by the driver.

        B0082M2E25
  5. FAIL-SAFE


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

    2. If a system malfunction occurs, the power steering ECU changes control mode to fail-safe mode.

    3. Fail-safe control is as follows:

      Malfunction Item Control
      Power Steering Torque Sensor Malfunction Disables the assist.
      Power Steering Motor Overheating Limits the assist torque until the motor temperature drops to a normal level.
      Power Steering Motor Short (Including mechanical malfunctions) Disables the assist.
      Power Steering Motor Over-current Disables the assist.
      Power Steering ECU Assembly Overheating Limits the assist.
      Power Steering ECU Assembly Malfunction Disables the assist.
      Vehicle Speed Signal Malfunction

      Temporarily disables the assist.

      (Normal assist is provided after the vehicle speed signal recovers.)

      Power Supply Voltage Malfunction

      Temporarily disables the assist.

      (Normal assist is provided after the voltage recovers.)

  6. DIAGNOSIS


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