POWER STEERING SYSTEM DETAILS


  1. FUNCTION OF MAIN COMPONENTS


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

      Component Function
      Power Steering ECU Assembly Power Steering ECU 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.
      Power Steering Motor Generates assist torque in accordance with a signal received from the power steering ECU.
      Rotation Angle Sensor Outputs the rotation angle of the power steering motor to the power steering ECU.
      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.
      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.
      Skid Control ECU Outputs the vehicle speed signal to the power steering ECU.
      Air Conditioning Amplifier Assembly Receives a signal from the power steering ECU to limit electrical use.
      Combination Meter Assembly EPS Warning Light

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

      • If the power steering ECU 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.

  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 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 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.
      Electric Load Control Prevents a reduction of assist torque if the battery voltage decreases.

    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 and power steering ECU assembly.

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

      3. The power steering motor is integrated into the power steering ECU assembly.

        B0001W3E01
        Text in Illustration
        *1 Electrical Power Steering Column Sub-assembly *2 Power Steering ECU Assembly
        *3 Torque Sensor *4 Reduction Mechanism
        *5 Power Steering Motor - -
        *a A-A Cross Section *b B-B Cross Section

      4. The power steering torque sensor is built into the electric power steering column sub-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.

      5. 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.

        B0001PYE01
        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 - -

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

      7. The power steering motor 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 is transmitted via the joint to the reduction mechanism.

      10. 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.

      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.

        B000260E01
        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.

        B00028VE02
        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.

        B000208E02
        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
        B00023F 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.

        B0001ZWE02

  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 to inform the driver.

    2. If a system malfunction occurs, 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 DTC can be accessed through the use of an intelligent tester. For details, refer to the Repair Manual.