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 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.
      Steering Column Assembly Power Steering Motor Generates assist torque in accordance with a signal received from the power steering ECU assembly.
      Rotation Angle Sensor (Built in Power Steering Motor) Outputs the rotation angle of the power steering motor to the power steering ECU 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 through the use of a worm gear and a wheel gear and transmits it to the column shaft.
      Brake Actuator Assembly Skid Control ECU Sends the vehicle speed signal to the power steering ECU assembly.
      Air Conditioning Amplifier Assembly Receives a signal from the power steering ECU assembly to limit electrical use.
      Accessory Meter Assembly P/S Warning Light Illuminates to alert the driver when the power steering ECU assembly detects a malfunction in the EPS system.
      ECM Outputs the engine speed signal to the power steering ECU assembly.

  2. SYSTEM CONTROL


    1. Assist Control


      1. The EPS system has the following controls:

        Control Outline
        Basic Control Based on the input torque signal, the motor current and motor control device are controlled to achieve target assist power.
        Compensation Control Assist power is adjusted to improve steering feeling during steering operation.

    2. Electrical Load Control


      1. Electrical load control stops the operation of the rear defogger, mirror heaters and wiper deicer if the steering wheel assembly is turned when the battery voltage is low. This enables the EPS system to ensure its power supply even if the battery voltage is low.

      2. The power steering ECU assembly detects the battery voltage, and if the voltage drops to approximately 10 V it sends an electrical load control request to the air conditioning amplifier assembly.

      3. Electrical load control is performed as shown below:

        Control start conditions

        Electrical load control starts when both of the following conditions are met.


        • Battery voltage is 10 V or less.

        • Steering wheel assembly is being turned.
        Control cancellation conditions

        Electrical load control stops when either of the following conditions is met.


        • Steering wheel assembly is not being turned.

        • Battery voltage has recovered to more than 12 V.

  3. CONSTRUCTION


    1. Steering Column Assembly


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

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

        A01F0QYE01
        Text in Illustration
        *1 Steering Column Assembly *2 Electric Power Steering Column Sub-assembly
        *3 Torque Sensor *4 Reduction Mechanism
        *5 Power Steering Motor - -
        *a A-A Cross Section *b B-B Cross Section

      3. The 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.

      4. The 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.

        A01F0VRE01
        Text in Illustration
        *1 Input Shaft *2 Torsion Bar
        *3 Output Shaft *4 Torque Sensor
        *5 Hall IC 2 *6 Hall IC 1
        *7 Multipole Magnet *8 Yoke

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

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

      7. 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.

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

      9. 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.

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

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

        A01F0VXE01
        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 assembly is not turned


      1. If the vehicle is driven straight and the driver does not turn the steering wheel assembly, 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.

        A01F0OUE02
        Text in Illustration
        *1 Yoke Tab *2 Multipole Magnet
        *3 Hall IC 2 *4 Hall IC 1

    2. When the steering wheel assembly is turned right or left


      1. When the driver turns the steering wheel assembly 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 assembly is being turned in accordance with the direction of the magnetic flux that passes between the Hall ICs. Hall IC 1 and Hall IC 2 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 assembly to the power steering ECU assembly.

        A01F0SUE04
        Text in Illustration
        *1 Yoke Tab *2 Multipole Magnet
        *3 Hall IC 2 *4 Hall IC 1
        *a When the steering wheel assembly is turned right *b When the steering wheel assembly is turned left
        A01F0UV Direction of the Magnetic Flux - -

    3. Torque Sensor Output Characteristics


      1. When the driver does not turn the steering wheel assembly, the 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 assembly.

      2. When the driver turns the steering wheel assembly to the right or left, the voltage output from the 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.

        A01F0IUE04

  5. FAIL-SAFE


    1. If the power steering ECU assembly detects a malfunction in the EPS system, the power steering ECU assembly illuminates the P/S warning light 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 Global TechStream (GTS). For details, refer to the Repair Manual.