POWER STEERING SYSTEM DETAILS

  1. FUNCTION OF MAIN COMPONENTS

    Component Function
    Steering Column Assembly Power Steering Motor Generates power assist in accordance with a signal received from the power steering ECU assembly.
    Rotation Angle Sensor (Built into Power Steering Motor) Outputs the rotation angle of the power steering motor 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.
    Power Steering Torque Sensor Detects the amount of twist of the torsion bar. Based on the torque applied to the torsion bar, the sensor creates an electrical signal, and outputs this signal to the power steering ECU assembly.
    Combination Meter Assembly Power Steering Warning Light Lights up to alert the driver when the power steering ECU assembly detects a malfunction in the EPS system.
    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.
    ECM Outputs the engine speed signal to the power steering ECU 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 the electrical usage.

  2. SYSTEM CONTROL

    Controls of EPS system
    Control Outline
    Basic Control Calculates the assist current and assist direction 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.
    Friction Compensation Control Reduces steering wheel operation friction, improving steering feeling.
    Recovery Control During the short interval between the time 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.
    Voltage Boost Control Boosts the battery voltage in the power steering ECU assembly, and issues the amount of voltage necessary to drive the power steering motor.
    System Overheat Protection Control Estimates the power steering motor temperature based on the amperage and the current duration. If the temperature exceeds the standard, it limits the amperage to prevent the power steering motor from overheating.
    EPS Electric Load Control

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

    The air conditioning amplifier assembly limits operation of the rear window defogger, quick heater assembly*, and mirror heater until the air conditioning amplifier assembly releases the limitation demand.

    • *: Models with quick heater assembly

    1. Basic Control

      1. The power steering ECU assembly receives the vehicle speed signal and signals from various sensors. Based on these signals, the power steering ECU assembly judges the current vehicle condition, and determines the assist current to be applied to the power steering motor.

      2. The diagram below describes the relationship between the steering torque and the assist power current.

        A0076GDE01

    2. EPS Electric Load Control

      1. EPS electric load control stops the operation of the rear window defogger, quick heater assembly*, and mirror heater if the steering wheel is turned when the battery voltage is low. This enables the EPS system to ensure its power supply even if the battery is low.

        • *: Models with quick heater assembly

      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. The EPS electrical load control is performed as shown below:

        Control Start Conditions

        EPS electrical load control starts when both of the following conditions are met:

        • Battery voltage is 10 V or less.

        • Steering wheel is being turned.
        Control Cancellation Conditions

        EPS electrical load control cancels when both of the following conditions are met:

        • Steering wheel is not being turned.

        • Battery voltage has recovered to more than 12 V.

  3. CONSTRUCTION

    1. Steering Gear Assembly

      1. Operability has been improved through a lightweight, compact rack and pinion type steering gear assembly.

      2. A light weight has been achieved through a unified aluminum housing unit.

      3. To keep the steering rigidity, the steering gear assembly is assembled in the front suspension crossmember sub-assembly directly without elastic material.

        A0076A0E01
        Text in Illustration
        *1 Steering Gear Assembly *2 Front Suspension Crossmember Sub-assembly

    2. Steering Column Assembly

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

        A0076MQE01
        A0076O3E01

    3. Power Steering Motor

      1. A low-inertia, low-noise, high-power output brushless motor is used.

      2. The operating range of the power steering motor is 8 V to 16 V.

      3. The power steering motor consists of the rotor, stator, motor shaft and rotation angle sensor.

      4. The torque generated by the power steering motor is transmitted via the joint to the worm gear. This torque is transmitted then via the wheel gear to the column shaft.

      5. The rotation angle sensor consists of the resolver sensor, which excels in reliability and durability. The rotation angle sensor detects the rotation angle of the power steering motor and outputs it to the power steering ECU assembly. As a result, it ensures efficient EPS control.

        A0076P4E01
        Text in Illustration
        *1 Wheel Gear *2 Column Shaft
        *3 Worm Gear *4 Motor Shaft
        *5 Rotation Angle Sensor *6 Rotor
        *7 Stator - -

    4. Reduction Mechanism

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

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

      3. A worm gear supported by ball bearings is used. Also, a spring is provided to ensure the optimal meshing of the gears at all times.

        A0076RLE01
        Text in Illustration
        *1 Spring *2 Ball Bearing
        *3 Wheel Gear *4 Column Shaft
        *5 Worm Gear *6 Ball Bearing

    5. Power Steering Torque Sensor

      1. A type of torque sensor with Hall ICs is used. This makes it possible to shorten the axial length of the sensor, thus increasing the extent of contraction of the steering column assembly with the energy absorbing mechanism.

      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 the torsion bar. A magnetic convergence ring assembly is placed outside of the yoke.

        A0076A6E01

      3. The magnetic convergence ring assembly 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 the 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.

        A0076MAE01

  4. OPERATION

    1. Straightline Driving

      1. If the vehicle is driven straight and the driver does not turn the steering wheel, the yoke is located in the center 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 the steering wheel is in the neutral position. Therefore, it does not apply current to the power steering motor.

        A0076FHE01

    2. When Steering

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

        • At this time, the magnetic flux from the N to S pole of the multipole magnet passes between the Hall ICs. The system detects the steered direction of the steering wheel 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.

        • The magnetic flux density becomes higher as it gets closer to the center of the respective pole. A Hall IC converts these magnetic flux fluctuations into voltage fluctuations, in order to transmit the turning torque of the steering wheel to the power steering ECU assembly.

      2. Upon receiving the signals from the power steering torque sensor, the power steering ECU assembly calculates the required assist torque and outputs it to the power steering motor.

        A0076JZE01
        A0076IGE01

  5. FAIL-SAFE

    1. Fail-safe operation modes are as follows:

      Detection Item Fail-safe
      Power Steering Torque Sensor Malfunction Disables the assist.
      Power Steering Motor Overheat Disables the assist.
      Power Steering Motor Short (Including Drive System Malfunction) Disables the assist.
      Rotation Angle Sensor (Built into Power Steering Motor) Malfunction Disables the assist.
      Power Steering ECU Assembly Internal Temperature Sensor Malfunction Limits the assist force.
      Power Steering ECU Assembly System Malfunction Disables the assist.
      Vehicle Speed and Engine Speed Signal Malfunction Limits the assist force.
      Power Supply Voltage Malfunction Pauses the assist. (Provides normal assist after the voltage recovers.)

  6. DIAGNOSIS

    1. If the power steering ECU assembly detects a malfunction in the EPS system, the power steering ECU assembly lights up the power steering warning light in order to alert the driver. At the same time, Diagnostic Trouble Codes (DTCs) are stored in memory.

    2. The DTCs can be read by connecting the intelligent tester II to the DLC3. For details, refer to the AVENSIS Repair Manual.