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Home >> Lexus >> 2008 >> IS F >> Repair and Diagnosis >> Steering >> Manual Steering >> Power Assist Systems >> Power Steering System >> System Description
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System Description

  1. DESCRIPTION 
    1. The power steering system uses a power steering motor (DC motor) and a reduction mechanism that are built into the steering gear housing to generate assist torque, in order to assist the driver's steering effort
      1. The power steering system actuates the motor only during steering when torque assist is required. As a result, the power steering system does not consume any energy during straight line driving, thus realizing fuel economy.
      2. The power steering ECU assembly controls this system.
  2. FUNCTIONS OF COMPONENTS 
    FUNCTIONS OF COMPONENTS

    Component Function
    Power Steering ECU Assembly
    • Actuates the power steering motor (DC motor) to provide power assist, based on the signals received from various sensors
    • When the power steering ECU assembly detects a power steering system malfunction, it outputs a warning signal to the combination meter
    Steering Gear Assembly Reduces the speed of the motor and transmits it to the rack shaft
    Torque Sensor (built into steering gear assembly) By detecting the twist of the steering torsion bar, the sensor indicates the torque that is applied to the torsion bar using an electrical signal, it sends to the power steering ECU assembly
    Power Steering Motor (built into steering gear assembly) Generates power assist in accordance with the actuation current received from the power steering ECU assembly
    Motor Rotation Angle Sensor (built into steering gear assembly) Transmits the rotation angle of the power steering motor (DC motor) to the power steering ECU assembly
    Combination Meter
    (P/S Warning Light)    		 Upon receiving a signal from the power steering ECU assembly in the event of a system malfunction, the meter ECU illuminates the P/S warning light
    Skid Control ECU Transmits speed sensor signals to power steering ECU assembly
    ECM Transmits engine speed signals to power steering ECU assembly
  3. CONSTRUCTION AND OPERATION 
    1. Steering Gear Assembly
      1. The steering gear assembly consists of the rack shaft, reduction mechanism, pinion gear, torsion bar, power steering motor, rotation angle sensor and torque sensor.
        • The reduction mechanism consists of balls and a ball nut The balls and ball contact surfaces are processed with a high degree of accuracy to achieve high efficiency and low noise
        • The power steering motor is a high power output and brushless type
        • The power steering motor is located around the rack shaft The motor consists of a magnet, coil, stator and rotor
        • The ball screw nut secured to the rotor transmits rotational torque from the rotor to the balls The balls then transmit the rotational torque to the rack shaft
      Fig 1: Exploded View Of Power Steering Gear Assembly
      G05752923Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
    2. Torque Sensor
    1. The torque sensor consists of resolver 1 and resolver 2.
    • Resolver 1 is mounted to the main shaft on the input side Resolver 2 is mounted to the pinion shaft on the output side.
    • These resolvers convert twisting force on the input and output sides of the torsion bar into electric signals and transmit these signals to the power steering ECU assembly.
    • The signals sent from the input and output sides differ depending on the amount of twisting force. The power steering ECU assembly calculates the torque value based on the differences in these signals.
    Fig 2: Identifying Main Shaft, Resolver And Pinion Shaft
    G05752924Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  4. SYSTEM CONTROL 

    The power steering system has the following controls

    POWER STEERING SYSTEM CONTROL CHART

    Control Outline
    Basic Control Calculates the assist current required baed on the steering torque value and the vehicle speed, and actuates the power steering motor
    Inertia Compensation Improves the starting movement of the power steering motor when the driver starts to turn the steering wheel
    Returnability Control Improves returnability during steering wheel operation
    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 It is maintained at 27 V when the driver is not turning the steering wheel or when the vehicle is being driven straight It effects variable control between 27 to 34 V in accordance with the load, when the driver is turning the steering wheel
    System Overheat Protection Control Estimates the motor temperature based on the amperage and the current duration If the temperature exceeds the standard, it limits the amperage to prevent the motor from overheating
    Fail-safe When the power steering ECU assembly detects a malfunction in the power steering system, the ECU changes the control mode to a fail-safe mode, thus enabling the vehicle to be driven
    Diagnosis If the power steering ECU assembly detects a malfunction in the system, the power steering ECU assembly makes the P/S warning light flash to warn the driver and stores DTCs (Diagnostic Trouble Codes) in memory
    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 motor
        • The diagram below describes the relationship between the steering torque and the assist power current
      Fig 3: Steering Input Torque Graph
      G05752925Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
    2. Fail-safe
      1. Fail-safe operation modes are as follows
      FAIL-SAFE OPERATION MODES CHART

      Item Control
      Torque Sensor System Malfunction Disables the assist
      Rotation Angle Sensor Malfunction Disables the assist
      Power Steering Motor Overheating Limits the assist force
      Power Steering Motor Short (including drive system malfunction) Disables the assist
      Power Steering Motor Over-current Disables the assist
      Power Steering ECU Overheating Limits the assist force
      Power Steering ECU Internal Temperature Sensor System Malfunction Limits the assist force
      Power Steering ECU System Malfunction (including boost circuit system malfunction) Disables the assist
      Vehicle Speed Signal Malfunction Limits the assist force
      Power Supply Voltage Malfunction Pauses the assist (Provides normal assist after the voltage recovers)
    3. Diagnosis
    1. If the power steering ECU assembly detects a malfunction in the power steering system, the power steering ECU assembly flashes the P/S warning light in order to alert the driver. At the same time, DTCs (Diagnostic Trouble Codes) are stored in memory DTCs can be read by the use of the Techstream
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