Steering: Description and Operation
POWER ASSIST SYSTEMS: POWER STEERING SYSTEM: SYSTEM DIAGRAM
POWER ASSIST SYSTEMS: POWER STEERING SYSTEM: SYSTEM DESCRIPTION
1. DESCRIPTION
(a) The EPS (Electric Power Steering) system uses a motor and reduction mechanism built into the steering gear housing to generate assist torque for the driver's steering effort. The amount of assist torque is calculated by the power steering ECU.
(b) This system offers excellent fuel economy as the motor mounted on the steering column consumes energy only when power assist is required.
(c) This system excels in serviceability because it does not require pipes, a vane pump, pulleys or power steering fluid.
2. FUNCTION OF MAIN COMPONENTS
3. COMPONENTS DESCRIPTION
(a) Steering Column Assembly
(1) A power steering torque sensor, power steering motor and a reduction mechanism are mounted on the steering column assembly.
(b) Power Steering Torque Sensor
(1) General
* The power steering torque sensor is built into the steering column. 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. A magnetic convergence ring assembly is placed outside of the yoke.
* Hall IC torque sensors are used. This makes it possible to shorten the axial length of the sensor, increasing the extent of steering column contraction through the energy absorbing mechanism.
* The magnetic convergence ring assembly contains two Hall IC, 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 monitors the torque sensor signals output by the two Hall ICs to detect malfunctions.
(2) When the steering wheel is not turned:
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, to indicate that the steering wheel is in the neutral position. Therefore, current is not applied to the motor.
(3) When the steering wheel is turned:
When a driver turn 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.
* 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 two 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 the Hall IC 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.
* Upon receiving the signals from the torque sensor, the power steering ECU calculates the required assist torque and outputs it to the motor.
(c) Power Steering Motor
(1) A low inertia, low noise, and high power output motor is used.
(2) The motor consists of the rotor, stator, and motor shaft.
(3) Torque generated by the motor is transmitted via a joint to the worm gear. This torque is then transmitted via the wheel gear to the column shaft.
(4) 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. As a result, it ensures efficient EPS control.
(d) Reduction Mechanism
(1) This mechanism reduces the speed of the motor via the worm gear and the wheel gear, and transmits steering effort to the column shaft.
(2) The wheel gear is made of a high strength, low friction, and low wear plastic, to realize low noise and a lightweight construction.
(3) A warm gear supported by ball bearings is used. Also, a torsion spring is provided to ensure optimal gear at all times.
(e) Power Steering ECU
(1) The power steering ECU receives signals from various sensors, judges the current vehicle condition, and determines the assist current to be applied to the motor accordingly.
(2) In the event of a system malfunction, the fail-safe function stops current from being output and reverts the power steering system to manual steering. At this time, the P/S warning light illuminates to alert the driver of the malfunction.
(3) The power steering ECU has the following functions:
4. FAIL-SAFE
(a) If the power steering ECU detects a malfunction in the power steering system, it turns the warning light in the combination meter on to inform the driver and stops the assist control. As a result, the power steering system operates in the same way as manual steering.
(b) In case of a malfunction, the fail-safe function activates and the power steering ECU performs various controls.
5. DIAGNOSIS
(a) If the power steering ECU detects a malfunction in the power steering system, the P/S warning light lights up to alert the driver of the malfunction.
(b) At the same time, DTCs (Diagnosis Trouble Codes) are stored in memory. DTCs can be read by connecting the Techstream to the DLC3, and observing the blinking pattern of the P/S warning light.