Steering: Description and Operation

POWER ASSIST SYSTEMS: POWER STEERING SYSTEM: SYSTEM DIAGRAM

















SYSTEM DESCRIPTION

1. DESCRIPTION

(a) 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





3. CONSTRUCTION AND OPERATION

(a) 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.





(b) 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.





4. SYSTEM CONTROL

The power steering system has the following controls:





(a) 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.

(b) Fail-safe

(1) Fail-safe operation modes are as follows:





(c) 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.