Electronic Power Steering System
ELECTRONIC POWER STEERING SYSTEMDESCRIPTION
The electronic power steering (EPS) system includes the same components of conventional power steering system.
In addition, it has a solenoid valve on power steering gear box, and a control unit underneath the audio of the center facia. To control the oil flow of steering gear box, a solenoid is provided and it functions by the current from control module which receives signals from VSS (Vehicle Speed Sensor) and TPS.
EPS performs the conventional power steering function in case a failure has occurred in the system. EPS electronically controls the current to the solenoid of by-pass valve by inputting sensor's signals to control the hydraulic amount in cylinder chamber and thereby varying the steering effort versus the hydraulic pressure according to vehicle speed.
ELECTRONIC POWER STEERING CONTROL MODULE
The EPSCM is installed underneath the audio inside the center facia.
EPS CONTROL MODULE CIRCUIT
EPS SOLENOID
SOLENOID VALVE
A solenoid valve is provided in the gear box to control the flow of power steering oil. The solenoid valve is composed of a releasing spring, a piston and a plunger. The input current is varying from OA to 1A according to the vehicle speed and is controlled by EPS control module. When the ignition switch is turned on, current is sent to the solenoid valve to push up the plunger and accordingly the piston contacting the plunger is pushed up while overcoming the spring force. As the vehicle speed increases, the current flow going into the solenoid decreases and the piston is pushed down by the spring force. When the piston is pushed up, it closes the oil passing hole, and the power steering oil pressure is transferred into the cylinder without any interruption. But when the hole is opened as the piston is going down, some of the oil coming from the rotary valve is drained into the reservoir via the hole inside the rotary valve.
NOTE: When it is necessary to remove the EPS gear box, be sure to disconnect the connector to prevent damage.
OPERATION
LEFT TURN (COUNTERCLOCKWISE)
The power steering oil pump generates the oil pressure and the oil enters into the rotary valve. And the torsion bar is twisted by the force between the steering wheel rotation and the resistance of the tire to the ground. At a high speed of vehicle, the oil passing the rotary valve comes into the solenoid valve through the lower hole and goes out through the upper hole flowing the connecting space of piston. At this time the piston is pushed back away due to the spring force overcoming the solenoid plunger. The oil flows through the space between the piston and side wall. And then the oil flows again into the rotary valve and finally is drained into the reservoir through the drain hole between the rotary valve and the torsion bar. Therefore, the oil Pump pressure does not affect the steering effort.
As the vehicle speed decreases, the hole of piston connecting the left chamber and right chamber is gradually closed while the oil pressure produced by oil pump is being transferred to the left chamber. Therefore, the steering effort is reduced.
RIGHT TURN (CLOCKWISE)
The operation procedure of right turn is the reverse of left turn.