Description And Operation
The Electric Power Steering (EPS) system is a power assisted steering gear designed to reduce the effort required to steer the vehicle by using torque from an electric motor.
The EPS module manages and controls the system by processing the input information and using a pre-programmed operating logic, the EPS module determines the direction of rotation and the supply current for the electric motor producing the desired servo-assistance effect.
In conjunction with the Electronic Stability Control (ESC) the power steering system also manages the Alfa Steering Torque (AST) function. In critical situations (braking on surfaces with different grip conditions), through the AST function the ESC system controls the steering to implement an additional torque contribution on the steering wheel to suggest the most correct maneuver to the driver.
There is a EPS Failure warning light and an AST warning light on the Instrument Panel Cluster (IPC).
Steering Column
The steering column along the steering wheel shaft (collapsible coupling) transfers steering wheel movement though the steering gear to the front wheels.
The steering column allows adjustment both in depth and height to the steering wheel using on the lever on the left side of the column.
- (1) Steering wheel stroke depth adjustment
- (2) Lever release for steering wheel adjustment
- (3) Steering wheel height adjustment
Steering Wheel Shaft
The steering wheel shaft is made of steel and connects the steering column to the steering gear pinion. The steering wheel shaft has two grooved U-joints which prevent it from being mounted in the wrong position. A bolt at the steering gear end of the steering shaft secures the shaft to the steering gear pinion.
- (1) Grooved U-joint to the pinion steering gear
- (2) Grooved U-joint to the steering column
EPS System
The vehicle is equipped with the latest generation steering gear with electric servo assist. This advanced technology of servo driving assist increases and improves control of the vehicle reducing fuel consumption and optimizing the precision performance and unmatched steering feel. The response of the torque applied to the steering system varies depending on the position of the steering selector.
The steering gear is divided into four main groups:
- (1) Steering gear pinion
- (2) Wire harness from the steering gear pinion housing to the EPS module
- (3) EPS module and electric motor for power assist
- (4) Electric motor to gear with ball screw connection
Steering Gear Pinion
The pinion of the steering gear is physically connected the rack of the steering gear. If the servo assist is absent the driver always has the ability to control the vehicle.
Two sensors are located in the pinion housing:
- Torque sensor
- Angle reference sensor
The feedback from the torque sensor aids the EPS module in managing the servo assist.
The angle reference sensor signals the 0 degrees position of the steering during each revolution.
The angle reference sensor is used by the EPS module for the calibration of the servo assist system and the plausibility of steering position and to be able to generate the steering angular value.
In calculating the plausibility of the steering position (and thus of the front wheels), there are many other values used including vehicle speed and the speed of the individual wheels.
EPS Module and Electric Motor
- (1) Electric motor
- (2) EPS module
The EPS module is attached to the electric motor.
The EPS module receives information from the sensors in the pinion housing of the steering gear to activate the electric motor. The EPS module controls the position of the steering rack receiving the number of revolutions performed by the rotor.
The EPS modules micro-processor processes the data coming from the sensors together with the values of vehicle speed and wheel speeds to identify the effective value of the steering angle and to send it through the Controller Area Network (CAN).
If battery power is lost the EPS module loses the angle reference sensor value coming from the pinion. On re-connection of the battery power it will be necessary to manually re-initialize the sensor.
EPS Module Strategies
During key ON:
The EPS module stores values for the rotor sensor RPM, reference sensor and the calculated steering angle value.
The checks on plausibility among the synchronization of the calculated steering angle, the reference sensor and the wheel speed are continuous.
Whenever the deviation between the calculated steering angle and the reference sensor value is less than 2° degrees the EPS module performs a new initialization.
During key OFF:
Cyclically the part of the EPS module dedicated to the control of the rotor RPM sensor is activated to check the integrity of the system. If the rotor RPM sensor senses a rotation, the EPS module wakes to receive and store the new values sent from the sensor.
When the rotor speed sensor ceases to communicate movements of the rotor, the system will return to making cyclical control.
During key off the EPS module does not send a signal of steering angle and does not provide power assist.
Manual Calibration of the Servo Assist System
If battery power is lost the EPS module loses the sensor reference value coming from the pinion. When power and ground is reconnected, it will be necessary to manually re-initialize the sensor.
Manual Procedure
With the engine in both, engine RUN and engine OFF key ON:
Turn the steering wheel from the center wheels straight ahead one revolution in each direction and back to the center position.
Automatic Procedure
The system is able to calibrate itself automatically while driving:
- Vehicle in motion to acquire the individual wheel speeds
- Steering wheel centered
- Steering wheel turned in the both directions
- Low rotation speed of the steering wheel (less than 200 degrees/sec)
- Low torque applied by the driver to the steering less than 4 Nm (35 In. Lbs.)
- Low torque provided by the engine max. 3.5 Nm (31 In. Lbs.)
The wire harness (1) connecting the EPS module to the torque and angle reference sensors (located in the pinion housing) MUST NOT be disconnected from the pinion housing.
Mechanical Control of the Servo Steering Assist
- (1) Rack
- (2) Ball screw
- (3) Toothed belt
- (4) Bearing
- (5) Ball screw crown
- (6) Electric motor pinion
The torque created by the electric motor, based on the commands received from the EPS module, is transferred to the rack of the steering gear through the use of a ball screw and a toothed rubber belt. The rotation of the ball screw crown enables the ball screw to rotate the balls sliding inside the screw to exert force on the rack. The force generated by the balls on the rack enables the rack to move to the right or to the left according to the direction of rotation of the screw.
The use of this technology results in efficiency significantly higher than similar technologies that use other types of electro-servo assist.