As technology advances, modern vehicles increasingly incorporate electric devices, with electric power steering (EPS) becoming widely used across a range of vehicles, from budget cars to those costing tens of thousands. The steering system has undergone three major evolutions: mechanical steering, hydraulic power steering, and now, electric power steering.

Advantages of Electric Power Steering

1.Reduced Fuel Consumption: The system operates only during steering, thereby conserving energy.
2.Simple Structure: The system is compact and occupies minimal space.
3.Lightweight: The components are designed to be light, reducing the overall weight of the vehicle.
4.Ease of Installation and Maintenance: The system is straightforward to install and maintain.
5.Variable Steering Assistance: The system adjusts the level of assistance based on vehicle speed—offering heavier steering at higher speeds and lighter steering at lower speeds.

Components of Electric Power Steering

Electric power steering builds upon the traditional mechanical steering system by adding an electric component. This electric system calculates various parameters and uses a motor to drive the steering rack, thereby providing assistance. The key components, excluding the mechanical parts, include sensors, a controller, and an actuator.

Sensors:
The main sensors include the steering angle sensor, torque sensor, and vehicle speed sensor.

1. Steering Angle Sensor: Mounted on the steering column between the combination switch and the steering wheel, this sensor consists of two coded discs with code rings and optical sensors. It measures the steering angle based on the grating principle, generating a pulse signal to indicate the current steering angle and position. If the steering angle sensor fails, an emergency operation program activates, maintaining steering assistance while illuminating a fault light.

2. Torque Sensor: This sensor connects the steering column to the steering gear. A magnetic wheel with alternating polarity zones rotates as the steering wheel turns, creating a changing signal that is sent to the EPS-ECU. If the torque sensor fails, the EPS-ECU calculates a substitute torque signal based on the steering angle and motor rotation angle. The fault light also illuminates.

3. Engine Speed Sensor: This sensor, also known as the crankshaft position sensor, measures the engine's rotational speed.

Controller:
The EPS-ECU is the core of the electric power steering system, receiving data from various sensors, analyzing and calculating the optimal steering assist torque, and controlling the assist motor accordingly.

Actuator:
The Steering Assist Motor is a DC motor that powers the steering system (no further explanation provided here).

Types of Electric Power Steering System Structures

There are three main types of electric power steering structures, categorized by the location of the assist mechanism:

Steering Column Assist
Gear Assist
Rack Assist

How EPS-ECU Controls the System

The EPS-ECU calculates the optimal steering assist torque based on parameters such as vehicle speed, steering torque, and steering angle. It then controls the assist motor to provide the best steering assistance.

The following is a control circuit diagram for an EPS motor (a1 and a2 are signal voltages from the EPS-ECU).

When a1 receives a signal, VT3 turns on, grounding the base of VT2 through R2, which also turns on VT2. In this case, positive voltage flows from the top, through VT2, the motor (with VT4 off), and finally grounds through VT3, causing the motor to rotate forward (red arrow).

When a2 receives a signal, VT4 turns on, grounding the base of VT1, which also turns on VT1. Positive voltage flows through VT1 to the motor, grounding through VT4, causing the motor to rotate in reverse (blue arrow).

It's important to understand the different control (operating) modes of these transistors—one with a base voltage and the other with a base ground.

Repairing the Electric Power Steering

In addition to mechanical faults, electric power steering systems can experience electric failures, such as issues with the steering angle sensor, motor damage, or control module malfunctions.

When diagnosing vehicles with EPS, it is essential first to read the fault codes and then repair the system based on these codes. electric failures often involve the steering angle sensor.

After repairs (such as disconnecting the battery, replacing sensors, or controllers), it is crucial to perform a calibration for the system to function correctly. Otherwise, error codes, such as "unreliable steering angle sensor signal," may appear.

Here is a basic calibration method for the Volkswagen Sagitar steering angle sensor:

1. Ensure the steering wheel is centered (with the front wheels pointing straight ahead).
2. Start the engine.
3. Turn the steering wheel approximately 10° to the left, pause for 2 seconds, and then return it to the center. Repeat the same process to the right.
4. Release the steering wheel, pause for 2 seconds, then turn it fully to the left, pause for 2 seconds, then turn it fully to the right, pause for 2 seconds, and return to the center.
5. Turn off the ignition.
Note: After calibrating the steering position sensor, you must read and clear any fault codes before completing the setup. It is not uncommon to require multiple calibration attempts to succeed.

Conclusion:
The electric power steering system is a significant technological advancement that offers numerous advantages, including improved fuel efficiency and ease of maintenance. Understanding its components and functionality is crucial for diagnosing and repairing potential issues effectively. Proper calibration and fault code management are essential steps in ensuring the system's reliability and performance.