How does Subaru electric power steering work?
Subaru's electric power steering (EPS) uses an electric motor to assist steering instead of a hydraulic pump, and it adjusts assist based on speed and steering input. The result is lighter steering at low speeds and steadier steering at highway speeds, with integration to Subaru's driver-assistance features such as EyeSight.
What EPS is and why Subaru uses it
EPS replaces hydraulic fluid-based systems, delivering variable assist and enabling tighter integration with safety systems. Here is how the system is built and operates.
Key components
The main parts that make Subaru’s EPS possible are listed below.
- Electric motor: a brushless DC motor mounted near the steering rack or steering column to provide assist torque.
- Torque sensor: detects how hard the driver is turning the wheel, forming the basis for how much assist to apply.
- Steering angle sensor: measures the wheel’s position and the rate of change for precise control.
- Vehicle speed sensor: provides real-time speed data to adjust assist level.
- EPS control module: the electronic brain that computes the necessary assist and drives the motor.
- Power electronics and wiring: manage energy from the vehicle’s electrical system (typically a 12V setup) to the motor and sensors.
- Steering rack/gear mechanism: transfers motor-assisted torque to the wheels through the steering gear.
Understanding these components helps explain how EPS can automatically vary steering effort across driving conditions.
How the system works in practice
The EPS uses a real-time feedback loop to turn driver input into steering force, while coordinating with other safety systems.
Step-by-step operation
- The driver applies torque to the steering wheel; a torque sensor in the steering column detects the magnitude of that input.
- A steering angle sensor records the wheel’s position and how quickly it’s turning.
- The EPS control unit reads the vehicle speed and combines it with the driver input data to determine the required assist torque.
- The electric motor is commanded to provide the calculated assist torque, transmitting it through the steering gear to help turn the wheels.
- Position and current sensors monitor the motor and steering gear, forming a closed loop that keeps actual assist in line with the target.
- The system continuously adjusts the torque in real time as speed, steering angle, or road conditions change; if a fault is detected, warning indicators light up and assist may be reduced or disabled to maintain control.
In Subaru vehicles, EPS often works alongside EyeSight and vehicle dynamics controls to enhance stability and steering feel without hydraulic fluid.
Safety, diagnostics and maintenance
EPS systems include built-in diagnostics and safety features to prevent loss of steering assist in abnormal situations.
Safety features and diagnostic steps
- Self-diagnostic routines monitor sensors, motor currents, and wiring for faults; a warning light may illuminate if a problem is detected.
- In some faults, the system can switch to a degraded mode that preserves steering with reduced assist.
- A service visit is typically required to repair faulty sensors, wiring, or motor components; routine maintenance does not involve fluid changes as with hydraulic systems.
- When available, Subaru’s driver-assist features may coordinate with EPS for features like lane-centering and adaptive steering behavior.
Potential issues can often be traced to sensor calibration, wiring harness damage, or motor temperature limits, and the system may require professional inspection and recalibration.
Summary
Subaru’s electric power steering uses a motor-driven, sensor-fed control loop to provide variable steering effort. By reading driver input, vehicle speed, and steering geometry, the EPS delivers more assistance at low speeds for easier parking and tighter turns, while reducing assist at higher speeds for improved highway stability. The system integrates with Subaru’s safety and driver-assistance technologies and relies on reliable sensors and electronics rather than hydraulic fluid for steering force.
