Electro-hydraulic power steering system (EHPS)
Because electric trucks have large loads and require stronger steering assistance, some models will adopt an electro-hydraulic power steering system (EHPS). This system introduces electronic control technology into the traditional hydraulic power steering (HPS) to improve energy efficiency and controllability.
1. Components of EHPS
The EHPS system mainly consists of the following components:
(1) Electric hydraulic pump (replacing the traditional engine-driven hydraulic pump)
Driven by an electric motor, it operates independently of the engine and is suitable for electric vehicles.
It typically uses a brushless DC motor (BLDC) or a permanent magnet synchronous motor (PMSM) to improve energy efficiency.
(2) Hydraulic power steering mechanism (rack and pinion or recirculating ball steering system)
Similar to HPS, but the oil pressure is precisely regulated by the electronic control system.
(3) Electronic control unit (ECU)
It regulates the speed and pressure of the hydraulic pump based on signals such as vehicle speed and steering torque.
(4) Oil reservoir, hydraulic lines, and steering valve
Similar to traditional HPS, it is responsible for the storage and flow control of hydraulic oil.
2. Working Principle of EHPS
(1) Signal Acquisition
When the driver turns the steering wheel, the steering wheel torque sensor detects the driver's steering intention (steering force magnitude and direction). The vehicle speed sensor provides current vehicle speed information (higher assist at low speeds, lower assist at high speeds).
(2) ECU Calculates Required Assist
The ECU calculates the appropriate hydraulic pressure requirement based on data such as torque, vehicle speed, and steering angle, and controls the speed of the electric hydraulic pump.
At low speeds (e.g., parking): The motor operates at high speed, providing a large flow of hydraulic oil, making steering easier.
At high speeds: The motor slows down, reducing assist, enhancing the "stable feel" of the steering wheel, and improving driving stability.
(3) Electric Hydraulic Pump Provides Pressure
The motor drives the hydraulic pump, pressurizing the hydraulic oil and delivering it to the steering control valve.
The control valve, according to the direction of the steering wheel torque, guides the hydraulic oil into the corresponding chamber of the hydraulic cylinder, pushing the rack or steering linkage to achieve assist.
At low speeds/heavy loads: Increased oil pressure enhances assist, making steering easier. High speed/no load: Reduce oil pressure, decrease power assist, and improve driving stability.
(4) Hydraulic oil circulation: After power assist is completed, the hydraulic oil flows back to the reservoir, forming a closed-loop circulation.
3. Advantages of EHPS
(1) High torque assist: Suitable for heavy-duty electric trucks and commercial vehicles, providing a stronger steering force than EPS.
(2) High reliability: The hydraulic system is mature and performs stably under extreme conditions (such as low temperature and high load).
(3) Moderate cost: More economical than EPS (high-power motor + reduction mechanism) and more energy-efficient than traditional HPS.
