New energy electric vehicle steering system and its working principle

New energy electric vehicle steering system and its working principle

As a key system of electric vehicles, the steering system of electric vehicles is quite different from that of fuel vehicles. The steering systems of electric vehicles are mainly divided into three categories: Electric Power Steering (EPS), Electro-Hydraulic Power Steering (EHPS) and Steer-By-Wire (SBW). Different types of steering systems have different working principles, advantages and disadvantages and applicable scopes. This article introduces the composition and working principles of these three electric vehicle steering systems.

1. Electric Power Steering System (EPS)

EPS is the most commonly used steering system in electric vehicles. It provides power through an electric motor, replacing the traditional hydraulic power steering (HPS)

1. EPS composition

EPS is mainly composed of the following components:

(1) Torque sensor: detects the torque and direction of the steering wheel rotation (driver's intention).

(2) Steering angle sensor: monitors the steering wheel angle (some systems are integrated in the torque sensor).

(3) Vehicle speed sensor: provides vehicle speed signal (used to dynamically adjust the power assist).

(4) Electronic control unit (ECU): processes sensor data in real time and calculates the required power assist.

(5) Power assist motor: usually a brushless DC motor (BLDC), which transmits torque to the steering column or rack through a reduction mechanism (such as a worm gear).

(6) Reduction mechanism: amplifies the motor torque and drives the steering system.

2. EPS working principle

(1) Detecting driver's intention

When the driver turns the steering wheel, the torque sensor measures the torsional torque of the steering shaft, the steering angle sensor records the angle, and sends a signal to the ECU.

The vehicle speed signal is input synchronously (for example: more power assist is required at low speeds, and less power assist is required at high speeds to enhance stability).

(2) ECU calculates the power assistance requirement

The ECU calculates the target power assistance based on torque, vehicle speed, and even vehicle body status (such as tilt angle, for some high-end models), and outputs a PWM signal to control the motor.

Algorithm example:

Low-speed parking: the power assistance motor outputs high torque (light steering).

High-speed driving: power assistance is reduced (improves road feel and avoids excessive sensitivity).

(3) The motor performs power assistance

The motor transmits power to the steering column or directly drives the rack through a reduction mechanism (such as a worm gear, belt, etc.) (different EPS types have different structures, see below).

The direction of the motor power assistance is consistent with the driver's steering direction (determined by the polarity of the torque sensor).

(4) Feedback and correction

The system continuously monitors the steering wheel torque and actual steering angle, dynamically adjusts the motor output, and implements closed-loop control to avoid excessive power assistance or lag.

3. Classification and application scope of EPS

According to the different installation positions of the motor, EPS can be divided into the following types:

Type: C-EPS (steering column type)

Motor position: installed on the steering column

Applicable models: small cars, micro cars

Features: simple structure, low cost, but small power

Type: P-EPS (pinion type)

Motor position: installed on the steering pinion

Applicable models: compact/mid-sized cars

Features: moderate power, good balance

Type: R-EPS (rack type)

Motor position: directly drive the rack

Applicable models: medium and large cars, SUVs

Features: large power, fast response, suitable for heavy vehicles

Type: DP-EPS (double pinion type)

Motor position: two motors drive the pinion and rack respectively

Applicable models: high-performance cars, luxury cars

Features: more precise steering, better dynamic response

4. Advantages and Disadvantages of EPS

Advantages

(1) High energy efficiency and endurance-friendly: EPS is directly driven by an electric motor, does not require a hydraulic pump, and has extremely low energy loss (traditional HPS will continue to consume engine power). For electric vehicles, the energy saved can indirectly increase the range (about 3%-5% energy efficiency optimization).

(2) Flexible and adjustable steering assistance: The size of the assistance can be dynamically adjusted through software to adapt to different scenarios (such as lightness at low speed and stability at high speed).

(3) Simple structure and low maintenance cost: It eliminates hydraulic oil, pumps, pipelines and other components, reduces the risk of oil leakage, and reduces the need for later maintenance.

(4) Strong environmental adaptability: It is not affected by extreme temperatures (the hydraulic system oil becomes viscous at low temperatures, resulting in steering hysteresis).

(5) Support for Advanced Driver Assistance Systems (ADAS): It supports advanced driver assistance functions such as lane keeping and automatic parking.

Disadvantages

(1) Weak road feel feedback: The simulated road feel of the electric power steering is not as natural as that of the hydraulic system.

(2) Dependence on the reliability of the electronic control system: If the motor, sensor or control software fails, the power assist may suddenly fail (although there is a redundant design, the risk still exists).

(3) Insufficient power assist in high-load scenarios: High-power motors are expensive, and some cheap models may not provide sufficient power assist in extreme working conditions (such as turning in place).

2. Electro-Hydraulic Power Steering System (EHPS)

Electric trucks have a large load and the steering system requires stronger power, so some models will use an electro-hydraulic power steering system (EHPS), which introduces electronic control technology on the basis of traditional hydraulic power steering (HPS) to improve energy efficiency and controllability.

1. Composition 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 runs independently of the engine and is suitable for electric vehicles.

Usually a brushless DC motor (BLDC) or a permanent magnet synchronous motor (PMSM) is used to improve energy efficiency.

(2) Hydraulic power mechanism (gear rack or recirculating ball steering gear)

Similar to HPS, but the oil pressure is precisely regulated by the electronic control system.

(3) Electronic control unit (ECU)

Regulates the speed and pressure of the hydraulic pump according to signals such as vehicle speed and steering torque.

(4) Oil storage tank, hydraulic pipeline, steering valve

Same as traditional HPS, 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 size and direction). The vehicle speed sensor provides information about the current vehicle speed (large assist at low speed and small assist at high speed).

(2) ECU calculates the 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 speed (such as parking): the motor runs at high speed, providing a large flow of hydraulic oil, making steering easier.

At high speed: the motor slows down, reducing assist, enhancing the "stable feeling" of the steering wheel, and improving driving stability.

(3) Electric hydraulic pump provides pressure

The motor drives the hydraulic pump to pressurize the hydraulic oil and deliver it to the steering control valve.

The control valve directs the hydraulic oil into the corresponding chamber of the hydraulic cylinder according to the direction of the steering wheel torque, pushing the rack or steering linkage to achieve assist.

At low speed/heavy load: increase the oil pressure, increase the assist, and make steering easier.

At high speed/no load: reduce the oil pressure, reduce the assist, and improve driving stability.

(4) Hydraulic oil circulation

After the power assist is completed, the hydraulic oil flows back to the reservoir to form a closed loop.

3. Advantages and disadvantages of EHPS

Advantages

(1) High torque assist: suitable for heavy-duty electric trucks and commercial vehicles, and can provide stronger steering force than EPS.

(2) High reliability: the hydraulic system is mature and performs stably under extreme working 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.

Disadvantages

(1) High energy consumption: the electric hydraulic pump works continuously, consuming more power than EPS (but more energy-efficient than traditional HPS).

(2) Complex structure: requires hydraulic pipelines, oil storage tanks, etc., and maintenance is slightly more troublesome than EPS.

(3) Slightly slow response: compared with EPS, the dynamic adjustment speed of the hydraulic system is slightly lower.

3. Steer-By-Wire System (SBW)

SBW (Steer-By-Wire) is the future development direction of steering systems. It completely eliminates the mechanical connection between the steering wheel and the wheels and relies entirely on electrical signals to control steering.

1. Composition of SBW

The SBW system mainly consists of the following components:

(1) Steering wheel module

Responsible for detecting the driver's steering intention and simulating road feel feedback, including: steering wheel torque/angle sensor, road feel feedback motor and steering wheel electronic control unit

(2) Steering execution module

Responsible for driving the wheel steering, replacing the traditional steering column and gear rack mechanism, including: steering execution motor, steering angle sensor and reduction mechanism (such as ball screw or gear set)

(3) Electronic control unit (ECU)

The "brain" of SBW, responsible for signal processing and system coordination, including: main ECU and redundant ECU

(4) Redundant safety system

To ensure safety, SBW must be equipped with multiple backups, including: dual power supply, dual communication channels and mechanical emergency backup

2. Working principle of SBW

(1) Signal acquisition (steering wheel module)

Torque/angle sensor: detects the force and angle of the driver's steering wheel rotation and converts it into an electrical signal

Vehicle speed, yaw rate and other signals: The vehicle status (such as ESP, ABS data) is obtained through the CAN bus to calculate the optimal steering response.

(2) Electronic control (ECU decision)

Main ECU: Calculates the target steering angle (dynamically adjusts the steering ratio in combination with vehicle speed, driving mode, etc.) and road feel feedback intensity (simulates the interaction force between tires and the road surface) based on sensor signals.

Redundant ECU: Monitors the main system in real time and immediately takes over or activates the emergency mode once a fault is detected (such as signal loss, motor abnormality).

(3) Steering execution (wheel drive)

The steering execution motor (usually a high-torque brushless motor) directly drives the rack or steering knuckle to push the wheel to turn.

The position sensor provides real-time feedback on the actual steering angle of the wheel to form a closed-loop control to ensure accurate execution of ECU instructions.

(4) Road feel simulation (steering wheel feedback)

The road feel feedback motor applies programmable resistance on the steering wheel to simulate the mechanical road feel of the traditional steering system (such as bumps, tire grip changes).

3. Advantages and Disadvantages of SBW

Advantages

(1) No mechanical connection: Improve the flexibility of chassis layout and increase the space inside the vehicle.

(2) Variable steering ratio: The steering angle and wheel steering angle can be freely adjusted (such as the steering wheel is more sensitive at low speed and more stable at high speed).

(3) Perfect adaptation to autonomous driving: Steering can be completely controlled by computer without driver intervention.

(4) Safer collision protection: No steering column, no intrusion into the cockpit in the event of a collision.

Disadvantages

(1) High cost: High-reliability electronic components and redundant systems are required.

(2) Regulatory restrictions: Currently, some countries require some mechanical backup.

(3) Consumer acceptance: Some users have doubts about the "no mechanical connection" steering method.

IV. Conclusion

This article introduces three different forms of steering systems for electric vehicles and their working principles. Due to the large steering load of electric trucks, EHPS (Electric Hydraulic Power Steering) is still the mainstream solution. It has achieved a good balance between power steering strength, reliability and cost. However, with the development of high-power EPS technology, some electric trucks may switch to EPS or hybrid steering systems in the future. At present, EPS is still the most cost-effective solution and is widely used. In the future, the steering system of electric vehicles will evolve from EPS (electric power steering) to SBW (steer by wire), and SBW will become mainstream with higher flexibility and compatibility with autonomous driving.