I. Development of Automotive Steering Systems
1. Assisting in the classification of steering
Mechanical Steering
Hydraulic power steering
Electro-hydraulic power steering
Electric power steering
2. Hydraulic Power Steering System (HPS)
The hydraulic power steering system employs a hydraulic servo control system, primarily composed of a V-belt, pressure and flow control valve body, oil lines, power cylinder, power steering pump, steering column, steering drive shaft, and reservoir.
advantage:
The mechanical components provide precise control, direct road feel, and rich information feedback.
The hydraulic pump is engine-driven, providing ample steering power, mature technology, high reliability, and low average manufacturing cost.
shortcoming:
It feels heavy when making sharp turns at low speeds.
Relying on engine power to drive the oil pump results in relatively high energy consumption.
The piping structure of hydraulic systems is very complex, and the oil circuits are often kept under high pressure, which affects their service life.
3. Electro-hydraulic power steering system (EHPS)
The main components of an electro-hydraulic power steering system (EHPS) include an oil reservoir, a control unit, an electric pump, a steering mechanism, and a power steering sensor.
Electro-hydraulic power steering system (EHPS)
advantage:
It uses an electric hydraulic pump, which outputs high torque at low speeds and low torque at high speeds.
shortcoming:
It has a complex structure, is inconvenient for installation, maintenance and testing, and is expensive. It also cannot overcome the shortcomings of hydraulic systems, such as oil leakage and low-temperature performance.
4. Electric Power Steering (EPS) system
Electric power steering (EPS) is a power steering system that directly relies on an electric motor to provide auxiliary torque. It can control the electric motor to provide different power according to different operating conditions, so that the steering assistance changes with the vehicle speed and only provides steering power when steering is needed, reducing fuel consumption and making steering easier.
Electric power steering (EPS) system structure
advantage:
Simple structure
Reduce fuel consumption
Low noise
Good assist effect
Achieve active return to center of steering system
Good environmental protection
II. Structure and Working Principle of Electric Steering System
1. Classification of Electric Power Steering (EPS) Systems
Electric power steering (EPS) systems can be classified into three types based on the different motor drive components and mechanical structures: steering shaft-assisted, gear-assisted, and rack-assisted.
Types of Electric Power Steering (EPS)
2. Structure of Electric Power Steering (EPS) System
Electric power steering (EPS) systems rely directly on an electric motor to provide auxiliary steering power.
EPS mainly consists of torque sensor, angle sensor, vehicle speed sensor, electric motor, electromagnetic clutch, reduction mechanism, electronic control unit (ECU), etc.
Schematic diagram of Electric Power Steering (EPS) system
(1) Torque sensor
Torque sensors are used to detect the magnitude and direction of the torque signal acting on the steering wheel.
A contact-type torsion bar potentiometer sensor adds a torsion bar at the steering shaft position. The torsion bar detects the relative torsional displacement between the input shaft and the output shaft and inputs this torsional change to the ECU.
Contact torque sensor
(2) Electric motor
EPS typically uses a brushless permanent magnet DC motor, which has the advantages of no excitation loss, high efficiency, and small size.
(3) Electromagnetic clutch
An electromagnetic clutch ensures that the electric power assist only works within a predetermined range.
When the vehicle speed or current exceeds the maximum limit, or when the steering system malfunctions, the clutch automatically cuts off the electric motor power and resumes manual steering control.
Single-plate dry electromagnetic clutch
(4) Reduction mechanism
The speed reduction mechanism is used to increase the torque transmitted from the electric motor to the steering gear.
Worm Gear Reducer
Electric EPS uses a DC motor as the power source. The electronic control unit controls the current intensity of the motor according to the vehicle speed and steering parameters, adjusts the working torque of the boost motor, and thus controls the steering assist intensity.
The power assist of an electric EPS is controlled by a computer. It provides the strongest power assist when turning at low speeds and gradually weakens as the vehicle speed increases.
Electric power steering (EPS) system
III. Steering-by-wire structure and principle
1. Steer-by-wire (SBW) system
Automotive steer-by-wire (SBW) eliminates the mechanical connection between the steering wheel and the steering wheels, using electric power to achieve steering. It breaks free from the limitations of traditional steering systems, allowing for the free design of the force transmission characteristics and angular transmission characteristics of vehicle steering. Through control algorithms, it enables intelligent vehicle steering and saves more installation space and is lighter than traditional steering systems.
Components of a Steer-by-Wire (SBW) System
1) Structural components of the steer-by-wire system
(1) Steering wheel sensor: When the steering wheel rotates, it drives the large gear of the angle sensor to rotate. The large gear drives the two small gears with magnets to rotate, generating a changing magnetic field. The angle signal generated by this change is detected by the sensitive circuit and the data is sent out through the CAN bus.
(2) Road feel motor: converts the return signal from the main controller into a return torque to provide road feel to the driver.
Steering actuator assembly: Quickly responds to the angle signal from the main controller to complete the vehicle's steering.
(3) Main controller: Collects information from sensors including steering wheel angle, steering wheel torque, vehicle speed, etc., calculates the appropriate front wheel angle according to the internal program and sends it to the steering actuator motor to realize vehicle steering, calculates the appropriate return torque and transmits it to the road feel motor to provide road feel to the driver.
2) Differences between steer-by-wire system and traditional electric power steering
The main difference between EPS and steer-by-wire is that steer-by-wire eliminates the mechanical connection between the steering wheel and the wheels. It uses sensors to obtain the steering wheel angle data, which is then converted into specific driving force data by the ECU, and the motor drives the steering gear to turn the wheels.
EPS, on the other hand, increases steering force based on the driver's steering angle.
The drawback of steer-by-wire is that it requires simulating the force feedback of a steering wheel. Since the steering wheel is not connected to the mechanical parts, the driver cannot feel the resistance transmitted from the road surface and will lose road feel. However, this is not a concern in autonomous vehicles.
The Q50L retains a mechanical steering mechanism to ensure normal steering even if the electronic system fails completely.
IV. Typical Applications of Steer-by-Wire Systems
Nissan's Infiniti Q50 DAS system
The three ECUs are connected in parallel and are responsible for different things (from left to right: left front wheel, steering wheel, and right front wheel), and they also monitor the operation of the other two ECUs at the same time.
When any ECU is detected to have a problem, the backup mode will be immediately activated via a clutch to restore the traditional mechanical steering mode, ensuring that nothing goes wrong.
However, under normal circumstances, the steering wheel is kept disconnected from the steering rack and front wheels by the backup clutch.
The steering force motor provides appropriate steering force feedback to the steering wheel/driver.
The biggest difference between the Q50's Active Steering Bypass (DAS) and traditional steering systems is that, under normal conditions, there is no mechanical connection between the steering wheel and the wheels; instead, signals are transmitted via electrical circuitry.
While ensuring the same precision and speed as traditional steering systems, steer-by-wire systems can also filter out unnecessary road information and reduce driver fatigue.
Handling: Thanks to the use of electronic signal control, which eliminates the lag in the transmission of steering force, DAS is significantly more sensitive and precise than traditional steering, allowing ordinary drivers to have unprecedented control over the vehicle's steering status in both spirited and everyday driving, and enjoy more driving pleasure.
