With the deepening of the degree of electronicization of automobiles and the acceleration of the electronic penetration rate of steering systems, electric power steering has gradually occupied the mainstream, and the arrival of the future autonomous driving era has further entered steer-by-wire steering.
Under the trend of electronicization of modern automotive systems and modules, the penetration rate of electronic control execution systems has continued to increase; with the development of electric vehicles, the disappearance of traditional engines has caused fundamental changes in the power sources and execution methods of transmission, steering, and braking. The transformation of electric control execution system has become a basic configuration. In the era of autonomous driving, the control system collects information from a large number of sensors in the perception layer, processes and analyzes it, senses the surrounding environment, plans driving routes, and finally executes the system through the wire control Maneuver the vehicle.
The traditional purely mechanical steering system was almost replaced. After upgrading from mechanical hydraulic power steering (HPS) to electro-hydraulic power steering (EHPS), electric power-assisted electric power steering (EPS) gradually occupied the mainstream. With the deepening of the degree of electronicization of automobiles and the acceleration of the electronic penetration rate of steering systems, electric power steering has gradually occupied the mainstream, and the arrival of the future autonomous driving era has further entered steer-by-wire steering.
1. Mechanical hydraulic power steering system (HPS)
Mechanical hydraulic power-assisted steering uses the force of the human body to turn the steering wheel and the mechanical energy of the engine, and combines the hydraulic force to amplify the force to push the steering rod to complete the steering action. The mechanical hydraulic power assistance system mainly includes two parts: a rack and pinion steering structure and a hydraulic system (hydraulic power booster pump, hydraulic cylinder, piston, etc.). The working principle is to provide oil pressure to push the piston through the hydraulic pump (driven by the engine belt), and then generate auxiliary force to push the steering rod to assist the steering of the wheel.
Car steering system, electric vehicle, steering wheel
Firstly, the mechanical valve body (which can be rotated with the steering column) on the steering gear. When the steering wheel is not rotated, the valve body remains in place, and the oil pressure on both sides of the piston is the same, in a balanced state. When the steering wheel is turned, the steering control valve will open or close accordingly. The oil on one side will directly return to the oil storage tank without passing through the hydraulic cylinder. The oil on the other side will continue to be injected into the hydraulic cylinder. The pressure difference is pushed, which in turn generates auxiliary force to push the steering rod, making steering easier.
Mechanical hydraulic assist technology is mature and stable. The complete mechanical structure does not rely on electronic equipment, high reliability, clear road feel, and convenient for the driver to determine the steering angle. Therefore, it is widely used. But its shortcomings are also obvious, the structure is more complicated, it takes up a lot of space, and the design, manufacturing and maintenance costs are higher. Moreover, the power of the pure mechanical hydraulic power assistance system cannot be adjusted, and it is difficult to take into account the different requirements for pointing accuracy at low and high speeds, and it cannot meet the needs of autonomous driving.
2. Electro-hydraulic power-assisted steering system (EHPS)
The difference between electronic hydraulic boost and mechanical hydraulic boost is mainly the driving method of the oil pump. The mechanical hydraulic boost hydraulic pump is directly driven by the engine belt, while the electronic hydraulic boost uses the ECU to detect the steering angle of the steering wheel and is powered by electric power. Driving the electronic pump to exert pressure on the hydraulic cylinder can design the steering wheel to be “light” for the driver’s convenience.
The electronic pump powered by electro-hydraulic does not need to be driven by the power of the engine itself, and the electronic pump is controlled by the electronic system. When the steering is not required, the electronic pump is turned off to further reduce energy consumption. The electronic control unit of the electronic hydraulic power steering system uses information processing of sensors such as the vehicle speed sensor and the steering angle sensor to change the strength of the steering assist by changing the flow of the electronic pump.
3. Electric power steering (EPS)
The electric power steering system (EPS) is mainly composed of a steering wheel sensor, a control unit, and a power assist motor. Since the hydraulic pump, hydraulic pipeline, and steering column valve body of the hydraulic power assistance system can be avoided, the design and construction are simple.
Car steering system, electric vehicle, steering wheel
The working principle of EPS is that when the steering wheel rotates, the steering wheel sensor transmits the rotation signal to the control unit. The control unit calculates the appropriate voltage to the motor, drives the torque output by the motor, and then reduces the speed of the reducer to increase the torque and then pushes the steering rod. Power steering.
The main advantages of EPS are that it does not contain any mechanical structure, is simple in design and construction, and has nothing to do with engine speed. It can make the steering wheel lighter at low speeds and more stable at high speeds. The disadvantage is that the mechanical devices need to be retained for a long time to ensure redundancy, otherwise, in case of failure of the electronic equipment, it will easily cause adverse consequences.
In addition, there are four types of EPS depending on the position of the auxiliary motor. They are column-assisted (C-EPS), gear-assisted (P-EPS) and rack-assisted (R-EPS).
The column-assisted C-EPS booster motor is installed on the steering column. A mechanical steering gear is connected below the steering column. The motor assisted torque acts on the steering column. The advantages of the C-EPS system are: compact structure, its motor, reduction mechanism, sensors and controllers are usually integrated design, arranged in the cockpit, the working environment is better, does not take up space in the engine compartment, convenient engine compartment layout, cost Lower. The disadvantage is that the power of the drive motor is transmitted to the steering gear through the steering column and steering gear. The steering column components are subject to large forces, and the amount of available power is limited. In addition, the motor and the reduction mechanism are arranged in the cockpit, which is more likely to cause driving. Noise is generated in the cabin; because the reduction mechanism is installed on the steering wheel, it is not conducive to the design of the energy absorbing structure of the steering shaft. Therefore, C-EPS is suitable for small and medium-sized passenger cars.
The gear-assisted P-EPS booster motor and reduction mechanism are arranged on the steering gear, and the output torque of the drive motor is transmitted to the steering gear through the worm gear reduction mechanism. P-EPS assisting torque directly acts on the steering gear, so it can provide greater steering assistance, and the assisting effect is more rapid and accurate. The power-assisted motor and reduction mechanism are arranged in the engine compartment, which helps reduce the cockpit noise level. The disadvantages of P-EPS are: its motors and sensors are installed in the engine compartment, and the components have high environmental and heat resistance requirements, such as high cost. Therefore, P-EPS is suitable for medium-sized passenger cars that require greater assistance.
Rack-type R-EPS, power-assisted motor and reduction mechanism are arranged on the steering rack, and motor assist torque acts on the steering rack. R-EPS assist torque acts directly on the steering rack, so it can provide greater steering assistance, and the assistance effect is the most rapid and accurate. The power-assisted motor and reduction mechanism are arranged in the engine compartment, which helps reduce the cockpit noise level. The disadvantages of R-EPS are: its motors and sensors are installed in the engine compartment, and the heat and water resistance of the device are high, and the cost is high. Therefore, R-EPS is suitable for large and medium-sized passenger cars that require greater assistance.
4. Steer-by-wire (SBW)
The steer-by-wire is to cancel the mechanical connection between the steering wheel and the steering wheel and replace it with a road sense feedback assembly, a steering execution assembly, a controller and related sensors. Simply use the sensor to obtain the steering wheel rotation angle data, then the ECU converts it into specific driving force data, and uses the motor to push the steering gear to rotate the wheels.
It can be said that steer-by-wire steering completely breaks away from the various limitations of traditional steering. Not only can it design the force transmission characteristics of the car’s steering, but also the angular transmission characteristics of the car’s steering. It is more convenient to integrate with other autonomous driving subsystems (such as perception, power, chassis, etc.). It has advantages in improving the active safety performance, driving characteristics, maneuverability, and driver’s road feel of the vehicle. It is the path tracking and avoidance of autonomous vehicles. Key technologies necessary for obstacle avoidance.
The 2014 Infiniti Q50 is the first production car with steer-by-wire. To ensure redundancy, it has three ECUs and a mechanical steering system. If a problem occurs with the electronic control, the driver can resume mechanical control.