The traditional pure mechanical steering system is almost replaced. After upgrading from mechanical hydraulic power steering system (HPS) to electro-hydraulic power steering system (EHPS), electric power steering system (EPS) driven by electricity gradually occupied the mainstream. With the deepening of automobile electronization, the popularization rate of electronic steering system is accelerated, and electric power steering gradually occupies the mainstream, and with the arrival of the future automatic driving era, it will enter the steering by wire.
1, mechanical hydraulic power steering system (HPS)
Mechanical hydraulic power steering combines the force of human body turning the steering wheel with the mechanical energy of the engine, and combines the amplification of hydraulic pressure to push the steering rod to complete the steering action. Mechanical hydraulic power-assisted system mainly includes rack-and-pinion steering structure and hydraulic system (hydraulic power-assisted pump, hydraulic cylinder, piston, etc.). ). The working principle is that the hydraulic pump (driven by the engine belt) provides oil pressure to push the piston, and then generates auxiliary force to push the steering rod to assist the wheel steering.
First of all, the mechanical valve body located on the steering wheel (which can rotate with the steering column) remains in its original position when the steering wheel does not rotate, and the oil pressure on both sides of the piston is the same, which is in a balanced state. When the steering wheel rotates, the steering control valve will be opened or closed accordingly, and the oil on one side will directly flow back to the oil storage tank without passing through the hydraulic cylinder, and the oil on the other side will continue to be injected into the hydraulic cylinder, so that the pressure difference between the two sides of the piston will be pushed, and then the auxiliary force will be generated to push the steering rod, making steering easier.
The mechanical hydraulic power-assisted technology is mature and stable, the mechanical structure is completely independent of electronic equipment, the reliability is high, the road feeling is clear, and it is convenient for the driver to judge the steering angle, so it is widely used. However, its disadvantages are also obvious, such as complex structure, large space occupation and high cost of design, manufacture and maintenance. Moreover, the strength of the mechanical hydraulic power-assisted system is not adjustable, so it is difficult to take into account the different requirements of pointing accuracy at low speed and high speed, and it cannot meet the requirements of automatic driving.
2. Electro-hydraulic power steering system (EHPS)
The difference between electro-hydraulic booster and mechanical hydraulic booster lies in the different driving modes of oil pump. The hydraulic pump assisted by mechanical hydraulic power is directly driven by the engine belt, while the electronic hydraulic power is used to detect the steering angle of the steering wheel by ECU, and the electronic pump is driven by electricity to exert force on the hydraulic cylinder, so that the steering wheel can be designed to be "portable" and convenient for drivers to use.
The electro-hydraulic assisted electronic pump is not driven by the power of the engine itself, but is controlled by the electronic system. When steering is not needed, the electronic pump is turned off, further reducing energy consumption. The electronic control unit of the electro-hydraulic power steering system can change the flow rate of the electronic pump by processing the information of sensors such as speed sensor and steering angle sensor, thus changing the power of steering assistance.
3. Electric Power Steering System (EPS)
Electric power steering system (EPS) is mainly composed of steering wheel sensor, control unit and power-assisted motor. Because the structure of the hydraulic pump, hydraulic pipeline and steering column valve body of the hydraulic power-assisted system can be avoided, its design and structure are simple.
The working principle of EPS is that when the steering wheel rotates, the steering wheel sensor transmits the rotation signal to the control unit, and the control unit provides an appropriate voltage to the motor through calculation, drives the motor to output torque, and then drives the steering rod to provide steering assistance after decelerating and increasing the torque through the reducer.
The main advantage of EPS is that it does not contain any mechanical structure, and its design and structure are simple. Power assist has nothing to do with engine speed, which can make the steering wheel lighter at low speed and more stable at high speed. The disadvantage is that mechanical equipment needs to be kept for a long time to ensure redundancy, otherwise it will easily lead to adverse consequences once electronic equipment fails.
In addition, there are four types of EPS according to the position of the auxiliary motor. They are column assist (C-EPS), gear assist (P-EPS) and rack assist (R-EPS).
The assist motor of steering column assist C-EPS is installed on the steering column, and the mechanical steering machine is connected below the steering column, and the motor assist torque acts on the steering column. C- EPS system has compact structure, integrated design of motor, deceleration mechanism, sensor and controller, good working environment, no occupation of engine compartment space, convenient layout of engine compartment and low cost. The disadvantage is that the power of the driving motor is transmitted to the steering machine through the steering column and steering gear, and the steering column components are stressed, so the power that can be provided is limited; In addition, because the motor and deceleration mechanism are arranged in the cockpit, it is easier to cause noise in the cockpit; Because the reduction mechanism is installed on the steering wheel, it is not conducive to the energy-absorbing structure design of the steering shaft. Therefore, C-EPS is suitable for small and medium-sized buses.
The gear-assisted P-EPS motor and reduction mechanism are arranged on the steering gear, and the output torque of the driving motor is transmitted to the steering gear through the worm gear reduction mechanism. P-EPS torque directly acts on the steering gear, so it can provide more steering power, and the power-assisted effect is faster and more accurate. The booster motor and deceleration mechanism are arranged in the engine compartment, which is beneficial to reduce the noise level in the cockpit. The disadvantage of P-EPS is that its motor and sensor are installed in the engine compartment, which requires high heat resistance, waterproof and other environments and has high cost. Therefore, P-EPS is suitable for medium-sized buses with large demand.
Rack-type R-EPS, the booster motor and reduction mechanism are arranged on the steering rack, and the booster torque of the motor acts on the steering rack. R-EPS torque directly acts on the steering rack, so it can provide more steering power, and the power-assisted effect is the fastest and most accurate. The booster motor and deceleration mechanism are arranged in the engine compartment, which is beneficial to reduce the noise level in the cockpit. The disadvantage of R-EPS is that its motor and sensor are installed in the engine compartment, which has high requirements for heat resistance, waterproof and other environments and high cost. Therefore, R-EPS is suitable for large and medium-sized buses with large demand.
4. Steering by Wire (SBW)
Steering-by-wire refers to canceling the mechanical connection between steering wheel and steering wheel, and replacing it with road sense feedback assembly, steering actuator assembly, controller and related sensors. The steering wheel angle data is obtained only by sensors, and then the ECU converts it into specific driving force data, and the motor drives the steering wheel to rotate the wheels.
It can be said that steer-by-wire completely gets rid of the limitations of traditional steering. It can design not only the force transmission characteristics of automobile steering, but also the angle transmission characteristics of automobile steering, which brings more space for the design of automobile steering characteristics and is more convenient to integrate with other subsystems of automatic driving (such as perception, power, chassis, etc.). It has advantages in improving active safety performance, driving characteristics, maneuverability and driver's sense of road, and is the necessary key to realize path tracking and obstacle avoidance in automatic driving.
20 14 finidi Q50 is the first production car equipped with steer-by-wire steering. To ensure redundancy, it has three ecus and a mechanical steering system. If there is a problem with the electronic control device, the driver can resume mechanical control.