The earliest electric power steering system appeared in the mid-1970s. At that time, the purpose of using this system was to prevent the sudden loss of control of the car when the engine suddenly stopped working when the car was running and the hydraulic steering assist was lost. At this time, the battery is used for power supply The electric power steering system was put into work. Modern electric power steering systems have been further developed. For pure electric cars, the use of electric power steering (EPS) is an inevitable choice, because it does not have an internal combustion engine, and the source of power for the power steering system can only come from electric motors, so the choice of pure electric car power steering system can only be EPS or Electronically controlled hydraulic power steering system one (EHPS), most tend to choose EPS.
1. Features of electric power steering system
The steering system of modern cars has evolved from the original mechanical hydraulic power steering system (HPS, see Figure 1) to EPS technology. The status quo of modern automotive EPS technology is that with the widespread use of microelectronic control technology in the automotive field and the promotion of the world’s two major themes of energy conservation and environmental protection, the superiority of EPS is becoming more and more prominent as the future development direction of automotive steering systems, which will surely replace Existing mechanical steering system, hydraulic power steering system and electronically controlled hydraulic power steering system.
This is mainly because electric power steering has the following advantages compared with hydraulic power steering.
①When not turning, it does not consume power, which can reduce fuel consumption by 3%~5% compared with hydraulic steering system.
②Improve the handling performance of the car. The magnitude of the assist can be changed with the change of the car speed through the software in the control unit.
③Compact structure and light weight.
④The noise is small during work.
③Compared with hydraulic steering system, the structure is simpler, without oil pump, hydraulic oil, tubing, oil can, etc.
⑥ It meets environmental protection requirements. When the car is scrapped, there is no need to deal with hydraulic oil, tubing, etc., and there is no hydraulic oil leakage.
⑦Simplified installation (especially for rear-mounted and mid-engine cars), which can save space during assembly.
2. Classification of electric power steering systems
Electric power steering systems can be divided into four categories: electronically controlled hydraulic power steering systems (EHPS), electric power steering systems (EPAS or EPS), active front-wheel electric power steering systems (AFS), and wire-controlled electric power steering systems (SBW).
The hydraulic oil pump is driven by a high-performance DC brushless motor controlled by an intelligent electronic control unit (ECU), which can provide pressure oil to the hydraulic steering booster according to the steering needs. The oil pump module can be specially developed for the system, so as to integrate the motor, oil pump, electronic control unit, fuel tank, etc. into a whole, which can facilitate the arrangement on the car.
At present, pure electric passenger cars mostly use electric motors to drive steering pumps to assist the recirculating ball power steering system. Generally, EHPS control units are not installed to control the motor speed and control the magnitude of power assistance, so they are always in a state of energy consumption.
EPS is an all-electric power steering assist system that has nothing to do with the engine. This system eliminates traditional parts such as hydraulic oil pumps, hoses, hydraulic oil, belts and pulleys. Compared with hydraulic steering system, energy consumption can be reduced by about 4%. The comparison of HPS, EHPS, EPS three steering systems is shown in Figure 2.
Compared with various steering assist systems, AFS not only reduces the steering wheel control force, but also is an electronic, variable rotation ratio system that can work with dynamic stability control (DSC) when necessary, and its rotation ratio It is directly related to car speed, driving mode, and road conditions. Under normal road conditions, when driving at low and medium speeds, the ratio of the input angle of the steering wheel to the steering angle of the front wheels is small, for example (1:10), and the steering feel becomes more direct. The driver only needs to turn the steering wheel at a small angle to turn the front wheel to a large angle without any effort, so as to improve the maneuverability of the car on the busy urban road, especially to provide the agility of driving the car when parking and turning. When driving at high speed, the turning ratio becomes larger (for example, 1:20) and the steering becomes “sluggish” to increase the stability of operation.
In order to maintain the familiar driving style and driving feeling, the system does not use a fully electronic control-by-wire system. The steering wheel and the front wheels are still mechanically connected, only between the steering column and the rack-and-pinion steering mechanism. It is installed with a planetary gear mechanism driven by a steering wheel and a motor (the steering wheel is connected to the sun gear, and the motor is connected to the ring gear). There are also two sensors in AFS, one is the steering wheel angle sensor, used to measure the speed and angle of turning the steering wheel, and the other is the slip sensor used to measure the sideslip rate of the actual car. These two signals are calculated by the computer, By comparison, we can know whether the car is over-steered or under-steered to determine the amount of correction to the steering angle of the wheels, and then adjust the speed of the motor connected to the ring gear of the planetary gear mechanism to change the speed ratio between the input shaft and the output shaft. AFS can also work closely with DSC. When the car is driving on a smooth road or turning too fast, and there is a serious understeer, the computer will make the inner rear wheel brake appropriately to correct the steering characteristics (some systems, such as Bosch’s The system will also reduce the engine output for coordination); if it is severely oversteered, the external front wheels will be appropriately braked, thereby improving the safety and stability of the car when turning.
SBW represents the development direction of the next-generation car steering system. This is because compared with the traditional steering system, it removes the direct mechanical connection between the steering wheel and the wheel, and can realize intelligent car steering through the control algorithm, making the car operation safe It is more guaranteed, saves installation space and is lighter than traditional steering systems, making the design and layout of the whole car extremely flexible.
3. Types of electric power steering system
Electric power steering system (EPAS or EPS) can be divided into steering column type, rack type, pinion type, direct drive type, and recirculating ball type according to the different installation positions of the electric power steering unit in the electric power steering system.
(1) Steering column type EPS
As shown in Figure 3, this type of EPS, its power assist unit, controller, torque sensor, etc. are all installed on the steering column, the system is compact, whether it is a fixed steering column or a tilted steering column, and other All types of steering columns can be installed. This structure is suitable for medium-sized cars.
(2) Rack type EPS
As shown in Figure 4, for this type of EPS, the power assist unit is installed on the rack of the steering mechanism, and the power assist unit can be installed at any position of the rack, which increases the flexibility of structural design and arrangement. The large reduction ratio of the power assist unit makes its inertia very small, while turning the steering wheel feels very good. The typical “double-own-wheel” electronically controlled mechanical power steering system is a rack-and-pinion EPS, which consists of two gears (steering gear and driving gear) that can provide sufficient steering power to the steering rod.
(3) Pinion type EPS
As shown in Figure 5, for this type of EPS, the power assist unit is installed on the pinion shaft of the steering mechanism. Because the power assist unit is outside the car, even if the assist torque is greatly increased, it will not increase the inside of the car. Noise. If it is combined with a variable-speed steering gear, the handling characteristics of the system will be very good.
(4) Direct drive EPS
The EPS steering rack and the power assist unit form a part, making the system very compact and easy to arrange in the engine compartment. Because the rack is directly assisted, the friction and inertia are very small, and then the steering wheel feels like turning Very ideal.
(5) Recirculating ball type EPS
The electric power steering system consists of a rack-and-pinion steering gear and a rack-and-pinion motor mounted concentrically. The motor drives the rack to move left and right through a recirculating ball drive mechanism. The steering sensor is installed in the input shaft gear housing.
Electric recirculating ball EPS is more suitable for passenger car steering system. The EPS deceleration mechanism is connected with the motor, which plays the role of reducing speed and increasing torque, often using worm gear mechanism, and some use planetary gear mechanism. The EPS adopts an electromagnetic clutch mounted on the side of the deceleration mechanism to ensure that the EPS only works within the preset driving speed range; when the car speed reaches a certain value, the clutch release motor stops working, and the steering system is converted to manual steering. , When the motor fails, the clutch will also be automatically disconnected.