During the operation of a car, it often needs to change its direction. The steering system, as one of the main assemblies of the entire vehicle, is a key system that affects the handling stability, comfort, and driving safety of the car. In the design of the steering system, in order to ease the conflict between the convenience and sensitivity of car steering, most commercial vehicles and 50% of passenger cars adopt the power steering system [1]. Since the 1950s, the power steering system has gone through three stages of development: conventional hydraulic power steering (HPS), electro-hydraulic power steering (EHPS), and electric power assisted steering (EPAS or EPS). It continues to develop towards electrification and intelligence. Due to its advanced technology and superior performance, EPAS will replace other power steering technologies in the future and become the mainstream of power steering technology.
1. Working Principle of EPS
The EPAS mainly consists of a torque sensor, a vehicle speed sensor, an electronic control unit (ECU), a motor, a reducer, and a clutch, as shown in the figure below. Its basic steering principle is: when the driver turns the steering wheel, the torque sensor connected to the steering column transmits the relative angular displacement of the torsion bar under the action of input torque into electrical signals and sends them to the ECU. The ECU controls the rotation direction of the motor and the size of the assist current according to the signals from the vehicle speed sensor and the torque sensor, realizing steering assistance. Therefore, it can easily provide different assist effects to the motor at different vehicle speeds, ensuring that the car is agile and flexible when steering at low speeds and has sufficient road feel when steering at high speeds, and is stable and reliable.
l. Steering Wheel 2. Torque Sensor 3. Reducer 4. Rack-and-pinion Steering Gear 5. Clutch
6. DC Motor 7. Vehicle Speed Signal 8. Control Current 9. Torque Signal 10. Wheel
2. Classification of EPS
EPS can be divided into four types according to the arrangement method of the assist motor: Column-assist type EPS, Pinion-assist type EPS, Rack-assist type EPS, Direct-drive type EPS.
(1) Column-assist type EPS (C-EPS)
In the column assist type steering system, the torque sensor, motor, clutch, and steering assist mechanism are integrated together and installed on the steering column. Its characteristics are compact structure and good responsiveness of the motor assist. However, due to the installation of the assist motor inside the cockpit, it is limited by space and noise, so the motor volume is small and the output torque is not large. Generally, it is only used on small and compact vehicles.
(2) Pinion-assist type EPS (P-EPS)
In the pinion assist type steering system, the torque sensor, motor, clutch, and steering assist mechanism are still integrated, but the whole assembly is installed at the pinion gear, directly assisting the pinion gear, which can obtain a larger steering force. It can be used for medium-sized vehicles and provides a larger assist value. This form allows each component to be arranged more conveniently, but when there is a universal joint transmission device between the steering wheel and the steering gear, the torque signal acquisition and the assist wheel part are not on the same straight line, making it difficult to ensure the accuracy of the assist control characteristics.
(3) Rack-assist type EPS (R-EPS)
In the rack assist type steering system, the torque sensor is independently installed at the pinion gear, and the motor and steering assist mechanism are installed at the rack end opposite the pinion gear, providing assistance to the rack. The power assist unit of the R-EPS system is installed on the rack, with relatively free specific installation positions, making it very convenient during the layout of the car chassis. In addition, compared to the pinion assist type, it can provide a larger assist value, suitable for larger vehicles, mainly used in heavy-duty vehicles.
(4) Direct-drive type EPS (D-EPS)
In the direct drive type EPAS system, the power assist mechanism and the rack of the steering gear form an independent unit, which is easy to arrange in the engine compartment. As shown in Figure 5, it is the direct drive type EPAS system designed by Danaher Motion company. This system adopts the IP67 sealing standard, reducing the complexity of the system, bringing significant advantages to vehicle design units, manufacturers, and users. The flexibility of the electric system can achieve zero replacement of input devices such as steering wheels, multi-turn wheels, control rods, and wired guidance components. The configurable AC drive device in the system realizes convenient parameter performance adjustment, and when used with the electrical traction system, users can control the traction speed according to the load and steering angle, improving the driving performance, stability, and overall safety of electric vehicles.
3. Characteristics of EPAS
Compared with traditional mechanical steering systems and currently widely used HPS and EHPS, EPAS has the following characteristics:
Provides optimal assistance under various driving conditions, reduces disturbances caused by road irregularities to the steering system, improves the steering characteristics of the car, reduces the steering effort when the car is traveling at low speeds, enhances the steering stability when the car is traveling at high speeds, thereby improving the automatic safety of the car. Moreover, different steering assist characteristics can be set to meet the needs of different users.
Energy-saving and environmentally friendly. The EPAS system only provides assistance when steering, reducing fuel consumption. Compared with the hydraulic power steering system, it can save 3%~5% of fuel, and can save energy by 80%-90% under various driving conditions. The EPAS eliminates the problem of hydraulic oil leakage in hydraulic power steering, reduces noise, exhaust emissions, and improves environmental friendliness.
The steering system is powered by an electric motor, which is powered by a battery, so whether the EPAS can provide assistance is irrelevant to whether the engine is started. Even when the engine is off or faulty, it can still provide assistance.
Good production line assembly, short development cycle. The EPAS system does not have a hydraulic circuit, the number of whole machines is greatly reduced, it is easier to adjust and detect than HPS, and the level of automated assembly is higher. Different programs can be set to quickly match different models, thus shortening the development and production cycle.
Good low-temperature working performance. When starting the engine at low temperatures, the hydraulic power steering system has a higher steering force due to the greater viscosity of the oil at low temperatures. However, the EPAS system does not increase the steering force and engine load at low temperatures, showing better low-temperature working performance than HPS and EHPS.
Provides variable steering torque. During mechanical steering and hydraulic power steering, the steering torque obtained by the steering mechanism is proportional to the torque provided by the steering wheel and is unrelated to the state of the car. The steering assistance provided by EPAS is not only related to the size of the torque provided by the steering wheel but also associated with the state of the car, capable of providing different steering assistance at different vehicle speeds.
Improves maneuverability and enhances steering road feel. Due to the use of microcomputer control, the car has higher stability, and the driver feels more comfortable.
Enhances steering tracking and improves steering return characteristics. The EPAS motor is directly connected to the assist mechanism, allowing its energy to be directly used for wheel steering. Using the effect of the inertial damper, the reverse rotation and oscillation of the front wheels before turning are greatly reduced, and the anti-disturbance ability of the steering system is greatly enhanced. Because the rotational torque is generated by the motor, there is no steering hysteresis effect of HPS, enhancing the tracking performance of the steering wheels to the steering wheel. When the driver rotates the steering wheel by a certain angle and then releases it, the EPAS system can automatically adjust and make the car loop back to the center. Through software programming, the torque characteristics of the motor under different vehicle conditions can be obtained, providing steering return characteristics matching the vehicle's dynamic performance, significantly improving steering ability. At the same time, engineers can use software to adjust design parameters within the maximum limit to obtain the best return characteristics.