I. Introduction
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The steering performance of the car is one of the main performances of the car. The performance of the steering system directly affects the steering stability of the car. It ensures the safe driving of the vehicle, reduces traffic accidents, protects the driver's personal safety and improves the working conditions of the driver. An important role. How to design the steering system reasonably and make the car have good maneuvering performance is always an important research topic for designers. In today's high-speed vehicles, non-professional drivers, and intensive traffic, the design of the car's easy-to-maneuver is particularly important for more different levels of driving. The development of Steering – By-Wire System (SBW) is precisely to meet this objective need. It is a new-generation steering system developed after EPS, which has better stability than EPS steering, and it no longer uses mechanical connection between the steering wheel and the steering wheel, completely eliminating the limitations inherent in the traditional steering system. It also improves the safety of the car while bringing convenience to the driver.
First, the development of the line-controlled steering system
Mercedes-Benz Germany began research on front-wheel-steering steering in 1990 and applied its line-controlled steering system to the concept car F400Carving. Koyo Japan has also developed a line-controlled steering system, but in order to ensure the safety of the system, the mechanical part between the steering wheel and the steering wheel is still retained, that is, through the clutch connection, when the wire-controlled steering fails, the clutch is combined to return to the mechanical steering. BMW's concept car, the BMW Z22, uses SteerByWire technology, and the steering wheel's range of rotation is reduced to 160°, which greatly reduces the driver's busyness during emergency steering. The concept car "FILO" designed and developed by Bertone in Italy, the "C-Crosser" of the Citroen off-road vehicle and the "R129" of the Daimlerchrysler concept car all adopt the line-controlled steering system. In 2003, Honda Japan introduced the Lexus HPX concept car at the New York International Auto Show. The car also uses a line-controlled steering system that integrates various control functions on the instrument panel to achieve automatic vehicle control. It is estimated that after a few years, the mechanical system will be replaced by cables and electronic signals.
Second, the structure and working principle of the wire-controlled steering system
(1) Structure of the wire-controlled steering system
The automobile remote control steering system is mainly composed of a steering wheel module, a front wheel steering module, a main controller (ECU) and an automatic fail-safe system, and its structure is shown in FIG. 1 .
Steering wheel module
The steering wheel module includes a steering wheel assembly, a steering wheel angle sensor, a torque sensor, and a steering wheel returning positive torque motor. Its main function is to convert the driver's steering intention (by measuring the steering wheel angle) into a digital signal and transmit it to the main controller, while the main controller sends a control signal to the steering wheel to the positive torque motor to generate the steering wheel returning positive torque. To provide the driver with the corresponding road feeling information.
2. Front wheel steering module
The front wheel steering module includes a front wheel angle sensor, a steering execution motor, a motor controller, and a front wheel steering assembly. Its function is to feedback the measured front wheel angle signal to the main controller, and accept the command of the main controller to control the steering wheel to complete the required front wheel angle to achieve the driver's steering intention.
3. Main controller
The main controller analyzes and processes the collected signals, determines the motion state of the car, sends commands to the steering wheel and the positive torque motor and the steering motor to control the coordination of the two motors. The main controller can also recognize the driver's operation command and determine whether the driver's steering operation is reasonable in the current state. When the car is in an unstable state or the driver gives an error command, the front wheel steer steering system will automatically perform stable control or shield the driver from the wrong steering operation, and automatically drive the vehicle in a reasonable manner to restore the car to a stable state as soon as possible. .
4. Automatic fail-safe system
The fail-safe system is an important module of the steer-by-wire system. It includes a series of monitoring and implementation algorithms to deal with different fault forms and fault levels in order to maximize the normal driving of the car. Wire-steering technology uses rigorous fault detection and processing logic to maximize vehicle safety.
(2) Working principle of the wire-controlled steering system
The working process: after the information from the steering wheel sensor and the current state of various vehicles is sent to the electronic control subsystem, the computer is used to control the information, and then the vehicle steering subsystem is commanded to turn the vehicle. At the same time, the information given by the steering resistance sensor in the wheel steering subsystem is also transmitted to the components of the steering wheel system that simulate the road feel via the electronic control subsystem. The principle is shown in Figure 2.
Third, the performance characteristics of the wire-controlled steering system
Since the steering wheel and the steering wheel in the steer-by-wire system have no mechanical connection and are disconnected, the necessary information is transmitted through the bus, so the system is also called a flexible steering system. Has the following performance characteristics:
Flexible steering eliminates steering interference problems and provides significant prerequisites for versatile and versatile automatic control, as well as system integration of automotive dynamics control systems and vehicle ride comfort control systems.
For front-wheel drive cars, the rigid steering shaft space needs to be considered when installing the engine. The steering shaft must be installed near the engine depending on whether the car is driving on the left or right. The designer must coordinate the various components to be arranged. The flexible steering removes the rigid mechanical connection between the various functional modules of the original steering system, which greatly facilitates the overall layout of the system.
Comfort is improved. In the rigid steering system, the unevenness of the road surface and the imbalance of the steering wheel can be transmitted back to the structure diagram of the steering control system of the steering diagram of Fig. 1. The working principle diagram of the line-controlled steering system of Fig. 2, and the flexible system cannot.
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The steering return torque can be adjusted by the driver according to the driver's request. Therefore, it is possible to personally adapt to a specific driver and driving environment without changing the design, and the steering-related driving behavior can be realized by software.
The possibility of injuring the driver caused by the steering column in the collision accident is eliminated, and it is not necessary to provide a steering anti-twist mechanism.
The driver's leg activity space is increased, and access is more convenient and free.
Fourth, the key technology of wire-controlled steering
(1) Sensor technology
One of the characteristics of modern automotive technology development is that more and more components are electronically controlled. The control effect of the automotive electronic control system depends on the accuracy of the sensor's information acquisition and feedback. The sensor technology content directly affects the performance of the entire automotive electronic control system. Related sensors required for automotive SBW systems include angular displacement sensors, torque sensors, vehicle speed sensors, lateral acceleration sensors, and yaw rate sensors.
(two) bus technology
With the development of automotive bus technology, there are a variety of automotive bus standards, and some bus standards and protocols with high-speed real-time transmission characteristics will be used in the future. This type of bus standard mainly includes TTP, Bytef-light and FlexRay. TTP (Time Trigger Protocol) is a complete communication protocol for distributed real-time control systems. It can support multiple fault-tolerant strategies with node recovery and integration functions. BMW Byte-light can be used for communication in automotive line control systems. The network is characterized by the ability to meet certain high-priority messages that require time triggering to ensure that delays are determined, and that certain messages are required to trigger events, requiring interrupt processing requirements; other automotive manufacturers currently plan to use FlexRay. This is a network communication system that is especially suitable for next-generation automotive applications. It has fault-tolerant functions and a defined message transmission time to meet the high-speed communication requirements of automotive control systems. BMW, Daimler-Chryler, Motorola and Philips jointly developed and established the FlexRay standard. GM, Boseh and Volkswagen also joined the Joint Development Association, and now have seven core members working together to develop automotive distributed control systems. The standard for high speed bus systems. The physical layer standard of the FlexRay standard has been developed by Philips, and the communication protocol is under development. The introduction of this standard not only improves the consistency and reliability of information transmission, but also simplifies the process of information development and use, and reduces costs. From the current development point of view, because FlexRay is a time and event based triggering protocol, it is better than TTP. The SBW system based on bus technology turns the traditional mechanical steering system into an electrical system connected by a high-speed fault-tolerant communication bus to realize automation, intelligence, networking and informationization of the system.
(3) Power supply
The power supply is responsible for the power supply of the electronic control unit and four motors in the SBW system (two redundant torque feedback motors and two redundant steering motors). The two torque feedback motors have a power of about 50-80W and two steering. The motor power is about 500-800W, and the power load is quite heavy. Therefore, to ensure the stable operation of the entire system, the performance of the power supply is crucial. As electronic components and their high-power components continue to increase, the load on automobiles has doubled. If you continue to maintain the 12V power supply system, you must increase the current to get more power, but too high current will bring safety hazards to the whole system. The thermal energy consumption on the car circuit is greatly increased, so the car power supply system must increase the voltage. Meet the growing needs of modern automotive electrical systems. Thus, the 42V power supply system came into being. The use of 42V power supply also created conditions for the development of SBW system: the quality of the motor is reduced by 20%; the diameter of the wire harness is reduced, the design and use cost is reduced, the installation is convenient, the load current is reduced, and the integration of electronic components is improved. . These advantages have a decisive influence on their development and will greatly promote the development of motors and related components of the SBW system.
(4) Reliability technology
The biggest problem in the development of the wire-controlled steering system is the reliability problem. Since there is no direct mechanical connection between the steering wheel and the steering wheel in the steer-by-wire system, when the electronic control system fails, the vehicle will not be able to guarantee the steering function and is out of control. With the development of technology, the reliability of electronic control systems has been continuously improved, and the concept of “redundant design†has been introduced in the system design, such as: redundancy of sensors, redundancy of motors, redundancy of vehicle power systems, etc. The reliability of the steer-by-wire system has been significantly improved. Figure 3 shows a typical representation of the redundant design of a steer-by-wire system.
To ensure adequate power supply to the steer-by-wire system, and to prevent power failures, a safer 42V power system must be used. Two steering sensors are placed under the steering wheel to ensure that the driver's intention to operate can be identified. The steering wheel motor is powered by two redundant designs; to ensure that the steering wheel motor is damaged, positive torque can be applied, a torsion spring is installed under the steering wheel or a second steering wheel motor is installed. In order to ensure the steering capacity of the front wheel of the vehicle, two steering motors are used, and two steering sensors are provided accordingly. The idea of ​​redundant design is also adopted in the design of the ECU design and control software. Due to the above various measures, the reliability of the steer-by-wire system is greatly improved. It provides a guarantee for the application of the SBW system in automobiles.
V. Prospects of the line-controlled steering system
The design of the vehicle's remote control steering system is to reduce the driver's physical strength and mental work, and to improve the active safety of the vehicle. The vehicle performance is suitable for more non-professional drivers and has great appeal to consumers. . The following is a description of its prospects in several ways:
In terms of production cost, as the cost of electronic chips and electronic components is reduced, the processing power and reliability are greatly improved, which will make the cost of the wire-controlled steering system reach the acceptance level of consumers in the near future.
From the conditions of its realization, it is expected that the 42V power supply will be developed rapidly, the accuracy of various sensors will be improved, the cost will be reduced, and the motor vibration control technology for analog road sense will be more mature. The application in the car created the conditions.
In addition, from the development trend of Hyundai Motor, the main vehicles of the future are low-emission vehicles (LEV), hybrid electric vehicles (HEV), fuel cell vehicles (FCEV), electric vehicles (EV), four EV vehicles, assisted driving systems and Driverless cars are an emerging hot research field. The best solution for achieving intelligent steering of automobiles is to adopt a line-controlled steering system, which brings a broader application prospect to the line-controlled steering system.
In short, the steer-by-wire system is a new technology for consumers, especially for the future realization of car unmanned vision.
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