I. Introduction
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With the rapid development of the automobile industry, a large number of electronic control devices are used in modern cars. Many mid-to-high-end cars use more than a dozen or even twenty electronic control units, and each electronic control unit needs to be related. The sensors and the actuators communicate, and the information exchange between the control units is also required. If each piece of information is transmitted through its own independent data line, this will result in an increase in the number of pins of the electronic control unit, and the wiring harness of the entire electronic control system. The connectors will also increase, and the failure rate will increase.
In order to simplify the line, improve the communication speed between the electronic control units, and reduce the fault frequency, a new type of data network CAN data bus came into being. The CAN bus has strong real-time performance, long transmission distance and strong anti-electromagnetic interference capability. In automotive electronics control components, sensors, anti-slip systems and other applications in the field of automation electronics, CAN's bit rate can be as high as 1Mbps. At the same time, it can be used inexpensively in traffic vehicle electrical systems.
Second, CAN bus introduction
CAN, the full name of "Controller Area Network", is the controller LAN, which is a serial communication bus defined by ISO. It is mainly used to realize the information exchange between the electronic control units on the vehicle, forming an in-vehicle network system. The CAN data bus is also called For the CAN-BUS bus. It has information sharing, reduces the number of wires, greatly reduces the weight of the wiring harness, minimizes the control unit and control unit pins, and improves reliability and maintainability.
CAN is designed to communicate as a microcontroller in an automotive environment, exchanging information between the vehicle's electronic control unit ECUs to form an automotive electronic control network. Its work uses a single-chip microcomputer as a direct control unit for direct control of sensors and actuators. Each MCU is a node on the control network. No matter how many electronic control units a car has, no matter how big the information capacity is, each electronic control unit only needs to lead two wires together to connect to the node. A wire is called a data bus (Bus). The data transfer in the CAN data bus is like a conference call. A telephone user is equivalent to the control unit. It “talks†the data into the network, and other users “answer†the data through the network. Users who are interested in this group of data will With data, users who are not interested can ignore the data.
In a single network composed of CAN bus, theoretically, there can be countless nodes, but in actual applications, the number of nodes connected will be limited by the electrical characteristics or delay time of the network hardware. The premise of using a computer network for communication is that each electronic control unit must use and interpret the same "electronic language", a language called "agreement." There are many common transmission protocols for automotive computer networks. In order to implement data exchange with numerous control and test instruments, a standard communication protocol must be established. With the application and promotion of CAN in various fields, in September 1991, Philips Semiconductors developed and released the CAN specification (Version 2.0). The technology consists of two parts, A and B. 2.0A gives the standard format of CAN messages, while 2.0B gives both standard and extended formats. In November 1993, ISO promulgated the road traffic vehicle—data information exchange—the international standard ISO 11898 for high-speed communication LAN, paving the way for the standardization and standardization of LAN. J 1939, proposed by the American Society of Automotive Engineering SAE in 2000, became the general standard for controller LANs in trucks and buses.
Third, the composition and structure of the CAN-BUS data bus
The CAN-BUS system mainly includes the following components: CAN controller, CAN transceiver, CAN-BUS data transmission line and CAN-BUS termination resistor. :
1. CAN controller, CAN transceiver
A CAN controller and a CAN transceiver are provided in each control unit on the CAN-BUS. The CAN controller is mainly used to receive the information from the microprocessor, process the information and transmit it to the CAN transceiver, and the CAN controller also receives the data from the CAN transceiver, processes the data, and transmits the data. A microprocessor to the control unit.
The CAN transceiver is used to receive the data sent by the CAN controller and send it to the CAN data transmission bus. The CAN transceiver also receives the data on the CAN data bus and passes it to the CAN controller.
2. Data bus termination resistor
Both ends of the CAN-BUS data bus are connected through a terminating resistor. The terminating resistor prevents the data from returning like an echo after reaching the line terminal, and thus interferes with the original data, thereby ensuring the correct transmission of the data, and the terminating resistor is mounted in the control unit.
3. Data transmission bus
Most of the data transmission buses use two bidirectional data lines, which are divided into high-level [CAN-H] and low-level [CAN-L] data lines. In order to prevent external electromagnetic interference and external radiation, the two data lines are entangled, and it is required to be twisted at least once every 2.5 cm. The potentials on the two lines are opposite, and the sum of the voltages is always equal to the constant value.
Fourth, the application classification of the vehicle network
The vehicle network is divided according to the application and can be roughly divided into four systems: body system, power transmission system, safety system, and information system.
1. Motion transfer system
In the powertrain, the powertrain modules are more concentrated and can be fixed in one place, and the modules installed in the engine compartment are connected by a network. High-speed networks are needed when connecting the main factors of the car—running, stopping, and turning—with a network.
The power CAN data bus is generally connected to three computers, which are engines, ABS/EDL and automatic transmission computers (power CAN data bus can be connected to airbags, four-wheel drive and combination meters). The bus can transmit 10 sets of data at the same time, 5 sets of engine computer, 3 sets of ABS/EDL computer and 2 sets of automatic transmission computer. The data bus transmits data at a rate of 500 Kbit/s. Each data group transmits approximately 0.25 ms, and each electronic control unit transmits data once every 7 to 20 ms. The priority order is ABS/EDL electronic control unit→engine electronic control unit→automatic transmission electronic control unit.
In the powertrain, data transfer should be as fast as possible in order to use the data in time, so a high-performance transmitter is needed. The high-speed transmitter speeds up the data transfer between the ignition systems, so that the received data is immediately applied to the next one. Go to the ignition pulse. The CAN data bus connection point is usually placed in the wiring harness outside the control unit. In special cases, the connection point may also be located inside the engine electronic control unit.
2. Body system
Compared to the powertrain, parts of the body system are placed throughout the car. Therefore, the wire harness becomes long and is susceptible to interference. In order to prevent interference, the communication speed should be reduced as much as possible. In the body system, because the number of modules and nodes of the human-machine interface increases, the communication speed control will not be a problem, but the cost is relatively increased. For this, people are exploring a cheaper solution, and currentlyadays, a direct-connect bus and an auxiliary bus are often used. .
The comfort CAN data bus connection is typically connected to seven control units, including the central control unit, one controlled unit behind the front of the vehicle, and four door control units. Comfort CAN data transfer has seven functions: central control door lock, power window, light switch, air conditioner, combination meter, rear view heating and self-diagnosis function. The transmission lines of the control unit converge in a star shape. The advantage of this is that if one control unit fails, the other control units can still send their own data. This system reduces the number of wires passing through the door and makes the wiring simple. If a short circuit to ground occurs somewhere in the line, short circuit to the positive pole or short circuit between the lines, the CAN system will immediately switch to emergency mode operation or switch to single line mode operation.
The data bus transfers data at a rate of 62.5 Kbit/s. Each set of data transfer takes about 1 ms, and each electronic control unit sends data once every 20 ms. The priority order is: central control unit → driver side door control unit → front passenger side door control unit → left rear door control unit → right rear door control unit. Since the data in the comfort system can be delivered at a lower rate, the transmitter performance is lower than that of the powertrain transmitter.
The entire automotive body system circuit has three main blocks: the main control unit circuit, the controlled unit circuit, and the gate unit circuit.
After the main control unit receives the switch signal, it first analyzes and processes the control command, and then sends the control command to each controlled terminal through the CAN bus, and each controlled terminal responds and then performs corresponding action. The front and rear control terminals only receive the commands from the main control terminal, execute according to the requirements of the main control terminal, and feed back the results of the execution to the main control terminal. The gating unit not only receives the command from the main control terminal through the CAN, but also receives the switch signal input on the door. According to the command and the switching signal, the gating unit will act accordingly and send the execution result to the main control unit.
(1) Security system
This refers to the system that activates the airbag based on the information of multiple sensors. Because the safety system involves people's life safety, plus the number of airbags in the car, collision sensors and other reasons, the safety system must have fast communication speed and communication. High reliability.
(2) Information system
Information systems are widely used in vehicles, such as the application of systems such as car phones and stereos. The requirements for the information system communication bus are: large capacity and very high communication speed. Communication media generally uses optical fiber or copper wire, because the two media transmission speed is very fast, which can meet the high speed requirements of information systems.
V. Key technologies for CAN bus technology in automotive applications
To construct an in-vehicle network using the CAN bus, the key technical issues that need to be solved are:
(1) Technical issues such as rate, capacity, priority level, and node capacity of bus transmission information
(2) Reliable data transmission in high electromagnetic interference environment
(3) Determine the delay time of the maximum transmission
(4) Network fault tolerance technology
(5) Network monitoring and fault diagnosis
(6) Time characteristics of real-time control network
(7) Wiring in installation and maintenance
(8) Network node addition and software and hardware update (scalability)
Sixth, the conclusion
As a reliable automotive computer network bus, CAN bus has been applied in advanced vehicles, enabling each computer control unit to share all information and resources through the CAN bus to simplify wiring and reduce the number of sensors. Avoid the duplication of control functions, improve system reliability and maintainability, reduce costs, better match and coordinate the purpose of each control system, with the development of automotive electronics technology, with high flexibility, simple scalability, excellent resistance The CAN bus communication protocol with interference and error correction capability will be more widely used in automotive electronic control systems.
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