introduction
The modern automobile industry and electronic technology are developing rapidly, and there are more and more electronic devices on automobiles. The number of electrical nodes of a high-end car has reached thousands. If the traditional method is used for wiring, the number of wiring is very amazing and there is a great potential for failure. In order to solve this problem, major automobile manufacturers have started the research of automotive networks since the 1970s, and have made great progress, forming a variety of network protocols suitable for different transmission rates and special purposes, such as: CAN bus , LIN bus, KWP2000 for diagnosis, TTP for X-by-wire, MOST protocol for multimedia applications, etc. Among them, CAN (Controller Area Network, Controller Area Network) was proposed by BOSCH in the 1980s. A serial data communication protocol developed to solve the data exchange between many control and test instruments in modern automobiles can solve the above problems well. Now many automobile companies in the world, such as Mercedes-Benz, BMW, Volkswagen and other companies have adopted CAN bus to realize the internal data communication of the car.
China's research on vehicle networks, buses , and communication protocols started late, but it has developed rapidly in recent years, and bus networks have been widely used, especially in electric vehicle projects.
2 Technical characteristics of automotive bus
Car bus transmission must ensure the following points: ensure that information can be accurately transmitted; bus nodes can access the bus at any time; nodes perform bus access according to predetermined priorities; have the ability to solve bus access competition based on information content and win the site after competition resolution Can access the bus and continue to transmit information; the node successfully accesses the bus in the shortest possible time; the optimal transmission rate (baud rate); the fault diagnosis ability of the node; the bus has a certain scalability and so on.
2.1 Encoding of digital signals
In order to ensure the reliability of information transmission, it is very important to encode digital signals correctly. Automobile LAN data signals mostly use pulse width modulation (PWM) and non-return-to-zero (NRZ). When PWM is used as the coding scheme, the upper bound of the baud rate is 3 & TImes; 105Kbps, which is used in the occasions where the transmission rate is low. Using NRZ for information transmission, it can reach 1Mbps and is used for occasions with high transmission rate.
2.2 Network topology
Practical automotive LAN is a bus topology, such as CAN, SAEJ1850, ADVANCED PALMNET, etc. Its advantages are: short cables and easy wiring; simple bus structure and passive components, high reliability; easy to expand, adding new nodes only Connect it at a certain point on the bus. If you need to increase the length, you can add an additional section through the repeater.
2.3 Bus access protocol
The access protocol of the car bus is generally a contention protocol, and each node can independently determine the transmission of information frames. If two or more nodes send information at the same time, an error will occur, which requires each node to have the ability to determine whether a conflict has occurred. When a conflict occurs, wait for a certain time interval and then resend it to avoid recurrence. conflict. The anti-collision monitoring measures used in the network protocol are mostly carrier monitoring multiple access. For example, CAN, SAEJ1850, ADVANCED PALMNET, etc. all use carrier monitoring multiple access / conflict detection + lossless arbitration (C *** A / CD + NDA ).
3 Characteristics of CAN bus
The CAN bus communication protocol is formulated in consideration of the industrial field environment. It uses the three layers of the open system interconnection ISO-OSI model developed by the International Organization for Standardization ISO, namely the physical layer, the data link layer, and the application layer. The CAN bus specification has been formulated by the International Organization for Standardization as an international quasi-ISO11898, and is recognized as one of the most promising field buses, which has been widely used in the industrial field. Supported by famous semiconductor device manufacturers such as MOTOROLA, PHILIPS, Intel, SIEMENS, etc., and then quickly launched a variety of integrated CAN protocol products. Compared with the general bus, the CAN bus has the advantages of reliability, flexibility and strong real-time.
(1) The CAN bus uses a multi-master structure, any node on the network can send information to other nodes at any time, and the communication method is flexible.
(2) Depending on the priority of the bus access (depending on the message identifier), the nodes on the network can get a response within 134 μs at the fastest.
(3) The use of non-destructive bus arbitration technology can greatly save the time of bus conflict arbitration, and the network will not be paralyzed under crowded conditions.
(4) CAN protocol abolishes the station address encoding, but encodes the communication data, which allows different nodes to receive the same data at the same time, and can easily realize the point-to-point, point-to-multipoint and global broadcast transmission and reception Data is easy to form a redundant structure to improve system reliability and system flexibility.
(5) CAN adopts NRZ coding, the direct communication distance can be up to 10km (speed 5Kbps), and the communication speed can be up to 1Mbps (at this time, the longest communication distance is 40m).
(6) The short frame structure is adopted, the transmission time is short, and the probability of interference is low. Each frame of CAN information has CRC verification and other error detection measures to ensure that the data error rate is extremely low.
(7) The communication medium can be twisted pair, coaxial cable or optical fiber, and the choice is flexible.
(8) The CAN node has the function of automatically turning off the output when the error is serious, so that the operation of other nodes on the bus is not affected.
4 Application of CAN bus in pure electric vehicles
The biggest difference between a pure electric car and a traditional car is that it is driven by electrical energy, that is, a combination of a battery and a motor is used to replace the traditional engine. In this way, the drive control of the motor and the management of the battery have become the key technologies of electric vehicles. In addition, the control system of pure electric vehicles also includes power steering control, body system control, instrument clusters, etc. Each part has an independent control unit (ECU), and some advanced cars are also equipped with anti-lock brake control systems (ABS), airbag control system (SRS), cruise control system, drive anti-skid control system (ASR), suspension control, air conditioning control, anti-theft and other control units. In addition, various comfort control devices and communication systems are also increasing, and there is a close relationship between each ECU, which forms a network of automobile control systems based on CAN bus.
The vehicle control system of a pure electric vehicle is composed of two buses, namely a high-speed CAN bus and a low-speed bus. The high-speed CAN bus and the low-speed bus are two independent bus systems. In order to facilitate the management of all functions of the car, the two bus networks are connected through a gateway, and data between different buses is shared through the gateway. In this way, the two buses operate independently, and only data that needs to be exchanged between the two buses is transmitted through the gateway. This method can separate different types of information, reducing the burden on each network bus. The high-speed CAN bus is mainly connected to the drive system of electric vehicles. It is composed of various subsystems of the drive system and the node of the fault analysis and recording system. It can achieve rapid control of key systems such as motors, batteries, steering, and braking.
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