Control Area Network CAN (ControllerAreaNetwork) is a communication protocol developed by Bosch in Germany to solve the data exchange between many control and test instruments in modern automobiles. In foreign countries, especially in Europe, CAN networks have been widely used in automobiles, such as BENZ, BMW, PORSCHE, ROLLSROYCE, JAGUAR and other vehicles.
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Typical control units for modern vehicles include electronically controlled fuel injection systems, electronically controlled transmission systems, anti-lock braking systems (ABS), anti-skid control systems (ASR), exhaust gas recirculation control, cruise systems and air conditioning systems.
In a sophisticated automotive electronic control system, much of the dynamic information must be synchronized with the speed of the vehicle. In order to meet the real-time requirements of each subsystem, it is necessary to share the public data of the vehicle, such as engine speed, wheel speed, and accelerator pedal position. However, the real-time requirements of each control unit are different due to different data update rates and control cycles. For example, if an 8-cylinder diesel engine is running at 2400r/min, the electronic control unit controls the interval between two injections to be 6.25ms. Among them, the crank angle (2ms) with an injection duration of 30° requires a series of processes such as speed measurement, oil quantity measurement, A/D conversion, working condition calculation, and actuator control in the remaining 4ms. This means that data transmission and reception must be completed within 1ms to meet the real-time requirements of diesel electrical control. This requires that the data exchange network is based on a priority competition mode, and has a very high communication rate, CAN field bus is designed to meet these requirements. Different parameters should have different communication priorities.
Typical parameters allow response time
Engine fuel injection amount 10ms
Engine speed 300ms
Wheel speed 1s~100s
Intake air temperature 20s
Coolant temperature 1min
Fuel temperature ≈10min
3CAN bus characteristics and communication protocol
3.1 CAN bus features
As a multi-master bus, CAN supports a distributed real-time control communication network. The communication medium can be twisted pair, coaxial cable or fiber optic. In automotive engine control components, sensors, anti-slip systems and other applications, the bit rate of the bus can be up to 1 Mbit/s. CAN light has the following main features:
a. Non-destructive bus arbitration based on priority competition.
b. Can be transmitted by means of receive filtered multi-address frames.
c. With error detection and error frame automatic resend function.
d. Data transfer methods can be divided into data broadcast and remote data request.
3.2CAN bus frame format
CAN and OSI seven-layer reference mode, according to IEEE802.2 and IEEE802.3 standards, its communication interface integrates the physical layer and data link layer functions of the CAN protocol, and can complete the framing processing of communication data, including bit filling and data. Block coding, cyclic redundancy check, and priority levels. In the system, data can be divided into four frame formats according to the type of information carried:
a. Data frame.
Used to transfer data between nodes, which is the main body of network information. A data frame consists of seven different bit fields: frame start, arbitration field, control field, data field, CRC field, ACK field, and frame end. The length of the data segment can be programmed from 0 to 8 bytes.
Data frame format
b. Remote frame. Sent by the online unit to request to send a data frame with the same identifier, the frame format is basically the same as the data frame, but there is no data field.
c. Error frame. An error frame is a signal flag that detects a bus error and consists of two different fields. The first field is superimposed by error flags from different nodes, and the second field is the error delimiter. The CAN protocol uses CRC checking and provides corresponding error handling functions to ensure the reliability of data communication.
d. Overload frame. It consists of an overload identifier and an overload delimiter indicating the internal overload status required by the logical link control layer and will be initiated by some error conditions of the media access control layer. The delay time used to extend the sequence of frames.
3.3CAN data link control
In the system, the CAN bus transmits data in units of messages, and the node accesses the bus by bit arbitration. The message start sending node identifier is divided into a function identifier (such as a speed signal) and an address identifier (such as a control unit node address). The biggest feature of the CAN protocol is that it breaks the traditional node address encoding method and expands the encoding method of the communication data block. The identifier of the data block can be represented by 11-bit or 29-bit binary, which can define 211 or 229 different data types. Even for the more complex car control network in the future, its capacity is sufficient. The smaller the value of the identifier, the higher the priority of the frame data. Through the data link control, each receiver completes the frame reception filtering to determine whether the frame data is valid. In actual automotive applications, non-redundant communication lines are generally used, and the CAN protocol provides a powerful error diagnosis mechanism to ensure reliable data communication. Sex has played an important role.
The circuit that the electronic control unit (ECU) is connected to the bus is implemented by the CAN physical layer. In practical applications, the total number of ECUs will be limited by the electrical load on the bus. The physical layer is divided into three functions according to the network standard specification model: physical signal completion and bit representation, timing and synchronization related functions; physical media attachment device completes bus transmission/reception function and provides bus fault detection method; media related interface completes physical layer Mechanical and electrical interfaces.
Application of 4CAN bus and its interface design
4.1 Automotive Network Design
In addition to command and clearing information, some basic state information of the car (such as engine speed, wheel speed, cooling water temperature, etc.) is the data that most control units must acquire. The control unit sends the broadcast to the bus. If all control units send data to the bus at the same time, a bus data collision will occur. At this point, the CAN bus protocol proposes a bus arbitration that uses the identifier to identify the data priority. Table 2 lists the types of data generated and transmitted by each electronic control unit of the vehicle, and the procedures for sharing the information with other units.
The oil position and speed signals have a higher priority because of their real-time requirements and directly affect the engine's power, economy and emissions performance.
4.2CAN interface design
In this study, the CAN bus was successfully used in the electronically controlled diesel engine calibration system, and the single-chip system and the CAN controller were used to form the CAN standard interface.
At present, there are many CAN bus chips, such as PHILIPSSJA1000, INTEL82526, MOTOROLA68HC05, SIEMENSC167C, and so on.
In this circuit design, SJA1000 is selected as the CAN controller chip, and the application layer of the ECU is provided by the microprocessor. The CAN controller SJA1000, which connects various types of microprocessors, performs the functions of the physical layer and the data link layer. It is suitable for automotive and general industrial environments, which not only reduces wire connections, but also enhances diagnostic and monitoring capabilities.
The hardware design of the CAN node communication interface is shown in Figure 3. In the design, the address line, data line and control line of the microprocessor are respectively taken out, and the CAN controller SJA1000 is operated by address allocation and chip selection. The bus data signal is isolated by high-speed linear optocoupler 6N137, and the power signal is +5V DC-DC isolation module, which enhances the system hardware to use anti-interference measures. The 82C250 is the interface between the CAN controller and the physical layer bus. It has the ability to resist transient interference and protect the bus in the automotive environment. The device provides differential transmit capability and differential receive capability to the bus and is fully compatible with the ISO/DIS11898 standard.
3CAN interface hardware circuit design
When the data message accepted by the filter is received, there will be two modes of operation. One is the query mode. The query receiving status bit is set high to indicate that the receiving buffer has data; the other is the interrupt mode. If the receiving interrupt open bit is enabled, a trigger interrupt is generated. Since the SJA1000 has a 64-bit receive buffer internally, it has a certain buffering capability for bus data. Generally, the system uses the main program query mode to process the received data, and broadcasts it by means of a remote frame application method for special data, which is more conducive to the structured management of multiple tasks by the program.
Communication program flow
CAN bus has many advantages such as high communication speed, high reliability, convenient connection and high performance-price ratio. And the CAN application system is designed according to the international standard (ISO11898). The controllers of each manufacturer have standard input/output interfaces, so the network is an open and flexible system that can not require all nodes and Freely add or subtract controller nodes if the application layer changes any software and hardware.
5 Conclusion
In order to give full play to the role of the electronic control unit in vehicle control, the CAN communication network provides conditions for global optimization control. Through practical application, the CAN bus has significant advantages over other communication methods:
a. Free networking, strong scalability, and a strong advantage for complex automotive networks;
b. The communication priority can be determined according to the data content, and the problem of real-time speed and sharing of the speed is solved;
c. Automatic error definition function simplifies the operation of the electronic control unit for communication.
d. Due to the standardization and openness of the data communication protocol, the interface circuit in this paper has certain promotion intentions, and is adopted by many industrial control systems, especially the high transmission rate and high real-time and reliability requirements. On the occasion, it is a very effective means of communication.
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