Application Research of CAN Communication Network in Automobile
Control Area Network CAN (Controller Area Network) is a communication protocol developed and applied by the German Bosch company to solve the data exchange between many control and test instruments in modern automobiles. In foreign countries, especially in Europe, the CAN network has been widely used in automobiles, such as BENZ, BMW, PORSCHE, ROLLSROYCE, JAGUAR and other vehicles.
Typical control units of modern automobiles include electronically controlled fuel injection system, electronically controlled transmission system, anti-lock brake system (ABS), anti-skid control system (ASR), exhaust gas recirculation control, cruise system and air conditioning system.
In a perfect automotive electronic control system, many dynamic information must be synchronized with the vehicle speed. In order to meet the real-time requirements of each subsystem, it is necessary to share car public data, such as engine speed, wheel speed, accelerator pedal position, etc. 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 runs at 2400r / min, the time interval between two injections controlled by the electronic control unit is 6.25ms. Among them, the crank angle (2ms) with an injection duration of 30 Â° requires a series of processes including speed measurement, fuel quantity measurement, A / D conversion, working condition calculation, and actuator control to be completed within the remaining 4ms. This means that the data transmission and reception must be completed within 1ms to meet the real-time requirements of diesel electronic control. This requires that its data exchange network is based on the priority competition mode and has a very high communication rate. The CAN field bus is designed to meet these requirements. Different parameters should have different communication priorities.
Typical parameters allow response time
Engine fuel injection 10ms
Engine speed 300ms
Wheel speed 1s ~ 100s
Inlet temperature 20s
Coolant temperature 1min
Fuel temperature â‰ˆ10min
3CAN bus characteristics and communication protocol
3.1 Characteristics of CAN bus
As a multi-master bus, CAN supports a communication network for distributed real-time control. The communication medium can be twisted pair, coaxial cable or optical fiber. In applications such as automotive engine control components, sensors, and anti-slip systems, the bus's bit rate can be up to 1 Mbit / s. CAN light has the following main characteristics:
a. Non-destructive bus arbitration based on priority competition.
b. Multi-address frame transmission with the aid of receive filtering.
c. With error detection and automatic retransmission of error frames.
d. Data transmission method can be divided into data broadcasting type and remote data request type.
3.2 CAN bus frame format
CAN and OSI seven-layer reference mode, according to the IEEE802.2 and IEEE802.3 standards, its communication interface integrates the physical layer and data link layer functions of the CAN protocol, which can complete the framing of communication data, including bit filling, data Work such as block coding, cyclic redundancy check and priority level. In the system, data can be divided into four frame formats according to the type of information carried:
a. Data frame.
It is used to transfer data between nodes and is the main body of network information. A data frame consists of 7 different bit fields: start of frame, arbitration field, control field, data field, CRC field, ACK field, and end of frame. The length of the data segment is programmable from 0 to 8 bytes.
Data frame format
b. Remote frame. It is sent by the online unit and is used to request the transmission of 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. The error frame is a signal flag for detecting bus errors and is composed of two different fields. The first field is superimposed by error signs from different nodes, and the second field is the error delimiter. The CAN protocol uses CRC check and can provide 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 and sent by some error conditions of the media access control layer. Used to extend the delay time of the frame sequence.
3.3CAN data link control
In the system, the CAN bus transmits data in units of messages, and the node uses the bit arbitration method to access the bus. The identifier of the sending node at the beginning of the message is divided into a function identifier (such as a speed signal) and an address identifier (such as the node address of the control unit). The biggest feature of the CAN protocol is that it breaks the traditional node address coding method and expands the coding 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 more complex car control networks in the future, its capacity is sufficient. The smaller the value of the identifier, the higher the priority of the frame data. Through 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 the reliability of data communication. Sex has played an important role.
The circuit of the electronic control unit (ECU) connected to the bus is implemented through the CAN physical layer. In actual 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: the physical signal completes the functions related to bit representation, timing and synchronization; the physical media accessory device completes the bus transmission / reception function and provides a bus fault detection method; the media-related interface completes the physical layer Mechanical and electrical interfaces.
4CAN bus application and interface design
4.1 Automotive network design
In addition to commands and clearing information, some basic status information of the car (such as engine speed, wheel speed, cooling water temperature, etc.) is the data that most control units must obtain. The control unit uses broadcast to send to the bus. If all control units send data to the bus at the same time, bus data conflicts will occur. At this time, the CAN bus protocol proposes a bus arbitration that uses identifiers to identify data priorities. Table 2 lists the types of data generated and sent by each electronic control unit of the car, and the procedures for other units to share these information.
The fuel position and speed signals have a higher priority because their real-time requirements are strong and directly affect the power, economy and emission performance of the engine.
4.2 CAN interface design
In this study, the CAN bus was successfully used in the calibration system of electronically controlled diesel engines, using a single-chip system and a CAN controller to form a CAN standard interface.
At present, there are many kinds of 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 connected to various types of microprocessors can complete the functions of the physical layer and the data link layer. It is suitable for automobiles and general industrial environments. It can not only reduce wire connections, but also enhance 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 led out respectively, and the CAN controller SJA1000 is operated through address allocation and chip selection. The bus data signal adopts high-speed linear optocoupler 6N137 isolation, and the power signal is + 5V DC-DC isolation module, which enhances the use of anti-interference measures in the system hardware. 82C250 is the interface between the CAN controller and the physical layer bus. It has the ability to resist instantaneous interference in the automotive environment and protect the bus. The device can provide differential transmission capability and differential reception capability to the bus, and is fully compatible with the ISO / DIS11898 standard.
3CAN interface hardware circuit design
When the data message passed the filter acceptance 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 receive buffer has data; the other is the interrupt mode. If the receive interrupt open bit allows, a trigger interrupt is generated. Since the SJA1000 has a 64-bit receive buffer, it has a certain buffer capacity for bus data. Usually, the system uses the main program query method to process the received data and sends it by broadcast, and the special data is applied by remote frame application, which is more conducive to the program's structured management of multiple tasks.
Communication program flow
CAN bus has many advantages such as high communication rate, high reliability, convenient connection and high performance-price ratio. And the design of the CAN application system is based on international standards (ISO11898). The controllers of various manufacturers have standard input / output interfaces, so the network is an open and flexible system that can be used without requiring all nodes and their The application layer changes any software and hardware, and freely increases or decreases the controller node.
In order to give full play to the role of electronic control unit in car control, CAN communication network provides conditions for global optimal control. The practical application shows that the CAN bus has significant advantages compared with other communication methods:
a. Free networking, strong expansibility, and strong advantages for complex automotive networks;
b. The communication priority can be determined according to the content of the data, which solves the problem of real-time and shared speed;
c. The automatic error definition function simplifies the communication operation of the electronic control unit.
d. Due to the standard and open nature of the data communication protocol, the interface circuit in this article has a certain promotion intention? The system is adopted by many industrial control systems, especially with high transmission rate and high requirements for real-time and reliability Occasions, it is a very effective way of communication.
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