Application of CAN Bus in Automobile Body Control

I. Introduction

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Since the 1980s, with the widespread use of integrated circuits and microcontrollers in automobiles, there are more and more electronic control units on automobiles, such as electronic fuel injection devices, anti-lock braking devices (ABS), airbag devices, Electronically controlled door and window devices and active suspensions, etc. In this case, if the conventional wiring method is still used, that is, one end of the wire is connected to the switch and the other end is connected to the electric device, the number of wires on the car will increase sharply, so that the quality of the wire accounts for the mass of the whole vehicle. %about. In addition, although the increase of the electronic control system improves the power, economy and comfort of the car, the complicated circuit that is added also reduces the reliability of the car and increases the difficulty of maintenance. To this end, the call for reform of automotive electrical technology is growing. Therefore, a new concept - the car controller area network CAN came into being.

CAN is the abbreviation of Controller Area Network, which was developed by Bosch and several semiconductor manufacturers in Germany. CAN bus is a serial multi-master controller LAN bus. It has high network security, communication reliability and real-time performance, and is simple and practical, and the network cost is low. It is especially suitable for automotive computer control systems and industrial environments with harsh ambient temperatures, strong electromagnetic radiation and high vibration.

Second, the technical characteristics of CAN bus

The CAN bus can effectively support distributed control or real-time control. The communication medium of the bus can be twisted pair, coaxial cable or optical fiber, and its main features are as follows:

• The CAN bus is a multi-master bus, and each node can send information to other nodes on the network at any time, regardless of the master and slave;
• The CAN bus adopts a unique non-destructive bus arbitration technology, and high-priority nodes preferentially transmit data, so the real-time performance is good;
• CAN bus has the function of transmitting data to point-to-point, point-to-multipoint and global broadcast;
• The CAN bus adopts a short frame structure, and the maximum number of valid bytes per frame is up to 8, the data transmission time is short, and there are CRC and other verification measures, and the data error rate is extremely low;
• When a node on the CAN bus has a serious error, it can automatically leave the bus, and other operations on the bus are not affected;
• When the CAN bus system is expanded, the new node can be directly hung on the bus, so the wiring is small, the system is easy to expand, and the modification is flexible;
• The maximum transmission rate of CAN bus can reach 1Mb/s, and the direct communication distance can reach 10km (the rate is below 5kbps);
• The number of nodes on the CAN bus depends on the bus driver circuit. Up to 110 in standard frames (11-bit message identifiers) and unlimited in extended frames (29-bit message identifiers).

Third, the CAN control design of the body system

1. CAN bus network system architecture

The control unit of modern automobile pawn has engine control module, transmission control module, multimedia control module, airbag control module, air conditioning control module, cruise control module, body control module (including lighting instructions and window, wiper, etc.), defense Locked brake system (ABS) anti-skid control system (ASR). The perfect car CAN bus network system architecture is shown in Figure 1.

Figure 1 Automotive CAN bus network system architecture

2. Hardware architecture of the CAN node

In this system, the CAN node adopts:

The circuit structure of ECU (AT89C51) + CAN controller (SJA1000) + CAN transceiver (PCA82C250) is the following is a summary of its core chip:

(1) CAN controller

In order to further expand the system, a chip SJA1000 supporting the CAN 2.0B communication protocol can be selected. The SJA1000 is a CAN controller manufactured by PHILIPS that supports both CAN 2.0B and CAN 2.0A. It is fully compatible with the CAN controller PCA82C200, which only supports CAN 2.0A, in hardware and software.

(2) CAN transceiver

The PCA82C250 is a CAN controller and physical bus interface chip from PHILIPS that provides differential transmission and reception of the bus. It is fully compatible with the ISO 11898 standard and has three different modes of operation: high speed, slope control and standby, which can be selected according to the actual situation.

(3) Single-chip AT89C51

The AT89C51 is a microcontroller from ATMEL. It is a low power, high performance, 8-bit CMOS microcontroller with 4KB of flash memory and is fully compatible with the industry standard MCS-51 command system and pins. The superiority of the AT89 series is that its on-chip flash memory can be programmed and erased 1000 times, and the data is not easily lost, and the data can be stored for 10 years.

The basic method of CAN bus controller, bus driver and single chip connection is shown in Figure 2.

Figure 2 CAN bus controller, bus driver and microcontroller connection diagram

Third, the CAN application layer protocol in the body control module

1. Principle of agreement

This protocol follows the CAN2.0B specification. According to the characteristics of the body control module, the source→purpose method is adopted. Each node has its own fixed identification address, and the number of nodes is less than 64. The central control module can be set as the master node during design. The door, the electric seat sub-module and the self-test sub-module are set as slave nodes. This agreement can complete the following functions:

(1) Broadcasting of specific information;
(2) the connection between the master and slave nodes;
(3) Information exchange between the master and slave nodes (including fault information).

This protocol uses the frame first principle to assign identifiers. The upper four bits of each frame identifier indicate the frame type. Different frame types have different priorities. The priority determines the order in which various information frames are transmitted under the same conditions. The assignment of the 29-bit identifier in is as follows:

Frame type (4 bits) + destination address (6 bits) + source address (6 bits) + command (or status, report) attribute (13 bits) [or data attribute + segment flag + segment number (13 bits total) ].

In principle, all commands or status, data, report attributes, and data collected in addition to the timing are required to be acknowledged (send an acknowledgement frame to ensure normal communication).

2. Frame format arbitration field and control field definition

The arbitration field consists of a 29-bit identifier ID28-ID0 and SRR, IDE and RTR. Registers 17-21 in SJA1000 are used to store the identifier of the extended frame format frame information. When transmitting, SRR=1, IDE=1, RTR=1/0 (remote frame/data frame). ID28-ID25 in the identifier is the frame type (4 digits in total) of the body control module exchange message. ID24-ID19 is the address of the frame information user (or the destination address, a total of 6 bits) in the body control module. ID18-ID13 is the address of the sender of the frame information in the body control module or called the source address (a total of 6 bits). ID12-ID5 is the command, status, data or report attribute (8 bits in total) exchanged in the body control module. The ID4 bit needs to be appended with a command or status, data, and segmentation flag when reporting attributes. ID3-ID0 is the segment number (4 digits in total) of the additional command or status, data, and report attributes. When ID4=0, the ID3-ID0 control field and data register 0-7 are valid. For remote frames, ID4-ID0 and the value of the control field can be ignored. The lower four bits of DLC3-DLC0 of register 16 of SJA1000 can form a control field to determine the data length of the data frame.

3. Body control module CAN2.0B communication message convention

According to the node requirements of the body control module, the communication information frames are divided into six types listed in Table 1. The priority levels in Table 1 are arranged from high to low according to the serial number. The allocation of destination address and source address is listed in Table 2.

Table 1 Body Control Module Frame Model

Table 2 Address assignment of each node of the body control module

It works as follows:

(1) After power-on or wake-up, the slave node sends status information to the master node, and the master node transmits a broadcast information remote frame (twice), and the broadcast information is shared information, including a vehicle speed signal, a gear position signal, an ignition switch position signal, and the like.

(2) Under normal circumstances, the status of each port is patrolled from the inside of the node. If there is a fault, the fault code is sent to the master node three times. After the master node receives three fault alarms, it starts to respond, and the slave node stops transmitting. Once the fault disappears, The master node sends normal information. There should be a fault table in the master node for communication with the diagnostic module.

(3) After the main node sends the self-test information separately, if the slave nodes are normal, the normal information, status and data frame are sent. If there is a fault, the fault alarm frame is sent through the segmented data frame.

(4) After monitoring from the node to the change of the normal input signal (including the change of the switching quantity and the analog sampling stage), the information is sent to the primary node through the report frame, and the primary node sends the command frame to indicate the response.

4. Communication message definition

Table 3 lists the communication message definitions of the central control module and the diagnostic module. In the table, aaaa is a segment number, which can be set when there are more than 8 fault codes, and can transmit up to 16×8 byte codes; bbbbbb is each sensor code, and the response frame adopts non-segmented data frames, cccccccc The code for executing the corresponding action, such as the window rising to 00000001, is reduced to 00000010, and the response can perform up to 256 actions. The response frame uses a remote frame and the request frame is a remote frame.

Table 3 Communication between the central control module and the diagnostic module

Table 4 Communication protocol agreement of each node during normal operation

When the system works normally, the communication protocol conventions of each node are listed in Table 4. The dddd in the table is the total number of segments included in the segment start command; eeee is a segment number of the broadcast information, the data length in the control field is the data length in the segment, and the actual data of a certain segment of the data broadcast in the data field , the data defined in order has:

• Data register 1 = car speed information is 8 bits high;
• Data register 2 = the lower 8 bits of the vehicle speed signal;
• Data register 3 = engine speed signal is 8 bits high;
• Data register 4 = engine speed signal low 8 bits;
• Data register 5 = ignition switch position, where 0 means the key is pulled out; 1 means the key is in OFF; 2 means the key is in ACC; 3 means the key is in RUN; 4 means the key is in START;
• Data register 6 = gear position signal, 0 means neutral; 1 means drive gear; 2 means drive gear; 3 means reverse gear; 4 means parking gear;
• Data register 7 = remote control signal, 0 means remote control unlocks the main driving door; 1 means remote control locks the main driving door; 2 means remote control unlocks all the doors; 3 means remote control locks all the doors; 4 means remotely unlocks the trunking box;
• The data register 8 is used for the anti-theft mode; 0 means to enter the anti-theft mode, and 1 means to cancel the anti-theft mode;
• Data Register 9-16: Reserved.

Fourth, the software process

Each controller shall send data to the bus in the specified format and cycle, and also accept information from other controllers. The other controllers on the bus take the required messages as needed. For receiving data, the system is implemented in an interrupt mode. Once the interrupt occurs, the data to be received is automatically loaded into the corresponding message register. At this time, the mask filtering method can also be adopted, and the identifier of the received message and the identifier set in advance when the receiving buffer is initialized by the mask filter register can be used. The characters are selectively compared bit by bit, and only the packets whose identifiers match can enter the receiving buffer, and those that do not meet the requirements will be shielded from the receiving buffer, thereby reducing the burden on the CPU to process the message. In addition, different data should be placed in different message registers. Therefore, in the receiving interrupt service program, it can be easily judged which receiving message is caused by the interrupt, and the program flow chart is shown in FIG.

Figure 3 program flow diagram

V. Conclusion

As a reliable car calculator network bus, CAN bus has been applied in advanced cars, enabling each car computer control unit to share all information and resources through the CAN bus to simplify wiring and reduce the number of sensors. Avoiding the duplication of control functions, improving system reliability and maintainability, reducing costs, better matching and coordinating the purpose of each control system, so that the power, operation stability and completeness of the car are raised to new heights. With the development of automotive electronics technology, CAN bus communication protocol with high flexibility, simple scalability, excellent anti-interference and error correction capability will be more widely used in automotive capacitor systems.