1 Introduction Semi-trailer refers to "an unpowered road vehicle whose design and technical characteristics require vehicle traction in order to be used normally" for carrying people and goods "(GB / T3730.1-2001 standard). There are many types of semi-trailers, including van semi-trailers, tank semi-trailers, flatbed semi-trailers, container semi-trailers, vehicle transport semi-trailers and so on. With the development of China's road logistics transportation industry, semi-trailers are more and more widely used due to their advantages of high transportation efficiency, large carrying capacity, good fuel economy, and low transportation costs. Along with this is the safety issue of semi-trailers. According to the statistics of the public security transportation department in 2006, the number of semi-trailers in the country accounts for about 5% of the total motor vehicle ownership. Among major traffic accidents, accidents involving large freight motor vehicles such as semi-trailers account for about 47%, resulting in personnel The mortality rate accounts for about 30% of the total. The safety hazards of semi-trailers mainly come from two aspects: one is overloading. The overload phenomenon not only damages the highway and other transportation facilities, but also makes the car tires under extreme load, which is easy to cause a flat tire accident; overloading the truck also makes the cargo more likely to become unbalanced, causing the vehicle to overturn and overturn. . The second is tire safety, which lacks a tire pressure and temperature monitoring system; therefore, it is necessary to establish a safety warning system for semi-trailers.
2 Overall design The body length of the semi-trailer is generally 6-14 meters, the body length is long, the electromagnetic interference is large, and the automotive electronic units are many and complicated. The independent TPMS scheme of the traditional four-wheeled car is used for signal transmission, which is easily shielded by interference and is also The unit cannot exchange information, and the reliability and scalability of the system are not good. Therefore, the design is based on the CAN bus, and each tire pressure, temperature monitoring device and other automotive electronic units such as ABS, engine, and instrument panel are used as nodes on the CAN bus, which are monitored and controlled by a central early warning module. The system reliability and monitoring efficiency can be improved, and the repeated development cost of each module can be saved. As shown in Figure 1.
3 System hardware design
3.1 Characteristics of CAN bus
CAN is called Controller Area Network, or Controller Area Network, and is one of the most widely used field buses in the world.
Compared with other bus structures, CAN has superior characteristics [1]: (1) The CAN bus is a multi-master bus, and each node can actively send information to other nodes on the network at any time, regardless of the master and slave , Flexible communication; (2) The maximum number of valid bytes per frame on the CAN bus is 8, the transmission time is short, the probability of interference is low, and there are CRC and other check measures, with excellent error detection effect, data error rate Very low. (3) The CAN bus has only two wires. When the system is expanded, there is no need for any changes in the application layer and any node software and hardware, and the new node can be directly hung on the bus, so there are fewer wires and the system is easy to expand. , Flexible modification; Low network construction cost and extremely high bus utilization; (4) The data transmission distance can meet the automotive system of 5-15 meters, the data transmission rate is up to 1Mbit / s, and it can also meet industrial applications up to 10km.
3.2 How does CAN bus address allocation distinguish each node on the bus? Each message sent on the CAN bus has a unique 11-bit (standard frame) or 29-bit (extended frame) digital ID. The method of statically assigning addresses can be used to define the first 3-4 bits of the ID as The node address is defined by the rest of you and written into the frame message sent by each node. After receiving a CAN data frame, the central node checks the previous 3-4 to determine the node from which the data was sent, so as to respond to the processing. The CAN bus state depends on the binary number "0" instead of "1", so the smaller the ID number, the higher the priority of the message. Generally, the load of the rear tires is greater than that of the front wheels. Because of the frequent right-hand drive, the load on the right tire may be greater than that on the left [2]. Therefore, when assigning the address of the tire node, the tire with a larger load can be assigned to a small address. Send the node data first.
3.3 Central early warning module The hardware design of the central control module is shown in Figure 2, which is divided into five parts: One is the selection and use of the processor. The processor requires high-performance, small size, low price, low power consumption, easy installation, safe and reliable products. At present, the more commonly used 32-bit S3C44B0X processor based on the ARM7 framework in embedded systems is more in line with the requirements. The second is the expansion of storage components, mainly used for program burning and running. The third is to have input and output components, using touch screen as input device, LCD and audio player as output device, to achieve the input of tire pressure and temperature threshold and the actual monitoring data output warning. The fourth is the design of the CAN interface circuit. If the selected processor comes with the CAN interface circuit, of course it is better. If the selected processor does not have the CAN interface function, data can be sent and received through an external CAN controller (Figure 2 uses S3C44B0X processing SPI synchronous serial interface is connected with MCP2510 controller and TJA1050 transceiver to realize CAN interface function). The fifth is to have a module debugging circuit, mainly the design of serial port and JTAG interface. 
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