Research on EPS Communication System Based on CAN Bus

　　The development of modern automotive electronic technology has made the degree of automobile electrification higher and higher. Although the electronic control system has improved the power and economy of the car, the increased complex circuits will inevitably lead to large and complex body wiring. Therefore, improving the reliability, real-time and safety of communication between control units has become an urgent problem to be solved. Bosch, a German company famous for the research and development and production of automotive electronic products, has developed the CAN bus protocol for this purpose and made it an international standard. Electric power steering (EPS) is a steering system that provides power according to the driver's intention and the operating conditions of the vehicle. The control process of EPS is a comprehensive control process of the power steering system, so the communication and coordinated control of the EPS electronic control unit with other electronic control units in the car are very important. Introducing CAN bus technology in the EPS system enables the EPS electronic control unit to communicate with other on-board electronic control units, which can realize data sharing and coordinate EPS with other system controls.

　　1 Introduction to CAN Bus

　　CAN, or controller area network, is a field bus communication structure developed by Bosch to solve many control and data exchange problems in modern automobiles. It has a maximum rate of 1 Mbps (within 40 m) and works in a multi-master mode. Compared with general communication buses, CAN bus data communication has outstanding reliability, real-time and flexibility, and is currently the most widely used automotive network. CAN bus has the following characteristics:

　　①The hardware connection is simple and has a good performance-price ratio.

　　②It has fast response capability and is very suitable for applications with high real-time requirements.

　　③ High reliability. The CAN bus has a very strong error correction capability. Each frame of data in the protocol uses CRC and other verification measures, and the data error rate is extremely low. If a serious error occurs in a node, it can automatically leave the bus, and the operation of other nodes on the bus will not be affected.

　　2 System Hardware Design

　　2.1  Introduction to LPC2129 (＄7.9484)

　　The LPC2129 microcontroller is one of the 32-bit microcontrollers of NXP. Its internal basic structure includes: central processing unit (CPU), two 16C550 industrial standard UARTs, high-speed I2C interface (400 kHz), two SPI interfaces, 8-channel input capture/output comparison timer, one 8-channel pulse width modulation module and 46 independent digital I/O ports. There are also 256KB Flash ROM and 16 KB RAM on the chip. The CAN function block includes two CAN controllers compatible with the CAN2.0B protocol. These rich internal resources and external interfaces can meet the requirements of ECU (electronic control unit) for processing various data and sending and receiving CAN network data. The chip integrates two CAN modules, which can realize the gateway node function of high-speed and low-speed CAN networks.

　　The CAN module complies with the CAN2.0B protocol and integrates all the functions of the CAN bus controller with the acceptance filter. In addition, it uses advanced buffer layout to improve real-time performance and simplify the design of application software.

　　2.2 CAN module design

　　The communication system of the car is composed of EPS control system, ABS system, engine system, electric window system, headlight control system, etc. These electronic control systems have different requirements for real-time response in the entire control system. In addition, a large amount of real-time data exchange is required between many nodes during the actual operation of the car. If all nodes of the entire car are connected to a CAN network, and many nodes communicate through a CAN bus, once the information management configuration is slightly improper, it is easy to have an excessive bus load, resulting in a decrease in the real-time response speed of the system. This is not allowed in a real-time system. Therefore, according to different requirements, the car network can be divided into two rate levels: high-speed CAN network and low-speed CAN network. The number of electronic control units of nodes such as ABS and EPS is small, and the real-time and stability requirements are high, forming a high-speed CAN network with a transmission rate of 500 kbps. The number of electronic control units of numerous body motors and headlight nodes is large, and the transmitted data is complex. The requirements for accuracy and stability are better than real-time performance, forming a low-speed CAN network with a transmission rate of 125 kbps. For communication between networks with different rates, there must be corresponding gateways for data filtering and rate conversion to achieve data communication between network nodes with different rates. The high-speed and low-speed CAN gateway is implemented using LPC2129, and its communication network is shown in Figure 1.

　　2.3 CAN node hardware design

　　The CAN node hardware circuit mainly includes a microcontroller with a CAN controller and a CAN transceiver for data transmission and reception. This design uses the 32-bit microcontroller LPC2129 from NXP, which has a CAN controller and is mainly responsible for CAN initialization and data processing. There are many types of CAN transceivers. This design uses the CAN high-speed transceiver TJAl050 from Philips. The basic CAN node structure is shown in Figure 2.

　　2.4 Hardware circuit design of high-speed and low-speed CAN gateway

　　The main function of the gateway is to coordinate the sharing of data between networks and to be responsible for the communication between nodes. Its hardware structure is very similar to that of a CAN node. Since it is responsible for sharing data between high-speed and low-speed networks, it must bridge both networks at the same time. The hardware structure of a CAN bus gateway is shown in Figure 3.

　　3 Software Design of High-Speed ​​and Low-Speed ​​CAN Gateway

　　The main function of the gateway software design is to send and receive data at each node, especially the gateway can realize the conversion of high-speed and low-speed network data. The light control and window control circuits with low real-time requirements use low-speed CAN network with a baud rate of 125 kbps; the ABS system and EPS system with high real-time requirements use high-speed CAN network with a baud rate of 500 kbps.

　　The CAN initialization procedure is as follows:

　　In order to reduce the occupancy rate of network resources and improve the real-time performance of network communication, in addition to the necessary communication between the high-speed and low-speed CAN networks, the respective messages are transmitted independently. This requires the use of identifiers in CAN to filter messages through acceptance filters, which can be achieved by setting the filter registers. The settings of the acceptance registers and mask registers of CANO and CAN4 registers are as follows:

　　Due to the different transmission rates, the data transmission between high-speed and low-speed CAN networks is different. When the high-speed CAN network data is transmitted to the low-speed CAN, it is necessary to add a soft buffer for temporary storage; when the low-speed CAN network data is transmitted to the high-speed CAN network, it can be directly transmitted. The communication process is shown in Figure 4.

　　Conclusion

　　The EPS control system using CAN bus technology can not only reduce the number of sensors, reduce costs, and achieve data sharing, but also improve the performance of EPS. This plan is an improvement plan proposed for the National Natural Science Foundation project "Research on Automobile Chassis System Control Methods and Key Technologies Based on Generalized Integration". Experiments have proved that the EPS control system using CAN network has good real-time performance, high reliability, and good operation.