Design of electric vehicle control system based on CAN bus

　　1. Introduction

　　CAN bus is a communication protocol developed by the German BOSCH company in the early 1980s to solve the problem of data exchange between numerous control and test instruments in automobiles. Due to its outstanding reliability, real-time performance and flexibility, the CAN bus has been widely recognized and used by the industry. It officially became an international standard and industry standard in 1993 and is known as one of the "most promising field buses". one. The application of bus technology represented by CAN in automobiles not only reduces the body wiring harness, but also improves the reliability of the automobile. In the design of modern foreign cars, CAN has become a must-have technology. Mercedes-Benz, BMW, Volkswagen, Volvo and Renault and other cars all use CAN as a means of controller networking. There is currently a big gap in the application of CAN bus technology in automobiles in my country, and research on the application of CAN bus technology in electric vehicles is still in its infancy.

　　Electric vehicles incorporate many electronic control systems, such as battery management systems, motor control systems, drive control systems, regenerative braking systems, and ABS systems. The extensive application of electronic equipment will inevitably lead to the growth and complexity of vehicle body wiring, reduced operational reliability, increased power loss on the lines, and increased difficulty in fault repair. Especially with the introduction of a large number of electronic control units, in order to improve signal utilization, a large amount of data information is required to be shared among different electronic units. A large number of control signals in the automotive integrated control system also need to be exchanged in real time. Traditional wiring harnesses are far from being able to meet this need. Introducing CAN bus technology into electric vehicles can overcome the above shortcomings and has broad application prospects. In this article, CAN bus technology is applied to the electric vehicle control system, and a universal expansion unit is used to solve the problem of circuit design complexity of the electric vehicle electronic control system, and the information of each electronic control unit is optimally combined to achieve full information sharing and improve the electric vehicle control system. performance purposes.

　　2. Characteristics of CAN bus

　　CAN belongs to the field bus category and is a serial communication network that effectively supports distributed control or real-time control. CAN bus is widely used in the field of industrial control thanks to its own technical characteristics.

　　(1) Data can be transmitted and received in several ways such as point-to-point, point-to-multipoint and global broadcast simply through message filtering, without the need for special "scheduling".

　　(2) Flexible communication methods. CAN works in a multi-master mode. Any node on the network can actively send information to other nodes on the network at any time without distinguishing between master and slave and does not require node information such as site address.

　　(3) CAN uses non-destructive bus arbitration technology. When multiple nodes send information to the bus at the same time, the node with lower priority will actively quit sending, while the node with the highest priority can continue to transmit data without being affected, thus This greatly saves the bus conflict arbitration time, especially when the network load is heavy, the network will not be paralyzed.

　　(4) Using short frame format communication, the transmission time is short, the probability of interference is low, and it has excellent error detection effect. The number of bytes per frame is up to 8, which can meet the general requirements for control commands, working status and test data in the usual industrial field. At the same time, 8B will not occupy too long bus time, thus ensuring the real-time nature of communication.

　　(5) Each frame of CAN information has CRC check and other error detection measures to ensure the reliability of data communication.

　　3. Application of CAN bus in electric vehicles

　　CAN bus has the following advantages when applied to electric vehicles.

　　(1) Reduce the number and volume of wiring harnesses required for each functional module.
　　(2) Reduce vehicle mass and cost, have high data transmission reliability and installation convenience, and expand vehicle functions.
　　(3) Some data such as vehicle speed, motor speed and SOC can be shared on the bus, so redundant sensors are removed, sensor signal lines are minimized, and the control unit can achieve high-speed data transmission.
　　(4) Functions can be expanded by adding nodes. If the data expansion adds new information, just upgrade the software.
　　(5) Monitor and correct transmission errors caused by electromagnetic interference in real time, and store fault codes after detecting faults.

　　There are currently a variety of automotive network standards that focus on different functions. To facilitate research, design and application, the SAE Vehicle Network Committee divides automotive data transmission networks into categories A, B, and C.

　　Class A is a low-speed network for sensor/actuator control, and the data transmission bit rate is usually only 1 to 10kb/s. Mainly used in electric doors and windows, seat adjustment and lighting control.

　　Class B is a medium-speed network for data sharing between independent modules, with bit rates generally ranging from 10 to 100kb/s. Mainly used in electronic vehicle information centers, fault diagnosis, instrument displays and airbag systems to reduce redundant sensors and other electronic components.

　　Class C is a multi-channel transmission network for high-speed, real-time closed-loop control, with a maximum bit rate of up to 1Mb/s. It is mainly used in systems such as suspension control, traction control, advanced engine control and ABS to simplify distributed control and further reduce vehicle body wiring harness. So far, the only automotive control LAN that meets the requirements of Class C network is CAN protocol.

　　4. Scheme design

　　1. System schematic diagram

　　Figure 1 is the schematic diagram of the CAN bus control system of an electric vehicle.

　　Figure 1 System schematic diagram The system mainly consists of a drive control module, a regenerative braking control module, a motor control module, an energy management module, a battery control module, an instrument display module and a fault diagnosis module. Information communication between various control modules is realized through CAN. In addition to the sending and receiving of instructions, some basic status information of the car (such as motor speed, battery state of charge, vehicle speed, etc.) is the data that most control units must obtain. The control unit uses broadcasting to send data to the bus.

　　If all control units send data to the bus at the same time, data conflicts on the bus will occur. Therefore, the CAN bus protocol proposes bus arbitration that uses identifiers to identify data priorities. Table 1 shows the data types received and sent by the electric vehicle electronic control unit and the procedures for other units to share this information.

　　Table 1 Data types received and sent by electric vehicle electronic control units

　　Note: T-send, R-receive 2. Module unit circuit block diagram

　　Universal expansion units (UDU) are used in hardware design of nodes on high-speed CAN. In this way, different functions of each node can be realized only by changing the software, thus simplifying the hardware system design.

　　The general expansion unit structure is shown in Figure 2.

　　Figure 2 Universal expansion unit. AT89C52 is selected as the microcontroller in the universal expansion unit. It is a low-voltage, high-performance CMOS 8-bit microcontroller. The chip contains 8kB of rewritable read-only program memory (EPROM) and 256B of random access memory. Access data memory (RAM), compatible with the standard MCS251 instruction system, with a built-in general-purpose 8-bit central processor and Flash storage unit, which can be applied to many more complex system control applications.

　　The CAN controller uses SJA1000 produced by Philips. It is an independent CAN controller used in automobiles and general industrial environments. It has all the necessary features required to complete the CAN high-performance communication protocol. The SJA1000 with simple bus connection can complete the physical layer and All functions of the data link layer. It can store a complete message that will be sent or received on the CAN bus. It also has a 64-byte extended receive buffer REFIFO. The receive buffer is larger. While the microcontroller is processing a message, it can continue to receive other incoming messages. message.

　　The bus transceiver uses PCA82C250, which provides a direct interface between the protocol controller and the physical transmission line, and can transmit data on two bus cables with differential voltage at a rate of up to 1Mb/s.

　　The maximum number of attached nodes can reach 110. Using PCA82C250 can increase the communication distance, improve the system's instant anti-interference ability, and reduce radio frequency interference. PCA82C250 and SJA1000 together form the control and interface circuit of the CAN bus.

　　3. Battery management control system design

　　The battery is a key factor affecting the performance of the entire vehicle for electric vehicles. It has a direct impact on driving range, acceleration performance, maximum gradeability and other performance. The battery control system mainly monitors the working status of the battery (battery voltage, current and temperature) and manages the working status of the battery (avoiding over-discharge, overcharge, overheating and serious voltage imbalance between single cells) in order to maximize the Take advantage of the battery's storage capacity and cycle life. Its structure is shown in Figure 3.

　　Figure 3 Structural diagram of the battery management control unit. The system mainly implements the following functions.

　　(1) Real-time monitoring of the main and auxiliary batteries. Use UDU to collect the battery voltage, current and battery temperature during the charging and discharging process of the main and auxiliary batteries to monitor the working conditions of the batteries and perform fault diagnosis.

　　(2) The UDU receives the vehicle driving status data from the bus and adjusts the motor speed and power output in real time according to the vehicle power demand; when receiving the braking information, the control unit regulates the actions of the inverter and motor, and starts the regenerative braking system recovery system. kinetic energy.

　　(3) Predict the remaining battery power and the corresponding remaining driving range. The control unit uses the collected charging and discharging current parameters and uses the corresponding algorithm to predict the remaining power. At the same time, the vehicle speed information received from the bus is used to estimate the remaining driving range, and the estimation result is sent to the instrument display unit through the bus.

　　4. System feasibility design

　　Due to the wide range of temperature changes in the car (-45~100℃), strong electromagnetic interference and other electronic noise, and harsh environment, to ensure the reliability of the system operating in the car, it is necessary to improve the fault tolerance and anti-interference capabilities of the network structure itself ability.

　　During design, a combination of software and hardware is used for anti-interference.

　　The hardware adopts electromagnetic compatibility design, focusing on dealing with interference introduced by electrostatic fields, magnetic fields, transmission lines and circuits, using methods such as filtering, decoupling, isolation, shielding and grounding, and adding power supply voltage detection, watchdog and other circuits. Specific measures are as follows.

　　(1) The transmission line uses shielded twisted pair.
　　(2) Use watchdog timer for timeout reset.
　　(3) A photoelectric isolation circuit composed of high-speed isolation device 6N137 is added between the CAN controller SJA1000 and the CAN transceiver PCA82C250, and the power supply is also isolated using a micro DC/DC module.
　　(4) Connect CANH and CANL of PCA82C250 to the CAN bus through a 5Ω resistor, which can limit the current and protect PCA82C250 from overcurrent impact. CANH and CANL are connected in parallel with a 30pF capacitor to ground, which can also filter the bus. high frequency interference.
　　(5) Damage to the transmission medium or damage to the bus driver will destroy the reliable communication of CAN. If these faults cannot be automatically detected and corresponding measures are taken to eliminate them, the system will partially or even completely lose its communication capabilities. An effective way to solve this problem is to use redundant communication control to ensure the normal operation of the main functions of the communication system, thereby improving the reliability of the system.

　　In terms of software, technologies such as fault comparison and fault tolerance are used to perform software filtering on signals, design power-on reset anti-interference procedures, and use effective insurance and other technologies to design anti-instant interference procedures.

　　5. Conclusion

　　Introduces the characteristics of CAN bus and its application in electric vehicles, designs the node settings of the electric vehicle vehicle control system based on CAN bus, and introduces a universal expansion unit to simplify the system hardware design, and also controls the battery management control unit that affects the performance of electric vehicles. Optimized design was carried out. The system has the advantages of compact structure, high reliability, complete functions and low cost, and can better meet the working requirements of electric vehicles.