1. Introduction Fieldbus technology has become a new communication method due to its low cost and ability to meet the communication requirements of industrial field environments. Among them, CAN (Controller Area Network) bus is a popular, advanced, and high-performance fieldbus technology. Electric vehicle energy management systems need to detect and exchange large amounts of data. Using hard-wired signal lines is difficult to solve this problem, and is cumbersome, complex, and costly. Using CAN bus to realize its internal data communication is an effective method. This paper studies a CAN network node—an electric vehicle energy recovery module—and introduces the design of a DC-DC controller based on the CAN bus. 2. CAN Bus The CAN bus is a serial communication network designed by Bosch, Germany, for automotive monitoring and control systems. It effectively supports distributed and real-time control and is a fully digital interconnect bus for field control devices, becoming an international standard (ISO 11898). Semiconductor manufacturers such as Philips, Intel, and Motorola have developed integrated devices supporting the CAN protocol, such as the 82526, SJA1000, and 68HC05X16. Because the CAN bus has strong error correction capability and supports differential transmission and reception, it is suitable for high-noise environments and has a long transmission distance. Therefore, the CAN bus is very attractive for distributed measurement and control systems in many fields, especially for small distributed control systems based on microcontrollers. It has been widely used in many fields such as industrial automation, machine tools, and automobiles. CAN has excellent performance and high reliability. The communication medium can be twisted pair, coaxial cable or optical fiber. The CAN bus has the following outstanding features: (1) It is a multi-master bus that can work in a multi-master mode, enabling the various modules of the system to achieve multi-master communication. In the multi-master mode, any node on the network can actively send information to other nodes at any time, without master and slave distinction, and the communication mode is flexible; (2) The node information on the CAN network is divided into different priorities, which can meet different real-time requirements; (3) Non-destructive priority-based bus arbitration and error definition; (4) The communication distance can reach 10km (rate 5kb/s), and the rate can reach 1mb/s (distance can reach within 40m). The most significant feature of the CAN protocol is that it breaks away from the traditional node address encoding method and instead encodes communication data blocks. This method allows different nodes to receive the same data simultaneously, and it can define 2^11 or 2^29 different data types, resulting in a huge network capacity. Simultaneously, it avoids bus "collisions." 3. Research on the application of CAN communication networks in domestic fuel vehicles has been documented in relevant literature. Our proposed CAN bus-based electric vehicle energy management system is shown in Figure 1, where each module becomes a CAN node in the system and can communicate with each other. It consists of a main controller, a battery management module, a motor drive module, an energy recovery module, and a vehicle condition monitoring module. It is mainly responsible for maintaining all battery components of the electric vehicle in optimal working condition; monitoring and controlling the motor; recovering instantaneous energy during braking; collecting operating data from various subsystems of the vehicle for monitoring and diagnosis; controlling the charging method; and providing a display of remaining energy. [align=center] Figure 1 Functional diagram of the energy management control system[/align] 4. Design of a CAN bus-based DC-DC controller 4.1 Introduction to the system functions of the energy recovery module High-level energy recovery is an important topic in electric vehicle research. Regenerative braking is a method of recovering and utilizing the kinetic energy of a vehicle, which is stored in the electric vehicle's energy storage device by the generator of the electric motor. We propose a method for regenerative braking energy recovery using supercapacitors. Currently, due to the limitations of batteries, the driving range and performance characteristics of electric vehicles on a single charge are still difficult to compare with those of gasoline vehicles. The common practice is to charge the battery to absorb the energy recovered during regenerative braking, but this method has disadvantages such as difficulty in achieving high-power charging in a short time, limited charge-discharge cycles, and high cost. Supercapacitors, on the other hand, have advantages such as high specific power, high specific weight, and large single-charge capacity, which can significantly increase the driving range of electric vehicles and effectively improve their performance characteristics during vehicle start-up, acceleration, and hill climbing. Furthermore, the use of supercapacitors can significantly extend the lifespan of the power battery, even by up to 1.5 times. In the regenerative braking experiment, a domestically produced permanent magnet brushless DC motor (18kW/288V) and two supercapacitors (350V/0.7F/400A, 400V/0.58F/400A) were used. A low-power chopper (DC/DC converter) capable of bidirectional step-up and step-down voltage conversion was designed. The DC/DC converter is a periodically switching control device between the supercapacitor and the motor. Its function is to control the charging or discharging of the supercapacitor and provide the rated voltage required by the load or supercapacitor by changing the duty cycle of the four IGBT transistors in its main circuit. When the electric vehicle starts, accelerates, and runs at constant speed, the supercapacitor discharges, supplying electrical energy to the motor, which is in a motoring state, realizing the conversion of electrical energy into mechanical energy and driving the vehicle forward. When the electric vehicle decelerates, the DC motor is required to be in regenerative braking state, that is, in regenerative braking state, charging the supercapacitor, which serves as the power source, to realize the conversion of mechanical energy into electrical energy. 4.2 Design of DC-DC Controller Hardware System To achieve the above control requirements, the hardware schematic diagram of the DC-DC converter controller is shown in Figure 2. [align=center] Figure 2 Hardware schematic diagram of DC-DC controller[/align] The main functional modules are: (1) Measurement and control module The CPU adopts an 80C196KC microcontroller. The voltage and current signals are calibrated by the sensor and signal conditioning circuit to be suitable for the microcontroller A/D converter to collect. It mainly measures and monitors the accelerator pedal, brake pedal, main circuit voltage and current, and supercapacitor voltage and current signals of electric vehicles. (2) Storage information module Expanded EPROM 32K×8-bit UV erasable electrically programmable read-only memory 27256. (3) Signal output module This system requires the output of four PWM waveforms and six control modes for bidirectional boost and buck conversion of the main circuit. The programmable logic device gal16v8 is directly connected to the PWM port of 80c196kc to realize four PWM outputs. The duty cycle of the four IGBT transistors in the main circuit is controlled in a time-division manner to regulate the voltage. (4) Communication interface module The communication interface extended by the controller is the CAN bus interface. The CAN bus interface extension adopts the CAN communication controller SJA1000 + high-speed optocoupler 6N137 + CAN bus transceiver 82C250 circuit. It can be connected to the RS232C serial port of the host computer through the MAX232 chip to realize bidirectional communication between the host computer and the controller. Its circuit schematic is shown in Figure 3. [align=center] Figure 3 CAN bus interface circuit schematic[/align] 4.3 DC-DC controller software system design The function of the system software is to judge the operating status of the electric vehicle. If the car accelerator pedal is pressed, the supercapacitor works in the discharge mode and the electric boost and buck subroutines are adjusted; if the car brake pedal is pressed, the supercapacitor works in the charging mode and the electric boost and buck subroutines are adjusted. [align=center]Figure 4 Main Program Flowchart[/align] [align=center]Figure 5 Interrupt Service Routine Flowchart 1[/align] [align=center]Figure 6 Interrupt Service Routine Flowchart 2[/align] To facilitate software writing and debugging, as well as the modification and analysis of control algorithms, the software adopts a modular structure. The system software consists of a main program, subroutines, and interrupt service routines. Figures 4, 5, and 6 show the software flowcharts of the main program and two interrupt service routines, respectively. Initialization should set initial values ​​and global variables, initialize the interrupt vectors used by each interrupt service routine, set the software structure, and reset the priority order. The A/D sampling interrupt service routine uses the CAM lock bit of the 80C196KC's high-speed output HSO to periodically start the ACH0 channel. The acceleration/deceleration interrupt service routine uses the 80C196KC's high-speed input HIS to record the time of an external event, used to determine acceleration and deceleration signals, making the program writing very concise. 5 Conclusion The hardware and software of this system adopt modular design, which has good versatility and flexibility. It can be used as a development platform and is easy to expand. It is an open distributed control system that is easy to realize human-machine dialogue and remote communication. 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