Electric vehicle AC charging pile system design

Abstract: This paper briefly introduces the content and research status of AC charging piles for electric vehicles. A control system solution based on STM32F107VCT6 microcontroller is designed, and the software and hardware design of the control system and the electrical part design of the pile are introduced in detail. The system provides a variety of functions including human-computer interaction, charging control, energy metering, IC card payment, ticket printing, operation status monitoring, charging protection and charging information storage and uploading to meet the requirements of the charging process.
Key words: electric vehicle; AC charging pile; STM32F107VCT6; vehicle charger

0 Introduction With the deepening of the global energy crisis, the depletion of petroleum resources and the increasing harm of air pollution and global temperature rise, governments and automobile companies generally recognize that energy conservation and emission reduction are the future direction of automotive technology development, and the development of electric vehicles. It will be the best way to solve these two problems. China attaches great importance to the development of electric vehicles, and the state has successively issued a series of standards to support and standardize the development of electric vehicles. However, there is still a long way to go to realize the large-scale popularization of electric vehicles in China. There are still many problems to be solved. In the recently released draft of the "Energy Conservation and New Energy Vehicle Industry Plan", it is pointed out that pure electric vehicles will be the main strategic orientation. Experts pointed out that there are three major bottlenecks in the development of pure electric vehicles: one is the lack of standards, the other is imperfect supporting policies, and the third is the orderly advancement of infrastructure planning and construction. The electric vehicle AC charging pile studied in this paper is of great significance for promoting the popularization of electric vehicles as part of the charging infrastructure.

1 Electric vehicle AC charging pile Introduction AC charging pile, also known as AC power supply unit, is fixed on the ground or wall, installed in public buildings (office buildings, shopping malls, public parking lots, etc.) and residential community parking lots or charging stations. The transmission mode provides a human-machine interaction interface and an AC charging interface for an electric vehicle having an in-vehicle charger, and has a dedicated device for corresponding measurement and control protection functions. The AC charging pile adopts a large-screen LCD color touch screen as a human-computer interaction interface, and can select four modes of charging, constant time, fixed amount, and automatic (full), with running status monitoring, fault condition monitoring, charging time-sharing, Historical data recording and storage functions. The AC working voltage (220±15%)V of the charging pile and the rated output current (AC) are 32 A (seven-core socket). It takes about 6-8 hours for the AC charging pile of ordinary pure electric car to be fully charged. The charging pile is more suitable. Charge at slow speed. The AC charging pile is generally composed of a pile body, an electrical module, a metering module, and a account management module. Depending on the installation method, the pile can be divided into two types: floor type and wall type. Floor-standing charging piles are suitable for ground installation in various parking lots and on-street parking spaces; wall-mounted charging piles are suitable for wall mounting in fixed buildings such as crowded spaces and surrounding walls, such as underground parking lots or garages.

2 AC charging pile system working principle According to the requirements of GB/T 20234.2-2011 "electric vehicle conduction charging connection device; AC charging interface", the control guiding circuit is adopted as the connection state of the charging connection device. And the judgment device of the rated current parameter. Its typical control and guiding circuit is shown in Figure 1.

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After the power supply device plug is connected to the socket, the power supply control device determines whether the power supply plug and the power supply socket are completely connected by the voltage value of the detection point 4 shown in FIG. At the same time, the electric vehicle control device determines whether the vehicle plug and the vehicle socket are completely connected by measuring the resistance value between the detection point 3 and the PE. After the connection state detection of the plug and the socket is completed, the operator completes the charging start setting of the power supply device, and the switch S1 is switched from the connection +12 V state to the PWM connection state, and the power supply control device issues a PWM signal. The power supply control device determines whether the charging connection device is fully connected by measuring the voltage value of the detection point 1. After the vehicle control terminal detects the error, the S2 is closed. The power supply control device determines whether the vehicle is ready by measuring the voltage value of the detection point 1 again. If the requirement is met, the AC power supply circuit is turned on by closing K.

3 AC charging pile system scheme system consists of LCD touch screen, printer, RS 485 interface energy meter, leakage protection circuit breaker, AC contactor, card reader and LED lights. The LCD touch screen can provide a friendly man-machine interface and quick and easy operation to meet the customer's requirements for charging electric vehicles in different ways. It can display the current charging status, charging power and charging cost. The friendly user interface allows customers to Make the appropriate selection. When the collected voltage exceeds the overvoltage protection setting or is lower than the undervoltage protection setting, the charging post stops charging. The leakage protection circuit breaker can ensure that the charging is stopped in the event of an emergency such as a leakage during charging. When an unexpected situation requires emergency stop charging, the emergency stop button can be used to interrupt charging. The electrical connection diagram of the system is shown in Figure 2.

4 control system unit circuit
4.1 The main controller selects the main controller to select STMicroelectronics' STM32F107VCT6 microcontroller. The STM32F107VC interconnect family uses the high-performance ARM Cortex-M3 32-bit RISC core operating at 72 MHz. The device includes two 12-bit ADCs, four general-purpose 16-bit timers, and a PWM timer. It also includes standard and advanced communication interfaces: up to two I2Cs, three SPIs, two I2Ss, and five USARTs. One USB and two CANs, the device also provides an Ethernet interface, which greatly facilitates circuit design.
4.2 Serial Interface Circuitry A total of four serial interfaces are used to communicate with the energy meter of the LCD touch screen, thermal printer, card reader and RS 485 interface. The LCD touch screen and thermal printer are RS 232 level, and the level conversion and MCU communication. The communication protocol of LCD touch screen and MCU adopts Modbus RTU communication protocol, MCU as the host and LCD touch screen as the slave. The thermal printer communicates according to the protocol provided by the printer module. The card reader is TTL level and can be directly connected to the MCU and communicated using the protocol provided by the card reader module. The meter for charging metering uses a multi-function single-phase meter. The meter uses a 2.0-level energy meter with a current specification of 5 (40) A. The meter provides an RS 485 interface and communicates with the MCU via the DL/T 645-2007 communication protocol. By reading the electric energy value of the electric energy meter as the electric energy metering value of the charging pile, the current and voltage values ​​of the electric meter are read to judge whether overcurrent and overvoltage occur during the charging process, and are processed. The circuit diagram of the energy meter interface is shown in Figure 3.

4.3 CAN bus interface circuit According to the relevant instructions in the draft of the "Electric Vehicle Car Charger and AC Charging Pile Communication Protocol", the consultation draft recommends that the communication system between the car charger and the AC charging pile adopts the CAN bus. So design the CAN bus interface. The data link layer provides reliable data transmission between the physical connections. The data frame format between the in-vehicle charger and the AC charging post of the system conforms to the CAN bus 2.0B version, and uses the 29-bit identifier of the CAN extension frame. The corresponding definition of each bit allocation and the transmission protocol and other functions are in accordance with SAE J1939-21.
4.4 Charging Voltage Measurement Circuit Voltage measurement first requires the measurement of the transformer to convert the voltage and current into small signals that can be measured. For example, for the measurement of the voltage signal of 220 V, the transformer ratio is 2 mA/5 mA. Using the circuit shown in Figure 4, the output of the transformer is exactly 5 mA at 220 V. Ignoring the effect of large resistance shunt, 27 Ω is equivalent to a sampling resistor. Since the sampled signal is AC, the signal has positive and negative points, and the input range of the A/D converter is 0 to 3.6 V, so the sampling voltage cannot be directly input to the A/D converter. Connect a positive reference voltage to the positive input of the op amp and select the appropriate amplification factor so that the output can be resolved in the input range of the A/D converter. After quasi-synchronous sampling, the data is calculated using a rectangular self-convolution window.

4.5 Control Guide Circuit The control guide circuit completes the connection confirmation of the charging pile and the electric vehicle before charging, the identification of the power supply and the current carrying capacity of the charging connection device, and the monitoring of the charging process. The MCU judges the state by detecting different voltage values ​​of the points, and its circuit schematic is shown in FIG. 5.

5 The electric part of the pile is designed. The electrical part of the AC charging pile mainly completes the functions of charging control and protection of the charging process. It has protection functions such as leakage protection, short circuit protection, over current, over voltage and under voltage protection. In addition to short circuit and leakage protection, other protection functions are realized by the charge controller control contactor to achieve self-recovery; short circuit and leakage protection are realized by miniature circuit breaker with leakage protection. In addition, the system also has a lightning protection module. The lightning protection module has a nominal discharge current of not less than 20 kA and a protection voltage level of less than or equal to 1.5 kV. When the single-phase power supply is used, the wiring mode of the lightning protection module is P+N. The charging post has an emergency stop button to force the termination of charging in an emergency.

6 Software design The charging pile is interactively controlled through the touch screen. If the card is swiped during operation, the interrupt is triggered to read the card, and the type of the card is determined to perform related operations. The charging mode provides a variety of options to set the time, battery, amount of charge, or set to fill directly. The overall flow chart of the program is shown in Figure 6.

7 Conclusions This paper analyzes the hardware design and software design of the AC charging pile control system, and describes the design of the electrical part of the charging pile. The system uses STM32F107VCT6 as the control core, and realizes various perfect functions such as human-computer interaction, charging control, energy metering, IC card payment, ticket printing, operation status monitoring, charging protection, and charging information storage and uploading. The system can meet the general slow charging requirements of electric vehicles, and it is of great significance to promote the popularization of electric vehicles as part of the charging infrastructure.