Automotive RFID system based on short-range wireless communication technology
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This system is a wireless identification system based on the principle of digital communication, using an integrated single-chip narrowband UHF transceiver. The basic working principle and hardware design ideas of the radio frequency identification system are described, and the flow chart of the program design scheme is given. From the perspective of low power consumption, high-efficiency identification and practicality, the RFID tag suitable for vehicles is designed. The test results show that the system can achieve effective identification within a range of 300m under the conditions of complex road conditions (busy roads), and can achieve effective identification within a range of 500m under the condition of line-of-sight.
The Internet of Things refers to the use of various information sensing equipment, such as sensors, radio frequency identification (RFID) technology, global positioning system, infrared sensors, laser scanners, gas sensors and other devices and technologies to collect any necessary monitoring, The connected and interactive objects or processes collect various required information such as sound, light, electricity, biology, location, etc., combined with the Internet to form a huge network. Its purpose is to realize the connection between things and things, things and people, and all things and the network, so as to facilitate identification, management and control. This project focuses on the key issues of data acquisition, transmission and application in the vehicle Internet of Things, and conducts research to design a new generation of vehicle radio frequency identification system based on short-range radio frequency communication technology. The system consists of a short-range wireless communication on-board unit (On-Board Unit, OBU) and a base station system (Base StaTIon System, BSS) to form a point-to-multipoint wireless identification system (Wireless idenTIficaTIon system, WIS), which can be used in the coverage of the base station Vehicle recognition and intelligent guidance.
1 System hardware design
The system hardware is mainly composed of a control part, a radio frequency part and an external extension application part. With low-power MCU as the control unit, integrated single-chip narrow-band UHF transceiver, built-in optimized design antenna. It is powered by advanced photovoltaic batteries, and is a highly integrated short-range wireless identification radio frequency terminal (OBU). The terminal is small in size, low in power consumption, and has a wide range of adaptability, and establishes an open protocol and standard interface, which is convenient for docking with existing systems or other systems.
The system working diagram is shown in Figure 1
1.1 Control circuit design
The control unit adopts the MSP430 series produced by TI, which is relatively mature in low-power applications in the industry. This series is a 16-bit ultra-low-power mixed signal processor (Mired Signal Proessor) that TI began to market in 1996. Application requirements integrate many analog circuits, digital circuits and microprocessors on a single chip to provide a “single chip” solution. In the WIS system, the working principles of OBU and BSS are the same, so the OBU part of the design is introduced, and the principle diagram of the control part is shown in Figure 2.
The input voltage of MSP430F2274 is 1.8 ~ 3.6V voltage. When running under the 1mHz clock condition, the power consumption of the chip is about 200 ~ 400μA, and the minimum power consumption of the clock shutdown mode is only 0.1μA. Due to the different functional modules that are turned on when the system is running, three different working modes of standby, running and dormancy are adopted, which effectively reduces system power consumption.
The system uses two clock systems; the basic clock system and the digital oscillator clock system (Digitally Controlled Oscillator, DCO), using an external crystal oscillator (32 768 Hz). After the power-on reset, the MCU (Microprogrammed Control Unit) is first started by DCOCLK to ensure that the program is executed from the correct position and that the crystal oscillator has sufficient start-up and stable time. Then the software can set the appropriate register control bits to determine the final system clock frequency. If the crystal oscillator fails when it is used as the MCU clock MCLK, the DCO will automatically start to ensure the normal operation of the system; if the program runs away, the watchdog can be used to reset it. This design uses the on-chip peripheral module watchdog (WDT), analog comparator A, timer A (Timer_A), timer B (Timer_B), serial port USART, hardware multiplier, 10-bit/12-bit ADC, SPI bus, etc. .
1.2 RF circuit
The radio frequency part uses TI’s CC1020 as the radio frequency control unit. This chip is the industry’s first true single-chip narrow-band UHF transceiver. There are three modulation modes: FSK/GFSK/OOK. The minimum channel interval is 50 kHz, which can meet multiple channels. Strict requirements for narrowband applications (402-470mHz and 804-94OmHz frequency bands), multiple working frequency bands can be switched freely, and the working voltage is 2.3-3.6 V, which is very suitable for integration and extension to mobile devices as wireless data transmission or electronic tags. The chip complies with EN300 220.ARIB STD-T67 and FCC CFR47 part15 specifications.
The carrier frequency frequency 430mHz is selected as the working frequency band. This frequency band is the ISM frequency band, which conforms to the standards of the National Wireless Management Committee. There is no need to apply for frequency points. The FSK modulation method is adopted, which has high anti-interference ability and low bit error rate, and adopts forward error correction channel coding technology to improve the ability of data to resist burst interference and random interference. The bit error rate in the channel is 10-2 When, the actual bit error rate can be 10-5～10-6. The data transmission distance can be up to 800m under the condition of open field of sight, the baud rate is 2A Kbs, and the large suction cup antenna (length 2m, gain 7.8 dB and height of 2m from the ground). The standard configuration of the RF chip can provide 8 channels to meet a variety of communication combinations. Due to the use of narrowband communication technology, communication stability and anti-interference are enhanced. The schematic diagram of the radio frequency part is shown as in Fig. 3.
1.3 System power supply
The power supply part of the system is powered by a combination of photovoltaic batteries as daily work power and lithium sub-batteries as backup batteries. The energy storage battery is charged by solar energy under better light conditions, and a certain amount of light is guaranteed every day to basically meet the daily work needs of the OBU, which greatly extends the service life of the backup battery and at the same time extends the working life of the OBU. It is suitable for vehicles that often run outdoors, and can collect enough sunlight for photovoltaic cells to work.
1.4 System development environment
The system development environment is as follows:
1) IAR Embedded Workbench formSP430 compiler;
2) PADS PCB Design Solutions 2007 Bisi circuit board design tool.
2 System programming
The program adopts modular design and is written in C language. It is mainly composed of 4 parts: main program module, communication program module, peripheral circuit processing module, interrupt and storage module. The main program mainly completes the initialization of the control unit, the configuration of various parameters, and the configuration and initialization of various peripheral modules; the communication program module mainly deals with the configuration of the RF chip and the 433mHz transceiver processing; the peripheral circuit processing module mainly controls the external LED instructions and voltage of the system Detection, sound prompt and other processing; interrupt and storage module mainly deal with system interruption and record storage. The main program flow is shown in Figure 4.
3 RF communication process
The communication process between OBU and BSS is divided into 3 steps: link establishment, information exchange and link release, as shown in Figure 5.
Step 1: Establish a connection The coordinate information of the location where the OBU is located and its ID code are stored in the Flash of the control unit MCU through preset parameters, and are stored for a long time. BSS (base station system) uses the downlink to cyclically broadcast positioning (base station identification frame control) information to the OBU to determine the frame structure synchronization information and data link control information, and the OBU that enters the effective communication area is activated and then requests to establish a connection And confirm the validity and send the response message to the corresponding OBU, otherwise it will not respond;
Step 2: Information exchange This design uses the method of detecting the strength of the radio frequency signal to determine whether the OBU has entered the service area. When the detected signal strength is greater than 1/2 of the maximum signal, the sender and receiver realize a wireless handshake. At this time, the OBU is considered to have entered service. Area. In this stage, all frames must carry the private link identification of the OBU and implement error control. For the judgment of the uplink and downlink of the OBU, the ID number can be used to determine whether it belongs to the same system, and the OBUs that are not the ID number of the same system are automatically deleted from the record. When OBU reports information, it uses a frequency hopping mechanism to randomly select a fixed channel in the service area for handshake communication to prevent channel congestion.
Step 3: When the connection is released and the detection signal strength is less than 1/2 of the maximum strength, it is considered that the car has left the station. After the RSU and OBU complete all applications, delete the link identification and issue a dedicated communication link release command, and the connection release timer will release the current connection according to the application service confirmation.
4 Development of the communication process between OBU and BSS
The communication protocol establishes a three-layer simple protocol structure based on the seven-layer protocol model of the Open System Interconnection Architecture, namely the physical layer, the data link layer and the application layer.
1) Physical layer The physical layer is mainly a communication standard. As a unified standard for 433mHz short-distance wireless communication has not yet been formed in the world, the physical layer defined by various standards is not the same, as shown in Table 1. Figure 6 shows the Manchester encoding method.
2) Data link layer The data link layer controls the information exchange process between OBU and BSS, establishes and releases the data link connection, defines the data frame and synchronizes with the frame, controls the frame data transmission, fault tolerance control, and data The link layer control and the parameter exchange of the link connection have been stipulated. Data transmission is carried out by data frame transmission, as shown in Figure 7.
3) Application layer The application layer formulates standard user function programs, defines the format of communication messages between various applications, and provides an open message interface for other databases or applications to call.
5 concluding remarks
The radio frequency identification system designed in this article uses TI’s low-power series MSP430 microcontroller, which is specifically designed by TI for low-power battery-powered equipment. The RF chip is also TI’s CC1020, which has a high level of integration, which can achieve small size, low power consumption, and easy installation, and is suitable for building a vehicle parking-free monitoring and monitoring system.The test results show that effective recognition can be achieved within 300m in complex road conditions (busy roads), and recognition can be achieved within 500m in line-of-sight conditions.
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