Automotive RFID system with short-range wireless communication technology

This system is a wireless identification system based on the principle of digital communication and using an integrated single-chip narrowband ultra-high frequency transceiver. The basic working principle and hardware design ideas of the radio frequency identification system are explained, and the flow chart of the program design scheme is given. Design radio frequency identification tags suitable for vehicles from the perspectives of low power consumption, efficient identification and practicality. The test results show that this system can achieve effective recognition within a range of 300m under complex road conditions (busy road conditions), and can achieve effective recognition within a range of 500m under line-of-sight conditions.

The Internet of Things refers to the real-time collection of any information that needs to be monitored, through various information sensing equipment, such as sensors, radio frequency identification (RFID) technology, global positioning systems, infrared sensors, laser scanners, gas sensors and other devices and technologies. Connecting and interacting objects or processes collects various required information such as sound, light, electricity, biology, location, etc., and combines it 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 collection, transmission and application in the vehicle Internet of Things, and designs a new generation of vehicle radio frequency identification system based on short-range wireless radio frequency communication technology. The system consists of a short-distance 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 within the coverage area of the base station. Vehicle identification and intelligent guidance.

1. System hardware design

The system hardware is mainly composed of the control part, the radio frequency part and the external expansion application part. It uses a low-power MCU as the control unit, integrates a single-chip narrow-band ultra-high frequency transceiver, and has a built-in optimized design antenna. It is powered by advanced photovoltaic cells and is a highly integrated short-range wireless identification radio frequency terminal (OBU). This terminal has small size, low power consumption, wide adaptability, and established open protocols and standard interfaces to facilitate docking with existing systems or other systems.

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 Processor) launched by TI on the market in 1996. It is aimed at practical applications. Application requirements integrate many analog circuits, digital circuits and microprocessors on one chip to provide a "monolithic" solution. In the WIS system, the working principles of OBU and BSS are the same, so we focus on the design of OBU part.

The input voltage of MSP430F2274 is 1.8~3.6V. When running under the clock condition of 1mHz, the power consumption of the chip is about 200~400μA, and the lowest power consumption in clock shutdown mode is only 0.1μA. Since the functional modules opened when the system is running are different, three different working modes of standby, running and hibernation are adopted, which effectively reduces the system power consumption.

The system uses two clock systems; the basic clock system and the Digitally Controlled Oscillator (DCO) clock system, which uses an external crystal oscillator (32 768Hz). After power-on reset, DCOCLK first starts the MCU (Microprogrammed Control Unit) to ensure that the program starts executing from the correct position and that the crystal oscillator has sufficient start-up and stabilization time. Software can then set the appropriate register control bits to determine the final system clock frequency. If the crystal oscillator fails when used as the MCU clock MCLK, the DCO will automatically start to ensure the normal operation of the system; if the program runs away, a 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 narrowband ultra-high frequency transceiver. It has three modulation modes: FSK/GFSK/OOK. The minimum channel spacing is 50 kHz, which can meet the needs of multi-channel Strict requirements for narrowband applications (402~470mHz and 804~94OmHz frequency bands), multiple operating frequency bands can be freely switched, and the operating voltage is 2.3~3.6 V. It is very suitable for integration and expansion into mobile devices for use as wireless data transmission or electronic tags. The chip complies with EN300 220.ARIB STD-T67 and FCC CFR47 part15 specifications.

Select the carrier frequency 430mHz as the working frequency band. This frequency band is the ISM band and complies with the standards of the National Wireless Management Committee. There is no need to apply for a frequency point. Using FSK modulation method, it has high anti-interference ability and low bit error rate. It adopts forward error correction channel coding technology to improve the data's ability to resist burst interference and random interference. The channel bit error rate is 10-2 When, the actual bit error rate can be obtained from 10-5 to 10-6. The data transmission distance can reach 800m under line-of-sight conditions in an open field, a baud rate of 2A Kbs, and a large suction cup antenna (length 2m, gain 7.8 dB, height 2m above the ground). The standard configuration of this RF chip can provide 8 channels to meet various communication combination methods. 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 in Figure 3.

1.3 System power supply

The power supply part of the system is powered by a combination of photovoltaic cells as daily power supply and lithium sub-battery as backup battery. Charging the energy storage battery through solar energy under good lighting conditions, ensuring a certain lighting time every day can basically meet the daily work needs of the OBU, greatly extending the service life of the backup battery, and at the same time extending the working life of the OBU. It is suitable for vehicles that often operate outdoors and can collect sufficient 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, the configuration and initialization of each peripheral module, etc.; the communication program module mainly handles the configuration of the RF chip and 433mHz transceiver processing; the peripheral circuit processing module mainly handles the external LED indication and voltage of the system. Detection, sound prompts are handled by keystrokes and other processing; the interrupt and storage module mainly handles system interrupts 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 three steps: link establishment, information exchange and link release, as shown in Figure 5.

Automotive RFID system with short-range wireless communication technology

Step 1: Establish a connection. The coordinate information of the OBU location and its ID code are Stored in the Flash of the control unit MCU through preset parameters and are saved for a long time. The BSS (Base Station System) uses the downlink to cyclically broadcast and send positioning (base station identification frame control) information to the OBU, determine the frame structure synchronization information and data link control information, and request the establishment of a connection after the OBU in the effective communication area is activated. Confirm the validity and send response information 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 sending and receiving parties implement a wireless handshake. At this time, the OBU is considered to have entered the service area. district. In this phase, all frames must carry the OBU's private link identification and implement error control. For the judgment of OBU upstream and downstream, you can use the ID number to determine whether it belongs to the same system. OBUs with ID numbers that are not the same system will be automatically deleted from the record. The OBU uses a frequency hopping mechanism when reporting information and randomly selects a fixed channel in the service area for handshake communication to prevent channel congestion.

Step 3: Release the connection. When the detection signal strength is less than 1/2 of the maximum strength, the car is considered to have left the station. After the RSU and OBU complete all applications, they delete the link identifier and issue a dedicated communication link release command. The connection release timer releases the connection according to the application service confirmation.

4. Development of 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, data link layer and application layer.

1) Physical layer The physical layer is mainly a communication signal standard. Since there is currently no unified standard for 433mHz short-distance wireless communication in the world, the physical layer defined by various standards is also different, 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, the establishment and release of data link connections, the definition and frame synchronization of data frames, the control of frame data transmission, fault tolerance control, and data transmission. Link layer control and parameter exchange of link connections are specified. Data transmission is performed 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 calls by other databases or applications.

5 Conclusion

The radio frequency identification system designed in this article uses TI's low-power series MSP430 microcontroller, which is specially designed by TI for low-power consumption of battery-powered equipment. The radio frequency chip is also TI's CC1020. It has high integration, can achieve small size, low power consumption, and is easy to install. It is suitable for building vehicle parking-free monitoring and surveillance systems. The test results show that in complex road conditions (busy roads), effective recognition can be achieved within a range of 300m, and in line-of-sight conditions, recognition can be achieved within a range of 500m.