Electronic tag identification system using ZigBee and RFID technology

1 Introduction

RFID (Radio Frequency Identification) is an automatic identification technology. Its basic principle is to use radio frequency signals and spatial coupling transmission characteristics to automatically identify the identified object. Compared with the existing barcode technology, radio frequency identification technology has the advantages of high temperature resistance, waterproofness, repeated data writing, high security, and large data storage space. In recent years, with the rapid development of computer technology, chip technology and wireless communication technology, RFID technology has also developed rapidly. Its size, cost and power consumption are getting lower and lower. Application systems based on RFID technology are widely used in various fields of life, such as transportation, physical management, access control, positioning systems, second-generation ID cards, etc. RFID systems are generally composed of antennas, readers and electronic tags. Traditional RFID systems use readers and PC host computers to communicate through wired forms (Ethernet, RS232), which have the disadvantages of poor flexibility, short data transmission distance, and high cost. Compared with wired transmission systems, ZigBee wireless transmission technology can realize wireless two-way transmission of data information, eliminating the trouble of wiring, and ZigBee networking is efficient, fast and simple. In order to improve the transmission distance and flexibility of the RFID system and reduce the system cost, an electronic tag identification system was designed by combining ZigBee and RFID technology. System tests show that the system has the advantages of low cost, high flexibility, long transmission distance, and low power consumption, which expands the application of ZigBee technology in wireless RFID systems.

2 Overall system design

The system hardware structure mainly consists of five parts: active electronic tags, master-slave RF modules with nRF24LE1 chip as microprocessor, ZigBee terminal nodes, ZigBee coordinator nodes and PC host computer. Figure 1 shows the overall structure of the system. Active electronic tags: record the ID number of the electronic tag and other item data information; master-slave RF modules: RFID readers, responsible for identifying the electronic tag data information within the antenna radiation range, and transmitting the received electronic tag information to the ZigBee terminal node through the serial port, and can also receive control commands transmitted by the ZigBee terminal node. The master RF module receives the electronic tag ID information identified by the slave RF module through SPI to achieve dual-channel transmission, which has better data accuracy and reliability; ZigBee terminal node: sends the data information of the electronic tag identified by the master and slave RF modules to the ZigBee coordinator node wirelessly, and the ZigBee terminal node controls the master and slave RF modules according to the control instructions transmitted by the coordinator, so as to achieve corresponding processing of the electronic tag; Coordinator node: transmits the electronic tag data information sent by the ZigBee terminal node to the host computer through the serial port RS232, and forwards the control instructions of the host computer to the ZigBee terminal node; PC host computer: has corresponding application software, processes the tag information from the ZigBee coordinator node and sends control information to the ZigBee coordinator node.

Figure 1 System overall structure diagram

3 System Hardware Design

3.1 System master and slave RF module circuit design

The master-slave RF module of the system is the core part of the RFID reader. It receives the control instructions sent by the host computer from the ZigBee coordinator node through the serial port, thereby controlling the RF chip to communicate with the electronic tag to complete the reading and writing of the electronic tag. The RF chip is responsible for the encoding and decoding, modulation and demodulation of wireless signals; the electronic tag is the application terminal of the system, which carries the data information of the object and the tag's own information. The wireless pulse sent from the reader antenna receives the control information sent by the reader, and then returns the data information of the electronic tag to the reader through the antenna to complete the reading and writing of the electronic tag data by the reader. The design of the master-slave RF module circuit ensures the accuracy and reliability of the electronic tag information recognized by the reader. The RF module circuit uses the nRF24LE1 chip, which is a wireless transceiver chip with an enhanced 8051 core launched by Nordic. It can work in the 2.4-2.5GHz ISM frequency band, does not require any channel communication fees, and users do not need to apply for a frequency use license, which is convenient for user application and development. The maximum air transmission rate is 2Mbps, the sensitivity is -94dBm, and the maximum signal transmission power is 0dBm. Under ideal conditions, the indoor transmission distance can reach 30-40 m, and the outdoor transmission distance can reach 100-200 m. The operating voltage is 1.9~3.3V, which greatly reduces the power consumption of the system. The combination of processor power, memory, low-power crystal oscillator, real-time real-name, counter, AEC encryption accelerator, random number generator and power saving mode provides an ideal platform for implementing RF protocols. For the application layer, nRF24LE1 provides a wealth of peripherals such as SPI, IIC, UART, 6 to 12-bit ADC, PWM and an ultra-low power analog comparator for voltage level system wake-up. One master SPI and one slave SPI realize dual-channel data communication of RFID system. nRF24LE1 integrates Enhanced ShockBurst technology, in which parameters such as communication channel, output power and automatic retransmission times can be set programmatically. The master and slave RF module circuits of the system are basically the same, and can be set as the master RF module by software, as shown in Figure 2.

Figure 2 RF circuit hardware structure diagram

3.2 ZigBee terminal node circuit design

The ZigBee terminal node is the hardware core of the contactless RFID reader and ZigBee wireless module in the system. It mainly controls the data exchange between the electronic tag and the master and slave radio frequency modules and the data communication with the ZigBee coordinator node. The terminal node circuit uses a 32MHz crystal oscillator as the clock signal and realizes data communication with the master and slave radio frequency modules through a serial port connection. The ZigBee terminal node uses the CC2530 chip, which is a chip launched by TI that can realize 2.4GHz IEEE 802.15.4 radio frequency transceiver. It has the characteristics of high sensitivity and strong anti-interference ability. In particular, the CC2530 chip has ultra-low power consumption. In passive mode (RX), the current consumption is 24mA, and in active mode (TX), the current consumption is 29mA. It has three modes. The current consumption of mode 1, mode 2 and mode 3 are 0.2mA, 1uA and 0.4uA respectively, which is particularly suitable for those occasions requiring low power consumption. It also has a wide power supply voltage range of 2V-3.6V. It contains an 8-bit MCU (8051), 8KB of RAM, a 12-bit analog-to-digital converter (ADC) with 8 inputs and configurable resolution, a MAC timer that complies with the IEEE 802.5.4 specification, a conventional 16-bit timer and an 8-bit timer, an AES-128 coprocessor, a watchdog timer, a sleep mode timer with a 32kHz crystal oscillator, a power-on reset circuit, a power-down detection circuit, and 21 programmable I/0 pins. Figure 3 shows the hardware circuit diagram of the ZigBee terminal node.

Figure 3 ZigBee terminal node hardware structure diagram

3.3 ZigBee coordinator node circuit design

The ZigBee coordinator node is responsible for communicating the data sent by the ZigBee terminal node with the host computer through the RS232 serial port line, and at the same time, it receives the control instructions transmitted by the host computer and sends them to the ZigBee terminal node. The ZigBee coordinator circuit diagram is consistent with the ZigBee terminal node circuit, as shown in Figure 3. It only needs to be set as a coordinator in the Z-stack protocol stack. Since CC2530 uses TTL level and PC communication uses EIA level, the system uses MAX232 chip to achieve level conversion to ensure effective communication of the system, as shown in Figure 4.

Figure 4 MAX232 level conversion circuit diagram

4 System Software Design

4.1 ZigBee terminal node software design

The main function of the terminal acquisition node is to collect electronic tag data information after receiving data acquisition instructions from the host computer, and send the collected data information to the coordinator node. First, the ZigBee terminal node is powered on and initialized, and applies to join the established ZigBee network. If the joining network is successful, it enters the low-power mode, that is, the sleep state, to reduce the power consumption of the terminal node. Waiting for the timing interrupt to be generated, the ZigBee terminal node microprocessor controls the master-slave RF module to read the electronic tag information, and transmits the identified tag data information to the ZigBee coordinator node through the ZigBee wireless module, and then transmits it to the host computer through the serial port RS232 for processing. The program flow chart of the terminal acquisition node is shown in Figure 5. [page]

Figure 5 ZigBee terminal acquisition node software flow chart

4.2 ZigBee Coordinator Node Software Design

The system uses the Z-STACK protocol stack of the ZigBee network for wireless communication. The Z-STACK protocol is implemented based on a round-robin query operating system. After the coordinator node is powered on, the hardware and protocol stack are initialized, the channel is searched and the idle channel is evaluated, the channel is selected and the ZigBee network is established. If the node applies to join the network, it is allowed to join and a 16-bit network short address is assigned, waiting for the data collection instruction sent by the host computer, and then the RFID reader recognizes the electronic tag and sends all received data packets to the PC host computer through serial communication for data processing. The software flow chart of the ZigBee coordinator node is shown in Figure 6.

Figure 6 ZigBee coordinator software flow chart

4.3 Host computer application software design

The host computer application software of this system is written in Visual Basic, which is a structured, modular, object-oriented, and visual programming language developed by Microsoft that includes an event-driven mechanism to assist in the development environment. The host computer application software interface is shown in Figure 7. By using the host computer application software to send command data to the electronic tag, it can realize the reading of the electronic tag ID information, the modification of the signal transmission power, and the switching of the working state.

The source code for setting the tag transmission signal power is as follows:

ReDim bytbyte(1)

bytbyte(0) = 221

bytbyte(1) = 17 - 2 * Val(Form3.Combo_rssi.Text)

Form3.MSComm1.Output = bytbyte()

The source code of the program for setting the label working status is as follows:

ReDim bytbyte(1)

bytbyte(0) = 221

bytbyte(1) = 17 * (Val(Form3.Combo_sta.ListIndex) + 1)

Form3.MSComm1.Output = bytbyte()

5 Test Results

In order to verify the reliability and stability of the experimental results, the system was tested indoors and outdoors. The indoor test mainly detected the transmission distance of the system penetrating the wall, and the outdoor test mainly detected the transmission distance of the system without obstacles. The host computer software sends control instructions to the electronic tag to change the signal transmission power of the electronic tag to achieve the farthest transmission distance of the electronic tag signal, so as to better achieve the balance point of reducing the power consumption of the electronic tag and maximizing the transmission distance. Under different signal transmission power conditions, the electronic tag signal transmission distance is shown in Table 1.

From the test results in Table 1, it can be seen that when the electronic tag signal transmission power is 0dBm (maximum signal transmission power), the electronic tag signal transmission distance is 30-65m outdoors and 25-50m indoors. Under the condition that the electronic tag signal transmission power is 0dBm, the electronic tag ID numbers 1 and 2 represent indoor and outdoor respectively, and the test results are shown in Figure 7.

Figure 7 System test results

Under different indoor and outdoor conditions, the system ZigBee wireless module can effectively transmit tag data information within a range of 200 meters, which improves the system transmission distance and has broad application prospects. The test results are shown in Table 2.

6 Conclusion

An electronic tag identification system was designed by using ZigBee and RFID technologies. A low-power design method was adopted in the system hardware and software design. The low-power design of the ZigBee node was realized by using CC2530 as the microprocessor of the ZigBee node. The nRF24LE1 was used as the electronic tag chip, achieving a balance between reducing power consumption and maximizing the signal transmission distance. The host computer application software developed based on Visual Basic language can read, write and control the electronic tag. The system test shows that under different indoor and outdoor environments and different signal transmission power conditions of the electronic tag, the signal transmission distance of the electronic tag that can penetrate the wall is 25-50m indoors, and the signal transmission distance of the electronic tag outdoors is 30-65m. The ZigBee wireless module based on the ZigBee protocol stack can effectively transmit data within a range of 200 meters, which improves the transmission distance of the system. At the same time, the ZigBee technology networking is simple and efficient, which not only reduces power consumption and cost, but also saves the trouble of wiring, so that ZigBee technology can be applied in wireless radio frequency identification and expands the application scope of ZigBee technology in wireless RFID systems.