"In recent years, with the continuous development and progress of information technology, the Internet of things technology has developed by leaps and bounds and has broad application prospects. As an important technology in the Internet of things, radio frequency identification (RFID) has attracted more and more attention with the in-depth development of the Internet of things. With the continuous introduction of low-power and high-performance integrated RF transceiver chips, miniaturized, modular and high-performance UHF RFID readers came into being.
This paper mainly designs a high-performance multi antenna structure reader based on R2000. Like the general UHF RFID system product development process, it analyzes, determines the index parameters, formulates the system scheme, hardware circuit design_ The upper computer software design, system integration, whole machine performance test and product formation are carried out in sequence. The product development process of the whole project is shown in Figure 2-1.
(1) Application requirements analysis
At this stage, we mainly understand the market demand through market research. The reader product involved in this paper is designed according to the needs of a company in Zhongshan City, Guangdong Province. It mainly faces two application fields: warehouse management and logistics supply chain management. These two application fields require readers and writers to realize long-distance leak free identification of electronic tags, and require high leak free identification speed.
(2) Determine index parameters
After understanding the application needs of market units, design a reader / writer that can realize high-speed, leak free and remote identification of electronic labels. Therefore, it is necessary to first determine the overall parameter index requirements of the circuit, and refine the index into various parts, so as to determine the selection range of devices and facilitate the overall scheme design.
(3) Formulate system scheme
After determining the circuit index parameters, it is necessary to select appropriate chip devices according to the requirements of each circuit module, mark the key pins of the devices, and connect the circuit block diagrams of each part to form the whole system scheme. In this process, we need to consider the enforceability of the system scheme and optimize the design and R & D cost as much as possible.
(4) Design hardware circuit
When formulating the system scheme, it only roughly describes the block diagram of each part of the circuit. In the process of hardware circuit design, it is necessary to design the schematic diagram of each circuit and the interface form of circuit module in detail, so as to make preparations for the composition of the circuit system.
(5) System integration
At this stage, it is mainly to connect the circuit as a whole, design the schematic diagram and PCB layout of the whole circuit, gradually weld the circuit and debug the circuit to ensure that each part of the circuit can work normally, and finally combine each circuit module to form a circuit system.
(6) Overall performance test
After the circuit debugging is passed, place the circuit module in the grinding tool of the reader writer, burn and configure the corresponding driver and control program, simulate the actual application environment and test the performance of the whole machine.
(7) Form products
After the performance test of the whole machine is completed, the stability and high and low temperature test shall be carried out. After all the tests are passed, it shall be pasted_ Put the product in the packing box and prepare for shipment.
2.2 composition of UHF RFID system
UHF RFID system has four important components: host computer system, reader, antenna and electronic tag. The working process of this system is 0-1:. The upper computer sends instructions to the reader / writer. Under the control of the upper computer's instructions, the reader / writer turns on the RF module to send high-frequency signals, and forms a magnetic field around the RFID electronic tag through the antenna. When the magnetic field strength is large enough, the chip circuit inside the tag is activated, and then loads the information it carries into the electromagnetic wave and returns it to the reader / writer. The reader / writer receives and converts the data information, Then through the processing of the host computer, the acquisition process of electronic label information is completed. When the tag leaves the RF magnetic field, the tag is dormant because there is not enough energy to activate the chip circuit. The composition of the whole system is shown in Figure 2-2:
2.2.1 upper computer
The host computer in this paper mainly refers to the PC master computer integrated in the whole equipment and communicating with the RF module. The UHF RFID reader / writer needs the upper computer to send control instructions to start the reader / writer to send RF signals. The reader / writer also needs the upper computer to process and store the signals returned by the tag, so as to realize the normal operation of the whole UHF RFID system.
2.2.2 reader writer
Ultra high frequency reader (Reader), namely RF module, is the key component of the whole RFID system. Firstly, the RF module determines the working frequency band of the whole system; Secondly, the transmitting power and receiving sensitivity of the reader directly affect the maximum recognition distance of the whole system. RF transceiver circuit, digital baseband control circuit and power management module are the main components of the reader. The RF transmission link mainly generates high-frequency carrier signals to complete the modulation and power amplification of baseband signals, so as to provide sufficient energy for electronic tags. The RF receiving circuit realizes the demodulation and low-noise amplification of the return signal of the electronic tag. The digital baseband circuit is used to analyze the control instructions of the upper computer in order to control the work of the RF transceiver circuit. The power management part mainly supplies power to each circuit module to make each circuit module work normally.
2.2.3 antenna
Antenna is a passive device used to realize the mutual conversion of electromagnetic wave and current signal. Antenna is mainly used to transmit and receive high-frequency carrier signals in UHF RFID system. The antenna configuration of UHF RFID system can adopt one or two forms of separation of transceiver and combination of transceiver. Transceiver separation mainly uses two antennas to separate RF signals, - one is used to send the RF signal output by the reader and the other is used to receive the RF signal returned by the tag. In this way, the mutual interference between RF transceiver signals is small and the cost is relatively high. In this scheme, the mutual interference between RF transceiver signals is more serious, so it is necessary to effectively suppress the interference of RF carrier leakage on the received signal. Considering the actual design cost and simplified circuit structure, the reader in this design uses a directional coupler to build a transceiver separation circuit structure, and uses a circularly polarized antenna with a gain of 8dbi.
2.2.4 electronic label
Electronic tag is the carrier of data information, which is mainly encapsulated by dipole antenna, resonant capacitor and micro IC chip! 41。 The electromagnetic wave sent by the RFID reader forms a magnetic field in space. When the tag enters the magnetic field range and the magnetic field energy is large enough, the power management voltage in the tag chip relies on the electromagnetic wave energy to induce the voltage to power the tag chip, load the internal information into the electromagnetic wave and return to the reader to complete the wireless communication between the reader and the electronic tag.
The sensitivity of tag has an important influence on the reading and writing distance of RFID system. The UHF RFID reader in this design adopts e4ib warehouse logistics tag and monza4qt tag. The receiving sensitivity of the two tags is about - 15dBm (30uw), which can meet the requirements of long-distance identification.
2.3 system function analysis
Considering the limited inventory range of conventional single antenna or four antenna reader writer, this paper proposes to design a reader writer scheme with multi antenna structure, so that the antenna can cover every corner of the warehouse and ensure that the RFID electronic tags on all goods in the warehouse can be identified by the reader writer without leakage.
Since the RF transceiver system built by discrete circuit elements is too complex and the circuit volume is huge, when designing the circuit scheme, it is considered to use the highly integrated chip as the main RF transceiver chip to realize the circuit scheme. On the premise of ensuring the circuit performance, the circuit structure is simplified, and the RF transceiver circuit module is designed, which can be used independently with other upper computers, The circuit module structure of the whole set of equipment is further simplified, and the stability of the whole circuit system is improved.
At present, information security problems in RFID technology often occur, leading to the leakage of important information of businesses or individuals, which hinders the further development and popularization of RFID technology. In order to eliminate the hidden danger of information security in RFID technology, in the design process, combined with special technology, the tag encryption processing is realized through the host computer software. The password needs to be input to access the private information content of businesses or individuals. Combine the above functional requirements with the current market_ According to the characteristics of the upper reader and writer, the required performance indexes of the reader and writer designed in this paper are as follows:
(1) Operating frequency band: 840 MHz ~ 960 MHz;
(2) Maximum output power: ≥ 30 DBM;
(3) Receiver sensitivity: ≤ - 80 DBM;
(4) Identification label distance: ≥ 15m;
(5) Number of identification labels without omission: 2200 (within 10s);
(7) Power consumption of RF module: ≤ 10W;
(8) Real time display of equipment working status;
(9) Label information security.
In order to realize the above functions, the design of software and hardware circuits is considered from four aspects: high reading and writing performance, multi antenna structure, work instruction function and reading and writing information security.
2.3.1 high reading and writing performance
When designing the hardware circuit structure of UHF RFID reader, it is necessary to adopt impedance matching network and low insertion loss circuit structure to maximize the transmission of RF signal power from the source end to the load end. The maximum output power of the RF port of the designed UHF RFID reader and writer needs to reach 31 DBM, which can be fast and leak free within 10s
Identify 240 UHF RFID electronic tags, and the sensitivity of the whole reader / writer receiver is less than - 80 DBM. During the test, the antenna with 8dbi gain and e4ib electronic logistics tag with reception sensitivity of - 15dBm are used. The maximum recognizable distance of the electronic tag exceeds 15m. The identification process of UHF RFID reader is shown in Figure 2-3.
RFID reader transmits high-frequency carrier signal, which is converted into electromagnetic wave through antenna, and electromagnetic wave generally has transmission loss in space medium. The whole reader adopts a circularly polarized antenna. It is assumed that the antenna radiates energy uniformly in all directions in the curved area surrounded by the direction angle, and the energy density is the same everywhere on the curved surface. For this kind of isotropic antenna, its received power is directly proportional to the effective area of the antenna. L meters away from the antenna, the radiation area of the antenna is s, then
2.3.2 multi antenna structure
At present, most of the common UHF RFID readers on the market are single antenna or four antenna structures. These two readers are only suitable for electronic tags in a small range of inventory. For a large range of electronic labels, using multiple readers to complete the inventory task will not only bring high cost, but also solve the problem of mutual interference between signals when multiple readers work at the same time. Based on the above considerations, this paper designs a multi antenna UHF RFID reader, which can not only complete the inventory of electronic tags in a wide range, but also use time division multiplexing for each antenna port. Therefore, there is no mutual interference between the signals of each antenna port of the reader. The multi antenna circuit structure of the reader / writer is shown in Figure 2-4.
In the design process, sixteen antenna ports are configured through two-level switches. In the way of time division multiplexing, each antenna port can work independently to inventory and initialize the electronic tag. The multi antenna structure can cover a larger space area to ensure that labels in all directions can be recognized in the process of warehouse management. In order to realize the leak free identification of the tag by the reader / writer, the antenna shall be placed evenly in different directions to ensure that the antenna can cover all tags. During the test, the antenna positions of the two multi antenna readers and writers are shown in Figure 2-5.
In the process of practical application, the number of UHF RFID reader antennas and the placement position of antennas can be selected according to the actual needs, so as to ensure that the radiation range of the selected antenna can cover all RFID electronic tags, so as to ensure that all tags can be recognized by at least one antenna in the process of warehouse management, so as to realize high-speed and leak free identification in warehouse management.
2.3.3 work indication function
The conventional reader / writer cannot grasp the working state of each antenna port in real time. When an abnormal condition occurs at an antenna port, it is difficult to find it in time. Such operation for a long time will reduce the working efficiency and affect the performance of the reader / writer. In order to avoid this situation, the work indication function circuit is added when designing the hardware circuit structure of the reader / writer, In this way, the working condition of the whole reader / writer can be known in real time through the status of the indicator light.
The work indication function includes power on indication, power on indication, RF module power supply indication and working status indication of each antenna port. The power on indication is used to judge whether the whole reader is powered on; The power on indicator is used to indicate whether the upper computer system of the reader / writer is started; The RF module power supply indication is used to indicate whether the upper computer software successfully sends the drive signal to let the power management module supply power to the RF module; The antenna port indication circuit is mainly to make the sixteen indicator lights correspond to the sixteen antenna terminals one by one, so as to realize the indicator light to display the working state of each antenna port in real time.
2.3.4 read and write information security
At present, UHF RFID technology is in the development stage, and the requirements for information security in relevant application fields are not high, which leads many manufacturers and users to pay less attention to the information security in RFID system. With the continuous development and maturity of UHF RFID technology, information security will become particularly important.
In this paper, monza4qt electronic tag of Impinj company is used to consider the safe communication between reader and writer and electronic tag. The electronic tag includes two modes: private configuration and public configuration. In the case of high confidentiality and privacy, the label can be set to the private configuration mode through the host computer program. In the private configuration mode, the access password is required to read and write the private information of the label. When the UHF RFID reader / writer needs to identify the private information of the electronic tag, it needs to input the access password through the upper computer program first, and the reader / writer can identify the tag user area information only after the authentication is passed; If the reader recognizing the tag needs to change the content of the tag user area, it is necessary to enter the password for writing the tag in the upper computer program interface for verification. Using this private configuration encryption method, the tag information can be prevented from being illegally obtained, modified or even maliciously destroyed by other readers and writers.
2.4 hardware circuit design scheme
For the general UHF RFID reader hardware circuit, its circuit can be divided into data interface circuit, digital baseband control circuit and RF transceiver circuit. In this paper, the host computer main control platform is added in the reader, so the designed reader can work independently from other control platforms. The basic circuit scheme composition is shown in Figure 2-6.
The data interface circuit is connected with the upper computer, which is mainly used to burn the control program and related configuration to the digital baseband module, so as to realize the communication between the reader and the upper computer; The digital baseband circuit receives and analyzes the control command sent by the upper computer and sends information to the RF transceiver circuit (201; After receiving the signal sent by the baseband control circuit, the RF transceiver circuit generates a high-frequency carrier, performs various transformation processing on the baseband signal, radiates outward through amplification processing and receives and processes the signal returned from the electronic tag, so as to realize the acquisition of the electronic tag by the reader / writer.
2.4.1 interface circuit
Interface circuit is an important part of UHF RFID reader. It is generally composed of baseband control circuit, program configuration interface circuit and host computer communication interface circuit. Common baseband circuit program configuration interface circuits include UART, J-TAG, etc. These interface circuits realize the configuration and burning of baseband circuit programs of readers and writers. The communication interface between the reader and the upper computer is generally
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