chrispeng [ad_1]
1 Introduction
Radio frequency identification is a non-contact automatic identification technology that uses radio frequency technology. It has the advantages of fast transmission rate, anti-collision, mass reading, and movement process reading. Therefore, RFID technology is used in logistics and supply chain management, production management and control. , Anti-counterfeiting and security control, traffic management and control and other fields have great application potential. At present, the working frequency band of radio frequency identification technology includes low frequency, high frequency, ultra high frequency and microwave range, among which high frequency and ultra high frequency are the most widely used.
2 Principles of RFID technology
The RFID system is mainly composed of a reader (target), a transponder (RFID tag) and a background computer. Among them, the reader implements the reading and writing and storage of data on the tag. It is composed of a control unit, a high-frequency communication module and an antenna. It is mainly composed of an integrated circuit chip and an external antenna. The circuit chip usually contains circuits such as radio frequency front-end, logic control, and memory. According to the power supply principle, tags can be divided into active (acTIve) tags, semi-active (semiacTIve) tags and passive tags. Passive tags are popular because of their low cost and small size.
The basic working principle of the RFID system is: after the tag enters the reader to transmit the radio frequency field, the induced current obtained by the antenna is used as the power supply of the chip after the boost circuit, and the induced current with information is transformed into a digital signal through the radio frequency front-end circuit. Enter the logic control circuit for processing, and the information that needs to be replied is sent from the tag memory, sent back to the RF front-end circuit via the logic control circuit, and finally sent back to the reader through the antenna.
3 Antenna in RFID system
From the perspective of RFID technology, the key to RFID tag performance lies in the characteristics and performance of the RFID tag antenna. The key role in the data communication between the tag and the reader is the antenna. On the one hand, the chip start circuit of the tag starts to work, and it needs to obtain enough energy in the electromagnetic field generated by the reader through the antenna; on the other hand, the antenna determines The communication channel and communication method between the tag and the reader. Therefore, the research on the antenna, especially the internal antenna of the tag, has become the focus.
3.1 Types of RFID system antennas
According to the power supply mode of the RFID tag chip, RFID tag antennas can be divided into active antennas and passive antennas. Active antennas have lower performance requirements than passive antennas, but their performance is greatly affected by battery life: passive antennas can overcome the limitations of active antennas due to battery limitations, but require high antenna performance. At present, the research focus of RFID antennas is passive antennas. Divided from the working frequency band of the RFID system, in the LF, HF section f such as 6.78MHz, 13.56MHz) I make the RFID system, the transmission of electromagnetic energy is completed in the induction field area (similar to a stable field), also known as an inductive coupling system In the UHF section (such as 915MHz, 2400Mttz) Z system, the transmission of electromagnetic energy is completed in the far field area (radiation field), also known as the microwave radiation system. Because the energy generation and transmission methods of the two systems are different, the corresponding RFID tag antennas and front-end parts have their own particularities, so the tag antennas are divided into near-field induction coil antennas and far-field radiation antennas. The inductive coupling system uses a near-field induction coil antenna, which is composed of a multi-turn inductance coil. The inductance coil and a capacitor connected in parallel form a parallel resonant loop to couple the maximum radio frequency energy; the far-field radiation antenna used in the microwave radiation system is mainly They are dipole antennas and slot antennas. The far-field radiating antenna is usually a resonant type, usually half-wavelength. The shape and size of the antenna determine the performance of the frequency range it can capture. The higher the frequency, the more sensitive the antenna and the less area it occupies. A higher operating frequency can have a smaller tag size, and compared with a near-field induction antenna, the radiation efficiency of a far-field radiating antenna is higher.
3.2 Design requirements of RFID tag antenna
The design requirements of the RFID tag antenna mainly include: the physical size of the antenna is small enough to meet the needs of tag miniaturization; it has the directivity of omnidirectional or hemispherical coverage; it has high gain and can provide the largest signal to the chip of the tag; impedance matching Well, no matter what direction the tag is in, the polarization of the tag antenna can match the signal of the reader; it is robust and low-cost. When choosing an antenna, the main considerations are: the type of antenna, the impedance of the antenna, the RF performance applied to the article, and the RF performance when there are other articles surrounding the label article.
4 Categories and research status of RFID tag antennas
Tag antennas are mainly divided into three categories: coil type, dipole, and slot (including microstrip patch) type. Coil type antenna is to coil the metal wire into a plane or wind the metal wire on the magnetic core; the dipole antenna is composed of two straight wires of the same thickness and length arranged in a straight line, and the signal is fed from the two end points in the middle. The length of the antenna determines the frequency range; the slot antenna is composed of grooves cut out of the metal surface, and the microstrip patch antenna is composed of a circuit board with a rectangle at the end, and the length and width of the rectangle determines the frequency range.
RFID antennas for medium and low frequency short-range application systems with a recognition distance of less than 1m generally use simple, low-cost coil antennas; long-distance application systems for high-frequency or microwave frequency bands above 1I1 need to use dipole and slot antennas.
4.1 Coil antenna
When the tag coil antenna enters the alternating magnetic field generated by the reader, the interaction between the tag antenna and the reader antenna is similar to a transformer. The two coils are equivalent to the primary coil and the secondary coil of the transformer.
The carrier frequency used for the two-way communication between the tag and the reader is when the antenna coil of the tag is required to have a small appearance, that is, the area is small, and a certain working distance is required. The mutual inductance of the antenna coil between the RFID tag and the reader is obviously not satisfactory. If required, a ferrite material with high magnetic permeability can be inserted into the tag antenna coil to increase the mutual inductance, thereby compensating for the problem of the small cross-section of the coil.” Currently, the realization technology of coil antennas is very mature and widely used in In RFID systems such as identification and cargo tags, it is difficult to achieve corresponding performance indicators with coil antennas for RFID applications with high frequency, large amount of information, and uncertain working distance and direction.
4.2 Dipole antenna
The dipole antenna has the advantages of good radiation ability, simple structure and high efficiency. It can be designed as an RFID system suitable for omnidirectional communication, and is widely used in the design of RFID tag antennas, especially in long-distance RFID systems.
The biggest problem with traditional half-wave dipole antennas is the impact on tag size, such as the 915MHz half-wave dipole. Studies have shown that a terminated, tilted, and folded dipole antenna can obtain the required input impedance by selecting appropriate geometric parameters. It has the advantages of high gain, wide frequency coverage, and low noise. It has excellent performance and is compatible with Compared with the traditional half-wave dipole antenna, the size is much smaller. If it is matched with the brazed electrical terminals and the balun, it can also maximize the gain, impedance matching and bandwidth. It is known that increasing the number of bends of the antenna is beneficial to reduce the size of the antenna without reducing the efficiency of the antenna. Then, how to “bend” in a limited space? The specific parameters of “bend” affect the resonance of the tag antenna What is the effect of frequency and input impedance? How to “bend” the RF efficiency most?
we know. Objects with a fractal structure generally have the characteristics of proportional self-similarity and space filling. Application to antenna design can achieve antenna multi-band characteristics and size reduction characteristics. A lot of research work has been done on antennas with fractal structures at home and abroad, and it is proved that antennas with fractal structures have good size reduction characteristics and can greatly improve antenna efficiency in a limited space.
Using Hilbert fractal transformation for the different positions and dimensions of the half-wave oscillator, and using the moment method to simulate the Hilbert tag antenna, the simulation results of the tag antenna’s resonant frequency and input impedance with different fractal dimensions and orders can be obtained. In the analysis results It is a common research method to determine which dimension and order of the tag antenna best meets the design requirements of the actual tag antenna, further fabricate the physical antenna, and test the RF identification distance.
4.3 Slot (including microstrip patch) antenna
Slot antenna has the characteristics of low profile, light weight, simple processing, easy to conform to objects, mass production, diversified electrical performance, broadband and active devices and circuits integrated into a unified component, etc., suitable for mass production and can simplify the whole machine The production and debugging, thus greatly reducing the cost.
The microstrip patch antenna is composed of a radiating patch conductor attached to a dielectric substrate with a metal bottom plate. According to the radiation characteristics of the antenna, the patch conductor can be designed in various shapes. It is generally used in low profile structures with frequencies higher than 100MHz. It is usually composed of a thin layer of dielectric (called a substrate) with a rectangular or square metal patch placed on the ground plane. The patch can be made by photolithography. Manufacturing makes it low cost and easy to mass produce.
As mentioned above, the bent antenna is beneficial to reduce the physical size of the tag antenna and meet the design requirements of miniaturization of the tag. For slot antennas, the concept of bending can also be used. In fact, the bent slot antenna is suitable for RFID tags in the high-frequency microwave range, which can effectively reduce the size of the antenna and has excellent performance. Has broad market prospects. The research method is similar to that of the bent dipole antenna. The method of moment is used to study the influence of the number of times, height, position, width of the slot bending and the size of the slot antenna on the resonance characteristics of the rectangular antenna.
For the bent slot antenna, the size of the flat plate is LxW, the bending width and height of the slot are s and h respectively, and the distance between the slot and the center of the feeding point is. The following discusses the changes of these parameters on the resonance characteristics, reflection coefficient, and antenna efficiency of the slot antenna And so on.
Based on the influence of various bending parameters on the performance of the slot antenna, the slot antenna for the UHF radio frequency identification tag can be designed according to actual needs to make a specific physical antenna. It can be predicted that the bent slot antenna will be a more promising development direction in the field of UHF tag antenna design.
5 Hot issues of RFID tag antenna
In the design of RFID tag antennas, in addition to the problem of reducing the physical size that has always been paid attention to, the bandwidth and gain characteristics of the miniaturized antenna are further improved to expand its practical application range, and the cross-polarization characteristics of the miniaturized antenna are analyzed to clarify it. Polarization purity is also an important research direction. In addition, composite antenna design covering various frequencies, multi-tag antenna optimized distribution technology, reader smart beam scanning antenna array technology, design simulation software and platform, tag antenna and attached medium matching technology , Consistent anti-interference and safety and reliability technologies are all worthy of continued research.
Among them, the on-chip antenna technology is a hot issue of recent research. The continuous expansion of the application field of RFID technology makes the requirements of RFID tags for miniaturization, light weight, multi-function, low power consumption and low cost continue to increase. However, the current RFID tags still use off-chip independent antennas, and their advantages are antennas. The Q (quality factor) value is high, easy to manufacture, and the cost is moderate. The disadvantage is that it is large in size, easy to break, and cannot be used for anti-counterfeiting or implantation in animals in the form of biological tags. If the antenna can be integrated on the tag chip, it can work without any external components, which will make the entire tag smaller and more convenient to use, which has led to the research of on-chip antenna technology.
Integrating the antenna on the chip not only simplifies the original label production process, reduces the cost, but also improves the reliability. The on-chip antenna as an energy receiver and signal sensor determines the performance of the entire system. Its basic starting point is to use the Faraday electromagnetic induction principle. The external magnetic field energy is converted into the on-chip power supply voltage, which is used as the working power supply of the entire chip. At the same time, the on-chip current or voltage changes caused by the electromagnetic field changes are used to identify the received signal. The signal is transmitted to the receiving end by changing the external magnetic field caused by its own output impedance. So far, on-chip antennas implemented on standard CMOS processes still use silicon-based integrated spiral inductors as the main structure.
In addition to the internal design of RFID tags, research in areas such as RFID smart platform (smarttable) antennas has also received increasing attention.
[ad_2]
