RFID, RFID technology introduction
Radio frequency identification is a non-contact automatic identification technology using radio frequency technology. It has the advantages of fast transmission rate, anti-collision, large-volume reading, and motion reading. Therefore, RFID technology in logistics and supply chain management, production management and control , anti-counterfeiting and security control, traffic management and control and other fields have significant application potential. At present, the working frequency band of radio frequency identification technology includes low frequency, high frequency, ultra high frequency and microwave segments, among which the application of high frequency and ultra high frequency is the most extensive.
The RFID system is mainly composed of a reader, a transponder (RFID tag) and a background computer. The reader/writer realizes reading, writing and storing data of the tag, and 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 includes a circuit such as a radio frequency front end, a logic control, and a memory. Labels can be classified into active (acTIve) tags, semi-active (semiacTIve) tags, and passive tags according to the power supply principle. Passive tags are favored 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 RF field, the induced current obtained by the antenna is used as the power supply of the chip through the boosting circuit, and the induced current with information is converted into the digital signal through the RF front-end circuit. Into the logic control circuit for processing, the information that needs to be replied is sent from the tag memory, sent back to the RF front-end circuit through the logic control circuit, and finally sent back to the reader through the antenna.
3 Antennas in RFID systemsFrom the principle of RFID technology, the key to the performance of RFID tags lies in the characteristics and performance of RFID tag antennas. In the data communication process between the tag and the reader, the key function is the antenna. On the one hand, the chip start circuit of the tag starts to work, and the antenna needs to obtain sufficient energy in the electromagnetic field generated by the reader; on the other hand, the antenna determines The communication channel and communication method between the tag and the reader. Therefore, the research of antennas, especially the internal antennas of tags, has become the focus.
3.1 Types of RFID system antennas
According to the power supply mode of the RFID tag chip, the RFID tag antenna can be divided into two types: an active antenna and a passive antenna. The performance requirements of active antennas are lower than those of passive antennas, but their performance is greatly affected by battery life: passive antennas can overcome the shortage of active antennas due to batteries, but the performance requirements of antennas are very high. At present, the research focus of RFID antennas is passive antennas. From the RFID system operating frequency band, in the LF, HF segment f such as 6.78MHz, 13.56MHz) I for the RFID system, the electromagnetic energy transmission is done in the induction field (like the stable field), also known as induction Coupling system; in systems with UHF segments (eg 915 MHz, 2400 Mttz) Z, the transfer of electromagnetic energy is done in the far field region (radiation field), also known as the microwave radiation system. Since the energy generation and transmission modes of the two systems are different, the corresponding RFID tag antenna and the front end portion have their own particularities, so the tag antenna is divided into a near field induction coil antenna and a far field radiation antenna. The inductive coupling system uses a near-field induction coil antenna composed of a multi-turn inductor coil. The inductor coil and its parallel capacitor form a parallel resonant loop to couple the maximum RF energy; the type of far-field radiating antenna used in the microwave radiation system is mainly It is a dipole antenna and a slot antenna. The far field radiating antenna is usually resonant, and generally takes half a wavelength. The shape and size of the antenna determine the range of frequencies it can capture. The higher the frequency, the more sensitive the antenna and the smaller the area occupied. Higher operating frequencies can have smaller tag sizes, and far field radiating antennas have higher radiation efficiency than near field sensing antennas.
3.2 Design requirements for RFID tag antennas
The design requirements of the RFID tag antenna mainly include: the physical size of the antenna is small enough to meet the requirements of miniaturization of the tag; the directionality with omnidirectional or hemispherical coverage; the chip with high gain, which can provide the largest signal to the tag; impedance matching Well, regardless of the direction of the tag, the polarization of the tag antenna can match the signal of the reader; it is robust and low cost. The main considerations when selecting an antenna are: the type of antenna, the impedance of the antenna, the RF performance applied to the item, and the RF performance when other items surround the label item.
4 RFID tag antenna category and research statusTag antennas are mainly divided into three categories: coil type, dipole, and slit (including microstrip patch) type. The coil type antenna is obtained by winding a metal wire into a plane or winding a metal wire around a core; the dipole antenna is composed of two straight wires of the same thickness and equal length, and the signal is fed from two intermediate terminals. The length of the antenna determines the frequency range; the slot antenna is composed of a groove cut out from the metal surface, wherein the microstrip patch antenna is formed by a rectangular circuit board at the end, and the length and width of the rectangle determine the frequency range.
RFID antennas for medium-low frequency close-range applications with a recognition distance of less than 1 m generally use coil antennas with simple process and low cost; long-distance application systems with high frequency or microwave frequency bands above 1I1 require 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 coils of the two are equivalent to the primary and secondary coils of the transformer.
The carrier frequency used for bidirectional communication between the tag and the reader is when the tag antenna coil is required to have a small outer shape, that is, the area is small, and a certain working distance is required, and the mutual inductance of the antenna coil of the RFID tag and the reader/writer is obviously not satisfactory. In the demand, a ferrite material with high magnetic permeability can be inserted inside the tag antenna coil to increase the mutual inductance, thereby compensating for the problem that the cross section of the coil is small." At present, the implementation technology of the coil antenna is mature and widely used in RFID systems such as identification and cargo tags, but for RFID applications with high frequency, large amount of information, and uncertain working distance and direction, it is difficult to achieve corresponding performance indicators by using coil antennas.
4.2 Dipole antenna
The dipole antenna has the advantages of good radiation capability, simple structure and high efficiency. It can be designed into an RFID system suitable for all-round 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 effect on the size of the tag, such as the 915 MHz half-wave dipole. Studies have shown that the terminated, tilted, folded dipole antenna can obtain the required input impedance by selecting appropriate geometric parameters, with the advantages of high gain, wide frequency coverage and low noise, and the performance is excellent, and Traditional half-wave dipole antennas are much smaller in size, and can be used to maximize gain, impedance matching, and bandwidth if used with brazed electrical terminals and baluns. It is known to increase the number of bending of the antenna to reduce the size of the antenna without reducing the efficiency of the antenna. Then, how to "bend" under the limited space, the specific parameters of the "bending" to the resonance of the tag antenna What is the effect of frequency and input impedance? How to "bend" the highest RF efficiency?
we know. Objects with fractal structures generally have the characteristics of proportional self-similarity and space filling. The multi-band characteristics and size reduction characteristics of the antenna can be realized by applying to the antenna design. A lot of research work has been done on antennas with fractal structures at home and abroad. It is confirmed that the antennas of the fractal structure have good size reduction characteristics, and the antenna efficiency network can be greatly improved in a limited space.
The Hilbert fractal transform is used for different positions and dimensions of the half-wave oscillator, and the Hilbert tag antenna is simulated by the moment method. The simulation results of the resonant frequency and input impedance of the tag antenna with fractal dimension and order can be obtained. Antenna gain and efficiency, to determine which dimension and order of the tag antenna best meet the design requirements of the actual tag antenna, further make a physical antenna, and test the RF recognition distance, which is a commonly used research method.
4.3 gap (including microstrip patch) antenna
The slot antenna has the characteristics of low profile, light weight, simple processing, easy conformity with objects, mass production, diversified electrical performance, integration of broadband and active devices and circuits, and is suitable for mass production and simplifies the whole machine. Production and commissioning, which greatly reduces costs.
The microstrip patch antenna is composed of a radiation patch conductor attached to a dielectric substrate having a metal substrate. The patch conductor can be designed in various shapes depending on the radiation characteristics of the antenna. It is commonly used in low-profile structures with frequencies above 100MHz. It is usually composed of a rectangular or square metal patch placed on the ground plane of a thin layer of dielectric (called a substrate). The chip can be photolithographically processed. Manufactured to make it low cost and easy to mass produce.
As mentioned above, the bent antenna is advantageous for reducing the physical size of the tag antenna and meeting the design requirements of the miniaturization of the tag. For slot antennas, the concept of bending can also be utilized. In fact, the bent slot antenna is suitable for RFID tags of high-frequency microwave segments, which can effectively reduce the size of the antenna and has excellent performance. Has a broad market prospects. The research method is similar to the bent dipole antenna. The moment method is used to study the effect of the number, height, position and width of the slot bending and the size of the slot antenna on the resonant characteristics of the rectangular antenna.
The slot antenna is bent, the flat piece size is LxW, the slot bending width and height are s and h respectively, and the gap is away from the center of the feed point. The variation of these parameters on the resonant characteristics, reflection coefficient and antenna efficiency of the slot antenna are discussed below. And so on.
Based on the influence of various parameters of the bending 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 produce a specific physical antenna. It can be expected that the bent slot antenna will be a relatively optimistic development direction in the field of UHF tag antenna design.
5 RFID tag antenna hot issuesIn the design of the RFID tag antenna, in addition to the problem of reducing the physical size that has been highly valued, the antenna bandwidth and gain characteristics of the miniaturized antenna are further improved to expand the practical application range, and the cross-polarization characteristics of the miniaturized antenna are analyzed to clarify the Polarization purity is also an important research direction. In addition, composite antenna design covering various frequencies, multi-tag antenna optimization distribution technology, reader intelligent beam scanning antenna array technology, design simulation software and platform, tag antenna and attached media matching technology , consistent anti-interference and safety and reliability technologies are all worthy of further research.
Among them, the on-chip antenna technology is a hot issue in recent research. The ever-expanding application of RFID technology has made RFID tags increasingly demanding for miniaturization, lightweight, multi-function, low power consumption and low cost. However, current RFID tags still use off-chip independent antennas, and the advantage is that the antenna The Q (quality factor) value is high, easy to manufacture, and cost-effective. The disadvantage is that it is bulky, easy to break, and cannot be used for tasks such as anti-counterfeiting or bio-labeling into animals. If the antenna can be integrated on the tag chip, it can work without any external device, which will make the whole tag smaller and more convenient to use, which leads to the research of the on-chip antenna technology.
Integrating the antenna onto the chip not only simplifies the original labeling process, reduces costs, but also increases reliability. The on-chip antenna acts as an energy receiver and signal sensor to determine the performance of the entire system. Its basic starting point is to use the Faraday electromagnetic induction principle. The externally changed magnetic field energy is converted into the on-chip power supply voltage as the operating power of the entire chip, and the received signal is discriminated by the change of the on-chip current or voltage caused by the electromagnetic field change. The signal is transmitted to the receiving end by changing the external magnetic field change due to its own output impedance. To date, on-chip antennas implemented on standard CMOS processes still use silicon-based integrated spiral inductors as the primary structure.
In addition to the internal design of RFID tags, research in the field of RFID smart platform antennas has also received increasing attention.
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