The absorbing material is a composite material with excellent absorbing ability to electromagnetic waves. This kind of absorbing material is a composite material formed by physically thinning the alloy and treating it with a magnetic field to form a magnetic alloy with high magnetic permeability and dispersing it evenly in a polymer.
How can RFID be used in the field of absorbing materials
Primary structure of absorbing material
The principle of the absorbing material is based on the magnetic microwave absorber, which converts the electromagnetic waves emitted by electronic equipment into heat energy by means of insulation loss, magnetic loss and impedance loss to achieve the effect of reducing electromagnetic radiation. It has high magnetic permeability and selectable frequency bands. Broad and other characteristics, and can be targeted for specific frequency band development.
The absorbing material has good absorption characteristics in the range of 10MHz~6GHz, which can avoid electromagnetic interference or leakage caused by secondary reflection. The products are mainly absorbing patch types, which can also be processed into various shapes according to customer needs. Absorbers can be used in electronic equipment cavities such as notebook computers, mobile phones, and communication cabinets.
The absorbing material has good electromagnetic wave absorption effect and wide absorption frequency. It can be customized according to the frequency band required by the customer. It is thin and cost-effective, and has a wide range of uses and applications.
After more than ten years of development in the RFID industry in my country, the technology is now relatively mature, especially in the past two years. Under the background that the country actively encourages and vigorously promotes the healthy development of the industry, as various factors continue to advance the Internet of Things, it has been Maintain a steadily rising development trend.
The industry has also reached a consensus on the formulation of RFID frequency standards. At present, the internationally common frequencies are 13.56MHz, 13.56MHz high-frequency RFID technology because of its stable performance and reasonable price, and its reading distance range matches the distance range of practical applications, so it is used in bus card and mobile phone payment. Widely used, especially in South Korea, Japan and other places.
RFID electronic tags are often used in metal environments. When the RFID electronic tags are close to metal, because the metal has strong reflectivity to electromagnetic waves, it will be accompanied by weakening of the signal, and the reading distance will become closer, and serious interference will occur. There is a phenomenon of card reading failure. The current general solution is to paste a layer of absorbing material on the back of the electronic label.
Absorbing materials are widely used in various aspects such as noise reduction, wave absorption and EMC of electronic equipment. Experts have also made many models to explain the basic principles of their work, forming a lot of theoretical knowledge, but these shortcomings are these The theory is more complicated, and it is difficult for some readers who are not in the field to understand.
Combining with the many problems encountered by many engineers in the use, this article will take the 13.56MHz passive RFID system using absorbing materials as an example, using simple, simple and popular language to explain it, hoping to bring some help to the readers.
1. The composition of the RFID system
The RFID system consists of an electronic tag or non-contact smart card (such as a smart phone with swiping function) placed on the identified object and a device that issues instructions to the electronic tag and collects feedback information from the electronic tag. This device is also called RFID card reader or reader/writer consists of two parts. As shown in Figure 1, in order to allow other devices to display or use these data, it is generally possible to set an external RS232 protocol interface on the reader, so that information can be transferred with external devices.
Therefore, the passive electronic tag, so the energy required for the chip and memory in the electronic tag needs to be provided by the reader. The communication between the reader and the electronic tag is realized through the principle of electromagnetic coupling. The energy of the electronic tag Produced by the reader coil antenna through electromagnetic coupling.
The high-frequency electromagnetic field is generated by the antenna coil of the reader, and then the magnetic field passes through the cross section of the coil and the space around the coil. According to the tag's use frequency of 13.56MHz, its wavelength is 22.1m, which is much longer than the distance between the reader antenna and the electronic tag, so the electromagnetic field between the reader and the antenna can be treated as a simple alternating magnetic field.
By adjusting the antenna coil and capacitor of the electronic tag to form a resonant circuit, it is tuned to the transmission frequency of 13.56MHz specified by the reader, so that according to the resonance of the circuit, the voltage generated by the coil inductance in the tag reaches the maximum value. The power transmission efficiency between the antenna coil of the reader and the electronic tag is proportional to the number of turns of the coil in the tag, the area enclosed by the coil, the relative angle between the two placed and the distance between each other. This is also The reason why the RFID tag reading distance has a certain limit.
For the RFID electronic tag used under 13.56MHz, its maximum reading and writing distance is usually about 10 cm, and the current consumption of the chip is roughly 1 mA. Because as the frequency increases, the required inductance of the electronic tag coil is shown as a decrease in the number of turns of the coil, usually at this frequency, the typical number of turns is 3 to 10 turns.
The RFID tag reading distance is not only related to itself, but also to the environment in which it is located. When using an inductively coupled radio frequency identification system, such a requirement is often put forward: the antenna of the reader or electronic tag is directly mounted on the metal surface. However, it is impossible to mount the magnetic antenna directly on the metal surface.
Because the antenna magnetic flux passing through the metal surface will produce induced eddy currents. According to Lenz's law, the eddy current will react to the antenna field and cause the magnetic field on the metal surface to rapidly attenuate, so that the data between the reader and the electronic tag The reading distance will be severely affected, and even misreading or reading failures may occur. Regardless of the magnetic field generated by the coil itself installed on the metal surface or the field approaching the metal plate from the outside (the electronic tag is on the metal surface), the result is the same.
2. The absorbing principle of absorbing materials in RFID
The absorbing material is a kind of magnetic functional material with high magnetic permeability. It is usually made by filling some absorbents uniformly on the polymer material and making it through a special process. Compared with absorbing materials in the traditional sense, this kind of high-performance absorbing materials for 13.56MHz is different in performance characterization and application principles.
Traditional absorbing materials are mainly used in military countermeasures. Some aircraft, battleships and armored tanks that cover and confuse the opponent's radar reconnaissance have a very high frequency of microwave range, and the application analysis is also a far-field model.
The absorbing materials mentioned in this article are mainly for the permeable magnets used to provide paths for the magnetic field in civil electronic equipment. They have high magnetic permeability and low magnetic loss at the frequency of use. However, when the frequency is higher than the frequency of use, the loss will increase. Large and other characteristics, with the nature of a low-pass filter. However, due to its advantages of flexibility and convenient installation, it has been favored by more and more R&D engineers.
Let's compare the special applications of absorbing materials in electronic tag products, and solve the above-mentioned problem of improving the failure of normal communication when electronic tags encounter metal plates.
a) It means that a non-metallic and non-magnetic object has basically no effect on the propagation of the electromagnetic field. It still follows the direction from the plateau, which is equivalent to the propagation of electromagnetic waves in free space, so the energy and direction of the electromagnetic field are not disturbed. In Figure 3(b), a metal plate with good electrical conductivity is laminated on the basis of Figure 3(a). It can be clearly seen in the figure that the direction of the magnetic field lines has changed greatly. The main manifestation is that the magnetic field before and after the metal plate changes, which is the so-called shielding phenomenon.
There is no magnetic field behind the metal plate, and the direction facing the incident electromagnetic field will generate an electromagnetic field opposite to the incident electromagnetic field due to the eddy current generated by the metal plate, thereby weakening the magnetic field and even completely canceling the highland magnetic field. This problem can be solved from the solution shown in Figure 3(c), that is, after attaching the absorbing material (sheet) to the surface of the metal plate facing the direction of the incident electromagnetic field, it can effectively provide an effective path for the magnetic field transmission. The existence of the absorbing material effectively avoids the eddy current effect of the metal plate.
In the same way, when the RFID electronic tag is close to the metal sheet, as shown in Figure 4(a), the above similar effects will also occur, and the resonant frequency fr of the coil will also change, and fr will move to the low-frequency direction. At this time, The communication ability of the electronic tag is greatly reduced, and the card reading distance is severely disturbed.
By inserting a high-permeability magnetic material between the coil and the metal surface, as shown in Figure 4(b), the generation of eddy currents can be avoided to a large extent, so that the electronic tag can be safely used on the metal surface Up. When installing the antenna on the magnetic sheet, it should be noted that the inductance of the loop coil antenna will become significantly increased due to the high permeability of the magnetic material, so that it is necessary to re-adjust the resonance frequency or together with the matching network (in reading and writing). The interior of the device needs to be re-determined).