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 most common frequency in the world is 13.56MHz. Due to the stable performance and reasonable price, the 13.56MHz high-frequency RFID technology has a reading distance range that matches the actual application distance range, so it is widely used in public transportation cards and mobile phone payments. Application, 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 stick a layer of magnetic 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 their working principles, forming a lot of theoretical knowledge, but the disadvantage is that these theories It is more complicated and 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 RFID system composition
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.
Because it is a passive electronic tag, the energy required by 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 operating 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 at 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.
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 through a special process. Compared with the traditional absorbing materials, this kind of high-performance absorbing materials for 13.56MHz is different in performance characterization and usage 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.
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 original 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). Need to be re-determined inside the device).
As the international standards for electromagnetic interference control become more and more stringent, our country is also in line with the international standards, speeding up the control of electromagnetic noise, especially electronic products. Therefore, how to achieve electronic products to meet these requirements will be an important course. After these years of development, microwave absorbing materials have made great progress. However, as the requirements for electronics become higher and higher, the use of microwave absorbing materials will become increasingly thinner and thinner under the premise that the frequency of use will become higher and higher. The development of high performance and light weight is also the driving force for the progress of materials.