Radio Frequency Identification (RFID) technology is a contactless automatic identification technology that uses electromagnetic waves to achieve automatic identification of items. At this stage, RFID technology has played an important role in many areas.
At present, radio frequency identification is most widely used in high frequency and ultra high frequency. However, radio frequency in the UHF band is sensitive to the environment, especially metal, which makes the passive tag of this working frequency unable to work on objects with metal surfaces, and the most widely used logistics industry for radio frequency identification is metal environment. Therefore, the shortcoming of metal sensitivity greatly limits its application in the logistics industry. This paper analyzes the influence of metal on RFID system from two aspects of reader and tag, and combines testing and simulation to verify.
1 Metal impact on the readerWhen RFID systems are used in metal environments, the impact of metals on readers is mainly reflected in two aspects: reflectivity and shielding.
When an electromagnetic wave is incident on a metal, a large portion thereof is reflected, and the phase of the reflected wave and the incident wave are opposite. When the electric field generated by the reflection of electromagnetic waves on the metal is exactly the same as the original electric field at a certain position, then the intensity of the electric field on the label is enhanced at this position, and the reading rate of the label can be improved; when the phase of the electric field is reflected When the phase of the original electric field is opposite, it cancels out, thereby reducing the reading rate of the tag. As shown in Figure 1.
Figure 1 Relationship between reflected waves and incident waves
In order to test this effect, the design experiment is shown in Figure 2. A metal plate is placed directly in front of the reader antenna. The distance between the metal plate and the reader is fixed at 2.5 m. The reader works in the UHF band. ISO18000--6 standard.
Figure 2 Metal reflection test scenario
Change the distance between the tag and the reader. The test tag read rate results are shown in Figure 3. It can be seen that there is a significant read hole in the case of a long distance. The electromagnetic wave emitted by the reader is reflected by the attenuation of 2.5 m, and the reflected wave is superimposed with the human wave. However, because of the attenuation, the electromagnetic waves reflected to the reader are not enough to cancel out the electromagnetic waves just emitted by the reader, so the read and write holes only appear far away from the reader. If the label is placed just in the place where the hole is read, it cannot be read.
Figure 3 Test results of metal reflection
Metal also has a shielding effect on the electromagnetic field. Since the electric field causes the movement of free charges inside the metal, energy is lost. The depth at which electromagnetic waves reach the interior of the metal is expressed in terms of skin depth:
Assume that the metal is iron (K = 1.06 & TImes; 106 S / m, μ = 300), and the skin depth is 2.2 μm at 868 MHz, so in general, electromagnetic waves cannot propagate directly through the metal. Will leave an unreadable area behind the metal. When the metal size is not large, this area becomes small due to the diffraction of electromagnetic waves. In order to test the influence of the metal shield, the metal plate of Fig. 2 was placed between the reader and the tag, and two metal plates of 200 mm & TImes; 200 mm and 400 mm & TImes; 400 mm were respectively studied, and the metal plate was kept at a distance of 1 m from the label. The read rate test results are shown in Figure 4. It can be seen that the shielding range of the small metal plate is much smaller than that of the large metal plate, and the farthest read/write distance is also far. Although the label behind the conductor is still possible to read, in practical applications, metal conductors should be avoided between the label and the reader.
Figure 4 Test results of metal shielding
2 The effect of metal on the labelAdjust the settings in Figure 2, change the distance between the label and the metal plate, test the read/write distance and read rate of the label. The test results are shown in Figure 5. When the tag is very close to the metal, it is completely unreadable. As the distance increases, the read rate and read/write distance increase. The reasons for this phenomenon are discussed below.
Figure 5 read rate and read and write distance test results
When the metal is close to the reader antenna, due to electromagnetic induction, the RF energy is absorbed into its own electric field energy, thus reducing the total energy of the original RF field strength, and also generating an induced magnetic field, which is perpendicular to the metal surface. The distribution of the RF field strength is deformed on the metal surface, and the magnetic curve tends to be gentle. Therefore, when the label is attached to the metal surface or very close to the metal surface, there is actually no RF field strength distribution in the space, and the tag antenna cannot cut the magnetic field line to obtain the electromagnetic field energy, and the label cannot work normally. In addition to the effect on the field, the metal also detunes the antenna. Study a commonly used bent dipole antenna, 30 mm & TImes; 51 mm, which can be placed in a standard identification card package. Using finite element method simulation, at 915 MHz, the input impedance is 29.1+208.9j, which is in accordance with the general tag chip impedance characteristics, the gain is 1.36 dBi, and the antenna structure is shown in Fig. 6.
Figure 6 antenna structure
Place a 200 mm × 200 mm metal plate parallel to the antenna. The simulation results of the 1 mm and 150 mm patterns from the metal plate are shown in Fig. 7. When the distance is 1 mm, the gain is 5.38 dBi, the gain is high, but there is almost no radiation field. When the distance is 150 mm, the gain is 2.45 dBi. The radiation field is strong. It can be seen that when the tag antenna and the metal are close together, the directivity of the antenna is enhanced.
Figure 7 Comparison of patterns at different distances
The simulation results of the input impedance change caused by changing the distance from the metal are shown in Fig. 8. The closer the metal is to the metal, the smaller the real part of the input impedance of the antenna is, and it is almost 0 when it is less than 1 mm. When the distance is greater than 40 mm, the imaginary part decreases with increasing distance. When it is less than 40 mm, the imaginary part It decreases with distance and changes rapidly. When the distance is less than 10 mm, the antenna impedance changes from inductive to capacitive. It can be seen that under close range conditions, the bent dipole antenna is completely inoperable and will improve after being larger than 50 mm.
Therefore, the metal near the antenna changes the input impedance of the bent dipole antenna, which causes the label to be detuned and also increases the antenna gain. If the gain increase effect of the antenna is greater than the effect of the antenna detuning, then the read/write distance of the antenna is Will increase, but will decrease.
3 Conclusion
Figure 8 Input impedance change with metal distance change
This paper studies the effects of metals on RFID systems through theoretical analysis and experimental testing combined with simulation. The metal will have reflection and shielding on the field of the reader. The reflection will cause read and write holes, and the shielding will reduce the reading rate, but it is not completely unreadable. When the tag is placed near the metal, it will be difficult to receive the energy of the reader and the impedance and gain of the tag antenna will change, causing detuning. The results of this paper can provide a reference for the use of RFID systems in metal environments.
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