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 for high frequency and ultra-high frequency applications. However, UHF radio frequency bands are sensitive to the environment, especially metals, which has caused the current passive tags of working frequencies to fail to work on objects with metal surfaces. The most widely used logistics industries for radio frequency identification are metal environments. Therefore, the shortcomings of metal sensitivity greatly limit its application in the logistics industry. This article theoretically analyzes the influence of metal on RFID systems from two aspects of readers and tags, and combines test and simulation to verify.
1, the impact of metal on the reader When the RFID system is used in a metal environment, the impact of metal on the reader is mainly reflected in two aspects: reflective and shielding.
When the electromagnetic wave is incident on the metal, a large part of it is reflected, and the reflected wave and the incident wave have opposite phases. When the electric field generated by the reflection of electromagnetic waves by metal is exactly the same as the original electric field at a certain position, the electric field will increase the induction intensity of the tag at this position, which can increase the reading rate of the tag; when the phase of the reflected electric field When the phase of the original electric field is reversed, it cancels out, which reduces the read rate of the tag. As shown in Figure 1.
To test this effect, the design experiment is shown in Figure 2. A metal plate is placed in front of the antenna of the reader/writer. The distance between the metal plate and the reader is fixed at 2.5 m. The reader operates at the UHF band. ISO18000--6 standard.
Changing the distance between the tag and reader The test tag read rate result is shown in Figure 3. It can be seen that there are obvious reading holes in the distance. The electromagnetic wave emitted by the reader is reflected after being attenuated by 2.5 m, and the reflected wave is superposed with the human wave. However, because of the attenuation, the electromagnetic waves reflected by the reader are not sufficient 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 tag is placed exactly where the hole is read, it cannot be read.
Metals also shield electromagnetic fields. Because the electric field will cause the free charge inside the metal to move, thus losing energy. The depth of the electromagnetic wave energy reaching the inside of the metal is represented by the skin depth:
Assuming that the metal is iron (K = 1.06 × 106S/m, μ = 300), the skin depth is 2.2 μm at a frequency of 868 MHz, so under normal circumstances, 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 smaller due to the diffraction of electromagnetic waves. In order to test the influence of metal shielding, the metal plate of Figure 2 was placed between the reader and the tag. Two metal plates of 200 mm x 200 mm and 400 mm x 400 mm were studied respectively, and the metal plate was kept 1 m away from the tag. 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 reading and writing distance is also farther away. Although it is possible to read the label behind the conductor, in practice, metal conductors should be avoided between the label and the reader.
2, the impact of metal on the label Adjust the settings in Figure 2, change the distance between the label and the metal plate, test the label reading and writing distance and read rate, the test results in Figure 5. The tag is completely inaccessible when it is close to the metal. As the distance increases, the read rate and read/write distance increase. The reasons for this phenomenon are discussed below.
When the metal is close to the reader/writer antenna, due to electromagnetic induction, it will absorb the radio frequency energy and convert it into its own electric field energy. Therefore, the total energy of the original radio frequency field is weakened, and the induced magnetic field is also generated. The magnetic field lines are perpendicular to the metal surface. The distribution of the RF field intensity is deformed on the metal surface, and the magnetic force curve tends to be gentle. Therefore, when the tag is attached to a metal surface or is very close to a metal surface, there is no real field distribution of the radio frequency in the space. The tag antenna cannot cut magnetic lines to obtain electromagnetic field energy, and the tag cannot work normally. In addition to the impact on the field, metal also detunes the antenna. A commonly used bent dipole antenna, measuring 30 mm by 51 mm, can be placed in a standard identification card package. Using the finite element method simulation, at 915 MHz, the input impedance is 29.1+208.9j, which is in line with the general tag chip impedance characteristics, and the gain is 1.36 dBi. The antenna structure is shown in Figure 6.
Place a 200 mm x 200 mm metal plate parallel to the antenna. The results of the pattern simulation with distances of 1 mm and 150 mm from the metal plate are shown in Fig. 7. With a distance of 1 mm, the gain is 5.38 dBi and the gain is high, but there is almost no radiation field. The gain is 2.45 dBi when the distance is 150 mm. The radiation field is strong. It can be seen that when the distance between the tag antenna and the metal is close, the antenna's directionality will be enhanced.
The simulation result of the input impedance change caused by changing the distance from the metal is shown in Fig. 8. The closer the distance to the metal, the smaller the real input impedance of the antenna, and almost 0 in the case of less than 1 mm; and the imaginary part decreases with increasing distance when the distance is greater than 40 mm, and the imaginary part decreases when the distance is less than 40 mm. Decreases with distance and decreases 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 does not work at all and will improve after more than 50 mm.
So the metal near the antenna will change the input impedance of the bent dipole antenna, so that the label detuning, but also increase the antenna gain, if the antenna gain increase effect is greater than the antenna detuning, then the antenna read and write distance It will increase, but it will decrease.
3. Conclusions This paper studied the influence of metal on RFID systems through theoretical analysis and experimental testing combined with simulation. The metal will reflect and shield the reader's field. The reflection will cause read and write holes. The shield will reduce the read rate, but it is not completely unreadable. Placing a tag near a metal can make it difficult to receive the energy of the reader while the impedance and gain of the tag antenna will change, causing detuning. The results of this study can provide a reference for the use of RFID systems in metal environments.
At present, radio frequency identification is most widely used for high frequency and ultra-high frequency applications. However, UHF radio frequency bands are sensitive to the environment, especially metals, which has caused the current passive tags of working frequencies to fail to work on objects with metal surfaces. The most widely used logistics industries for radio frequency identification are metal environments. Therefore, the shortcomings of metal sensitivity greatly limit its application in the logistics industry. This article theoretically analyzes the influence of metal on RFID systems from two aspects of readers and tags, and combines test and simulation to verify.
1, the impact of metal on the reader When the RFID system is used in a metal environment, the impact of metal on the reader is mainly reflected in two aspects: reflective and shielding.
When the electromagnetic wave is incident on the metal, a large part of it is reflected, and the reflected wave and the incident wave have opposite phases. When the electric field generated by the reflection of electromagnetic waves by metal is exactly the same as the original electric field at a certain position, the electric field will increase the induction intensity of the tag at this position, which can increase the reading rate of the tag; when the phase of the reflected electric field When the phase of the original electric field is reversed, it cancels out, which reduces the read rate of the tag. As shown in Figure 1.
To test this effect, the design experiment is shown in Figure 2. A metal plate is placed in front of the antenna of the reader/writer. The distance between the metal plate and the reader is fixed at 2.5 m. The reader operates at the UHF band. ISO18000--6 standard.
Changing the distance between the tag and reader The test tag read rate result is shown in Figure 3. It can be seen that there are obvious reading holes in the distance. The electromagnetic wave emitted by the reader is reflected after being attenuated by 2.5 m, and the reflected wave is superposed with the human wave. However, because of the attenuation, the electromagnetic waves reflected by the reader are not sufficient 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 tag is placed exactly where the hole is read, it cannot be read.
Metals also shield electromagnetic fields. Because the electric field will cause the free charge inside the metal to move, thus losing energy. The depth of the electromagnetic wave energy reaching the inside of the metal is represented by the skin depth:
Assuming that the metal is iron (K = 1.06 × 106S/m, μ = 300), the skin depth is 2.2 μm at a frequency of 868 MHz, so under normal circumstances, 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 smaller due to the diffraction of electromagnetic waves. In order to test the influence of metal shielding, the metal plate of Figure 2 was placed between the reader and the tag. Two metal plates of 200 mm x 200 mm and 400 mm x 400 mm were studied respectively, and the metal plate was kept 1 m away from the tag. 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 reading and writing distance is also farther away. Although it is possible to read the label behind the conductor, in practice, metal conductors should be avoided between the label and the reader.
2, the impact of metal on the label Adjust the settings in Figure 2, change the distance between the label and the metal plate, test the label reading and writing distance and read rate, the test results in Figure 5. The tag is completely inaccessible when it is close to the metal. As the distance increases, the read rate and read/write distance increase. The reasons for this phenomenon are discussed below.
When the metal is close to the reader/writer antenna, due to electromagnetic induction, it will absorb the radio frequency energy and convert it into its own electric field energy. Therefore, the total energy of the original radio frequency field is weakened, and the induced magnetic field is also generated. The magnetic field lines are perpendicular to the metal surface. The distribution of the RF field intensity is deformed on the metal surface, and the magnetic force curve tends to be gentle. Therefore, when the tag is attached to a metal surface or is very close to a metal surface, there is no real field distribution of the radio frequency in the space. The tag antenna cannot cut magnetic lines to obtain electromagnetic field energy, and the tag cannot work normally. In addition to the impact on the field, metal also detunes the antenna. A commonly used bent dipole antenna, measuring 30 mm by 51 mm, can be placed in a standard identification card package. Using the finite element method simulation, at 915 MHz, the input impedance is 29.1+208.9j, which is in line with the general tag chip impedance characteristics, and the gain is 1.36 dBi. The antenna structure is shown in Figure 6.
Place a 200 mm x 200 mm metal plate parallel to the antenna. The results of the pattern simulation with distances of 1 mm and 150 mm from the metal plate are shown in Fig. 7. With a distance of 1 mm, the gain is 5.38 dBi and the gain is high, but there is almost no radiation field. The gain is 2.45 dBi when the distance is 150 mm. The radiation field is strong. It can be seen that when the distance between the tag antenna and the metal is close, the antenna's directionality will be enhanced.
The simulation result of the input impedance change caused by changing the distance from the metal is shown in Fig. 8. The closer the distance to the metal, the smaller the real input impedance of the antenna, and almost 0 in the case of less than 1 mm; and the imaginary part decreases with increasing distance when the distance is greater than 40 mm, and the imaginary part decreases when the distance is less than 40 mm. Decreases with distance and decreases 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 does not work at all and will improve after more than 50 mm.
So the metal near the antenna will change the input impedance of the bent dipole antenna, so that the label detuning, but also increase the antenna gain, if the antenna gain increase effect is greater than the antenna detuning, then the antenna read and write distance It will increase, but it will decrease.
3. Conclusions This paper studied the influence of metal on RFID systems through theoretical analysis and experimental testing combined with simulation. The metal will reflect and shield the reader's field. The reflection will cause read and write holes. The shield will reduce the read rate, but it is not completely unreadable. Placing a tag near a metal can make it difficult to receive the energy of the reader while the impedance and gain of the tag antenna will change, causing detuning. The results of this study can provide a reference for the use of RFID systems in metal environments.