Analyzing the feasible solutions for using RFID on metal based on its physical characteristics

JasonYoo

Analyzing the feasible solutions for using RFID on metal based on its physical characteristics [Copy link]

At present, most manufacturers are mainly in the experimental stage for the metal problem, and most of them do not have mature products. Low frequency and high frequency do not have much to do with metal conductors. The only way is to increase the magnetic flux through ferrite or leave a certain distance from the metal, or to reduce the negative effects of metal on the RF signal, which is determined by their physical properties. The wavelength of ultra-high frequency (UHF) is shorter. When the distance between the metal surface and the tag is appropriate, the signal can be reflected by the metal surface to increase the signal strength. Only a few companies such as Intermec have announced their metal-specific tags.

According to the working principles and physical characteristics of several frequency bands, we will analyze the feasible solutions on the metal surface:

1. Low frequency band (125KHz, 134KHz)

This frequency mainly works by inductive coupling, that is, there is a transformer coupling effect between the reader coil and the sensor (tag) coil. The voltage induced in the sensor antenna by the alternating field of the reader is rectified and can be used as a power supply voltage. The magnetic field area can be well defined, but the field strength drops too quickly. The wavelength of this frequency band is about 2200m.

RFID technology was first widely used and promoted in this frequency band. The magnetic field of this frequency band drops too quickly, which determines that its recognition distance is very short, but it can produce a relatively uniform reading and writing area, and the fast attenuation will not affect other magnetic fields. Therefore, there are no special licensing restrictions for readers working at low frequencies in the world; the longer wavelength enables this frequency band to pass through any material except metal and liquid without reducing its reading distance, but the data transmission rate is relatively slow. The technology of this frequency band is relatively mature, and the sensor can be packaged in different forms, but the price of ordinary tags is more expensive than that of readers. Sensors in this frequency band generally do not provide anti-collision algorithms, and most of them on the market are read-only and only provide ID numbers; Philips has a Hitag S sensor that can be written. Although its reader module supports anti-collision algorithms, not many of them are actually used.

The working mode of low frequency is based on the law of electromagnetic induction, and coupling is achieved through spatial alternating magnetic fields. If the sensor is directly attached to the metal surface, it will interfere with the alternating magnetic field formed by the reader and the sensor, thereby affecting the reading of data. The solution is to add an isolation layer between the sensor and the metal. A popular material called ferrite on the market can isolate the metal well and increase the magnetic flux near the sensor coil. However, the cost of ferrite is high, and it is not suitable for mass use at present.

Main application areas: animal husbandry, access control, identity recognition, etc.

2. High frequency (13.56MHz)

Sensors at this frequency no longer need to be wound with coils, and antennas can be made by attachment or printing. Sensors generally work by load modulation. That is, the voltage on the reader antenna changes by connecting and disconnecting the load resistor on the sensor, so that the antenna voltage can be amplitude modulated by a long-distance sensor. If the connection and disconnection of the load voltage is controlled by data, then the data can be transmitted from the sensor to the reader. The wavelength of this frequency is about 22m.

Due to the long wavelength of this frequency band, RF signals can pass through most materials except metal materials, but the reading distance is often reduced, and the sensor needs to be away from the metal for a distance. Although the magnetic field area of this frequency drops quickly, it can still produce a relatively uniform read and write area; this frequency band is recognized worldwide and has no special restrictions. High-frequency sensors are generally in the form of electronic tags. The system has anti-collision characteristics and can read multiple electronic tags at the same time. The signal transmission rate is faster than that of low frequency, the price is not very expensive, and it generally provides a certain amount of storage. The working principle of this frequency band is roughly the same as that of low frequency, and the sensitivity to metal is higher than that of low frequency, so an isolation layer must be established to isolate the sensor from the metal, otherwise the reader cannot read the sensor at all.

High-frequency systems are used for access control and applications that require the transmission of large amounts of data, such as one-card systems (which can be used in libraries and canteens, because the storage space of the sensor chip is divided into several sectors, the information in the library is stored in one sector, and the information used as a meal card is stored in another sector), management of gas cylinders, management of medical logistics systems, etc.

3. Ultra-high frequency (working frequency is between 860MHz and 960MHz)

Ultra-high frequency systems transmit energy through electric fields. The coupling type of the RF signal between the reader and the electronic tag is electromagnetic backscatter coupling, which is based on the principle of the radar model. The electromagnetic wave emitted is reflected after hitting the target, and carries back the target's information at the same time. It is based on the spatial propagation law of electromagnetic waves; the energy of the electric field does not drop very quickly, but the reading area is not well defined. The reading distance of this frequency band is relatively far, and the passive reading distance can reach about 10m. It is mainly achieved through capacitive coupling.

This frequency band is currently expected to have the greatest application potential. Its standard formulation and technological progress will have an impact on the RFID industry. The current focus is on the EPC standard and Gen 2. UHF Gen 2 is just one of the multiple standards developed by EPCglobal. The frequency and energy used are fully in line with various current laws and regulations. The standard involves middleware in the information system and the communication method between information systems, etc. (it may not be fully formulated now). Simply put, it is like various domain name resolutions, various network communication protocols, etc. on the Internet; in addition, in order to better protect the data stored in the tag and the corresponding database, products based on the UHF Generation 2 standard will adopt advanced encryption technology, password protection and authentication mechanisms. EPC encoding is generally 96 bits, and the China Standardization Committee prefers EPC. The definition of UHF is not the same around the world. The frequency defined in Europe and parts of Asia is 868MHz, the frequency defined in North America is between 902 and 905MHz, and the frequency band recommended in Japan is between 950 and 956. The reason for paying attention to EPC is that EPC emphasizes communication, network protocols and various interfaces, which will be very useful for the current Boeing manufacturing model. It may be used in warehouse management and transportation management in assembly plants, and this model can better solve the traceability of products.
The wavelength of this frequency band is about 30cm, and the signal cannot pass through many materials, especially suspended particles such as water, dust, and fog. Compared with high-frequency electronic tags, electronic tags in this frequency band do not need to be separated from metal. It can use the principle of radar. If the distance between the tag and the metal surface is appropriate, it will not only not affect the reading of the tag, but also strengthen the signal. This frequency band

has a high data transmission rate, and a large number of electronic tags can be read in a very short time. Alien claims that they have been able to reach 360 pieces/second and can read correctly at a moving speed of 40km/second. The reason why RFID technology has been valued decades after its creation is that it has strong support from the US Department of Defense and Walmart. The two typical applications, logistics and supply chain management, are the advantages of the RFID UHF band. UHF has the advantages of reading distance and speed, and is very good for batch reading, mobile reading and long-distance reading compared to low frequency and high frequency.

Main application areas: management and application in the supply chain, management and application of production line automation, management and application of air parcels, management and application of containers, and application of logistics management systems.

In summary, there are three methods for metal parts identification at this stage. One is the method of isolating metal from tags used in high and low frequencies, which is relatively expensive; the other is to reasonably design the tag so that the tag antenna is at an appropriate distance from the metal surface; the third is to attach the tag, including hanging a sign, marking the non-metallic tray or container carrying the identified object, etc. In the future, with the advancement of manufacturing technology, the tag antenna and metal can be made together, and the identified metal conductor can be used as part of the antenna to easily solve the conductor sensitivity problem; but now for direct identification of metal parts, if there is no reading distance requirement, it can be directly adhered to the surface. If there is a distance requirement, it is better to use a reasonably designed UHF tag or an attached tag.