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1 Introduction
Radio frequency identification technology (RFID) is favored by the container transportation industry due to its fast identification speed, high accuracy, no contact, and strong environmental adaptability, and has been rapidly developed. He has gradually been used in transportation, air parcel management, logistics management, automatic production line control, access control management, material handling, medical care and other fields. The application of RFID technology in the management of pallets, containers, transportation vehicles, etc., the identification of commodities and the overall management of the supply chain have promoted the development of the logistics industry. Electronic tags have the characteristics of long reading distance (generally within 20 m), suitable for harsh environments, strong penetrability, and recognizable moving targets.
The application of RFID technology in the container field makes it possible to accurately control and track the who, where, and when information of the container. Electronic tags for containers mostly use ultra-high frequency (UHF) segments.This frequency band has strong penetrating ability and shows superiority in dynamic reading[1]. Compared with applications in industrial automation and other fields, the application of electronic tags in container logistics transportation has a strong particularity. RFID tags flow with containers at sea, docks, storage yards and other places. The working environment has the characteristics of large temperature difference, high ultraviolet radiation intensity, and sea water corrosive to tags. The packaging of RFID tags for containers must consider these factors. At the same time, the reflection of electromagnetic waves from the metal surface of the container has a great impact on data transmission. Under the influence of the metal surface, the originally matched antenna becomes mismatched. It is necessary to redesign the antenna and perform impedance matching again.
2 Introduction to the working principle of RFID
RFID is a technology that uses radio frequency signals through spatial coupling (alternating magnetic or electromagnetic fields) to achieve contactless information transmission and achieve identification through the transmitted information. His core component is an electronic tag, which reads the stored information in the electronic tag through the radio waves emitted by the reader within a distance of several centimeters to several meters or even ten meters. Passive RFID tags convert the radio frequency energy sent by the reader into a DC power supply to supply power to the chip circuit. Figure 1 is a schematic diagram of the working principle of the communication between the tag and the reader.
3 Problems in the application of electronic tags in the container field
3.1 The impact of the container transportation environment on the application performance indicators of electronic tags
Compared with applications in industrial automation and other fields, the application of electronic tags in container logistics transportation has a strong particularity.
(1) The working environment is relatively harsh
Electronic tags flow with containers on the sea, docks, storage yards and other places, which is different from the ordinary electronic tag working environment. The following factors must be considered during label design and packaging:
The particularity of temperature The container working environment has high temperature and large temperature difference, and the operation is all-weather.
High-humidity working environment Most of the time during container transportation is completed at sea (or in inland rivers), so the electronic tag must be able to work in an environment with a humidity of 35% to 80%.
Intense ultraviolet radiation for a long time and high-intensity ultraviolet radiation accelerate the aging of the label shell plastic.
Acid-base corrosion and vibration shock Containers work most of the time in the docks and at sea. This environment requires the label to be resistant to acid and alkali corrosion, and the packaging structure and materials must be resistant to vibration and shock.
(2) Dynamic label reading
Due to the fast circulation of containers, the container information must be accessible from a long distance. Therefore, the identification speed of electronic tags is required to be high (moving speed>100 km/h), long distance (>6 m), high accuracy rate (>99%), etc. .
Analysis of the influence of metal surface on electronic tags for containers
3.2 Analysis of the influence of metal surface on electronic tags for containers
Ordinary RFID tags are directly attached to the metal surface. Due to the reflection of incident electromagnetic waves on the metal surface, strong electromagnetic waves in the opposite direction will also pass through the electronic tag. After the incident wave and the reflected wave are superimposed in phase, a part of it will be offset, and the intensity will be greatly weakened, which will seriously affect the reading distance of the reader to the RFID tag, and even the data on the RFID tag cannot be read, as shown in Figure 2. At the same time, the magnetic flux generated between the reader and the RFID tag will induce eddy currents on the metal surface. According to Lenz’s law, the eddy currents have a reaction to the magnetic field of the reader, causing the magnetic field on the metal surface to be strongly attenuated.
The solution to eliminate the influence of the metal surface of the container is to increase the relative height of the antenna and the metal surface. As the relative height increases, the composite field signal vector will gradually increase. When the relative height reaches 1/4 of the wavelength, the composite field signal vector reaches the maximum and a gain of 3 dB can be obtained. When the RFID tag is attached to the metal surface of the container, the distance between the tag and the metal surface is very small, and the space that can be raised is limited, which cannot meet the requirements. For example, the 1/4 wavelength of a 900 MHz radio frequency is approximately 8 cm, while the actual depth of the groove on the outer surface of the container is only 2 cm. It is necessary to insert an isolation medium between the ordinary electronic tag and the metal surface, according to the wavelength formula of the electromagnetic wave in the medium: Among them: λ0 is the free space wavelength, μ and ε are the permeability and permittivity of the medium respectively.
The usual practice is to encapsulate the isolation medium and the electronic tag into a whole, thereby forming an anti-metal electronic tag. The research that needs to be done includes medium research (medium selection, test and process determination, etc.), data analysis of specific medium parameters (μ, ε, etc.), and the choice of packaging form.
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