RFID Arena


RFID gates – The technology behind them

RFID gates are an effective way of tracking assets automatically without manual labor. But what are the technical aspects that need to be taken into account at deployment and how do we make the most out of the system?

What materials affect the performance of RFID readers? How do the amount of tags and the reading purpose affect antenna and tag positioning? What RFID antennas are optimal for what reading environment? And, how do we reduce the power consumption of automated systems? These questions among others will be answered in this article.

RFID gates - What are they?

An RFID gate is a physical gate with a fixed RFID reader and antenna(s) attached to it. The system registers everything that moves through the gate as long as it is equipped with an UHF RFID tag. RFID gates use the same technology as RFID mobile computers, except for that everything works automatically and no manual labor is required. The automated operations make RFID gates a faster and more accurate way of checking goods - especially compared to checking them manually.

Material choice effects antenna positioning

The positioning of the RFID readers and antennas depend a lot on the material used on the loading cages, pallets or containers as well as the material surrounding the gates. Different materials call for different positioning solutions.

When it comes to items loaded on wooden pallets and plastic crates, the positioning of the antennas isn't that demanding. Wood and plastic do not interfere remarkably with the RFID radio waves. But when we are talking about metal roller cages there are some issues to be considered. Firstly, the metal roller cages often have at least two sides covered with "metal net". If this net is very thick, reading the tags through the net can be a challenge. This can be solved by using a material that RF signals can pass through in the bottom plate of the metal roller cage and by installing one antenna on the top of the gate and another under the gate. This is the most reliable solution when it comes to reading accuracy with "metal challenges". When registering movement of people or other "non-metallic and non-liquid elements", the antennas can preferably be placed on both sides of the gate for optimal reading.

As with RFID in general, liquids and metals pose challenges on reading accuracy and may call for creativity for finding a well-performing solution. The reflective nature of metal can be used as a benefit: for instance one manufacturer of large drink containers has placed the RFID tags in a location where they are read well by using the reflection of the metal around them. Tags on liquid items are read most successfully by optimizing the placement of the tag so that the radio signals can reach them without passing through the liquid. They can e.g. be placed high up on a bottle's neck if read from above/side, or underneath the bottle it if the antennas are in the floor. The more distance between the bottles, the better passage for the radio signals.

rfid gates

The more tags - the more antennas

Generally the amount and density of goods, i.e. tags, on a transport platform define both the needed amount and the positioning of the antennas, as well as the optimal output power level of the reader. The more tags that pass through the gate simultaneously, the more antennas are needed - which should be taken into account when planning the system.

A high density of tags may cause a so called "shadowing effect" (the tags shadow each other) or/and a "coupling effect" (the tags mesh as a result of being in to close contact with each other) that decreases the reading accuracy. The solution to these problems is simply to use a higher output power. "Increase the volume and the tags shall hear you!"

The intended "reading level" - whether the purpose is to read tags on item-level or box/container-level - also affects the positioning of the tags and antennas. Maximum reading capacity is achieved by optimizing the positioning of the tags so that their radiation hit the antenna from the right angle. If the tags are placed in random angles and positions in a box, reading accuracy can be improved by installing several antennas, preferably with circular polarization, that read from as many angles as possible.

A tag passed by - but where did it come from and where did it go?

RFID gates can tell us whether a tag is getting closer to or moving away from an antenna. The strength of the signal (RSSI value) and the "phase difference" tell the reader how far away the tag is. By interpreting the phase difference and measuring whether the strength of the signal goes stronger or weaker, the reader knows whether the tag is approaching or retreating from the antenna. It can roughly tell the distance, but it can't necessary tell whether the tag is approaching from the right or the left. If the system administrator wants to know exactly to and from where the target is moving, two antennas are required. This ensures reliable information about the direction as well as the distance.

  1. RSSI value (strength of signal): the changes in the RSSI value are examined over time. When a tag approaches, the RSSI value increases. When the tag retreats from the antenna, the value decreases. There is one problem related to RSSI based systems, and that is the fact that the RSSI value can change as a cause of other factors than just distance. For instance, when placing a hand between the tag and the antenna the strength of the signal decreases and the system interprets the event as the tag moving away from the antenna.
  2. Phase difference: This method is a bit more "high tech", but it's less prompt to be affected by environmental influences. The system compares the phase difference between the reader-to-tag signal to the tag-to-reader signal. The phase difference between the signals lets the system know how far away from the antenna the tag is. By examining the phase difference data over time, it's possible to determine whether the tag is moving towards or further away from the antenna. The signal leaving and returning to the antenna can be compared to waves on the water surface with their "peaks". At the reader antenna, the gap between the peaks of the reader-to-tag and the tag-to-reader wave gets bigger the further away the tag is from the antenna.

Specific tag reading

In some spaces it isn't possible to have a long distance between the gates and the tags (e.g. in warehouses with shelves next to the gate). In these areas the reading distance and angle should be limited to ensure correct reading. If lowering the output power is not an option, the use of attenuating materials and directing the antennas optimally at the gate might solve this issue. This helps the reader pay attention to the right tags and ONLY the right tags.

What antenna for what purpose?

When it comes to the preferred type of antenna for an particular application, circular antennas provide more accurate results in situations when the tag position and angle is unknown, for instance when the tags are higgledy-piggledy thrown in a sealed boxes. A linear or a cross-dipole antenna is preferred when the tag position and angle is known AND the same for all tags.

Constant tag reading or not?

Automatized RFID reading systems may raise questions about the reader constantly being on sending "RF pollution" to the environment and consuming power. However, different solutions do exist to solve this challenge.

The RFID reader can be triggered to switch on with the help of clapping or light sensors. Or, a "light curtain" can be used to activate the reading of tags in large areas. E.g. the curtain recognizes a forklift approaching and tells the reader to wake up. This avoids blocking the nearby air waves when nothing interesting is happening at the gate.

Fixed RFID readers should be supplied with constant electricity at installation and they should have a fixed Internet connection to ensure a smooth and well-performing system. There are also fixed RFID readers with Ethernet on the market, where the PoE feature is supported. This enables powering the reader via the Ethernet cable.

A reliable system calls for qualitative connections and cables

The antenna cabling is something that should be taken into account when planning the system. It's not uncommon that half of the output power from the reader is lost due to the attenuation properties of the cable in certain applications. Therefore, to insure the best possible performance, the cables have to be of good quality and the connection from the antenna to the RFID reader should be as short as possible.

2 comments on “RFID gates – The technology behind them”

  1. Posted 29 July 2014 at 16:08:18

    Great RFID description, we understand well what is RFID, and nice solution too. And it's right, don't forget the cables ! Industrial wireless power supply doesn't exist yet !

  2. Gravatar of Kirsikka DrägerKirsikka Dräger
    Posted 30 July 2014 at 09:46:15

    Thank you for your comment and we are glad you liked the article! If you want to read more on RFID gates, check this article: http://www.rfidarena.com/2012/10/23/rfid-gates-%E2%80%93-where-are-they-and-why.aspx

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