Determining Directionality with Passive RFID

Here is the situation my team faced: A large retailer in the Southwest wants to track items as they move between the sales floor and storeroom. To solve this challenge we had to answer a very practical question, "How do we know if something is coming or going?"

Whether your team is working in a distribution center, manufacturing facility or cross-docks, determining directionality is critical for many companies. Most portal solutions simply tell you the tag is there and you have to assume directionality. Although determining directionality is not as difficult as it may first appear, there are some challenges to overcome.Some background: To begin, I need to define a couple terms for clarity. A RFID "read point" refers to a specific antenna connected to a reader. A "read zone" is a logical grouping of one or more read points. In order to determine direction, two separate read zones must be created. "Read events" are generated by the reader based on the occurrence of tag activity. Software makes decisions based on the read events.

Here is the solution we put into play: The following diagram will help illustrate the solution. It is an overhead view of a doorway, or as it's more commonly called, a portal. Read zone 1 is a store room and read zone 2 is the sales floor. There are two RFID interrogators (readers). Reader A has four antennas: A1, A2, A3 and A4. Reader B also has four antennas: B1, B2, B3 and B4.

Determining Directionality with passive RFID
Click the image to enlarge.

This configuration will work with the majority of the stationary RFID readers on the market, but for this specific solution, we used Motorola (Symbol) readers. They have built-in capabilities that help determine direction.

Using the reader configuration settings, two read zones are defined:

Read zone 2 is the area covered by antennas A1, A2, B1 and B2.

Read zone 1 is the area covered by antennas A3, A4, B3 and B4.

It is important to point out that the antennas comprising a read zone are NOT connected to the same reader. Reader A is serving all the antennas on the left, and reader B is serving all the antennas on the right. This eliminates the need to install long antenna cables over the doorway. Long antenna cables can degrade performance. Keep in mind, holes must be drilled in the walls for the cables connecting reader A to antennas A3 and A4 and reader B with antennas B3 and B4.

The composite materials of the walls affect read performance. RFID tags can be read through dry wall. We found that having a 10° angle for the antennas in read zone 1 helped with the differentiation of the zones. The angle helps keep more signal in the back room and less on the sales floor.

Other RFID products often traveled past this portal. Using 90 angles for the antennas in read zone 2 helped to keep the number of RFID tags read in the travel lanes at a minimum. Reading tags traveling adjacent to a zone isn't necessarily bad. Just be sure you account for those events in your software. For instance, you may want to update the inventory system as to the last location the product was seen (i.e., the toothpaste just went by the storeroom door). It does make interpreting the data more complicated if a product is passing by at the same time another product is going though the portal.

Once all of the equipment is installed, the readers are connected to a central computer that monitors the read events that occur for each zone. There are three different types of events we need to understand for this solution:

  1. A "New Tag" event occurs when tag appears to a reader for the first time.
  2. A "Visibility Changed" event occurs when the visibility of a tag changes for a read point.
  3. A "Tag Not Visible" event occurs when a tag is no longer visible to any read point.

For simplicity in this example, we'll describe what happens when one tag travels through the portal. (Please go to the chalk board). The same events occur when working with multiple tags. As goods (X) travel from the store room (read zone 1) to the sales floor (read zone 2), here's what happens:

X1. New tag event occurs for zone 1.
X2. Visibility changed event reports tag is now in zone 2.
X3. Tag not visible event occurs.

The same series of events occur when moving products in the other direction.

A tag may have transient periods when it is experiencing a temporary moment of invisibility. The reader provides settings to smooth out such fluctuations so the number of "visibility changed" events are kept to a minimum. Of course, testing is always required when designing read points and read zones.

Finally, software would then make a decision based on these series of events being reported. If read zone 1 is a store room, and read zone 2 is the sales floor, then the inventory system would update the location for each of the RFID tagged products to reflect the change of location.

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