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Detecting presence (arrival/departure) with active RFID tags

Actually arrival is pretty simple, tag gets into a range of receivers antenna, but the departure is what is causing the problems.

First some information about the setup we have. Tags: They work at 433Mhz, every 1.5 seconds they transmit a "heartbeat", on movement they go into a transmission burst mode which lasts for as long as they are moving. They transmit their ID, transmission sequence number(1 to 255, repeating over and over), for how long they have been in use, and input from motion sensor, if any. We have no control over them whatsoever. They will continue doing what they do until their battery dies. And they are sealed shut.

Receiver forwards all that data + signal strength of a tag to our software. Software can work with several receivers. Currently we are using omnidirectional antennas.

How can we be sure that the tag has departed from premises?

Problems:

  • Sometimes two or more tags transmit "heartbeat" at the same time and no signal is received. With number of tags increasing these collisions happen more often, this problem is solved by tags randomly changing their heartbeat rate (in several milliseconds) to avoid collisions. Problem is I can't rely on tags not "checking in" for a certain period of time as sign of departure. It could be timeout because of collisions. Because of these collisions we cannot rely that every "heartbeat" will be received.
  • Tag manufacturer advised that we use two receivers and set them up as a gate for tags to pass through. Based on the order of tags passing through "gates" we can tell in which direction they are going. The problem with our omnidirectional antennas is that sometimes tag signal bounces of building and then arrives to receiver. So based on signal strength it looks like its farther away then it is.

Does anybody have a solution of what we can do to have a reliable way of determining if tags are coming or leaving? Also we can setup antennas in different way as well.

I wrote the software that interprets data from receivers, so that part can be manipulated in any way. But I'm out of ideas of how to interpret information to get reliability we need.

Right now the only idea is to try out with directional antennas? But I would like to tryout all the options with the current equipment we have.

Also any literature suggestion that deals with active RFID tags is more than welcome, most of books I've found deal with passive tag solutions.

Solution

  • As a top level statement, if you need to track items leaving your site, your RFID technology is probably the wrong one. The technology you have is better suited to the positional tracking tags within a large area - eg a factory floor. Notwithstanding the above, here is my take:

    A good approach to active RFID is to break your area down into zones that are tied to your business processes, for example:

    • Warehouse
    • Loading bay
    • Packing

    Entry of a tag into a zone represents the start of a new process or perhaps the end of a process the tag is currently in. For example, moving from warehouse to the packing represents assembling a shipment, and movement into the loading bay initiates a shipment.

    The crux of many RFID implementations is the installation and configuration of the RFID intrastructure to:

    1. Map tag -> asset (which you have done)
    2. Map tag read -> zone (and by inference asset -> zone)
    3. Map movements between zones to steps in a business processes (and therefore understand when an asset leaves the site, your goal)

    There are a number of considerations: the physical characteristics of 433MHz signals, position of antennae, sensitivity of antennae and some tricks that some vendors have. After an optimal site configuration, then you may need to have some processing tricks on the tag reads that will pour in.

    Dirty data

    Always keep in mind that tag read data is dirty - that RF interference (from unshielded motors, electric wiring, etc), weather conditions and physical manipulation of tags (eg covering with metal) happen all the time.

    RSSI's are like stock tickers - there is a lot of random/microeconomic noise on top of broad macroeconomic trends. To interpret movement, compute the linear regression of groups of reads rather then rely on a specific read's RSSI.

    If you do see a tag broadcasting with a high RSSI, which then falls to medium then low and then disappears, you really can interpret that as the tag is leaving the range of the receiver. Is that off-site? Well, you need to consider the site's layout (the zones) and the positioning of receivers within the zones.

    TriangulationTrilateration

    EDIT I had incorrectly used the term 'triangulation'. This refers to determining the position of something by known the angle it subtends from two or three known locations. In RFID, you use the distance and as such it is called 'trilateration'.

    In my experience, vendors selling the tag technology you describe have server software that determines the absolute position of the tags using the received RSSI. You should be able to obtain the position of the tag within 1-10m using such software. Determining if the tag is moving off-site is then easy.

    To code this yourself:

    First, each tag is pinging away when moving. These pings hit the receivers at almost the same time and sent to the server. However the messages can sometimes arrive out of order or interleaved with earlier and later reads from other receivers. To help correlate pings, the ping contains a sequence number. You are looking for tag reads from the same tag, with the same sequence number, received by three (or more) receivers. If more than three, pick the three with the largest RSSI.

    The distance is approximated from RSSI. This is not linear and subject to non-trivial random variation. A quick google turns up:

    enter image description here enter image description here

    Given three approximate distances from three known points (the receivers' locations), you can then resolve the approximate position of the tag using Trilateration using 3 latitude and longitude points, and 3 distances.

    Now you have the absolute position of the tag. You can use these positions to track the absolute movement of the tag.

    To make this useful, you should position receivers so that you can reliably detect tags right up to the physical site boundaries. You should then determine a 'geofence' around your site, within receiver range. I would write a business rule that states:

    • If the last known position of a tag was outside the geofence, and
    • A tag read from the tag has not been detected in (say) 10s, then
    • Declare the tag has left the site.

    By using the trilateration and geofence, you can focus the business logic on only those tags close to going awol. If you fail to receive your 1.5s ping only a few times from such a tag, it's highly likely that the tag has gone outside your receiver's range, and therefore off-site.

    You're already aware that tag reads can sometimes come from reflections. If you have a lot of these, then your trilateration will be pretty poor. So this method works best when there are fairly large open spaces and minimal reflectors.

    Some RFID vendors have all this built into their servers - processing this by writing your own code is (clearly) non-trivial.

    Zone design using wide-area receivers

    Logical design of zones can help the business logic layer. For example, suppose you have two zones (A and B) with two receivers (1 and 2):

         A           B
+----------+----------+
|          |          |
|    1     |     2    |
|          |          |
+----------+----------+

    If you get tag reads from the tag at receiver 1, then one at receiver 2, how do you interpret that? Did tag T move into zone B, or just get a read at the extreme range of 2?

    If you get a later read at 1, did the tag move back, or did it never move?

    A better physical solution is: 

         A           B
+----------+----------+
|          |          |
|   1      2      3   |
|          |          |
+----------+----------+

    In this approach, a tag moving from A to B would get reads from the following receivers:

    1      1   1   2  1  2  2 3 2  2 3  2  3  3  3  3      3  
             -------> time

    From a programming logic point of view, a movement from A -> B has to traverse reads 1 -> 2 -> 3 (even though there is a lot of jitter). It gets even easier to interpret when you combine with RSSI.

    Portal design with directional receivers

    You can create quite a good portal using two directional receivers (you will need to spend some time configuring the antenna and sensitivity carefully). Mount a receiver well above the door on both sides. Below is a schematic from the side. R1 and R2 are the receivers (and the rough read field is shown), and on the left is a worker pushing an asset through the door:

                ----> direction of motion

        -------------------+----------------
                     R1    |    R2
                    /  \   |   /  \
           o       /    \     /    \
           |-++   /      \   /      \
           |\++  /        \ /        \
  ------------------------------------------

    You should get a pattern of reads like this:

     <nothing>    1   1   1 1  1  12  1 21 2 12 2 1 2 2   2  2   2   <nothing>
             -------> time

    This indicates a movement from receiver 1 to receiver 2.

    "Signposts"

    Savi implementations often use "sign posts" to assist with location. The sign post emits beam that illuminates a small area (like a doorway) in a 123KHz beam. The signpost also transmits a unique number identifying itself (left door might be 1, while the right door might be 2). When the tag passes through the beam, it wakes up and re-broadcasts the number. The reader now knows which door the tag passed through.

    Watch out for any metal in the surrounding area. 123KHz travels extremely well down rebar in concrete walls, metal fences and rail tracks. We once had tags reporting themselves hundreds of meters from a signpost due to such effects.

    With this approach you can implement a portal much like you would for passive.

    Simulating signposts

    If you don't have the ability to use signposts, then there is a dirty hack:

    1. Stick a passive RFID tag to your active RFID tag
    2. Install a passive RFID reader on each doorway

    Passive RFID is actually very good in restricted spaces, so this implementation can work very well. This solution may be the same cost (or cheaper) than with your active RFID vendor.

    If you're clever, you can use the EPC GIAI namespace for the passive tag ID and so burn it with the active tag ID. Both active and passive tags would then be identically named.

    Physical considerations

    433MHz tags have some interesting characteristics. Well-constructed receivers can get a read of tags within about 100m, which is a long way for RFID. In addition, 433MHz wraps itself around obstacles very well, especially metal ones. We could even read tags in the boot (trunk) of a car travelling at 50km/h - the signal propagates from the rubber seal.

    When installing a reader to monitor a zone, you need to adjust its location and sensitivity very carefully to maximize the reads from tags within your zone, but also to minimize reads from outside your zone. This might be done in HW or in SW configuration (like dropping all reads below a particular RSSI).

    One idea might be to move the receiver away from the area where your tags are exiting as in the layout below (R is the reader):

    +-------------------------+-----------+
|       Warehouse         |    Exit   |
|                         .           |
|                         .            
|  R                      .          R   --->
|                         .            
|                         .           |
|                         |           |
+-------------------------+-----------+

    It pays to do a RF site survey and spend enough time to properly understand how tags and readers work in an area. Getting the physical installation right is critical.

    Other thing to do is to consider physical constrictions such as corridors and doorways and treat them as choke-points - map logical zones to them. Put a reader (with directional receiver tuned to cover the constriction) and lower sensitivity in to cover the constriction.

    What no tag-reads actually means

    If my experience of RFID has taught me anything, it is that you can get spurious reads at any time, and you need to treat everything with a degree of suspicion. For example, you might have a few seconds of missing reads from a given tag - this can mean anything:

    1. A user accidentally putting a metal tin over the tag
    2. A fork lift truck getting between tag and reader
    3. An RF collision
    4. A momentary network congestion
    5. The battery dying or fading out (remember to check the low-battery flag in tag reads and ensure the business has a process to replace old tags).
    6. Tag destroyed by a pallet being pushed into it
    7. Stollen by someone wanting to resell it for scrap (Not a joke - this actually happened)
    8. Oh yeah, it may be that the tag moved off-site.

    If the tag has not been heard of in, say, 5 minutes, odds are that it's off site.

    In most business processes that you would use this active tag technology for, a short delay before the system decides the tag is off-site is acceptable.

    Conclusions

    1. Site survey: spend time experimenting with readers in different locations. Walk around the site with a tag and see what reads you are actually getting. Use this to:
    2. Logically segment your site into zones and locate receivers to most accurately position tags in zones
    3. It's easier to determine movement between zones using several receivers; if possible, instrument physical constrictions such as doors and corridors as portals. As part of your RFID implementation, you might even want to install new walls or fences to create such constrictions. Consider a passive RFID for portals.
    4. Beware of metal, especially large expanses of it.
    5. You have dirty data. You need to compute linear regressions on the RSSIs to spot trends over short periods; you need to be able to forgive a small number of missing tag reads
    6. Make sure that there are business processes to handle dying batteries and sudden disappearances of tags.

    Above all, this problem is best solved by getting the receivers installed in the best locations and configuring them carefully, then getting the software right. Trying to solve a bad site installation with software can cause premature ageing.

    Disclosure: I worked 8 years for a major active RFID vendor.