Implementing RTLS & RSSI with TagNet
Real-Time-Locating-System (RTLS) is a method to automatically determine the relative location of tags such as an inventory room or staging area. When RTLS is used with passive UHF technology a grid of antennas are deployed to cover the inventoried area providing a reasonable level of granularity. TagNet has a configuration method for this type of tracking as explained further on in this topic.
Received Signal Strength Indicator or ‘RSSI’ is the measurement of power received (Rx) in a radio signal, in this case the signal backscattered by a tag and measured by the reader at its antenna port. Implementing RSSI into the overall TagNet configuration can more accurately determine a tag’s relative position by using RSSI as an indicator of distance from a given read point (Antenna). When multiple read points are involved that can see the same tag, the triangulation algorithm becomes increasingly more accurate and thus RSSI can be used to determine tag position, direction, and velocity as well as providing the ability to filter out tags not of consequence to the event. Listed below are business scenarios where RSSI can provide a greater accuracy of the tag movements. In fact RSSI can provide “Real-Time Locating System” (RTLS) capabilities in a passive RFID environment.
Passive RFID readers are limited to 1 watt EIRP (Effective Isotropic Radiated Power) at the antenna port thus 4-port read can output 4 watts EIRP total. Based on the gain of the antenna (e.g. 6.5 dBic), the power emitting from the antenna can radiate no more than 36 dBm EIRP per FCC Part 15.247 regulations. When the tag is powered up, the RSSI can run anywhere from -0 dBm (tag touching antenna) to -84 dBm (tag farthest away).
1.Zone based Room Coverage
2.Choke Point Coverage
3.Simple RSSI Determination
4.Determining the closest Location Tag
Zone based Room Coverage
In this use case, deploying a number of (distributed) read points in a given room can provide an RTLS inventory of what tagged objects are where. In order to achieve this, Zone based rules are setup in TagNet based on taking RSSI readings of tags at each read point. In the floor layout below, there are (4) shaded zones with green squares representing read points showing their approximate power range and red squares are tags. Based on where the tags are, some may be seen by only a single read point or upwards to (4). The more read points that can see the tag the better the location accuracy will be as there is more data to make a logical determination.
Essentially TagNet’s reader management module will continue to take sampling of tag data as seen by each read point in the given area. At set intervals (e.g. 30 seconds), the sampling data will be summarized by Tag and what read points saw that tag and at what RSSI levels. These are then compared to the Zone rules and a winner is declared via a combined score (discussed later on). It is important to note that if a given tag if seen by (3) read points it can only be inventoried in one zone and the RSSI is the determining factor. Because of the of volume of sampling data it would be rare that 2 or more zones would have the same score however there are ‘tie breakers’ that kick-in in this eventuality.
There are a number of physical factors that can affect the consistency of RSSI readings:
1.Different tags in the same exact location (e.g. Rack slot) can give different RSSI readings based on the size of their antenna and chip efficiency.
2.The same tags on different object types (metallic, wood, glass) can give different RSSI readings at the same distances.
3.How the tag is oriented on the similar object types (in same location) can give different RSSI readings at the same distances.
4.Tagged objects sitting in metal racks as opposed to an open table or floor area can give different RSSI readings at the same distance.
5.Rogue ‘Signal bounce’ can give misleading RSSI however this is mostly mitigated based on the volume of sampling data that averages this out.
What does this all mean? It means that one size doesn’t fit all and you will need incorporate a degree of forgiveness in your TagNet RSSI Zone rules.
Choke Point Coverage
In this use case, where a number of RFID enabled doorways (Portals) are in close proximity to each other, RSSI can enable filtering out of tag read events that are being seen in the proximity but are not traveling through that given doorway portal. Additionally to determine directionality, RSSI can aid in determining which side of the portal threshold is seeing the tags thus answering the question “is the tag entering the room or leaving the room”.
There are a number of physical factors that can affect the consistency of RSSI readings when seen at portal:
1.How the tagged objects are transported through the portal can affect RSSI (e.g. hand carried, on cart, in box via hand truck, etc.).
2.The same tags on different object types (metallic, wood, glass) can give different RSSI readings as they pass through the portal.
3.Multiple objects with different tag types can give different RSSI readings based on the size of their antenna and chip efficiency. Example: hand tool with tiny ‘dash’ tag as opposed to equipment box with large asset tag.
Simply putting in an RSSI ‘cutoff’ will not suffice in this use… why? In a perfect world each tag would be seen at a given RSSI as it goes through the portal, but if you have a difficult to read tags that are being obscured or by the factors above, you could eliminate a valid read event. As long as the RSSI for the intended portal (e.g. Door 1) is stronger than that of any nearby doorways (e.g. Door 2 or 3) then it can be determined that Door 1 is the winner.
Simple RSSI Determination
In those use cases where a tagged object needs to be logged when the RSSI reaches a given strength and the reaction time needs to be very quick, a simple RSSI threshold calculation can be used instead of a more complex Zone based evaluation. In the example of workstations in a manufacturing environment whereas a given tagged Item is brought into the proximity of Station 2, as soon as the RSSI is greater than X, the event is logged and any higher order business logic is evoked. It would be highly unlikely that other Stations in the proximity would see the tag at the same RSSI strength at the same time.
The setup in TagNet would be:
a)*FILTER used at reader level with the RSSI cutoff set to say -30. Note All antennas would use this setting
b)The reader would not be part of a Zone calculation because *ZONE not enabled and set to *NO
c)As soon as a tag is seen stronger than the RSSI cutoff, the PHLOG is written which triggers the higher order binding (this could be at the read point level to initiate a PEV push to the display at that workstation). Note that Net Change is still applicable as well as IMOVE.
Location Tag Determination
In those use cases where the strongest location tag needs to be assessed based on a matrix in ceiling. In this case more than one location tag could be seen however only the strongest RSSI is the winner. When the read cycle ends, the winning location tag is logged and any higher order business logic is evoked.
The setup in TagNet would be:
A)*MAXIMUM used at reader level and *FILTER for RSSI strength. Note All antennas would use this setting
B)The reader would not be part of a Zone calculation because *ZONE not enabled
C)As soon as event cycle completes, the location tag with the strongest RSSI is written to PHLOG which triggers the higher order binding (this could be at the read point level to initiate a PEV push to the operator). Note that Net Change is still applicable as well as IMOVE.
This section covers the steps needed to configure TagNet for RTLS
2)Configure Logical Reader for RSSI Sampling Interval (must be greater than the read cycle of reader but still low if using directional portal). Why? When a tag is seen at outside read zone it must be read and evaluated and IMOVE rules done before being seen on inside read zone. If not then its read event at that read zone is bypassed and IMOVE/NET Change doesn’t kick in.
Tuning Tips & Techniques
Here are some tips to make your testing more accurate:
•Determine your lowest and strongest RSSI values and use that range to spread out your Zone from/to values. If your ranges are too far apart then competing read zones will get the same score, thus the tag will not change location even it is closer to one of read zones because they both register in the same level. in the example below if Zone A has a an RSSI of -47 and Zone B -49, they both get the same score. What is the tiebreaker logic ??
•For directional portals, you need to consider the velocity of tagged objects and the time it takes to pass through both read zones. Look at how you set your read cycle and sampling interval with respect to each other. If you make the sampling interval too short it many not have enough read events to get a good average. Conversely, if the sampling interval is too long the tag can simply pass through both read zones and not have enough time to change inventory states. In other words, if can be seen at at same strength in both opposing read zones A & B, thus when moving A --> B the system could determine that A was strongest even though the tag has now passed the B zone.
•Sampling too long example; If a tag dwells in read zone A for say 3 seconds and then takes 2 seconds to pass through Zone B, then the average could be stronger for A.
•It is important that *YES RSSI is not enabled as the tag can be registered by both read zones, not simply the strongest. To get an RSSI value to display in logging and the EV, use *FILTER with a very low RSSI of say -82 just to force an RSSI value.
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