Calibrate a Read Zone

Calibrating a Read Zone or Read Point

The term calibrating (aka 'tuning') is the process of configuring a read point/zone to recognize the tags you want and ignore those tags you don't want. This is a combination of both physical installation know-how as well as TagNet software configuration steps. This process can be anywhere from simple to complex based upon the density/population of tags and the proximity of your read zone relative to other read zones.

Note: Ideally you would have gone through a Phase I implementation with Stratum guiding the way and the correct antennas and tags have been chosen and implemented. This is the important starting point so that calibration can have an impact and you are not fighting against a bad foundation. This may seem simple but there are many antennas out there such as linear, circular and specialty so refer to the RFID 101 section to ensure you are clear about the terminology and what options are available for passive UHF equipment.

Key Steps to follow:

1.Physically adjusting the antenna(s) so they cover the the area where tags are being placed or passing through.

2.Adjusting the Antenna(s) power known as 'attenuation'. Simply put it means increasing/decreasing the transmit power (Tx) so it does intrude/radiate into areas you don't want.

❖This is enabled in the antenna configuration section of the Physical Reader properties.

3.Introducing a signal strength cut-off based on the RSSI (Received Signal Strength Indicator).

❖In order for this feature to be most effective all the tags need to read at the same RSSI so a common cut-off value can be used. Unless the tags are on an assembly line where there are no variables with respect to distance, orientation or blockage it is rare that that tags will return the same RSSI. This being the case, the best approach is to implement an RSSI cut-off based on the 'weakest' tag. Take the example of a cart of tagged WIP parts going through a choke point, you can have tags facing up, down, sideways, partially blocked by material etc. and you have tagged inventory sitting in storage say 10-15 ft away. You want to recognize the tags going through the doorway on the cart but not the tags in storage.

❖It is important to note that if your RSSI cut-off threshold is set too high, you risk tossing out tags that you actually want to see as they pass through your read zone. Example: a cart of tagged parts are going through a portal. Most of them read at (-55) however some of are being read at -(62) because of reasons mentioned above. If you put in a threshold of say (-58) you will ignore those weaker (-62) tags and they will not be processed in your business logic. If the tags you don't want to see nearby are also being registered at say -(60) this will cause contention. There has to be a reasonable RSSI differential between the tags you want to recognize and those you don't, otherwise the filtering will simply not work. This is where you would need to take a look a moving the offending material farther away from the read zone or putting up RF shielding shown below.

❖You would first enable the RSSI feature within the reader properties so you can log what is actually happening (and visualize in the the Event Viewer) to get a baseline of the RSSI values being returned by tags on the cart as well as the RSSI of tags in storage. This would be your starting baseline.

❖If in fact the variability of the tag's RSSI is based (or partially based) on the physical properties and/or orientation of the part on a fixture, there is a more advanced feature Implementing Variable RSSI Filters that can be considered in addition to the above tuning steps.

Important Note: Calibration should not be performed with Session Control enabled (S1-S3) as this will suppress the tags from responding in a timely manner and can easily cause confusion (e.g. is the RSSI limit set to high or is the tag dwelling in 'B' state?). Refer to the Performance Considerations section on how and when Session control should be used.

Shown Below is a Matrix of Typical Use Cases and the Tuning Considerations for each:

Use Cases	Complexity	Goal	Tuning Activity
Doorway choke-point or machinery feed point where there are no close-by tags that need to be filtered out.	Simple	Read the tag(s) only when they approach the desired position to be recognized.	#1 Antenna positioning and #2 Attenuation.
Doorway choke-point, machinery feed point, or RFID enabled bench/table where there are close-by tags that need to be filtered out.	Medium	Read the tag(s) only when they approach the desired position to be recognized and ensure that nearby tags are being ignored	#1 Antenna positioning, #2 Attenuation and #3 RSSI Cut-off
Doorway choke-point or machinery feed point where there are very close-by tags that need to be filtered out.	Complex	Read the tag(s) only when they approach the desired position to be recognized and ensure that very close-by tags (moving or not moving) are being ignored	#1 Antenna positioning, #2 Attenuation, #3 RSSI Cut-off and physical repositioning of nearby material.

Implementing the above calibration activities require that you physically be at the read zone to gauge cause and effect. Trying to to this remotely can lead to frustration and wasted time as it is vital that you see the difference with every configuration change. Note that every time you make a Reader property change you must restart the schedule so that the new properties are sent to the reader. For Server managed Readers this it it is done by a submitting a manual Run command and for SRC managed Readers it is done via Rebuild Core Directories switch within the SRC edit page. Also note that TagNet can capture before/after values of reader property changes which is invaluable to see where you started and what has been tested already. This configuration logging can be found here.

NOTE: A powerful way to ignore the most difficult tags is to use intelligent filtering based on the inventory state of what you want to recognize and those that you don't. For Example: if you are loading WIP through an assembly line, and their inventory location is say Line 1, Station A, Station B, etc., and you have pallets of material stored very closely to that line that have an inventory location different (e.g. Bin01, Bin 02, etc.) then you can simply use the power of the IMOVE rule to only consider tagged material that is on Line 1. Refer to IMOVE rules on how this is done.

RF Blocking Methods

Mitigating signal bounce and RF intrusion into areas that are not desired is a key factor to a successful implementation. There are a number of ‘screening’ methods to block/diffuse RF bleeding into an area such as parked carts nearby an RFID covered doorway portal.

RF Blocking Mesh

This fabric has a -100 dB isolation factor and is see-through enabling a safety factor so that cart or human movement is still visible and not blocked prior to entering high traffic areas. Additionally, this will not impede air flow as with a solid sheet of metal. This material is ~$16 per square foot and can be sewn into panels for blinds as shown below in Figure RF1.

Figure RF1 – Vendor Produced RF Blind Figure RF2– RF Blocking Mesh Example

Wire Mesh Screening

A metal mesh or screen (Figure RF2) would be the next best alternative to the specialized RF blocking material. Where the RF bleed is relatively weak another option to consider is a wire mesh material shown here that is much less expensive. This can be cut to any panel size and tried out for its suitability and if accomplishes the goals, can be deployed elsewhere.