Tag Placement

To receive the best possible performance from your tag, you must not only choose the right tag for your product, but also determine its best placement on a particular item or its packaging. Sometimes the tags are already preselected, and then it is up to you to make them work. There are several general rules that you should follow to do this successfully:

• Use Physics and Scientific Tools to Test Your Products There are several tools, from signal generators and spectrum analyzers to specialized tag-testing software and RFID-designed probes, that can be used to leverage science rather than trial and error to find the optimal tag and location. One such tool is EasyTag (available at www.odintechnologies.com/store), which allows someone with very little training to accurately test for the RF response of a particular product and tag combination. Figure 4.2 shows the starting screen for EasyTag.

• Utilize Air Gaps as Much as Possible This is especially important when you are tagging products or packaging containing RF-difficult materials, such as cans of soup (metal) or bottles of soft drinks (liquid).

  When you are tagging cases of products, look at how the items are positioned in a case. For instance, the lean bottlenecks create large air gaps at the top of the case. This will be the best space in which to place a tag. These air gaps are called static air gaps because their location in the case does not change with movement.

  When tagging some items, you will often encounter dynamic air gaps, which are air gaps that change with product movement and orientation. For instance, an air gap created at the top of a box of Milk-Bone dog biscuits will disappear when my Newfoundland helps herself by flipping the box off the counter and dropping it onto the floor. The air gap could be on any one of the sides depending on how it lands and where the bones settle. Dynamic air gaps are impossible to utilize, but you can make them somewhat static by stabilizing the product orientation (for instance, ensure that the bottles are standing upright when passing through an interrogation zone).

  When tagging a single bottle, one of the best practices is to embed the tag into the bottle cap; however, you have to make sure that the cap or bottle spout does not contain a metal lining. Placing the tag into a bottle cap has several advantages. The tag is protected from external damage, it has a slight distance from the contents of the bottle (even if it is just a couple of millimeters of plastic), and it can be integrated into the cap during its manufacturing.

  If you do not have air gaps large enough to accommodate a tag or if you cannot integrate it into packaging that would not restrict the tag's performance but rather support it, you will have to use a tag intended for the tagged material. Be aware that specialized tags are usually a lot more expensive than generic tags, so you have to evaluate carefully all the possibilities of how to make a generic tag work before resorting to these.

• Build a Pallet Correctly This rule ties in closely with the air gap rule. When you are building your pallet, you should try to position the items so that the tags are facing outside the pallet. If this is not possible, try to stack the products to create air gaps in between them. Do not forget that the products may have air gaps inside their packaging. When building a pallet or stacking items or cases on shelves, you should not only utilize air gaps, but also try to avoid shadowing.

  Shadowing occurs when one tag is positioned in front of another tag. When the tags are interrogated, the first tag collects the signal from the reader and uses it to respond, while the second tag receives only very little or no signal and therefore does not have enough energy to respond to interrogation. The second tag is in the shadow of the first tag. To avoid shadowing, you should position tags so that they are not placed right behind one another, although this may be quite difficult, especially when there is a high density of tags on the pallet or a shelf is high.

  Tip 

  Possible shadowing can be overcome by motion, especially by rotation. Pallets are commonly stretch-wrapped by using a stretch-wrapping machine that turns the pallet around its axis. If the interrogation zone is placed at this station, pallet rotation will often eliminate the shadowing problem and you will receive high reads from your tags. Plus, the tags will be in the interrogation zone longer so you'll have a higher probability of reading them.

  Stacking tags close to each other could also cause interference between them. This could be avoided by the already discussed techniques, but also by using tags that have been tested to perform well in close proximity to other tags.

• Ensure Proper Tag Orientation You must make sure that you know what types of antennas are used as well as their polarization in the interrogation zones the product will travel through, because this will affect your tag choice.

  Single dipole tags will be easily accommodated by a circularly polarized antenna, regardless of the tag's orientation, as long as the tag is facing the antenna (see Figure 4.3). However, if the antenna is linearly polarized, you will have to make sure that the tag's antenna is parallel to the reader's antenna and is in the same plane as the reader's antenna wave propagation (see Figures 4.4 and 4.5). You will also have to ensure that the tag's orientation is the same on every product that crosses the discussed interrogation zone. The constant product orientation and position can be achieved mainly on manufacturing lines.

  A dual dipole tag will be easier to implement, because it is less orientation sensitive. This tag will perform well at any orientation with a circular antenna as well as a linear antenna. Of course, the tag should face the antenna. Dual dipole tags are well suited for use on items that will be traveling on conveyor belts and whose position cannot be unified.

  Tags using near-field coupling (LF, HF, and some UHF tags) are also orientation insensitive, unless you approach perpendicular to the reader's antenna, in which case the tags "disappear" altogether.

Figure 4.2: EasyTag tag-testing software

Figure 4.3: Orientation with circular antenna

Figure 4.4: Correctly oriented tag

Figure 4.5: Incorrectly oriented tag

Warning 

The first thing you should do before you jump into tag testing is to check out the environment and make sure it is "clean." I suggest setting up a spectrum analyzer and looking at the area being tested, making sure you are reading in the entire frequency range. If no interference is recorded when using the spectrum analyzer, the second thing you should do is make sure that you have ample room (20 feet) behind the product, on all sides, and above. This will limit the possibility of standing waves and nulls.

Package Contents

This RFID stuff is awfully confusing I know: not only does it matter what type of material the tag is actually affixed to, but it also matters what's inside the box. So cardboard is easy and a great product to tag, but liquid or metal inside that cardboard can really throw a wrench in the works. Although the packaging may be an easily penetrated corrugated cardboard, the product within can contain metals, water-based liquids, or other RF-difficult materials, which could either detune the tag or prevent the interrogation signal from reaching it. There are three main groups of products and packaging: product or packaging that does not include any RF-difficult materials, product or packaging that does not include liquids or metals but includes other RF-difficult materials, and product or packaging that includes liquids and metals.

Product or packaging that does not include any RF-difficult materials usually consists of corrugated cardboard, paper, foam, or dry food such as cereal, tea bags, or flour. You will be able to choose from a broad range of tags to tag this group of products and packaging. Generic tags will perform relatively well and their placement on the product can be chosen for convenience of application or space, but it will not be critical for tag's performance.

Product or packaging that does not include liquids and metals but includes other RF-challenging materials could contain glass, wood, thick layers of plastic or paper (which are normally RF-friendly), dry dog food in high volumes, and various chemicals (including washing powder). All of these materials and products will affect the RF field distribution, and therefore you will have to perform tag placement testing, utilize possible air gaps, or choose tags that are either specially tuned or work in a wider frequency band.

Product or packaging that contains liquid or metal is the most difficult to tag because not only do these substances detune the tags, but they also are opaque to RF waves. Such products include bottled drinks, cans, food in metal-lined packaging, liquid hair products, electrical equipment, and laptops and PCs. When tagging such items, you will have to heavily utilize the air gaps in the packaging, test the tag placement, and ensure that the product will go through the interrogation in a correct position. This principle has to be applied to pallets as well; because of the product's impenetrable nature, the items in the middle of the conventionally built pallet will not be read.

Tip 

Make sure that you place the whole tag, including the antenna, into the air gap.

Note 

Although the Generation 2 tags have enhanced capabilities and perform much better with RF-difficult materials than the previous generation, you should still follow these best practices and perform all necessary testing to ensure the best possible performance of your tags and the whole system.

Tag Placement Testing

To locate and verify the best possible spot to place your tag, you will have to perform scientific-based testing. You can do this by yourself if you have the location and testing equipment, or you can send the products to a specialized testing facility. Testing labs differ by the range of available testing equipment, testing methods, and services they provide. ODIN Technologies has been testing products for years and has completed literally hundreds of thousands of tests, so what you are about to learn is without a doubt industry best practices.

Some labs perform testing in a "clean" environment without RF noise and interference and under ideal environmental conditions; this is done in an anechoic chamber. The performance results coming from these facilities will be the best possible results you can achieve with that type of tag and your product, and are the best way to ensure 100 percent read rates. If you don't get 100 percent read rates, the data you will get from you, and your partner's, RFID networks will be incomplete, and therefore less valuable. However, with testing in an anechoic chamber you are not getting the "real world" impact on the tag and product. That's why this type of testing is not really applicable to the real world; however, it can be a helpful first step, especially for tough to tag products. A tag's ability to handle outside interference and noise is a critical performance factor. If you have a product of paper, Styrofoam, rubber, or similar RF-friendly characteristics, however, this test is entirely unnecessary.

If you know that your RFID system and tagged items will reside in an environment with high levels of RF noise, interference, high humidity, extreme temperatures, or other conditions that are far from ideal, your best bet is to choose a lab that performs the testing in a "dirty" or "noisy" environment. This means that the lab simulates the conditions that will be encountered in the customer's facility, and the testing results will be very similar to results achieved in reality.

The steps to identify the perfect tag and location are as follows:

1. Use a spectrum analyzer with a tracking generator and a probe to determine the RF signature of the product under test.

2. Select 4–5 different tags to test and obtain quantities of at least 100 tags of each.

3. Test each tag and note the minimum effective power to receive a signal back from the tag.

4. Plot the distribution of tags to determine the standard deviation and the power of an "average" tag.

5. Select six average tags of each model and test those on the sweet spots using a reader with a scientific testing tool such as EasyReader.

6. Verify results by incorporating dynamic testing in real-world use cases.

The first step in static testing is using a spectrum analyzer with a built-in tracking generator and a special probe or probe table that has a very small antenna (to simulate the tag) connected to the test gear. From this test equipment you can get an RF profile of the sides of the case to be tested. This will help you determine where to put the tags.

After you identify these sweet spots by using the scientific testing equipment, you can test the tag types to find an "average" performing tag. Make sure you test at least 100 tags of each type so that you will have statistical significance. This is important for two reasons: first, you need to find out which tag types have the lowest performance variability (or the highest manufacturing quality) and second, you need to make sure that you are not testing with a particularly good or bad tag, which will throw off your results. Using a reader and a set of different tags with tag-testing software will make this go very quickly. A tested product (or case) is placed a certain distance from a reader's antenna (usually around 2 meters). The tag-testing software will then use an algorithm to lower the power on the read to determine the lowest possible power that gets a tag response. This is superior to a methodology of moving the case, because you will always be in the same location relative to the RF wavelength and outside influences. All results are recorded for later comparison. This process is repeated for every evaluated tag on every type of product, and results in a narrow selection of best performing tags.

Tip 

Each of the testing facilities has developed various automated processes to achieve the same goal of finding the best tag for a particular product. If you have an RF-friendly product such as paper towels and would like to do this yourself, you can achieve reasonable results with minimal equipment: a reader, an antenna, tag-testing software, and time. (I can already see some folks from the test labs grinding their teeth when they are reading this, but the truth is some materials do not need the expertise of an RFID lab.)

After the most suitable tags are selected, their best possible placement has to be identified and verified. This is referred to as dynamic testing. Dynamic testing is usually performed on a conveyor and through a dock door. Maybe you are surprised about the conveyor use. The reason for this is not only a discovery of a good way of evaluating the performance of tags in motion, but also a necessity to verify compliance to various mandates. (Some retailers, such as Wal-Mart, require 100 percent tag reads on a 600-feet-per-minute conveyor.)

When testing a tag placement on a case, the case is divided into numbered sections forming a grid. Several sections will be left out of testing because they carry a bar code, product name, warnings, or other information that cannot be covered for merchandizing, marketing, or safety reasons. The tags are then placed one by one into every remaining section on the case (or into the sections that are presumed the most suitable), which is then circulated on a conveyor. The number of reads per each pass through the interrogation zone is captured in a testing log. After all tags are tried out with selected grids, the log should verify which location is optimal.