Understanding the elements of RFID and mastering the techniques will go a long way to achieving a high performance RFID system. This article presents practical guidelines for selecting and applying RFID tags, based on expertise developed from numerous production deployments across many industries.
Get Your Bearings: Develop the Use Case and Conduct a Site Survey
Start by developing the use case for RFID in your organization, and then conduct a thorough site survey.
Through the use case, you will define the business objectives, the process, and metrics for success for RFID. Do you plan to track assets or locate high-value equipment? Do you want to track work-in-progress or improve warehouse operations? Do you want to improve shipping accuracy? Do you want to reduce out-of-stocks? Is your application targeted at containment, addressing market diversion or alleviating counterfeits? As organizations move beyond customer mandate projects, they are seeing firsthand that RFID can deliver process efficiencies with a clear return on investment.
In developing the use case, identify the processes involved and determine which assets should be tagged, such as individual items, cases and/or pallets. Also consider the evolution of the application, so that the application can be designed to scale to future business needs. For instance, the use case may call for reading cases on a conveyor, but a longer term goal is to read all cases on a pallet. Building to the minimum requirements now can make it more expensive to scale the application as the requirements increase later.
A site survey is essential to successful RFID system design. Through a site survey, issues related to RF communications and sources of electromagnetic interference are identified. A site survey is also instrumental in mapping out the antenna RF coverage, power and network architecture. Through the site survey process, identify equipment requirements, such as readers, antennas and applications, and determine ideal placement of these components. The site survey is also a good time to consider the amount and content of data needed in a tag and where and how to apply the tags.
2. Material Composition of the Tagged Items
Consider the material composition of the items to be tagged. Paper, corrugate, plastics and metal have different requirements. Identify whether the material transmits, reflects or absorbs the RF signal from the reader. Since RF powers the tag, this is critical to performance.
- Paper, cloth and cardboard are transparent to RF, as the RF energy penetrates them relatively easily. Tagging paper and corrugate is straightforward.
- Plastics are reasonably transparent to RF, and are generally easy to tag. But some types of plastics--most notably polycarbonate--can be difficult to tag. Polycarbonate, which is very common, has a high carbon content, which absorbs the RF. Be on the lookout for black plastic, which usually means it contains carbon. In this case, consider a broadband tag like a "World Tag," which has a flatter frequency response and better performance.
- Metal was once considered very difficult to tag. Metal reflects RF, which can be both good and bad. If a standard tag is placed on metal, the conductive surface shunts (shorts) the available energy from the tag. In-phase reflections or re-enforcing waveguides can enhance the signal--or, if out of phase, they can cancel the signal and create a null or dead spot. However, you can successfully tag metal by using an on-metal tag that uses an insulator to space the tag slightly from the metal surface [note: in and out of phase signals depend upon the spacing distance]. In this way, you can reinforce the reflective properties of metal and get better response from the tag.
- Metal or foil packaging also can be RF-difficult, because the metal or foil packaging reflects the RF. Similar to tagging metal, you can use techniques to take advantage of the reflective properties of foil or metal packaging to successfully tag the item. For example, you can design the tag into the foil packaging of a razor or CD such that the metal in the packaging acts as an antenna to focus the RF and get a good response from the tag.
When tagging metal or other RF-difficult items, it's advisable to work with a well-trained solution provider or with the tag manufacturer to design a tag that will work in this application. They will be able to take advantage of unique material properties to optimize the available signal to the tag.
3. Contents of the Tagged Items
The contents of the items to be tagged can also impact read performance.
- Dry materials can be tagged without issue, for the most part.
- Liquid. Water based products are regarded as challenging. Water acts as a large "RF sponge," absorbing most of the available RF energy, leaving little to activate the tag and return data to the reader. But not all liquids react the same: Water absorbs a lot, but contrary to what one may believe (given that it's carbon based), oil doesn't absorb much. In general, oil-based products are RF transparent. Subtle tag antenna designs and tag placement are the keys to performance optimization. Move the tag off the surface, as when tagging metals. For instance, you can tag a wine bottle by placing the tag above the wine or where the glass is thick (alcohol has the same absorptive properties as water).
- Semi-liquid. Semi-liquids or pastes can be challenging, depending on the percent of the contents that are water based. For instance, jelly can be difficult to tag, but peanut butter may be found to be easier. Items that contain a lot of sugar can be difficult to tag, because the carbon in sugar absorbs the RF.
- Wood or moist products. Less so today, with more sensitive tags, but some wooden pallets with high moisture content have been know to be challenging as well, because the wood typically used for pallets tends to have a lot of moisture. That's compounded with the reality that pallets are often left out in the rain and the wood absorbs even more moisture.
4. Geography of Operations
Consider geographic compliance. Quite often global tags are needed for items of export so that guaranteed reads can take place at both the country of origin and the point of sale. Global tags typically encompass a very wide frequency band, from 860MHz to 960MHz. A World Tag is a variant of a global tag that has a relatively flat frequency response (+/-1.5dB) in addition to a broad bandwidth. This makes them ideal for difficult read environments or challenging applications.
5. Size of Tagged Items
How large are the items to be tagged? Keep in mind that smaller isn't necessarily better when it comes to tags. Tag performance generally decreases with size, so it's advisable to use the largest size possible that fits the object. Consider high volume, general purpose mainstream tags, and then migrate to high-end or custom options if necessary. If the item is difficult to tag, you may want to select a tag designed for high performance. If dimensionally restricted, and a vertical orientation is desired, then consider use of a 2x2-inch square tag.