RFID - The Technology
How does it work?
In the last section we mentioned many things that you are relevant to an RFID system. In this section we will try to explain all of them.
The diagram below explains the basic schematic of all RFID systems.
The Tag or Transponder can be either active or passive. It responds to a signal from the Interrogator (reader/writer/antenna) which in turn sends a signal to the Computer.
Taking each piece in turn:
The Tag comes in a variety of shapes. It is made up from a chip (IC) and an antenna. Depending on your application it may be embedded in glass, or epoxy, or it may be in a label, or a card. See below for a selection of shapes.
The tag can be passive, battery assisted, or active.
Passive tags get all their power from the signal sent by the interrogator. As well as using this radio wave to carry the data, the tag is able to convert it into power. This means that the tag is only powered when it is in the beam of the interrogator. The tag then uses a technique called backscatter to reply to the interrogator. This does not involve a transmitter on the tag, but is a means of "reflecting" the carrier wave and putting a signal into that reflection.
Battery assisted tags are just like passive tags (they use backscatter) but they have a battery to provide the power to the chip. This provides a big advantage, because the tag is not dependent on the strength of the carrier from the interrogator to provide the power it needs. Now it can use all the power from the battery and so is able to work at a greater distance from the interrogator.
Active tags, have not only a battery, but also some form of transmitter on the tag. Now we can really talk about long range.
The disadvantage of having a battery is two fold. One, it adds cost to the tag, and two they run out of power eventually. The decision on which one is right for you will depend on your application.
The tag is made of an IC and an antenna. The IC will include memory and some form of processing capability. The memory may be read only or read/write, the type selected will depend on the application.
The tag talks to the interrogator using what is called the air-interface. This is a specification for how they talk to each other and includes the frequency of the carrier, the bit data rate, the method of encoding and any other parameters that may be needed. ISO 18000 is the standard for the air interface for item management.
Also a part of this air interface is what is commonly called the anti-collision protocol (if the tag support it). This is a means of allowing many tags in the field to talk "at the same time". There are several ways of doing this, and each manufacturer has developed their own way of implementing it. Simplistically, consider a first grade teacher talking to his/her class. She says "Call out your name if you are here today". What she hears is 20 (or more) kids all shouting at the same time. So she says, "If your name begins with an A, shout out your name". Maybe she only hears one name now, or maybe she hears several. If she hears several, she refines the command, "If you name begins AA". By telling a child to keep quiet after she is able to record the name, she is now able to collect all the names.
Two other terms you may hear are "Reader talks first" (RTF) and "Tag talks first" (TTF). With a RTF system, the tag just sits there, until it hears a request from the interrogator. This means that even though a tag may be illuminated (receiving power) from the interrogator, it does not talk until it is asked a question. With TTF the tag talks as soon as it gets power, or in the case of a battery assisted tag or active tag, it talks for short periods of time, all the time. This gives you a much faster indication of a tag within sight of the interrogator, but it also means that the airwaves have constant traffic.
The antenna in a tag is the physical interface for the RF to be received and transmitted. Its construction varies depending on the tag itself and the frequency it operates on. Low frequency tags often use coils of wire, whereas high frequency tags are usually printed with conducting inks.
Another form of tag is often called the smart label. This is really a paper (or similar material) label with printing, but also with an RFID tag embedded in it. Examples are shown below (with the antenna structure shown in the corner).
RFID - The Frequencies
RFID operates in several frequency bands. The exact frequency is controlled by the Radio Regulatory body in each country.
The generic frequencies for RFID are:
125 - 134 kHz
13.56 MHz
UHF (400 – 930 MHz)
2.45 GHz
5.8 GHz
Although there are other frequencies used, these are the main ones.
In the UHF band, there are two areas of interest. Several frequencies in the 400 MHz band and then the band 860 – 930 MHz
Each of the frequency bands have advantages and disadvantages for operation. The lower frequencies 125-134kHz and 13.56MHz work much better near water or humans than do the higher frequency tags. Comparing passive tags, the lower frequencies usually have less range, and they have a slower data transfer rate. The higher frequency ranges have more regulatory controls and differences from country to country.
The various bodies that control frequencies include the FCC in the USA and CEPT/ETSI in Europe.
But can’t bar codes do everything I want?
A common question heard here, followed by "RFID is so expensive". As with most questions there are two answers for both of these statements. The use of bar code is something we have become very familiar with, and it is difficult to find an application that has not benefited from the use of bar code. However, bar codes do have some very important limitations that RFID can overcome. Using the two hand in hand can give some significant benefits to a system. The second question is a difficult one to answer, but in many questions the answer is "how can you afford to NOT implement RFID".
Let me explain further. Two areas that bar codes do not perform well at are: changing data and harsh environments. Let’s face it, a bar code does not stand up to an automobile paint show very well (there are of course always exceptions). And once you have printed a bar code, that’s it. You can always print a new one, but you cannot change the first one.
RFID tags have the capability to have the data changed on them many times depending on the specific type. You can store information and update it as a tag moves through a process, keeping the important information with the tag (and the item) and so making it available at any point in its life. Now some items do not need information changed frequently and this may well be an example of when a bar code will do the job needed.
Let’s consider an operation where an item is moved through a series of processes along a "conveyer belt" system. At each station, the item is taken off the belt and an operation is performed on the item. It is then returned to the belt to move on to the next station. The item has a bar code on it, and at each station the bar code is read as it enters the station and again as it leaves. The work done is recorded on the central database system. The operator has to move the item to the reader (or the reader to the item) twice at each station.
Now let’s think about an RFID system in this situation. The items all have a tag attached instead of a bar code label. The reader is setup to cover the access to the belt, such that the operator must remove the item through the readers window when it is removed and replaced. Because the reader is able to sense the tag in any orientation, it is not necessary for the operator to swivel the item to get a good read. As the item is replaced on the belt, the tag is updated with the latest information about the work in progress.
Think about the time savings alone when the operator does not have to manipulate the item to a scanner. Think about the benefits of using a portable reader to interrogate the work in progress at any time.
Now think about the item being used in a harsh environment. The same tag will work in most situations, hot, cold, dirty etc. where a standard bar code label might have problems (direct thermal printed labels don’t like the heat too much) and so give the flexibility needed to incorporate into a total system.
Where do I use RFID?
A generalization of another favorite question I get asked – "will RFID work in my application" The answer is usually a resounding yes. There are obviously situations where it doesn’t make sense, either practically or economically, to use RFID. Tagging low value items with a high cost single use tag is probably not a good idea. Using an RFID system in an application with large amounts of ferrous metal may pose a challenge. But in those situations where the tag is re-used the answer is almost always a big YES. Asset tracking, reusable containers, harsh environments, and non line-of-sight applications for example are classic candidates for an RFID system and you will find that initial cost issues will disappear when you look at the return on investment.
RFID – Is the time now or yesterday?
A year ago, I along with many others were looking forward to 2001 being the year that RFID really "took off". Did it happen? Is this the start of a new era? As usual the answer is not a simple yes or no, but we can honestly say that we did see a step in the right direction.
RFID has taken a major step forward. You can search the web and find hundreds of web sites that deal with this as a topic. Hundreds of manufacturers making RFID systems. We can read about organizations like the UCC and EAN supporting an initiative called GTAG to produce an RFID solution for global supply chain applications involving returnable containers, asset management, transport and logistics.
We see MIT’s (Massachusetts Institute of Technology) Auto-Id center working on developing a standard system to identify objects using RFID [Radio Frequency Identification]. RFID tags are built into objects like food, clothes, drugs or auto-parts, and ‘read’ by devices in the environment, e.g. in shelves, floors, doors. Their goal is a tag that costs under 5 cents and a reader for ~$100 with ~4 feet range using multiple frequencies (e.g. 13.56 MHz and 915MHz)
But what of NOW. The answer is easy. For those companies that are looking for a solution to their needs, especially if those needs involve a reusable tag, DON'T WAIT. Do it now. We have seen some recent cases where the return on investment has been 4-6 months for some fairly large systems and this is a saving that your company could be having now. With most manufacturers participating in the standards work, you can easily get assurances from your vendor as to their intended conformance with the proposed standards and with this knowledge the path forward should be clear.
