RFID readers perform important operations that make using an RFID system easy
RFID readers are used to read from, and in most cases write data to, an RFID tag. Since RFID tag systems utilize radio waves to exchange information between an RFID tag and some kind of control system (PC, PLC, DCS), RFID readers are transmitters and receivers of radio waves. Frequently, RFID readers are also called read/write heads and some people have called them RFID sensors. Independent of what they are called, RFID readers perform several very important operations that make using an RFID system easy.
The air interface
Because RFID is a noncontact technology, meaning the RFID reader does not have to touch the RFID tag to exchange information, the data exchange occurs over an air interface. The reader emits a radio field and the tag responds to that field. Just like AM and FM modulation operate different “on-the-air interfaces,” the air interface is typically very different from tag to tag, even if those tags work at the same operating frequency. It is thus the job of the RFID reader to know about those tags' specific differences and emit the necessary RF field so that the user does not have to worry about these details. Clearly, an RFID reader is more than an antenna.
Most modern designs are microprocessor-based devices that receive commands – for instance, to read a certain amount of data or to write a particular string of information to the tag – then translate this into the proper RF field modulations such that the tag executes the desired operation. Think of the RFID reader as a “translating agent” that takes in data on a cable connection and turns it into a string of data on the air interface.
The cable interface
RFID readers are typically connected to an RFID controller (in some cases, those two functional entities are located in the same housing). Modern designs utilize a standardized communication interface between the RFID reader and RFID controller. This has many advantages with respect to the RFID controller. By establishing a standardized communication interface between the RFID controller and the RFID readers, it is easily possible to develop new RFID readers and/or give them new functions without having to modify the RFID controller.
Example: Imagine an installation is using an RFID tag based on the iCode SLI ship from NXP. Years later, NXP decides to get out of the business (we certainly hope not), obsolescing this chip. At first glance, this looks like bad news and a big problem for the user. Fortunately, the installation uses IDENTControl from Pepperl+Fuchs. The worst case scenario is that the obsolete tags are replaced by new tags using chips from one of the competing vendors. There is actually a very good chance that this chip will work without any further modifications to the system or the PLC code. But even if a totally new tag is required, the RFID controller would most likely not have to be touched and only a slight modification to the PLC program may be necessary.
RFID reader size and operating range
The operating range between an RFID reader and tag depends on the operating frequency. Readers utilizing the microwave or UHF band will have significantly more range than readers operating in the low- or high-frequency band. Looking at low- and high-frequency-based RFID readers, the size of both the tag and reader – or more precisely, the antenna structure in the tag and the antenna in the reader – are the main contributing factors to the operating range. Larger antenna structures result in longer operating ranges.
The form factor of an RFID can have a significant impact on the application. For factory automation applications, it is most common to utilize housing designs that are identical to housings used for proximity sensors. The advantage for the user is that manufacturers have extensive experience using these housings, resulting in products with high IP ratings at an attractive price.
Read-only and read/write RFID readers
In the past, manufacturers designed readers that were capable of exchanging data with read/write RFID tags (those with memory that can be altered by the user) and readers that could only communicate with read-only (R/O) tags. Cost was the sole reason for doing this. Today, this is no longer necessary as most readers use highly integrated ASICs that allow communication with either style, making separate development not only unnecessary but needlessly expensive.
The read zone
When selecting an RFID reader, it is important to understand the effect of the read zone. The read zone is the area in front of a reader where a specific tag can be read with certainty or at least a very high probability for success. Most typically, this information is provided graphically. RFID suppliers should be able to present this information; without it, it is very difficult to determine how precisely a tag needs to be moved past the reader, how quickly a tag can move by the reader and what happens at the fringes of the read zone. The following will discuss how read zones are determined and what type of analysis is used to end up with useful information for the user.
It should be intuitively clear that not all tags from a batch behave exactly the same. Some have more and other have less read range. Similarly, there are variations from one reader to the next.
The best way to quantitatively evaluate these variations is to take a number of tags T and a number of readers R. Next, the read field for every possible combination is determined. A read curve is determined by moving the tag at different distances past the reader. At each measurement point, the test equipment records whether the read operation was successful or not. Then the whole process is repeated at a slightly increased tag-to-read separation. The results are R•T distinct read curves.
The information for all these read curves can then be consolidated into a single graphic that shows how many of those possible R•T reads are successful. Figure 1 shows such a consolidated curve for 8 readers and 8 tags, resulting in a total of 64 curves. The green area is where every reader/tag combination was successful. The red area shows locations where no reader/tag combination was able to read. The yellow area shows those separations and offsets where some combinations worked and others did not.
Figure 1: This graphic shows how many read attempts were successful at a specified distance and offset between the reader and the tag. Of those distance and offset values that are within the green area, every one of the 8 tags and 8 readers used for this analysis resulted in a successful read. The red area indicates no successful reads while the yellow area represents those distance offset values where at least 1 but not all tag/reader combinations were successful.
Using this information and some statistical analysis (the number of successful reads at each location follows a Gauss Distribution), it is possible to determine a “safe read zone” where the probability for a successful read is xx.x%.
Figure 2 shows such a graph. This graph shows the maximum range that one of the tag/reader combinations was able to reach. It also shows the minimum measured range amongst any tag/reader combination. Since it can not be assumed that the measured minimum is the shortest range we should expect, assuming a Gaussian distribution allows one to determine a read curve where 99% (orange curve) or 99.9% (red curve) of all combinations will be successful, Pepperl+Fuchs always specifies the 99.9% curve as the read curve. When looking at this data from other suppliers, it is important to understand what is being presented.
Figure 2: These curves show the maximum, minimum, and average read distance measured for any tag/reader combination in the set. Applying Gaussian statistics to this data results in the orange and red curve. The red curve is what Pepperl+Fuchs specifies to be the read zone of this tag type and read type combination. It indicates that with 99.9% certainty any arbitrary tag/reader combination will be successful.
Using the read curves
Having access to such read zone data is important when selecting RFID hardware or designing a machine using RFID. Looking at the red curve in figure 2, it should be clear that a setup where the tag is 120 mm away from the reader is not a good idea. While every single measured tag/reader combination was read at this distance, one must expect other combinations not to work here. Also, at this distance, the allowable lateral offset is basically zero. This means that even the slightest left/right movement due to vibration, wear, and general mechanical tolerances, would result in a no read.
Generally speaking, it is best to design a setup such that the tag is roughly between 40% and 80% of the maximum read range in this case. At this distance the read zone is the widest, providing maximum tolerance. Figure 3 shows this setup superimposed onto an arbitrary read curve.
Figure 3: The ideal tag-to-reader separation is roughly between 40% and 80% of the maximum read range in this case. At these distances the zone is quite wide, resulting in significant “forgiveness” in case of mechanical inaccuracies. This is also the distance range necessary when tags must be read on the fly.
Having access to this curve, one can calculate the maximum safe passing speed in situations where reading on the fly is necessary. In the above example, the read zone is (at least) 84 mm wide. The next parameter refers to the time it will take to read from the tag. This amount of time depends on the chip used to build the tag and the amount of data that must be read. The read speed can be obtained from your RFID system supplier. For example, reading 16 bytes from a tag using a Futjitsu FRAM chip takes 15.5 ms, resulting in a maximum speed of 8 m/s. At this speed, the reader ends up with exactly one chance to exchange tag data; there is no room for error whatsoever. In RFID, it is strongly suggested to consider a safety factor. For HF tags, one should use a safety margin of 3 while LF systems need a safety factor of 2. For our above sample, this results in a maximum suggested safe passing speed of 2.6 m/s.