What factors lead to such failure, and how can they be minimized?
It depends on what you mean by “failure rate.” That phrase could refer to the percentage of time that RFID readers fail to capture data from a tag, or the percentage of applications that fail to deliver business benefits. Each type of failure is different, and the explanation of how to reduce the failure rate is complex.
I will focus my answer on the inability to capture tag data. There are many potential reasons that this could occur, which might differ depending on the type of RFID system you deploy. With an active system, failure to read tags is rarely an issue since the tags broadcast their own signal. Evidence of this reliability can be found in toll-collection systems, such as E-ZPass (see RFID News Roundup: E-ZPass Celebrates 20th Anniversary). No transportation agency would use active RFID for automated toll collection if the technology could only read tags 90 percent—or even 99 percent—of the time. These systems need to be nearly perfect, which they are.
Failure to read tags is a passive RFID issue, and often a passive ultrahigh-frequency (UHF) issue, since high-frequency (HF) and low-frequency (LF) solutions have a shorter read range and utilize a different means of tag-reader communication. Listed below are some common issues that prevent tags from being read consistently.
Incorrect tag orientation: Some tags are designed to be read regardless of orientation, and some reader antennas (those that are circular-polarized) are built to read tags in any orientation. But if you are using a dipole UHF tag and a linear-polarized antenna, for example, and if a tag is improperly oriented, it will not capture sufficient energy from the reader.
Electromagnetic interference: “Noise,” or electromagnetic interference from other RF devices, as well as factory machines or fluorescent lights, can either block waves from getting to the tag (if they are employing the same frequency) or corrupt transmission.
Absorption of RF energy: Materials that contain a large amount of water absorb RF energy at the frequency used by passive UHF RFID systems. The energy absorbed is unavailable for use by the tag, which will fail to receive enough energy to reflect back a strong signal to an interrogator. In addition to water, other dense materials can prevent RF energy from reaching the tag as well.
Reflection of RF energy: Metal can reflect energy away from a tag, or reflect the tag’s signal away from a reader antenna, thereby preventing normal tag-to-reader communication. If energy bounces off a floor, a ceiling, metal shelving or other materials, it can cancel out waves and cause null spots in the read field. Tags located within any such dead zones will not be read.
Detuning the tag antenna: Antennas on a UHF RFID tag have been tuned to receive signals within the UHF spectrum. If a tag is placed on or very close to metal, its antenna will become detuned and it will no longer receive a signal. This is similar to touching a radio antenna with a piece of metal—the channel being clearly received suddenly becomes static since the radio can no longer receive the signal.
Shadowing: If tags are lined up one behind another, the first tag captures the reader’s energy, but the tags behind it do not. This effect is sometimes called shadowing.
All of these issues can usually be overcome with good system design.
—Mark Roberti, Founder and Editor, RFID Journal
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