Trial by Tire

By Pat King

Singulation and the RFID Label
The dilemma presented above emphasizes the field nature of RFID. Even though the handheld reader employed had a linearly polarized antenna, it still projected the RF beam as an expanding field. The best way to visualize this is to imagine the RF reader as a flashlight projecting a beam of light that becomes wider as the distance from the flashlight increases, and that any tag within the field of the light can be read. The farther the tag is from the reader, the wider becomes the field of reference that includes the tag (and, potentially, many other tags).

This phenomenon is in stark contrast to the use of a bar code reader. Utilizing a narrow laser beam, a bar code reader traditionally scans only one bar code at a time. As a simple test, compare a pointer laser and a conventional flashlight. Imagine the laser precision as it individually scans over an individual bar code, while the RFID tag would be read as a result of being in a field similar to that emitted by the flashlight. One way to bridge this difference would be to find a way to cause the RFID system to behave more like the bar code system.

Also tested were tires with sidewall labels, placed in stacks.

Unfortunately, many RFID implementations today try to replace bar code systems directly with RFID systems, ignoring the process issue that RFID projects a wide field while the bar code scanner is more linear. The real goal is to enable the interrogator to read each RFID tag individually. There are many ways to approach this goal, but once it is accomplished, a reliability in excess of Six Sigma becomes possible—even routine—as does a high degree of automation, because RFID does not have to find the tag, as a bar code reader does. While this appears to be a disadvantage for the RFID system, that's only because most people imagine RFID as a direct replacement for bar codes. If the RF field has only one tag in the field, not only can RFID be at least as effective as a bar code, but it can be far more automated since the bar code needs to be specifically oriented and scanned by the narrow beam of a laser, whereas the RFID tag need only be within the field. This means an RFID system, when properly designed, can be faster, more reliable and more automated.

The sidewall of a tire is physically more uniform than the tread area of a tire. In fact, it is generally possible to attach an RFID label to a uniform surface on a sidewall, as there are more smooth surfaces available. Imagine a typical tread pattern; now imagine the probability of centering the tag over an identical location on an adjacent tire. On the first tread tag, the chip portion could bridge an air space, while on the other, that chip might rest on the tread rubber.

Furthermore, when the tags are located on the sidewalls of stacked tires, the surface of the opposite sidewall in the same stack acts as a filter or barrier to RF propagation. The adjacent sidewalls of two tires, with an RFID tag on one sidewall, forms a sort of rubber sandwich with an air gap in the middle. This acts as the only channel for the reader beam to read the "singulated" RFID tag.

In the course of this test, tags placed on the exterior surface of a sidewalls provided 100 percent reliable reads. The test results did not suggest that retail solutions should include such an RFID label, but they did reveal that the same tags and tires can be organized so that either 0 percent or 100 percent reliability is attainable. This is a remarkable example of singulation theory. In the case of the tire-tread label, no amount of practical efforts would net a high-speed reliable material management system. By contrast, as demonstrated in this test, a sidewall tag location could be employed for high-speed reliable material management.

Pat King is Michelin's global electronics strategist and the company's representative to EPCglobal.