Tracking wireless

InTech, Apr 2008 by Fuhr, Peter, Hedroug, Nacer

Experts answer users' questions on tracking wireless assets in industrial settings

Looking for your car keys, the remote, or the cat is fast becoming analogous in an industrial facility to finding a vehicle, sensor, or tank car. What was originally associated with the radio frequency identification (RFID) world has evolved into the more appropriate real-time location service (system), or RTLS, arena. While you could think of RFID as a database management system with readers who could inform the database as to where it last observed a tag, RTLS brings the realm of tracking devices into the industrial world.

Vendors are touting several technologies as optimal for industrial asset tracking, especially end users in petrochemical (on-shore/off-shore), pharmaceutical manufacturing, power systems (generation and distribution), and pulp and paper processing.

Industrial setting

An industrial setting presents specific radio frequency (RF) and environmental ambient conditions that may be more taxing of a wireless system than seemingly any other site. In each of these locales, the amount of reflective and absorptive surfaces associated with the aptly named canyons of metal lead to variable attenuation and multipath conditions, which are frequently dovetailed into requirements for the wireless devices to operate in a non-line-of-sight situation. In practice, a radio signal may encounter many objects in its transmission path and undergoes additional attenuation depending on the absorption characteristics of the objects. There are many types of objects, including fixed, mobile, and transient objects that absorb RF energy and cause RF attenuation. Similar to the free-space propagation loss, higher frequencies attenuate much faster than lower frequencies. Therefore, 5 GHz RF signals typically have higher attenuation than 2.4 GHz, though there are a few exceptions.

The table shows how various objects introduce attenuation to RF signals at the 2.4 GHz and 5.8 GHz bands. Note the attenuation values for concrete. The large disparity in the values conies from different types of concrete materials in use in different parts of the world. In addition, the thickness and coating differ depending on whether it is used in floors or interior or exterior walls. Brick walls usually have attenuation at the lower end of the range shown here.

Not included in the table are attenuation values for a number of other materials commonly found on site. Measurements of RF signal attenuation caused by water reveal higher levels of relative attenuation of signal transmissions in the 5.7 GHz ISM bands than in the 2.4 GHz band. However, rain, snow, and fog attenuations are very small for frequencies under 10 GHz. The rain attenuation at 5 GHz is barely noticeable (

Four candidate technologies associated with industrial RTLS include: RFID, GPS-based, chirped frequency-based, and received signal strength indicator (RSSI)-based. Each technology has its own unique requirements for a supporting infrastructure to provide a somewhat comparable location resolution with, of course, an accompanying system cost.

RFID-based asset tracking

Different flavors of RFID range from systems with no active components within the tag to batterypowered semi-active tags, to fully active tags (microcontroller-based wireless devices, including the possibility for sensing). The key aspect of RFID systems is they require a reader, or interrogator, and the tags themselves. By using ISM (licensefree) frequencies ranging from 125 kHz to 5.8 GHz, it is possible to tailor the entire system for specific needs. The separation distance between the reader and tag plays a significant role in the actual deployment of RFID systems. Directional antennas frequently see use to limit the field-of-view, and therefore the location resolution zone, for the readers.

Numerous variations exist to the above classic RFID system. In a passive system, the tag has no battery but reduces the reader-tag separation distance to less than 1 meter. Adding a battery into the tag provides an onboard source to power a radio, which leads to a situation where the separation distance may be 100 ( /-) meters. While innumerable matters might arise with an active tag (what radio to use, what frequency to use, what power to output, what protocol to use, and directional antennas), in this instance, the tags and readers topology is similar to that of any sensor and gateway topology. Such higher power active tags typically have considerable on-tag memory and a processor that allows for more information, perhaps even sensorrelated information, to transmit upon receipt of an interrogation signal from the reader. Having a battery implies at some time you will probably have to replace it, hence the increased servicing needs, particularly in an industrial facility that may be hazardous or even present an explosive environment.

Hundreds of companies are currently offering RFID components and systems, but the general situation for an RFID-based system is the database can inform the user about which reader last observed the tagged asset. A requirement for higher resolution as to the asset's location requires more readers to deploy.