Asset Tracking

Asset Tracking in Industrial Settings—A Review of Wireless Technologies Part 2: RFID-, GPS-, and Chirped Frequency-Based Systems

July 1, 2008 By: Dr. Peter Fuhr, Oak Ridge National Laboratory

Real-time location systems require accuracy and reliability. Will RFID-, GPS-, or chirped frequency-based systems meet your minimum requirements?


Radio-frequency identification (RFID), GPS, and chirped frequency wireless are part of the constellation of technologies that can enable industrial real-time location systems (RTLS). Each has its own strengths, weaknesses, and unique infrastructure requirements. A quick review of the fundamental principles of these wireless technologies may help you to evaluate their potential.

RFID-Based Asset Tracking
In general, RFID-based systems include a database application, asset tag readers (sometimes referred to as interrogators or gateways), and the tags themselves. A key aspect of these systems is that they require a reader, or interrogator, to obtain data from the tags. The reader-tag topology is similar to that of a sensor-gateway topology. All the system components follow a highly choreographed bidirectional communication sequence, as shown in Figure 4, to deliver the data to back-end systems.

 

Click for larger image Figure 4. Interrogation procedure for elements in the RFID system (Click image for larger version)

By using license-free (ISM) frequencies, ranging from 125 kHz to 245 GHz (Figure 5), it's possible to tailor a system to meet specific needs. For example, if the goal is to track livestock, a system operating at ±125 kHz is preferable to one that functions at 2.4 GHz because of the significantly lower signal attenuation caused by the animal at that frequency.

 

Figure 5. Frequencies typically used in RFID

figure

Each band has its own drawbacks. For example, the bandwidth allocated to the 125 kHz ISM band is substantially less than that associated with the 2.4 GHz band and therefore transmits less information (bits). Another drawback for both readers and tags is that the lower the frequency the larger the antenna must be.

There are many flavors of RFID, ranging from systems with no active components on the tag to battery-powered semi-active and fully active tags (which are essentially microcontroller-based wireless devices that sometimes include sensing capabilities). Passive tags have no battery and receive their energy from the interrogating signal, a feature that reduces the reader-tag separation distance to <1 m. By adding a battery to the tag, the design can change to incorporate a radio transceiver, extending the separation distance to as much as 100 m.

As depicted in Figure 6, the separation distance between the reader and the tag plays a significant role in the actual deployment of RFID-based systems. Directional antennas are frequently used to limit the field of view, and therefore the location resolution zone, for the readers.

 

figure
Figure 6. A typical RFID-based system, consisting of readers, tags, and a connection to the database management system that is recording where the tagged asset was last observed

Active tags, with their onboard power supply, typically support substantial on-tag memory and a processor, all of which allows more information to be transmitted. Having onboard power, however, implies that at some point you'll have to replace the battery, which translates into increased servicing needs.

A basic RFID-based system can tell you which reader the tagged asset was most recently near. To obtain higher spatial resolution, where the system provides the asset's location, you must deploy more readers.

After the deployment of RFID-based asset tracking technology in an industrial facility, the next step is usually to integrate it with IT-centric systems at the site. User surveys conducted by Mann Consulting (for ISA) and WINA have shown that users are interested in deploying WiFi access points throughout their facilities to allow operators to access the plant's HMIs via tablet and laptop PCs or handheld units. This WiFi backbone can also be used to integrate RFID readers into the overall IT and network infrastructure.

The use of multiple wireless systems for multiple applications is even more evident when ISA100.11a (or similar) wireless field transmitters are deployed. In this situation, the gateway device for the wireless field transmitters may be using wired/fiber or wireless links to the facility's communication infrastructure. This enables transmission of the data from the field transmitters to back-end systems where the data is used. A depiction of this integrated environment is shown in Figure 7.

 

figure
Figure 7. Integrated wireless applications deployed at an industrial facility

GPS-Based Asset Tracking
The near-ubiquitous presence of GPS handheld units and automotive navigation systems raises the prospect of using this asset tracking technology in an industrial setting. A quick review of the fundamental principles of GPS may help in evaluating the technology's potential.

To track an asset using GPS, you must determine the asset's position coordinates, Ax, Ay, Az, and the time of the determination, At•At, which is related to a value referred to as clock bias Cb. GPS-based tracking requires the receiver on the asset to pick up signals from four satellites and measure the time it took the signals to arrive. Using the equation R = Vt, where V is the velocity of the signal (in this case a constant, namely the speed of light) and t is the time it took to receive the signal, you can calculate R, the range from the receiver (asset) to the satellite.

Within the signal transmitted from the GPS satellite are its X, Y, and Z coordinates at the time the signal was sent. Having this positional information for four GPS satellites, coupled with the range between the asset's receiver and each satellite, as well as the clock bias information, yields the four equations necessary to determine the position of the asset using Equation 2:

equation (2)

With simultaneous data received from four satellites and ideal conditions—direct line of sight to the satellites (no multipath) and minimal ionosphere-induced variations—you can calculate the asset's location (e.g., latitude, longitude, and altitude).

The physical environment of an industrial setting is often far from ideal. It's easy to envision situations where there isn't visibility to all four satellites and the multipath environment presented by the "canyons of metal" within a plant plays havoc with the GPS signals, particularly when trying to ascertain the clock bias.

Nonrestricted GPS signals are transmitted at 1.575 GHz, a frequency blocked by steel and concrete structures and attenuated when the signals pass through trees. The GPS specification for minimum detectable signals renders reception marginal when the signal is attenuated by foliage. As such, receivers that just barely meet the specification are not reliable for use in forests or even tree-lined streets. Add the potential complexities of tracking mobile assets, either within an industrial site or going in and out of buildings, and you begin to see the fundamental limitations associated with using traditional GPS for industrial asset tracking.

Numerous techniques associated with locally generated differential corrections have been developed to enhance overall system performance. The net result is that GPS-based asset tracking may be of use in outdoor industrial settings but with the full understanding of the possible limitations on performance.

Chirped Frequency–Based Asset Tracking
There has been a significant call from the end-user community to identify a relatively easy method of obtaining location and sensing information from a wireless field transmitter. While discussions pertaining to the actual need for such capability continue (e.g., "The transmitter is bolted onto the tank; it's not going anywhere."), there is a variant of the ISA100.11a radio technology that may readily provide such multifunction capability. The 802.15.4a radio relies on a technique of changing the center frequency of the transmission in a linear manner. The output begins at one frequency, but over time, at a predetermined rate of change (slope), it changes linearly to another frequency, as shown in Equation 3:

equation (3)

where:

F(t)  =  output of the radio at time t
F0  =  radio's starting frequency
a  =  change of frequency with time (i.e., the slope)
T  =  time

The system performance and range determination are easy to see using the analogy of chirped frequency radar. Consider the case where a chirped frequency radar transceiver outputs a certain frequency F1, at time T1. The signal proceeds a distance X, taking time T, until it encounters a surface and is backscattered toward the radar transceiver. After another time T, the reflected signal is received by the radar transceiver. Because distance equals velocity multiplied by time (X = V•T) it has taken the signal a time interval 2T to travel down and back from the reflector. Because the speed of the electromagnetic signal is C, the speed of light, it is easy to find the distance, or range, from the radar transceiver to the reflector.

In the chirped frequency case, the situation is similar, but involves recording the frequency (F2 at time T2) that the radar transceiver is current outputting. The difference in frequency is calculated using Equation 4:

equation (4)

By knowing the slope of the linear frequency ramp function a, the time difference ΔT is determined. Knowing ΔT allows you to determine X, the distance (range) to the reflector (Figure 8).

 

figure
Figure 8. Chirped frequency-based range determination

Using a chirped frequency radio to obtain location information relaxes the time-resolution requirements for range-measurement resolution by shifting the measurement into the frequency domain. In the industrial field transmitter realm, there is a requirement for multiple devices to have connectivity to the asset under measurement (to provide the X, Y, and potentially Z coordinates of the asset). You can achieve this by having the asset within range of multiple gateways or by using relative location information for other field transmitters within range of the asset.

There must be an infrastructure, or network/system, connection in place that will transport the range information to the appropriate software application. It's worth reiterating that the asset tag must be within range of multiple gateways to achieve X, Y, and Z positional information. Typical location accuracy is in the cm range, with an overall asset-to-asset or asset-to-gateway separation in the 1–100 m range. The infrastructure support system is therefore similar to that shown in Figure 6.

Next Month
Part 3 of this series, which will appear next month, continues the technological overview of wireless asset tracking systems with an examination of received signal strength, RuBee (IEEE P1902), and ultra-wide bandwidth technologies.

This article is extracted from a document submitted to ISA100 by the ISA100.21 Working Group.

EDITOR'S NOTE
The figures in Parts 1–3 of this series are numbered consecutively.


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