Real-time locating systems basics

You may be able to use RTLS technology to gain visibility and find efficiency in your operation.

By Sara Pearson Specter, Editor at Large -- Modern Materials Handling, 6/1/2009

 Real-time locating systems (RTLS) can offer visibility and efficiency to supply chain and manufacturing operations.

They work by leveraging radio frequency identification (RFID) tags attached to the assets being tracked—tag signals are read automatically and continuously. Data collected about the tag location, as well as the speed of movement and information related to the asset, is processed by software that determines positional coordinates.

These systems address some common challenges:

• Finding assets on pallets or in containers stored in the wrong location in a warehouse or yard, and preventing the expiration of perishable goods.

• Locating tools or component parts when needed to prevent production slowdowns and the unnecessary purchase of replacements.

• Locating the correct work-in-process items quickly.

• Monitoring personnel movement through high-security areas.

• Eliminating theft.

Given the inherent limitations of RFID technology in areas with a high concentration of metal—such as racking systems that impede signal transmission—RTLS technology has principally been gaining ground in large, work-in-process manufacturing and job shop facilities with high ceilings and open spaces as well as in marine terminals and outdoor yard management applications.

"The biggest challenge with RTLS is accuracy," says Andy Bennett, RFID solutions consultant for HK Systems (262-860-7000, www.hksystems.com). "If a solution's read rate reliability is 99.99%, then you can probably justify the investment."

The type of RTLS system selected, however, can vary dramatically depending on the application and its unique demands. RTLS systems can calculate an asset's location down to a 3-foot radius—and in some cases less—to more of a ballpark estimate of within 20 feet. The best choice of technology for a given application depends on a variety of factors.

"RTLS is appropriate if you have a lot of small things in a small area or a lot of big things in a big area that you need to locate—and speed is of the essence," advises David Shannon, senior vice president for marketing strategy at Savi Technology (888-994-7284, www.savi.com). "When it comes to putting up an RTLS solution, before you get too worried about precise RTLS granularity, you should have an area big enough where something can actually get lost." Here we examine RTLS technologies, their component parts, and the most appropriate application for each.

RTLS Technologies

As previously mentioned, RTLS is driven by RFID tags. At their most basic level, tags are comprised of a silicon chip that stores pertinent information and an antenna to relay and receive data. While the tags themselves come in a variety of shapes and sizes, three kinds have become the most prevalent in RTLS applications:

Active tags include a battery to power signal transmission. While the battery yields a longer read range and precision to within 3 to 10 feet, it also makes the tags more expensive. The life of the tag is limited by the life of the battery, with most lasting five to seven years. Adjusting the frequency of the signal transmission is effective in prolonging battery life.

Passive tags are battery-free. They use energy from RFID readers to trigger and power data transmission—meaning they are significantly less expensive than active tags, but limited in their read range and not exactly real time. Subsequently, their level of precision is more in the 20-foot range, making them most appropriate for outdoor, yard management use.

Ultra-wideband (UWB) tags are the most expensive but also the most accurate. These solutions use active tags that send short pulse radio waves at high bandwidths for maximum precision locating capabilities, down to a few inches. This makes them ideal for high value items or processes, such as just-in-time manufacturing.

"Precision is the No. 1 benefit of UWB technology over the others," says Greg Clawson, senior vice president of worldwide sales and marketing for Time Domain Corp. (256-922-9229, www.timedomain.com), who notes that the technology has even proven to be capable of transmitting signals from inside metal coolers and warehouses with metal parts and bins.

Regardless of the tag technology, the signals from the tag are captured by an RFID reader or readers located in multiple areas. For some applications, a very basic choke point RTLS system may be all that's needed to determine the approximate location of an asset. The tags are read when they pass fixed (choke) points in facility or yard—such as through a doorway or opening, indicating where the asset has been, although not necessarily where the asset currently is located. In more complex operations, multiple readers are often located overhead indoors or on vehicles both indoors and outside.

Additionally, sensors may be used in conjunction with the readers to monitor the asset's physical condition, including ambient temperature or humidity.

Two methodologies for RTLS data transmission are principally in use: traditional proprietary systems (more expensive), and systems that piggyback on a facility's existing WiFi 802.11 wireless infrastructure (less expensive). The tags and readers are specially configured to accommodate either method.

How RTLS Finds Assets

Information is gathered from the readers and transmitted wirelessly to a host computer. The collected data is then transferred to a host computer where software algorithms crunch the information to determine the item's location as precisely as the base tag technology permits. There are a few approaches to analyzing the signal from a tag.

Time difference of arrival (TDOA), also called triangulation, uses data from three RFID readers to receive the tag signals. The time each signal is recorded is forwarded to the software, which triangulates the asset's location based on the time difference between each of the three signal points.

Alternately, some RTLS systems calculate asset location based on received signal strength indication (RSSI). This method is based on a map of radio signal strengths at various points throughout the facility. Algorithms compare the most current information about a received RF signal with historic data archived within the system to determine the location.

Global positioning systems (GPS), which rely on triangulation from satellite data, are also employed in outdoor applications as an additional layer of coordinate information. GPS data is transmitted to the receiver along with other information about the asset from specially configured tags.

While the options may seem endless, there are a few rules of thumb to follow when determining which methodology is most appropriate for a given operation. Define the process that will be implemented and how RTLS information will be used in that process, recommends David Phillips, director of sales engineering for Zebra Enterprise Solutions (866-230-9494, www.zebra.com).

"If the location of an asset is needed in inches, not feet, that will drive you toward RTLS utilizing UWB. If the location of an asset needs to be located to a work cell or a parking spot in a multi-acre parking lot, that would drive the technology toward a traditional RFID solution. If an asset needs to be tracked and located in a general office area, 802.11 is a preferred solution," says Phillips.

The right RTLS for a given operation often comes down to return on investment, adds Michelle Kiang, founder and vice president of marketing at PINC Solutions (510-845-4900, www.pincsolutions.com). "Think hard about your requirements in terms of operations and processes," she says. "Are you willing to make changes to your infrastructure, or would you prefer an approach that's compatible with your current process to maximize cost effectiveness and get a faster ROI?"

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