Infrared: Traditionally, one of the most precise ways to get room-level accuracy has been to use a system that combines traditional RF technology to identify the asset, while using an IR signal (like that of your TV remote) to provide location information. IR works in this application because it does not go through walls or floors, thus enabling a sensor in each room to provide the exact room-level location of the tag. On the other hand, that inability to go through solid objects also means the signal is easily blocked by a nurse’s uniform, a work glove or any other misplaced object. While the cost of these tags is reasonable, the IR power requirements often make for short battery life and the cost of placing a receiver in each room can be high.
Ultrasound: Like IR-based systems, ultrasound systems provide room-level location accuracy. They work by using small tags with speakers that emit ultrasound at around 40kHZ, which is above the range of human hearing. Once again, ultrasound does not go thru walls in any real way, so placing a sensor in each room provides a high degree of positional accuracy. Ultrasound has the additional benefit of being much harder to shield with clothing or misplaced objects. Maximum distance is fine for a hospital environment, but can be an issue for larger rooms and is likely not a good application for warehouse space. The largest downside to these systems tends to be the cost of “gridding” the building with enough receivers to cover each room.
Conventional Active RFID: When RFID first started being used for asset tracking, this was the technology employed. There were and are a lot of tradeoffs, however, because of the laws of physics. Pick a low frequency like 125KHz and the signal easily passes through objects made of metal and containing fluid, like the human body; however, power requirements go up and maximum distances tend to drop. At the other end of the spectrum, the use of 900MHz signals provides distance and reasonable battery life, but placing a hand over the tag will block all output.
Location finding is also a problem. Since these signals move through walls, they either need another technology like IR to provide location information, or they need to compute the location using multiple receivers. This computing process measures either relative signal strength to multiple receivers or the time the signal takes to arrive at multiple receivers. The latter is becoming the preferred technology, but is complex and can still be affected by signals reflecting off nearby objects and producing multiple ghost signals that the receiver has to find a way to ignore. Overall, newer technologies find ways to offset one or more of these issues in ways that make these systems less popular at this point.
Wi-Fi: Wi-Fi-based systems are radio-based and use the common technology invented to provide wireless connections for laptops. The key advantage of these systems is the ability to take advantage of the existing Wi-Fi infrastructure that many companies already have in place to lower the cost of the installation.
Unfortunately, there are some counterbalancing issues. In most cases, the Wi-Fi equipment already in a building was not selected or positioned with location finding in mind; therefore, location accuracy can be poor unless you add or relocate the Wi-Fi infrastructure — which raises the cost substantially. Tag cost also tends to be higher than competing technologies and battery life shorter. Still, this is a technology worth considering if your accuracy needs are low or you have happen to have the right infrastructure in place. For more information about WiFi-based systems, visit Ekahau (www.ekahau.com).
Ultra Wide Band (UWB): Ultra Wide Band is an intriguing technology. By using very short pulses of RF spread across a very wide spectrum, UWB systems avert some of the shortcomings of conventional RF systems. Short pulses make the tags much easier to reliably find in spite of reflections from nearby objects (think of listening for a hand clap in the middle of a noisy room and you start to get the idea). The wide spectrum also makes the technology relatively immune to interference.
Put all of this together and you get a reliable system with long battery life that can locate assets down to 2-foot accuracy. The downside is the cost, which can be among the highest — owing to the need to use a large number of precisely installed receivers to get that accuracy. Time Domain Corp. (www.timedomain.com) is a source of UWB systems.