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Passive vs Active RFID: Advantages and Disadvantages

Posted by Kapil Asher on Oct 1, 2015 9:53:45 AM

In IoT Technology, RFID, Internet of Things, RTLS, WiFi

This year marks my 10th anniversary working on wireless tracking technologies. As a young electrical engineering student intrigued by all things wireless, my first encounter with radio frequency tracking was during a research assistantship at the University of Maryland, where I evaluated technologies for improving supply chain automation. I was blown away by the fact that a Band-Aid-sized electronic sensor could essentially make anything in physical space identifiable in the digital world. The terms "Internet of Things" and “IoT” were not commonly used back then.

From a ten-thousand-foot view, the technological nuances of wireless tracking can be intimidating, but once the basics are understood, it can be an extremely powerful tool. Notice the use of the word “tool”, because that’s exactly what wireless tracking is—an enabling tool and not an entire solution. In this blog, I break down the concepts of RFID and RTLS, the advantages and disadvantages of both, and their role in automation, specifically how it pertains to hospital workflow.

Understanding Passive Tags

RFID is the blanket term that refers to Ultra High Frequency (UHF) Gen2 Passive radio frequency identification technology. It’s usually called “passive RFID” because in a normal state the tags are not powered by an external source such as a battery. They are energized only when they are within the RF (radio frequency) field of a device typically called the RFID reader or interrogator. Passive RFID operates in the FCC (Federal Communications Commission)approved Industrial Scientific Medical band that spans from 902 - 928MHz, typically the size of a network router powered by AC supply or over the Ethernet (PoE). The readers are connected to antennas which disperse the RF field that energizes the tags. Since the passive tags are energized only when they pass through the RF field from these antennas, selecting the locations for installing them is one of the most important design criteria in passive RFID deployment. Different antennas have varying shapes of RF fields (RF bubble), which is published by each manufacturer.

The limited coverage of a single antenna point warrants the need for installing the RFID readers and antennas at every location where tracking is required. The advantage of the limited coverage is that a particular antenna can be tied to a single location, and as tags are read the system updates their last known location. The nature of coverage provided by an RFID antenna allows solution architects to design a system with varying degrees of location granularity, achieving facility, floor and room-level accuracy. Depending on the nature of the installation, the RFID readers can continuously detect the presence (or absence) of tagged items, such as used equipment in soiled rooms, or act as a checkpoint for loss and theft prevention at critical chokepoints, like laundry chutes and dock doors. RFID enabled chokepoints can also be used to detect equipment entering or leaving large rooms such as central distribution or a biomed shop. Another advantage of passive RFID is that the tags can be read using iOS and Android-based mobile devices with a handheld attachment, allowing users to locate tagged equipment hidden (intentionally or unintentionally) inside closed bins and cabinets, behind ceiling tiles, or under desks, sinksyou name it.

Passive RFID has grown in popularity within the hospital community since the tags typically cost $2 to $5 per unit, and it eliminates the need for battery management completely. However, location accuracy depends heavily on system design. Being batteryless, the tag response is not very sensitive to the reader signals. Solution architects must make sure that the designed system is able to energize tags in all locations, keeping the RF field pattern in mind. The response of the tag may also be dampened by environmental conditions, such as the presence of metal and liquid between the antenna and the tags, which makes placement of tags on the assets themselves critical. In a few cases, the system can suffer from "overdesigning", like when the RF fields of two antenna read-pointsintended to cover two different locationsoverlap in space, causing a cross-reading of the tagged asset.

Understanding Active Tags

RTLS (real-time location system) is a term that typically refers to tracking using active tags. It is called active because the tags are constantly powered up by an external source such as a battery or AC power. The tags constantly ping their locations within a facility providing real-time tracking visibility. RTLS tags have multiple ways to identify their locations within a facility, one of which is triangulation used by the network of WiFi access points. A basic RTLS system with floor or area-level accuracy does not require installation of additional infrastructure within the hospital, provided the WiFi network is supported by the RTLS technology. However, if room or zone-level accuracy is required, most RTLS systems require additional sensors that must be installed in strategic locations. These sensors use technologies such as infrared light, low frequency radio,  ultrasonic sound, and BLE (Bluetooth Low Energy). Similar to passive RFID, these sensors are tied to a single location. As active tags pass through their field, the system updates their last read location. These location sensors are typically powered by their own batteries.

As the name suggests, RTLS provides continuous location information of all tagged items within the entire facility. The technology is particularly popular for highly mobile assets such as wheelchairs and patient beds—larger assets that typically do not have a permanent station. RTLS provides facility-wide coverage which is cost prohibitive for passive RFID, as it requires installation of readers and antennas throughout the hospital. RTLS has gained momentum in recent years for tracking staff, including nurses, doctors and even wandering patients. RTLS tags also provide buttons that can be programmed to trigger on-demand alerts and notifications. Another common application for RTLS tags is temperature monitoring of vaccines, and other medications—alerting staff in case something is out of compliance.

Hospitals can benefit tremendously from RTLS, but they must also understand the limitations of WiFi-based location tracking. Access-point-based triangulation is not accurate and can compromise data integrity, especially in closed-loop equipment distribution workflows where soiled and clean rooms may be right next to each other. Without installing additional location sensors as described, it is very difficult to guarantee that tagged assets are in a given room. Erroneous alerts can be generated by the system assuming equipment presence in the wrong category of rooms. Solution architects must therefore analyze the workflow and design the system using the right combination of location sensors for high accuracy use cases. In addition, the deployed solution must factor in the usable battery life of the tags and location sensors, and alert users well in advance. Unlike passive tags, active tags are completely undetectable if the battery needs a replacement, rendering them completely useless. In some cases, tag size and operational temperature of active tags is also a limiting factor for their usage.

In the past decade I have seen dramatic progress in this technology. Performance improvements, new features, a reduction in device size and cost are a testament to the considerable investment companies have made in the technology— say nothing of all the hard work done by some pretty smart engineers. With the revolution of smart devices and the Internet of Things, I’m excited to see what the next 10 years has in store for us. Powerful hardware platforms like Raspberry PI and Arduino combined with sensors are able to collect more and more data from the physical space, and intelligent algorithms and software workflows generate a level of automation and accuracy that is staggering to comprehend. As they continue to streamline workflow processes, hospitals will do well to keep a finger on the pulse of these growing IoT technologies, for their advancement has already begun to shape the future.

 
  Successful Hospital Implementation of RFID