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RFID Positioning Explained

Everything you need to know about RFID positioning before deciding to implement it

RFID Positioning

Radio Frequency IDentification (RFID) is widely used for electronic identification and RFID positioning. RFID offers substantial advantages for businesses allowing automatic inventory and tracking on the supply chain. This technology plays a key role in pervasive networks and services. Data can be stored and remotely retrieved on RFID tags enabling real-time identification of devices and users. However, the usage of RFID could be hugely optimized if identification information was linked to location. 

The venerable RFID tag traces its origin to the “friend or foe” transponder systems developed for military aircraft beginning in WWII. Since then, RFID has earned and retained its status as a reliable asset tracking system. Recently, RFID positioning technology has been marketed as a solution for real-time indoor tracking of people. In this article, we’ll explore how RFID location tracking works, how it might be used for indoor asset tracking (people monitoring), and how it compares to alternative solutions. 

How does a RFID tracking system work? 

A RFID tracking system is composed of three different entities, RFID tags, readers, and servers. All RFID  tags use radio frequency energy to communicate with the readers. However, the method of powering the tags varies. An active tag embeds an internal battery which continuously powers it and its RF communication circuitry. Readers can thus transmit very low-level signals, and the tag can reply with high-level signals. An active tag can also have additional functionalities such as memory, and a sensor, or a cryptography module. 

On the other hand, a passive tag has no internal power supply. generally speaking, it backscatters the carrier signal received from a reader. Passive tags have a smaller size and are cheaper than active tags, but have very limited functionalities. The last type of RFID tags is semi passive tags. These tags communicated with the readers like passive tags, but they embed an internal battery that constantly powers their internal circuitry. 

RFID position tracking readers have two interfaces. The first one is a RF interface that communicates with the tags in their read range in order to retrieve tags’ identities. The second one is a communication interface, generally IEEE 802.11 or 802.3, for communicating with the servers. 

FInally, one or several servers constitute the third part of an RFID system. They collect tags’ identities sent by the reader and perform calculations such as applying a localization method. They also embed the major part of the middleware system and can be interconnected between each other. 

The specific means by which the RFID tags and readers communicate (i.e. their coupling mechanism) determines the range, complexity, and cost of the specific system. (“Coupling” in this context refers to an energy transfer between tag and reader). Currently, three types of coupling mechanisms compete in the market: inductive, capacitive, and backscatter. 

Inductive Coupling

Inductive coupling has been present since the early days of RFID when the systems involved bulky tags with complicated antenna mechanisms used to track large objects (e.g. cars or cattle). An inductively coupled tag draws energy from the magnetic field created by the reader and modulates it. The reader then measures the perturbation produced by the tag and decodes it as data. The magnetic fields used in these RFID localization systems drops off rapidly, affording inductive coupling an RFID  tracking range of about 1cm to 1m. 

Capacitive Coupling Systems 

Capacitive Coupling systems were created to lower the cost and size of RFID when large inductive systems were the only RFID positioning option on the market. They employ conductive patches on both reader and tag to form a capacitor and signal data by varying the capacitance of the circuit. These systems are extremely close range –1cm– and the orientation of the patches matters, so a typical application would be an ID card that must be inserted into a reader. 

As inductive circuits shrank, so too did the market for the more limited capacitive systems. Indeed, most RFID indoor positioning systems today use some version of inductive coupling. However, they’re still limited by magnetic fields’ rapid drop in strength at a distance. To achieve longer range reliably, RFID positioning systems must use higher frequency signals and rely on the electric side of the electromagnetic signal.

Backscatter Coupling 

Backscatter coupling employs a reader that sends out a UHF or microwave signal that impinges on a tag and then reads patterns in the reflected energy. Whether the increased range is an advantage or disadvantage depends, of course, on the use case. Scanning pallets as they pass through a large warehouse gate? Great. Unlocking doors or disseminating payment information? Probably less desirable. 

Using RFID location tracking to track assets

Before assessing RFID positioning’s merits as an indoor asset tracking technology, we need to clarify what is meant by the term “tracking”. RFID positioning has, since its inception, been used to track assets in a sort of spreadsheet sense. It makes it simple to identify and log which tracked items are nearby. If your goal is to make sure all the train cars that went through gate A also made it through gate B, or whether an employee swiped into a building, then RFID indoor positioning is a well-tested and proven solution. 

In such use cases, RFID competes most directly with barcodes or QR codes. It offers the obvious advantage of being readable at a distance. Active or semi-active RFID positioning tags can provide valuable sensor information. On the other hand, passive readers are very expensive, and powered tags are costly and have a limited lifespan. 

A more challenging type of tracking is knowing the (nearly) real-time location of a tracked asset. Although this is a relatively recent use case for RFID localization, there are already quite a few commercially available solutions on the market. 

The way these systems work varies. Some systems use RFID positioning purely for object identification while leveraging another technology for ranging. Those that rely purely on RFID almost exclusively use active RFID tags. There’s some exciting research that uses passive RFID tags, but the cost of passive readers and the low range of these systems makes them commercially prohibitive. 

Real-time location systems (RTLS) that use active RFID positioning tags behave similarly to competing technologies- Bluetooth, Bluetooth Low Energy (BLE), WiFi, Ultrasonic, and Ultra-Wideband (UWB). The RFID location tracking versions are largely based on the LANDMARC system, which determines location by comparing the Received Signal Strength (RSS) of an active tag’s signals with the RSS of reference tags with a known location.

Active RFID position tracking has a much greater range than BLE. It’s capable of spanning a kilometer in open air. Compare that to BLE’s ~70m. This is less important in indoor environments with obstructions (e.g. walls or floors), but in warehouses or barns, active RFID’s range might allow businesses to make do with fewer readers, cutting costs and reducing potential failure points. 

RFID positioning tag selection considerations

In spite of the numerous RFID positioning tag and inlay options available tady, tagging items remains as one of the most significant challenges to implementing a successful RFID system, Before engaging in the effort to tag your items, you should take some time to consider the workflow and business processes associated with the items you wish to tag.

For example, if you are tagging a medical instrument, at some point in the workflow it may go into an autoclave. It is important that the tag can survive the autoclave process. Another example is a tool on a bench that may have many areas that a tag can be attached and will read, but when the tool is used to maintain a piece of equipment, there may be a clearance that is no longer achieved because of the addition of the tag. 

Understanding the lifecycle of the items(s) will ensure that the selected RFID positioning tag will enhance, rather than interfere with those processes. Additionally, taking into consideration the five factors of tag selection will further ensure a successful system deployment: Read Range, Environment, Application, data requirements, and size. 

Read Range

Read range is simply the physical distance between the tagged assets and an RFID tracking system, each RFID tag or inlay has a specified read distance when used in its optimized environment. Today, Passive RFID positioning tags vary in read range from just a few inches up to a hundred feet. 

Tags can be read using fixed readers or handheld readers. Fixed readers are permanently installed in a defined area and are always ready to detect any tag that moved into its read zone.

Handheld readers are chosen to allow a user to bring the device to the point of work or the tagged items, rather than moving tagged items past a fixed reader. As a general rule, fixed readers will have a 25% greater read distance than handheld devices. 

Environment

The environment in which your tagged asset will live is very important. If it will primarily be housed outdoors or exposed to moisture, chemicals, low or high temperatures, then the encasement of a tag or the label material and adhesive will play a big factor in selection. 

A passive RFID tracking system tag’s performance can be significantly decreased if there is metal or liquid in the surrounding environment, as both substances interfere with RF signals operating in the 860-960 MHz band-water absorbs the radio waves while metal reflects them. 

There are many specialty tags on the market today built to perform in these RF-unfriendly environments. Likewise, label materials and adhesives can be customized to survive in these harsher environments. 

Application

Application refers to the actual material makeup of the items to be tagged or the surface to which the tag will be attached, the method of attachment, and where the tag will be attached. 

Surfaces such as plastic are considered to be RF friendly surfaces, so a standard pressure-sensitive RFID tracking system should work well when tagging these types of products. One exception is when the item being tagged contains liquids. There are some inlays that perform better than others in the presence of liquids. Just as the presence of metal and water in RFID positioning systems’s surrounding environment may pose challenges, so will the actual tagging of those types of items. In these instances, tags that have been specially constructed to minimize the RF interference of these substances must be chosen.

Data Requirements

When selecting an RFID tag, data requirements are another important consideration. You will need to consider the amount of data that needs to be encoded in the chip as well as any other requirements, such as having the data human readable or including a barcode. This may dictate the type of tag and the encoding/imprinting method.

Size

The size of the tag you select is dictated by the size of the asset being tagged and is dependent on the space available to place the RFID tag. On many assets such as shipping containers and vehicles, there is plenty of available space to successfully affix a tag, but available real estate on smaller assets can be very limited. A tag must not be placed in an area that could potentially compromise a product’s functional purpose.

Over the last several years, RFID positioning tags and inlay manufacturers have responded to the need to tag very small, high-value assets such as prescription drug bottles, tools, and electronic equipment where available space to place the tag is limited. As a result, there are many tag options now available on the market, but it is important to remember: the smaller the tag, the shorter the read distance.

Marketing2/26/2021
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