UHF RFID : From passive identifiers to passive sensor-tags

The passive UHF-RFID technology is nowadays widely used to provide simplicity in logistic processes. However, its functions are limited to transmit an identifier and/or a limited amount of stored data. In this article, we will explore how we can improve a basic tag by adding sensing capabilities to it.

What is RFID

RFID (Radio Frequency IDentification) is a general term which designate several technologies. In this article, we will talk about the passive UHF-RFID technology. It is the most widespread form of RFID in industrial logistic, because it offers a better read-range (around 20m max in the air) than LF or HF-RFID. Basically, we could define this technology as an electromagnetic barcode. It offers the same possibilities than a barcode but it can be read over long distances (up to 100 meters with active RFID), and behind many materials. Some retail companies, such as Decathlon in France, replaced the traditional barcode by passive UHF-RFID tags. Thereby, the cashier spend less time identifying each client’s products and the whole logistic process is improved.

How does it work ?

Passive UHF-RFID is a wirelessly powered technology. The tag exploits the electromagnetic power, transmitted by the reader, using RF energy harvesting so as to power itself. The following figure shows a classic block diagram of an UHF-RFID tag.

There are two main parts :

• The antenna : It converts the electromagnetic waves into data and energy for the chip. Its properties depend on the environment and on his dimensions. This sensitivity to the environment impose a specific antenna design for each application, otherwise the tag’s performance could decrease.
• The chip : It turns the antenna into a smart tag by acting on its properties to communicate with the reader. It’s an electronic device composed of an Analog Front End (AFE) and a digital part. The memory contains the tag identifier but also has user registers, which can be used to store more informations about the product (price, weight, …)

Joshua R. Smith (University of Washington) was apparently the first searcher to imagine UHF-RFID as a sensor. He began by associating two RFID chip with two mercury switches in an opposite position so as to create a binary accelerometer. The α-WISP (Wireless Identifying and Sensing Platform) was born.

We will now see how, by acting whether on the antenna or the chip, we can turn a passive identifier into a passive smart sensor-tag.

Antenna as a sensor

The first way is to take advantage of the antenna’s environment dependency. As an exemple, we will now consider a tag connected to a simple dipole antenna. The resonant-frequency of this antenna geometry is given by the following approximation :

Where c is the celerity of light, L the antenna length and ε the relative permittivity of the medium.

Permittivity is a materials property which indicates the capacity of the medium to polarize itself, when an electric field is applied on it. This property depends on the frequency, the temperature, the molecular structure, the moisture, etc.. of the medium. As the antenna resonant-frequency relies on it, a change in the environment implies a change on the resonant-frequency, and thus constitute a sensor. The following figure shows a practical exemple of a dipole antenna mounted on a dielectric, exposed to different conditions. Values taken by the relative permittivity are exaggerated and not relevant of the reality.

As we can see, there is not a unique resonant-frequency for each condition. It implies that the designer must know the environnement he wants to sense to be able to choose the right medium. Using specific dielectrics, we can obtain moisture or temperature sensitive tags. Many laborites such as the LCIS (Valence, France) or the IES (Montpellier, France) work on this technology and provide proof of concepts.

Advantages :

• You can use any basic RFID chip
• In some cases, it can be simple and cheap to produce (it depends on the dielectric)
• The tag’s energy-consumption remains the same

Drawbacks :

• You need a specific reader which will determine the resonant-frequency of the tag
• The read-time is longer, so it is less adapted for inventory
• It is more difficult to design than a basic RFID tag

RFID sensor chip

With the wide deployment of UHF-RFID in the industry, chip manufacturers started to develop ASICs (Application Specific Integrated Circuits) which include serial communication protocols and/or Analog to Digital Converters (ADC). Thereby, you just need to associate the sensor of your choice to the UHF-RFID ASIC to obtain a passive UHF-RFID sensor tag. As the sensor will be wirelessly powered by the chip, the energy consumption of the system is highly increased, thus limiting the read-range.

Components such as ams SL900A or Farsens Rocky100 are very simple to implement into a design but are a way more expensive than a basic RFID chip. Moreover, most of the time, specific command need to be used to access the sensor data.

Advantages :

• It is easy to develop, most of the work is to be done on the applicative layer
• You don’t need a specific reader
• You can use many different sensors and create more precise applications

Drawbacks :

• It is expensive (regarding the price of a basic tag)
• It is a way less integrated than a basic tag (bigger packages)
• Limited read-range

Conclusion

In this article, the two existing solutions to turn a passive UHF-RFID tag into a passive UHF-RFID sensor tag were presented.

The first one consist of using a basic tag and to associate a specific material, sensitive to the environnement property you want to sense, as a dielectric. However, you’ll need a specific reader to detect the tag’s resonant-frequency changes and it might be expensive.

The second one is the most popular in the industry. It is pure embedded system development based on an ASIC connected to the sensor of your choice. It is easier to develop and might be cheaper for high volumes production. However, the tag is generally a way bigger and need more components to work properly.

The following bibliography is quiet comprehensive and will allow you to get deeper into the subject.

Bibliography

• “Wirelessly Powered Sensor Networks and Computational RFID”, Joshua R. Smith
• “RFID at Ultra and Super High Frequencies”, Dominique Paret
• http://www.rfidjournal.com/articles/view?13815, Decathlon
• http://www.grenoble-inp.fr/newsletter-grenoble-in-press-/lcis-transforms-rfid-labels-into-sensors-658605.kjsp, LCIS
• https://www.ies.univ-montp2.fr/edr/m2a/index.php/9-cat-thematiques/75-rfid-et-capteurs-flexibles, IES
• http://www.farsens.com/en/products/Rocky100/, Farsens
• https://ams.com/sl900a, Ams