INTRODUCTION TO RFID
How do RFID tags work?
In addition to the way they are used, RFID tags can also be classified according to the presence or absence of a radio signal transceiver and of an embedded power supply:
Active RFID tags: They embed a radio signal transceiver and thereby a battery to power it. Thanks to the integrated power supply, active RFID tags can activate themselves regardless of the presence of a reader/interrogator in proximity and provide, at the same time, longer read ranges in comparison to passive RFID tags that have no battery and integrated transceiver.
Because of these characteristics, active RFID tags are commonly used when Real Time Location Systems (RTLS) need to be deployed.
Often, active RFID tags feature extra sensors such as those for the measurement of humidity, temperature and pressure.
Usually, active RFID tags are more expensive than passive RFID tags and have environmental limitations due to the presence of the battery.
Passive RFID tags: They do not have an embedded transceiver and battery. When passive RFID tags enter the field generated by a reader, they wake up and respond to it by “reflecting” modulated its signal; this technique is called “backscatter”. Then the reader receives and decodes the response.
Passive RFID tags are the most widespread in the market because of their low prices that make them suitable for a wide range of applications. Moreover, passive tags can withstand challenging environmental conditions that can restrict the use of RFID tags with integrated batteries.
Passive RFID tags usually provide shorter read ranges in comparison to active RFID tags and Battery-Assisted Passive RFID tags.
Given that passive RFID tags communicate with a reader only when they enter its action field, they are rarely used for RTLS applications.
Battery-Assisted Passive (BAP) RFID tags: These are essentially passive RFID tags and use backscatter to work but, unlike passive RFID tags, BAP RFID tags have an integrated battery that keeps the integrated chip in a stand-by status. Passive RFID tags read ranges are often limited by the need of providing their ICs with the minimum power threshold to “wake up” when distance makes the signal from the reader too weak. BAP RFID tags have a battery to help with the “wake up” problem increasing significantly the read distances. When the battery is discharged, BAP RFID tags continue working as common passive RFID tags.
BAP tags are usually more expensive then “comparable” passive tags and cheaper than active tags.
As for active RFID tags, battery can be a limiting factor for usage in extreme environmental conditions.
Passive RFID tags with Light panel and Ultracapacitor: These work in a way which is very similar to the way BAP RFID tags work, but use the combination of the built-in light panel and ultracapacitor instead of battery to extend the typical read ranges of passive mode. In lighted conditions, the solar panel provides power to enable longer read ranges. In extreme low light conditions or in total darkness, the ultracapacitor maintain high read performance for many hours. If these RFID tags remain in darkness for a too long time and the ultracapacitor is discharged, they continue to operate as normal passive RFID tags till they are exposed to light again and the ultracapacitor is recharged.
Semi-passive RFID tags: Like passive RFID tags and BAP RFID tags, semi-passive RFID tags use the backscatter technique to communicate with the reader. They have a battery that is used to power on-board microcontrollers and extra sensors, i.e. a temperature logger. When the battery is discharged, semi-passive RFID tags stop transmitting any signal.
Note: When a choice between active, passive and other types of RFID tags has to be made, also the cost of readers is very important. It depends on many factors such as reader types, models and quantities needed for the goals and requirements of any specific RFID system.