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 an embedded power supply:
Active RFID tags: They embed a radio signal transceiver and 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 than passive RFID tags that have no battery and integrated transceiver. Furthermore active RFID tags provide excellent orientation insensitivity. Because of all these characteristics, active RFID tags are commonly used to develop Real Time Location Systems (RTLS).
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 embedded transceiver and battery. They work only when the energy of the EM field generated by a reader is sufficient to wake them up. When activated, passive RFID tags elaborate the reader signal and reflect it modulated back to the interrogator, accordingly to a technique called “backscatter”. The operation is then concluded by the reader that receives and decodes the response.
Passive RFID tags are likely the most widespread RFID tags 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 featuring integrated batteries.
Usually, passive RFID tags provide shorter read ranges than active RFID tags and Battery-Assisted Passive RFID tags.
Most passive UHF RFID tags feature linearly polarized antennas that provide high directionality and therefore high orientation sensitivity with reduced read range when the relative orientation of the tag and reader antennas become less favorable. For this reason, most passive UHF RFID tags are not suggested when their orientation cannot be controlled during reading operations. However, some models of passive RFID tags feature antennas specifically designed to provide very low orientation sensitivity.
As 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: Also these RFID tags use backscatter to work but, unlike passive RFID tags, they have an integrated battery that allows longer read range. In fact, while the read range of passive RFID tags can be strongly limited by the lack of the minimum power threshold necessary to “wake up” the IC when the reader signal is too weak, the battery in BAP RFID tags keeps the chip always on and therefore increases the read range. When the battery is discharged, BAP RFID tags continue working as common passive RFID tags.
BAP tags are usually more expensive than “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 like BAP RFID tags, but use the combination of a built-in light panel and a ultracapacitor instead of battery to extend the typical read ranges of passive mode. In lighted conditions, the solar panel harvests energy to provide extended read range and charge the ultracapacitor that allows high performance also for a limited time period of extreme low light or total darkness. If these RFID tags stay in darkness for too long time and the ultracapacitor is discharged, they continue to work like 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 leverage a battery that can be used to power on-board microcontrollers and extra sensors, i.e. a temperature logger, or improve read performance not only in terms of read distance but also in terms of read speed, i.e. for high-speed Automatic Vehicle Identification purposes. 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 to meet the goals and requirements of any specific RFID system.