RFID battery-free sensors are a very good alternative to natural source energy harvesting solutions for those applications in need of a reliable wireless energy source.
Energy harvesting from natural sources is the optimum solution. However, the efficiency of devices commercially available is still a bit limited. On top of that, natural energy sources are generally not reliable (i.e.: photovoltaics depend on a sunny day).
As an alternative solution, RFID battery free sensors are dependent on a reliable power source: an RFID reader. These devices can be fixed or handheld (the size of a PDA) and can power up and communicate with battery free sensors located in their read range. Generally limited to around 2 meters (Personal Area Network or PAN) if using UHF technology and some centimeters when using HF technology, the information collected by the reader can then be shared via Wi-Fi, Ethernet, Bluetooth or similar means.
RFID sensor tags can be placed in hardly accessible locations to forget about wiring and battery changes forever. As examples of use cases to easily understand the system we have:
Structural Health Monitoring in construction. Embedding RFID strain gages in beams of a bridge can give you information about its status. Maintenance staff will periodically visit the site with a handheld reader and check the sensor that’s embedded in the concrete.
Rotor contact temperature monitoring in electrical engines. Placing a thermistor or thermocouple in contact with the rotor surface to monitor the temperature of the rotor material. The RFID sensor will continuously send information to a nearby reader and the engine will never have to be stopped in order to change batteries.
RFID sensor tags harvest the energy from the RF field created by the UHF RFID reader. Figure 1 shows the typical architecture of a battery free RFID sensor tag.
The antenna receives the signal emitted by the reader. In order to achieve the maximum power transference from the antenna to the voltage multiplier, a matching network is required. Typically this matching network is implemented together with the antenna. The voltage multiplier rectifies the incoming signal charging the supply capacitor CSUPPLY. This capacitor is used to supply power to the rest of the tag. The analog front-end provides the signals that the rest of the tag requires to work properly, such as regulated voltages, clock and reset signals. It is also in charge of demodulating the incoming ASK signal and modulating the tag answer. The digital core communicates with the EEPROM and, when present, the external sensors or devices. It also realizes the required actions to answer the reader queries using the EPC C1G2 standard.
The sensors embedded in RFID tags require a lot more power than standard RFID identification tags need. For this reason, a proper power management must be implemented at the complete system level:
1.Reader output power optimization. Not only the power level is important for passive sensor solutions (generally limited by country regulation) but also the time during which the RFID reader is actually transmitting this power should be taken into account.
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