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Battery-free temp sensors based on RFID tags

Posted: 28 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:RFID? UHF? sensor? Fenix2? EPC C1G2?

Nowadays, there is an indisputable and unstoppable trend to use RF identification (RFID) in a number of applications, such as supply chain management, public transportation, access control and many more. The use of this technology entails a number of advantages over barcode technologies such as tracking people, items and equipment in real time, non-line of sight requirement, long reading range, standing harsh environments, etc.

Recently, the combination of RFID with sensory systems has extended the applications of RFID to environmental monitoring or healthcare applications. Those existing sensors usually operate in the 13.56MHz and 134.2kHz frequency bands. However, these sensors have the inconvenience of a limited reading range (a few centimetres) and high cost. To overcome this limitation, battery free RFID sensor development has been focused on RFID tags using the UHF bands (868MHz, 900MHz and higher) which offer temperature monitoring and higher reading distances than previous ones. On the other hand, the popularity of the standard EPC C1G2 (ISO 18000-6C) makes it possible the use of a universal communication standard compatible with many readers from many vendors.

Therefore, there is a strong motivation to develop optimised RFID tags able to support different sensor types, such as temperature sensors, pressure sensors, humidity sensors, strain gages, etc. and able to use standardised communication protocols such as EPC C1G2.

Farsens is developing full passive UHF sensor tags that can communicate to over a metre without the need of batteries on the sensor tags. The Fenix2 is a wireless passive temperature sensor that works under these premises. Figure 1 shows the typical architecture of a battery free RFID sensor tag.

Figure 1: Architecture of an RFID tag with external sensor.

The antenna receives the signal emitted by the reader. In order to achieve the maximum power transfer 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 analogue 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 temperature sensor. It also realises the required actions to answer the reader queries using the EPC C1G2 standard. The EPC C1G2 is specifically designed for identification applications and does not include support for sensors. Nevertheless there are some possible workarounds to integrate sensors in the C1G2 standards.

The most straightforward method is to choose user commands for this purpose. A new user command can be defined to access the sensor. The new command may contain configuration parameters for the sensor, so that the behaviour of the sensor can be controlled externally. The answer from the tag to this user command may contain the value of the sensor. The standard allows the use of user commands which makes such an RFID sensor C1G2 compliant. Nevertheless, compliance with the standard may not be good enough. In order to integrate these battery-less RFID sensors in any commercial C1G2 network, compatibility between all the elements in the network must be ensured. Including user commands limits the compatibility, as every item in the network needs to support the new command. If one or more items in the communication chain do not support this user command, the end user will not be able to retrieve the information from the sensor. Therefore, in order to solve this incompatibility problem, mandatory commands of the standard shall be used.

Memory mapping
A good solution to obtain a fully EPC C1G2 compatible tag, is to implement the memory mapping method. This method consists in redirecting the user memory bank (defined in the standard) to the temperature sensor value. This way, the reader only needs to request the value contained in a specific memory location. If the requested address contains the direction assigned to the sensor, the answer of the tag will contain the value of the measurement. The main advantage of this solution is that any C1G2 network does support it, and as the EPC code is not altered, none of the advanced features of the EPC networks are damaged.

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