Carbon Nanotube Loaded Passive UHF RFID Sensor Tag with Built-in Reference for Wireless Gas Sensing
Adhur Kutty, Ajith (2016)
Adhur Kutty, Ajith
2016
Master's Degree Programme in Electrical Engineering
Tieto- ja sähkötekniikan tiedekunta - Faculty of Computing and Electrical Engineering
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Hyväksymispäivämäärä
2016-03-09
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201602243565
https://urn.fi/URN:NBN:fi:tty-201602243565
Tiivistelmä
Radio Frequency IDentification (RFID) technology, which uses communication by means of reflected power, is used for the wireless identification of objects. Individual objects are identified by the RFID tag placed on them. An RFID tag consists of a microchip and an antenna. An RFID reader transmits radio frequency (RF) waves to identify the tagged objects. It transmits identification information, which is stored in its memory, through back scattered radio waves to the RFID reader. Passive RFID tags harvests RF energy from the reader device to power its microchip, enabling battery free operation.
Passive wireless sensors based on UHF RFID technology are a promising prospect in the realm of ubiquitous sensing and Internet of Things (IoT). The sensing principles and methods used depend on the variation of the tag antenna gain, the impedance match between the tag antenna and the RFID chip, or both, with respect to the sensed parameter. The RFID reader uses back scattered RF signal properties to perform sensing. Usually, threshold power, the power at which an RFID tag harvests enough power to turn itself ON, or back scattered signal power, is used for sensing measurements. These measurements depend heavily on the environment, where the tag is placed, and the distance at which it is measured by a reader. This poses severe restrictions in sensing measurements. To maintain sensor accuracy, precise calibration of the measurement setup is required. Any disturbance in the measurement setup or the RF propagation environment affect the sensor measurement.
This thesis presents a novel architecture of inkjet-printed passive UHF RFID based sensor tag that allows a reference measurement and sensing measurement for wireless gas sensing. In this work, an RFID tag is made with Silver (Ag) ink, and is loaded with carbon nanotube (CNT) ink for sensing purpose. Carbon nanotubes (CNT) have a property that it modifies its conductivity in the presence of certain gases. This property is exploited for sensing (CO2) gas. A switch, used in the sensor tag’s structure, provides two modes of operation. They are, sensor on (SON) or sensing mode operation, and sensor off (SOFF) or reference mode operation. In SON mode, the sensor tag modifies its backscatter properties in the presence of gas. In SOFF mode, the realized gain of the sensor tag remains constant in the presence of gas, which provides a reference measurement. The difference in threshold power, between SON mode and SOFF mode is used as the sensing parameter. This sensing paradigm allows sensor measurements that do not depend on the RF propagation conditions, or the distance of the reader. The fabricated sensor tags, when exposed to CO2, show a threshold power variation of up to 2dB, with a read range of about 4m at 915MHz. This means, threshold power difference between SON and SOFF mode provides unambiguous detection of CO2 at all measurement conditions.
Study and measurements done in this work prove the feasibility of gas detection by placing CNT very close to the tag, instead of, on the tag. More importantly, the concept of using a switch in the sensor tag to provide reference measurement is proven. Several possibilities exist in the realization of the switch including, but not limited to, incorporating the switch within the RFID chip. These ideas will be explored in future work.
Passive wireless sensors based on UHF RFID technology are a promising prospect in the realm of ubiquitous sensing and Internet of Things (IoT). The sensing principles and methods used depend on the variation of the tag antenna gain, the impedance match between the tag antenna and the RFID chip, or both, with respect to the sensed parameter. The RFID reader uses back scattered RF signal properties to perform sensing. Usually, threshold power, the power at which an RFID tag harvests enough power to turn itself ON, or back scattered signal power, is used for sensing measurements. These measurements depend heavily on the environment, where the tag is placed, and the distance at which it is measured by a reader. This poses severe restrictions in sensing measurements. To maintain sensor accuracy, precise calibration of the measurement setup is required. Any disturbance in the measurement setup or the RF propagation environment affect the sensor measurement.
This thesis presents a novel architecture of inkjet-printed passive UHF RFID based sensor tag that allows a reference measurement and sensing measurement for wireless gas sensing. In this work, an RFID tag is made with Silver (Ag) ink, and is loaded with carbon nanotube (CNT) ink for sensing purpose. Carbon nanotubes (CNT) have a property that it modifies its conductivity in the presence of certain gases. This property is exploited for sensing (CO2) gas. A switch, used in the sensor tag’s structure, provides two modes of operation. They are, sensor on (SON) or sensing mode operation, and sensor off (SOFF) or reference mode operation. In SON mode, the sensor tag modifies its backscatter properties in the presence of gas. In SOFF mode, the realized gain of the sensor tag remains constant in the presence of gas, which provides a reference measurement. The difference in threshold power, between SON mode and SOFF mode is used as the sensing parameter. This sensing paradigm allows sensor measurements that do not depend on the RF propagation conditions, or the distance of the reader. The fabricated sensor tags, when exposed to CO2, show a threshold power variation of up to 2dB, with a read range of about 4m at 915MHz. This means, threshold power difference between SON and SOFF mode provides unambiguous detection of CO2 at all measurement conditions.
Study and measurements done in this work prove the feasibility of gas detection by placing CNT very close to the tag, instead of, on the tag. More importantly, the concept of using a switch in the sensor tag to provide reference measurement is proven. Several possibilities exist in the realization of the switch including, but not limited to, incorporating the switch within the RFID chip. These ideas will be explored in future work.