Mid Sweden University

The paper presents how materials with a highability of absorbing moisture can be used with microstripantennas and passive RFID chips for constructing remotely readmoisture sensors. The concept is based on pairs of microstripantennas, applied on the same piece of PCB and each equippedwith one RFID chip, where one antenna is covered by themoisture absorbent material and the other is left open. In ahumid or wet environment the moisture concentration is higherin the absorbent material than in the surrounding environmentwhich causes degradation to the embedded antenna both interms of dielectric losses and change of input impedance. Thelevel of relative humidity or the amount of water in theabsorbent material is determined for a passive RFID system bycomparing the difference in RFID reader output power requiredto power up respectively the open and embedded tag. While ithas previously been shown how this can be accomplished withsimple one-layer antennas we here look into using microstripantennas to get around the problem with high influence frombackground material. Typical applications include moisturedetection in buildings, especially from leaking water pipeconnections hidden beyond walls or simply to measure the levelof relative humidity at hidden locations.

This paper presents a printed UHF RFID sensor solution that indicates whether a passive RFID tag has been exposed to a certain degree of humidity. The printed sensor operates as a Write Once Read Many (WORM) resistive memory as it permanently changes its resistance from about 2 kÙ to less than 50 Ù after exposure to humidity or water. The change of the sensor resistance is used to modulate the properties of a UHF RFID tag antenna by changing antenna input impedance and also introducing or removing ohmic losses in the antenna structure. The final result is reflected in the change of the minimum transmit power required to power-up the RFID tag. Both the sensor and the tag antenna are printed horizontally on a paper substrate. Different geometric combinations of sensor and antenna structures are investigated. Typical applications include non-invasive methods for detecting wetness or humidity levels in hidden locations such as within construction structures. The sensor setup can also serve to provide information as to whether a package sent over a supply chain has exceeded a certain level of humidity during its route.

This letter presents a printed UHF RFID sensor solution that indicates if a passive RFID tag has been exposed to a certain degree of moisture. The printed sensor operates as a write-once-read-many (WORM) resistive memory device as it permanently changes its resistance from about 10 to 10 after exposure to moisture or water. A printed coupling loop with an embedded WORM sensor is horizontally placed just above the surface of an ordinary UHF RFID tag. Electromagnetic coupling is used to modulate the properties of the tag antenna by changing its input impedance and introducing ohmic losses in proportion to the embedded sensor values. The passive RFID tag can change state from readable to unreadable when the WORM bit is set, i.e., is put in a low-resistance state. The proposed concept verifies that commercial RFID tags can be used as sensor tags by simply adding an electromagnetically coupled sensor as a sticker or by similar means, without the need for ohmic contacts between the sensor and the original RFID tag. © 2006 IEEE.

This paper presents an analytical model for electromagnetically coupled UHF RFID sensor tags where a coupling loop with an embedded sensor is attached to an ordinary UHF RFID tag with a small gap. Electromagnetic coupling is used, in this case, to modulate the properties of the tag antenna in proportion to the values of the embedded sensor. The antenna together with the coupling loop are represented as an equivalent circuit and the analysis of the sensor tag becomes a circuit-level calculation after extracting parameters from full-wave simulations for, respectively, the separated dipole antenna and coupling loop. The results calculated from the equivalent circuit model are compared with the results from full-wave simulations and show good agreement. The presented model can thus be used for analyzing and predicting the behavior of electromagnetically coupled sensor tags. Based on the analysis with the presented model, the methods for optimizing the sensory performance of this kind of RFID sensor tags are also presented in this paper.

This work discuss the possibilities of integrating passive sensor components to passive UHF RFID tags. The sensor system works by degrading a tag's communication performance in proportion to a sensed quantity. Two approaches are studied, sensors directly integrated to tag antenna structures and sensors electromagnetically coupled to tag antennas. The em coupled sensors provide the possibility to produce small sensor components as easily applied add-ons to ordinary commercial RFID tags.

Thin inkjet-printed tracks of silver nanoparticles have previously been observed to show a non-reversible decrease in resistance when exposed to a high degree of relative humidity and thus providing sensor functionality with a memory effect. This paper provides a more in-depth explanation of the observed humidity sensor effect that originates from inkjet-printed silver nanoparticle sensors on a paper substrate. It is shown that the geometry of the sensor has a large effect on the sensor's initial resistance, and therefore also on the sensor's resistive dynamic range. The importance of the sensor geometry is believed to be due to the amount of solvent from the ink interacting with the coating of the paper substrate, which in turn enables the diffusion of salts from the paper coating into the ink and thus affecting the silver ink.

High speed manufacturing processes are commonly associated with high mechanical tolerances. For RFID antennas, mechanical manufacturing tolerances imply uncertainties in the antennas outer dimensions as well as in the surface- and line-roughness of the antennas geometry. Rough edges can for example be caused by high speed dry patterning processes. This work investigates the impact that rough patterns along the edges of an antenna structure have on UHF RFID tags' communication capabilities. The work characterizes the negative influence that edge roughness of different levels has on standard RFID dipoles. Results show that, as expected, the performance degradation for an RFID tag antenna increases with increased edge roughness amplitude. The results also show that the performance degradation due to edge roughness is almost identical for the investigated antennas' different line widths when the roughness level is normalized to its specific antenna line width, i.e., an antenna with a wider line width is more robust and can withstand a higher degree of edge roughness.