Jinlan Gao

Printed Electromagnetic Coupler With an Embedded Moisture Sensor for Ordinary Passive RFID Tags

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 kΩ to 10 Ω 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.

The influence of paper coating content on room temperature sintering of silver nanoparticle ink.

The resistance of inkjet printed lines using a silver nanoparticle based ink can be very dependent on the substrate. A very large difference in resistivity was observed for tracks printed on paper substrates with aluminum oxide based coatings compared to silica based coatings. Silica based coatings are often cationized with polymers using chloride as a counter ion. It is suggested that the precipitation of silver salts is the cause of the high resistivity, since papers pretreated with salt solutions containing ions that precipitate silver salts gave a high resistance. Silver nitrate has a high solubility and paper pretreated with nitrate ions gave a low resistivity without sintering. The results obtained show that, by choosing the correct type of paper substrate, it is possible to manufacture printed structures, such as interconnects on paper, without the need for, or at least to reduce the need for, post-print sintering. This phenomenon is, of course, ink specific. Inks without or with a low silver ion content are not expected to behave in this manner. In some sensor applications, a high resistivity is desired and, by using the correct combination of ink and paper, these types of sensors can be facilitated.

Investigation of Humidity Sensor Effect in Silver Nanoparticle Ink Sensors Printed on Paper

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 sensors initial resistance, and therefore also on the sensors 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.

Microstrip antennas for remote moisture sensing using passive RFID

The paper presents how materials with a high ability of absorbing moisture can be used with microstrip antennas and passive RFID chips for constructing remotely read moisture sensors. The concept is based on pairs of microstrip antennas, applied on the same piece of PCB and each equipped with one RFID chip, where one antenna is covered by the moisture absorbent material and the other is left open. In a humid or wet environment the moisture concentration is higher in the absorbent material than in the surrounding environment which causes degradation to the embedded antenna both in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. While it has previously been shown how this can be accomplished with simple one-layer antennas we here look into using microstrip antennas to get around the problem with high influence from background material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls or simply to measure the level of relative humidity at hidden locations.

Home care with NFC sensors and a smart phone

The worlds aging population and limited hospital resources together with advancements in information technology provides a thorough basis for further development of tele-healthcare and assisted living. For ordinary people, and especially elderly people, to be diagnosed and treated in their homes there is a definite need for technical equipment that is extremely easy to handle. One way to facilitate tele-healthcare is to take advantage of modern cell phone technology and the infrastructure surrounding these units. Many elderly people are today comfortable with using cell phones and even advanced smart phones can be purchased to a cost that is low compared to corresponding specialized medical equipment. The concept presented in this work is based upon Near Field Communication (NFC), a wireless feature that is currently being widely launched in cell phones. Amongst other capabilities, NFC technology allows a phone to act as a 13.56 MHz RFID reader. This work shows how NFC-equipped phones can be used to read passive NFC tags that in turn are equipped with general resistive sensors and how sensor data are read and communicated to a web server without the need to push a single button.

Chemically programmed ink-jet printed resistive WORM memory array and readout circuit

In this paper an ink-jet printed write once read many (WORM) resistive memory fabricated on paper substrate is presented. The memory elements are programmed for different resistance states by printing triethylene glycol monoethyl ether on the substrate before the actual memory element is printed using silver nano particle ink. The resistance is thus able to be set to a broad range of values without changing the geometry of the elements. A memory card consisting of 16 elements is manufactured for which the elements are each programmed to one of four defined logic levels, providing a total of 4294 967 296 unique possible combinations. Using a readout circuit, originally developed for resistive sensors to avoid crosstalk between elements, a memory card reader is manufactured that is able to read the values of the memory card and transfer the data to a PC. Such printed memory cards can be used in various applications.

An analytical model for electromagnetically coupled UHF RFID sensor tags

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.

Electric and electromagnetic coupled sensor components for passive RFID

This work discuss the possibilities of integrating passive sensor components to passive UHF RFID tags. The sensor system works by degrading a tags 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.