Resumen de Battery-less near field communications (nfc) sensors for internet of things (iot) applications
Streamline industrial processes have been always a priority to reduce time hence reducing costs. Identification of goods is an important factor to consider in almost any process since it is necessary to track and get specific information from the goods, thus, the importance of automatize it. The most common method is to use barcode systems based on a printed label with an identification number which is read by an optical device connected to a computer. Radiofrequency identification (RFID) arrived to overcome barcode limitations. An RFID system consist of 2 differentiated parts: a reader (also called interrogator) and a tag. The reader generates a signal which is received by the tag, modulated by the tags transponder based on its internal information (e.g. an ID), and sent back to the reader to be interpreted. Within the RFID technology, there is a subset of protocols working in the high frequency band at 13.56 MHz, called Near Field Communication (NFC) which operational range it’s a few centimetres. Even though NFC has the shortest range among radiofrequency technologies, it has been widely extended due its security, compatibility, user friendly interface, large amount of applications, and low-cost components. Furthermore, unlike other RFID systems, NFC does not need specific and expensive readers. The main reason of the current growing implementation of NFC in almost any smartphone is due its use for contactless payments, making it increasingly indispensable for the end-user, as happens with other wireless technologies such as Wi-Fi or Bluetooth.
The present work is motivated by the possibilities that NFC brings, to create sensing applications without batteries, thus, avoiding toxic waste while extending devices life-time. To accomplish that goal, some issues must be deal with. The main constraint is to ensure, beyond the communication, an efficient and dependable wireless power transfer (WPT). Guarantee a reliable power supply is pivotal to develop sensors embedded on passive tags. On the other hand, the fact of avoiding batteries open the doors to applications which otherwise would not be feasible. For instance, when using sensing tags to control food quality, the toxicity of a battery should be dealt with by using expensive coating methods. Likewise happens to implantable devices, which have the same problem with battery toxicity. Furthermore, when developing implantable devices, the size is vitally important. Hence the benefit of evade the use of batteries thus reducing tags dimensions.
The main objectives of this thesis have been:
1) Study the operation and constrains of WPT applied to NFC in real-case scenarios using commercial smartphones as a reader. 2) Study, design and implementation of custom passive tags, and the effects of different antenna topologies, and manufacturing materials as well as compare the performance of different NFC chips and different power requirements.
3) Demonstration of NFC capabilities and advantages for passive sensing devices, using both, analog and digital sensors.
4) Study of the read range and the energy harvesting range.
5) Design of new NFC sensing applications not present in the literature.
6) Analyse the effects of the body in implantable NFC tags and evaluate techniques to improve the maximum implantable depth.
To accomplish these goals, both, simulations and measurements, have been done putting special interest on the measurements made to analyse it in real-case scenarios, which, considering the huge variety of NFC enabled devices present in the market, is pivotal to ensure that the designed devices could be used with any commercial reader.
This work presents, first of all, an introduction to NFC technology, followed by an extensive analysis of WPT applied to NFC. The working principle is introduced, analysing the main constraints and followed by simulations and measurements of the most relevant design considerations such as the antenna modelling and design, the coupling coefficient, the magnetic field received by the tag, the energy harvesting capability of three different chips, the backscattering modulation, and the effects produced by the presence of metal and body mass near the antennas. All the previous considerations have been used to design several sensing tags, which are presented in this thesis.
The presented battery-less sensing tags in dis works are:
A device that is capable to measure volumetric water content (soil moisture), temperature and relative humidity and show it on a smartphone application or upload it to the cloud to be shared or stored. The proposed solution combines commercial sensors for temperature and relative humidity along with a specific method to measure the soil’s volumetric water content, based on a capacitive measure which has been selected between the different possible methods found in the literature considering the NFC constraints. A printed interdigital capacitor using conventional PCB technology has been designed, fabricated, and measured for this purpose. The effect of an insulator layer for corrosion protection has been analysed. An accurate capacitance measurement method based on a low-power oscillator and a diode-based detector is employed. A procedure for the calibration of the sensor has been presented based on a simple expression whose coefficients can be experimentally obtained.
A prototype for a smart diaper implemented in flexible substrate that measures moisture by measuring the change in capacitance between electrodes. Capacitance is noninvasively determined by measuring its charge time when driven by a high-value resistor using a microcontroller. The tag is based on an NFC IC with energy harvesting and can be read with a mobile phone. As an alternative to urine detection based on a resistance switch, a tag based on capacity sensing that can be adhered to the outer layer of commercial diapers is proposed.
A batteryless, low-cost colorimeter integrated into an NFC tag has been presented. While this device has several applications, first it has been applied to the measurement of pH using pH-sensitive paper strips. Raw RGB measurements of the colorimeter IC are transformed into HSV colour space. A simple linear relationship is found between pH and Hue for typical commercial strips. The pH is obtained from this model and can be easily calibrated using two known pH solutions. The repeatability and reliability of the measurement is higher than for methods based on smartphone cameras because the measurements are taken in controlled light conditions. A smartphone application to read the data from the NFC tag and send the pH to a cloud database has been designed.
Another application for the colorimeter tag is presented for classifying the fruit ripeness grade based on its colour. Experimental results show that the ripeness grade is a function of time and environment conditions (especially the storage temperature). HSV colour space is used for classification. It is observed that the main parameters that change are the hue and saturation, which are used as features for the classification. Different classification algorithms have been compared in order to show the robustness of the system. Linear Discriminant Analysis and nearest neighbour work well in all cases. The proposed system is a low-cost solution compared with expensive spectrometers. In addition, the measurement is not influenced by external illumination, and therefore the measurements are repeatable in comparison with computer vision systems based on mobile cameras. A simple table-based method is proposed to avoid increasing the complexity of implementing the software in the mobile application when the boundary decision regions are not described by analytical functions, such as the case of the nearest neighbour classifier.
The last part of this work is analyse the feasibility of using battery-less NFC tags for implants that can be read using commercial mobiles. The use of standardized NFC readers and devices presents several challenges compared to custom-designed WPT systems. Two of these are the size of the implanted antenna and the reader loop antenna. Others arise in connection with the limitations in power and the high bandwidth of the communication that limit the system’s overall quality factor. The effects of the tag’s nonlinearity must also be considered. The IC impedance decreases with the power received and, as a consequence, the loaded quality factor decreases, and efficiency cannot achieve high values compared with other WPT cases in which this limitation does not exist. Thus, the quality factor of the tag (implanted antenna) is not critical. Finally, other effects are the detuning due to the metallic cases of the mobile and the high permittivity of body materials. Two WPT systems have been theoretically and experimentally studied, these being the 2-coil and 3-coil systems. The latter is implemented using a relay antenna placed on top of the skin that makes it possible to reduce the effects of the low coupling coefficient between the reader and tag antennas, which can be of different sizes and enable the magnetic field received at the tag to increase. The result is an increase in the reliability of the link and a greater depth for the implant. The experimental results show that when using a standard 2-coil system, a maximum depth of 12 mm can be achieved with the mobile close to the skin, whereas depths of up to 16 mm can be achieved with the proposed 3-coil system when placing the mobile phone at distances of 1-2 cm from the skin. The maximum distances obtained for activating the harvesting mode of the NFC IC and feeding the sensors are similar when considering the implant both inside the phantom and in the air, because the body is non-magnetic and thus barely attenuates the magnetic field at the operating frequency. However, both the implanted and the relay antennas need to be tuned for each case. The effect of detuning is studied by means of electromagnetic simulations, and an improved model has been proposed for modelling the quality factor of the antennas. The detuning effects due to high permittivity on the loop antennas are avoided by installing a low permittivity spacer in the order of 0.5 to 1 mm. The relay antenna can then be placed on top of the skin using adhesive patches. The use of low permittivity coating (e.g. a biocompatible material such as silicone) reduces degradation of the quality factor of the coils and sensitivity.
This Doctoral Thesis has studied the application of sensors in battery-less NFC tags powered by the radiofrequency generated by smartphones. Circuit models for the reader and the tag have been proposed to be subsequently analysed in order to achieve an efficient wireless power transfer as well as taking into account the standards related to NFC to assure the data communication. Several sensors have been successfully embedded in battery-less tags.
