LORAWAN BATTERY-FREE WIRELESS SENSORS NETWORK DESIGNED FOR STRUCTURAL HEALTH MONITORING IN THE CONSTRUCTION DOMAIN - MDPI
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sensors Article LoRaWAN Battery-Free Wireless Sensors Network Designed for Structural Health Monitoring in the Construction Domain Gaël Loubet 1, * , Alexandru Takacs 1 , Ethan Gardner 2 , Andrea De Luca 2 , Florin Udrea 2 and Daniela Dragomirescu 1 1 LAAS-CNRS, Université de Toulouse, CNRS, INSA, UPS, 31400 Toulouse, France; alexandru.takacs@laas.fr (A.T.); daniela.dragomirescu@laas.fr (D.D.) 2 Department of Engineering, University of Cambridge, Cambridge CB3 0FF, UK; elwg2@cam.ac.uk (E.G.); ad597@cam.ac.uk (A.D.L.); fu10000@cam.ac.uk (F.U.) * Correspondence: gael.loubet@laas.fr Received: 25 January 2019; Accepted: 21 March 2019; Published: 28 March 2019 Abstract: This paper addresses the practical implementation of a wireless sensors network designed to actualize cyber-physical systems that are dedicated to structural health monitoring applications in the construction domain. This network consists of a mesh grid composed of LoRaWAN battery-free wireless sensing nodes that collect physical data and communicating nodes that interface the sensing nodes with the digital world through the Internet. Two prototypes of sensing nodes were manufactured and are powered wirelessly by using a far-field wireless power transmission technique and only one dedicated RF energy source operating in the ISM 868 MHz frequency band. These sensing nodes can simultaneously perform temperature and relative humidity measurements and can transmit the measured data wirelessly over long-range distances by using the LoRa technology and the LoRaWAN protocol. Experimental results for a simplified network confirm that the periodicity of the measurements and data transmission of the sensing nodes can be controlled by the dedicated RF source (embedded in or just controlled by the associated communicating node), by tuning the radiated power density of the RF waves, and without any modification of the software or the hardware implemented in the sensing nodes. Keywords: wireless power transmission (WPT); wireless sensor network (WSN); simultaneous wireless information and power transfer (SWIPT); cyber-physical systems (CPS); structural health monitoring (SHM); Internet of Things (IoT); communicating material 1. Introduction By using Internet of Things (IoT) technologies that are now widely available, cyber-physical systems (CPS) can be implemented to respond to the needs of various applications. In the heart of these applications lies structural health monitoring (SHM) [1] for buildings, and civil and transportation infrastructures (e.g., railway and subway stations, bridges, highways, etc.) [2,3], which are part of the Smart City concepts and paradigms. In parallel, the building information modelling (BIM) concept [4] has provided new tools to more efficiently plan, design, construct and manage buildings and infrastructures during the different stages of their lifecycle. At the border between these concepts (SHM and BIM) the idea of ‘communicating material’ has arisen [5]. A ‘communicating material’ is intrinsically able to generate, process, store and exchange data from its environment to dedicated digital systems as a virtual model. A few works have already proposed implementations of communicating materials based on radio frequency identification (RFID) technology for various Sensors 2019, 19, 1510; doi:10.3390/s19071510 www.mdpi.com/journal/sensors
Sensors2019, Sensors 19,x1510 2019,19, FOR PEER REVIEW 2 2ofof26 26 Sensors 2019, 19, x FOR PEER REVIEW 2 of 26 materials and applications. The main targeted application is the storage and access, at short ranges, materials materials and and applications. applications.The Themain maintargeted targetedapplication applicationisisthe thestorage storageand andaccess, access,at atshort shortranges, ranges, of some disseminated or measured data in a material like wood [6], textile [7], or concrete [8]. of of some some disseminated disseminated or or measured measured data data in in aa material like wood [6], textile [7], or concrete [8]. The research project McBIM [9]—communicating material at the disposal of the building The The research research project project McBIM McBIM [9]—communicating [9]—communicating material materialofat the atthe disposal theconcept disposal of of the the building building information modelling—aims to provide a practical application of communicating information information modelling—aims modelling—aims to to provide provide aa practical practical application application of of the the concept concept of of communicating communicating material in the construction domain through communicating reinforced precast concrete. Currently, material material in the the construction inconcrete construction domain domain through through communicating communicating reinforced reinforced precast concrete. Currently, reinforced is the most common construction material thanks to itsprecast concrete. scalability, Currently, durability and reinforced reinforced concrete concrete is the most common is the mostconcrete common construction construction material material thanksthanks to its scalability, to its scalability,durability and durability cost [10]. The communicating must be intrinsically able to generate, process, store and cost [10]. [10]. and cost The communicating concrete must be be intrinsically able totogenerate, process, store and exchange data The overcommunicating several meters from concrete must its environment intrinsically able (i.e., the reinforced generate, precastprocess, concretestore and element exchange exchange data over several meters from its environment (i.e., the reinforced precast concrete element that is the data over several monitored meters structure) to from its environment dedicated systems. These (i.e., the reinforced dedicated precast systems may concrete include element other that that is is the the monitored monitored structure) structure) to to dedicated dedicated systems. systems. These These dedicated dedicated systems systems may may include include other other structural elements made of communicating concrete (e.g., a floor or a wall) and a unique virtual structural structural elements elements made made of of communicating communicating concrete concrete (e.g., (e.g., aa floor floor oror aa wall) wall) and and aa unique unique virtual virtual model (a BIM), shared or transmitted between all the stakeholders. The system must be functional model model (a (a BIM), BIM), shared or shared ortransmitted transmittedbetween betweenallallthe thestakeholders. stakeholders. The system must be functional for the entire lifespan of the element (i.e., several decades) as part ofThe system a global must structure beand functional for by itself. for thethe entire entire lifespan lifespan of of the the element element (i.e., (i.e., several several decades) decades) as as part part of a ofglobal a global structure structure and and by by itself. itself. As presented in Figure 1, the properties of precast concrete elements change regularly during As As presented presented in Figure in Figure 1,1,the theproperties propertiesofofprecast precast concrete concrete elements elements change change regularly during regularly the construction process of a structure, but often virtual/digital information is not shared during or can the be the construction construction process process of of acannota structure, structure, but often virtual/digital information is not shared or can be lost. Thus, a new owner havebut often virtual/digital a complete view of the information history of their is not shared or structural can elements.be lost. By lost. Thus,Thus, a anew a new owner owner cannot cannot have havea(BIM) a complete complete view viewelement, of the of the history history of their ofstructural their structural elements. elements. By By joining joining unique virtual model at each it becomes possible to conserve the entire joining a unique a unique virtual model (BIM) at each element, it becomes possible to conserve the entire history ofvirtual an element modeland (BIM) at eacha element, to create virtual model it becomes possible to of a structure and conserve each ofthe itsentire history of components. an The history element ofandan toelement create aand to create virtual model aofvirtual a modeland structure of each a structure of its and each ofThe components. its components. virtual model The can virtual model can also be updated at each modification (e.g., for each step in the lifecycle, for each virtual model also bechange, updated canatalso bemodification updated at (e.g., each modification step (e.g.,thefor each step for in theowner lifecycle, for each owner etc.) each and be partially stored in fortheeach element initself. lifecycle, each change, etc.) owner change, etc.) and be partially and be partially stored in the element itself. stored in the element itself. Changesofofowner Figure1.1.Changes Figure ownerfor fora aprecast precastconcrete concreteelement elementduring during the the construction construction phase of a building. building. Figure 1. Changes of owner for a precast concrete element during the construction phase of a building. As As presented presented in Figure2, 2, the riskrisk of losing information is increased by that the fact that many As presentedininFigure Figure the 2, the of losing risk information of losing is increased information by the fact is increased by the many industries fact that many industries work work together together on a project and have similar tasks, but do not follow the same methods or industries workon a project together on and have and a project similar havetasks, buttasks, similar do not butfollow do notthe samethe follow methods or process same methods or process monitoring monitoring conventions. conventions. process monitoring conventions. Representationof Figure2.2.Representation Figure ofthe theinteractions interactionsbetween betweenthe thestakeholders stakeholdersworking workingon onthe thedifferent differenttasks tasks Figure for a 2. Representation common building of the interactions between the stakeholders working on the different tasks project. for a common building project. for a common building project. Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
Sensors 2019, 19, 1510 3 of 26 Concerning the McBIM project, three main stages of the life cycle of a precast concrete element are targeted: (i) the manufacture and storage of each element; (ii) the construction of a structure by the combining of elements; and (iii) the exploitation of the structure, particularly through structural health monitoring. For instance, the monitoring of the curing process of a fluid screed or a precast element in terms of temperature and humidity could be a relevant use of communicating concrete during the manufacturing stage to help with the decision to continue the works (e.g., to unmould, to tile, etc.). The storage of environmental information of the element within itself and in the BIM allows the creation of a precise and real-time plan of a structure during the construction phase, whilst also continuously sharing information between all the stakeholders and track elements. The monitoring of the temperature at each side of a wall could be relevant during the exploitation phase to quantify the real thermal resistance of this wall and to certify some standards. Other uses, such as investigating maturity or applying preventative treatment through the detection of cracks, are also relevant in the construction domain. Thus, with a unique WSN several different applications can be met during the entire lifespan of the element according to the use of specific sensors. Finally, literature that provides solutions for some specific applications with concrete is outlined. In [8] a solution based on RFID technology is proposed for monitoring moisture and temperature into concrete. The communication is short-range (centimetres) and the system only works when deployed and monitored by a user. This solution requires human interactions and therefore answers only a task during manufacturing and exploitation stages and cannot be fully automatized. A solution to monitor mechanical stress—through a load cell—and temperature of a concrete element is provided in [11]. This solution uses a battery which can be charged in the near-field thanks to an inductive wireless power transfer system and uses the M-BUS wireless protocol to communicate over a few metres. Again, human interactions are needed, here for the charging process. The tracking of precast concrete elements is provided in [12,13] by using embedded RFID tags in the concrete elements. In these cases, no measurement is performed, human interactions are needed for the short range reading, a smartphone connected to the Internet is required and only the construction phase of a structure is targeted. Several companies provide solutions to monitor concrete structures. For instance, [14] proposes a wireless temperature and humidity sensor with a battery based on SigFox (Labège, France) technology to be embedded into concrete and monitor its curing process for the two cases of in situ and precast casting. Some sensor nodes using batteries and middle range wireless communication are provided by [15] to characterize concrete in-situ. Finally, [16] provides an external and generic sensor to monitor concrete structure. All these solutions have a lifespan estimated at a few years, which is far less than the lifespan of a reinforced concrete structure. In this paper, a smart wireless micro-node mesh network is proposed to design cyber-physical systems for SHM applications in the construction domain. This is the first step towards designing fully autonomous communicating concrete. The issues concerning installation and maintenance costs are considered, as well as the need of having a reliable system for decades. The wireless sensor network (WSN) presented in Figure 3 is composed of two kinds of nodes: sensing nodes (SNs) and communicating nodes (CNs). The sensing nodes are designed to sense the physical world (i.e., the monitored structure) and communicate data collected from their environments to the communicating nodes over a distance ranging from several meters to kilometres. These nodes must be energy autonomous and reliable for the entire lifespan of the monitored structure because they cannot be accessed once deployed. Thus, to avoid any maintenance, and for increasing their lifetime, the SNs are designed to be battery-free and wirelessly powered by a RF source driven by (or even integrated into) the CNs through a far-field wireless power transmission (WPT) system. As communication and WPT work on the same frequency band, this architecture answers the paradigm of simultaneous wireless information and power transfer (SWIPT) [17,18].
Sensors 2019, 19, 1510 4 of 26 Sensors 2019, 19, x FOR PEER REVIEW 4 of 26 Figure 3. Figure 3. Schematic Schematic diagram diagram of of the the architecture architectureof ofthe thewireless wirelesssmart-nodes smart-nodesmesh meshnetwork. network. The Thecommunicating communicating nodes nodes are are designed designedto aggregate, process,process, to aggregate, store andstoreexchangeand data exchangegenerated data by the SNs with generated by the theSNs other withCNstheand with other CNstheand virtual withmodel through the virtual the Internet. model throughAs thethey are accessible, Internet. As they they do not need are accessible, to be they doenergy not need autonomous to be energy for the entire lifetime autonomous for of thethe element. entire Moreover, lifetime of the the CNs element. are able to power the SNs by controlling the RF source dedicated to Moreover, the CNs are able to power the SNs by controlling the RF source dedicated to the WPT. the WPT. In Inthis thispaper, paper,an animplementation implementationof ofaa LoRaWAN LoRaWANbattery-free battery-freewireless wirelesssensing sensingnode nodeisispresented presented and andcharacterized. characterized. It It is is powered powered via via far-field far-field WPT WPT and and measures measures temperature temperature and and relative relativehumidity humidity which which are are then communicated communicated by by using using the theLoRa LoRatechnology technologyand andthethe LoRaWAN LoRaWAN protocol protocol to to an an ‘exploded’ ‘exploded’ communicating communicating node node(LoRa(LoRa gateway gateway andand WPT WPT system system are are currently currently separated). separated). Its Its periodicity periodicity of measurement of measurement andand communication communication is controlled is controlled by thebyWPT the system WPT system through through the the tuning tuning of the of the transmitted transmitted power.power.Thus, Thus, as theasneedthe need of measurements of measurements cannot cannot be fully be fully specified specified during during the the design design phase phase and andcancan change change during during thethedifferent differentphases phasesofofthe thelifecycle lifecycle of of the the element, the the control control of ofthe theperiodicity periodicity through through the WPT the WPTsystem allows allows system the hardware and the software the hardware and thetosoftware remain unmodified. to remain Moreover, unmodified. some experiments Moreover, somewere conducted experiments werewith a networkwith conducted of two SN prototypes a network of two of SNsensing prototypesnodes. of Although sensing nodes.our main objective is to provide a communicating reinforced precast concrete, the first experiments Although were our carried out forisgeneric main objective to provide in-the-air SHM applications a communicating reinforced and not yet precast in reinforced concrete, the first concrete. experiments The current objective were carried outis to forprovide generica proof-of-concept of a wirelesslyand in-the-air SHM applications powered not yet andinbattery-free reinforced wireless concrete.sensing node designed The current objective is fortocyber-physical systems dedicated provide a proof-of-concept to structural of a wirelessly health and powered monitoring battery- applications free wirelessinsensing the construction node designeddomain. forThis proof-of-concept cyber-physical systemsmust be powered dedicated wirelesslyhealth to structural over several monitoringmetres, must senseinsome applications the relevant constructionenvironmental domain. parameters (in this case,must This proof-of-concept temperature be powered and relative humidity, but other types of sensors could be integrated according wirelessly over several metres, must sense some relevant environmental parameters (in this case, to the targeted application, such as mechanical temperature stress humidity, and relative sensors) and butmust otherexchange data wirelessly types of sensors could be(in this caseaccording integrated for long range, to the but some application, targeted communication suchtechnologies as mechanical arestress investigated sensors)according and musttoexchange the targeted dataapplication). wirelessly (in In the this near case future, for longthe prototypes range, but some willcommunication be embedded into a reinforced technologies are concrete investigatedbeamaccording to experiment to thewith the targeted targeted environment application). In the near which is a the future, more difficult medium prototypes for RF propagation. will be embedded into a reinforced concrete beam to Recently, experiment several with implementations the targeted environment of energy whichautonomous LoRaWAN is a more difficult medium sensing nodes for RF dedicated to propagation. various applications, Recently, several essentially for SHM,of implementations with various energy software or autonomous hardware defined LoRaWAN sensing periodicity, nodes dedicated from seconds to various to applications, hours, and with various essentially forkinds SHM,ofwith sensors, various such as temperature software or hardware anddefined humidity [19,20], periodicity, mechanical from seconds stress [20,21], to hours, pH [19] and with variousor electrical currentsuch kinds of sensors, [22],as have been presented. temperature and humidity Some are [19,20], battery-free mechanical[20,22,23], stress [20,21],some harvest pH [19] ambient energy (e.g., or electrical solar[22], current [19,21,23], have thermal [19], mechanical been presented. Some[20], are inductive battery-free [22],[20,22,23], etc.) and otherssome are basedambient harvest on backscattering energy (e.g.,[23,24].solar Some solutions thermal [19,21,23], that combine [19], mechanical [20], inductive [22], etc.) and others are based on backscattering [23,24]. Some solutions that combine multiple sources are proposed, too [19,21,23]. These projects are listed in Table 1. Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
Sensors 2019, 19, 1510 5 of 26 multiple sources are proposed, too [19,21,23]. These projects are listed in Table 1. Nevertheless, and although the feasibility of WPT for powering a LoRaWAN node was demonstrated theoretically in [25], no complete implementation was provided until [26]. The advantage of WPT over ambient energy harvesting is that it is not dependent on the availability of the fluctuating ambient sources. Some estimates of available harvestable energy in buildings are shown in [27]. Table 1. Comparative study of implementations of LoRaWAN sensing nodes supplied by an energy-harvesting system. Energy Periodicity of [Reference] Energy Storage Sensor Type Measurement and/or Application(s) Year Source(s) Capacitance Transmission Temperature [20] Mechanical 100 mF Humidity (rain) 3 h 30 min (estimation) Bridges SHM 2016 (vibrations) Vehicles counter SHM; Precision agriculture; Smart [24] contact lens; Backscattering N/A N/A N/A 2017 Flexible epidermal patch sensor Electricity [22] Electrical 22 mF consumption/AC Seconds (few) Smart grids 2018 induction current [21] Solar and 2 F and 1400 SHM of construction Crackmeter N/A 2018 battery mAh materials Temperature [19] Solar and environment (water) 20 Ah Humidity Hour 2018 thermal monitoring pH [23] Backscattering 33 mF N/A 20 min (estimation) N/A 2018 and solar Communicating Controlled by the WPT Temperature material This work system RF WPT 22 mF Relative SHM in harsh 2019 (minutes to hours, or humidity environments more) CPS A complete specification of the implemented prototype of the sensing node will be the core of Section 2. Section 3 will highlight the obtained experimental results for a SN prototype and for a network of two prototypes of sensing nodes over the air. Discussions and future improvements will be discussed in Section 4. 2. Design and Implementation of the Prototype of Sensing Nodes Due to its need for complete energy autonomy, the design of the sensing node is more challenging than the design of the communicating nodes. As previously described, the SNs must be battery-free and wirelessly powered, generate data by sensing their environment and transmitting these data wirelessly to the CNs. 2.1. Topology of the Sensing Nodes Figure 4 shows the architecture chosen for the sensing node prototype. The SNs use a temperature and relative humidity sensor to sense their environment, which will be, in the end, reinforced precast concrete elements. The collected data are recorded by a microcontroler unit (MCU) in a LoRaWAN frame to be transmitted to the CNs through a wireless unidirectional radiofrequency communication based on LoRa technology. In order to help achieve energy autonomy, the SNs must be low power and as simple as possible. Therefore, they must carry out minimal data processing and should not store the collected data once transmitted. To provide the energy autonomy for decades without access, the SNs are battery-free and wirelessly powered through a far-field RF WPT system, which is controlled by
Sensors 2019, 19, x FOR PEER REVIEW 6 of 26 communication based on LoRa technology. In order to help achieve energy autonomy, the SNs must be low power and as simple as possible. Therefore, they must carry out minimal data processing and should2019, Sensors not19,store 1510 the collected data once transmitted. To provide the energy autonomy for decades 6 of 26 without access, the SNs are battery-free and wirelessly powered through a far-field RF WPT system, which is controlled by the CNs. A rectenna is used in the SNs to harvest the dedicated generated RF the powerCNs. A rectenna density is used initthe and transform SNs into DCtopower harvestwhich the dedicated generated is supplied RF power to a power density unit management and transform it into DC power which is supplied to a power management unit (PMU). (PMU). This PMU has the role of efficiently managing the energy provided by the rectenna. It needs This PMU has the role of to store thisefficiently energy inmanaging the energy a supercapacitor andprovided by the when there rectenna. is enough It needs energy to storeallthis to power theenergy active in a supercapacitor and when there is enough energy to power all the active components (sensor, MCU and LoRa transceiver), deliver this energy to perform a measurement components (sensor, and MCU and LoRa transceiver), deliver this energy to perform a measurement and a complete wireless transmission. As the SNs are inaccessible once deployed, no software or a complete wireless transmission. As the SNscan hardware modifications arebe inaccessible once deployed, made. Nevertheless, no software by controlling the or RFhardware modifications power density illuminatingcan be made. Nevertheless, by controlling the RF power density illuminating the SNs, the SNs, the activation and periodicity of the measurement and transmission can be managed by the activation and periodicity the CNs. of the measurement and transmission can be managed by the CNs. Figure 4. Figure Schematic diagram 4. Schematic diagram of of the the architecture architecture of of the the battery-free battery-free wireless wireless sensing sensing node. node. 2.2. Rectennas 2.2. Rectennas To ensure the energy autonomy of the SNs, different strategies can be employed. As presented To ensure the energy autonomy of the SNs, different strategies can be employed. As presented in Table 1, there are some published works on LoRaWAN sensing nodes based on energy harvesting in Table 1, there are some published works on LoRaWAN sensing nodes based on energy harvesting techniques. The available energy sources and the best strategies to scavenge them in buildings and techniques. The available energy sources and the best strategies to scavenge them in buildings and for SHM applications are highlighted in [27]. Unfortunately, these sources are too low, too fluctuating for SHM applications are highlighted in [27]. Unfortunately, these sources are too low, too fluctuating and sometimes unavailable for our targeted applications. For instance, light sources, air flows and and sometimes unavailable for our targeted applications. For instance, light sources, air flows and thermal gradients are not available in concrete. The mechanical sources (e.g., vibrations) are fluctuating thermal gradients are not available in concrete. The mechanical sources (e.g., vibrations) are and hardly related to the uses of the structure (e.g., bridge, residential building, etc.). And the RF fluctuating and hardly related to the uses of the structure (e.g., bridge, residential building, etc.). And power densities measured in a laboratory in [27] are very low, specifically between 1.69 pW/cm2 in the RF power densities measured in a laboratory in [27] are very low, specifically between the 2.45 GHz ISM band and 57.37 nW/cm2 in the 900 MHz GSM band. Finally, the strategy of the RF 1.69 pW/cm² in the 2.45 GHz ISM band and 57.37 nW/cm² in the 900 MHz GSM band. Finally, the wireless power transmission was chosen because of the complete control of the energy source and the strategy of the RF wireless power transmission was chosen because of the complete control of the possibility of managing the activation and periodicity of measurement and data transmission by tuning energy source and the possibility of managing the activation and periodicity of measurement and the transmitted power. There are two different approaches for RF WPT: the magnetic or near-field data transmission by tuning the transmitted power. There are two different approaches for RF WPT: WPT as presented in [8,28,29] in the case of SHM of reinforced concrete structures applications, and the the magnetic or near-field WPT as presented in [8,28,29] in the case of SHM of reinforced concrete far-field RF WPT used in this project. The far-field solution can increase the useful range between the structures applications, and the far-field RF WPT used in this project. The far-field solution can RF source and the sensing nodes. This maximum distance is related to the chosen frequency through increase the useful range between the RF source and the sensing nodes. This maximum distance is the free space losses and to propagation medium through its dielectric properties. It should be noted related to the chosen frequency through the free space losses and to propagation medium through its that the capabilities of the far-field WPT systems are limited today mainly by regulations (i.e., the dielectric properties. It should be noted that the capabilities of the far-field WPT systems are limited maximum effective isotropic radiated power (EIRP) that can be radiated in the ISM bands [30]) and today mainly by regulations (i.e., the maximum effective isotropic radiated power (EIRP) that can be not by the technology itself. For instance, the maximum EIRP for the ISM 868 MHz frequency band is radiated in the ISM bands [30]) and not by the technology itself. For instance, the maximum EIRP for 2 W (or +33 dBm). the ISM 868 MHz frequency band is 2 W (or +33 dBm). In order to scavenge the generated RF power densities, specially designed rectennas (the acronym for “rectifying antenna”) are used. They are tuned to operate efficiently around the ISM 868 MHz band. Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors The choice of this frequency is justified by a compromise between the free space losses and the size of the required antenna. The used rectennas, presented in Figure 5, are composed of:
(2D) rectenna. For the second rectenna, the RF rectifier is orthogonally connected with the antenna to have a three-dimensional (3D) rectenna. (iii) a metallic reflector (15 cm × 9 cm) positioned at 5 cm behind the rectennas (2D and 3D), which can increase the antenna gain (respectively from +2.2 dBi to +6.6 dBi) while having a three-dimensional structure comparable in terms of volume with the three-dimensional rectenna Sensors 2019, 19, 1510 7 of 26 used in [32]. The manufactured rectennas are represented in Figure 6. The use of the metallic reflector increases the rectenna performance by improving the directivity and the gain of the (i) a compact (11antenna cm × 6 in cm)theand opposite direction broadband of the dipole reflector. antenna The adding enclosed of a metallicring by a rectangular reflector [31], increases the DC power manufactured on FR4 harvested. substrate (thickness: 0.8 mm, relative electric permittivity: 4.4 and loss Undesirable tangent: 0.02) harmonics are generated which captures during the and converts the rectifying generatedprocess, both at the electromagnetic input and energy fieldoutput into a portsRF of the diode, signal which around theis ISM a nonlinear 868 MHz component. band. Its An impedance simulated matching circuit (HFSS—High composed Frequency of a Structure bent short-circuited Simulator—from stub and an Ansys Inc.inductor of 33 nH (Canonsburg, PA,isUSA)) inserted before the maximal gainRFis rectifier +2.2 dBito atbe a band-pass this frequency. filter aand (ii) to avoid single the re-radiation Schottky diode (Broadcom by the Inc. antenna (SanofJose, the generated CA, USA) harmonics. HSMS 2850Thus, mountedthe RF in power series conversion efficiencyhigh-frequency configuration) is maximized. Again, a low-pass RF rectifier filter is inserted (manufactured after the on Rogers RF rectifier (Chandler, Corporation to provide a DCAZ, signal to the USA) PMU. RT/Duroid 5870 substrate, thickness: 0.787 mm, relative electric permittivity: 2.3 The results depicted and loss tangent: 0.0012) in Figurewhich7 demonstrate converts the that the manufactured guided RF signal into rectennas operate DC power. Forefficiently the first for low power densities of the incident RF waves. These generated power rectenna, the RF rectifier is connected with the antenna in an overlapping manner to have densities are higher than a thoseplanar available in buildings [27] and, consequently, a dedicated RF power source two-dimensional (2D) rectenna. For the second rectenna, the RF rectifier is orthogonally is required. Thus, DC powers connectedgreater withthan 50 µW can the antenna be harvested to have if the incident a three-dimensional power (3D) density exceeds 0.42 µW/cm² rectenna. and 0.50 µW/cm² respectively for the 2D and 3D rectenna. (iii) a metallic reflector (15 cm × 9 cm) positioned at 5 cm behind the rectennasMoreover, these rectennas (2D are relatively and 3D), broadband, covering the ISM 868 MHz and 915 MHz bands, and which can increase the antenna gain (respectively from +2.2 dBi to +6.6 dBi) whilea part of the GSM 900 MHz bands. having a Regarding Figure 7, the 2D rectenna is superior to the 3D rectenna with three-dimensional structure comparable in terms of volume with the three-dimensional rectenna the use of the metallic reflector usedatin868 MHz. [32]. The Nevertheless, manufacturedboth designs rectennas areare within one represented inorder Figureof6.magnitude in terms of DC power. More details concerning the rectennas used in this SN prototype are reported in [33]. Sensors 2019, 19, x FOR PEER Figure Figure 5. REVIEW 5. (A) Block (A) Block diagram diagram and and (B) (B) schematic schematic of of the the manufactured manufactured rectenna. rectenna. 8 of 26 Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors Figure 6. Photograph of the manufactured rectennas. rectennas.
Sensors 2019, 19, 1510 8 of 26 Sensors 2019, 19, x FOR PEER REVIEW 8 of 26 The use of the metallic reflector increases the rectenna performance by improving the directivity and the gain of the antenna in the opposite direction of the reflector. The adding of a metallic reflector increases the DC power harvested. Undesirable harmonics are generated during the rectifying process, both at the input and output ports of the diode, which is a nonlinear component. An impedance matching circuit composed of a bent short-circuited stub and an inductor of 33 nH is inserted before the RF rectifier to be a band-pass filter and to avoid the re-radiation by the antenna of the generated harmonics. Thus, the RF power conversion efficiency is maximized. Again, a low-pass filter is inserted after the RF rectifier to provide a DC signal to the PMU. The results depicted in Figure 7 demonstrate that the manufactured rectennas operate efficiently for low power densities of the incident RF waves. These generated power densities are higher than those available in buildings [27] and, consequently, a dedicated RF power source is required. Thus, DC powers greater than 50 µW can be harvested if the incident power density exceeds 0.42 µW/cm2 and 0.50 µW/cm2 respectively for the 2D and 3D rectenna. Moreover, these rectennas are relatively broadband, covering the ISM 868 MHz and 915 MHz bands, and a part of the GSM 900 MHz bands. Regarding Figure 7, the 2D rectenna is superior to the 3D rectenna with the use of the metallic reflector at 868 MHz. Nevertheless, both designs are within one order of magnitude in terms of DC power. More details concerning the rectennas Figure used inofthis 6. Photograph the SN prototyperectennas. manufactured are reported in [33]. Figure 7. Measured Figure 7. rectenna performances: Measured rectenna performances:(A) (A)DC DCvoltage voltageversus versusfrequency frequency(load: (load:1010 kΩ, kΩ, input input power: +0 dBm) power: dBm) and and (B) (B) DC DC power powerversus versusilluminating illuminatingpower powerdensities (frequency: densities (frequency: 868868 MHz, MHz,load: load: 10kΩ); 10 kΩ); both both the 2D-rectenna with with reflector reflector (red) (red)and andthe the3D-rectenna 3D-rectennawith withreflector reflector(blue). (blue). 2.3. 2.3. Power Power Management Unit Management Unit To Toefficiently efficiently manage manage and and store store the the DC DCpower powerprovided providedby bythe therectenna, rectenna,a apower power management management unit unit (PMU) is used. The Texas Instruments (Dallas, TX, USA) bq25504 [34] is chosen because of of (PMU) is used. The Texas Instruments (Dallas, TX, USA) bq25504 [34] is chosen because itsits minimum minimum required input DC required input DC power power (15 (15 µW approximately).This µW approximately). ThisPMU PMUalsoalsocontains contains a DC-to-DC a DC-to-DC convertor. Figure 8 shows the PMU operational behaviour. The PMU convertor. Figure 8 shows the PMU operational behaviour. The PMU must efficiently charge must efficiently charge thethe supercapacitor with the DC power provided by the rectenna and discharge supercapacitor with the DC power provided by the rectenna and discharge the supercapacitor tothe supercapacitor to power all the active power all thecomponents (the temperature active components and relative (the temperature and humidity sensor, the relative humidity MCUthe sensor, andMCUthe LoRaWAN and the transceiver) when enough LoRaWAN transceiver) whenstored energy enough is available. stored To optimize energy is available. To its storageitsoperation, optimize a maximum storage operation, a power maximumpointpower tracking point(MPPT) hardware tracking (MPPT)function hardware is implemented in the PMU.inRegarding function is implemented the PMU. its operation, Regarding its operation, the PMU stores thethe PMU stores the available available power untilpower until the the voltage at voltage the ports at of thethe ports of the supercapacitor supercapacitor exceeds a exceeds threshold selected a selected threshold (Vmax =and (Vmax = 5.25V) 5.25V) thenand then supplies supplies the active the components active components untilsame until this this voltage same voltage goes down under another selected threshold (V = 2.30V). Moreover, goes down under another selected threshold (Vmin = 2.30V). Moreover, both over- and under-voltage min both over- and under- voltage protections protections are usedare byused by the When the PMU. PMU. theWhen the (under-) (under-) over-voltage over-voltage threshold threshold is attained is attained the the active active components components and theand the supercapacitor supercapacitor are disconnected are disconnected in orderintoorder to prevent prevent a deep (dis)charge. a deep (dis)charge. The PMU The PMU aintegrates integrates cold-starta procedure. cold-start procedure. Thus, if aThus, if a minimal minimal voltage voltage of around of around 350 mV350 andmV and a a minimal Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
Sensors 2019, 19, 1510 9 of 26 Sensors 2019, 19, x FOR PEER REVIEW 9 of 26 DC power minimal of 15 µW DC power areµW of 15 provided to the to are provided PMU, it can itbecan the PMU, self-powered be self-poweredand begin the storage and begin of the the storage of the input power in the supercapacitor. Once enough energy is stored to reach almost 1.3 V voltage at input power in the supercapacitor. Once enough energy is stored to reach almost 1.3 V voltage at the the ports of the supercapacitor, supercapacitor, thethePMU PMUswitches switcheson oncompletely, completely,optimizes optimizesitsitsoperation operation and and more more efficiently charges the supercapacitor. When efficiently charges the supercapacitor. When a voltagea voltage higher than 1.3 1.3 V is available at the ports ofthe V is available at the ports of supercapacitor, the cold-start procedure is no longer necessary, the supercapacitor, the cold-start procedure is no longer necessary, even even if there was was if there no DC noinput power DC input provided power for a long provided for a period. The PMU long period. intrinsic The PMU consumption—announced intrinsic consumption—announced around 15 µW—is around limiting 15 µW—is regarding the minimum power that must be scavenged by the rectenna to allow limiting regarding the minimum power that must be scavenged by the rectenna to allow the complete the complete charge of theofsupercapacitor. charge the supercapacitor. Figure Figure 8. 8. Voltage Voltage waveforms waveforms at at thethe ports ports of of thethe supercapacitor supercapacitor (blue) (blue) and and rectenna rectenna output output voltage voltage (red), (red), forfor a –8 a –8 dBm dBm RFRF power power at at thethe input input ofof the the RFRF rectifier. rectifier. 2.4. Supercapacitor 2.4. Supercapacitor An AVX Corporation (Greenville, SC, USA) BZ01CA223ZSB supercapacitor [35] of 22 mF of An AVX Corporation (Greenville, SC, USA) BZ01CA223ZSB supercapacitor [35] of 22 mF of capacitance (noted as C) is chosen in order to store enough energy to power all the active components capacitance (noted as C) is chosen in order to store enough energy to power all the active components during the required time for measurements (temperature and relative humidity) and the LoRaWAN during the required time for measurements (temperature and relative humidity) and the LoRaWAN transmission of a data frame. The supercapacitor exhibits a low loss current of 10 µA which must be transmission of a data frame. The supercapacitor exhibits a low loss current of 10 µA which must be take into consideration to estimate properly the required power that must be scavenged by the rectenna take into consideration to estimate properly the required power that must be scavenged by the to power the PMU and to allow the complete use of the supercapacitor to store energy. The cycles of rectenna to power the PMU and to allow the complete use of the supercapacitor to store energy. The charges and discharges are managed by the PMU between Vmax and Vmin voltages. The energy stored cycles of charges and discharges are managed by the PMU between Vmax and Vmin voltages. The (noted as E) by the supercapacitor can be estimated as: energy stored (noted as E) by the supercapacitor can be estimated as: C 2 2 E== 2∙ · Vmax − Vmin (1)(1) 2 ByBy using using Equation(1), Equation (1),the theenergy energystored storedbybythe the2222 mF mF supercapacitor supercapacitor is estimated estimated toto 245 245mJmJininits itsvoltage voltagerange range(between (betweenVVmax = 5.25 max = 5.25VVand andVVmin min== 2.30 2.30 V).V). Figure Figure 9 shows 9 shows the the consumption consumption ofof thethe SNSN prototype prototype forfor a complete a complete operation operation (measurement (measurement andand wireless wireless communication). communication). ItsIts consumption consumption is is estimated at 214 mJ. Thus, enough energy is stored by the PMU in the supercapacitor estimated at 214 mJ. Thus, enough energy is stored by the PMU in the supercapacitor to power the to power the sensor, sensor, the the MCU MCU and and the the LoRa LoRa transceiver transceiver duringa acomplete during completeoperation. operation. Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
Sensors2019, Sensors 19,x1510 2019,19, FOR PEER REVIEW 1010ofof26 26 Figure Figure9.9.Supply Supplyvoltage voltage(blue), (blue),current currentconsumption consumption(red), (red),and andpower powerconsumption consumption(green) (green)of ofthe the SN SNprototype. prototype. 2.5.Microcontroller 2.5. MicrocontrollerUnit Unitand andLoRa LoRaTransceiver Transceiver ToTo manage manage the the temperature temperature and and relative relative humidity humiditysensor, sensor, to tocapture capturethe themeasured measureddata datain inaa LoRaWAN data frame and to control the LoRa transceiver, a microcontroller LoRaWAN data frame and to control the LoRa transceiver, a microcontroller unit is used. The LoRa unit is used. The LoRa frame generated frame generated by by thethe MCU MCU is is then then transmitted transmitted by by aa LoRa LoRa transceiver. transceiver. These These processes processes are are accomplished thanks to a Murata Manufacturing Co., Ltd. (Nagaokakyo-shi, accomplished thanks to a Murata Manufacturing Co., Ltd. (Nagaokakyo-shi, Kyoto Prefecture, Japan) Kyoto Prefecture, Japan) CMWX1ZZABZ-091 CMWX1ZZABZ-091 all-in-one all-in-one LoRaWAN LoRaWAN communication communication modulemodule [36][36] (composedofof aa (composed STMicroelectronics(Schiphol, STMicroelectronics (Schiphol,Amsterdam, Amsterdam,Netherlands) Netherlands)STM32L072CZ STM32L072CZ[37] [37]MCU MCUbased basedon onan anARM ARM Cortex M0+ and a Semtech (Camarillo, CA, USA) LoRa transceiver Cortex M0+ and a Semtech (Camarillo, CA, USA) LoRa transceiver SX1276 [38]). A B-L072Z- SX1276 [38]). A B-L072Z-LRWAN1 development LRWAN1 board [39]board development by STMicroelectronics [39] by STMicroelectronics(Schiphol, (Schiphol, Amsterdam, Netherlands) Amsterdam, including Netherlands) the aforementioned hardware was chosen. As stated, the transceiver including the aforementioned hardware was chosen. As stated, the transceiver manages the manages the LoRa physical LoRa layer protocol, whereas the MCU manages the sensor, the physical layer protocol, whereas the MCU manages the sensor, the LoRaWAN protocol stack andLoRaWAN protocol stack and the the applicationsoftware. application software. As presented As presented in inFigure Figure9,9,onceoncepowered, powered,the theMCUMCUswitches-on switches-onand andinitializes initializesitselfitselfand andall allthe the other active components (the temperature and relative humidity sensor other active components (the temperature and relative humidity sensor and the LoRa transceiver) and the LoRa transceiver) (‘Init’ and ‘D’ (‘Init’ stages). and Then it manages ‘D’ stages). Then it the measurement manages from the sensor the measurement from(providing the sensortwo measurements (providing two encoded on two bytes) (‘M’ stage), processes the data and creates measurements encoded on two bytes) (‘M’ stage), processes the data and creates a LoRaWAN a LoRaWAN frame (17 bytes long, frame whose (17 bytes 13 long, bytes whose are induced13 bytes by theare LoRaWAN induced byprotocol) the LoRaWANand drives the LoRa protocol) andtransceiver drives thetoLoRa send the LoRaWAN data frame (17 bytes long) (‘P and T’ stage). Once transceiver to send the LoRaWAN data frame (17 bytes long) (‘P and T’ stage). Once the complete the complete data frame is sent, the system goes into a deep-sleep mode (‘D–S’ stage). All these operations data frame is sent, the system goes into a deep-sleep mode (‘D–S’ stage). All these operations are are performed each time the SN prototype performed each time is supplied and the entire the SN prototype loop lasts is supplied andnearly 2.13 s.loop the entire The lasts largest amount nearly 2.13ofs. DC Thepower largestis required by the LoRa transceiver during the transmission of the LoRaWAN amount of DC power is required by the LoRa transceiver during the transmission of the LoRaWAN data frame. Nevertheless, for our data frame.applications, Nevertheless, the for measurements our applications, and thethewireless communication measurements must not and the wireless be performed communication continuously must (e.g., the temperature not be performed continuously and thethe (e.g., humidity shouldand temperature be measured the humidity and transmitted should be measured hourly or once a day/week, etc.). The transmission respects LoRaWAN class and transmitted hourly or once a day/week, etc.). The transmission respects LoRaWAN class A A standard (the node transmits data but (the standard cannot node betransmits interrogated), data but with a +2 be cannot dBm radio frequency interrogated), with aoutput +2 dBmpower at 868 MHz radio frequency (ISM output power at 868 MHz (ISM band), an activation by personalization (ABP) and without causing the band), an activation by personalization (ABP) and without causing an acknowledgment from an network. This allfrom acknowledgment aids tothelimit the number network. This allofaids exchanges to limitandthe reduce numberthe of global exchanges power and consumption. reduce the The bandwidth global is 125 kHz and power consumption. Thethe spreading bandwidth is factor 125 kHz is 12, and allowing a data-rate the spreading factor ofis293 12,bps. It is probable allowing a data- that this configuration can be further optimized to consume less energy. rate of 293 bps. It is probable that this configuration can be further optimized to consume less energy. Forthe For thetargeted targetedapplication, application,the thecommunication communicationbetween betweenSNs SNsand andCNsCNsmustmustbe bewireless wirelessand and unidirectional(only unidirectional (onlyuplink) uplink)in inorder orderto toreduce reducethe theDC DCconsumption consumptionby bydeleting deletingthe theactive activereception reception process. The targeted distance between SN and CN is in the range of tens of metres (middle range) Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
Sensors 2019, 19, 1510 11 of 26 process. The targeted distance between SN and CN is in the range of tens of metres (middle range) or kilometres (long range). LoRa technology and the LoRaWAN protocol are chosen for implementing our SN prototype because of: (i) the intrinsic long-range capabilities and (ii) the availability of a reliable infrastructure in the laboratory. Thus, a long-range (1.3 km) wireless communication between the SN prototype (located at LAAS-CNRS Toulouse, France, GPS coordinates: 43◦ 330 45.300 N 1◦ 280 38.400 E) and a LoRa gateway (installed on the INSA campus Toulouse, France, GPS coordinates: 43◦ 340 14.800 N 1◦ 270 58.500 E) can be performed. The data measured and transmitted by the SN are then routed by the TheThingsNetwork [40] network, up to servers, where these can be recovered and processed to obtain a complete cyber-physical system. However, the choices of LoRa technology and the LoRaWAN protocol are not necessarily the best regarding the targeted application. Table 2 shows a short comparison between three technologies (LoRaWAN, Bluetooth Low Energy (BLE) and RuBee) identified as possible solutions to implement the wireless communication between the SNs and the CNs for different communication ranges (long and middle). It should be noted that the RuBee technology that is based on near-field communication, is not yet commercially available. It defines intrinsically an auxiliary channel for the WPT and is defined as insensitive to reinforced concrete and water. Conversely, BLE used the ISM 2.45 GHz band which is very sensitive to water and thus to concrete. Finally, LoRa is the most consuming solution among the three presented but with, in return, the longest reach. To estimate properly the power consumption of each standard, a hardware module must be selected and the duration of data communication (only uplink or bidirectional as function of the selected standard and/or application) must be considered. Table 2. Comparative study of three wireless communication technologies identified as possible solutions for the wireless communication between the SNs and the CNs. Minimal Data Frequency Band Technology Standard Directionality Range Frame Size (ISM) (byte) Uplink LoRaWAN 433/868/915 MHz Long LoRa (Downlink 14 [41] (far-field) (km) restricted) Bluetooth Low Bidirectional IEEE 802.15.1 2.4 GHz Middle Energy “Uplink-only” 11 [42] (far-field) (tens of m) (BLE) available 131 kHz IEEE 1902.1 Middle RuBee Bidirectional (near-field) 5 [43] (tens of m) (65 kHz for WPT) 2.6. Temperature and Relative Humidity Sensor To measure the temperature and relative humidity, an Adafruit Industries (New York City, NY; USA) DHT22 [44] active sensor is used. Each measurement is coded on 2 bytes and has a resolution respectively of 0.1 ◦ C and 0.1%. The measured average consumption of this sensor for its initialization and a unique measurement is 30 mW for a voltage varying from Vmax to Vmin or, in other words, nearly 64 µJ are needed to initialize and use this sensor for temperature and relative humidity measurements. This represents nearly 30% of the consumed energy and 26% of the stored and available energy. This sensor needs a ‘stabilized’ supply voltage between 3 V and 5 V during at least one second before performing its first measurement. Additionally, two seconds are needed between two consecutive measurements. The communication with the MCU is assured by a one-wire homemade protocol. Thus, this active sensor is not low-power, rather slow and is selected mainly by its availability and ease of use, with the objective to have a complete proof of concept. In addition, it is not dedicated to harsh environment and could not be embedded in concrete. More optimal sensing solutions are introduced and discussed in Section 4.3 as well as other kinds of sensor that could be relevant to integrate in the sensing node to monitor concrete elements.
Sensors 2019, 19, 1510 12 of 26 2.7.Sensors 2019, 19, Complete x FOR PEER REVIEW Operation 12 of 26 The 2.7. supercapacitor Complete Operation is initially empty and the PMU is switched on by using a cold start-up procedure, which requires an input voltage higher than 350 mV and a DC power higher than 15 µW. The supercapacitor is initially empty and the PMU is switched on by using a cold start-up When the cold-start is achieved and a DC power higher than 15 µW is available, the PMU charges procedure, which requires an input voltage higher than 350 mV and a DC power higher than 15 µW. efficiently the supercapacitor providing that its current loss is lower than the provided power. Once When the cold-start is achieved and a DC power higher than 15 µW is available, the PMU charges enough energy efficiently the is stored, the active supercapacitor components providing are supplied. that its current Thus, loss is lower thethe than supercapacitor provided power. is charged Once andenough discharged between two threshold voltages, as presented in Figure 8. Specifically, energy is stored, the active components are supplied. Thus, the supercapacitor is charged the software initializes the MCU, and discharged the sensor—with between two thresholda voltages, delay of asalmost one second presented in Figure before being usable—and 8. Specifically, the softwarethe LoRa transceiver. Then the SN prototype senses temperature and relative initializes the MCU, the sensor—with a delay of almost one second before being usable—and humidity and performsthe a LoRaWAN transmission, LoRa transceiver. Then thebefore being deactivated SN prototype and shifting senses temperature to a deep-sleep and relative humidity and mode and then performs a completely LoRaWAN powering off. Asbefore transmission, presented being in deactivated Figure 9, theand complete shiftingoperation lasts approximately to a deep-sleep mode and then 2.13 s (830 ms to initialize completely poweringthe system off. As and make aintemperature presented Figure 9, theand relative complete humidity operation measurement lasts approximately and2.13 1.30 s s (830 msato to transmit initialize the LoRaWAN datasystem frame).andThemake a temperature system and 214 needs at least relative mJ tohumidity measurement work properly, where and 64 mJ are1.30 s to sensor, for the transmitasa detailed LoRaWAN data The earlier. frame). The system recovering, needs at least processing 214 mJ to and storing of work properly, data on where the servers has 64 mJ are for the sensor, as detailed earlier. The recovering, processing not been currently implemented. A visual check through a web-browser of the reception of the dataand storing of data on the servers frame has not is done been on the currently implemented. TheThingsNetwork A visual check through a web-browser of the reception website. of A the data frame of photograph is done on the the two SN TheThingsNetwork prototypes manufactured website. is shown in Figure 10. At this stage of the A photograph of the two SN prototypes manufactured research the SN prototypes are either integrated or miniaturized is shown but allowin Figure 10. At this stage a proof-of-concept of the validation research the SN prototypes are either integrated or miniaturized but allow of the architecture highlighted in Figure 4. Moreover, the choices in terms of sensor and communication a proof-of-concept validation of the architecture highlighted in Figure 4. Moreover, the choices in terms of sensor and technology do not provide a real low-power solution. Nevertheless, having a functional complete communication technology do not provide a real low-power solution. Nevertheless, having a SN prototype with the available sensor (not low power) and using LoRa technology and LoRaWAN functional complete SN prototype with the available sensor (not low power) and using LoRa protocol means that a more optimal system in terms of energy efficiency can be developed in the future technology and LoRaWAN protocol means that a more optimal system in terms of energy efficiency for in the air applications. Moreover, its adaptation to be embedded in reinforced concrete must take can be developed in the future for in the air applications. Moreover, its adaptation to be embedded intoinconsideration new hard reinforced concrete mustconstraints take into especially regarding consideration new hardthe attenuation induced byregarding constraints especially this particular the medium. The following functionalities are currently implemented: attenuation induced by this particular medium. The following functionalities are (i) currently implemented: the energy autonomy is performed by the WPT system (RF power source and rectennas), (i) the energy autonomy the supercapacitor is performed by the WPT system (RF power source and rectennas), and the PMU; the supercapacitor and the PMU; (ii) the measurement is performed by a temperature and relative humidity sensor; (ii) the measurement is performed by a temperature and relative humidity sensor; (iii) the(iii) unidirectional wireless communication is warranted by the developed software implemented the unidirectional wireless communication is warranted by the developed software in the MCUinand implemented the the MCU LoRa andtransceiver; and the LoRa transceiver; and (iv) the(iv) reconfigurability of the periodicity of measurement the reconfigurability of the periodicity andand of measurement data communication data communicationisispossible possibleby bytuning tuning the the amount of transmitted amount of transmittedpower powerviaviathe the RFRF source. source. Figure 10.10.Photograph Figure Photographofofthe thesensing sensing node node prototypes usedfor prototypes used forexperiments. experiments. Sensors 2019, 19, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors
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