Article A Simple Node System for Electricity Monitoring Applications: Design, Integration, and Testing with Different Piezoelectric Energy Harvesters †

Zongxian Yang 1 , Sid Zarabi 1, Egon Fernandes 2, Maria-Isabel Rua-Taborda 3,Hélène Debéda 3, Armaghan Salehian 2, David Nairn 1 and Lan Wei 1,*

1 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada; [email protected] (Z.Y.); [email protected] (S.Z.); [email protected] (D.N.) 2 Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada; [email protected] (E.F.); [email protected] (A.S.) 3 IMS Laboratory, University of Bordeaux, 33400 Talence, France; [email protected] (M.-I.R.-T.); [email protected] (H.D.) * Correspondence: [email protected]; Tel.: +1-519-888-4567 (ext. 31423) † This paper is an expanded version of “Design and development of a self-contained and non-invasive integrated system for electricity monitoring applications” published in the Proceedings of the 2017 IEEE 12th International Conference on ASIC (ASICON), Guiyang, China, 25–28 October 2017.

 Received: 7 September 2018; Accepted: 31 October 2018; Published: 2 November 2018 

Abstract: Real time electricity monitoring is critical to enable intelligent and customized energy management for users in residential, educational, and commercial buildings. This paper presents the design, integration, and testing of a simple, self-contained, low-power, non-invasive system at low cost applicable for such purpose. The system is powered by piezoelectric energy harvesters (EHs) based on PZT and includes a unit (MCU) and a central hub. Real-time information regarding the electricity consumption is measured and communicated by the system, which ultimately offers a dependable and promising solution as a wireless . The dynamic power management ensures the system to work with different types of PZT EHs at a wide range of input power. Thus, the system is robust against fluctuation of the current in the electricity grid and requires minimum adjustment if EH unit requires exchange or upgrade. Experimental results demonstrate that this unit is in a position to read and transmit 60 Hz alternating current (AC) sensor signals with a high accuracy no less than 91.4%. The system is able to achieve an operation duty cycle from <1 min up to 18 min when the current in an electric wire varies from 7.6 A to 30 A, depending on the characteristics of different EHs and intensity of current being monitored.

Keywords: energy management; self-contained system; electricity monitoring; wireless sensor node; power conditioning circuit; energy harvester; PZT; screen-printing; dynamic duty cycle

1. Introduction In contemporary urban growth and development, the problem of extensive energy utilization in residential, educational, and commercial buildings has become crucial and urgent. About one third of total civil energy is consumed in buildings at present around the world [1]. The growing concern over enhancing the efficiency of energy consumption within residential, educational, and commercial buildings [2] has therefore attracted keen research and industry interests in developing novel intelligent power management systems based on sensors and actuators [3,4]. While an electric meter provides the utility company with information regarding the total energy consumption, no information is provided to the consumers with respect to the energy consumed by individual appliances [5]. Such information

Sensors 2018, 18, 3733; doi:10.3390/s18113733 www.mdpi.com/journal/sensors Sensors 2018, 18, 3733 2 of 15 could enable consumers to better control and manage their energy usage, achieving a reduced overall energy consumption [6]. By using the emerging smart technology, the outdated electric distribution network within buildings can be replaced with automated and smart systems to increase the power consumption efficiency and optimize the power distribution [7]. Among these new technologies, wireless sensor networks (WSNs) have been widely emphasized because of their flexible and decent way to establish an intelligent power management system [8,9]. This system is able to gather, analyze, and transmit the sensory information regarding the power information, providing better efficiency and power management. Not only in continuous environmental monitoring [10–12] including widespread air quality monitoring and monitoring, WSNs have also received increasing attentions for other various applications, such as animal tracking [13,14] and health monitoring of civil structures [15–17]. The applications of WSNs in different fields are based on abundant state-of-the-art sensors adopted for monitoring systems [18], human-machine interactive systems [19], wearable electronic systems [20], etc. However, WSNs are still facing some inherent challenges such as slow data transmission speed, high power consumption, insufficient data storage capacity, narrow wireless communication bandwidth, and short life time [21–23]. Such limitations ahead extensively restrict the development and popularity of modern WSN systems. Therefore, developing low cost, self-contained, and non-invasive wireless sensor nodes has become a major challenge to establish such infrastructure. In this work, integrated with our piezoelectric (based on PZT) energy harvesters (EHs) and power conditioning circuitry with dynamic power management scheme, the proposed system is able to solve some of the existed critical issues in order to design and develop a simple, low-cost, self-powered, robust, and non-invasive WSN for electricity monitoring applications. Tested with two different kinds of EHs in our experiments under various conditions, the robustness of the WSN system is verified. We believe that other available harvesters, sensor devices, and wireless communication units are able to be conveniently integrated with our system as a result of its increased flexibility and robustness. In particular, this allows minimum efforts and cost for potential replacement and upgrade of the harvester and sensor units. Furthermore, the design approach introduced in this paper provides a systematic approach that is easily extended to other similar applications. The rest of the paper is organized as follows: Section2 describes the major components of the system and their corresponding functionalities. Section3 introduces our methodology in circuit design and embedded development. Section4 presents the experimental implementation and testing method to validate the system’s performance. Results and comparisons are discussed in Section5 including analysis with respect to performance, error, transmission duty cycle, and reliability. Finally, Section6 concludes the work.

2. System Description The schematic of the self-contained system shown in Figure1 consists of a PZT-based piezoelectric EH, a power conditioning circuit, a non-invasive piezoelectric-based current sensor, a wireless microcontroller unit (MCU), and a wireless transceiver in central monitoring system. Each component plays an indispensable role in the operation of the unit. They are selected considering the factors such as power consumption, size, cost, and reliability so that each of them operates and functions most effectively. The WSN system is powered by the energy harvested from the electromagnetic field across the electric wires, thus independent of battery supply. Furthermore, interfaces and the power conditioning circuitry are designed in a flexible way to boost the adjustability of the system. In this way, various EHs and sensor devices can be integrated with the system as well. In this work, we realize and test the system with two different PZT EHs in order to validate its functionality, flexibility and robustness. The wide range of input power provided by the two EHs indicates the potential of the WSN to work with other low-power, state-of-the-art devices. Sensors 2018, 18, 3733 3 of 15 Sensors 2018, 18, x FOR PEER REVIEW 3 of 15 Sensors 2018, 18, x FOR PEER REVIEW 3 of 15

FigureFigure 1. 1.Overall Overall schematic schematic ofof thethe self-containedself-contained WSN WSN monitoring monitoring system system independent independent of ofbattery battery.. Figure 1. Overall schematic of the self-contained WSN monitoring system independent of battery. 2.1.2.1. Piezoelectric Piezoelectric Energy Energy Harvesters HarvestersBased Based onon PZTPZT 2.1. Piezoelectric Energy Harvesters Based on PZT InIn the the case case of of non-invasivenon-invasive wireless wireless sensor sensor nodes nodes for for electricity electricity monitoring monitoring applications, applications, the In the case of non-invasive wireless sensor nodes for electricity monitoring applications, the theelectromagnetic electromagnetic energy energy is the is dominant the dominant and most and accessible most accessible source available source availableto the EH [24 to]. the Through EH [24 ]. electromagnetic energy is the dominant and most accessible source available to the EH [24]. Through Throughthe PZT the-based PZT-based piezoelectric piezoelectric harvester harvester with permanent with permanent magnets, magnets,the ambient the electromagnetic ambient electromagnetic energy the PZT-based piezoelectric harvester with permanent magnets, the ambient electromagnetic energy energygenerated generated from the from current the-carrying current-carrying wire is finally wire converted is finally into converted electrical into energy electrical in a non energy-invasiv ine a generated from the current-carrying wire is finally converted into electrical energy in a non-invasive non-invasivemanner after manner an intermediate after an intermediate form of mechanical form of mechanical vibrational vibrational energy. The energy. transformed The transformed electric manner after an intermediate form of mechanical vibrational energy. The transformed electric electricenergy energy can be can stored be stored in thein storage the storage unit (e.g. unit, super (e.g.,- super-capacitor)capacitor) and provides and provides the required the required energy energy for energy can be stored in the storage unit (e.g., super-capacitor) and provides the required energy for the WSN. This eliminates the need of external power supply such as batteries. forthe the WSN. WSN. This This eliminates eliminates the the need need of ofexternal external power power supply supply such such as batteries as batteries.. A number of structures have been reported for PZT-based EHs. One of the most popular AA number number of of structures structures have have been been reported reported for for PZT-based PZT-based EHs. One One of of the the most most popular popular configurations is the cantilever beam structure [25–27]. In our work, a harvester with bulk PZT and configurationsconfigurations is is the the cantilevercantilever beam structure structure [25 [25–27–27]. ].In our In our work, work, a harvester a harvester with bulk with PZT bulk and PZT permanent magnet tip mass is firstly used with the structure of a simple piezoelectric bimorph andpermanent permanent magnet magnet tip tip mass mass is is firstly firstly used used with with the the structure structure of of a a simple simple piezoelectric piezoelectric bimorph bimorph cantilever beam. In the experiment, the distance between our cantilever EH (EH1) and the electric cantilevercantilever beam. beam. In In the the experiment, experiment, thethe distancedistance between our our cantilever cantilever EH EH (EH1) (EH1) and and the the electric electric wire that carries a 60-Hz AC current is 6 mm. The surrounding electromagnetic energy from the field wirewire that that carries carries a 60-Hza 60-Hz AC AC current current is is6 6 mm.mm. TheThe surrounding electromagnetic electromagnetic energy energy from from the the field field across the wire is collected by the harvester non-invasively. The voltage Frequency Response acrossacross the the wire wire is collected is collected by the by harvester the harvester non-invasively. non-invasively The voltage. The voltage Frequency Frequency Response Response Function Function (FRF) of EH1 is presented as in Figure 2 with the peak voltage at 60.2 Hz resonance Function (FRF) of EH1 is presented as in Figure 2 with the peak voltage at 60.2 Hz resonance (FRF)frequency of EH1 which is presented matches as the in North Figure American2 with the 60 peak Hz power voltage line at frequency 60.2 Hz resonance. frequency which matchesfrequency the Northwhich Americanmatches the 60 North Hz power American line frequency.60 Hz power line frequency.

FigureFigure 2. 2. Measured voltage frequency response function of the simple PZT-based cantilever beam Figure Measured2. Measured voltage voltage frequency frequency response response function function of of the the simple simple PZT-based PZT-based cantilever cantilever beam beam EH EH (EH1 shown in the inset (dimension unit in mm)) with the peak voltage at 60.2 Hz resonance (EH1EH shown (EH1 shown in the insetin the (dimension inset (dimension unit in mm))unit in with mm)) the with peak the voltage peak voltage at 60.2 Hzat 60.2 resonance Hz resonance frequency frequency which matches the North American 60 Hz power line frequency. whichfrequency matches which the Northmatches American the North 60 American Hz power 60 lineHz power frequency. line frequency. The second PZT-based piezoelectric EH (EH2) shown in Figure 3a for further system testing is TheThe second second PZT-based PZT-based piezoelectric piezoelectric EHEH (EH2)(EH2) shown in in Figure Figure 3a3a for for further further system system testing testing is is more advanced and fabricated using screen-printing technology on a stainless steel (SS) substrate. moremore advanced advanced and and fabricated fabricated usingusing screen-printingscreen-printing technology technology on on a astainle stainlessss steel steel (SS) (SS) substrate. substrate. The design employs a so-called meandering geometry which is centrally-supported [28]. This TheThe design design employs employs a so-called a so-called meandering meandering geometry geometry which which is centrally-supported is centrally-supported [28]. This [28]. structure This structure is known to be capable of achieving better power efficiency than that of a zigzag structure is knownstructure to is be known capable to of be achieving capable of better achieving power better efficiency power than efficiency that of than a zigzag that of structure a zigzag owing structur toe the owing to the reduced torsional mode effects in the clamped beam [29]. As presented in Figure 3b, reducedowing torsionalto the reduced mode effectstorsional in mode the clamped effects beamin the [clamped29]. As presented beam [29]. in As Figure presented3b, during in Figure thecyclic 3b,

Sensors 2018, 18, 3733 4 of 15 Sensors 2018, 18, x FOR PEER REVIEW 4 of 15 Sensors 2018, 18, x FOR PEER REVIEW 4 of 15 motion,during each the individual cyclic motion, beam experienceseach individual either beam tension experiences (shown in either red) or tension compression (shown (shown in red) in blue) or alongduringcompression its entire the cyclic length. (shown motion, The in adjacent blue) each along individual beams its experience entire beam length. experiences regularly The adjacent alternating either beams tension strains experience (shown to generate in regularly red) periodic or alternatingcompressionalternating output strains (shown currents. to generate in blue) Moreover, periodic along its by alternating entireusing the length. output novel The screen-printingcurrents. adjacent Moreover, beams techniques, experience by using one the regularly is novel able to alternating strains to generate periodic alternating output currents. Moreover, by using the novel obtainscreen more-printing advantages techniques, such as one relatively is able lower to obtain investment more advantagescosts for both such equipment as relatively and materials lower screen-printing techniques, one is able to obtain more advantages such as relatively lower comparedinvestment to others costs and for high both volume equipment manufacturing and materials with compared automatic to process others line. and The high experimental volume investment costs for both equipment and materials compared to others and high volume andmanufacturing simulated (using with COMSOL automatic model) process tip line. displacement The experimental and open-circuit and simulated voltage (using FRFs are COMSOL shown in manufacturing with automatic process line. The experimental and simulated (using COMSOL Figuremodel)4 when tip displacement the distance betweenand open EH2-circuit and voltage the electric FRFs wire are shown with 1 in A ACFigure current 4 when is approximately the distance model) tip displacement and open-circuit voltage FRFs are shown in Figure 4 when the distance 12.5between mm. The EH2 experimental and the electric resonant wire with frequency 1 A AC current is shown is approximat to be 60.3 Hz,ely 12. matching5 mm. The that experimental of the North betweenresonant EH2frequency and the is electricshown wireto be with 60.3 1Hz, A AC matching current that is approximat of the Northely 12.American5 mm. The power experimental grid. The American power grid. The simulation shows just a negligible error as well where the COMSOL model resonantsimulation frequency shows just is showna negligible to be error60.3 Hz,as well matching where thatthe COMSOLof the North model American gives us power a prediction grid. The of gives us a prediction of the resonant frequency to be 60.2 Hz. During the subsequent test of the system, simulationthe resonant shows frequency just a tonegligible be 60.2 errorHz. During as well the where subsequent the COMSOL test of model the system, gives usthis a predictionEH harvests of this EH harvests energy from the electric wire at a distance of 4.5 mm (the minimum) for a higher theenergy resonant from thefrequency electric towire be at60.2 a distance Hz. During of 4. 5the mm subsequent (the minimum) test of for the a higher system, output this EH power. harvests outputenergy power. from the electric wire at a distance of 4.5 mm (the minimum) for a higher output power.

(a) (b) (a) (b) Figure 3. (a) Fabricated printed PZT energy harvester with Au electrodes (EH2 (dimension unit in FigureFiguremm)); 3. (3b(.) a (Strain)a) Fabricated Fabricated contour printed printedplot showing PZT PZT energy energytension harvester harvester(red) and with compression with Au Au electrodes electrodes (blue) (EH2 in EH2. (EH2 (dimension The (dimension structure unit inof unit in mm));meandering (b) Strain PZT contour contour-based piezoelectric plot plot showing showing energy tension tension harvester (red) (red) and and (EH2) compression compression with a central (blue) (blue)ly in-supported inEH2. EH2. The The meanderingstructure structure of of meanderingmgeometryeandering fabricated PZT-based PZT-based us piezoelectricin piezoelectricg screen-printing energy energy technology harvesterharvester [2 (EH2)8]. with with a a central centrally-supportedly-supported meandering meandering geometrygeometry fabricated fabricated using using screen-printing screen-printing technologytechnology [[2288].].

Figure 4. Tip displacement and voltage Frequency Response Function of the EH2 with the peak voltage Figure 4. Tip displacement and voltage Frequency Response Function of the EH2 with the peak at 60.3 Hz resonance frequency [28]. Figurevoltage 4at. Tip60.3 displacementHz resonance andfrequency voltage [2 8 Frequency]. Response Function of the EH2 with the peak voltage at 60.3 Hz resonance frequency [28]. The average power output and performance of a vibrational EH are determined by a variety of The average power output and performance of a vibrational EH are determined by a variety of factors such as size, structure, input stimulus, operating frequency, operating environment, and the factorsThe such average as size, power structure output, input and performancestimulus, operating of a vibrational frequency, EH operating are determined environment, by a variety and the of impedance of the interface circuitry responsible for energy regulation and storage [30]. This power factorsimpedance such of as the size, interface structure circuitry, input stimulusresponsible, operating for energy frequency, regulation operating and storage environment, [30]. This and power the outputimpedanceoutput should should of be the be able interfaceable to to fulfil fulfil circuitry the the power power responsible requirementsrequirements for energy of electronic regulation devices devicesand storage within within [30 the]. the This integrated integrated power system.outputsystem. The should The intensity intensity be able of of to power power fulfil outputtheoutput power fromfrom requirements oneone designated of electronic electromagnetic electromagnetic devices withinPZT PZT EH EHthe in in integratedthis this work work variessystem.varies mainly mainly The because intensity because of of the of power inputthe input output stimulus, stimulus, from i.e., one its i.e., distancedesignated its distance from electromagnetic thefrom conducting the conducting PZT wire EH and wire in thethis and intensity work the of currentvarintensityies mainly carried of current because by thecarried wire.of the by Figure inputthe wire. 5 stimulus, presents Figure i.e.,5 the presents its measured distance the measured RMS from power the RMS conducting output power of output wire EH1 and andof EH1 the EH2 withintensity regard of to current the current. carried EH2by the has wire. much Figure lower 5 presents output the power measured since RMS it uses power screen-printing output of EH1 PZT,

Sensors 2018, 18, x FOR PEER REVIEW 5 of 15 Sensors 2018, 18, 3733 5 of 15 and EH2 with regard to the current. EH2 has much lower output power since it uses screen-printing whichPZT, resultswhich inres aults thinner in a layerthinner of PZTlayer withof PZT lower with densification. lower densification. However, However, screen-printing screen-printing technique hastechnique major advantages has major advantages of low cost, of possibility low cost, topossibility add electrodes to add electrodes with different with shapesdifferent and shapes suitable and for highsuitable volume for production,high volume asproduction mentioned, as in mentioned previous in section. previous section.

(a) (b)

FigureFigure 5. 5(.a ()a Power) Power output output ofof EH1;EH1; ((bb)) PowerPower output of of EH2. EH2. Measured Measured RMS RMS power power output output of oftwo two harvestersharvesters with with regard regard to to the the current current passing passing through through the the conducting conducting wire. wire. (The (The distance distance between between EH1 andEH1 the and wire the is wire 6 mm is and6 mm 4.5 and mm 4. for5 m EH2m for for EH2 a higher for a higher output output power) power) EH2 hasEH2 much has much lower lower output poweroutput than power EH1 than as a EH1 result as ofa result screen-printed of screen-printed PZT versus PZT versus bulk PZT. bulk PZT.

2.2.2.2. Interface Interface and and Power Power Conditioning Conditioning CircuitCircuit TheThe AC AC signal signal acrossacross the the load load connected connected to tothe the EH EHhas hasto be to processed be processed (rectified, (rectified, regulated, regulated, and andconditioned conditioned)) before before it goes it goes to the to load the circuitry load circuitry to power to power the WSN the system WSN. Thesystem. rectified The current rectified currentcharges charges a unit for a unitenergy for storage energy purpose storage such purpose as a battery such aswhich a battery is rechargeable which is or rechargeable a widely-used or a widely-usedsuper capacitor super which capacitor can whichbe subsequently can be subsequently conditioned conditioned to drive the to integrated drive the electronic integrated devices. electronic devices.Some Someimportant important and common and common interface interface circuitries circuitries intended intended for powerfor power conditioning conditioning have have been been exploredexplored and and available available in in the the literature literature [[3131––33] within energy energy harve harvestingsting systems systems.. Popular Popular circuities circuities varyvary from from simplest simplestbridge bridge rectifiers rectifiers inin simple simple and low low-cost-cost application applicationss to to complicated complicated active active converterconverter circuits circuits adopted adopted inin thethe designdesign of intelligent control control systems systems for for high high power power density density and and low noise [34]. Different power conditioning technologies are considered regarding their design low noise [34]. Different power conditioning technologies are considered regarding their design complexity, conditioning efficiency, power consumption, and they are usually designed for complexity, conditioning efficiency, power consumption, and they are usually designed for operation operation within a unique environment [35,36]. In this work, considering the application within a unique environment [35,36]. In this work, considering the application requirements, we use requirements, we use the combination of a simple bridge rectifier and a dynamic power the combination of a simple bridge rectifier and a dynamic power management circuit to minimize the management circuit to minimize the cost and maximize the efficiency and flexibility. cost andThe maximize changes the in the efficiency power and output flexibility. from EHs result in variations in the amount of electrical energyThe changesgatheredin in thethe powerstorage output unit during from EHsthe same result period, in variations making init difficult the amount to accurately of electrical predict energy gatheredthe performance in the storage of the unitEH after during the WSN the same is already period, installed making and it difficultin use. We to design accurately and present predict a the performancedynamic power of the management EH after the scheme WSN is in already this work installed where and the in b use.ehaviour We design of the andcircuits present and aWSN dynamic is poweradjusted management and regulated scheme according in this workto the whereperformance the behaviour of the EH. of theThe circuits efficiency and and WSN effectiveness is adjusted of and regulatedthe sensor according node can to be the considerably performance improved of the EH. compared The efficiency with other and schemes effectiveness [37]. of the sensor node can beMost considerably EHs including improved the two compared used in withthis wo otherrk are schemes only able [37 ].to provide a very small amount of outputMost power EHs includingon the order the of two10 μW used to 100 inthis μW work[38–40 are] at onlyan electricity able to usage provide level a veryfrom smalla few amps amount ofto output a few powertens of onamps, the which order ofputs 10 aµ constraintW to 100 µonW[ the38 energy–40] at available an electricity to operate usage the level control from and a few ampscommunic to a fewation tens units ofamps, [41]. Therefore, which puts the a entire constraint WSN on is theexpected energy to availablebe in proper to operate motion theunder control a andsubstantially communication restricted units power [41]. Therefore,budget below the milliwatts. entire WSN Considering is expected this to berestriction in proper associated motion underwith a substantiallythe operation restricted of EH units, power a cold budget start belowcircuit milliwatts.is necessary Consideringto regulate the this unst restrictioneady and associatedweak output with theof operation the EH. of EH units, a cold start circuit is necessary to regulate the unsteady and weak output of the EH.

Sensors 2018, 18, 3733 6 of 15

2.3. Wireless MCU The microcontroller is one of the most fundamental and important elements in the operation of wireless sensor nodes and it is responsible for consuming the main portion of the total energy available to a single node. Thus, it is essential for the MCU to sustain a high level of functionality and performance while simultaneously minimizing the energy consumption. Selecting the proper MCU for a particular application is mainly dictated by factors such as computational requirements of the sensor node, power consumption, size, cost, and reliability. Through a detailed and comprehensive investigation of our application requirements, the CC2640R2F Wireless MCU (Texas Instruments, Dallas, TX, USA) is selected to complete the operation of analog-digital converter (ADC) and data transmission. Ultra-low power consumption, the availability of development tools and libraries, ease of integration with the EH, and the available on-board wireless technology are the important factors to make it the most suitable candidate for our WSN system. The CC2640R2F Wireless MCU contains an onboard 2.4-GHz RF Transceiver that supports low energy (BLE) connectivity [42]. The unit’s BLE controller and its host libraries are embedded in the read only memory (ROM) which improves the overall power consumption of the unit. The CC2640R2F can be manipulated under several power modes (active, idle, standby, shutdown), making designers utilize the restrictive energy collected by the low-power harvesters more effectively. By programming the MCU according to the requirements of our intended application, we further enhance the efficiency of the system by disabling functions available in the MCU that are not necessary. Additionally, the MCU has an internal DC-DC converter that we fully utilize as a voltage regulator, enabling the power consumption efficiency of the unit to be further enhanced. The on-chip DC-DC converter powers the MCU as long as its input voltage is within the operational range of the device, which serves as another essential reason that we choose this device for energy-harvesting applications.

3. Hardware and Software Design As previously shown, the power output of the EH is a function of the current intensity in the conducting wire and is susceptible to the variations in the current. As a consequence, the rectified output is unstable to provide a power supply to the WSN system. In addition, some inevitable power outages could completely disrupt the operation of the EH, which may result in a system shutdown. It is important that the system can automatically recover after the power outage. The practical interface circuitry must be compatible with both the EH and the control and communication unit while being functional over a wide range of operating conditions. Meanwhile, the MCU is supposed to complete operations (in active mode) to interact with the WSN most effectively within the shortest time to lower the limited power budget. These above anticipations have been realized by (1) designing and developing a dynamic power management interface circuitry and (2) developing and implementing embedded software algorithm used to operate the Wireless MCU for controlling the wireless sensor node’s operation, i.e., gathering, analyzing, and transmitting the sensory information to a central hub for post-processing purposes.

3.1. Circuit Design Figure6 presents the WSN system design architecture (the arrows show the directions of different signals passing on within the system) of the proposed single sensor node, where the power conditioning module is shown encompassed within the dashed outline. In the proposed design architecture, the voltage detector within the power conditioning module plays a significant role in the system’s behavior as it directly regulates the MCU’s operation by triggering the ADC. The triggering signal delivered by the voltage detector in turn adjusts the operating frequency of the wireless sensor node to transmit data packet on the basis of the performance of the EH. (Any input voltage within the range of 1.8 V to 3.8 V connected to the VDDS pin of the CC2640R2F effectively powers the unit.) Sensors 2018, 18, 3733 7 of 15 SensorsSensors 2018 2018, ,18 18, ,x x FOR FOR PEER PEER REVIEW REVIEW 77 ofof 1515

Figure 6. WSN system design architecture of a single sensor node. The arrows show the directions of FigureFigure 6. 6.WSN WSN system system design design architecturearchitecture of of a a single single sensor sensor node node.. The The arrows arrows show show the directions the directions of different signals passing on within the system. The dashed box indicates the power conditioning of different signals passing passing on on within within the the system system.. The The dashed dashed box box indicates indicates the the power power conditioning conditioning module including storage unit, voltage detector, and cold start circuit. modulemodule including including storage storage unit, unit, voltage voltage detector,detector, andand coldcold start circuit. Figure 7 shows the detailed circuit schematics of the power conditioning module, with storage FigureFigure7 shows 7 shows the the detailed detailed circuit circuit schematics schematics ofof thethe powerpower conditioning module, module, with with storage storage unit, voltage detector, and cold start circuit encompassed in the dashed boxes. A 330 μF capacitor is unit,unit, voltage voltage detector, detector, and and cold cold start start circuit circuit encompassedencompassed in in the the dashed dashed boxes. boxes. A A330 330 μFµ Fcapacitor capacitor is is chosen to be the storage unit which is suitable to store enough energy for one and only one single chosenchosen to beto be the the storage storage unit unit which which is suitableis suitable to storeto store enough enough energy energy for for one one and and only only one one single single data datadata measurement measurement and and transmission transmission activity. activity. During During the the capacitor’s capacitor’s charging charging process, process, due due to to the the measurement and transmission activity. During the capacitor’s charging process, due to the connection connectionconnection between between the the ground ground pin pin of of the the MCU MCU an andd the the drain drain of of the the far far right right NMOS NMOS transistor transistor between the ground pin of the MCU and the drain of the far right NMOS transistor which works in OFF whichwhich worksworks inin OFFOFF region,region, thethe onboardonboard groundground nodenode (GND)(GND) ofof thethe wirelesswireless MCUMCU isis disconnecteddisconnected region, the onboard ground node (GND) of the wireless MCU is disconnected from the real ground fromfrom the the real real ground ground (i.e., (i.e., GND GND is is floating) floating) so so that that the the MCU MCU is is always always shut shut down down before before one one (i.e., GND is floating) so that the MCU is always shut down before one threshold point. This scheme thresholdthreshold point.point. ThisThis scheschememe suppressessuppresses anyany unintendedunintended andand nonidealnonideal currentcurrent leakageleakage fromfrom MCUMCU suppressesandand allowsallows any thethe unintended storagestorage unitunit and toto benonidealbe continuouslycontinuously current charged/rechargedcharged/recharged leakage from MCU beforebefore and allows thethe voltagevoltage the storage onon thethe storageunitstorage to be continuouslycapacitorcapacitor approachesapproaches charged/recharged thethe threshold.threshold. before DuringDuring the voltage thethe charging/rechargingcharging/recharging on the storage capacitor periods,periods, approaches thethe CC2640R2FCC2640R2F the threshold. MCUMCU Duringremainsremains the disabled.disabled. charging/recharging periods, the CC2640R2F MCU remains disabled.

FigureFigureFigure 7. Detailed7.7. DetailedDetailed circuit circuitcircuit schematic schematicschematic of of theof thethe power ppowerower conditioning conditioningconditioning module. module.module. The TheThe voltage voltagevoltage detector detectordetector detects detectsdetects the triggeringthethe triggeringtriggering information informationinformation by actively byby activelyactively monitoring monitormonitor theinging charging thethe chargingcharging voltage voltagevoltage across across across the thethe storage storagestorage unit. unit. unit. Once OnceOnce this voltagethisthis voltage voltage approaches approaches approaches the circuit the the cir circuit thresholdcuit threshold threshold (3 V( (33 inV V in thisin this this circuit), circuit) circuit), the ,the the circuitcircuit circuit generates generates a a atrigger trigger trigger signal signal signal activingactivingactiving the the the Wireless Wireless Wireless MCU MCU MCU by by by starting starting starting the the the cold cold cold startstart start circuit.circuit. circuit.

Meanwhile,Meanwhile,Meanwhile, the thethe output outputoutput of ofof the thethe voltage voltagevoltage divider dividerdivider (a(a 22 MΩMMΩΩ resistorresistor and and aa a 66 6 MΩMΩ MΩ resistor)resistor)resistor) isis isdelivereddelivered delivered directlydirectlydirectly to to to the the the gate gate gate of of of thethe the farfar far left left transistor transistor with with with the the the addition addition addition of of a ofa small small a small 10 10 μF μF 10 capacitor capacitorµF capacitor to to stab stab toilizeilize stabilize the the thenode.node. node. AsAs As thethe the voltagevoltage voltage acrossacross across thethe storagestorage the storage unitunit reachreach unites reacheses thethe circuitcircuit the circuitthresholdthreshold threshold voltage,voltage, voltage,33 VV inin thisthis 3 circuit, Vcircuit, in this circuit,thethe voltage voltage the voltage detector detector detector detects detects detects this this triggering thistriggering triggering information information information by by turning turning by turning on on the the on far far the left left far transistor. lefttransistor. transistor. As As a a As a consequence,consequence,consequence, the thethe output output of of of thethe the detector detector detector triggers triggers triggers the the thecold cold cold start start start circuit circuit circuit with with witha a multistagemultistage a multistage amplifieramplifier amplifier and and andturnsturns turns thethe the farfar far rightright right transistortransistor transistor on,on, on, abruptlyabruptly abruptly activatingactivating activating thethe theCC2640R2FCC2640R2F CC2640R2F byby reconnectingreconnecting by reconnecting thethe GNDGND the GND pinpin ofof pin ofthe the MCU MCU to to thethe realreal real groundground ground node.node. node. AtAt At thisthis this time,time, time, thethe the MCUMCU MCU willwill will turnturn turn onon and onandand followfollow follow itsits instructionsinstructions its instructions toto

Sensors 2018, 18, 3733 8 of 15

to completeSensors 2018 the, operations18, x FOR PEER of REVIEW measurement and data communication. It is worth mentioning8 that of 15 the simple circuit does not consider about different impedance from different EHs. On one hand, the simplecomplete circuit is the able operations to work withof measurement the two different and data designs commu at anication. wide current It is worth range mentioning with duty that cycle the from <1simple min upcircuit to 18 does min, not acceptable consider forabout the different target application. impedance Onfrom the different other hand, EHs. oneOn one of the hand, main the applicationsimple requirements circuit is able is to to work design with a cheap the two and different robust systemdesigns withat a wide flexibility current to workrange withwith differentduty cycle EHs withoutfrom <1 adjusting min up to the 18 circuit.min, acceptable for the target application. On the other hand, one of the main application requirements is to design a cheap and robust system with flexibility to work with 3.2. Embeddeddifferent Software EHs without Development adjusting the circuit.

The3.2. MCU Embedded must Software be programmed Developmen accordingt to the application requirements in order for the wireless sensor node to undertake function properly. Figure8 shows the application flow chart diagram and The MCU must be programmed according to the application requirements in order for the algorithm developed for running the system and controlling the wireless sensor node’s operation. wireless sensor node to undertake function properly. Figure 8 shows the application flow chart The process begins by enabling the MCU’s active mode through a trigger generated by the voltage diagram and algorithm developed for running the system and controlling the wireless sensor node’s detector. At this point, task initialization including the following steps begins: operation. The process begins by enabling the MCU’s active mode through a trigger generated by • Clearingthe voltage all the detector. registers At this point, task initialization including the following steps begins: • Pin initializationClearing all the registers • Turning Pin on initializa the GPIOtion Module • Selection Turning of the on ADC the GPIO input Module • Scheduling Selection the firstof the task ADC execution input  Scheduling the first task execution • Setting up the ADC parameters  Setting up the ADC parameters • Setting up the parameters  Setting up the Radio parameters • Requesting Requesting access access to ADC to ADC • Requesting Requesting access access to the to Radio the Radio

Start

Stay in No Power

Trigger Shutdown Mode ?

Yes

Task RF Transmission Initialization and Shut down

No

Assign 1 to Count counter

Yes

ADC sampling Increment counter by 1

FigureFigure 8. Wireless 8. Wireless MCUprocess MCU process flow chart. flow Algorithmchart. Algorithm usedto used operate to operate the Wireless the Wireless MCU MCU designed designed for controllingfor controlling the wireless the sensor wireless node’s sensor operation. node’s operation.

Once taskOnce initialization task initialization is completed, is completed, the ADC the module ADC starts module to sample starts theto sample input signal the input generated signal from thegenerated sensor devicefrom the by sensor obtaining device a finiteby obtaining number a offinite samples numbe thatr of are samples equidistant that are (with equidistant a constant (with samplinga constant frequency) sampling to cover frequency) one complete to cover period one complete of the alternating period of signal. the alternating Since the signal. ADC’s Since lower the boundADC’s of the operationlower bound voltage of the of operation this MCU voltage is −0.3 of V,this the MCU program is −0.3 instructs V, the program the ADC instructs to only the register ADC to only register samples from the positive portion of the sensor signal. Lastly, upon finishing collecting

Sensors 2018, 18, 3733 9 of 15

samplesSensors 2018 from, 18, the x FOR positive PEER REVIEW portion of the sensor signal. Lastly, upon finishing collecting9 desired of 15 signal samples instructed by the program, the MCU creates data packets and the onboard RF module transmitsdesired thesignal sampling samples information instructed by to the the program receiver, the hub MCU for post-processing.creates data packets After and thesethe onboard operations, RF allmodule the on-chip transmit moduless the will sampling be disabled information andthe to MCU the receiver device hub will for remain post- inprocessing a complete. After shutdown these modeoperations, to furthest all the manage on-chip the modules small amount will be ofdisabled energy and harvested the MCU for device the system will remain until in another a complete power triggershutdown signal mode arrives. to furthest manage the small amount of energy harvested for the system until another power trigger signal arrives. 4. Implementation and Testing 4. Implementation and Testing To validate the circuit’s behavior and the performance of the self-contained integrated WSN, the systemTo validate is tested the circuit’s according behavior to the and application the performance requirements. of the self- Numerouscontained integrated different WSN, testing scenariosthe system are is developed tested according to ensure to the the application anticipated requirements. performance Numerous of the system different under testing all scenari conditions.os Additionally,are developed various to stress ensure tests the areanticipated performed performance in order to confirm of the the system sensor under node’s all stability conditions. under abnormalAdditionally, conditions various including stress tests power are performed outages, disruptionsin order to confirm or rapid the changes sensor node’s in the stability current under passing throughabnormal the wire,conditions or loss including of connection power with outages, the centraldisruptions hub. or rapid changes in the current passing through the wire, or loss of connection with the central hub. The EH is mounted on an electric wire, mimicking the electricity grid in the The EH is mounted on an electric wire, mimicking the electricity grid in the residential/commercial buildings. A combination of several common appliances (such as a space residential/commercial buildings. A combination of several common appliances (such as a space heater and a hair drier) is used to draw current through the wire during testing at different levels heater and a hair drier) is used to draw current through the wire during testing at different levels between 5 A to 30 A. The current intensity in the electric wire is measured using an AC clamp-on between 5 A to 30 A. The current intensity in the electric wire is measured using an AC clamp-on meter (i400 s A Current Clamp, Fluke, Everett, WA, USA) separately before EH starts to harvest meter (i400 s A Current Clamp, Fluke, Everett, WA, USA) separately before EH starts to harvest energy.energy As. As introduced introduced inin thethe previous section, section, the the distance distance between between EH1 EH1and the and wire the is wire 6 mm is and 6 mm EH2 and EH2harvest harvestss energy energy from from the the wire wire at at a adistance distance of of 4.5 4.5 m mmm for for a a higher higher power power output output.. (Since (Since th thee amplitudeamplitude of of vibration vibration of of EH1 EH1 is is reasonably reasonably greatergreater thanthan EH2’s, EH1 EH1 is is mounted mounted relatively relatively farther farther aboveabove the the wire wire so so as as to to protect protect it it from from permanent permanent damagedamage during vibration vibration and and also also to to decrease decrease the the nonlinearnonlinear effects effects when when the the distance distance between the the magnet magnet an andd the the wire wire is small.) is small.) The Thesensor sensor chosen chosen for forthis this project project was was designed designed at atthe the University University of Waterloo’s of Waterloo’s Energy Harvesting Laboratory Laboratory [43] and [43] its and itsoutput (converted (converted from from AC AC peak peak-to-peak-to-peak to totheir their corresponding corresponding Root Root Mean Mean Square Square (RMS) (RMS) values) values) is is linearlylinearly proportionalproportional to the current intensity intensity in in the the wire wire (shown (shown in in Figure Figure 9).9). During During testing, testing, this this sensorsensor is placedis placed approximately approximately 10 10 mm mm above above thethe wirewire withwith its outputs connected connected to to the the designated designated pinspins (ADC (ADC input) input) of of the the MCU MCU so so as as to to be besampled. sampled. A TDS 2114C 2114C oscill oscilloscopeoscope (Tektronix, (Tektronix, Beaverton, Beaverton, OR,OR, USA) USA is) usedis used to to monitor monitor the the signals signals from from sensor sensor outputoutput (sinusoidal signal) signal) and and circuit circuit nodes nodes..

FigureFigure 9. Sensor’s9. Sensor’s output output (RMS (RMS voltage) voltage)with with regardregard to the current passing passing through through the the wire wire (Output (Output voltagevoltage is linearlyis linearly proportional proportional to to the the current currentintensity). intensity). Different plots plots represent represent different different distances distances betweenbetween the the wire wire and and the the sensor sensor [43 [43].].

Furthermore,Furthermore, in in the the central central monitoring monitoring system,system, a CC2650 Wireless Wireless MCU MCU LaunchPad™ LaunchPad™ KitKit (TI’s (TI’s SimpleLinkSimpleLink™™ Ultra-Low Ultra-Low Power Power Portfolio, Portfolio Texas, Texas Instruments, Instruments Dallas,, Dallas, TX, USA) TX, USA is employed) is employed to receive to thereceive transmitted the transmitted data packets data and packets to validate and to the validate data transmission the data transmission process. The process. receiver The is connectedreceiver is to a desktopconnected computer to a desktop (as the computer central hub) (as usingthe central its onboard hub) using emulator its onboard and USB emulator serial port. and The USB hub serial is at a

Sensors 2018, 18, 3733 10 of 15 Sensors 2018, 18, x FOR PEER REVIEW 10 of 15 distanceport. The of onehub meteris at a (withindistance maximum of one meter distance (within around maximum 100 m distance supported around by the 100 BLE m 4.2supported specification) by awaythe BLE from 4.2 the specification) sensor node. away from the sensor node.

5.5 Results. Results and and Discussion Discussion OnceOnce the the unit unit is is mounted mounted inin place,place, sensor data data is is transmitted transmitted to to the the receiver receiver hub hub by bythe the wireless wireless MCUMCU in in order order to to validate validate the the sensor sensor node’s node’s operation. operation. Moreover, Moreover, key key performance performance parameters parameters includingincluding the the initial initial charging charging time, time, recharging recharging time, time, operating operating frequency, frequency, and duty and cycle duty are cycle presented are andpresented their dependency and their dependency to the EH’s performance to the EH’s performance is investigated. is investigated. Comparative analysisomparative of circuit analysis behavior of usingcircuit different behavior EHs using is alsodifferent illustrated. EHs is also Lastly, illustrated. stress testsLastly, are stress performed tests are toperformed verify the to reliabilityverify the of reliability of the system. the system.

5.1.5.1. Performance Performance Validation Validation In order to generate data packets to represent a full cycle of the sensors’ signal during each data In order to generate data packets to represent a full cycle of the sensors’ signal during each data transmission to decrease the error rate and to verify the consistency and reliability of the device’s transmission to decrease the error rate and to verify the consistency and reliability of the device’s operation over time, multiple full-wave rectified sinusoidal signals replicating the sensor’s output operation over time, multiple full-wave rectified sinusoidal signals replicating the sensor’s output for for different experiments are fed into the sensor node and the transmitted ADC values are captured different experiments are fed into the sensor node and the transmitted ADC values are captured and and compared to the peak voltage values of the input signal. In Figure 10, the captured data points comparedfor two of to the the arbitrary peak voltage signals values at the of receiver the input node signal. are plotted In Figure of a10 read, the and captured transmit data process points (with for two of15 the points arbitrary representing signals at a thefull receivercycle of nodethe sensor’s are plotted signal). of aThe read nominal and transmit errors associated process (with with 15 peak points representingmeasurement a full are cycle−2.7% of and the −6 sensor’s.4% respectively signal). The as a nominalresult of errorsdiscrete associated sampling, with which peak are measurement caused by arethe− 2.7% limited and number−6.4% of respectively points available as a result from of discrete discrete sampling sampling, to coverwhich the are sensor’s caused by signal. the limited The numbermeasurement of points result availables here validate from discrete the sensor sampling node’s tooperation cover the as sensor’sthe peak signal.measurement The measurement errors are resultsless than here the validate calculated the sensor worst case node’s error operation of −8.6% as (i.e. the, with peak an measurement accuracy of errors91.4%) are as a less result than of the calculateddiscrete sampling. worst case (If error one of uses−8.6% 15 equidistant (i.e., with anpoints accuracy to represent of 91.4%) one as cycle a result (with of discrete period T) sampling. of a (Ifsinusoidal one uses 15 waveform, equidistant the points best case to represent is to collec onet the cycle point (with with period peak T) value of a assinusoidal the measur waveform,ed data, the bestideally case iswith to collectzero error. the point Whereas with the peak worst value case as thehappens measured when data, the point ideally T/15 with away zero from error. peak Whereas point the worstis chosen case happens to stand whenfor the the peak point value. T/15) away from peak point is chosen to stand for the peak value).

(a) (b)

FigureFigure 10. 10(.a ()a Captured) Captured ADC ADC values values for for a a 60 60 Hz, Hz, 0.36 0.36 V V peak-peak peak-peak signalsignal ((bb)) CapturedCaptured ADCADC valuesvalues for a 60for Hz, a 60 1.0 Hz, V peak-peak 1.0 V peak signal.-peak signal. Transmitted Transmitted ADC valuesADC values (red dots) (red for dots) two for arbitrary two arbitrary signal inputssignal to theinputs wireless to the sensor wireless node sensor (with node 15 positive (with 15 points positive representing points representing a full cycle a full of thecycle sensor’s of the sensor’ sinusoidals signal).sinusoidal The blue signal). dashed The curve blue dashed shows the curve fitting shows result the of fitting the sensor’s result signal of the with sensor’s absolute signal values. with absolute values. 5.2. Transmission Duty Cycle Analysis 5.2. Transmission Duty Cycle Analysis An essential characteristic of self-contained wireless sensor nodes is their duty cycle or operating frequency.An essential In the proposed characteristic design, of the self- dutycontained cycle wireless(the frequency sensor of nodes operation) is their of duty the sensor cycle nodeor isoperating directly regulated frequency. by In the the triggering proposed signals design, provided the duty by cycle the (the voltage frequency detector of circuit operation) introduced of the in Sectionsensor 3.1 node, and is isdirectly self-adjusted regulated according by the triggering to the output signals of theprovided EH. by the voltage detector circuit introduced in Section 3.1, and is self-adjusted according to the output of the EH.

Sensors 2018, 18, x FOR PEER REVIEW 11 of 15

In the beginning, it needs an initial charging period to accumulate enough energy from none to Sensors 2018, 18, 3733 11 of 15 complete its very first read and transmit operation. The dynamic behavior of the storage unit with EH1 harvesting from 7.6 A grid current is shown in Figure 11a. It takes 242 s for the storage capacitor Into reach the beginning, 3 V from 0 itV needsto initiate an initialthe wireless charging MCU’s period operation. to accumulate Upon completion enough of energy a data sampling from none to and transmission process instructed by the program, the voltage across the capacitor will drop complete its very first read and transmit operation. The dynamic behavior of the storage unit with EH1 abruptly to below 2 V due to the partial energy consumed by MCU’s operation (the total MCU active harvesting from 7.6 A grid current is shown in Figure 11a. It takes 242 s for the storage capacitor to time for ADC measurement and wireless transmission is measured to be approximately 4.8 ms). At reachthis 3 V moment, from 0 Vthe to MCU initiate returns the wirelessto the shutdown MCU’s mode operation. until it Upon re-accumulates completion enough of a dataenergy sampling to send and transmissionthe consecutive process data instructed packet. Nevertheless, by the program, the relatively the voltage long charging across the time capacitor only appears will in drop the very abruptly to belowfirst period. 2 V due As to long the as partial the initial energy charging consumed process of by the MCU’s storage operation capacitor is (the completed, total MCU the EH active only time for ADCneeds measurement to make up for andthe voltage wireless drop transmission (around one isthird measured of the threshold to be approximately voltage) caused 4.8by the ms). data At this moment,packing the MCUand transmission returns to the of shutdown the WSN, modeand the until time it re-accumulates between subsequent enough data energy transmission to send the consecutive(recharging data per packet.iod) isNevertheless, much reduced. the Figure relatively 11b shows long the charging similar timedynamic only charg appearsing/discharging in the very first period.behavior As long of asthe the system initial when charging the WSN process is integrated of the storagewith EH2. capacitor is completed, the EH only needs As the current intensity in the wire increases, the EH’s output power rises accordingly. Higher to make up for the voltage drop (around one third of the threshold voltage) caused by the data packing power supply produces same amount of electric charge/energy within a shorter period of time, and transmission of the WSN, and the time between subsequent data transmission (recharging period) implying a lesser charging time of the storage capacitor. Similarly, the recharging time for following is muchprocesses reduced. will Figurebe reduced 11b showsas well. theIn this similar way, dynamicthe frequency charging/discharging of operation is dynamically behavior adjusted of the systemto whenincrease the WSN with is the integrated incremental with current EH2. passing through the wire.

(a) (b)

FigureFigure 11. Charging/discharging11. Charging/discharging behaviorbehavior of the storage storage capacitor capacitor for for WSN WSN operations operations when when the the currentcurrent in the in wirethe wire is (isa) (a 7.6) 7. A6 A (with (withEH1) EH1) and ( (bb) )23. 23.33 A A(with (with EH2), EH2), respectively. respectively. In the In beginning, the beginning, it needs an initial charging period to accumulate enough energy to complete its very first read and it needs an initial charging period to accumulate enough energy to complete its very first read and transmit operation. Upon completion of a single ADC read and transmit process, the voltage will transmit operation. Upon completion of a single ADC read and transmit process, the voltage will drop drop abruptly to below 2 V. At this moment, the MCU returns to the Shutdown mode until it abruptly to below 2 V. At this moment, the MCU returns to the Shutdown mode until it re-accumulates re-accumulates enough energy to send the consecutive data packet. enough energy to send the consecutive data packet. Figure 12 presents the relationship between the duty cycle of the system and current intensity in Asthe the wire current tested intensity with EH1 in and the EH2 wire respectively. increases, the As EH’s expected, output the power duty cyclerises accordingly. of operations Higher is powercompressed supply produces by the increasing same amount current intensity. of electric The charge/energy measurement with within the EH1 a shorter demonstrates period that of time, implyingthe WSN a lesser system charging is capable time of achieving of the storage a duty capacitor. cycle from Similarly, <1 min to 2. the5 m rechargingin when the timecurrent for ranges following processesfrom will7.6 A be toreduced 30 A, easily as well. meeting Inthis the way,need thein the frequency applications of operation of monitor ising dynamically electricity usage adjusted in to increaselarge with buildings. the incremental The measurement current resultspassing of through EH2 indicate the wire. this system powered by the advanced Figureweaker 12 harvester presents is the in a relationship position to bring between about the du dutyty cycle cycle from of the<4 m systemin to 1 and8 min current for a current intensity in the wirevarying tested from with 8.4 EH1 A up and to EH2 30 A respectively.. In this case, As the expected, duty cycles the becomeduty cycle much of operations longer than is those compressed of previous test due to the weaker output power supplied by the meandering PZT-based low-power by the increasing current intensity. The measurement with the EH1 demonstrates that the WSN system harvester. In fact, if the current intensity continuously goes lower than 8 A, the power output of the is capableEH2 will of achievingreach the system’s a duty cyclelower frompower <1 supply min tolimit 2.5 at min some when point, the i.e., current the EH’s ranges power from output 7.6 A to 30 A,tends easily to meetingbe not powerful the need enough in the to applicationscharge and recharge of monitoring the storage electricity capacitor usageeffectively in large because buildings. of The measurementthe leakage current results and of static EH2 power indicate dissipation this system of the powered system. by the advanced weaker harvester is in a position to bring about duty cycle from <4 min to 18 min for a current varying from 8.4 A up to 30 A.

In this case, the duty cycles become much longer than those of previous test due to the weaker output power supplied by the meandering PZT-based low-power harvester. In fact, if the current intensity continuously goes lower than 8 A, the power output of the EH2 will reach the system’s lower power supply limit at some point, i.e., the EH’s power output tends to be not powerful enough to charge and Sensors 2018, 18, 3733 12 of 15

recharge the storage capacitor effectively because of the leakage current and static power dissipation of theSensors system. 2018, 18, x FOR PEER REVIEW 12 of 15

Sensors 2018, 18, x FOR PEER REVIEW 12 of 15

(a) (b)

FigureFigure 12. Duty 12. Duty cycle cycle of operation of operation with with regard regard to to current current intensity intensity passing passing throughthrough the wire wire when when the the system is tested with (a) EH1 and (b) EH2. Higher current indicates a shorter duty cycle because system is tested with (a) EH1 and (b) EH2. Higher current indicates a shorter duty cycle because of a of a weaker output power supplied by the EHs. weaker output power supplied(a) by the EHs. (b)

5.3. Reliability5.3. ReliabilityFigure Verification 12 Verification. Duty cycle of operation with regard to current intensity passing through the wire when the system is tested with (a) EH1 and (b) EH2. Higher current indicates a shorter duty cycle because For theForof sake thea weaker sake offurther outputof further power verifying verify supplieding the bythe the robustness robustness EHs. and dependability dependability of ofWSN WSN system’s system’s operation operation over time under various conditions, stress tests are designed and performed: over time under various conditions, stress tests are designed and performed: 5.3. Reliability Verification – The system is tested in the case of current intensity varying rapidly passing through the electric – The systemwireFor tothe is validatesake tested of further inits the robustness. verify caseing of currentthe Experimental robustness intensity and results varyingdependability show rapidly that of theWSN passing unit system’s have through operation the correctthe electric wireoverbehavior to time validate under under itsvarious robustness.this abnconditionormal Experimentals circumstance, stress tests are resultsand designed changing show andthat theperformed current the unit: intensity have the just correct influences behavior under– theThe this duty system abnormal cycle is of tested the circumstance operation. in the case of andcurrent changing intensity thevarying current rapidly intensity passing through just influences the electric the duty cycle– The ofwire the system to operation. validate is then its tested robustness. under Experimentalthe circumstance results of showpower that outage. the unit Figure have 13 the presents correct the – Thesystem’s systembehavior isbehavior under then this tested when abn ormala under power circumstance the outage circumstance occurs and changingduring of powera therecharging current outage. intensity period. Figure justThe influencesstorage13 presents unit the system’sstartsthe behaviordutycharging cycle againof when the afteroperation. a power the power outage outage occurs is recovered. during aThe recharging desired data period. sample The packets storage are unit – successfullyThe system created is then and tested transmitted under the by circumstance the WSN in this of power accidental outage. situa Figuretion, demonstrating 13 presents the the starts charging again after the power outage is recovered. The desired data sample packets are system’ssystem’s automatic behavior recoveringwhen a power from outage outages. occurs during a recharging period. The storage unit successfully created and transmitted by the WSN in this accidental situation, demonstrating the – Otherstarts stress charging tests again including after the disruptions power outage in the is recovered.current and The the desired sudden data disconnection sample packets with are the system’scentralsuccessfully automatic controlling created recovering hub and are transmitted also from performed outages. by the so WSN as to in confirmthis accidental that the situa selftion-powered, demonstrating system isthe able – Otherto system’s stressrecover tests fromautomatic including these recovering abnormal disruptions fromconditions. outa inges. the current and the sudden disconnection with the

central– Other controlling stress tests hub including are also disruptions performed in sothe as current to confirm and the that sudden the self-powereddisconnection with system the is able central controlling hub are also performed so as to confirm that the self-powered system is able to recoverto recover from from these these abnormal abnormal conditions. conditions.

Figure 13. Charging/discharging behavior of the storage capacitor for WSN operations when a

power outage occurs. Tested with EH2 when the current in the wire is 23.3 A before/after the outage. Figure 13. Charging/discharging behavior of the storage capacitor for WSN operations when a SuccessfulFigure 13 completion. Charging/discharging of data transmission behavior of the demonstrates storage capacitor the system’s for WSN automaticoperations whenrecovering a power outage occurs. Tested with EH2 when the current in the wire is 23.3 A before/after the outage. powerfrom outage outages. occurs. Tested with EH2 when the current in the wire is 23.3 A before/after the outage.Successful Successful completion completion of of data data transmission transmission demonstrates demonstrates the system’s the system’s automatic automatic recovering recovering from outages. from outages.

Sensors 2018, 18, 3733 13 of 15

6. Conclusions The design, development, and testing of a smart, low-power wireless sensor node system for real-time electricity monitoring applications within residential, educational, and commercial buildings are presented in this paper. By harvesting energy from the wire being monitored, the unit is battery-independent and is able to wirelessly gather, process and transmit the data packets sampled from the non-invasive sensor via Bluetooth connectivity with a low error less than 8.6%. Enabled by a dynamic power management scheme, the intelligent sensor node can self-adjust its duty cycle depending on the characteristics of different EHs and the current being monitored. Without any need of modification or adjustment, the circuit is able to operate with two different EHs at a wide range of input power and pass different stress tests, proving the flexibility and robustness of the system. This simple and state-of-the-art integrated system serves as a promising solution to the target applications.

Author Contributions: Conceptualization, Z.Y. and S.Z.; Data curation, Z.Y. and S.Z.; Formal analysis, Z.Y., S.Z., E.F. and M.-I.R.-T.; Funding acquisition, L.W.; Investigation, Z.Y., S.Z., E.F. and H.D.; Methodology, Z.Y., S.Z. and M.-I.R.-T.; Project administration, A.S., D.N. and L.W.; Resources, A.S. and L.W.; Software, Z.Y. and S.Z.; Supervision, H.D., A.S., D.N. and L.W.; Visualization, Z.Y. and S.Z.; Writing—original draft, Z.Y.; Writing—review & editing, H.D. and L.W. Funding: The work was supported by University of Waterloo International Research Partnership Grants (IRPG) and grants from Waterloo Institute of Sustainable Energy (WISE). Acknowledgments: The authors would like to thank Armughan Al-Haq for his administrative support. Conflicts of Interest: The authors declare no conflict of interest.

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