Review of Power Converter Impact of Electromagnetic Energy Harvesting Circuits and Devices for Autonomous Sensor Applications

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Review Review of Power Converter Impact of Electromagnetic Energy Harvesting Circuits and Devices for Autonomous Sensor Applications

Mahidur R. Sarker 1,2,*, Mohamad Hanif Md Saad 1, José Luis Olazagoitia 2 and Jordi Vinolas 2

1 Institute of IR 4.0, Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Selangor, Malaysia; [email protected] 2 Industrial Engineering and Automotive, Nebrija University, Campus de la Dehesa de la Villa, Calle Pirineos, 55, 28040 Madrid, Spain; [email protected] (J.L.O.); [email protected] (J.V.) * Correspondence: [email protected]

Abstract: The demand for power is increasing due to the rapid growth of the population. There- fore, energy harvesting (EH) from ambient sources has become popular. The reduction of power consumption in modern wireless systems provides a basis for the replacement of batteries with the electromagnetic energy harvesting (EMEH) approach. This study presents a general review of the EMEH techniques for autonomous sensor (ATS) applications. Electromagnetic devices show great potential when used to power such ATS technologies or convert mechanical energy to electrical energy. As its power source, this stage harvests ambient energy and features a self-starting and self-powered process without the use of batteries. Therefore, it consumes low power and is highly stable for harvesting energy from the environment with low ambient energy sources. The review highlights EMEH circuits, low power EMEH devices, power electronic converters, and controllers Citation: Sarker, M.R.; Saad, M.H.M.; utilized in numerous applications, and described their impacts on energy conservation, beneﬁts, Olazagoitia, J.L.; Vinolas, J. Review of and limitation. This study ultimately aims to suggest a smart, low-voltage electronic circuit for a Power Converter Impact of low-power sensor that harvests electromagnetic energy. This review also focuses on various issues Electromagnetic Energy Harvesting and suggestions of future EMEH for low power autonomous sensors. Circuits and Devices for Autonomous Sensor Applications. Electronics 2021, 10, 1108. https://doi.org/10.3390/ Keywords: electromagnetic device; energy harvesting; autonomous sensors; low power electronics10091108

Academic Editor: Noel Rodriguez 1. Introduction Received: 21 March 2021 The harvesting of electromagnetic energy from ambient environments has attracted Accepted: 28 April 2021 much interest recently. Micro energy harvesters are small, vibration-driven electromagnetic Published: 8 May 2021 transducers that harvest ambient energy for conversion into electrical charge. The features of sample electromagnetic energy harvesting (EMEH) are investigated in [1–3], and the Publisher’s Note: MDPI stays neutral achievement levels of some energy harvesting (EH) techniques are explained in Table1[4] . with regard to jurisdictional claims in These techniques can usually produce charge for low power wireless sensor networks published maps and institutional affil- (WSNs) applications [5,6]. As an aim to facilitate the natural and causal interaction between iations. such devices and humans, the embedding of ubiquitous computation into environments has drawn significant research attention along with the concepts of sensation and percep- tion. Examples of such systems are WSNs, which show potential applicability in many areas [7–9]: piezoelectric conversion technology [10–12], solar system [13], thermoelectric- Copyright: © 2021 by the authors. powered [14], and wind power systems [15,16]. These systems are capable of harvesting Licensee MDPI, Basel, Switzerland. energy from ambient environments, which activate devices directly and finally store har- This article is an open access article vested energy in infinite capacity batteries for future use [17]. Conventional, chemical distributed under the terms and batteries provide the required power to each sensor node. However, they are large, ex- conditions of the Creative Commons pensive, and often tough to change frequently. Thus, the development of such systems is Attribution (CC BY) license (https:// greatly limited. Various energy sources are used to power micro-level unmanned systems. creativecommons.org/licenses/by/ 4.0/).

Electronics 2021, 10, 1108. https://doi.org/10.3390/electronics10091108 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 1108 2 of 35

Table 1. Energy harvesting approach and their veriﬁed capabilities.

Harvesting Method Performance Radio frequency 10 µW Wind 20 mW Temperature (∆T = 5 ◦C) 60 µW Mechanical vibration 3.35 mW Automotive light sensor 1.03 mW

Figure1 shows a block diagram of a dual-source EH system [ 18,19]. In the dual- energy source energy harvesting system, two energy harvesters get ambient energy from the surroundings, after processing provides the output to the load. Because there are dual-energy sources, there are two levels of output voltages. Conventional electromagnetic batteries are normally utilized to power WSNs, which have gained signiﬁcant attention because of their applicability in monitoring plans, resources, and infrastructures [20,21]. Although the power requirements of electronic devices decrease as they become energy efﬁcient, batteries are still their main power source. Batteries need to be recharged or replaced, and the proper disposal and recycling of batteries is a global challenge [22,23]. They are also heavy, especially when compared with microelectronic devices to which they supply power. A typical battery has a short life span and is thus not practical for devices, which are expected to last more than 10 years [24,25]. In addition, some electronic devices are located in hard-to-reach areas: embedded in people or animals, installed inside structures (bridges, roads, and buildings), placed in satellite systems, etc. EH is a potential

Electronics 2020, 9, x FOR PEER REVIEWsolution that can provide power to these applications. 3 of 46

FigureFigure 1. 1.Block Block diagram diagram of of a a dual-source dual-source energy energy harvesting harvesting system. system. A single EMEH transducer can obtain energy from multiple natural sources. The

harvesting electrical charge needs to monitor, control, and storage to utilize it in the load. Thus, a constant energy source is necessary for some WSNs applications. In 2011, around 150–200 million WSNs were utilized in the remote industrial areas to monitor and control the health of the equipment, safety, bio-medical application, defence application, and intelligent gathering [26,27]. Thus, the present study ultimately aims to suggest low-power EMEH devices with low power electronic circuit, which can be integrated into WSNs nodes for ATS [28,29]. The overall architecture of a typical block diagram of EMEH is shown in Figure2; it contains three core components: EMEH sources, energy conversion devices/power electronic converters, and energy storage with the load.

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FigureFigure 2. The 2.typicalThe typical block diagram block diagram of EMEH of EMEH system. system.

2. PhenomenonThe rest of thisof Electromagnetic paper is organized as follows: Section2 describes the phenomenon of electromagnetic system. Section3 describes the relation between electromagnetic energy The fundamental characteristics of electromagnetic devices are the generation of en- harvesting and autonomous sensors. Section4 describes detailed power electromagnetic ergy through electromotive force as shown in Figure 3 [30–32]. The electromagnetic sys- technologies for electromagnetic energy harvesting systems. Section5 presents an overview tem is based on Faraday’s law of electromagnetic induction. The conduct of Faraday’s law of electromagnetic energy harvesting techniques and prototypes. Section6 highlights the is associated with the voltage or electromotive force (emf) that induces magnetic flux link- issues and challenges and, ﬁnally, conclusions and recommendations are presented in age in a circuit, according to the following equation: [31]. Section7.

2. Phenomenon of Electromagnetic The fundamental characteristics of electromagnetic devices are the generation of energy through electromotive force as shown in Figure3[ 30–32]. The electromagnetic system is based on Faraday’s law of electromagnetic induction. The conduct of Faraday’s law is associated with the voltage or electromotive force (emf) that induces magnetic ﬂux linkage in a circuit, according to the following equation: [31].

dφ V = − (1) dt where V is induced emf, φ is ﬂux linkage, and t is time. Usually, the circuit consists of various tune wires of coil during implementation to a generator, shown by:

dΦ dφ V = − = −N (2) dt dt where Φ is total ﬂux linkage and N is turns of coil. Basically, the total ﬂux linkage for a various turn coil is estimated by:

N Z Φ = ∑ B.dA (3) i=1 Ai

dB V = −NA sin(α) (4) dt

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where B is density of the magnetic ﬂux at each coil segment, i is the current of the coil tune, and Φ = NBA sin (α) where α is the angle between the coil area. Equation (4) can be expressed by: Electronics 2020, 9, x FOR PEER REVIEW dΦ dx dΦ dx 5 of 46 V = − = −N (5) dx dt dx dt

FigureFigure 3. 3. FeatureFeature of of the the electromagnetic electromagnetic energy energy harves harvesterter [30]. [30]. (Reprinted (Reprinted from from ref. ref. [30]). [30]).

The magnetic force on the magnet is given by: dφ V = − (1) dx dt dx V2 Fmag = Dmag ,Fmag = = (6) dt dt RL + RC + jωLC where V is induced emf, φ is flux linkage, and t is time. Usually, the circuit consists of variouswhere Ftunemag iswires generator of coil indication,during implRementationL is the load to resistances,a generator,R shownC is the by: coil resistances, j is the complex number and time variable, and LC is the coil inductance. The following equation can be expressed utilizingd EquationΦ (5):dφ V = − = −N (2) dt dt 1 dΦ Φ Dmag = (7) where is total flux linkage and N RisL turns+ RC of+ coil.jωL CBasically,dx the total flux linkage for a various turn coil is estimated by: where Dmag is the electromagnetic damping. The following equation can express the N maximum power extracted withΦ the= electromagnetic force:   B.dA (3) i=1 Ai (t)dx(t) Pmax = Fmag (8) dB dt V = −NA sin(α) (4) The performance of the EMEH generatordt is depending on the combination of the material. Therefore, the material selection plays an important role to boost up the efficiency whereof power B is in density the EMEH of the system. magnetic The flux category at each of magnetcoil segment, and which i is the types current of material of the usedcoil tune,in the and magnet Φ = define NBA thesin magnetic(α ) where field α power.is the angle between the coil area. Equation (4) can be expressed by: 2.1. Mounting of Electromagnetic Vibration Generators dΦ dx dΦ dx The electromagneticV vibration= − generators= −N (EMVG) produce power(5) from the ambient sources through mechanicaldx vibrationaldt force,dx dt where structure acts as a magnetic circuit [33,34]. To extract electrical charge from the EMVG, this depends on the number The magnetic force on the magnet is given by: of coil turns, damping, flux connection slope, and load impedance [35–37]. However, as compared to conventional electromagneticdx dx generators,V 2 the micro-electromagnetic power F = D F = = generator producesmag highmag power.mag The study in [33+,38] developed+ ω an electromagnetic(6) generator using a circular stabledt magnet, tuning.dt R TheL R workC inj [L39C ] proposed a piezoelectric bimorph cantilever beam for harvesting energy, utilizing magnetic attraction. In [40], the where Fmag is generator indication, RL is the load resistances, RC is the coil resistances, j is authors described a magneto-electric harvester based on a stable magnet and a piezoelec- the complex number and time variable, and LC is the coil inductance. The following equation can be expressed utilizing Equation (5): 1  dΦ  D = mag + + ω   (7) RL RC j LC  dx 

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tric harvester utilizing a metal ball. The structure of the electromagnetic power generator and EH based on vibration is shown in Figure4a,b [ 41,42]. This feature includes a coil sandwiched between magnets, where two sets, oppositely separated, comprise the higher and lower magnets. This reverse split generates a ﬂux slope in the direction of motion for the coil, which is in the x-direction in this case. Figure4a shows a ﬂux generator axial rotor, normally attached to the tube and covered by a ball bearing that retains the small gap between the magnets and the copper windings to increase the output power. The basic structure of the linear vibration-based EH, which maintains a small gap between the winding and magnet array, is shown in Figure4b. The power and dimension of a mass that produces NdFeB magnetic material with a vibration rate of 100 Hz and a vibration acceleration level of 1m/s2 is shown in Figure5. The graph shows a power density of almost 2.3 µW/mm3 (2.3 mW/cm3) for a system with a size of 1 cm3. In Figure5, the Electronics 2020, 9, x FOR PEERgraph REVIEW is representing the power on the y-axis and the dimension of the electromagnetic 7 of 46 vibrational generator on the x-axis.

(a) (b) Electronics 2020, 9, x FOR PEER REVIEW 8 of 46 Figure 4.FigureStructure 4. Structure electromagnetic electromagnetic vibration vibration generator: generator: (a) ﬂux generator, (a) flux generator, (b) structure (b) ofstructure the linear of vibration-basedthe linear vibration-based energy energy harvesting [41,42]. harvesting [41,42].

FigureFigure 5. 5.Power Power density density with with maximum maximum available available power power dimension dimension for for the the electromagnetic electromagnetic vibration vibration generator structure [42]. generator structure [42].

The following equation indicates that the average power generated via the mass damping force is: T 1 Z (ma)2 P = F .Udt = (9) D T D 2D 0

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ma x = (10) m Dω Therefore, the average power can be expressed as:

m.a.x ω ρ.x y.z.(x − x )ω P = m = mass mass (11) D 2 4 here, D is the displacement; a is the acceleration; m is the mass; F is the force; ρ is the density of the material; xm is the peak displacement of mass; and xmass, y, and z are the dimensions of the mass of materials. From Equation (11), after differentiating with respect to xm, the following equation is obtained for the optimum length of mass for maximum power: Electronics 2020, 9, x FOR PEER REVIEW 9 of 46 x xoptmass = (12) 2

2.2.2.2. Mounting Mounting of of Electromagnetic Electromagnetic Damping Damping TheThe electromotiveelectromotive forceforce isis induced induced by by motion motion relative relative to to the the magnetic magnetic ﬁeld, field, which which causes an eddy current [43]. Eddy currents arise when a magnetic ﬁeld shifts inside a causes an eddy current [43]. Eddy currents arise when a magnetic field shifts inside a conductor that causes electrical current loops in the conductor. Eddy currents can strain conductor that causes electrical current loops in the conductor. Eddy currents can strain the motion that is involved, a fact that is known as magnetic damping [44,45]. Figure6 the motion that is involved, a fact that is known as magnetic damping [44,45]. Figure 6 shows the schematic diagram of electromagnetic damping [46]. shows the schematic diagram of electromagnetic damping [46].

FigureFigure 6. 6.Schematic Schematic diagram diagram of of electromagnetic electromagnetic damping damping [ 46[46].]. (Reprinted (Reprinted from from ref. ref. [ 46[46].].

TheThe following following equation equation can can express express the the magnitude magnitude of of magnetic magnetic damping damping [ 47[47,48]:,48]: 2 (NBL C ) 2 ξ = (NBLc cC ) e ξe = ω + (13) (13) 2me2m1(eRωc1(RRc +L )RL)

wherewherem me eis is the the mass mass of of electromagnetic electromagnetic resonator, resonator,N Nis theis the number number of turnsof turns of theof the coil, coil,B is B theis the strength strength of the of magneticthe magnetic ﬁeld, field,Lc is theLc is effective the effective length length of the of coil, theC coil,is the Ccoil is the ﬁll coil factor fill andfactor the and value the of valueC should of C beshould high, beR chigh,is the R coilc is resistance,the coil resistance, and RL isand the R loadL is the resistance. load re- Usually,sistance. there Usually, are three there kinds are three of electromagnetic kinds of electromagnetic damping asdamping shown inas Equationshown in (14):Equation (14): ζl = ζm + ζh + ζa (14) ζ = ζ + ζ + ζ l m h a (14) where ζm is the materials damping, ζh is the thermoelastic damping, and ζa is the air ζ ζ ζ damping.where m Figureis the 7materials shows effectivedamping, power h is variation,the thermoelastic which candamping, be generated and a through is the air numerousdamping. coilFigure technologies 7 shows effective [48]. The power graph variation, is representing which can the be power generated on the through y-axis and numer- the dimensionous coil technologies of the wire [48]. coil The and graph planar is micro repres coilenting of thethe generatorpower on onthe the y-axis x-axis. and the dimension of the wire coil and planar micro coil of the generator on the x-axis.

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FigureFigure 7. 7.Highest Highest possible possible electrical electrical power power generated generated from from the the wire wire coil coil and and a micro-fabricateda micro-fabricated coil. coil. 2.3. Maximum Power from an Electromagnetic Generator 2.3. TheMaximum maximum Power power from generationan Electromagnetic depends Generator on the maximum displacement of the mass of theThe electromagnetic maximum power (EM) generation generator. depends The maximum on the maximum electricalpower, displacement the resonance of the mass of theof EMthe electromagnetic generator, is shown (EM) by generator. Equation (15)The [maximum40,49]: electrical power, the resonance of the EM generator, is shown by Equation (15) [40,49]: (ma)2 Pmax(ma= )2 (15) = 8DP Pmax (15) 8DP where Pmax is the maximum electrical power, DP is the parasitic mechanical damping, m is

thewhere mass, Pmax and is atheis maximum the acceleration. electrical The power, following DP is Equation the parasitic (16) mechanical denotes EM damping, damping m to is parasiticthe mass, damping and a is withthe acceleration. readjusting The the loadfollowing to get Equation the optimum (16) denotes resistance EM of damping the load, to whichparasitic produced damping maximum with readjusting power: the load to get the optimum resistance of the load, which produced maximum power: 2 N2 dφ φ dx2 RI =2  d  − RC (16) N  PD =  dx  − (16) However, in the real conditionRI based on the availabilityRC to produce power, the load is P ﬁxed. The following Equation (17) showsD the optimum load resistance that increases the highestHowever, power: in the real condition based on the availability to produce power, the load 2 2 dφ is fixed. The following Equation (17) shows thNe optimumdx load resistance that increases the highest power: RI = RC + (17) PD 2  dφ  3. Relation between Electromagnetic EnergyN 2  Harvesting and Autonomous Sensors Usually, in wireless sensor networks, ATS dx is utilized for conducting(17) real-time data R = R + and active low power devices.I InC the real-time data, the ATS senses any variation in PD the parameters and transmits that variation in the form of a voltage signal at the same time. On the other hand, active low power devices are basically logic gates acting like a3. switch Relation and between attached Electromagnetic with sensors [50 Energy,51]. WSNs Harvesting have differentand Autonomous applications Sensors such as bio-medicalUsually, [52 in,53 wireless], Internet sensor of Things networks, (IoT) [ATS54], andis utilized environmental for conducting monitoring real-time [55]. Andata ATSand can active run low automatically power devices. with lowIn the power real-time mode data, when the it isATS required senses to any sense variation the change in the inparameters any parameter, and transmits processing that the variation change in in the the form form of of the a voltagevoltage signal, signal andat the transmitting same time. thatOn signalthe other [56 ].hand, Figure active8 shows low power the general devices architecture are basically of thelogic ATS, gates in acting which like the powera switch sourceand attached is delivering with sensors energy based[50,51]. on WSNs the demand have different of the load.applications The purpose such as of bio-medical the power conditioning[52,53], Internet circuit of isThings to provide (IoT) a[54], stabilized and environmental voltage, and anmonitoring enhanced [55]. and An ﬁltered ATS outputcan run toautomatically the load [57,58 with]. The low system power for mode storing when purposes it is required can use a to battery sense or the capacitor. change However,in any pa- therameter, battery processing has a limitation the change in terms in of the recharging form of requiredthe voltage after signal, a certain and period. transmitting Therefore, that ansignal EMEH [56]. system Figure can 8 shows be an alternativethe general solution. architecture of the ATS, in which the power source

Electronics 2020, 9, x FOR PEER REVIEW 11 of 46

is delivering energy based on the demand of the load. The purpose of the power conditioning circuit is to provide a stabilized voltage, and an enhanced and filtered output to Electronics 2021, 10, 1108 the load [57,58]. The system for storing purposes can use a battery or capacitor. However,8 of 35 the battery has a limitation in terms of recharging required after a certain period. There- fore, an EMEH system can be an alternative solution.

FigureFigure 8. 8.Block Block diagram diagram of of the the autonomous autonomous sensors sensors [ 51[51].]. 3.1. Autonomous Sensors Power Source 3.1. Autonomous Sensors Power Source Battery and EMEH systems can be the power sources for the ATS [59,60]. The battery is an easyBattery solution and EMEH to be used systems as a can power be the supply power [61 sources]. To avoid for the replacement ATS [59,60]. of theThe battery battery tois poweringan easy solution the ATS, to thebe used following as a power Equation supply (18) [61]. can be To expressed: avoid replacement of the battery to powering the ATS, the following Equation (18) can be expressed: E=bat = Pc,av.Top (18) Ebat Pc, av .Top (18)

where Ebat is the sufﬁcient amount of energy for power in the ATS, Pc; av is average power where Ebat is the sufficient amount of energy for power in the ATS, Pc; av is average power demand from the power sources; and Top is the operation for a lifetime period. To sustain demand from the power sources; and Top is the operation for a lifetime period. To sustain a maintainable process of the ATS when using EMEH approach, the average produced a maintainable process of the ATS when using EMEH approach, the average produced power (Pg,av) is given by the following Equation (19): power (Pg,av) is given by the following Equation (19):

Pg≥,av ≥ Pc,av (19) Pg,av Pc,av (19) where P , av is the ambient condition and the efﬁciency of transducer monitoring. P is where Pg g, av is the ambient condition and the efficiency of transducer monitoring. cP,avc,av is thethe energy, energy, power, power, and and leakage leakage of of storage storage unit unit monitoring. monitoring. The The storage storage element element enhances enhances the shifted ﬂow of power to the load Pg < Pc; the arbitrary value will take a long time T, to the shifted flow of power to the load Pg < Pc; the arbitrary value will take a long time T, to storage (Estorage), with the following Equation (20): storage (Estorage), with the following Equation (20):    Z  ≥  −  EstorageEstoragemax≥ max (Pc (Pcg −).dtPg) .dt (20) (20)    T T 

wherewhereP cPisc is the the consumed consumed power, power,Pg isPg the is instantaneousthe instantaneous generated generated power, power, and T andis the T time.is the time.The capacitor, supercapacitor, and rechargeable batteries are used as storage devices in theThe EMEH capacitor, system supercapacitor, [62,63]. The capacitor and rechargeable is one of thebatteries elements are used of the as EMEHstorage system devices forin storagethe EMEH [64 ].system The advantage [62,63]. The of capacitor the capacitor is one is of availability the elements and of low the cost.EMEH However, system for a drawbackstorage [64]. of theThe capacitor advantage is thatof the it iscapacitor not suitable is availability for the EMEH and low system cost. in However, terms of a power draw- densityback of (poor) the capacitor and also is takes that it a longis not time suitable to charge for the and EMEH discharge. system On in theterms other of power hand, minordensity internal(poor) and impedance also takes and a long higher time lifetimes to charge (in and terms discharge. of the On number the other of charging/discharge hand, minor internal cycles)impedance areindicated and higher as lifetimes the merit (in ofterms the of supercapacitor the number of [charging/dis65,66]. Nevertheless,charge cycles) superca- are in- pacitorsdicated areas the able merit to accurately of the supercapacitor count the amount [65,66]. of Nevertheless, residual energy superc suchapacitors as charging are able and to dischargingaccurately count capacity the andamount states of ofresidual energy energy density such (i.e., as upper charging or lower). and discharging The drawback capacity of theand supercapacitor states of energy has density a much (i.e., higher upper cost or perlower). energy The unit. drawback However, of the various supercapacitor problems has are a associatedmuch higher with cost batteries, per energy when unit. used However, as a power various supply problems for autonomous are associated sensors, with especially batteries, inwhen remote used areas. as a power Some ofsupply these fo limitationsr autonomous are: sensors, size, poor especially storage in capacity, remote areas. and limitedSome of lifecyclesthese limitations [67]. A are: typical size, graph poor storage of the power capacity, density and limited and energy lifecycles density [67]. A of typical the capacitor, graph of supercapacitor, and storage unit batteries is shown in Figure9.

Electronics 2020, 9, x FOR PEER REVIEW 12 of 46 Electronics 2021, 10, 1108 9 of 35 the power density and energy density of the capacitor, supercapacitor, and storage unit batteries is shown in Figure 9.

FigureFigure 9. 9.Power Power density density and and energy energy density density of of capacitor, capacitor, supercapacitor, supercapacitor, and and batteries batteries [51 [51].].

OnOn the the other other hand, hand, the the ATS ATS is is able able to to run run through through harvest harvest energy energy from from the the ambient ambient sourcessources withoutwithout oror with little maintenance maintenance [68–70]. [68–70 ].The The EMEH EMEH transduc transducer,er, energy energy condition- condi- tioninging circuit, circuit, and and energy energy storage storage are are required required to to run run ATS ATS fromfrom ambientambient energyenergy sources, sources, as as Electronics 2020, 9, x FOR PEER REVIEWshownshown in in Figure Figure 10 10.. However, However, the the drawback drawback of of this this system system is is the the complexity complexity of of the the circuit 13circuit of 46 model.model. Therefore,Therefore, a vibration-based vibration-based EMEH EMEH syst systemem has has been been proposed proposed to overcome to overcome the thehur- hurdlesdles in the in theautomatic automatic running running of the of system. the system.

FigureFigure 10. 10.Block Block diagram diagram power power supply supply source source of an ATS.of an ATS. 3.2. Autonomous Sensor in Electromagnetic Energy Harvesting Systems 3.2. Autonomous Sensor in Electromagnetic Energy Harvesting Systems ATS plays an important role to harvest energy from natural sources, and it works efﬁcientlyATS plays without an theimportant need for role batteries to harvest [50 ,71energy]. Usually, from natural this system sources, is user-friendly and it works andefficiently can operate without the the process need infor the batteries EMEH [50,71]. system Usually, easily. The this system system is is developed user-friendly based and oncan maximum operate the performance process in the as EMEH maximum system energy easily. can The be system harvested is developed from environmental based on max- sourcesimum performance [72,73]. However, as maximum the drawback energy of ATScan be in theharvested EMEH systemfrom environmental can work only sources when the[72,73]. ambient However, sources the are drawback available. of ATS Therefore, in the EMEH the advantage system can to work use ATS only in when the EMEHthe am- systembient sources is their are lifetime, available. storage Therefore, capability, the adva charging,ntage to anduse ATS discharging in the EMEH performances system is their are unlimitedlifetime, storage [74]. The capability, block diagram charging, of anand autonomous discharging harvesting performances system are (AHS)unlimited is shown [74]. The in Figureblock diagram11. The AHSof an consists autonomous of an harvesting EH module sy (EHM),stem (AHS) an energy is shown converter in Figure module 11. The (ECM), AHS andconsists an energy of an EH dissipation module (EHM), module an (EDM). energy The converter EH time module interval (ECM), can be and addressed an energy by dissi- the followingpation module Equation (EDM). (21): The EH time interval can be addressed by the following Equation t (21): Z 2 EH(t1, t2) = PH(t)dt (21)

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where PH(t) is power over time, t is the time, and EH is the energy harvesting. The merit of this system requires only ECM. Equation (22) describes the efﬁciency of energy transfer from the harvesting module to the EDM:

t Z 2 ED(t1, t2) <= PH(t).η(u, i)dt (22)

t1 Electronics 2020, 9, x FOR PEER REVIEW 14 of 46 where η(u, i) is a nonlinear efﬁciency coefﬁcient of the ECM, u(t) is voltage, and u(i) is current.

FigureFigure 11. 11.Block Block diagram diagram of of an an autonomous autonomous harvesting harvesting system system [ 75[75].].

3.3. Autonomous Sensor in Hybrid Electromagnetic Energy Harvesting Systems t2 One of the most popular conventional= types of EMEH systems is the autonomous EH (t1,t2 ) PH (t)dt (21) hybrid [76,77]. They use a rechargeable battery and ultra-capacitor as a storage device, and t1 they collect energy to operate the system [78,79]. The advantage of this system is a long lifetimewhere P andH(t) is high power storage over capacity time, t is due the time, to its and small EH size is the [80 energy]. The performanceharvesting. The of energymerit of managementthis system requires and controlling only ECM. system Equation is faster (22) in termsdescribes of rising the efficiency and settling of time.energy However, transfer thefrom limitation the harvesting of this module system isto thatthe EDM: it spends more energy than harvested from natural sources because of the system using a secondary battery to store harvested energy [81]. t2 Therefore, to overcome this problem, a supercapacitor can be used to replace the battery. E (t ,t ) <= P (t).η(u,i)dt (22) The block diagram of an autonomousD 1 2 hybrid H EH (ATHEH) system is shown in Figure 12. t The system consists of three modules1 such as (EHM, ECM, and EDM), and an energy storagewhere moduleƞ(u, i) is (ESM). a nonlinear The following efficiency Equations coefficient (23) of and the (24)ECM, show u(t) ﬁnite is voltage, time interval and u( fori) is thecurrent. ATHEH system [82]: t Z 2 3.3. Autonomous Sensor in Hybrid PElectromagneticH(t)dt ≥ ρ(t2 − Energyt1) − σHarvesting1 Systems (23) One of the most populart 1conventional types of EMEH systems is the autonomous

wherehybridP H[76,77].(t) is an They applicable use a rechargeable source in the ATHEHbattery and system ultra-capacitor and (t2 − t1 )asreal-time a storage interval. device, and they collect energy to operate the system [78,79]. The advantage of this system is a t long lifetime and high storageZ capacity2 due to its small size [80]. The performance of energy management and controllingPH (systemt)dt ≤ ρis( t2faster− t1) in + termsσ2 of rising and settling time.(24)

However, the limitation of thist1 system is that it spends more energy than harvested from natural sources because of the system using a secondary battery to store harvested energy [81]. Therefore, to overcome this problem, a supercapacitor can be used to replace the battery. The block diagram of an autonomous hybrid EH (ATHEH) system is shown in Figure 12.

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where PH(t) is the harvested power of ambient sources, σ1 and σ2 are the energy storage capacity coefﬁcients, and ρ is the power density. The energy storage capacity of the ATHEH system is shown by the following Equations (25) and (26):

≤ ( ) ≤ EθL ES t EC (25)

EC ≥ σ1 + σ2 (26)

where EC is the energy storage capacity in the supercapacitor or rechargeable battery, ES

is the energy storage, and EθL is the limit threshold of energy. An optimal energy storage device in the ATHEH system can be expressed by the following Equations (27) and (28):

t t Z 2 Z 2 PD(t) dt ≤ PH(t)η1(t)dt + ES (27) η2(t) t1 t1

t t t Z 2 Z 2 Z 2 PD(t) dt ≤ PH(t)η1(t)dt + ES − PL(t)dt (28) η2(t) t1 t1 t1

where PD is the waste power, and σ1 and σ2 are the coefﬁcients of the ECM1 and ECM2 correspondingly. The EHM (η1) and EDM (η2) present the efﬁciency voltage and current of Electronics 2020, 9, x FOR PEER REVIEWthe system operating point. 15 of 46

FigureFigure 12. 12.Block Block diagram diagram of of autonomous autonomous hybrid hybrid electromagnetic electromagnetic energy energy harvesting harvesting system system [75 [75].].

3.4. AutonomousThe system Sensors consists Electromagnetic of three modules Energy such Harvesting as (EHM, Systems ECM, and with EDM), Radio Frequencyand an energy storageOne module of the (ESM). most largely The following utilized Equations radio frequency (23) and (24) EH show (RFEH) finite systems time interval for power for the ATSATHEH has addressed system [82]: many researchers [83–85]. The advantage of the RFEH system from

ambient sources is power densityt2 is high [86,87]. The RFEH system consists of an antenna, impedance matching, filters, and a rectifier,≥ ρ which− is−σ shown in Figure 13. To harvest energy PH (t)dt (t2 t1) 1 (23) from an RF signal, an antenna is required [88]. t1 The equivalent circuit model of an antenna is shown in Figure 14. Where S is the powerwhere density, PH(t) is PanAV applicableis the available source power in the thatATHEH the antenna system canand supply(t2 − t1) toreal-time a matched interval. load, A is the antenna’s effective field, R is radiation resistance, R is the loss resistance, and e t2 S loss Xant is the reactive part. P (t)dt ≤ ρ(t − t ) +σ  H 2 1 2 (24) t1

where PH(t) is the harvested power of ambient sources, σ1 and σ2 are the energy storage capacity coefficients, and ρ is the power density. The energy storage capacity of the ATHEH system is shown by the following Equations (25) and (26):

Eθ ≤ E (t) ≤ E L S C (25) ≥ σ +σ EC 1 2 (26)

where EC is the energy storage capacity in the supercapacitor or rechargeable battery, ES is the

energy storage, and Eθ is the limit threshold of energy. An optimal energy storage device in L the ATHEH system can be expressed by the following Equations (27) and (28):

t2 t2 PD (t) dt ≤ P (t)η (t)dt + E (27)  η (t)  H 1 S t1 2 t1

t2 t2 t2 PD (t) dt ≤ P (t)η (t)dt + E − P (t)dt (28)  η (t)  H 1 S  L t1 2 t1 t1

where PD is the waste power, and σ1 and σ2 are the coefficients of the ECM1 and ECM2 correspondingly. The EHM (ƞ1) and EDM (ƞ2) present the efficiency voltage and current of the system operating point.

3.4. Autonomous Sensors Electromagnetic Energy Harvesting Systems with Radio Frequency

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One of the most largely utilized radio frequency EH (RFEH) systems for power ATS has addressed many researchers [83–85]. The advantage of the RFEH system from ambi- Electronics 2021, 10, 1108 ent sources is power density is high [86,87]. The RFEH system consists of12 an of 35antenna, impedance matching, filters, and a rectifier, which is shown in Figure 13. To harvest energy from an RF signal, an antenna is required [88].

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The equivalent circuit model of an antenna is shown in Figure 14. Where S is the power density, PAV is the available power that the antenna can supply to a matched load, Ae is the antenna’s effective field, RS is radiation resistance, Rloss is the loss resistance, and Xant is the reactive part. FigureFigure 13. Block 13. Block diagram diagram of radio of radio frequency freque energyncy energy harvesting harvesting system. system.

FigureFigure 14. 14. EquivalentEquivalent electrical electrical model model for for an an antenna antenna [20]. [20]. Reprinted Reprinted from from ref. ref. [20]. [20].

InIn this this EMEH EMEH system system where where the the battery battery is used is used as a as storage a storage source source of energy, of energy, on the on otherthe other hand, hand, the harvesting the harvesting device device such as such a transducer as a transducer plays the plays main the role. main The role. energy The controllingenergy controlling sector plays sector an plays essential an essential role in rolesuch in a such system a system to manage to manage the discharging the discharging ca- pacitycapacity of the of the battery battery and and increase increase the the lifetime lifetime of of the the battery battery [89–91]. [89–91]. The The advantage advantage of of this this systemsystem is is a a model model with with two two kinds kinds of of battery battery st storageorage such such as as a a primary primary and and secondary secondary battery. battery. Usually,Usually, the the load load can can run run using using primary primary battery battery storage, storage, on the on theother other hand, hand, secondary secondary also worksalso works as a backup as a backup of a load of [92]. a load Hence, [92]. the Hence, load thecan loadwork can suitably. work However, suitably. However,the drawback the ofdrawback such a system of such is ait systemis not convenient is it is not convenientfor the environment for the environment EH system due EH systemto the limited due to lifetimethe limited of battery lifetime storage. of battery The system storage. is Thealso systemcostly in is terms also costlyof replacing in terms the of battery replacing and there- quiresbattery more and storage requires as more a backup storage for as a aload. backup Therefore, for a load. an automated Therefore, EMEH an automated system can EMEH be utilizedsystem to can overcome be utilized such to a overcomeproblem. Figure such a15 problem. shows an Figure EMEH 15 system shows using an EMEHthe additional system battery.using the The additional system consists battery. of Thethe systemcomponents consists EHM, of theECM, components EDM, ESM, EHM, a primary ECM, EDM,(non- Electronics 2020, 9, x FOR PEER REVIEWchargeable)ESM, a primary battery (non-chargeable) module (BM), batteryand a power module multiplexer (BM), and a(PM). power The multiplexer following (PM).equation18 The of 46 showsfollowing the additional equation showsbattery the energy additional storage battery system. energy storage system.

FigureFigure 15. 15.Electromagnetic Electromagnetic energy energy harvesting harvesting system system using using an an additional additional battery battery [75 [75].].

4. Power Electronic Technologies in Electromagnetic Energy Harvesting This segment describes the characteristics, efficiency, and limitation of power electronic converters EMEH system. The controller mathematical model, formation, and methods are also presented [93–95].

4.1. A Standard Circuit for Energy-Harvesting Usually, the EMEH transducer generates an AC voltage [96,97]. Most of the low power sensors require DC voltage to run micro devices such as WSNs. The general EMEH circuit is contained in a coil array such as sensors, transducer, a full-wave rectifier circuit, low pass filter, and load as shown in Figure 16. The basic principles of the EMEH circuit are that input sources such as a transducer or sensors on both sides are connected in series with a rectifier circuit, and the output of the rectifier is connected with the capacitor, which filters and stores energy [98,99]. A nonlinear EMEH and a nonlinear interface circuit are integrated into the device [100–103]. In Figure 17, the equivalent circuit of the nonlinear EMEH system is shown. The structure equivalent to the EMEH transducer circuit is constructed with internal resistance RW and internal inductance LW. The nonlinear EMEH system’s governing equations can be expressed as Equation (29).

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4. Power Electronic Technologies in Electromagnetic Energy Harvesting This segment describes the characteristics, efﬁciency, and limitation of power electronic converters EMEH system. The controller mathematical model, formation, and methods are also presented [93–95].

4.1. A Standard Circuit for Energy-Harvesting Usually, the EMEH transducer generates an AC voltage [96,97]. Most of the low power sensors require DC voltage to run micro devices such as WSNs. The general EMEH circuit is contained in a coil array such as sensors, transducer, a full-wave rectifier circuit, low pass filter, and load as shown in Figure 16. The basic principles of the EMEH circuit are that input sources such as a transducer or sensors on both sides are connected in series with a rectifier circuit, and the output of the rectifier is connected with the capacitor, which filters and stores energy [98,99]. A nonlinear EMEH and a nonlinear interface circuit are integrated into the device [100–103]. In Figure 17, the equivalent circuit of the nonlinear EMEH system is shown. The structure equivalent to the EMEH transducer circuit is constructed with internal resistance RW and internal inductance LW. The nonlinear EMEH system’s governing equations can be expressed as Equation (29).  . . x(t) ..  Mx(t) + Cmx(t) + k(|x(t)| − S) + . µMg + kI(t) = −Mx (t) |x(t)| b (29) . .  Lw I(t) + Rw I(t) + V(t) = kx(t)

here, µ is the constant of revolving irritation, M is the mass of the central edge, CM is the damping coefﬁcient, S is the elementary holes between the open ends of the springs and the other part of the exterior edge, and k is the actual stiffness of the springs. The product of the electromechanical coupling of the dynamics of the harvester can be written as:

. . kVr . . kI(t) = Cex(t) − sgn(x(t)) + krCrVL(t)sgn(x(t)) (30) Rw + RL

where sgn(·) is the signum function, RW is the inner resistance, LW is the internal inductance, Electronics 2020, 9, x FOR PEER REVIEW 19 of 46 t is the time, I(t) is similar with x(t), Vr is the voltage drop of the rectiﬁer, and VL is the voltage across the controlling capacitor.

FigureFigure 16.16. StandardStandard energy-harvestingenergy-harvesting circuitcircuit [[28].28].

The basic principle of the EMEH transducer generates AC voltage with ﬂuctuation and

harmonic [104,105]. The rectifier is required to convert AC voltage to DC, and a low pass filter is required to reduce the noise of the rectifier signal. Usually, the performance of the EMEH circuit depends on the outcome of energy conversion between mechanical to electrical through proper impedance matching [106,107]. In [108], the authors indicated a resistive technique for impedance-matching of EMEH. However, the limitation of this technique is old and low conversion efficiency. Therefore, to overcome this problem, synchronous switch harvesting on inductor (SSHI) technique is required. The study in [109,110] addressed a two-stage power management circuit (PMC) for AC to DC converter and DC to DC converter in EMEH. The PMC consists of a rectifier module, buck–boost DC-DC

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converter module, a switch control module, and a charging and discharging module for energy storage devices, as shown in Figure 18. The overall operation principle of the PMC is that the output of the EMEH transducer coil is connected with the input of the full-wave diode bridge rectifier circuit [111,112]. The EMEH transducer’s harvester output voltage is rectified and directly linked to the DC-DC converter circuit supply [113]. The NMOS switch is controlled by the pulse produced by the pulse-width modulation (PWM) process. The control circuit consists of a controller, a relational operator, a modulator for PWM, and a generator for sawtooth voltage. The outcomes of the DC-DC converter are connected in parallel with the resistance and storage devices in the capacitor [114]. However, the drawback of this model is lacking efficiency of voltage. The lack of efficiency Electronics 2020, 9, x FOR PEER REVIEWof voltage is basically the voltage loss in the converter due to the internal resistance20 ofof 46 the converter. This loss of efficiency can be reduced by decreasing the internal resistance of the AC-DC converter.

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converter. This loss of efficiency can be reduced by decreasing the internal resistance of FigureFigurethe AC-DC 17. 17. CorrespondingCorresponding converter. symmetric symmetric diagram diagram of electromagnetic of electromagnetic energy energy harvesting harvesting system [sys-28]. tem [28].

 . . x(t) .. Mx(t) + C x(t) + k(| x(t) | −S) + μMg + kI(t) = −M x (t)  m . b  | x(t) | (29)  . .  + + = Lw I(t) Rw I(t) V (t) k x(t)

here, µ is the constant of revolving irritation, M is the mass of the central edge, CM is the damping coefficient, S is the elementary holes between the open ends of the springs and the other part of the exterior edge, and k is the actual stiffness of the springs. The product of the electromechanical coupling of the dynamics of the harvester can be written as:

. . . . = − kVr + kI(t) Ce x(t) sgn(x(t)) krCr VL (t)sgn(x(t)) (30) R + R FigureFigure 18. 18.Power Power management management circuitw circuitL of electromagnetic of electromagnetic energy energy harvester harvester [28]. [28]. where Insgn(·)In [ 115[115,116], is,116 the], signum the the authors authors function, proposed proposed RW is the an an inner EMEH EMEH resistance, circuit circuit with LwithW is close theclose internal loop loop boost boostinductance, converter converter t is switchtheswitch time, control control I(t) is throughsimilar through with PWM PWM x(t for), for𝑉 feedforwardr is feedforward the voltage and dropand feedback feedbackof the rectifier, control control as and shown as V shownL is the in Figure voltagein Figure 19 . acrossThe19. workingThethe controlling working principle principle capacitor. of this of this model model when when the the output output voltage voltage of of the the rectiﬁer rectifier is is bigger bigger thanthanThe the the basic forwarding forwarding principle voltage voltage of the of EMEHof the the diode diodetransducer and and boost boost generates converter converter AC isvoltage is that that the thewith voltage voltage fluctuation directly directly andconnectsconnects harmonic to to the [104,105].the storage storage unitThe unit (i.e.,rectifier (i.e., capacitor, capacitor, is requ battery,ired battery, to supercapacitor)convert supercapacitor) AC voltage [116 ,[116,117].117to DC,]. However, and However, a low the passlimitationthe filter limitation is ofrequired this of modelthis to model reduce is complex is the complex noise in terms ofin termsthe of rectifier its of need its needsignal. for afor mathematical Usually, a mathematical the performance model, model, and and it ofis theit also is EMEHalso time-consuming time-consuming circuit depends and and noton not the suitable suitable outcome for fo lowofr low energy power power conversion devices devices such such between as as EH EH mechanical applications. applications. toTherefore, electricalTherefore, through an an optimization optimization proper impedance technique technique matching can can be be utilized utilized[106,107]. to to overcomeIn overcome [108], the the the authors limitation limitation indicated of of this this a controllerresistivecontroller technique model. model. The The for system systemimpedance-matching can can be be expressed expressed of by EMEH.by the the following following However, equations: equations: the limitation of this technique is old and low conversion efficiency. Therefore, to overcome this problem, syn- V R (R + R ) R C chronous switch harvesting(1 − D )Ton= inductor1 (SSHI)2 technique3 4 is 1 required. The study (31)in [109,110] addressed a two-stage powerVes (managemeR1 + R2) nt circuitR4 (PMC)(1 − D pfor)T AC to DC converter and DC to DC converter in EMEH. The PMC consists of a rectifier module, buck–boost DC-DC converter module, a switch control module, and a charging and discharging module for energy storage devices, as shown in Figure 18. The overall operation principle of the PMC is that the output of the EMEH transducer coil is connected with the input of the full-wave diode bridge rectifier circuit [111,112]. The EMEH transducer’s harvester output voltage is rectified and directly linked to the DC-DC converter circuit supply [113]. The NMOS switch is controlled by the pulse produced by the pulse-width modulation (PWM) process. The control circuit consists of a controller, a relational operator, a modulator for PWM, and a generator for sawtooth voltage. The outcomes of the DC-DC converter are connected in parallel with the resistance and storage devices in the capacitor [114]. How- ever, the drawback of this model is lacking efficiency of voltage. The lack of efficiency of voltage is basically the voltage loss in the converter due to the internal resistance of the

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V R (R + R ) R C − = 0 = 1 2 3 4 1 1 D D 2 (32) Ves (R1 + R2) R4 (1 − Dp)T V R (R + R ) R C = − 1 2 3 4 1 D 1 2 (33) Electronics 2020, 9, x FOR PEER REVIEW Ves (R1 + R2) R4 (1 − Dp)T 22 of 46 where D is the duty ratio, T is the whole period of time, R is the parasitic series resistor, C

is the capacitor, V is the voltage, and Ves is the voltage of the energy storage element.

Figure 19. Energy harvesting circuit with boost converter for feedforward and feedback control [115]. Figure 19. Energy harvesting circuit with boost converter for feedforward and feedback controlIn [[115].118], the author proposed a vibration-based EMEH interface circuit. Figure 20 shows the four categories of electrical circuits, used to extract the energy from ambient sources and store the harvestingV chargeR in(R the+ EHR ) applications.R C Figure 20a shows the (1− D)T = 1 2 3 4 1 ordinary EH and energy storageV ( circuit;R + R Figure) R 20b shows(1− D the) synchronousT (31) electric charge extraction (SECE) circuit; Figurees 201 c shows2 the4 parallel SSHIp circuit; and Figure 20d shows the series SSHI circuitV [118–120R ]. However,(R + R the) drawbackR C of such a circuit for EH application is that1− theD = conversionD′ = 1 efﬁciency2 is poor3 between4 1 mechanical to electrical energy + − 2 (32) in terms of impedance matchingVes mechanism(R1 R2 ) usingR4 SECH(1 andDp SSHI)T methods. Therefore, to overcome this problem in the EH system, optimization techniques can be utilized to make Electronics 2020, 9, x FOR PEER REVIEW V R (R + R ) R C 23 of 46 the system simpleD and=1 automated− 1 without2 time-consuming.3 4 1 + − 2 (33) Ves (R1 R2 ) R4 (1 Dp )T where D is the duty ratio, T is the whole period of time, R is the parasitic series resistor, C is the capacitor, V is the voltage, and Ves is the voltage of the energy storage element. In [118], the author proposed a vibration-based EMEH interface circuit. Figure 20 shows the four categories of electrical circuits, used to extract the energy from ambient sources and store the harvesting charge in the EH applications. Figure 20a shows the ordinary EH and energy storage circuit; Figure 20b shows the synchronous electric charge extraction (SECE) circuit; Figure 20c shows the parallel SSHI circuit; and Figure 20d shows the series SSHI circuit [118–120]. However, the drawback of such a circuit for EH application is that the conversion efficiency is poor between mechanical to electrical energy in terms of impedance matching mechanism using SECH and SSHI methods. Therefore, to overcome this problem in the EH system, optimization techniques can be utilized to make the system simple and automated without time-consuming.

FigureFigure 20.20. ConventionalConventional vibration-basedvibration-based energy energy harvesting harvesting circuit: circuit: (a )(a standard,) standard, (b )(b SECE,) SECE, (c) ( parallelc) par- SSHI,allel SSHI, and and (d) series(d) series SSHI SSHI circuit circuit [118 [118].]. Reprinted Reprinted with with permission permission fromfrom ref. [118]. [118]. Copyright Copyright 20152015 Elsevier.Elsevier.

The traditional vibration-based EMEH interface circuit and optimal harvested power are derived by the following equation:

2  π 1  a 2 .R . +  N N  2 R  Ph =  N  .. 2 2 (34)  2 2   π   M . | y |   1  2 2. +  + α .R      N N  D   2 R N      

.. M 2.| y |2 where Ph is the input resonant power, is the reference power, and aN and RN D are the electromagnetic harvester. In [2,121], the authors described the block of spring, mass, damper for the EMEH system as shown in Figure 21. This is an analogous system generally used for vibration-based EH systems. The system is important for energy conversion from mechanical to electrical and vice versa.

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The traditional vibration-based EMEH interface circuit and optimal harvested power are derived by the following equation:

2 2 π 1 a .RN. + Ph N 2 RN .. = (34) 2 M2.|y|2 2 2. π + 1 + α2 .R D 2 RN N N

.. M2.|y|2 where Ph is the input resonant power, D is the reference power, and aN and RN are the electromagnetic harvester. In [2,121], the authors described the block of spring, mass, damper for the EMEH system as shown in Figure 21. This is an analogous system generally used for vibration- Electronics 2020, 9, x FOR PEER REVIEW 24 of 46 based EH systems. The system is important for energy conversion from mechanical to electrical and vice versa.

(a) (b)

FigureFigure 21. Block 21. diagramBlock diagram of the spring, of the spring,mass, damping mass, damping method methodfor EMEH for system EMEH (a system,b). Schematic (a,b). Schematic of the spring, of the mass, spring, damping mass, systemdamping [93,121]. system [93,121].

TheThe study study reported reported in in [122] [122] indicated indicated the the n- n-stagestage rectifier rectiﬁer circuit circuit model model for for the the EMEH EMEH systemsystem also also shown in Figure 2222.. TheThe authorsauthors alsoalso proposedproposed multiple multiple diodes diodes connected connected in in a Electronics 2020, 9, x FOR PEER REVIEWaseries series to to increase increase the the power power of theof harvestingthe harvesting signal. signal. However, However, the limitation the limitation of this of model25 thisof 46 modelis the biasis the voltage bias voltage of the diode. of the Therefore,diode. Therefore, the Schottky the Schottky diode can diode be used can be to overcomeused to over- this problem due to its low bias voltage. come this problem due to its low bias voltage.

Figure 22. n-stage rectiﬁerrectifier circuit modelmodel ofof electromagneticelectromagnetic energyenergy harvestingharvesting systemsystem [[122].122].

The authors in [123] suggested an electromagnetic damper cum energy harvester (EMDEH) circuit model as shown in Figure 23. The basic principle of this circuit model

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The authors in [123] suggested an electromagnetic damper cum energy harvester (EMDEH) circuit model as shown in Figure 23. The basic principle of this circuit model comparedcompared to to other other EH EH is, is, it it proposes proposes a aresistor resistor circuit, circuit, which which includes includes a afull-wave full-wave bridge bridge rectifier.rectiﬁer. The The presence presence of ofthis this resistor resistor circuit circuit provides provides a basi a basiss for forevaluating evaluating the theoutput output of theof functional the functional EH device, EH device, which which inevitably inevitably includes includes power power loss and loss input and inputresistance resistance var- iationvariation [124,125]. [124,125 However,]. However, the thedrawback drawback of this of this model model is, is,it is it isused used in in many many electronic electronic components,components, and and those those components components have have their their own own biasing biasing voltage voltage that that is is not not suitable suitable for for thethe low low power power model model [126]. [126]. Therefore, Therefore, a asimple simple circuit circuit model model with with low low biasing biasing electrical electrical componentscomponents such such as as a alow low power power resistor, resistor, Schott Schottkyky diode diode is is utilized utilized to to overcome overcome the the issue. issue.

Figure 23. Symmetry of EMDEH in energy harvesting for vibration control [124]. Figure 23. Symmetry of EMDEH in energy harvesting for vibration control [124]. The authors in [127–129] reported an EMEH circuit based on active and passive mode, The authors in [127–129] reported an EMEH circuit based on active and passive shown in Figure 24. Figure 24a shows the passive mode EH circuit model, and Figure 24b mode, shown in Figure 24. Figure 24a shows the passive mode EH circuit model, and shows the active mode EH circuit model. The study in [128] presented the phenomenon Figure 24b shows the active mode EH circuit model. The study in [128] presented the phe- of circuits namely the passive and active techniques’ components. The authors referred nomenon of circuits namely the passive and active techniques’ components. The authors to an active technique in [127] and selected its components such as MOSFET, thyristor, referred to an active technique in [127] and selected its components such as MOSFET, and transistors to design the EH circuit. The authors also proposed EH through active thyristor, and transistors to design the EH circuit. The authors also proposed EH through components that are suitable for ultra-low-power application in terms of low biasing active components that are suitable for ultra-low-power application in terms of low bias- voltage. In [129], the authors suggested a passive technique to design an EH circuit model. ing voltage. In [129], the authors suggested a passive technique to design an EH circuit However, the passive technique is not suitable for low power applications, especially in model. However, the passive technique is not suitable for low power applications, espe- the EH system, to increase the low input voltage in terms of high biasing voltage. cially in the EH system, to increase the low input voltage in terms of high biasing voltage. 4.2. Converter Technology for Electromagnetic Energy Harvesting System Generally, electronic converters topology has four classiﬁcations, such as i. AC to AC, ii. AC to DC, iii. DC to DC, and iv. DC to AC [130,131]. Many applications have already made use of these converters. The AC to DC converters convert AC waveform to DC, which is suitable for EMEH transducer. The basic application of DC to DC converters is to boost up DC to DC voltage of vibration, wind, solar PV that is suitable for low power EH applications. The DC to AC converter is utilized to convert DC signal to AC waveform, which is suitable for thermoelectric EH application. With reference to Figures1 and2, the different topologies of power converters are explained below:

4.2.1. AC to DC Converter AC-DC converters transform the alternating current to a constant DC output voltage. Small-, medium-, and high-power applications are all possible with these converters. Uncontrolled rectifiers, also known as diode rectifiers or unidirectional AC-DC converters, consist of diodes. On the other hand, a diode rectifier causes significant input current distortion. As a result, a significant amount of effort is required to develop a filter that reduces ripple. Figure 25 shows an AC-DC converter with multi-input, bridgeless resonant- circuit design concepts [130]. The conversion of low amplitude voltages (alternating) from

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multiple electromagnetic-reed (EMR) generators, to a controlled DC output voltage, is also shown in Figure 25. Without the use of a diode bridge, the topology is capable of interfacing many, individual, input sources. Voltages with very low amplitude may be scaled up to moderately high voltage. Multiple inductive sources can be controlled with this topology. In the EMR generator, N is the number assigned to each inductive source. A resonant-inductor, diode, and several MOSFET-capacitor-bridges make up the multi-input circuit. Each input source is wired into one of two MOSFET-capacitor-bridges (M(N)r1, Electronics 2020, 9, x FOR PEER REVIEWC(N)r1 , and M(N)r2, C(N)r2) sharing a resonant inductor with a diode. 27 of 46

FigureFigure 24. 24. OperationOperation modes modes and and the theassociated associated electrical electrical circuits circuits for the for (a the) passive (a) passive EMEH EMEH system; system; (b) active (b )EMEH active system EMEH [127].system Reprinted [127]. Reprinted with permission with permission from Hasheminejad, from Hasheminejad, Rabiee and Rabiee Markazi and Markazi (2018). Copyright (2018). Copyright 2018 ASCE 2018 Library. ASCE Library.

4.2. ConverterIn the literature, Technology a variety for Electromagnetic of multilevel Energy converter Harvesting topologies System with various features have beenGenerally, deﬁned [131 electronic–133]. Figureconverters 26 shows topology the classiﬁcationhas four classifications, of a multilevel such AC-DCas i. AC converter.to AC, ii. AC to DC, iii. DC to DC, and iv. DC to AC [130,131]. Many applications have already made use of these converters. The AC to DC converters convert AC waveform to DC, which is suitable for EMEH transducer. The basic application of DC to DC converters is to boost up DC to DC voltage of vibration, wind, solar PV that is suitable for low power EH applications. The DC to AC converter is utilized to convert DC signal to AC waveform, which is suitable for thermoelectric EH application. With reference to Figures 1 and 2, the different topologies of power converters are explained below:

4.2.1. AC to DC Converter AC-DC converters transform the alternating current to a constant DC output voltage. Small-, medium-, and high-power applications are all possible with these converters. Un- controlled rectifiers, also known as diode rectifiers or unidirectional AC-DC converters, consist of diodes. On the other hand, a diode rectifier causes significant input current distortion. As a result, a significant amount of effort is required to develop a filter that reduces ripple. Figure 25 shows an AC-DC converter with multi-input, bridgeless resonant-

Electronics 2020, 9, x FOR PEER REVIEW 28 of 46

circuit design concepts [130]. The conversion of low amplitude voltages (alternating) from multiple electromagnetic-reed (EMR) generators, to a controlled DC output voltage, is also shown in Figure 25. Without the use of a diode bridge, the topology is capable of interfacing many, individual, input sources. Voltages with very low amplitude may be scaled up to moderately high voltage. Multiple inductive sources can be controlled with this topology. In the EMR generator, N is the number assigned to each inductive source. Electronics 2021, 10, 1108 A resonant-inductor, diode, and several MOSFET-capacitor-bridges make up the19 multi- of 35 input circuit. Each input source is wired into one of two MOSFET-capacitor-bridges (M(N)r1, C(N)r1, and M(N)r2, C(N)r2) sharing a resonant inductor with a diode.

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FigureFigure 25. 25Multi-channel. Multi-channelIn the literature, of of EMR EMR a generators varietygenerators of system systemmultilevel [130 [130].]. Reprinted co Reprintednverter with topologieswith permission permission with from from various ref. ref. [130 ].features Copyright[130]. Copyrighthave 2015 been IEEE. 2015 defined IEEE. [131–133]. Figure 26 shows the classification of a multilevel AC-DC converter. Multilevel Converter

AC-DC converter

AC-DC switching AC-DC boost AC-DC buck-boost AC-DC switch- converters converters converter inductor boost converter

Figure 26. ClassiﬁcationFigure 26. of multilevelClassification converter. of multilevel converter.

4.2.2. AC4.2.2. to DCAC Switchingto DC Switching Converter Converter In switch-modeIn switch-mode power power supply supply circuits, circuits, AC to DCAC convertersto DC converters are used are to used transform to transform AC voltageAC voltage to DC outputto DC (controlled).output (controlled). The important The important characteristics characteristics are i. input are i. voltage input voltage scale, ii.scale, maximal ii. maximal input current, input current, iii. DC iii. output-voltage, DC output-voltage, iv. maximal iv. maximal peak current, peak current, and v. and v. continuouscontinuous current current of the load. of the The load. work The in work [131] in reported [131] reported a direct a AC-DC direct AC-DC converter converter that that is suitableis suitable for low for voltage low voltage EH applications. EH applications.

4.2.3. AC4.2.3. to DCAC Boostto DC and Boost Buck–Boost and Buck–Boost Converter Converter A boost-converterA boost-converter is a step-up is a step-up that generates that generate a highers a outputhigher output voltage voltage as compared as compared to to the inputthe voltage. input voltage. There are There two statesare two of operationstates of operation for boost-converters: for boost-converters: ON and OFF, ON and and OFF, share the same characteristics as the buck-converter. A step-down or step-up converter and share the same characteristics as the buck-converter. A step-down or step-up con- can be used with a buck–boost. When the service period is less than 0.5, the converter verter can be used with a buck–boost. When the service period is less than 0.5, the con- behaves like a buck-converter. Whereas, it acts like a boost-converter when the service verter behaves like a buck-converter. Whereas, it acts like a boost-converter when the ser- cycle is set between 0.5 and 1. The study in [132] reported that i. AC-DC boost converter vice cycle is set between 0.5 and 1. The study in [132] reported that i. AC-DC boost con- and ii. AC-DC buck–boost converters are favorites for power processing circuits for (a) verter and ii. AC-DC buck–boost converters are favorites for power processing circuits for electromagnetic, (b) electrostatic and (c) piezoelectric inertial energy harvesting. Figure 27a (a) electromagnetic, (b) electrostatic and (c) piezoelectric inertial energy harvesting. Figure 27a depicts the two boost-converter sub-circuits: one that produces the upper half of the output voltage with positive generator voltage, and the other that generates the lower half with negative generator voltage. The parasitic diodes of the MOSFETS cannot be forward biased because the generator voltage is too low. Figure 27b shows the buck converter that was first investigated because it was the initial circuit that easily converted high to low voltage.

Electronics 2021, 10, 1108 20 of 35

depicts the two boost-converter sub-circuits: one that produces the upper half of the output voltage with positive generator voltage, and the other that generates the lower half with negative generator voltage. The parasitic diodes of the MOSFETS cannot be forward biased because the generator voltage is too low. Figure 27b shows the buck converter that was Electronics 2020, 9, x FOR PEER REVIEWﬁrst investigated because it was the initial circuit that easily converted high to low voltage.30 of 46

(a)

(b)

Figure 27. A double-splitdouble-split boost converterconverter ((aa),), buckbuck converterconverter ((bb)[) [132].132]. ReprintedReprinted withwith permissionpermission from ref. [[132].132]. CopyrightCopyright 2007 SpringerSpringer Nature.Nature. 4.2.4. AC to DC Switch-Inductor Boost Converter 4.2.4. AC to DC Switch-Inductor Boost Converter For effective EH, with the low voltage inertial micro generators, a direct AC-DC boost For effective EH, with the low voltage inertial micro generators, a direct AC-DC boost converter is used. To avoid using a front-end bridge rectifier, the converter makes use of converter is used. To avoid using a front-end bridge rectifier, the converter makes use of MOSFETs’ bidirectional current-conduction capability. It provides a resistive load, and MOSFETs’ bidirectional current-conduction capability. It provides a resistive load, and the mode of operation is discontinuous. The distinct topologies of active AC-DC switch the mode of operation is discontinuous. The distinct topologies of active AC-DC switch converters are employed in EH applications such as i. dual-capacitor boost-converter, converters are employed in EH applications such as i. dual-capacitor boost-converter, ii. ii. dual-capacitor buck–boost converter, iii. boost-converter secondary switches, and iv. dual-capacitor buck–boost converter, iii. boost-converter secondary switches, and iv. bridgeless boost rectifier as shown in Figure 28[ 133]. For separate +ve and –ve voltage bridgeless boost rectifier as shown in Figure 28 [133]. For separate +ve and –ve voltage conductions, both bidirectional switches and dual-capacitors are used in such topologies, conductions, both bidirectional switches and dual-capacitors are used in such topologies, or parallel DC-DC converters (two) are used. Due to the low frequency in micro-generators, or parallel DC-DC converters (two) are used. Due to the low frequency in micro-genera- dual-capacitor topologies are used as shown in Figure 28a,b, which required large enough totors, repress dual-capacitor ripple-voltage topologies below are the used desired as shown amount. in Figure Besides, 28a,b, the which alternative required of dual-large capacitorsenough to isrepress two synchronous ripple-voltage MOSFETs below the as de shownsired inamount. Figure Besides,28c. Figure the alternative28d shows of a bridgelessdual-capacitors boost-rectifier is two synchronous that combines MOSFETs in a very as shown special in way Figure a boost 28c. Figure with buck–boost 28d shows converters.a bridgeless Withboost-rectifier +ve input-voltage, that combines the circuitry in a very works special in way boost a boost mode, withM1 isbuck–boost switched on,converters. and diode With (D1 +ve) is input-voltage, reverse-biased. the Table circuitry2 shows works the in comparison boost mode, of M1 different is switched AC-DC on, converterand diode topologies(D1) is reverse-biased. famous in the Table EH system. 2 shows the comparison of different AC-DC converter topologies famous in the EH system.

Electronics 2021, 10, 1108 21 of 35

Electronics 2020, 9, x FOR PEER REVIEW 31 of 46

Figure 28. ACAC to to DC DC switch switch converter converter topologies: topologies: (a) dual (a) dual capacitor capacitor boost boost converter, converter, (b) dual (b) ca- dual capacitorpacitor buck–boost buck–boost converter, converter, (c) (boostc) boost converter converter with with secondary secondary switches, switches, and ( andd) bridgeless (d) bridgeless boost-rectiﬁerboost-rectifier [[133].133].

Table 2.2. ComparisonComparison ofof differentdifferent AC-DCAC-DC convertersconverters topologytopology forfor energyenergy harvesting.harvesting.

Refs. Topology Advantages Disadvantages Frequency Load Efficiency (%)Appli- Applications Refs. Topology Advantages Disadvantages Frequency Load Efficiency (%) cations Not High efficiency AC-DC High efficiency recommendedNot recom- [130] AC-DC con- with low Low Resistive 86.3% EH [130] converter with low fre- mendedfor use for at highuse at Low Resistive 86.3% EH verter frequency quency highpower power levels levels -Biasing -Biasingvoltage voltage of AC-DC Suitability for AC-DC Suitability for ofdiode diode high high [131] switching low power 50-kHz Resistive High EH [131] switching con- low power mi- -Difficult-Difficult to over- to 50-kHz Resistive High EH converter micro devices verter cro devices comeovercome low input low inputvoltage voltage AC-DCAC-DC boost boost Capable toto i.i. -Complex -Complex design andand buck– step-upstep-up andand ii.ii. design [132] [132] -Difficult to im- 50-kHz50-kHz Resistive Resistive High High EH EH buck–boostboost con- step-downstep-down volt- -Difficult to plement converterverter voltageage levels levels implement -Small size AC-DCAC-DC switch- -Small size -High noise -Easy to -High noise [133] inductorswitch-induc- boost -Easy to imple- -Low power 50-kHz Resistive 71% EH [133] implement -Low power capa- 50-kHz Resistive 71% EH convertertor boost con- ment capability -Low cost bility verter -Low cost

4.2.5. DC to DC Converter 4.2.5. DC to DC Converter A single-direction DC-DC converter is one of the most common power electronic convertersA single-direction for EMEH applications DC-DC converter [113–115 is ].on Thee of outputthe most voltage common magnitude power ofelectronic such a converterconverters increasesfor EMEH or applications decreases in[113–115]. comparison The output to the voltage input voltage magnitude magnitude of such a from con- ambientverter increases sources. or The decreases DC-DC in converter comparison has to weak the voltage-regulationinput voltage magnitude with an from inadequate ambient activesources. response The DC-DC when converter operated has in anweak open-loop, voltage-regulation so it is used with as an a closed-loop inadequate feedbackactive re- controlsponse systemwhen operated for output in an voltage open-loop, regulation. so it is To used achieve as a theclosed-loop best output feedback voltage control from thesystem controller, for output some voltage researchers regulation. suggest To achi switchingeve the the best converter output voltage on and from off (duty the control- cycle). Forler, some switching researchers control suggest of the MOSFETs switching in the the co DC-DCnverter converteron and off circuitry, (duty cycle). high For frequency switch- PWMing control is the of best the option MOSFETs to regulate in the powerDC-DC ﬂow converter [115]. circuitry, The beneﬁts high of frequency single-phase PWM DC-DC is the convertersbest option includeto regulate the needpower for flow only [115]. one MOSFET The benefits to be of switched, single-phase low DC-DC losses, and converters suitability in- forclude EMEH the need applications. for only Basedone MOSFET on their to operating be switched, modes, low DC-DC losses, convertersand suitability can befor divided EMEH intoapplications. three categories: Based on (i)their linear operating mode, modes, (ii) hard DC-DC switching converters mode, can and be (iii) divided soft-switching into three modecategories: converter (i) linear as shown mode, in(ii) Figure hard switchin29. The switchingg mode, and mode (iii) converters soft-switching are categorizedmode converter into as shown in Figure 29. The switching mode converters are categorized into two groups namely non-isolated and isolated-converters. The non-isolated topologies are classified into

Electronics 2021, 10, 1108 22 of 35

Electronics 2020, 9, x FOR PEER REVIEWtwo groups namely non-isolated and isolated-converters. The non-isolated topologies32 of are 46 classiﬁed into four groups such as (a) boost, (b) buck, (c) buck/boost, and (d) Cuk converter, whichfour groups consists such of as a diode, a) boost, inductors, (b) buck, capacitors, (c) buck/boost, and aand switch (d) Cuk as shown converter, in Figure which 30 consists[ 134]. Theyof a diode, can produce inductors, satisfactory capacitors, outcomes and a switch by reducingas shown noise,in Figure simplifying 30 [134]. They the process, can produce and providingsatisfactory a outcomes quick response by reducing with good noise, regulation. simplifying However, the process, they and have providing some drawbacks, a quick re- suchsponse as with the factgood that regulation. they dissipate However, power they have under some any drawbacks, working conditions, such as the resultingfact that they in lowdissipate performance. power under any working conditions, resulting in low performance.

Electronics 2020, 9, x FOR PEER REVIEW 33 of 46 FigureFigure 29.29. ClassiﬁcationClassification ofof DC-DCDC-DC converters.converters.

Figure 30. Numerous conventional non-isolated DC-DC converters: (a) buck, (b) boost, (c) buck– Figure 30. Numerous conventional non-isolated DC-DC converters: (a) buck, (b) boost, boost, and (d) Cuk [134]. (c) buck–boost, and (d) Cuk [134]. OnOn thethe otherother hand,hand, thethe isolatedisolated topologiestopologies areare classifiedclassified into into four fourtypes types suchsuch asas boost,boost, forward,forward, flyback,flyback, andand full-bridgefull-bridge converter,converter, whichwhich consistsconsists ofof a a transformer transformer or or coupled coupled inductorinductor as as shown shown in in Figure Figure 31 31[ 134[134].]. These These converters converters can can solve solve the the problem problem of of unstable unstable lowlow power power sources sources in thein the EMEH EMEH system system [135 ].[135]. Vibration-based Vibration-based EMEH EMEH produces produces low power low frompower a varietyfrom a variety of sources, of sources, making making it unsuitable it unsuitable for micro-electronic for micro-electronic devices devices unless unless the voltagethe voltage is increased. is increased. As As a result, a result, the the primary primary reason reason for for employing employing aa boost converter converter is isto to enhance enhance the the output output from from the theinput input voltage voltage in order in order to achieve to achieve the desired the desired output. output.In com- In comparison to the closed-loop stage, the boost’s controlling efficiency is very low in parison to the closed-loop stage, the boost’s controlling efficiency is very low in the open-loop the open-loop stage. The operating condition for an open-loop device is that the input stage. The operating condition for an open-loop device is that the input and output voltages and output voltages be fixed depending on the application. Therefore, the advantages of be fixed depending on the application. Therefore, the advantages of isolated converters are i. isolated converters are i. run at a low frequency, ii. storing energy in a coupled inductor run at a low frequency, ii. storing energy in a coupled inductor per cycle, and iii. transferring per cycle, and iii. transferring to load in subsequent cycles that are suitable for EMEH to load in subsequent cycles that are suitable for EMEH applications because of low power applications because of low power loss, easy to implement, and low cost. Preceding loss, easy to implement, and low cost. Preceding research has shown that a duty cycle-con- research has shown that a duty cycle-controlled switch-mode DC-DC boost converter will trolled switch-mode DC-DC boost converter will achieve the best results in the EMEH system. achieve the best results in the EMEH system. Table3 shows the comparison of various Table 3 shows the comparison of various DC-DC converters topologies for EMEH. DC-DC converters topologies for EMEH.

ElectronicsElectronics2021 2020,,10 9,, x 1108 FOR PEER REVIEW 2334 ofof 3546

(a) (b)

(c)

Figure 31. Isolated DC-DCDC-DC converter:converter: ((aa)) ﬂybackflyback converter, converter, ( b(b)) forward forward converter, converter, and and (c )(c full-bridge) full- converterbridge converter [134]. [134].

Table 3. ComparisonTable 3. Comparison of different of DC-DCdifferent converters DC-DC converters topology fortopology energy for harvesting. energy harvesting.

Implementation Imple- Effi- Refs. Converter Con- Switch TimeSwitch Frequency Fre- Load EfficiencyApplica- (%) Applications Refs.Difficulty mentation Load ciency verter Time quency tions [136] Buck SimpleDifficulty Low 53 Hz Resistive (%) 59% EH Resis- [115] Boost[136] Simple Buck Simple Low Low 40 53Hz Hz Resistive 59% HighEH EMEH tive Buck–boost Resis- [28] [115]Complex Boost Simple High Low 8 Hz 40 Hz Resistive High 41.7%EMEH EMEH converter tive Flyback Buck– [39] Complex High 50 Hz Resis-Resistive Low EH converter [28] boost Complex High 8 Hz 41.7% EMEH tive Full-bridge converter [96] Complex Low 30kHz Resistive 87–88% EMEH converter Flyback High Resis- [39] Complex 50 Hz Low EH converter tive Full- 5. Overview of Electromagnetic Energy HarvestingResis- Techniques and Prototypes [96] bridge Complex Low 30kHz 87- 88% EMEH The EMEH research has been conducted fortive several decades, and it presents different converter ideas, functions, and utilities. Generally, the overall EMEH process is composed depending on energy source, power electronic circuit, and storage devices [75,137]. These three 5. Overview of Electromagnetic Energy Harvesting Techniques and Prototypes basic components must always be present to ensure the harvest of convenient electrical charges.The EMEH The following research sectionhas been summarizes conducted for the several common decades, energy and sources it presents and different reviews EMEHideas, functions, prototypes. and utilities. Generally, the overall EMEH process is composed depending on energy source, power electronic circuit, and storage devices [75,137]. These three 5.1.basic Electromagnetic components must Energy always Harvester be present Devices to ensure the harvest of convenient electrical charges.Electromagnetic The following harvesters section summarizes represent an the EH approachcommon energy that uses sources a magnetic and reviews field to alterEMEH mechanical prototypes. energy to electrical energy [138–140]. An AC voltage is induced by the analogous signal between the mass and the pick-up coil, according to Faraday’s Law; the5.1. fluxElectromagnetic of a magnetic Energy field Harvester varies withDevices time. When a conductor is put in a magnetic fieldElectromagnetic according to Faraday’s harvesters induction represent law, an which EH approach is often referred that uses to asa magnetic electromagnetic field to induction,alter mechanical an electromotive energy to forceelectrical is induced energy [ 141[138–140].,142]. The An coil AC moves voltage by is various induced amounts by the ofanalogous magnetic signal flux; thus,between an ACthe voltagemass and is generatedthe pick-up in coil, the coil.according When to an Faraday’s outer mechanical Law; the forceflux of pushes a magnetic an electrically field varies conductive with time. coil When to cut a the conductor magnetic is inductionput in a magnetic field between field magnets,according anto Faraday’s electric charge induction is produced law, which in the is often coil. referred This explains to as electromagnetic a combined principle induc- totion, generate an electromotive electrical energy. force is Ininduced [142], the[141 authors,142]. The indicated coil moves a magnetic by various field, amounts which isof producedmagnetic byflux; current thus, resonantan AC voltage electrodes is genera in the lowted powerin the devices,coil. When which an harvestouter mechanical electricity toforce run pushes micro-device an electrically sensors. conductive Table4 presents coil to the cut available the magnetic products induction in the market, field between which aremagnets, generated an electric using thecharge EMEH is produced technique. in the coil. This explains a combined principle to

Electronics 2020, 9, x FOR PEER REVIEW 35 of 46 Electronics 2020, 9, x FOR PEER REVIEW 35 of 46 Electronics 2020, 9, x FOR PEER REVIEW 35 of 46 generate electrical energy. In [142], the authors indicated a magnetic field, which is pro- Electronics 2020, 9, x FOR PEER REVIEW 35 of 46 Electronics 20212020,, 109, ,x 1108 FOR PEER REVIEWducedgenerat bye electricalcurrent resonant energy. electrodesIn [142], the in authorsthe low indicatedpower devices, a magnetic which field, harvest which electricity3524 isof 46 35pro- ducedgenerat bye electricalcurrent resonant energy. electrodesIn [142], the in authorsthe low indicatedpower devices, a magnetic which field, harvest which electricity is pro- generateto run electrical micro-device energy. sensors. In [142], Table the 4 authors presents indicated the available a magnetic products field, in the which market, is pro- which generattoduced rune micro-deviceelectricalby current energy. resonant sensors. In [142],electrodes Table the 4 presentsauthors in the low indicatedthe poweravailable adevices, magnetic products which field, in the harvest which market, electricityis pro-which ducedare by generated current resonantusing the electrodes EMEH technique. in the low power devices, which harvest electricity ducedareto run generated by micro-devicecurrent using resonant thesensors. EMEHelectrodes Table technique. 4in presents the low thepower available devices, products which harvestin the market, electricity which toTable runare micro-device 4. generatedMarket survey using sensors. of the magnetic EMEHTable field-based 4 technique. presents items the available for energy products harvesting. in the market, which to Tablerun micro-device 4. Market survey sensors. of magnetic Table 4field- presentsbased itemsthe available for energy products harvesting. in the market, which are generated using the EMEH technique. areTable generated 4.Product Market using survey the EMEHof magnetic technique. field-based items for energy harvesting. Table 4. Market survey of magneticDescription/Functionality field-based items for energy harvesting. Shape (Company)Product Product Description/Functionality Shape TableTable 4. 4. Market(Company) Market survey survey of of magnetic magnetic field- field-basedbased items items for for energy energy harvesting. harvesting. Product Description/Func- Sends informationtionality using Shape (Company) -Description/Func Sends informationtionality using Shape Product(Company) general packet radio service. Product Description/Funcgeneral- Sends packet information tionalityradio service. using Shape (Company) - DrivenDescription/Func- Sends by a two-piece, informationtionality specially using built Shape (Company) - Driven generalby a two-piece, packet radio specially service. built trans- transformer- Sendsgeneral information packet that controls radio using service. from the Power Line former- Driven that- Sends bycontrols a two-piece, information from the specially magneticusing builtd fiel along trans- Power Line - Drivenmagneticgeneral by packet a field two-piece, along radiothe service.specially transmission built trans- PowerSensor Line former thatgeneralthe controls transmissio packet from radion linethe service. magneticscavenges. field along PowerSensor Line - Drivenformer by that a two-piece,controlsline from scavenges. specially the magnetic built trans- field along (Protura)(Protura)Sensor - Driven- Powered- Powered bythe aby transmission two-piece, a by harvesting a harvesting specially line circuit scavenges. circuit built when whentrans- the line Power LineSensor former that controlsthe transmission from the magnetic line scavenges. field along Power(Protura)[143] [Line143 ] formercurrent- Powered thatthe iscontrols line more by current a thanharvesting from is55 the more A. magnetic Thecircuit than auxiliary 55 when field A. The thesupplyalong line Sensor(Protura) - Poweredthe transmission by a harvesting line scavenges. circuit when the line Sensor[143] powercurrenttheauxiliary donut is transmission more (USi)supply than powers 55 line power A. scavenges. Thethe donut deviceauxiliary (USi) when supply the (Protura)[143] - Poweredcurrent by is a more harvesting than 55 circuit A. The when auxiliary the line supply (Protura) - Poweredpowercurrentpowers donut byis lower a theharvesting (USi) devicethan powers the when circuit provided the the whendevice current value the when isline[144]. the [143] currentpower is more donut than (USi) 55 A.powers The auxiliary the device supply when the [143] currentcurrent islower more is- lowerTransmits than than thethan 55 provided A.data the The provided on auxiliary demand value value [144 supply]. [144]. powercurrent donut is-(USi) Transmitslower powers than data the the on provideddevice demand when value the [144]. power- Transfers donut -on-demand (USi)Transmits powers data data the on through device demand whena global the mo- current is lower- Transmits than the provided data on demand value [144]. Power currentbile- Transfers communications- is Transfers lower on-demand than on-demand the wireless dataprovided roughth data cell throughvaluephone a global [144]. technol- a mo- - Transfers- Transmits on-demand data on data demand through a global mo- DonutPowerPower bile communicationsglobal- Transmits mobileogy communications data framework. wireless on demand cell phone wireless technol- Power - Transfersbile communications on-demand data wireless through cell a globalphone mo-technol- (USi)DonutDonut [145] - Transfers- Operatescell on-demand using phone ogyharvested technology dataframework. through energy framework. agiven global line mo- cur- PowerDonut bile communications ogywireless framework. cell phone technol- Power(USi)(USi) [145] [145] bile- Operatescommunications- Operates usingrents harvested usingwireless of above harvested energycell 50 phone A. energygiven technol- line cur- DonutDonut(USi) [145] - Operatesgiven usingogy lineogyrents framework. harvested currentsframework. of above of energy above50 A. given 50 A. line cur- (USi)(USi) [145] [145] - Operates- OperatesPDMS usingare using largely harvested harvestedrents utilized of energyabove energy electronegativity 50 given given A. line line cur- cur- poly- Polydimethylsilox- merPDMS films are in rentslargelyPDMS the oftriboele utilized areabove largelyctric 50 electronegativity A. nano-generator. utilized poly- The PDMS are largelyrents of utilized above 50electronegativity A. poly- Polydimethylsilox-ane (PDMS) [146] merPDMS filmselectronegativity dimension in the triboele is 50 polymer ctricmm ×nano-generator. 50 films mm inand the is de- The PolydimethylsiloxanePolydimethylsilox-PDMSmer are triboelectricfilms largely in the utilized triboele nano-generator. electronegativityctric nano-generator. The PDMS poly- The ane (PDMS) [146]PDMS signedPDMS are aslargelydimension a support utilized is case 50 electronegativity mm for soft× 50 PDMSmm and springs. poly- is de- × Polydimethylsilox-Polydimethylsilox-ane(PDMS) (PDMS) [146 [146]]mermer filmssigned PDMSfilms indimension in as thedimension the a triboelesupport triboele is 50ctricis casectric mm50 nano-generator.mm fornano-generator. 50soft× 50 mm PDMS mm and and springs. isThe Theis de- designed as a support case for soft aneane (PDMS) (PDMS) [146] [146] PDMSPDMSsigned dimension dimension as a support is is 50 50 mm casemm × for×50 50 mmsoft mm andPDMS and is is de-springs. de- PDMS springs. 5.2. Other Relatedsigned Workssigned as on as a Electromagneticasupport support case case for for Energysoft soft PDMS PDMS Harvesters springs. springs. Devices 5.2. Other Related Works on Electromagnetic Energy Harvesters Devices 5.2. OtherThe following Related Works section on Electromagneticin Table 5 denotes Energy various Harvesters types Devicesof EMEH devices, their fea- 5.2.tures, OtherThe specifications, Related following Works section dimensions, on Electromagnetic in Table and 5 functionality.denotes Energy various Harvesters types Devices of EMEH devices, their fea- 5.2.5.2. Other OtherThe Related Related following Works Works onsection on Electromagnetic Electromagnetic in Table 5 Energydenotes Energy Harvesters Harvestersvarious typesDevices Devices of EMEH devices, their features,The specifications, following section dimensions, in Table5 denotesand functionality. various types of EMEH devices, their features, tures,The following specifications, section dimensions, in Table 5 anddenotes functionality. various types of EMEH devices, their fea- specifications,TableThe 5. following Electromagnetic dimensions, section energy-harvesting in and Table functionality. 5 denotes devices’ various specifications types of and EMEH figures. devices, their features,tures,Table specifications, specifications, 5. Electromagnetic dimensions, dimensions, energy-harvesting and and functionality. functionality. devices’ specifications Figures and figures. TableReferences 5. Electromagnetic Description/Functionality energy-harvesting devices’ specifications andFigures figures. Table 5. ElectromagneticReferences energy-harvestingDescription/Functionality devices’ specifications and figures.Figures TableTable 5. 5. Electromagnetic Electromagnetic energy-harvesting energy-harvesting devices’ devices’ specifications specifications and and figures. figures. References SeveralDescription/Functionality magnetic field designs for Figures References Description/Functionality Figures References Description/FunctionalitySeveralharvesting magnetic techniques field designshave been for Figures References SeveralDescription/Functionality magnetic field designs for proposedharvesting in techniquesprevious studies. have been The [147] Severalharvesting magnetic techniques field designs have for been Severalproposedpower magnetic line in sensor previous field (Protura) designs studies. foris Thean Several magnetic[147] harvesting fieldproposed designs techniques forin previous harvesting have studies. been The [147] harvestingpower line techniques sensor (Protura) have been is an techniques have beenavailable proposed item in that previous extracts energy proposedproposedavailablepower in in lineprevious itemprevious sensor that studies. extractsstudies.(Protura) The energy The is an [147] studies.[147][147] The power linefrom sensor electromagnetic (Protura) is an fields. powerpoweravailable fromline line sensor electromagnetic sensor item (Protura)that (Protura) extracts is fields. is an energyan Power line sensor (Protura). available itemavailable that extractsfrom item electromagnetic energy that extracts from energy fields. Power line sensor (Protura). electromagneticavailable item fields. that extracts energy Power line sensor (Protura). from electromagnetic fields. from electromagnetic fields. Power line sensor (Protura). It is a hybrid power-producing Power lineline sensorsensor (Protura).(Protura). methodIt is a forhybrid sensors. power-producing In this system, a It is a hybrid power-producing methodnumber for of sensors.coil-grappled In this magnets system, a It ismethod a hybrid for power-producing sensors. In this system, a Itjoined numberis a hybrid to aof cantilever coil-grappled power-producing are utilized magnets to It is a hybrid[70] power-producing methodnumber for sensors. of methodcoil-grappled In this for system, magnets a sensors. In thismethodproduce system,joined for to apowersensors. a number cantilever from In of this an are electromag-system, utilized a to [70] numberjoined of tocoil-grappled a cantilever magnetsare utilized to coil-grappled[70] magnets numberproducenetic joined generator.of topower coil-grappled a cantilever from With an the aremagnets electromag- system, joinedproduce to a cantilever power from are utilized an electromag- to [70] utilized[70] to producejoinedaroundnetic to power agenerator. cantilever25.45 from mW anWith arecan utilizedbethe obtained system, to [70] producenetic power generator. from an With electromag- the system, electromagneticproduce generator.aroundwith power With 25.45 a frequency from the mW system, an can electromag-of be 60 obtained Hz. around 25.45 mWneticnetic canaround generator. generator. bewith obtained 25.45 a frequency With WithmW with the can the a ofsystem, besystem, 60 obtained Hz. Electromagnetic hybrid energy harvester. around 25.45with mWa frequency can be obtained of 60 Hz. Electromagnetic hybrid energy harvester. frequencyaround of 6025.45 Hz. mW can be obtained Electromagnetic hybrid energy harvester. withwith a afrequency frequency of of 60 60 Hz. Hz. ElectromagneticElectromagnetic hybrid hybrid energy energyenergy harvester. harvester.harvester.

Electronics 2020, 9, x FOR PEER REVIEW 36 of 46

This model generates a power of 58 μW combined with a resistive load Electronics 2020, 9, x FOR PEER REVIEW 36 of 46 of 15 kΩ. In the form of electrical Electronics 2021, 10, 1108 25 of 35 power with a resonant frequency of 50 Hz, an acceleration of 0.59 m/s2, This model generates a power of 58 Electronics 2020, 9, x FOR PEER REVIEW [148] and a power density of 307 μW/m3, 36 of 46 μW combined with a resistive load the generator may deliver 30% of Electronics 2020, 9, x FOR PEER REVIEW of 15 kΩ. InTable the form 5. Cont. of electrical 35 of 45 the total input power to the load. power with a resonant frequency of References Description/FunctionalityThisThe model prototype generates geometry a power was ofset 58 Figures 50 Hz, an acceleration of 0.59 m/s2, withμW combined an applied with volume a resistive of 0.15 load cm3 [148] Thisand model a power generates density a power of 307 ofμW/m 58 3, Micro generator cantilever with magnet of 15 kΩ. In the form of electrical 3 μWand combined a component with a volume resistive of load 0.1 cm . scale. This model generatespowerthe generator with a power a resonant may of 58 deliverµ Wfrequency 30% of of of 15the k Ωtotal. In inputthe form power of electrical to the load. 2 combined with a resistiveIt50 is Hz, an electromagnetican load acceleration of 15 kΩ. Inofpiezoelectric the0.59 m/s , form of electricalpowerThe power with prototype a withresonant a geometry resonant frequency was of set 3 [148] andhybrid a power harvester’s density power of 307 dynamicμW/m , frequency of 50 Hz,50 Hz,with an accelerationan an acceleration applied volume of 0.59of 0.59 m/s of m/s0.152, 2, cm3 device.the generator On the maybasis deliver of the elemen-30% of Micro generator cantilever with magnet [148] and[148] a power and density a power of 307 densityµW/m of3 307, the μW/m3, 3 andthe a total component input power volume to theof 0.1 load. cm . scale. generator maythe delivertary generator mathematical 30% of may the deliver total model, input 30% the of pro- The prototype geometry was set power to the load.posed The prototype model was geometry designed was and ex- theIt totalis an inputelectromagnetic power to the piezoelectric load. 3 set with an appliedhibitedwith volume an with applied of the 0.15 simulation.volume cm3 and of a0.15 The cm ini- Micro generator cantilever with magnet [149] Thehybrid prototype harvester’s geometry power was dynamic set 3 componentandtial volume a results component of show 0.1 cm volumethat3. 46.2 ofμ W0.1 and cm . scale. withdevice. an applied On the volume basis ofof the0.15 elemen- cm3 3.6 μW output powers can be ob- and tarya component mathematical volume model, of 0.1 the cm pro-3. Micro generatorgenerator cantilevercantilever withwith magnetmagnet scale.scale. Ittained is an electromagneticby the portable systempiezoelectric from posed model was designed and ex- hybridelectromagnetic harvester’s and power piezoelectric dynamic It is an electromagneticIt ishibited an electromagnetic piezoelectricwith the simulation. piezoelectric hybrid The ini- [149] device.transducers, On the respectively, basis of the withelemen- an Prototype of a hybrid harvester. harvester’s powerhybrid dynamictial resultsharvester’s device. show powerOn that the 46.2 dynamic basis μW and of the elementarytary mathematicalexcitation mathematical frequency model, model, the of the5 Hz. pro- 3.6device. μW outputOn the powersbasis of canthe be ob- proposed model wasposed designed model was and designed exhibited andex- elementarytainedA similar by mathematical the axial–flux portable model, permanentsystem the from with the simulation.hibited The initial with resultsthe simulation. show that The [149] [149] magnets generator with eight poles µ proposedµelectromagnetic model was and designed piezoelectric and 46.2 W and 3.6 tialwithW results output a diameter show powers ofthat 5 can mm.46.2 be μTheW andcoil Prototype of a hybrid harvester. obtained byexhibited thetransducers, portable withsystem the respectively, simulation. from with The an was3.6 fabricatedμW output differently, powers can given be ob- the electromagnetic[149] initial and results piezoelectricexcitation show frequency that transducers, 46.2 ofμW 5 Hz.and fourtained layers by the of copperportable wound system around from respectively, with3.6 anA μ excitationsimilarW output axial–flux frequencypowers permanentcan of be theelectromagnetic stator. NdFeB andmagnets piezoelectric were uti- obtainedmagnets5 Hz. by generator the portable with system eight poles lizedtransducers, in the rotor. respectively, These fabrications with an Prototype of a hybrid harvester. withfrom a electromagnetic diameter of 5 mm. and The coil ini- resultedexcitation in a totalfrequency coil resistance of 5 Hz. of A similar axial–fluxwaspiezoelectric fabricated permanent differently,transducers, magnets given the PrototypePrototype ofof aa hybridhybrid harvester.harvester. generator with eight30A polesΩsimilar. When with axial–flux the a diameter rotor permanentwas of rotated respectively,four layers of with copper an excitation wound around 5 mm. The coil wasmagnetsusing fabricated a spindle, generator differently, given with an given eight air gap poles of [150] the stator.frequency NdFeB of magnets 5 Hz. were uti- the four layers ofwith copper1 mm, a diameter woundthe generator around of 5 mm. delivered the The coil a Alized similar in the axial rotor.–flux These permanent fabrications stator. NdFeB magnetswasmaximum fabricated were utilizedpower differently, of in 0.412 the rotor. mW given at the2.2 These fabricationsmagnetsresulted resulted generator in a in total a with total coil eight coil resistance poles of fourkrpm. layers The of authors copper alsowound indicated around resistance of 30 withΩ. When30 a Ω diameter. When the rotor the of 5wasrotor mm. rotated was The rotatedcoil thethe stator. device NdFeB later generatedmagnets were stator uti- [150] using a spindle,was givenusing fabricated an a airspindle, gapdifferently, of given 1 mm, angiven the air gapthe of Stator with filament windings shown in one [150] coilslized usingin the lowrotor. temperature These fabrications co-fired generator deliveredfour layers1 mm, a maximum of the copper generator powerwound delivered of around a layer. resultedceramic materials.in a total coil With resistance the use ofof 0.412 mW at 2.2themaximum krpm.stator. TheNdFeB power authors magnets of 0.412 also weremW at 2.2 this30 Ωtechnique. When the and rotor with was the rotated modifi- indicated the devicekrpm.utilized later generatedThe in theauthors rotor. stator also These coilsindicated using low temperatureusingcation co-fired a of spindle, the ceramic test given setup, materials. an the air output gap of [150] fabricationsthe device resulted later generated in a total coilstator Stator with filament windings shown in one With the use of thispower1 mm, technique levelsthe generator andreached with delivered 1.89 the mW ataStator with filament windings shown in one layer. resistancecoils using of 30low Ω .temperature When the rotor co-fired modification of themaximum test setup, power the output of 0.412 power mW at 2.2 layer. was rotated using13,325 a spindle, rpm. given levels reachedceramic 1.89krpm. mW The materials. at authors 13,325 rpm.With also indicatedthe use of [150] anAthis air generator gaptechnique of 1 with mm, and four the with generatormagnets, the modifi- with the device later generated stator Stator with filament windings shown in one deliveredacation volume of a thelengthmaximum test ofsetup, 1.1 power cm, the a outputof width A generator withcoils four magnets,using low with temperature a volume co-fired layer. 0.412of 0.9mW cm, at 2.2and krpm. a height The 0.85 authors cm. The length of 1.1 cm, a widthceramicpower of levels materials. 0.9 cm, reached and With a height1.89 the mW use atof outcomealso indicated of this the generator device later fromis 1V 0.85 cm. The outcomethis technique of this13,325 generator and rpm.with is the 1 V modifi- generatedan input stator range coils of 150 using mV lowat reso- from an input rangeAcation generator of 150of the mV with test at resonancefoursetup, magnets, the output withStator with filament windings shown in one layer. [151] frequency 106 Hztemperaturenance with an frequency acceleration co-fired 106 levelHzceramic with of an ac- [151]2 apower volume levels length reached of 1.1 1.89cm, amW width at 2.6 m/s . The generatormaterials.celeration attached Withlevel theof with 2.6 use m/s a of car2 this. The gen- of 0.9 cm, and13,325 a height rpm. 0.85 cm. The engine and from theerator system attached produces with a a peak car engine techniqueoutcome and of withthis generator the modification is 1V from power of 3.9 mW,of A whichthe generatorand test from can setup, be thewith obtained the system four output magnets, withproduces power a with a an input rangeµ of 150 mV at reso- peak powerapeak volume ofpower 157 length ofW. 3.9 of mW, 1.1 cm, which a width can levelsnance reached frequency 1.89 106mW Hz at with13,325 an ac-Electromagnetic generator powered by vibration. [151] ofbe 0.9 obtained cm, and with a height a peak 0.85 power cm. The of celeration levelrpm. of 2.6 m/s2. The gen- Electromagnetic generator powered by vi- outcome of this157 generator μW. is 1V from bration. anerator input attached range of with 150 a mV car atengine reso- nanceand fromfrequency the system 106 Hz produces with an ac-a [151] Electromagnetic generator powered by vi- celerationpeak power level of of3.9 2.6 mW, m/s which2. The gen-can bration. beerator obtained attached with with a peak a car power engine of and from the157 system μW. produces a peak power of 3.9 mW, which can Electromagnetic generator powered by vi- be obtained with a peak power of bration. 157 μW.

Electronics 2020, 9, x FOR PEER REVIEW 37 of 46

An electromagnetic generator (EG) Electronics 2021, 10, 1108 with discrete coil fabricated with sil- 26 of 35 Electronics 2020, 9, x FOR PEER REVIEW Anicon electromagnetic materials. The generatordimensions (EG) of 37 of 46 [152] withthe EG discrete are 525- coilμm-thick fabricated silicon with wa- sil- iconfer and materials. 25-μm-thick The dimensions copper wire. of Table 5. Cont. Electronics 2020, 9, x FOR PEER REVIEW [152] theThe EG EG are produces 525-μm-thick 0.38V withsilicon a fre-wa- 37 of 46

References Description/Functionalityfer and quency25-μm-thick of 6.4 copperkHz. wire. Figures An electromagnetic generator (EG) Diagram of silicon electromagnetic genera- The EG produces 0.38V with a fre- with discrete coil fabricated with sil- tor with discrete coil. quency of 6.4 kHz. icon materials. The dimensions of Diagram of silicon electromagnetic generator with discrete coil. [152] theAn electromagneticEG are 525-μm-thick generator silicon (EG) wa- An electromagneticwithThis generatorfer discretestructureand 25- (EG)μ coil m-thickcontains withfabricated discretecopper three with spring- wire. sil- coil fabricated withmassiconThe silicon EGmaterials.systems produces materials. and The the0.38V dimensions Theproof with masses a fre- of [152] dimensions of the EG are 525-quencyµm-thick of 6.4 siliconkHz. [152] Thistheare patternedEG structure are 525- withcontainsμm-thick double-layer three silicon spring- wa-spi- Diagram of silicon electromagnetic genera- wafer and 25-µm-thick copper wire. The EG massral-shapedfer and systems 25- aluminumμm-thick and the copper proofcoils with masses wire. di- produces 0.38V with a frequency of 6.4 kHz. tor with discrete coil. areThemensions patterned EG produces of 10with μm 0.38Vdouble-layer in width with anda fre- spi- 1

μral-shapedm in depth.quency aluminum On of the 6.4 other kHz.coils hand, with di-the Diagram of silicon electromagnetic genera- mensionsdimensions of 10of theμm MEMSin width EMEH and Diagram1 of silicon electromagnetic generator with This structure contains three spring- tor with discrete coil. [153] μchip,m in 3depth. mm in On diameter, the other and hand, 2 mm the discrete coil. mass systems and the proof masses This structure containsindimensions height, three are of spring-mass mounted the MEMS through EMEH a are patterned with double-layer spi- systems and[153] the proofsupportingchip, masses 3 mm are beamin patterneddiameter, on top withandof the 2 mmhar- Working principle illustration of electro- ral-shaped aluminum coils with di- double-layer spiral-shapedThisvesterin height, structure array. aluminum are The contains mounted downside coils three through with tospring- such a magnetic MEMS energy harvester (a); setup µ mensions of 10 μµm in width and 1 dimensions of 10 mmasssupportinga inscheme widthsystems is,and beam itand 1is complicated onthem intop proof depth. of the masses and har- ofWorking electromagnetic principle MEMS illustration energy of harvester electro- On the other hand,μ them in dimensions depth. On ofthe the other MEMS hand, the arecostly.vester patterned Therefore,array. Thewith properdownside double-layer selection to such spi- of magnetic MEMS energy(b). harvester (a); setup EMEH chip, 3 mm in diameter, and 2 mm in [153] materialsral-shapeddimensions at aluminum low of costthe MEMSis coils the best withEMEH solu- di- of electromagnetic MEMS energy harvester height, are mounteda through scheme ais, supporting it is complicated beam and [153] mensionschip, 3 mm of in 10 diameter, μm in width and and2 mm 1 on top of the harvestercostly.tion array. Therefore,to overcome The downside proper this problem. selection to of (b). in height, are mounted through a such a scheme is,μmaterials itm is in complicated depth. at low On cost the and otheris costly. the hand,best solu- the supporting beam on top of the har- Working principle illustration of electro- Therefore, proper selectiondimensionstion to overcome of materials of the MEMSthis at problem. low EMEH Working principle illustration of electromagnetic vester array. The downside to such magnetic MEMS energy harvester (a); setup cost is[153] the best solutionchip, 3 mm to overcomein diameter, this and 2 mm MEMS energy harvester (a); setup of Ita schemeis a meso is, scale it is complicatedvibration-based and of electromagnetic MEMS energy harvester problem.in height, are mounted through a electromagnetic MEMS energy harvester (b ). EMEH devices with double-layer costly.supporting Therefore, beam properon top ofselection the har- of Working principle (illustrationb). of electro- spring FR4 materials. The basic materialsvesterIt is a array. meso at low scaleThe cost downside vibration-based is the best to such solu- magnetic MEMS energy harvester (a); setup principle of this vibration trans- It is a meso scale vibration-basedaEMEH tionscheme to devices overcome is, it EMEHis with complicated this double-layer devices problem. and of electromagnetic MEMS energy harvester ducer is one side fixed and another with double-layercostly. springspring Therefore, FR4FR4 materials.materials. proper The Theselection basic of (b). sideprinciple open free of thisto move vibration and generate trans- basic principle[154,155] of thismaterials vibration at low transducer cost is the is onebest solu- side ﬁxed and anotherelectricalducer side is onecharge open side freevia fixed pressure. toanother moveand There- Ittion is a to meso overcome scale vibration-based this problem. [154,155] and generate electricalsidefore, open the charge advantage free to via move pressure. of suchand devices [154,155] EMEH devices with double-layer Therefore, the advantageelectricalis that they of charge such can devices runvia low-power is that sen- (a) Measurement setup of MEMS electro- spring FR4 materials. The basic they can run low-powerfore,sors thatthe advantageare sensors currently that of are suchpowered by magnetic vibration energy harvester, (b) currently powered byprinciple the battery; of this such vibration devices trans- theisIt that battery;is a theymeso suchcan scale run devices vibration-based low-power are suitable sen- Meso-scale prototype of Electromagnetic vi- are suitable forducer the IoTis one applications. side fixed and another(a) generate Measurement setup of MEMS electromagnetic sorsEMEH thatfor devices theare IoTcurrently with applications. double-layer powered by bration energy harvester. side open free to move and generatevibration energy harvester, (b) Meso-scale prototype [154,155] thespring battery; FR4 such materials. devices The are basicsuitable of Meso-scale Electromagnetic prototype vibration of Electromagnetic energy harvester. vi- electrical charge via pressure. There- (a) Measurement setup of MEMS electro- principlefor the of IoT this applications. vibration trans- devic bration energy harvester. It is a nonlinear vibration-based magnetic vibration energy harvester, (b) ducerfore, the is one advantage side fixed of suchand another devices es EMEH structure. The model consists (a) Measurement setup of MEMS electro- It is a nonlinear vibration-basedsideis that open they free can EMEH to run movepr low-power structure.essu andre generate sen- [154,155] of a combination .of There Polyethylene- magnetic vibration energy harvester, (b) The model consistselectricalsorsIt is that ofa nonlinear acharge combinationare currently via vibration-based pressure. ofpowered There- by terephthalate film, magnets, copper Meso-scale prototype of Electromagnetic vi- Polyethylene terephthalateEMEHthefore, battery; the structure.ﬁlm, advantage such magnets, devicesThe of model such copper are devices consistssuitable coils, and aluminum shells. The ad- bration energy harvester. coils, and aluminumisof that shells.a combination forthey the The can IoT advantagerun applications. of low-power Polyethylene of sen- (a) Measurement setup of MEMS electro- [156,157] this model is the upperterephthalatevantage and of lower this film, model copper magnets, is coils the copperupper [156,157] sors that are currently powered by magnetic vibration energy harvester, (b) are connected in series to gain a maximum coils,and and lower aluminum copper coils shells. are The con- ad- Meso-scale prototype of Electromagnetic vi- voltage outcome.the TheIt battery; is restriction a nonlinear such of devices thisvibration-based model are suitable nectedvantage in of series this modelto gain isa themaximum upper bration energy harvester. was[156,157] the production EMEHfor ofstructure. the a cost-effective IoT applications.The model consists voltageand lower outcome. copper The coils restriction are con- of combination ofof composite a combination materials. of Polyethylene nectedthis model in series was tothe gain production a maximum of a Schematic of the double-clamped beam terephthalate film, magnets, copper Schematic of the double-clamped beam voltagecost-effectiveIt is a nonlinear outcome. combination vibration-basedThe restriction of com- of electromagnetic vibration energy harvester. coils, and aluminum shells. The ad- electromagnetic vibration energy harvester. EMEHthis model structure.posite was thematerials. The production model consists of a vantage of this model is the upper Schematic of the double-clamped beam [156,157] cost-effectiveof a combination combination of Polyethylene of com- and lower copper coils are con- electromagnetic vibration energy harvester. terephthalateposite film, materials. magnets, copper coils,nected and in aluminumseries to gain shells. a maximum The ad- voltage outcome. The restriction of vantage of this model is the upper [156,157] this model was the production of a and lower copper coils are con- Schematic of the double-clamped beam cost-effective combination of com- nected in series to gain a maximum electromagnetic vibration energy harvester. voltage outcome.posite materials. The restriction of

this model was the production of a Schematic of the double-clamped beam cost-effective combination of com- electromagnetic vibration energy harvester. posite materials.

Electronics 2021, 10, 1108 27 of 35

Electronics 2020, 9, x FOR PEER REVIEW 38 of 46

It is an EMEHTable device, 5. Cont. which consists of aluminum, pure iron, and References Description/Functionalitycopper winding in the top side, and Figures It is an EMEHcoil device, bobbins, which copper consists windings, of and aluminum, pure iron,aluminum and copper in windingthe bottom in theside re- top side, and coil bobbins,spectively. copper The windings, dimension and of this aluminum in the bottommodel side is 3.7 respectively. mm copper The winding dimension of thispitch, model which is 3.7 matches mm copper the width of winding pitch, whichthe magnet. matches The the widthminimum of the distance magnet. The minimumbetween distance the magnet between array the and the magnet array and the coil is 0.2 mm, which is the coil is 0.2 mm, which is the alumi- aluminum housing’s minimum thickness. The num housing’s minimum thickness. [41] schematic diagram and fabrication of the electromagnetic[41] energyThe schematic harvester. diagram The dimension and fabrication of the electromagnetic energy of each magnet after fabrication is 3.18 × 12.7 × 3 harvester.× × The dimension3 of each 1.59 mm and 12.7 12.7 0.2 mm . The ElectromagneticElectromagnetic energy energy harvester harvester fabrication fabrica- of the restriction of this modelmagnet manufacturing after fabrication process, is 3.18 × tion(unit: of the mm): (unit: ( amm):) top side;(a) top (b side;) bottom. (b) bot- however, is time12.7 consuming × 1.59 mm with3 and low 12.7 output × 12.7 × 0.2 tom. power generation.mm Therefore,3. The restriction the optimal of this model materials and themanufacturing required dimensions process, of however, the is systems can be fine-tuned to overcome this time consuming with low output problem optimization technique. power generation. Therefore, the optimal materials and the required 6. Issues anddimensions Challenges of the systems can be The mainfine-tuned goal of to EMEH overcome for autonomousthis problem sensors plays an important role in powering up the devices, however,optimization it technique. has some problems such as availability of power sources, storage, energy dissipation, large size, converter for boost up power, and cost [141,158]. 6.Modifications Issues and Challenges are required to make less energy dissipate, small size with low cost sensors, andThe overall main management goal of EMEH [159 for,160 autonomous]. A small amount sensors of plays power an forimportant low-energy role electronicsin powering is upgiven the bydevices, the EMEH however, system. it has some problems such as availability of power sources, storage, energyThe power dissipation, source large comprises size, converter a combination for boost of voltageup power, and and current, cost [141,158]. frequency Modi- pro- ficationsfile, and are harmonics. required to There make are less a energy number dissi of issuespate, small that maysize with arise low with cost environmental sensors, and overallambient management sources, such [159,160]. as distortion A small of amount voltage of due power to current for low-energy harmonics, electronics flicker, is voltage given bysag, the and EMEH voltage system. surge [161,162]. Moreover, because of the geometric shapes and proper materialThe power selection source problem comprises of the a EMEH combination devices, of morevoltage research and current, is necessary frequency to improve profile, andthe efficiencyharmonics. power There [are163 –a165 number]. Many of issues studies that are may focused arise on with increasing environmental the efficiency ambient of sources,EMEH systemssuch as todistortion improve of power voltage electronic due to current circuits harmonics, such as the flicker, converter, voltage inverter, sag, and and voltagecontroller surge [96 ,[161,162].110,111,113 Moreover,]. because of the geometric shapes and proper material selectionGenerally, problem standalone of the EMEH micro devices, EMEH systemsmore research are useful is necessary in areas where to improve electricity the is effi- not ciencyavailable, power especially [163–165]. in remote Many areas. studies There are arefocu sosed many on methodsincreasing and the materials, efficiency for of example, EMEH systemsphotovoltaic, to improve thermoelectric, power electronic piezoelectric, circuits radio such frequency,as the converter, electrostatic, inverter, and and capacitive, control- which can be used for micro EMEH systems [39,144,166]. Many methods have been ler [96,110,111,113]. developed to extract energy from the different resources available, such as wind, vibration, Generally, standalone micro EMEH systems are useful in areas where electricity is solar, chemical, thermal, wave, radiofrequency, biological, etc. [15,57,95]. However, it has not available, especially in remote areas. There are so many methods and materials, for some issues such as generating low power, instability, fluctuation, harmonics, and it is example, photovoltaic, thermoelectric, piezoelectric, radio frequency, electrostatic, and ca- costly. Further researches are required to create stable, controlled, low-cost maintenance, pacitive, which can be used for micro EMEH systems [39,144,166]. Many methods have and long-term sustainability. been developed to extract energy from the different resources available, such as wind, Currently, WSNs and the IoT are getting more and more attention for low-power vibration, solar, chemical, thermal, wave, radiofrequency, biological, etc. [15,57,95]. How- EMEH devices [54,167]. WSNs and IoT are the favorites in industrial, remote areas as well ever, it has some issues such as generating low power, instability, fluctuation, harmonics, as in commercial applications, because it has modern and fast communication processors and it is costly. Further researches are required to create stable, controlled, low-cost as well as low power usage [26,55,168]. This network can be used in a poor and harsh maintenance, and long-term sustainability. environment where the wired system is not possible. IoT is a physical network that assignsCurrently, IP addresses WSNs and to devices the IoT inare the getting network more for and internet more attention connectivity, for low-power and the EMEH output devicesinformation [54,167]. of theseWSNs devices and IoT canare bethe sentfavorites to any in placeindustrial, in the remote world. areas The as drawback well as in ofcom- the mercialWSNs application,applications, however,because it ishas that modern is far fromand fast the communicatio power line andn processors those devices as well powered as low power usage [26,55,168]. This network can be used in a poor and harsh environment where the wired system is not possible. IoT is a physical network that assigns IP addresses to devices in the network for internet connectivity, and the output information of these devices can be

Electronics 2021, 10, 1108 28 of 35

by the battery currently. However, the battery life is limited, it must be recharged regularly, and the cost of replacing the battery is typically high.

7. Conclusions and Recommendations This paper presented a comprehensive review of EMEH systems for ATS, giving a special focus on recent technological developments and existing barriers. There are various applications based on EMEH, such as WSNs, wireless building automation, battery-less networks, low-power switches, automotive, low power electronics devices, IoT, consumer electronics, and bio-medical that are presented in this study. It is necessary to achieve the ultimate purpose of avoiding replacing batteries or providing ATS with free perpetual energy. In this system, batteries are not used, thereby reducing the expenditure for buying batteries and the labor cost required for battery replacement. The batteries are used to power the nodes of the sensors but having a limited life, which restricts the performance and application of WSNs. To overcome these problems and to make the sensors self- powered, the best option is harvesting energy from the ambient energy resources. Finally, harvesting energy from ambient energy resources offers several advantages, including the supply of potential energy to common low-power devices such as motion sensors. Many past research works are summarized along with the numerous approaches that are acceptable for the research based on the literature survey. The review helps in realizing the basics of geometrical dimensional outcomes, parameter values, and scalability to model EMEH devices, which is suitable for low power electronic devices. The phenomenon of devices, such as the low power harvesting interface circuit methods, converter, and controller technique, is also presented in this review. Furthermore, this review indicates the numerous types of power electronic topologies for the betterment of EMEH. The study outlines a comprehensive survey of power electronic circuits, converters, controllers, and identified implementation in low-power applications to associate their significant contribution in the improvement. The conduction and switching losses of an AC-DC converter are minimal, but the power factor and harmonic distortion are high. A DC-DC converter has a fast response time and high performance, but it suffers from high switching loss. Finally, various methods are suggested in this study to overcome the limitations of the controller, converter, and low-power electronics devices for EMEH application. There are some recommendations for the improvement of the EMEH system. A power electronic circuit and converter, inverter, and controller have been reported in this review, such as: • More studies should be carried to improve energy density, power efficiency, and low resonance frequency for EMEH deceives. • A switching device with the PWM technique can control the power in AC-DC converters but reduces the power factor and adds harmonics, which can be avoided by a low pass filter and PWM switch. Whereas, in a DC-DC converter for power factor improvement, a power factor correction capacitor is a good option. • With regards to gaining accurate system performance and market acceptance, security, flexibility, long life, and stability problems of the EH transducer need to be addressed. • Optimization techniques can be utilized to get the optimal parameter geometry shape of the EMEH devices such as length, width, and thickness. • An intelligent real-time interface controller board can be performed to implement the EMEH circuit prototype for the robust, smart, and efficient converter, controller, and inverter. • It is important to address a capable energy storage scheme such as supercapacitor charging/discharging, protection, reliability, scale, expense, life cycle, and overall management. • Further research on the power, safety, control interface, energy management, and features of an advanced EMEH device are needed to improve the efficiency of energy storage in EH applications. Electronics 2021, 10, 1108 29 of 35

Author Contributions: Conceptualization, methodology, original draft writing, visualization, editing and funding acquisition. M.R.S., M.H.M.S.; J.L.O.; and J.V. contributed by writing of sections, reviewing, visualization and editing. All authors have read and agreed to the published version of the manuscript. Funding: Universidad Antonio de Nebrija for funding under Grant Code the Comunidad de Madrid [grant SEGVAUTO 4.0-CM-P2018EEMT-4362] and the Agencia Estatal de Investigación [grant RETOS 2018-RTI2018-095923-B-C22]. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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