Design of Kinetic-Energy Harvesting Floors "2279

Article † Design of Kinetic-Energy Harvesting Floors

Thitima Jintanawan 1, Gridsada Phanomchoeng 1,2,*, Surapong Suwankawin 3 , Phatsakorn Kreepoke 1, Pimsalisa Chetchatree 1 and Chanut U-viengchai 3 1 Department of Mechanical Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [email protected] (T.J.); [email protected] (P.K.); [email protected] (P.C.) 2 Smart Mobility Research Unit, Chulalongkorn University, Bangkok 10300, Thailand 3 Department of Electrical Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [email protected] (S.S.); [email protected] (C.U.-v.) * Correspondence: [email protected]; Tel.: +66-2-218-6630 This paper is an extended and revised article presented at the International Conference on Sustainable † Energy and Green Technology 2019 (SEGT 2019) on 11–14 December 2019 in Bangkok, Thailand.

Received: 18 September 2020; Accepted: 12 October 2020; Published: 16 October 2020

Abstract: Alternative energy generated from people’s footsteps in a crowded area is sufficient to power smart electronic devices with low consumption. This paper aims to present the development of an energy harvesting floor—called Genpath—using a rotational electromagnetic (EM) technique to generate electricity from human footsteps. The dynamic models of the electro-mechanical systems were developed using MATLAB®/Simulink to predict the energy performances of Genpath and help fine-tune the design parameters. The system in Genpath comprises two main parts: the EM generator and the Power Management and Storage (PMS) circuit. For the EM generator, the conversion mechanism for linear translation to rotation was designed by using the rack-pinion and lead-screw mechanism. Based on the simulation analysis, the averaged energy of the lead-screw model is greater than that of the rack-pinion model. Thus, prototype-II of Genpath with 12-V-DC generator, lead-screw mechanism was recently built. It shows better performance when compared to the previous prototype-I of Genpath with 24-V-DC-generator, rack-pinion mechanism. Both prototypes have an allowable displacement of 15 mm. The Genpath prototype-II produces an average energy of up to 702 mJ (or average power of 520 mW) per footstep. The energy provided by Genpath prototype-II is increased by approximately 184% when compared to that of the prototype-I. The efficiency of the EM-generator system is ~26% based on the 2-W power generation from the heel strike of a human’s walk in one step. Then, the PMS circuit was developed to harvest energy into the batteries and to supply the other part to specific loads. The experiment showed that the designed PMS circuit has the overall efficiency of 74.72%. The benefit of the design system is for a lot of applications, such as a wireless sensor and Internet of Thing applications.

Keywords: energy harvesting; electromagnetic generator; energy ﬂoor tile; power management system; footstep energy harvesting; piezoelectric; energy harvesting paver

1. Introduction The smart internet of things (IoT) has been brought to the world’s attention lately. The smart IoT-devices, e.g., Radio-frequency identification (RFID) sensor, Global Positioning System (GPS) tracking, etc., are widely used in the crowded areas such as airports and public stations, commercial buildings, and department stores [1,2]. Seeking an alternative energy for such devices, it was noticeable that a large population can generate power from their footsteps, grabbing the authors’ attention. Therefore, the ultimate goal was to develop an affordable Vibration Energy Harvesting (VEH) system—called Genpath—the smart floor capable of conversing kinetic energy from thousands

Energies 2020, 13, 5419; doi:10.3390/en13205419 www.mdpi.com/journal/energies Energies 2020, 13, 5419 2 of 19 of footsteps into electrical energy. Genpath can be installed in places with a crowded population for harvesting energy. Energy harvesting from human motions is an interesting and applicable issue, thanks to the ultra-low power consumption of electronic devices lately [3]. Focused on walking, the energy produced by the heel strike of a person’s walk is 1–5 J or 2–20 W per step [4]. There are various commercial products which harvest energy from people’s walks, such as energy storage shoes [5] and the energy floor [6–11]. Pavegen and Energy Floors have produced a commercial system that generates power from footsteps [6,7]. The Pavegen system, using electromagnetic generators, can produce 2 to 4 joules, or around 5 watts of power of off-grid electrical energy per step. The Energy Floors focuses on harvesting energy from humans dancing and playing games. Dutch Railways built a novel phone charger for Utrecht Central Station using a swing set called Play for Power [8]. The system turns kinetic energy from the swings into power dispensed through charging cables. However, few technical details of those products are published thus far. Kinetic energy, among practical energy-harvesting sources, comes in various forms, e.g., seismic noise, vibration of rotating machinery, motions of vehicles and humans, etc. Vibrational energy harvesting (VEH) is the concept of converting kinetic energies present in the environment into electrical energy. There are two main techniques of kinetic-to-electrical energy conversion applicable to the design of the VEH floor: i.e., piezoelectric generators and electromagnetic generators [12]. The piezoelectric generator produces an electrical charge and energy when deformed under mechanical stress. Piezoelectric generators can provide high voltage levels up to several volts. With its compact size, the piezoelectric generator gives high-power density per unit of volume [13]. The electromagnetic (EM) generator generates an electromotive force (EMF) and thus energy when the permanent magnet relatively moves through the coil. Both techniques of VEH discussed are suitable for generating energy for low-power electronic instruments. However, the piezoelectric generator effectively gives out maximum energy at around its natural frequencies, i.e., higher than 200 Hz, whereas the bandwidth of human motion is between 1–10 Hz. In contrast, the electromagnetic generator is effective in the frequency range of 2–20 Hz. Hence, it is more suitable for harvesting energy from human movements. In addition, the electromagnetic generators also yield higher power density and their costs are much lower than the piezoelectric generators [14]. For the piezoelectric generator, it needs a structural design not only to optimize the power density but also to protect the piezo element itself due to its fragility, while the structural design of the EM generator is much simpler for the application of a VEH floor. There are also two types of the EM generator in terms of energy-conversion mechanisms: linear oscillation to electrical energy and rotation to electrical energy [15]. The mechanism of linear oscillation conversion is simpler, but it requires the larger amplitude of excitation to produce electricity, whereas human pedals are random in vibration with low amplitude. Consequently, the rotational EM generator was chosen for the VEH floor to convert kinetic energy from people’s footsteps into electrical energy. The objective of the paper is to design a simple but efficient VEH floor embedded with the rotational electromagnetic (EM) generator. To achieve this purpose, the dynamic models of the electro-mechanical systems were developed using MATLAB®/Simulink for predicting the energy performances of the VEH floors and fine-tuning the design parameters. The entire system consists of two main parts of (1) the EM generator, including the translation-to-rotation conversion mechanism, and (2) the Power Management and Storage (PMS) circuit. For simplicity, a direct-current (DC) generator was used in the design to produce electricity. The rack-pinion and lead-screw mechanisms were adopted to converse a linear motion from a human’s pedal to a rotation of the generator’s rotor. The PMS circuit with extra low energy consumption was designed to simultaneously convert and store electrical energy. The paper is organized into the following sections. In Section2, the design of each sub-system is described in detail. Then, the installation and demonstration of application are presented in Section3. Finally, the conclusion is stated in Section4. Energies 2020, 13, 5419 3 of 19

2.Energies Design 2020 of, 13 Subsystems, x FOR PEER REVIEW 3 of 19

The development of the VEH floor floor—called—called Genpath—capableGenpath—capable of harvesting kinetic energy from people’s footsteps footsteps and and converting converting it to it electrical to electrical energy energy is presented. is present In theed process. In the of process energy generation, of energy thegeneration, rotational the EM rotational generator EM is deployedgenerator asis deployed it is independent as it is independent from the resonant from frequencythe resonant and frequency achieves higherand achiev energyes higher density energy when compareddensity when to the compared linear EM to generatorsthe linear EM [15]. generators Figure1 shows [15]. Figure the diagram 1 shows of the completediagram of system the complete in Genpath. system In Figurein Genpath.1, when In the Figure force 1, from when a footstepthe force is from applied a footstep on the is floor-tile, applied theon the mechanism floor-tile, for the a mechanism movement-converter for a movement changes-converter the translation changes of thethe floor-tiletranslation to theof the rotation floor- oftile the to EMthe rotation generator of tothe induce EM generato voltage.r Withto induce the connected voltage. With PMS the circuit, connected the electrical PMS circuit, voltage the and electrical power generatedvoltage and by power the EM generated generator by is the processed. EM generator The harvested is processed power. The is harvested then stored power in the is rechargeable then stored batteriesin the rechargeable so it can be supplied batteries to so the it smart can beIoT-devices supplied with to thelow smartenergy IoT consumption.-devices with Two low main energ parts,y i.e.,consumption. (1) the system Two of main EM generator, parts, i.e., including (1) the system the movement-converter of EM generator, and including (2) the the system movement of PMS- circuit,converter are and presented (2) the insystem detail of in PM theS followingcircuit, are subsections. presented in detail in the following subsections.

Figure 1. Genpath Concept Diagram. Figure 1. Genpath Concept Diagram. 2.1. The System of EM Generator 2.1. The System of EM Generator 2.1.1. Conceptual Design 2.1.1. Conceptual Design Figure2 shows the two proposed designs of the generator system in Genpath, with the only differenceFigure in 2 theshow mechanismss the two proposed for the movement designs of converter. the generator A set system of Genpath in Genpath, comprises with a floor-tilethe only blockdifference embedded in the withmechanisms the translation-to-rotation for the movement conversion converter. mechanism A set of Genpath which iscomp connectedrises a tofloor the- DCtile generatorblock embedd anded the with PMS the circuit translation board.- Theto-rota entiretion dimensionconversion is mechanism 40 40 10 which cm3 withis connected the maximum to the × × allowableDC generator displacement and the PM ofS circuit 15–20 mm.board. Two The typesentire ofdimension mechanisms, is 40  rack 40  pinion 10 cm3 and with lead the maximum screw, are usedallowable to convert displacement the translation of 15–20 from mm. aTwo footstep types toof themechanisms rotation of, rack the pinion generator. and lead For thescrew, rack-pinion are used mechanismto convert the in Figure translation2a, the from pinion a connected footstep to to the the floor-tile rotation drivesof the the generator. DC-generator For the shaft rack through-pinion themechanism additional in Figure pinion 2 gearsa, the whichpinion helpconnected transform to the low-speed floor-tile drives power the to DC high-speed-generator power. shaft through For the lead-screwthe additional mechanism pinion gear in Figures which2b, help the nuttransform fixed to low the-speed floor-tile’s power center to high moves-speed up andpower. down For and the driveslead-screw thelead mechanism screw to in rotate Figure about 2b, the its axis.nut fixed The setto the of bevelfloor-tile’s gears center transmits move thes up rotation and down from and the leaddrives screw the lead to the screw DC generatorto rotate about and changes its axis. the The direction set of bevel of rotation gears transmit by 90◦.s The the rotation rotation of from the DCthe generatorlead screw in to both the DC designs generator then inducesand change thes voltage. the direction The springsof rotation with by the 90 maximum. The rotation displacement of the DC ofgenerator 15–20 mm in both connected designs to then the induces four corners the voltage of the. blockThe springs help restore with the the maximum top floor-tile displacement back to the of equilibrium15–20 mm connected position. Considering to the four thecorners limits of of the the block dimension help restoreand the the displacement, top floor-ti thus,le back the to small the sizeequilibrium of 12/24-V-DC position motor. Considering was decided the forlimits the of DC the generator. dimension and the displacement, thus, the small size of 12/24-V-DC motor was decided for the DC generator.

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Energies 2020, 13, x FOR PEER REVIEW 4 of 19 Energies Energies2020 2020, ,13 13,, 5419 x FOR PEER REVIEW 4 of4 19 of 19

(a) Rack-Pinion Design (b) Lead-Screw Design

((aa)) RackRack-PinionFigure DesignDesign 2. (a) Rack -Pinion Design; (b) Lead-(Screwb()b Lead) Lead Design-Screw-Screw. Design Design

2.1.2. Analysis Figure 2. 2. ((aa)) RackRack Rack-Pinion--PinionPinion Design Design Design;; (;b ( )b Lead) Lead Lead-Screw-Screw-Screw Design Design.Design. .

2.1.2.2.1.2. ToAnalysis Analysis predict the energy performances of the EM-generator designs, the dynamic models of the two electro-mechanical systems, shown in Figure 2, were developed using MATLAB/Simulink. ToTo predictpredictpredict thethethe energyenergy performancesperformances of ofof the thethe EM EM-generatorEM-generator-generator designs, designs, the the dynamic dynamic models models of the of the Figure 3a,b show the physical models corresponding to the two systems in Figure 2a,b, respectively.® twotwo electro-mechanical electro-mechanical systems,systems, shown shown in in Figure Figure 2,2, were developed developed using using M MATLABATLAB /Simulink. /Simulink. twoIn Figure electro 3a-mechanical, the system systemsconsists, ofshown the elements in Figure of rack,2, were pinion developed and gear using on the M mechanicalATLAB /Simulink. side, and FigureFigure3 a,b3a,b show show the the physicalphysical models corresponding to to the the two two systems systems in inFigure Figure 2a2,ba,b,, respectively. respectively. Figurethe DC 3 agenerator,b show the (with physical its own models resistance corresponding, 푅 , and inductance to the two ,systems 퐿) connected in Figure to the 2a, bload, respectively. 푅 on the InIn Figure Figure3 a,3a the, the system systemconsists consists ofof the elements퐺 of of rack, rack, pinion pinion and and gear gear on on the the mechanical mechanical side, side,퐿 and and Inelectrical Figure 3a side., the Contrastively, system consists the of elementsthe elements of rack, of rack, pinion pinion and and gear gear are on replaced the mechanical by the side, nut, and lead thethe DC DC generatorgenerator (with its own own resistance resistance,, 푅R퐺G, ,and and inductance inductance,, 퐿)L connected) connected to tothe the load load 푅퐿 RonL onthe the thescrew, DC andgenerator bevel gears (with for its the own system resistance in Figure, 푅퐺, 3b and. The inductance footstep force, 퐿) connected 퐹(푡) is modeled to the load as the 푅 퐿arbitrary on the electrical side. Contrastively, the elements of rack, pinion and gear are replaced by the nut, lead electrical side. side. Contrastively, Contrastively, the the elements elements of ofrack, rack, pinion pinion and and gear gear are replaced are replaced by the by nut, the lead nut, screw, lead functionscrew, and repor bevelted gears in [9 for] and the presentedsystem in Figure in Figure 3b. The 4. The footstep spring force with 퐹 (a푡 )maximum is modeled compression as the arbitrary of 15– andscrew, bevel and gears bevel for gears the systemfor the system in Figure in3 Figureb. The 3b footstep. The footstep force F (forcet) is modeled퐹(푡) is modeled as the arbitrary as the arbitrary function 20function mm provides reported the in [restoring9] and presented force 퐹 푠in to Figure restore 4. theThe floorspring-tile with back a maximumto the equilibrium compression position. of 15– The reportedfunction repor in [9]ted and in presented [9] and presented in Figure in4. Figure The spring 4. The with spring a maximum with a maximum compression compression of 15–20 of mm 15– dynamic20 mm provides equations the restoring governing force the 퐹 electro to restore-mechanical the floor models-tile back of to both the equilibrium designs are position. formulated The as provides20 mm provides the restoring the restoring force Fs forceto restore 퐹푠 to the restore ﬂoor-tile the floor back- totile the back equilibrium to the equilibrium position. position. The dynamic The followsdynamic. equations governing the electro푠 -mechanical models of both designs are formulated as equationsdynamic equations governing governing the electro-mechanical the electro-mechanical models of both models designs of both are formulated designs are as follows.formulated as follows. follows.

(a) Rack and Pinion Model (b) Lead-Screw Model (a) Rack and Pinion Model (b) Lead-Screw Model Figure 3. Physical models: (a) Rack and Pinion Model; (b) Lead-Screw Model. (a) Rack and Pinion Model (b) Lead-Screw Model FigureFigure 3. Physical models: models: (a (a) )Rack Rack and and Pinion Pinion Model Model;; (b ()b Lead) Lead-Screw-Screw Model Model.. Figure 3. Physical models: (a) Rack and Pinion Model; (b) Lead-Screw Model.

Figure 4. Input Footstep Force. FigureFigure 4. 4. Input Input Footstep Footstep Force Force. .

Figure 4. Input Footstep Force.

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For the electrical side in Figure3, Kirchho ﬀ’s voltage law yields Energies 2020, 13, x FOR PEER REVIEW 5 of 19 VL + V + VRL V = 0, (1) RG − G For the electrical side in Figure 3, Kirchhoff’s voltage law yields where VG is the back emf of the generator; i.e., VG = KtωG where Kt is the back emf (torque) constant. 푉퐿 + 푉푅퐺 + 푉푅퐿 − 푉퐺 = 0, (1) In addition, VL, VRG and VRL are the voltages across the generator’s inductor, generator’s resistor and whereload’s resistor,푉퐺 is the respectively. back emf With of the the generator; voltage-current i.e., 푉퐺 relations,= 퐾푡휔퐺 (1)where becomes 퐾푡 is the back emf (torque) constant. In addition, 푉 , 푉 and 푉 are the voltages across the generator’s inductor, generator’s 퐿 푅퐺 . 푅퐿 R + R K resistor and load’s resistor, respectively.i + WithG theL i voltaget ω-current= 0, relations, (1) becomes (2) L − L G 푅퐺+푅퐿 퐾푡 푖̇ + ( ) 푖 − ( ) 휔퐺 = 0, (2) where i is the current, Kt is the back emf (torque)퐿 constant,퐿 ωG is the generator rotational speed, L is the inductance of the generator, R is the resistance of the generator and R is the resistance of the load. where 푖 is the current, 퐾푡 is theG back emf (torque) constant, 휔퐺 is the generatorL rotational speed, 퐿 Equation (2) is the diﬀerential equation governing the armature winding of the generator. is the inductance of the generator, 푅퐺 is the resistance of the generator and 푅퐿 is the resistance of the load.From Eq theuation free body(2) is diagramthe differential (FBD) ofequation the mechanical governing system the armature with the rackwinding and pinionof the generator. in Figure5 , Newton’sFrom secondthe free law body and diagram the law (FBD) of angular of the momentum mechanical describe system with the translation the rack and of thepinion rack, in and Figure the 5,rotations Newton’s of thesecond pinion law and and the the generator law of angular rotor, respectively, momentum asdescribe the translation of the rack, and the rotations of the pinion and the generator.. rotor, respectively, as mx = F(t) Fr Fs, (3) 푚푥̈ = 퐹(푡) − 퐹− − −퐹 , (3) .. 푟 푠 . x Jpωp = J푥p̈ = (Fr fr)r2, (4) 퐽 휔̇ = 퐽 r=2 (퐹 − 푓−)푟 , (4) 푝 푝 푝 푟 푟 푟 2 2 .. . x JGωG = J푥G̈ = ( fr τG)r1, (5) 퐽퐺휔̇ 퐺 = 퐽퐺 =r1 (푓푟 − 휏−퐺)푟1, (5) 푟1 where m is the mass of the plate and rack, and Jp and JG are the moments of inertia of the pinion and where 푚 is the mass of the plate and rack, and 퐽푝 and 퐽퐺 are the moments of inertia of the pinion the gear. x is the displacement of the rack, ωp is the angular velocity of the pinion, ωG is the angular and the gear. 푥 is the displacement of the rack, 휔푝 is the angular velocity of the pinion, 휔퐺 is the velocity of the gear. In (3)–(5), F(t) is the input force, Fs is the restoring spring and damper force, Fr angular velocity of the gear. In (3)–(5), 퐹(푡) is the input force, 퐹푠 is the restoring spring and damper is the friction force between the rack and pinion, fr is the friction force between the pinion and gear. force, 퐹푟 is the friction force between the rack and pinion, 푓푟 is the friction force between the pinion In addition, τG is the electromagnetic torque of the DC generator, where τG = Kti, and r1 and r2 are the and gear. In addition, 휏 is the electromagnetic torque of the DC generator, where 휏 = 퐾 푖, and 푟 radius of the gear and pinion,퐺 respectively. 퐺 푡 1 and 푟2 are the radius of the gear and pinion, respectively.

Figure 5. Rack and Pinion Free Body Diagram. Figure 5. Rack and Pinion Free Body Diagram.

Eliminating Fr and fr from (3)–(5), the diﬀerential equation governing dynamics of the mechanical Eliminating 퐹 and 푓 from (3)–(5), the differential equation governing dynamics of the system with the rack푟 and pinion푟 is obtained as mechanical system with the rack and pinion is obtained as .. 퐾 K 푀푥M̈ +x +푡 푖 +t i퐹+=F 퐹=(푡F),( t),(6) (6) 푟 푠 s 1 r1 퐽푝 퐽퐺 where 푀 = (푚 + 2 + 2). Then the governing equations of the electro-mechanical system from 푟2 푟1 combining (2) and (6) are

푅 +푅 퐾 − ( 퐺 퐿) 0 푡 0 (7) 푖̇ 퐿 퐿푟1 푖 [푥̇] = 0 0 1 [푥] + [ 0 ], 퐹(푡)−퐹푠 퐾푡 푥̈ − 0 0 푥̇ 푀 [ 푀푟1 ]

Energies 2020, 13, 5419 6 of 19

J where M = m + p + JG . Then the governing equations of the electro-mechanical system from r2 r2 2 1 combining (2) and (6) are

.    RG+RL Kt      i   L 0 Lr  i   0   .   − 1       =   x  +  0   x   0 0 1    , (7)  ..   K  .   F(t) Fs   x   t 0 0  x   −  Energies 2020, 13, x FOR PEER REVIEW − Mr1 M 6 of 19

Similarly, from the the FBD FBD of of the the mechanical mechanical system system with with lead lead and and screw screw as shown as shown in Figure in Figure 6, the6, equationsthe equations govern govern the translation the translation of the of nut the and nut the and rotations the rotations of the of lead the screw lead screwand the and generator the generator rotor, rotor, respectively, are respectively, are .. mx = F(t) Fa Fs, (8) 푚푥̈ = 퐹(푡) − 퐹− − −퐹 , (8) .. 푎 푠 2πJ1 .. J1θ1 = x = Γ TB, (9) 2휋퐽1 l − 퐽1휃̈1 = 푥̈ = 훤 − 푇퐵, (9) .. 푙 2πJG .. JGθ2 = x = TB τG, (10) 2휋퐽 l − 퐽 휃̈ = 퐺 푥̈ = 푇 − 휏 , (10) 퐺 2 푙 퐵 퐺 where m is the mass of the ﬂoor-tile and the nut, J1 is the moment of inertia of the lead screw and JG is wherethe moment 푚 is the of inertia mass of of the the floor bevel-tile gear. andl theis the nut, pitch 퐽1 is of the the moment lead screw, of inertiax is the of displacement the lead screw of and the plate퐽퐺 is the and moment nut, θ1 isof theinertia angular of the position bevel gear. of the 푙 leadis the screw pitch and of theθ2 islead the screw, angular 푥 positionis the displacement of the bevel ofgear. the Inplate addition, and nut,F( 휃t)1 isis the angular applied position force from of the the lead footstep, screwF sandis the 휃2 restoringis the angular spring position and damper of the bevelforces, gear.Fa is theIn addition, friction force 퐹(푡) between is the applied nut and force lead screw,from theTB isfootstep, the friction 퐹푠 is torque the restoring of the bevel spring gear, andτG damperis the electromagnetic forces, 퐹푎 is the torque friction of the force DC generator;between nut where andτ leadG = Kscrew,ti. Note 푇퐵 that is theΓ in friction (9) is the torque transmitted of the beveltorque gear, from 휏 the퐺 is nut the to electromagnetic the lead screw torque which isof proportionalthe DC generator; to Fa aswhere 휏퐺 = 퐾푡푖. Note that  in (9) is the transmitted torque from the nut to the lead screw which is proportional to 퐹푎 as Γ = ∆Fa, 훤 = ∆퐹푎, l푙 where ∆ = , with ηthread and ηthrust are the eﬃciencies of the thread and the thrust bearing, where ∆= 2πηthreadηthrust , with 휂푡ℎ푟푒푎푑 and 휂푡ℎ푟푢푠푡 are the efficiencies of the thread and the thrust respectively.2휋휂푡ℎ푟푒푎푑휂푡ℎ푟푢푠푡 bearing, respectively.

Figure 6. Lead-Screw Free Body Diagram. Figure 6. Lead-Screw Free Body Diagram. By rearranging (8)–(10), the equations become By rearranging (8)–(10), the equations become .. ̈ 퐾푡 Kt 퐽푒푞J휃eq1θ+ + 푖 +i퐹+푠 =Fs퐹=(푡F),( t),(11) (11) 1 ∆ ∆

2휋퐽푒푞 퐾푡 2π푥Jeq̈ +.. 푖K+t 퐹푠 = 퐹(푡), (12) 푙 x +∆ i + Fs = F(t), (12) l ∆ 푚푙 (퐽1+퐽퐺) where 퐽푒푞 = + is the equivalent moment of inertia corresponding to the mass of the plate ml2휋 (J1∆+JG) where Jeq = 2π + ∆ is the equivalent moment of inertia corresponding to the mass of the plate and nut m, and the mass moments of inertia of the lead screw and bevel gear J1 and JG, respectively. and nut m, and the mass moments of inertia of the lead screw and bevel gear J and J , respectively. By combining (2) and (11)–(12), the equations governing the electro-mechanical1 systemG with the lead and screw design are obtained as

푅 +푅 퐾 − ( 퐺 퐿) 0 푡 0 (13) 푖̇ 퐿 퐿 푖 0 [휃1̇ ] = 0 0 1 [휃1] + [ ], 퐹(푡)−퐹푠 ̈ 퐾푡 ̇ 휃1 − 0 0 휃1 퐽푒𝑔 [ 퐽푒푞∆ ] or

Energies 2020, 13, 5419 7 of 19

By combining (2) and (11)–(12), the equations governing the electro-mechanical system with the lead and screw design are obtained as

.    RG+RL Kt      i   0  i  0   .   − L L           0   θ1  =  0 0 1  θ1  +  , (13) Energies 2020, 13, x FOR PEER REVIEW ..    .   ( )  7 of 19    Kt    F t Fs  θ 0 0 θ1 − 1 − Jeq∆ Jeg 푅 +푅 퐾 or − ( 퐺 퐿) 0 푡 0 (14) . 푖̇ 퐿 퐿 푖    RG+RL Kt   0  i  0 L 00 1 L  i  0   [푥̇] =  [푥] + [푙(퐹(푡)−퐹 )],   .   − 푙퐾    푠   x  =  0푡 0 1 푥̇  x  +  0 , (14)  푥̈  − 0 0   2휋퐽푒𝑔   ..  [ 2휋퐽lK푒푞∆ ]  .   l(F(t) Fs)   x   t 0 0  x   −  − 2πJeq∆ 2πJeg The MATLAB/Simulink models corresponding to (7) and (14) were developed to predict the The MATLAB®/Simulink models corresponding to (7) and (14) were developed to predict the voltages and currents for various load resistance 푅퐿 that both the EM generator systems with rack pinionvoltages and and lead currents screw could for various generate. load The resistance SimulinkR Lmodelthat bothof the the system EM generatorwith lead screw systems is presented with rack inpinion Figure and 7. lead With screw the could selected generate. parameters The Simulink shown inmodel Tables of the 1 and system 2, the with simulation lead screw results is presented were comparedin Figure7. to With the thecorresponding selected parameters test results shown as illustrated in Tables1 and in Figures2, the simulation 8 and 9 (The results test were procedure compared will beto thedescribed corresponding in Section test 2.1.4.) results. Figures as illustrated 8 and 9 inshow Figures that8 theand analytical9 (The test models procedure accurately will be predict described the magnitudesin Section 2.1.4 of.). the Figures voltages8 and and9 show currents that thegenerated analytical by modelsthe EM accurately generator. predict The voltage the magnitudes and current of signalsthe voltages in Figures and currents 8 and 9 generated can be divided by the EMinto generator.two stages The according voltage to and the current movement, signals i.e., in Figuresforward8 and 9return can be stages. divided In intothe forward two stages stage, according 0.2–0.8 to s, the the movement, floor-tile moves i.e., forward downwards and return when stages. the footstep In the- forceforward applied, stage, causing ~0.2–0.8 the s, generator the floor-tile to rotate moves in downwards one direction when and thehence footstep-force induce negative applied, voltage causing and currentthe generator as shown to rotate in Figures in one 8 directionand 9. During and hence0.2–0.8 induce s, the negativefloor-tile voltagemight reach and currentthe lowest as shownposition, in causingFigures8 the and generator9. During to 0.2–0.8 stop the s, the rotation floor-tile and might induce reach no voltage the lowest and position, current before causing the the return generator stage begins.to stop the In the rotation return and stage, induce 0. no8–1 voltage.4 s, the and floor current-tile moves before up thewards return to stage the equilibrium begins. In the position return accordingstage, ~0.8–1.4 to the s, the restoring floor-tile spring moves forces upwards and drive to thes equilibriumthe generator position to rotate according backward to the and restoring induce positivespring forces voltage and and drives current the generatoras seen in to Figures rotate backward8 and 9. Note and that induce in Figure positive 9, voltagethere exist ands currentthe small as humpsseen in of Figures the predicted8 and9. Notevoltage that and in current Figure9 ,during there exists0.6–0.8 the s. These small humpssignals ofcorrespond the predicted to the voltage applied forceand current at the same during interval 0.6–0.8 when s. These the floor signals-tile correspond is moving dow to thenwards applied. The force discrepancy at the same of this interval analytical when predictionthe floor-tile and is movingthe test downwards.result might Thebe b discrepancyecause of the of difference this analytical between prediction the actual and force the test and result the forcemight function be because in Figure of the 4 di. ff erence between the actual force and the force function in Figure4.

Figure 7. SimulinkSimulink model model for for the the electromagnetic electromagnetic ( (EM)EM) generator with lead lead-screw-screw design design..

Energies 2020, 13, x FOR PEER REVIEW 8 of 19

Energies 2020, 13, 5419 Table 1. Rack-Pinion Parameters. 8 of 19 Parameters Value Mass Tableof Rack 1. Rack-Pinionand Plate (푚 Parameters.) 3.016 kg −2 Radius of Pinion (푟1) 0.72 × 10 m Parameters Value−2 Radius of Gear (푟2) 3 × 10 m −7 MassMoment of Rack of inertia and Plate of (bevelm) gear (퐽퐺) 8.67563.016 × kg10 2 r −5 MomentRadius of of Pinion Inertia ( 1 )of Pinion (퐽푝) 10.72 × 10 10 kg− mm × 2 Radius of Gear (r2) 3 10− m Spring Coefficient (푘) 20,500× N/m 7 Moment of inertia of bevel gear (JG) 8.6756 10− Damping Coefficient (푑) 900 N·×5s/m Moment of Inertia of Pinion (Jp) 1 10 kg m × − SpringResistance Coeﬃ cientof Generator (k) (푅퐺) 20,50042 Ohm N/ m Damping CoeInductanceﬃcient (d )(퐿) 19.6900 × N10s−/3m H · ResistanceGenerator of Generator constant (RG) (퐾푡) 0.585442 OhmVs/rad 3 Inductance (L) 19.6 10− H Resistance of Load (푅퐿) 30 ×Ohm Generator constant (Kt) 0.5854 Vs/rad Resistance of Load (RL) 30 Ohm Table 2. Lead-Screw Parameters.

ParametersTable 2. Lead-Screw Parameters. Value Pitch of Lead Screw (푙) 8 mm Parameters Value Mass of Nut and Plate (푚) 2.16 kg Pitch of Lead Screw (l) 8 mm −7 Moment of inertia of bevel gear (퐽퐺) 8.6756 × 10 Mass of Nut and Plate (m) 2.16 kg Moment of Inertia of lead screw (퐽 ) 2.5536 × 10−6 kg m2 Moment of inertia of bevel gear (J ) 푙 8.6756 10 7 G × − Moment of InertiaLead of leadangle screw (J ) 2.553645 degree10 6 kg m2 l × − SpringLead angleCoefficient (푘) 40,00045 degree N/m SpringDamping Coeffi Coefficientcient (k) (푑) 13,60040,000 N N·s/m/m Damping Coefficient (d) 13,600 N s/m Resistance of Generator (푅퐺) 37 Ohm· 37 Ohm Resistance of Generator (RG) −3 Inductance (퐿) 19.6 × 10 3H Inductance (L) 19.6 10− H 퐾 0. × GeneratorGenerator constant constant (Kt) ( 푡) 0.392392 Vs/rad Vs/rad ResistanceResistance of Load of Load (RL) (푅퐿) 3030 Ohm Ohm FrictionFriction coe fficoefficientcient (µ) (µ) 00.21.21 Efficient of thrust bearing ηthrust 0.6529 Efficient of thrust bearing 휂푡ℎ푟푢푠푡 0.6529 Efficient of thread ηthread 0.8132 Efficient of thread 휂푡ℎ푟푒푎푑 0.8132

FigureFigure 8. 8.Voltage Voltage and and Current Current of of Rack-Pinion Rack-Pinion Model Model from from Experiment Experiment and and Simulation. Simulation. ( a(a)) Voltage Voltage of of b Rack-PinionRack-Pinion Model. Model. ( (b)) Current Current of of Rack-Pinion Rack-Pinion Model. Model. Energies 2020, 13, 5419 9 of 19 EnergiesEnergies 2020 2020, ,13 13, ,x x FOR FOR PEER PEER REVIEW REVIEW 99 of of 19 19

FigureFigureFigure 9. 9. 9.Voltage Voltage Voltage and and and Current Current Current of of of Lead-Screw Lead-Screw Lead-Screw Model Model Model from from from Experiment Experiment Experiment and and and Simulation. Simulation. Simulation. ((a (a)a) Voltage)Voltage Voltage of of of Lead-ScrewLead-ScrewLead-Screw Model. Model. Model. (b( (b)b) Current) Current Current of of of Lead-Screw Lead-Screw Lead-Screw Model. Model. Model.

InInIn summary, summary, summary, the the the induced induced induced voltages voltages voltages and and and currents currents currents for for for both both both EM-generator EM-generator EM-generator designs designs designs predicted predicted predicted by by by the the the analyticalanalyticalanalytical models modelsmodels were werewere compared comparedcompared in Figureinin FigureFigure 10. 10. Then10. ThenThen the performancesthethe performancesperformances of the ofof two thethe EM-generators twotwo EM-generatorsEM-generators were predictedwerewere predicted predicted and summarized and and summarized summarized in Table in3 in. Table The Table results 3. 3. The The mainly results results show mainly mainly that show show the designed that that the the systems designed designed generate systems systems angenerategenerate averaged an an 216–886averaged averaged mJ 21 21 of66–– electrical886886 mJ mJ of of energyelectrical electrical per energy energy footstep, per per or footstep, footstep, the averaged or or the the poweraveraged averaged of 216–590power power of of mW. 21 2166– It–590590 is sumW.mW.fficient It It is is tosufficient sufficient power electronicto to power power electronic deviceselectronic with devices devices low powerwithwith low low consumption power power consumption consumption in the vicinity, in in the the suchvicinity, vicinity, as sensors such such as as andsensorssensors communication and and communication communication instruments. instruments. instruments. This finding This This assures finding finding the assures assures possibility the the possibilityof possibility building of bothof building building of Genpath’s both both ofof prototypes.Genpath’sGenpath’s prototypes. Moreover,prototypes. the Moreover, Moreover, verified analyticalthe the verified verified models analytical analytical were models usedmodels in were thewere parametric used used in in the the design parametric parametric as presented design design inasas Section presented presented 2.1.3 in in. Section Section 2.1.3. 2.1.3.

Figure 10. Simulation of Rack-Pinion and Lead-Screw Model. (a) Voltageof Rack-Pinion and Lead-Screw Figure 10. Simulation of Rack-Pinion and Lead-Screw Model. (a) Voltage of Rack-Pinion and Lead- Model.Figure (10.b) CurrentSimulation of Rack-Pinion of Rack-Pin andion and Lead-Screw Lead-Screw Model. Model. (a) Voltage of Rack-Pinion and Lead- ScScrewrew Model Model. .( (bb)) Current Current of of Rack-Pinion Rack-Pinion and and Lead-Screw Lead-Screw Model. Model. Energies 2020, 13, 5419 10 of 19

Table 3. Performances of the EM-generator designs.

Rack-Pinion Design Lead-Screw Design Variables Values per Footstep Values per Footstep Maximum voltage 9.92 V 10.13 V Average voltage 0.99 V 4.16 V Maximum current 330.9 mA 337.9 mA Average current 33.15 mA 138.8 mA Maximum power 3.28 W 3.42 W Average power 216.4 mW 590.3 mW Wave duration 1.00 s 1.50 s Average energy 216.4 mJ 885.8 mJ

2.1.3. Design of Elements The critical elements of the EM generator systems in Figures2 and3 were decided with the use of the analytical models to tune for the optimized parameters. Design of the elements such as the rack pinion and lead screw, the springs, the transmitted gears and the DC generator is summarized as follows. The rack pinion and lead screw shown in Figures2 and3 are used to change the translation to rotation. A mechanism of the rack-pinion was first adopted in the first prototype [16] because of its availability and economy cost. The drawback of the rack-pinion mechanism arises from its coarse tolerance, resulting in large friction loss. In addition, only the rough-pitch models were found. Thus, for the rack’s allowable displacement of 15 mm, the angular displacement of the pinion is very limited. Therefore, the EM generator system using the rack and pinion is inefficient for harvesting energy from the footstep with limited displacement. To improve the design of the movement converter, the rack-pinion mechanism is replaced by the lead screw in the second prototype. The lead screw has more variety in dimensions for the selection. With finer pitch or smaller lead angles, the angular displacement of the lead can be extended with the limited stroke of the nut. Two sets of the lead angles, i.e., 45◦ and 60◦, were selected and installed to the 24-V-DC-generator system for the comparison. Table4 presents the energy produced by the generator, when connected to 49- Ω resistance load, for three different designs of the movement converters: the rack pinion, the lead screw with 60◦ lead angles and the lead screw with 45◦ lead angles. It was found that the EM generator system with the 45◦ lead screw produces the highest level of energy among the three designs. With the smaller value of the lead angles, the lead proceeds to larger angular displacement within the same limited stroke of 15 mm, resulting in the greater period for the generator to spin. Therefore, the accumulative energy the generator provides is higher.

Table 4. Comparison of Rach-Pinion, Lead-Screw (60◦ lead angles) and Lead-Screw (45◦ lead angles) Average Energy.

Design Averaged Energy (mJ) Rack pinion 319 60◦ lead angles Lead screw 353 45◦ lead angles Lead screw 488

The spring and transmitted gears are also the parts critical to harvest the energy. The softer spring is preferable in the design. For the explanation, Figure 11 shows the predicted voltages and currents for the two EM-generator systems varying in the stiffness coefficients. The softer spring, with less value of stiffness coefficient, results in the higher levels of the voltage and current in the forward stage, and hence yields the greater power in harvesting. The softer spring inserts less restoring forces and causes the floor-tile to move down with higher speed that is converted to a higher rotational speed of the generator. With more speed, the generator can produce more power. Although the softer Energies 2020, 13, 5419 11 of 19

Energies 2020, 13, x FOR PEER REVIEW 11 of 19 springs are theoretically desirable, they should be suﬃciently hard enough to restore the system back tosystem equilibrium back to due equilibrium to the friction due ofto the system.friction of To the satisfy system. such To conditions, satisfy such the conditions, optimized the springs optimized with thesprings wire diameterwith the wire of 2.2 diameter mm or the of sti2.2ﬀ mmness or of the 40 kNstiffness/m were of selected.40 kN/m were selected.

Figure 11. Simulation of Lead-Screw Model with Soft and Hard Springs. (a) Voltage of Lead-Screw ModelFigure with 11. Soft Simulation and Hard of Springs.Lead-Screw (b) CurrentModel with of Lead-Screw Soft and Hard Model Springs. with Soft(a) Voltage and Hard of Springs.Lead-Screw Model with Soft and Hard Springs. (b) Current of Lead-Screw Model with Soft and Hard Springs. Moreover, the sets of gear train and bevel gears in Figures2 and3, respectively, are used to transmitMoreover, the rotation the fromsets of the gear movement train and converters bevel gears to the in generator’s Figures 2 rotor.and 3, The respectively, gear ratio largerare used than to 1:1transmit can help the increase rotation the from speed the of movement the generator. converters However, to the the generator’s increase of therotor. gear The ratio gear is limitedratio larger by thethan amount 1:1 can of help the increase resistance the force speed in of the the system. generator. The However, increase ofthe the increase gear ratio of the leads gear toratio the is greater limited frictionsby the amount and the of greater the resistance resistance force torque in the provided system. byThe the increase generator. of the If gear the resistance ratio leads force to the exceeds greater thefrictions applied and force the fromgreater the resistance footstep, torque the ﬂoor-tile provided will by not the move. generator. Consequently, If the resistance the maximum force exceeds gear ratiothe applied of 4:1 is designedforce from for the the footstep, rack-pinion the floor-tile system and will originated not move. from Consequently, a pinion’s 6the cm maximum diameter andgear transmittedratio of 4:1 tois adesigned gear’s 1.5 for cm the diameter rack-pinion as shown system in and Figure originated2. In addition, from a the pinion’s maximum 6 cm geardiameter ratio ofand thetransmitted bevel gears to ina gear’s Figure 1.53 for cm the diameter lead-screw as shown system in is Figure set to 1:1.2. In addition, the maximum gear ratio of the Thebevel DC gears generator in Figure was 3 for used the in lead-screw the design system for simplicity. is set to 1:1. In order to generate at least 3.3 V for operatingThe theDC micro-controller, generator was used the typical in the 12-V design or 24-V-DC-motorfor simplicity. generatorIn order to was generate selected at to least ensure 3.3 suchV for criteria.operating To choosethe micro-controller, a proper DC generator’s the typical speed,12-V or the 24-V- kinematicDC-motor relation generator of the was transmission selected to system ensure wassuch analyzed. criteria. WithTo choose the maximum a proper value DC ofge 20nerator’s mm displacement speed, the forkinematic safely walking, relation the of maximumthe transmission value ofsystem the angular was analyzed. velocity is Withobtained the at maximum 210 rpm. valueHence, of the 20 model mm displacement of a motor generator for safely with walking, a 300 rpm the ratedmaximum speed value was selected. of the angular Two types velocity of the is obtained DC generators, at 210 rpm. 12 V Hence, and 24 the V with model the of properties a motor generator shown inwith Table a 5300, were rpm installed rated speed in the wa seconds selected. prototype Two types of Genpath of the DC for comparisongenerators, 12 of V their and performance. 24 V with the Theproperties induced shown voltage in andTable current 5, were for installed both types in the of second the DC prototype generators of are Genpath compared for comparison in Figure 12 of. Thetheir energy performance. per footstep The produced induced voltage by the DC and generators current for for both various types rated of the load DC resistancesgenerators are showncompared in Tablein Figure6. The 12-V 12. The motor energy provides per the footstep energy produced as much as by 2.5 the times DC that generators of the 24-V for motor various because rated of the load resistanceresistances during are shown the transience. in Table 6. The 12-V motor provides the energy as much as 2.5 times that of the 24-V motor because of the resistance during the transience. Table 5. Comparison of 12- and 24-V-DC-Generator Parameters. Table 5. Comparison of 12- and 24-V-DC-Generator Parameters. Voltage Resistance RG Inductance L Kt (V) Voltage Resistance(Ω) RG Inductance(mH) L Kt (Vs/rad) 12(V) 37() 3.6(mH) (Vs/rad) 0.2903 2412 4237 19.63.6 0.2903 0.5854 24 42 19.6 0.5854 EnergiesEnergies2020 2020, 13, 13, 5419, x FOR PEER REVIEW 1212 of of 19 19

Figure 12. Comparison Voltage and Current of Lead-Screw Model with 12– and 24–V-DC Generator. (aFigure) Voltage 12. of Comparison Lead-Screw Voltage Model. and (b) CurrentCurrent ofof Lead-ScrewLead-ScrewModel. Model with 12– and 24–V-DC Generator. (a) Voltage of Lead-Screw Model. (b) Current of Lead-Screw Model. Table 6. Energy per footstep produced by the DC generators for various rated load resistances.

Table 6. Energy per footstepLoad produced by theAverage DC generators Energy for (mJ) various rated load resistances. (Ω) Load Average12 V Energy (mJ) 24 V 30() 12 798.2 V 24 V 321.5 3930 798.2 750.0 321.5 313.2 4939 750.0 745.5 313.2 488.2 49 745.5 488.2 In conclusion, the analytical models developed in the previous section were utilized to obtain the In conclusion, the analytical models developed in the previous section were utilized to obtain best-fit design. First, the simulation results show that both the rack-pinion and lead-screw models the best-fit design. First, the simulation results show that both the rack-pinion and lead-screw models yield about the same maximum power, according to the same levels of both voltage and current yield about the same maximum power, according to the same levels of both voltage and current magnitudes as seen in Figure 10. However, the simulation results in Figure 10 also indicate that the magnitudes as seen in Figure 10. However, the simulation results in Figure 10 also indicate that the lead screw provides the longer time in movement and yields the larger angular displacement of the lead screw provides the longer time in movement and yields the larger angular displacement of the rotor within the limited stroke. This results in more time for the generator in the lead-screw design rotor within the limited stroke. This results in more time for the generator in the lead-screw design to generate power. It was also found that the lead-screw design with the finer pitch or, i.e., 45 lead to generate power. It was also found that the lead-screw design with the finer pitch or, i.e., 45◦  lead angles, provides the highest energy per step. Second, although the simulation results show that the angles, provides the highest energy per step. Second, although the simulation results show that the softer spring could provide higher power in the forward stage, the harder spring with the optimum softer spring could provide higher power in the forward stage, the harder spring with the optimum stiffness value of 40 kN/m was selected to enable to restore the system back to equilibrium. Finally, the stiffness value of 40 kN/m was selected to enable to restore the system back to equilibrium. Finally, 12-V-DC generator, compared to the 24-V-DC generator, gives a better performance probably because the 12-V-DC generator, compared to the 24-V-DC generator, gives a better performance probably of the lower resistance during the transient, as the properties show in Table5. because of the lower resistance during the transient, as the properties show in Table 5. 2.1.4. Development of the Prototypes 2.1.4. Development of the Prototypes Figures 13 and 14 show the two prototypes of Genpath built with the key components as listed Figures 13 and 14 show the two prototypes of Genpath built with the key components as listed in Table7. Prototype-I [ 16] was installed with the 24-V-DC generator and uses the rack pinion for in Table 7. Prototype-I [16] was installed with the 24-V-DC generator and uses the rack pinion for the the movement converter. Prototype II is the improved prototype built with the lead screw for the movement converter. Prototype II is the improved prototype built with the lead screw for the movement converter and the 12-V-DC generator. The experiment was then performed to test the movement converter and the 12-V-DC generator. The experiment was then performed to test the prototypes’ performances as shown in Figure 15. First, each prototype was connected to the rated prototypes’ performances as shown in Figure 15. First, each prototype was connected to the rated resistor RL to provide the maximum power output. Then the voltage across RL, the current i and the resistor RL to provide the maximum power output. Then the voltage across RL, the current i and the corresponding electrical power when a normal footstep is applied were measured using an oscilloscope corresponding electrical power when a normal footstep is applied were measured using an and a current probe. The test results are shown in Figure 16 and summarized in Table8. Genpath oscilloscope and a current probe. The test results are shown in Figure 16 and summarized in Table 8. prototype-II with 12-V-DC generator and lead-screw mechanism was significantly improved when Genpath prototype-II with 12-V-DC generator and lead-screw mechanism was significantly improved when compared to the prototype-I [16]. It stated in Table 8 that the latest Genpath Energies 2020, 13, 5419 13 of 19

Energies 2020, 13, x FOR PEER REVIEW 13 of 19 Energies 2020, 13, x FOR PEER REVIEW 13 of 19 compared to the prototype-I [16]. It stated in Table8 that the latest Genpath prototype produces an average energy of 702 mJ (or average power of 520 mW), the maximum voltage of 9.5 V and the prototype produces an average energy of 702 mJ (or average power of 520 mW), the maximum maximum current of 285 mA per footstep in the duration of 1.35 s. The energy provided by the voltage of 9.5 V and the maximum current of 285 mA per footstep in the duration of 1.35 s. The energy EM-generator in Genpath’s prototype-II was increased by approximately 184% when compared to provided by the EM-generator in Genpath’s prototype-II was increased by approximately 184% when that of the prototype-I [16]. The eﬃciency of the EM-generator system is 26% based on the power compared to that of the prototype-I [16]. The efficiency of the EM-generator system is 26% based on generation from the heel strike of a human’s walk of 2 W per step. This amount of energy could the power generation from the heel strike of a human’s walk of 2 W per step. This amount of energy suﬃciently power typical low-power electrical devices, as previously described. could sufficiently power typical low-power electrical devices, as previously described.

Figure 13. Photograph of Rack and Pinion Prototype.

Figure 14. PhotographPhotograph of Lead-Screw Prototype. Table 7. Components of rack and pinion and lead-screw prototypes. Table 7. Components of rack and pinion and lead-screw prototypes. Prototype I (Rack and Pinion) Prototype II (Lead Screw) Prototype I (Rack and Pinion) Prototype II (Lead Screw) Item Dimensions # Item Dimensions # Item Dimensions # Item Dimensions # AcrylicAcrylic plate 400400 ×400 400 10× 10 mm mm 22 WoodWood plate plate 400400400 × 4005 mm× 5 mm 2 2 Acrylic plate 400× × 400× × 10 mm 2 Wood plate 400× × 400× × 5 mm 2 DiaDia 12 12 DiaDia 12 12 LinearLinear g guideuide Dia 12 44 LinearLinear guide guide Dia 12 4 4 Linear guide Length 90 mm 4 Linear guide Length 90 mm 4 Length 90 mm Length 90 mm Linear bearing Inner dia 12 mm 4 Linear bearing Inner dia 12 mm 4 Linear bearing Inner dia 12 mm 4 Linear bearing Inner dia 12 mm 4 ShaftShaft c couplingoupling InnerInner dia dia 12 12 mm mm 44 ShaftShaft coupling coupling InnerInner dia 12dia mm 12 mm 4 4 LengthLength 60 60 mm mm LengthLength 60 mm 60 mm Coil spring Length 60 mm 4 Coil spring Length 60 mm 4 Coil spring Dia 1.6 mm 4 Coil spring Dia 2.2 mm 4 Dia 1.6 mm Dia 2.2 mm Dia 8 mm Dia 8 mm ShaftShaft to generator to Dia 8 mm 1 Shaft to generator Dia 8 mm 1 Length 60 mm 1 Shaft to generator Length 60 mm 1 generator Length 60 mm Length 60 mm Dia 8 mm Rack and pinion Pinion radius 3 cm 1 NutNut and leadand screw lead Dia 8 mm 1 Rack and pinion Pinion radius 3 cm 1 Nut and lead PitchDia 2 mm 8 mm 1 Rack and pinion Pinion radius 3 cm 1 screw Pitch 2 mm 1 Flexible coupling 8 mm 1 Flexibles couplingcrew 8Pitch mm 2 mm 1 Flexible coupling 8 mm 1 Flexible coupling 8 mm 1 Gear Radius 0.75 cm 1 Bevel gear Inner dia 8 mm 2 Gear Radius 0.75 cm 1 Bevel gear Inner dia 8 mm 2 Ball bearing Inner dia 8 mm 3 Ball bearing Inner dia 8 mm 3 Ball bearing Inner dia 8 mm 3 Ball bearing Inner dia 8 mm 3 Generator ZGA37RG 24V 300 rpm 1 Generator ZGA37RG 12V 300 rpm 1 ZGA37RG 24V 300 ZGA37RG 12V 300 Generator 1 Generator 1 rpm rpm Energies 2020, 13, x FOR PEER REVIEW 14 of 19

EnergiesEnergies 20202020,, 1313,, x 5419 FOR PEER REVIEW 1414 of of 19 19

Figure 15. Test Set up Diagram for the Prototypes. Figure 15. Test Set up Diagram for the Prototypes. Figure 15. Test Set up Diagram for the Prototypes. a

Figure 16. Comparison Voltage and Current of Prototype I and Prototype II. (a) Voltage of Prototype I and PrototypeFigure 16. II. (Comparisonb) Current of Voltage Prototype and I Current and Prototype of Prototype II. I and Prototype II. (a) Voltage of PrototypeTable I and 8. PrototypePerformances II. (b of) Current Genpath of prototypes Prototype II andand II.Prototype II. Figure 16. Comparison Voltage and Current of Prototype I and Prototype II. (a) Voltage of Prototype I Tableand Prototype 8. PerformancesPrototype II. (b) ICurrent (Rack-Pinion)of Genpath of Prototype prototypes Prototype I and I and Prototype II II. (Lead-Screw) II. Variables PrototypeValues I per(Rack Footstep-Pinion) Prototype Values II per (Lead Footstep-Screw) VariablesTable 8. Performances of Genpath prototypes I and II. Maximum voltageValues per 7.5 VFootstep Values per 9.5 VFootstep MaximumAverage voltage voltage Prototype I (Rack7.5 1.26 V V -Pinion) Prototype II 9.5 2.88(Lead V V -Screw) MaximumVariables current 246 mA 285 mA Values per Footstep Values per Footstep Average current voltage 1. 42.526 mAV 2.88 88 mA V MaximumMaximum voltage power current 7.5246 1.85 V mA W 9.5285 2.71 V mA W AverageAverage voltage power current 1.42.526 216 V mA mW 2.8888 520 VmA mW MaximumMaximumWave duration current power 2461.85 mA 1.14 W s 2852.71 1.35mA W s AverageAverageAverage current energy power 42.5216 247mA mW mJ 885 20 702mA mW mJ MaximumWave duration power 1.851.14 W s 2.711. 35W s 2.2. The systemAverageAverage of Power power Managementenergy and Storage2162 47mWCircuit mJ 520702 mW mJ The powerWave management duration and storage (PMS)1.14 s circuit was designed to1. convert35 s and store electrical energy2.2. The at system theAverage same of Power time. energy Management Figure 17 shows and Storage2 the47 mJ circuit Circuit diagram and the real-world702 mJ circuit is depicted in FigureThe 18 power. From management the performance and storage test of Genpath(PMS) circuit as shown was designed in Figure to16 convert, it is clearly and store seen thatelectrical the 2.2. The system of Power Management and Storage Circuit generatedenergy at voltage the same and time. current Figure waveform 17 shows are the AC circuit signals. diagram Negative and portions the real-world occur when circuit a footstepis depicted is applied,in FigureThe causingpower 18. From management the generatorthe performance and to rotate storage test inone (PMS)of Genpath direction. circuit as Positivewas shown designed portionsin Figure to convert occur 16, it duringis and clearly store the seen restoration electrical that the energyperiod, at resulting the same in time. the opposite Figure 17 direction shows ofthe the circuit generator’s diagram rotation. and the real-world circuit is depicted in Figure 18. From the performance test of Genpath as shown in Figure 16, it is clearly seen that the Energies 2020, 13, x FOR PEER REVIEW 15 of 19 Energies 2020, 13, x FOR PEER REVIEW 15 of 19 generated voltage and current waveform are AC signals. Negative portions occur when a footstep is generatedapplied, causingvoltage and the current generator waveform to rotate are inAC one signals. direction. Negative Positive portions portions occur when occur a duringfootstep the is Energies 2020, 13, 5419 15 of 19 applied,restoration causing period, the resulting generator in the to opposite rotate indirection one direction. of the generator’s Positive rotation. portions occur during the restoration period, resulting in the opposite direction of the generator’s rotation. Genpath protypes II Battery Active Bridge Rectifier Buck-Boost Converter Genpath12-V-DC protypes-generator II 6VBattery 4.5 Ah Active Bridge Rectifier Buck-Boost Converter 12-V-DC-generator 6V 4.5 Ah pT pi po pT pi po R L iT ii io a a i i i Ra La T i o    G     G  e v  a T vi Vo  e v  a T vi Vo  G   G       

* D vi  ˆ * 1 Ra vi  x D  1 Rˆ n  x a n d d 1 Pulse Width Modulation 1 Pulse Width Modulation

Impedance-Matching Control Scheme Impedance-Matching Control Scheme

Figure 17. Circuit Diagram of Power Management and Storage System. Figure 17. CircuitCircuit Diagram Diagram of Power Management and Storage System System..

Connect to Genpath; Voltage Current P-Channel Schottky Connect to ConnectDC to GeneraGenpathtor; DetectionVoltage DetectionCurrent PMOSFET-Channel SchottkyDiode Connect6 V 4.5 Ah to Battery DC Generator Detection Detection MOSFET Diode 6 V 4.5 Ah Battery

Active DC Bus MCU Schottky N-Channel Inductor ActiveBridge DCCapacitor Bus MCUPIC16F1776 SchottkyDiode NMOSFET-Channel Inductor BridgeRectifier Capacitor PIC16F1776 Diode MOSFET Rectifier Figure 18. Real-World Circuit of Power Management and Storage System. Figure 18. Real-World Circuit of Power Management and Storage System. Figure 18. Real-World Circuit of Power Management and Storage System. The energy from the generator is stored to a 6-V, 4.5-Ah battery through a two-stage power converter.The energy First, thefrom active the generator bridge rectifier is stored converts to a 6- theV, 4.5 AC-Ah voltage battery to through the DC voltage.a two-stage Since power the converter.The energy First, thefrom active the bridgegenerator rectifier is stored converts to a the 6-V AC, 4.5 voltage-Ah battery to the throughDC voltage. a two Since-stage the powermetal– converter.metal–oxide–semiconductor First, the active bridge field-e rectifierffect transistors converts (MOSFETs)the AC voltage used to in the the DC rectifier voltage. have Since extremely the metal low– turn-onoxide–semiconductor resistances and field junction-effect voltage transistor drops,s (MOSFETs this kind of) used rectifier in the can rectifier perform have with highextremely efficiency. low oxideturn-–onsemiconductor resistances and field junction-effect voltage transistor drops,s (MOSFETs this kind )of used rectifier in the can rectifierperform havewith high extremely efficiency. low turnSecond,-on resistances the buck-boost and converterjunction voltage helps convert drops, thethis variable kind of DCrectifier voltage can from perform the rectifierwith high to theefficiency. battery. ThisSecond, buck-boost the buck converter-boost converter is operated helps in accordance convert the with variable a matching-impedance DC voltage from control the rectifier scheme which to the Second,battery. the This buck buck-boost-boost converter converter helps is operated convert in the accordan variablece DC with voltage a matching from - theimpedance rectifier control to the battery.paves the This way buck for- theboost maximum converter power is operated transfer. in In accordan this scheme,ce with the a reactance matching- termimpedance by inductance control isscheme assumed which to bepaves small the and way is for neglected the maximum from the power calculated transfer. impedance. In this scheme, This the assumption reactance isterm valid by schemeinductance which is assumedpaves the to way be smallfor the and maximum is neglected power from transfer. the calculated In this scheme, impedance. the reactance This assumption term by inductanceby investigating is assumed the value to be of small inductance and is neglected in Table5 fromand the the low-frequencycalculated impedance. AC voltage This exhibited assumption in Figureis valid 19 byb. investigatin (Figure 19ag shows the value the output of inductance voltage in and Table current 5 and without the low power-frequency management AC voltage and is valid by investigating the value of inductance in Table 5 and the low-frequency AC voltage a storage circuit.) The reactance is, therefore, insignificant in comparison with the resistance and

EnergiesEnergies 20202020,, 1313,, 5419x FOR PEER REVIEW 1616 of of19 19

exhibited in Figure 19b. (Figure 19a shows the output voltage and current without power itm isanagement neglected and in thea storage impedance-matching circuit.) The reactance control is, schemetherefore, for insignificant the sake of in simplicity. comparison This with control the schemeresistance is implementedand it is neglected with thein the microcontroller impedance-matching PIC16F1776 control which scheme includes for the the sake extreme of simplicity. low-power consumptionThis control scheme feature. is implemented (Note: the input with forcethe microcontroller to the prototype PIC16F1776 II in this which section includes is diﬀerent the extreme from the previouslow-power section consumption due to the feature. environment (Note: the setup.) input force to the prototype II in this section is different from the previous section due to the environment setup.) Pure R (8.7 Ω) Power Management Circuit

vt 2 V 2 V 0.3 A vt it

vi 2 V

i 0.3 A 0.3 A i i v t o 2 V 0.3 A io

(a) Output Voltage and Current without Power (b) Output Voltage and Current with Power Management and Storage Circuit Management and Storage Circuit

Figure 19. Experimental result result showing showing the the operation operation of of power power management management and and storage storage circuit circuit;; 푣푡v t and i푖t푡are are outputs outputs of of generator; generator;v i푣and𝑖 andii are푖𝑖 are outputs outputs of theof the rectiﬁer rectifier circuit; circuit;vo and푣표 andio are 푖표 outputsare outputs of the buck-boostof the buck- circuit.boost circuit.

The experiment is conducted to to evaluate evaluate the the performance performance of of the the two two-stage-stage converter. converter. The The AC AC voltagevoltage is efficiently efficiently rectified; rectified; the the efficiency efficiency of ofactive active bridge bridge rectifier rectifier is about is about 95.78%. 95.78%. Figures Figures 19b and 19 b andFigure 20 b20 showb show the operationthe operation of the of buck-boostthe buck-boost converter converter along along with thewith impedance the impedance matching matching control scheme.control scheme. The converter The converter helps charge helps thecharge power the into power the i batterynto the andbattery the controland the scheme control canscheme match can the impedancematch the impedance to achieve to the achieve maximum the maximum power transfer. power Thetransfer. power The management power management system is system capable is of gainingcapable theof gaining averaged the poweraveraged of 280power mW of from 280 mW each from footstep each andfootstep storing and the storing accumulative the accumulative energy of 302energy mJ intoof 302 the mJ battery into the at the battery output at stage.the output The estage.fficiency The of efficiency the buck-boost of the converterbuck-boost is aboutconverter 78.00%, is thusabout the 78.00%, overall thus effi theciency overall of theefficiency power of management the power management system is 74.72%. system Table is 74.72%.9 gives Table the detailed9 gives performancesthe detailed performances of power management of power management and storage and system storage for each system footstep. for each If comparingfootstep. If comparing the Genpath prototypethe Genpath II withprototype the commercial II with the product commercial such product as the Pavegen’s such as the system, Pavegen’s the Pavegen’s system, the system Pavegen’s which hassystem three which generators has three per generators tile can generate per tile can the energygenerate approximately the energy approximately 2 J per step 2 and J per the step Genpath and prototypethe Genpath II whichprototype has II one which generator has one per generator tile can per generate tile can the generate energy the approximately energy approximately 0.3 J per 0.3 step. TheJ per Genpath step. The prototype Genpath prototype II generates II generates the energy the approximately energy approximately 6 times less 6 times than thatless than of the that Pavegen’s of the system.Pavegen’s Even system. though Even it cannot though generate it cannot as much generate energy as as much the commercial energy as one, the commercialthe Genpath one, prototype the IIGenpath is developed prototype based II onis developed open hardware based which on open is easy hardware to access which and build.is easy The to access mathematical and buil modeld. The of themathematical system exists. model Thus, of itthe is possiblesystem exist to develops. Thus, Genpath’s it is possible system to develop in any community Genpath’s tosystem generate in any more energycommunity in the to future. generate more energy in the future.

Energies 2020, 13, 5419 17 of 19 Energies 2020, 13, x FOR PEER REVIEW 17 of 19

Pure R (8.7 Ω) Power Management Circuit

pi po

0.5 W pt 0.5 W

0.15 J 0.15 J Et Eo

(a) Power and Energy without Power and Storage (b) Power and Energy with Power Management and Management Circuit. Storage Circuit

FigureFigure 20. 20. ExperimentalExperimental result result showing showing the the operation operation of of the the power power management management system system regarding regarding thethe process process of of power power conversion conversion and and energy energy storage; storage; 푃푡P tandand 퐸E푡t are power and energy energy outputs outputs of of the the generator;generator; 푃P𝑖i isis power power output output of of the the rectifier rectiﬁer circuit circuit;; 푃P표 oandand 퐸E표o areare power and ener energygy outputs outputs of of the the buckbuck-boost-boost circuit. circuit.

TableTable 9. 9. PerformancesPerformances of of power power management management and and storage storage circuit circuit with with Genpath Genpath proto prototypestypes II and II and12- 12-V-DC generator. V-DC generator. Values per Footstep Variables Values per Footstep Variables Pure R푹 PowerPower Management Management and andStorage Storage Circuit Circuit (8.7(8.7Ω )) Maximum voltage; max (vt) 4.22 V 3.97 V Maximum voltage; max( vt ) 4.22 V 3.97 V Maximum current; max (it) 548 mA 635 mA

MaximumMaximum power; current; max ( pmax(t)it ) 548 2.29 mA W 635 mA 2.48 W Average power; pt 351 mW 374 mW Maximum power; max( pt ) 2.29 W 2.48 W Average power; pi - 359 mW Average power; p - 280 mW Average power;o pt 351 mW 374 mW Wave duration 1.18 s 1.08 s

Stored energyAverage at 1.4 s;power;Eo (t = 1.4pi s) 415-mJ 359 mW 302 mJ Efficiency of Active Rectifier - 95.78% Average power; p Efficiency of Buck-Boost Convertero -- 280 mW 78.00% OverallWave Effi ciencyduration 1.18 - s 1.08 74.72%s

Stored energy at 1.4 s; Eo (t = 1.4 s) 415 mJ 302 mJ 3. InstallationEfficiency and of Demonstration Active Rectifier - 95.78% EfficiencyTo demonstrate of Buck the-Boost application Converter of the developed- system, the system78.00 was% assembled as a floor tile with the dimensionOverall of Efficiency 40 40 15 cm and installed- by three sets of the74.72 floor% tiles side-by-side without × × wiring at an exhibition hall of a 100-year-old engineering building at Chulalongkorn University, 3.on Installation the mechanical and D engineeringemonstration project exhibition day. Figure 21 shows the photo of the system. ThenTo the demonstrate exhibitors werethe application invited to walkof the on developed the floor tiles.system, Once the they system stepped was assembled on the floor as tiles, a floor the tilesystem with generatedthe dimension power of to40 store 40 in 15 the cm battery and installed and lit upby thethree array sets ofof LEDs,the floor as seentiles side in Figure-by-side 21 . withoutThis demonstration wiring at an made exhibition exhibitors hall relive of a the100 importance-year-old engineering of green energy building and at energy Chulalongkorn harvesting University,in their daily on lives.the mechanical The benefit engineering of the designed project system exhibition can apply day. Figure for a lot 21 of shows applications, the photo such of asthe a syswirelesstem. Then sensor the and exhibitors Internet were of Thing invited applications. to walk on the floor tiles. Once they stepped on the floor tiles, the system generated power to store in the battery and lit up the array of LEDs, as seen in Figure 21. This demonstration made exhibitors relive the importance of green energy and energy harvesting in their daily lives. The benefit of the designed system can apply for a lot of applications, such as a wireless sensor and Internet of Thing applications.

Energies 2020, 13, 5419 18 of 19 Energies 2020, 13, x FOR PEER REVIEW 18 of 19

Figure 21. InstallationInstallation and and demonstration demonstration.. 4. Conclusions 4. Conclusions The paper presented a design of an energy harvesting floor capable of converting mechanical The paper presented a design of an energy harvesting floor capable of converting mechanical energy from people’s footsteps to electrical energy. The system, comprising the translation-to-rotation energy from people’s footsteps to electrical energy. The system, comprising the translation-to- conversion mechanism, the EM generator, and the power management circuit system, generates rotation conversion mechanism, the EM generator, and the power management circuit system, electricity from people’s footsteps. For the EM generator, the conversion mechanism for linear generates electricity from people’s footsteps. For the EM generator, the conversion mechanism for translation to rotation was designed by using the rack-pinion and lead-screw mechanism. Based on linear translation to rotation was designed by using the rack-pinion and lead-screw mechanism. simulation analysis, the averaged energy of the lead-screw model is more than that of the rack-pinion Based on simulation analysis, the averaged energy of the lead-screw model is more than that of the model. Moreover, the design of lead-screw model elements was studied. The results show that rack-pinion model. Moreover, the design of lead-screw model elements was studied. The results the lead-screw model with 45 lead angles can generate more averaged energy than others, and the show that the lead-screw model◦ with 45 lead angles can generate more averaged energy than others, 12-V-DC generator can provide more energy than the 24-generator due to the resistance during the and the 12-V-DC generator can provide more energy than the 24-generator due to the resistance transience, and the softer spring can convert translation motion to higher rotational speed of the during the transience, and the softer spring can convert translation motion to higher rotational speed generator which can produce more power. Although the softer springs are theoretically desirable, they of the generator which can produce more power. Although the softer springs are theoretically should be sufficiently hard enough to restore the system back to equilibrium due to the friction of the desirable, they should be sufficiently hard enough to restore the system back to equilibrium due to system. Then, Genpath prototype-II with 12-V-DC generator, lead-screw mechanism, and allowable the friction of the system. Then, Genpath prototype-II with 12-V-DC generator, lead-screw displacement of 15 mm was built. It was significantly improved when compared to the prototype-I [16]. mechanism, and allowable displacement of 15 mm was built. It was significantly improved when This Genpath prototype produces an average energy of up to 702 mJ (or average power of 520 mW). compared to the prototype-I [16]. This Genpath prototype produces an average energy of up to 702 The energy provided by Genpath prototype-II is increased by approximately 184% when compared mJ (or average power of 520 mW). The energy provided by Genpath prototype-II is increased by to that of the prototype-I [16]. The efficiency of the EM-generator system is 26% based on the power approximately 184% when compared to that of the prototype-I [16]. The efficiency of the EM- generation from the heel strike of a human’s walk of 2 W per step. Next, the power management generator system is 26% based on the power generation from the heel strike of a human’s walk of 2 and storage circuit were developed to harvested energy into the batteries and to supply other parts to W per step. Next, the power management and storage circuit were developed to harvested energy specific loads. The experiment showed that the circuit has the overall efficiency of 74.72%. The benefit into the batteries and to supply other parts to specific loads. The experiment showed that the circuit of the designed system can apply for a lot of applications, such as a wireless sensor and Internet of has the overall efficiency of 74.72%. The benefit of the designed system can apply for a lot of Thing applications. applications, such as a wireless sensor and Internet of Thing applications.

Author Contributions: Conceptualization,Conceptualization, T.J.,T.J.,S.S., S.S., and and G.P.; G.P.; methodology, methodology, T.J., T. S.S.,J., S.S., and and G.P.; G. software,P.; software, T.J., T S.S.,.J., and G.P.; validation, P.K., P.C. and C.U.-v.); formal analysis, T.J.; investigation, P.K., P.C. and C.U.-v.; resources, T.J.; S.S., and G.P.; validation, P.K., P.C. and C.U.-v.); formal analysis, T.J.; investigation, P.K., P.C. and C.U.-v.; data curation, G.P.; writing—original draft preparation, T.J., S.S., and G.P.; writing—review and editing, T.J., S.S., resources,and G.P.; visualization, T.J.; data curation, T.J.; supervision, G.P.; writing T.J.;— projectoriginal administration, draft preparation, G.P. T All.J., authorsS.S., and have G.P. read; writing and— agreedreview to and the editing,published T.J version., S.S., and of theG.P manuscript..; visualization, T.J.; supervision, T.J.; project administration, G.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ratchadaphiseksomphot Endowment Fund Chulalongkorn University, Funding:grant number This CU_GI_63_07_21_01.research was funded by Ratchadaphiseksomphot Endowment Fund Chulalongkorn University, grantAcknowledgments: number CU_GI_63_07_21_01We would like. to thank W. Lowattanamart, V. Suttisung, and S. Sintragoonchai who initiated and supported the idea on the project and built up Genpath prototype-I. Acknowledgments: We would like to thank W. Lowattanamart, V. Suttisung, and S. Sintragoonchai who initiatedConﬂicts and ofInterest: supportedThe the authors idea on declare the project no conﬂict and built of interest. up Genpath Also, theprototype funders-I. had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to Conflictspublish the of results. Interest: The authors declare no conflict of interest. Also, the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Energies 2020, 13, 5419 19 of 19

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