Distributing the Generation of Electricity to Extreme Level

Distributing the Generation of Electricity to Extreme Level

Tero Kivimäki, Janne Ruuskanen, David Blažević, Aki Halme, Turo Salminen, Jukka Vanhala, Paavo Rasilo Fa culty of Informa tion Technology a nd Communica tion Sciences, Electric Engineering

Ta mpere University Ta mpere, Finla nd

ema il:{tero.kivima ki, ja nne.ruuska nen, a ki.ha lme, turo.sa lminen, da vid.bla zevic, jukka .vanhala , pa avo.ra silo}@tuni.fi Abstract—Energy harvesting is the process of converting

low level ambient energy into electrical energy usually in the µW to mW range. Energy harvesting enables the distribution of the production of electricity to extremely localized in situ electricity generation. This paper briefly describes our research work and results in the field of energy harvesting.

Energy harvesting in general is first shortly discussed and then the design, implementation and testing of our inductive harvester are presented.

Keywords—distributed generation, energy harvesting, microgeneration, in situ generation

I. INTRODUCTION

The genera l interest towa rds the microgenera tion of electricity ha s grown rema rka ble during the la st ten yea rs.

Consumers a re willing to sma ll sca le production of electricity for their own needs. Susta ina ble sola r power genera tors ca n be found on the rooftops of single-fa mily houses in ever increa sing numbers [1]. Energy ha rvesting ta kes the idea further: the electricity is genera ted precisely where it is needed.

In situ electricity genera tion ena bles the consuming device to genera te the electricity it needs to opera te.

Consequently, ha rvester powered electrica l a ppa ratus can be used in a n environment where no grid electricity is a va ila ble. Therefore, the ha rvester technology can significa ntly reduce the costs a nd a mount of la bour needed for wiring conventional electric devices. Tha t ca n be a rema rka ble benefit for exa mple in the sma rt homes of the nea r future where severa l hundreds of sensors a nd a ctuators might exist, or in a ny situa tion where the insta lla tion of wires is pa rticula rly tough or ma y not be even possible.

The energy ha rvesting technology reduces the usa ge of a ccumula tors a nd batteries a nd the la borious repla cement of ba tteries ca n be a voided. Consequently, the a mount of produced toxic wa ste ca n be reduced.

This pa per presents the design, implementa tion and testing of a n electroma gnetic induction ha rvester system.

This pa per is orga nized a s follows. In Section II, the power sources, genera l a rchitecture, a nd technologies used with energy ha rvesting a re described. Section III expla ins the inductive ha rvester more ca refully. The simula tion tool used in the design a nd the results a chieved by simula tion a re presented in Section IV. The implementa tion a nd the construction of the prototype ha rvester a nd the other components of the system a re expla ined in Section V. The testing a nd the results a re described in Section VI. Section VII provides the conclusions a nd describes the future work.

II. ENERGY HARVESTING SCHEMES

Severa l power sources a nd technologies a re used in energy ha rvesting. The most used energy sources a re kinetic energy (vibra tion or rota tion), light a nd hea t. The most preferred technologies include electrodyna mic and

piezoelectric, photovolta ic, a nd thermoelectric, respectively [2].

The genera l a rchitecture of a ha rvesting system is shown in Fig. 1. The power source specific microgenera tor converts the source energy to electricity. The genera ted volta ge must be converted a nd rectified by the energy ma na gement electronics. The cha rging of the energy storage is a lso ma na ged by the electronics. A low-ca pa city energy stora ge is usua lly needed a nd implemented by a superca pa citor. The consuming device is electrified by the ma na gement electronics.

Figure 1. The architecture of a harvesting system.

The most widely used power source for ha rvesting is light. For deca des there ha s been ca lcula tors a nd wa tches powered by light to electricity conversion. Genera l shortcoming rela ted to photovolta ics is evident in interrupted supply of light energy i.e., light is not a va ila ble in a ll circumsta nces, a nd the surfa ce needed by the photocell is rela tively la rge [2].

A tempera ture gra dient ca n be used to genera te volta ge by thermoelectric effect. If a tempera ture difference is ma inta ined between two termina ls of a thermoelectric module, electrica l power is genera ted to the module’s output termina ls [3]. To get a sa tisfa ctory a mount of power, considera ble tempera ture differences a re needed.

Pressure cha nge ca n be used to ha rvest electricity by using three technologies: piezoelectric, electrosta tic and ma gnetostrictive. These technologies a re briefly presented in the following pa ra gra phs.

Piezoelectric effect refers to the ca pa bility of piezoelectric ma teria ls to genera te electric cha rge when a mecha nica l stra in is a ffecting to it. Piezoelectric ha rvesters a re utilizing this effect to convert mechanical source energy to electricity [4]. The mecha nica l source energy ca n be in the form of pressure, flow, rota tion, or vibra tion.

In the electrosta tic ha rvester a ca pa citor is subjected to pressure cha nges from the environment which va ry the dista nce between the ca pa citor’s conductive pla tes and consequently a lter the ca pa cita nce. The cha nge of the

ca pa cita nce is used to increa se the electrica l energy stored in the ca pa citor, a nd the energy ca n be collected by discha rging the ca pa citor [5]. In a ddition to pressure cha nge, vibra tion is a lso used a s a source energy for electrosta tic ha rvesters Although a n idea l ca ndida te for minia turiza tion, electrosta tic energy ha rvesters ca nnot function without a n initia l power source, a ba ttery, which ma inta ins the ca pa citor pla tes cha rge sepa ra tion .

When ma gnetostrictive ma teria ls a re deformed, they induce cha nges to the ma gnetic field. This phenomenon is known a s inverse ma gnetostriction a nd is used in energy ha rvesting. A time-va rying pressure is a pplied to a ma gnetostrictive ma teria l, ca using a time-va rying ma gnetic field. The time-va rying ma gnetic field induces current in a coil loca ted in the ra nge of the ma gnetic field [6].

Vibra tion a nd rota tion as a source energy ca n be utilized by inductive ha rvesters. Rota tion ca n be tra nsformed to electricity by a tra ditiona l, but mini-sized genera tors. The utiliza tion of vibra tion by a n inductive ha rvester is presented in the next cha pter. In a ddition to a bove mentioned power sources, a lso other sources of energy ha ve been resea rched. For exa mple, in [7] a ha rvester that uses the a ccelera tion of a fired projectile a s a power source is represented.

III. INDUCTIVE LINEAR HARVESTER

In our a pproa ch the electromagnetic induction is used to ha rvest electricity from mecha nical vibra tion. The grounds a re tha t the inductive ha rvester is quite uncomplica ted and inexpensive to construct. Further, the opera ting principle is ba sed on a phenomenon tha t ha s been recognized and resea rched for centuries. Furthermore, kinetic energy suita ble for ha rvester excita tion exists commonly as a consequence of running ma chines a nd human a ctivity.

Fa ra da y’s la w of induction, published a lrea dy in 1831 [8], is utilized by the inductive ha rvesters. In a ccordance the la w of induction, time-va rying ma gnetic flux genera tes electromotive force to a conductor tha t loca tes in the influence of the ma gnetic flux. The time-va rying ma gnetic flux is genera ted by ma king a perma nent magnet to move nea rby the conductor, or the other wa y a round, ma king the conductor to move nea rby the ma gnet [9]. In inertia l vibra tion ha rvesters the preferred solution is to ma ke the perma nent magnet to move rela tive to a coil. This solution results in fixed output wires from the coil, which simplifies the construction of the physica l structure of the ha rvester [10]. The source mecha nica l energy (vibra tion) is used to excite the ha rvester, i.e., to get the ma gnet to move.

Our ha rvester is opera ting linea rly, signifying tha t the ma gnet a nd the coil a re moving rela tive to ea ch other back a nd forth on one a xis. The ha rvester is composed of a moving ma gnet a nd a coil. A perma nent ma gnet rests on springs inside a pla stic tube. When insta lled to a loca tion where vibra tion occurs, the ma gnet sta rts to vibra te and consequently, current is induced to the coil winded a round the pipe. The structure a nd the circuit dia gra m of a typical inductive linea r ha rvester a re illustra ted in Fig. 2.

The ha rvester is used in loca tions where continuous vibra tion exists, like inside or on a ma chine ca sing, a vehicle or a living a nd moving crea ture. The external dimensions of the ha rvester ca n be a djusted, if needed it

ca n be implemented in considera ble sma ll sca le. However, the dimensions a re a lmost directly proportiona l to the electric genera ting ca pa city a s the genera ting ca pa city is dependent on the number of turns of the coil a nd the strength of the ma gnet, in a ddition to the velocity of the ma gnet. The ha rvester must be designed a nd a djusted based on the opera tion environment (a va ila ble kinetic energy, spa ce, a nd weight limita tions) a nd on the power requirements (output energy needed).

Figure 2. The structure(a) and the circuit diagram(b) of the harvester.

IV. SIMULATION Designing a n optima l device for a given a pplica tion

requires a ccura te simula tion tools. For this purpose, a finite element method (FEM) ba sed simula tion tool wa s developed a nd implemented on MATLAB.

In this section, the utilized simula tion a pproa ch a nd the simula tion results demonstra ting the electromecha nical beha viour of the designed ha rvester a re presented.

A. Approach

In the utilized simula tion a pproa ch, we use FEM to a ccura tely compute the electroma gnetic qua ntities present in the system. For a given geometry, spline interpola tion functions for induced volta ge a nd ma gnetic force a re crea ted ba sed on their pre-computed va lues a t releva nt ma gnet position a nd coil currents. This method ma kes solving the resulting time dependent problem extremely fa st for a given ha rvester excita tion.

In the FEM pa rt of the simula tion a pproa ch, a Maxwell’s magnetostatic problem is solved on an a xisymmetric modelling doma in. The problem to be solved rea ds

where (1) is the Amperes la w where h is the ma gnetic field strength rela ted to the ma gnetic flux density b by the permea bility µ, a s in (3). The ma gnetic flux density is required to be divergence free by (2). Furthermore, b is expressed by the curl of the ma gnetic vector potentia l a that is the unknown field of our ma gnetosta tic problem. The source fields of the problem a re the ma gnetic field hpm, that is due to the ma gnetiza tion of the perma nent magnet, and the coil current density jcoil. Moreover, a linea r rela tion between the magnet’s magnetic field hpm a nd ma gnetic flux

density bpm is a ssumed, i.e., hpm = µpmhpm, where µpm is the permea bility of the ma gnet.

Ba sed on the solution of the ma gnetosta tic problem, the tota l ma gnetic force Fm a cting on the moving perma nent ma gnet, due to the coil current or other ma gnetic objects, is computed using the virtua l work principle [11].

In order to model a n inductive vibra tion ha rvester, a coupled electrodyna mica l a nd mecha nica l problem is solved with a time-stepping procedure. The coupled problem thus consists of two equa tions to be solved with two unknows, i.e.,

(5)

where m is the ma ss of the moving ma gnet, c the da mping coefficient, k the spring consta nt a nd a the excita tion of the housing. The resista nces Rcoil a nd Rload a re the resista nce of the coil wire a nd the resista nce of the loa d, respectively.

The ma gnetic force Fm is pre-computed with FEM a s a function of the position of the ma gnet z, with respect to its housing, a nd the coil current I. The induced volta ge, i.e., the time-deriva tive of the coils’ tota l ma gnetic flux is expressed with the cha in-rule a s

(6)

where the deriva tives of coils’ ma gnetic flux Φ with respect to ma gnet position a nd coil current a re pre-computed with FEM. For solving the time dependent coupled problem (5), ode45 (Ma tla b’s ordina ry differentia l equa tions solver function) wa s utilized.

In rega rds of the development of the simula tion tool, as a next step, the simula tion tool will be ma de compa tible with a n electrica l circuit simula tor. Consequently, the FEM ba sed model describing the ma gnetomechanica l beha vior of the ha rvester ca n be coupled with the electronics consisting of cha rging the energy stora ge a nd the components mea suring a nd tra nsmitting informa tion a t the ha rvester loca tion.

B. Results

Using the simula tion tool developed ba sed on the presented simula tion a pproa ch, a ha rvester design wa s optimized for our a pplica tion of interest, i.e., to be pla ced inside a ca r tyre. The obta ined design pa ra meters a re shown in TABLE I. The ha rvester design is presented in more deta il in the following section.

TABLE I. OPTIMIZED DESIGN PARAMETERS

The ha rvester wa s optimized ba sed on the mea sured a ccelera tion da ta shown in the top gra ph of Fig. 3. The data is mea sured in the ra dia l direction a nd corresponds to car speed of 80 km/h. The ra dius of the tyre wa s 30 cm. The constra ints for optimizing the ha rvester were: minimize the ma ss a nd volume of the ha rvester such tha t the minimum a vera ge loa d power is 1 mW.

Fig. 3 shows the electromecha nica l beha viour of the ha rvester when exciting the ha rvester a ccording to the a ccelera tion shown in the top gra ph. The second gra ph from the top shows the simula ted displa cement of the ma gnet with respect to its housing, a s a function of time.

Below tha t, the gra ph shows the simula ted loa d volta ge.

The bottom gra ph shows the loa d power a s a function of time. The a vera ge of the loa d power wa s a round 50 mW.

Figure 3. Simulation results.

V. PROTOTYPE IMPLEMENTATION

On the grounds of the results of the simula tion, a prototype device wa s implemented. In a ddition to the actual ha rvester, a lso other components of the system (shown in Fig. 1) ha ve got to be implemented to permit the properly testing of the ha rvester.

The housing of the ha rvester is implemented with a progra mma ble turning tool with indexa ble insert. A Teflon ba r wa s first used a s a ma teria l, but it wa s noticed that polya ceta l is ea sier to process. With Teflon, the coefficient of friction (between the ma gnet a nd the tube) would have been minor, but by using polya ceta l, thinner wa ll thickness (between the ma gnet a nd the coil) wa s rea liza ble, a nd that results a more critica l effect to the a mount of power genera ted by the ha rvester. The housing with the springs a nd the ma gnet is shown in Figure 4a , a nd Figure 4b presents the housing with the two coils. The coils a re made of copper wire with a dia meter of 0,1 mm a nd the number of turns is 2000 in both. The coils a re winded in opposite directions.

Figure 4. The housing of a double-spring design with the springs and the magnet(a) and with the coils(b).

After the first prototype implemented it beca me a ware tha t a ha rvester with sma ller dimensions with the same power output would be possible to construct, if only one spring with a grea ter spring consta nt would be used. The va lues for the spring a nd the ma gnet were reca lcula ted and a new housing wa s la thed. The housing of the second prototype ha s dimensions 20,10 mm x 26,00 mm, which is 7,0 mm lower tha n the first version. The dimensions a re shown in Figure 5a , a nd the housing with the ma gnet and the spring in Figure 5b. The coils in single-spring version of the ha rvester a re simila r tha n in the double-spring version.

Figure 5. The dimensions(a) and the housing with the magnet and the spring(b) of the single-spring design. The coin is included to illustrate

the dimensions.

The spring used in the single-spring design is a normal commercia lly a va ila ble helica l spring ma de of sta inless- steel wire with a dia meter of 0,81 mm. The dia meter of the spring is 9,14 mm, the length of rest is 15,75 mm, a nd the spring consta nt is 2,17 N/mm [12].

The dimensions of the disc sha ped nickel-pla ted perma nent magnet, which is a lso a commercia l product, are 10,00 mm x 5,0 mm. The strength of the ma gnet is 3,3 kg [13].

The friction between the ma gnet a nd the housing of the ha rvester decelera tes the ma noeuvring speed of the ma gnet a nd consequently the electric production of the ha rvester.

During the construction of the third prototype, in pursuit to reduce friction between the moving ma gnet a nd the cylindrica l housing of the ha rvester, we turned to the precision design principles a nd the principles of ba ll bea ring opera tion. By employing three guides inside the ha rvester housing the ma gnet ca n be precisely a ligned in spa ce a nd by reducing the conta ct surfa ce between the ma gnet a nd the housing, like in a ba ll bea ring construction - the friction is minimized. The low-friction design is shown in Fig. 6. The method of ma nufacture wa s simila r to tha t of the previous version, likewise the dimensions, the

used ma gnet a nd the spring, a nd the coils a re identica l to the preceding version.

Figure 6. Low-friction harvester frame.

The energy ma na gement electronics wa s implemented ba sed on Texa s Instruments BQ25570 integra ted circuit (IC). The IC is designed to opera te with va rious ha rvesters, like for exa mple sola r pa nel or therma l energy collector [14], but in our a pproa ch it is optimized to work with an inductive ha rvester. Only a limited number of external components is needed, a nd due to the minia ture dimensions of the IC (3,50 mm x 3,50 mm), it is a ppropria te for size optimized a pplica tions.

As a n energy stora ge, a commercia lly a va ila ble superca pa citor is used. On the grounds of the power requirements of the consuming device, a superca pa citor with 220 mF ca pa cita nce wa s selected. Superca pa citor has a nomina l volta ge of 5,5 volts tha t is sufficient for the loa ding system. The dimensions of the cylindrica l ca pa citor a re 11,5 mm in dia meter a nd 5,0 mm in thickness [15].

Size of Printed Circuit Boa rd (PCB) is 20 mm x 30 mm including a ll a ctive a nd pa ssive components.

Superca pa citor is connected to the PCB by through hole pins. Other components a re interconnected by surface mount technology.

Electrica l current produced by the energy ha rvester is a lterna ting a nd ha s got to be rectified before used by the boost cha rger. As a rectifying component, a Schottky diode-ba sed bridge rectifier CBRHDSH1-40L is used. It is pa cka ged in one solid IC ca se, a nd ha s a forwa rd volta ge drop of 440mV with the ma ximum current of 1A [16].

As a n electric consuming device, we a re using an a ccelera tion sensor a nd a Bluetooth Low Energy (BLE) tra nsmitter to tra nsmit the da ta measured by the sensor. The mea surements a nd the tra nsmitter a re implemented by using Apollo3 Blue microcontroller, whose power consumption is rema rka bly low. Apollo3 Blue ha s Arm Cortex-M4F core running a t the frequency of 48 MHz, integra ted Bluetooth 5.0 low energy with a dedica ted processor a nd a fa st 14-bit a na log-to-digita l converter (ADC) on chip. According to the ma nufacturer’s da tasheet, the power consumption is 6 μA / MHz a nd 1 μA in microcontroller’s deep sleep state [17]. We tested the power consumption by ma king 900 mea surements at a frequency of 15 kHz a nd by sending the mea sured data to the receiver. This test process wa s performed 60 times, the minimum current wa s found to be 3,6 μA, the ma ximum 4500 μA, and the average 240 μA. On the grounds of the mea sured power consumption a nd the fea tures provided, Apollo3 Blue microcontroller wa s discovered to be a ppropria te for our purpose.

Requirements for the a ccelerometer consists of the frequency response to be a t minimum 15 kHz a nd the

mea surement ra nge to be plus-minus 200 g. The requirements a re ca used by the fina l testing environment (inside a ca r tyre a t velocities up to 100 km/h) a nd by the intentions to utilize the mea sured da ta . A single a xis a na logica l output a ccelerometer ADXL1003 [18] wa s selected grounds on the requirements.

The microcontroller wa s progra mmed with Ambiq Softwa re Development Kit [19], a nd a custom BLE profile Ambiq Micro Da ta Tra nsfer Protocol [20] is used for the da ta tra nsmission. The softwa re mea sures 900 sa mples with the a ccelerometer a t the frequency of 15 kHz once per second a nd tra nsmits this sa mple bundle to the receiver.

The receiver is connected to a seria l port of a PC, a nd the da ta is tra nsferred to the PC to be further processed a nd to be displa yed to a huma n user. The softwa re runs on FreeRea lTimeOpera tingSystem (FreeRTOS) [21] provided with the softwa re development kit, a nd utilizes services provided by the FreeRTOS in timing of the ta sks a nd to put the microcontroller into the deep sleep sta te when possible a nd profita ble.

VI. TESTING AND RESULTS

The prelimina ry testing of the ha rvester wa s performed by a muscle ma intena nce ha mmer. An a da pter to connect the ha rvester to the ha mmer wa s ma nufactured by 3D printing. The frequency a nd the a mplitude of the vibra tion provided by the ha mmer a re a djusta ble. This a ppa ratus, shown in Fig. 7, wa s used to quickly a nd unla boriously test the funda mental opera tion of the ha rvester.

Figure 7. The harvester connected to a muscle maintenance hammer.

The second pha se of the testing wa s completed in the la bora tory. The mea surements were a djusted and performed, a nd the results ga thered by Signa l Express mea surement a nd da ta -logging softwa re running on a la ptop computer. Na tiona l Instruments USB6251 Data Acquisition Module wa s used to connect a ll the test and control points. The device to genera te the source energy wa s Vibra tion exciter system 4805 ma nufactured by Brüel

& Kja er, a nd the control signa l to the exciter wa s a mplified by Venea ble Linea r Amplifier VLA 1000. The ha rvester wa s connected on the exciter between two pla stic pla tes.

The a ccelera tion of the ha rvester wa s mea sured by a n PCB 355B02 a ccelera tion sensor, with a 10 mV/g sensitivity, connected next to the ha rvester, a nd PCB 480C02 Signa l Conditioner wa s used to condition the a ccelerometer signa l. Adjusta ble electrica l loa d wa s implemented with three resistors a nd one potentiometer a nd connected to the ha rvester. The test configura tion a nd devices a re presented in Fig. 8.

The electric load was adjusted to 390 Ω, as this equals to the a ctua l loa d provided our consuming device. The low- friction version of the ha rvester wa s used, the prelimina ry tests provided tha t the electric production ca pa city of it is considera bly higher tha n the production ca pa city of the previous ha rvester versions.

Figure 8. The testing equipment.

The tests were performed by regula ting the frequency of the vibra tion used a s a source energy a nd by mea suring the volta ge a nd the current from the loa d powered by the ha rvester. The electric power rms va lue wa s ca lcula ted from the volta ge a nd the current. A representa tive sa mple of the tests results is shown in Fig. 9.

Figure 9. The output voltage (top graph) and power (bottom graph) at different frequency.

The test results strongly indica te the cha ra cteristic of frequency dependence of inductive linea r ha rvester. The output power ha s the ma ximum va lue a t the resona nce frequency, a nd the va lue of the output power decrea ses ra pidly when disloca ting from the resona nce frequency.

The resona nce frequency wa s observed to equa l 131 Hz, in the results of the simula tion the va lue wa s 134 Hz. The

difference between the simula ted a nd the mea sured va lues a re ma inly ca used by the ma ss of the sea ling compound used to secure the springs to the ma gnet, which wa s not a cknowledged in the simula tion.

The mea sured output power wa s on a n expected level on the grounds of simula tion a nd a dequa te to power our consuming device when opera ting in the vicinity of the resona nce frequency. Indeed, tests were a lso successfully performed to power the mea surement a nd the data tra nsmission by the ha rvester. The ha rvester produced a dequa tely electricity to power the consumer device a nd to loa d the energy stora ge simulta neously. The consuming device wa s ca pa ble to opera te five a nd ha lf minutes powered by the fully loa ded energy stora ge a fter the source energy wa s disa bled.

A ca r tyre will be used a s the fina l testing environment. More a ccura tely, the testing environment will be inside a tyre of a va n. A va n tyre wa s selected a s there is a dequa tely kinetic energy a va ila ble (grea ter a mount than in a hea vy-duty tyres), but the rota tion speed a nd exertion forces a re lower tha n in the tyre of a pa ssenger ca r. A tyre is a potentia l opera tion environment for a ha rvester a s it would be useful to be a ble to mea sure numerous qua ntities from the tyre, in pa rticula r when developing self-drivin g ca rs. Further, due to the tyre being in consta nt rota tion, it is dema nding to provide ca bling inside a tyre.

VII. CONCLUSIONS AND FUTURE WORK

The development of microelectronics during the la st yea rs ha s resulted with a significa nt drop in dissipa tion of electric energy [22]. This ena bles the implementa tion of self-powered devices, tha t a re genera ting the electricity needed to opera te by using energy ha rvesting. Energy ha rvesting provides environmenta lly friendly renewa ble electricity in situ. This is extremely profita ble in surroundings where no grid electricity is a va ila ble, and ba tteries a re not a n a ppropria te solution.

We ha ve designed a n inertia l inductive ha rvester using a simula tion tool. We ha ve a lso designed a nd implemented severa l prototypes of the ha rvester system on the strength of the results of the simula tion a nd performed initia l and functiona l testing with the prototypes in the la bora tory environment. The test results indica te tha t the ha rvester is ca pa ble to provide the opera ting electricity to a low powered device in suita ble environment. The next step will be the testing of the proto device in a rea l-world use ca se.

An a greement ha s been rea ched with a lea ding roa d vehicle tyre ma nufacturer to perform further tests. They will supply us with the tyres a nd the technology needed to mount the ha rvester system inside a tyre. Further they will provide us a testing environment, in which the tyre will be rota ted, and different kinds of roa d surfa ce ca n be simula ted.

Our future resea rch on inductive ha rvesters will deal with using ma gnetic springs, i.e. perma nent ma gnets instea d of mecha nica l springs. The resea rch will investiga te if a more robust structure is a chieva ble by using ma gnetic springs. Further, it will be investiga ted if a n a djustable ma gnetic spring force ca n be implemented, a nd how this a djusta bility could be utilized on the electricity output ca pa bility of a ha rvester.

VIII.ACKNOWLEDGEMENTS

This resea rch a nd work a re being conducted within the ENOMA-project, which is pa rtly funded by the Business Finla nd under the gra nt 6698/31/2018 together with several industria l pa rtners.

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[11] A. Kameari, Local force calculation in 3d fem with edge elements, International journal of applied electromagnetics in materials, vol.

3, no. 1, pp. 231–240, 1993.

[12] Spring C03600320620S datasheet, available at:

https://www.industrial-springs.com/c03600320620s

[13] Magnet S-10-05-N52N datasheet, available at:

https://www.supermagnete.fi/disc-magnets-neodymium/disc- magnet-10mm-5mm_S-10-05-N52N

[14] BQ25570 Integrated circuit datasheet, Texas Instruments, available at: https://www.ti.com/lit/ds/symlink/bq25570.pdf [15] DX-5R5H224U Electric double layer capacitor datasheet, Elna

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http://www.elna.co.jp/en/capacitor/double_layer/catalog/index.ht ml

[16] CBRHDSH1-40L Diode rectifier datasheet, Central

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[17] Apollo3 Blue microcontroller datasheet, Ambiqmicro, available at: https://ambiq.com/wp-content/uploads/2020/10/Apollo3- Blue-MCU-Datasheet.pdf

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https://www.analog.com/en/products/adxl1003.html

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https://learn.sparkfun.com/tutorials/using-sparkfun-edge-board- with-ambiq-apollo3-sdk

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Proceedings of the 34^th Symposium on Microelectronics Technology and Devices, 2019.

Authors’ Background

Name Email Position

(Prof , Assoc.

Prof. etc.)

Research Field Homepage URL

Tero

Kivimä ki tero.kivima ki@tuni.fi

Postdoctora l Resea rch

Fellow Energy ha rvesting https://www.tuni.fi/en/tero-kivima ki Ja nne

Ruuska nen

ja nne.ruuska nen@tuni .fi

Postdoctora l Resea rcher

Computa tional

electroma gnetics https://www.tuni.fi/en/ja nne-ruuskanen Da vid

Blažević

Da vid.bla zevic@tuni.

fi

Ma rie Curie

Postdoc Energy ha rvesting https://www.resea rchga te.net/profile/D a vid-Bla zevic

Aki Ha lme a ki.ha lme@tuni.fi Doctora l

Resea rcher Embedded systems Turo

Sa lminen turo.sa lminen@tuni.fi Resea rch Assista nt

Embedded systems

softwa re https://www.tuni.fi/en/turo-sa lminen Jukka

Va nha la jukka .va nhala@tuni.fi Professor

Embedded systems https://www.tuni.fi/en/jukka -vanhala Pa a vo

Ra silo pa a vo.ra silo@tuni.fi Associa te

Professor Electromecha nics https://www.tuni.fi/en/pa a vo-ra silo