VIBRATIONAL ENERGY SCAVENGING WITH SI TECHNOLOGY ELECTROMAGNETIC INERTIAL MICROGENERATORS

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1 Stresa, Italy, 6-8 Aril 6 VIBRATIONAL ENERGY SCAVENGING WITH SI TECHNOLOGY ELECTROMAGNETIC INERTIAL MICROGENERATORS C. Serre 1, A. Pérez-Rodríguez 1, N. Fondevilla 1, J.R. Morante 1, J. Montserrat, and J. Esteve. 1 EME/CEMIC/CERMAE Det. Eletrònia, Univ. Barelona, Martí Franquès 1, 88 Barelona, Sain. Centre Naional de Miroeletrònia CNM-CSIC, Camus UAB, 8193 Bellaterra, Sain ABSTRACT In this work, we resent the design and otimization of an eletromagneti inertial mirogenerator for energy savenging aliations, omatible with Si tehnology. It onsists of a fixed miromahined oil and a movable magnet (inertial mass) mounted on a resonant struture (Katon membrane). The modeling of the devie, based on a veloity damed resonator, inludes the losses related to the oil series resistane and has allowed the analysis of the design and loading onditions required to otimize both the generated ower and outut voltage. The haraterization of a first (not otimized) rototye has allowed the validation of the model, whih is then used as a roadma for a number of otimizations for the final devie design. For this design, the model develoed shows the ossibility to ahieve ower levels u to hundreds of μw s, with voltage levels omatible with the requirements of standard retifying iruits. 1. INTRODUCTION The autonomy requirements of modern mirosystems for wearable, ubiquitous and self-owered aliations have raised an inreasing demand for the develoment of ower sulies suitable for their integration with next generation of miro and nanosensors. On the other hand, owing to the miniaturization of the devies, the ower onsumtion have onsiderably dereased, allowing to onsider owering alternatives based on harvesting residual ambient energy. Among other soures, mehanial vibrations inherent in our environment -- from the movement of our bodies to the hum of a omuter -- an aount for a ermanent ower density. This residual ambient energy an be harnessed to generate eletrial ower. For suh aliations, an interesting otion is the use of inertial mirogenerators for energy savenging from the vibrations in the environment [1, ]. These devies onstitute eretual energy soures without the need for refilling, thus being well suited for abandoned sensors, wireless systems or mirosystems whih must be omletely embedded within the struture, with no outside hysial onnetions. This work desribes the design and otimization of an eletromagneti inertial mirogenerator for energy savenging aliations, omatible with Si tehnology. The design is based on a veloity damed resonator, whih is suitable for harvesting of mehanial energy from vibrations indued by oerating mahines and engines. These vibrations are haraterized by a well defined frequeny and low dislaement amlitudes [3]. Adjusting the resonant frequeny of the system to that of the vibrations allows to amlify these low amlitude dislaements. Moreover, for these aliations, the use of an eletromagneti devie has the otential advantages of a high level of omatibility with Si Mirosystem tehnology, as well as the ossibility of relatively high eletromehanial ouling with simle designs. The devie roosed in this work has a simlified struture formed by a fixed oil and a movable magnet (inertial mass) mounted on a resonant struture (Katon membrane). The modeling and otimization, based on the alulation of the em daming oeffiient [1], and erformed by Finite elements analysis (ANSYS) are resented. The mehanial and eletrial haraterization of a reliminary rototye is omared to those exeted with a next rototye inluding some of the roosed otimizations. The results obtained oint out the omatibility of this simle devie struture with the generation of ower values u to hundreds of μw s, with voltage levels omatible with the requirements of standard retifying iruits.. DEVICE MODELLING The modeling of the devie is based on a veloity damed resonator, as reresented in figure 1. This system is formed by an inertial mass m linked to the frame with an elasti onstant k. The movement is damed by two fores related to the eletromehanial transdutor (F g = D g ż) and to a arasiti daming (F = D ż) due to air resistane and hysteresis loss effets in the mehanial resonator.

2 k D g m D z(t) takes into aount the ower dissiated in the oil series resistane, whih determines that only a fration P L of the ower given by () is available at the load resistane. By deriving this ower in relation to R L, it is ossible to determine the otimum value of R L whih maximizes P L : 1 dφ R Lot = + R mω ζ dz n whih gives the following exression for the maximum ower dissiated at the resistive load: C (4) y(t) P Lot Y ω m ζ 3 o n = 16 ζ ζ + ζ (5) Figure 1. Shemati reresentation of a veloity damed resonator. Under an harmoni exitation y(t) = Y o os (ωt), the amlitude Z o of the resonse is given by the module of the transfer funtion as follows: Z o ω = (1) Y o 1 ω ) + (ζ ω ) ( where ζ reresents the total daming oeffiient ζ = ζ + ζ g, ζ g = D g /( m ω n ) and ζ = D /( m ω n ) are the normalized eletromagneti and arasiti daming fators, resetively, and ω is the angular frequeny normalized to the system natural frequeny ω n. A shemati reresentation of the roosed design is shown in figure. Assuming a resistive load, the devie behaves as an inertial resonator if the indutive omonent of the oil imedane is muh lower than the resistane in the iruit. In this ase, the ower generated at resonant onditions is given by: P res 3 ζ g Yo ωn m = () ( ζ + ζ ) 4 g In this design, the normalized eletromagneti daming fator ζ g an be exressed as follows: 1 dφ ζ = (3) g m( R + R ) ω dz C L n where R and R L are the oil series and load resistanes, resetively. (dφ/dz) is the magneti flux rate through the oil due to the magnet dislaement. This model also takes into aount the existene of a arasiti daming ζ, related to air resistane and hysteresis loss effets in the mehanial resonator. From (), it an be derived that one ondition leading to a maximum value of P res is ζ g = ζ. However, this doesn t where ζ orresonds to the eletromagneti daming obtained with R L =. This funtion inreases monotonously with ζ whih, in turn, is inversely roortional to R C. Aording to this, the maximum ower, obtained when R, is P Lmax = [(Y o ω n 3 m)/(16 ζ )]. Then, the otimum design in terms of the generated ower orresonds to a minimum value of both R C and ζ. On the other hand, the generated voltage is given by the time derivative of the magneti flux. At resonant onditions, the voltage amlitude at the load is given by: RL Yo ωn Vo = ( R + R ) ( ζ + ζ C L g dφ ) dz In this ase, the voltage inreases with the value of R L, and tends asymtotially to V omáx : V o max Yo ωn = ζ dφ dz This imlies that, in relation to R L, the onditions leading to a maximum voltage (R L ) are different from those orresonding to the maximum outut ower (R L = R Lot ). Planar oil Membrane h z (7) r nul r mag Figure. Shematis ross setion of the devie (6)

3 3. FIRST PROTOTYPE FABRICATION AND CHARACTERIZATION A first rototye has been designed and fabriated, using a oil with an area of about 1 m. The design of the devie has been based on the FE analysis (ANSYS) of the flux rate (dφ/dz) max as a funtion of the different devie arameters This has been erformed assuming oils formed by 3 μm wide, 1.5 μm thik Al metal traks, with a searation between traks of μm, and for both irular and square shaed onfigurations. The results indiate that i) (dφ/dz) inreases with the magnet size, and the otimum ase is obtained when the magnet fills the whole nuleus area and ii) that there is a maximum value of the flux rate when the uer surfae of the magnet is loated in the lane of the oil. This analysis also shows that the otimum flux rate value for this first rototye was obtained with square shaed oils, made of 9 turns. This determines an 8x8 mm oil nuleus. A struture formed by a square shaed membrane with an inertial mass orresonding to the magnet has been imlemented, using Katon membrane with a thikness of 17 μm. This olymer has a Young modulus signifiantly lower than that of Si related materials (E =.5 GPa), whih is better suited for the design of strutures with resonant frequenies in the range from few Hz s to few khz s. This has been fixed on a PCB square frame and the NdFeB ermanent magnet has been glued on the entre of the membrane. To avoid otential ollisions of the magnet with the edges of the oil nuleus, a magnet with a size a bit smaller than that of the nuleus has been used (7x7 mm ). The haraterization has been erformed with the following exerimental setu: exitation is rovided by a iezoeletri atuator (15 μm dislaement at 1 V), with a resonant frequeny at 69kHz (i.e. far from our d (μm) d (μm) user simulation f (Hz) Figure 3. Mehanial haraterization of the struture formed by a 7x7x mm 3 magnet fixed onto an 11x11 mm Katon membrane (thikness 17 μm). PL (mirow),5,4,3,,1,8 1 1, w Figure 4. Outut ower P L vs normalized angular frequeny ω, measured with a load resistane R L = 13 Ω and an exitation amlitude Y o =3.4 μm. range of interest). The mehanial resonse of the struture is measured with a Mirotak 7 dislaement sensor system ouled to a MT-5- laser head (MTI Instruments In.), allowing a vertial resolution of.17μm with a khz bandwidth. The laser head is mounted on a XYZ translation stage, and lamed to a damed mounting ost to avoid noise from interation with the exitation. The exitation signal, the analogue outut of the Mirotak 7, and the eletrial resonse of the devie are monitored by an osillosoe for easier analysis. The mehanial haraterization of this struture with an 11x11 mm membrane shows a resonant frequeny (figure 3) in agreement with that simulated by ANSYS (about 9 Hz). The fitting of the exerimentally measured resonant eak aording to (1) has allowed to estimate the arasiti daming oeffiient in this resonator to a value of ζ =,11. The eletrial haraterization of the mirogenerator with a 7x7x4 mm 3 magnet and a 13x13 mm membrane (resonant frequeny of 36 Hz, Y = 3.4 μm) is shown in figure 4. A eak ower of about 45 nw was obtained. By inreasing the exitation onditions u to Y o = 6.8 μm, an exerimental inrease in the generated ower u to. μw has been obtained (higher exitation amlitudes led to non linear resonant behavior). One reason of suh low ower values is that in this first rototye, the 1.5 μm thik Al metal traks determine a high value of the oil series resistane, R C 91 Ω, whih drastially limits the erformane of the devie. In figure 5 and 6, the obtained data are lotted as a funtion of the load resistane. These data orresond to the outut ower and voltage amlitude measured at resonant onditions using the same exitation amlitude as in the revious figure. The figures also show the fitting of the exerimental data using the theoretial equations

4 PL (mirow),45,4 ex,35 model,3,5,,15,1, RL (ohms) Figure 5. Outut ower P L vs load resistane R L, measured at resonant onditions. Vo (mv) RL (ohms) desribed in Setion. As shown in these figures, there is a good agreement between the exerimental data and the theoretial model develoed for these devies. The fitting of these data gives in this ase a value of the arasiti daming ζ =.3, taking into aount the magneti flux rate value of (dφ/dz) =.14 Wb/m alulated by ANSYS for this struture. This arasiti daming is higher than that obtained from the measurements shown in figure 3, where a magnet signifiantly smaller was used. These results orroborate the validity of the model develoed for the eletromagneti generator. The data also show the ability of this first (not otimized) rototye to generate owers in the range between nw s and μw s. 4. OPTIMIZATIONS ex model Figure 6. Outut voltage V o vs load resistane R L, measured at resonant onditions. Otimizations to this design an be made in three main ways: derease of the arasiti daming, geometri otimizations in order to maximize the magneti flux rate dφ/dz, and eletri otimizations aiming at maximizing the eletromagneti ouling reresented by the eletromagneti daming ζ g. Aording to equation (5) and (7), dereasing the arasiti daming in the struture allows to imrove both the generated ower and the outut voltage. In this sense, our estimated values of ζ for the Katon membranes are signifiantly higher than that reviously reorted in the literature for devies with a similar design struture (ζ =.37, [1, 4]). To minimize the arasiti daming effets, several otions are being imlemented, whih inlude the fabriation of the membrane by deosition of a olymeri film (suh as SU-8 or PMMA) onto a miromahined struture, and the relaement of the olyimide films by Si based membranes (whih have very low mehanial hysteresis losses). A further redution of the arasiti daming ould be ahieved by erforming the enasulation of the devies under vauum onditions. However, the otimization of the devie in terms of arasiti daming has also to take into aount that dereasing the total daming in the system also leads to a derease in the range of Y o values omatible with the devie design. This is determined by the existene of a higher limit Z L for the dislaement of the inertial mass in the devie, imosed by the otential ollision of the mass with fixed arts in the system. For a given value of Y o, this imoses the need to have a value of total daming (ζ g + ζ ) [Y o /(Z L )]. To quote this value, we have made a onservative estimation of Z L, by limiting the highest vertial osition of the magnet base to the osition of the lane of the oil. This gives Z L = z + h/ (see figure 1), whih leads to a lower limit value for the total daming of ζ =.15 for Y o 5 μm. The analysis of the flux rate as a funtion of the oil traks arameters reveals the ossibility to obtain a further inrease of the value of the magneti flux rate by dereasing both the width and searation, and inreasing the number of turns. With a value of 6 μm (minimum value omatible with high aset ratio traks, as (dφ/dz)max (Wb/m),9,8,7,6, number of turns Figure 7. Maximum flux rate for oils with 6 μm metal trak width and searation between metal traks vs number of turns

5 exlained later) for both width and searation of the traks, a maximum flux rate is obtained for 1 turns (figure 7). However, this tends to inrease the oil series resistane R, whih an omromise the otential inrease of the generated ower related to the otimization of the flux rate. Finally, as shown in equation (3), otimizing ζ g requires reduing the value of R C. This an be ahieved by using a thiker metal for the oil traks. For this, the seletive growth of thik Cu traks by eletrohemial deosition [5] is roosed, in ombination with a revious high aset ratio lithograhy roess. For 5 μm thik traks, the series resistane of a devie with the same design as our first rototye should dro down to a value of R C = 8.4 Ω. This would lead to an inrease of two orders of magnitude in the generated ower assuming the same onditions as in the revious ase, obtaining a value of P L 1 μw. Aording to these diretives, figure 8 shows the simulation of an otimized devie that inludes a oil made of 5 μm thik, 6 μm ith Cu traks. For this design, the signal generated by the devie has been alulated using the exitation onditions orresonding PL (mirow) Vo (mv) RL(ohms) RL (ohms) Figure 8. Generated P L and V o vs R L alulated for the design with the highest flux rate (n = 1, ζ =.15, f = 1 Hz, Y o = 4.4 μm) to the vibrations indued by a small mirowave oven, f = 1 Hz and Y o = 4.4 μm. As reorted in [3], these are reresentative of the low level vibrations tyially resent in domesti and offie environments, whih have frequenies between 7 and Hz and aeleration amlitudes between 1 and 1 m/s. In this ase, a maximum ower of P Lot = 8 μw is obtained with R L = R Lot = 6 Ω, with an amlitude of the outut voltage of V o =.58 V. Inreasing the load resistane u to R L = 135 Ω allows the generation of P L = 4 μw with a voltage amlitude V o =.8 V. It is interesting to remark that these voltage levels are omatible with the requirements related to the use of standard retifying iruits for the generation of a DC signal suitable for ower suly aliations. The omarison of these data with those reorted in the literature for the same exitation onditions oints out the ossibility to generate similar ower levels with the eletromagneti devie roosed in this aer, obtaining higher values than with other aroahes suh as the eletrostati one. In this last ase, Roundy et al [3] have reorted a value of 43 μw from the simulation of an otimized design of eletrostati generator. These authors have also develoed a rototye of iezoeletri generator, whih gives a higher ower of 7 μw. Simulations show that an otimized design would be aable of generating a ower of 5 μw for the same vibration soure, whih is still slightly lower than the maximum value of P Lot = 8 μw simulated for our otimized eletromagneti design. It is interesting to remark that the devies desribed in [3] orresond to designs with a total volume of.5 m 3, whih is of the same order of magnitude as the volume that an be estimated for our devie (in the range.6-.7 m 3 ). 5. CONCLUSIONS In this work, we have resented the design and otimization of an eletromagneti inertial mirogenerator for energy savenging aliations, omatible with Si tehnology. The roosed design onsists of a fixed miromahined oil and a movable magnet (inertial mass) mounted on a resonant struture (Katon membrane). The modeling of the devie is based on a veloity damed resonator. The inlusion in the model of the losses related to the oil series resistane has allowed to analyze the design and loading onditions required to otimize both the generated ower and outut voltage, exloring the aabilities of this simle devie struture for the develoment of devies suitable for ratial aliations. Aording to this struture, a first rototye (not otimized) has been designed and fabriated. The

6 exerimental mehanial and eletrial haraterization of this rototye has allowed the validation of the model develoed for the inertial eletromagneti mirogenerators, whih is then used as a roadma for a number of otimizations for the final devie design. These otimizations an be made in three main ways: derease of the arasiti daming, geometri otimizations in order to maximize the magneti flux rate dφ/dz, and eletri otimizations aiming at maximizing the eletromagneti ouling reresented by the eletromagneti daming ζ g. In relation to revious works roosing eletromagneti generators with a similar struture formed by a fixed oil and a movable magnet [3, 4], the simulation of the devies with the otimum design using the same exitation onditions as in these works reveals the ossibility to obtain a signifiant inrease of the flux rate (more than orders of magnitude), thus allowing to generate muh higher ower levels. For the otimum design, these results orroborate the ability of this devies to generate ower u to hundreds of μw s, with voltage levels omatible with the requirements related to the use of standard retifying iruits for the generation of a DC signal suitable for ower suly aliations. This oen interesting ersetives for the fabriation of ower mirogenerator suitable for their integration with advaned miro/nanosensors in mirosystems for autonomous oeration, wireless systems, or mirosystem that must be omletely embedded within the struture with no outside hysial onnetions, allowing to overome the limitations of onventional batteries in terms of miniaturization and devies lifetime. ACKNOWLEDGEMENTS The funding of this work by the IST rogram of the Euroean Commission under the SENSATION rojet (ref. FP6-5731) is aknowledged by the authors from the University of Barelona REFERENCES [1] P.D. Mitheson, T.C. Green, E.M. Yeatman, A.S. Holmes, Arhitetures for vibration-driven miroower generators, Journal of Miroeletromehanial Systems 13, (4). [] T. Sterken, K. Baert, C. Van Hoof, R. Puers, G. Borghs, P. Fiorini, Comarative modeling for vibration savengers, Proeedings IEEE Sensors, 4. [3] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a ower soure for wireless sensor nodes, Comuter Communiations 6, (3). [4] C.B. Williams, C. Shearwood, M.A. Harradine, P.H. Mellor, T.S. Birh, R.B. Yates, Develoment of an eletromagneti mirogenerator, IEE Pro. Ciruits, Devies and Systems 148, (1). [5] S. Martínez, N. Yaakoubi, A. Pérez-Rodríguez, C. Serre, P. Gorostiza, J.R. Morante, J. Esteve, Eletrohemial deosition of metal layers and strutures for Si-based mirosystems, Sensors and Atuators A 99, (). [6] W.-S. Huang, K.-E. Tzeng, M.-C- Cheng, R.-S. Huang, Design and fabriation of a vibrational miro-generator for wearable MEMS, Proeedings of the 17 th Euroean Conferene on Solid State Sensors Eurosensors XVII, (3).