Environmental Vibration-based MEMS

Piezoelectric Energy Harvester(EVMPEH)

Salem Saadon, Othman Sidek Collaborative Microelectronic Design Excellence Center (CEDEC), Universiti Sains Malaysia(USM), Engineering Campus,

14300 Nibong Tebal, Pulau Pinang, Malaysia School of Electrical & Electronic Engineering, Universiti Sains Malaysia(USM), Engineering Campus, 14300 Nibong Tebal,

Pulau Pinang, Malaysia

Abstract __The wireless sensor network market is growing quickly yet is limited by existing short lifetime batteries. Providing a green, virtually infinite alternative power source to traditional energy sources will significantly expand applications for WSNs and other technologies, the use of piezoelectric materials to capitalize on the ambient vibrations surrounding a system is one method that has seen a dramatic rise in use for power harvesting. The simplicity associated with the piezoelectric micro-generators makes it very attractive for MEMS applications, in which mechanical vibrations are harvested and converted to electric energy, these micro- generators were designed as an alternative to a battery-based solution especially for remote systems. In this paper we reviewed the work carried out by researchers during the last few years. The improvements in experimental results obtained in the vibration based MEMS piezoelectric energy harvesters show very good scope for MEMS piezoelectric harvesters in the fields of power MEMS and Green Technology in the near future. Keywords- PZT; Vibration; Cantilever beam; Energy harvesting; MEMS

the parallel triple layer bimorph the same as the previous one but the piezoelectric materials were connected in parallel. The parallel triple layer bimorph had the highest power under medium excited frequencies and load resistances, while the series triple layer bimorph produce a highest power when excited under higher frequencies and load resistances. A series connection will increases the device impedance as well as improve the output delivered power at higher loads. Other method of increasing the bimorph efficiency investigated by Jiang et al [4], their study focused on a bimorph cantilever with a proof mass attached to its end, their results showed that, by reducing the bimorph thickness and increasing the attached proof mass will decrease the harvester resonant frequency and produce a maximum harvested power. Similarly, Anderson and Sexton [5], found that, by varying the length and width of the proof mass will affect the output harvested power. Cantilever's geometrical structures also play an important aspect to improve the harvesters efficiency, however,

I. INTRODUCTION Rectangular shaped cantilever structures are the most There are several applicable methods to improve the

harvested power of MEMS micro-generators, the first method is by selecting a proper coupling mode of operation , practically there are two modes of operation, the first mode called 31mode, in which the excited vibration force is applied perpendicular to the poling direction (pending beam), while the other is called 33mode, in which the force is applied on the same as the poling direction. The second method for harvested power improvement is by changing the device configuration by adding a multiple pieces of piezoelectric materials to the harvester as shown in figure1. The unimorph cantilever beam configuration shown in figure1 (c), described by Johnson et al [1], by this configuration a highest power can be generated under lower excitation frequencies and load resistances. Series and parallel triple layer bimorph structures represented by Ng and liao [2,3] as shown in figure1 (a) and (b) respectively.

Figure 1. (a) A series triple layer type cantilever, (b) A parallel triple layer type cantilever, (c) A unimorph cantilever. The series triple layer bimorph constructed of a metallic layer sandwiched between two piezoelectric materials, the piezoelectric patches connected in series electrically. While

commonly used in MEMS based piezoelectric harvesters due to their easy implementation and effective to harvest energy from ambient vibrations. The study proposed by Mateu and Moll [6], showed that the triangular shaped cantilever beam with a small end free will maintain a higher strains and maximum deflections to produce a higher output power than a rectangular beam having width and length equal the base and height dimensions of the proposed triangular cantilever beam. A trapezoidal shaped cantilever beam discussed by Roundy et al [7], they found that, the strain can be more distributed throughout the trapezoidal structure, and stated that, for the same PZT volume a trapezoidal cantilever beam can deliver more than twice the energy than a rectangular shaped beam, similarly, Baker et al [8], experimentally tested a nearly triangular, trapezoidal shaped cantilever beam against with a rectangular shaped beam of the same volume, and found that 30% more power can be achieved by the trapezoidal beam than the rectangular beam. A 'symbal' called circular shaped structure developed by Kim et al [9], in such structure, two dome-shaped metal were bonded on a piezoelectric circular plate. Finally, other method of improving the efficiency of the power harvesters is by tuning the device so that it's resonant frequency matches the ambient vibrations resonant frequency, Shahruz [10,11], designed a power harvester resonate at various frequency range without any need of

2011 Developments in E-systems Engineering

978-0-7695-4593-6/11 $26.00 © 2011 IEEE

DOI 10.1109/DeSE.2011.87

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2011 Developments in E-systems Engineering

978-0-7695-4593-6/11 $26.00 © 2011 IEEE

DOI 10.1109/DeSE.2011.87

507

2011 Developments in E-systems Engineering

978-0-7695-4593-6/11 $26.00 © 2011 IEEE

DOI 10.1109/DeSE.2011.87

511

2011 Developments in E-systems Engineering

978-0-7695-4593-6/11 $26.00 © 2011 IEEE

DOI 10.1109/DeSE.2011.87

511

adjustments. His device consisted of different cantilever beams with different lengths and tip masses attached to common base frame, so each cantilever had it's resonant frequency, and the overall created a so called "mechanical band-pass filter", which results in an increasing of size and cost. Additionally, Rastegar et al [12], designed a passive tuning system, their system was a two-stage system in which a very low frequencies in the range of 0.2 to 0.5 Hz can be converted into potential energy, then transferred to the system with a higher natural frequency. The schematic diagram of their harvester Shown in figure2

Figure 3. Schematic of a PZT unimorph cantilever [16]

Figure 4. Cross-sectional sketch and the fabrication process of micro piezoelectric power generator [17].

figure4, the composite cantilever is made up of an upper Figure 2. Schematic of a typical energy harvesting power source using the

two-stage design [12].

Related works on the modeling, design and fabrication of MEMS-based piezoelectric power harvesters can be found in [13-23].

PZT thick film sandwiched between a pair of (Pt/Ti) metal electrodes and a lower non-piezoelectric element. As shown in figure4, when the base of the system is vibrated by the environmental groundwork, input force is feed to the mechanical parts, some parts move relatively to the base frame, cause the PZT material to be tensed or compressed which in turn induces charge shift due to piezoelectric effect,

II. MEMS-BASED PIEZOELECTRIC ENERGY the magnitude of this electric charge is proportional to the HARVESTERS mechanical stress induced by the displacement.

Most of previous researches done on piezoelectric energy harvesting micro generators are concentrated on bulk prototypes, however, only few of them have demonstrated MEMS micro generators capable of delivering useful power. Marzencki et al. [13], successfully designed and fabricated a thin film AlN cantilever micro generator, that can generate a power of 0.038μW from a 0.5g (g =9.81 m/s2) acceleration at 204 Hz resonant frequency, the output power is limited to low power levels due to the properties of AlN material. Moreover, Marzencki et al.[14], improved the power generated by increasing the vibration amplitude and frequency of their device to 4g at 1368 Hz resonant frequency to generate a power of 1.97μW. Dongna Shen et al [15], designed and fabricated a PZT cantilever MEMS-based micro generator with an integrated Si proof mass, that can generate 2.15μW from 2g (g = 9.81 m/s2) acceleration at it's resonant frequency of 461.15 Hz. Additional work done by M. Renaud et al. [16], they proposed a fabricated MEMS-based PZT cantilever micro generator with an integrated proof mass that can generate 40μW at 1.8 kHz vibration frequency. Their device unimorph cantilever described in figure3. Furthermore, Hua-Bin Fang et al. [17], designed a composite cantilever structure with nickel metal mass as shown in figure4. The metal mass on the tip of the cantilever is used to decrease the structure's natural frequency (w = √k/m, where k, is the material stiffness and m is the mass),for the application under low-frequency vibration. As described in

The micro-generator should be designed so that it can mechanically resonate at frequency tuned to the ambient vibration in order to generate maximum electrical power, and the structure natural frequency can be regulated by varying the moving parts dimensions. The device was micro-fabricated involve functional films preparation and pattern, bulk silicon micromachining. The prototype fabricated by MEMS technology which resulted in the level of 898 mV and 2.16μW output power, under resonant operation with about 609 Hz. Similar to the previous work, Jing-Quan Liu et al. [18] used the previous cantilever structure to construct a power generator array to improve power output and frequency flexibility. the cantilevers are designed working under low frequency range. The schematic configuration of a single cantilever was shown in figure5.

Figure 5. Schematic configuration of single cantilever beam. [18]

It is shown that the output voltage drops off when the excited frequency deviates from the resonant frequency as described in figure 6. The available bandwidth is only just 2-3 Hz. Normally the driving frequency should be determined before any design or fabrication of such devices, and this taken as advantageous to design a device that can perfectly operate over a range of frequencies.

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P.Muralt et al. [22], designed and fabricated a micro power generator of thin film PZT laminated cantilever with proof mass and interdigitated electrodes, can generates a 1.6 V and 1.4μW excited with 2g at 870Hz resonant frequency. The schematic structure of the generator is shown in figures 8.

Figure 6. Output voltage as a function of excited frequency [18]. After the serial connection, the dc voltage goes up to 3.93 V, and the maximum output dc power reached to about 3.98μW. The experimental results showed that, the arrayed device is promising in improving the operation bandwidth and the output power of the generator, and indicated that a potential in the development of the power generator meets applications in wireless/embedded sensor networks. The MEMS vibration-based harvesting devices have an AC output that needs to be rectified, since all rectifying semiconductor devices need at least 500 mV as dropped voltage consumed, in this case to overcome this point it is much better to use interdigitated electrodes instead of the proposed PZT parallel electrodes, those resulting in lower output ac voltage, as published by the following researches. Jeon et al [19] developed a {3-3} mode thin film PZT cantilever device with interdigitated electrodes that can generate 1.0μW from 10.8 g vibration at 13.9 kHz resonant frequency. B.S.Lee et al.[20], designed and fabricated piezoelectric MEMS micro generator with laminated {3-3}mode PZT cantilever and interdigitated electrodes that can generate 0.123μW under 2g (g = 9.81 m/s2) acceleration amplitude. Similarly B.S.Lee et al.[21] developed two piezoelectric MEMS generators with{3-1}mode and {3-3} mode, those have a cantilever type made by a silicon micromachining process , their experimental results showed that {3-1} mode micro generator can generate output power of 2.765μW excited at 2.5g amplitude and 255.9Hz resonant frequency, While the {3-3} mode generator can generate an output power of 1.288μW under 2g amplitude and 214Hz. the schematic diagram illustrated in figure7.

Figure 8. Schematic structure and operation principle of piezoelectric laminated cantilever for harvesting vibration energy coupled in through the vibration of the frame [22]. Finally, R. Elfrink et al. [23], designed and fabricated a MEMS-based AlN piezoelectric cantilever micro generator, that can generate an output power of 60μW under 2g (g =9.81m/s2) acceleration at 572Hz resonant frequency. Different devices with different cantilever beams and mass geometries were produced. Glass wafers are used for the top and bottom covers. Figure 9 left, shows the generator design and its package configuration at the rest position, while Figure 9 right, shows the motion of the mass when the generator at resonance.[23]. Figure 9. (top) Vibration energy harvester packaged in between glass substrates at the rest position. (bottom) displacement of the proof mass of the harvester [23]. The devices were packaged with top and bottom glass substrates within cavities to allow mass displacement up to about 400μm.

III. CONCLUSIONS From the previous studies, we can summarized all the previously designed harvesters and their experimentally obtained results as mentioned in the following two points. First point is that the maximum output power harvested is

Figure 7. Schematic diagram of the piezoelectric MEMS generators: (top){3-1}mode configuration, (bottom) {3-3} mode configuration. [21]. In the case of {3-3} mode, the interdigitated electrodes were

fabricated with 30μm widths and 30μm gaps. The proof masses for both MEMS generators were fabricated under the beam structure with dimensions of 500 × 1500 × 500 μm3 and 750 × 1500 × 500 μm3 for the {3-1 and {3-3}modes respectively. A different proof mass dimension was used to demonstrate the ability of the structure to adjust the resonant frequency.

about several microwatts without any interface power conversion circuit, which will produce power losses in the form of consumption power, will cause a reduction in the delivered power. Secondly, the absence of the vibration source control will affect the delivered output power. According to the previous background literature, It is clearly

appeared that the delivered power output of those MEMS micro generators is much less to be used as dc power supplies to a recently used electronic equipments as well as sensors and other medical monitoring devices.

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IV. FUTURE WORK [12]J. Rastegar, C. Pereira, and H. L. Nguyen, Our future work is to design a novel vibration-based MEMS "Piezoelectric-based power sources for harvesting energy micro power harvesting device consists of piezoelectric from platforms with low frequency vibration - art. no. cantilever array type that provides the optimal desired output 617101," Smart Structures and Materials 2006: Industrial power characteristics within high efficiency in order to meet and Commercial Applications of Smart Structures all the desired objects to maintain a satisfied output power Technologies, vol. 6171, pp. 17101-17101, 2006. required that can be employed to fed wireless sensor [13] M. Marzencki, S. Basrour, B. Charlot, A. Grasso ,M. networks instead of conventional batteries. Colin, L. Valbin,"Design and Fabrication of Piezoelectric

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