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Analysis of an in-plane electromagnetic energy harvester with integrated magnet array
Sensors and Actuators A 219 (2014) 38–46
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Analysis of an in-plane electromagnetic energy harvester with integrated magnet array Mengdi Han, Zhongliang Li, Xuming Sun, Haixia Zhang ∗ National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 15 January 2014 Received in revised form 13 August 2014 Accepted 14 August 2014 Available online 23 August 2014 Keywords: Electromagnetic energy harvester In-plane movement Magnet array CoNiMnP electroplating
a b s t r a c t In this paper, a novel MEMS electromagnetic energy harvester is designed, fabricated and tested. In-plane operation mode is utilized in the device to induce voltage in the coils, which enhances the changing rate of magnetic flux density across the coils. In order to produce larger magnetic flux density across the coil, magnetic properties of permanent magnets are simulated and optimized. Transient analysis of the induced voltage is conducted to prove the effectiveness of structural design. Comparison with the out-ofplane operation modes is carried out in the simulation, indicating that the in-plane operation mode not only enlarges the output, but also can make full use of the large vibration amplitude. In the fabrication process, instead of manually assembling bulk magnets, CoNiMnP hard magnetic alloy is electroplated onto the vibration plate. This method is MEMS compatible, which not only increases the production efficiency but also condenses the device’s volume to 67.5 mm3 . Through experimental measurement, the proposed structure with integrated magnet array can generate 0.98 mV peak voltage at the frequency of 48 Hz. The maximum peak power density of this device reaches to 0.16 W/cm3 with a 15.8 external resistance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As the power consumption of microelectronic devices is scaled down to microwatts [1], MEMS energy harvester becomes a promising power supply method for low power consumption applications, such as remote sensor networks, biomedical implantable devices, and portable electronics. Among various energy sources in the environment, such as vibrations, solar energy and wind energy, vibrational energy exhibits unique advantages [2]. It is almost available everywhere and can be easily converted to electrical power through many applicable methods. Energy harvesters utilizing piezoelectric [3–5], electrostatic [6–8], and electromagnetic [9–11] transduction mechanisms have been fabricated and investigated. Among them, piezoelectric energy harvesters use vibration-produced mechanical force to strain a piezoelectric material such as PZT (lead zirconate titanate) [4,5] or PVDF (polyvinylidene fluoride) [12,13]. The strained piezoelectric material will produce a potential difference, which can be utilized as electrical energy source. Electrostatic energy harvesters are based on the changing capacitance value on account of vibrations, which will cause a change of the electric field, thus generating current flow
∗ Corresponding author. Tel.: +86 10 62767742. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.sna.2014.08.008 0924-4247/© 2014 Elsevier B.V. All rights reserved.
in the external circuit. Compared with those two types, the internal resistance of electromagnetic energy harvester is relatively low, leading to larger output current. Till now, several methods have been conducted to improve the output performance of electromagnetic energy harvesters. For example, Sari et al. fabricated a cantilever array instead of a single one, which not only increases the total output power but also broadens the working frequency [14]. Apart from this, by utilizing the frequency upconversion technique [15], low frequency vibration is converted into high frequency movement, which also enhances the output power obviously. However, those methods increase the total volume of the device and complicate the structure, which not really enhance the power density. Through well-designed magnet structure, in-plane movement mode of the electromagnetic energy harvester can improve the output power density by orders of magnitude, due to the larger changing rate of magnetic field [16]. To further increase the output performance of in-plane electromagnetic energy harvesters, various structures and magnet arrays have been designed and optimized [17–20]. Typically, the magnet arrays are manually assembled using NdFeB permanent magnets. Traditional fabrication of NdFeB magnet requires high processing temperature, which brings difficulty to the integration in MEMS process. Additionally, the manually assembled bulk magnet array limits the application field of the device, due to the relatively large volume. To solve the problem, NdFeB
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Therefore, the movement of electrical connections can be avoided which enhances the reliability of the device. Besides, the permanent magnet array works as the masses of the vibration plate, which lowers the resonant frequency (fR ) of the device and enhances the power output (p) according to Eqs. (1) and (2) [26],
1 fR = 2
k m
(1) 3
p= Fig. 1. Schematic of the energy harvester with integrated magnet array.
micromagnets have been batch fabricated and successfully applied in energy harvesting devices [21–24]. In this paper, we propose an in-plane electromagnetic energy harvester with integrated CoNiMnP magnet array. The magnet array is batch fabricated using MEMS technology by electroplating CoNiMnP hard magnetic alloy [25]. Taking advantage of the photolithography, shape and distribution of the magnet array can be arbitrarily designed and fabricated. The electroplated CoNiMnP array are utilized as permanent magnets and vibration masses, which reduces the resonant frequency, greatly decreases the volume of the device and makes the fabrication processes compatible with other MEMS processes. Other components of the device such as coils and beams are also designed and mass-fabricated to effectively harvest the vibration energy. The mass-fabricated small size energy harvester is promising to be integrated with other micro/nano devices, making the entire system compactable and self-sustainable. In Section 2, simulations have been conducted to optimize and analyze its structure. Detailed fabrication process is introduced in Section 3 and experimental measurement is shown in Section 4. 2. Design and simulation 2.1. Structure of the device The electromagnetic energy harvester is designed to work with low frequency horizontal vibration input. In order to increase the output performance, in-plane movement mode is utilized in this device. As illustrated in Fig. 1, the device consists of four parts: fixed series coils, supporting pillars, vibration plate (including the folded beams) and permanent magnet array. The coils are placed in the bottom of the device and the vibration plate is supported by two copper pillars, leaving a 10 m gap between the coils and the vibration plate. The vibrating beams are designed to be winding shape, which is beneficial to respond to low frequency vibrations in the environment. Permanent magnet array are fabricated on to the vibration plate to produce magnetic field across the coils. In the device, the coils are fixed while the magnets are movable.
mY 2 ς e (f/fR ) (2f )
3
(2)
2 2
2
[2ς (f/fR )] + [1 − (f/fR ) ]
where m is the mass of the system, k is the effective spring constant, Y is the vibration amplitude, f is the vibration frequency, ς is the damping ratio which includes electrical (ς e ) and mechanical (ς m ) parts. 2.2. Comparison of the magnetic flux density distribution Given an external vibration source along the vibration plate, magnetic field over the coils will be changed, thus producing induced voltage in the coils. Compared with out-of-plane type, in-plane operation mode has larger changing rate of magnetic flux density, which leads to larger induced voltage. For traditional out-of-plane electromagnetic energy harvesters, the change of magnetic flux density is caused by varying the distance between the magnet and coil. Taking the common cylindrical magnet as an example, the magnetic flux density (B) along its central axis can be expressed as [27],
Br B(d) = 2
H+d
R2 + (H + d)
2
−
d
(3)
R2 + d2
where Br is the residual magnetic flux density determined by the material property, d is the distance from the magnet surface, H and R are the height and radius of the cylindrical magnet, respectively. For a CoNiMnP cylindrical magnet with the retentivity of 8887 Gs [28], height of 10 m and radius of 2000 m, the magnetic flux density and its gradient along the central axis are shown in Fig. 2(a) and (b), respectively. In this case, the maximum changing rate appears at 0.95 mm, with a value of 0.0095 T/mm. In the out-of-plane type, the gradient of magnetic flux density cannot be largely increased. However, in-plane operation mode offers opportunity to enhance the changing rate of magnetic flux density, because large magnetic field appears only at the edge of a permanent magnet and the value sharply decreases to zero at other region. Therefore, at the edge of a permanent magnet, large changing rate of magnetic flux density can be obtained. Replacing a whole block magnet with magnet array, the length of the magnet edge will increase, thus enhancing the changing rate of magnetic flux density. A finite element method (FEM) simulation is also
Fig. 2. Magnetic flux density and its changing rate for out-of-plane operation mode.
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Table 1 Geometry and magnetic parameters in the FEM simulation. Parameters
Permanent magnet Whole block
Retentivity (Gs) Coercivity (Oe) Thickness (m) Length (m) × width (m) Number Array spacing (m) Size of the air box (m)
8887 751 10 2300 × 1500 1 – 2800 × 2000 × 30
Square array 8887 751 300 × 300 6×4 100 2800 × 2000 × 30
conducted to compare the magnetic field distribution between a whole block and square array. Geometry and magnetic parameters are set according to the device size and our previous investigation [28,29], as shown in Table 1. In the simulation, the total volumes of the whole block and magnet array (including the interspace) are the same. Fig. 3(a) and (b) shows the magnetic field distribution of the whole block magnet and the 4 × 6 square array at a 10 m distance from the magnet surface. For both of the permanent magnets, largest magnetic flux density appears at the edge region, which is about 0.01 T. In other regions, the value of magnetic flux density is reduced almost to zero. To quantitatively investigate the field change in the in-plane movement mode, magnetic flux density and its changing rate along line 1 and line 2 in Fig. 3 is simulated and plotted in Fig. 4(a) and (b). In both cases, the magnetic flux density decrease from 0.01 T to 0 at 0.1 mm away from the edge. As shown in Fig. 4(a), for a whole block magnet, only two edges exist over the line, resulting two magnetic flux density peaks along line 1. Magnet array offers more edge regions (8 edges in this array) over the line, which contributes to more magnet peaks as indicated in Fig. 4(b). Changing rate of the magnetic flux density shows a similar trend. The largest changing rate in in-plane mode also appear near the magnet edge with a value of 0.42 T/mm (Fig. 4(b)), which is about 45 times larger than the traditional out-of-plane operation mode. 2.3. Transient analysis of the induced voltage Large magnetic flux density and its gradient in the magnet array offer opportunity to improve the induced voltage of
Fig. 3. Comparison of the magnetic flux density produced by (a) a whole magnet block and (b) magnet array with the same area. The inset shows the enlarged magnetic flux density distribution.
electromagnetic energy harvesters. However, coils in the device and distribution of the magnet array should also be carefully designed in order to effectively utilize the large magnetic field. To accumulate the induced voltage, rectangular spiral coils are adopted in our device. Due to the electroplating technology used in the fabrication process, all the permanent magnets have the same polarity. In this case, magnet array distributing over all region of the coil will cause cancelation of the induced voltage. Therefore, we place the single-pole magnet array at half region of the coil (effective coil), leaving the other region (ineffective coil) vacant, as shown in Fig. 5(a). Since magnetic flux density decreases sharply to zero away from the edge of the magnets, strong magnetic field can only cover the effective coil (Fig. 5(b)). When the magnets move with input vibration, the effective coils cut the magnetic lines and generate induced voltage, while the ineffective coils do not experience great magnetic flux density and produce much lower voltage since they are relatively far from the magnets. With the magnets moving in one direction, the effective coils can generate induced voltage in the same direction and the voltage can be added because
Fig. 4. Magnetic flux density and its gradient from (a) a whole magnet block and (b) magnet array.
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Fig. 7. FEM analysis of the peak voltage and normalized power density as a function of the distance between effective and ineffective coils. The inset illustrates the implication of the distance. (For interpretation of the references to color in text near the reference citation, the reader is referred to the web version of this article.)
Fig. 5. (a) Illustration of the series coils for enhanced output. (b) Magnetic flux density over the effective and ineffective coils. Table 2 Geometry parameters in the transient analysis. Parameters
Number of turns Width (m) Spacing (m) Thickness (m) Distance to X-axis (m) Moving amplitude (m)
Components Effective coils
Ineffective coils
Magnet array
8 20 20 10 0 0
8 20 20 10 0 0
– 20 20 10 20 200
the value in the ineffective coils is only 3.0 V. Due to the much smaller voltage in the ineffective coils, the overall output will not be dramatically weakened. As shown in Fig. 6(b), the overall output voltage in this case is 12.7 V. Based on the above analysis, we investigate the dependence of overall output voltage on the distance between effective and ineffective coils. As shown in the blue curves in Fig. 7, the simulated overall induced voltage increases monotonically when the distance varies from 0 to 400 m. This is because the magnetic flux density across the ineffective coils decreases at further distances. However, since the increase of distance will enlarge the volume of the device, the distance should be optimized to provide the maximum power density. The power density can be expressed as, PD =
of end-to-end connection. Consequently, the total output voltage (V) can be expressed as, V = Nlv(Beffective − Bineffective )
(5)
where N is the number of coils, l is the length of each rectangular coil, v is the velocity of the magnet, Beffective and Bineffective are the magnetic flux densities across the effective coils and ineffective coils, respectively. To prove the effectiveness of this design, transient analysis of the induced voltage is conducted. A 2D model of fixed copper coils with moving magnet (50 Hz Sinusoidal vibration) is established with the size parameters listed in Table 2 and illustrated in Fig. 6(a). When the magnet array moves at X-axis, voltage is induced in both the effective coils and the ineffective coils. Setting the distance between effective coils and ineffective coils as 150 m, the maximum induced voltage in the effective coils reaches to 14.8 V, while
U2 RWt(L + L)
(6)
where PD is the power density, U is the induced voltage, R is the inner resistance of the coil, L, W, t are the length, width and thickness of the device, L is the increased length. In our model, the total length (L) is 600 m if there is no distance (L) between effective and ineffective coils. Neglecting the change of the coils inner resistance, the power density is proportional to U2 /(L + L). As shown in the red curves in Fig. 7, the power density (normalized value) first increase with the distance and reaches maximum when the distance is 300 m. When distance further increases, the power density slightly drops due to the length increment. 2.4. Comparison with the out-of-plane operation mode Compared with out-of-plane mode, electromagnetic energy harvesters with in-plane operation mode can provide larger
Fig. 6. (a) Schematic diagram of the 2D model. The inset shows the corresponding top view of the coil. (b) Overall output voltage and the induced voltage in the effective and in effective coils.
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Fig. 10. Induced voltage of the out-of-plane mode at different vibration amplitudes. Fig. 8. Induced voltages of in-plane and out-of-plane operation mode with same geometry parameter at the amplitude of 350 m.
changing rate of magnetic flux density, thus enhancing the induced voltage. In this work, we carry out FEM analysis to compare the output performance of these two operation modes. Two 2D models with the same dimension but different vibration direction are applied. As an example, we set the vibration amplitude and frequency as 350 m and 50 Hz for both modes. When the magnet array moves along the Y-axis (i.e., out-of-plane mode), induced voltage with a maximum value of 59.5 nV is achieved, as shown in Fig. 8. By contrast, when the magnet array moves along the Xaxis (i.e., in-plane mode) with the same amplitude and frequency, much larger induced voltage is obtained. In the in-plane operation mode, the maximum induced voltage is 23.5 V, which is about 400 times larger than that of the out-of-plane operation mode. In addition, the vibration amplitude can greatly affect the induced voltage of in-plane mode electromagnetic energy harvesters. As shown in Fig. 9(a), keeping the frequency at 50 Hz, when the vibration amplitude increases from 50 m to 350 m, the primary peak induced voltage raises from 3.7 V up to 23.5 V. Interestingly, when the amplitude reaches a certain value (250 m in our model), a secondary peak appears, which also increases with the amplitude. A more detailed relationship between vibration amplitude and peak induced voltage is demonstrated in Fig. 9(b), showing a linear relationship between them. Therefore, by designing the mechanical structure that allowing large amplitude, output performance of the in-plane mode electromagnetic energy harvesters can be further enhanced. The raised voltage can be attributed to the increased velocity, because magnets with larger amplitude move at a faster speed. As for the out-of-plane operation mode, increasing the amplitude will inevitably cause the original position of magnets move away from the coils. Therefore, although larger amplitude increases the velocity, the further distance from coils greatly reduce the magnetic flux density, which is not conducive to the induced voltage. FEM analysis is also conducted to investigate the amplitude influence on the out-of-plane mode energy harvesters. To ensure the consistency, the frequency is also set as 50 Hz. As shown in Fig. 10, when the
amplitude is 15 m, peak voltage of 61.4 nV is obtained for the out-of-plane operation mode device. As the amplitude increases to 100 m, peak voltage drops to 22.6 nV, because of the reduced magnetic flux density. The combined effect of velocity and magnetic flux density is complex, as the peak value raises to 59.5 nV with 350 m amplitude. Compared with the out-of-plane mode, the in-plane operation mode can greatly enhance the induced voltage. In addition, the output can be further improved by increasing the amplitude through rational structural design. 3. Fabrication Compared to traditional electromagnetic energy harvesters with manually assembled magnets [30,31], this device is fabricated with MEMS batch fabrication process completely which is accurate, reliable and mass-productive. Electroplating technique is the main process in this fabrication. The coils, supporting pillar, vibration plate are all fabricated by copper electroplating. Innovatively, we applied the CoNiMnP electroplating technique in energy harvesting applications to batch fabricate the magnet array. Detailed fabrication process is illustrated in Fig. 11. First, 3000 A˚ SiO2 is grown on both sides of silicon wafer by thermal oxidation which serves as an insulating layer (Fig. 11(a)). Next, 300 A˚ titanium and 3000 A˚ copper are sputtered onto SiO2 as the seed layer for electroplating (Fig. 11(b)). Then, coils are patterned with 10 m thick AZ 4620 photoresist and electroplated on the seed layer with 10 m thickness (Fig. 11(c)). The resistance of electroplated coils is 11.6 , measured by the Handheld Digital Oscilloscope (SHS806). As the next step, the 10 m thick supporting pillars are electroplated onto the side of the coils (Fig. 11(d)). Afterwards, another 3000 A˚ copper seed layer is sputtered (Fig. 11(e)) and the vibrating plate is electroplated onto this seed layer with 10 m thickness (Fig. 11(f)). In the following, CoNiMnP hard magnetic alloy is batch-fabricated by electroplating technique (Fig. 11(g)). In this process, two external NdFeB magnets are placed at two sides of the container to make the magnetization vector of CoNiMnP magnets aligned in one direction [28]. Finally, the photoresist and seed layers are removed leaving a 10 m gap between the coils and vibration plate (Fig. 11(h)).
Fig. 9. (a) Transient analysis of induced voltage versus different amplitude. (b) Detailed relationship between peak induced voltage and amplitude.
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Fig. 11. Fabrication process: (a) deposition of SiO2 as insulting layer, (b) sputtering of copper seed layer, (c) electroplating of the copper coils, (d) electroplating of the supporting pillars, (e) sputtering the second copper seed layer, (f) electroplating of the vibration plate, (g) electroplating of the CoNiMnP magnet array, and (h) release the device by removal of the photoresist and seed layer.
Fig. 12. SEM images of the fabricated energy harvester: (a) top view of the device, (b) the folded beam and 10 m gap, (c) the electroplated copper coil, and (d) the electroplated CoNiMnP magnet array.
With the above MEMS batch fabrication process, a massproductive electromagnetic energy harvester is obtained and its SEM images are shown in Fig. 12. From Fig. 12(a), a concise view of the device is given, showing the coils, supporting pillar, vibration plate with folded beam and CoNiMnP magnet array. In Fig. 12(b), the 10 m gap between coils and vibration plate can be observed. Fig. 12(c) shows a tilted view of the electroplated coils with a 10 m thickness. The electroplated CoNiMnP permanent magnet array is uniformly arranged on the vibration plate, as shown in Fig. 12(d). Table 3 gives out the specific dimensions of the device. Due to the electroplated CoNiMnP micro magnet array, the total volume of the vibration plate and magnets is only about 5 mm3 , which is
Table 3 Geometry of the energy harvester. Parameters
Number of turns Number of units Thickness (m) Total area (mm2 )
Components Coils
Vibration plate
Magnet array
8 – 10 45
– – 10 56
– 12 × 4 10 4.5
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Fig. 13. EDS analysis of the composition. The inset shows the surface morphology of the electroplated CoNiMnP.
Fig. 14. (a) Measurement system. (b) Photo of the fabricated energy harvester.
The device is tested through a vibration system as shown in Fig. 14(a) and the enlarged photo of the prototype is shown in Fig. 14(b). The measurement system consists of five parts: a wave form generator (RIGOL DG1022), a power amplifier (SINOCERA YE5871A), a vibrator (SINOCERA JZK-5), an amplifier circuit and an oscilloscope (RIGOL DS1102E). The vibrator in the measurement system is used to simulate environmental vibrations with different frequencies, which can be controlled by the wave form generator. Acceleration of the vibration can be controlled by the power amplifier. The output voltage of the electromagnetic energy harvester
is amplified through the operational amplifier circuit and sent to oscilloscope to be recorded. A sweep frequency measurement is conducted under the acceleration of 1.2 g with a total time of 60 s. The output voltage as a function of vibration frequency is shown in Fig. 15. The maximum peak output voltage is 0.98 mV at the frequency of 48 Hz. The second and third voltage peaks may be caused by the high resonant mode, which is explained in Fig. S1 in the Supporting Information file. In comparison with the simulation results, larger measured voltage can be attributed to different moving state of the magnet array. On the one hand, larger amplitude may be achieved in the measurement, which can enhance the induced voltage. On the other hand, the vibration plate as well as the magnet array may be closer to the coil due to the imperfect vibration direction, leading to stronger magnetic flux density. Load response of this electromagnetic energy harvester is also measured. As shown in Fig. 16, the maximum peak power reaches to 11.2 nW under a load resistance of 15.8 . Thanks to the electroplated CoNiMnP permanent magnet array, the thickness of the magnet is only 10 m.
Fig. 15. Frequency response of the energy harvester.
Fig. 16. Output voltage and power under different load resistance.
greatly decreased compared with previous energy harvesters with manually assembled magnets [30]. As shown in Fig. 13, an energy dispersive spectrum (EDS) analysis is conducted to investigate the material composition of the electroplated CoNiMnP. The inset shows the surface morphology of electroplated CoNiMnP, which is uniform. 4. Measurement and discussion
M. Han et al. / Sensors and Actuators A 219 (2014) 38–46 Table 4 NPD comparison of electromagnetic energy harvesters.
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Acknowledgements
Energy harvesters
Magnet type
NPD (kgs m−3 )
This work Ref. [11] Ref. [30] Ref. [34]
CoNiMnP CoNiMnP NdFeB NdFeB
0.0012 0.0003 0.09 26.07
Considering a 500 m silicon substrate on which the device is fabricated, the volume of this electromagnetic energy harvester is 67.5 mm3 (16 mm × 8 mm × 0.54 mm), corresponding to a power density of 0.16 W/cm3 . Table 4 compares the normalized power density (NPD) [32,33] of our device with other electromagnetic energy harvesters. Compared with previous electromagnetic energy harvester with integrated CoNiMnP magnets [11], the output performance is improved. The in-plane operation mode and magnet array effectively utilizes the large magnetic flux density at the edge of the magnets, which contribute to the enhancement. Moreover, the series coils and arrangement of the magnet array accumulate the induced voltage in each conducting rod, thus further increasing the output. Compared with energy harvesters with NdFeB magnets [30,34], the NPD is still relatively low due to the weak magnetic properties of CoNiMnP. However, we can see that the output voltage and power of our device still have potential improvement by further optimization. For example, increasing the thickness of electroplated CoNiMnP will enhance the magnetic flux density, which is benificial to the output. Enlarging the aspect ratio of copper coils (i.e., increasing the thickness and reducing the width) will also contribute to larger induced voltage [11]. Moreover, by utilizing new structures such as 3D coils and frequency up-conversion technique, better output performance is expected to be achieved. With the small volume, low resonant frequency and enhanced power output, this electromagnetic energy harvester is promising to be utilized to generate energy from environmental vibrations.
5. Conclusions This paper reports a novel in-plane electromagnetic energy harvester with integrated single-pole CoNiMnP magnet array. Unlike traditional energy harvester with manually assembled magnets, the whole fabrication process of this device is using MEMS technology, which is accurate, reliable and mass-productive. The vibration plate, folded beams and permanent magnets are fabricated by electroplating technique using the thick photoresist as sacrificial layer. Through a series of FEM simulations, structure and size parameters of the device are designed and investigated. Firstly, distribution of the single-pole CoNiMnP magnet array is carefully optimized to generate large magnetic flux density across the coils. Besides, the fixed coils are designed as rectangular spiral shape to accumulate the induced voltage, which is verified by transient analysis. In addition, by comparing the induced voltage and amplitude dependence, the advantage of in-plane operation mode over out-of-plane operation mode is demonstrated, and the obtained results can be referred for further improvement of inplane electromagnetic energy harvesters. Through experimental measurement, maximum peak output voltage appears at 48 Hz, with a value of 0.98 mV. Power density of 0.16 W/cm3 is achieved with a 15.8 external resistance. Further enhancement, such as longer coils and stronger magnets, can be applied to increase the output voltage. Also, with this integrated process, more innovation structure design of this device, such as series coils arrays and three-dimension structures, can be considered in the future plan.
This work is supported by the National Natural Science Foundation of China (Grant nos. 61176103, 91023045 and 91323304), the National Hi-Tech Research and Development Program of China (“863” Project) (Grant no. 2013AA041102), the National Ph.D. Foundation Project (Grant no. 20110001110103) and the Beijing Natural Science Foundation of China (Grant no. 4141002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sna.2014.08.008. References [1] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Comput. Commun. 26 (2003) 1131–1144. [2] S.P. Beeby, M.J. Tudor, N.M. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175–R195. [3] R. Elfrink, T.M. Kamel, M. Goedbloed, S. Matova, D. Hohlfeld, Y. van Andel, R. van Schaijk, Vibration energy harvesting with aluminum nitride-based piezoelectric devices, J. Micromech. Microeng. 19 (2009), 094005-1–094005-8. [4] H. Liu, C.J. Tay, C. Quan, T. Kobayashi, C. Lee, M.E.M.S. Piezoelectric, energy harvester for low-frequency vibrations with wideband operation range and steadily increased output power, J. Microelectromech. Syst. 20 (2011) 1131–1142. [5] T. Galchev, E.E. Aktakka, K. Najafi, A piezoelectric parametric frequency increased generator for harvesting low-frequency vibrations, J. Microelectromech. Syst. 21 (2012) 1311–1320. [6] P.D. Mitcheson, P. Miao, B.H. Stark, E.M. Yeatman, A.S. Holmes, T.C. Green, MEMS electrostatic micropower generator for low frequency operation, Sens. Actuators A: Phys. 115 (2004) 523–529. [7] H. Lo, Y.C. Tai, Parylene-based electret power generators, J. Micromech. Microeng. 18 (2008), 104006-1–104006-8. [8] Y. Naruse, N. Matsubara, K. Mabuchi, M. Izumi, S. Suzuki, Electrostatic micro power generation from low-frequency vibration such as human motion, J. Micromech. Microeng. 19 (2009), 094002-1–094002-5. [9] P. Wang, K. Tanaka, S. Sugiyama, X. Dai, X. Zhao, J. Liu, A micro electromagnetic low level vibration energy harvester based on MEMS technology, Microsyst. Technol. 15 (2009) 941–951. [10] Ö. Zorlu, E.T. Topal, H. Külah, A vibration-based electromagnetic energy harvester using mechanical frequency up-conversion method, IEEE Sens. J. 11 (2011) 481–488. [11] M. Han, Q. Yuan, X. Sun, H. Zhang, Design and fabrication of integrated magnetic MEMS energy harvester for low frequency applications, J. Microelectromech. Syst. 23 (2014) 204–212. [12] M. Han, X. Zhang, W. Liu, X. Sun, X. Peng, H. Zhang, Low-frequency wide-band hybrid energy harvester based on piezoelectric and triboelectric mechanism, Sci. China Tech. Sci. 56 (2013) 1835–1841. [13] M. Han, W. Liu, X. Zhang, B. Meng, H. Zhang, Investigation and characterization of an arc-shaped piezoelectric generator, Sci. China Tech. Sci. 56 (2013) 2636–2641. [14] I. Sari, T. Balkan, H. Külah, An electromagnetic micro power generator for wideband environmental vibrations, Sens. Actuators A: Phys. 145 (2008) 405–413. [15] I. Sari, T. Balkan, H. Külah, An electromagnetic micro power generator for low-frequency environmental vibrations based on the frequency upconversion technique, J. Microelectromech. Syst. 19 (2010) 14–27. [16] Q. Zhang, E.S. Kim, Energy harvesters with high electromagnetic conversion efficiency through magnet and coil arrays, in: MEMS 2013 Proceedings, Taipei, Taiwan, January 20–24, 2013, pp. 110–113. [17] D. Zhu, S. Beeby, J. Tudor, N. Harris, Vibration energy harvesting using the Halbach array, Smart Mater. Struct. 21 (2012), 075020-1–075020-11. [18] H. Liu, Y. Qian, N. Wang, C. Lee, Study of the wideband behavior of an in-plane electromagnetic MEMS energy harvester, in: MEMS 2013 Proceedings, Taipei, Taiwan, January 20–24, 2013, pp. 829–832. [19] S. Roundy, E. Takahashi, A planar electromagnetic energy harvesting transducer using a multi-pole magnetic plate, Sens. Actuators A: Phys. 195 (2013) 98–104. [20] D. Zhu, S. Beeby, J. Tudor, N. Harris, Increasing output power of electromagnetic vibration energy harvesters using improved Halbach arrays, Sens. Actuators A: Phys. 203 (2013) 11–19. [21] D.P. Arnold, N. Wang, Permanent magnets for MEMS, J. Microelectromech. Syst. 18 (2009) 1255–1266. [22] N. Wang, B.J. Bowers, D.P. Arnold, Wax-bonded NdFeB micromagnets for microelectromechanical systems applications, J. Appl. Phys. 103 (2008), 07E109-1–07E109-3. [23] Y. Jiang, S. Masaoka, M. Uehara, T. Fujita, K. Higuchi1, K. Maenaka, Microstructuring of thick NdFeB films using high-power plasma etching for magnetic MEMS application, J. Micromech. Microeng. 21 (2011), 045011-1–045011-5. [24] Y. Jiang, S. Masaoka, T. Fujita, M. Uehara, T. Toyonaga, K. Fujii, K. Higuchi, K. Maenaka, Fabrication of a vibration-driven electromagnetic energy harvester with
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[25]
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Biographies
Mengdi Han received the B.S. degree in Electronic Science and Technology from Huazhong University of Science and Technology, China in 2012. He is a Ph.D. candidate in National Key Lab of Nano/Micro Fabrication Technology at Peking University, China. His research work is focused on hybrid energy harvesting and MEMS sensors.
Zhongliang Li received the B.S. degree in Institute of Microelectronics from Peking University, China in 2013. He is now a Ph.D. student in the Department of Biomedical Engineering in University of California, Davis, USA. His research focused on Power MEMS and microfluidics.
Xuming Sun received the B.S. degree in Electronic Science and Technology from Chongqing University, Chongqing, China, in 2009. He is currently working toward the Ph.D. degree in microelectronics and solid state electronics at Peking University, Beijing, China. His research interests mainly include three areas: (1) design and fabrication of micro energy devices; (2) wireless power transfer system; (3) RF MEMS devices.
Haixia Zhang, Professor, Institute of Microelectronics, Peking Universituy. She received Ph.D. degree in Mechanical Engineering from the Huazhong University of Science and Technology, China, 1998, joined the faculty of the Institute of Microelectronics in 2001 after her Post-Doc in Tsinghua University. She is the senior member of IEEE and was served on the general chair of IEEE NEMS 2013 Conference, the organizing chair of Transducers’11. As the founder of the International Contest of Applications in Network of things (iCAN), she organized this world-wide event since 2007. At 2006, Dr. Zhang won National Invention Award of Science & Technology. Her research fields include Micro-nano Design and Fabrication Technology, SiC MEMS and Micro Energy Technology.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Analysis of an in-plane electromagnetic energy harvester with integrated magnet array Mengdi Han, Zhongliang Li, Xuming Sun, Haixia Zhang ∗ National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 15 January 2014 Received in revised form 13 August 2014 Accepted 14 August 2014 Available online 23 August 2014 Keywords: Electromagnetic energy harvester In-plane movement Magnet array CoNiMnP electroplating
a b s t r a c t In this paper, a novel MEMS electromagnetic energy harvester is designed, fabricated and tested. In-plane operation mode is utilized in the device to induce voltage in the coils, which enhances the changing rate of magnetic flux density across the coils. In order to produce larger magnetic flux density across the coil, magnetic properties of permanent magnets are simulated and optimized. Transient analysis of the induced voltage is conducted to prove the effectiveness of structural design. Comparison with the out-ofplane operation modes is carried out in the simulation, indicating that the in-plane operation mode not only enlarges the output, but also can make full use of the large vibration amplitude. In the fabrication process, instead of manually assembling bulk magnets, CoNiMnP hard magnetic alloy is electroplated onto the vibration plate. This method is MEMS compatible, which not only increases the production efficiency but also condenses the device’s volume to 67.5 mm3 . Through experimental measurement, the proposed structure with integrated magnet array can generate 0.98 mV peak voltage at the frequency of 48 Hz. The maximum peak power density of this device reaches to 0.16 W/cm3 with a 15.8 external resistance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As the power consumption of microelectronic devices is scaled down to microwatts [1], MEMS energy harvester becomes a promising power supply method for low power consumption applications, such as remote sensor networks, biomedical implantable devices, and portable electronics. Among various energy sources in the environment, such as vibrations, solar energy and wind energy, vibrational energy exhibits unique advantages [2]. It is almost available everywhere and can be easily converted to electrical power through many applicable methods. Energy harvesters utilizing piezoelectric [3–5], electrostatic [6–8], and electromagnetic [9–11] transduction mechanisms have been fabricated and investigated. Among them, piezoelectric energy harvesters use vibration-produced mechanical force to strain a piezoelectric material such as PZT (lead zirconate titanate) [4,5] or PVDF (polyvinylidene fluoride) [12,13]. The strained piezoelectric material will produce a potential difference, which can be utilized as electrical energy source. Electrostatic energy harvesters are based on the changing capacitance value on account of vibrations, which will cause a change of the electric field, thus generating current flow
∗ Corresponding author. Tel.: +86 10 62767742. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.sna.2014.08.008 0924-4247/© 2014 Elsevier B.V. All rights reserved.
in the external circuit. Compared with those two types, the internal resistance of electromagnetic energy harvester is relatively low, leading to larger output current. Till now, several methods have been conducted to improve the output performance of electromagnetic energy harvesters. For example, Sari et al. fabricated a cantilever array instead of a single one, which not only increases the total output power but also broadens the working frequency [14]. Apart from this, by utilizing the frequency upconversion technique [15], low frequency vibration is converted into high frequency movement, which also enhances the output power obviously. However, those methods increase the total volume of the device and complicate the structure, which not really enhance the power density. Through well-designed magnet structure, in-plane movement mode of the electromagnetic energy harvester can improve the output power density by orders of magnitude, due to the larger changing rate of magnetic field [16]. To further increase the output performance of in-plane electromagnetic energy harvesters, various structures and magnet arrays have been designed and optimized [17–20]. Typically, the magnet arrays are manually assembled using NdFeB permanent magnets. Traditional fabrication of NdFeB magnet requires high processing temperature, which brings difficulty to the integration in MEMS process. Additionally, the manually assembled bulk magnet array limits the application field of the device, due to the relatively large volume. To solve the problem, NdFeB
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Therefore, the movement of electrical connections can be avoided which enhances the reliability of the device. Besides, the permanent magnet array works as the masses of the vibration plate, which lowers the resonant frequency (fR ) of the device and enhances the power output (p) according to Eqs. (1) and (2) [26],
1 fR = 2
k m
(1) 3
p= Fig. 1. Schematic of the energy harvester with integrated magnet array.
micromagnets have been batch fabricated and successfully applied in energy harvesting devices [21–24]. In this paper, we propose an in-plane electromagnetic energy harvester with integrated CoNiMnP magnet array. The magnet array is batch fabricated using MEMS technology by electroplating CoNiMnP hard magnetic alloy [25]. Taking advantage of the photolithography, shape and distribution of the magnet array can be arbitrarily designed and fabricated. The electroplated CoNiMnP array are utilized as permanent magnets and vibration masses, which reduces the resonant frequency, greatly decreases the volume of the device and makes the fabrication processes compatible with other MEMS processes. Other components of the device such as coils and beams are also designed and mass-fabricated to effectively harvest the vibration energy. The mass-fabricated small size energy harvester is promising to be integrated with other micro/nano devices, making the entire system compactable and self-sustainable. In Section 2, simulations have been conducted to optimize and analyze its structure. Detailed fabrication process is introduced in Section 3 and experimental measurement is shown in Section 4. 2. Design and simulation 2.1. Structure of the device The electromagnetic energy harvester is designed to work with low frequency horizontal vibration input. In order to increase the output performance, in-plane movement mode is utilized in this device. As illustrated in Fig. 1, the device consists of four parts: fixed series coils, supporting pillars, vibration plate (including the folded beams) and permanent magnet array. The coils are placed in the bottom of the device and the vibration plate is supported by two copper pillars, leaving a 10 m gap between the coils and the vibration plate. The vibrating beams are designed to be winding shape, which is beneficial to respond to low frequency vibrations in the environment. Permanent magnet array are fabricated on to the vibration plate to produce magnetic field across the coils. In the device, the coils are fixed while the magnets are movable.
mY 2 ς e (f/fR ) (2f )
3
(2)
2 2
2
[2ς (f/fR )] + [1 − (f/fR ) ]
where m is the mass of the system, k is the effective spring constant, Y is the vibration amplitude, f is the vibration frequency, ς is the damping ratio which includes electrical (ς e ) and mechanical (ς m ) parts. 2.2. Comparison of the magnetic flux density distribution Given an external vibration source along the vibration plate, magnetic field over the coils will be changed, thus producing induced voltage in the coils. Compared with out-of-plane type, in-plane operation mode has larger changing rate of magnetic flux density, which leads to larger induced voltage. For traditional out-of-plane electromagnetic energy harvesters, the change of magnetic flux density is caused by varying the distance between the magnet and coil. Taking the common cylindrical magnet as an example, the magnetic flux density (B) along its central axis can be expressed as [27],
Br B(d) = 2
H+d
R2 + (H + d)
2
−
d
(3)
R2 + d2
where Br is the residual magnetic flux density determined by the material property, d is the distance from the magnet surface, H and R are the height and radius of the cylindrical magnet, respectively. For a CoNiMnP cylindrical magnet with the retentivity of 8887 Gs [28], height of 10 m and radius of 2000 m, the magnetic flux density and its gradient along the central axis are shown in Fig. 2(a) and (b), respectively. In this case, the maximum changing rate appears at 0.95 mm, with a value of 0.0095 T/mm. In the out-of-plane type, the gradient of magnetic flux density cannot be largely increased. However, in-plane operation mode offers opportunity to enhance the changing rate of magnetic flux density, because large magnetic field appears only at the edge of a permanent magnet and the value sharply decreases to zero at other region. Therefore, at the edge of a permanent magnet, large changing rate of magnetic flux density can be obtained. Replacing a whole block magnet with magnet array, the length of the magnet edge will increase, thus enhancing the changing rate of magnetic flux density. A finite element method (FEM) simulation is also
Fig. 2. Magnetic flux density and its changing rate for out-of-plane operation mode.
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Table 1 Geometry and magnetic parameters in the FEM simulation. Parameters
Permanent magnet Whole block
Retentivity (Gs) Coercivity (Oe) Thickness (m) Length (m) × width (m) Number Array spacing (m) Size of the air box (m)
8887 751 10 2300 × 1500 1 – 2800 × 2000 × 30
Square array 8887 751 300 × 300 6×4 100 2800 × 2000 × 30
conducted to compare the magnetic field distribution between a whole block and square array. Geometry and magnetic parameters are set according to the device size and our previous investigation [28,29], as shown in Table 1. In the simulation, the total volumes of the whole block and magnet array (including the interspace) are the same. Fig. 3(a) and (b) shows the magnetic field distribution of the whole block magnet and the 4 × 6 square array at a 10 m distance from the magnet surface. For both of the permanent magnets, largest magnetic flux density appears at the edge region, which is about 0.01 T. In other regions, the value of magnetic flux density is reduced almost to zero. To quantitatively investigate the field change in the in-plane movement mode, magnetic flux density and its changing rate along line 1 and line 2 in Fig. 3 is simulated and plotted in Fig. 4(a) and (b). In both cases, the magnetic flux density decrease from 0.01 T to 0 at 0.1 mm away from the edge. As shown in Fig. 4(a), for a whole block magnet, only two edges exist over the line, resulting two magnetic flux density peaks along line 1. Magnet array offers more edge regions (8 edges in this array) over the line, which contributes to more magnet peaks as indicated in Fig. 4(b). Changing rate of the magnetic flux density shows a similar trend. The largest changing rate in in-plane mode also appear near the magnet edge with a value of 0.42 T/mm (Fig. 4(b)), which is about 45 times larger than the traditional out-of-plane operation mode. 2.3. Transient analysis of the induced voltage Large magnetic flux density and its gradient in the magnet array offer opportunity to improve the induced voltage of
Fig. 3. Comparison of the magnetic flux density produced by (a) a whole magnet block and (b) magnet array with the same area. The inset shows the enlarged magnetic flux density distribution.
electromagnetic energy harvesters. However, coils in the device and distribution of the magnet array should also be carefully designed in order to effectively utilize the large magnetic field. To accumulate the induced voltage, rectangular spiral coils are adopted in our device. Due to the electroplating technology used in the fabrication process, all the permanent magnets have the same polarity. In this case, magnet array distributing over all region of the coil will cause cancelation of the induced voltage. Therefore, we place the single-pole magnet array at half region of the coil (effective coil), leaving the other region (ineffective coil) vacant, as shown in Fig. 5(a). Since magnetic flux density decreases sharply to zero away from the edge of the magnets, strong magnetic field can only cover the effective coil (Fig. 5(b)). When the magnets move with input vibration, the effective coils cut the magnetic lines and generate induced voltage, while the ineffective coils do not experience great magnetic flux density and produce much lower voltage since they are relatively far from the magnets. With the magnets moving in one direction, the effective coils can generate induced voltage in the same direction and the voltage can be added because
Fig. 4. Magnetic flux density and its gradient from (a) a whole magnet block and (b) magnet array.
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Fig. 7. FEM analysis of the peak voltage and normalized power density as a function of the distance between effective and ineffective coils. The inset illustrates the implication of the distance. (For interpretation of the references to color in text near the reference citation, the reader is referred to the web version of this article.)
Fig. 5. (a) Illustration of the series coils for enhanced output. (b) Magnetic flux density over the effective and ineffective coils. Table 2 Geometry parameters in the transient analysis. Parameters
Number of turns Width (m) Spacing (m) Thickness (m) Distance to X-axis (m) Moving amplitude (m)
Components Effective coils
Ineffective coils
Magnet array
8 20 20 10 0 0
8 20 20 10 0 0
– 20 20 10 20 200
the value in the ineffective coils is only 3.0 V. Due to the much smaller voltage in the ineffective coils, the overall output will not be dramatically weakened. As shown in Fig. 6(b), the overall output voltage in this case is 12.7 V. Based on the above analysis, we investigate the dependence of overall output voltage on the distance between effective and ineffective coils. As shown in the blue curves in Fig. 7, the simulated overall induced voltage increases monotonically when the distance varies from 0 to 400 m. This is because the magnetic flux density across the ineffective coils decreases at further distances. However, since the increase of distance will enlarge the volume of the device, the distance should be optimized to provide the maximum power density. The power density can be expressed as, PD =
of end-to-end connection. Consequently, the total output voltage (V) can be expressed as, V = Nlv(Beffective − Bineffective )
(5)
where N is the number of coils, l is the length of each rectangular coil, v is the velocity of the magnet, Beffective and Bineffective are the magnetic flux densities across the effective coils and ineffective coils, respectively. To prove the effectiveness of this design, transient analysis of the induced voltage is conducted. A 2D model of fixed copper coils with moving magnet (50 Hz Sinusoidal vibration) is established with the size parameters listed in Table 2 and illustrated in Fig. 6(a). When the magnet array moves at X-axis, voltage is induced in both the effective coils and the ineffective coils. Setting the distance between effective coils and ineffective coils as 150 m, the maximum induced voltage in the effective coils reaches to 14.8 V, while
U2 RWt(L + L)
(6)
where PD is the power density, U is the induced voltage, R is the inner resistance of the coil, L, W, t are the length, width and thickness of the device, L is the increased length. In our model, the total length (L) is 600 m if there is no distance (L) between effective and ineffective coils. Neglecting the change of the coils inner resistance, the power density is proportional to U2 /(L + L). As shown in the red curves in Fig. 7, the power density (normalized value) first increase with the distance and reaches maximum when the distance is 300 m. When distance further increases, the power density slightly drops due to the length increment. 2.4. Comparison with the out-of-plane operation mode Compared with out-of-plane mode, electromagnetic energy harvesters with in-plane operation mode can provide larger
Fig. 6. (a) Schematic diagram of the 2D model. The inset shows the corresponding top view of the coil. (b) Overall output voltage and the induced voltage in the effective and in effective coils.
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Fig. 10. Induced voltage of the out-of-plane mode at different vibration amplitudes. Fig. 8. Induced voltages of in-plane and out-of-plane operation mode with same geometry parameter at the amplitude of 350 m.
changing rate of magnetic flux density, thus enhancing the induced voltage. In this work, we carry out FEM analysis to compare the output performance of these two operation modes. Two 2D models with the same dimension but different vibration direction are applied. As an example, we set the vibration amplitude and frequency as 350 m and 50 Hz for both modes. When the magnet array moves along the Y-axis (i.e., out-of-plane mode), induced voltage with a maximum value of 59.5 nV is achieved, as shown in Fig. 8. By contrast, when the magnet array moves along the Xaxis (i.e., in-plane mode) with the same amplitude and frequency, much larger induced voltage is obtained. In the in-plane operation mode, the maximum induced voltage is 23.5 V, which is about 400 times larger than that of the out-of-plane operation mode. In addition, the vibration amplitude can greatly affect the induced voltage of in-plane mode electromagnetic energy harvesters. As shown in Fig. 9(a), keeping the frequency at 50 Hz, when the vibration amplitude increases from 50 m to 350 m, the primary peak induced voltage raises from 3.7 V up to 23.5 V. Interestingly, when the amplitude reaches a certain value (250 m in our model), a secondary peak appears, which also increases with the amplitude. A more detailed relationship between vibration amplitude and peak induced voltage is demonstrated in Fig. 9(b), showing a linear relationship between them. Therefore, by designing the mechanical structure that allowing large amplitude, output performance of the in-plane mode electromagnetic energy harvesters can be further enhanced. The raised voltage can be attributed to the increased velocity, because magnets with larger amplitude move at a faster speed. As for the out-of-plane operation mode, increasing the amplitude will inevitably cause the original position of magnets move away from the coils. Therefore, although larger amplitude increases the velocity, the further distance from coils greatly reduce the magnetic flux density, which is not conducive to the induced voltage. FEM analysis is also conducted to investigate the amplitude influence on the out-of-plane mode energy harvesters. To ensure the consistency, the frequency is also set as 50 Hz. As shown in Fig. 10, when the
amplitude is 15 m, peak voltage of 61.4 nV is obtained for the out-of-plane operation mode device. As the amplitude increases to 100 m, peak voltage drops to 22.6 nV, because of the reduced magnetic flux density. The combined effect of velocity and magnetic flux density is complex, as the peak value raises to 59.5 nV with 350 m amplitude. Compared with the out-of-plane mode, the in-plane operation mode can greatly enhance the induced voltage. In addition, the output can be further improved by increasing the amplitude through rational structural design. 3. Fabrication Compared to traditional electromagnetic energy harvesters with manually assembled magnets [30,31], this device is fabricated with MEMS batch fabrication process completely which is accurate, reliable and mass-productive. Electroplating technique is the main process in this fabrication. The coils, supporting pillar, vibration plate are all fabricated by copper electroplating. Innovatively, we applied the CoNiMnP electroplating technique in energy harvesting applications to batch fabricate the magnet array. Detailed fabrication process is illustrated in Fig. 11. First, 3000 A˚ SiO2 is grown on both sides of silicon wafer by thermal oxidation which serves as an insulating layer (Fig. 11(a)). Next, 300 A˚ titanium and 3000 A˚ copper are sputtered onto SiO2 as the seed layer for electroplating (Fig. 11(b)). Then, coils are patterned with 10 m thick AZ 4620 photoresist and electroplated on the seed layer with 10 m thickness (Fig. 11(c)). The resistance of electroplated coils is 11.6 , measured by the Handheld Digital Oscilloscope (SHS806). As the next step, the 10 m thick supporting pillars are electroplated onto the side of the coils (Fig. 11(d)). Afterwards, another 3000 A˚ copper seed layer is sputtered (Fig. 11(e)) and the vibrating plate is electroplated onto this seed layer with 10 m thickness (Fig. 11(f)). In the following, CoNiMnP hard magnetic alloy is batch-fabricated by electroplating technique (Fig. 11(g)). In this process, two external NdFeB magnets are placed at two sides of the container to make the magnetization vector of CoNiMnP magnets aligned in one direction [28]. Finally, the photoresist and seed layers are removed leaving a 10 m gap between the coils and vibration plate (Fig. 11(h)).
Fig. 9. (a) Transient analysis of induced voltage versus different amplitude. (b) Detailed relationship between peak induced voltage and amplitude.
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Fig. 11. Fabrication process: (a) deposition of SiO2 as insulting layer, (b) sputtering of copper seed layer, (c) electroplating of the copper coils, (d) electroplating of the supporting pillars, (e) sputtering the second copper seed layer, (f) electroplating of the vibration plate, (g) electroplating of the CoNiMnP magnet array, and (h) release the device by removal of the photoresist and seed layer.
Fig. 12. SEM images of the fabricated energy harvester: (a) top view of the device, (b) the folded beam and 10 m gap, (c) the electroplated copper coil, and (d) the electroplated CoNiMnP magnet array.
With the above MEMS batch fabrication process, a massproductive electromagnetic energy harvester is obtained and its SEM images are shown in Fig. 12. From Fig. 12(a), a concise view of the device is given, showing the coils, supporting pillar, vibration plate with folded beam and CoNiMnP magnet array. In Fig. 12(b), the 10 m gap between coils and vibration plate can be observed. Fig. 12(c) shows a tilted view of the electroplated coils with a 10 m thickness. The electroplated CoNiMnP permanent magnet array is uniformly arranged on the vibration plate, as shown in Fig. 12(d). Table 3 gives out the specific dimensions of the device. Due to the electroplated CoNiMnP micro magnet array, the total volume of the vibration plate and magnets is only about 5 mm3 , which is
Table 3 Geometry of the energy harvester. Parameters
Number of turns Number of units Thickness (m) Total area (mm2 )
Components Coils
Vibration plate
Magnet array
8 – 10 45
– – 10 56
– 12 × 4 10 4.5
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Fig. 13. EDS analysis of the composition. The inset shows the surface morphology of the electroplated CoNiMnP.
Fig. 14. (a) Measurement system. (b) Photo of the fabricated energy harvester.
The device is tested through a vibration system as shown in Fig. 14(a) and the enlarged photo of the prototype is shown in Fig. 14(b). The measurement system consists of five parts: a wave form generator (RIGOL DG1022), a power amplifier (SINOCERA YE5871A), a vibrator (SINOCERA JZK-5), an amplifier circuit and an oscilloscope (RIGOL DS1102E). The vibrator in the measurement system is used to simulate environmental vibrations with different frequencies, which can be controlled by the wave form generator. Acceleration of the vibration can be controlled by the power amplifier. The output voltage of the electromagnetic energy harvester
is amplified through the operational amplifier circuit and sent to oscilloscope to be recorded. A sweep frequency measurement is conducted under the acceleration of 1.2 g with a total time of 60 s. The output voltage as a function of vibration frequency is shown in Fig. 15. The maximum peak output voltage is 0.98 mV at the frequency of 48 Hz. The second and third voltage peaks may be caused by the high resonant mode, which is explained in Fig. S1 in the Supporting Information file. In comparison with the simulation results, larger measured voltage can be attributed to different moving state of the magnet array. On the one hand, larger amplitude may be achieved in the measurement, which can enhance the induced voltage. On the other hand, the vibration plate as well as the magnet array may be closer to the coil due to the imperfect vibration direction, leading to stronger magnetic flux density. Load response of this electromagnetic energy harvester is also measured. As shown in Fig. 16, the maximum peak power reaches to 11.2 nW under a load resistance of 15.8 . Thanks to the electroplated CoNiMnP permanent magnet array, the thickness of the magnet is only 10 m.
Fig. 15. Frequency response of the energy harvester.
Fig. 16. Output voltage and power under different load resistance.
greatly decreased compared with previous energy harvesters with manually assembled magnets [30]. As shown in Fig. 13, an energy dispersive spectrum (EDS) analysis is conducted to investigate the material composition of the electroplated CoNiMnP. The inset shows the surface morphology of electroplated CoNiMnP, which is uniform. 4. Measurement and discussion
M. Han et al. / Sensors and Actuators A 219 (2014) 38–46 Table 4 NPD comparison of electromagnetic energy harvesters.
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Acknowledgements
Energy harvesters
Magnet type
NPD (kgs m−3 )
This work Ref. [11] Ref. [30] Ref. [34]
CoNiMnP CoNiMnP NdFeB NdFeB
0.0012 0.0003 0.09 26.07
Considering a 500 m silicon substrate on which the device is fabricated, the volume of this electromagnetic energy harvester is 67.5 mm3 (16 mm × 8 mm × 0.54 mm), corresponding to a power density of 0.16 W/cm3 . Table 4 compares the normalized power density (NPD) [32,33] of our device with other electromagnetic energy harvesters. Compared with previous electromagnetic energy harvester with integrated CoNiMnP magnets [11], the output performance is improved. The in-plane operation mode and magnet array effectively utilizes the large magnetic flux density at the edge of the magnets, which contribute to the enhancement. Moreover, the series coils and arrangement of the magnet array accumulate the induced voltage in each conducting rod, thus further increasing the output. Compared with energy harvesters with NdFeB magnets [30,34], the NPD is still relatively low due to the weak magnetic properties of CoNiMnP. However, we can see that the output voltage and power of our device still have potential improvement by further optimization. For example, increasing the thickness of electroplated CoNiMnP will enhance the magnetic flux density, which is benificial to the output. Enlarging the aspect ratio of copper coils (i.e., increasing the thickness and reducing the width) will also contribute to larger induced voltage [11]. Moreover, by utilizing new structures such as 3D coils and frequency up-conversion technique, better output performance is expected to be achieved. With the small volume, low resonant frequency and enhanced power output, this electromagnetic energy harvester is promising to be utilized to generate energy from environmental vibrations.
5. Conclusions This paper reports a novel in-plane electromagnetic energy harvester with integrated single-pole CoNiMnP magnet array. Unlike traditional energy harvester with manually assembled magnets, the whole fabrication process of this device is using MEMS technology, which is accurate, reliable and mass-productive. The vibration plate, folded beams and permanent magnets are fabricated by electroplating technique using the thick photoresist as sacrificial layer. Through a series of FEM simulations, structure and size parameters of the device are designed and investigated. Firstly, distribution of the single-pole CoNiMnP magnet array is carefully optimized to generate large magnetic flux density across the coils. Besides, the fixed coils are designed as rectangular spiral shape to accumulate the induced voltage, which is verified by transient analysis. In addition, by comparing the induced voltage and amplitude dependence, the advantage of in-plane operation mode over out-of-plane operation mode is demonstrated, and the obtained results can be referred for further improvement of inplane electromagnetic energy harvesters. Through experimental measurement, maximum peak output voltage appears at 48 Hz, with a value of 0.98 mV. Power density of 0.16 W/cm3 is achieved with a 15.8 external resistance. Further enhancement, such as longer coils and stronger magnets, can be applied to increase the output voltage. Also, with this integrated process, more innovation structure design of this device, such as series coils arrays and three-dimension structures, can be considered in the future plan.
This work is supported by the National Natural Science Foundation of China (Grant nos. 61176103, 91023045 and 91323304), the National Hi-Tech Research and Development Program of China (“863” Project) (Grant no. 2013AA041102), the National Ph.D. Foundation Project (Grant no. 20110001110103) and the Beijing Natural Science Foundation of China (Grant no. 4141002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sna.2014.08.008. References [1] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Comput. Commun. 26 (2003) 1131–1144. [2] S.P. Beeby, M.J. Tudor, N.M. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175–R195. [3] R. Elfrink, T.M. Kamel, M. Goedbloed, S. Matova, D. Hohlfeld, Y. van Andel, R. van Schaijk, Vibration energy harvesting with aluminum nitride-based piezoelectric devices, J. Micromech. Microeng. 19 (2009), 094005-1–094005-8. [4] H. Liu, C.J. Tay, C. Quan, T. Kobayashi, C. Lee, M.E.M.S. Piezoelectric, energy harvester for low-frequency vibrations with wideband operation range and steadily increased output power, J. Microelectromech. Syst. 20 (2011) 1131–1142. [5] T. Galchev, E.E. Aktakka, K. Najafi, A piezoelectric parametric frequency increased generator for harvesting low-frequency vibrations, J. Microelectromech. Syst. 21 (2012) 1311–1320. [6] P.D. Mitcheson, P. Miao, B.H. Stark, E.M. Yeatman, A.S. Holmes, T.C. Green, MEMS electrostatic micropower generator for low frequency operation, Sens. Actuators A: Phys. 115 (2004) 523–529. [7] H. Lo, Y.C. Tai, Parylene-based electret power generators, J. Micromech. Microeng. 18 (2008), 104006-1–104006-8. [8] Y. Naruse, N. Matsubara, K. Mabuchi, M. Izumi, S. Suzuki, Electrostatic micro power generation from low-frequency vibration such as human motion, J. Micromech. Microeng. 19 (2009), 094002-1–094002-5. [9] P. Wang, K. Tanaka, S. Sugiyama, X. Dai, X. Zhao, J. Liu, A micro electromagnetic low level vibration energy harvester based on MEMS technology, Microsyst. Technol. 15 (2009) 941–951. [10] Ö. Zorlu, E.T. Topal, H. Külah, A vibration-based electromagnetic energy harvester using mechanical frequency up-conversion method, IEEE Sens. J. 11 (2011) 481–488. [11] M. Han, Q. Yuan, X. Sun, H. Zhang, Design and fabrication of integrated magnetic MEMS energy harvester for low frequency applications, J. Microelectromech. Syst. 23 (2014) 204–212. [12] M. Han, X. Zhang, W. Liu, X. Sun, X. Peng, H. Zhang, Low-frequency wide-band hybrid energy harvester based on piezoelectric and triboelectric mechanism, Sci. China Tech. Sci. 56 (2013) 1835–1841. [13] M. Han, W. Liu, X. Zhang, B. Meng, H. Zhang, Investigation and characterization of an arc-shaped piezoelectric generator, Sci. China Tech. Sci. 56 (2013) 2636–2641. [14] I. Sari, T. Balkan, H. Külah, An electromagnetic micro power generator for wideband environmental vibrations, Sens. Actuators A: Phys. 145 (2008) 405–413. [15] I. Sari, T. Balkan, H. Külah, An electromagnetic micro power generator for low-frequency environmental vibrations based on the frequency upconversion technique, J. Microelectromech. Syst. 19 (2010) 14–27. [16] Q. Zhang, E.S. Kim, Energy harvesters with high electromagnetic conversion efficiency through magnet and coil arrays, in: MEMS 2013 Proceedings, Taipei, Taiwan, January 20–24, 2013, pp. 110–113. [17] D. Zhu, S. Beeby, J. Tudor, N. Harris, Vibration energy harvesting using the Halbach array, Smart Mater. Struct. 21 (2012), 075020-1–075020-11. [18] H. Liu, Y. Qian, N. Wang, C. Lee, Study of the wideband behavior of an in-plane electromagnetic MEMS energy harvester, in: MEMS 2013 Proceedings, Taipei, Taiwan, January 20–24, 2013, pp. 829–832. [19] S. Roundy, E. Takahashi, A planar electromagnetic energy harvesting transducer using a multi-pole magnetic plate, Sens. Actuators A: Phys. 195 (2013) 98–104. [20] D. Zhu, S. Beeby, J. Tudor, N. Harris, Increasing output power of electromagnetic vibration energy harvesters using improved Halbach arrays, Sens. Actuators A: Phys. 203 (2013) 11–19. [21] D.P. Arnold, N. Wang, Permanent magnets for MEMS, J. Microelectromech. Syst. 18 (2009) 1255–1266. [22] N. Wang, B.J. Bowers, D.P. Arnold, Wax-bonded NdFeB micromagnets for microelectromechanical systems applications, J. Appl. Phys. 103 (2008), 07E109-1–07E109-3. [23] Y. Jiang, S. Masaoka, M. Uehara, T. Fujita, K. Higuchi1, K. Maenaka, Microstructuring of thick NdFeB films using high-power plasma etching for magnetic MEMS application, J. Micromech. Microeng. 21 (2011), 045011-1–045011-5. [24] Y. Jiang, S. Masaoka, T. Fujita, M. Uehara, T. Toyonaga, K. Fujii, K. Higuchi, K. Maenaka, Fabrication of a vibration-driven electromagnetic energy harvester with
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Biographies
Mengdi Han received the B.S. degree in Electronic Science and Technology from Huazhong University of Science and Technology, China in 2012. He is a Ph.D. candidate in National Key Lab of Nano/Micro Fabrication Technology at Peking University, China. His research work is focused on hybrid energy harvesting and MEMS sensors.
Zhongliang Li received the B.S. degree in Institute of Microelectronics from Peking University, China in 2013. He is now a Ph.D. student in the Department of Biomedical Engineering in University of California, Davis, USA. His research focused on Power MEMS and microfluidics.
Xuming Sun received the B.S. degree in Electronic Science and Technology from Chongqing University, Chongqing, China, in 2009. He is currently working toward the Ph.D. degree in microelectronics and solid state electronics at Peking University, Beijing, China. His research interests mainly include three areas: (1) design and fabrication of micro energy devices; (2) wireless power transfer system; (3) RF MEMS devices.
Haixia Zhang, Professor, Institute of Microelectronics, Peking Universituy. She received Ph.D. degree in Mechanical Engineering from the Huazhong University of Science and Technology, China, 1998, joined the faculty of the Institute of Microelectronics in 2001 after her Post-Doc in Tsinghua University. She is the senior member of IEEE and was served on the general chair of IEEE NEMS 2013 Conference, the organizing chair of Transducers’11. As the founder of the International Contest of Applications in Network of things (iCAN), she organized this world-wide event since 2007. At 2006, Dr. Zhang won National Invention Award of Science & Technology. Her research fields include Micro-nano Design and Fabrication Technology, SiC MEMS and Micro Energy Technology.