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Microelectronics Journal 43 (2012) 154–159

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Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo

Design, simulation, fabrication and characterization of a micro electromagnetic vibration energy harvester with sandwiched structure and air channel Peihong Wang a,n, Huiting Liu b, Xuhan Dai c, Zhuoqing Yang c, Zhongzhu Wang a, Xiaolin Zhao c a

School of Physics and Material Science, Anhui University, Hefei, 230039, China School of Computer Science and Technology, Anhui University, Hefei 230039, China c Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2011 Received in revised form 30 September 2011 Accepted 10 October 2011 Available online 12 December 2011

This paper presents the design, simulation, fabrication and characterization of a novel electromagnetic vibration energy harvester with sandwiched structure and air channel. It mainly consists of a top coil, a bottom coil, an NdFeB permanent magnet and a nickel planar spring integrated with silicon frame. The prototype is fabricated mainly using silicon micromachining and microelectroplating techniques. The tested natural frequency of the magnet–spring system is 228.2 Hz. The comparison between the simulation and the tested results of the natural frequency shows that the Young’s modulus of microelectroplated Ni film is about 163 GPa rather than 210 GPa of bulk Ni material. Experimental results indicate that the sandwiched structure and the air channel in the silicon frame of the prototype can make the induced voltage increase to 42%. The resonant frequency of the prototype at 8 m/s2 acceleration is 280.1 Hz, which results from the nonlinear behavior of the magnet–spring system. The load voltage generated by the prototype is 162.5 mV when the prototype is at resonance and the input vibration acceleration is 8 m/s2 and the maximal load power obtained is about 21.2 mW when the load resistance is 81 O. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Energy harvester Vibration energy Electromagnetic Sandwiched structure Air channel

1. Introduction There has been a great advance in microelectronics technology and ultra low power Very Large Scale Integration (VLSI) design over the past decades. This advance has led to the generation and application of various new miniature sensor/actuators and microsystems. They have very small volume and low power consumption; they are wireless, portable and even embeddable. So they can be used in intelligent monitoring, health care, automotive industry, medical implants, wireless sensor networks, Microelectromechanical systems (MEMS), etc. However, power requirements place important limits on the capability of these devices, since the developing speed of conventional battery technology is far slower than that of integrated circuit technology described by Moore’s law [1]. Batteries having limited lifetime, are bulky compared with microsystems, and need to be recharged. Moreover, batteries cannot power some embedded devices, which does not have any physical connections to the outside and it is hardly possible to periodically replace batteries for thousands of sensor nodes in wireless sensor network. Therefore, a renewable power source

n

Corresponding author. E-mail addresses: [email protected], [email protected] (P. Wang). 0026-2692/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2011.10.003

must be developed to substitute batteries or electrical cable to power these wireless microsystems. Harvesting energy from the ambient environment and then converting it into electrical power is a very promising alternative to conventional power sources. Energy can be harvested from many ambient sources such as light, heat and vibration [2–5]. Solar cells supply excellent power in direct sunlight, but it is not convenient for embedded systems and not efficient in dim environment. Thermal energy can be scavenged from the environment with high thermal gradient; however, thermal gradient over MEMS scale is very small. Mechanical vibrations seem to be the most promising energy source since they are abundant in many environments. As a typical kind of energy harvesting techniques, various electromagnetic vibration energy scavengers have been developed [6–14] since Williams et al. reported the analysis of a micro-electric generator for microsystems in 1996. However, the structures of these reported energy harvester prototypes are same and consist of only one induced coil, a permanent magnet and a spring. In this structure, only the magnetic field close to the coil can be used to convert energy, although the magnetic field exists around the magnet. So the magnetic field is not used sufficiently. We have presented an electromagnetic vibration energy harvester with an electroplated metal spring and a two-layer copper coil previously [15,16]. In this paper, a micro electromagnetic vibration energy harvester with sandwiched structure and

P. Wang et al. / Microelectronics Journal 43 (2012) 154–159

2. Design and simulation The 3-D schematic and cross-section view of the sandwiched electromagnetic vibration energy harvester are shown in Fig. 1. It mainly consists of a top coil, a bottom coil, an NdFeB permanent magnet and a Nickel (Ni) planar spring integrated with silicon frame. The magnet–spring system is used to harvest the mechanical vibration energy and the magnet–coil system is used to convert vibration energy into electrical energy. When outside vertical vibration is applied on the vibration energy harvester, the magnet will vibrate up and down relative to the coil, which results in the change of the magnetic flux through the coil. According to Faraday’s law of induction, induced voltage is generated in the coil and induced current is generated in the circuit if the coil is connected into an outside circuit. In the sandwiched structure, the top coil and the bottom coil are located symmetrically on each side of the magnet, respectively. So when the energy harvester is working, the top coil and the bottom coil can generate same electrical energy. As a result, the generated electrical energy of the energy harvester is increased after top coil and bottom coil are connected in series. If the sandwiched structure is sealed, the inner air will generate air pressure on the magnet–spring system to damp their vibration and so decrease the amplitude of the magnet when the device is working. If the structure is with air channels, the inner air will flow freely and so will not damp the vibration of magnet–spring system. So the air channel in the silicon frame can decrease the air damping effect and then the output performance of the energy harvester can be increased further. Moreover, the gap between the magnet and the coil can be controlled by using silicon frame with different thickness or multiple silicon frames with same thickness. The glass substrate with top coil is like a cap so that it can protect the magnet–spring system and does not increase the total volume additionally. The design, modeling and simulation details of vibration energy harvester with single coil structure have been reported in our previous work [15,16]. Here we simulate and discuss the influence of the Young’s modulus of Ni on the natural frequency

of the mass–spring system, since there is a great difference between the Young’s modulus of bulk Ni material and that of microelectroplated Ni film. In the simulation, the Young’s modulus of bulk Ni material is defined as 210 GPa [17] and that of microelectroplated Ni film 163 GPa [18]. Fig. 2 shows the simulation results about the relationship between the natural frequency of Ni spring and the spring’s thickness under different Young’s modulus of Ni material. As seen from Fig. 2, the natural frequency of Ni spring increases with the thickness of the Ni spring. Meanwhile, the natural frequency of Ni spring with 210 GPa Young’s modulus is always bigger than that with 163 GPa Young’s modulus. Moreover, the thicker is the Ni spring, the bigger is the difference between the natural frequencies under different Young’s modulus.

3. Fabrication The presented sandwiched electromagnetic vibration energy harvester is fabricated using MEMS micromachining technique. The fabrication process of the micro two-layer copper coil has been described previously [15,16]. The nickel planar spring on the silicon frame with air channel is fabricated using microelectroplating and silicon micromachining technique. The detailed fabricated process is shown in Fig. 3 (a)–(h). Fig. 3(a) shows that the photoresist (AZ P4903) is spin coated on the backside of a doubleside polished silicon wafer, which is thermally oxidized on both sides and then patterned by photolithography. Fig. 3(b) shows that the exposed SiO2 layer is wet etched using HF solution and then the photoresist is removed. Fig. 3(c) shows that the exposed

E=210GPa (Bulk Ni) E=163GPa (Microelectroplated Ni)

500 Natural frequency (Hz)

air channel is presented. Its sandwiched structure is clearly different with our previous work and other published prototypes. Its symmetrical arrangement of two coils can use the magnetic field more efficiently and the air channel in the silicon frame can decrease the air damping efficiently, both of which can increase the output performance of energy harvester. The prototype is fabricated mainly using microelectroplating and silicon micromachining techniques. Tested results show that the prototype has better performance compared with our previous work and other single coil structures.

155

400

300

200

100 20

30

40 50 60 70 Ni spring thickness (μm)

80

Fig. 2. Natural frequency of the magnet–spring system versus spring thickness with different Young’s modulus of Ni material.

Fig. 1. Schematic of the sandwiched vibration-based power generator: (a) 3-D view, (b) cross-section view.

156


Fig. 3. Fabrication process of Ni planar spring integrated with silicon frame.

Fig. 4. Photographs of fabricated Ni spring (a), Cu coils (b) and the assembled prototype (c).

Si in the center of the silicon frame is wet etched to about 300 mm using KOH solution (the etching rate is about 1 mm/min). Fig. 3(d) shows that the photoresist is spin coated on the backside again and patterned. And then the SiO2 layer on the air channel is wet etched for next Si etching. Fig. 3(e) shows that the Cu/Cr seed layer is sputtered on the upside of the silicon wafer and the photoresist is coated and patterned for next electroplating. Fig. 3(f) shows that the Ni planar spring is electroplated using nickel electroplating bath (Ni [NH2SO3]2) (600 g/L, H3BO3 (25 g/L), NiCl2 6H2O (10 g/L)) and then the photoresist is stripped. Fig. 3(g) shows that the silicon wafer is wet etched through to release the Ni spring. At the same time, the air channel in the silicon frame is obtained. Fig. 3(h) shows that the SiO2 layer and Cu/Cr seed layer under the Ni spring is wet etched out. The fabricated Ni planar spring on the silicon frame with and without air channel is shown in Fig. 4(a). The dimension of the air channel is about 3 0.75 0.3 mm3. The thickness of the Ni planar spring is 50 mm, the width of the spring beam 500 mm and the gap between the beams 200 mm. The fabricated Cu planar square coil and it SEM picture are shown in Fig. 4(b). The inner/outer side lengths of the coil are 0.7/2.5 mm, respectively. The linewidth of the coil is 15 mm, the thickness 15 mm, the number of turns of every layer 30, and the resistance 40.4 O. Finally, two coils on glass substrate with thickness of 2 mm, a Ni planar spring on silicon

frame with air channel, a silicon frame with air channel, two NdFeB permanent magnets with dimension of 2 2 1 mm3, are assembled into the sandwiched electromagnetic vibration energy harvester prototype. Its dimension is about 9 7 5 mm3 and so the volume is about 0.32 cm3. The gap between the magnet and the coil is about 600 mm. The photograph of the prototype is shown in Fig. 4 (c).

4. Results and discussion The fabricated prototypes are tested by the experimental setup shown in Fig. 5. The vibrator (SINOCERA JZK-5) is used to supply mechanical vibration to the prototype. The power amplifier (SINOCERA YE5872) incorporated with the waveform generator (Agilent 33220A) is used to drive the vibrator and regulate its vibration intensity (acceleration, displacement or velocity). The prototype is fixed on the top of the vibrator and connected with an adjustable load resistance in series. The accelerometer (SINOCERA CA-YD-1107) is tightened on the vibrator by a screw and is connected with a vibration monitor. The vibration monitor (SINOCERA YE5932A) can measure the acceleration, velocity and displacement of the vibration signal from the vibrator. The oscilloscope (Agilent, MSO6034A) is used to measure the load


157

Fig. 5. Photograph of experimental setup.

120

120

Amplitude (μm)

Amplitude (μm)

228.2Hz 99.8μm

80

40

80

40

0

0 150

200

250

300

A

A/ 2

220

f1 fn f2

240

Frequency (Hz)

Frequency (Hz)

Fig. 6. (a) Amplitude–frequency curve of the magnet–spring system at constant input amplitude of 1 mm and (b) the enlarged picture marked in (a).

voltage of the resistance. The laser displacement sensor (KEYENCE LK-G10) is used to measure the displacement of the magnet. The natural frequency of the energy harvester is first measured using the experimental setup since it is a very important parameter for energy harvester. The tests are carried out at a constant input displacement of 1 mm and by sweeping the frequency from 150 to 300 Hz. So the acceleration can be calculated and changes from 0.89 to 3.55 m/s2. The laser displacement sensor can measure the relationship between the magnet’s displacement and the time. The amplitude versus frequency is shown is Fig. 6, which is obtained by applying FFT on the displacement–time curve. Fig. 6 shows that the resonant amplitude of the magnet is 99.8 mm and the natural frequency of the magnet–spring system is 228.2 Hz. Comparing with the simulation results shown in Fig. 2, we can see that the measured natural frequency 228.2 Hz is closer to 235.8 Hz (simulated result using Young’s modulus of microelectroplated Ni film) other than 267.6 Hz (simulated result using Young’s modulus of bulk Ni). So the Young’s modulus of the microelectroplated Ni material in this work is about 163 GPa. On the other hand, Eq. (1) gives the method to calculate the quality factor Q and the damping ratio x of the magnet–spring system. In this equation, fn is the natural frequency and Df is the half-power bandwidth. The meanings of fn and Df are indicated in Fig. 6(b). Q¼

fn

Df

¼

1 2x

ð1Þ

Using Eq. (1) and the data in Fig. 6(b), the quality factor Q of the magnet–spring system is calculated as 98 and the damping ratio is

0.0051. These two parameters are very important in the dynamic simulation of the energy harvester, otherwise the transient analysis of the damped vibration of the magnet–spring system cannot be carried out. In order to compare the output performance of sandwiched structure with that of single coil structure, testing is performed twice before and after the top coil and bottom coil are connected in series. The testing conditions are same, which are (1) the input vibration acceleration is 8 m/s2 during the swept frequency process; (2) the load resistance is 1 MO. Tested result of the induced voltage in both coils is shown in Fig. 7 when the top coil and the bottom coil in the prototype are not connected. This figure shows that there is a maximal voltage generated in each coil during the swept frequency process. It also can be seen from Fig. 7 that the induced voltage through the coil is an AC signal and the waveform is triangular other than sinusoidal. The magnetic flux rate is different when the permanent magnet moves in positive half period and negative half period, which results in the non-sinusoidal waveform of the AC signal. The relevant simulations have been made and are shown in our previous work. The enlarged picture in Fig. 7 indicates that the maximum of load voltage generated in top coil and bottom coil are same and its peak–peak value is 114.5 mV, and there is a phase difference between these two voltage–time curves. Tested result of the load voltage is shown in Fig. 8 when the top coil and bottom coil are connected in series. It shows that the generated voltage signal through the coil in series is also an AC signal but the waveform is almost sinusoidal, which is different

158


Fig. 7. Induced voltage on the load resistance versus time in the top and bottom coil before these two coils are connected in series.

with the result in Fig. 7. This sinusoidal waveform shown in the enlarged picture in Fig. 8 is the synthesis results of two triangular waveforms shown in the enlarged picture in Fig. 7 but not the simple addition of them. As seen from Fig. 8 that the load voltage generated by the coil in series is 162.5 mV (peak–peak value). Fig. 7 also shows that there is a phase difference between the waveforms of voltage signal in both coils, so the induced voltage 162.5 mV through the coils in series is smaller than the sum of voltages in two coils (namely 229 mV). According to the above analysis about Figs. 7 and 8, the load voltage of 162.5 mV generated by the energy harvester prototype with sandwiched structure and air channel is increased 42% comparing with that of 114.5 mV by a single coil. So the sandwiched energy harvester prototype with air channel has higher output performance and so higher energy conversion efficiency. The relationship between the load voltage and input vibration frequency at 1 m/s2 and 8 m/s2 accelerations for the sandwiched prototype are shown in Fig. 9. This figure shows that the prototype has different resonant frequency and different load voltage under input vibration with different accelerations. The resonant frequency at 1 m/s2 acceleration is 229.4 Hz, which is almost same to the natural frequency of the magnet–spring system shown in Fig. 6. However, the resonant frequency 280.1 Hz at 0.8 g acceleration is much higher than the natural frequency of the magnet–spring system, and the shape of the curve also has great difference with that curve at 1 m/s2 acceleration shown in Fig. 9. The load voltage increases to the maximum gradually and then decreases promptly after resonance point. This is a significant nonlinear phenomenon which also existed in the reference [9]. We think this nonlinear

Peak-peak value of load voltage (mV)

Fig. 8. Induced voltage on the load resistance versus time in the coil after the top and bottom coil are connected in series.

160 0.8g acceleration 0.1g acceleration

280.1Hz 162.5mV

120

80

40

229.4Hz 32mV

0 150

200 250 300 Vibration frequency (Hz)

350

Fig. 9. Peak–peak value of load voltage versus input vibration frequency at 0.1 g and 0.8 g acceleration and 1 MO load resistance.

behavior comes from the hard spring effect and have measured the nonlinear relationship between the deflection of the Ni spring and the applied force. The relative work about how to use this nonlinear effect to increase the frequency bandwidth of energy harvester is ongoing. Fig. 10 shows the load voltage versus the load resistance when the prototype is resonant at 8 m/s2 vibration acceleration. It indicates

160 20

Output voltage Output power

120

20

120

16

15

10

80

80

12

8

40 0

100

200

300

400

Load power (μW)

Peak-peak value of load voltage (mV)


5

0 1000

2000 3000 4000 Load resistance (Ω)

Acknowledgments

5000

Fig. 10. Load voltage and load power versus load resistance measured at resonant condition at 0.8 g acceleration.

that the load voltage increases with the load resistance dramatically when the load resistance is smaller than 1 kO and then slowly increases to a stable value of 162.5 mV. The relationship between the load power, load resistance and load voltage can be described by the following equation: P load ¼

V 2pp 4 Rload

the sandwiched prototype with air channel at 8 m/s2 acceleration is increased 42% comparing with the load voltage of 114.5 mV generated by single coil. The nonlinear behavior of the magnet–spring system at 8 m/s2 acceleration results in a big change in the shape of voltage–frequency curve and the resonant frequency of the prototype is 280.1 Hz. This nonlinear behavior may be used to increase the bandwidth of energy harvester. The prototype can generate maximal load voltage of 162.5 mV under load resistance of above 1 KO and load power of 21.2 mW corresponding to 66.25 mW/cm3 under the optimum load resistance 81 O, when the input vibration acceleration is 8 m/s2 and the prototype is at resonance.

500

40 0

159

ð2Þ

where Pload is the load power, Vpp is the peak–peak value of load voltage, Rload is the load resistance. The calculated load power using Eq. (2) versus load resistance is also given in Fig. 10. It can be seen from the enlarged curve in Fig. 10 that the load power is maximal when the load resistance is equal to the coil’s resistance 81 O and the maximum is 21.2 mW. In one of our published papers [19], the performance of a sandwiched electromagnetic vibration energy harvester without air channels has been presented. It showed that the maximal output power is 13.2 mW. So the air channel is very helpful to increase the output power of the presented sandwiched energy harvester in this paper.

5. Conclusion A micro electromagnetic vibration energy harvester with sandwiched structure and air channel is presented. It consists of a top coil, a bottom coil, an NdFeB magnet and a planar Ni spring on the silicon frame with air channel. The sandwiched structure and the air channel are very helpful to increase the output performance. Finite element method is used to simulate the influence of Young’s modulus of Ni material on the natural frequency of magnet–spring system. The Ni planar spring on silicon frame with air channel is fabricated using silicon micromachining and microelectroplating technique. The assembled prototype is about 0.32 cm3. The amplitude–frequency curve of the magnet–spring system shows its natural frequency is 228.2 Hz, which indicates that the Young’s modulus of microelectroplated Ni film is about 163 GPa but not the one of bulk Ni material. Experimental results show that the load voltage of 142.5 mV generated by

This work was supported by the National Natural Science Foundation of China (51007001), the 211 Project of Anhui University and start up grant for doctor’s research of Anhui University.

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