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Power Variation with External Load in Vertical Vibration based Electret-Cantilever Micro-Power Generation

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Power Variation with External Load in Vertical Vibration based Electret-Cantilever Micro-Power Generation
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   International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.10, pp : 1287-1291 1 Oct 2014 IJSET@2014 Page 1287 Power Variation with External Load in Vertical Vibration based Electret-Cantilever Micro-Power Generation Akin-Ponnle A.E. 1, 2 , Ponnle A.A. 1  and Falaki S.O. 1   1 Department of Electrical and Electronics Engineering, Federal University of Technology, Akure, Nigeria. 2 Research and Development Department, Engineering Materials Development Institute, Akure, Nigeria. nicebikky@yahoo.co.uk, ponnleakinlolu@yahoo.co.uk, swolefalaki@yahoo.com   Abstract   —   We present a generator model to describe the generated electrical output power variation with external load for vertical vibration based electret-cantilever micro-power generator. The generator, being electrostatic, is represented by an equivalent current source model. A vertical vibration based electret-cantilever micro-power generator was set up in which CYTOP was used as the electret and the upper electrode was made in form of an A-shaped cantilever fabricated from materials of copper with embedded glass epoxy. Experiments were conducted on the set up in the laboratory to validate the model. The results (generated electrical output power variation) from experiments agree fully with expected characteristics from the mathematical model. Also, maximum average power was expended in the external load when its value is equal to the equivalent internal source resistance of the micro-power generator. Electret-cantilever micro-power generator is best suited for high impedance loads. Keywords   —   electret, cantilever, micro-power generator, average output power, external load. I.   Introduction Micro-power generation is a broad term that describes the work that various researchers are doing to develop very small electric generators or devices that convert heat or motion to electricity, for use close to the generator or the device. These devices provide a power source for portable electronic devices with very light weight, and have a longer operating time than currently existing batteries (Beeby et al  ., 2006). Much attention has been drawn towards devices that can harvest vibration energy from the environment to produce electricity, and being able to replace batteries in handheld devices (Li et al  ., 2000; Mitcheson et al  ., 2004; Beeby et al  ., 2007; Wischke and Woias, 2008; Kibong et al  ., 2012). Mechanical energy in the form of vibrations is commonly available in low frequencies up to 100Hz, and can be converted into electrical forms by means of energy harvesting techniques (Sardini and Serpelloni, 2011). There are four techniques that can be used to generate electricity from vibrations; electromagnetic (inductive),  piezoelectric, magnetostrictive and electrostatic (capacitive) methods of generation. An electret is the electrostatic equivalent of a permanent magnet. Cyclic Transparent Optical Polymer (CYTOP), a perfluoronated amorphous (non-crystalline)  polymer with ultra-high light transparency level, which was developed by Asahi Glass Company, Japan (Asahi Corporate Brochure) has been demonstrated to be a good electret material with high charge density (Sessler, 1998; Tsutsumino et al. , 2006; Lo et al. , 2007; Nagasawa et al  ., 2008). In recent times, there has been effort geared towards the use of the electrostatic approach of harvesting vibration energy. Also, electret-based electrostatic method and use of CYTOP as electret material is on the increase (Roundy et al  ., 2002; Boland et al  ., 2003; Boland and Tai, 2004; Mitcheson et al.,  2004; Tsutsumino et al  ., 2006; Lo et al,.  2007; Hoffmann et al. , 2008;  Naruse et al  ., 2008; Nagasawa et al  ., 2008; Edamoto et al  ., 2009; Miki et al  ., 2009; Altena et al  ., 2011; Akin-Ponnle et al  ., 2014). In the concept of vertical vibration based electret-cantilever micro power generator, an electret dielectric with air or other medium is placed between two plate electrodes. The electret is attached to the lower electrode (base electrode) while the upper electrode (counter electrode) is allowed to vibrate freely in the vertical direction. Free charges are deposited on the electret by external charging and when placed in the arrangement described, charges are induced to the upper electrode and there is transport of charges if connected to an external load due to changes in the capacitance between the two electrodes as the cantilever vibrates. In this type of micro power generator, maximum output  power is realized towards the contact point of the upper and lower electrodes, after which noise will set in and there is distortion of signals (Akin-Ponnle et al  ., 2014). The aim of this work is to investigate and develop a model for output power variation with load for electret-cantilever micro-power generator with vibration in the vertical plane. The expected results from the model are compared with experimental results. CYTOP was chosen as the electret material. II.   Proposed Model to describe Output Power with Varying External Load A diagrammatic model of the vertical vibration based electret-cantilever generator is represented in Figure 1. Upper ElectrodeLower Electrode i s ( t  ) Internal Source Resistance R  s External Load R  L R  Electret +++++ +++++ AirVertical Vibration  Figure 1: Diagrammatic model of the vertical vibration based electret-cantilever micro power generator.   International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.10, pp : 1287-1291 1 Oct 2014 IJSET@2014 Page 1288 The upper electrode (in form of a cantilever) is allowed to vibrate freely in the vertical direction as shown in the figure. The arrangement is connected to a load R comprising of parallel combination of the equivalent internal source resistance R  s  of the generator, and externally connected load R  L . Due to the electrostatic nature of the generator, it can be represented as a current source with internal source resistance  R  s . An a.c. equivalent circuit model of the generator model (current source) in Figure 2 is developed and the load circuit is further analysed. The electret-cantilever arrangement produces current i  s  which flows through  R  s  and  R  L ; and produces an output voltage V  o . R  s  R  L i s ( t  )i in ( t  ) i o ( t  )V o ( t  )  Figure 2: Representative equivalent circuit model of the electret-cantilever micro-power generator. From Figure 1 and Figure 2,  L s L s  R R R R R   (parallel combination of  R  s  and  R  L ) (1) Output voltage, V  o    Ri Ri RiV   s sin Loo   ,  L s L s s  R R R Ri  , (2)  L s s s  R R Ri  1 . (3) If  R  L  = 0 (output short circuited), from equation 2, short circuited output voltage, V  os  = 0. As  R  L   → ∞ (output open circuited), from equation 3, open circuited voltage,  s soo  RiV     (4) If  R  L  =  R  s , from equation 2 or 3, 22 oo s soL V  RiV    . (5) Equations 3 to 5 show that with varying external load R  L  from zero to infinity, the output voltage should vary non-linearly from zero to a limiting value corresponding to the open-circuited voltage; and when the external load is equal to the internal source resistance of the micro-power generator, the output voltage will be equal to half the open-circuited voltage. Average output power in the external load  R  L ,  Loavo  Ri P   2   (6) where  L s s so  R R Rii   (7) Substituting for i o  in equation 6,  L L s s savo  R R R Ri P  2       (8) 222 21       L s L s L s s  R R R R R Ri  (9) If  R  L  = 0, from equation 8,  P  avo  = 0. As  R  L   → ∞, from equation 9,    P  avo  = 0.       222222 2  L L s s  L s s s  Lavo  R R R R  R R Ri dRdP    (10) At maximum power, 0   Lavo dRdP  . In equation 10, this occurs when 0 22   L s  R R  i.e.  R  s  =  R  L  (11) Equations 8 to 11 show that the average output power in external load  R  L  should vary quadratically as it varies from zero to infinity, with a maximum value when the load is equal to the equivalent internal source resistance of the generator. This is maximum power transfer theorem. III.   Materials and Methods The concept of the cantilever-electrostatic micro power generator is based on placing an electret dielectric with air or other medium between two plate electrodes. The electret is attached to the lower electrode while the upper electrode (cantilever) is allowed to vibrate freely in the vertical direction. The micro power electrostatic generator was developed by first fabricating the electret and the cantilever, and then setting up the generator. Cyclic Transparent Optical Polymer (CYTOP (CTL-809A)) from Asahi Glass Company, Japan, was used as the dielectric material for the electret. The substrate for the lower electrode was fabricated by making a sample into size of 20 mm  by 20 mm from a wide copper plate with a thickness of 1.5 mm. The sample substrate was washed with ethanol and distilled water using a vibrator, after which the sample was dried using  Nitrogen gas. Few drops of S330 Silane Coupling Agent were deposited on the copper substrate and spin-coated. Then, CYTOP was applied to the substrate, spin-coated and soft- baked. This process was repeated four times to obtain 8µm  –   thick film, and then fully cured. Electrical connection was then made to the sample substrate to complete it as the lower electrode, and was then charged using a corona discharge set up. The upper electrode was made in form of a cantilever fabricated from a material of double-sided copper with embedded glass epoxy. A-shaped cantilever was fabricated and thin film of laser reflector was pasted around its tip to aid displacement sensing of the cantilever. Electrical connection was then made to the cantilever. The fabricated cantilever and its dimensions are shown in Figure 3.   International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.10, pp : 1287-1291 1 Oct 2014 IJSET@2014 Page 1289 35mm75mm40mm10mm13mm   (a) (b)  Figure 3: (a) The fabricated A-shaped cantilever, and (b) dimensions of the A-shaped cantilever. The experimental setup of the micro power generator is shown in Figure 4. The micro power generator was mounted on a shaker (Vibropet PET-05) and connected to a precision stage which was controlled by a computer. The shaker is controlled by a laser vibrometer (ONOSOKKI LV-1710) that sets the vibration waveform, amplitude, acceleration, and the frequency via a vibration exciter (Asahi APD-200FCG).The two electrodes of the generator were connected, through a coaxial cable, to the external load resistor across which the output of the generator was obtained. A laser displacement projector/sensor was mounted close to the top of the generator to sense and measure the vibrations of the cantilever. The laser displacement sensor works in conjunction with a Laser Impedance/Gain Phase Analyser (Yokogawa-Hewlett Packard 4194A), from which the vibration waveforms were obtained. The movement of the cantilever was observed with a high frame rate digital motion camera. Experiments were conducted on the generator to validate the proposed model by vibrating the upper electrode in the vertical direction in non-contact mode using the shaker representing mechanical vibration from the environment, and the mean distance between the electrodes was fixed by the precision stage. The experiments involved the generated output of the A-shaped cantilever generator with varying external load; at a fixed frequency of vibration of 100Hz, acceleration of 30.2m/s 2 , displacement amplitude of 0.15mm (pk-pk), and electret surface voltage of -101V. Output data was acquired by a digital oscilloscope (Tektronix TDS2014B) and were analysed using MATLAB software. ShakerVibration ExciterLaser VibrometerLaser projector/displacement sensorLaser AnalyserOscilloscopeElectric micro power generator Precision Stage Stage ControllerComputer  Figure 4: Experimental setup for the micro-power generation. IV.   Results and Discussion Figure 5 shows the generated electrical output voltage waveforms of the A-shaped cantilever generator for different values of external load resistor of 100kΩ, 470kΩ, 1MΩ, 10MΩ, 100MΩ and 1GΩ in non -contact mode of operation. Frequency of operation was maintained at 100Hz, acceleration at 30.2 m/s 2 , amplitude of vibration at 0.15mm (pk-pk), and the electret surface voltage at -101V. The load values indicated are those for which clean waveforms were obtained. -20020-505Time [ms]     A   m   p .    [   m    V    ] R:100000 Ohm-20020-10-50510Time [ms]     A   m   p .    [   m    V    ] R:470000 Ohm-20020-10010Time [ms]     A   m   p .    [   m    V    ] R:1000000 Ohm-20020-20-1001020Time [ms]     A   m   p .    [   m    V    ] R:10000000 Ohm-20020-20-1001020Time [ms]     A   m   p .    [   m    V    ] R:100000000 Ohm-20020-20-1001020Time [ms]     A   m   p .    [   m    V    ] R:1000000000 Ohm  Figure 5: Waveforms of the generated electrical output voltage of the A-shaped cantilever generator with varying external load in non-contact mode of operation. In the course of the experiment, it was found out that noise amplitudes overwhelmed the generated signals significantly for external load values below 47kΩ, and is indicated in Figure 6 which shows the frequency spectra of the output electrical waveforms including those with load values below 47kΩ.   00.500.51R:1000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:10000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:47000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:100000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:470000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:1000000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:10000000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:100000000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]00.500.51R:1000000000 Ohm     N   o   r   m .    A   m   p . Freq [kHz]  Figure 6: Frequency spectra of the waveforms of the generated output of the A-shaped cantilever generator with varying external load. Figure 7 shows the output r.m.s. voltage of the generated electrical output waveforms with varying external load. The   International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.10, pp : 1287-1291 1 Oct 2014 IJSET@2014 Page 1290 voltage rises with increasing load and approaches a limiting value as the load increases further. 10 5 10 6 10 7 10 8 10 9 051015Load [Ohm]     R    M    S    V   o    l   t   a   g   e    [   m    V    ] RMS Voltage with Load  Figure 7: RMS voltage of the generated electrical output waveforms of the A-shaped cantilever generator with varying load. The scale of the horizontal axis is logarithmic. Considering Figure 7, output r.m.s. voltage rises non- linearly; and as the external load is reaching 1GΩ (almost open -circuited), it is being limited to about 16mV. This conforms to the analysis in Section II, and equations 3 to 5. At a load of 1MΩ, output r.m.s . voltage is about half of 16mV. This gives an indication of the equivalent internal source resistance of the micro-power generator. Figure 8 shows the average power in the external load as it varies from 100kΩ to 1GΩ. It can be observed that the average  power in the external load rises to a maximum, and then decreases to almost zero when the load is very high. Of course, when the external load is infinitely high (open circuited), there will be no output current, and thereby average power in the external load will be zero. For the set up micro-power generator used in the experiment, the average power reached a maximum value at a load of 1MΩ.  This further indicates that the internal source resistance of the micro-  power generator is 1MΩ.  The  power  –   load curve varies in accordance with the analysis in Section II (equations 9 to 11). 10 5 10 6 10 7 10 8 10 9 00.010.020.030.040.050.06Load [Ohm]     A   v   e   r   a   g   e    P   o   w   e   r    [   n    W    ] Average Power with Load  Figure 8: Average power in the external load of the A-shaped cantilever generator with varying load. The scale of the horizontal axis is logarithmic. Therefore, for this particular generator set up, maximum  power i s transferred to the external load when it is 1MΩ. The high value of the internal source resistance is due to the electrostatic nature of the electret-cantilever generator, as against its electromagnetic counterpart which has low internal source resistance. From the results of this experiment, it can be concluded that electret-cantilever micro-power generator is best suited for (to drive) high impedance loads. V.   Conclusion The fabrication, set up and experimental operations of a vertical vibration-based electret-cantilever micro-electric power generator with varying external load was researched in this work. Maximum average power is transferred to the external load when it is equal to the equivalent internal source resistance of the micro-power generator. It is concluded that electret-cantilever micro-power generator is best suited for high impedance loads. Acknowledgement We express our gratitude to Professor H. Kuwano and Associate Professor M. Hara, of the department of Nano-Mechanics Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan, in whose laboratory the experimental aspect of this work was carried out. References i.    Altena G., Hohlfeld D., Elfrink R., Goedbloed M.H., and Van Schaijk R. (2011): “Design, Modelling, Fabrication a nd Characterization of an Electret-based MEMS Electrostatic Energy  Harvester”. Proceedings 16th International IEEE Conference on (Transducers); Solid-State Sensors, Actuators and Microsystems,  Beijing, China, 2011, pp. 739-742. ii.    Beeby S. P., Tudor M. J., and White N. M. (2006): “Energy Harvesting Vibration Sources for Micro Systems  Applications,” Measuring Science Technology, School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Vol. 17, pp.175-195. iii.    Beeby S. P., Torah R. N., Tudor M. J., Glynne-Jones  P., O’ Donell T., Saha C. R., and Roy S. (2007): “A Micro  Electromagnetic Generator for Vibration Energy Harvesting”.  Journal of Micro-mechanics and Micro-engineering Vol. 17, pp.1257-1265. iv.    Akin-Ponnle A. E, Ponnle A. A., and Falaki S.O. (2014): “Vertical Vibration based Electret  -Cantilever Method of  Micro-  Power Generation for Energy Harvesting”. International  Journal of Engineering and Innovative Technology, Vol. 3, Issue 12,  pp. 218-223. v.    Akin-Ponnle A. E, Ponnle A. A., Falaki S.O., Hara  M., and Kuwano H. (2014): “Cantilever Shapes on Power Output in Vertical Vibration based Electret Micro-  Power Generation”.  International Journal of Emerging Technology and Advanced  Engineering, Vol. 4, Issue 4, pp. 775-780. vi.    Boland J.S., Chao Y-H., Suzuki Y., and Tai Y-C. (2003): “Micro Electret Power Generator,” Proceedings 16th IEEE  Annual International Conference on MEMS, Kyoto, Japan, January 19-23, 2003, pp. 538-541. vii.    Boland J. S. and Tai Y. - C. (2004): “Liquid  -Rotor  Electret Micropower Generator”, Solid  -State Sensor, Actuator, and  Microsystems Workshop, Transducers Research Foundation, Inc.,  Hilton Head Island, South Carolina, June 6-10, 2004, pp. 133-136.
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