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The nanostructured super-oleophobic liquid-floated rotor gyroscope
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Contents lists available at ScienceDirect
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The nanostructured super-oleophobic liquid-floated rotor gyroscope
3 6
Q1
Rui Weng a, Haifeng Zhang a,⇑, Xiaowei Liu a,b a
7 8
b
10 9 11
MEMS Center, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin 150001, People’s Republic of China
a r t i c l e
1 5 3 2 14 15 16 17 18
i n f o
Article history: Received 30 October 2013 Received in revised form 20 July 2014 Accepted 2 August 2014 Available online xxxx
19 20 21 22 23 24
Keywords: Nanostructured Oleophobic Liquid-floated rotor gyroscope Drag reduction
Q2
a b s t r a c t Liquid-floated rotor gyroscopes can provide high accuracy at small volume and low cost. In rotor gyroscopes, the rotating speed of rotor can significantly affect the detection accuracy. This work presents a new kind of liquid-floated rotor gyroscope using super-oleophobic surface processing, which can improve the performance of driving system. In this work, an unconventional anodization in oxalic-acid electrolyte under high current was employed to fabricate diverse nanostructured alumina surfaces. The top-view SEM image shows the nanowire pyramid structure on the surface of processed sample. Modification of the rough surfaces was achieved by dipping substrates in 0.5 wt% 1H,1H,2H,2H-perfluoro-octadecyltrichlorosilane-in-hexane and then curing then at 120 °C for 1 h. The maximum contact angle (CA) of the aluminum rotor with nanowire pyramid structure was measured to be 156° in average. The oleophobicity of the rotor surface was used to reduce the resistance in the floating liquid. The test results show that, under the normal working state, the rotating speed of super-oleophobic rotor can be reached up to 3200 rpm. While a similar system without the micro-nano composite structure can only reach 2860 rpm. Thus, the nanostructured super-oleophobic alumina surface processing can greatly higher the rotating speed, thereby improving the performance of the gyro system. Ó 2014 Published by Elsevier B.V.
26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
43 44
1. Introduction
45
Liquid-floated gyroscopes have the advantages of accurate, stable and impact resistance, however, they usually have large volume and high weight. While scaling down the component, the sensitivity and resolution are also reduced. In order to deal with this problem, the rotating speed of rotor must be increased to maintain adequate Coriolis force [1]. Under the same driving power, a higher rotating speed can be obtained if there is drag reduction treatment on the rotor surface. Therefore, the drag reduction treatment of the rotor surface is one of the important means to improve the performance of liquid-floated rotor gyroscope. The device presented here is a new kind of liquid-floated rotor gyroscope, which can provide high accuracy at smaller volume and lower cost [2,3]. In this device, the mechanical structure is manufactured by precision machinery processing technology. The rotor in this device is directly driven by stator coils, it is just similar to the structure reported by Shearwood and Damrongsak, even though they may have completely different processes [4,5]. But this work is also different from both theirs, the aluminum alloy rotor of this system is hollow to match the density of floating
46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
⇑ Corresponding author. Tel.: +86 451 86413451; fax: +86 451 86413441. E-mail address: [email protected] (H. Zhang).
liquid, and no separate levitation coils are needed. The liquidfloated design of this system can avoid using bearings, signifying that this system can reduce frictional drag while avoiding device damage caused by wear [6]. The whole mechanical system consists of two plexiglass covers, two sets of detecting nodes, two stator rings, a set of driving coils, a stator made of silicon-steel sheets and the hollow aluminum alloy rotor placed in the center of the stator. In this structure, floating liquid’s buoyancy can support the rotor Q3 thereby eliminating the mechanical supporting structure [7]. At the same time, the anodized nanoporous alumina nanostructured super-oleophobic surface is employed, which greatly reduced the resistance between the rotor and the floating liquid [8]. Therefore, this design can make the rotor achieve higher speed in the same driving power. In other words, the new design can reduce the power consumption of liquid-floated gyroscope, signifying that the device is more suitable for battery powered applications.
64
2. System structure and driving mechanism
81
The mechanical structure of liquid-suspended rotor gyroscope is shown in Fig. 1. It can be seen that the liquid-suspended gyroscope contains the following components: stator, driving coils, stator ring, cover, hollow rotor and capacitance detecting electrodes.
82
http://dx.doi.org/10.1016/j.mee.2014.08.001 0167-9317/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: R. Weng et al., Microelectron. Eng. (2014), http://dx.doi.org/10.1016/j.mee.2014.08.001
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Fig. 1. The structure of liquid-suspended gyroscope. 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117
The stator is composed of a set of stacked silicon steel sheets, it acts as the coil core, and it is also a part of the closed cavity. The closed cavity is filled with a floating liquid to provide buoyancy to the hollow rotor. Alternating driving current flows through the driving coil and produces a rotating magnetic field, which generates eddy currents in the aluminum rotor and drives it. The floating liquid is 3# industrial white oil, a kind of petroleum product, the main components of which are aromatic hydrocarbons. The density of 3# industrial white oil is around 880 kg/m3, and the kinematic viscosity ranges from 2.9 to 3.5 mm2/s at Q4 40 °C. The 3# industrial white oil also has good electrical insulation and chemical stability making the liquid-suspended gyroscope quite suitable. In the gyroscope driving control system, there is a microcontroller unit (MCU). The MCU can speed-up the rotor smoothly during system startup, and keep the rotor rotating uniformly at the user-set speed. To realize the closed-loop controlling, the rotor’s actual rotating speed is obtained by the speed detection circuit. When the control system is connected to a personal computer via a USB cable, the detailed operating status of the system can also be viewed through the gyroscope control program. The block diagram of driving control system is shown in Fig. 2. The system uses the differential Sine Pulse Width Modulation (SPWM) driving waveform to keep the driving torque smooth. Assume the stator plane is the x–o–y plane and the direction perpendicular to the stator plane is z axis, the direction is from lower side to upper side. Ignore the magnetic coupling between different phase driving coils, and according to the vector superposition principle, when a single coil is powered up or serval coils are powered up together, the total magnetic moment can be expressed as follows:
121
~ ¼~ M P ~ B
ð2Þ
130
Since the value of magnetic induction intensity ~ B is determined by the rotor, here it can be considered as a constant, then the driving torque is only related to the magnetic moment generated by the driving coils. When several driving coils are energized at the same time, the generated magnetic moment is larger, but the power consumption is also higher. In order to reduce the computational workload, we estimate the driving torque through the finite element analysis method [9]. In the condition where the rotor diameter is 15 mm, the driving coil ampere-turns is 100, the numerical simulation results show that, the average torque of 6 phase 12 coil structure can reach 506.8076 lN * m. When the system is balanced, the driving torque and drag torque are equal in value, and the output power of a drive system can be described by the following formula:
131
~ x ~ Pdriv e ¼ M
ð3Þ
148
~ is total driving torque, and x ~ is the angular velocity of In which, M rotor. For a particular drive system, the maximum output power is limited. Therefore, the rotating speed of the rotor can be improved by reducing the drag torque [10].
149
3. Theory of drag reduction
154
~ Psingle ¼ i~ S N 1 X ~ in~ Sn Ptotal ¼
ð1Þ
n¼0
122 123 124 125 126 127
128
132 133 134 135 136 137 138 139 140 141 142 143 144 145
146
150 151 152 153
155 The gyroscope structure can be considered as a variation of 156 hydrodynamic bearing, and the load is very light. According to Spikes’ work, if one slipping surface the bearing is a slipping sur- Q5 157 158 face, the frictional resistance can be significantly reduced [11,12]. 159 Fig. 3 shows the simplified sectional view of the gyroscope. 160 Slightly different from Spikes’ work, there is no need to entrain 161 fluid into the bearing. In this system, the outer surface of the hol162 low rotor is processed to be an oleophobic surface, which presents 163 slipping characteristics in the floating liquid – 3# industrial white 164 oil. While the inner surface of the closed cavity is not processed, it 165 is still an ordinary non-slipping surface. 166 The drag reduction characteristics of oleophobic surface are 167 generally explained using Navier’s model [13]. According to the 168 formula of wall slipping, the relationship of boundary slip velocity 169 and shear stress can be described as: 170
us ¼ Ls
118
120
In which, i is the driving current, ~ S is the cross-sectional area of driving coil, its direction follows the right-hand screw, N is the number of driving coils. The driving coils’ magnetic moment and the rotor’s magnetic field generate a torque. In the previously described orthogonal Cartesian coordinate system, the value of magnetic torque can be expressed as:
@u @ywall
ð4Þ
In which, us is the slip velocity, Ls is the slip length while @u @y
172
is
173
the surface shear strain rate. Slipping wall reduces the shear forces between the wall surface and the fluid, which reduces the frictional
174
wall
175
User Interface
USB Cable
Driving Power Stage
Coils
Speed Detection
Rotor
Hollow Rotor
MCU
Floating Liquid Oleophobic Surface
Fig. 2. The block diagram of gyroscope driving system.
Ordinary Surface
Fig. 3. The simplified structure of the slipping surface.
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Q7
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Fig. 4. The simplified structure of the slipping surface.
Fig. 8. The relationship between the CA and the current density.
Fig. 5. The experimental set to prepare micro-nano composite surfaces.
interface. The solid–liquid–gas composite interface presents a drag reduction effect similar to tiny ball bearings.
186
4. Surface processing
188
Using a large current, the preparation of micro-nano composite structure can be achieved by a single anodization step [18]. In this paper, an unconventional anodization was employed to fabricate micro–nano composite structured alumina surfaces in oxalic-acid electrolyte, the structure of experimental set is shown in Fig. 5. The preparation of micro–nano composite surface can be divided into the following three steps:
189
1. Cleaning: Wash aluminum alloy pieces with ethanol and then deionized water in an ultrasonic cleaning machine. 2. Anodizing: Do anode oxidation in oxalic acid solution to construct nanowire pyramids, where the aluminum alloy piece is anode and the carbon rod is the cathode. 3. Modifications with low surface energy material: Modify the anodized aluminum pieces with low surface energy materials, and after drying, the oleophobic surface is obtained.
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190 191 192 193 194 195
197 198 199 200 201 202 203 204
Fig. 6. The relationship between the CA and the anodic oxidation time.
resistance [14,15]. After chemical etching, the aluminum surface has a large number of micron and nano scale pits and protrusions on it. These micro-nano composite structures can capture and keep air, thus forming Cassie contact, as is shown in Fig. 4 [16]. According to Cassie theory, when the solid–liquid contact area is smaller, the oleophobicity is better. Under floating liquid’s pressure, the air captured by the micro–nano composite structure is compressed to form a cushion of air [17]. Due to the surface tension of the floating liquid, air is trapped in the micro–nano composite structure. This greatly reduces the area of the solid–liquid
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140°
(a) 4 minutes Q8
In the initial stage of anodizing, the primary ionization of oxalic-acid in the electrolyte begins. The process can be described by the following chemical equation: þ
H2 C2 O4 ! H þ
HC2 O4
ð5Þ
At this time, the following reaction occurs on the two electrodes: Anode:
2Al þ 6OH ! Al2 O3 þ 3H2 O þ 6e
ð6Þ
Cathode: þ
206 207
208 210 211 212 213
214 216 217
2H þ 2e ! H2 "
ð7Þ
153°
(b) 8 minutes
205
153°
(c) 14 minutes
Fig. 7. The top-view SEM images of samples at different anodic oxidation time.
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124°
(a) 300mA/cm 2
155°
(b) 500mA/cm 2
153°
(c) 1000mA/cm 2
Fig. 9. The top-view SEM images of samples at current density.
221 222 223 224 225 226 227 228
During the reaction, OH in the electrolyte gathered at the anode, aluminum anodic oxidation reaction occurs here, producing a dense Al2O3 film. As the reaction proceeds, this Al2O3 film can be dissolved in H+ ions later under electrolysis conditions, forming micro–nano composite structures. Modification of the alumina surfaces was achieved by dipping substrates in 0.5 wt% 1H,1H,2H,2H-perfluoro-octadecyltrichlorosilane-in-hexane, and then curing at 120 °C for 1 h.
229
5. Discussion
230
According to Young’s equation, the contact angle of the liquid (h) is related to the surface tensions of solid–gas (cSG), solid–liquid (cSL) and liquid–gas (cLG).[19] Namely,
231 232
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h cSG ¼ cSL þ ccos LG :
ð8Þ
Young’s equation is the basis of solid surface invasive studies, the contact angle is generally defined as the standard to determine the degree of invasion of solid surface [20]. Comparative experiments of different anodic oxidation times were performed at room temperature and a current density of 500 mA/cm2. Then the contact angle (CA) of oil droplet was measured using an optical contact-angle meter system (Data Physics Instrument GmbH, Germany) at room temperature. The diameter of oil droplet is about 5 lm, and the average CA value was obtained by measuring the same sample at five different positions. Fig. 6 shows the contact angles at different anodic oxidation times. It can be seen from the figure that during the anodic oxidation process, the CA of 3# industrial oil on the surface first goes up and then drops, the maximum CA is 156° at 12 min. In order to explore the reasons, the surface morphologies of samples at different anodic oxidation times were observed by a field-emission scanning electron microscope (FE-SEM, TESCAN VEGA). And Fig. 7 shows the top-view SEM images of anodized alumina surface whose anodic oxidation times are 4, 8 and 14 min. When anodic oxidation time is 4 min, some small holes appear on the surface of sample, the apertures are around 120 nm, but the micron structure is not formed. Under the effect of capillary forces, the oil droplet can be adsorbed onto the surface, forming Wenzel contact [21]. When the anodic oxidation time is 8 min, the holes of some walls begin to dissolve. Since the vertical dissolution rate is greater than the transverse one, a nanowire pyramid structure appears. The peak-to-peak distance of the nanowire pyramids is about 3 lm, and the apertures of small holes are about 150 nm. When anodic oxidation time is 14 min, the size of pyramids becomes larger than 4 lm, but the number of nano holes is significantly reduced. The micro-nano composite structure can capture
air more easily than micro or nano structure alone. According to the previous conclusion, micro–nano composite structure has a better drag reduction effect. Change the current density in the range of 300–1200 mA/cm2, and repeat the above experiments at 10 min anodic oxidation time. The results and top-view SEM images are shown in Fig. 8 and Fig. 9. When the current density is 300 mA/cm2, there are only tiny holes on the surface, whose size is slightly smaller than 100 nm. For the alumina is dissolved quite slowly, there is no micron structure produced, its oleophobic effect is poor. When the current density is 500 mA/cm2, the alumina is dissolved faster, forming micro–nano composite pyramid structure. The structure greatly reduced the solid–liquid contact area, thereby increasing the contact angle. When the current density continues to increase, Joule heat generated by the current heats of the solution. The alumina is dissolved quickly, but the vertical depth of the structure decreases. Then the oleophobic effect becomes worse. In order to validate the drag reduction effect, two sets of liquidfloated gyroscope with 15 mm diameter rotor are assembled. One Q6 of the rotors is processed under 500 mA/cm2 for 12 min, and the other’s rotor is not processed [22]. The maximum rotating speeds under different RMS driving currents are shown in Fig. 10. Under low driving current of 600 mArms/phase, the system with nanowire pyramids structure can reach a rotating speed up to 2000 rpm, while the other one is only 1480 rpm, the rotating speed is enhanced by around 35.1%. Under normal working driving current of 1000 mArms/phase, a system with nanowire pyramid
Fig. 10. Maximum rotating speed vs RMS driving current.
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of China (No. 61071037), and the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2014040) for financial support.
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structure can reach a rotating speed up to 3200 rpm, while that of the other one is only 2860 rpm, the rotating speed is enhanced by around 11.9%.
297
6. Conclusions
References
318
298
311
This paper presents an attempt to use oleophobic drag reduction principle on liquid floating rotor gyroscope. Nanowire pyramid structure is fabricated by anodic oxidation on the surface of aluminum alloy rotor. The contact angle of oil droplets on the surface can be controlled by changing the anodization time or the current density. The maximum contact angle is 156° at the current density of 500 mA/cm2 and the anodization time of 12 min. In the gyroscope system, in case of 60% working current underdriving, the micro-nano composite structure can higher the maximum rotating speed of rotor from 1480 rpm to 2000 rpm, increasing by even up to 35.1%. While in case of normal driving, the processed surface can increase the maximum rotating speed by around 11.9%. The tie between the drag reduction effect and the driving current is still left to be further studied.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
312
Acknowledgements
313
The authors would like to thank National Basic Research program of China (No. 2012CB934104), National Science Foundation
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299 300 301 302 303 304 305 306 307 308 309 310
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[19] [20] [21] [22]
Xingang Li, Jianping Yuan, Adv. Mech. 03 (2003) 23–28. M. Kraft, IMC Meas. Control 33 (2000) 164–168. N. Barbour, G. Schmidt, IEEE Sens. J. 1 (4) (2001) 332–339. C. Shearwood, K.Y. Ho, C.B. Williams, H. Gong, Sens. Actuators 83 (2000) 85–92. M. Kraft, B. Damrongsak, Sensors (2010) 23–28. Lei Jiang, Zhiyong Zhong, J. Sci. Instrum. 5 (2008) 28–31. Bin Xiong, Xiaozhen Huang, et al., J. Sci. Instrum. 9 (2001) 38–41. A. Santos, L. Vojkuvka, et al., Electron Devices (2009) 436–439. Zhouqing Li, Electr. Eng. Technol. 04 (2012) 36–39. J.T. Chen, Z.Q. Zhu, et al., IEEE Trans. Ind. Appl. 47 (2011) 1681–1691. H.A. Spikes, Proc. Inst. Mech. Eng. J 217 (2003) 1–14. H.A. Spikes, Proc. Inst. Mech. Eng. J 217 (2003) 15–26. Xiuqing Hao, Li Wang, et al., Lubr. Eng. 34 (2009) 25–28. C. Neto, D.R. Evans, et al., Rep. Prog. Phys. 68 (2005) 2859–2897. Zheng Liu, Yu. Zhijia, Liaoning Chem. Ind. 39 (2010) 897–900 (in Chinese). A. Cassie, S. Baxter, Transl. Faraday Soc. 40 (1944) 546–551. J. Ou, B. Perot, J.P. Rothstein, Phys. Fluids 16 (2004) 4635–4673. Xinhua Chen, Yongmei Ma, et al., Chem. J. Chin. Univ. 25 (2004) 2304–2307 (in Chinese). T. Young, Philos Trans. R. Soc. Lond. 94 (1804) 1–16. Thierry Darmanin, Frederic Guittard, Am. Chem. Soc. 25 (2009) 5463–5466. J.D. Eick, R.J. Good, A.W. Neumann, J. Colloid Interface Sci. 53 (1975) 235–238. J.Y. Leong, T. Reddyhoff, et al., Tribol. Lett. 49 (2013) 217–225.
Please cite this article in press as: R. Weng et al., Microelectron. Eng. (2014), http://dx.doi.org/10.1016/j.mee.2014.08.001
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12 August 2014 Microelectronic Engineering xxx (2014) xxx–xxx 1
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee 4 5
The nanostructured super-oleophobic liquid-floated rotor gyroscope
3 6
Q1
Rui Weng a, Haifeng Zhang a,⇑, Xiaowei Liu a,b a
7 8
b
10 9 11
MEMS Center, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin 150001, People’s Republic of China
a r t i c l e
1 5 3 2 14 15 16 17 18
i n f o
Article history: Received 30 October 2013 Received in revised form 20 July 2014 Accepted 2 August 2014 Available online xxxx
19 20 21 22 23 24
Keywords: Nanostructured Oleophobic Liquid-floated rotor gyroscope Drag reduction
Q2
a b s t r a c t Liquid-floated rotor gyroscopes can provide high accuracy at small volume and low cost. In rotor gyroscopes, the rotating speed of rotor can significantly affect the detection accuracy. This work presents a new kind of liquid-floated rotor gyroscope using super-oleophobic surface processing, which can improve the performance of driving system. In this work, an unconventional anodization in oxalic-acid electrolyte under high current was employed to fabricate diverse nanostructured alumina surfaces. The top-view SEM image shows the nanowire pyramid structure on the surface of processed sample. Modification of the rough surfaces was achieved by dipping substrates in 0.5 wt% 1H,1H,2H,2H-perfluoro-octadecyltrichlorosilane-in-hexane and then curing then at 120 °C for 1 h. The maximum contact angle (CA) of the aluminum rotor with nanowire pyramid structure was measured to be 156° in average. The oleophobicity of the rotor surface was used to reduce the resistance in the floating liquid. The test results show that, under the normal working state, the rotating speed of super-oleophobic rotor can be reached up to 3200 rpm. While a similar system without the micro-nano composite structure can only reach 2860 rpm. Thus, the nanostructured super-oleophobic alumina surface processing can greatly higher the rotating speed, thereby improving the performance of the gyro system. Ó 2014 Published by Elsevier B.V.
26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
43 44
1. Introduction
45
Liquid-floated gyroscopes have the advantages of accurate, stable and impact resistance, however, they usually have large volume and high weight. While scaling down the component, the sensitivity and resolution are also reduced. In order to deal with this problem, the rotating speed of rotor must be increased to maintain adequate Coriolis force [1]. Under the same driving power, a higher rotating speed can be obtained if there is drag reduction treatment on the rotor surface. Therefore, the drag reduction treatment of the rotor surface is one of the important means to improve the performance of liquid-floated rotor gyroscope. The device presented here is a new kind of liquid-floated rotor gyroscope, which can provide high accuracy at smaller volume and lower cost [2,3]. In this device, the mechanical structure is manufactured by precision machinery processing technology. The rotor in this device is directly driven by stator coils, it is just similar to the structure reported by Shearwood and Damrongsak, even though they may have completely different processes [4,5]. But this work is also different from both theirs, the aluminum alloy rotor of this system is hollow to match the density of floating
46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
⇑ Corresponding author. Tel.: +86 451 86413451; fax: +86 451 86413441. E-mail address: [email protected] (H. Zhang).
liquid, and no separate levitation coils are needed. The liquidfloated design of this system can avoid using bearings, signifying that this system can reduce frictional drag while avoiding device damage caused by wear [6]. The whole mechanical system consists of two plexiglass covers, two sets of detecting nodes, two stator rings, a set of driving coils, a stator made of silicon-steel sheets and the hollow aluminum alloy rotor placed in the center of the stator. In this structure, floating liquid’s buoyancy can support the rotor Q3 thereby eliminating the mechanical supporting structure [7]. At the same time, the anodized nanoporous alumina nanostructured super-oleophobic surface is employed, which greatly reduced the resistance between the rotor and the floating liquid [8]. Therefore, this design can make the rotor achieve higher speed in the same driving power. In other words, the new design can reduce the power consumption of liquid-floated gyroscope, signifying that the device is more suitable for battery powered applications.
64
2. System structure and driving mechanism
81
The mechanical structure of liquid-suspended rotor gyroscope is shown in Fig. 1. It can be seen that the liquid-suspended gyroscope contains the following components: stator, driving coils, stator ring, cover, hollow rotor and capacitance detecting electrodes.
82
http://dx.doi.org/10.1016/j.mee.2014.08.001 0167-9317/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: R. Weng et al., Microelectron. Eng. (2014), http://dx.doi.org/10.1016/j.mee.2014.08.001
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Fig. 1. The structure of liquid-suspended gyroscope. 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117
The stator is composed of a set of stacked silicon steel sheets, it acts as the coil core, and it is also a part of the closed cavity. The closed cavity is filled with a floating liquid to provide buoyancy to the hollow rotor. Alternating driving current flows through the driving coil and produces a rotating magnetic field, which generates eddy currents in the aluminum rotor and drives it. The floating liquid is 3# industrial white oil, a kind of petroleum product, the main components of which are aromatic hydrocarbons. The density of 3# industrial white oil is around 880 kg/m3, and the kinematic viscosity ranges from 2.9 to 3.5 mm2/s at Q4 40 °C. The 3# industrial white oil also has good electrical insulation and chemical stability making the liquid-suspended gyroscope quite suitable. In the gyroscope driving control system, there is a microcontroller unit (MCU). The MCU can speed-up the rotor smoothly during system startup, and keep the rotor rotating uniformly at the user-set speed. To realize the closed-loop controlling, the rotor’s actual rotating speed is obtained by the speed detection circuit. When the control system is connected to a personal computer via a USB cable, the detailed operating status of the system can also be viewed through the gyroscope control program. The block diagram of driving control system is shown in Fig. 2. The system uses the differential Sine Pulse Width Modulation (SPWM) driving waveform to keep the driving torque smooth. Assume the stator plane is the x–o–y plane and the direction perpendicular to the stator plane is z axis, the direction is from lower side to upper side. Ignore the magnetic coupling between different phase driving coils, and according to the vector superposition principle, when a single coil is powered up or serval coils are powered up together, the total magnetic moment can be expressed as follows:
121
~ ¼~ M P ~ B
ð2Þ
130
Since the value of magnetic induction intensity ~ B is determined by the rotor, here it can be considered as a constant, then the driving torque is only related to the magnetic moment generated by the driving coils. When several driving coils are energized at the same time, the generated magnetic moment is larger, but the power consumption is also higher. In order to reduce the computational workload, we estimate the driving torque through the finite element analysis method [9]. In the condition where the rotor diameter is 15 mm, the driving coil ampere-turns is 100, the numerical simulation results show that, the average torque of 6 phase 12 coil structure can reach 506.8076 lN * m. When the system is balanced, the driving torque and drag torque are equal in value, and the output power of a drive system can be described by the following formula:
131
~ x ~ Pdriv e ¼ M
ð3Þ
148
~ is total driving torque, and x ~ is the angular velocity of In which, M rotor. For a particular drive system, the maximum output power is limited. Therefore, the rotating speed of the rotor can be improved by reducing the drag torque [10].
149
3. Theory of drag reduction
154
~ Psingle ¼ i~ S N 1 X ~ in~ Sn Ptotal ¼
ð1Þ
n¼0
122 123 124 125 126 127
128
132 133 134 135 136 137 138 139 140 141 142 143 144 145
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150 151 152 153
155 The gyroscope structure can be considered as a variation of 156 hydrodynamic bearing, and the load is very light. According to Spikes’ work, if one slipping surface the bearing is a slipping sur- Q5 157 158 face, the frictional resistance can be significantly reduced [11,12]. 159 Fig. 3 shows the simplified sectional view of the gyroscope. 160 Slightly different from Spikes’ work, there is no need to entrain 161 fluid into the bearing. In this system, the outer surface of the hol162 low rotor is processed to be an oleophobic surface, which presents 163 slipping characteristics in the floating liquid – 3# industrial white 164 oil. While the inner surface of the closed cavity is not processed, it 165 is still an ordinary non-slipping surface. 166 The drag reduction characteristics of oleophobic surface are 167 generally explained using Navier’s model [13]. According to the 168 formula of wall slipping, the relationship of boundary slip velocity 169 and shear stress can be described as: 170
us ¼ Ls
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In which, i is the driving current, ~ S is the cross-sectional area of driving coil, its direction follows the right-hand screw, N is the number of driving coils. The driving coils’ magnetic moment and the rotor’s magnetic field generate a torque. In the previously described orthogonal Cartesian coordinate system, the value of magnetic torque can be expressed as:
@u @ywall
ð4Þ
In which, us is the slip velocity, Ls is the slip length while @u @y
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is
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the surface shear strain rate. Slipping wall reduces the shear forces between the wall surface and the fluid, which reduces the frictional
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wall
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User Interface
USB Cable
Driving Power Stage
Coils
Speed Detection
Rotor
Hollow Rotor
MCU
Floating Liquid Oleophobic Surface
Fig. 2. The block diagram of gyroscope driving system.
Ordinary Surface
Fig. 3. The simplified structure of the slipping surface.
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Fig. 4. The simplified structure of the slipping surface.
Fig. 8. The relationship between the CA and the current density.
Fig. 5. The experimental set to prepare micro-nano composite surfaces.
interface. The solid–liquid–gas composite interface presents a drag reduction effect similar to tiny ball bearings.
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4. Surface processing
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Using a large current, the preparation of micro-nano composite structure can be achieved by a single anodization step [18]. In this paper, an unconventional anodization was employed to fabricate micro–nano composite structured alumina surfaces in oxalic-acid electrolyte, the structure of experimental set is shown in Fig. 5. The preparation of micro–nano composite surface can be divided into the following three steps:
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1. Cleaning: Wash aluminum alloy pieces with ethanol and then deionized water in an ultrasonic cleaning machine. 2. Anodizing: Do anode oxidation in oxalic acid solution to construct nanowire pyramids, where the aluminum alloy piece is anode and the carbon rod is the cathode. 3. Modifications with low surface energy material: Modify the anodized aluminum pieces with low surface energy materials, and after drying, the oleophobic surface is obtained.
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Fig. 6. The relationship between the CA and the anodic oxidation time.
resistance [14,15]. After chemical etching, the aluminum surface has a large number of micron and nano scale pits and protrusions on it. These micro-nano composite structures can capture and keep air, thus forming Cassie contact, as is shown in Fig. 4 [16]. According to Cassie theory, when the solid–liquid contact area is smaller, the oleophobicity is better. Under floating liquid’s pressure, the air captured by the micro–nano composite structure is compressed to form a cushion of air [17]. Due to the surface tension of the floating liquid, air is trapped in the micro–nano composite structure. This greatly reduces the area of the solid–liquid
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140°
(a) 4 minutes Q8
In the initial stage of anodizing, the primary ionization of oxalic-acid in the electrolyte begins. The process can be described by the following chemical equation: þ
H2 C2 O4 ! H þ
HC2 O4
ð5Þ
At this time, the following reaction occurs on the two electrodes: Anode:
2Al þ 6OH ! Al2 O3 þ 3H2 O þ 6e
ð6Þ
Cathode: þ
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2H þ 2e ! H2 "
ð7Þ
153°
(b) 8 minutes
205
153°
(c) 14 minutes
Fig. 7. The top-view SEM images of samples at different anodic oxidation time.
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124°
(a) 300mA/cm 2
155°
(b) 500mA/cm 2
153°
(c) 1000mA/cm 2
Fig. 9. The top-view SEM images of samples at current density.
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During the reaction, OH in the electrolyte gathered at the anode, aluminum anodic oxidation reaction occurs here, producing a dense Al2O3 film. As the reaction proceeds, this Al2O3 film can be dissolved in H+ ions later under electrolysis conditions, forming micro–nano composite structures. Modification of the alumina surfaces was achieved by dipping substrates in 0.5 wt% 1H,1H,2H,2H-perfluoro-octadecyltrichlorosilane-in-hexane, and then curing at 120 °C for 1 h.
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5. Discussion
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According to Young’s equation, the contact angle of the liquid (h) is related to the surface tensions of solid–gas (cSG), solid–liquid (cSL) and liquid–gas (cLG).[19] Namely,
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h cSG ¼ cSL þ ccos LG :
ð8Þ
Young’s equation is the basis of solid surface invasive studies, the contact angle is generally defined as the standard to determine the degree of invasion of solid surface [20]. Comparative experiments of different anodic oxidation times were performed at room temperature and a current density of 500 mA/cm2. Then the contact angle (CA) of oil droplet was measured using an optical contact-angle meter system (Data Physics Instrument GmbH, Germany) at room temperature. The diameter of oil droplet is about 5 lm, and the average CA value was obtained by measuring the same sample at five different positions. Fig. 6 shows the contact angles at different anodic oxidation times. It can be seen from the figure that during the anodic oxidation process, the CA of 3# industrial oil on the surface first goes up and then drops, the maximum CA is 156° at 12 min. In order to explore the reasons, the surface morphologies of samples at different anodic oxidation times were observed by a field-emission scanning electron microscope (FE-SEM, TESCAN VEGA). And Fig. 7 shows the top-view SEM images of anodized alumina surface whose anodic oxidation times are 4, 8 and 14 min. When anodic oxidation time is 4 min, some small holes appear on the surface of sample, the apertures are around 120 nm, but the micron structure is not formed. Under the effect of capillary forces, the oil droplet can be adsorbed onto the surface, forming Wenzel contact [21]. When the anodic oxidation time is 8 min, the holes of some walls begin to dissolve. Since the vertical dissolution rate is greater than the transverse one, a nanowire pyramid structure appears. The peak-to-peak distance of the nanowire pyramids is about 3 lm, and the apertures of small holes are about 150 nm. When anodic oxidation time is 14 min, the size of pyramids becomes larger than 4 lm, but the number of nano holes is significantly reduced. The micro-nano composite structure can capture
air more easily than micro or nano structure alone. According to the previous conclusion, micro–nano composite structure has a better drag reduction effect. Change the current density in the range of 300–1200 mA/cm2, and repeat the above experiments at 10 min anodic oxidation time. The results and top-view SEM images are shown in Fig. 8 and Fig. 9. When the current density is 300 mA/cm2, there are only tiny holes on the surface, whose size is slightly smaller than 100 nm. For the alumina is dissolved quite slowly, there is no micron structure produced, its oleophobic effect is poor. When the current density is 500 mA/cm2, the alumina is dissolved faster, forming micro–nano composite pyramid structure. The structure greatly reduced the solid–liquid contact area, thereby increasing the contact angle. When the current density continues to increase, Joule heat generated by the current heats of the solution. The alumina is dissolved quickly, but the vertical depth of the structure decreases. Then the oleophobic effect becomes worse. In order to validate the drag reduction effect, two sets of liquidfloated gyroscope with 15 mm diameter rotor are assembled. One Q6 of the rotors is processed under 500 mA/cm2 for 12 min, and the other’s rotor is not processed [22]. The maximum rotating speeds under different RMS driving currents are shown in Fig. 10. Under low driving current of 600 mArms/phase, the system with nanowire pyramids structure can reach a rotating speed up to 2000 rpm, while the other one is only 1480 rpm, the rotating speed is enhanced by around 35.1%. Under normal working driving current of 1000 mArms/phase, a system with nanowire pyramid
Fig. 10. Maximum rotating speed vs RMS driving current.
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of China (No. 61071037), and the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2014040) for financial support.
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structure can reach a rotating speed up to 3200 rpm, while that of the other one is only 2860 rpm, the rotating speed is enhanced by around 11.9%.
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6. Conclusions
References
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This paper presents an attempt to use oleophobic drag reduction principle on liquid floating rotor gyroscope. Nanowire pyramid structure is fabricated by anodic oxidation on the surface of aluminum alloy rotor. The contact angle of oil droplets on the surface can be controlled by changing the anodization time or the current density. The maximum contact angle is 156° at the current density of 500 mA/cm2 and the anodization time of 12 min. In the gyroscope system, in case of 60% working current underdriving, the micro-nano composite structure can higher the maximum rotating speed of rotor from 1480 rpm to 2000 rpm, increasing by even up to 35.1%. While in case of normal driving, the processed surface can increase the maximum rotating speed by around 11.9%. The tie between the drag reduction effect and the driving current is still left to be further studied.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Acknowledgements
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The authors would like to thank National Basic Research program of China (No. 2012CB934104), National Science Foundation
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[19] [20] [21] [22]
Xingang Li, Jianping Yuan, Adv. Mech. 03 (2003) 23–28. M. Kraft, IMC Meas. Control 33 (2000) 164–168. N. Barbour, G. Schmidt, IEEE Sens. J. 1 (4) (2001) 332–339. C. Shearwood, K.Y. Ho, C.B. Williams, H. Gong, Sens. Actuators 83 (2000) 85–92. M. Kraft, B. Damrongsak, Sensors (2010) 23–28. Lei Jiang, Zhiyong Zhong, J. Sci. Instrum. 5 (2008) 28–31. Bin Xiong, Xiaozhen Huang, et al., J. Sci. Instrum. 9 (2001) 38–41. A. Santos, L. Vojkuvka, et al., Electron Devices (2009) 436–439. Zhouqing Li, Electr. Eng. Technol. 04 (2012) 36–39. J.T. Chen, Z.Q. Zhu, et al., IEEE Trans. Ind. Appl. 47 (2011) 1681–1691. H.A. Spikes, Proc. Inst. Mech. Eng. J 217 (2003) 1–14. H.A. Spikes, Proc. Inst. Mech. Eng. J 217 (2003) 15–26. Xiuqing Hao, Li Wang, et al., Lubr. Eng. 34 (2009) 25–28. C. Neto, D.R. Evans, et al., Rep. Prog. Phys. 68 (2005) 2859–2897. Zheng Liu, Yu. Zhijia, Liaoning Chem. Ind. 39 (2010) 897–900 (in Chinese). A. Cassie, S. Baxter, Transl. Faraday Soc. 40 (1944) 546–551. J. Ou, B. Perot, J.P. Rothstein, Phys. Fluids 16 (2004) 4635–4673. Xinhua Chen, Yongmei Ma, et al., Chem. J. Chin. Univ. 25 (2004) 2304–2307 (in Chinese). T. Young, Philos Trans. R. Soc. Lond. 94 (1804) 1–16. Thierry Darmanin, Frederic Guittard, Am. Chem. Soc. 25 (2009) 5463–5466. J.D. Eick, R.J. Good, A.W. Neumann, J. Colloid Interface Sci. 53 (1975) 235–238. J.Y. Leong, T. Reddyhoff, et al., Tribol. Lett. 49 (2013) 217–225.
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