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Mitigation of lunar dust adhered to mechanical parts of equipment used for lunar exploration
Journal of Electrostatics 69 (2011) 365e369
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Mitigation of lunar dust adhered to mechanical parts of equipment used for lunar exploration H. Kawamoto*, T. Miwa Dept. of Applied Mechanics and Aerospace Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2010 Received in revised form 19 February 2011 Accepted 29 April 2011 Available online 26 May 2011
A unique cleaning system has been developed utilizing electrostatic force to remove lunar dust adhered to the mechanical parts of equipment used for lunar exploration. A single-phase voltage is applied to parallel electrodes printed on a flexible substrate to remove the dust. More than 90% of adhered dust was repelled from the surface of the slightly inclined device in a vacuum, and the cleaning performance of the system would be further improved in the low-gravity environment of the Moon. This technology is expected to increase the reliability of equipment used in long-term manned and unmanned activities on the lunar surface. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Aerospace engineering Lunar dust Electrical equipment Lunar exploration
1. Introduction The lunar surface is covered by a regolith (soil) layer; approximately 20% of this material by volume consists of particles less than 20 mm in diameter [1]. Because of its small size and the low lunar gravity, lunar dust easily becomes airborne when any disturbance occurs. From the Apollo era, it has been known that the dust on the Moon can cause serious problems for exploration activities [2,3]. There are three main concerns regarding lunar dust. The first problem is that dust brought into the lunar module after moonwalks by astronauts makes breathing without a helmet difficult and particles present in the cabin atmosphere affect the astronauts’ vision [4]. The second problem is that the dust covers solar panels and optical elements, such as lenses and mirrors, degrading their performance [5e8]. The third problem is that airborne dust adheres to mechanical parts of equipment coming into contact with bearings and seals; such a situation could lead to catastrophic damage. In order to mitigate the first problem, in previous work we have developed an automatic cleaning system that utilizes alternating electrostatic force [8]. It consists of a particle-repelling device, parallel wire electrodes stitched into the insulating fabric of a spacesuit, and a power supply to generate a single-phase rectangular voltage. If the system is operated intermittently, the dust
* Corresponding author. Tel./fax: þ3 5286 3914. E-mail address: [email protected] (H. Kawamoto). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.04.015
adhering to the fabric of spacesuits is removed. On the other hand, if the system is operated continuously, the dust that comes close to the fabric is repelled, and thus, the system can protect spacesuits against the adhesion of dust. To overcome the second problem, the electrostatic particle transport system was developed. The idea of transporting particles using electrostatic traveling-waves was first proposed by Masuda [9] and many investigations have since been conducted on this technology, mainly as a toner supplier of electrophotography [10e24] and recently as a cleaner of lunar dust during lunar exploration missions [5,6]. We have developed the similar system for removing lunar dust using electrostatic traveling waves generated by a four-phase rectangular voltage applied to a transparent conveyer consisting of vortical pattern transparent ITO (indium tin oxide) electrodes printed on a glass substrate [7]. It was demonstrated that more than 98% of the dust could be removed in a vacuum, and the transmission rate of light was reduced by only a few percent by the application of ultrasonic vibrations in addition to traveling waves. This paper describes a countermeasure against the third issue: adhesion of dust to mechanical parts. To overcome this problem, we developed a barrier system that employs an alternating electrostatic field to repel and remove the lunar dust from the surface of mechanical parts of equipment. The principle of the proposed system is similar to that of the cleaning system for spacesuits [8]. That is, a single-phase rectangular voltage was applied to parallel electrodes printed on a flexible substrate. Since a traveling wave is not generated by the application of a single-phase voltage, particles
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
are not transported in one direction but rather repelled from the device. An alternating electrostatic field acts as a barrier against the dust based on the same principle as the cleaning system for spacesuits [8]. This technology is expected to improve the reliability of equipment used for long-term manned and unmanned activities on the lunar surface. 2. System configuration
100 abundance ratio %
366
10 initial 1
residual dust after operation
0.1
The cleaning system shown in Fig. 1 consists of a particlerepelling plate with parallel line electrodes printed on a substrate and a power supply used to generate a single-phase rectangular high voltage. Due to the Coulomb force and dielectrophoresis force acting on the particles on the plate, the particles turn over on the plate along the electrostatic flux lines [8]. 2.1. Electrostatic particle-repelling device The particle-repelling device consists of parallel copper electrodes, 18 mm thick and 0.3 mm wide, etched by photolithography on a flexible polyimide substrate having a thickness, width, and length of 100 mm, 150 mm, and 52.5 mm, respectively. An acrylic plate was attached to the back of the polyimide substrate as a rigid support. In actual application, the flexible substrate will be attached on the surface of the critical part where dust must be eliminated. To determine a suitable configuration of electrodes for efficient cleaning, particle-repelling devices with three different pitches were prepared: 0.6 mm, 1.2 mm, and 1.8 mm. The surface of the device was covered with an insulating film made of polyimide (12.5 mm thickness) to prevent insulation breakdown between the electrodes. 2.2. Power supply We used the power supply originally designed and used for the cleaning system of spacesuits [8]. A single-phase rectangular voltage was generated by utilizing a set of positive and negative amplifiers switched by semiconductor relays that were controlled by a microcomputer. The power supply is designed to be simple, small, and lightweight for space applications. 2.3. Lunar dust simulant The lunar dust simulant FJS-1 (Shimiz Corp) [25], which is almost identical to the simulant JSC-1A (Orbital Technologies Corporation), was used in the present experiments. The specifications of the simulant are summarized in the literature [7]. Because large particles can be easily dislodged, particles larger than 53 mm in diameter were removed from the bulk of particles by using
Fig. 1. Principle of electrostatic removal of lunar dust.
0
20 particle diameter
40 m
60
Fig. 2. Particle size distributions of lunar dust simulant used in experiments (initial) and residual dust on particle-repelling plate after operation (p ¼ 1.8 mm, 3.0 kVpep, 10 Hz).
a sand sieve. Fig. 2 shows the particle size distribution of the lunar dust simulant used in the present experiments. 3. Cleaning performance The particle-repelling plate was inclined at an angle of 40 , and a lunar dust simulant (20 mg) with a particle diameter smaller than 53 mm was uniformly scattered on the particle-repelling plate in a circular area of 30 mm in diameter (0.028 mg/mm2). Because the dust particles used in the experiment were small, they adhered to the plate and the dust did not run off the plate even when the plate was inclined. A single-phase rectangular voltage was applied to the parallel electrodes. Particles that were repelled from the plate onto the floor were weighed on an electronic balance (SAG105, Mettler), and the cleaning rate was determined, that is, the ratio of repelled particles to the initial weight of particles. The cleaning experiments were conducted in a dry air environment (20 C dew point, 4.4% RH at 20 C) created by a clean air unit (P4-QD10, IAC Co.). 3.1. Effect of electrode pitch, applied voltage and frequency Fig. 3(a) and (b) show the cleaning rate versus the applied voltage and the frequency, respectively. The pitch of the parallel electrodes was selected as a parameter. Dust on the particlerepelling plate was removed at a threshold voltage. The cleaning rate increased with applied voltage, but saturated at high voltage. Because the applied voltage is limited by the insulation breakdown that is determined by the electrostatic field, the system performance at the voltage limit is almost independent of the electrode pitch. Electrostatic field calculation revealed that at the voltage limit the peaks of the electrostatic field at the edges of the line electrodes are approximately the same for the different pitches. This feature is the same as that of the cleaning system of spacesuits [8]. The maximum cleaning rate was about 80% at a low frequency less than 10 Hz. In conclusion, a particle-repelling plate with a long pitch is preferable for simplicity. The performance will be further improved in vacuum, because gas discharge does not take place in a vacuum, and therefore, the voltage limit can be increased. Although a high cleaning rate was realized, not all the small particles were removed, and a small amount of particles remained on the plate. The residual particle size distribution after the operation is shown in Fig. 2. Particles smaller than 10 mm were not efficiently removed. Observation using an optical microscope after operation revealed that particles adhered on the electrode edges of the particle-repelling plate. Although it is expected that small particles can be removed more efficiently in a vacuum environment with application of mechanical vibration, a very small amount of dust will remain on the plate [7,8]. Thus, the system performance
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
a
367
100
60
p = 1.2 mm
cleaning rate %
%
80
cleaning rate
100 p = 0.6 mm
p = 1.8 mm
40 20
80 in vacuum 60
20 0 0
0 0
b
in air
40
1 2 applied voltage kVp-p
3
0.5 1 1.5 applied voltage kVp-p
Fig. 5. Cleaning performance in dry air (dew point 20 (p ¼ 0.6 mm, 10 Hz).
C)
2
and vacuum (5 Pa)
cleaning rate %
100 80 p = 0.6 mm (1.2 kVp-p)
60 40
p = 1.2 mm (2.1 kVp-p)
20 0 0
20
40 60 frequency Hz
80
100
Fig. 3. Cleaning performance.
Although the maximum vacuum that can be achieved in this desiccator is 5 Pa (0.04 Torr), it is sufficient to eliminate the air drag acting on airborne particles. Fig. 5 shows the experimental results. The cleaning rate in the vacuum was higher than that in air, as in the case of electrostatic transport of dust by means of a travelling wave [7] and cleaning of spacesuits by means of an alternating electrostatic field [8]. A higher cleaning rate was achieved in the vacuum because the air drag was eliminated, even though the attained degree of vacuum was insufficient to simulate the environment on the Moon. 3.4. Effect of inclination
needs to be further improved or additional means are necessary for complete cleaning. 3.2. Effect of amount of initially adhered dust Fig. 4 shows the cleaning rate with respect to the amount of initially adhered dust on the particle-repelling plate. At a low initial weight, because the residual dust on the plate after operation did not increase proportionally to the initially adhered dust but remained almost constant, the apparent cleaning rate increased with the initial weight up to 30 mg (0.042 mg/mm2). The cleaning rate saturated at an initial weight of dust greater than 30 mg. 3.3. Effect of vacuum The experiments were conducted not only in air (1 atm) but also in a vacuum. The particle-repelling device was placed in a conventional vacuum desiccator (GLD-136C, Sanplatec), and the desiccator was evacuated by using an oil-sealed rotary vacuum pump (GLD136C, ULVAC). The evacuation time was approximately 1 h.
Because particles on the particle-repelling plate are not transported in one direction by the application of the single-phase voltage, particles on the completely flat plate are not removed from the surface and they merely flip-flop on the plate. Consequently, the effect of the plate inclination was investigated to clarify the plate inclination needed to realize sufficient cleaning performance. Fig. 6 shows the experimental results. A cleaning experiment using a four-phase traveling wave was also conducted for comparison with single-phase cleaning. Contrary to our expectation, high performance was achieved even when the plate was slightly inclined. It is also surprising that the performance is almost the same as that attained for traveling wave cleaning. This feature is desirable for space applications; by adopting the single-phase system, not only the power supply but also the particle-repelling device can be simplified. (If the four-phase system is adopted, the ends of the electrodes must be three-dimensional to prevent the overlapping of phases.) 4. Shield performance If the system is operated continuously, it is expected that the dust that comes close to the device is flicked out, and thus, the 100
cleaning rate 80
%
80 60
cleaning rate
cleaning rate % weight of residual dust mg
100
1 Hz 10 Hz
40 20 residual dust 0 0
20 40 weight of initially adhered dust mg
60
Fig. 4. Cleaning rate and weight of residual dust after operation with respect to weight of initially adhered dust (p ¼ 0.6 mm, 1.4 kVpep).
60 40
single-phase 1 Hz single-phase 10 Hz 4-phase traveling wave 1 Hz 4-phase traveling wave 10 Hz
20 0 0
20
40 60 plate inclination deg
80
Fig. 6. Cleaning performance of inclined plate (p ¼ 0.6 mm, 1.6 kVpep).
368
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
system can protect mechanical parts against the adhesion of dust. This barrier effect was confirmed by the experiment. The device was inclined in 20 , and the dust was drop down from the upper side of the device by a vibrating feeder after 2 kVpep voltage was applied to the device. A feed rate of the dust was 0.04 g/min in 340 mm2 area, and the dust was supplied for 4 min. Fig. 7 shows the measured shield rate versus the frequency. The shield rate was determined as the ratio of repelled particles to the weight of fed particles. It was observed that high performance was realized by applying high voltage of low frequency as the case of the operation after the dust adhered to the device. Because it is difficult to clean small dust particles that adhere to the device or get into the clearance, the continuous operation scheme is preferable to protect sensitive parts. 5. Observations and calculation of particle motion The dynamic behavior of the particles was observed with a high-speed microscope camera (Fast-cam-max 120K model 1, Photoron) from the lateral side of the plate, and the observed behavior was compared with the calculation results. The numerical calculation was based on a three-dimensional hard-sphere model using the Distinct Element Method [23]. The basic equation of motion of a particle i of a system with six degrees of freedom is as follows:
€ ¼ 6phRx_ i þ qi E þ F image þ F dipole þ F adhesion mi x €i ¼ 0 þmi ðcosq; 0; sinq g; Ii 4
(1)
where x ¼ (x, y, z), j ¼ (j x,j y,j z); m denotes the mass of the particle; h, the viscosity of air; R, the radius of the particle; q, the charge of the particle; E, the electrostatic field; g, the gravitational acceleration; q, the inclination of the plate, and I, the inertia of the particle. The electric field E in terms of the Coulomb force consists of the electrostatic field E0 generated by the power supply and the electrostatic field Eq generated by other charged particles.
E ¼ E 0 þ E q ¼ Vf þ
1 4pe0
N X n¼i
qn
r jrj
(2)
3
where e0 is the permittivity of free space; 4, the electric potential; N, the number of particles; and r ¼ (xi xn, yi yn, zi zn). The potential distribution j is calculated by a two-dimensional Finite Element Method (FEM) in a cyclic domain. The image force Fimage is calculated by the following equation:
F image ¼
N X q1 qn 2 d 4pe0 n ¼ 1 2d jdj
1
(3)
100
shield rate %
80
Fig. 8. Observed and calculated motion of particles in alternating electrostatic field (p ¼ 0.6 mm, 1.6 kVpep, 10 Hz).
where d is the effective distance to the electrode including the film thickness. The charge density q and the adhesion force Fadhesion is determined by the experiment. The dielectrophoresis force Fdipole acting on a polarized particle in the gradient field is determined by the following equation [26]:
F dipole ¼ 4pe0
e1 3 R E 0 VE 0 eþ2
(4)
where e is the relative permittivity of the particle. The particle is assumed to be a dipole, and the force from other polarized particles is neglected, because the particles are not distributed densely. Fig. 8 shows the observed and calculated particle motion of particles on the earth in air and in vacuum. In the case of operation in air, the dust forms a cloud at a height of several millimeters, owing to the air drag acting on the airborne dust particles. However, the dust reaches higher heights in a vacuum. This feature was confirmed from the calculation. Although the dynamic motion of particles cannot be measured from the still images, we confirmed that the calculated and observed motion are in qualitative agreement. Because experiments on the Moon are not possible at present, the expected system performance on the Moon was evaluated from simulation. The gravity was reduced to g/6, and air drag was eliminated. It was confirmed that particles would reach higher altitudes on the Moon [6], and that the cleaning performance of the system would be improved.
60
6. Concluding remarks
40 20 0 1
10 100 frequency Hz
1000
Fig. 7. Shield performance. (p ¼ 1.2 mm, 2 kVpep, in air).
A unique technology based on electrostatic force has been developed for lunar dust removal. The system is designed to remove lunar dust adhered to mechanical parts of equipment during longterm lunar surface operations. The system consists of a particlerepelling device, parallel line electrodes printed on a flexible substrate, and a power supply for generating a single-phase
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
rectangular voltage. More than 90% of adhered dust was removed from the surface of a slightly inclined particle-repelling plate in vacuum. The cleaning performance of the system is expected to improve further with the application of mechanical vibration and operation in the low-gravity environment of the Moon. However, it is difficult to separate and remove small particles of less than 10 mm in diameter. Although complete cleaning will not be realized, the system will be effective in mitigating problems caused by the adhesion of lunar dust. Acknowledgement The authors would like to express their gratitude to Yusuke Oizumi (Waseda University) for his support in carrying out the experiments and to Dr. David S. McKay (NASA, Johnson Space Center) and Dr. Bonnie L. Cooper (Oceaneering Space Systems) for their beneficial advice and encouragement. Lunar dust simulant was provided by Shimiz Corp. A part of this study was supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science. References [1] D.S. McKay, et al., in: G. Heiken, D. Vaniman, B.M. French (Eds.), The Lunar Regolith, Lunar Sourcebook, Cambridge University Press, Cambridge (1991, pp. 285e356. [2] NASA Manned Spacecraft Center, APPOLO 12 Preliminary Science Report (1970) NASA SP-235. [3] R. Christoffersen, J.F. Lindsay, S.K. Noble, M.A. Meador, J.J. Kosmo, J.A. Lawrence, L. Brostoff, A. Young, T. McCue, Lunar Dust Effects on Spacesuit Systems, Insights from the Apollo Spacesuits (2009) NASA/TP-2009e214786. [4] B.L. Cooper, D.S. McKay, L.A. Taylor, H. Kawamoto, L.M. Rifrio and C.P. Gonzalez, Extracting Respirable particles from lunar regolith for Toxicology Studies, Proc.. Of 12th Int. Conf. On Engineering, Science, Construction, and Operation in Challenging Environment, Earth & Space 2010, ASCE, Honolulu HI, (2010) 66e73. [5] C.I. Calle, J.L. McFall, C.R. Buhler, S.J. Snyder, E.E. Arens1, A. Chen, M.L. Ritz, J.S. Clements, C.R. Fortier1 and S. Trigwell, Dust Particle Removal by Electrostatic and Dielectrophoretic Forces with Applications to NASA Exploration Missions, Proc. ESA Annual Meeting on electrostatics, ESA, Minneapolis, MN, O1 (2008). [6] C.I. Calle, C.R. Buhler, J.L. McFall, S.J. Snyder, Particle removal by electrostatic and dielectric forces for dust control during lunar exploration missions, J. Electrostat. 67 (2009) 89e92.
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[7] H. Kawamoto, M. Uchiyama, B. L. Cooper and D. S. McKay, Mitigation of lunar dust on solar panels and optical elements utilizing electrostatic travelingWave, J. Electrostat. submitted for publication(submitted). [8] H. Kawamoto and N. Hara, Electrostatic cleaning system of lunar dust adhered to spacesuits, J. Aerospace Eng. (to be published). [9] S. Masuda, K. Fujibayashi, K. Ishida, H. Inaba, Confinement and transportation of charged aerosol clouds via electric curtain, Trans. Inst. Elec. Eng. Jpn. 92-B (1) (1972) 9e18. [10] J.R. Melcher, E.P. Warren, R.H. Kotwal, Traveling-wave delivery of singlecomponent developer, IEEE Trans. Ind. Appl. 25 (1989) 956e961. [11] J.R. Melcher, E.P. Warren, R.H. Kotwal, Theory for finite-phase traveling-wave boundary-guided transport of triboelectric particles, IEEE Trans. Ind. Appl. 25 (1989) 949e955. [12] F.W. Schmidlin, A new nonlevitated mode of traveling wave toner transport, IEEE Trans. Ind. Appl. 27 (1991) 480e487. [13] F.W. Schmidlin, Modes of traveling wave particles transport and their applications, J. Electrostat. 34 (1995) 225e244. [14] F. Schmidlin, Advances in Traveling Wave Toner Transport. IS&T’s NIP15: Int. Conf. on Digital Printing Technologies, IS&T, Orlando, FL, 1999, pp. 302e305. [15] F.M. Moesner, T. Higuchi, Electrostatic devices for particle microhandling, IEEE Trans. Ind. Appl. 35 (1999) 530e536. [16] M.D. Thompson, Y. Gartstein, J.T. LeStrange, Aspect of Toner Transport on a Traveling Wave Device. IS&T’s NIP15: Int. Conf. on Digital Printing Technologies, IS&T, Orlando, FL, 1999, pp. 262e265. [17] K. Taniguchi, S. Morikumi, S. Watanabe, Y. Nakano, T. Sakai, H. Yamamoto, T. Yagi, Y. Yamamoto, Improved Driving Characteristics for the Toner Transportation System. IS&T’s NIP16: Int. Conf. on Digital Printing Technologies, Vancouver, Canada, 2000, pp. 740e742. [18] R. Kober, Traveling Wave Transport of Conductive Toner Particles. IS&T’s NIP16: Int. Conf. on Digital Printing Technologies, IS&T, Vancouver, Canada, 2000, pp. 736e739. [19] R. Kober, Simulation of Traveling Wave Toner Transport. IS&T’s NIP18: Int. Conf. on Digital Printing Technologies, IS&T, San Diego, CA, 2002, pp. 453e457. [20] R. Kober, Transport geladener Partikel in elektrischen Wanderfeldern, dissertation, Achen University of Technology (2003). [21] K. Taniguchi, H. Yamamoto, Y. Nakano, T. Sakai, S. Morikuni, S. Watanabe, Y. Yamamoto, A new technique for measuring the distribution of charge-tomass ratio for toner particles with on-line use, J. Imaging Sci. Technol. 47 (2003) 224e228. [22] H. Kawamoto, N. Hasegawa, Traveling wave transport of particles and particle size classification, J. Imaging Sci. Technol. 48 (2004) 404e411. [23] H. Kawamoto, K. Seki, N. Kuromiya, Mechanism on traveling-wave transport of particles, J. Phys. D: Appl. Phys. 39 (2006) 1249e1256. [24] H. Kawamoto, Some techniques on electrostatic classification of particle size utilizing electrostatic traveling wave field, J. Electrostat. 66 (2008) 220e228. [25] H. Kanamori, S. Udagawa, T. Yoshida, S. Matsumoto and K. Takagi, Properties of lunar soil stimulant manufactured in Japan, Proc.. Of the 6th Int. Conf. On Engineering, Construction and operations in space, ASCE, Albuquerque NM (1998) 462e468.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Mitigation of lunar dust adhered to mechanical parts of equipment used for lunar exploration H. Kawamoto*, T. Miwa Dept. of Applied Mechanics and Aerospace Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2010 Received in revised form 19 February 2011 Accepted 29 April 2011 Available online 26 May 2011
A unique cleaning system has been developed utilizing electrostatic force to remove lunar dust adhered to the mechanical parts of equipment used for lunar exploration. A single-phase voltage is applied to parallel electrodes printed on a flexible substrate to remove the dust. More than 90% of adhered dust was repelled from the surface of the slightly inclined device in a vacuum, and the cleaning performance of the system would be further improved in the low-gravity environment of the Moon. This technology is expected to increase the reliability of equipment used in long-term manned and unmanned activities on the lunar surface. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Aerospace engineering Lunar dust Electrical equipment Lunar exploration
1. Introduction The lunar surface is covered by a regolith (soil) layer; approximately 20% of this material by volume consists of particles less than 20 mm in diameter [1]. Because of its small size and the low lunar gravity, lunar dust easily becomes airborne when any disturbance occurs. From the Apollo era, it has been known that the dust on the Moon can cause serious problems for exploration activities [2,3]. There are three main concerns regarding lunar dust. The first problem is that dust brought into the lunar module after moonwalks by astronauts makes breathing without a helmet difficult and particles present in the cabin atmosphere affect the astronauts’ vision [4]. The second problem is that the dust covers solar panels and optical elements, such as lenses and mirrors, degrading their performance [5e8]. The third problem is that airborne dust adheres to mechanical parts of equipment coming into contact with bearings and seals; such a situation could lead to catastrophic damage. In order to mitigate the first problem, in previous work we have developed an automatic cleaning system that utilizes alternating electrostatic force [8]. It consists of a particle-repelling device, parallel wire electrodes stitched into the insulating fabric of a spacesuit, and a power supply to generate a single-phase rectangular voltage. If the system is operated intermittently, the dust
* Corresponding author. Tel./fax: þ3 5286 3914. E-mail address: [email protected] (H. Kawamoto). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.04.015
adhering to the fabric of spacesuits is removed. On the other hand, if the system is operated continuously, the dust that comes close to the fabric is repelled, and thus, the system can protect spacesuits against the adhesion of dust. To overcome the second problem, the electrostatic particle transport system was developed. The idea of transporting particles using electrostatic traveling-waves was first proposed by Masuda [9] and many investigations have since been conducted on this technology, mainly as a toner supplier of electrophotography [10e24] and recently as a cleaner of lunar dust during lunar exploration missions [5,6]. We have developed the similar system for removing lunar dust using electrostatic traveling waves generated by a four-phase rectangular voltage applied to a transparent conveyer consisting of vortical pattern transparent ITO (indium tin oxide) electrodes printed on a glass substrate [7]. It was demonstrated that more than 98% of the dust could be removed in a vacuum, and the transmission rate of light was reduced by only a few percent by the application of ultrasonic vibrations in addition to traveling waves. This paper describes a countermeasure against the third issue: adhesion of dust to mechanical parts. To overcome this problem, we developed a barrier system that employs an alternating electrostatic field to repel and remove the lunar dust from the surface of mechanical parts of equipment. The principle of the proposed system is similar to that of the cleaning system for spacesuits [8]. That is, a single-phase rectangular voltage was applied to parallel electrodes printed on a flexible substrate. Since a traveling wave is not generated by the application of a single-phase voltage, particles
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
are not transported in one direction but rather repelled from the device. An alternating electrostatic field acts as a barrier against the dust based on the same principle as the cleaning system for spacesuits [8]. This technology is expected to improve the reliability of equipment used for long-term manned and unmanned activities on the lunar surface. 2. System configuration
100 abundance ratio %
366
10 initial 1
residual dust after operation
0.1
The cleaning system shown in Fig. 1 consists of a particlerepelling plate with parallel line electrodes printed on a substrate and a power supply used to generate a single-phase rectangular high voltage. Due to the Coulomb force and dielectrophoresis force acting on the particles on the plate, the particles turn over on the plate along the electrostatic flux lines [8]. 2.1. Electrostatic particle-repelling device The particle-repelling device consists of parallel copper electrodes, 18 mm thick and 0.3 mm wide, etched by photolithography on a flexible polyimide substrate having a thickness, width, and length of 100 mm, 150 mm, and 52.5 mm, respectively. An acrylic plate was attached to the back of the polyimide substrate as a rigid support. In actual application, the flexible substrate will be attached on the surface of the critical part where dust must be eliminated. To determine a suitable configuration of electrodes for efficient cleaning, particle-repelling devices with three different pitches were prepared: 0.6 mm, 1.2 mm, and 1.8 mm. The surface of the device was covered with an insulating film made of polyimide (12.5 mm thickness) to prevent insulation breakdown between the electrodes. 2.2. Power supply We used the power supply originally designed and used for the cleaning system of spacesuits [8]. A single-phase rectangular voltage was generated by utilizing a set of positive and negative amplifiers switched by semiconductor relays that were controlled by a microcomputer. The power supply is designed to be simple, small, and lightweight for space applications. 2.3. Lunar dust simulant The lunar dust simulant FJS-1 (Shimiz Corp) [25], which is almost identical to the simulant JSC-1A (Orbital Technologies Corporation), was used in the present experiments. The specifications of the simulant are summarized in the literature [7]. Because large particles can be easily dislodged, particles larger than 53 mm in diameter were removed from the bulk of particles by using
Fig. 1. Principle of electrostatic removal of lunar dust.
0
20 particle diameter
40 m
60
Fig. 2. Particle size distributions of lunar dust simulant used in experiments (initial) and residual dust on particle-repelling plate after operation (p ¼ 1.8 mm, 3.0 kVpep, 10 Hz).
a sand sieve. Fig. 2 shows the particle size distribution of the lunar dust simulant used in the present experiments. 3. Cleaning performance The particle-repelling plate was inclined at an angle of 40 , and a lunar dust simulant (20 mg) with a particle diameter smaller than 53 mm was uniformly scattered on the particle-repelling plate in a circular area of 30 mm in diameter (0.028 mg/mm2). Because the dust particles used in the experiment were small, they adhered to the plate and the dust did not run off the plate even when the plate was inclined. A single-phase rectangular voltage was applied to the parallel electrodes. Particles that were repelled from the plate onto the floor were weighed on an electronic balance (SAG105, Mettler), and the cleaning rate was determined, that is, the ratio of repelled particles to the initial weight of particles. The cleaning experiments were conducted in a dry air environment (20 C dew point, 4.4% RH at 20 C) created by a clean air unit (P4-QD10, IAC Co.). 3.1. Effect of electrode pitch, applied voltage and frequency Fig. 3(a) and (b) show the cleaning rate versus the applied voltage and the frequency, respectively. The pitch of the parallel electrodes was selected as a parameter. Dust on the particlerepelling plate was removed at a threshold voltage. The cleaning rate increased with applied voltage, but saturated at high voltage. Because the applied voltage is limited by the insulation breakdown that is determined by the electrostatic field, the system performance at the voltage limit is almost independent of the electrode pitch. Electrostatic field calculation revealed that at the voltage limit the peaks of the electrostatic field at the edges of the line electrodes are approximately the same for the different pitches. This feature is the same as that of the cleaning system of spacesuits [8]. The maximum cleaning rate was about 80% at a low frequency less than 10 Hz. In conclusion, a particle-repelling plate with a long pitch is preferable for simplicity. The performance will be further improved in vacuum, because gas discharge does not take place in a vacuum, and therefore, the voltage limit can be increased. Although a high cleaning rate was realized, not all the small particles were removed, and a small amount of particles remained on the plate. The residual particle size distribution after the operation is shown in Fig. 2. Particles smaller than 10 mm were not efficiently removed. Observation using an optical microscope after operation revealed that particles adhered on the electrode edges of the particle-repelling plate. Although it is expected that small particles can be removed more efficiently in a vacuum environment with application of mechanical vibration, a very small amount of dust will remain on the plate [7,8]. Thus, the system performance
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
a
367
100
60
p = 1.2 mm
cleaning rate %
%
80
cleaning rate
100 p = 0.6 mm
p = 1.8 mm
40 20
80 in vacuum 60
20 0 0
0 0
b
in air
40
1 2 applied voltage kVp-p
3
0.5 1 1.5 applied voltage kVp-p
Fig. 5. Cleaning performance in dry air (dew point 20 (p ¼ 0.6 mm, 10 Hz).
C)
2
and vacuum (5 Pa)
cleaning rate %
100 80 p = 0.6 mm (1.2 kVp-p)
60 40
p = 1.2 mm (2.1 kVp-p)
20 0 0
20
40 60 frequency Hz
80
100
Fig. 3. Cleaning performance.
Although the maximum vacuum that can be achieved in this desiccator is 5 Pa (0.04 Torr), it is sufficient to eliminate the air drag acting on airborne particles. Fig. 5 shows the experimental results. The cleaning rate in the vacuum was higher than that in air, as in the case of electrostatic transport of dust by means of a travelling wave [7] and cleaning of spacesuits by means of an alternating electrostatic field [8]. A higher cleaning rate was achieved in the vacuum because the air drag was eliminated, even though the attained degree of vacuum was insufficient to simulate the environment on the Moon. 3.4. Effect of inclination
needs to be further improved or additional means are necessary for complete cleaning. 3.2. Effect of amount of initially adhered dust Fig. 4 shows the cleaning rate with respect to the amount of initially adhered dust on the particle-repelling plate. At a low initial weight, because the residual dust on the plate after operation did not increase proportionally to the initially adhered dust but remained almost constant, the apparent cleaning rate increased with the initial weight up to 30 mg (0.042 mg/mm2). The cleaning rate saturated at an initial weight of dust greater than 30 mg. 3.3. Effect of vacuum The experiments were conducted not only in air (1 atm) but also in a vacuum. The particle-repelling device was placed in a conventional vacuum desiccator (GLD-136C, Sanplatec), and the desiccator was evacuated by using an oil-sealed rotary vacuum pump (GLD136C, ULVAC). The evacuation time was approximately 1 h.
Because particles on the particle-repelling plate are not transported in one direction by the application of the single-phase voltage, particles on the completely flat plate are not removed from the surface and they merely flip-flop on the plate. Consequently, the effect of the plate inclination was investigated to clarify the plate inclination needed to realize sufficient cleaning performance. Fig. 6 shows the experimental results. A cleaning experiment using a four-phase traveling wave was also conducted for comparison with single-phase cleaning. Contrary to our expectation, high performance was achieved even when the plate was slightly inclined. It is also surprising that the performance is almost the same as that attained for traveling wave cleaning. This feature is desirable for space applications; by adopting the single-phase system, not only the power supply but also the particle-repelling device can be simplified. (If the four-phase system is adopted, the ends of the electrodes must be three-dimensional to prevent the overlapping of phases.) 4. Shield performance If the system is operated continuously, it is expected that the dust that comes close to the device is flicked out, and thus, the 100
cleaning rate 80
%
80 60
cleaning rate
cleaning rate % weight of residual dust mg
100
1 Hz 10 Hz
40 20 residual dust 0 0
20 40 weight of initially adhered dust mg
60
Fig. 4. Cleaning rate and weight of residual dust after operation with respect to weight of initially adhered dust (p ¼ 0.6 mm, 1.4 kVpep).
60 40
single-phase 1 Hz single-phase 10 Hz 4-phase traveling wave 1 Hz 4-phase traveling wave 10 Hz
20 0 0
20
40 60 plate inclination deg
80
Fig. 6. Cleaning performance of inclined plate (p ¼ 0.6 mm, 1.6 kVpep).
368
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
system can protect mechanical parts against the adhesion of dust. This barrier effect was confirmed by the experiment. The device was inclined in 20 , and the dust was drop down from the upper side of the device by a vibrating feeder after 2 kVpep voltage was applied to the device. A feed rate of the dust was 0.04 g/min in 340 mm2 area, and the dust was supplied for 4 min. Fig. 7 shows the measured shield rate versus the frequency. The shield rate was determined as the ratio of repelled particles to the weight of fed particles. It was observed that high performance was realized by applying high voltage of low frequency as the case of the operation after the dust adhered to the device. Because it is difficult to clean small dust particles that adhere to the device or get into the clearance, the continuous operation scheme is preferable to protect sensitive parts. 5. Observations and calculation of particle motion The dynamic behavior of the particles was observed with a high-speed microscope camera (Fast-cam-max 120K model 1, Photoron) from the lateral side of the plate, and the observed behavior was compared with the calculation results. The numerical calculation was based on a three-dimensional hard-sphere model using the Distinct Element Method [23]. The basic equation of motion of a particle i of a system with six degrees of freedom is as follows:
€ ¼ 6phRx_ i þ qi E þ F image þ F dipole þ F adhesion mi x €i ¼ 0 þmi ðcosq; 0; sinq g; Ii 4
(1)
where x ¼ (x, y, z), j ¼ (j x,j y,j z); m denotes the mass of the particle; h, the viscosity of air; R, the radius of the particle; q, the charge of the particle; E, the electrostatic field; g, the gravitational acceleration; q, the inclination of the plate, and I, the inertia of the particle. The electric field E in terms of the Coulomb force consists of the electrostatic field E0 generated by the power supply and the electrostatic field Eq generated by other charged particles.
E ¼ E 0 þ E q ¼ Vf þ
1 4pe0
N X n¼i
qn
r jrj
(2)
3
where e0 is the permittivity of free space; 4, the electric potential; N, the number of particles; and r ¼ (xi xn, yi yn, zi zn). The potential distribution j is calculated by a two-dimensional Finite Element Method (FEM) in a cyclic domain. The image force Fimage is calculated by the following equation:
F image ¼
N X q1 qn 2 d 4pe0 n ¼ 1 2d jdj
1
(3)
100
shield rate %
80
Fig. 8. Observed and calculated motion of particles in alternating electrostatic field (p ¼ 0.6 mm, 1.6 kVpep, 10 Hz).
where d is the effective distance to the electrode including the film thickness. The charge density q and the adhesion force Fadhesion is determined by the experiment. The dielectrophoresis force Fdipole acting on a polarized particle in the gradient field is determined by the following equation [26]:
F dipole ¼ 4pe0
e1 3 R E 0 VE 0 eþ2
(4)
where e is the relative permittivity of the particle. The particle is assumed to be a dipole, and the force from other polarized particles is neglected, because the particles are not distributed densely. Fig. 8 shows the observed and calculated particle motion of particles on the earth in air and in vacuum. In the case of operation in air, the dust forms a cloud at a height of several millimeters, owing to the air drag acting on the airborne dust particles. However, the dust reaches higher heights in a vacuum. This feature was confirmed from the calculation. Although the dynamic motion of particles cannot be measured from the still images, we confirmed that the calculated and observed motion are in qualitative agreement. Because experiments on the Moon are not possible at present, the expected system performance on the Moon was evaluated from simulation. The gravity was reduced to g/6, and air drag was eliminated. It was confirmed that particles would reach higher altitudes on the Moon [6], and that the cleaning performance of the system would be improved.
60
6. Concluding remarks
40 20 0 1
10 100 frequency Hz
1000
Fig. 7. Shield performance. (p ¼ 1.2 mm, 2 kVpep, in air).
A unique technology based on electrostatic force has been developed for lunar dust removal. The system is designed to remove lunar dust adhered to mechanical parts of equipment during longterm lunar surface operations. The system consists of a particlerepelling device, parallel line electrodes printed on a flexible substrate, and a power supply for generating a single-phase
H. Kawamoto, T. Miwa / Journal of Electrostatics 69 (2011) 365e369
rectangular voltage. More than 90% of adhered dust was removed from the surface of a slightly inclined particle-repelling plate in vacuum. The cleaning performance of the system is expected to improve further with the application of mechanical vibration and operation in the low-gravity environment of the Moon. However, it is difficult to separate and remove small particles of less than 10 mm in diameter. Although complete cleaning will not be realized, the system will be effective in mitigating problems caused by the adhesion of lunar dust. Acknowledgement The authors would like to express their gratitude to Yusuke Oizumi (Waseda University) for his support in carrying out the experiments and to Dr. David S. McKay (NASA, Johnson Space Center) and Dr. Bonnie L. Cooper (Oceaneering Space Systems) for their beneficial advice and encouragement. Lunar dust simulant was provided by Shimiz Corp. A part of this study was supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science. References [1] D.S. McKay, et al., in: G. Heiken, D. Vaniman, B.M. French (Eds.), The Lunar Regolith, Lunar Sourcebook, Cambridge University Press, Cambridge (1991, pp. 285e356. [2] NASA Manned Spacecraft Center, APPOLO 12 Preliminary Science Report (1970) NASA SP-235. [3] R. Christoffersen, J.F. Lindsay, S.K. Noble, M.A. Meador, J.J. Kosmo, J.A. Lawrence, L. Brostoff, A. Young, T. McCue, Lunar Dust Effects on Spacesuit Systems, Insights from the Apollo Spacesuits (2009) NASA/TP-2009e214786. [4] B.L. Cooper, D.S. McKay, L.A. Taylor, H. Kawamoto, L.M. Rifrio and C.P. Gonzalez, Extracting Respirable particles from lunar regolith for Toxicology Studies, Proc.. Of 12th Int. Conf. On Engineering, Science, Construction, and Operation in Challenging Environment, Earth & Space 2010, ASCE, Honolulu HI, (2010) 66e73. [5] C.I. Calle, J.L. McFall, C.R. Buhler, S.J. Snyder, E.E. Arens1, A. Chen, M.L. Ritz, J.S. Clements, C.R. Fortier1 and S. Trigwell, Dust Particle Removal by Electrostatic and Dielectrophoretic Forces with Applications to NASA Exploration Missions, Proc. ESA Annual Meeting on electrostatics, ESA, Minneapolis, MN, O1 (2008). [6] C.I. Calle, C.R. Buhler, J.L. McFall, S.J. Snyder, Particle removal by electrostatic and dielectric forces for dust control during lunar exploration missions, J. Electrostat. 67 (2009) 89e92.
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[7] H. Kawamoto, M. Uchiyama, B. L. Cooper and D. S. McKay, Mitigation of lunar dust on solar panels and optical elements utilizing electrostatic travelingWave, J. Electrostat. submitted for publication(submitted). [8] H. Kawamoto and N. Hara, Electrostatic cleaning system of lunar dust adhered to spacesuits, J. Aerospace Eng. (to be published). [9] S. Masuda, K. Fujibayashi, K. Ishida, H. Inaba, Confinement and transportation of charged aerosol clouds via electric curtain, Trans. Inst. Elec. Eng. Jpn. 92-B (1) (1972) 9e18. [10] J.R. Melcher, E.P. Warren, R.H. Kotwal, Traveling-wave delivery of singlecomponent developer, IEEE Trans. Ind. Appl. 25 (1989) 956e961. [11] J.R. Melcher, E.P. Warren, R.H. Kotwal, Theory for finite-phase traveling-wave boundary-guided transport of triboelectric particles, IEEE Trans. Ind. Appl. 25 (1989) 949e955. [12] F.W. Schmidlin, A new nonlevitated mode of traveling wave toner transport, IEEE Trans. Ind. Appl. 27 (1991) 480e487. [13] F.W. Schmidlin, Modes of traveling wave particles transport and their applications, J. Electrostat. 34 (1995) 225e244. [14] F. Schmidlin, Advances in Traveling Wave Toner Transport. IS&T’s NIP15: Int. Conf. on Digital Printing Technologies, IS&T, Orlando, FL, 1999, pp. 302e305. [15] F.M. Moesner, T. Higuchi, Electrostatic devices for particle microhandling, IEEE Trans. Ind. Appl. 35 (1999) 530e536. [16] M.D. Thompson, Y. Gartstein, J.T. LeStrange, Aspect of Toner Transport on a Traveling Wave Device. IS&T’s NIP15: Int. Conf. on Digital Printing Technologies, IS&T, Orlando, FL, 1999, pp. 262e265. [17] K. Taniguchi, S. Morikumi, S. Watanabe, Y. Nakano, T. Sakai, H. Yamamoto, T. Yagi, Y. Yamamoto, Improved Driving Characteristics for the Toner Transportation System. IS&T’s NIP16: Int. Conf. on Digital Printing Technologies, Vancouver, Canada, 2000, pp. 740e742. [18] R. Kober, Traveling Wave Transport of Conductive Toner Particles. IS&T’s NIP16: Int. Conf. on Digital Printing Technologies, IS&T, Vancouver, Canada, 2000, pp. 736e739. [19] R. Kober, Simulation of Traveling Wave Toner Transport. IS&T’s NIP18: Int. Conf. on Digital Printing Technologies, IS&T, San Diego, CA, 2002, pp. 453e457. [20] R. Kober, Transport geladener Partikel in elektrischen Wanderfeldern, dissertation, Achen University of Technology (2003). [21] K. Taniguchi, H. Yamamoto, Y. Nakano, T. Sakai, S. Morikuni, S. Watanabe, Y. Yamamoto, A new technique for measuring the distribution of charge-tomass ratio for toner particles with on-line use, J. Imaging Sci. Technol. 47 (2003) 224e228. [22] H. Kawamoto, N. Hasegawa, Traveling wave transport of particles and particle size classification, J. Imaging Sci. Technol. 48 (2004) 404e411. [23] H. Kawamoto, K. Seki, N. Kuromiya, Mechanism on traveling-wave transport of particles, J. Phys. D: Appl. Phys. 39 (2006) 1249e1256. [24] H. Kawamoto, Some techniques on electrostatic classification of particle size utilizing electrostatic traveling wave field, J. Electrostat. 66 (2008) 220e228. [25] H. Kanamori, S. Udagawa, T. Yoshida, S. Matsumoto and K. Takagi, Properties of lunar soil stimulant manufactured in Japan, Proc.. Of the 6th Int. Conf. On Engineering, Construction and operations in space, ASCE, Albuquerque NM (1998) 462e468.