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Experiments on dust removal performance of a novel PLZT driven lunar dust mitigation technique
Acta Astronautica 165 (2019) 17–24
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Research paper
Experiments on dust removal performance of a novel PLZT driven lunar dust mitigation technique
T
Jing Jiang1, Yifan Lu∗,1, Hongyue Zhao, Lei Wang State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, 150001, China
ARTICLE INFO
ABSTRACT
Keywords: Lunar exploration Lunar dust mitigation PLZT Dust removal efficiency
Lunar dust removal has proved to be one of the most important issues to be solved in lunar exploration. A novel lunar dust removal technique based on the anomalous photovoltaic effect of lanthanum-modified lead zirconate titanate (PLZT) has been developed in our previous research which could be used for conductor surface dust mitigation. In this study, the working performance of this novel dust removal technique is investigated. Since the particle size of the dust has a strong influence on the lunar dust removal performance, the particle size range which is applicable for this technique is explored first. Then the relation between the dust particle size and charge is discussed theoretically and experimentally. An experimental platform is built up for testing the working performance of this technique and the results indicate that a 95% dust removal efficiency can be achieved on a 320 mm × 125 mm dust covered surface, which is very promising for future lunar dust mitigation.
1. Introduction The lunar dust is composed of particles comprising approximately 20% by volume of the weathered layer which covers the surface of the moon. Under external disturbance such as human lunar exploration, those dust are easy to be suspended in the air [1]. Due to the environment on the moon surface and the small size property of the lunar dust, the lunar dust is apt to be charged and possess strong adhesion [2]. Therefore, the lunar dust can easily attach to the surface of lunar exploring instruments. This will not only contaminate and abrade the equipment but also give rise to a reduction of the optical properties, solar absorptivity and infrared emissivity [3,4]. Therefore, reliable dust removal techniques are of great necessity for future lunar explorations. The physical, chemical, biological and electrical characteristics of lunar dust have been widely investigated in the past few decades [5–7]. The charging mechanism of lunar dust grains under UV irradiation and dynamic models for electrostatic transport on the moon surface were also analyzed theoretically [8–10]. Afshar-Mohajer et al. reviewed the existing dust mitigation/removal technologies including fluidal, mechanical, electrical and passive self-cleaning methods for lunar and Martian missions and suggested the most suitable dust cleaning approach for different situations [11]. Specifically, an electrodynamic dust shield which not only removes the dust adhering to surface but also prevents the dust from accumulation onto the surface was designed [12]. A dust
cleaning system utilizing the electrostatic traveling wave with small energy consumption was developed and improved to remove the dust deposited on solar panels and other optical elements with an over 90% dust removal efficiency [13–15]. The Electrostatic Lunar Dust Collector (ELDC) [16] and Electrostatic Lunar Dust Repeller (ELDR) [17] were designed to collect or to repel the charged lunar dust particles on the protected surface. The dust cleaning efficiency in vacuum condition was tested [18] and the influence of back electrostatic field on the dust cleaning efficiency of an ELDC was investigated [19]. Electric curtains were also investigated for dust mitigation for solar panels and the dynamics of dust particles under the effect of traveling waves were analyzed by using an electrostatic discrete-element method [20–22]. Carbon nanotube technology was employed to construct a Spacesuit Dust Removal system and a scaled knee joint prototype of the self-cleaning spacesuit was fabricated to demonstrate the feasibility of this technique in future planetary exploration missions [23,24]. A new photovoltaic lunar dust mitigation system driven by PLZT ceramics was proposed for the first time by Jiang et al., which was very promising for non-contact dust removing for precision optical instrument surface [25,26]. In this paper, the range of the dust particle size which is applicable for this lunar dust removal technique is explored first. A high-speed camera is used to track the trajectory of dust particles between the photovoltaic lunar dust removal (PLDR) and the protected surface. The time needed for dust particles to be lifted up from the protected surface
Corresponding author. E-mail address: [email protected] (Y. Lu). 1 These authors contributed equally to this work. ∗
https://doi.org/10.1016/j.actaastro.2019.08.023 Received 7 June 2019; Received in revised form 7 July 2019; Accepted 28 August 2019 Available online 30 August 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
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and to be absorbed to the PLDR is tested respectively. The relationship between the size and the charge of dust particles is also investigated both theoretically and experimentally. Finally, an experimental platform is established to evaluate the dust removal performance of this new technique. The experimental results indicate that with a 100 mm × 100 mm PLDR, a satisfactory dust removal efficiency up to 95.3% can be achieved for 5 g of lunar dust simulants on a 320 mm × 125 mm aluminum protected surface. 2. The principle of photovoltaic lunar dust removal technique 2.1. Light-thermal-electrical properties of PLZT
Fig. 2. Diagram of forces acting on a lunar dust particle.
PLZT is a type of photoelectric ceramic which has photovoltaic, pyroelectric and piezoelectric properties. When illuminated by the ultraviolet light, a photovoltage of several kilovolts per centimeter will be generated between two electrodes of the PLZT, which is known as the anomalous photovoltaic effect. The photovoltage output of PLZT under ultraviolet light climbs gradually with the increasing of the illumination time, and finally reaches to a certain saturation value. Our previous study has verified that the time-history response of the photovoltage generated by the PLZT under UV illumination can be simulated by an RC charging curve. The equivalent electrical model of the PLZT can be described as a current source Ip in parallel with a capacitance Cp and a resistance Rp. The photovoltage output of the PLZT is given by Ref. [25].
Up = Ip Rp 1
exp
t Rp Cp
= U pS 1
exp
3. Analysis of the influence of lunar dust particle size 3.1. Force analysis of the lunar dust particle Lunar dust particles deposited on the protected surface are charged and then subjected to a resultant force consisting of the electrostatic force FE, the gravitational force G, the van der Waals adhesion force FvdW and the static-electric-image force FI, as shown in Fig. 2. The lunar dust grains covering upon the protected surface are assumed to be identical in charge and size. The particle is assumed to be a sphere with a diameter of dp. The density ρ of the lunar dust ranges from 2.3 to 3.2 g/cm3 [27]. The lunar gravitational acceleration gm is assumed to be 1.62 m/s2. The gravitational force G can be estimated as
t p
G=
(1)
1 g dp3 6 m
(2)
The van der Waals force FvdW can be approximately calculated by Ref. [28].
where τp = RpCp is the time constant; UpS is the saturation photovoltage, and Ip is the equivalent photocurrent.
FvdW =
2.2. Photovoltaic lunar dust removal system
1 Adp 12 s 2
(3)
where A is the Hamaker constant (including both dispersion and polar energies) of the lunar dust; s is the minimum distance between two contacted particles which is suggested to be 0.4 nm [5]. When the dust particle is located in a uniform electric field with intensity E, the charge of the particle will finally reach a saturation value qs, which can be approximately calculated by Ref. [29].
When the PLZT is illuminated by ultraviolet light, a high photovoltage will be generated between its two electrodes. When the PLDR and the protected surface are connected to two electrodes of the PLZT respectively, a strong electrostatic field (approximately 500 V/mm) is built up between the PLDR and the protected surface, which can be used to remove the lunar dust. In order to prevent the dust particles from being charged repeatedly, an insulating polyimide film is covered on the surface of the PLDR to ensure the dust removal efficiency. A triaxial sliding table is employed to achieve the movement of the PLDR over the protected surface. The schematic diagram of the dust removal system is shown in Fig. 1.
qs =
2 0 dp E
3 +2
1 0
(4)
sv −12
F/m); ε is the where ε0 is the permittivity of the space (8.854 × 10 permittivity of the particle, and ϕsv is the shape coefficient. For spherical particles such as limestone and sand, ϕsv = 11.43–12.5 [30]. The electrostatic force acting on the particle can then be described as (5)
FE = Eqs
The static-electric-image force FI on the dust particle is given approximately by Ref. [31].
FI =
qs2 4
2 0 dp
(6)
where α is a correction factor which depends on the polarization of the dielectric particle. One can find that the gravitational force G of the particle is proportional to the cubic of dp, the electrostatic force FE and the staticelectric-image force FI are proportional to the square of dp, and the van der Waals force FvdW is proportional to dp. Therefore, the dust particle larger than a certain value may not be removed since the gravitational force increases faster than the electrostatic force; similarly, the dust particle smaller than a certain value may not be easily removed since the van der Waals force decreases slower than the electrostatic force. When the electrostatic force is strong enough to overcome the
Fig. 1. Schematic diagram of the dust removal system. 18
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gravitational force, the static-electric-image force, and the van der Waals force, namely
Table 1 Taking-off time of different particles.
(7)
FE > G + FI + FvdW
the dust particle will be lifted up from the protected surface. The lunar dust particle is ultimately absorbed by the PLDR and therefore can be removed. However, there is another circumstance that the dust particle is not deposited on the surface but suspended in the gap. In this case, the van der Waals and static-electric-image force can be neglected. More details can be found in the research of Afshar-Mohajer et al. [19]. When the lunar dust particle has been lifted up from the protected surface, it is influenced only by the electrostatic force and the gravitational force, and the force condition can be simply described by Newton's second law as
ma = FE
where m is the mass of the particle and a is the acceleration. From Eqs. (2), (5) and (8) it can be deduced that
4 3
1 dp 2
3
(a + gm) =
U q d s
Average taking-off time (s)
Standard deviation (s)
> 0.18 0.1–0.18 0.075–0.1 0.025–0.05 0.01–0.025 < 0.01
∞ 12 3 8 15 31
N/A 0.61 0.39 0.48 0.99 1.82
Chinese Academy Sciences according to the 14 lunar dust samples of the Apollo Programs [32]. It has been reported that range distribution of lunar dust particle size is very wide: the particle diameter is between 10 μm and 1 mm, and the median diameter is between 46 μm and 110 μm, with an average mean particle size of 72 μm [33]. Therefore, dust particles of six different sizes are tested in the experiments with the following diameters: > 0.18 mm, 0.1 mm– 0.18 mm, 0.075 mm–0.1 mm, 0.025 mm–0.05 mm, 0.01 mm–0.25 mm and < 0.01 mm. The distance between the PLDR and the protected surface is 4 mm. The intensity of the ultraviolet light used to illuminate the PLZT (15 mm × 15 mm × 1 mm) is 596 mW/cm2. When the dust removal system starts to work, namely, the UV light source is turned on, the time needed for dust particles to take off from the protected surface are recorded in this experiment. The experiment is repeated for five times for dust particles of each size. The average value and the square deviation of the taking-off time of dust particles are shown in Table 1. The experimental results reveal that when the diameter of the particle is larger than 0.18 mm or less than 0.01 mm, this photovoltaic dust removal technique is not applicable. Therefore, the proposed lunar dust removal technique is able to provide satisfactory performance for dust particle within the size range of 0.01–0.18 mm, which counts for almost 90% of the lunar dust size range [33,34]. From Table 1 one can also find that when the size of the dust particle is too large or too small, the taking-off time becomes much longer. When the particle size is larger than 0.18 mm, the taking-off time becomes infinite, which means that the dust cannot be removed in this case. This is qualitatively consistent with the discussion in Section 3.1. This experiment shows the feasibility of this dust removal technique for a certain range of lunar dust particles. In the following section, the relation between the charge and size of the dust particles in the electrostatic field will be investigated both theoretically and experimentally.
(8)
G
Particle size (mm)
(9)
where U is the dust removal photovoltage; d is the distance between PLDR and protected surface; and a = 2d/t2 is the acceleration of the dust particle. It is noteworthy that the force analysis in this section is only used for qualitatively analyzing the effective dust removal condition of the dust particle. The specific calculations of forces acting on the particle are not performed. The feasibility of this proposed dust removal technique will be demonstrated by a series of experiments next. 3.2. Analysis of the applicable lunar dust particle size range In this section, an experimental platform (see Fig. 3) is built up to analyze the suitable range of dust particle size for this dust removal method. A UV light source UVEC8-144A (central wavelength is 365 nm) which could provide adjustable and uniform intensity ultraviolet light is used to illuminate the PLZT. A high-resistance voltmeter (Trek.820) is employed for measuring the photovoltage between the PLDR and the protected surface. The PLDR is an aluminum plate with the dimension of 192 mm × 75 mm × 0.8 mm. In order to avoid the loss of charge, the PLDR is pasted on a glass epoxy board with a thickness of 10 mm. Then, the PLDR is mounted on the triaxial sliding table with a displacement accuracy of 0.1 mm to achieve the movement over the protected surface. A steel plate (420 mm × 420 mm × 5 mm) is taken as the protected surface and four micro-displacement regulators with a precision of 0.01 mm are adopted to ensure the parallelism of the PLDR and the protected surface. It is noteworthy that in the middle sub-image of Fig. 3, the steel plate (protected surface) is not presented. The details of the steel plate, the PLDR, and the micro-displacement regulators are shown in the right sub-image. The lunar dust simulant used in the experiment is CLDS-i (ρ = 2.7 g/cm3) which is developed by the Institute of Geochemistry
3.3. Analysis of the relation between dust particle size and charge In this section, the relation between the charge and size of the dust particles is investigated. The lunar dust simulant is uniformly distributed on the protected surface. The dimension of the PLDR used in this experiment a 60 mm × 60 mm square and the distance between the PLDR and the protected surface is 3 mm. The intensity of the ultraviolet
Fig. 3. The experimental platform. 19
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Fig. 4. The trajectories of dust particles of different sizes. Table 2 Voltages and time spent on absorbing dust particle. Particle size (mm)
Time (ms)
Voltage (V)
0.1–0.18 0.075–0.1
30 17
1550 1480
Table 3 The charges of particles. Particle size (mm)
Charges (10−2pC) from Eq. (10)
Charges (10−2pC) from Eq. (4)
0.1–0.18 0.075–0.1
4.51–26.28 1.99–4.72
3.59–11.64 1.93–3.43
Fig. 6. The output of SPCs (S1~S6). Table 5 The average response voltages of SPCs.
Fig. 5. Silicon photocell. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 4 The main performance parameter of SPC. Characteristics
Parameter
Package size Back pin length Size of receiving surface Peak voltage (700 nm) Spectral range
16.5 mm × 15 mm 14 mm 10 mm × 10 mm 1.5 V 300 nm–700 nm
SPCs
Voltage (V)
S1 S2 S3 S4 S5 S6 Total
0.354 0.335 0.325 0.323 0.388 0.335 2.07
light is 400 mW/cm2. A high-speed camera (10000 Frames/s) is used to track the trajectories of the particles. Two kinds of dust particles with different diameters, i.e., 0.1 mm–0.18 mm and 0.075 mm–0.1 mm are tested to explore the effects of particle size on its charge. The images captured by the high-speed camera indicate that the dust particles can be absorbed from the protected surface to PLDR in a few milliseconds. The trajectories of a selected dust particle are shown in Fig. 4. The time spent on absorbing a dust particle from the protected surface to the PLDR (not including the taking-off time) and the dust removal voltages are shown in Table 2. 20
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Fig. 7. The experimental platform of the dust removal system. Fig. 8. The control block diagram of the dust removal system.
Fig. 10. The protected surface covered with dust.
The acceleration of the dust particle can be obtained by a = 2d/t2. Substituting the acceleration into Eq. (9) yields
qs =
1 3 d 2d dp +g 6 U t2
(10)
The charges of two kinds of particles can be obtained from Eq. (10) experimentally and from Eq. (4) theoretically, as shown in Table 3. Note that g is the gravitational acceleration on earth which is 9.8 m/s2 and ϕsv is set to be 12. From Table 2 one can find that the time for dust particles being absorbed to the PLDR from the protected surface is about several milliseconds. Although the bigger particles carry greater charges, the gravitational force G changes faster than the electrostatic force FE. According to the experimental results, the accelerations are approximate to be 6.67 m/s2 (dp = 0.1 mm–0.18 mm) and 20.76 m/s2 (dp = 0.075 mm–0.1 mm), which prove that the smaller particles are more quickly to be absorbed to PLDR after taking off. From Table 3, it can be concluded that the particle charge derived from experimental results agree with its theoretical results in the same order of magnitude.
Fig. 9. The control system flow chart. 21
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Fig. 11. The dust removal performance after different working cycles.
will decrease evidently compared to the one with no dust covering on it. Therefore, the output voltage of the SPC could be used to reflect the dust removal performance of this technique. A piece of SPC (ZL-G010) is shown in Fig. 5, and the main performance parameters are listed in Table 4. The six SPC sensors are in series to give a total output signal which will be analyzed and processed by the controller. Firstly, the output characteristics of the SPC covered with no dust are tested. Six SPCs are embedded evenly on the protected surface and illuminated by fluorescent lamps. The time-history responses of the output voltage of the SPCs (S1~S6) are shown in Fig. 6. The average output of each SPC and the total output are shown in Table 5. Note that the Data Acquisition Card (Advantech PCI-1710) is used in this step to collect and record the output voltages of SPCs in real time. Table 5 indicates that the voltage of each SPC is about 0.35 V and the total voltage output is 2.07 V. The experimental platform of the dust removal system is established as shown in Fig. 7. In the experiment, the UV light source is still selected to provide energy for PLZT. The size of the PLDR is 100 mm × 100 mm. It is mounted on a triaxial sliding table to move over the protected surface and the distance between the PLDR and the protected surface is 4 mm. The protected surface is a 320 mm × 125 mm × 8 mm aluminum plate with six SPCs embedded on it, which is parallel to the PLDR by adjusting the z-axis micro-displacement regulators. The SPCs are connected to the analog signal input ports of PCI-1710, which is used to collect the output signals of SPCs in real time. The digital signal output ports of PCI-1710 are connected to the driver of the stepper motor to control the motion of the PLDR.
Table 6 The output voltages of the SPCs. Cycle
n=0 n=1 n=2 n=4 n=6
Voltage (V) S1
S2
S3
S4
S5
S6
Total
0.243 0.258 0.288 0.320 0.355
0.231 0.248 0.263 0.293 0.333
0.152 0.193 0.215 0.266 0.313
0.204 0.220 0.242 0.269 0.306
0.253 0.285 0.324 0.354 0.396
0.238 0.256 0.293 0.298 0.322
1.323 1.460 1.625 1.800 2.025
Table 7 The dust removal efficiency. Cycle
η(n)
n=1 n=2 n=4 n=6
18.6% 41.0% 64.7% 95.3%
The charge is approximately proportional to the square of particle diameter, which is also consistent with Eq. (4). 4. Experiments on the photovoltaic lunar dust removal performance In this section, an experimental platform is established to verify the feasibility of the photovoltaic lunar dust removal technique.
4.2. Control strategy of the lunar dust removal system
4.1. The experimental platform setup
As has been noted, the output of SPCs can mirror the mass of the dust on the protected surface. Therefore, the output signals of SPCs can be used to control the dust removing process. When the total output voltage reaches 2 V, it can be considered that the dust on the surface has been cleaned up. Then the motor will receive the instruction from the controller that the current dust removal cycle will be completed, and
In order to evaluate the dust removal performance, six photosensitive silicon photocells (SPCs) which have a quick response speed to light change are used in the experimental platform as sensors. When the SPC is partially or completely covered by lunar dust, its output voltage 22
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the PLDR will return to the initial position driven by the stepping motor. The control block diagram is shown in Fig. 8. The linear interpolation method is used during the motion of the PLDR to subdivide the distance between the starting point (x0) and the end point (x1). Based on the current position and the target position, a pulse signal is sent to the driver to make the PLDR moves along the xdirection according to each pulse. When the PLDR reaches the target position, the starting point and the end point are switched, and the PLDR moves back in the opposite direction. When the PLDR finishes a round trip, we can regard it as one dust removal cycle. With the moving of the PLDR, the dust covered on the SPCs will be removed gradually. When the dust is cleaned up, namely, the total output voltage of the SPCs is above the set value (2 V), the dust removal is completed. The PLDR will return to the initial position after completing the current working cycle and the motor stops running. The control system flow chart is shown in Fig. 9. Through the open LabVIEW environment and integrated software, the data acquisition and control system is designed to achieve the above functions.
lunar dust particles is presented to reveal the dust removal process. Lunar dust simulants of different sizes have been tested through experiments to verify the particle size range applicable for this dust removal technique. The influence of particle size on its charge is analyzed theoretically and verified experimentally. An experimental platform has been established to prove the feasibility and evaluate the working performance of this photovoltaic lunar dust removal technique. By theoretical analysis and experimental validation, this study leads to the following conclusions: (1) The photovoltaic dust removal technique proposed is mainly suitable for the dust particles of size ranging from 0.01 mm to 0.18 mm which covers most of the lunar dust. The dust particle larger than 0.18 mm cannot be lifted by the electrostatic force mainly due to the effect of the gravitational force; while the one smaller than 0.01 mm tends to stay on the protected surface because of the domination of the Van der Waals force. (2) The charge of the lunar dust particle is proportional to the square of the diameter. Although the bigger particles carry greater charges, the smaller particles are absorbed to the PLDR more quickly after taking off. (3) When 5 g of lunar dust simulants (particle size d = 0.1 mm–0.18 mm) are uniformly distributed on the 320 mm × 125 mm protected surface, a dust removal efficiency up to 95.3% can be achieved after six working cycles with a 100 mm × 100 mm PLDR. The experimental results indicate that this technique could be very promising for precision optical surface dust mitigation in future lunar exploration.
4.3. Implementation and experimental results The lunar dust sample with particle size d = 0.1 mm–0.18 mm is used in this experiment. Totally five grams of the dust is spread uniformly over the protected surface, as shown in Fig. 10. According to the dimension of the protected surface and the PLDR, the distance between x0 and x1 is 300 mm, and the speed of the PLDR is set to be 300 mm/min. When the dust removal starts, the output voltages of the SPCs are recorded after each dust removal cycle. Once the recorded total voltage is higher than 2 V, the dust removal will stop automatically after finishing the current working cycle. The dust removal performance after different working cycles is shown in Fig. 11. From Fig. 11 one can find that with the increasing of the working cycle, the dust on the protected surface decreases gradually. After the sixth working cycle, the dust has almost been cleaned up. The dust removing experiments were repeated for three times in the same experimental condition. The time interval of each group of the experiment is 45 min. During the experiments, the temperature is 25 °C and the relative humidity is 32%. The average output voltages of the SPCs after each working cycle of dust removal are shown in Table 6. Let us define the dust removal efficiency as
(n) =
Ua U0
Ub Ub
Acknowledgments This research is supported, in part, by a grant from the National Natural Science Foundation of China (No. 51575125, 51175123) and the China Postdoctoral Science Foundation (2014M561358). References [1] T.J. Stubbs, R.R. Vondrak, W.M. Farrell, A dynamic fountain model for lunar dust, Adv. Space Res. 37 (2006) 59–66, https://doi.org/10.1016/j.asr.2005.04.048. [2] S.B. Rakesh Chandran, G. Renuka, C. Venugopal, Plasma electron temperature variability in lunar surface potential and in electric field under average solar wind conditions, Adv. Space Res. 51 (2013) 1622–1626, https://doi.org/10.1016/j.asr. 2013.01.016. [3] N. Khan-Mayberry, The lunar environment: determining the health effects of exposure to moon dusts, Acta Astronaut. 63 (2008) 1006–1014, https://doi.org/10. 1016/j.actaastro.2008.03.015. [4] N.C. Orger, J.R. Cordova Alarcon, K. Toyoda, M. Cho, Lunar dust lofting due to surface electric field and charging within Micro-cavities between dust grains above the terminator region, Adv. Space Res. 62 (2018) 896–911, https://doi.org/10. 1016/j.asr.2018.05.027. [5] O.R. Walton, Review of adhesion fundamentals for micron-scale particles, KONA Powder Part. J. 26 (2008) 129–141, https://doi.org/10.14356/kona.2008012. [6] D.S. McKay, B.L. Cooper, L.A. Taylor, J.T. James, K. Thomas-Keprta, C.M. Pieters, S.J. Wentworth, W.T. Wallace, T.S. Lee, Physicochemical properties of respirablesize lunar dust, Acta Astronaut. 107 (2015) 163–176, https://doi.org/10.1016/j. actaastro.2014.10.032. [7] I.A. Kuznetsov, A.V. Zakharov, G.G. Dolnikov, A.N. Lyash, V.V. Afonin, S.I. Popel, I.A. Shashkova, N.D. Borisov, Lunar dust: properties and investigation techniques, Sol. Syst. Res. 51 (2018) 611–622, https://doi.org/10.1134/s0038094617070097. [8] M.M. Abbas, D. Tankosic, P.D. Craven, J.F. Spann, A. LeClair, E.A. West, Lunar dust charging by photoelectric emissions, Planet. Space Sci. 55 (2007) 953–965, https:// doi.org/10.1016/j.pss.2006.12.007. [9] A. Champlain, J.-C. Matéo-Vélez, J.-F. Roussel, S. Hess, P. Sarrailh, G. Murat, J.P. Chardon, A. Gajan, Lunar dust simulant charging and transport under UV irradiation in vacuum: experiments and numerical modeling, J. Geophys. Res. Sp. Phys. 121 (2016) 103–116, https://doi.org/10.1002/2015JA021 738. [10] Z. Mao, G.R. Liu, A smoothed particle hydrodynamics model for electrostatic transport of charged lunar dust on the moon surface, Comput. Part. Mech. 5 (2018) 539–551, https://doi.org/10.1007/s40571-018-0189-4. [11] N. Afshar-Mohajer, C.Y. Wu, J.S. Curtis, J.R. Gaier, Review of dust transport and mitigation technologies in lunar and Martian atmospheres, Adv. Space Res. 56 (2015) 1222–1241, https://doi.org/10.1016/j.asr.2015.06.007. [12] C.I. Calle, C.R. Buhler, M.R. Johansen, M.D. Hogue, S.J. Snyder, Active dust control and mitigation technology for lunar and Martian exploration, Acta Astronaut. 69 (2011) 1082–1088, https://doi.org/10.1016/j.actaastro.2011.06.010. [13] H. Kawamoto, T. Shibata, Electrostatic cleaning system for removal of sand from
(11)
where Ua is the total output voltage of the SPCs after the nth working cycle; Ub is the total output voltage of the SPCs covered by the dust before dust removal (Ub = 1.323 V); U0 is the total output voltage of SPCs before covered by the dust (U0 = 2.07 V). The dust removal efficiency after each working cycle is presented in Table 7, from where one can find that the efficiency can reach up to 95.3% after sixth working cycles of dust removal, which could prove the feasibility of this noncontact PLZT driven dust removal technique. It is noteworthy that although the influence of the size of the protected surface on the dust removal performance is not investigated in this paper, we can infer that this technique is also effective for a protected surface of different dimensions. That is because, if the protected surface is small in size, the number of working cycles needed for dust removal is small. Otherwise, if the protected surface is relatively large, more working cycles will be needed to remove the dust. However, the dust removal ability and/or efficiency of the system is the same for the large or small protected surface. The only difference is the total time needed for dust removal. 5. Conclusions In this study, a novel lunar dust removal technique based on the photovoltaic effect of PLZT is introduced. Analysis of forces acting on 23
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[14] [15]
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solar panels, J. Electrost. 73 (2015) 65–70, https://doi.org/10.1016/j.elstat.2014. 10.011. H. Kawamoto, B. Guo, Improvement of an electrostatic cleaning system for removal of dust from solar panels, J. Electrost. 91 (2018) 28–33, https://doi.org/10.1016/j. elstat.2017.12.002. H. Kawamoto, S. Hashime, Practical performance of an electrostatic cleaning system for removal of lunar dust from optical elements utilizing electrostatic traveling wave, J. Electrost. 94 (2018) 38–43, https://doi.org/10.1016/j.elstat.2018. 05.004. N. Afshar-Mohajer, B. Damit, C.Y. Wu, N. Sorloaica-Hickman, Electrostatic particle collection in vacuum, Adv. Space Res. 48 (2011) 933–942, https://doi.org/10. 1016/j.asr.2011.04.030. N. Afshar-Mohajer, C.Y. Wu, R. Moore, N. Sorloaica-Hickman, Design of an electrostatic lunar dust repeller for mitigating dust deposition and evaluation of its removal efficiency, J. Aerosol Sci. 69 (2014) 21–31, https://doi.org/10.1016/j. jaerosci.2013.11.005. N. Afshar-Mohajer, C.Y. Wu, N. Sorloaica-Hickman, Efficiency determination of an electrostatic lunar dust collector by discrete element method, J. Appl. Phys. 112 (2012), https://doi.org/10.1063/1.4739734. N. Afshar-Mohajer, Y. Thakker, C.Y. Wu, N. Sorloaica-Hickman, Influence of back electrostatic field on the collection efficiency of an electrostatic lunar dust collector, Aerosol Air Qual. Res. 14 (2014) 1333–1343, https://doi.org/10.4209/aaqr.2013. 12.0353. G.Q. Liu, J.S. Marshall, Effect of particle adhesion and interactions on motion by traveling waves on an electric curtain, J. Electrost. 68 (2010) 179–189, https://doi. org/10.1016/j.elstat.2009.12.007. D. Qian, J.S. Marshall, J. Frolik, Control analysis for solar panel dust mitigation using an electric curtain, Renew. Energy 41 (2012) 134–144, https://doi.org/10. 1016/j.renene.2011.10.014. J. Zhang, C. Zhou, Y. Tang, F. Zheng, M. Meng, C. Miao, Criteria for particles to be levitated and to move continuously on traveling-wave electric curtain for dust mitigation on solar panels, Renew. Energy 119 (2018) 410–420, https://doi.org/10. 1016/j.renene.2017.12.009.
[23] K.K. Manyapu, P. De Leon, L. Peltz, J.R. Gaier, D. Waters, Proof of concept demonstration of novel technologies for lunar spacesuit dust mitigation, Acta Astronaut. 137 (2017) 472–481 https://doi.org/10.1016/j.actaastro.2017.05.005. [24] K.K. Manyapu, L. Peltz, P. De Leon, Self-cleaning spacesuits for future planetary missions using carbon nanotube technology, Acta Astronaut. 157 (2019) 134–144, https://doi.org/10.1016/j.actaastro.2018.12. 019. [25] J. Jiang, H. Zhao, L. Wang, H. Yue, X. Hou, Analysis of influence factors on dust removal efficiency for novel photovoltaic lunar dust removal technology, Smart Mater. Struct. 26 (2017) 125025, https://doi.org/10.1088/1361-665X/aa975b. [26] Y. Lu, J. Jiang, X. Yan, L. Wang, A new photovoltaic lunar dust removal technique based on the coplanar bipolar electrodes, Smart Mater. Struct. 28 (2019) 085010, , https://doi.org/10.1088/1361-665X/ab28da. [27] C.K. Goertz, Dusty plasmas in the solar system, Rev. Geophys. 27 (1989) 271, https://doi.org/10.1029/RG027i002p00271. [28] P. Atten, H.L. Pang, J.-L. Reboud, Study of dust removal by standing-wave electric curtain for application to solar cells on mars, IEEE Trans. Ind. Appl. 45 (2009) 75–86, https://doi.org/10.1109/TIA.2008.2009723. [29] Z. Luo, J. Jiang, L. Zhao, H. Chen, M. Fang, K. Cen, Research on the charging of fine particulate in different electric fields, Zhongguo Dianji Gongcheng Xuebao/ Proceedings Chinese Soc. Electr. Eng. 34 (2014) 3959–3969, https://doi.org/10. 13334/j.0258-8013.pcsee.2014.23.017 (in Chinese). [30] J. Gao, G. Xie, Characteristics of electric charges carried by dust particles and their effects on connector contact failure, Chin. J. Electron. 21 (2012) 559–565. [31] O.R. Walton, Adhesion of lunar dust, Natl. Aeronaut. Sp. Adm. (2007) 11–12. [32] H. Tang, X. Li, S. Zhang, S. Wang, J. Liu, S. Li, Y. Li, Y. Wu, A lunar dust simulant: CLDS-i, Adv. Space Res. 59 (2017) 1156–1160, https://doi.org/10.1016/j.asr.2016. 11.023. [33] W.D. Carrier, Particle size distribution of lunar soil, J. Geotech. Geoenviron. Eng. 129 (2003) 956–959, https://doi.org/10.1061/(asce)1090-0241(2003) 129:10(956). [34] Y. Liu, L.A. Taylor, Characterization of lunar dust and a synopsis of available lunar simulants, Planet, Space Sci 59 (2011) 1769–1783, https://doi.org/10.1016/j.pss. 2010.11.007.
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Research paper
Experiments on dust removal performance of a novel PLZT driven lunar dust mitigation technique
T
Jing Jiang1, Yifan Lu∗,1, Hongyue Zhao, Lei Wang State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, 150001, China
ARTICLE INFO
ABSTRACT
Keywords: Lunar exploration Lunar dust mitigation PLZT Dust removal efficiency
Lunar dust removal has proved to be one of the most important issues to be solved in lunar exploration. A novel lunar dust removal technique based on the anomalous photovoltaic effect of lanthanum-modified lead zirconate titanate (PLZT) has been developed in our previous research which could be used for conductor surface dust mitigation. In this study, the working performance of this novel dust removal technique is investigated. Since the particle size of the dust has a strong influence on the lunar dust removal performance, the particle size range which is applicable for this technique is explored first. Then the relation between the dust particle size and charge is discussed theoretically and experimentally. An experimental platform is built up for testing the working performance of this technique and the results indicate that a 95% dust removal efficiency can be achieved on a 320 mm × 125 mm dust covered surface, which is very promising for future lunar dust mitigation.
1. Introduction The lunar dust is composed of particles comprising approximately 20% by volume of the weathered layer which covers the surface of the moon. Under external disturbance such as human lunar exploration, those dust are easy to be suspended in the air [1]. Due to the environment on the moon surface and the small size property of the lunar dust, the lunar dust is apt to be charged and possess strong adhesion [2]. Therefore, the lunar dust can easily attach to the surface of lunar exploring instruments. This will not only contaminate and abrade the equipment but also give rise to a reduction of the optical properties, solar absorptivity and infrared emissivity [3,4]. Therefore, reliable dust removal techniques are of great necessity for future lunar explorations. The physical, chemical, biological and electrical characteristics of lunar dust have been widely investigated in the past few decades [5–7]. The charging mechanism of lunar dust grains under UV irradiation and dynamic models for electrostatic transport on the moon surface were also analyzed theoretically [8–10]. Afshar-Mohajer et al. reviewed the existing dust mitigation/removal technologies including fluidal, mechanical, electrical and passive self-cleaning methods for lunar and Martian missions and suggested the most suitable dust cleaning approach for different situations [11]. Specifically, an electrodynamic dust shield which not only removes the dust adhering to surface but also prevents the dust from accumulation onto the surface was designed [12]. A dust
cleaning system utilizing the electrostatic traveling wave with small energy consumption was developed and improved to remove the dust deposited on solar panels and other optical elements with an over 90% dust removal efficiency [13–15]. The Electrostatic Lunar Dust Collector (ELDC) [16] and Electrostatic Lunar Dust Repeller (ELDR) [17] were designed to collect or to repel the charged lunar dust particles on the protected surface. The dust cleaning efficiency in vacuum condition was tested [18] and the influence of back electrostatic field on the dust cleaning efficiency of an ELDC was investigated [19]. Electric curtains were also investigated for dust mitigation for solar panels and the dynamics of dust particles under the effect of traveling waves were analyzed by using an electrostatic discrete-element method [20–22]. Carbon nanotube technology was employed to construct a Spacesuit Dust Removal system and a scaled knee joint prototype of the self-cleaning spacesuit was fabricated to demonstrate the feasibility of this technique in future planetary exploration missions [23,24]. A new photovoltaic lunar dust mitigation system driven by PLZT ceramics was proposed for the first time by Jiang et al., which was very promising for non-contact dust removing for precision optical instrument surface [25,26]. In this paper, the range of the dust particle size which is applicable for this lunar dust removal technique is explored first. A high-speed camera is used to track the trajectory of dust particles between the photovoltaic lunar dust removal (PLDR) and the protected surface. The time needed for dust particles to be lifted up from the protected surface
Corresponding author. E-mail address: [email protected] (Y. Lu). 1 These authors contributed equally to this work. ∗
https://doi.org/10.1016/j.actaastro.2019.08.023 Received 7 June 2019; Received in revised form 7 July 2019; Accepted 28 August 2019 Available online 30 August 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
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and to be absorbed to the PLDR is tested respectively. The relationship between the size and the charge of dust particles is also investigated both theoretically and experimentally. Finally, an experimental platform is established to evaluate the dust removal performance of this new technique. The experimental results indicate that with a 100 mm × 100 mm PLDR, a satisfactory dust removal efficiency up to 95.3% can be achieved for 5 g of lunar dust simulants on a 320 mm × 125 mm aluminum protected surface. 2. The principle of photovoltaic lunar dust removal technique 2.1. Light-thermal-electrical properties of PLZT
Fig. 2. Diagram of forces acting on a lunar dust particle.
PLZT is a type of photoelectric ceramic which has photovoltaic, pyroelectric and piezoelectric properties. When illuminated by the ultraviolet light, a photovoltage of several kilovolts per centimeter will be generated between two electrodes of the PLZT, which is known as the anomalous photovoltaic effect. The photovoltage output of PLZT under ultraviolet light climbs gradually with the increasing of the illumination time, and finally reaches to a certain saturation value. Our previous study has verified that the time-history response of the photovoltage generated by the PLZT under UV illumination can be simulated by an RC charging curve. The equivalent electrical model of the PLZT can be described as a current source Ip in parallel with a capacitance Cp and a resistance Rp. The photovoltage output of the PLZT is given by Ref. [25].
Up = Ip Rp 1
exp
t Rp Cp
= U pS 1
exp
3. Analysis of the influence of lunar dust particle size 3.1. Force analysis of the lunar dust particle Lunar dust particles deposited on the protected surface are charged and then subjected to a resultant force consisting of the electrostatic force FE, the gravitational force G, the van der Waals adhesion force FvdW and the static-electric-image force FI, as shown in Fig. 2. The lunar dust grains covering upon the protected surface are assumed to be identical in charge and size. The particle is assumed to be a sphere with a diameter of dp. The density ρ of the lunar dust ranges from 2.3 to 3.2 g/cm3 [27]. The lunar gravitational acceleration gm is assumed to be 1.62 m/s2. The gravitational force G can be estimated as
t p
G=
(1)
1 g dp3 6 m
(2)
The van der Waals force FvdW can be approximately calculated by Ref. [28].
where τp = RpCp is the time constant; UpS is the saturation photovoltage, and Ip is the equivalent photocurrent.
FvdW =
2.2. Photovoltaic lunar dust removal system
1 Adp 12 s 2
(3)
where A is the Hamaker constant (including both dispersion and polar energies) of the lunar dust; s is the minimum distance between two contacted particles which is suggested to be 0.4 nm [5]. When the dust particle is located in a uniform electric field with intensity E, the charge of the particle will finally reach a saturation value qs, which can be approximately calculated by Ref. [29].
When the PLZT is illuminated by ultraviolet light, a high photovoltage will be generated between its two electrodes. When the PLDR and the protected surface are connected to two electrodes of the PLZT respectively, a strong electrostatic field (approximately 500 V/mm) is built up between the PLDR and the protected surface, which can be used to remove the lunar dust. In order to prevent the dust particles from being charged repeatedly, an insulating polyimide film is covered on the surface of the PLDR to ensure the dust removal efficiency. A triaxial sliding table is employed to achieve the movement of the PLDR over the protected surface. The schematic diagram of the dust removal system is shown in Fig. 1.
qs =
2 0 dp E
3 +2
1 0
(4)
sv −12
F/m); ε is the where ε0 is the permittivity of the space (8.854 × 10 permittivity of the particle, and ϕsv is the shape coefficient. For spherical particles such as limestone and sand, ϕsv = 11.43–12.5 [30]. The electrostatic force acting on the particle can then be described as (5)
FE = Eqs
The static-electric-image force FI on the dust particle is given approximately by Ref. [31].
FI =
qs2 4
2 0 dp
(6)
where α is a correction factor which depends on the polarization of the dielectric particle. One can find that the gravitational force G of the particle is proportional to the cubic of dp, the electrostatic force FE and the staticelectric-image force FI are proportional to the square of dp, and the van der Waals force FvdW is proportional to dp. Therefore, the dust particle larger than a certain value may not be removed since the gravitational force increases faster than the electrostatic force; similarly, the dust particle smaller than a certain value may not be easily removed since the van der Waals force decreases slower than the electrostatic force. When the electrostatic force is strong enough to overcome the
Fig. 1. Schematic diagram of the dust removal system. 18
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gravitational force, the static-electric-image force, and the van der Waals force, namely
Table 1 Taking-off time of different particles.
(7)
FE > G + FI + FvdW
the dust particle will be lifted up from the protected surface. The lunar dust particle is ultimately absorbed by the PLDR and therefore can be removed. However, there is another circumstance that the dust particle is not deposited on the surface but suspended in the gap. In this case, the van der Waals and static-electric-image force can be neglected. More details can be found in the research of Afshar-Mohajer et al. [19]. When the lunar dust particle has been lifted up from the protected surface, it is influenced only by the electrostatic force and the gravitational force, and the force condition can be simply described by Newton's second law as
ma = FE
where m is the mass of the particle and a is the acceleration. From Eqs. (2), (5) and (8) it can be deduced that
4 3
1 dp 2
3
(a + gm) =
U q d s
Average taking-off time (s)
Standard deviation (s)
> 0.18 0.1–0.18 0.075–0.1 0.025–0.05 0.01–0.025 < 0.01
∞ 12 3 8 15 31
N/A 0.61 0.39 0.48 0.99 1.82
Chinese Academy Sciences according to the 14 lunar dust samples of the Apollo Programs [32]. It has been reported that range distribution of lunar dust particle size is very wide: the particle diameter is between 10 μm and 1 mm, and the median diameter is between 46 μm and 110 μm, with an average mean particle size of 72 μm [33]. Therefore, dust particles of six different sizes are tested in the experiments with the following diameters: > 0.18 mm, 0.1 mm– 0.18 mm, 0.075 mm–0.1 mm, 0.025 mm–0.05 mm, 0.01 mm–0.25 mm and < 0.01 mm. The distance between the PLDR and the protected surface is 4 mm. The intensity of the ultraviolet light used to illuminate the PLZT (15 mm × 15 mm × 1 mm) is 596 mW/cm2. When the dust removal system starts to work, namely, the UV light source is turned on, the time needed for dust particles to take off from the protected surface are recorded in this experiment. The experiment is repeated for five times for dust particles of each size. The average value and the square deviation of the taking-off time of dust particles are shown in Table 1. The experimental results reveal that when the diameter of the particle is larger than 0.18 mm or less than 0.01 mm, this photovoltaic dust removal technique is not applicable. Therefore, the proposed lunar dust removal technique is able to provide satisfactory performance for dust particle within the size range of 0.01–0.18 mm, which counts for almost 90% of the lunar dust size range [33,34]. From Table 1 one can also find that when the size of the dust particle is too large or too small, the taking-off time becomes much longer. When the particle size is larger than 0.18 mm, the taking-off time becomes infinite, which means that the dust cannot be removed in this case. This is qualitatively consistent with the discussion in Section 3.1. This experiment shows the feasibility of this dust removal technique for a certain range of lunar dust particles. In the following section, the relation between the charge and size of the dust particles in the electrostatic field will be investigated both theoretically and experimentally.
(8)
G
Particle size (mm)
(9)
where U is the dust removal photovoltage; d is the distance between PLDR and protected surface; and a = 2d/t2 is the acceleration of the dust particle. It is noteworthy that the force analysis in this section is only used for qualitatively analyzing the effective dust removal condition of the dust particle. The specific calculations of forces acting on the particle are not performed. The feasibility of this proposed dust removal technique will be demonstrated by a series of experiments next. 3.2. Analysis of the applicable lunar dust particle size range In this section, an experimental platform (see Fig. 3) is built up to analyze the suitable range of dust particle size for this dust removal method. A UV light source UVEC8-144A (central wavelength is 365 nm) which could provide adjustable and uniform intensity ultraviolet light is used to illuminate the PLZT. A high-resistance voltmeter (Trek.820) is employed for measuring the photovoltage between the PLDR and the protected surface. The PLDR is an aluminum plate with the dimension of 192 mm × 75 mm × 0.8 mm. In order to avoid the loss of charge, the PLDR is pasted on a glass epoxy board with a thickness of 10 mm. Then, the PLDR is mounted on the triaxial sliding table with a displacement accuracy of 0.1 mm to achieve the movement over the protected surface. A steel plate (420 mm × 420 mm × 5 mm) is taken as the protected surface and four micro-displacement regulators with a precision of 0.01 mm are adopted to ensure the parallelism of the PLDR and the protected surface. It is noteworthy that in the middle sub-image of Fig. 3, the steel plate (protected surface) is not presented. The details of the steel plate, the PLDR, and the micro-displacement regulators are shown in the right sub-image. The lunar dust simulant used in the experiment is CLDS-i (ρ = 2.7 g/cm3) which is developed by the Institute of Geochemistry
3.3. Analysis of the relation between dust particle size and charge In this section, the relation between the charge and size of the dust particles is investigated. The lunar dust simulant is uniformly distributed on the protected surface. The dimension of the PLDR used in this experiment a 60 mm × 60 mm square and the distance between the PLDR and the protected surface is 3 mm. The intensity of the ultraviolet
Fig. 3. The experimental platform. 19
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Fig. 4. The trajectories of dust particles of different sizes. Table 2 Voltages and time spent on absorbing dust particle. Particle size (mm)
Time (ms)
Voltage (V)
0.1–0.18 0.075–0.1
30 17
1550 1480
Table 3 The charges of particles. Particle size (mm)
Charges (10−2pC) from Eq. (10)
Charges (10−2pC) from Eq. (4)
0.1–0.18 0.075–0.1
4.51–26.28 1.99–4.72
3.59–11.64 1.93–3.43
Fig. 6. The output of SPCs (S1~S6). Table 5 The average response voltages of SPCs.
Fig. 5. Silicon photocell. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 4 The main performance parameter of SPC. Characteristics
Parameter
Package size Back pin length Size of receiving surface Peak voltage (700 nm) Spectral range
16.5 mm × 15 mm 14 mm 10 mm × 10 mm 1.5 V 300 nm–700 nm
SPCs
Voltage (V)
S1 S2 S3 S4 S5 S6 Total
0.354 0.335 0.325 0.323 0.388 0.335 2.07
light is 400 mW/cm2. A high-speed camera (10000 Frames/s) is used to track the trajectories of the particles. Two kinds of dust particles with different diameters, i.e., 0.1 mm–0.18 mm and 0.075 mm–0.1 mm are tested to explore the effects of particle size on its charge. The images captured by the high-speed camera indicate that the dust particles can be absorbed from the protected surface to PLDR in a few milliseconds. The trajectories of a selected dust particle are shown in Fig. 4. The time spent on absorbing a dust particle from the protected surface to the PLDR (not including the taking-off time) and the dust removal voltages are shown in Table 2. 20
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Fig. 7. The experimental platform of the dust removal system. Fig. 8. The control block diagram of the dust removal system.
Fig. 10. The protected surface covered with dust.
The acceleration of the dust particle can be obtained by a = 2d/t2. Substituting the acceleration into Eq. (9) yields
qs =
1 3 d 2d dp +g 6 U t2
(10)
The charges of two kinds of particles can be obtained from Eq. (10) experimentally and from Eq. (4) theoretically, as shown in Table 3. Note that g is the gravitational acceleration on earth which is 9.8 m/s2 and ϕsv is set to be 12. From Table 2 one can find that the time for dust particles being absorbed to the PLDR from the protected surface is about several milliseconds. Although the bigger particles carry greater charges, the gravitational force G changes faster than the electrostatic force FE. According to the experimental results, the accelerations are approximate to be 6.67 m/s2 (dp = 0.1 mm–0.18 mm) and 20.76 m/s2 (dp = 0.075 mm–0.1 mm), which prove that the smaller particles are more quickly to be absorbed to PLDR after taking off. From Table 3, it can be concluded that the particle charge derived from experimental results agree with its theoretical results in the same order of magnitude.
Fig. 9. The control system flow chart. 21
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Fig. 11. The dust removal performance after different working cycles.
will decrease evidently compared to the one with no dust covering on it. Therefore, the output voltage of the SPC could be used to reflect the dust removal performance of this technique. A piece of SPC (ZL-G010) is shown in Fig. 5, and the main performance parameters are listed in Table 4. The six SPC sensors are in series to give a total output signal which will be analyzed and processed by the controller. Firstly, the output characteristics of the SPC covered with no dust are tested. Six SPCs are embedded evenly on the protected surface and illuminated by fluorescent lamps. The time-history responses of the output voltage of the SPCs (S1~S6) are shown in Fig. 6. The average output of each SPC and the total output are shown in Table 5. Note that the Data Acquisition Card (Advantech PCI-1710) is used in this step to collect and record the output voltages of SPCs in real time. Table 5 indicates that the voltage of each SPC is about 0.35 V and the total voltage output is 2.07 V. The experimental platform of the dust removal system is established as shown in Fig. 7. In the experiment, the UV light source is still selected to provide energy for PLZT. The size of the PLDR is 100 mm × 100 mm. It is mounted on a triaxial sliding table to move over the protected surface and the distance between the PLDR and the protected surface is 4 mm. The protected surface is a 320 mm × 125 mm × 8 mm aluminum plate with six SPCs embedded on it, which is parallel to the PLDR by adjusting the z-axis micro-displacement regulators. The SPCs are connected to the analog signal input ports of PCI-1710, which is used to collect the output signals of SPCs in real time. The digital signal output ports of PCI-1710 are connected to the driver of the stepper motor to control the motion of the PLDR.
Table 6 The output voltages of the SPCs. Cycle
n=0 n=1 n=2 n=4 n=6
Voltage (V) S1
S2
S3
S4
S5
S6
Total
0.243 0.258 0.288 0.320 0.355
0.231 0.248 0.263 0.293 0.333
0.152 0.193 0.215 0.266 0.313
0.204 0.220 0.242 0.269 0.306
0.253 0.285 0.324 0.354 0.396
0.238 0.256 0.293 0.298 0.322
1.323 1.460 1.625 1.800 2.025
Table 7 The dust removal efficiency. Cycle
η(n)
n=1 n=2 n=4 n=6
18.6% 41.0% 64.7% 95.3%
The charge is approximately proportional to the square of particle diameter, which is also consistent with Eq. (4). 4. Experiments on the photovoltaic lunar dust removal performance In this section, an experimental platform is established to verify the feasibility of the photovoltaic lunar dust removal technique.
4.2. Control strategy of the lunar dust removal system
4.1. The experimental platform setup
As has been noted, the output of SPCs can mirror the mass of the dust on the protected surface. Therefore, the output signals of SPCs can be used to control the dust removing process. When the total output voltage reaches 2 V, it can be considered that the dust on the surface has been cleaned up. Then the motor will receive the instruction from the controller that the current dust removal cycle will be completed, and
In order to evaluate the dust removal performance, six photosensitive silicon photocells (SPCs) which have a quick response speed to light change are used in the experimental platform as sensors. When the SPC is partially or completely covered by lunar dust, its output voltage 22
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the PLDR will return to the initial position driven by the stepping motor. The control block diagram is shown in Fig. 8. The linear interpolation method is used during the motion of the PLDR to subdivide the distance between the starting point (x0) and the end point (x1). Based on the current position and the target position, a pulse signal is sent to the driver to make the PLDR moves along the xdirection according to each pulse. When the PLDR reaches the target position, the starting point and the end point are switched, and the PLDR moves back in the opposite direction. When the PLDR finishes a round trip, we can regard it as one dust removal cycle. With the moving of the PLDR, the dust covered on the SPCs will be removed gradually. When the dust is cleaned up, namely, the total output voltage of the SPCs is above the set value (2 V), the dust removal is completed. The PLDR will return to the initial position after completing the current working cycle and the motor stops running. The control system flow chart is shown in Fig. 9. Through the open LabVIEW environment and integrated software, the data acquisition and control system is designed to achieve the above functions.
lunar dust particles is presented to reveal the dust removal process. Lunar dust simulants of different sizes have been tested through experiments to verify the particle size range applicable for this dust removal technique. The influence of particle size on its charge is analyzed theoretically and verified experimentally. An experimental platform has been established to prove the feasibility and evaluate the working performance of this photovoltaic lunar dust removal technique. By theoretical analysis and experimental validation, this study leads to the following conclusions: (1) The photovoltaic dust removal technique proposed is mainly suitable for the dust particles of size ranging from 0.01 mm to 0.18 mm which covers most of the lunar dust. The dust particle larger than 0.18 mm cannot be lifted by the electrostatic force mainly due to the effect of the gravitational force; while the one smaller than 0.01 mm tends to stay on the protected surface because of the domination of the Van der Waals force. (2) The charge of the lunar dust particle is proportional to the square of the diameter. Although the bigger particles carry greater charges, the smaller particles are absorbed to the PLDR more quickly after taking off. (3) When 5 g of lunar dust simulants (particle size d = 0.1 mm–0.18 mm) are uniformly distributed on the 320 mm × 125 mm protected surface, a dust removal efficiency up to 95.3% can be achieved after six working cycles with a 100 mm × 100 mm PLDR. The experimental results indicate that this technique could be very promising for precision optical surface dust mitigation in future lunar exploration.
4.3. Implementation and experimental results The lunar dust sample with particle size d = 0.1 mm–0.18 mm is used in this experiment. Totally five grams of the dust is spread uniformly over the protected surface, as shown in Fig. 10. According to the dimension of the protected surface and the PLDR, the distance between x0 and x1 is 300 mm, and the speed of the PLDR is set to be 300 mm/min. When the dust removal starts, the output voltages of the SPCs are recorded after each dust removal cycle. Once the recorded total voltage is higher than 2 V, the dust removal will stop automatically after finishing the current working cycle. The dust removal performance after different working cycles is shown in Fig. 11. From Fig. 11 one can find that with the increasing of the working cycle, the dust on the protected surface decreases gradually. After the sixth working cycle, the dust has almost been cleaned up. The dust removing experiments were repeated for three times in the same experimental condition. The time interval of each group of the experiment is 45 min. During the experiments, the temperature is 25 °C and the relative humidity is 32%. The average output voltages of the SPCs after each working cycle of dust removal are shown in Table 6. Let us define the dust removal efficiency as
(n) =
Ua U0
Ub Ub
Acknowledgments This research is supported, in part, by a grant from the National Natural Science Foundation of China (No. 51575125, 51175123) and the China Postdoctoral Science Foundation (2014M561358). References [1] T.J. Stubbs, R.R. Vondrak, W.M. Farrell, A dynamic fountain model for lunar dust, Adv. Space Res. 37 (2006) 59–66, https://doi.org/10.1016/j.asr.2005.04.048. [2] S.B. Rakesh Chandran, G. Renuka, C. Venugopal, Plasma electron temperature variability in lunar surface potential and in electric field under average solar wind conditions, Adv. Space Res. 51 (2013) 1622–1626, https://doi.org/10.1016/j.asr. 2013.01.016. [3] N. Khan-Mayberry, The lunar environment: determining the health effects of exposure to moon dusts, Acta Astronaut. 63 (2008) 1006–1014, https://doi.org/10. 1016/j.actaastro.2008.03.015. [4] N.C. Orger, J.R. Cordova Alarcon, K. Toyoda, M. Cho, Lunar dust lofting due to surface electric field and charging within Micro-cavities between dust grains above the terminator region, Adv. Space Res. 62 (2018) 896–911, https://doi.org/10. 1016/j.asr.2018.05.027. [5] O.R. Walton, Review of adhesion fundamentals for micron-scale particles, KONA Powder Part. J. 26 (2008) 129–141, https://doi.org/10.14356/kona.2008012. [6] D.S. McKay, B.L. Cooper, L.A. Taylor, J.T. James, K. Thomas-Keprta, C.M. Pieters, S.J. Wentworth, W.T. Wallace, T.S. Lee, Physicochemical properties of respirablesize lunar dust, Acta Astronaut. 107 (2015) 163–176, https://doi.org/10.1016/j. actaastro.2014.10.032. [7] I.A. Kuznetsov, A.V. Zakharov, G.G. Dolnikov, A.N. Lyash, V.V. Afonin, S.I. Popel, I.A. Shashkova, N.D. Borisov, Lunar dust: properties and investigation techniques, Sol. Syst. Res. 51 (2018) 611–622, https://doi.org/10.1134/s0038094617070097. [8] M.M. Abbas, D. Tankosic, P.D. Craven, J.F. Spann, A. LeClair, E.A. West, Lunar dust charging by photoelectric emissions, Planet. Space Sci. 55 (2007) 953–965, https:// doi.org/10.1016/j.pss.2006.12.007. [9] A. Champlain, J.-C. Matéo-Vélez, J.-F. Roussel, S. Hess, P. Sarrailh, G. Murat, J.P. Chardon, A. Gajan, Lunar dust simulant charging and transport under UV irradiation in vacuum: experiments and numerical modeling, J. Geophys. Res. Sp. Phys. 121 (2016) 103–116, https://doi.org/10.1002/2015JA021 738. [10] Z. Mao, G.R. Liu, A smoothed particle hydrodynamics model for electrostatic transport of charged lunar dust on the moon surface, Comput. Part. Mech. 5 (2018) 539–551, https://doi.org/10.1007/s40571-018-0189-4. [11] N. Afshar-Mohajer, C.Y. Wu, J.S. Curtis, J.R. Gaier, Review of dust transport and mitigation technologies in lunar and Martian atmospheres, Adv. Space Res. 56 (2015) 1222–1241, https://doi.org/10.1016/j.asr.2015.06.007. [12] C.I. Calle, C.R. Buhler, M.R. Johansen, M.D. Hogue, S.J. Snyder, Active dust control and mitigation technology for lunar and Martian exploration, Acta Astronaut. 69 (2011) 1082–1088, https://doi.org/10.1016/j.actaastro.2011.06.010. [13] H. Kawamoto, T. Shibata, Electrostatic cleaning system for removal of sand from
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where Ua is the total output voltage of the SPCs after the nth working cycle; Ub is the total output voltage of the SPCs covered by the dust before dust removal (Ub = 1.323 V); U0 is the total output voltage of SPCs before covered by the dust (U0 = 2.07 V). The dust removal efficiency after each working cycle is presented in Table 7, from where one can find that the efficiency can reach up to 95.3% after sixth working cycles of dust removal, which could prove the feasibility of this noncontact PLZT driven dust removal technique. It is noteworthy that although the influence of the size of the protected surface on the dust removal performance is not investigated in this paper, we can infer that this technique is also effective for a protected surface of different dimensions. That is because, if the protected surface is small in size, the number of working cycles needed for dust removal is small. Otherwise, if the protected surface is relatively large, more working cycles will be needed to remove the dust. However, the dust removal ability and/or efficiency of the system is the same for the large or small protected surface. The only difference is the total time needed for dust removal. 5. Conclusions In this study, a novel lunar dust removal technique based on the photovoltaic effect of PLZT is introduced. Analysis of forces acting on 23
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