A PROTOTYPE OF PERCUSSIVE ROCK SURFACE CRUSHER USING SOLENOID FOR LUNAR AND PLANETARY EXPLORATION

Katsushi Furutani (1), Hisashi Kamiishi (1), Yasuo Murase (1), Takashi Kubota (2), Makiko Ohtake (2), Kazuto Saiki (3), Tatsuaki Okada (2), Hisashi Otake (2), Chikatoshi Honda (4), Hirohisa Kurosaki (5),

Takamitsu Sugihara (6), Tomokatsu Morota (7)

(1) Toyota Technological Institute, 12-1, Hisakata 2-chome, Tempaku-ku, Nagoya 468-8511 Japan, Email: furutani@toyota-ti.ac.jp

(2) Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 1-1, Yoshinodai 3-chome, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan (3) Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 Japan

(4) The University of Aizu, Ikki-machi, Aizuwakamatsu, Fukushima 965-8580 Japan (5) Japan Aerospace Exploration Agency,

44-1, Jinidaiji-higashi-machi 7-chome, Chofu, Tokyo 182-8522, Japan (6) Japan Agency for Marine-Earth Science and Technology,

3173-25, Showa-machi, Kanazawa-ku, Yokohama 236-0001 Japan (7) Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602 Japan

ABSTRACT

This paper deals with a percussive rock surface crusher driven with a solenoid. For a future lunar exploration, an unmanned mission is planned to investigate rock samples automatically. The weathered rock surface should be removed and smoothed to investigate its structure and composition precisely. A solenoid was used to vibrate a tool bit made of tungsten carbide because of its long stroke and large generative impulsive force. The crusher measures 59 mm × 60 mm × 129 mm and weighs 490 g. The specimen was reciprocally fed with a slider-crank mechanism in the horizontal direction. The peak of the generative force was 170 N driven by a sinusoidal voltage of 50 Hz. In order to decrease the temperature rise at the solenoid, heat should be conducted to the structure, especially in vacuum. Smoothing basalt was demonstrated in air and vacuum environment. 1. INTRODUCTION

In order to investigate more amounts of rock samples from various aspects, in-situ inspection methods are useful. As future lunar, planet and asteroid explorations, unmanned missions are planned for in-situ automatic investigation of rock samples. For example, Science Integrated Package (SIP) has been developing [1]. Bullet type samplers are sometimes used to obtain a sample from an asteroid in sample-return missions [2] because the gravity is very small there. However, rock fragments cannot be raised under a larger gravity. Many drilling or coring devices have been developed for robotic mining missions [3]. The weathered rock surface should be removed and smoothed to investigate its structure and composition precisely. To remove the unevenness, larger stress than breaking stress must be act on the rock because the fracture of brittle rocks fits

Griffith theory. Augers, twisted drills or dual pegs are inappropriate for machining hard, brittle rocks [4, 5]. Shaving with rotary blades has been used in Mars exploration [6]. Some device used vibration to crush or remove the weathered sample surface because precision finishing has not been strongly required in the missions yet [7-10]. Because the pressure on Mars is about 7×103 Pa, the air keeps the ambient temperature below 20ºC and works as a coolant. Consequently, the tool performance is almost the same as that on Earth. On the moon and asteroids, the ambient pressure is 10-6 Pa or lower. The temperature exceeds 100ºC during the day by solar radiation. However, missions are usually executed during the day to generate electric power with solar cells. Then tools for mechanical removal process such as the blades or grinding wheel will be easily damaged and their tool life will be shortened in vacuum due to heat and tribological problems. A small gravity seldom affects the tool life and cutting performance. Saw wires on which diamond grits are fixed by electroplated nickel have been increasingly used in semiconductor production industry. Although some wire-sawing systems have been proposed for lunar explorations [11-14], they have not been applicable in vacuum environment, yet [15]. In order to decrease the tool damage, sharp or thin tools are inappropriate. Therefore, the percussive crushers are preferable. Large amplitude of the tool vibration is needed in the case of smoothing a rough surface. Although the ultrasonic vibration acts a large impulsive force, its vibration amplitude is too small [9]. A solenoid is good for the generation of the tool vibration because of its long stroke and large generative impulsive force. For a non-contact drilling method, an application of electric pulse power has been proposed [16]. However, this method needs much power and cannot be used in

vacuum. This paper describes a prototype of a percussive crusher driven with a solenoid to remove a rock surface. At first, a structure of the crusher is introduced. Then, some basic performance of the crusher such as generated force and temperature rise during machining in vacuum is measured. Finally, machining some rock specimens are demonstrated in vacuum. 2. STRUCTURE OF DEVICE

Fig. 1 shows an appearance of a prototype of the percussive crusher system. It measures 370 mm in length, 65 mm in width and 208 mm in height. The structure was mainly made of duralumin (JIS A2017). A percussive crusher device driven with a solenoid is lifted or pressed onto a workpiece with a combination of a stepping motor and a slide screw in the vertical direction. The slide screw and a nut are made of stainless steel (JIS SUS304) and brass, respectively. The step angle is 0.72º, and the reduction gear ratio is 50:1. The screw lead and stroke are 2 and 39 mm, respectively. The maximum feeding speed was 0.2 mm/s. The workpiece was held with screws from its side and was reciprocated with a slider-crank mechanism driven another stepping motor in the horizontal direction. The stroke is 9 mm. The slide guide was made of polytetrafluoroethylene (PTFE). The maximum feeding speed was 2.8 mm/s. No grease was used in this system. The whole unit was mounted through 4 wire rope isolators (load capacity: 2.4 N, spring constant for continuous vibration: 2.8 kN/m, spring constant for impulse: 1.2 kN/m) not to transmit the vibration to the base. Fig. 2 shows a structure of the crusher device. It measures 59 mm × 60 mm × 129 mm excluding a sensor attachment, and weighs 490 g. The frames were made of aluminum alloy (JIS A2014). The solenoid core

was divided as E- and I-cores. 15 non-oriented electrical steel sheets with each thickness of 0.3 mm were stacked for the core. Polyurethane-coated copper wires were bifilarly wound 200 turns onto the core covered with a bobbin made of 6-nylon with a thermal conductivity of 0.2 W/(m·K). A sinusoidal voltage was applied at an amplitude of 24 Vp-p and a mechanical resonant frequency of 50 Hz. The current varied from 1.5 to 2.0 A by the initial core position. The average and peak power consumption was 6.0 and 25.7 W, respectively. A pressing load was applied with two coil springs with each spring constant of 0.5 kN/m, and measured load was 0.96 kN/m. The load was detected with an optical sensor as the distance between the solenoid and outer frame. A 10-mm square tool bit with 1- or 2-mm pitch pyramids was made of tungsten carbide (JIS V3, nominal hardness: 1250 Hv, toughness KIC: 471 N/mm3/2, heat conduction: 62.8 W/(m·K)). The tool holder was skewed by 11.3º not to overlap the trails of the pyramids. Fig. 3 illustrates the working principle of the device. When the solenoid current is small, the I-core is lifted with a coil spring with a spring constant of 5.9 kN/m and its left side is detached from the E-core as shown in Fig. 3 (a). The gap length was set about 0.5 mm, which is equal to the tool vibration amplitude. With an increase of the current, the I-core is attracted toward the E-core as shown in Fig. 3 (b). Then, the shaft is pressed downward and the tool bit hits the workpiece. By repeating these motions, the tool bit is oscillated at the

Figure 1. Appearance of prototype of percussive crusher system

Reciprocation motor

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Figure 2. Structure of device

second harmonic of the driving current. 3. BASIC PERFORMANCE

3.1. Generated force

The most important performance of this device is the generated force by the tool oscillation. The impulsive force was measured with a piezoelectric load cell (Kistler 9021A, force range: 35 kN and stiffness: 3.5 kN/μm) through a 3-mm carbon steel plate. Fig. 4 (a) shows an example of applied voltage and generated force. Although the peaks of the generated force were varied, they were periodic and their maximum was 170 N. Fig. 4 (b) shows a result of the frequency analysis. The main frequency component of the impulsive force was 100 Hz. Some harmonics, especially the second one, were also observed. Fig. 5 shows vibration of the platform and the mount measured with piezoelectric acceleration sensors with a frequency range of 1 Hz to 10 kHz. The main peaks were observed when the tool bit hit and detached from the workpiece. The maximum peak was reduced from 250 to 12 m/s2. The calculated peak vibrations were 134 μmp-p on the platform and 24 μmp-p on the mount. This vibration may disturb other instruments. 3.2. Temperature of device

The temperature at various points was measured during machining. Fig. 6 illustrates measurement positions of temperature. The surface temperature was measured at the stepping motors with TO-92 type temperature IC sensors (National semiconductor, LM35DZ) with a range of 0 to 100ºC, a rise time of 7.5 s for stirred oil, and the solenoid and the tool holder just above the tool bit with thermistors (Semitec, 103JT-025) with a range

of -50 to 125ºC, a resistance of 10 kΩ and thermal time constant of 5 s. In order to improve the linearity of the thermistors, a resistor of 7.5 kΩ was inserted in series.

Tool bit

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Figure 5. Vibration isolation

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(b) Frequency domain Figure 4. Applied voltage and generated force

The sensors were glued with cyanoacrylate instantaneous adhesive. In order to prevent the damage of the coil coating due to heat, the machining was stopped when the coil temperature reached about 80ºC. The total machining time was 10 min. Fig. 7 shows an example of measured temperatures during machining a basalt block at 4.3×10-3 Pa and in air. The coil temperature reached about 75ºC after 8 min. in vacuum. Then, the machining was paused and the device was cooled only by the heat conduction through the frame for 30 min. After that, the machining was resumed for 2 min. The coil temperature rose by 47ºC. Because the temperature rise was proportional to the square of the current and heat flux was isolated with the bobbin, Joule heat at the coil caused it. The temperature at the tool rose by 2ºC though the impact between the tool and workpiece generally generates heat. Because the tool temperature gradually rose even after stopping machining in fact, the heat conduction to the shaft and

frame decreased the temperature rise. The temperature rise by the driving current of the stepping motors, which is always constant, was almost the same each other. Next, the gap between the outer teeth of the E-core and the coil was filled with woven copper wires to conduct the heat to the frame as shown in Fig. 8. Fig. 9 shows a comparison of the temperature rise with and without the woven copper wires, and in air and vacuum. The temperature rise was decreased to 26ºC after machining for 10 min by filling the gap with the woven copper wires. In air, air convection cooled the solenoid, and the heat conduction did not significantly affect the decrease of the temperature rise. 4. MACHINING PERFORMANCE

4.1. Machining conditions

In order to evaluate the strength of rocks, their Vickers

Frame

Solenoid

Wiring to thermistorCore

Woven copper wires

Figure 8. Filling into gap between housing and solenoid coil with woven copper wires

Figure 6. Measurement points of temperature

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Figure 7. Temperature rise during machining in vacuum

hardness was measured prior to the experiments. A terrestrial basalt block, which has a similar structure and strength to a lunar mare basalt rock, with 20 mm in length, 10 mm in width and 20 mm in height was mainly used for a workpiece. Fig. 10 shows the Vickers hardness of workpiece and tool materials. The hardness was fluctuated by mineral crystals and pores dispersed in the matrix. For example, basalt was from 500 to 970, and an average of 740 Hv. Because the hardness of tungsten carbide was 900 Hv at a measuring load of 19.6 N, which is harder than rock specimens, it could be used for the tool. The total machining time was 10 min. When the solenoid temperature reached about 80ºC, machining was paused once. The tool was renewed for each experiment. The maximum and arithmetical surface roughness, Rz and Ra, was measured along 3 lines both in the parallel and perpendicular directions to the workpiece reciprocation. The workpiece and tool weights were measured with a top-loading precision balance with a resolution of 0.1 mg before and after machining to measure the machining amount.

4.2. Surface roughness

Fig. 11 shows an example of machined basalt in vacuum. The appearance of machined surface in vacuum was almost the same as that in air. Trails crushed with pyramids on the tool bottom were observed because the stroke of workpiece reciprocation was short. Fig. 12 shows the relationship between machining amount and number of machining in air. At the beginning of the machining, the sharp pyramids on the tool bottom wore heavily and machining amount was steeply decreased until 6th machining. The vibration amplitude will be decreased under an over pressure. However, the machining amount was increased with an increase of the pressing load up to 7.5 N in these cases. Consequently, the pressing load was set to 7.5 N. Fig. 13 shows examples of surface profiles machined in air and vacuum. Ridges due to the reciprocating motions of the pyramids were observed in the perpendicular direction to the reciprocation in both cases. The profiles in the parallel direction to the reciprocation were smoother than that in the perpendicular one. In order to make the surface more smooth, in-plane motion will be preferable. Table 1 shows the arithmetical and maximum surface roughness after machining, machining amount and tool wear. Pores originally contained in rock specimens affected the maximum roughness. Consequently, the arithmetical roughness is preferable to evaluate machining performance of the crusher. Although the machining amount in vacuum was smaller than that in air, the roughness was the same each other. The tools seldom wore under the both conditions. Only the pyramid tips wore. This caused the initial decrease of the machining amount. Fig. 14 shows debris generated during machining in vacuum. It was gathered around the specimen. Its diameter fluctuated from several to several tens of micrometers. This trend was the same as that in air. Micro cracks, which starting points were grain boundaries, caused the machining principle with the

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Figure 10. Hardness of workpiece and tool materials

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Figure 12. Relationship between machining amount and number of machining against pressing load

Figure 11. Result of machining in vacuum (10×20mm)

crusher. Because much debris worked as shock absorber, it should be removed from the specimen surface for further machining.

Fig. 15 shows an appearance of the tool after machining in vacuum. The debris was adhered on the white area. The dark area was the exposed tool surface. Materials are easily charged by electrostatics in vacuum. Basalt is mainly composed of SiO2 (50%), Al2O3 (15%) and CaO (10%). Although insulated minerals are electrically neutral, they contain both positive and negative electric charges. When the minerals are fractured or rubbed with other materials, the electric charges released so that both polarity independently observed [17]. In addition, SiO2 shows piezoelectricity and generate charges when an external stress is applied [18, 19]. Consequently, the surface force such as electrostatic force due to electrification by the fracture of the minerals, friction or piezoelectricity caused the adhesion of debris. Granite was also machined by 45 mg in 5 minutes with the 2-mm pitch tool. Anorthosite was machined by 53 mg with the 1-mm pitch tool. 5. CONCLUSIONS

In this paper, a prototype of a percussive crusher was described. It was driven with a solenoid to remove a rock surface. Conclusions can be drawn as follows.

Figure 15. Debris on tool bit after machining in vacuum (1 mm pitch)

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(a) Machining in air

(b) Machining in vacuum Figure 13. Surface roughness of machined surface

Table 1. Surface roughness after machining, machining amount and tool wear

(a) Air Average Max. Min.

Ra μm Perpendicular 15.6 16.5 15.2Parallel 14.6 20.8 11.5

Rz μm Perpendicular 92.2 127.9 72.4Parallel 125.6 248.2 63.5

Amount: workpiece/tool mg 71.3/0.5

(b) Vacuum Average Max. Min.

Ra μm Perpendicular 15.5 25.2 7.0 Parallel 13.6 17.8 10.6

Rz μm Perpendicular 108.2 190.9 46.9Parallel 110.2 127.8 86.6

Amount: workpiece/tool mg 43.0/0.4

100 μm

Figure 14. Debris machined in vacuum

(1) This percussive crusher removed the surface of basalt.

(2) The temperature rise should be cared especially in vacuum to prevent the damage of the solenoid coil coating. The heat conduction to the frame was good for the decrease of the temperature rise at the solenoid coil.

(3) The debris from the rock adhered on the tool surface by electrostatics. It should be removed from the gap between the tool and rock for the further machining.

For the future work, the rock specimen will be fed in the x- and y-directions to obtain the isotropic surface roughness.

ACKNOWLEDGEMENTS

A part of this study was financially supported by the Support Program for Forming Strategic Research Infrastructure for Private Universities, “Sustainable Mechanical Systems Research Center,” a matching fund subsidy by Ministry of Education, Culture, Sports, Science and Technology, Japan.

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