Scholarly article on topic 'Design and experimental performance verification of a thermal property test-bed for lunar drilling exploration'

Design and experimental performance verification of a thermal property test-bed for lunar drilling exploration Academic research paper on "Materials engineering"

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{"Drilling test bed" / "Lunar drill" / "Lunar environment simulator" / "Temperature measurement" / "Thermal property"}

Abstract of research paper on Materials engineering, author of scientific article — Tao Zhang, Zeng Zhao, Shuting Liu, Jinglin Li, Xilun Ding, et al.

Abstract Chinese Chang’e lunar exploration project aims to collect and return subsurface lunar soil samples at a minimum penetration depth of 2m in 2017. However, in contrast to those on the Earth, automated drilling and sampling missions on the Moon raise the risk of burning bits. Test-beds are required for testing the thermal properties of drill tools in a lunar environment. In this paper, a novel temperature measuring method based on thermocouples and a slip ring was proposed. Furthermore, a data acquisition system for a drilling process was designed. A vacuous, cryogenic, and anhydrous soil environment simulating the lunar surface was established. A drilling test-bed that can reach a depth of 2.2m was developed. A control strategy based on online monitoring signals was proposed to improve the drilling performance. Vacuum and non-vacuum experiments were performed to test the temperature rising effect on drill tools. When compared with the non-vacuum experiment, the vacuum temperature rise resulted in a 12°C increase. These experimental results provide significant support for Chinese lunar exploration missions.

Academic research paper on topic "Design and experimental performance verification of a thermal property test-bed for lunar drilling exploration"

JOURNAL OF

AERONAUTICS

Chinese Journal of Aeronautics, (2016), xxx(xx): xxx-xxx

Chinese Society of Aeronautics and Astronautics & Beihang University

Chinese Journal of Aeronautics

cja@buaa.edu.cn www.sciencedirect.com

Design and experimental performance verification of a thermal property test-bed for lunar drilling exploration

Zhang Tao a, Zhao Zengb, Liu Shutinga, Li Jinglina, Ding Xiluna*, Yin Shenb, Wang Guoxin b, Lai Xiaoming b

a School of Mechanical Engineering and Automation, Beihang University, Beijing 100083, China b China Academy of Space Technology, Beijing 100094, China

Received 9 November 2015; revised 28 December 2015; accepted 20 January 2016

KEYWORDS

Drilling test bed; Lunar drill;

Lunar environment simulator;

Temperature measurement; Thermal property

Abstract Chinese Chang'e lunar exploration project aims to collect and return subsurface lunar soil samples at a minimum penetration depth of 2 m in 2017. However, in contrast to those on the Earth, automated drilling and sampling missions on the Moon raise the risk of burning bits. Test-beds are required for testing the thermal properties of drill tools in a lunar environment. In this paper, a novel temperature measuring method based on thermocouples and a slip ring was proposed. Furthermore, a data acquisition system for a drilling process was designed. A vacuous, cryogenic, and anhydrous soil environment simulating the lunar surface was established. A drilling test-bed that can reach a depth of 2.2 m was developed. A control strategy based on online monitoring signals was proposed to improve the drilling performance. Vacuum and non-vacuum experiments were performed to test the temperature rising effect on drill tools. When compared with the non-vacuum experiment, the vacuum temperature rise resulted in a 12 °C increase. These experimental results provide significant support for Chinese lunar exploration missions.

© 2016 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

* Corresponding author.

E-mail addresses: buaazt@gmail.com (T. Zhang), xlding@buaa.edu. cn (X. Ding).

Peer review under responsibility of Editorial Committee of CJA.

Our understanding of extraterrestrial regolith properties can be greatly enhanced by analyzing soil samples collected from celestial bodies. The method of drilling and sampling is the most direct approach to explore subsurface soil layers, and has been widely used to explore extraterrestrial soils for decades.1 Automated drilling on the Moon and samples return missions were first accomplished in 1970 by the Soviet's robotic Lunar 16 lander.2 In 1971, the Apollo Lunar Surface

http://dx.doi.org/10.1016/j.cja.2016.03.008

1000-9361 © 2016 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Drill (ALSD) was first deployed to extract soil column samples by American astronauts on Apollo 15. The Sampler, Drill and Distribution System (SD2) was developed in 2001 by the European Space Agency (ESA) for a comet exploration mission named Rosetta.5-7 The Curiosity rover was equipped with the Mars Science Laboratory (MSL) drill to collect Martian rocks and sand, and successfully landed on the Mars in 2012.8-1° For the first mission of the ESA's Aurora Exploration Program, a 2-m long multi-auger drilling sampler will be employed to collect soil at a specified depth. NASA's Mars2020 Sample Acquisition and Caching Technologies and Architectures, currently under development, will be used to acquire Mars rock samples in 2020. For Chinese extraterrestrial exploration project, lots of drilling samplers have been proposed in recent years.14-17

Most planetary drilling will be performed blindly, i.e., without any precursor seismic imaging of substrates typically used on the Earth when drilling for hydrocarbons.18 On the Earth, geologic formation drilling is a mature technology; however, extraterrestrial drilling entails challenges that are significantly more complex. In the former Soviet Union's three lunar soil sampling and return missions, Luna-20 encountered a hard formation and rebooted three times during drilling to depths of 90, 150, and 340 mm (drilling was abandoned at this depth). Similarly, Luna-24 was obstructed twice during drilling and coring.19 To reduce the risk of mission failure, a number of tests should be performed to improve the safety and reliability of automated extraterrestrial drilling samplers. Test rigs and methods have been proposed and developed in recent years. However, each testing equipment and the corresponding test method are only applicable to specific tests. For example, the LunarVader was developed to test the drilling performance of water-saturated soils that may occur in lunar polar regions.20 CRUX was designed to demonstrate the efficiencies in breaking through hard ice-soil layers.21 MARTE was designed to verify remote command sequences for future Mars explorations.21'22 DAME was developed for fully hands-off drilling, including fault dictation, recovery, and resumption of drilling.21,22 A drilling and coring test-bed was developed to validate the flexible tube coring method, which may be used in Chinese Chang'e lunar exploration.23 A drilling and sampling test-bed was designed to test various drilling and sampling parameters.24 An on-line temperature-measuring system based on fiber Bragg grating sensors was employed to test the drill bit temperature against rocks in vacuum.25

The Moon has an atmosphere so tenuous as to be considered nearly a vacuum. The gas conduction effect does not exist in lunar soil, and the thermal conductivity of the soil is much lower than that on the Earth.26,27 The temperature of the top 300-mm layer of the lunar regolith varies greatly with time in a lunar day. The maximum temperature difference of the surface nearly reaches 300 0C. However, the temperature of the lower layer dose not fluctuate and is approximately —30 0C at the lunar equator.28,29 For Chinese Chang'e lunar exploration, a 2-m sample of subsurface lunar soil will be acquired with an automated drilling sampler.30 The lunar exploration drill will work in extreme environments, i.e., airless (without convection cooling), dry (without drilling muds or lubricants), blind (no prior local or regional seismic or other surveys), and weak (very downward force on the bit).1 There is no effective way to dissipate the heat generated during the drilling process due to the poor thermal conductivity of lunar soil. Therefore,

drilling in the lunar environment may lead to higher equipment temperatures, thereby greatly increasing the risk of burning the drill bit. The temperature performance of the drill bit must be confirmed by ground tests.

This paper aims to describe the design and experimental verification of a thermal property test-bed. Section 2 shows the test-bed scheme. Sections 3 and 4 discuss the designs of a lunar environment simulator and a drilling test-bed, respectively. A control system is introduced in Section 5. Section 6 presents experimental results. Finally, Section 7 summarizes the findings of this study.

2. Scheme of the test-bed

The thermal property test-bed was designed based on the drilling requirements of a simulated lunar environment and involves three components: the lunar environment simulator, the drilling test-bed, and the control system, as shown in Fig. 1. The lunar environment simulator contains a vacuum chamber and a pump system (to simulate a lunar vacuum environment), a cooling and heating system (to control the environment temperature), and a specimen holder. The drilling test-bed was developed for testing drilling in a simulated lunar environment. The control system is divided into three subcontrollers, i.e., the vacuum pump controller, the cooling and heating controller, and the drilling controller. Each subcontroller is individually controlled.

3. Lunar environment simulator

The thermal properties of a powder or granular material are highly dependent on ambient gas pressures. This is because a significant amount of heat within the powder is transferred by gaseous conduction within the inter-particle space. Only a relatively small amount of heat is transferred by conduction through soil particles.31,32 However, the approximately 10—10 Pa pressure on the surface of the Moon is impossible to achieve on the Earth.26 For pressures below 10 Pa, Bernett et al.,33 Wechsler and Glaser,34,35 Fountain and West,36 and Tien and Nayak37 showed that the thermal properties of a powdered material were independent of the ambient pressure. Therefore, to simulate the vacuum conditions of the Moon, thermal property measurements were conducted at pressures below this 10 Pa.

A general view of the lunar environment simulator design is presented in Fig. 2. The simulator is comprised of a vacuum pump system, a vacuum chamber, a specimen holder (inside the chamber), and a cooling and heating system. Two vacuum gauges were used for the measurement of the pressure inside the chamber.

3.1. Vacuum pump system and vacuum chamber

The vacuum pump system is comprised of an oil diffusion pump (1.8 x 104 L/s pumping speed at 6 x 100Pa), a roots pump (6 x 102L/s pumping speed at 5 x 102Pa), and three rotary-vane vacuum pumps (7 x 101 L/s pumping speed at 1 x 105 Pa).

The vacuum chamber is comprised of three components as shown in Fig. 3: the top component holds the drill tool, the middle component holds the specimen holder for drilling

Fig. 1 Scheme of the thermal property test-bed.

Fig. 2 Lunar environment simulator.

experiments, and the bottom pedestal component supports the chamber. The vacuum chamber is evacuated from the pedestal, which has been shown to be more efficient in reducing dust clouds when operating.

3.2. Specimen holder, cooling and heating system

Fig. 3 Diagram of the vacuum chamber (perspective view).

The temperature on the Moon equator surface varies from — 180 °C to 130 °C, while the soil temperature at a 300 mm depth is nearly constant at —30 °C. A 7.5 kW refrigerating machine utilizing silicone oil as refrigerant has a minimum temperature of —55 °C. A hermetically sealed copper cooling coil was interwoven on the outside of the specimen holder except the top 300 mm. Three 200 W iodine-tungsten lamps were mounted just above the specimen holder to heat the surface of soil samples up to a maximum temperature above 250 °C. Details of the specimen holder are shown in Fig. 4.

The diameter and depth of the specimen holder are 500 mm and 2050 mm, respectively. The holder was mounted above a slewing bearing that can rotate 120°. Therefore, there are at least three drilling positions, each separated by 60°.

To minimize errors due to heat conduction along temperature sensors, very thin (1 mm in diameter) platinum resistance thermometer elements (Pt100) were strung lengthwise through the specimen holder. To ensure the accuracy of the temperature field measurement, three temperature strings were placed in the simulated lunar soil. Each string consists of ten platinum resistors, at 200 mm intervals along the string. The first temperature string is located at the center of the specimen holder, the second string at a 150 mm radial distance from the center, and the third string at a 250 mm radial distance from the center (i.e., on the inside wall of the specimen holder).

Holes of a considerable size were drilled on the side wall of the specimen holder to more readily evacuate gas molecules from the soil. Three layers of stainless steel mesh, 40, 600,

Fig. 4 Details of the specimen holder.

Fig. 5 Schematic diagram of the drilling test-bed.

and 800 mesh (from inside to outside), were used on the inside wall of the specimen holder to prevent leaking of lunar soil simulant.

4. Drilling test-bed

A drilling test-bed scheme was designed based on test requirements. Fig. 5(a) shows the mechanical system of the drilling test-bed, which is comprised of a rotary-percussive drilling mechanism, a penetrating mechanism, a magnetic fluid and metallic bellow sealing mechanism, a drill tool, and a slip ring.

The combination of a magnetic fluid and a metallic bellow sealing transmission (as shown in Fig. 5(b)) was used to ensure penetrating and rotary sealing performances. This sealing approach has great advantages because it separates the penetrating motion from the rotary motion. The magnetic fluid sealing element consists of an inner ring, an outer ring, and a magnetic fluid between them. The outer ring was mounted to the vacuum chamber and the inner ring to the bottom end of the metallic bellow. The drill pipe was enveloped by the metallic bellow, which could be stretched over 2.6 m and compressed to less than 0.6 m. The top end of the metallic bellow

Table 1 Signals monitored and variables of the drilling test-bed.

Source Variables/signals

Penetrating motor encoder Rotary motor encoder Percussive motor encoder Thermocouple Axial-Torsion load cell Position switch Penetrating depth Rotary speed Percussive frequency Temperature on the drill tool Penetrating force and torque on the drill tool Penetrating start position

Fig. 6 Scheme diagram of the rotary-percussive mechanism.

was mounted to and sealed with the drill pipe, while the bottom end was sealed with the inner ring of the magnetic fluid sealing element. During the drilling process, the top end penetrated and rotated with the drill pipe, but the bottom end only rotated with the drill pipe. Thus, the metallic bellow was compressed by the motion of the top end, and the penetrating motion was sealed. The rotary motion of the inner ring was sealed by the magnetic fluid inside the magnetic fluid sealing element.

As referenced in the drilling test-bed scheme, Table 1 shows the monitored signals and variables. The rotary-percussive drilling mechanism, the penetrating mechanism, and the drilling toll are introduced.

4.1. Rotary-percussive drilling mechanism

The rotary-percussive drilling mechanism is comprised of two independent actuators for the rotation of the auger and the drive of the percussive system. The use of two actuators enables three distinct drilling modes: rotary, rotary-percussive, and percussive. This flexibility is particularly advantageous when drilling in a variety of formations with different strengths. In softer formations, the rotary-only mode could be used to save energy. In harder formations, the rotary-percussive mode could be engaged for a lower weight on bit (WOB).38,39

Fig. 6 shows the rotary-percussive mechanism of the drilling test-bed. The rotary motor provides rotary torque to the drill tool via a pair of spur gears and a spline. The percussive mechanism produces motion via a rocker bearing and an air cylinder. The rocker bearing driven by the percussive motor and the air cylinder pushes the percussive mass by compressing air inside the cylinder. The percussive frequency can be controlled by modulating the rotational speed of the percussive motor. The rotary and percussive torque signals are collected by the control system, and their motions can be terminated when a fault occurs.

4.1.1. Rotary mechanism

To test the adaptability and safety for drilling in extreme lunar rock environments, the rotary motor capability was extended to satisfy different drilling conditions. The 3 kW rotary motor uses a pair of gears (2:1) with a gearhead ratio of 5:1. For highspeed outputs, the motor drives the auger at a 200 r/min rated speed with a torque of 112.5 N-m. For high-torque outputs, the motor drives the auger at a 160 r/min rated speed with a

torque of 179.0 N-m. A high-torque output is sufficient for drilling in lunar soil, lunar rock, or a mixed environment. The following equations are used to calculate the driving torque and rotary speed of the auger:

Td = Tr i'si'rg

where Td is the driving torque of the auger, Tr is the rated torque of the rotary motor, is is the reduction ratio of the spur gears, ir is the reduction ratio of the gearhead, na is the rated rotary speed of the auger, nrr is the rated rotary speed of the rotary motor, and g is the efficiency of the gear drive.

4.1.2. Percussive mechanism

The percussive mechanism reduces the WOB and energy consumption. This is especially important in a low-g environ-ment.40 As shown in Fig. 7, the percussive mechanism is comprised of the following components: a percussive motor, a rocker bearing, and an air cylinder.

The percussive motor directly drives the rocker bearing from 0 r/min to 3000 r/min with a percussive frequency varying from 0 Hz to 50 Hz. The air cylinder was pulled by a pendulum rod moving up and down. When the air inside the cylinder was compressed, the percussive mass impacted the top of the drill tool. The power of the percussive motor was

Fig. 7 Scheme diagram of the percussive mechanism.

400 W. The air cylinder had a 50% efficiency; therefore, a maximum of 200 W could be delivered to the drill tool (the remaining 200 W was lost through friction and dissipated as heat).

4.2. Penetrating mechanism

As shown in Fig. 8, the penetrating mechanism is comprised of a penetrating motor, a ball screw, four sliders, and two rolling guides. The ball screw was employed to advance and retract the drill bit in and out of the regolith surface. The test-bed enabled a penetration of at least 2 m to sample subsurface soil in a simulated lunar environment; therefore, the nominal stroke of the rolling guide was 2.2 m. The maximum penetrating force and speed are 2000 N and 500 mm/min, respectively.

The penetrating speed and maximum output torque of the penetrating mechanism are calculated as follows:

Tmax Tnipgggb

where vp is the actual penetrating speed, nrp is the rotational speed of the penetrating motor, p is the ball screw lead, ip is the reduction ratio of the gearhead, Tmax is the maximum output torque of the ball screw, Tn is the nominal output torque of the penetrating motor, gg is the efficiency of the planetary gear-head, and gb is the efficiency of the ball screw.

4.3. Drill tool

The drill tool was tested inside the vacuum chamber, whereas the other components of the drilling test-bed were tested outside. As shown in Fig. 9, a special drill tool construction was designed to transmit motion and sensor signals. The components of the drill tool include: (1) a slip ring — to pass electrical power or signals across a rotating interface (from sensors inside the drill tool to a data acquisition system); (2) a drill pipe — to transmit penetrating, rotary, and percussive motions and to seal the vacuum chamber (in combination with the magnetic fluid and metallic bellow sealing mechanism); (3) a seal joint — to separate vacuum and non-vacuum environments (the drill pipe is hollow to enable signal wires to pass through; the inside of the drill pipe is a non-vacuum environment and three avia-

Fig. 8 Scheme diagram of the penetrating mechanism.

Fig. 9 Components of the drill tool.

tion plugs are used to seal the end of the drill pipe); (4) an axial torsion load cell — to measure the penetrating force and rotary torque in the same cell; (5) a drill joint — to connect the auger and the axial torsion load cell; and (6) an auger — with a drill bit to penetrate up to a 2-m depth.

The auger length is just over 2.5 m and the internal diameter of the coring pipe is 15.4 mm. Because the auger penetrates up to 2 m, the total volume of soil sampled is up to approximately 3.7 x 105 mm3. The drill pipe is 3.245 m in length and is hollow to pass signal wires.

The distance between the auger and the coring pipe is only 1.75 mm. Special customized nickel chromium-nickel silicon thermocouples were adopted for measuring the temperature of the drill tool. The customized thermocouples have a 0.5 mm diameter and are adhered to the inside wall of the auger using a phosphoric acid-cuprous oxide inorganic adhesive. The maximum temperature resistance of this adhesive is 1000 0C, which is sufficient for lunar drilling conditions. The sensors made continuous contact with the auger during drilling operation, and a continuous temperature distribution could be obtained. The coring pipe rotated with the auger during the drilling process.

To measure the temperature of the drilling bit, three fine 1 mm holes were drilled to insert thermocouples. The intervals were filled with the adhesive for better heat transfer. Eight thermocouples were fixed at different positions to obtain a temperature distribution of the drill tool (as shown in Fig. 10): Temp. 1 is near the cutting edge of the bit and the maximum tool temperature likely occurs at this position; Temp. 2 is to measure the temperature along the inside wall of the bit; Temp. 3 is to measure the temperature along the contact surface of the auger and the bit; Temp. 4 is to measure the temperature of the lowest point of the coring pipe; Temp. 5 is to measure the temperature of the lowest point of the inside wall of the auger; Temp. 6 is located 100 mm from Temp. 5 to measure the temperature along the inside wall of the auger; Temp. 7 is located 50 mm from Temp. 6 to measure the temperature along the inside wall of the auger; and Temp. 8 is located 50 mm from Temp. 7 to measure the temperature along the inside wall of the auger.

_ nrpp

v„ =

Fig. 10 Structure and temperature measuring positions of the drill tool.

5. Control system

The control system of the thermal property test-bed is divided into three subsystems: the vacuum pump control system, the cooling and heating control system, and the drilling control system. For convenience, each subsystem was designed to run independently.

5.1. Vacuum pump control system

The pump controller roughly follows the structure shown in Fig. 11. The controller integrates a PLC for multiple arrangements of pumps and electromagnetic valves, an air compressor to drive valves, a forced air cooler to supply water coolant, and an interface for communication with a PC. A diffusion pump is combined with rotary-vane pump 3 and used for maintaining a high vacuum. Rotary-vane pump 2 serves as a backup for rotary-vane pump 1 to compensate for damages caused by soil dust at pump startup. During the rough vacuum phase, only the rotary-vane pumps were used, and valves 1, 2, and 4 were opened. During the high vacuum phase, the rotary-vane pumps and the diffusion pump were engaged, and valves 1, 3, and 4 were opened. The roots pump was employed during the transition between the rough and high vacuum phases; valves 1 and 2

Fig. 11 Vacuum pump control system.

were opened during the transition. The temperature of the coolant was maintained by the forced air cooler at 22 0C, and the water flow rate for cooling the pumps was 70 L/min. The vacuum pump system specifications are shown in Table 2.

5.2. Cooling and heating control system

Fig. 12 shows the cooling and heating control system of the test-bed used to simulate the lunar temperature environments. The components of the controller include: a PLC — central control; a vacuum gauge and a vacuum head — for measuring the air pressure inside the chamber; three temperature strings — for measuring the soil temperature distribution; a temperature data logger — for acquiring temperature signals; three iodine-tungsten lamps — for heating the soil surface to simulate the lunar temperature at mid-day; a fluid reservoir — for coolant storage; a circulator pump — to circulate the coolant in the closed copper coil; a heat exchanger — to transfer heat between the coolant and the refrigerator; a heating element — to raise the coolant to room temperature at the end of an experiment; a refrigerator — to transfer heat from inside the fridge to the external environment so that the inside

of the fridge is cooled to a temperature below the ambient temperature of the room; and a forced air cooler — to cool the refrigerator.

The specifications of the cooling and heating system are shown in Table 3.

5.3. Drilling control system

Fig. 13 shows the drilling control system of the test-bed, which is mainly consisted of motors and sensors in the drilling test-bed, a controller, a PC, and a power supply. The motors in the drilling test-bed include a penetrating motor, a percussive motor, and a rotary motor. The sensors in the test-bed include an axial-torsion load cell to measure the WOB and torque, a position switch, and thermocouples for measuring the drilling temperature. Corresponding motor drivers, a power source, an A/D converter, and a data logger are also included in the controller. The motion coordinator was programmed to control and monitor the corresponding motors during the drilling process. Force, displacement, temperature, and motor current signals were delivered to the PC and stored in Excel data files. The specifications of the drilling test-bed are shown in Table 4.

Table 2 Specifications of the vacuum pump

system.

Item Range

Pump capacity (Pa) 2.5 x 10-4

Pumping speed (L/s) 1 x 104

Air pressure (MPa) 0.8

Water flow rate (L/min) 70

Coolant temperature (OC) 22

Table 3 Specifications of the cooling and heating system.

Item Range

Minimum temperature (°C) -55

Maximum temperature (°C) 250

Refrigerating power (kW) 7.5

Heating power (W) 600

Water flow rate (L/min) 70

Coolant temperature (°C) 15

Fig. 12 Cooling and heating control system.

Fig. 13 Drilling control system.

Table 4 Specifications of the drilling test-bed.

Item Range

Penetrating speed (mm/min) 0- 500

Rotary speed (r/min) 0- -500

Percussive frequency (Hz) 0- -50

Percussive energy (J) 0- -3

Penetrating force (N) 0- 2200

Rotary torque (N m) 0- -80

Penetrating power (W) 200

Percussive power (W) 400

Rotary power (kW) 3

6. Experimental study

A lunar soil simulant, supplied by the China Academy of Space Technology (CAST), was used as the drilling medium. Using safe drilling practices, a flexible drilling control algorithm was proposed based on adapting the penetrating force to the complex drilling medium.

6.1. Lunar soil simulant

The lunar soil simulant is a terrestrial material synthesized to approximate the chemical, mechanical, and engineering properties, and the mineralogy and particle size distribution of lunar regolith.41 The primary ingredients of the soil are anorthosite, basalt, pyroxene, ilmenite, and volcanic glass. The lunar soil simulant was stored in closed containers, and the moisture content was maintained below 0.1%. The chemical and mineral compositions of the lunar soil simulant are shown in Table 5.

The particle sizes of the lunar soil simulant are shown in Table 6. The density of the loose lunar soil simulant is 1265 kg/m3. Previous studies indicated that packing density had a significant effect on the temperature properties of drill tools.42,43 Thus, to simulate drilling in an extreme lunar regolith, a high packing density of 1910 kg/m3 was used. The temperature strings were fixed inside the specimen holder prior to drilling.

To control the packing density of the lunar soil simulant, a certain amount of material was loaded in the specimen holder

Table 5 Chemical and mineral compositions of the lunar soil simulant.

Item Percentage (%)

Chemical composition SiO2 TiO2 Al2O3 FeOT MnO MgO CaO Na2O K2O P2O5 Others

48.32 2.38 16.01 12.50 0.15 6.95 7.39 0.19 2.12 0.54 0.19

Mineral composition Anorthosite Peridotite Opaque mineral Pyroxene Volcanic glass

59.4 16.5 14.1 4.5 5

Table 6 Particle size range of lunar soil simulant.

Particle size (mm) <0.01 0.01-0.025 0.025-0.05 0.05-0.075 0.075-0.1 0.1-1 1-2 2-4

Percentage (%) 13.36 11.84 12.73 7.89 5.08 32.35 5.43 3.68

Particle size (mm) 4-10 10-11 12-13 14-15 16-17 18-19 20-29 30-41

Percentage (%) 4.18 0.4 0.33 0.34 0.26 0.21 0.98 0.94

Fig. 14 Lunar soil simulant for drilling experiments.

and vibrated for an hour with a 3-dimensional vibrator until no further height decrease of the material could be noted. This operation was repeated until the specimen holder was full of the lunar soil simulant. Fig. 14 shows the lunar soil simulant prepared for the drilling experiments.

6.2. Control principle

The torque and penetrating force are the most important signals in an autonomous drilling process. In most circumstances, rotary-only drilling is used in softer formations, and the percussive actuator is activated when encountering harder formations. The drilling algorithm was based on the drilling signals monitored by a real-time data acquisition (DAQ) system, as

shown in Fig. 15. A filter was designed to suppress noise in the signals. The drilling WOB Ft, the motor current Q and torque T, the penetrating speed vpi, the rotary speed nri, and the percussive frequency f were stored in an on-line database.

For the WOB routine, the penetrating force was fed into the drilling algorithm. Ten data points per second, Ft, Fi_1,..., F_9, of the penetrating force were acquired, and a running average Faver of the 10 most recent data points was calculated to reflect variations in the drilling formation. When Faver was greater than the expected 250 N, the drilling strategy would stop the penetrating actuator for 10 s to maintain the WOB below a safe value. The motor torque T (a running average Taver) and current Ci (a running average Caver) were also fed into the control strategy for similar protection strategies.

6.3. Experiment and result analysis

The drilling parameters in a preliminary experiment were as follows: a rotary speed of nr = 120 r/min and a penetrating speed of vp = 130mm/min. These drilling parameters were optimized in an earlier study by CAST. The preliminary experiment studied the differences between temperature rises in vacuum and non-vacuum environments, and thus the lunar soil simulant was not cooled or heated.

When the lunar soil simulant was not placed inside the vacuum chamber, 55 min were required to achieve 9.5 x 10_2 Pa (vacuum gauge 2) and 1.7 x 10_1 Pa (vacuum gauge 1) pressures, as shown in Fig. 16. The pressures in vacuum gauges 1 and 2 were nearly the same as before, i.e., 2 x 10_1 Pa. An obvious change was observed when a new pump was started.

Fig. 15 Process of the autonomous drilling control.

\ ---Vacuum gauge 1 \ -Vacuum gauge 2 \ Rotary-vane pump

Roots pump

Diffusion pump

10 20 30 40 Time (min)

Fig. 16 Pressure curve inside the vacuum chamber without the soil simulant.

6.3.1. Non-vacuum experiment

A 2000.92 mm depth was reached in 16.45 min. Fig. 17 shows the mechanical data during the experiment. The sampling frequency was 10 Hz, which reflected an accurate force variation during the entire drilling process.

Fig. 18 shows the temperature data of the drill tool in the non-vacuum experiment. The initial temperature was approximately 25 0C, and the bit temperature (Temps. 1-3) rapidly increased to 33 0C within two min. During the entire process, the temperature difference between the bit and the auger was below 5 0C except for certain peak points. The coring pipe temperature (Temp. 4) and the auger temperature (Temps. 5-8) changed with the bit temperature (Temps. 1-3) and always

0 2 4 6 8 10 12 14 16

Time (min)

Fig. 17 WOB and torque of the drill tool in the non-vacuum experiment.

Temp. 1

----Temp. 2

_.......Temp.3

-----Temp. 4 , t\ ¿Nw^ I\ / A \ / A 1 __/' Ij Vs7..'

i i i i i i 1 1

0 2 4 6 8 10 12 14 16

Time (min)

0 2 4 6 8 10 12 14 16

Time (min)

Fig. 18 Temperatures of the drill tool in the non-vacuum experiment.

105 10" 103 10; 10' 10"

--Vacuum gauge 1 -Vacuum gauge 2

|Rotary-vane pump

■ I Roots pump

Ditfusion pump

120 180 Time (min)

Fig. 19 Pressure curve inside the vacuum chamber with the soil stimulant.

lagged behind. This trend followed the heat transfer trend. In this experiment, the maximum temperature was 45 oc as observed for Temp. 1.

6.3.2. Vacuum experiment

The chamber was evacuated to 8 Pa in approximately 4.5 h (vacuum gauge 1), as shown in Fig. 19. The atmosphere was always maintained below 9 Pa during the drilling process. In this experiment, a turning point was not observed when the diffusion pump was started.

Fig. 20 shows the mechanical data series of the drill tool in the vacuum experiment. In the non-vacuum experiment, a 2000.92-mm depth was reached within 16.45 min, and the average penetrating speed was 121.64 mm/min. However, a 1857.12 mm depth took 20.55 min to reach, and the average

Time(min)

Fig. 20 WOB and torque of the drill tool in the vacuum experiment.

Fig. 21 Temperatures of the drill tool in the vacuum experiment.

penetrating speed decreased to 90.37 mm/min in this vacuum experiment. According to the control principle, when the WOB was greater than the expected 250 N, the drilling controller would stop the penetrating actuator for 10 s to maintain the WOB below the safe value. When compared with the non-vacuum experiment, more time was taken to reach the final depth in this experiment because the WOB was higher in the final 4 min, necessitating more frequent stops of the penetrating motor.

Fig. 21 shows the thermal data of the drill tool in the vacuum experiment. The initial temperature was approximately 24 oc. However, the temperature of the bit (Temps. 1-3) increased to 45 0C within two min. The coring pipe temperature (Temp. 4) increased slowly with depth during the entire drilling process. In the final 5 min, the WOB increased due to the presence of hard formations (likely a rock inclusion), and hence, the specific energy increased. This increased the heat input from the drilling process, which could not be efficiently dissipated by the lunar soil simulant. During the drilling experiments, the maximum temperature was 136 0C for Temp. 1. In the vacuum experiment, when compared with the non-vacuum environment results, the average temperature of the drilling bit was approximately 12 oc higher regardless of the presence of a hard formation.

7. Conclusions

(1) To verify the thermal properties of drill tools for lunar exploration, this paper proposed a thermal property test-bed. The test-bed is consisted of a lunar environment simulator for the vacuous, cryogenic, and anhydrous Moon surface environment, and a drilling test-bed for penetrating at least 2 m in a simulated lunar environment.

(2) A method for simulating the lunar environment was proposed. A vacuum chamber and pump system was designed to achieve a near-vacuum environment, iodine-tungsten lamps were employed to heat the soil surface to 250 oc, a refrigerating machine utilized silicone oil to cool the soil to _40 oc, and temperature strings were embedded into the lunar soil simulant to measure the soil temperature online.

(3) A new drilling testing method was proposed. In this method, a drill pipe was designed to transmit penetrating, rotary, and percussive motions, as well as to seal the vacuum chamber (when combined with a metallic bellow and a magnetic fluid). The penetrating and rotary motions are sealed by the metallic bellow and the magnetic fluid, respectively. A control strategy based on the online monitoring signals was employed to enhance the performance of the drill.

(4) A novel temperature measuring method for drill tools was proposed. The proposed temperature measuring system used thermocouples integrated with the bit or on the inside auger wall. The thermocouples were positioned as close as possible to the cutting edge or in other positions to measure the temperature of the drill tool. To collect and transmit data, aviation plugs, a slip ring, and a temperature data logger were incorporated into the test-bed.

(5) Preliminary experiments results showed that the temperature of the drill tool was influenced by the WOB. In a vacuum experiment, the average temperature of the drilling bit was approximately 12 oC greater than that in a non-vacuum environment regardless of the presence of a hard formation. Vacuum must be considered as one of the important factors in the drilling process on the lunar surface.

Acknowledgments

The research was supported by the China Academy of Space Technology (CAST). The authors would like to thank CAST for their help and kind cooperation on the system designs and tests.

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Zhang Tao is a Ph.D. student in the School of Mechanical Engineering

and Automation at Beihang University. He received his B.S. degree in

Aircraft Manufacturing Engineering from Beihang University in 2008.

His research interests include extraterrestrial planet unmanned sampling.