UNIVERSITY OF OKLAHOMA GRADUATE COLLEGE
DESIGN AND IMPLEMENTATION OF A LOW-COST ROCK ABRASION TOOL
A THESIS SUBMITTED TO THE GRADUATE FACULTY in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE
By MAYANK MURARKA Norman, Oklahoma 2006 DESIGN AND IMPLEMENTATION OF A LOW-COST ROCK ABRASION TOOL
A THESIS APPROVED FOR THE SCHOOL OF AEROSPACE & MECHANICAL ENGINEERING
BY
Prof. David P. Miller
Prof. Yunjun Xu
Prof. Sesh Commuri c Copyright by MAYANK MURARKA 2006 All Rights Reserved. DEDICATION
This dissertation is dedicated to my parents for their mentorship, love and support all through my life.
To Dr. Miller, for his patience with me throughout this work and its documentation.
iii ACKNOWLEDGEMENTS
I would like to thank my parents for giving me moral support and instilling in me values which kept me on course. I want to thank Dr. Miller without who’s guidance this thesis would not have taken the present shape. He helped me think creatively to develop new ideas and also made me realize how important theoretical and practical backing is in the real world. I also want to thank my friends Matt Roman, Tim Hunt, Alois Winterholler and Amit Iyer who in their own way have impacted this work giving bits and pieces here and there. I am also thankful to Billy Mays and Greg Williams for providing the machine shop facilities and being very flexible with it. Also want to thank NASA Ames Research Center for supporting this thesis work in part.
iv CONTENTS
DEDICATION iii
ACKNOWLEDGEMENTS iv
LIST OF TABLES viii
LIST OF FIGURES ix
ABSTRACT xi
1 Introduction 1
1.1 Scientific Instruments for Mars Exploration ...... 2
1.2 The Role of Rock Abrasion in Planetary Science ...... 3
1.3 Previous Rock Abrasion Systems ...... 4
1.4 Creating the OU RAT ...... 7
1.5 Organization of This Thesis ...... 8
2 Design Requirements for the OU RAT 10
v 2.1 Low-Cost System ...... 11
2.2 Time & Energy Efficient Abrasion ...... 12
2.3 Passive Compliance ...... 19
2.4 Summary of Requirements ...... 21
3 Design Concepts for OU RAT Components 22
3.1 Modules of the OU RAT ...... 22
3.2 The Abrasion Bit ...... 23
3.3 The Locking Compliant Joint ...... 24
3.3.1 Tooling Ball and Solenoid ...... 25
3.3.2 Spherical Bearing and Solenoid ...... 27
3.3.3 Split ball and Solenoid ...... 29
3.3.4 Spherical Bearing with Link Lock Mechanism ...... 31
3.4 The Force on Bit Module ...... 32
3.4.1 Flexishaft and Spring ...... 32
3.4.2 Dremel Motor and Spring ...... 34
3.4.3 Servo Motor and Lead Screw ...... 35
4 Final Design of OU RAT 37
4.1 The Abrasion Bit ...... 37
4.2 Locking Compliant Joint ...... 39
4.2.1 Flexishaft Vs Dremel Motor ...... 40
4.2.2 Tooling Ball Clamp ...... 43
vi 4.3 Force on Bit Module ...... 44
4.4 Calculations for Implemented Design ...... 46
4.4.1 Centering Spring ...... 47
4.4.2 Locking of the Ball Joint ...... 48
4.4.3 Lead Screw Mechanism ...... 52
4.5 Testing of OU RAT ...... 55
4.5.1 Revised Design Calculations ...... 57
4.6 Controlling the OU RAT ...... 61
5 Comparison of OU RAT to other RATs 63
5.1 The MER RAT ...... 63
5.2 The Ng RAT ...... 65
5.3 Performance of the OU RAT and other RATs ...... 66
6 Conclusions and Recommendations for Future Work 69
6.1 Conclusions ...... 69
6.2 Recommendations for Future Work ...... 70
Bibliography 71
vii LIST OF TABLES
2.1 Performance of Dremel Bits as compared to Ng RAT ...... 17
5.1 Comparison of different RATs with OU RAT ...... 68
viii LIST OF FIGURES
1.1 Rock Abrasion Tool developed by Honeybee Robotics [18] ...... 5
1.2 Rock Abrasion Tool developed by Dr. T. C. Ng ...... 6
2.1 Paint removed by Ng RAT with rock sitting on it ...... 13
2.2 Paint removed by Ng RAT sitting on the rock ...... 14
2.3 Paint removed by 4.8V cordless dremel ...... 15
2.4 Dremel Bits used in tests ( From L to R):Silicon Carbide Bit, Tungsten
Carbide Cutter, Diamond Tip, Chain Saw Sharpening Tool ...... 16
2.5 Illustration of advantage of Passive Compliance in RAT ...... 20
3.1 Conceptual Designs for Abrasion Bit ...... 25
3.2 Tooling Ball and Solenoid used for Locking Compliant Joint . . . . . 26
3.3 Ball-Joint Spherical Bearing and Solenoid used for Locking Compliant
Joint ...... 28
3.4 Split Ball and Solenoid used for Locking Compliant Joint ...... 29
3.5 Spherical Bearing with Link Lock Mechanism actuated by Solenoid . . 31
3.6 Flexishaft with solenoid ...... 33
ix 3.7 Dremel motor with solenoid ...... 34
3.8 Servo Motor applying grinding force ...... 36
4.1 Abrasion Bit ...... 39
4.2 Schematic Diagram for force on bit Module for Flexishaft and Dremel
Motor ...... 41
4.3 Model of the implemented RAT system ...... 45
4.4 Free body diagram of the Centering Spring ...... 47
4.5 Free body diagram for clamping the ball joint ...... 49
4.6 Experiment to determine the Stall Torque of Dremel ...... 50
4.7 Force from solenoid to lock the ball joint ...... 52
4.8 Force Duty Cycle graph of solenoid (reproduced from [5]) ...... 53
4.9 Lead Screw Mechanism ...... 54
4.10 Revised Calculation for Centering Spring ...... 58
4.11 Revised Calculation for Locking Ball Joint ...... 59
4.12 Modification for Locking Ball Joint ...... 61
5.1 Mechanical Design of HB RAT (reproduced from [20]) ...... 64
x ABSTRACT
This thesis describes the design and fabrication of a Low-cost Rock Abrasion Tool (RAT). The RAT on the Mars rovers Spirit and Opportunity used in the MER mission were developed for performing on Mars and were quite expensive. Such RATs would be cost-prohibitive for prototype testing on earth for experimentation and follow-on research. So, the OU RAT was mainly developed as a low cost tool which can be used for research and experimentation. The rocks on Mars have the key to its geology and studying it can throw light on whether life existed on Mars or not. But the surface on the rocks, over the years, develop rock varnish and this can be removed by the RAT so that fresh rock can be exposed and examined. The implemented OU RAT was cheap and took less time to remove rock varnish than some other low-cost RAT and also had some passive compliance which in turn allowed better contact with the rock while removing varnish. The work ends with some improvements and further research suggestions.
xi Chapter 1
Introduction
Space and Planetary exploration has been of great interest to mankind right from the days of Yuri Gagarin to Neil Armstrong to present missions on Mars. In 2000
NASA outlined its vision for Mars exploration for the next two decades [23]. It expressed its focus on sending orbiters, rovers, landers and sample return missions to Mars and is poised to “unravel the secrets of the red planet’s past environments, the history of its rocks, the many roles of water and, possibly, evidence of past or present life.” In lieu of this vision it planned the Mars Odyssey Orbiter in 2001
[22], twin Mars Exploration Rovers (Spirit and Opportunity) in 2003 [16] and Mars
Reconnaissance Orbiter in 2005 [13]. In another document released, NASA detailed its 2003 strategic plan [15] in which one of the goals was “Explore the solar system and the universe beyond, understand the origin and evolution of life, and search for evidence of life elsewhere.” These documents express increasing focus for research on explorational devices for planets in general and Mars in particular.
1 1.1 Scientific Instruments for Mars Exploration
The 2003 MER mission sent two rovers, Spirit and Opportunity, to Mars which were instrumented for the purpose of Mars exploration. They carried the Athena science investigation package for probing into the geology of Mars [1]. The primary objec- tives of having such a science payload was to understand the geological and biological environment of the earth’s neighbor. The MER rovers were supposed to determine the morphology and mineralogy of the surface and understand the nature and origin of volcanic deposits which might also help understanding the ambient conditions.
The Athena payload comprised of a suite of scientific instruments for geological ex- ploration. It had a Panoramic Camera (Pancam) and a Miniature Thermal Emission
Spectrometer (Mini-TES) for remote sensing capabilities and an Instrument Deploy- ment Device for in-situ testing. The Instrument Deployment Device in itself had a
Microscopic Imager (MI) for close-up imaging, an Alpha Particle X-Ray Spectrome- ter (APXS) for elemental chemistry, a Mssbauer Spectrometer (MB) for mineralogy of ferric materials, and a Rock Abrasion Tool (RAT) for removing dust and exposing fresh rock surfaces. The Pancam and Mini-TES would identify key sites or surfaces of interest and then the instrument deployment device would collect more data - close-up images using the MI and spectral data from APXS and MB. This collec- tion of data would be done before and after using the RAT. From the differences in readings observed, the formation of crust on the rock could be inferred.
2 1.2 The Role of Rock Abrasion in Planetary Science
Mars is largely dominated by volcanic materials and these deposits are not stable in the presence of water. With years of aeolian erosion and deposition the spectral data obtained from orbital missions is also blurred. Chemical alterations occur only locally by ground water interactions which cannot be derived by analysis of spectral data from orbiters.
The Athena science package has a number of instruments which closely take im- ages of the surface of the rock and process the data for determining the composition and properties of the surface but none of them “see” the inside of the rock. The
Martian rocks contain a surface of dust similar to rust found on steel and, just as rust is not same as steel, the dust on Martian rocks has a composition different from that of the rocks. Previous Mars missions, Viking and Pathfinder, have shown that
Mars is a dirty place. One of the major discoveries of Pathfinder was that the rocks on Mars have weathered with time from exposure to the atmosphere and the strong martian winds [2]. In this case, even if there was no dust, the composition of the surface of the rock would not be the same as that of the interior of the rock. It’s the interior composition which has the key to the geology of martian soil.
Geologists on earth have for years been using a rock hammer to get to the inside of a rock. On Mars the Rock Abrasion Tool does the job of the rock hammer.
Bringing back samples of rock from Mars for examination on Earth or sending a human geologist on Mars is expensive and cost prohibitive. So having a tool like the
3 RAT is the next best thing. The RAT uses a grinding wheel to expose fresh rock by removing the dust, varnish and weathering rinds from the rock surfaces. The interior on the rock is the part of the rock which hasn’t changed since it was formed many years ago. Accessing this can provide information about the geology of Mars and the formation of rocks.
1.3 Previous Rock Abrasion Systems
One of the earlier rock surface removal tools was demonstrated in 1991 as part of the Rocky IV rover at the Jet Propulsion Laboratory. It had a rock chipper which impacted on the rock surface in an oscillatory motion to remove weathered coverings on rocks [21]. Though this process removed the outer crust on the rock effectively it marred the surface finish so much that further examination by a science package was not feasible. So further research was not pursued after its demonstrations in
1991 [11].
The Twin Rover MER Mission in 2003 carried a Rock Abrasion Tool (RAT) developed by Honeybee Robotics in New York as part of the Athena science payload.
These RATs were the size of a soda can and cost a little over 1.5 million dollars to put one, on each of the rovers Sprit and Opportunity. The Honeybee RAT [18] used a diamond tipped grinding tool which could remove dust, rock varnish or other outer layers off the rock to expose fresh rock on the inside. It had three actuators: one of which rotated at high speed and was used for the two diamond tipped grinding bits;
4 another motor rotated the two grinding bits about a central axis so as to make a circular finished surface on the rock; a third motor controlled the movement in the z-direction which determined the speed and force of penetration into the rock. It had 2 spikes on either sides and a butterfly ring with knurled knob which was used to hold the RAT in position while grinding. Once the rover identified a location or rock of interest, the RAT approached the surface and made contact on the small spikes. The butterfly ring then closed in, to further hold ground. The actuators were then actuated to lower the bit on to the surface and grind the top surface off. The brush near the cutting bit prevented dust to accumulate and hamper the process of rock abrasion. During the grinding process the rover monitored currents, encoder readings and temperatures for further control.
Figure 1.1: Rock Abrasion Tool developed by Honeybee Robotics [18]
5 Figure 1.2: Rock Abrasion Tool developed by Dr. T. C. Ng
The experiments conducted with the Honeybee RATs were very promising and provided much scientific data [3], [9]. In spite of its effectiveness the Honeybee RATs are cost prohibitive when it comes to using them for rover prototype demonstrations on Earth. So, in 2002 NASA Ames Research Center acquired a low-cost RAT systems developed by Dr. T. C. Ng. The Ng RAT had two actuators. Though the actuators were functionally similar to the Honeybee RAT’s actuators the grinding bit and
6 mechanism were different. The Ng RAT had a shell milling cutter which was used as the grinding bit. The cutter was actuated in the z-direction for plunging into the rock surface and would also be rotated to get a circular grind finish on the surface.
The cutter was also spring loaded in the z-direction to absorb shocks. It had four solid spikes which made contact with the grinding surface and kept the RAT in position.
1.4 Creating the OU RAT
Understanding the surface of the planets goes a long way in providing knowledge about the scientific phenomenon on these planets. To examine such planetary rock surfaces fresh rock needs to be exposed. For this purpose the Rock Abrasive Tool was developed. The Honeybee RATs used in the MER mission [18] cost 875,000 dollars a piece. It is not economically viable to use such RATs for rover testing and data accumulation. The OU RAT project was funded to make a low-cost prototype
RAT which can be mounted on a prototype rover, the K-9 rover, to serve as a basis for follow-on research [11]. This thesis describes a Low-cost Rock Abrasive Tool which can be used to enhance research and experimentation in this area.
The Ng RAT was a low-cost abrasion tool possessed by NASA Ames Research
Center that could be mounted on the K-9 rover for experimentation. Various tests conducted by Brandon DeKock & Carol Stoker in the summer of 2002 highlighted several shortcomings of the Ng RAT [4]. Prominent among them were:
7 1. Grinding was too slow taking more than an hour to remove a mm of rock of a
basalt sample
2. The system would get jammed easily by the rock chips generated during abra-
sion
3. The grinding bit stalled very easily increasing power consumption
4. The system was not stable, experiencing large vibrations making it unsuitable
for mounting on the arm of the K-9 rover
The OU RAT, apart from being low cost, was aimed at improving on these shortcomings. This prototype RAT had to increase power efficiency, decrease the time to remove rock varnish and do it without jamming the system. The OU RAT was also to induce a passively compliant mount for the RAT system which would be an additional feature as compared to the Honeybee RATs. The Honeybee RAT was mounted rigidly to the robot arm and this posed certain limitations while it was being used. Having passive compliance will provide better contact between the rock and the grinder and, might also consume less power. The issues that were considered while designing the system (in order of preference) were: low-cost; reduce time of cutting; include passive compliance.
1.5 Organization of This Thesis
This thesis is organized as follows:
8 Chapter 2 goes through the various design requirements and parameters consid- ered while designing the OU RAT.
Chapter 3 discusses the various concepts generated for implementation of the system.
Chapter 4 defines the final system and theoretical calculations for arriving at
final design.
Chapter 5 compares the OU RAT with other RATs in terms of working and performance.
Chapter 6 describes improvements and potential future work
9 Chapter 2
Design Requirements for the OU RAT
This chapter describes the parameters that were taken into consideration and the goals which were to be achieved by the Rock Abrasion Tool being developed. This basis for design crops from some of the earlier tests done on other RATs and possible nuances that could be added in later systems. It discusses why these aspects are important for the application for a system such as a RAT.
The basic need for another prototype rock abrasive tool was because a low-cost system was needed for experimentation and research purposes. The MER RAT by
Honeybee Robotics was too expensive and could not serve the purpose. So, the main criterion for the OU RAT was to be fairly inexpensive as it is to be used for experimentation purposes on prototype rovers. Tests conducted by Stoker and
DeKock showed that one of the main drawbacks of the Ng RAT was that it was too slow in removing varnish. Their report also states that the Ng RAT had very short time between failures because of the rock chips jamming the system. So, to improve on this aspect, the design of the OU RAT should be shielded in some way so that
10 rock dust does not jam the system or halt it. A change in cutting bit or cutting method might be needed to decrease the time required for varnish removal.
The MER RATs were attached to an instrument deployment device which had a flexibility of about 5 degrees. [14] shows that this limited freedom of the robotic manipulator limited their accessibility. So, the grinding of the surface was mostly normal to the approach vector of the rover arm and not normal to the surface being cut. This made for less effective abrasion as the grinding had to be done over a larger area to expose the same area for examination. To overcome this, the OU
RAT was proposed to have passive compliance so that the grinder orients itself to remove minimal material to expose the same area for examination.
The following sections discuss these issues in detail.
2.1 Low-Cost System
As discussed earlier the motivation for this project was to build a prototype Rock
Abrasive Tool for experimental purposes which can be mounted on the K-9 rover for testing and data collection. As this RAT had to be low-cost it was desirable to use off-the-shelf parts wherever plausible. The KISS, keep-it-simple-stupid, approach was used in the design process. This would lead to ease in manufacturing so that the components can be produced in-house given the facilities at the OU AME Machine
Shop.
11 2.2 Time & Energy Efficient Abrasion
The tests conducted by DeKock and Stoker with Ng RAT showed that it was too slow in removing surface material taking about an hour to remove rock varnish on most rocks. The tests conducted in our labs showed a similar response. On the other hand using a standard dremel tool for abrasion showed much faster material removal.
So, more exploratory experiments were conducted to formulate some trends in the observations. The rock used for experiments was a quartz sandstone. A reference of occurrence of some different kinds of sandstone in Maridiani Planum on the Martian
Surface is given in [6]. The weight of the rock was eight and half pounds. For rock varnish silver paint was used as analogy. Both the Ng RAT and the Dremel Tool were used to remove the paint and the time and energy taken were compared.
The Ng RAT was used in two configurations to remove the silver paint - first with the rock sitting on the RAT and then with the RAT sitting on the rock. The
first run simulated the situation where the RAT was connected to a rigid arm and points of contact between the rock at the RAT were always maintained. No external force was applied for grinding and the only internal force aiding in the process of grinding came from the spring loaded grinder and its vertical motion. The progress of abrasion can be seen through the pictures in Figure 2.1. These pictures were taken after certain time intervals to assess how much time is required to remove the varnish. The rock was checked after small intervals to see that only the paint is removed. Care was taken that after taking every shot the rock was placed back in
12 the same orientation on the RAT. It was observed that the Ng RAT barely starts removing material after 10 minutes and takes about 60 minutes to expose enough surface for examination.
(a) In the beginning (b) After 10 minutes
(c) After 30 minutes (d) After 60 minutes
Figure 2.1: Paint removed by Ng RAT with rock sitting on it
The grinder of the Ng RAT was running at an rpm of 4700 and the surface of paint removed could be approximated to an ellipse of major axis 42mm and minor axis 37mm, drawing current ranging from 0.07 to 0.52 amps. So, in 60 minutes
13 Figure 2.2: Paint removed by Ng RAT sitting on the rock
12.2cm2 of area was exposed drawing an estimated average current of 0.3A with an estimated power consumption of 3.6W.
After the first set of grinding, the RAT had to be blown with air to remove rock chips as the system had reached very close to failure. This was probably because with the RAT sitting below the rock, all the rock chips were falling right into the grinding system. In the second run the RAT was placed on the rock and it was hard to keep it at the same spot as it kept bouncing around. This case simulated the RAT being attached as an end effector to a rigid robot arm which provided a contact force equal to the weight of the RAT at the legs. The force on bit in this case was provided by the motion of the grinder transmitted through the springs and also by the weight of the reciprocating member. The robot arm would have to have more holding force that just the weight of the RAT to keep it in position. Also,
14 Figure 2.3: Paint removed by 4.8V cordless dremel a time progressive grinding could also not be generated as the grinding system got jammed after 15 minutes. Figure 2.2 shows the abrasion resulted in this phase (red circle). The brown pocket seen at lower part of the circle is because of a part of rock breaking away and does not correspond to the grinding process. These two runs are in accordance with DeKock and Stoker’s observation of Ng RAT getting jammed very easily. Clearly the weight of the RAT wasn’t enough to hold the rock but the force required by the robot arm will definitely be less than that of the weight of the rock (8.5 lbforce = 37.8N).
A quick test with a 4.8V cordless dremel tool (4.8V, 2 speed cordless rotary tool- model 750) on the other hand removed the same amount of varnish as the Ng RAT in less than 15 minutes running at 8270rpm with a silicon carbide bit. Figure 2.3 shows the rock surface after using this dremel.
15 Figure 2.4: Dremel Bits used in tests ( From L to R):Silicon Carbide Bit, Tungsten Carbide Cutter, Diamond Tip, Chain Saw Sharpening Tool
This triggered more exploratory experiments with a higher power 9.6V cordless dremel tool and various grinding heads. The heads used were Aluminum Oxide,
Silicon Carbide, Tungsten Carbide cutter, Chain Saw Sharpening Tool and Diamond
Tip (Figure 2.4). Each of the grinding bits were used to remove varnish for similar area as that removed by the Ng RAT and the readings were taken. Each bit was used twice and an average of both the cases was taken for comparison. The aluminum oxide bit was too soft to remove the paint on the rock and wore out instead of eroding the rock. The Silicon Carbide bit and the Chain sharpening tool were used to remove the same amount of material as that removed by the Ng RAT. The speeds of the dremel was also played with to get the operational rpm which was slightly different for different bits. It was seen during the experiment that at a certain speed the varnish removal started to be quick and stayed pretty much at the same rate
16 even when power was increased. Increase in power after this speed only resulted in increased tool wear. Table 2.1 shows the time taken and the energy used in each pass to expose 12.2cm2 of rock surface along with the bit speeds. The tungsten carbide cutter and the diamond tip were too small to establish a comparison with the other bits.
Table 2.1: Performance of Dremel Bits as compared to Ng RAT
The rows of table 2.1 lists the type of tool used for abrasion and their corre- sponding specifics. The speed at which grinding was done was measured using a stroboscope upto an accuracy of 10rpm. The voltage was measured using a volt- meter accurate to first decimal and the current was measured using an ammeter.
The current which was measured, was the estimated current used during the cutting process and does not reflect either the stall current or the no load current. It is the current which was mostly being drawn by the grinder during removal of varnish.
17 The time taken was measured using a stop watch with the abraded surface being checked at regular intervals to remove just enough material to equal that removed by the Ng RAT (12.2cm2). The last column lists the energy consumption which is defined as the energy required to remove varnish from one unit area of rock surface.
So, energy consumption in terms of varnish removal is
Energy Consumption = Energy used in abrasion / Area of rock abraded
The energy consumption values show that the dremel tool is pretty much as efficient in removing rock varnish as the Ng RAT, if not more so. These energy consumption values can only be treated as an indication of the trend and not the exact values. A thorough energy comparison would require specialized microscopic imaging equipment like the one used in [3] for measuring “grind energy index”. But, what is striking is the time taken by the dremel tool to remove varnish. It takes less than one-third of the time as that of the Ng RAT to grind the same area on the rock. It does consume a higher wattage of power but that is consumed for a much lesser time period giving it an efficiency slightly better than that of the Ng RAT.
From the tests with various dremel bits it also seemed that as the hardness of the material of the bit increased, the abrasion became faster. The Aluminium Oxide bit was poor in grinding and Silicon Carbide and Chain Saw Sharpening bits were much better. In summation, abrasion bit should be selected such that it is made up of a hard material and the RAT should be capable of removing rock varnish in
18 lesser time than the Ng RAT. From the experiments, a time frame of about 15 to
20 minutes looked a reasonable and achievable goal.
2.3 Passive Compliance
The motivation for introducing passive compliance was to have varnish removal over the surface of rock such that the grinder removes just enough material to expose a
flat surface on the rock. This means that the grinding surface would be determined by the terrain of the rock rather than the orientation of the rover arm on which the
RAT is mounted. The abrasion tool would have the ability to orient itself such that the grinding surface stands normal to the rock surface at the time of rock varnish removal. In other words when the angle of approach and the rock surface normal are within few degrees of misalignment, the flat spot will be created along the rock surface. This would facilitate efficient use of energy as the least amount of material would have to be removed to expose an area of fresh rock. Figure 2.5 shows that a compliant system would remove material ‘A’ to expose an area equal to that exposed by a non-compliant system by removing material ‘B’. Clearly a compliant system would have to remove less material which means faster varnish removal and less power usage.
The compliance can be achieved actively or passively. In an active application the system would have to be loaded heavily with sensors - proximity sensors, touch sensors, force sensors, torque sensors and so on - to asses the contour on the rock
19 Figure 2.5: Illustration of advantage of Passive Compliance in RAT and then orient the grinding bit for best performance. On the other hand a pas- sive compliance would have some spring loaded mechanism which allows temporary misalignments.
Though an active compliance would be the robust way to go and might make the system more reliable, efficient and rigid, going by the kiss approach the implementa- tion of a passive system was considered more suitable and cost effective. Once, the alignment of the grinder with the rock is achieved the grinder needs to be locked in place during the grinding process. Otherwise, the vibrations generated by grinding could dislodge the alignment and the varnish might not be removed from the desired location. A locked configuration would also be good in arresting grinding vibrations within the RAT itself giving minimum shiver to the rover arm.
20 2.4 Summary of Requirements
Based on the details in this chapter all the requirements and aspects of consideration for designing the OU RAT are summarized below:
Requirement R1: The OU RAT should cost much less than the MER rat
Requirement R2: The OU RAT should not get jammed easily
Requirement R3: The OU RAT should clean a square inch of rock in lesser time
than the Ng RAT (about 15 to 20 minutes)
Requirement R4: The OU RAT should have some kind of passive compliance
built into it
Requirement R5: The passive compliant mechanism should be able to lock itself
in a particular orientation
Requirement R6: The OU RAT should have some control system to avoid stalling
of the grinder
Requirement R7: The OU RAT should be able to create a flat spot at one location
and not bounce around while grinding.
21 Chapter 3
Design Concepts for OU RAT Components
This chapter describes the process of evolution of the design of OU RAT based on the design basis. The whole system is broken into three modules based on functionality and the requirements to be fulfilled. Various design ideas have been brainstormed for implementation of each module and the advantages and disadvantages of each of these design ideas are discussed.
3.1 Modules of the OU RAT
Based on the design requirements there are three components which need to be determined to satisfy the expectations of OU RAT. One would be the grinding bit including the design of the bit and the material to be used for it. This should be decided so that it aids the achievement of requirements R1, R2 and R3 listed in section 2.4. A proper design of the bit should provide exit for rock chips produced during grinding. The size of the bit should be big enough to clear the required area
22 and it should be done quick. Then the supporting components which will make the bit work, like the grinding motor and supporting mechanism has to be designed. This design needs to have some way of applying force on to the bit during the grinding process. This should be designed to satisfy the goals of R1, R2 and R4. The RAT also needs some passive compliance (R4) with a locking mechanism (R5) capable of locking itself in different orientations.
So, the OU RAT was divided into 3 modules based on functionality and require- ments for design purposes. They are:
• The Abrasion Bit
• Locking Compliant Joint
• Force on Bit Module
3.2 The Abrasion Bit
From the experiments described in section 2.2 it seemed that a harder bit removed material faster than a softer bit. So the material of the bit should be hard like a diamond bit or likewise. For requirement R7 the bit would have a pilot drill in the center. During the grinding process the pilot drill would be the first to make contact with the rock surface and dig into the rock. So, when the flat grinding surface comes in contact with the rock the grinding bit would center the flat spot around the pre-drilled hole made by the pilot drill. This design is shown in Figure 3.1(a).
23 The straight slots on the flat portion of the grinding bit are for the escape of rock chips. Having a pilot would also help in reducing the power consumption as power will not be lost in removing material at stray locations on the rock.
Another variation of this design is having a pilot grind instead of a pilot drill. In
Figure 3.1(b) the grinding bit has a protruding surface in the center. The protruding center is supposed to act as a grinding pilot which would dig into the rock at the beginning of grinding. When the flat spot comes in contact with the rock the pilot would wedge into the rock avoiding the RAT from bounding around during grinding.
This design has spiral slots for escape of rock chips.
Figure 3.1(c) shows a drill bit with a similar pilot grinding head but the slots cut all the way through the width of the grinding bit. This would provide more room for removal of chips.
3.3 The Locking Compliant Joint
To have compliance (requirement R5) in the RAT system the grinder should have
2 degrees-of-freedom for it to take different orientations from a fixed arm position.
To achieve this angular or pointing compliance it was proposed to split the RAT such that the part which is attached to the instrument deployment device and the part which contained the actual grinding bit are linked with some kind of ball joint.
Design concepts for locking this type of a joint are described in this section discussing the pros and cons.
24 (a) Bit with pilot drill and (b) Bit with grinding pilot (c) Bit with grinding pilot and straight slots and spiral slots through slots
Figure 3.1: Conceptual Designs for Abrasion Bit
3.3.1 Tooling Ball and Solenoid
The first conceptual design was to use a ball end shaft for the joint and solenoid for locking it. Figure 3.2 shows a CAD model of the proposed design. The upper housing which is attached to the arm of the rover carries the locking system and the ball joint at the bottom. A standard tooling ball which is available through catalog serves as the ball end shaft. The upper housing has a spherical contour built in the bottom with a hole in the center. The spherical surface of the tooling ball would rest on this side of the housing making a sliding contact ball joint. To the shaft end of the tooling ball is attached the lower casing which accommodates the grinding system along with the abrasion bit. The solenoid is contained in the upper housing
25 Figure 3.2: Tooling Ball and Solenoid used for Locking Compliant Joint and is spring loaded. The plunger of the solenoid contacts with the ball end of the tooling ball. The lower casing would have three legs at the bottom which would serve as points of contact with the rock surface during the abrasion process. The shaft of the tooling ball also has a coaxial spring sandwiched between two washers.
When the RAT in resting (i.e. not grinding) the solenoid is not activated and the ball joint is free. As the RAT approaches the rock surface the legs make contact with the rock and the RAT settles on the plane of the rock. The tooling ball rotates for this to happen and the spring gives stiffness so the lower casing is not just dangling on the other end. Once the RAT is in position the solenoid is activated to lock the ball and grinding process is started. After the rock varnish is removed the solenoid
26 is deactivated and the RAT retracted. The coaxial spring now acts as a centering spring to bring back the tooling ball in alignment with the rover arm.
The plunger has to be of steel because it contacts with the tooling ball and has to provide force for locking. The end of the steel plunger needs to be designed such that it can provide enough friction to keep the tooling ball in place. The good thing about using a solenoid is that the locking mechanism would be quick avoiding any time delays during transition from one phase to another. The housing of the tooling ball would be complicated with manufacturing of the spherical surface in the upper housing.
3.3.2 Spherical Bearing and Solenoid
This system is similar to the previous one except that the ball-joint has been changed.
A Ball-Joint Spherical Bearing is available from standard catalog and this was used as the ball joint. The spherical bearing consists of two coaxial rings. The inner ring has a cylindrical hollow surface on the inside and a spherical surface on the outside.
This inner ring sits inside an outer ring which has a spherical surface on the inside and cylindrical surface on the outside. The radius of curvature of the spherical surface of the inner ring is greater than that of the outer ring and this provides for smooth contact between the two rings. Figure 3.3 gives a visualization of the design. The upper housing has the spherical bearing pressed in at the bottom and the solenoid with its spring loaded plunger is mounted above the bearing. The inner
27 Figure 3.3: Ball-Joint Spherical Bearing and Solenoid used for Locking Compliant Joint race of the bearing has a steel shaft passing through it which has a spherical head on one side. This side wedges into the inner race of the spherical bearing forming the ball joint. The plunger of the solenoid makes contact with the round end of the steel shaft for the purpose of locking it. The other end of the shaft attaches to the grinding system which has legs at the bottom for rock contact. The shaft also has a coaxial spring accommodated for centering the grinding system when the RAT is not in use.
28 The working of this system will be similar to the ball-joint and solenoid system.
During the approach of the RAT towards the rock the solenoid is deactivated and the ball joint is free. The contact between the legs and rock makes the RAT align to the rock surface. The solenoid is activated to lock the joint and remove varnish.
The ball-joint spherical bearing can be easily installed in the upper housing and gives very smooth functionality. The plunger again needs to be of a hard material like steel and the end used for locking needs to provide sufficient friction. Taking manufacturing aspect into consideration, the spherical head on the steel shaft might be an issue.
3.3.3 Split ball and Solenoid
Figure 3.4: Split Ball and Solenoid used for Locking Compliant Joint
29 In this design the ball joint is made up of half of tooling ball and half of a steel ball. The locking actuator is again solenoid. Figure 3.4 illustrates this idea. The tooling ball’s ball end can be machined to half ball end. The steel ball can also be machined to half and have the shaft of the tooling ball pass through it. The two half balls together would make a full ball. This is enclosed in a circular race which is pressed into the lower casing. The steel ball half is rigidly attached to the circular race. When the tooling ball is pulled upwards the two spherical halves mate and the ball is free to take any orientation. When the tooling ball is pushed downwards, the ball end of it butts against the circular race locking the ball joint. The motion of the tooling ball is provided by the solenoid. When the RAT is not grinding the solenoid is deactivated and to remove the varnish the solenoid is activated to lock the joint.
The tooling ball’s shaft has a coaxial spring for centering the grinding system.
The main advantage of this setup is that the ball joint was attached to the lower casing instead of the upper housing. This gives huge advantage in terms of decreasing moment arm of the grinder. So, less force is required to lock the compliant joint.
But, implementation of this design had many complications. There has to be a way to hold the plunger inside the solenoid otherwise it could just fall out. Nothing could be figured out for installation of the circular race the way it has been described.
30 Figure 3.5: Spherical Bearing with Link Lock Mechanism actuated by Solenoid
3.3.4 Spherical Bearing with Link Lock Mechanism
The idea of this locking system was to increase the force for locking from the solenoid.
In the Figure 3.5 it can be seen that the ball joint used here is same as that described in section 3.3.2. It has a spherical bearing with a shaft running through it which has a spherical end. But the mechanism to lock the ball joint is through mechanical linkages. The upper link is pivoted on one end along an axis perpendicular to the axis on the outer ring of the spherical bearing. The other side of the link is connected to the core of the solenoid. Another link goes from the core to the ball end of the shaft. When the solenoid is not energized the links are not vertical and the ball is free. Once the RAT has complied on the rock the core is pulled in by activating the solenoid, the links become vertical and the contact between the lower link and the shaft puts pressure on the joint to lock it.
31 This system gives the advantage of the grinding system being more sturdy as the ball joint is locked by rigid linkages and does not directly depend on the force of the solenoid. One of the disadvantages of this system could be that the vibration might travel more easily through the linkages increasing tremors at the robot arm.
3.4 The Force on Bit Module
This section discusses the evolution of the mechanism for applying force on the bit for grinding. It was seen during the experiments with the Ng RAT and the dremel tool that the grinding bit was getting stalled easily when force was applied during the grinding process. This is because the motors were running at high speed but providing very low torque. So, the force module had to have some method of applying force to the bit and also be resilient enough so as not to stall the grinding motor. Design concepts for these are discussed below.
3.4.1 Flexishaft and Spring
The force required to lock the compliant joint is directly proportional to the weight of the grinding system at the far end of the passive joint. So, decreasing the weight at this end would imply easy locking of the joint. For this purpose flexishaft was considered to be used in the force on bit module. Flexishaft is a flexible cable which can mechanically transfer torque with an efficiency of 50%-80%. This would allow the grinding motor to be placed on the arm of the rover instead of on the far end of
32 the passive joint reducing the weight of the bit-force module. The reduced weight would in turn require less force to lock the compliant joint.
Figure 3.6: Flexishaft with solenoid
Figure 3.6 shows the flexishaft with solenoid for inducing force onto the bit. The solenoid will be activated in default position with the flexishaft held in position. The grinding bit is surrounded by three legs which are used to rest firmly onto the rock.
The link connecting to the ball joint is spring loaded and together with the three legs it forms a pivoted tripod. This orients itself to seat the RAT in a favorable position for grinding on the rock. Once the configuration is formed the solenoid is deactivated and the flexishaft is pushed against the rock by the preloaded spring coaxial to the solenoid core. The grinding force is provided by this spring. The spring can also absorb the vibrations caused during the grinding. It would also help
33 in avoiding the stalling of the motor. When the solenoid is activated the core is pulled in, thus, stopping the grinding process.
3.4.2 Dremel Motor and Spring
Figure 3.7: Dremel motor with solenoid
Instead of using flexishaft for attaching the grinding bit it could as well be at- tached to the dremel motor directly. The motor might increase the weight of the force on bit module but might make it less bulky. This arrangement is shown in Fig- ure 3.7. The motor is attached to the solenoid on one side and has a support guide on the other side so that it can slide easily along the z-direction without getting cocked. The grinding process will be similar to the previous case where the grinding force is provided by the preloaded spring in the solenoid. Activating the solenoid
34 will lift the bit off the rock and stop the grinding. The three legs surrounding the bit are used for sitting on the surface of the rock.
3.4.3 Servo Motor and Lead Screw
The solenoid and spring arrangement is not a rigid way of obtaining vertical move- ment so a lead screw mechanism was considered. The lead screw will be actuated by servo motor so there is more control over the position of the bit. The servo motor can be actuated in pulses lowering the grinding bit enough to provide force for grinding.
If the grinding motor gets stalled, the increase in current can be monitored and the servo can be backed up accordingly. This gives a closed feedback control loop. The exact depth at which the surface is being exposed can also be observed. Figure 3.8 shows two variations of this implementation. The ability to move away form the rock also gives more room for the rock chips to scatter.
The motor is a heavy object and moving it in the z-direction held at a distance induces a tendency of it to jam the lead screw. So, some kind of support needs to be provided on the other side. Figure 3.8.a shows steel pins being used as sliding support and Figure 3.8.b shows rails having rollers for vertical movement. The module also has legs for seating the RAT on the rock.
35 (a) Sliding Contact
(b) Rolling Contact
Figure 3.8: Servo Motor applying grinding force
36 Chapter 4
Final Design of OU RAT
Chapter 3 outlined some of the brainstormed designs which might be suited given the goals to be achieved and the limitations discussed in Chapter 2. This chapter will explain the RAT specifications using a theoretical analysis. The chosen design for each module will be explained and some theoretical calculations will be performed to arrive at the exact specifics of the final design. This chapter is divided into different sections for each of the three modules, followed by detailed design calculations and then the control circuit developed for demonstration of the instrument is discussed.
4.1 The Abrasion Bit
There were three designs discussed for the grinding bit in the earlier chapter. These designs were forwarded to few vendors who manufactured such type of custom dia- mond tools and were willing to quote a price. Most of the vendors did not entertain such type of custom orders for just a couple of parts and only two of them agreed
37 to manufacture them. The vendor which seemed more promising was chosen and further discussion on the design of the bit was carried. In the discussion it was ob- served that the designs that were thought of earlier might be difficult to manufacture and even if it were the bits would be exorbitantly expensive. In conclusion, the bit shown in figure 4.1(a) was suggested by the contractor and that was the bit which was chosen.
This bit was made out of CBN bonded diamond tip. The base matrix was tool steel which had been hardened for extra strength. The bit was 1” in diameter with
12-18 grit size diamond bonded matrix extending upto a depth of 1/2”. It had a pilot tip protruding out in the center, of diameter 1/4” and height 1/4”. This grinding pilot was supposed to grind into the rock and wedge into the rock to avoid the grinding bit from bouncing around at high speeds. This would give the flat grinding surface better precision in creating a better surface for examination. The
flat surface of the bit also had 6 straight radial slots running from the base of the pilot to the circumference for better cutting of the rocks and escape of rock chips.
But, when this bit with straight slots and pilot grinding surface was used, the pilot instead of digging into the rock proved as a hinderance in grinding. It did not remove enough material to dig into the rock and hindered the flat surface of the bit from touching the rock. The main reason for this was probably because the grinding speed goes to zero at the center of the bit and so no grinding was possible. Other accentuating factors could be not having any provision for the rock dust to escape
38 (a) Diamond bonded (b) Diamond bonded grinding bit with pilot grinding bit with counter- bore
Figure 4.1: Abrasion Bit at the central region. So the pilot grinder was got rid off and instead a counter bore was done to get the same effect of preventing the bit from bouncing. The idea was that once the flat cutting surface starts getting into the surface of the rock a protrusion at the center would be left on the rock which would wedge in the counter bore. Figure 4.1(b) shoes the final bit that was used.
4.2 Locking Compliant Joint
Some of the designs for having a locking ball joint have been discussed in Section 3.3.
In this section these aspects will be discussed in more detail to take design decisions to finalize the implementation of the compliant joint. The first subsection pertains
39 to the question as to which component should be used for holding the grinding bit and the second subsection talks about the joint locking system.
4.2.1 Flexishaft Vs Dremel Motor
Chapter 3 explained that the force required for locking the compliant joint will greatly depend on the weight of the force-on-bit module at the far end of the compli- ant joint. Two choices for transferring the bit motion had been discussed - flexishaft and dremel motor. The designs of these two modules are illustrated in Section 3.4.1 and 3.4.2 respectively.
These two configurations were studied for force required to lock the ball joint.
To determine which option would induct less load on the clamping mechanism two identical force on bit modules were chosen. For simplicity the solenoid and spring arrangement was used as the force on bit module. The dimensions and weights of the assembly for this module when a flexishaft is used and when the dremel motor is used was calculated.
Figure 4.2 shows a schematic diagram of the force on bit module. The first figure is the configuration in normal position and the second figure is the configuration in locked position at an angle of approach ‘θ’ between the robot arm and the normal on the rock surface. The clamping force at joint A is a function of the weight of the force on bit module and the lever arm ‘l’ of the center of gravity.
Flexishaft module:
40 Figure 4.2: Schematic Diagram for force on bit Module for Flexishaft and Dremel Motor
The weight of the various components for the flexishaft module are listed below.
These weights were calculated based on the detail of the design at that stage of the design process.
Mass of Flexishaft = 100 g
Acrylic hold for flexishaft + rail = 39 g
Steel rails = 23 g
Brass Guide = 160 g
Solenoid = 132 g
Aluminum = 229 g
Total Mass = 683 g (=1.50 lbs)
41 As the module is symmetric about the grinding axis the whole weight can be assumed to be on the center of gravity of the module lying along this axis. Based on the weight distribution the center of gravity was estimated to be 115mm from A.
So,
Restoring Torque = l *W Sin(θ) = 0.798gSin(θ)
Dremel Motor Module:
Similarly, for the dremel motor module the weight of the various components are listed below. These weights were calculated based on the detail of the design at that stage of the design process.
Mass of motor = 130 g
Acrylic slider = 35 g
Steel rails = 23 g
Brass Guide = 117 g
Solenoid = 132 g
Aluminum = 207 g
Total Mass = 644 g (=1.41 lbs)
Again, as the module is symmetric about the grinding axis the whole weight can be assumed to be on the center of gravity of the module lying along this axis. The motor is bulkier closer to the point A so the center of gravity for this module will be closer to the rotation point. This was estimated based on the mass distribution to be around 50mm.
42 So,
Restoring Torque = l *W Sin(θ) = 0.694gSin(θ)
It has to be noted that both the modules have similar materials used and the component designs are also similar to a good extent. It is seen that the weights of both the modules are pretty much similar but the dimensions are different. The dremel motor is heavier than the flexishaft but due to the bulkier size of the flexishaft, its supporting components are bulkier and heavier. Again from the arrangement of the motor and the flexishaft it shows that the center of gravity of the motor would be closer to the ball joint than that of the flexishaft. This is because the flexishaft’s weight is uniformly distributed along its length whereas the motor is beefier on the side mounted closer to the joint. So, the restoring torque for the dremel motor module would be less than that for the flexishaft module. So, this option was chosen.
It was also seen that the installation process for this module would be simpler than the former.
4.2.2 Tooling Ball Clamp
In Section 3.3 different brainstormed ideas for the compliant joint had been dis- cussed along with issues that would be encountered to implement them. After all consideration, it was decided that the tooling ball would be the best alternative to create a ball joint. The tooling ball was a component which was easily available in the catalog and didn’t require any major modification to fit in the design. To clamp
43 the tooling ball, solenoid was used but it was not connected directly to the clamp.
This system is shown in Figure 4.3.
The tooling ball rests between two flat steel plates. The plates are machined to make spherical pockets for the ball to sit in. These two steel plates or fingers act as a clamp. The tooling ball material is made up of stainless steel and the fingers are also made of steel to have strong contact surfaces clamping the ball. The flange on the tooling ball was removed and the extended shaft on that is passed through a hole in the bottom finger of the clamp. The extended shaft is connected to the force on bit module and has a centering spring housed coaxial to it. This spring is held in place between two washers and a bolt running through it. The clamping force is achieved by a solenoid mounted at the far end of the clamp fingers. This increases the force on the ball joint giving lever advantage from the moment arm.
The dimensions of these components were a result of theoretical calculations which will be discussed in Section 4.4.
4.3 Force on Bit Module
Section 4.2.1 shows that for reducing the clamping force for locking the compliant joint the dremel motor should be used directly in the force on bit module. For getting the motor to apply force onto the rock during the grinding process two methods were discussed in Chapter 3. Using a spring loaded solenoid would be good for absorbing the vibrations during grinding but not uniform in applying force onto
44 Figure 4.3: Model of the implemented RAT system the rock. Moreover, there is no control on how much force is being applied. The lead screw mechanism on the other hand is rigid and has more control of the position.
Some milling machines like, Haas Vertical Milling Machine VF-4SS, implement lead screws for precision and control [7]. The lead screw is actuated by a servo motor which is controlled electronically. And depending on the pulses given to the motor the increment on the lead screw can be controlled which controls the position of the
45 abrasion bit. From the rotation of the servo the depth of varnish removal can be obtained.
Section 3.4.3 described the lead screw mechanism for plunging into the rock and because the lead screw is off-center rails with sliding or rolling support have been incorporated. However, it was observed that the system would be more rigid and symmetrical if lead screws are used on either side of the dremel. Both the lead screws are driven by the same servo with motion transferred using pulleys and a timing belt. This removes the hassle of trying to synchronize both the lead screws or servos and avoids any tilt in the dremel. The complete system is shown in Figure
4.3. Detailed calculations of the components is discussed in Section 4.4.
4.4 Calculations for Implemented Design
The final design has few components whose specifications were decided based on the physics of the system. There were mainly three of these:
1. Centering Spring
2. Locking of Ball Joint
3. Lead Screw Mechanism
46 4.4.1 Centering Spring
The centering spring was supposed to bring the grinding assembly to a default center position when the RAT is not being used. The forces acting on the system are shown in the free body diagram in Figure 4.4.
Figure 4.4: Free body diagram of the Centering Spring
When the RAT is about to come in contact with the grinding surface one of the legs will come in contact with the surface and an external reaction force ‘N’ will be experienced at a distance ‘D/2’ from the central axis. This will trigger restoring action from the centering spring with force ‘Fs’ being applied along the diameter of the spring ‘d’. ‘W’ is the weight of the rotating part i.e the force-on-bit module. For the system to be in equilibrium,
Fs * (d/2) = N * (D/2)
47 The maximum value of the reaction force can be equal to the weight of the RAT.
Selecting a spring of mean diameter 0.6in (15.24mm) from the MSC catalog [12] and taking D and W from Section 4.2.1,
Fs * (15.24/2) = 0.644 * (120/2)
So, Fs = 5.07 N
Based on this a spring was selected from the catalog [ wireD(0.045in)(1.12mm),
OD(0.6in)(15.24mm), k(10.5lbf/in)(1.84N/mm), l(1.0in)(25mm), maxLoad(7lb)(31.1N)
]. The spring will be prestressed when it is installed. If the prestressed length is say
20mm then,
Fs = 1.84 * 5.4 = 9.93 N
So, choosing this spring gives a factor of safety of about 2. The centering spring will be compressed to its maximum when the force-on-bit module is deflected to its maximum i.e. by 150. So,
Total compression = 5.4 + d/2 * Tan 150 = 8.55 mm
And, Max Fs = 15.7 N
4.4.2 Locking of the Ball Joint
Once the centering spring was decided, the torque required to clamp the ball joint was to be calculated. Figure 4.5 illustrates the free body diagram of the RAT during grinding. The only external forces on this system will be the reactional frictional torque from grinding ‘Tf ’ and the normal reaction on the bit ‘R’ which is same as the
48 normal force for cutting. The normal reaction on the legs ‘N’ will be balanced by the weight of the RAT system when all three legs are in contact with the rock surface.
The normal cutting force will be provided by the robot arm on which the grinder will be mounted and the torque to counteract the frictional torque will be provided by the clamping mechanism. So, the clamping force should provide enough frictional force to produce torque equal or greater than the frictional torque experienced at the bit due to grinding.
Figure 4.5: Free body diagram for clamping the ball joint
From the earlier experiments with the dremel tool in Section 2.2 the running conditions of the dremel have been observed. From this and using the power equation
P = V * A = T * ω
the running torque can be calculated. Two of those conditions are:
49 For an rpm = 6750 : Voltage = 2.9V : Current = 3.2A ⇒ Trunning = 13.0 Nmm
For an rpm = 7850 : Voltage = 3.2V : Current = 3.3A ⇒ Trunning = 12.8 Nmm
But, the maximum torque will be experienced when the dremel is stalled. To determine the stall torque a simple experimental setup was used as shown in Figure
4.6. The dremel was held fixed and a cantilever beam was attached to the spinning bit with a pan to add weights at the far end. The operational voltage was applied and the dremel was stalled gradually increasing the weights in the pan. It was seen that the dremel stalled when weight in the pan was 97 grams. The length of the cantilever arm was 57 mm.
So, Stall Torque Tstall = 57 * 0.097 * 9.81 = 54.18 Nmm
Figure 4.6: Experiment to determine the Stall Torque of Dremel
Assuming dry steel-on-steel contact between the ball and clamp fingers, Coeffi- cient of Friction µ = 0.42 [8].
Diameter of the ball db= 12.7 mm (0.5 in)
50 So, Normal force to be provided at contact Fb = Tstall /(µ * db/2)
⇒ Fb = 20.31 N
Now, this is the force needed at the contact between the ball and the clamp
fingers. From this the force to be provided by the solenoid at the far end of the
finger can be calculated from Figure 4.7. Balancing the moments about the hinge of the clamp fingers gives
Fc *Dc = Fb *Db
A solenoid was selected from the catalog [5] which was small in dimensions and could provide force similar to the order needed in this application. From the force- duty cycle graph in Figure 4.8 of the selected solenoid it is seen that while running at 25% duty cycle the solenoid can give 40 ounces of force at 50 thousands of an inch of displacement. This translates to 11.12 N at 1.3 mm and taking Db = 12 mm the length of the lever arm is calculated as,
Dc = 20.31 * 12 / 11.12
⇒ Dc = 21.9 mm
Taking a factor of safety of about 2, Dc = 42 mm
So, the solenoid was mounted in the upper housing such that the plunger of the solenoid sits 42 mm away from the clamp fingers hinge.
51 Figure 4.7: Force from solenoid to lock the ball joint
4.4.3 Lead Screw Mechanism
For the force on bit module the lead screw mechanism was selected. The purpose of the lead screw and servo motor was to provide movement in the direction perpen- dicular to the plane of the rock and be able to apply enough force for cutting of the rock. During the initial tests with the dremel tool, the weight of the dremel itself was being used as the contact force which was about 8.9 N (2 lbs). The Honeybee RAT operated at a contact force of about 5 N [18]. So going along similar lines, a force of
8.9 N was used as the desired force for grinding. The honeybee RAT was designed to remove varnish upto a depth of about 5 mm [20] and it’s historic cut into Mars rock on February 5, 2004 recorded a hole of depth 2.85mm over a three-hour period
[19]. So, the depth till which varnish removal was expected should be around 5 mm and this translated into the maximum displacement required in the z-direction.
A standard servo motor rated for a movement range of 0 to 180 degrees [17] was chosen to power the lead screw. Though unofficially it was capable of providing
52 Figure 4.8: Force Duty Cycle graph of solenoid (reproduced from [5]) upto 2100 of rotation in one single turn, the lead screw needed have a lead of about
10 mm so that a safe displacement of 5 mm is achieved for the rated 1800 turn.
The best choice under these constraints for a lead screw from existing catalogs [24] was a 3/8” - 11 thread/in lead screw with 4 thread starts. With the lead screw selected, the rating of the servo was decided based on the torque requirements. The free body diagrams for the lead screw while lowering i.e. providing force on bit and while raising the bit are in Figure 4.9.
Here, ‘P’ is the horizontal force from the servo;
‘F’ is the normal force on the bit for grinding (8.9 N);
‘dm’ is the mean diameter of the lead screw (8.2 mm);
‘l’ is the lead on the screw (9.5 mm);
53 Figure 4.9: Lead Screw Mechanism
‘µ’ is the coefficient of friction between steel and brass (0.2) [8];
‘W’ is the weight of the force on bit module (644 g);
For the first case, balancing the forces in perpendicular directions gives,
P + N ∗ Sinλ = µ ∗ N ∗ Cosλ
F − µ ∗ N ∗ Sinλ = N ∗ Cosλ
F (sin λ + µ cos λ) ⇒ P = (4.1) cos λ + µ sin λ)
F (l/πd + µ) P = m (4.2) 1 + (µl/πdm)
Putting the force in terms of torque,
l + µπdm Tlower = F dm/2( ) (4.3) πdm + µl
Putting in the values, the Torque of the servo Tlower = 19.3Nmm
54 For the second case, when the servo lifts the bit and the dremel motor off the surface of the rock balancing the forces gives,
W (sin λ + µ cos λ) P = (4.4) cos λ − µ sin λ)
F (l/πd + µ) P = m (4.5) 1 − (µl/πdm)
Writing in terms of torque,
l + µπdm Traise = W dm/2( ) (4.6) πdm − µl
Calculating from the values, Traise = 18.4Nmm
A standard servo from the catalog has a torque rating of 42 ounce.in which translates to 296 Nmm. So, this servo motor was selected for the application.
The pulley and timing belt were then selected based on the minimum distance that could be kept in between the two lead screws. The biggest concern was to keep the system as compact as possible.
4.5 Testing of OU RAT
After the OU RAT was built it was tested in experiments to see the performance of the whole system. The diamond grinding bit seemed pretty good with varnish removal on the sandstone rock which was used earlier. The speed of the grinding bit
55 was about 7950 rpm. The straight slots on the abrasion bit provided easy removal of rock chips but the counterbore in the center of the bit did not fulfill what it was expected to do. There was no uncut center projection left on the rock. This might possibly be because the vibrations of the bit cut the center region of the rock too.
The lead screw mechanism showed promise with the dremel being quite sturdy and in control. The response of the servo motor to the inputs given by the handyboard were quick and the dual lead screw raised and lowered the dremel grinding motor without any noticeable play.
The centering spring however, did not work in all cases. The centering spring as designed in section 4.2.2 for misalignment of +/- 15 degrees but due to the bulkiness of the force on bit module the centering spring could not handle the torque.
Moreover, a review of the calculations for the centering spring exposed certain cases which had not been taken into consideration. Revised calculations for the spring is done in Section 4.5.1.
The tooling ball which was used as a ball joint was smooth in its application and gave full freedom to the force on bit module to orient itself while sitting on the rock.
But, the locking mechanism to clamp the ball joint fell short of its expectation. From the force duty cycle specification graph of the solenoid (Figure 4.8) it was expected to provide a force of 60 ounces (16.68 N) at 6% duty cycle 0.1 inch away from closed position and about 40 ounces (11.12 N) at 25% duty cycle 0.05 inch away from the closed position. But, when the solenoid was run at 6% duty cycle it was getting
56 heated very rapidly so solenoid was run at 25% duty cycle. Even at this duty cycle the solenoid was heating rapidly and again the force delivered was much less that the specification. And moreover, for some reason the response was not uniform.
The plunger of the solenoid instead of holding stable position was vibrating with the square wave fed to it from the handyboard. Careful review of this situation led to another way of locking the ball joint which is discussed in 4.5.1.
4.5.1 Revised Design Calculations
When the earlier calculations for the centering spring were done the case when the instrument deployment device is moving horizontally was not taken into considera- tion. In this configuration the force-on-bit module is horizontal and the spring will experience maximum restoring torque for centering. This case is shown in Figure
4.10.
In the figure, ‘W’ is the weight of the force-on-bit module;
‘l’ is the distance of the center of gravity of the force-on-bit module;
‘Fs’ is the restoring force from the centering spring;
‘d’ is the mean diameter of the spring;
The force-on-bit module is beefier closer to the ball joint and has less weight away from it and the heaviest component is the dremel motor. So, the center of gravity of the module will be on the dremel motor. It was assumed to be along the axis on the dremel along two-thirds of the distance away from the top of the dremel
57 Figure 4.10: Revised Calculation for Centering Spring motor. Which comes to l (=72 mm). Weight of the force-on-bit module is 635 g and assuming 10% pseudo-weight W (=699 g). If d is assumed to be 18.29 mm (0.72 in),
‘Fs’ is given by balancing the torque as
Fs ∗ d/2 = W ∗ l
⇒ Fs = 53.9N
If the spring is prestressed about 5.4 mm the the spring constant k = 129.98N/mm
(=56 lbf/in). So, from the catalog the spring with the following specifications was selected [ wireD(0.08in), OD(0.72in), k(51.1), l(1.0in), maxLoad(24.89lbs) ].
With the new centering spring the OU RAT system is more stable and the centering works the way it was supposed to. This reduces the pressure needed to lock the ball also. Consider the forces shown in Figure 4.11, in this configuration
58 the RAT is being used to grind a vertical rock surface. The reaction forces that are observed on the legs and the grinding bit will be completely taken by the force from the arm carrying the RAT. The weight will be balanced by the frictional forces on the legs which in turn will be provided by the normal reaction force at the legs.
And the torque needed to clamp the ball joint will be equal to the frictional torque experienced during grinding.
Figure 4.11: Revised Calculation for Locking Ball Joint
So,
FA = 3N + R
W = 3f
⇒ FA = W/µ + R
And TA = Tf
59 The coefficient between aluminum and grinding rock is taken to be 0.5 based on data in [10] which says that most rocks have coefficient of friction between 0.5 and
0.9. Assuming the same force-on-bit, R (= 8.9 N), the actual built weight of the force-on-bit, W (= 635 g) the force from the robotic arm carrying the RAT will be
FA = 21.36N
For locking the ball joint instead of using a solenoid or some other actuator, the clamp fingers itself can be used to do the job. If the clamp fingers are kept in position such that they just have only few microns of space with the ball then this can be used to lock the ball. Figure 4.12 shows how this can be done by putting two bolts from the upper finger to the lower. The holes in the upper finger being through holes and the bolts not being completely tightened will allow the tooling ball to move and align itself for the rock surface. And when the grinding is to be done a force from the robot arm will force the ball against the upper finger giving a contact frictional force. This frictional force can provide the torque needed for locking the ball joint.
TA = f ∗ dball/2 and f = µ ∗ FA
⇒ TA = µ ∗ FA ∗ dball/2
TA = 56.97Nmm
This is greater than Tf which can upto a maximum be equal to the stall torque of the dremel motor Tstall (=54.18 N).
60 Figure 4.12: Modification for Locking Ball Joint
4.6 Controlling the OU RAT
The control circuit of the OU RAT was developed mainly to demonstrate the working of the system. Detailed autonomous control system was not in the scope of this work. The demonstration of the OU RAT was done using direct power from battery and simple signals from a handyboard [25]. A handyboard is a Motorola 68HC11 based, microcontroller which can be used to control mobile robots for experimental purposes. The OU RAT has three actuators. The servo motor was controlled by the resistance knob on the handyboard for movement in the z-direction. In an autonomous system this movement could be controlled by monitoring the current drawn by the dremel and making sure that the dremel does not stall. The actuation of the solenoid to clamp the ball joint was done using a mosffet which was triggered
61 by a square wave generated from the handyboard. The grinding dremel motor was directly hooked to the battery power.
62 Chapter 5
Comparison of OU RAT to other RATs
This chapter describes the testing of the OU RAT and some comparison of it to other rock abrasion devices. Some of the shortcomings of the OU RAT are discussed and certain design modifications are suggested to get the system to working condition.
An overview of the Ng RAT and MER RAT are given and then their performance is compared to that of the OU RAT.
5.1 The MER RAT
The MER RAT was designed and developed by Honeybee Robotics to be part of the Athena Science Payload sent on Sprit and Opportunity in 2003. The Honeybee
RAT [20] was a diamond tipped grinding tool which can remove dust, rock varnish or other outer layers off the rock to expose fresh rock as it is on the inside. It had three actuators. One of the actuators rotated at high speed rotating a grinding bit with two diamond tips embedded in a phenolic resin pad at close to 3000 rpm.
63 Another motor rotated the the grinding bit about a central axis so as to make a circular finished surface on the rock giving more surface area of varnish removal. A third motor controlled movement in the z-direction which determined the speed and force of penetration into the rock. It had 2 spikes on either sides and a butterfly ring with knurled knob which were used to hold the RAT in position while grinding.
Figure 5.1: Mechanical Design of HB RAT (reproduced from [20])
Once the rover identifies a location or rock of interest, the RAT approaches the surface and makes contact on the small spikes. The butterfly ring then closes in, to further hold the position. The actuators are then actuated to lower the bit on to the surface and grind the top surface off. The penetration into the rock in generally done in discrete steps once or twice per revolution. The total depth of penetration varies depending on the duration of grinding and the material properties but goes
64 to a maximum of about 5 mm. The brush near the cutting bit prevents dust to ac- cumulate and hamper the process of rock abrasion. During the grinding process the rover can monitor currents, encoder readings and temperatures for further control.
A more comprehensive summary of the results of MER RAT on Mars exploration can be found in [3].
5.2 The Ng RAT
The Ng RAT was developed to act as a low cost prototype rock abrasive tool for the purpose of experimentation and research. The Ng RAT had two actuators as compared to three in honeybee RAT. Though the actuators were functionally similar to the honeybee RAT’s actuators, the grinding bit and mechanism were different.
The Ng RAT had a shell milling cutter which was used as the grinding bit. Another actuator rotated the cutter to get a larger circular finished region on the surface.
This actuator was also engaged with a cam which moved the grinding cutter in the z-direction for plunging into the rock surface. The cutter was spring loaded in the z-direction to absorb shocks and also provided force for grinding. It had four solid spikes which made contact with the grinding surface and kept the RAT in position.
The Ng RAT could be attached to the instrument deployment device at the top of the system where the spikes were attached. When the Ng RAT is placed on the rock from which varnish has to be removed, the 4 spikes first make contact with the rock and seat the RAT in position. Both, the actuators are turned on. The
65 grinding cutter rotates about a horizontal axis being lowered by the other actuator.
As soon as it comes in contact with the rock surface it starts grinding, the force being applied by motion in the z-direction but transferred through the spring along that direction. So, the penetration force is not direct but is damped through the springs. The motion in z-direction is very important as the grinding cutter would stall very easily and need to be backed up to regain the momentum for cutting again.
5.3 Performance of the OU RAT and other RATs
The MER and Ng RATs are unique in themselves and were designed for their specific application. The OU RAT has some requirements similar to the Ng RAT but it is more of a step towards exploring additions, variations, improvements for another low-cost RAT. So, an apples-to-apples comparison of the specifics might not be possible but some similar parameters can be discussed. These are listed in Table
5.1. The OU RAT is significantly bigger than the other two RATs.
Net weight of OU RAT = 0.835 kg
Net wt of pivoting force-on-bit module = 0.635 kg
As the functional requirements of the MER RAT were different from that of the Ng RAT and OU RAT, it would be more appropriate to compare these two in more detail. The MER RAT was made to be sent on Mars so the design was very critical whereas for the Ng RAT and OU RAT, the system needed to be operational to be used for experimentation and research. Going back to the report of DeKock
66 and Stoker [4], some of the shortcomings of Ng RAT are discussed as compared to changes in OU RAT.
The cost of the OU RAT was estimated based on the cost of the material used and an estimate of the research and manufacturing hours gone into it. It was estimated at about $5500, which is much less that the MER RAT. It would be good to compare the cost with the Ng RAT but many specifics of the Ng RAT were not available.
The abrasion bit on the OU RAT runs at a much higher speed, close to twice that of the Ng RAT. The time taken to grind the surface of rock by OU RAT is much less, almost one-third, than the Ng RAT.
And with the power consumptions being similar, the net energy dissipation of grinding of OU RAT for the same amount of varnish removal will be less that the
Ng RAT.
The OU RAT was tested with rock abrasion in three different configurations. One with the RAT on top of the rock another case was when the RAT was horizontal and the third with the RAT pointing vertically upwards. In the third case there was highest probability of rock chips entering the RAT and jamming it. But, because of the closed structure of the force-on-bit module the dust only collected on the outside and there was also no jamming of the OU RAT.
As for compliance, the OU RAT could align itself to make better contact with the rock surface upto + 15 degrees where as the MER RAT had to be oriented by the robot arm at entry angles of + 15 degrees to have the same configuration.
67 Table 5.1: Comparison of different RATs with OU RAT
68 Chapter 6
Conclusions and Recommendations for Future
Work
This thesis described the OU RAT system, its detailed design and final implementa- tion. It also described some of the testing that was done on the OU RAT and some of the observed shortcomings in the design. The thesis then compared the OU RAT to other related systems.
6.1 Conclusions
As was mentioned in chapter 1, the goal of this work was to develop a low-cost rock abrasion tool which could be used for experimentation purposes and also explore some features like compliance which can improve power efficiency of the system.
The as built OU RAT was pretty cheap with an estimated cost of about $5500.
The abrasion bit removed about a square inch of varnish which was achieved by using a 1 inch diameter bit. It took about 20 minutes to remove this area of varnish
69 running at a speed of 7950 rpm and 2.2 volts and drawing about 2.8 amps of current.
The energy consumption was about 0.4 watt-hr/cm2. The maximum displacement of the bit in the normal-to-grinding direction was 10mm. Because of the closed system there was no jamming of the RAT. The revised centering mechanism with the compliant joint worked well giving a freedom of + 15 degrees.
In summary, the three most important aspects achieved by the OU RAT over the Ng RAT were that it took much less time in removing rock varnish, it did not get jammed easily and had passive compliance built into it.
6.2 Recommendations for Future Work
In retrospect, the outcomes and possible improvements for future prototypes can be discussed with reference to the three modules.
The Abrasion bit which was used was used worked well in terms of time taken to create to remove varnish from a rock. The relief slots helped in getting rid of dust avoiding any jamming, but the counterbore did not work as well. More discussion on the type of bit needs to done with rigorous design brainstorming. The best cutting method should be chosen; for instance, the MER RAT and OU RAT had a vertical grinder as apposed to a horizontal grinder used on the Ng RAT.
The compliance of the system was well implemented by the ball joint but the solenoid system designed for locking the joint did not work well. However, the
70 alternate design described in 4.5.1 was checked for plausibility and this design mod- ification showed good promise. This could be implemented and further use of the
RAT on the K9 rover might bring in more modification parameters to fine tune the design.
The force-on-bit module using the lead screw mechanism having a closed design was good in shielding the grinder and the supporting accessories. The dremel motor was fast in removing rock varnish but more scrutiny could be done to determine what factors - torque, speed, power - affect the grinding process more, rather than using an off-the-shelf part. Though the servo motor provided the movement and force to grind the rock, a full fledged control system will be needed before it could be effectively used for further experiments. This might include monitoring of the current to the grinding motor to avoid stalling of the motor when it is in contact the rock and grinding.
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