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AN AUTOMATED SAMPLE ACQUISITION & PREPARATION SYSTEM FOR LORAX

M. Murarka, A. Iyer, D. P. Miller, T. Taber, and T. Hunt

University of Oklahoma, 865 Asp Ave Rm 212, Norman OK 73019 USAmurarka@ou.edu, itsamit@ou.edu, dpmiller@ou.edu, taber@ou.edu & timhunt@ou.edu

ABSTRACT

The LORAX project will perform a micro-biological as-say of large ice fields such as those found in the Antarc-tic, the polar regions of Mars or the surface of Europa.To do this, it will make use of a rover which carries anew UV spectrometer which scans over specially pre-pared samples of ice and snow. The system for acquiringand preparing those samples is the subject of this paper.The LORAX Sample & Acquisition System (SAS) willacquire samples from the upper 10 cm of ice/snow. Thesample will be fragmented and flattened prior to examina-tion by the spectrometer. This paper presents the designfor the SAS as well as experimental results from a partialimplementation.

Key words: robot, drill.

1. MOTIVATION

Polar surfaces like the ones found on Mars and Europahave always been an object of interest for Astrobiologists.A close resemblance of such ice surfaces are found in theAntarctic region. Preliminary path-breaking studies haveshown microorganisms to be present in these ice sheetsbelow the surface (in the Vostok Ice Core) [1, 5] and atthe surface (near South pole) [2].

The Life On ice Robotic Antarctic eXplorer (LORAX)project proposes to perform a robotic biological survey ofAntarctica to advance our understanding of polar micro-bial ecology and also enhance data for further planetaryexploration missions. The field campaign will determinethe biodiversity of microorganisms in the region using aspectrometer which will be carried on a robot (see [4] fora description of the rover) along with a sampling deviceto retrieve ice samples on and below surface and other en-vironment and imaging sensors. Many biomolecules ob-serve auto-fluoresce at particular UV wavelengths whichcan be used for detecting and identifying biological parti-cles. An instrument called BioSpectralLogger-2 (BSL-2)is being assembled by U. C. Berkley. The core extraction

and sample formation is done by the Sample AcquisitionSystem (SAS).

Ice core drilling is a very well research and implementedfield. These drilling technologies can be done using:

• manual,

• electromechanical,

• electrothermal,

• hot water drill systems or

• thermal mechanical drill

technologies. A survey of these techniques can be foundin [3]. However, most of these drilling systems are forvery large depths (10s or 100s of meters) and the presentapplication only requires core from a depth of about 10cm.

The following sections talk about the design of SAS withits components, cross contamination issues, a partial im-plementation of the original design with control box, in-termediate test results and activities to be done in the nearfuture.

2. SAMPLE ACQUISITION SYSTEM DESIGN

The LORAX Sample Acquisition System (SAS) shouldenable productive science exploration, without human in-tervention, while performing automated coring and sam-pling operations at multiple locations. The SAS will ex-tract a vertical core sample from a chosen spot, fragmentthe sample into crystals and then fill a sample examina-tion container with those crystals, flattening the top sur-face, so that the sample may be inspected by the UV spec-trometer. One objective of the mission is to characterizemicrobial populations in the ice at 1cm, 5cm and 10cmdepth.

The SAS consists of the following subsystems

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1. Core Extraction System

2. Core Ejection System

3. Core Fragmenter

4. Sample Collection Plate

5. Sample Flattening System

6. Turntable & Sample Disposal System

2.1. Core Extraction System

The SAS uses an ice core drill (Figure 2) which is 39mmin diameter and collects a core sample of 25mm diameterto a depth of about 100mm. The core was sized in orderproduce samples with 2 cm of vertical resolution; assum-ing 50% of the sample being lost in the sample prepara-tion process, and producing a final sample of statisticallyrelevant size (see section 2.4).

The core extraction and breaking system on the ice coredrill is similar to that of the minicorer designed by Hon-eybee Robotics [6]. It has a pair of concentric cylindri-cal tubes the inner holes of which are off-axis (Figure 1).The hole of the inner core tube is off-center by the sameamount as the hole of the outer core tube which makesthe centers align in one configuration and completely outof phase when the inner core tube is rotated by half aturn. In the drilling mode the outer core tube rotates ina clockwise direction cutting through the ice. The innercore tube also rotates along with it as it is engaged in thisdirection by a pin and groove arrangement. Needless tosay the axes of both the tubes align in this mode.

Figure 1. Pictorial illustration of a Nested Drill

Once the coring is done to the desired depth the innercore tube is rotated clockwise forcing the ice core to gooff axis. This is done using a half gear mounted on thecore breaking motor (Figure 2) which is not engaged inthe drilling mode but can rotate the inner core tube byhalf a turn. The outer core tube in this mode is fixed beinglocked by a ratchet and the ice core comes in contact witha sharp knife edge at the bottom which parts and breaks

the core (similar to that of [6]). The knife edge whichis part of the outer core tube also supports the core to belocked-in when the drill assembly is lifted up. The tip ofthe drill is designed based on previous ice coring drills [7]having sharp notches at relief angles of 150 and 450 (Fig-ure 3). It has wide spiral flutes to let out the ice chips. Asthe core drill is slender and subject to a lot of vibrations,it has been supported by needle and thrust bearings in theradial and axial directions. The inner and the outer coretube are separated by a thin teflon ring on one end and anO-ring on the other. This reduces friction and facilitateseasier relative motion between the outer and inner coretubes. The O-ring also serves as a seal for any chips thatmight find their way between the tubes. Aluminium hasbeen used for the inner core tube but the outer tube wouldbe out of stainless steel for durability. The core drill alsoaddresses the concern on cross contamination which willbe discussed in section 3.

Figure 2. Ice Core Extraction System

2.2. Core Ejection System

The Ice core drill, after extracting a sample core and de-taching from the place of interest, is lifted on a rack-pinion-follower arrangement out of the hole. This systemalso helps in the drilling process by providing a recip-rocatory motion for drilling into the ice. The rack andpinion arrangement lifts the core (in the core-break orcore-lock position) following along a groove which tiltsthe drill and orients it horizontally. Bearings and follow-ers are provided along the way to ensure this transition issmooth. The inner core tube is then rotated keeping theouter core tube stationary so that both the tubes becomeconcentric for core ejection. In the horizontal position

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Figure 3. Cutting tip of a typical ice core bit (reproducedfrom [7])

the ice core drill assembly is locked by two levers whichprevents the assembly from tipping off when the plungerpushes it forward. After this, the plunger is activated. Theplunger ejects the ice core towards the fragmenter to pre-pare samples at different depths of ice. The plunger com-prises of a rack and pinion assembly with a round headslightly lesser in diameter to the inner core tube. The en-coders on the plunger motor helps keep track of the depthfrom which the sample analyzed is being recovered. (Fig-ure 4)

Figure 4. Core Lift and Ejection System

2.3. Core Fragmenter

The core fragmenter breaks the solid core sample intosmaller fragments in preparation for examination by theUV spectrometer. The UV spectrometer needs a slightlyloose sample of ice with a flat surface. The fragmenterhas to be of a material which can be be easily sterilized

as it is in direct contact with the sample to be inspected.Wire brush was used to fragment the ice as the metal bris-tles can be easily sterilized by heating and also the iceshaving which are sprayed by it provide an ideal samplefor the spectrometer. The wirebrush used is 25 mm wideso that the entire diameter of the sample could be pre-sented. The wire brush is rotated at high speeds allowingvery fine fragments to be recovered. The speed and posi-tion of the fragmenter is adjusted so that most of the icefragments are collected in the sample tray placed belowit.

2.4. Sample Collection Plate

Based on tests conducted by [2] in the Amundsen-ScottSouth Pole Station in January 2000 it was discovered thatabout 200 to 5000 cells/cm3 of bacteria were present inthe surface snow. To have a statistically significant sam-ple to examine, it has been determined that the samplechamber must have a surface area of at least 10cm2 anda depth of at least 5mm. The sample plate is sized at40× 25× 5mm in order to fulfill these requirements.

Figure 5. Flattener with Linear Actuator

2.5. Sample Flattening System

When the wirebrush fragments a 2cm core of ice the snowoverflows the collection plate forming a lump of ice. TheUV spectrometer requires a flat surface for inspection andat the same time there should be no cross contamination.This requires a sterile mechanism to flatten the surface.This is done by the sample flattening system which con-sists of the flattener and a linear actuator (Figure 5). Theflattener is a frame that holds a nichrome wire at a tem-perature of about 2000oK which is used to cut the frag-mented ice. A spring loaded plunger is used to push theice off the collection tray as the nichrome wire completesits pass through the ice.

The flattener is mounted on the linear actuator. The speedof the linear actuator has to be maintained above a certainminimum so as not to melt the sample and also main-tain certain speed depending upon the rate at which the

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nichrome wire cuts the ice. The speed of the linear ac-tuator can be adjusted so that it synchronizes with theamount of heat dissipated by the wire. It is also neces-sary that the nichrome wire is not in contact with anyother sub-assembly. To maintain a relatively flat surfacethe nichrome wire should have a tensile force at its ends.Nichrome wire when heated becomes slack due to coeffi-cient of linear expansion and plastic deformation causedby the force exerted by the ice on the wire. This is takencare by mounting the nichrome wire on pins which arespring loaded. This spring pulls back the nichrome wirewhen it is heated maintaining a tensile force at its ends.The pins holding the nichrome wire are insulated from therest of the system and are connected to a power source.

2.6. Turntable & Sample Disposal System

Once the sample is prepared it has to be transferred to thespectrometer for inspection. The flattener is moved backso that it facilitates the rotation of the inspection door.The Collection plate is fixed to a rotating door (Figure6). This door is maintained closed when the sample iscollected and prepared for inspection. The door is a partof the wall which seperates the SAS and the spectrome-ter. The door is mounted on a bevel gear which rotatesthe door through 180 degrees in either direction. Afterthe sample has been flattened the door revolves through180 degrees in the clockwise direction, so that the sampleis presented for inspection on the other side of the SAS.After the door revolves through 180 degrees it hits a me-chanical stop which prevents it from moving any further.The door maintains a light tight seal as ambient light willinterfere with the spectrometer data.

After the inspection the door is rotated through 180 de-grees in the anticlockwise direction. This again closesthe spectrometer door and brings back the collection platein a position to acquire a new sample. But before a newsample is acquired the collection plate has to get rid of theold sample.This is accomplished by mounting the collec-tion plate on an actuator which tilts it through 160 degreesso that the sample is thrown away and fresh sample canbe acquired for a new inspection. (Figure 6)

3. CROSS CONTAMINATION

One of the key issues addressed throughout the designwas to avoid contamination from one sample to the next.Several steps have been taken both in the design of thehardware and in the operational procedures to reduce thecontent carried over from one sample to the next. Thesample container is to remain in the spectrometer cham-ber except when receiving a sample. The sample con-tainer is of a depth greater than the penetration of thespectrometer – so any contamination stuck to the bottomof the container will not be viewed by the spectrometer.The flattener physically touches the sample only with thehot nichrome wire; and that wire is self sterilizing.

Figure 6. Turntable & Sample Disposal System

To avoid cross contamination in the drill and the frag-menter the plan is to overwhelm any contamination. Thiswill be done operationally by drilling and fragmentingseveral samples at each sampling site, but only collectingand testing the snow from the last sample drilled at thatsite. It is hoped that this flushing of the drill system andthe fragmenter will reduce, to the point of insignificance,any bio-load carried from one sample site to another.

Figure 7. CAD Implementation of SAS

4. MECHANICAL IMPLEMENTATION

To test out some of the cross contamination remedia-tion strategies and several parts of the sample preparationstrategy a prototype SAS has been constructed. In thisphase of testing a temporary core drill which is a modi-fied hole saw chucked to a hand drill is used to acquirethe sample. A mounting bracket has been fixed near thefragmenter that allows the unchucked drill to be mounted,where upon a plunger can be inserted to force the sample

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out against the fragmenter. The remainder of the systemis very close to what we expect for the final SAS (Figures7, 8 and 9).

Figure 8. Physical Implementation of SAS

Figure 9. Detail of turntable and tray in sample disposalorientation

5. PRELIMINARY CONTROL SYSTEM IMPLE-MENTATION

In this preliminary system implementation, control andmanipulation of the drill and core ejection is handledcompletely manually. The rest of the system runs througha control box. This box has knobs that adjust speed anduses a circuit, making use of the limit sensors installed inthe mechanical systems, to ensure automatic terminationof most motor operations. The initiation of each motor isdone manually by depressing a switch on the control box.The control box is shown in Figure 10.

The wirebrush is a simple on/off motor which is turnedon when the fragmentation is to be done and turned offwhen enough sample collects in the collection plate.

The motor for linear actuator which has the flattenermounted on it and the motor for the turntable and for

Figure 10. Control box for preliminary implementationof SAS

the flipping of the sample plate are controlled by rockerswitches as all these motors have to run in both directions.The control circuit consists of a double pole double throwrelay which allows the motor to run in both directions,the extremes of which are limited by limit switches. Thelimit switches, when depressed prevent further motion inthat direction, and only allow power to flow in the circuitto drive the actuator in the opposite direction. The motorshave 5 different speed levels at which they can be run andcircuit breakers for safety.

The Nichrome wire is to be heated to about 2000K anddraws a high load of current. To efficiently control thisand have minimum heat dissipation a voltage controlledcurrent circuit was used. A Pulse-Width Modulated sig-nal for the voltage was generated by op-amps generatinga triangular wave followed by a comparator which givesa square wave. The square wave is fed to an N-channelMOSFET which acts as an electronic switch between thenichrome wire and the power supply. By changing theratio of resistors the duty cycle of the pulse voltage sig-nal can be varied giving different heating levels of thenichrome wire. The current is adjusted so as to get bestcutting performance.

6. TESTING & INTERMEDIATE RESULTS

Initial tests of SAS were conducted independent of theUV spectrometer. The first set of these tests were con-ducted at room temperature. This was mainly to ascertainthe relative positions of the fragmenter, the test drill andflattener for best sample preparation. It was seen that thewirebrush worked better at higher speeds and the prob-ability of fragmented ice collecting in the sample platewas higher when the fragmentation was done closer tothe plate with the center of the plate lying along the verti-cal tangent to the wirebrush. A large portion of the frag-mented sample melted in the collection tray before com-ing in contact with the nichrome wire. This was due to

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the temperature difference between the sample and theassembly and probably due to the heat generated by fric-tion between the fragmenter and the ice core.

The next set of tests were conducted in a walk-in freezerso that the temperature of the system went below freez-ing. The control box was kept outside the freezer at roomtemperature and the mechanical system was maintainedat below 0oC. The quality of samples recovered duringthese tests greatly exceeded the quality of samples ac-quired at room temperature. The fragments of ice col-lected in the sample tray were finer and more uniform innature. Considerably less amount of ice core was usedfor preparation of the sample as the fragmenter was moreefficient at sub freezing temperature. The speed of the lin-ear actuator driving the sample flattener had to be reducedas the sample ice encountered was harder due to the lowertemperature and it therefore offered more cutting resis-tance. Also, the current passing through the nichromewire had to be increased as more heat was required to cutthe ice. The sample dump mechanism and turntable func-tioned almost the same under both conditions. These ex-periments allowed us to optimize the position of the frag-menter in relation to the sample container and the coredrill to maximize sample yield.

Operating the flattener required more finesse than wasoriginally suspected. The actuator speed had to be tightlylinked to the cutting wire temperature. If the speed wastoo low, the sample would be melted by the hot wire andif it was too fast, then the wire would break. Automatingthis control will probably require the addition of forcesensors on the tensioning system of the nichrome wireand/or on the linear actuator. No other control difficultieswere discovered during these experiments.

7. FUTURE ACTIVITIES

An integrated testing of the SAS along with the UV spec-trometer is scheduled for late Summer 2005. These testswill give more reliable data of cross contamination fromsample to sample and/or core to core. It will also pro-vide evidence of any interaction incompatibilities whichmight occur in the actual system of SAS with the spec-trometer. This will help improve the final design of theCore Extraction and Ejection System.

8. ACKNOWLEDGMENTS

This work was supported in part by a grant from NASAAmes Research Center.

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