MICRO ROBOTS FOR SCIENTIFIC APPLICATIONS 2 – DEVELOPMENT OF A ROBOTIC SAMPLING SYSTEM

Suomela J., Saarinen J., Halme A., *

Anttila M., Laitinen S., ** Kaarmila P., *** Visentin G., ****

* Helsinki University of Technology, ** Space Systems Finland Ltd, *** VTT, ****ESA/ESTEC

Contact: jussi.suomela@hut.fi

P.O.BOX 5400 FIN-02015 HUT Tel: +358–9–451 3312. Fax: +358-9-451 3308

An invited session: Mechatronic systems for space Abstract: This paper describes design, manufacturing and testing of a small scaled robotic drilling and sampling system called RoSA. This project was launched by ESTEC in order to study the feasibility of a drilling rover in exobiological exploration on planets, especially Mars. RoSA is a tracked tethered vehicle, which can drive in harsh off-road conditions. The payload of the rover is a drill with dimensions of 110x110x350mm. Despite of its size drill can drill down to 2 meters and sample totally 10 samples during one sample trip. The mission consists of three sample trips up to 15 meters away from the lander. After one trip the 10 samples are brought back to the lander. Keywords: Space robotics, mobile robots, mechanisms, automatic control, teleoperation

1. INTRODUCTION

In the past few years the world has witnessed the discovery of so-called extremophiles. These are life forms thriving in extreme environments (such as rocks several kilometers underground, or underwater thermal vents where temperatures exceed +100 degrees Celsius), which were previously considered to be so hostile as not to be able to sustain any form of life. One of the possible implication of this unexpected proliferation and survivability of life is that some sort of life could have possibly also evolved on Mars or Europa. It is now commonly believed that Mars had once atmosphere and surface water, and recent discovery of possible fossils in SNC-meteorites suggests that life did evolve on Mars during that wet period. As the extremophiles show, life might even have survived deep underground till the present day. The search for possible extinct or extant life is the goal of the exobiology investigations to be undertaken during future Mars missions. As it has been learned from the NASA Viking and Pathfinder missions, sampling of surface soil and rocks can gain only limited scientific information. In fact, possible organic signatures tend to be erased by surface processes (weathering, oxidation and exposure to UV). The only sensible Martian exobiology investigation must be performed on pristine samples that have never been exposed to the surface environment (see [ESA SP-1231]). Two types of samples have this characteristic:

• samples extracted from surface stones/rocks by

coring at a depth of a few centimeters • deep soil samples acquired vertically from a

depth of more than 1 meter. Therefore a Robotic Sample System to be used as part of an exobiology investigation facility has to accommodate the following list of operational requirements (ESA, 1998): • Reach sampling locations in a 15m of radius

around a lander spacecraft • Acquire samples and deliver them to the lander • Drill up to 2 meters into regolith • Drill up to several centimeters into surface

rocks/stones • Drill into non-homogeneous regolith of

unknown hardness • Sample at a certain depth, material of that

specific layer • Allow investigation of soil layering • Acquire pristine samples of unknown hardness

and coherence • Preserve morphology of the sample In order to design system fulfilling such challenging requirements, the European Space Agency (ESA) has funded a technology research contract called “Micro Robots for Scientific Applications 2”.

2. SYSTEM OVERVIEW The Robotic Sampling System (RSS) consists of small rover called Rover Functional Mock-Up (RFMU), which have Drilling and Sampling Subsystem (DSS) as payload. The rover will carry the drill around the landing area. The DSS performs the actual sampling and stores the samples before they are delivered to Sample Delivery Port (SDP). Mission is illustrated in Fig. 1.

Fig. 1. System mission The lander has a stereovision system onboard, which is used to control the rover. The communication and power for the rover is going trough tether.

3. ROVER FUNCTIONAL MOCK-UP (RFMU) 3.1 Design The requirements for the RFMU were that it should be Nanokhod-like robot (Fig. 2) that is able to carry the DSS and enable drilling in all angles. Furthermore the RFMU has to be able to make multiple sample acquisition trips on radius of 15m around the lander.

Fig. 2. Nanokhod robot Nanokhod is the newest version of the well-known IDD family. It was studied in the Micro RoSA project, which was the predecessor of this project. The only “improvement” to the Nanokhod design

from the requirements side was that the tether should be rewindable. The size of the DSS required that RFMU had to be scaled up from the original Nanokhod size. During the design phase it was also noticed that the constant bridge in the rear end of Nanokhod limited the clearance and formed a U-shaped “trap” for stones and other possible obstacles. This reduces dramatically the roving ability of rover on the stony surface of the Mars. The design of the RFMU was made together with Russian RCL-company, which is a spin-off of the VNIITRANSMASH institute – the original designer of the IDD family. RCL also manufactured the RFMU. 3.2 Structure The structure of the RFMU is illustrated in Fig. 3. Rover has two tracks and the equipment and payload departments between the tracks. The whole midpart of the rover can be lifted thus the clearance can be controlled between –20cm to 20cm. The symmetrical structure of the rover allows the use of negative clearance in the case of capsizing i.e. there is no need for recovery. The payload part of the rover can be tilted full 360 degrees in order to allow the drilling in all angles.

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system on Earth i.e. they were little over dimensioned. However, Micro RoSA2 was a study project where only the feasibility of a robotic drilling and sampling rover was examined. Therefore the actuators were dimensioned to provide sufficient performance on earth instead of optimizing them for the Martian conditions. Tracks with adjustable clearance provide extremely good off-road locomotion capability. Only loose sand hills and big step like obstacles stopped rover during the locomotion tests. 3.4 Tether According to the requirements RSS should get its energy from the lander via a rewindable tether. Sample acquisition trips should have minimum radius of 15m. When manouvers were allowed during the trip, the needed length of the tether was decided to be 40m. A Kapton type flat foil cable was chosen because of easy handling as flat cable and wide temperature area.

Fig. 4. Tether unit. Tether system (Fig. 4) consists of reel and feed rollers. In the tether control the cable tightness on the reel is the main issue. Kapton foil is extremely slippery and loose cable will “explode” on the reel. Tension is kept by controlling the cable speed with the feed rollers and keeping the reel torque always to the “inside” direction. There is totally 41 meters of Kapton cable on the reel. Cable has 5 wires, 3 for communication and two for power supply.

4. THE DRILLING AND SAMPLING SUBSYSTEM

The Drilling and Sampling subsystem (DSS) was restricted in volume 110 x 110 x 350mm and in mass 5kg. It should be able to drill into 2m with maximum of 6W power consumption. The DSS is similar to conventional automatic drilling machines. In order to meet the 2m-penetration depth and still be able to meet the volume and size restrictions, the DSS features an extendable drill string. The string is assembled from up to 10 pipes (20cm each). There are two carousels in DSS called

the pipe carousel and tool carousel. The tool carousel has space for ten tool bits that also work as sample containers.

Fig. 5. The MDP in drilling configuration There are totally seven actuators in DSS. Drilling is done with two independent motors. The rotation actuator (30 rpm, 1Nm) is mounted on a sledge moved in and out by a spindle, which is propelled by thrust actuator (0-30N). Both carousels have one actuator for rotation. Two actuators are used to clamp the drill pipe while new pipe is added (or removed from) to drill string. Further more there is one solenoid-operated actuator, which grips into pipe that is currently handled.

Fig. 6. DSS 4.1 Spindle The spindle is mounted on a sledge that moves up and down along a linear slide. The motor is located aside the spindle and below the sledge and the motion is transmitted with train of gears. When feeding the string down inside diameter of carousel support allows the spindle motor to penetrate below lower end of the pipe carousel. Slip ring is used to transfer electricity from sledge to the rotating spindle for the spindle solenoid and active drill tool functions. There are five isolated lines in the slip ring. Absolute spindle position is detected with the aid of a split in the slip ring. The split in the ring is detected as a break in electrical current. After this signal spindle position is

calculated from incremental encoder mounted to the end of the spindle motor.

Slide top support

Pinion and sledge on slides

Decoupling mechanism

Pipe carousel motor

Pipe top support

Top disk arc

Pipes in carousel

Clamp motor

Clamper frame

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Fig. 7. DSS Mechanical layout 4.2 Solenoid Spindle solenoid is used to operate a wedge that can prohibit contraction of the locking ring and thus control disconnection of the spindle coupling when separating pipes from each other and from the spindle. The solenoid is a flip-flop-type using a permanent magnet core to maintain each of its two positions and thus does not require any springs, separate locking mechanisms or continuous power input. Power storage is realized with a capacitor that can produce a high short-term output power for solenoid, while collecting energy for the next operation with low input power.

Fig. 8. Solenoid actuated pipe gripper

4.3 Pipes The drill pipes are located in a carousel. Inside diameter of the pipes is 13 mm, outside diameter is

15 mm, and three-ended helical flute outside the pipe has 17 mm outer diameter. Electrical connections for active tools are located concentric in the middle of the pipe cone using a coaxial plug. The coupling between the pipes and between the string and spindle is realized with a conical connection that also provides a geometric constraint with triangular cross section to transfer torque. Coupling is locked with a split ring, or a C-ring, on the spindle part (male), the female coupling on the pipe upper ends has a mating groove for the ring. Shape of the groove is made non-symmetric with different conical angles such that coupling by pushing happens easily, but de-coupling by pulling requires a force close to 100 N, which is close to linear drive capacity. It was soon learned that the C-rings needed to have a special design to operate with desired forces. Several different designs were incorporated and tested. Groove for the C-ring has angled surfaces so that the slightly contracted ring causes some pre-load for the coupling. 4.4 Tools The tool is designed to drill a 17 mm diameter hole into rock material and to contain ~9 mm diameter and ~15 mm long sample core inside it. A crown that carries out the cutting is constructed of several cutting bits made of hard alloy. In order to penetrate deep into soil the upper end of the core is ground into dust with a secondary cutter blade and ejected outside the tool through small holes in the side of tool. Drilling does not utilize any hammering action and relies only on cutting and grinding. Therefore its ability to penetrate into rock is limited to softer materials like limestone. Acquired sample is stored and transferred inside the tool. A hard core is broken with the aid of a core lifter (a ring shaped wedge) that grasp on the core when the tool is elevated. The tool bits are made of hard alloy and have a sharp cutting edge, instead of dull point that would be suitable for a hammering drill.

Fig. 9. Tools and pipe The tools are connected to the tool carousel with a flexible spring member. The tool is inserted to the carousel with combined and controlled feed and tool rotation. With the aid of an aligning pin the tool finds correct position in the carousel and the spring finds an edge to hold the tool. The helical flute of the tool provides the alignment with the alignment pin. For removing the tool it is rotated so that a bulge in the side of the tool lifts the spring above the edge and sets the tool free. The string connection between the tool and string is similar to pipe connections. The locking between the carousel and tool makes it

possible to disconnect the tool from the string by pulling. After a sample is captured inside the tool the string is retracted from the borehole. The coring tool where the sample is stored is transferred to the tool carousel in the end of a pipe. The tool containing the sample is locked into the tool carousel with the procedure described above. 4.5 Tool design test The tool design was tested in test bench. Tests were done with a gear motor installed into vertical linear slide. The weight of the motor gave a 30 N thrust force and the motor speed was set to 30 RPM. The drilling was done into limestone. Tests show that penetration deep into limestone was 14 mm/hour (at 3 Watts electrical power, 30 N thrust and 30 RPM) coring (10 mm diameter core), and 3 mm/hour during drilling deep (making a bore hole 17 mm in diameter, holding a 20 mm core inside the tool). Up to 8 cm deep was drilled in 24 hours. Dust exists the borehole properly and flow-through design of the tool is functional. The 20 mm core remains inside the tool during drilling as planned.

Fig. 10. Drill tool and a core sample from limestone

Grabbing, breaking and holding of the rock core with given wedge-design was not reliable. The wedge was too stiff to properly grab on core. Accidentally a core broke by itself, and then the wedge was able to grab it and hold it stiffly. In this case the core inside the tool prevented any deeper penetration. A new design of a wedge is being developed.

5. SOFTWARE DEVELOPMENT Much of the on-board software development and testing had to be done before the actual rover computer and mechanics were available. The functions of the rover control program are quite complex: • it obeys high-level commands from the lander to

move the rover and run the DSS, • it controls the rover and DSS for automatic

drilling, mobility, and sample delivery, using several motors and mechanisms concurrently,

• it sends housekeeping, monitoring and reporting telemetry to the lander.

5.1 Component selection criteria To keep the cost low, commercial or free parts and tools were chosen, while keeping in mind eventual upgrading to flight quality. The programming was made in Ada language, which enables portable code between different processors and operating systems in the development phase and in possible flight model upgrade. Operating system was Linux, which is not only low-cost but also makes it easy to write new device drivers. A common PC104 stack was chosen for the rover computer, which includes a PC board with the Intel 486DX processor and required boards. This computer is powerful enough to run Linux, the rover control program, and several other programs that may be required for testing purposes. These software and hardware choices have interesting synergies: • Since the Ada code is portable, the prototype can

be equipped with a powerful processor without impact on the software.

• The powerful processor allows the use of a powerful operating system. Since concurrency is a standard feature of Ada, the choice of operating system has little impact on the Ada code.

The rover computer boots a small Linux system from a Disk-On-Chip (DoC) solid-state device, which simulates a small hard disk. It also loads the rover software from the DoC.

6. SOFTWARE DETAILS 6.1 Overview The rover and DSS are controlled from a Control Unit PC that simulates lander. The Control Unit provides a graphical user interface (GUI) for control and monitoring. The GUI and the rover control program allow manual control of all rover and DSS operations. A testing interface and a rover simulation program can be used to test the software without the rover hardware. Figure 11 illustrates the overall system.

Fig. 11. System and software units.

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6.2 Lander simulation software The GUI is written in Java. It sends user commands to the Control Unit TC/TM program through a UDP socket. The Control Unit TC/TM program is written in Ada and reuses TC/TM code from the rover. It formats the command as a TC and sends it to the rover through the tether, again using UDP. TM from the rover is received over the tether and passed through the Control Unit TC/TM software to the GUI for display. 6.3 General autonomy requirements The control of the robot is made with so-called “move and wait” principle (Sheridan, 1992), which is typical for space robots with long communication delay. Due to the demanding task the rover has to teleoperated. However, teleoperation is carried by short tasks – like “drive 20cm forward” – and feedback will then be waited for over the delay. Adequate autonomy of the rover-lander system is an essential task for move and wait teleoperation. In-situ image processing for rover control must be included in the mission, since the time delay of one-way telecommunication between Earth and Mars varies from 6 to 22 minutes, depending on the relative positions of the planets. In the worst case, the ground receives the reply over 40 minutes after sending a message. Navigating the rover from Earth would be very difficult and time-consuming, and therefore the lander and rover must control the short navigation tasks and all drill functions autonomously. The rover-lander tether is 40 meters in length, and the TC/TM bandwidth is limited (the prototype uses RS232 signals). Therefore, even without the Earth-Mars time delay, the rover must perform the lower level tasks autonomously, including the actual drilling and sampling.

7. SYSTEM TESTS Tests of each subsystem tests are reported in the chapter in question. The objective of the system tests was to imitate the mission and drive to a sampling place and perform automatic drilling and drive back. The tests with the integrated system were carried in hurry when the time and small budget were already used. Integration and testing were mainly concentrating to the software-DSS cooperation in order to achieve automatic drilling. This was caused by a remarkable delay in the drill manufacturing. The rover control was tested by teleoperating it with the planned “move and wait” method in off-road conditions. System worked well and even the most critical part, tether with automatic control, ran without any problems. The performance of drill tools was already tested in the bench but there was no time for the bench testing

of the ready drill. Instead, the drill was supplied to the integration. Integration and testing were carried simultaneously. Automatic drilling was achieved in the laboratory conditions when plastic foam was used as material. Real materials were not tested. The main reason for difficulties in the automatic drilling was the solenoid, which was used to grip and release the topmost pipe to/from spindle. Lack of power caused a function probability of few percent, which made the reliable automatic function impossible. The pity was that solenoid would have been easily replaced with a motor and linear screw combination which would have provided enough force. However, the budget didn’t anymore allow this.

8. CONCLUSION The project proved that a small sized robotic sampling and drilling vehicle is feasible. A 2-meter long drill can be built in relatively small dimension and still be functional. The innovative idea to use several drill heads, which also work as sample containers, provides possibility to drill different materials and collect multiple, separated samples during one trip. Long rewindable tether makes it possible to minimize the size of the rover and still provide enough power for the locomotion and drilling. Its much more easy to produce and store the electric power in the lander than in the rover. However, there were also matters to be developed further or replaced before the concept is totally approved. The size and mass of the rover should be bigger in order to provide a rigid base for drilling, especially for deep holes. The existing power of the drill, 6W, is too low for hard materials (MOH > 4). The mechanical structure of the DSS is good in principle but some details need tuning. The automatic drilling was extremely difficult. However, this results mainly from low budget, which did allow only fast prototyping and final manufacturing. An iterated second version should have been done in case of complicated mechanical system like DSS. Anyway, the basic idea is functional and it will be developed further in two projects. First concentrating to the development of the drill system and another to the supporting rover.

REFERENCES ESA/IPC (1998)

Invitation to Tender AO/1-3477/98/NL/PA, Micro Robots for Scientific Applications 2

ESA SP-1231 Exobiology in the Solar System and The Search for Life on Mars. ISBN 92-9092-520-5

Sheridan T. B. (1992) Telerobotics, Automation, and Human Supervisory Control, The MIT Press 1992