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Instrumentation and vehicle platform of a miniaturized submersible for exploration of terrestrial and extraterrestrial aqueous environments
Acta Astronautica 79 (2012) 203–211
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Instrumentation and vehicle platform of a miniaturized submersible for exploration of terrestrial and extraterrestrial aqueous environments$ Jonas Jonsson n, Johan Sundqvist, Hugo Nguyen, Martin Berglund, Sam Ogden, Kristoffer Palmer, Katarina Smedfors, Linda Johansson, Klas Hjort, Greger Thornell ˚ Angstr¨ om Space Technology Centre, Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden
a r t i c l e i n f o
abstract
Article history: Received 29 February 2012 Accepted 26 April 2012
An example of an extraterrestrial environment likely to support life is the vast liquid body believed to hide underneath the frozen crust of Jupiter’s moon Europa. The hypothetical exploration of this, as well as the more accessible subglacial lakes on Earth, has been used as model applications for the development of a heavily miniaturized, yet qualified, submersible with the potential to be deployable either in itself through a long and narrow borehole or as the daughter craft of an ice-penetrating cryobot. Onboard the submersible, which is only 20 cm in length and 5 cm in diameter, accommodation of a versatile set of sensors and instruments capable of characterizing and imaging the surroundings, and even collecting water samples with microorganisms for return, is facilitated through the use of miniaturization technologies. For instance, together with a small camera, a laser-based, microoptic device enables the 3-D reconstruction of imaged objects for topographical measurements. As a complement, when the water is turbid or a longer range is wanted, the world’s smallest side-scanning sonar, exhibiting centimeter resolution and a range of over 30 m, has been developed. The work on miniaturizing a CTD, which is a widely employed oceanographic instrument used to measure and correlate conductivity, temperature, and depth, has commenced. Furthermore, a device employing acoustics to trap microscopic particles and organisms, and, by this, enrich water samples, is under development. To ensure that the gathered samples are pristine until analyzed at the end of a mission, the device is equipped with high-pressure, latchable valves. Remote operation and transfer of measurement data and images, or even live streaming of video, is made possible through a kilometer-long fiber optic cable being reeled out from the vehicle underway and tethering it to a terminal. To extend the missions, the same fiber shall also be capable of charging the onboard batteries. In this paper, the vehicle and its subsystems are summarized. Subsystems essential for the vehicle’s operation, e.g., hull structure, communication and power management, are treated separately from those of more mission-specific nature, like the instruments mentioned above. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Submersible Micro Imaging Water sampler Subglacial Sensor
1. Introduction $ n
This paper was presented during the 62nd IAC in Cape Town. Corresponding author. Tel.: þ 46 18 471 72 35; fax: þ 46 18 471 35 72. E-mail address: [email protected] (J. Jonsson).
0094-5765/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2012.04.048
Astrobiology and the search for extraterrestrial life is of increasing interest as new signs are revealed of a
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Mars that was seemingly wetter in the past [1], and due to the continuous discovery of Earth-like exoplanets situated in the habitable zone around other stars in our galaxy [2]. In addition to these Earth-like planets, ice-covered moons orbiting gas giants, such as Europa and Enceladus in our own solar system, could outnumber these habitable alien Earth’s, and potentially have environments to support extraterrestrial life, even though situated outside the habitable zone. They are thought to shelter liquid environments underneath their frozen crusts, which could host extraterrestrial ecosystems [3]. To find out, these extraterrestrial water bodies will have to be accessed and explored. This, however, necessitates a lander mission with an ice-penetrating cryobot, or a long and narrow borehole, and the subsequent deployment of a small submersible—a hydrobot. Due to the nature of such a mission, there will be severe size and weight restrictions imposed on the hydrobot and its scientific payload. As a stepping stone towards a suitable vehicle, a small but well-instrumented submersible is currently under development. The Deeper Access, Deeper Understanding (DADU) project aims to develop a small submersible concept using miniaturization technologies to provide high functionality, despite the vehicle’s limited size (Fig. 1). The submersible is smaller than two soda cans placed together end-to-end, and is designed to fit on or inside an ice borehole probe [4]. The main objective, and the size-limiting application for the submersible, is to explore the otherwise inaccessible subglacial lakes on Earth, which are environments analogous to those thought to exist on the frozen moons. These lakes, some of which are believed to have been isolated by ice for millions of years [5], can be found under up to several kilometer thick ice sheets in the polar regions. The exploration of the lakes can possibly teach us about Earth’s climate changes, since they are seen as environmental fossils, and may testify to life’s start and evolution on Earth. However, the vehicle could of course also be used for the exploration of any water-filled environment, such as pipe and cave systems. As for payloads, the vehicle will be equipped with a camera, illuminating LEDs and a compact laser system in its bow (Fig. 1). This system will provide the images seen by the camera with topographical information through
the projection of specific reference patterns. Along the corpus assembly of the submersible, ports and pockets can house devices such as the miniaturized side-scanning sonar to be used for mapping of the surroundings, having a range of over 30 m, and centimeter resolution. A conductivity– temperature–depth (CTD) sensor, which is a fundamental oceanographic instrument, enables depth- and temperature-corrected measurements of the salinity. A water sampler with a particle and microbe enrichment feature enables concentrated samples to be returned for laboratory analysis. A two-dimensional flow sensor records the velocity of the submersible through the water, or the flow around its hull when the vehicle is stationary. In this paper, the status of this highly functional submersible concept and its subsystems, enabled through forceful miniaturization technology, is reported. 2. Background 2.1. Subglacial lakes Subglacial lakes are bodies of water found underneath ice sheets up to several kilometers thick. They can be as big as Lake Ontario in North America [5]. The largest of the more than 145 subglacial lakes found in Antarctica is Lake Vostok [6], which is covered by a layer of ice over 3 km thick. It is believed to have been isolated by ice for the last 15 million years. An ice core with frozen subglacial lake water on it has revealed microorganisms [7]. However, no water sample has yet been obtained although current drilling operations have just penetrated the lake–ice boundary. In the northern hemisphere, lakes underneath the ¨ Vatnajokull ice cap in Iceland have been reached by drilling and sampled for microbes [8]. Geothermal heating from underlying volcanoes melts the ice of the glacier to create these lakes, which are the sources of periodically occurring glacial outburst floods. In in-situ investigations, a hot-water drill penetrated the 300 m thick glacier. At this depth the drill created holes from 6.6 to 14.7 cm in diameter, depending on the speed used for drilling [9]. Also, a simple sampler, with a diameter of 4 cm and a length of 185 cm, was built to collect 400 ml water samples at certain depths to characterize the water and search for microbial life in it [10]. These samples could, however, be taken only straight underneath the boreholes. 2.2. Miniaturization technology
Fig. 1. Hull model of the DADU submersible.
For designing a small submersible with high functionality, commercial components and instruments are generally not suitable, as they tend to be too large. Therefore, miniaturization technologies have been used where applicable in the development of this vehicle. The microelectromechanical systems (MEMS) technology, also known as microsystems technology, is a field where devices with features and even components measured by the micrometer, are developed [11]. Most of the processing technology for MEMS is inherited from the semi-conductor industry, and often employs photolithography, thin-film deposition and etching processes, to create structures on or in a silicon or glass wafer.
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With features being comparable with dust in size, the processing is performed in cleanroom facilities under controlled ambient conditions. Since the surface-to-volume ratio is large for MEMS devices, behavior and performance may not be according to intuition. For instance, surface effects such as wetting dominate over volume effects such as inertia. These scaling effects can be used to manufacture devices not only smaller than their macroscopic equivalents, but with, for instance, higher sensor sensitivity. In addition, through batch processing, the components’ cost can be very competitive.
unit (IMU) uses accelerometers and gyros to monitor the orientation of the submersible. Simultaneous data and command transfer is enabled through a fiber optic cable, connecting the submersible with a control station. The brain of the vehicle is the microcontroller unit, which controls the peripheral units, data and command flow. Power to the submersibles subsystems is provided by rechargeable battery packs. Further descriptions of the subsystems and the instrumentations of the DADU vehicle follow below in their respective sections.
3. Submersible design
3.1. Control system
Early considerations set the size of the submersible to 5 cm in diameter and 20 cm in length. The primary reason was to enable deployment through narrow ice boreholes, by itself or together with an ice probe [4]. With future, extraterrestrial missions in mind, a low mass was another reason. Studying other ROVs, both large and small, a common feature is that components off the shelf (COTS) are often used. In applications where no strict size restrictions are employed, or a high degree of functionality is not required, this works fine. However, when size and weight are of essence, and more instrumentation and functionality are needed, tailored miniaturized subsystems are required. The final design of the submersible was derived after a concept generation and evaluation phase, attempting to maximize the submersible’s maneuverability in tight and confined waters, and to provide some maneuver redundancy in case of thruster malfunction. The vehicle is semi-autonomous and equipped with eight small thrusters, enabling movement with five degrees of freedom. The vehicle will be supervised by an operator, but could be pre-programmed to perform simple maneuvers and attitude controls automatically. An inertial measurement
A single low-power, 8-bit microcontroller was chosen to perform the control and monitoring of the on-board data handling (Fig. 2). Compared with many 32-bit architectures, this preserves power and reduces system complexity and size. A dynamic prioritization of different tasks enables a highly configurable interrupt- and event-system, ensuring system responsiveness and stability. Furthermore, a compact way of obtaining and storing measurement data from the various instruments is provided by the microcontroller’s on-chip memory and analog-to-digital converters. The control system is passive in that it replies to, and takes action on, requests from the control station. However, collection of on-board housekeeping data is an exception to this, and is performed continuously. If anomalies are detected, the control system will signal to the control station at highest priority. 3.2. Propulsion and attitude control The propulsion system has not yet been fully miniaturized, but eight small electric engines and propellers have been custom fitted and integrated into the hull of the submersible. For the main, strong forward and backward
Fig. 2. System architecture of the DADU vehicle.
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locomotion, four thrusters are placed in the back end of the stern assembly (Fig. 3). By using four thrusters with propellers rotating oppositely in each pair, the center of the stern is free for feeding out, or retrieving, the optic fiber from the reel located in the stern. A certain redundancy is also built in with this configuration, since as long as all of the thrusters do not fail at the same time, loss of thrusters will not substantially cripple the submersible’s locomotive ability. On each end of the corpus assembly is an attitude control unit with one vertical and one horizontal thruster (Fig. 3). These can be used for translation in vertical and horizontal directions as well as for changing the submersible’s pitch and yaw by activating them in opposite directions. Compared with a conventional design based on external rudders and propellers, there are three main advantages with the current thruster setup. First, the submersible does not need a simultaneous forward or backward movement to perform attitude adjustments. Instead, the vehicle can, e.g., change its field of view while maintaining its position. In addition, this increases the submersible’s maneuverability in confined spaces. Second, the risk of getting tangled up in the fiber optic cable or any objects in the environment is reduced since there are no large protruding objects from the hull. Third, there are no space-demanding mechanisms in the stern as would be required for controlling a rudder. With the current configuration, only a rolling function is not available. However, this is not desired. 3.3. Navigation and attitude sensing A commercially available six-axis IMU (SD755, Sensor Dynamics, Austria) was chosen. Due to the availability of suitable COTS accelerometers and gyros, commercial sensors have been used instead of tailor made ones, in order to fully monitor the translational and rotational movements of the submersible. Although an IMU is not useful for long-range navigation, since even the smallest offsets and errors will accumulate over time, it is able to accurately reveal the submersible’s current orientation, and can be used to track movements for shorter periods of time, such as during sonar sweeps. 3.4. Power handling Low power consumption is of utmost importance, for a miniaturized system designed to be operational in
hard-to-reach environments over extended periods of time. The power consumption has been estimated for each subsystem (Table 1). The motors account for most of the power consumption of the submersible, but not all of them will be active at the same time. The water sampler is activated only when sampling is being done, and the valves only when they are opening and closing, since they are latchable. It is estimated that the sonar element uses 1 W when transmitting, but only at a duty cycle of 1%, meaning 10 mW/element. The LEDs are the four diodes used to illuminate the camera’s view. The power figure is for the maximum effect, 150 mW each; however, the LEDs are dimmable. Power, in the form of light, will be transferred through the fiber optic cable simultaneously with data and commands. Operating as a solar cell, a photodiode will collect this light, converting it into electrical power. It has been estimated that 0.1 W of power can be transferred through the fiber. For extended missions in environments where resurfacing is not an option, such as deployment in an extensive cave system, the submersible could go into hibernation mode, resulting in power consumption in the sub-milliwatt range, while the batteries are recharged through the fiber, after which the submersible can recommence its mission. Because of their flexibility in shape and their high energy density, lithium-ion polymer type batteries were chosen.
3.5. Communication When the vehicle ventures through narrow and winding openings, the fiber optic cable must enable noninterrupted communication and high bi-directional data rates. By using a thin, single-strand optic fiber, reeled out gradually, the drag, which is usually the main opposing force a ROV experiences, will be minimized. Three wavelengths will be used concurrently in the fiber to simultaneously transmit the uplink and downlink signals and power. Evanescent couplers will couple these wavelengths into and out of the fiber at each end. According to simulations, a coupling efficiency of over 90% can be reached for separating the wavelengths of 940 nm, 1060 nm and 1550 nm. The waveguides and guide structures, used for precise alignment of the fiber core to the waveguide core, which is only 5 5 mm2 in cross section, are made from microstructured polymer materials (Fig. 4). Table 1 Estimated power consumption of subsystems.
Fig. 3. Exploded CAD view of the submersible.
Subsystem
Estimated power consumption [mW]
Microcontroller Motor Communication Water sampler Sonar element IMU Flow and speed log Laser and Camera LEDs CTD
50 1000/each 100 10 10 250 100 500 600 10
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3.6. Hull structure The bow, corpus and stern assemblies comprise the three main sections of the submersible’s hull, Fig. 3, and contain guiders, fixtures and supports for the subsystems inside. In this way, the numbers of interfaces and connectors are reduced, saving, in turn, passive material and mass. Also, it increases the strength of the hull. To enable iterative design and testing of the hull and the locomotion modules, rapid 3-D prototyping (Dimension Elite, Stratasys) was utilized, using fused deposition of ABS-plus plastic. As a design goal for the submersible, a maximum operating depth of 1000 m was set. Leakage elimination is an important issue in the design, due to the high pressures experienced at these large depths. Because of the external locations of sensors and thrusters, the many feed-throughs in the hull make this a great challenge. To further decrease the risk of leakage between the three main hull sections when connected, each section is sealed from the others. The hull can be pre-pressurized, for example to the half of the maximum operation pressure, in order for a fairly thin, and thus not too heavy, hull to withstand high pressures experienced at larger depths. However, while mitigating the leakage problem, such a hull can become a safety issue at shore. Another option is to use a flexible hull section or bladder, and filling the submersible with a more or less incompressible liquid in order to equalize the internal pressure with the external [12]. This puts fewer requirements on the hull itself, but transfers the pressure tolerance requirements to the internal subsystems. In the hull design, the overall density of the vehicle will have to be considered since the submersible should be neutrally buoyant in water. Empty or gas-filled compartments have to be used to compensate for the higher-density components and subsystems, including the intended hull. 3.7. Buoyancy Operation in different environments and at different depths requires that the density of the submersible is
Fig. 4. Optical fiber guided in place by two ridges (top of image), and a waveguide on the chip (bottom of image). The diameter of the fiber is about 100 mm and the width of the waveguide core is 5 mm.
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adjustable. The submersible is designed to be neutrally buoyant in water, and the subsystems inside the submersible are placed so that the submersible is stable, i.e. the mass center is located right below the geometrical center. To be able to change the buoyancy during a deployment, a solution with two tanks, each close to one end of the corpus, was chosen. The buoyancy and the pitch angle of the submersible can be adjusted by loading different amounts of water in the tanks. It is preferable to position the tanks in the horizontal plane going through the overall mass center. If the position of the tanks is not in this plane, the overall mass center will be shifted up or down when filling or emptying the tanks, and thus there will be a risk of instability. A ballast of small lead balls was used to obtain the right buoyancy for tests with the prototype in shallow water. 4. Instrumentation The instrumentation of the submersible covers standard functions of a submersible vehicle and more missionspecific devices. The primary placement of the instruments is in specific bays on the hull. 4.1. Side-scanning sonar A matchstick sized side-scanning sonar, 50 2 3 mm3 in size, to map the surroundings of the submersible, for scientific as well as navigational purposes, and complement the camera, has been developed and characterized [14]. Mapping of the bottom, sidewalls, or the ceiling of an enclosure around the vehicle can be performed by mounting side-scanning sonars around the submersible in various configurations. One of the strengths of the side-scanning sonar compared with a camera system is that it is not affected by murky and dark waters, due to the longer wavelength used for ultrasonic devices compared with visual light. The resolution in the direction along the track is set by the pulse frequency of the sonar electronics. However, the resolution in the direction perpendicular to the path of travel is inversely proportional to the length of the sonar and directly proportional to the frequency used. Unfortunately, the higher the frequency used, the larger the attenuation. But for the application of the DADU submersible, a range of only 10 m was required. A trade-off resulted in 600 kHz. The finite element analysis software Comsol Multi-physics was used to predict different sonar design performances and the effect of the mounting structure on the beam pattern due to their tight mounting in the hull (Fig. 5). Measurements were performed in a water tank setup using a needle hydrophone to characterize the performance of the sonar. Stepping motors were used to sweep the sonar in front of the hydrophone, and a profile of the beam pattern was acquired. Finally, field tests in a nearby river, with dark and murky waters, were performed. This was thus a good test for the ability of the sonar to map the bottom, for which the camera system could not be used. Objects such as stones and rocks, mudflats, sunken trees, cables, and even schools
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of fish, could be imaged from the side of a boat traveling along the river (Fig. 6). The small side-scan sonar, to fit on the miniaturized DADU submersible has been designed, manufactured and tested. The sonar has been used to image river bottoms and objects at least 30 m away, and was found to exhibit centimeter resolution.
4.2. Laser-camera topography measurement system A small high-resolution camera is used to capture images and record live streaming videos of the surroundings. The camera is placed in the nose cone with four LEDs to illuminate the scene in front of the submersible. One problem with images acquired underwater is that they tend to be flat and difficult to make measurements in. To solve this problem, laser illuminated diffractive optical elements (DOE) have been developed to project reference patterns, e.g. dots and grids, onto a scene viewed by the camera [13] (Fig. 7). The working principle of the camera-laser system is that the laser and DOE project a pattern onto an object viewed by the camera (Fig. 8). By knowing the geometrical setup of the camera, laser and DOE, and by tracking how the object has distorted the pattern, the position of each laser dot in 3-D space can be calculated. From this, the distance, shape and size of the object can be determined.
At a distance of 30–45 cm the system was able to determine the position of objects with 0.5 cm accuracy. Angles from 01 to 801 were measured on a screen at a distance of 45 cm within an error of 41. The performance in turbid conditions was tested by adding coffee (up to 600 ml brewed with 50 g fine-grained coffee per liter of water) to the tank, containing 25 l of water. At the higher concentrations, it was hard to discern the contours of the object by the naked eyes. The system, however, was still able to make measurements. The errors of these distance measurements with the turbid water were found to be within 0.5 cm at a set distance of 45 cm. 4.3. Conductivity, temperature and depth sensor system An essential instrument for the field of oceanography is a CTD sensor. It measures conductivity, temperature and depth, and is found on most research vessels. From these measurements, other characteristics of the water can be calculated, such as salinity. CTD instruments are usually large and cumbersome devices, hoisted down into the water using cranes. This necessitates the use of a ship large enough to handle these devices. Although with a penalty in sensitivity and precision, miniaturized CTD sensors are an alternative, allowing for mounting on small vehicles and even animals. Moreover, since several miniaturized CTD setups can be included on one small chip, redundancy can be achieved. The size of the CTD chip being developed here, to fit in one of the instrument bays on the outside of the
Fig. 5. Comsol simulation result showing the total acoustic pressure emanating from a sonar element.
Fig. 6. Acquired side-scan sonar field image of a river. The wavy pattern close to the midddle of the image is the bottom at the end of a boat launching ramp.
Fig. 7. Laser, DOE, and projected pattern on a screen.
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Object
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DOE Laser
Camera
CPU Fig. 8. Working principle of the topography measurement system, where a laser shines through a DOE, creating a pattern onto an object. The camera records this pattern, and software compares this with a reference image of the pattern to calculate the position of each dot in 3D space.
submersible’s hull, is 30 15 3 mm3. It accommodates six individual CTD sensor element sets of varying designs. The measuring electronics will fit inside the submersibles hull, or, for a stand-alone instrument, be fitted together with the chip in a package smaller than a matchstick box. Conventional MEMS technologies and processes were employed to miniaturize the sensor elements, and a silicon wafer was used as the base material. Photolithography and physical vapor deposition processes were used to form the sensor electrodes on one of the wafer surfaces (Fig. 9). To create membranes, which will be used to measure pressure together with strain gage structures, the back side of the wafer was structured using deep reactive ion etching. The CTD chips were glued to glass to create a sealed-off cavity underneath the pressure membrane (Fig. 10). The temperature sensor is an integrated platinum element, where a certain resistance shift can be correlated to a specific temperature close to linearly. To measure the conductivity, an AC potential is applied over two electrodes. Ions in the water will set up an electrical current, and the impedance can be measured and correlated to the conductance of the water. The pressure sensors have been simulated using finite element analysis software and designed in four different dimensions, each able to withstand a portion of the specified operating range of 0–1000 m depth, and yet provide membrane strains high enough to be measured by the integrated strain gauges. A protruding rigid center is used to halt further vertical movement of the membrane when its specified maximum pressure has been reached, in order to prevent rupture. A precision and accuracy of 0.01 1C, when cycling up and down in the range 5.00–40.00 1C, have been obtained in a tempered water bath. Although the instrument may not reach the sensitivity and precision of larger CTD systems, it enables coarser measurements in tight and confined environments, where larger systems cannot reach. This chip-based CTD could also be used separately as a bio-logger on small marine animals. 4.4. Flow and speed log A two-dimensional thermal flow and speed log is being developed to record the velocity of the submersible, or to
Fig. 9. Electrode layers making up two CTD sensors on part of a 30 15 mm2 chip.
record the water flow over the hull when the vehicle is stationary. The sensor uses the so called calorimetric principle where forced convection changes the temperature profile around a heater (Fig. 11). By measuring the temperature upstream and downstream of the heater from the change in resistance of similarly designed elements, the flow can be recorded. A prototype sensor, Fig. 12, has been developed with 0.5 mm thick Pyrex glass as a substrate material. It has a fairly low thermal conductivity, meaning that the heat loss and thermal crosstalk will be low compared with a sensor with silicon as substrate material. Four sensing elements are placed symmetrically around the heater in order to be able to measure the flow in two dimensions. Preliminary tests indicate that the sensor design is suitable for the typical velocity range of the submersible. 4.5. Water sampler In order to collect particles in the water, such as microbes, water samples are needed. Due to the size restrictions of the DADU vehicle, the large sample volumes necessary, especially in a sparsely populated oligotrophic environment, cannot be accommodated on board. To mitigate this, a compact sample enrichment system is under development. In this, water will be filtered for microbial organisms, 1–10 mm in size, in an acoustic standing wave field generated by miniaturized piezoelectric transducers [15]. After a tailored dwell time of the moving vehicle forcing water through the microfluidic acquisition system, while the transducers enrich the fluid, the sample will be sealed off by high-pressure latchable valves [16].
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Fig. 10. Cross section of the glued CTD sensor with a membrane structure etched in the top part, onto which the different electrode layers have been deposited.
Fig. 11. Comsol simulation of the two-dimensional flow sensor, with the heater in the middle. The color plot represents the temperature profile over the surface, and the arrows the velocity field of the water across the sensor casuing this profile to be asymmetrical. (The five squares represent the locations of the heater (middle) and the four orthogonally placed sensing elements.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Microscope image of a developed flow sensor. The field of view is approximately 1.7 1.3 mm2. (The copper of an underlying PCB is visible through the glass substrate.)
The sampler can then be taken to a laboratory facility for subsequent examination, even after an extended mission. The device, which is 30 mm long and 15 mm wide, consists of a stack of stainless steel and polyimide sheets (Fig. 13). Steel sheets make up the structure of the microfluidic channel and the valves. A polyimide sheet provides a membrane material for the valves. A cover glass is used as an acoustic reflecting layer for the piezoelectric transducers
Fig. 13. Sampler with a raised intake and the microfluidic channel extending behind it. Size of device is 30 15 mm2.
integrated on the microfluidic channel floor for the creation of acoustic trapping zones. The steel sheets were photochemically machined, and the polyimide sheets were structured using standard UV lithography and copper etching, followed by reactive ion etching of the polyimide. All sheets were then coated with Parylene C, used both as an insulator between the different sheets and as a bonding agent for the stack. The glass was glued on top of the bonded stack together with a steel sheet, forming a water-collecting inlet funnel. In total, the thickness of the stacked sheets measures less than 2 mm, excluding the funnel, which extends 5 mm above the top surface. Water-suspended fluorescent beads, 1.9 mm in diameter, have been successfully captured over the transducer areas when activated by a sinusoidal signal of 10 V peak-to-peak at a frequency of 10 MHz. The beads quickly gathered at the pressure nodes while letting pure water by, and could subsequently be released by switching off the transducers. Together with the high-pressure valves, DADU will be able to enrich and store samples taken from previously unreachable small and narrow habitats. 5. Discussion and conclusions The best way to learn how to search for life on alien worlds is probably by exploring extreme environments, such as the subglacial lakes, here on Earth. This, however, necessitates the development of miniaturized, yet capable, systems and instruments with the prospect of being deployable
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through kilometers of ice and performing a wide range of accurate measurements beneath. The small size of the DADU submersible enables deployment through boreholes, by itself or attached to an ice-penetrating probe. It also makes the vehicle easily transportable to explore remote water-filled environments, even those only accessible by foot. The submersible and its subsystems are under continuous development. Specifications, designs and results from the vehicle and many of its subsystems have been presented in this paper. Even though the vehicle itself has not been finalized, partial studies and subsystems show the possibility of such a vehicle. The vehicle, or its subsystems as stand-alone instruments, will enable the exploration of previously unreachable submerged environments.
Acknowledgment This work was supported by MISTRA—the foundation for strategic environmental research, Sweden—and by Vinnova—the Swedish Governmental Agency for Innovation Systems. References [1] R.P. Irwin, Timing, duration, and hydrology of the Eberswalde Crater Paleolake, Mars, in: Proceedings of the 42nd Lunar and Planetary Science Conference, March 7–11, 2011, No. 1608 p. 2748. [2] M. Mayor, M. Marmier, C. Lovis, S. Udry, D. Se´gransan, F. Pepe, W. Benz, J.-L. Bertaux, F. Bouchy, X. Dumusque, G. Lo Curto, C. Mordasini, D. Queloz, N.C. Santos, The HARPS Search for Southern Extra-Solar Planets, Astronomy and Astrophysics Manuscript No. HARPS˙stat, September 2011. [3] C.P. McKay, The search for life in our Solar System and the implications for science and society, Phil. Trans. R. Soc. A 369 (2011) 594–606.
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[4] A. Behar, F. Carsey, A. Lane, H. Engelhardt, The Antarctic Ice Borehole Probe, Aerospace Conference IEEE Proceedings, vol. 1, 2001, pp. 325–330. [5] A.P. Kapitsa, J.K. Ridley, G. de, Q. Robin, M.J. Siegert, I.A. Zotikov, A large deep freshwater lake beneath the ice of central East Antarctica, Nature 381 (1996) 684–686. [6] M.J. Siegert, S. Carter, I. Tabacco, S. Popov, D.D. Blankenship, A revised inventory of Antarctic subglacial lakes, Antarct. Sci. 17 (3) (2005) 453–460. [7] B.C. Christner, E. Mosley-Thompson, L.G. Thompson, J.N. Reeve, Isolation of bacteria and 16S rDNAs from Lake Vostok accretion ice, Environ. Microbiol. 3 (9) (2001) 570–577. [8] E. Gaidos, V. Marteinsson, T. Thorsteinsson, T. Jo´hannesson, A´.R. Ru´narsson, A. Stefansson, B. Glazer, B. Lanoil, M. Skidmore, S. Han, M. Miller, A. Rusch, W. Foo, An oligarchic microbial assemblage in the anoxic bottom waters of a volcanic subglacial lake, ISME J. 3 (2009) 486–497. ´ . Elefsen, E. Gaidos, B. Lanoil, T. Jo´hannesson, [9] T. Thorsteinsson, S.O V. Kjartansson, V. Marteinsson, A. Stefa´nsson, T. Thorsteinsson, A hot water drill with built-in sterilization: design, testing and ¨ performance, Jokull 51 (2008) 71–82. [10] E. Gaidos, B. Glazer, D. Harris, Z. Heshiki, N. Jeppsson, M. Miller, A simple sampler for subglacial water bodies, J. Glaciol. 53 (180) (2007) 157–158. [11] T.-R. Hsu, MEMS and Microsystems, 2nd edition, John Wiley & Sons, Inc., 2008. [12] A. Behar, J. Matthews, K. Venkateswaran, J. Bruckner, J. Jonsson, A deep sea hydrothermal vent bio-sampler for large volume in-situ filtration of hydrothermal vent fluids, Cah. Biol. Mar. 47 (2006) 443–447. [13] J. Jonsson, M. Berglund, H. Kratz, H. Nguyen, G. Thornell, A Compact System to Extract Topography Information From Scenes Viewed by a Miniaturized Submersible Explorer, Unpublished. [14] J. Jonsson, E. Edqvist, H. Kratz, M. Almqvist, G. Thornell, Simulation, manufacturing, and evaluation of a sonar for a miniaturized submersible explorer, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57 (2) (Feb 2010) 490–495. [15] M. Evander, L. Johansson, T. Lilliehorn, J. Piskur, M. Lindvall, S. Johansson, M. Almqvist, T. Laurell, J. Nilsson, Noninvasive acoustic cell trapping in a microfluidic perfusion system for online bioassays, Anal. Chem. 79 (7) (2007) 2984–2991. [16] S. Ogden, J. Jonsson, K. Hjort, G. Thornell, A latchable high-pressure composite valve actuator combining paraffin and a low melting point alloy, in: Proceedings of the 16th Transducers Conference, Beijing, China, 2011.
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Instrumentation and vehicle platform of a miniaturized submersible for exploration of terrestrial and extraterrestrial aqueous environments$ Jonas Jonsson n, Johan Sundqvist, Hugo Nguyen, Martin Berglund, Sam Ogden, Kristoffer Palmer, Katarina Smedfors, Linda Johansson, Klas Hjort, Greger Thornell ˚ Angstr¨ om Space Technology Centre, Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden
a r t i c l e i n f o
abstract
Article history: Received 29 February 2012 Accepted 26 April 2012
An example of an extraterrestrial environment likely to support life is the vast liquid body believed to hide underneath the frozen crust of Jupiter’s moon Europa. The hypothetical exploration of this, as well as the more accessible subglacial lakes on Earth, has been used as model applications for the development of a heavily miniaturized, yet qualified, submersible with the potential to be deployable either in itself through a long and narrow borehole or as the daughter craft of an ice-penetrating cryobot. Onboard the submersible, which is only 20 cm in length and 5 cm in diameter, accommodation of a versatile set of sensors and instruments capable of characterizing and imaging the surroundings, and even collecting water samples with microorganisms for return, is facilitated through the use of miniaturization technologies. For instance, together with a small camera, a laser-based, microoptic device enables the 3-D reconstruction of imaged objects for topographical measurements. As a complement, when the water is turbid or a longer range is wanted, the world’s smallest side-scanning sonar, exhibiting centimeter resolution and a range of over 30 m, has been developed. The work on miniaturizing a CTD, which is a widely employed oceanographic instrument used to measure and correlate conductivity, temperature, and depth, has commenced. Furthermore, a device employing acoustics to trap microscopic particles and organisms, and, by this, enrich water samples, is under development. To ensure that the gathered samples are pristine until analyzed at the end of a mission, the device is equipped with high-pressure, latchable valves. Remote operation and transfer of measurement data and images, or even live streaming of video, is made possible through a kilometer-long fiber optic cable being reeled out from the vehicle underway and tethering it to a terminal. To extend the missions, the same fiber shall also be capable of charging the onboard batteries. In this paper, the vehicle and its subsystems are summarized. Subsystems essential for the vehicle’s operation, e.g., hull structure, communication and power management, are treated separately from those of more mission-specific nature, like the instruments mentioned above. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Submersible Micro Imaging Water sampler Subglacial Sensor
1. Introduction $ n
This paper was presented during the 62nd IAC in Cape Town. Corresponding author. Tel.: þ 46 18 471 72 35; fax: þ 46 18 471 35 72. E-mail address: [email protected] (J. Jonsson).
0094-5765/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2012.04.048
Astrobiology and the search for extraterrestrial life is of increasing interest as new signs are revealed of a
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Mars that was seemingly wetter in the past [1], and due to the continuous discovery of Earth-like exoplanets situated in the habitable zone around other stars in our galaxy [2]. In addition to these Earth-like planets, ice-covered moons orbiting gas giants, such as Europa and Enceladus in our own solar system, could outnumber these habitable alien Earth’s, and potentially have environments to support extraterrestrial life, even though situated outside the habitable zone. They are thought to shelter liquid environments underneath their frozen crusts, which could host extraterrestrial ecosystems [3]. To find out, these extraterrestrial water bodies will have to be accessed and explored. This, however, necessitates a lander mission with an ice-penetrating cryobot, or a long and narrow borehole, and the subsequent deployment of a small submersible—a hydrobot. Due to the nature of such a mission, there will be severe size and weight restrictions imposed on the hydrobot and its scientific payload. As a stepping stone towards a suitable vehicle, a small but well-instrumented submersible is currently under development. The Deeper Access, Deeper Understanding (DADU) project aims to develop a small submersible concept using miniaturization technologies to provide high functionality, despite the vehicle’s limited size (Fig. 1). The submersible is smaller than two soda cans placed together end-to-end, and is designed to fit on or inside an ice borehole probe [4]. The main objective, and the size-limiting application for the submersible, is to explore the otherwise inaccessible subglacial lakes on Earth, which are environments analogous to those thought to exist on the frozen moons. These lakes, some of which are believed to have been isolated by ice for millions of years [5], can be found under up to several kilometer thick ice sheets in the polar regions. The exploration of the lakes can possibly teach us about Earth’s climate changes, since they are seen as environmental fossils, and may testify to life’s start and evolution on Earth. However, the vehicle could of course also be used for the exploration of any water-filled environment, such as pipe and cave systems. As for payloads, the vehicle will be equipped with a camera, illuminating LEDs and a compact laser system in its bow (Fig. 1). This system will provide the images seen by the camera with topographical information through
the projection of specific reference patterns. Along the corpus assembly of the submersible, ports and pockets can house devices such as the miniaturized side-scanning sonar to be used for mapping of the surroundings, having a range of over 30 m, and centimeter resolution. A conductivity– temperature–depth (CTD) sensor, which is a fundamental oceanographic instrument, enables depth- and temperature-corrected measurements of the salinity. A water sampler with a particle and microbe enrichment feature enables concentrated samples to be returned for laboratory analysis. A two-dimensional flow sensor records the velocity of the submersible through the water, or the flow around its hull when the vehicle is stationary. In this paper, the status of this highly functional submersible concept and its subsystems, enabled through forceful miniaturization technology, is reported. 2. Background 2.1. Subglacial lakes Subglacial lakes are bodies of water found underneath ice sheets up to several kilometers thick. They can be as big as Lake Ontario in North America [5]. The largest of the more than 145 subglacial lakes found in Antarctica is Lake Vostok [6], which is covered by a layer of ice over 3 km thick. It is believed to have been isolated by ice for the last 15 million years. An ice core with frozen subglacial lake water on it has revealed microorganisms [7]. However, no water sample has yet been obtained although current drilling operations have just penetrated the lake–ice boundary. In the northern hemisphere, lakes underneath the ¨ Vatnajokull ice cap in Iceland have been reached by drilling and sampled for microbes [8]. Geothermal heating from underlying volcanoes melts the ice of the glacier to create these lakes, which are the sources of periodically occurring glacial outburst floods. In in-situ investigations, a hot-water drill penetrated the 300 m thick glacier. At this depth the drill created holes from 6.6 to 14.7 cm in diameter, depending on the speed used for drilling [9]. Also, a simple sampler, with a diameter of 4 cm and a length of 185 cm, was built to collect 400 ml water samples at certain depths to characterize the water and search for microbial life in it [10]. These samples could, however, be taken only straight underneath the boreholes. 2.2. Miniaturization technology
Fig. 1. Hull model of the DADU submersible.
For designing a small submersible with high functionality, commercial components and instruments are generally not suitable, as they tend to be too large. Therefore, miniaturization technologies have been used where applicable in the development of this vehicle. The microelectromechanical systems (MEMS) technology, also known as microsystems technology, is a field where devices with features and even components measured by the micrometer, are developed [11]. Most of the processing technology for MEMS is inherited from the semi-conductor industry, and often employs photolithography, thin-film deposition and etching processes, to create structures on or in a silicon or glass wafer.
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With features being comparable with dust in size, the processing is performed in cleanroom facilities under controlled ambient conditions. Since the surface-to-volume ratio is large for MEMS devices, behavior and performance may not be according to intuition. For instance, surface effects such as wetting dominate over volume effects such as inertia. These scaling effects can be used to manufacture devices not only smaller than their macroscopic equivalents, but with, for instance, higher sensor sensitivity. In addition, through batch processing, the components’ cost can be very competitive.
unit (IMU) uses accelerometers and gyros to monitor the orientation of the submersible. Simultaneous data and command transfer is enabled through a fiber optic cable, connecting the submersible with a control station. The brain of the vehicle is the microcontroller unit, which controls the peripheral units, data and command flow. Power to the submersibles subsystems is provided by rechargeable battery packs. Further descriptions of the subsystems and the instrumentations of the DADU vehicle follow below in their respective sections.
3. Submersible design
3.1. Control system
Early considerations set the size of the submersible to 5 cm in diameter and 20 cm in length. The primary reason was to enable deployment through narrow ice boreholes, by itself or together with an ice probe [4]. With future, extraterrestrial missions in mind, a low mass was another reason. Studying other ROVs, both large and small, a common feature is that components off the shelf (COTS) are often used. In applications where no strict size restrictions are employed, or a high degree of functionality is not required, this works fine. However, when size and weight are of essence, and more instrumentation and functionality are needed, tailored miniaturized subsystems are required. The final design of the submersible was derived after a concept generation and evaluation phase, attempting to maximize the submersible’s maneuverability in tight and confined waters, and to provide some maneuver redundancy in case of thruster malfunction. The vehicle is semi-autonomous and equipped with eight small thrusters, enabling movement with five degrees of freedom. The vehicle will be supervised by an operator, but could be pre-programmed to perform simple maneuvers and attitude controls automatically. An inertial measurement
A single low-power, 8-bit microcontroller was chosen to perform the control and monitoring of the on-board data handling (Fig. 2). Compared with many 32-bit architectures, this preserves power and reduces system complexity and size. A dynamic prioritization of different tasks enables a highly configurable interrupt- and event-system, ensuring system responsiveness and stability. Furthermore, a compact way of obtaining and storing measurement data from the various instruments is provided by the microcontroller’s on-chip memory and analog-to-digital converters. The control system is passive in that it replies to, and takes action on, requests from the control station. However, collection of on-board housekeeping data is an exception to this, and is performed continuously. If anomalies are detected, the control system will signal to the control station at highest priority. 3.2. Propulsion and attitude control The propulsion system has not yet been fully miniaturized, but eight small electric engines and propellers have been custom fitted and integrated into the hull of the submersible. For the main, strong forward and backward
Fig. 2. System architecture of the DADU vehicle.
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locomotion, four thrusters are placed in the back end of the stern assembly (Fig. 3). By using four thrusters with propellers rotating oppositely in each pair, the center of the stern is free for feeding out, or retrieving, the optic fiber from the reel located in the stern. A certain redundancy is also built in with this configuration, since as long as all of the thrusters do not fail at the same time, loss of thrusters will not substantially cripple the submersible’s locomotive ability. On each end of the corpus assembly is an attitude control unit with one vertical and one horizontal thruster (Fig. 3). These can be used for translation in vertical and horizontal directions as well as for changing the submersible’s pitch and yaw by activating them in opposite directions. Compared with a conventional design based on external rudders and propellers, there are three main advantages with the current thruster setup. First, the submersible does not need a simultaneous forward or backward movement to perform attitude adjustments. Instead, the vehicle can, e.g., change its field of view while maintaining its position. In addition, this increases the submersible’s maneuverability in confined spaces. Second, the risk of getting tangled up in the fiber optic cable or any objects in the environment is reduced since there are no large protruding objects from the hull. Third, there are no space-demanding mechanisms in the stern as would be required for controlling a rudder. With the current configuration, only a rolling function is not available. However, this is not desired. 3.3. Navigation and attitude sensing A commercially available six-axis IMU (SD755, Sensor Dynamics, Austria) was chosen. Due to the availability of suitable COTS accelerometers and gyros, commercial sensors have been used instead of tailor made ones, in order to fully monitor the translational and rotational movements of the submersible. Although an IMU is not useful for long-range navigation, since even the smallest offsets and errors will accumulate over time, it is able to accurately reveal the submersible’s current orientation, and can be used to track movements for shorter periods of time, such as during sonar sweeps. 3.4. Power handling Low power consumption is of utmost importance, for a miniaturized system designed to be operational in
hard-to-reach environments over extended periods of time. The power consumption has been estimated for each subsystem (Table 1). The motors account for most of the power consumption of the submersible, but not all of them will be active at the same time. The water sampler is activated only when sampling is being done, and the valves only when they are opening and closing, since they are latchable. It is estimated that the sonar element uses 1 W when transmitting, but only at a duty cycle of 1%, meaning 10 mW/element. The LEDs are the four diodes used to illuminate the camera’s view. The power figure is for the maximum effect, 150 mW each; however, the LEDs are dimmable. Power, in the form of light, will be transferred through the fiber optic cable simultaneously with data and commands. Operating as a solar cell, a photodiode will collect this light, converting it into electrical power. It has been estimated that 0.1 W of power can be transferred through the fiber. For extended missions in environments where resurfacing is not an option, such as deployment in an extensive cave system, the submersible could go into hibernation mode, resulting in power consumption in the sub-milliwatt range, while the batteries are recharged through the fiber, after which the submersible can recommence its mission. Because of their flexibility in shape and their high energy density, lithium-ion polymer type batteries were chosen.
3.5. Communication When the vehicle ventures through narrow and winding openings, the fiber optic cable must enable noninterrupted communication and high bi-directional data rates. By using a thin, single-strand optic fiber, reeled out gradually, the drag, which is usually the main opposing force a ROV experiences, will be minimized. Three wavelengths will be used concurrently in the fiber to simultaneously transmit the uplink and downlink signals and power. Evanescent couplers will couple these wavelengths into and out of the fiber at each end. According to simulations, a coupling efficiency of over 90% can be reached for separating the wavelengths of 940 nm, 1060 nm and 1550 nm. The waveguides and guide structures, used for precise alignment of the fiber core to the waveguide core, which is only 5 5 mm2 in cross section, are made from microstructured polymer materials (Fig. 4). Table 1 Estimated power consumption of subsystems.
Fig. 3. Exploded CAD view of the submersible.
Subsystem
Estimated power consumption [mW]
Microcontroller Motor Communication Water sampler Sonar element IMU Flow and speed log Laser and Camera LEDs CTD
50 1000/each 100 10 10 250 100 500 600 10
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3.6. Hull structure The bow, corpus and stern assemblies comprise the three main sections of the submersible’s hull, Fig. 3, and contain guiders, fixtures and supports for the subsystems inside. In this way, the numbers of interfaces and connectors are reduced, saving, in turn, passive material and mass. Also, it increases the strength of the hull. To enable iterative design and testing of the hull and the locomotion modules, rapid 3-D prototyping (Dimension Elite, Stratasys) was utilized, using fused deposition of ABS-plus plastic. As a design goal for the submersible, a maximum operating depth of 1000 m was set. Leakage elimination is an important issue in the design, due to the high pressures experienced at these large depths. Because of the external locations of sensors and thrusters, the many feed-throughs in the hull make this a great challenge. To further decrease the risk of leakage between the three main hull sections when connected, each section is sealed from the others. The hull can be pre-pressurized, for example to the half of the maximum operation pressure, in order for a fairly thin, and thus not too heavy, hull to withstand high pressures experienced at larger depths. However, while mitigating the leakage problem, such a hull can become a safety issue at shore. Another option is to use a flexible hull section or bladder, and filling the submersible with a more or less incompressible liquid in order to equalize the internal pressure with the external [12]. This puts fewer requirements on the hull itself, but transfers the pressure tolerance requirements to the internal subsystems. In the hull design, the overall density of the vehicle will have to be considered since the submersible should be neutrally buoyant in water. Empty or gas-filled compartments have to be used to compensate for the higher-density components and subsystems, including the intended hull. 3.7. Buoyancy Operation in different environments and at different depths requires that the density of the submersible is
Fig. 4. Optical fiber guided in place by two ridges (top of image), and a waveguide on the chip (bottom of image). The diameter of the fiber is about 100 mm and the width of the waveguide core is 5 mm.
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adjustable. The submersible is designed to be neutrally buoyant in water, and the subsystems inside the submersible are placed so that the submersible is stable, i.e. the mass center is located right below the geometrical center. To be able to change the buoyancy during a deployment, a solution with two tanks, each close to one end of the corpus, was chosen. The buoyancy and the pitch angle of the submersible can be adjusted by loading different amounts of water in the tanks. It is preferable to position the tanks in the horizontal plane going through the overall mass center. If the position of the tanks is not in this plane, the overall mass center will be shifted up or down when filling or emptying the tanks, and thus there will be a risk of instability. A ballast of small lead balls was used to obtain the right buoyancy for tests with the prototype in shallow water. 4. Instrumentation The instrumentation of the submersible covers standard functions of a submersible vehicle and more missionspecific devices. The primary placement of the instruments is in specific bays on the hull. 4.1. Side-scanning sonar A matchstick sized side-scanning sonar, 50 2 3 mm3 in size, to map the surroundings of the submersible, for scientific as well as navigational purposes, and complement the camera, has been developed and characterized [14]. Mapping of the bottom, sidewalls, or the ceiling of an enclosure around the vehicle can be performed by mounting side-scanning sonars around the submersible in various configurations. One of the strengths of the side-scanning sonar compared with a camera system is that it is not affected by murky and dark waters, due to the longer wavelength used for ultrasonic devices compared with visual light. The resolution in the direction along the track is set by the pulse frequency of the sonar electronics. However, the resolution in the direction perpendicular to the path of travel is inversely proportional to the length of the sonar and directly proportional to the frequency used. Unfortunately, the higher the frequency used, the larger the attenuation. But for the application of the DADU submersible, a range of only 10 m was required. A trade-off resulted in 600 kHz. The finite element analysis software Comsol Multi-physics was used to predict different sonar design performances and the effect of the mounting structure on the beam pattern due to their tight mounting in the hull (Fig. 5). Measurements were performed in a water tank setup using a needle hydrophone to characterize the performance of the sonar. Stepping motors were used to sweep the sonar in front of the hydrophone, and a profile of the beam pattern was acquired. Finally, field tests in a nearby river, with dark and murky waters, were performed. This was thus a good test for the ability of the sonar to map the bottom, for which the camera system could not be used. Objects such as stones and rocks, mudflats, sunken trees, cables, and even schools
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of fish, could be imaged from the side of a boat traveling along the river (Fig. 6). The small side-scan sonar, to fit on the miniaturized DADU submersible has been designed, manufactured and tested. The sonar has been used to image river bottoms and objects at least 30 m away, and was found to exhibit centimeter resolution.
4.2. Laser-camera topography measurement system A small high-resolution camera is used to capture images and record live streaming videos of the surroundings. The camera is placed in the nose cone with four LEDs to illuminate the scene in front of the submersible. One problem with images acquired underwater is that they tend to be flat and difficult to make measurements in. To solve this problem, laser illuminated diffractive optical elements (DOE) have been developed to project reference patterns, e.g. dots and grids, onto a scene viewed by the camera [13] (Fig. 7). The working principle of the camera-laser system is that the laser and DOE project a pattern onto an object viewed by the camera (Fig. 8). By knowing the geometrical setup of the camera, laser and DOE, and by tracking how the object has distorted the pattern, the position of each laser dot in 3-D space can be calculated. From this, the distance, shape and size of the object can be determined.
At a distance of 30–45 cm the system was able to determine the position of objects with 0.5 cm accuracy. Angles from 01 to 801 were measured on a screen at a distance of 45 cm within an error of 41. The performance in turbid conditions was tested by adding coffee (up to 600 ml brewed with 50 g fine-grained coffee per liter of water) to the tank, containing 25 l of water. At the higher concentrations, it was hard to discern the contours of the object by the naked eyes. The system, however, was still able to make measurements. The errors of these distance measurements with the turbid water were found to be within 0.5 cm at a set distance of 45 cm. 4.3. Conductivity, temperature and depth sensor system An essential instrument for the field of oceanography is a CTD sensor. It measures conductivity, temperature and depth, and is found on most research vessels. From these measurements, other characteristics of the water can be calculated, such as salinity. CTD instruments are usually large and cumbersome devices, hoisted down into the water using cranes. This necessitates the use of a ship large enough to handle these devices. Although with a penalty in sensitivity and precision, miniaturized CTD sensors are an alternative, allowing for mounting on small vehicles and even animals. Moreover, since several miniaturized CTD setups can be included on one small chip, redundancy can be achieved. The size of the CTD chip being developed here, to fit in one of the instrument bays on the outside of the
Fig. 5. Comsol simulation result showing the total acoustic pressure emanating from a sonar element.
Fig. 6. Acquired side-scan sonar field image of a river. The wavy pattern close to the midddle of the image is the bottom at the end of a boat launching ramp.
Fig. 7. Laser, DOE, and projected pattern on a screen.
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Object
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DOE Laser
Camera
CPU Fig. 8. Working principle of the topography measurement system, where a laser shines through a DOE, creating a pattern onto an object. The camera records this pattern, and software compares this with a reference image of the pattern to calculate the position of each dot in 3D space.
submersible’s hull, is 30 15 3 mm3. It accommodates six individual CTD sensor element sets of varying designs. The measuring electronics will fit inside the submersibles hull, or, for a stand-alone instrument, be fitted together with the chip in a package smaller than a matchstick box. Conventional MEMS technologies and processes were employed to miniaturize the sensor elements, and a silicon wafer was used as the base material. Photolithography and physical vapor deposition processes were used to form the sensor electrodes on one of the wafer surfaces (Fig. 9). To create membranes, which will be used to measure pressure together with strain gage structures, the back side of the wafer was structured using deep reactive ion etching. The CTD chips were glued to glass to create a sealed-off cavity underneath the pressure membrane (Fig. 10). The temperature sensor is an integrated platinum element, where a certain resistance shift can be correlated to a specific temperature close to linearly. To measure the conductivity, an AC potential is applied over two electrodes. Ions in the water will set up an electrical current, and the impedance can be measured and correlated to the conductance of the water. The pressure sensors have been simulated using finite element analysis software and designed in four different dimensions, each able to withstand a portion of the specified operating range of 0–1000 m depth, and yet provide membrane strains high enough to be measured by the integrated strain gauges. A protruding rigid center is used to halt further vertical movement of the membrane when its specified maximum pressure has been reached, in order to prevent rupture. A precision and accuracy of 0.01 1C, when cycling up and down in the range 5.00–40.00 1C, have been obtained in a tempered water bath. Although the instrument may not reach the sensitivity and precision of larger CTD systems, it enables coarser measurements in tight and confined environments, where larger systems cannot reach. This chip-based CTD could also be used separately as a bio-logger on small marine animals. 4.4. Flow and speed log A two-dimensional thermal flow and speed log is being developed to record the velocity of the submersible, or to
Fig. 9. Electrode layers making up two CTD sensors on part of a 30 15 mm2 chip.
record the water flow over the hull when the vehicle is stationary. The sensor uses the so called calorimetric principle where forced convection changes the temperature profile around a heater (Fig. 11). By measuring the temperature upstream and downstream of the heater from the change in resistance of similarly designed elements, the flow can be recorded. A prototype sensor, Fig. 12, has been developed with 0.5 mm thick Pyrex glass as a substrate material. It has a fairly low thermal conductivity, meaning that the heat loss and thermal crosstalk will be low compared with a sensor with silicon as substrate material. Four sensing elements are placed symmetrically around the heater in order to be able to measure the flow in two dimensions. Preliminary tests indicate that the sensor design is suitable for the typical velocity range of the submersible. 4.5. Water sampler In order to collect particles in the water, such as microbes, water samples are needed. Due to the size restrictions of the DADU vehicle, the large sample volumes necessary, especially in a sparsely populated oligotrophic environment, cannot be accommodated on board. To mitigate this, a compact sample enrichment system is under development. In this, water will be filtered for microbial organisms, 1–10 mm in size, in an acoustic standing wave field generated by miniaturized piezoelectric transducers [15]. After a tailored dwell time of the moving vehicle forcing water through the microfluidic acquisition system, while the transducers enrich the fluid, the sample will be sealed off by high-pressure latchable valves [16].
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Fig. 10. Cross section of the glued CTD sensor with a membrane structure etched in the top part, onto which the different electrode layers have been deposited.
Fig. 11. Comsol simulation of the two-dimensional flow sensor, with the heater in the middle. The color plot represents the temperature profile over the surface, and the arrows the velocity field of the water across the sensor casuing this profile to be asymmetrical. (The five squares represent the locations of the heater (middle) and the four orthogonally placed sensing elements.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Microscope image of a developed flow sensor. The field of view is approximately 1.7 1.3 mm2. (The copper of an underlying PCB is visible through the glass substrate.)
The sampler can then be taken to a laboratory facility for subsequent examination, even after an extended mission. The device, which is 30 mm long and 15 mm wide, consists of a stack of stainless steel and polyimide sheets (Fig. 13). Steel sheets make up the structure of the microfluidic channel and the valves. A polyimide sheet provides a membrane material for the valves. A cover glass is used as an acoustic reflecting layer for the piezoelectric transducers
Fig. 13. Sampler with a raised intake and the microfluidic channel extending behind it. Size of device is 30 15 mm2.
integrated on the microfluidic channel floor for the creation of acoustic trapping zones. The steel sheets were photochemically machined, and the polyimide sheets were structured using standard UV lithography and copper etching, followed by reactive ion etching of the polyimide. All sheets were then coated with Parylene C, used both as an insulator between the different sheets and as a bonding agent for the stack. The glass was glued on top of the bonded stack together with a steel sheet, forming a water-collecting inlet funnel. In total, the thickness of the stacked sheets measures less than 2 mm, excluding the funnel, which extends 5 mm above the top surface. Water-suspended fluorescent beads, 1.9 mm in diameter, have been successfully captured over the transducer areas when activated by a sinusoidal signal of 10 V peak-to-peak at a frequency of 10 MHz. The beads quickly gathered at the pressure nodes while letting pure water by, and could subsequently be released by switching off the transducers. Together with the high-pressure valves, DADU will be able to enrich and store samples taken from previously unreachable small and narrow habitats. 5. Discussion and conclusions The best way to learn how to search for life on alien worlds is probably by exploring extreme environments, such as the subglacial lakes, here on Earth. This, however, necessitates the development of miniaturized, yet capable, systems and instruments with the prospect of being deployable
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through kilometers of ice and performing a wide range of accurate measurements beneath. The small size of the DADU submersible enables deployment through boreholes, by itself or attached to an ice-penetrating probe. It also makes the vehicle easily transportable to explore remote water-filled environments, even those only accessible by foot. The submersible and its subsystems are under continuous development. Specifications, designs and results from the vehicle and many of its subsystems have been presented in this paper. Even though the vehicle itself has not been finalized, partial studies and subsystems show the possibility of such a vehicle. The vehicle, or its subsystems as stand-alone instruments, will enable the exploration of previously unreachable submerged environments.
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