Feature Articles—December 2009 Issue
Designing a Deep-Towed Camera Vehicle Using Single Conductor CableCost-Effective Approach Uses Telecom Modems And Existing Winch and Cable
By Peter Hill
Electronics Engineer
National Institute of Water
& Atmospheric Research
Wellington, New Zealand
Several large marine institutes around the world have developed deep-towed camera vehicles capable of reaching 6,000-meter depths. These vehicles often use an armored fiber optic tow cable to transmit live video back to the ship and to send system control data down to the towed vehicle. With their enormous bandwidth, fiber optic cables are well suited to this task. However, because they are normally considered to be less robust than copper cables, best practice cable handling and dedicated winches are required. Since many of the smaller marine institutes do not have these expensive resources, New Zealand’s National Institute of Water & Atmospheric Research (NIWA) set out to see if existing winches with cheap and robust single conductor conductivity, temperature, depth (CTD) cables could be utilized to transmit live video images.
For many years NIWA had operated an armored cable winch with 9,000 meters of 10.4-millimeter-diameter single conductor cable (Culpeper, Virginia-based Rochester Wire & Cable 1-H-422A) for CTD operations from their 70-meter research vessel Tangaroa. To survey New Zealand’s enormous Exclusive Economic Zone, NIWA had a pressing need to replace their old film drop camera with a modern towed system with both still and video digital cameras. They set about development work and, in less than a year, had produced the Deep Towed Imaging System (DTIS).
At first, the data transmission capability of the single conductor tow cable was disappointing: The CTD modem achieved only about 60 kilobits per second, not nearly enough for a live video uplink.
Unlike coaxial cables, single conductor cables have no separate braided copper shield; instead, the steel armor is used to complete the electrical circuit. Because of this, attenuation is very high, especially at higher frequencies. Moreover, the cable is necessarily unbalanced as the armor is unavoidably grounded both to the seawater and to the ship’s hull through contact with the winch drum.
Data Communications Link
Peter Hill, the NIWA engineer charged with designing the new camera vehicle, surveyed modern telecom modems and decided on a variant of digital subscriber line (DSL) technology, specifically symmetrical high-bit-rate DSL, developed for improved long distance transmission.
Like all telecom modems, these generally operate at the typical telephone cable impedance of 130 ohms, whereas the CTD cable is 43 ohms. This large impedance mismatch limited the power that could be coupled into the cable.
To overcome this, the modems were modified to reduce their output impedance, thus enabling a data speed of 448 kilobits per second over a 9,000-meter-long cable. Next, the data volume requirement of the video signal was reduced with the use of an off-the-shelf video encoding device that applies MPEG-4 data compression.
This device is programmable to allow selection of the video resolution and frame rate. The end result was a live color video frame rate of two frames per second—not fast, but still good enough to allow real-time control of the vehicle and to allow observers an adequate first look at the seafloor.
In addition to providing data compression for the video uplink, the video encoder has a transparent 9.6-kilobit-per-second downlink that is used for sending system control commands to the towed vehicle, allowing operators to control cameras and lights from the surface.
Video and Still Cameras
The video camera is a Sony (Tokyo, Japan) HDR-HC1 high-definition camcorder that records onto MiniDV tapes, providing images with exceptional clarity and detail.
The camera is remotely operated via its built-in Control-L interface. This is an industry standard introduced by Sony for controlling audio and video equipment. It uses a bit serial data format that requires the controller to be synchronized to the controlled equipment. This is achieved with an Elm Electronics (Toronto, Canada) Elm624, a special-purpose device based on a PIC microcomputer that will accept ASCII commands into an RS-232 input, generate Control-L commands and synchronize with the camcorder. The system uses two DeepSea Power & Light (San Diego, California) Deep Multi Sealites, totaling 450 watts, for video lighting.
The still camera is a 10-megapixel Canon (Tokyo, Japan) 400D single lens reflex with a 24-millimeter lens. Its small physical size permits the use of a reasonable diameter housing, and its automatic flash exposure system works well underwater, while the wired shutter release allows easy remote control. A USB 2.0 connection permits images to be downloaded quickly on deck without opening the housing. Full manual camera and lens control is available, and the “sleep” and “wake up” features provide a long auxiliary battery life.
A relay feature on the video encoder device is used to trigger the still camera, which, at its best JPEG resolution, can take 580 shots on a two-gigabyte compact flash card. Lighting is provided by a cluster of three Canon 580EX strobes that recycle in four seconds.
Other Underwater Equipment
Power is provided by two DeepSea Power & Light 12-volt, 80 ampere-hour pressure-compensated oil-filled batteries that can power the vehicle for 2.3 hours. Vehicle height above the seabed is measured with a Tritech (Aberdeen, Scotland) altimeter, while a Sea-Bird Electronics (Bellevue, Washington) sensor measures depth below the surface.
A Parallax (Rocklin, California) Stamp microcomputer accepts RS-232 data from these instruments and formats it for connection to a Decade Engineering (Turner, Oregon) video overwriter that superimposes the altitude and depth text data onto the video image. The Stamp also handles commands coming down the tow cable to control the video camera and lights.
Pairs of parallel lasers are used to provide scaling dots onto the still and video camera images. Various configurations have been used, including green lasers running off the main 24-volt line from the still camera and red lasers that use internal batteries.
A Honeywell (Morristown, New Jersey) digital compass module in a titanium housing is used to provide heading, pitch, roll and magnetic data. These data are transmitted up the tow cable on an auxiliary low-speed channel.
A transponder for a Simrad (Horten, Norway) HPR acoustic navigation system is normally also included, and a Sea-Bird SeaCat CTD is attached as required.
Since the DTIS has been used in Antarctica and the camera electronics are rated down to only 0° Celsius, thermostatically controlled heaters have been fitted in the housings. For redundancy, there are two thermostats, both of which close at 5° Celsius and open at 15° Celsius. The heaters are thick film power resistors and small fans circulate air around the interior to even out temperature gradients.
Topside Equipment
A topside modem and video encoder accepts data from the tow cable and provides a composite video output, which is cabled to a Sony liquid crystal display television screen. It has a picture-in-picture facility that allows it to double as a monitor for the topside PC. This PC runs the National Instruments (Austin, Texas) LabView program, which provides a serial output to the topside video encoder’s 9.6-kilobit-per-second transparent data link, sending commands to the video camera and lights. The PC uses a printer port output to trigger the video encoder electromechanical relay that fires the still camera. There is also an Ethernet output to the vessel network with data relating to each firing of the still camera.
The live video image is also cabled to a display for the winch driver. Using the live video and the overwritten vehicle altitude and depth data, the operator flies the vehicle close to the seafloor by winching tow cable in and out. The towing speed is less than one knot and the target vehicle altitude is two to three meters, but useful images are still possible at six meters above the seafloor. The winch operators have become very skilled at maintaining a reasonably even vehicle altitude. When the vehicle is back on deck, the videotape is changed and the still camera downloaded. The system can be back in the water in fewer than 20 minutes.
Mechanical Design
The underwater housings are made from a duplex stainless steel, type SAF2205, that has two times the yield strength of type 316 and has excellent corrosion resistance. The housings, designed for a depth of 6,000 meters, are comprised of cylinders machined from hollow bar, and the end caps are made from slices of solid round. The cluster of three electronic strobes would have required a flat window of too large a diameter and thickness if they were in a cylindrical housing. They are instead housed in a Teledyne Benthos (North Falmouth, Massachusetts) glass sphere, the curved walls of which provide the incidental advantage of preserving the wide beam angle of the strobes. The result is a very even light field that gives excellent exposure across the full width of the still image. All of the cameras and lights are on pivoting mounts that allow an adjustable inclination away from the vertical of up to 38°.
The DTIS vehicle is 2.2 meters long and has a total weight of 280 kilograms. It is a welded assembly made from type 316 stainless steel with a rectangular inner part made from 50 by 50 millimeter angle. This is surrounded by a curved frame with steeply sloping top members that join at a high towing point. Made from 16-millimeter-diameter solid round, this outer frame is designed to resist snagging on the bottom. Some years ago, NIWA had the unfortunate experience of losing a complete Benthos film camera system after it became wedged on the bottom, and they were keen to avoid this. Contacts with the seabed are inevitable with towed camera deployments, but the design of the DTIS has proved its worth on the precipitous terrain of seamounts, where the vehicle has rarely become snagged and each time has come free and continued the transect.
Results
The DTIS has been used on 11 research voyages since 2006, both inside and outside the New Zealand Exclusive Economic Zone. During an eight-week voyage into the Ross Sea in Antarctica, the DTIS took 12,500 still images and 55 hours of video. In a collaborative program with Amy Baco-Taylor from Woods Hole Oceanographic Institution and Lisa Levin from Scripps Institution of Oceanography, the DTIS explored new chemosynthetic habitats off the east coast of New Zealand. The most recent voyage was with Australian collaborators along the chain of seamounts forming the Macquarie Ridge to the southwest of New Zealand. Results from the DTIS are a major contributor to New Zealand’s participation in the Census of Marine Life, the global network of researchers from 80 nations engaged in a 10-year scientific initiative to assess the diversity of marine life in the oceans.
The DTIS has become a workhorse for NIWA, improving vessel efficiency by allowing both DTIS and CTD operations from a single winch and cable.
Peter Hill is an electronics and software engineer with New Zealand’s National Institute of Water & At-mospheric Research, where he focuses on designing equipment for oceanographic and atmospheric research. He is a veteran of more than 80 research voyages with an interest in diver-held and remote underwater photography.
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