Feature ArticleDoppler Velocity Log Navigation For Observation-Class ROVs
By Jeff Snyder
SeaVision Underwater Solutions Inc.
Little Compton, Rhode Island
For more than a decade, Doppler velocity logs (DVLs) have been used to provide navigation information for ships, remotely operated vehicles (ROVs), human-occupied vehicles (HOVs) and autonomous underwater vehicles (AUVs). By using multiple transducers to measure velocity (either bottom tracking or water-column tracking), these observations can be integrated to generate displacements. These relative displacements, when oriented to real-world coordinate systems through heading, pitch and roll sensors, can generate absolute displacements relative to geographic coordinate systems. Although DVL systems have a tendency to drift over time (or distance traveled), they offer a unique opportunity to generate self-contained navigation solutions that can be used in confined access settings or for input into subsea dynamic positioning control systems.
Historically, DVL systems could only be carried on large underwater platforms such as work-class ROVs, large AUVs and HOVs. The size and weight of a typical DVL makes these systems impractical for the mini-ROVs that are frequently used in confined-space or confined-access underwater inspection applications. These applications (e.g., tunnel inspections or under ship hulls) require subsea positioning in conditions where ultrashort-baseline (USBL) or long-baseline positioning may not be feasible.
Within the past two years, sensor manufacturers have reduced the size and weight of commercial off-the-shelf (COTS) DVL systems to the point that their use on mini-ROVs has become feasible.
Recognizing this opportunity, SeaVision Underwater Solutions Inc. began working on an integrated COTS system, which resulted in a self-contained dead-reckoning navigation solution deployed on a mini-ROV. For this system, SeaVision selected a VideoRay (Phoenixville, Pennsylvania) Pro 4 ROV and the Explorer DVL, which is manufactured by Poway, California-based Teledyne RD Instruments (RDI). Despite the addition of peripheral instruments, the Pro 4 still maintains effective maneuverability. It can drag its own tether and it has full maneuverability in all directions.
Top view of the VideoRay Pro 4 ROV alongside the Teledyne RDI Explorer DVL.
Self-Contained Phased-Array DVL
The Explorer DVL, a self-contained unit with a 600-kilohertz phased-array transducer, weighs less than four kilograms in air and measures approximately 11.6 by 30.5 centimeters. Teledyne RDI custom-built a self-contained housing for the navigation solution and provided power and communications ports to specs requested by SeaVision.
The Explorer is housed in a machined aluminum pressure hull that also contains the necessary control electronics. It includes an internally mounted Honeywell (Morris Township, New Jersey) HMR-3000 digital compass that provides heading, pitch and roll outputs. The compass can be self-calibrated for local soft-iron effects and programmed for magnetic variation or hard-iron effects. Communication via standard RS-232 and power (either 12 or 24 volts direct current) are supplied via a nine-pin Teledyne Impulse (San Diego, California) connector on the subsea housing.
The DVL features an onboard firmware suite that allows a variety of user-configurable control commands for system performance settings, data output and integration with other sensors. Of particular value is the self-contained dead-reckoning navigation algorithms that capture onboard heading, pitch and roll observations, and output displacements (in northing and easting) from a '0,0' starting point.
The Pro 4 features a high-resolution color camera on a tilt-platform, high-intensity light-emitting diode (LED) lighting, three brushless thrusters, a depth rating of 300 meters and tether lengths up to 600 meters. Command communications for the Pro 4 are via RS-485 protocol, and its full-copper tether contains a spare twisted-pair conductor set that can be used for additional sensors.
The Pro 4 was selected for several reasons, including its small form-factor, available payload capacity, available 12 and 24 volts direct current power supplies, and a simple but effective communications capability. An additional factor the Pro 4 provides is the standard installation of a microelectromechanical system (MEMS)-gyro-stabilized compass that offers high-quality heading, pitch and roll data onboard the vehicle. The data are passed topside to a graphical user interface in the VideoRay Cockpit control software and can be tapped via the software interface for third-party use.
Ground-truth comparison between dead-reckoning DVL navigation (dashed line) and RTK GPS position (solid line).
Integration of the DVL to an ROV
The integration of the Explorer DVL to the VideoRay Pro 4 includes clamping the subsea housing of the DVL to the BlueView Technologies Inc. (Seattle, Washington) P900-45 imaging sonar skid of the Pro 4. With all ballast weights removed from the Pro 4's skid, the combined system is just slightly negatively buoyant. When neutrally buoyant (by adding some syntactic foam), the ROV has the thrust available to fly in all horizontal (forward, reverse, twist) and vertical (up/down) directions.
The engineering team at VideoRay developed a methodology for low-bandwidth communications between the Pro 4 and RS-232 devices (such as the Explorer). Using a technology called a protocol adaptive multiplexer (PAM), low-bandwidth communications for an RS-232 device can be assembled and transmitted along the excess bandwidth that is available in the Pro 4's RS-485 channel. The PAM board is preprogrammed for its application and sealed into a subsea communications cable between the DVL and the ROV. Topside, generic software running on a PC in the background of the VideoRay Cockpit control interface software 'grabs' the RS-232 communications string from the PAM unit and distributes it to a virtual or actual serial port on the host PC. This allows the RS-485 channel to carry the DVL's RS-232 communications. More importantly, it leaves the spare conductor pair in the umbilical open for use with higher-bandwidth instruments such as scanning or multibeam imaging sonars.
Field Test Results
SeaVision conducted field tests in Sakonnet Harbor, which is a few miles southwest of its Rhode Island office. Initial testing of the DVL consisted of attaching it to a single-pole mount over the side of a four-meter skiff. SeaVision conducted closed-loop survey courses in different orientations in the harbor while collecting real-time kinematic (RTK) global positioning system (GPS) information. After the DVL was integrated with the BlueView sonar and the Pro 4 ROV, the system was deployed again in the harbor to perform ROV missions under and around the pier, under moored boats, directly under hulls and along recreational boat docks.
The DVL performance, in testing both on board the Pro 4 and independently against an RTK GPS receiver, suggested a minor soft-iron error in the resulting navigation solution. The calculated accuracy of the solution during a 160-meter round-trip mission was between 0.8 and 3.3 percent of distance traveled, depending on whether the return to start position or the total distance traveled was used as the metric. The published accuracy for velocity measurements of the phased-array DVL is 0.75 percent; the magnitude and character of the error in the solution indicates a soft-iron error in the HMR-3000 onboard compass.
Conclusions and Future Uses
Teledyne RDI has developed a new, self-contained phased-array DVL with an integrated internal compass that provides heading, pitch and roll. The compass can be self-calibrated to remove soft-iron and hard-iron effects, and it offers approximate true-north heading information to the dead-reckoning navigation algorithms in the internal firmware.
Initial testing of the Explorer DVL has demonstrated a qualitatively effective navigation solution that can suffer from errors that are consistent with magnetic compass deviations. These errors are to be expected when working with any magnetic compass, so care must be taken to properly calibrate and correct these compasses for soft-iron and hard-iron effects. Care must also be taken to deploy such a system in conditions where the DVL, with its magnetic compass, is not as likely to be adversely effected by magnetic influences.
The Explorer DVL can be effectively carried by the VideoRay Pro 4 ROV, which has the payload capacity, the power supply and the communications availability to utilize the DVL as its navigation sensor. An advancement in low-bandwidth communications on the Pro 4 using a new multiplexing technique maintains the availability of the spare conductor pair on the ROV tether for higher bandwidth instrumentation.
As the accuracy of the DVL navigation solution improves (by orienting the DVL with the Pro 4's MEMS-gyro-stabilized compass), the applicability of the system for observation-class ROVs will increase. Having access to geographically referenced positioning information in real-time—whether in open water or in confined spaces—will grant users the ability to track the ROV with a high-degree of accuracy and precision in environments previously not possible.
The concept of integrating a DVL to an observation-class ROV is not a terminal endeavor. Rather, this demonstration should serve as a stepping stone to other endeavors where traditional acoustic positioning methods are not feasible (e.g., tunnel inspections, hull inspections, salvage operations, cave mapping, etc.).
Future integrated COTS systems could eventually include a USBL, a smart tether and a DVL. GPS integrated with USBL would track the ROV to its initial deployment location; a smart tether modified to accept DVL inputs would track initial egress from the deployment location to the mission; and as missions become more complicated, the navigation would weigh more heavily toward the DVL. Such a proposed system could one day provide observation-class ROVs with the high-quality navigation data available to work-class ROVs without sacrificing all of the available sensor bandwidth.
The author would like to thank the navigation products group at Teledyne RDI, particularly Omer Poroy, Jeff Barnes and Matt Burdyny. The author would also like to thank VideoRay, particularly Scott Bentley, Marcus Kolb and Andy Goldstein.
Jeff Snyder is the president of SeaVision Underwater Solutions Inc. A former U.S. Navy diving and mine countermeasures officer, Snyder is a certified hydrographer who has been working since 2005 to develop new remote underwater survey and inspection techniques that rely on traditional hydrographic survey principles.