Underwater robot hits the mainstreamUnderwater robot hits the mainstream

Since deep-ocean pressures create a hostile environment for human divers, remote-controlled underwater robots make a lot of sense for completing many underwater tasks. These remote operated vehicles, or ROVs, come in all shapes and sizes. Complex work-class systems with several dexterous manipulators, cameras, tools, and other equipment have recently gained fame for recovering massive treasures from long-lost shipwrecks. But they can cost up to $500,000. Smaller vehicles have recently achieved notoriety for filming the Titanic's wreckage, and providing a glimpse of the "swimming tanks" that sunk during the Normandy Invasion. Yet even systems limited to simple inspection and observation tasks can cost as much as $100,000.

Improvements in electrically powered systems over the last two decades have spawned a surge in the development of smaller, more affordable ROVs in the $20,000 to $50,000 range. However, according to ROV distributor Brock Rosenthal, president of a marine supply house Ocean Innovations (La Jolla, CA), lower cost systems aren't particularly useful. "The one or two sub-$20,000 underwater vehicles that have been developed are so underpowered and poorly designed that they really aren't very useful for many applications."

Hydrovision's HyBall ROV cost $50,000 when it was introduced in 1986, and today it's about the same price, according to its inventor, Don Rodocker-a price, that, in his opinion, is still too costly for many everyday jobs. Nine years ago Rodocker conceptualized a much more integrated design approach that he hoped would yield a better ROV for the money. His concept debuts this year as LBV, or Little Benthic Vehicle from SeaBotix (San Diego). In fact, the ROV designed as a platform capable of delivering a wide variety of instruments, is just going into production now.

Thanks to innovative packaging and advances in power conversion, electronics, and motion control, the little, 22-lb diving robot is making big waves. Its $7,495 base price opens up the opportunity of buying an ROV for all kinds of people from researchers and divers, to recreational yachtsmen. "LBV is doing to the ROV industry what the PC did to the computer industry in the 1980s," Rodocker explains, "making the technology affordable for almost anyone."

Though a typical order might cost approximately $10,000 with a bit more umbilical, a bumper, and manipulator, that's still less than most entry-level cars on the market today. "In a lot of ways, we're in the information technology business like so many other companies today," he explains. "We provide our clients with a flexible platform that enables them to acquire a variety of data depending on their individual needs."

LBV's small size gives it more than just a cost advantage over larger systems. It eliminates the need for bulky launch systems because you can pick it up with one hand and toss it over the side of a smaller watercraft, making it more suitable for inland operations. It also fits in tighter spaces where larger ROVs can't.

3,000m under the sea. Despite its small size, LBV is capable of achieving significant depths, attaching a line to a structure, and picking up small objects. Available in 150-, 300-, 1,500-, and 3,000m depth versions, LBV consists of five major subsystems: the ROV itself, the control console, the power supply, the umbilical cable, and accessories such as bumper frame, grabbers and manipulators, sonar, advanced tracking systems, and a tether management system for deeper dives.

Low-cost yet powerful thrusters are one of the many keys to the vehicle's success. "We spent a lot of time researching the propulsion system motors," Rodocker explains. "Every component had to be capable, reliable, compact, and affordable." Rodocker says that's why he chose the brush dc motors from Pittman (Harleysville, PA). "They were the best suited for our application."

In all, the LBV propulsion system uses four bi-directional thrusters that are all mounted directly to a central backplane. The four thrusters provide up/down, fore/aft, starboard/port, and rotate thrust to propel the vehicle through the water. Two horizontal thrusters mounted in the rear of the vehicle, one port and one starboard, provide fore/aft thrust, and rotate thrust when run in opposite directions. A lateral thruster provides sideways movement, and a vertical thruster moves the vehicle up and down. Using an intuitive, three-axis joystick and membrane-switch control console, pilots fly LBV like a helicopter through the water at speeds up to 3 knots.

Each thruster has its own 1.5-inch diameter, four-layer PWM (pulse width modulated) controller with surface mount components. The controller mounts inside the motor's pressure vessel, opposite the motor's output shaft. Power, control signals, and motor current feedback travel through a 4-pin connector, two for power and two for serial data, from the pressure vessel directly to the backplane. In air, Rodocker says, "we are able to run the motors at 25V, 4.5A, achieving 3 lb-6 oz of thrust. We can drive these motors at 180W in oil, achieving greater than 6 lb of thrust for deeper applications that require a longer umbilical which produces more drag."

Wagging the dog . "The umbilical is LBV's biggest advantage, but it's also its biggest disadvantage," Rodocker explains. "With it, LBV is lighter and more compact, and has unlimited diving time because it never needs recharging. However, the deeper the dive the longer the umbilical, and the more drag it creates. That's why we use Kort nozzles, which essentially surround the propeller to focus the thrust and keep the kinetic energy from being wasted in a radial direction. Higher power keeps the tail [umbilical] from wagging the dog, so to speak."

For even deeper, more capable systems Rodocker considered using recently introduced Pittman brushless dc motors. "We may eventually offer a brushless dc thruster. It might add $1,000 to the total cost of the vehicle, but brushless motors would provide up to 30% more continuous torque in the same size package to help the vehicle overcome the additional drag of a longer umbilical."

Using SolidWorks CAD software, consulting engineer Jeff Krause developed the LBV's industrial- and detail-design models from Rodocker's concept sketches. "We were actually trying to make a quasi-consumer product, and Don was the driving brilliance behind the integrated packaging that got the cost into the $8,000 range," Krause notes. This integrated approach to low-cost design was carried throughout all of LBV's subsystems.

For example, LBV uses its backplane to support six of the seven pressure vessels designed using Cosmos FEA software from SRAC. Integral connections in each pressure vessel provide an electromechanical link to the backplane, eliminating the need for separate connectors, according to Krause. "Reducing the number of discrete components helps with reliability which is always a problem with such complex systems," Rodocker notes. "That's why we gave each thruster an independent pressure vessel, so that if one should fail, it won't take the others out."

Many have already purchased an LBV, although at the time of this writing the first LBV had yet to roll off of the assembly line. Ocean Innovations' Rosenthal is anticipating delivery of his first LBV. "We have two types of customers that are very excited about the LBV. People who work in the offshore business and commercial divers that know how useful these robotic vehicles are but have never been able to afford one. And other people who have never thought about having a tool like this. I think it will sell itself when I hand someone the controller and say, 'fly it.' "