About the size and weight of a small compact car, the Benthic Rover moves very slowly across the seafloor, taking photographs of the animals and sediment in its path. Every three to five meters (10 to 16 feet) the Rover stops and makes a series of measurements on the community of organisms living in the seafloor sediment.
MBARI researchers have been working on the Benthic Rover since 2005, overcoming many challenges along the way. The most obvious challenge was designing the Rover to survive at depths where the pressure of seawater is about 420 kilograms per square meter (6,000 pounds per square inch). To withstand this pressure, the engineers had to shield the Rover's electronics and batteries inside custom-made titanium pressure spheres.
To keep the Rover from sinking into the soft seafloor mud, the engineers outfitted the vehicle with large yellow blocks of buoyant foam that will not collapse under extreme pressure. This foam gives the Rover, which weighs about 1,400 kilograms (3,000 pounds) in air, a weight of only about 45 kilograms (100 pounds) in seawater.
Other engineering challenges required less high-tech solutions. In constructing the Rover's tractor-like treads, the design team used a decidedly low-tech material—commercial conveyor belts. After watching the Benthic Rover on the seafloor using MBARI's remotely operated vehicles (ROVs), however, the researchers discovered that the belts were picking up mud and depositing it in front of the vehicle, where it was contaminating the scientific measurements. In response, the team came up with a low-tech but effective solution: they removed the heads from two push brooms and bolted them onto the vehicle so that the stiff bristles would clean off the treads as they rotated. The team also discovered that whenever the Rover moved, it stirred up a cloud of sediment like the cloud of dust that follows the character "Pig-Pen" in the Charlie Brown comic strip. This mud could have affected the Rover's measurements. To reduce this risk, the engineers programmed the Rover to move very, very slowly—about one meter (3 feet) a minute. The Rover is also programmed to sense the direction of the prevailing current, and only move in an up-current direction, so that any stirred-up mud will be carried away from the front of the vehicle.
In its basic configuration, the Benthic Rover is designed to operate on batteries, without any human input. However, during its month-long journey this summer, the Rover was connected by a long extension cord to a newly completed underwater observatory. This observatory, known as the Monterey Accelerated Research System (MARS), provided power for the robot, as well as a high-speed data link back to shore.
According to Sherman, "Hooking up the Rover to the observatory opened up a whole new world of interactivity. Usually when we deploy the Rover, we have little or no communication with the vehicle. We drop it overboard, cross our fingers, and hope that it works." In this case, however, the observatory connection allowed MBARI researchers to fine tune the Rover's performance and view its data, videos, and still images in real time. Sherman recalls, "One weekend I was at home, with my laptop on the kitchen table, controlling the vehicle and watching the live video from 900 meters below the surface of Monterey Bay. It was amazing!"
The Rover is constructed entirely of plastics and titanium for long-term corrosion resistance. The frame is assembled using transverse bulkheads built from 1" thick sheets of polypropylene shaped with a water-jet cutter. Longitudinal stiffening comes from four 2-1/2” titanium tubes bolted through the bottom of the plates as well as a large titanium lifting bail assembly bolted through the top of the transverse plates. Eight standard 1 Cu-Ft syntactic foam blocks are bolted between the polypropylene plates and six “football” style floats are mounted on two 2” titanium tubes that are positioned at the top of the vehicle. These football floats can be slid fore and aft to adjust the pitch trim of the vehicle. Lead weights will be bolted to the bottom of the transverse plates to adjust roll trim.
The 100-pound negatively buoyant vehicle is propelled using two, 18” wide reinforced rubber tank treads that can be independently driven through motors driving the rear wheels. The wheels are built using six, 18” diameter, 1.5” thick disks spaced 2” apart and stacked on a 2” diameter drive shaft. The inside surface of the tank tread has several longitudinal bosses bonded to it that fit into the gaps between the disks to keep the belt from working off. Transverse bosses are bonded to the outside of the rubber tank tread to aid traction. A low-friction, plastic pressure plate is used to distribute vehicle loading to the drive belt in the span between the wheels. Non-metallic glass-ball/Delrin bearings support the drive shaft.
Main Drive Motor
The main drive motors are brushless DC gear motors made by Maxon Motors. The motors are mounted in delrin housings that are pressure compensated with Tellus 10 hydraulic fluid that is pressurized to 5 psi above ambient using a spring-energized hydraulic compensator. Both drive motors share a single compensator, which we wouldn't normally do since it reduces system redundancy, i.e. if one motor leaks, both are affected, but since both motors are required at all times, we chose to combine systems, which actually reduces system complexity and maintenance.