The DEPTHX AUV
The DEPTHX AUV is egg-shaped, with a long axis of 4.26 meters (13.97 feet), a short axis of 3.04 m (9.97 ft), and a weight of 1.3 metric tons (1.43 short tons). It can move freely, hover and wall track in three dimensions without any external commands. DEPTHX has a minimum cruising speed of 0.2 meters per second (0.65 feet per second) for mapping and can go as deep as 1,000 m (3,280 ft). To operate autonomously, DEPTHX must be able to do the following:
- Navigate and map
- Process information
- Power itself
- Take environmental measurements and samples
DEPTHX has six thrusters (four horizontal and two vertical), which allow it to maneuver in three dimensions. The vehicle can be maneuvered with just two of the horizontal and one of the vertical thrusters -- the extras are for backup.
DEPTHX can also hover. To hold its position, it could use the thrusters, but this would rapidly consume valuable battery power. Instead, the vehicle is equipped with two variable-buoyancy engines (VBE). A VBE computer senses the pressure, temperature and salinity (salt content) of the surrounding water and calculates the craft's buoyancy. The computer then opens or closes valves that allow pumps or pressurized gas to let water in or out. As with the thrusters, there is built-in redundancy and the DEPTHX can hover using just one VBE.
Navigation and Mapping
DEPTHX has 54 sonar sensors spaced around its frame and controlled by two sonar arrays. The sonar uses pulses of high-energy sound waves and their reflections to locate objects within a 250- to 300-meter radius of the vehicle. The information it collects is relayed to on-board computers for navigation control.
In addition to sonar, DEPTHX navigates using accelerometers, depth gauges and an inertial guidance unit. A Doppler velocity logger determines how fast the vehicle is moving and feeds this information to the primary computer, which adjusts the vehicle's speed.
As DEPTHX moves, the computers use the sonar information to build up 3-D images, which are overlaid in the computer memory to make a progressive geometrical map. The technique is called Simultaneous Localization and Mapping (SLAM). Bill Stone and his colleagues developed a diver-propelled SLAM device that they used to map the Wakulla Springs aquifer in Florida. DEPTHX uses its internalized map for navigation so that it can move to any specific location without the assistance of external navigation, such as GPS.
DEPTHX has several onboard computers dedicated to specific tasks. One controls vehicle operations, SLAM and navigation; one controls the VBEs; and one controls experiments and analyzes data. In total, these computers contain more than 30 microprocessors, from 8-bit controllers to Pentium 4 chips. DEPTHX is programmed for autonomous functioning and analysis.
Internal Power and Communication
DEPTHX has two battery towers comprising rechargeable lithium-ion batteries. The batteries provide up to eight hours of power.
Environmental Measurements and Samples
DEPTHX will use its SLAM technologies to map and image as it explores. It has a wide-field imaging camera and will also be able to make environmental measurements such as temperature, pressure and concentrations of various chemicals. It also an extendable arm with a science probe that will allow it to collect both liquid and solid samples and a pressure-rated microscope onboard to aid in the detection of microbial life.