Robotic Operations
	Introduction The Three Sisters AUVs Superman AUV Tug ROV Drillette ROV Distant Docking Station Spiral Sensor Array Sensor Fleet Sensors Tools Docking with Habitat Power Mobility Structure Navigation

Navigation

A quick look at Buoyancy

It is usual to make vehicles that are positively buoyant. This buoyancy is usually 2.3 kg for small vehicles and 22.7 kg for larger vehicles. Positive buoyancy reduces power, and the need to thrust up, disturbing the sediment below.

The moment equation assesses the stability of vehicles:
m = (W)BGsintheta
m = moment w = weight of vehicle BG= distance between the centre of buoyancy an d the centre of gravity theta = pitch angle or roll angle

A high BG, low weight, and high buoyancy produces a stable vehicle. Heavy components such as motors tend to be at the bottom, while the syntactic foam and light chambers are at the top of the ROV. Syntactic foam is described as a fixed ballast. AUVs can use the design of their pressure hulls as a means of providing buoyancy. For a drilling ROV it will have to have variable buoyancy, the ability to move right down to the ocean floor and back up. This can be achieved by using air compressors, or flooding tanks similar to those found in submarines.

Navigation

One top of the line AUV developed in 2000 is called Martin (see references for link). A key feature to achieving this autonomy is Martin's unique high-precision positioning system, Marpos, developed jointly by Maridan and the Kearfott Navigation and Guidance Corporation of New Jersey. The system centers on Kearfott's KN5053 monolithic ring-laser gyro coupled with their accelerometers and integrated with an RDI Doppler velocity log. Additional inputs handled by the system come from a Digiquartz pressure sensor, an altitude sensor, CTD sensor and clock and, on the surface before and after each survey, a DGPS receiver. Input from these sensors is processed by sophisticated software developed by Maridan that ensures the extraordinary precision of the system.

Hybrid Navigation System

This is the best method of navigation. Sensor beacons are going to be built into the communications table set-up, so it makes sense to use these, in conjunction with maps, to triangulate position. We will also use a transponder independent system when the AUV is exploring regions out of range with the beacons, or completely undiscovered. This is a Long-Base Line (LBL) navigation method. In LBL navigation, acoustic beacons(transponders) are laid on the surface. Four transducers is an optimum numbers at the four corners of the entire region. The underwater vehicles are equipped with acoustic transceivers to interact with the acoustic beacons.

Inside the pressure hull of the AUV will be the inertial navigation unit consisting of ring laser gyro, coupled with velocity logs from our ADCP. It also consists of a filter to collect all the data from the pressure sensor, altitude sensor and CTD.

References