The design of an underwater vehicle depends entirely on its mission. The type and scope of the mission determine the necessary capabilities of the vehicle so that it may successfully complete its tasks. Once the mission is clearly defined, the specific objectives and constraints can be established; a design can then be chosen to best meet those requirements. Underwater vehicles are broadly classified by three categories: type of control system, Degrees of Freedom (DOF) of movement, and design configuration. Any combination of these three items can be used to classify an underwater vehicle design.
The control system continuously analyzes the vehicle’s current position, heading, depth, speed, and condition. It compares this data to its assigned mission and determines whether a variation is present from the expected values. If a variation is present, the control system calculates the corrective action necessary to eliminate the variation. A signal is then sent by the control system to the vehicle’s propulsion system and control surfaces to enact the corrective action. There are two categories of control: manned and unmanned. Manned systems use human operators to directly control the vehicle from within the vessel. Unmanned systems, or Unmanned Underwater Vehicles (UUVs), use mechanical systems and/or electronic computers & transducers to either control the vehicle or relay controls received from signals external to the vehicle; they do not have any humans onboard. Within the category of UUVs, there are two types of control systems: Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs).
ROVs are controlled remotely by human operators by sending a signal from a control station to the ROV. The ROV then interprets these signals and actuates the propulsion system & control surfaces. The signals are either sent through a signal wire (called a “tether”) linking the control station and the ROV, or through the water using acoustic or electromagnetic waves. ROVs are commonly supplied power externally through the tether. An AUV operates under its own control, without continuous communication between it and a control station. It receives instructions either directly, in the form of programming and instructions installed onto its control system before it is launched, or through acoustic or electromagnetic signals sent during its mission. AUVs require control systems with the ability to adapt to changing circumstances but do not necessarily require any artificial intelligence. AUVs are usually under their own power but could use external power through a tether or other means, if so designed.
With regards to UUV control, the DOF denotes the number of independently variable kinematic and positional states that can be directly changed by the vehicle using its propulsion systems and control surfaces. The maximum amount of kinematic DOF in three-dimensional space is six; three DOF for translation on the three horizontal and vertical directional axes, and three DOF for rotation about those axes. A seventh addition to the DOF, time response, may be considered in fourth-dimensional space, but it is easiest to distinguish different designs by their directional motion capabilities rather than by more complicated concepts. When concerned with the maneuvering capabilities of a UUV, the DOF of its design will indicate whether it is applicable to the mission. The UUV’s DOF is related to its design configuration, but certain categories of design configuration can have varying levels of DOF. The design configuration of a UUV is the general description of the types, numbers, & positions of the vehicle’s buoyant elements, propulsion systems, pressure & hydrodynamic hulls, control surfaces, and structural members. Common categories of design configuration include Underwater Gliders, Torpedo-type UUVs, Amphibious Unmanned Aerial/Underwater Vehicles (UUV/UAVs), Open Space Frames, and Underwater Rovers. Below are brief descriptions of each type and its associated properties. Some designs are a combination of these types, and this list is not all-encompassing.
Underwater Gliders utilize changes in buoyancy and hydrodynamic lift to generate motion. By altering their buoyancy, they alternate between diving and surfacing. When combined with a hydrofoil, this vertical motion produces lift and results in a forward gliding motion. Common buoyancy engines—devices to change the vehicle’s mass, volume, or both—are ballast tanks, compressible bladders, or devices that use changes in water temperature to melt “phase-change” materials that expand & contract, doing work to power ballast systems (Buis). They have few moving parts, are capable of extreme endurance, and are relatively cheap. Most Underwater Gliders are AUVs, and typically have two to three DOF—Pitch, diving, and yaw or roll (depending on mechanism). The disadvantages of this configuration are its low speed, inability to hold a constant depth or station-keep (maintain position relative to another object), and difficulty operating in shallow waters.
Torpedo-type UUVs consist of a simple cylindrical-shaped hydrodynamic shell, with at least one thruster and two control surfaces. They commonly use a ballast tank for vertical motion but can also use hydrofoil elevators. This UUV type typically has four DOF—thrust, pitch, yaw, and diving. They are often quite fast and hydrodynamic, use very few motors and control surfaces, and can resist subsurface currents. However, they cannot move laterally without yawing and may have difficulty maneuvering in tight spaces.
Open Space Frame
Open Space Frames are a broad category of design configurations, with the common feature of having a skeletal frame with thrusters, sensors, pressure vessels, ballasts, weights, and other accessories attached to it. They are often very maneuverable and compact, almost always having six DOF, which makes them good for surveying tunnels, marine infrastructure & machinery, ship hulls, and other tight or enclosed spaces. They excel at station-keeping. They can carry a wide variety of payloads and are very modular. They typically use multiple thrusters for movement and are either neutrally buoyant or have a ballast tank. Most open space frame UUVs use a tether.
Underwater Rovers function by using buoyancy or some gripping force to interface with a surface such as the seafloor, cliff faces, or the underside of ice sheets. They then use traction from wheels or tracks, or mechanical legs, to propel themselves on that surface. The advantage of this design is that it can stay in one spot for a long time and is very stable relative to the ground. It requires no power to remain stationary and is mostly resistant to underwater currents. The disadvantage of this design configuration is its slow speeds, and most designs have difficulty traversing extreme underwater terrains such as cliffs, ravines/trenches, reefs, thick vegetation, and extremely soft soils.