AUV Subsystems
A perspective into components of an AuV would bring out the technological complexities involved in the design and competence required for its manufacture. An AuV consists of subsystems like pressure and hydrodynamic hull, ballast system, masts, power and energy system, propulsion system, obstacle-avoidance system, manoeuvring system, communications system, sensors, navigation system, host interfaces and combat payload. uuV structure makes the vehicles rigid and provides strong points for control surfaces, thrusters, batteries and other uuV components while permitting internal components to be accessed. Pressure hull enables uuV to withstand sea pressure as it descends into the ocean. The pressure to which an uuV is subjected increases linearly with depth. At 6,000 m, the sea pressure is about 4.4 tonnes per square inch (psi), whereas at 300 m sea pressure, it is about 441 psi. for relatively shallow operation, therefore, the hulls of AuVs can be fabricated from lighter materials such as aluminium.
Hydrodynamic hull design reduces drag as uuV moves through the ocean. minimising drag to maximise speed and endurance is one of the design objectives along with fRQWuROOlQJ flRZ RyHu WKH 889 eRGy IRu HIfiflHQW SuRSXOVlRQ. 6WDelOlWy DQG PDQRHXyuDelOlWy DW ORZ VSHHGV DuH GlIfifXOW. 6HQVRu operation stabilisation at higher speeds is problematic. Stability in AuV design scores over speed and endurance.
Ballast System: Neutral or near-neutral buoyancy in submerged uAV is achieved WKuRXJK eDOODVWlQJ. /HDG Ru IRDP fixHG buoyancy systems are used which can be adjusted depending upon the changes in role/payload of the AuV. Variable ballast system is used for diving or surfacing the AuV and drop ballast system is used for surfacing the AuV in case of an emergency.
The power requirements of an AuV until recently were being met with by the use of silver zinc batteries. However, due to their higher costs and limited shelf and cycle lives, they are being replaced with lithium ion or lithium polymer batteries. Solar powered AuVs, which surface during the day for recharging, have also been developed. Bus system is used for uniform power distribution.
Brushless DC motors with propellers are favoured for AuVs of the torpedo type since they are better than the motors with brushes lQ IDfWRuV RI uHOlDelOlWy, HIfiflHQfy DQG power density.
Obstacle avoidance is carried out by use of single or multiple acoustic beam systems to detect and avoid obstacles. Avoidance manoeuvres are pre-programmed in the AuV. HowHyHu, WKHVH QHHG PXfK PRuH fiQH-WXQlQJ DV they continue to be an area of concern.
Control surfaces, vectored propulsors or thrusters are generally used for AuV manoeuvres. for hovering, lateral or vertical movement multi thrusters are utilised.
Communication is essential for an AuV when submerged or surfaced. Acoustic systems are usually used underwater. emergency beacons and locating mechanisms in emergency or on completion of mission are also built in an AuV.
This is a complicated design feature as it impacts the launch and recovery of an AuV. The masts are used for mounting sensors, communication and navigation antennas.
AUVs are free-swimming autonomous underwater vehicles characterised by modularity, reliability and possibility for custom design
Sensors can be put into following groups: conductivity, temperature, and depth (CTD) sensors, acoustic sensors; electromagnetic sensors; magnetic sensors; optical sensors; and chemical, biological, radiological, nuclear and explosives (CBNRe) sensors.
GPS can only be used by AuVs when they are near the surface. for underwater navigation they use inertial navigation system (INS) and doppler velocity log navigation systems. Gyroscopes for orientation, accelerometers for velocity changes and propeller turn rates for speed measurements form the main components of the INS. Doppler shift provided by the down-track and cross-track sonar returns give accurate speed and course.
Host interfaces form one of the most important aspects of AuV design. These include both the software and the hardware interfaces with which the AuV communicates with its controlling vessel. launch and recovery, signal, control and power are some of the interfaces required.
This may include specially designed torpedoes, missiles or mines.
AUVs in the US Navy
The uuV programme was launched in the uS Navy with the aim of shaping, control-