لخّصلي

Most of the earth's surface is covered with water in the form of oceans, rivers and lakes, many of which remain unexplored till date. These environments contain some of the most natural resource-rich habitats. These habitats directly or indirectly affect humans. Deployment of underwater robotic vehicles can help to study these environments to ensure their safety against environmental pollution and use the available natural resources for human development.

Manned underwater vehicles have humans on board which increase the risk as well as operational cost, so underwater unmanned systems are getting very popular. These systems can be used in greater depth and extremely harsh conditions. “Autonomous underwater vehicle (AUV)” and “Remotely operated underwater vehicle (ROV)” are two categories of unmanned underwater robotic systems. ROVs are controlled from the surface, generally by a wired connection. These can do a variety of tasks, but the wired connection limits its manoeuvrability as well as accessibility to remote locations. AUVs navigate autonomously relying on its navigation algorithm and surrounding information. Once deployed, they collect data and come back to the surface after completion of the predefined task. As AUVs are not connected to the ground they have high manoeuvrability, can travel to remote locations, narrow complex pathways, involve no human fatigue and operation cost is very less. Underwater wireless communication has its limitations so AUVs have seen an increase in interest from the underwater research community. It has always been a challenge to make a robotic device to explore these hostile territories. With advancements in AUV research, materials, manufacturing techniques, sensors, computational power and battery technology, autonomous decision making underwater robots have become more reliable and practical. A reliable, fully autonomous decision making robotic system is the objective of current AUV research.

The first AUV “SPURV (The Self Propelled Underwater Research Vehicle)” was developed by Stan Murphy and Bob Francois in 1957 in the Applied Physics Laboratory at the University of Washington (Remotely Operated Vehicle Committee of the Marine Technology Society). “SPURV” operated at 2–2.5 m/s up to a depth of 3600 m Widditsch (1973). In the 1970s few AUVs were developed in MIT and also in the Soviet Union (Remotely Operated Vehicle Committee of the Marine Technology Society). These early underwater robots were bulky, expensive and inefficient. AUVs have come a long way since then. The modern-day AUVs can have six degrees of freedom, can travel faster than 20 m/s, accurately detect obstacles and map ocean floors. These are getting compact, less expensive, yet sophisticated and accessible to the general population for exploration, fishing, and entertainment etc.. These systems have yet to go a long way in terms of autonomy till fully autonomous robotic systems help us explore and protect these deep and hazardous habitats.

AUVs have a large number of applications in hazardous underwater environments. Still, AUVs have to overcome some limitations to have large-scale adoption. Some of the key challenges are low price, underwater wireless communication, long lasting batteries, advanced manufacturing techniques, smart materials, compact on-board computers with high computational power for better decision making, on-board energy generation and its efficient use. In this paper, the major subsystems of AUVs are presented in different sections. The different sections are applications, AUV structure, propulsion techniques, kinematics and dynamics, planning and control, navigation and localization and underwater communication. The objective of this paper is to present recent developments and challenges in the field of AUV. The next section discusses different useful applications of the AUVs.

AUVs are becoming very popular for underwater exploration in commercial, military and industrial applications. Over the years a large number of AUVs have been developed for various application. Table 1 lists the potential applications along with AUVs used for these purposes.

With the help of different sensors, these vehicles can collect a variety of useful scientific information like temperature, depth, pH level, chemical composition, turbidity etc. This information can help in environmental monitoring and scientific study. Cameras can be used to take pictures of the environment to study underwater ecosystems, different aquatic animals and underwater ground structure (Desa et al. (2007); Ferguson (2009); Kaminski et al. (2010); Shome and Das (2012) Kongsberg Maritime. 2018). With multiple cameras and sonars, 3D mapping and reconstruction of the sea floor can be done which can be used in site selection for constructions like tidal energy plant, ports; claim the maritime borders with continental shelf data (Shome and Das (2012); Thompson et al. (2012a); Morice et al. (2009); Kim et al. (2013)). Allotta et al. (2018) presented AUVs “MARTA” and “A-Size” used for 3D mapping of underwater archaeological sites and a turtle-inspired bio-mimetic AUV “U-CAT” used for ship wreckage penetration and survey. AUVs are being used for inspection of cracks and damages in underwater structure Jacobi (2015), track and discover ore Tri-ton (2013), oil, natural gas reserves. Underwater vehicles can be used to track oil leakages from oil mines, gas leakage from under-sea gas pipelines to protect the underwater ecosystem and avoid pollution Kimura et al. (2013). Intervention-AUVs with autonomous manipulator systems are being used for various intervention tasks such as self-docking, search and retrieve objects (Sanz et al. (2012)), payload delivery to the ocean floor, pipeline and cable deployment etc. Such I-AUVs can be used for black-box search and retrieval during air crash investigations. Various co-operative tasks such as pipeline (Simetti et al., 2017) and cable deployment, transportation of long and heavy payload to the ocean floor can be carried out by a fleet of AUVs and I-AUVs. Some bio-mimetic underwater robot like a snake and fish robots and other AUVs are being used for inspection and surveillance Shome and Das (2012). Bio-mimetic robots (SoFi Katzschmann et al. (2018)) are being developed with soft flexible materials to create lifelike motion. Such AUVs can be used for close-up observation of aquatic life without disturbance. Apart from these scientific and commercial applications, AUVs are being extensively used for military purpose. Bio-mimetic, as well as other AUVs, can be used for surveillance and reconnaissance. Using sonar AUVs can be used for mine countermeasures without engendering human life. Other applications may include anti-submarine warfare, search and rescue and site inspection etc. Generally, these robotic vehicles are used in oceanographic applications. Underwater robots are also being used to study underwater environments in rivers and lakes and carry out surveys. In recent years AUVs have gained much more popularity because of its potential applications in the fields of scientific research, military and industries. AUVs have been initially developed by military and research establishments for specific applications. Later multi-purpose as well as application specific industrial AUVs have been developed. “REMUS-6000” (Fig. 1 (a)) developed by Jun et al., 2012, Jun et al., 2013 is such a multi-purpose industrial AUV. REMUS-6000 weighs 862 kg with a maximum depth range of 6 km and travel velocity up to 2.3 m/s. The AUV houses acoustic modem for communication, side-scan sonar for bathymetric data collection and acoustic underwater positioning and navigation system with IMU(Inertial Measurement Unit) and DVL (Doppler Velocity Log) sonar for navigation. This AUV can be used for fisheries research, habitat mapping, under ice exploration, marine archaeology, deep-sea ecology, seabed investigation, deep-sea mining, mine countermeasures, surveillance and reconnaissance etc. “Bluefin-21” (Fig. 1(b)) developed by General Dynamics Mission Systems. 2012 is another multi-purpose industrial AUV rated for 4.5 km depth. This AUV can be used for the oceanographic study, mine countermeasures, anti-submarine warfare and underwater exploration etc. Maintenance and repair of underwater gas and oil pipelines is a major potential industrial application for AUVs. An AUV (Fig. 1(d)) with a robotic arm for pipeline inspection is under development by Kawasaki Subsea (UK) Limited, to be launched in the year 2020. Table 2 presents some examples of AUVs being used in the field for various applications encountered during this literature survey. More examples of such AUVs used for different applications can be found in the AUV database AUVAC. The following section talks about the structural design of the AUVs.