ROV framework
The frame of ROV is used to install and protect various underwater equipment. Most manufacturers use open frame structure to improve the stability of the whole machine. This structure is also convenient to add auxiliary equipment, and the water flow can pass through the body unimpeded. Some ROV manufacturers also integrate the frame with the electronic warehouse, even the integrated structure. There is no obvious distinction between the propulsion system, pressure shell, floating block and frame. If the shape design is good, it is easy to cause turbulence, increase resistance and poor stability. It is suitable for those small observation level ROVs.
The type of material used for the rack is an important consideration for the design engineer. The modified material must be able to withstand harsh conditions and provide sufficient structural strength and stiffness to support the carrying equipment of ROV. 6 series aluminum alloy or stainless steel are generally used for large or super large ROV. These two materials are rarely used for small ROV, but they are not absent. Through reasonable structural design, a balance point between weight and strength can be found.
After long-term contact with seawater, the weight of metal will increase due to corrosion. Here, the impact of weight increase on buoyancy balance is mainly considered. At the same time, if the metal structure is too thin, it is easy to corrode and decompose. Therefore, polymers have increasingly become the main flow choice of small ROV frame materials. Acrylonitrile butadiene styrene (ABS), high density polyethylene (HDPE) and polypropylene (PP) are widely used by ROV manufacturers. The use of polypropylene on ROV has been studied, and the results are OK. Mainstream observation grade ROV manufacturers have been using polymer material frames.
Carbon fiber reinforced plastics (CFRP) is a technological progress. It has better stiffness, strength and material damping than conventional polymers. Recently, this material has been used in some projects, and the use of CFRP in the underwater robot industry is expected to grow in the next few years.
Since most observation level ROVs are portable, their weight in the air is usually no more than 70kg. The modified weight can be supported by polymer structure, which is light, difficult to decompose in seawater and low cost. Therefore, polymer materials will be more and more widely used in the underwater robot industry.
4. Hydrodynamic force
For large work level ROVs, hydrodynamics is hardly considered in appearance design, because they emphasize stability rather than rapidity when performing heavy operations. The application scenarios of small ROV are more diverse, so hydrodynamic forces need to be considered in the design.
The medium-sized observation level ROV is usually box type, just like the observation level ROV in oceaneering at the beginning of the article. There is a buoyancy block on the top, and the propeller and equipment are suspended under the rack. This layout provides good stability. The center of gravity and floating center can be on the same vertical line, but the spacing is reserved. In addition, stability is very important in acoustic imaging measurement. Therefore, the pitch and roll amplitude of ROV with this shape are small, and the measurement results are easy to process. There are also cases where the box observation level ROV is designed to coincide with the center of gravity, so that it can quickly respond to interference in any direction. However, it is necessary to rely on the active control system to maintain the body stability. The mooring force, surge and flow force need to be automatically compensated by the control system. The design of underwater robot and UAV is more reasonable.
Generally speaking, many micro ROVs are only used as an “underwater camera”, and their requirements for stability are lower than those of acoustic equipment. Of course, professional photography is another matter. The shape design of this ROV can adopt more novel shapes, even eye-catching shapes. Universal joints and eccentric mechanisms can be used to control the attitude inside the body.
In recent years, there has been some progress in the research of bionic robots. Using underwater robots with this shape to observe marine organisms will not scare them. The propulsion efficiency of some fish is as high as 90%, while the efficiency of traditional propellers is 50%, and the highest in practice is no more than 70% – 80%. Most bionic fish robots can imitate the swimming or maneuvering of a certain fish in a specific swimming mode. For more advanced motion control, the robot should be able to switch freely in a variety of swimming modes, because the real fish can randomly adjust the fin and tail according to the water flow and motion requirements to achieve countless optimal combinations. There is a measuring line on the side of the fish body. What is a measuring line?
Lateral line is a special sensory organ of fish, which can be seen from the outside of fish body. This is the line formed by many small holes on both sides of the fish body. These small holes are called side line tube holes. The small holes are distributed on some scales. The lower parts of the small holes are connected with each other to form a long tube, which is called side line tube. The tube is full of mucus. The stimulation of external water flow is transmitted to the sensory cells in the lateral tube through the lateral tube hole on the scale, which can produce excitement. The excitement is transmitted to the brain through the nerve, which can produce water flow sensation. Fish can sense the size of water pressure, water flow direction, water flow speed, the position of objects in the water and other changes through the measuring line.
Lateral line system is the nervous system of fish. It grows on the scales on the side of the body, which is called lateral line scale. There is one on both sides. The lateral line organ is located on the scale and forms a small hole. The small hole contains nerve endings, which transmit external information to the brain nerve through its connected sensory organs,. The lateral line system is usually the same length as the fish body, and sometimes extends to the fish nose through the gill cover. It can be used for detection and induction in contact with the outside world, and can feel the vibration and water flow velocity. Therefore, many fish can swim freely in dark waters, or even fish whose eye function has degenerated to invisible can detect obstacles in the water with a side line system.
The main function of pectoral fin and ventral fin is to maintain the balance of fish body, cooperate with the steering of fish body and adjust the rise and fall of fish body. But the pectoral fin is more widely used than the ventral fin. It can paddle like an oar to make the fish move forward slowly.
Inspired by this bionic principle, MEMS strain sensors can be installed on the side of the bionic fish robot to imitate the measuring line of the fish, and then the sensing signal can be output into the action required by the fish to swim. In addition, there are also robots that simulate the movement of sea turtles for sunken ship investigation. In 2016, the US Navy also approved the research of swordfish robot for underwater mine clearance. Sepios is a squid like underwater robot driven by four fins. One advantage of using fin drive is that the robot can shuttle through seaweed vegetation without worrying about the propeller being entangled.
If the current is too large, you can try crawling or walking robots. At the school of marine science and technology of the University of Newcastle in the UK, they have developed a multi legged robot that can inspect pipelines in tidal currents.
Hydrodynamics software is usually used in the design of ROV. CFD is used to analyze the effectiveness of the parameter prediction model based on gray box, so as to better control ROV.
Commercial products focus more on the functions and applications of ROV. In the near future, ROV design will pay more attention to hydrodynamic analysis. With the growth of marine renewable energy industry, observation level ROV is expected to operate in more extreme environments, which means that the resistance of ROV must be reduced in addition to other design standards. However, for bionic robots, a lot of research is still needed to make them have commercial value. So far, their functions are too thin. How can human beings imitate the organism that has evolved for hundreds of millions of years.
