TECHNICAL FIELD

The present invention relates to the field of underwater vehicles, and in particular to those vehicles often used for subsea tasks, such as inspecting, cleaning or repairing equipment located underwater.

STATE OF THE ART

Current remotely operated underwater vehicles (also referred to as ROVs) are electrically powered via an umbilical from a control ship or platform, which supplies power to a plurality of thrusters located at the ROV. The thrusters are generally in the form of impellers configured to operate in forward and reverse directions. US patent application US2007/0283871A1 describes a ROV having four thrusters pivotally mounted on the vehicle.

International patent application WO2013/060693A2 discloses different configurations of a exoskeleton device. One of these configurations has six thrusters grouped in two sets, each set comprising three thrusters. Other conventional ROVs achieve 4 or 5 degrees-of-freedom (three linear movements and one or two orientations) using typically 4, 5 or 6 thrusters in different configurations, such as the ones commercialized by SeaBotix (www.seabotix.com). Traditional ROV design places the buoyancy material at the top of the vehicle and ballast at the bottom, in order to create a naturally stable platform. This is an easy solution, but leads to a number of drawbacks in real-world operational scenarios, where environmental forces, tether drag and limited maneuverability contribute to making operation difficult or even impossible under common conditions.

There also exists a ROV which achieves 6 d eg rees-of-f reed om (movement capability in any direction and any angle) using 8 thrusters. An example of such vehicle is the V8 ROV model offered by Ocean Modules Sweden AB (www.ocean-modules.com). Using 8 thrusters for achieving the 6 spatial degrees-of-freedom implies an actuation redundancy of value 2. This means that this system includes a higher number of thrusters than theoretically necessary for controlling the system in the 6 degrees of freedom.

Therefore, there is a need of improved remotely operated underwater vehicles capable of being controlled in 6 deg rees-of-f reed om while reducing the number of thrusters to six, thus achieving a less complex and more compact and lightweight platform.

DESCRIPTION OF THE INVENTION

It is therefore an object of the invention to provide a remotely operated underwater vehicle which can be controlled in 6 degrees-of-freedom using 6 thrusters.

According to an aspect of the present invention, there is provided an underwater vehicle comprising a structure holding six thrusters each defining a thrust vector. The thrust vector of each of the six thrusters is oriented as follows: a first thrust vector and a second thrust vector are disposed on respective first and second planes, said first and second planes being parallel to each other; a third thrust vector and a fourth thrust vector are disposed on respective third and fourth planes, said third and fourth planes being parallel to each other and perpendicular to said first and second planes; and a fifth thrust vector and a sixth thrust vector are disposed on respective fifth and sixth planes, said fifth and sixth planes being parallel to each other and perpendicular to said first, second, third and fourth planes, such that the vehicle is enabled to move in a controlled way along its 6 spatial d eg re es-of-f re ed o m .

In a particular embodiment, each of said thrust vectors forms a respective angle a with respect to a reference vector defined in the plane at which the corresponding thrust vector is located. The reference vectors for said first thrust vector and second thrust vector are parallel to an Y axis, the reference vectors for said third thrust vector and fourth thrust vector are parallel to a Z axis, and the reference vectors for said fifth thrust vector and sixth thrust vector are parallel to an X axis, said X, Y and Z axis defining a cartesian coordinate system.

More particularly, said six respective angles a are substantially of the same value. Alternatively, at least one of said respective angles a is different from the other angles.

In a particular embodiment, said first, second, third, fourth, fifth and sixth planes correspond to the six faces of a rectangular cuboid or to the six faces of a cube.

More particularly, said thrust vectors pass by the geometrical centre of the corresponding face on which they are respectively located.

In a particular embodiment, in order to minimize the collision of flux jets from the thrusters, the thrust vector of at least one of the thrusters is displaced in parallel with respect to its original position, such that convergence of several fluxes in a single point is avoided.

In a particular embodiment, in order to minimize the collision of flux jets from the thrusters, at least one of the thrusters is rotated with a certain angle, such that convergence of several fluxes in a single point is avoided.

The vehicle preferably comprises at least one payload or mission sensor. The sensor is more preferably a camera.

In a particular embodiment, the structure holding six thrusters is a frame comprising a plurality of rods.

In a particular embodiment the vehicle comprises a plurality of floating structures.

In a particular embodiment, the thrusters are bidirectional.

In a particular embodiment, the vehicle comprises a plurality of covers located at the inner volume of the vehicle in order to isolate from each other the points at which different thrust fluxes converge.

In another aspect of the invention, a system is provided. The system comprises a vehicle like the previously escribed. The vehicle is a remotely operated vehicle (ROV) or an autonomous underwater vehicle (AUV) or a hybrid remotely operated vehicle (HROV). The vehicle comprises a control center from which the vehicle is controlled.

Additional advantages and features of the invention will become apparent from the detail description that follows and will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 shows a remotely operated underwater vehicle according to an embodiment of the present invention.

Figure 2 A shows the remotely operated underwater vehicle of figure 1 , in which some of the outer floating structures and parts have been taken out, in order to allow the view of the inner elements.

Figures 2B and 2C show the remotely operated underwater vehicle of figure 2A, in which some parts have been taken out, in order to allow the view of the six thrusters. Figure 3A shows a scheme in which the six thrust vectors corresponding to the six thrusters are designed to be placed on respective faces of a parallelepiped, in particular a cube. A reference frame located in the geometric center of the cube is included, and the thrust vectors (1 ) and (2) are parallel to the Y axis of such frame, the (3) and (4) are parallel to the Z axis, and the (5) and (6) are parallel to the X axis. Figure 3B shows the six thrust vectors (1 -6) of Figure 3A placed on the same faces of the cube but in this case their directions have been changed. For example, thrust vector (1 ) has been oriented an angle a with regard to the Y direction. In figure 3C, the thrust vectors are aligned with the diagonals of the faces of the cube.

Figure 4 shows another schematic drawing illustrating the location of the six thrusters comprised in a remotely operated underwater vehicle according to the invention. Figures 5 A and 5B show two different arrangements of thrusters according to the invention

Figure 6 also represents in detail an arrangement of thrusters.

Figures 7 A and 7B show two possible views of the remotely operated underwater vehicle of the invention.

Figure 8 shows another view of the remotely operated underwater vehicle of the invention.

DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION

In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc. In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the terms "about" and "around" and "substantially".

The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Next embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing apparatuses and results according to the invention.

An underwater vehicle is described. The underwater vehicle can be a remotely operated underwater vehicle (ROV). ROVs are controlled by a person from a remote location, such as a boat connected to the ROV via an umbilical. Alternatively, the umbilical is connected from the ROV to an unmanned boat or platform, which is wirelessly connected to a control center. The umbilical provides power to the ROV and transmits/receives data between the ROV and the manned control center. It is possible to remove the umbilical from the ROV, in which case the vehicle is powered by means of batteries. Moreover, the vehicle may be programmed for developing a mission in an autonomous way. These vehicles are called AUVs (Autonomous Underwater Vehicles) when they always work autonomously (they need no remote operation at all) and hybrid ROVs (HROVs) when they can either be remotely controlled via an umbilical or be autonomous, for which the umbilical is removed. This invention applies to ROVs, AUVs and HROVs.

Figure 1 shows an underwater vehicle according to an embodiment of the present invention. The vehicle comprises a frame 1 1 which in turn holds six thrusters 12 and can be driven or controlled in 6 d eg rees-of-f reed om (movement capability in any direction and any angle). It is therefore omnidirectional. In the view of figure 1 only five thrusters 12 can be seen. Only certain parts of the frame 1 1 and the thrusters 12 of the vehicle, as well as some other elements, are shown in figure 1. In normal circumstances, the vehicle can be loaded with other parts, such as fittings, sensors, actuators and/or grabbers that do not form part of the invention and therefore are not exhaustively shown in the figures. In figure 1 , several modules can be seen. In this particular embodiment, there are several modules 13 14, which are floating elements used to increase the floatability and counteract the weight of the vehicle once it is submerged. The vehicle provides 4 natural fixing surfaces (pointed by arrows in Fig. 4) for mounting payload sensors or other equipment, such as manipulator arms. Non- limiting examples of typical payload sensors used in these vehicles are altimeter, obstacle avoidance sonar, multi-beam sonar, acoustic Doppler current profiler, USBL and sensors for water ambient conditions (such as temperature, salinity, pH, 02, chlorophyll and fluoride). It is also possible to assemble two cameras 16A 16B instead of only one, as depicted in the particular embodiment of figure 7B. In this embodiment, the space left in the centre of one floating module has been used for assembling the second camera. This can be used for achieving stereovision or 3D vision. These fixing surfaces are used for assembling the floating modules 13 14, and in the centre of these modules 13 14 the payload sensors are fixed. An example of equipment (sensor) fixed to one of the floating modules is a camera or main camera 16, which is usually necessary, as the main and basic function of these vehicles is normally visual inspection. In figure 1 , a camera 16 is fixed on floating module 14.

The ability of being omnidirectional with only six thrusters 12 is achieved thanks to the special disposition of the six thrusters 12, which is described next.

Figure 2A shows a particular implementation of a vehicle according to figure 1 , in which some of the outer floating structures and parts have been taken out, in order to allow the view of the inner elements. In this implementation, there is a frame 1 1 formed by a plurality of bars or rods, which has an upper end and a lower end opposite the upper end. A non-limiting example of the material of which the bars are made is stainless steel. The six thrusters 12 are held at different fixing points, plates or holders 17 disposed at the frame 1 1 . In general, the components that form the vehicle platform, including the floating structures, are made of rust resistant materials. Non-limiting examples of such materials are plastics, stainless steel, anodized aluminum and titanium. The design has to beware also of galvanic corrosion. Therefore, putting two different metals in electric contact needs to be avoided. The outer body of each thruster 12 may be covered by a protection tube 15. This tube 15 is preferably made of a plastic material. The disposition of each thruster 12 is explained with reference to figures 3A- 3C.

In a preferred embodiment, a container carrying the electronics 19 as well as the camera 16 is solidary to the frame 1 1 . In the particular embodiment of figure 2A, this element 19 is solidary to the upper part of the frame 1 1. A camera 16 is held in the container 19. In this embodiment, the camera 16 may be surrounded by a lens hood for protecting the camera lens against direct sun rays.

The thrusters 12 are bi-directional and can operate in forward or reverse mode. The thrusters 12 are out of the scope of the present invention. As a matter of example, they can be motors with attached propellers or water pumping turbines.

Figures 2B and 2C shows an alternative implementation of a vehicle according to figure 2A, in which some of the outer floating structures and parts have been taken out, in order to allow the view of the inner elements. In figures 2B and 2C the container 19 has been drawn transparent in order to leave sight to the six thrusters 12-1 12-2 12-3 12-4 12-5 12-6 (or at least the protection tubes 15-1 15-2 15-3 15-4 15-5 15-6 which preferably cover the thrusters). Figure 2C is a 180° rotation of figure 2B. Additional elements, not shown, such as sensors, buoys or others, can be fixed to the frame 1 1 or to the fixing points, plates or holders 17.

Next, the approach followed in the design of preferred implementations of the location of the thrusters is explained. Each of the six thrusters is meant to be at the plane defined by each of the faces of an imaginary parallelepiped. In a preferred embodiment, all the six faces of the parallelepiped are rectangular or square. In other words, the imaginary parallelepiped is preferably a rectangular cuboid (six rectangular faces) or a cube (six square faces). In other words, each thruster (the vector defined thereby) is located on a plane (face) and there are three pairs of planes (faces) which are parallel to each other, while the non-parallel planes (faces) are perpendicular to each other.

Figure 3A shows a scheme in which the six thrusters are designed such that their thrust vectors are each placed on respective faces of a parallelepiped, which in particular is a cube (but could be a rectangular cuboid instead). One or more of the six thrust vectors may be at the geometrical centre of the face of the cube at which it is located (one thruster per face). In a more general implementation, each of the six thrust vectors can be located at any geometrical position within its face (of the cube). The thrust vector of each thruster has a certain magnitude which can vary with time and is placed onto the corresponding face of the cube with an angle a to be set (angle defined with respect to a reference direction). The thrusters are bidirectional, so the thrust vectors are reversible. In figure 3A a reference frame located in the geometric center of the cube is included, and the thrust vectors (1 ) and (2) are parallel to the Y axis of such frame, the (3) and (4) are parallel to the Z axis, and the (5) and (6) are parallel to the X axis. The X, Y and Z axis define a cartesian coordinate system. These vectors are named from now on "reference vectors '.

Figure 3B shows the six thrust vectors (1 -6) of Figure 3A placed on the same faces of the cube but in this case their directions have been changed. For example, thrust vector (1 ) has been oriented an angle a with regard to the Y direction. This configuration, in which every thrust vector is placed on a face of a cube, and oriented in any possible direction inside the plane defined by the face with an angle a with regard to direction of the reference vectors as described in Figure 3A, is the most generic configuration.

Thanks to this configuration (each thrust vector disposed on a corresponding plane defined by the faces of an imaginary rectangular cuboid or cube), six degrees-of- freedom may be controlled in the movement of the vehicle. This is obtained because there are potential force components (produced by the thrusters) that may counteract any external force or torque applied to the vehicle. This means that, in order to achieve six controlled degrees-of-freedom in the movement of the vehicle, any combination of six thrust directions on the faces of a parallelepiped is possible (the directions being defined by corresponding angles a), provided the following conditions are simultaneously held:

-there is at least one potential component of any of the thrust forces according to each of the three directions (x, y, z) (x, y, z being perpendicular to each other); and

-there is at least one pair of forces that may apply a torque in each of the three mentioned directions.

For example, in order to have a torque in an x-axis according to the reference system in figures 3A-3C, one pair of forces applying a torque in x is enough. This pair of forces does not necessarily correspond to parallel faces (of the parallelepiped), but can be originated at two perpendicular faces. For example, considering figure 3A, the two vectors in faces (3) and (4) would apply a torque in x, but also any pair of vectors located in other faces and having certain orientation could produce such torque.

In figure 3C, the thrust vectors are aligned with one diagonal of each face of the cube. This represents a particular embodiment of the disposition of the thrusters. In other words, in this preferred particular configuration, the angle is approximately +45deg for every thrust vector. Angle a taking a value of around 45deg represents a good configuration considering isotropy aspects. Note that positive angle is not always on the same direction. In this particular configuration (angle a is set to 45 degrees for every thrust vector and the thrust vectors are at the geometrical centre of the corresponding face of the cube), the thrusters are therefore located along the edges of a regular tetrahedron, as depicted in figure 4. In a preferred embodiment, the camera 16 is placed facing one of the four corners of the cube which are free from thruster flux (for instance, the corner formed by faces (1 )-(4)-(6) in figure 3C). With an angle different from 45deg, the fluxes are not convergent anymore. But with the angle at Odeg (configuration shown in Figure 3A), the capacity to support torques (typically from the umbilical) is more limited. As a matter of example, it has been observed that selecting angle a to be set to 32 degrees in all six faces of the cube, provides an optimal behavior in terms of isotropic behavior, but implies a less simple physical structure of the vehicle.

Figure 4 shows a schematic drawing illustrating the six thrusters 12 of the underwater vehicle of figures 1 and 2A-2C. In this figure 4, it is shown how each of the six thrusters is located at one of the six edges 42 of an imaginary tetrahedron. In a preferred embodiment, the thrust vector of each thruster 12 coincides with the edge 42 of the tetrahedron at which it is placed. In this context, a thrust vector represents the propulsion force produced by the corresponding thruster. In other words, the thrust vector of each thruster 12 is placed along a corresponding edge 42 of the tetrahedron. Each of the six edges of the tetrahedron is a diagonal of each of the six faces of the cube of figure 3C. Three of the four faces 41 of the tetrahedron are shown. The four faces 41 of the tetrahedron represent free space surfaces that may be used in a physical implementation for mounting sensors (also identified with arrows in figure 4). In other words, figure 4 shows an imaginary tetrahedron that surrounds the physical frame 1 1 of figures 2A-2C. In figures 2A-2C, it can be seen how the six thrusters 12 are located in tetrahedric disposition (on the edges of a tetrahedron) surrounding container 19 and being fixed thereto and/or to frame 1 1 by means of fixing points, plates or holders 17.

With respect to the cube of figures 3A-3C, it is remarked that the configuration still typically works on a random spatial parallelepiped instead of a cube, because it still provides the possibility to control the system in 6 degrees of freedom For example, if more thrust is desired on a direction, one could take the discussed preferred configuration and "translate ^' ' for example the face 2 transforming the cube into a parallelepiped; if one would still place the thrusters along the diagonal or, in general, along any angle from the reference vectors, the principle still works, with probably a less isotropic behavior. Any other deformation of the cube would work just as fine. However, the cube is the one that provides the best isotropic behavior, because the sum of the components of the thrust vectors in any cartesian direction might be the same; but something close enough to a cube is acceptable, and could be even better in some specific cases - the example of translating face 2 of the cube to have more thrust in this direction is just one of many examples one could think about. Figures 5A and 5B show two different arrangements of a group of three thrusters. In figure 5A, the three thrust vectors of the thrusters are concurrent in a point. In figure 5B, the three thrust vectors are not coincident, as the thrusters have been rotated a bit with respect to the configuration of figure 5A (that is to say, not every angle a is set to 45 degrees).

Figure 6 also represents in detail the arrangement of thrusters, which is discussed in detail later. Figure 7 A shows a view of the underwater vehicle of the invention, wherein a front view of the camera 16 incorporated in module 14 can be seen. The camera is preferably a HD camera. The vehicle preferably incorporates navigation sensors, such as inertia! measurement units (IMUs) or pressure sensors. The vehicle also includes illumination by means of leds, which can be remotely regulated (from the operator control station in land or in a ship or floating structure). In the embodiment of figure 7B, two cameras 16A 16B are mounted on the vehicle. Isotropic behaviour of underwater vehicles is a key issue to obtain a vehicle accurately controllable in every direction of space when it is working on an offshore structure. For this same reason, thrusters having thrust curves as symmetric as possible in both directions (forward and reverse) are preferably selected. On the other hand, in an underwater vehicle of relative small dimensions, when the six thrusters 12 are placed at the edges 42 of an imaginary tetrahedron (see figure 4), the flux jets may interfere at the theoretical vertices of the tetrahedron. The larger the dimensions of the vehicle, the less relevant the effect of these interferences are. In other words, this configuration of the inventive vehicle may result in conflictive water flows due to the fact that the groups of thrusters have four possible points of convergence. This feature tends to generate disturbances when the flows are expelled by the thrusters towards that point of convergence. This is even made worse because the flows are swirly due to the effect of the rotation of the propeller.

The inventors have studied the hydrodynamic behavior of the discussed configuration and have concluded that, if the vehicle is implemented having certain dimensions and certain characteristics of the thrusters, the mentioned interferences are not relevant, as long as the flux jets are free from any obstacle. Therefore the external casing of the vehicle has been optimized with the purpose of leaving free way to the fluxes. The area where the fluxes cross is also important for the fluxes interference effect. The larger it is without obstacles, the better. In a preferred embodiment, in order to limit interferences among thrusters, and in particular, among those which are not adjacent in a convergence point, the inner volume of the vehicle has been closed or confined, in such a way that a flux is unable to reach a second convergence point from a first convergence point. So, it is important to have the "inside closed from outside". For this purpose, a plug or cover 20 has been added, as shown for example in figures 1 and 6A, in order to prevent movement of water therein. There are three plugs or covers 20. There is a corner or place free of plug or cover, which is the corner at which the umbilical (not shown) leaves the vehicle. This uncovered corner corresponding to the umbilical exit 80 is shown in figure 8. This figure also shows several floating modules 13 of the vehicle. It has been analyzed that the uncovered corner does not cause relevant problems. Preferably, the container 19 (figure 2A) for keeping electronics is located at the entrance of this opening, thus becoming an obstacle to the eventual flow of water through this point.

In an alternative embodiment, in order to overcome the collision of flux jets, at least one of the thrusters needs to be displaced from its original theoretical position. In other words, at least one thrust direction is shifted, in order to avoid the convergence of several fluxes in a single point, thus causing the non-desired effects already mentioned. For example, at least one thruster can be displaced within its own plane, in such a way that the thrust vector of the displaced thruster (or thrusters) is parallel to the edge of the tetrahedron at which it is located (or they are located). In another example, instead of placing at least one thruster such that its thrust vector is parallel to the edge of the tetrahedron at which it is located, at least one thruster is rotated with a certain angle with respect to the axis of its corresponding edge (or the angle a is different from 45°), as in the arrangement of figure 5B. Said angle of rotation depends of several factors, such as the size of tetrahedron, the diameter of thruster and the geometry of the external elements of the ROV. On the other hand, a system is provided, comprising:

-a control center, that may be remotely placed either on land or on a boat or ship, from which the movement of the underwater vehicle is controlled and from which the images captured by its camera can be seen in real time; and

-the vehicle as described in this text.

Preferably, the system also comprises an element, which can be a floating element or a non-floating element (for example, in applications for inspecting rivers, this element can be deployed from a bridge), configured to be connected to the vehicle via an umbilical and to be wired or wirelessly connected to the control center. This element can be a boat, comprising the necessary equipment for transporting and deploying the vehicle where required and the control center, or alternatively if this control center is placed remotely, for transporting and deploying the communication means required to establish communication with the remote control center (preferably wirelessly) and with the vehicle (via an umbilical);

In sum, an underwater vehicle (ROV, AUV or HROV) which can be controlled in 6 d eg rees-of-f reed om using six thrusters has been described. The vehicle is light (typically less than < 15-20 kg) and easy to use and deploy. So, in the application in which the vehicle is a ROV, it is included in the mini-ROV category, also known as eyeball-class or observation-class ROVs.

Among its applications, we can mention: defence and civil protection (such as surveillance and examination of critical infrastructures, military areas, mine detection, hull inspection, emergency activities and rescue operations), inspection & diagnosis of submerged civil and industrial structures (such as dams, dykes, pillars, docks, sea energy and wind offshore infrastructures, aquaculture installations), oceanography, environmental surveillance and scientific research (such as depths studies, marine biomass supervision, environmental data measuring, underwater archaeology and geology) and others (such as cleaning, yatch maintenance, leisure, public aquariums).

On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.