WO2020002473A1

WO2020002473A1 - Robot arm, remotely operated vehicle and method of cleaning a sub-surface structure

Info

Publication number: WO2020002473A1
Application number: PCT/EP2019/067079
Authority: WO
Inventors: Don CLAUSEN EVERS; Michael Blom HERMANSEN
Original assignee: Subblue Robotics Aps
Priority date: 2018-06-26
Filing date: 2019-06-26
Publication date: 2020-01-02

Abstract

The invention relates to a robot arm (1) comprising at least one arm section (11, 12); a first articulated joint (21) formed at a proximal end (2) of said robot arm (1) and configured for mounting to a base (100); and a second articulated joint (22) formed at a distal end (3) of said robot arm (1) and configured for mounting a tool (200); wherein said first and second articulate joints (21, 22) comprises actuators (31, 32) configured for imparting movement between the base (100) and the robot arm (1), and the robot arm (1) and the tool (200), respectively, and wherein at least one element comprises a liquid displacement device (40), and wherein the elements are chosen from the tool (200), the distal joint (22), or the arm section (11,12).

Description

Robot arm, remotely operated vehicle and method of cleaning a subsurface structure

The present invention relates to a robot arm for use in subaquatic

environments, in particular for cleaning structures below the surface of the sea or other bodies of water, such as ship hulls, pillars of offshore platforms or ship propellers. The invention also concerns a remotely operated vehicle equipped with such a robot arm. The invention also concerns a method of cleaning a sub-surface structure.

Description of related art

Sub-surface ship hulls and ship-propellers are prone to marine growth, i.e. growth of algae and the like, also referred to as fouling. Fouling increases the friction, and results in power loss. Therefore, ship hulls and propellers need to be cleaned regularly. Remotely operated vehicles (ROV) have been used to clean ship hulls. However, ship propellers are difficult to clean, due to its rather complex three-dimensional form. Therefore, this task is performed manually by a diver when the ship is in port or at berth. So far, efficient cleaning of ship propellers using ROVs has not appeared to be possible. However, using divers is dangerous. Therefore, there is a need for an efficient and safer way of cleaning sub-surface ship propellers, and equipment therefore.

WO 2016/120071 discloses a snake-robot with a pinching tool arranged at a front end or at both ends. The snake robot has a plurality of links, and each link comprises thrusters arranged lateral to the longitudinal axis of the link. The thrusters may aid in moving the links relative to each other. When performing an operation on a sub-surface item, the snake robot may be kept in place by using the thrusters located at links remote from the ends carrying the pinching tool, Figs. 5, 7.

WO 2018/096214 A1 discloses an apparatus for underwater marine vessel maintenance having a chassis. One or two segmented tool arm may be attached to the chassis. The tool arms may be equipped with cleaning instruments, exemplified with ultrasonic cleaning devices. None of these tool arms are equipped with thrusters. In an embodiment, a brush unit is connected to the chassis via a single segment arm or holder. The brush unit comprises three rotatable brushes. A suction pipe is positioned to the middle of the brushes for removing the fouling brushed away by the brushes. The fouling is led to the surface via an umbilical cord attached to the chassis.

WO 2012/108765 A1 discloses a moveable arm comprising two segments, where laterally arranged thrusters may be arranged thrusters on a distalmost part of the distal segment.

One problem with these devices that it is difficult to maintain the tool at the distal end(s) of the multi segment arms in close contact with the sub surface structure, the tool is to operate on.

It is an object of the present invention to provide for more efficient equipment for performing operations on sub-surface structures, such as ship propellers. It is a further object of the invention to provide more efficient control and manoeuvring of a submerged robot arm. It is a further object of the invention to provide a more efficient and safer method of cleaning a sub-surface structure, such as a ship propeller. It is a further object of the invention to provide a more efficient method and more efficient equipment for pressing a cleaning tool towards a surface of a sub- surface structure - such as the surface of a ship propeller - and thereby provide more efficient cleaning of the surface. Summary of the invention The objects are - in a first aspect of the invention - achieved by robot arm system comprising

a robot arm; and

a control system configured for controlling said robot arm,

wherein the robot arm comprises

two or more arm sections including at least a proximal first arm section and a distal second arm section;

a first articulated joint formed at a proximal end of said robot arm and configured for mounting the robot arm to a base;

a second articulated joint formed at a distal end of said robot arm and configured for mounting a tool to the robot arm;

intermediary articulated joints between the two or more arm sections; an actuator provided at each of said first, second and intermediary articulate joints, and configured for imparting movement between the base and the robot arm, between the robot arm and the tool, and between the two or more arm sections, respectively, and

at least two liquid displacement devices arranged on an arm section and/or an articulated joint,

wherein the control system comprises

a controller or a network of controllers;

first communication connections to the actuators;

second communication connections to the liquid displacement devices; first detection means for measuring the torque of each actuator; and wherein the control system is configured to

forwarding a command signal to the actuators and to the liquid

displacement devices to obtain a desired movement of the robot arm relative to the base; receiving information signals from the first detecting means relating to the detected torque of each actuator;

calculating an expected torque in response to the command signal to the actuators and the liquid displacement devices;

comparing the detected torque with the expected torque;

determine, based on the compared detected torque and expected torque, any externally generated load;

calculating a revised command signal to the actuators and to the liquid displacement devices to provide an output therefrom, which balances the load applied by the actuators and by the liquid displacement devices with the externally generated loads such that power applied in each actuator is minimized; and

forwarding the revised command signal to the actuators and to the liquid displacement devices.

Thereby, a robot arm system is achieved, which, when applied in an aqueous environment (herein also called a sub-surface environment and similar), allows an improved control of a robot arm. The addition of a liquid

displacement device on two elements of the robot arm may further allow downscaling one or more of the actuators at the articulated joints of the robot arm.

In an embodiment, the actuators are electrically driven, and each liquid displacement device comprises a thruster having an electrical thruster motor; where the forwarded command signal is an electric signal; and where the control system is configured for

receiving information signals from the first detecting means relating to the detected torque current of each actuator;

calculating an expected torque current in response to the command signal to the actuators and the electrical thruster motors;

comparing the detected torque currents with the expected torque current; calculating a revised command signal to the actuators and to the electrical thruster motor; to provide an output therefrom, which balances the load applied by the actuators and by the liquid displacement devices with the externally generated loads, such that power applied in each actuator is minimized; and

In an embodiment a thrust direction of at least one liquid displacement device may be rotated relative to the element on which it is arranged. In a further embodiment the thrust direction of two or more liquid displacement devices may be rotated relative to the element on which it is arranged. In a further embodiment the thrust direction of each of the liquid displacement devices may be rotated relative to the element on which it is arranged. By element is meant the distal articulated joint, any intermediary articulated joints or any of the one or more arm sections.

The base of the robot arm may be attached to, or form part of, a remotely operated vehicle (ROV) or a submarine, a boat (surface vessel), or a fixed sub-surface structure, such as pillars, tubes, pipelines or cables.

In a second aspect, objects of the invention is achieved by a remotely operated vehicle comprising a robot arm system according to any one of the embodiments of the above mentioned first aspect of the invention, and where the remotely operated vehicle forms at least part of the base for the robot arm.

In an embodiment of the second aspect the remotely operated vehicle further comprises an attachment mechanism configured for attaching the remotely operated vehicle to a sub-surface structure. I an embodiment thereof the attachment mechanism is a gripping device comprising a set of jaws. The objects may - in a third aspect of the invention - be achieved by a method of controlling a robot arm, the robot arm forming part of a robot arm system further comprising a control system configured for controlling said robot arm, wherein the robot arm comprises:

a second articulated joint formed at a distal end of said robot arm and configured for mounting a tool) to the robot arm;

wherein the control system comprises

a controller or a network of controllers;

first communication connections to the actuators;

second communication connections to the liquid displacement devices; first detection means for measuring the torque of each actuator; and wherein the method comprises the steps of

forwarding a command signal to the actuators and to the liquid

displacement devices to obtain a desired movement of the robot arm;

receiving information signals from the first detecting means relating to the detected torque of each actuator;

comparing the detected torque with the expected torque; determining, based on the compared detected torque and expected torque, any externally generated load;

calculating a revised command signal to the actuators and to the liquid displacement devices to provide an output therefrom, which balances the load applied by the actuators and to the liquid displacement devices with the externally generated loads, such that power applied in each actuator is minimized; and

In an embodiment the of the method according to the third aspect of the invention, the actuators are electrically driven, and each liquid displacement device comprises a thruster having an electrical thruster motor, and the forwarded command signal is an electric signal; and the method comprises the steps of

comparing the detected torque currents with the expected torque current; calculating a revised command signal to the actuators and to the electrical thruster motors; to provide an output therefrom, which balances load applied by the actuators and by the liquid displacement devices with the externally generated loads, such that power applied in each actuator is minimized; and forwarding the revised command signal to the actuators and to the electrical thruster motors.

In the first, second and third aspect of the invention, the first and second actuators, and any further actuators, may each be chosen from electrical, hydraulic or pneumatic actuators, rotational or linear. All articulated joints may be provided with the same type of actuators, or different types of actuators may be used for each articulated joint, or groups thereof.

In the first, second and third aspect of the invention, the liquid displacement device may comprise a propeller or fan (jet) or be a device configured for ejecting a fluid such as air or water. In the latter case the device may be connectable to a source of air or water. The latter may be particularly advantages in case where the robot arm is utilized in an aqueous

environment which is contaminated with much debris or seaweed or the like

In the first, second and third aspect of the invention, the tool is preferably a cleaning device/tool such as a polishing tool, and may preferably comprise a scraper or a brush, such as a rotary brush, a polishing disc, or a flushing device (jet). Thereby, the robot arm may be particularly useful in cleaning ship hulls or ship propellers. However, in other embodiments, the tool may be other kinds of devices, e.g. for repairs - such as an underwater welding device - or a device for cutting, etc.

The objects may - in a fourth aspect of the invention - be achieved by a robot arm system according to any one of the embodiments of the first aspect of the invention, outlined above and further comprising a polishing tool mounted at the second articulated joint of the robot arm, wherein the control system is configured for

controlling a position of the polishing tool, by jointly controlling the liquid displacement devices and/or the actuators, to provide a zero or near-zero pressing force against a surface of a sub-surface structure along the rotation axis and zero bending moments between the polishing tool and the surface, when

a signal is provided to the the control system indicative of the polishing tool being in contact with a flat surface of a sub-surface structure, and when a signal is provided to control system indicative of keeping the polishing too stationary at a desired location relative to the sub-surface structure;

controlling a position of the polishing tool, by jointly controlling the liquid displacement devices and/or the actuators, to provide zero or near-zero pressing force against the surface along rotation axis and a non-zero bending moment between the polishing tool and the surface for moving the polishing tool along the surface, when

a signal is provided to the the control system indicative of the polishing tool being in contact with a flat surface of a sub-surface structure, and when

a signal is provided to control system indicative of moving the polishing tool along the surface of the sub-surface structure; or controlling a position of the polishing tool by jointly controlling the liquid displacement devices and/or the actuators, to provide a pressing force along rotation axis at a desired positive value, when a signal is provided to the the control system indicative of the polishing tool being in contact with a non-flat surface is detected.

In the context of the present invention a flat surface is defined in the following way. A flat surface is a surface which -when a polishing disc of a polishing tool is brought into contact therewith - the entire lower surface area of the polishing disc is in contact therewith. If the entire lower surface area of the polishing disc is not in contact with the surface, the surface will be considered to be non-flat. This means that dependent on the diameter and a flexibility of the polishing disc, a slightly curved surface of the sub-surface structure to be polished, may be considered flat. Non-flat surfaces are surfaces where the curvature is so large that the entire lower surface area of the polishing disc is not in contact with the surface. This occurs e.g. at an edge (trailing or leading edge of a ship propeller or a least at portions of the hub of a ship propeller. It is clear that as polishing disc may have a slight flexibility a bending thereof may be possible. However, the person skilled in the art will appreciated that the load on the polishing disc towards the surface should be kept within normal operating ranges for the given task. The objects may - in a fifth aspect of the invention - be achieved by a method of cleaning or polishing a sub-surface structure, said method comprising the steps of

mounting a polishing tool at a second articulated joint of a robot arm of a robot arm system according to any one of the embodiments of the first aspect of the invention described above;

bringing the base of the robot arm within range of at least a portion of a surface of a sub-surface structure;

bringing the polishing tool into contact with said portion of a surface of a sub-surface structure, by controlling at least the actuators of said robot arm; rotating a polishing disc of the polishing tool;

detecting if the polishing tool is located at

a flat surface of the sub-surface structure; or at

a non-flat surface of the sub-surface structure, and

if a flat surface is detected and the polishing tool is to be kept the same location relative to the sub-surface structure, controlling a position of the polishing tool relative to the surface, by jointly controlling the liquid

displacement devices and/or the actuators, to provide a zero or near-zero pressing force against the surface along rotation axis and zero bending moments between the polishing tool and the surface;

if a flat surface is detected, controlling a position of the polishing tool, by jointly controlling the liquid displacement devices and/or the actuators, to provide zero or near-zero pressing force against the surface along rotation axis and a non-zero bending moment between the polishing tool and the surface for moving the polishing tool along the surface, or if a non-flat surface is detected, controlling a position of the polishing tool by jointly controlling the liquid displacement devices and/or the actuators, to provide a pressing force along rotation axis to a desired positive value.

In an embodiment of the fifth aspect, the step of detecting if the polishing tool is located at

a flat surface of the sub-surface structure; or at

a non-flat surface of the sub-surface structure,

is provided by an operator inspecting the sub surface structure via one or more cameras on

the robot arm;

the base of the robot arm; or

on a remotely operated vehicle on which the robot arm is arranged.

The objects may - in a sixth aspect of the invention - be achieved by a remotely operated vehicle, comprising

a body;

a plurality of liquid displacement devices provided on the body for adjusting motion of the remotely operated vehicle;

an attachment mechanism configured for attaching the remotely operated vehicle to a sub-surface structure; and

(in some embodiments) a robot arm,

Where the attachment mechanism comprises two flanges having mutually facing surfaces arranged at a fixed distance relative to each other, and providing a gap between them.

The objects may - in a seventh aspect of the invention - be achieved by a method of attaching a remotely operated vehicle according to the sixth aspect of the invention to a sub-surface structure, wherein the method comprises manoeuvring said remotely operated vehicle, such that an edge of said sub-surface structure is located within the gap of the attachment mechanism; operating the plurality of liquid displacement devices to collectively tilt the remotely operated vehicle relative to the edge of the sub-surface structure to provide contact surfaces between portions of mutually facing surfaces and the sub-surface structure.

The objects are - in a eighth aspect of the invention - achieved by a robot arm system comprising

a robot arm; and

a control system configured for controlling said robot arm,

wherein the robot arm comprises

a second articulated joint formed at a distal end of said robot arm and configured for mounting a polishing tool to the robot arm, said polishing tool having a rotation axis;

intermediary articulated joints between the two or more arm sections; and at least two liquid displacement devices,

wherein said first, second and intermediary articulate joints each comprises an actuator configured for imparting movement between the base and the robot arm, between the robot arm and the polishing tool, and between the two or more arm sections, respectively,

wherein at least one of the at least two liquid displacement device is arranged on the distal second arm section or on the second articulated joint, and

wherein the other of the at least two liquid displacement devices is arranged on the distal second arm section or on the second articulated joint;

wherein the control system comprises a controller or a network of controllers;

first communication connections to the actuators; and

second communication connections to the liquid displacement devices, and wherein the control system is configured to control the liquid

displacement devices to jointly force the polishing tool in a direction along the rotation axis thereof.

In one embodiment, the robot arm system may comprise only a single arm section. However, in further embodiments, the robot arm may comprise two or three or four or more arm sections. By“any further arm sections”, as mentioned above is meant that the robot arm may comprise two, three four or more of such arm sections. If the robot arm system comprises two, three four or more of such arm sections, an intermediary articulated joint is provided in between each arm section. In embodiments, where the robot arm system comprises two or more arm sections, preferably, each of the intermediary articulated joints comprises an actuator configured for imparting movement between the respective arm sections.

In one embodiment, the liquid displacement devices of the robot arm system are located on the distal articulated joint, or on the distalmost arm section of the robot arm.

Thereby is achieved a robot arm system which, when applied in an aqueous environment, allows to hold a tool, attached at the second articulated joint of the robot arm, close to a surface or a portion of the surface to be cleaned.

In an embodiment, in addition to any of the embodiments described above, the robot arm system may have a tool attached at the second (distal) articulated joint, and in a further embodiment hereof, the liquid displacement device may instead, or in addition, be located on the tool or be integrated therewith. The first and second actuators, and any further actuators, may each be chosen from electrical, hydraulic or pneumatic actuators, rotational or linear. All articulated joints may be provided with the same type of actuators, or different types of actuators may be used for each articulated joint, or groups thereof.

The liquid displacement device may comprise a propeller or fan (jet) or be a device configured for ejecting a fluid such as air or water. In the latter case the device may be connectable to a source of air or water. The latter may be particularly advantages in case where the robot arm is utilized in an aqueous environment which is contaminated with much debris or seaweed or the like.

The tool is preferably a cleaning device, and may preferably comprise a scraper or a brush, such as a rotary brush, a polishing disc, or a flushing device (jet). Thereby, the robot arm may be particularly useful in cleaning ship hulls or ship propellers. However, in other embodiments, the tool may be other kinds of devices, e.g. for repairs - such as an underwater welding device - or a device for cutting, etc.

In further embodiments, the robot arm system may comprise one or more further liquid displacement devices, provided on any of the above mentioned elements of the robot arm, i.e. any of the arm sections, or any of the intermediary articulated joints between the arm sections. In this case, control of the motion of the elements may be achieved by a combination of displacement of liquid, using the liquid displacement device, and controlling the direction of the liquid displacement. This may further increase the control of the robot arm. Any or all of the liquid displacement devices may be fixedly arranged relative to the element on which it is mounted. By fixedly arranged is meant that it cannot rotate - or otherwise move - relative to the element on which it is arranged.

However, in an embodiment of the invention at least one liquid displacement device may be rotated relative to the element on which it is arranged, meaning that a direction of the displacement of fluid provided by the liquid displacement device may be adjusted relative to the element on which it is mounted. Thereby, the control of the motions of the element on which the liquid displacement device is arranged may be enhanced. The relative movement between the liquid displacement device and the element on which it is arranged may be controlled remotely by an operator, or by an automated control system, when the robot arm is submerged.

In a further embodiment, one liquid displacement device is arranged at said second articulated joint, a tool is arranged in extension of the second articulated joint, and the tool comprises a longitudinal axis extending through said articulated joint, and the liquid displacement device is arranged to displace liquid in parallel to said longitudinal axis. Thereby, a particularly stable operating condition may be achieved, because the tool in this case may be kept steadily, close to a surface which needs to be treated, e.g. cleaned.

Objects of the invention are - in a ninth aspect of the invention - achieved by a remotely operated vehicle (ROV), the ROV comprising a robot arm according to any one of the embodiments described in relation to the first aspect of the invention, above. The remotely operated vehicle may constitute the base for the robot arm. Alternatively, the base of the robot arm may be attachable to the remotely operated vehicle.

The ROV is preferably a submersible remotely operated vehicle, i.e. a vehicle to be operated under water. The operation of the ROV and the robot arm is preferably controlled by an operator/user from a position above the surface. This control may be achieved through electrical cables and/or wires and/or hydraulic or pneumatic connections, extending from a position above surface to the submerged ROV.

In an embodiment of the remotely operated vehicle according to the second aspect of the invention, the ROV further comprises an attachment

mechanism configured for attaching the remotely operated vehicle to a sub surface structure. The sub-surface structure may for example be a ship hull, a ship propeller, sub-surface portions of pillars, cables or tubes.

The attachment mechanism may, if the sub-surface structure has magnetic properties, be a magnetic device (not shown). In other, not shown

embodiments, the attachment mechanism may comprise a suction device.

However, in a preferred embodiment, attachment mechanism of the remotely operated vehicle is a gripping device comprising a set of jaws. This embodiment is particularly suitable for attachment to e.g. an edge of a ship propeller.

Objects of the invention are - in a tenth third aspect of the invention - achieved by a method of cleaning a sub-surface structure, said method comprising the steps of

providing a robot arm according to any one of the embodiments of the ninth aspect of the invention; mounting a tool at the second articulated joint at the distal end of the robot arm;

bringing the polishing tool into contact with said portion of a surface of a sub-surface structure controlling at least the actuators of said robot arm; forcing the polishing tool in a direction along the rotation axis thereof, by jointly controlling the liquid displacement devices, to provide a resulting thrust from the liquid displacement devices along the rotation axis

operating said polishing tool to polish the portion of the surface.

Thereby, an improved control of the robot arm is achieved, and the method of cleaning the sub-surface structure is thereby made more efficient.

Throughout the present application, by“within range”, we mean that the base of the robot arm is brought so close to the surface or to the surface portion of the subsurface structure, that the robot arm is able to reach a surface portion and move the tool at least over a smaller portion of the surface in such a way that the tool can be brought to clean the smaller portion of the surface.

An embodiment of the method according to the tenth aspect involves displacing fluid with liquid displacement device such that the tool is pressed against said surface.

Thereby, a method is achieved, when applied in an aqueous environment, allows to hold the tool of the robot arm close to a surface or a portion of the surface to be cleaned. The sub-surface structure may for example be a ship hull, a ship propeller, sub-surface portions of pillars, cables or tubes.

The mentioned tool is a tool configured for cleaning.

In an embodiment, the method according to the tenth aspect comprises the subsequent step of translating the base of the robot arm relative to the surface of the sub-surface structure to bring the robot arm into range of another portion of the surface of a sub-surface structure. Thereby, by repeating the above mentioned method, the other surface portion of the surface of the sub-surface structure may be cleaned. By repeating this, eventually the whole surface of the sub-surface structure may be cleaned.

Preferably, in the method according to any of the above described

embodiments according to the tenth aspect, the base for the robot arm forms part of - or is attached to - a remotely operated vehicle. In further

embodiment hereof, the remotely operated vehicle comprises an attachment mechanism configured for attaching the remotely operated vehicle to the sub surface structure. In a further embodiment hereof, the attachment

mechanism is a gripping device comprising a set of jaws, and the method comprises clamping the remotely operated vehicle to the sub-surface structure, using the gripping device comprising a set of jaws. Thereby the above mentioned step of “bringing the base of the robot arm within range of at least a portion of a surface of a sub-surface structure” may further involve clamping to the sub-surface structure at that location, where the robot arm is within range of the portion of the surface to be cleaned.

The sub-surface structure may in particular be a propeller of a ship. A gripping device comprising a set of jaws will allow the ROV to repeatedly attach to an edge of a ship propeller.

Objects of the invention are - in an eleventh aspect of the invention - achieved by a remotely operated vehicle (ROV) comprising a robot arm and an attachment mechanism configured for attaching the remotely operated vehicle to a sub-surface structure. Thereby, the ROV may provide a stable platform for providing operations, e.g. cleaning, on a sub-surface structure to which the ROV is attached, using the robot arm. The robot arm may comprise a tool. The tool is preferably a cleaning device, preferably comprising a scraper or a brush, such as a rotary brush or a rotary polishing disc, or a flushing device (jet). Thereby, the ROV may be particularly useful in cleaning ship hulls or ship propellers. However, in other embodiments, the tool may be other kinds of devices, e.g. for repairs - such as an underwater welding device - or a device for cutting, etc.

In a preferred embodiment of the eleventh aspect of the invention, the attachment mechanism is a gripping device comprising a set of jaws. This embodiment is particularly suitable for attachment to e.g. an edge of a ship propeller.

The ROV according to the eleventh aspect of the invention may further comprise a robot arm according to any one of the embodiments of the first aspect of the invention.

By“any further arm sections”, as mentioned above is meant that the robot arm may comprise two, three four or more of such arm sections. If the robot arm comprises two, three four or more of such arm sections an intermediary articulated joint is provided in between each arm section.

In a twelfth aspect, objects of the invention may be achieved by a remotely operated vehicle comprising

an elongate first arm having a first end and a second end;

an elongate second arm having a first end and a second end;

an elongate third arm having a first end and a second end,

a liquid displacement device arranged at the first end of the first arm; a liquid displacement device arranged at the second end of the first arm; a liquid displacement device arranged at the first end of the second arm; a liquid displacement device arranged at the second end of the second arm; a liquid displacement device arranged at the first end of the third arm; and a liquid displacement device arranged at the second end of the third arm; wherein the first, second and third arm intersect at an intersection.

In an embodiment of the twelfth aspect the remotely operated vehicle further comprises an attachment mechanism for reliably attaching the remotely operated vehicle to a sub-surface structure.

In an embodiment thereof, the remotely operated vehicle comprise a flange is arranged in parallel with the first arm form the attachment mechanism.

In an further embodiment of any of the above mentioned embodiments of the twelfth aspect the remotely operated vehicle comprises a robot arm system according to any one of the embodiments of the first aspect of the invention.

It should be emphasized that the term "comprises/comprising/comprised of" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Brief description of the drawings

Fig. 1 , in schematic form, shows a robot arm according to an aspect of the invention, with a base and a tool. The robot arm is in operation in relation to a sub-surface structure.

Fig. 2, shows a diagram of a robot arm control system according to an aspect of the invention; Fig. 3 shows a diagram of a method of controlling a robot arm according to an aspect of the invention;

Fig. 4 shows a diagram of a method of cleaning a sub-surface structure;

Figs. 5A-B, in schematic form, shows remotely operated vehicle with a clamp according to an aspect of the invention, and also illustrate a method of attaching a remotely operated vehicle to a sub-surface structure; and Fig. 6, in schematic form, shows a remotely operated vehicle according to an aspect of the invention.

In the following, the invention will be described in greater detail with reference to aspects and embodiments shown by the enclosed figures. It should be emphasized that the embodiments shown are used for example purposes only and should not be used to limit the scope of the invention.

Detailed description of the embodiments

Fig. 1 , in schematic form, shows a sub-surface structure 300 in the form of a ship propeller. In the figure is shown a propeller blade 301 and a hub 302 of the ship propeller. The ship propeller - or a sub-surface structure 300 in general - has a surface 310. The surface 310 is an outer surface of the ship propeller. Also shown in Fig. 1 , is a first portion 315 of the surface 310 of the sub-surface structure 300 and a second portion 316 of the surface 310 of the sub-surface structure 300.The first and second surface portions 315, 316 of the surface 310 of the sub-surface structure 300 are indicated by dashed lines in Fig. 1. The various aspects of the invention relates to a robot arm 1 , a remotely operated vehicle 110, and methods of performing operations, such as cleaning, on a sub-surface structure 300, exemplified by the ship propeller shown in Fig. 1.

Accordingly, Fig. 1 also shows a robot arm 1 attached - at a proximal end 2 of the robot arm 1 - to a base 100. At a distal end 3 of the robot arm 1 , a tool 200 is shown attached to the robot arm 1. The robot arm 1 comprises at least one arm section. The arm sections are elongate. The robot arm 1 allows a user to perform operations on the surface 310 of the sub-surface structure 300, when the base 100 is brought into range of the sub-surface structure 300. The operation, e.g. cleaning the surface 310 of the sub-surface structure 300, or a portion 315 thereof, may be performed by moving the tool 200 (when attached to the robot arm 1 ) relative to at least the portion 315 of the surface 310.

The first surface portion 315 may be reachable for a tool 200, provided at a distal end of the robot arm 1 when the base 100, is located at the position relative to the sub surface structure 300 shown in Fig. 1. In order for the tool 200 to be able to reach the second surface portion 316 it may be necessary to translate the base 100 relative to the sub surface structure 300 closer to the second surface portion 316.

The base 100 may be part of remotely operated vehicle 1 10 or may be attachable to a remotely operated vehicle 1 10 (ROV). The remotely operated vehicle 1 10 is a submersible ROV that may be operated from above surface via electric cable, wires, hydraulic cables, pneumatic cables, wirelessly, or combinations thereof.

In other, not shown aspects of the invention, the base 100 of the robot arm 1 may be part of ship or may be attachable to a ship. In yet further aspects (also not shown), the base 100 of the robot arm 1 may form part of an underwater structure, or may be attachable to an underwater structure.

In the embodiment shown, the robot arm has a first arm section 1 1 (proximal arm section) and a second arm section 12 (distal arm section), as well as a third arm section 13 and a fourth arm section 14. The third and fourth arm sections 13, 14 are arranged between the first and second arm sections 1 1 , 12.

A first articulated joint 21 is formed at the proximal end 2 of the robot arm 1 , and rotateably connects or mounts the first arm section 1 1 to the base 100.

A second articulated joint 22 is formed at the distal end 3 of the robot arm 1. The second articulated joint 22 rotateably connects or mounts the second arm section 12 to the tool 200.

Similarly, a third articulate joint 23 rotateably interconnects the first arm section 1 1 and the third arm section 13. A fourth articulate joint 24 rotateably interconnects the third arm section 13 and the fourth arm section 14. A fifth articulate joint 25 rotateably interconnects the fourth arm section 14 and the second (distal) arm section 12.

The first and second articulate joints 21 , 22 comprises actuators 31 , 32 configured for imparting movement between the base 100 and the first arm section 1 1 , and the second arm 12 and the tool 200, respectively.

Likewise, the third articulate joint 23 comprises an actuator for imparting movement between the first arm section 1 l and the third arm section 13. The fourth articulate joint 24 comprises an actuator for imparting movement between the third arm section 13 and the fourth arm section 14. The fifth articulate joint 25 comprises an actuator for imparting movement between the fourth arm section 14 and the second (distal) arm section 12.

It will be appreciated that the actuators 31 , 32, 33, 34, 35 may be

implemented in various ways at the articulate joints 21 , 22, 23, 24, 25. As indicated they may be motors installed in the articulate joints 21 , 22, 23, 24, 25, electrical, pneumatic or hydraulic. Alternatively, they may be linear actuators or the like, electrical, pneumatic or hydraulic, arranged between the elements (the element being the base 1 1 , first arm section 1 1 , second arm section 12, third arm section 13, fourth arm section 14 and the tool 200) across the respective articulate joints. In further embodiments, the actuators 31 , 32, 33, 34, 35 may be cables/wires extending at least from the base 100, the ROV 1 10 or from above surface. In yet further embodiments, the actuators 31 , 32, 33, 34, 35 may be hydraulically operated, e.g. via hydraulic cables from at least the base 100, the ROV 1 10, or from above surface.

In the case of electrical or hydraulic actuators, each of the five articulate joints 21 , 22, 23, 24, 25 and the tool 200 (end-effector) are actuated by motors within waterproof houses. The motor is mounted inside a motor housing. The motor itself may be connected to a gearing, which is connected to a compliant clutch (taking into effect misalignments), which is connected to an output shaft going through a bearing pair (which absorbs all loads except for the motor torque) and lip-seals (through which water cannot pass) out to the outside of the housing.

Preferably actuators may be Brushless Direct Current Motors (BLDC Motors), which actuate joint motions and the spinning of the polishing disc.

The actuators 31 , 32, 33, 34, 35 may all be of the same type. However, in some embodiments, the actuators 31 , 32, 33, 34, 35 may be chosen from the above mentioned types e.g. depending on the power needed to impart movement at the particular articulate joint 21 , 22, 23, 24, 25.

Even though Fig. 1 shows a robot arm 1 having four arm sections 1 1 , 12, 13, 14, it will be appreciated that the robot arm 1 in other embodiments may have only one arm section, with actuated articulate joints at both ends, or it may comprise two, three, five, six, or even more arm sections. More arm sections will increase the flexibility of the robot arm 1.

In the embodiment shown in Fig. 1 , the robot arm 1 has five degrees of freedom, and is inspired by the three degree of freedom SCARA (Selective Compliance Assembly Robot Arm) design. All joints are revolute. The first two joints 21 , 23 (the "shoulder"), as seen from the proximal end 2 of the robot arm 1 , form a pair that can move the rest of the robot arm 1 along spherical paths, providing a large workspace.

The next two joints 24, 25 (which are inspired by SCARA), seen in the outward direction from the proximal 2 to the distal end 3, provides a planar workspace (ideal for polishing almost locally planar surfaces of a ship propeller 301 ), which is rigid along the normal direction of the plane in question. This is intended to reduce vibrations.

The outermost or distalmost (second) articulate joint 22 allows for adjustment of orientation of the tool 200, which in the example shown in Fig 1 is a spinning polishing disc 201.

Normally, six degrees of freedom are chosen for design, since this gives the ability to control three translational positions and three orientations. But since this arm has a spinning polishing disc as its end-effector/tool 200, the orientation degree of freedom of this spinning polishing disc does not need to be controlled. As mentioned, the tool 200 may be a cleaning device, preferably a polishing tool 210. In Fig. 1 is shown an embodiment, where the tool 200/end-effector/ polishing device 210 of the robot arm 1 is a single polishing disc 201 provided with a motor 202 for rotating the polishing disc 201. The polishing disc rotates relative to a diving shaft 203, about a rotational axis A. The polishing disc 201 has a silicon carbide disc attached to a polishing disc base, such that the silicon carbide disc may be exchanged. The polishing disc 201 is attached to a driving shaft 203 of the motor 202. The silicon carbide disc may be attached to the polishing disc base through layers of the hook and loop type (Velcro), or a thick compliant sponge. This design allows the attachment and detachment of polishing discs of various grit sizes. The compliant sponge allows the polishing disc 201 to orient itself along the surface 310 of the propeller 301 or other sub-surface structure to be cleaned/polished, to avoid along-plane forces that would push the polishing disc away, and give rise to annoying limit cycle vibrations.

When parts of the carbide disc is not in contact with the surface 310 of the sub-surface structure, such as the propeller 301 surface 310, but is close, the venturi-effect sucks the disc 201 closer to the surface 310. All in all, the compliant (and high damping ) sponge, the venturi effect, and a rigidity of the clutch makes sure vibrations are small.

The robot arm further comprises at least one liquid displacement device 40, 41 , 42 arranged on either one of the following elements or parts of the robot arm 1 :

- the tool 200, in embodiments, where the tool is part of, or attached to, the robot arm 1 ,

- the second (distal) articulate joint 22,

- the second (distal) arm section 12,

- any further arm sections 1 1 , 13, 14, or - any intermediary articulated joints 23, 24, 25.

As shown in Fig. 1 a liquid displacement device 40 may be located at the second (distal) articulate joint 22. Fig. 1 further shows that the robot arm 1 may comprise alternative or additional one, two or more liquid displacement device 41 , 42, e.g. on the second (distal) arm section 12. It is to be noted that liquid displacement devices may be additionally or alternatively (to at least some of the previously mentioned liquid displacement devices) arranged on one or more of the first third or fourth arm sections 1 1 , 13, 14 or on one or more of the third, fourth, fifth articulate joints 22, 24,25.

The liquid displacement device(s) 40, 41 , 42 may comprise a propeller or fan (jet) or be a device configured for ejecting a fluid such as air or water. In the latter case the device may be connectable to a source of air or water.

Preferably, the liquid displacement device(s) 40, 41 , 42 is a thruster.

Thrusters may be used for high-load actuation but are imprecise. This means thrusters can be used to assist low-load motors, such as the above

mentioned actuators, whereby the actuator may be used for compensating for the thruster’s imprecision. The high load of the thrusters means that other motors/actuators need not be so large. Thereby, a more compact robot arm may be obtained. The thruster(s) is/are also used for pressing the tool 200, such as a polishing disc, into or towards the surface 310 of the sub-surface structure 300.

In an embodiment, each and any of the at least one liquid displacement device(s) 40, 41 , 42 may be rotateably controlled relative to the element on which it is arranged.

In a one embodiment, and as shown in Fig. 1 , one liquid displacement device 40 is arranged at said second articulated joint 22; and the tool 200 is arranged in extension of the second articulated joint 22. In this case the tool 200 comprises a longitudinal axis which is coinciding with the rotational axis, A, in this case also extending also through the articulated joint. As shown in Fig. 1. the liquid displacement device 40 on the second articulated joint 22 may be arranged to displace liquid in parallel to said longitudinal axis A. As mentioned above, the robot arm 1 - in any of its described embodiments - including the base 100 may be arranged on a remotely operated vehicle 110 (ROV). The base 100 of the robot arm 1 may also be/form part of the ROV 110 as such, i.e. where the base is formed integrated with a ROV 1 10.

The ROV may be controlled through an ethernet cable or the like, e.g. by use of an open-source software called QGroundControl or other suitable software. The ROV 110 may be of conventional type, where the movement of the ROV in the water is controlled using thrusters 140, provided on the ROV body 1 1 1.

In further embodiments, the remotely operated vehicle may comprise an attachment mechanism 50 configured for attaching the remotely operated vehicle to the sub-surface structure 300. The attachment mechanism 50 may if the sub-surface structure 300 has magnetic properties, be a controllable magnetic device (not shown), whereby the ROV 1 10 may be attached to the sub-surface structure 300 by turning on the magnetic device and detached by turning the magnetic device of again. In other not shown embodiments, the attachment mechanism 50 may comprise a suction device.

However, in a one embodiment, the attachment mechanism 50 of the remotely operated vehicle 1 10 is a gripping device 55 comprising a set of claws or jaws 56, 57. The jaws 56, 57 may open and close to grip onto an edge or the like of a sub-surface structure 300. This embodiment is particularly suitable for attachment to e.g. an edge 311 of a ship propeller 310. In further - not shown embodiments - the jaws 56, 57 may further be equipped with magnetic or suction devices to improve the grip on the subsurface structure 300 and/or to provide flexible possibilities for attaching the ROV 1 10 to the subsurface structure 300.

When the ROV 1 10 is attached to the subsurface structure 300, using the attachment mechanism 50, the ROV 110 provides a stable platform for performing operations on the subsurface structure, such as cleaning a surface 310, or at least a portion of the surface 315, 316, of the sub-surface structure 300, using the tool 200 mounted at the distal end 3 of the robot arm 1 .

Once the tool 100 has performed all desired operations on the surface portion 315, which is within reach (within the range) of the robot arm 1 at the location, where the ROV 1 10 is either clamped to the sub-surface structure (using the attachment mechanism 50), or where the ROV 100 is positioned using the steering means 140 (propellers, thrusters) of the ROV 110, the base 100 may be moved to another location within reach (within the range) of a second surface portion 316 and perform further operations there.

In further embodiments (not shown), the robot arm 1 and/or the ROV 1 10 may comprise cameras, connected to an above surface control post, and allowing an operator/user to control the positioning of the ROV 1 10 relative to the sub surface structure, and/or the operation of the robot arm 1 and the tool 200 attached thereto. Cameras mounted on the robot arm 1 allow for the operator to follow the movement of the robot arm 1 , and/or on the ROV 1 10. Having more than one camera will allow 3D imaging of the robot arm 1 and its surroundings through computer vision routines. This solves the

referencing problem of encoders, helps detect propeller edges 310 and in the long term allows increased automation of the subsurface structure 300 cleaning process. It can also give a reading of the orientation of the tool 200. This will be particularly useful when the sub-surface structure 300 is a ship propeller, and the tool 200 is e.g. a polishing tool/device 210, such as a polishing disc 201.

In further embodiments (not shown), the robot arm 1 and/or the ROV 1 10 may comprise other types of sensors, e.g. distance sensors, connected to an above surface control post, and allowing an operator/user to control the positioning of the ROV 1 10 relative to the sub surface structure 300 and/or to control the operation of the robot arm 1 and the tool 200 attached thereto. Such distance sensors may be laser distance measurement sensors: A complication problem may occur when operating the robotic arm 1 as in the embodiment shown in Fig. 1. Five degrees of freedom must be controlled. The operator/user only gets a video feed and a shoddy vision reading regarding the positions and orientations of the robot arm 1. Therefore, a laser distance measurement setup may be implemented, which allows a computer to measure the orientation of the sub surface structures surface 310 (such as the propeller’s 301 surface 310) in comparison to the polishing disc 201 and correct the robot arm 1 orientation, so that the polishing disc 201 is as closely aligned with the propeller surface's normal axis as possible.

In further embodiments (not shown), the robot arm 1 may further comprise means (a sensor) for measuring the pressure force between the tool 200 and the surface 310, which the tool 200 is pressed against. Such means/such a sensor may be an S-beam, a load cell which measures the pressing force of polishing. The thrust of the liquid displacement devices40, 41 , 42 may be controlled in response to the measured pressing force.

Using the robot arm 1 as described above, allows for a new method of cleaning a sub-surface structure 300, such as a ship hull or a ship propeller. Such a method would encompass - bringing the base 100 of the robot arm 1 within range of at least a portion 315 of the surface 310 of the sub-surface structure 300; and

- operating the tool 200 of the robot arm to clean the surface 310, while:

o displacing fluid with liquid displacement device 40, 41 , 42 such that the tool 200 is pressed against the surface 310; and o controlling movement of the robot arm 1 with said actuators 31 , 32 such that the tool 200 is displaced relative to at least the portion 315 of the surface 310 of the sub-surface structure 300 using said actuators 31 , 32.

By“within range”, we mean that the base 100 is brought so close to the surface 310 or surface portion 315 of the subsurface structure 300 that the robot arm is able to reach a surface portion 315 and move the tool 200 at least over a smaller portion of the surface 310.

When a surface portion 315 has been cleaned, the base 100 of the robot arm 1 may be translated relative to the surface 310 of the sub-surface structure 300 to bring the robot arm 1 into range of another, second portion 31 6 of the surface 310 of a sub-surface structure 300. Repeating the cleaning

procedure described above, the second portion 31 6 of the surface 310 may also be cleaned. By repeatedly translating the base 100, and cleaning the surface portion 315, 31 6, which has thereby been brought into reach of the robot arm 1 , the entire surface 310 may be cleaned.

As is obvious from the above, the base 100 for the robot arm 1 may form part of a remotely operated vehicle 1 10, or may be attached to a remotely operated vehicle 1 10. At each location (close to the surface portion 315, 31 6 to be cleaned), the remotely operated vehicle 1 10 may be attached to the sub-surface structure 300 using an attachment mechanism 50 configured therefore. As mentioned above the invention in its aspects is particularly useful for cleaning ship propellers. In various embodiments a method of cleaning a ship propeller may comprise some or all of the following steps:

1. An operator/user arrives at a harbor near the ship with the ROV 1 10 with the robot arm 1 and an electronic station for controlling the two, the electronic station containing computer and ROV/robot intelligence electronic hardware;

2. The operator connects his power supply to wherever he can get

power.

3. The operator sinks the ROV 1 10 into the water, and starts filming with the robots arm and/or ROV cameras.

4. The operator steers the ROV 110 down to the ship propeller 301

visually controlling the movement using a camera that feeds live video to him through a monitor in the electronic station;

5. The operator steers the ROV 1 10, so that the gripping device 55 may be attached to a propeller blade (This might require communication with the ship operators, who will be asked to turn the propeller blades into positions suitable for cleaning the propeller 301 );

6. The operator clamps the ROV 1 10 to the propeller 301 ;

7. The operator activates the robot arm 1. A brief software procedure (where the operator does nothing, but the robot calibrates) initiates and finishes;

8. The operator can now control the robot arm along the propeller

surface portion 315, turn on the polishing disc 201 on, and control the pressing force of the polishing disc 210;

9. The operator uses the robot arm 1 to clean the entire surface portion 315 of the propeller blade. Then he deactivates the polishing disc 201 , robot arm 1 and the gripping device 55;

10. The operator moves the ROV 110 away from the propeller; 11. Steps 5-10 are repeated until all surface portions 305, 306 of all the propeller blades are cleaned;

12. The operator steers the ROV back to the water surface and pulls up the ROV.

13. The operator stops filming with the robots cameras, and saves the video files.

14. The operator cleans the ROV and robot arm 1.

15. The operator can now edit and speed up the video files, documenting the cleaning of the propeller.

As mentioned above, in an aspect the invention also relates to a robot arm system comprising a robot arm 1 and a control system 400 configured for controlling said robot arm 1 ,

The robot arm 1 comprises two or more arm sections 1 1 , 12, 13, 14 including at least a proximal first arm section 1 1 and a distal second arm section 12. Further intermediate arm sections 13, 14 may be arranged there between.

A first articulated joint 21 is formed at a proximal end 2 of the robot arm 1. The first articulated joint 21 is configured for mounting the robot arm to a base 100 such as described above also.

A second articulated joint 22 is formed at a distal end 3 of the robot arm 1. The second articulated joint 22 is configured for mounting a tool 200 to the robot arm 1. The tool may be the same as described above.

Intermediary articulated joints 23, 24, 25 are provided between the two or more arm sections 1 1 , 12, 13, 14. Further, an actuator 31 , 32, 33, 34, 35 provided at each of the first, second and any intermediary articulate joints 21 , 22, 23, 24, 25. A first actuator 31 is configured for imparting movement between the base 100 and the robot arm 1. A second actuator 32 is configured for imparting movement between the robot arm 1 and the tool 200. A third actuator 33 is configured for imparting movement between the first arm section 1 1 and a neighbouring third arm section 13. A fourth actuator 34 is configured for imparting movement between third arm section 13 and a neighbouring fourth arm section 14. A fifth actuator 35 is configured for imparting movement between fourth arm section 14 and the neighbouring, distalmost second arm section 1 1.

In an embodiment at least two liquid displacement devices 40, 41 , 42 are arranged on an arm section 1 1 , 12, 13, 14 and/or an articulated joint 21 , 22, 23, 24, 25.

The liquid displacement devices 40, 41 , 42 may be as described above.

However, in one embodiment, each liquid displacement device 40, 41 , 42 comprises a thruster having an electrical thruster motor 40’, 41’, 42’.

As mentioned the robot arm system further comprises a control system 400.

Referring now to Fig. 2, the control system 400 comprises a controller 410 - which may comprise a CPU - or a network of controllers. In the following a single controller is shown and described for simplicity sake. However, it will be appreciated that the controller 410, may instead be a network of controllers, distributed in the robot arm system.

The controller 410 may be located in the robot arm 1 or at a base 100 (e.g. a remotely operated vehicle 1 10) to which the robot arm 1 is attached, or it may located in connection with a control panel located above surface (of the water in which the robot arm is to function). In either case suitable connections to the elements or parts (arm segments, articulated joints, actuators, liquid displacement devices, detection means (e.g. sensors)) of the robot arm 1 may be provided between the controller 410 and the element/part in order to transfer control signals, and power to the element/part and to obtain information about relevant parameters relating to the element/part.

In any case the controller 410 is connected to control the actuators 31 , 32,

33, 34, 35 via first communication connections 420. The communication connections 420 may be suitable cables or wires, electric cables, pneumatic or hydraulic tubes or a combination thereof.

Further, in any case the controller 410 is connected to control the liquid displacement devices 40, 41 , 42 via second communication connections 430. The communication connections 420 may be suitable cables or wires, electric cables, pneumatic or hydraulic tubes or a combination thereof.

The first and second communication connection may transfer information and/or power in the form of electrical energy, mechanical energy, or a pressure (pneumatic or hydraulic).

The control system 400 further comprises first detection means 440 for measuring the torque at each actuator 31 , 32, 33, 34, 35. Torque may be measured at each actuator 31 , 32, 33, 34, 35 by use of suitable sensors, e.g. strain gauges/load cells. Information regarding the measured torque may be transferred to the controller 410 via the mentioned first communication connections 420. The torque at the each actuator 31 , 32, 33, 34, 35 may - in embodiments where the actuator is pneumatic or hydraulic, be determined by measuring flow to the actuator 31 , 32, 33, 34, 35.

In an embodiment, where the liquid displacement devices 40, 41 , 42 are thrusters having an electrical thruster motor 40’, 41’, 42’ the first detection means 440 may be configured for measuring/determining the torque current of each actuator 31 , 32, 33, 34, 35. Herein, by torque current is meant: Electrical current sent to a motor/actuator, passing through inductive coils to generate a magnetic torque on the motor/actuator.

In some embodiments the control system 400 may further comprise second detection means 450 for measuring torque of each liquid displacement device 40, 41 , 42. Information regarding the measured torque may be transferred to the controller 410 via the mentioned second communication connections 430. In a further embodiment, where liquid displacement devices 40, 41 , 42 are electrically driven thrusters with electric thruster motors 40’,

41’, 42’, the second detection means 450 may be configured for measuring torque current of each one of the thruster motors 40’, 41 ', 42’ of the liquid displacement devices 40, 41 , 42. The thruster motors 40’, 41 ', 42’ may in this case be electrical motors, driving e.g. a fan or propeller. Alternatively to measuring/determining the torque/torque current of each liquid displacement devices 40, 41 , 42, the torque/torque current may be estimated in a model suitable for the applied liquid displacement device 40, 41 , 42.

The control system 400 thereby is configured to send command signals to the actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40,

41 , 42. Further, the control system 400 is thereby configured to receive information signals from at least the first detecting means 440 relating to the torque of each actuator 31 , 32, 33, 34, 35 (and in some embodiments also from the second detecting means 450 (relating to the torque of each liquid displacement device 40, 41 , 42)). Further in some embodiments, the control system 400 is thereby configured to receive information signals from the first and/or second detecting means 440, 450 relating to the torque current of each actuator 31 , 32, 33, 34, 35 and/or each thruster motor 40’, 41 ', 42’.

Further, the control system 400 may be configured for calculating an expected torque, i.e. a torque in the actuators (and in some embodiments, the liquid displacement devices) of the robot arm 1 , which should be expected in response to the command signal send to the actuators 31 , 32,

33, 34, 35 and the liquid displacement devices 40, 41 , 42.

When the actuators 31 , 32, 33, 34, 35 and the liquid displacement devices 40, 41 , 42 are electric devices, the control system 400 may be configured for calculating an expected torque current in response to the command signal to the actuators 31 , 32, 33, 34, 35, and to the thruster motors 40’, 41’, 42’ of the liquid displacement devices 40, 41 , 42.

Further, the control system 400 is configured for comparing the detected torque with the calculated expected torque.

When the actuators 31 , 32, 33, 34, 35 and the liquid displacement devices 40, 41 , 42 are electric devices, the control system 400 may be configured for comparing the detected torque current with the calculated expected torque current.

Further, the control system 400 is configured for determining, based on the compared detected torque and expected torque, any externally generated load.

When the actuators 31 , 32, 33, 34, 35 and the liquid displacement devices 40, 41 , 42 are electric devices, the control system 400 may be configured for determining, based on the compared detected torque current and expected torque current, any externally generated load.

Further, the control system 400 is configured for calculating a revised command signal to the actuators 31 , 32, 33, 34, 35, and to the liquid displacement devices 40, 41 , 42 to provide an output therefrom which balances the load applied by the actuators 31 , 32, 33, 34, 35 and by the liquid displacement devices 40, 41 , 42 with the external generated loads, where the balancing is done such that power applied to each actuator 31 , 32, 33, 34, 35 is minimized. This allows a switch from primarily using the actuators to control the robot arm1 to increased force applied through the liquid displacement devices.

Further, the control system 400 is configured for forwarding a revised command signal to the actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40, 41 , 42.

When the actuators 31 , 32, 33, 34, 35 and the liquid displacement devices 40, 41 , 42 are electric devices, the control system 400 may be configured for calculating a revised command signal to the actuators 31 , 32, 33, 34, 35, and to the liquid displacement devices 40, 41 , 42 to provide an output therefrom which balances the load applied by the actuators 31 , 32, 33, 34, 35 and by the liquid displacement devices 40, 41 , 42 with the external generated loads. Further, the control system 400 is configured for forwarding a revised command signal to the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices 40, 41 , 42.

In robot arms motors/actuators are controlled by motor/actuator controllers. The controllers receive a command position from a program with user inputs. Sensors on the motor sends back position feedback from the motor. The position command and feedback is processed in the motor controller to send an appropriate current to the motor. If the motor encounters external loads that resist motions, the controller has mechanisms that automatically ramps up current sent. There are multiple motors and motor controllers on the robot arm. In embodiments, the robot arm 1 has (first/second) detection means/sensors 440 measuring current sent to the actuators 31 , 32, 33, 34, 35 and the current joint angle position of all articulated joints 21 , 22, 23, 24, 25. These are used to determine first the external load on each actuator/motor and then the total force and moment on the robot arm 1. Then it is determined what set of thrust commands would generate a set of forces and moments that would generate a subset of the measured external loads. If liquid displacement devices 40, 41 , 42, such as thrusters were to be operated with these commands, the load on the robot arm actuators 31 , 32, 33, 34, 35 will be relieved, as the loads are transferred form the actuators 31 , 32, 33, 34, 35 to the thrusters.

The set of thrust commands will be chosen so as to achieve a tradeoff between reducing the load on the actuators 31 , 32, 33, 34, 35 and

constraining the power sent to the thrusters for efficiency. The real thrust commands sent to the thrusters will be slowly ramped up to the determined optimal set of thrust commands. The slow ramping up may be necessary to prevent stability issues.

The result is that thrusters slowly will take over loads from the electrical actuators 31 , 32, 33, 34, 35. External loads on the robot arm 1 that vary fast will be carried by the actuators 31 , 32, 33, 34, 35. Their quick oscillation gets filtered by the thrust command ramping, so that thrusters do not attempt to carry high frequency loads.

It is expected that the loads on the robot arm 1 consist of high amplitude slowly varying loads (like gravity), and small amplitude high frequency loads (like vibrations and high frequency waves). This is highly advantageous, since thrusters can apply large load, but do not have large bandwidth, while electrical actuators 31 , 32, 33, 34, 35 have high bandwidth but cannot apply large forces to objects displaced from the axis of the actuators 31 , 32, 33, 34, 35 (being caused by e.g. a long robot arm 1 ).

Fig. 2 shows a diagram of a robot arm control system.

Fig. 3 shows a diagram of a method 600 of controlling a robot arm1. In connection with this method, a robot arm 1 as illustrated in Fig. 1 may form of a robot arm system as described above and the certain steps of the method may carried out by a control system 400 configured for controlling such a robot arm 1 , preferably as described above.

The robot arm 1 comprises two or more arm sections 1 1 , 12, 13, 14, for example four as shown in Fig. 1. The robot arm 1 comprises at least a proximal first arm section 1 1 and a distal second arm section 12. However three, four, five or six arm sections may be applied.

As shown in Fig. 1 , a first articulated joint 21 is formed at a proximal end 2 of the robot arm 1. The first articulated joint 21 is configured for mounting the robot arm 1 to a base 100, which may be as described in connection with Fig. 1 , above. Further, a second articulated joint 22 is formed at a distal end 3 of the robot arm 1. The second articulated joint 22 is configured for mounting a tool 200 to the robot arm 1.

Intermediary articulated joints 23, 24, 25 are preferably formed between the two or more arm sections 1 1 , 12, 13, 14. An actuator 31 , 32, 33, 34, 35 is provided at each of said first, second and intermediary articulate joints 21 ,

22, 23, 24, 25. The actuator 31 , 32, 33, 34, 35 are configured for imparting movement

- between the base 100 and the robot arm,

- between the robot arm 1 and the tool 200, and

- between the two or more arm sections 1 1 , 12, 13, 14, respectively. Further, at least two liquid displacement devices 40, 41 , 42 are arranged on an arm section 1 1 , 12, 13, 14 and/or at an articulated joint 21 , 22, 23, 24, 25. The control system 400 comprises a controller 410 and first communication connections 420 formed between the actuators 31 , 32, 33, 34, 35 and the controller 410.

The control system 400 further comprises second communication

connections 430 provided to connect the liquid displacement devices 40, 41 , 42 and the controller 410.

The control system 400 further comprises first detection means 440 for measuring the torque of each actuator 31 , 32, 33, 34, 35; Information regarding the torque of each actuator 31 , 32, 33, 34, 35 may be conveyed to the controller 410 via the first communication connections 420.

The method 600 of controlling a robot arm comprises the following steps. In step 610 a command signal is forwarded to the actuators 31 , 32, 33, 34,

35 and to the liquid displacement devices 40, 41 , 42 to obtain a desired movement of the robot arm 1. The command signal may be provided as input from a user. Next, in a step 620, the torque resulting from the movement of imparted to the robot arm 1 in step 610 is detected by the first detecting means 440 at each actuator 31 , 32, 33, 34, 35.

In step 630 information signals containing information about the detected torques are sent to the controller 410, i.e. the information signals are received from the first detecting means 440. In step 640 an expected torque in response to the command signal to the actuators 31 , 32, 33, 34, 35 and the liquid displacement devices 40, 41 , 42 is calculated. This operation is carried out by the controller. The calculation may be based on model information.

In step 650 the detected torque received from the first detecting means 440 is compared with the calculated expected torque. Then in step 660, based on the compared detected torque and expected torque any externally generated load is determined. The externally generated load may be calculated as the difference between the detected torque and expected torque. In step 670 a revised command signal to the actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40, 41 , 42 is calculated by the controller 410. The revised command signal is intended to provide an output from actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40, 41 ,

42, which balances the load applied by the actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40, 41 , 42 with the externally generated loads. Preferably, this is done such that power applied in each actuator 31 ,

32, 33, 34, 35 is minimized.

In step 680 the revised command signal is forward to the actuators 31 , 32, 33, 34, 35 and to the liquid displacement devices 40, 41 , 42.

This adjustment may be carried out continuously, e.g. a regular time intervals, until new command signal is provided by a user (or by an automated control) as indicated by the choice 690 between continuing to the step 620 of detecting torque or returning to the step 610 of providing a new command signal. Thrusters and electrical actuators 31 , 32, 33, 34, 35 apply forces in the same degree of freedom. When the robot arm 1 is standing still, its actuators 31 ,

32, 33, 34, 35“feels” the net external load on the robot arm as torques T_e in each articulated joint 21 , 22, 23, 24, 25. This torque must be carried either by sending currents to the actuators 31 , 32, 33, 34, 35, generating an actuator torque T, , or by having thrusters apply thrust forces F, which generates torque loads T_f on the actuators 31 , 32, 33, 34, 35, T_f = J^rF, where J is a matrix that projects thrust forces into actuator loads. The torque from sending currents through the actuator 31 , 32, 33, 34, 35 and the torques from thruster forces must balance externally generated loads on each actuator 31 , 32, 33, 34, 35:

T, = T_f + T_e (1 ) If the robot arm 1 is not standing still, the actuator 31 , 32, 33, 34, 35 current torques and the thrust-generated torques must balance with the sum of externally generated torques and possible inertia torques. Inertia torques can be seen as externally generated torques and be dealt with in the same way. The thrusters will be used to ease actuator currents without needing user inputs, only measurements of actuator currents, joint angles, and parameters specific to the robot arm 1. Thereby, the electrical actuators 31 , 32, 33, 34,

35 carry loads that are changing rapidly, while in the long run, thrusters take over larger loads. This division is employed because thrusters can apply large torques due them being positioned with a large moment arm, while the thrusters cannot apply forces quickly due to their slow dynamics. Electrical actuators 31 , 32, 33, 34, 35 can apply forces quickly due to their fast dynamics. A concrete example (but not the only) of a thrust command that accomplishes the objectives is

where T is some number with dimensions of seconds that govern how fast external loads are taken over by the thrusters. The thrust that thrusters must then perform can be found as

If there are fewer thrusters than there are electrical actuators 31 , 32, 33, 34, 35, a set of thrust commands cannot in general be found as to transfer the entire external load to thrusters from the electrical actuators 31 , 32, 33, 34, 35. In that case, one can only minimize the weighted sum of squares of actuator currents. In the above relation, this would be reflected by, for equal weighting of all currents, by using a pseudoinverse of J⁷ instead of a pure inverse of J ⁷ in the above relation.

To see why a control choice such as that in (2) would get thrusters to take over long-run average loads, one can consider (1 ) and (2) in frequency domain, where they become

(5) from which it can be shown that

( ) This shows that the actuator motor current torques frequency components □,(w) are the high pass filtered frequency components of the external loads

□ _b(w), whereas the thrust-generated motor torques frequency components

□ _ί(w) are the low pass filtered frequency components of the external loads

□ _q(w).

In an embodiment of the robot arm system a thrust direction of one or more or all of the liquid displacement device 40, 41 , 42 may be rotated relative to the element/part, i.e. articulated joint 21 , 22, 23, 24, 25 or arm section 1 1 , 12, 13, 14, on which it is arranged.

In further embodiments also the intermediary arm sections 13, 14 may be equipped with liquid displacement devices (not shown), which may be of any of the types described above, and be connected to the control system as explained above. In yet further embodiments, the intermediary articulated joints 23, 24, 25 may alternatively or additionally be equipped liquid displacement devices (not shown), which may be of any of the types described above, and be connected to the control system as explained above.

The describe robot arm system may be attached to a remotely operated vehicle 1 10. In this case the remotely operated vehicle 1 10 may form the base 100 for the robot arm 1. In such cases, the remotely operated vehicle may in further embodiments be equipped with an attachment mechanism 50 configured for attaching the remotely operated vehicle to a sub-surface structure 300. Such an attachment mechanism 50 may be of the kind described above.

In a further aspect, the invention relates to a robot arm system as described above and where a polishing tool 210 is mounted at the second articulated joint 22 of the robot arm 1. Preferably the polishing tool 210 comprises a is a single polishing disc 201 provided with a motor 202 for rotating the polishing disc 201 , as also describe in connection with Fig. 1 above. The polishing disc 201 rotates relative to a diving shaft 203, about a rotational axis A. The polishing disc 201 may have a silicon carbide disc attached to a polishing disc base, such that the silicon carbide disc may be exchanged.

The control system 400 may then further be configured for controlling a position of the polishing tool 210 by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35, to provide a zero or near-zero pressing force against a surface 310 of a sub-surface structure 300 along rotation axis A, and zero bending moments between the polishing tool 210 and the surface 310, when

- a signal is provided to the the control system 400 indicative of the

polishing tool 210 being in contact with a flat surface of a sub-surface structure 300, and when

- a signal is provided to control system 400 indicative of keeping the polishing tool 210 stationary at a desired location relative to the sub- surface structure 300.

The control system 400 may further be configured for controlling a position of the polishing tool 210 by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35 to provide a zero or near-zero pressing force against the surface 310 along rotation axis A, and a non-zero bending moment between the polishing tool 210 and the surface (310) for moving the polishing tool 210 along the surface, when

- a signal is provided to the the control system 400 indicative of the

polishing tool 210 being in contact with a flat surface of a sub-surface structure 300, and when - a signal is provided to control system 400 indicative of moving the polishing tool 210 along the surface of the sub-surface structure 300.

The control system 400 may further be configured for controlling a position of the polishing tool 210 by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35, to provide a pressing force along rotation axis A at a desired positive value, when a signal is provided to the the control system 400 indicative of the polishing tool 210 being in contact with a non-flat surface is detected.

Fig. 4 shows a diagram of a method 700 of cleaning a sub-surface structure 300. The method 700 of cleaning a sub-surface structure 300 may be applied by a robot arm system according to any one of the aspects or embodiments described above. Preferably, the method 700 of cleaning a sub-surface structure 300 may be applied in a robot arm system where a polishing tool 210 is mounted at a second articulated joint 22 of the robot arm 1 of the robot arm system.

The method 700 of cleaning a sub-surface structure 300 comprises at least the following steps.

In a step 710 the base 100 of the robot arm 1 is brought within range of at least a portion 315 of a surface 310 of a sub-surface structure 300.

In step 720 the polishing tool 210 is brought into contact with the portion 315 of the surface 310 of a sub-surface structure 300 mentioned in step 710. The polishing tool 210 is brought into contact with the surface 310 by controlling at least the actuators 31 , 32, 33, 34, 35 of said robot arm 1. However as described above the liquid displacement devices 40, 41 , 42 may contribute as well. In step 730 a polishing disc 201 of the polishing tool 210 is rotated. The rotation may be started at desired time before the polishing tool 210 is brought into contact with the surface 310, preferably shortly before.

Alternatively, the rotation may be started at the moment when the polishing tool 210 is brought into contact with the surface 310. Alternatively, the rotation may be started shortly after the polishing tool 210 is brought into contact with the surface 310. Thus, the sequence of steps 720 and 730 in Fig. 4 may be reversed.

The rotation of the polishing disc 201 of the polishing tool 210 preferably is continued as long as the polishing tool 210 is in contact with the surface 310.

Preferably the polishing tool 210 comprises a is a single polishing disc 201 provided with a motor 202 for rotating the polishing disc 201 , as also describe in connection with Fig. 1 above. The polishing disc 201 rotates relative to a diving shaft 203, about a rotational axis A. The polishing disc 201 may have a silicon carbide disc attached to a polishing disc base, such that the silicon carbide disc may be exchanged.

In step 740 it is detected if the polishing tool 210 is located at a flat surface portion of the sub-surface structure 300 or at a non-flat surface of the sub surface structure 300.

In embodiments of the method, this detection may be provided by a human operator inspecting the sub surface structure 300 via one or more cameras 60, see Figs 1 and 2. Such cameras 60 may be provided on the robot arm 1 and/or the base 100. Cameras may be provided on a remotely operated vehicle 1 10 on which the robot arm 1 is arranged. Based on the visual information the user may input to the control system relating to the nature of the surface at which the polishing tool 210 is located, i.e. if it is flat or non-flat. Alternatively, the determination of whether or not the surface in the vicinity of the polishing disc 201 may be provided by robotic intelligence implemented in the control system 400. The robotic intelligence may use image processing methods on images of the surface in the vicinity of the polishing disc 201 obtained by cameras 60 as described above.

In step 745 it is determined if the surface contacted by the polishing disc 201 is flat or non-flat.

If the surface is flat, the method 700 continues to step 746. If on the other hand in step 745, it was decided that the surface, at the location at which the polishing tool 210 has been located, is non-flat, then the method 700 continues to step 770.

In step 746 it is decided if the polishing tool 210 should remain at the current position (for example if further cleaning/polishing is needed), or if the polishing tool should be moved. This may be determined e.g. from visual inspection -and input by an operator - or by image analysis in the control system 400.

If it is decided that the polishing tool 210 should remain at the current position the method continues to step 750.

In a step 750 if a flat surface is detected and the polishing tool 210 is to be kept the same location relative to the sub-surface structure 300, controlling a position of the polishing tool 210 relative to the surface 310 s provided by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35 to provide a zero or near-zero pressing force against the surface 310 along rotation axis A and zero bending moments between the polishing tool 210 and the surface 310. However if in step 746, it is decided that the polishing tool 210 should be moved from the current position the method continues to step 760.

In a step 760 if a flat surface is detected, controlling a position of the polishing tool 210 is provided by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35, to provide zero or near-zero pressing force against the surface 310 along rotation axis A and a non-zero bending moment between the polishing tool 210 and the surface 310 for moving the polishing tool 210 along the surface 310.

In a step 770, if a non-flat surface is detected, controlling a position of the polishing tool 210 is provided by jointly controlling the liquid displacement devices 40, 41 , 42 and/or the actuators 31 , 32, 33, 34, 35 to provide a pressing force along rotation axis A to a desired non-zero (positive) value.

Motors/actuators 31 , 32, 33, 34, 35 are controlled by motor/actuator controllers. The controllers receive a command position from a program with user inputs. One or more sensors on each actuator/motor sends back position feedback from the motor/actuator. The position command and feedback is processed in the motor/actuator controller to send an appropriate current to the motor. If the motor encounters external loads that resist motions, the controller has mechanisms that automatically ramps up current sent. There are multiple motors/actuators and motor/actuator controllers on the robot arm 1.

The robot arm 1 comprises sensors, such as first detection means 440 (e.g. measuring current sent to the actuators 31 , 32, 33, 34, 35, and sensors for detecting/measuring the current joint angle position of all articulated joints 21 , 22, 23, 24, 25, (and possibly thrust forces estimated form thrust commands) that estimate the loads on the polishing disc 201 of the robot arm 1. Simultaneously an operator gives a motion command.

The loads on the robot arm and the motion command from the operator can be divided into 6 degrees of freedom in the control system 400 of the robot arm system, which may be implemented as software running on the controller 410:

The control system 400 of the robot arm system can use the motion given by the operator to move the robot or use the forces estimated as inputs in an algorithm that provides a motion command that moves the robot arm 1 in the direction of the estimated forces. This is equivalent to force controlling to zero forces. The latter can be advantageous for polishing, because forces caused by polishing flat surfaces underwater are stabilizing the polishing disc to be normal to and sucked to the surface 310 of the sub-surface structure 310 to be polished.

Alternatively the robot arm 1 can be force controlled so that the polishing disc 201 is moved to generate a certain load on the polishing disc - this is advantageous for polishing non-flat surfaces, where polishing loads are not stabilizing, and the polishing disc must be pressed against the surface to be polished with an appropriate force.

The robot arm system either comprises sensors or an operator using a camera or cameras that can determine of the polishing disc is in contact with the surface 310 to be polished, and whether this surface 310 is flat or non flat, or an operator can use a camera image-feed to determine this.

If the polishing disc 201 is not in contact with the relevant surface 310, degrees of freedom no. 1 -5 are in position control mode, so the robot arm 1 moves as the operator commands.

If the polishing disc is in contact with a non-flat surface, degrees of freedom no. 2-5 are in position control mode, while degree of freedom no. 1 is in force control mode, controlling the polishing force to an appropriate value, resulting in the polishing disc 201 being pushed into the surface 310 to be polished.

If the polishing disc is in contact with a flat surface, two different methods are proposed that mix force and position control:

1. Version 1 : Degrees of freedom nos. 2 and 3 are in position control mode, while degrees of freedom 1 , 4 and 5 are in force control mode, and bending moments and polishing force being controlled to zero. The result is that the polishing disc gets sucked into being normal to the surface to be polished.

2. Version 2: Degrees of freedom nos. 1 -3 are force controlled to zero, while degrees of freedom 4-5 are force controlled to values

proportional to motion commands in degrees of freedom nos. 2-3 given by the operator. This means that when the operator commands the robot arm to move the polishing tool 210 along the surface 310 to be polished, instead of directly commanding the robot arm 1 to move in this fashion, it is instead commanded to bend the polishing disc 201 into the surface 310 to be polished. This generates a shear force on the polishing disc 201. The control system 400 of the robot arm system then force controls this shear force to zero, so that the robot arm 1 will continue to move along the surface 310 to be polished while bending moments nos. 4-5 are controlled to non-zero. The result is that the control system 400 of the robot arm system moves the polishing disc 201 along the surface to be polished in an indirect way that ensures a method of smoothly moving the disc along this surface 310.

Degree of freedom no. 6 is the polishing discs 201 spin degree of freedom and it is always in speed control mode, where its spin speed is controlled by the operator.

The method 700 is particularly useful for polishing/cleaning sub-surface structures 300 such as ships propellers.

When contacting flat parts of the propeller, rotation of the polishing disc 201 will generate a suction beneath If on the other hand in step 745, it was decided that the surface, at the location at which the polishing tool 210 has been located, is non-flat, then the method 700 continues to step 760.

it, which will eventually align the polishing disc 201 with the surface 310 for a high quality polish. However, this will only happen if the orientation of the robot arm systems polishing tool 210 (that holds the polishing disc 201 ) is lenient. The robot arm system must give in to the suction forces. Giving in to the suction forces corresponds to force controlling the robot to a near-zero force along the surface’s 310 normal axis, and a zero bending moment along one or a combination of the two axis' that align with the propellers surface 310. These load types will be controlled to zero, while the motion of the polishing disc 201 along the surface 310 of the propeller must be controlled. Hence associated forces must not in general be controlled to zero, and neither may be the torque around the axis normal to the surface 310 of the propeller (else there would be no polishing).

When contacting flat surface portions and for moving along the flat surface potions of a propeller, the rotating polishing disc 201 has a peculiar mechanic to it. Should a bending moment be applied to the polishing disc 201 from the surface 310 of the propeller around one or a combination of the two axes along the propeller surface while the polishing disc 201 is rotating, an additional force along the surface 310 of the propeller (but along the same axis as the bending moment) will be generated, analogous to the gyroscopic coupling of translation and rotation found in helicopters, while not being caused by the exact same physical phenomenon.

In this case, bending moments and along-surface forces between the surface 310 of the propeller are coupled to them being generated by the non-uniform distributions of pressure. This can result in a uniform high quality polish.

This will in addition involve regulating applied bending moments while moving the robot arm 1 , so that the robot arm motion is entirely caused by the forces on the polishing disc 201. This will result in the polishing disc 201 moving smoothly even if the surface is not entirely flat.

At non-flat surface portions of the propeller it is necessary to hold the polishing disc closely to the surface 310 of the propeller applying forces from the robot arm system’s actuators 31 , 32, 33, 34, 35, as there is no helping suction that would otherwise be generated from a rotating disc 201 at the surface 310 of a flat propeller surface portion. Propellers have inherent non flat parts, such as it’s trailing and leading edge 311 , concaves, curves, and the propeller hub 302. When pressing the polishing tool 210 against the propeller’s surface 310, the holding force must only be so great as to remove fouling and corrosion-induced imperfections, but not so great as to scrape off propeller material. This is especially crucial at the trailing edge of the propeller, where the thickness of the propeller vanishes in a sharp bend, and too large polishing forces can severely alter the shape and performance of the propeller blade. Hence, the holding force (the force aligned with the normal of the propeller surface) has to be controlled to a specific value, and movements of the polishing tool 210 along the propeller’s surface 310 must remain position controlled.

To perform these force controls, the robot arm system operator or a robotic intelligence implemented in the control system 400 of the robot arm system, will have to perform the following tasks, which hold in them the essence of the proposed method 700 for propeller cleaning/polishing:

1. By looking at the section of the propeller blade the operator or the robotic intelligence about to polish will choose to either engage expecting a at surface, or a non-flat surface.

2. The operator or the robotic intelligence will alert the relevant layer of the robots software that the propeller is touched and force control can now be used. (If used without contact with the propeller, force control can never work because there is no environment that can generate large enough forces on the polishing disc 201 ).

3. The relevant software layer will now take over, and activate its force control algorithms. For a flat surface, it will control for near-zero pressing force and zero bending moments. For movements along a flat surface, the bending moments will be controlled to non-zero values until the motion of the robot arm 1 is fully actuated by the resultant gyroscopic-like forces on the polishing disc 201. For non-flat surfaces, the relevant software layer will control the pressing force to an appropriate but non-zero value.

4. Should contact with the propeller’s surface 310 break, the operator or the robotic intelligence will cancel all force control algorithms. 5. All degrees of freedom of motion of the polishing disc 201 that are not currently in force control "mode" must be in position control mode and under the control of the operator or the robotic intelligence.

Figs. 5A-B, in schematic form, shows a base 100 in the form of a remotely operated vehicle 110 with a clamp 50 according to a further aspect of the invention, and also illustrates a method of attaching a remotely operated vehicle 1 10 to a sub-surface structure 300.

Figs. 5A-B, in a schematic form shows a sub-surface structure 300 in the form of a ship propeller. The ship propeller has a hub 302 comprising one or more propeller blades 305, only one of which is shown in Figs. 5A-B.

An outline of the remotely operated vehicle 1 10 (ROV) is shown in dashed lines and above the propeller blade in a direction perpendicular to the plane of the figure.

The remotely operated vehicle 1 10 comprises a body 1 1 1 and a plurality of liquid displacement devices 140 provided on or extending from the body 1 1 1. The liquid displacement devices 140 are configured for adjusting motion of the remotely operated vehicle 1 10.

The remotely operated vehicle 1 10 comprises further comprises an attachment mechanism 50 configured for attaching the remotely operated vehicle to the sub-surface structure 300.

In an embodiment the remotely operated vehicle 1 10 may further comprise a robot arm 1.

The attachment mechanism 50 comprises two flanges 51 , 52 having mutually facing surfaces 51’, 52’ arranged at a fixed distance relative to each other. A gap 53 with a fixed distance is thereby provided between the mutually facing surfaces 51 52’ of the flanges 51 , 52.

Elongate portions 59 of friction material may be provided on the mutually facing surfaces 51 52’ along edges of the flanges 51 , 52.

The attachment mechanism may be used in a method 800 for attaching a remotely operated vehicle 1 10 according to a further aspect of the invention.

The method 800 for attaching a remotely operated vehicle 1 10 to a sub surface structure 300 is particularly useful for attaching the remotely operated vehicle 1 10 to an edge 31 1 of a ship propeller. The method 800 comprises two steps.

The first step 810 comprises manoeuvring the remotely operated vehicle 1 10, such that an edge 31 1 of the sub-surface structure 300 is located within the gap 53 of the attachment mechanism 50. This situation is shown in Fig. 5A.

The second step 820 comprises operating the plurality of liquid displacement devices 140 such that they collective effort will tilt the remotely operated vehicle 1 10 relative to the edge 311 of the sub-surface structure 300.

Thereby, contact surfaces between portions of mutually facing surfaces 51 ', 52’ and the sub-surface structure 300 is provided. This situation is shown in Fig. 5B. The contact surfaces on the portions of mutually facing surfaces 51 ', 52’ are provided along the edges of the flanges 51 , 52. As shown in Fig. 5B the contact surfaces on the portions of mutually facing surfaces 51 ', 52’ are provided at the elongate portions 59 of friction material, which are provided on the mutually facing surfaces 51 ', 52’ along edges of the flanges 51 , 52.

The liquid displacement devices 140 may be electrically driven thrusters such as described in connection with the robot arm system above. Only two of the ROV's possibly many thrusters are shown in Figs. 5A-B. To attach itself, the ROV uses the same thrusters that it also uses for regular motion, so there is no need for additional thrusters. The ROV activates select thrusters that turn/tilt the ROV, and thereby the fixed flanges 51 , 52 so that the edges of each clamp touches the propeller blade.

Between the flanges 51 , 52 of attachment mechanism 50 and the propeller blade there is a frictional material 59 that transmits all forces between the propeller blade and the ROV.

Each frictional material 59 can transfer one normal force, two perpendicular friction forces, and additional moments, meaning the attachment has reaction loads corresponding to all three translations and rotations.

While the ROV is attached to the propeller blade, its robotic arm 1 can operate as if its base was rigidly mounted to the propeller blade.

The described attachment mechanism 50 and the method 800 sown in Figs. 5A-B may also be use in connection with the remotely operated vehicle 1 10, robot arm systems, and robot arms 1 , according to the aspects and embodiments thereof described above.

Fig. 6, in schematic form, shows a remotely operated vehicle 500 according to a further aspect of the invention.

The remotely operated vehicle 500 comprises three mutually intersecting elongate arms 510, 520, 530. The elongate first arm 510 has a first end 51 1 and a second end 512. The elongate second arm 520 has a first end 521 and a second end 522. The elongate third arm 530 having a first end 531 and a second end 532.

A liquid displacement device 540 is arranged at the first end 511 of the first arm 510 and another liquid displacement device 540 is arranged at the second end 512 of the first arm 510.

A liquid displacement device 540 is arranged at the first end 521 of the second arm 520, and another liquid displacement device 540 is arranged at the second end 522 of the second arm 520.

A liquid displacement device 540 is arranged at the first end 531 of the third arm 530; and another liquid displacement device 540 is arranged at the second end 532 of the third arm 530. ;

The first, second and third arms 510, 520, 530 intersect at an intersection 535.

In an embodiment, and as shown in Fig. 6, the remotely operated vehicle 500 further comprises an attachment mechanism 550 for reliably attaching the remotely operated vehicle 500 to a sub-surface structure 300.

The attachment mechanism 550 may in an embodiment be of the same type as described above in connection with Fig. 1. Alternatively, the attachment mechanism 550 may in an embodiment be of the same type as described above in connection with Figs. 5A-B.

As shown in Fig, 6 the attachment mechanism 550 may in an embodiment be formed as a flange 551 arranged in parallel with the first arm 510 and a in a uniform fixed distance therefrom. Thereby a gap 553 between the flange 551 and the first arm 510 is provided, similar to the gap 53 shown in Figs. 5A-B, the gap 553 having a uniform fixed distance there between. Thus, the attachment mechanism 550 shown in Fig. 6 may work in the same manner as the attachment mechanism 50 and attachment method 800 described in connection with Figs. 5A-B.

In an embodiment, the remotely operated vehicle 500 may comprise a robot arm system as described in connection with any of the above describe aspects and embodiments thereof.

The liquid displacement devices 540 may be electrically driven thrusters such as described in connection with the robot arm system above, or any other type as described in connection with the robot arm system above.

The remotely operated vehicle 500 shown in Fig. 6 also includes a first tube 560 for electronics, such as ROV control equipment.

The remotely operated vehicle 500 shown in Fig. 6 also includes a second tube 570 for other electronics, such as robot arm control equipment.

It will be appreciated that the remotely operated vehicle 500 may be equipped with a robot arm 1 of a robot arm system as described in

connection with any of the aspects above.

The remotely operated vehicle 500 shown in Fig. 6 may suffice with only six liquid displacement devices 540 (thrusters). Most prior art ROVs uses eight thrusters for full actuation, and if less are used, typically not all motions can be controlled by the ROV operator. In the remotely operated vehicle 500 shown in Fig. 6, the six thrusters are sufficient for performing all types of translatory and rotary motions. The relationship between necessary thrust commands and movement is fairly straightforward (there is no need for trigonometries to know what thrusters to activate to go in a particular direction). The thruster configuration can be divided into three parts, the three different beams or elongate arms 510, 520, 530, which all have a thruster pair pointing in the same direction, but displaced, so that there is one thruster at each end of the beam. A thruster pair moves the ROV in the direction that the thrusters point by activating both thrusters in the same direction. A thruster pair rotates the ROV 500 around an axis perpendicular to both the beam and the thrust direction by activating both thrusters in opposite directions. Since the three beams are mutually perpendicular, and so are the beams thruster pair directions, so are their respective net forces and rotation moments. Thus the ROV is fully actuated.

It is to be noted that the figures and the above description have shown the example embodiments in a simple and schematic manner. Many of the specific mechanical details have not been shown since the person skilled in the art should be familiar with these details and they would just unnecessarily complicate this description.

List of reference numbers

A longitudinal axis

1 robot arm

2 proximal end of robot arm

3 distal end of said robot arm

11 arm section, proximal arm section

12 arm section, distal arm section

13 third arm section, intermediary arm section

14 fourth arm section, intermediary arm section

21 first articulated joint

22 second articulated joint

31 actuator for first articulated joint, and configured for imparting movement between a base (100) and the first arm section (1 1 )

32 actuator for second articulated joint, and configured for imparting movement between the second arm (12) and a tool (200)

40 liquid displacement device

41 liquid displacement device

42 liquid displacement device

40’ thruster motor of liquid displacement device 40

41’ thruster motor of liquid displacement device 41

42’ thruster motor of liquid displacement device 42

50 attachment mechanism

51 jaw, fixed jaw

52 jaw, fixed jaw

53 gap between fixed jaws

55 gripping device

56 jaw

57 jaw

60 camera

100 base

110 remotely operated vehicle 11 1 body of remotely operated vehicle

140 liquid displacement devices on remotely operated vehicle

200 tool/”end effector”

201 polishing disc

202 motor

210 polishing tool/polishing device

300 sub-surface structure

301 propeller blade

302 hub

310 surface of the sub-surface structure

315 first portion of the surface of the sub-surface structure

316 second portion of the surface of the sub-surface structure

400 control system

410 controller, cpu

420 first communication connections to the actuators

430 second communication connections to the liquid displacement devices 440 first detection means for measuring the torque in actuators

450 second detection means for measuring the torque in liquid displacement means

460 connection for camera

490 display

491 connection for display

500 Submersible/Remotely operated vehicle

510 first arm of remotely operated vehicle

520 second arm of remotely operated vehicle

530 third arm of remotely operated vehicle

535 intersection of arms of remotely operated vehicle

540 liquid displacement devices

550 clamp

560 first tube for ROV control equipment

570 second tube for robot arm control equipment

Claims

1. A robot arm system comprising

- a robot arm (1 ); and

- a control system (400) configured for controlling said robot arm (1 ), wherein the robot arm (1 ) comprises

- two or more arm sections (1 1 , 12, 13, 14) including at least a proximal first arm section (1 1 ) and a distal second arm section (12);

- a first articulated joint (21 ) formed at a proximal end (2) of said robot arm (1 ) and configured for mounting the robot arm (1 ) to a base (100);

- a second articulated joint (22) formed at a distal end (3) of said robot arm (1 ) and configured for mounting a tool (200) to the robot arm (1 );

- intermediary articulated joints (23, 24, 25) between the two or more arm sections (11 , 12, 13, 14);

- an actuator (31 , 32, 33, 34, 35) provided at each of said first, second and intermediary articulate joints (21 , 22, 23, 24, 25), and configured for imparting movement between the base (100) and the robot arm (1 ), between the robot arm (1 ) and the tool (200), and between the two or more arm sections (1 1 , 12, 13, 14), respectively, and

- at least two liquid displacement devices (40, 41 , 42) arranged on an arm section (11 , 12, 13, 14) and/or an articulated joint (21 , 22, 23, 24, 25),

wherein the control system (400) comprises

- a controller (410) or a network of controllers;

- first communication connections (420) to the actuators (31 , 32, 33, 34,

35);

- second communication connections (430) to the liquid displacement devices (40, 41 , 42);

- first detection means (440) for measuring the torque of each actuator (31 , 32, 33, 34, 35); and wherein the control system (400) is configured to

- forwarding a command signal to the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices (40, 41 , 42) to obtain a desired movement of the robot arm (1 );

- receiving information signals from the first detecting means (440)

relating to the detected torque of each actuator (31 , 32, 33, 34, 35);

- calculating an expected torque in response to the command signal to the actuators (31 , 32, 33, 34, 35) and the liquid displacement devices (40, 41 , 42);

- comparing the detected torque with the expected torque;

- determine, based on the compared detected torque and expected

torque, any externally generated load;

- calculating a revised command signal to the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices (40, 41 , 42) to provide an output therefrom, which balances the load applied by the actuators

(31 , 32, 33, 34, 35) and by the liquid displacement devices (40, 41 , 42) with the externally generated loads such that power applied in each actuator (31 , 32, 33, 34, 35) is minimized; and

- forwarding the revised command signal to the actuators (31 , 32, 33,

34, 35) and to the liquid displacement devices (40, 41 , 42).

2. A robot arm system according to claim 1 ,

wherein the actuators (31 , 32, 33, 34, 35) are electrically driven, wherein each liquid displacement device (40, 41 , 42) comprises a thruster having an electrical thruster motor (40’, 41’, 42’);

wherein the forwarded command signal is an electric signal; and wherein the control system (400) is configured to

- receiving information signals from the first detecting means (440)

relating to the detected torque current of each actuator (31 , 32, 33, 34, 35); - calculating an expected torque current in response to the command signal to the actuators (31 , 32, 33, 34, 35) and the electrical thruster motors (40’, 41’, 42’);

- comparing the detected torque currents with the expected torque

current;

- calculating a revised command signal to the actuators (31 , 32, 33, 34, 35) and to the electrical thruster motor (40’, 41 ', 42’) to provide an output therefrom, which balances the load applied by the actuators

(31 , 32, 33, 34, 35) and by the liquid displacement devices (40, 41 , 42) with the externally generated loads, such that power applied in each actuator (31 , 32, 33, 34, 35) is minimized; and

- forwarding the revised command signal to the actuators (31 , 32, 33,

34, 35) and to the liquid displacement devices (40, 41 , 42).

3. A robot arm system according to claim 1 or 2, wherein a thrust direction of at least one liquid displacement device (40, 41 , 42) may be rotated relative to the element on which it is arranged.

4. A remotely operated vehicle (1 10), comprising a robot arm system according to any one of the claims 1 -3, and where the remotely operated vehicle (1 10) forms the base (100) for the robot arm (1 ).

5. A remotely operated vehicle (1 10) according to claim 4, further comprising an attachment mechanism (50) configured for attaching the remotely operated vehicle to a sub-surface structure (300).

6. A method of controlling a robot arm (1 ), the robot arm (1 ) forming part of a robot arm system further comprising a control system (400) configured for controlling said robot arm (1 ), wherein the robot arm (1 ) comprises:

- two or more arm sections (1 1 , 12, 13, 14) including at least a proximal first arm section (1 1 ) and a distal second arm section (12); - a first articulated joint (21 ) formed at a proximal end (2) of said robot arm (1 ) and configured for mounting the robot arm (1 ) to a base (100);

wherein the control system (400) comprises

- a controller (410) or a network of controllers;

- first communication connections (420) to the actuators (31 , 32, 33, 34, 35);

- first detection means (440) for measuring the torque of each actuator (31 , 32, 33, 34, 35); and

wherein the method comprises the steps of

- receiving information signals from the first detecting means (440)

relating to the detected torque of each actuator (31 , 32, 33, 34, 35); - calculating an expected torque in response to the command signal to the actuators (31 , 32, 33, 34, 35) and the liquid displacement devices (40, 41 , 42);

- comparing the detected torque with the expected torque;

- determining, based on the compared detected torque and expected torque, any externally generated load;

- calculating a revised command signal to the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices (40, 41 , 42) to provide an output therefrom, which balances the load applied by the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices (40, 41 , 42) with the externally generated loads, such that power applied in each actuator (31 , 32, 33, 34, 35) is minimized; and

- forwarding the revised command signal to the actuators (31 , 32, 33, 34, 35) and to the liquid displacement devices (40, 41 , 42).

7. A method according to claim 6,

wherein the forwarded command signal is an electric signal; and wherein the method comprises

- receiving information signals from the first detecting means (440)

relating to the detected torque current of each actuator (31 , 32, 33, 34, 35);

- calculating an expected torque current in response to the command signal to the actuators (31 , 32, 33, 34, 35) and the electrical thruster motors (40’, 41’, 42’);

- comparing the detected torque currents with the expected torque

current;

- calculating a revised command signal to the actuators (31 , 32, 33, 34, 35) and to the electrical thruster motors (40’, 41 ', 42’); to provide an output therefrom, which balances the load applied by the actuators (31 , 32, 33, 34, 35) and by the liquid displacement devices (40, 41 , 42) with the externally generated loads, such that power applied in each actuator (31 , 32, 33, 34, 35) is minimized; and

- forwarding the revised command signal to the actuators (31 , 32, 33,

34, 35) and to the electrical thruster motors (40’, 41’, 42’).

8. A robot arm system according to any one of the claims 1 -3, comprising a polishing tool (210) mounted at the second articulated joint (22) of the robot arm (1 ), wherein the control system (400) is configured for

- controlling a position of the polishing tool (210), by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide a zero or near-zero pressing force against a surface (310) of a sub-surface structure (300) along the rotation axis (A) and zero bending moments between the polishing tool (210) and the surface (310), when

o a signal is provided to the the control system (400) indicative of the polishing tool (210) being in contact with a flat surface of a sub-surface structure (300), and when

o a signal is provided to control system (400) indicative of keeping the polishing tool (210) stationary at a desired location relative to the sub-surface structure (300);

- controlling a position of the polishing tool (210), by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide zero or near-zero pressing force against the surface (310) along rotation axis (A) and a non-zero bending moment between the polishing tool (210) and the surface (310) for moving the polishing tool (210) along the surface, when

o a signal is provided to the the control system (400) indicative of the polishing tool (210) being in contact with a flat surface of a sub-surface structure (300), and when o a signal is provided to control system (400) indicative of moving the polishing tool (210) along the surface of the sub-surface structure (300); or

- controlling a position of the polishing tool (210) by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide a pressing force along rotation axis (A) at a desired positive value, when a signal is provided to the the control system (400) indicative of the polishing tool (210) being in contact with a non-flat surface is detected.

9. A method of cleaning a sub-surface structure (300), said method comprising the steps of

- mounting a polishing tool (210) at a second articulated joint (22) of a robot arm (1 ) of a robot arm system according to any one of the claims 1 -3;

- bringing the base (100) of the robot arm (1 ) within range of at least a portion (315) of a surface (310) of a sub-surface structure (300);

- bringing the polishing tool (210) into contact with said portion (315) of a surface (310) of a sub-surface structure (300), by controlling at least the actuators (31 , 32, 33, 34, 35) of said robot arm (1 );

- rotating a polishing disc (201 ) of the polishing tool (210);

- detecting if the polishing tool (210) is located at

o a flat surface of the sub-surface structure (300); or at o a non-flat surface of the sub-surface structure (300), and

- if a flat surface is detected and the polishing tool (210) is to be kept the same location relative to the sub-surface structure (300), controlling a position of the polishing tool (210) relative to the surface (310), by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide a zero or near zero pressing force against the surface (310) along rotation axis (A) and zero bending moments between the polishing tool (210) and the surface (310);

- if a flat surface is detected, controlling a position of the polishing tool (210), by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide zero or near-zero pressing force against the surface (310) along rotation axis (A) and a non-zero bending moment between the polishing tool (210) and the surface (310) for moving the polishing tool (210) along the surface (310), or

- if a non-flat surface is detected, controlling a position of the polishing tool (210) by jointly controlling the liquid displacement devices (40, 41 , 42) and/or the actuators (31 , 32, 33, 34, 35), to provide a pressing force along rotation axis (A) to a desired positive value.

10. A method according to claim 9, wherein the step of detecting if the polishing tool (210) is located at

- a flat surface of the sub-surface structure (300); or at

- a non-flat surface of the sub-surface structure (300),

- the robot arm (1 );

- the base (100) of the robot arm (1 ); or

- on a remotely operated vehicle (1 10) on which the robot arm is

arranged.

1 1 . A remotely operated vehicle (1 10), comprising

- a body (1 1 1 );

- a plurality of liquid displacement devices (140) provided on the body (1 1 1 ) for adjusting motion of the remotely operated vehicle (1 10);

- an attachment mechanism (50) configured for attaching the remotely operated vehicle to a sub-surface structure (300); and - a robot arm (1 )

wherein the attachment mechanism (50) comprises two flanges (51 , 52) having mutually facing surfaces (51 52’) arranged at a fixed distance relative to each other, and providing a gap (53) between them.

12. A method of attaching a remotely operated vehicle (1 10) according to claim 10 to a sub-surface structure (300), wherein the method comprises

- manoeuvring said remotely operated vehicle (1 10), such that an edge (31 1 ) of said sub-surface structure (300) is located within the gap (53) of the attachment mechanism (50);

- operating the plurality of liquid displacement devices (140) to

collectively tilt the remotely operated vehicle (1 10) relative to the edge (31 1 ) of the sub-surface structure (300) to provide contact surfaces between portions of mutually facing surfaces (51’, 52’) and the sub- surface structure (300).

13. A robot arm system comprising

- a robot arm (1 ); and

- a first articulated joint (21 ) formed at a proximal end (2) of said robot arm (1 ) and configured for mounting the robot arm (1 ) to a base (100); - a second articulated joint (22) formed at a distal end (3) of said robot arm (1 ) and configured for mounting a polishing tool (210) to the robot arm (1 ), said polishing tool (210) having a rotation axis (A);

- intermediary articulated joints (23, 24, 25) between the two or more arm sections (11 , 12, 13, 14); and

- at least two liquid displacement devices (40, 41 , 42), wherein said first, second and intermediary articulate joints (21 , 22, 23, 24, 25) each comprises an actuator (31 , 32, 33, 34, 35) configured for imparting movement between the base (100) and the robot arm (1 ), between the robot arm (1 ) and the polishing tool (210), and between the two or more arm sections (11 , 12, 13, 14), respectively,

wherein at least one of the at least two liquid displacement device (40, 41 , 42) is arranged on the distal second arm section (12) or on the second articulated joint (22), and

wherein the other of the at least two liquid displacement devices (40, 41 , 42) is arranged on the distal second arm section (12) or on the second articulated joint (22);

wherein the control system (400) comprises

- a controller (410) or a network of controllers;

- first communication connections (420) to the actuators (31 , 32, 33, 34, 35); and

- second communication connections (430) to the liquid displacement devices (40, 41 , 42),

and wherein the control system (400) is configured to control the liquid displacement devices (40, 41 , 42) to jointly force the polishing tool (210) in a direction along the rotation axis (A) thereof.

14. A robot arm system according to claim 13, wherein a thrust direction of at least one liquid displacement device (40, 41 , 42) may be rotated relative to the element on which it is arranged.

15. A remotely operated vehicle (1 10), comprising a robot arm system according to claim 13 or 14, and where the remotely operated vehicle (1 10) forms the base (100) for the robot arm (1 ).

16. A remotely operated vehicle (1 1 ) according to claim 15, further comprising an attachment mechanism (50) configured for attaching the remotely operated vehicle to a sub-surface structure (300).

17. A method of cleaning a sub-surface structure (300), said method comprising the steps of

- providing a robot arm system according to any one of the claims 12- 13;

- mounting a polishing tool (210) at said second articulated joint (22);

- bringing the polishing tool (200) into contact with said portion (315) of a surface (310) of a sub-surface structure (300) controlling at least the actuators (31 , 32, 33, 34, 35) of said robot arm (1 );

- forcing the polishing tool (210) in a direction along the rotation axis (A) thereof, by jointly controlling the liquid displacement devices (40, 41 , 42), to provide a resulting thrust from the liquid displacement devices (40, 41 , 42) along the rotation axis (A)

- operating said polishing tool (210) to polish the portion (315) of the surface (310).

18. A remotely operated vehicle (500) comprising

- an elongate first arm (510) having a first end (51 1 ) and a second end (512);

- an elongate second arm (520) having a first end (521 ) and a second end (522);

- an elongate third arm (530) having a first end (531 ) and a second end (532),

- a liquid displacement device (540) arranged at the first end (51 1 ) of the first arm (510); - a liquid displacement device (540) arranged at the second end (512) of the first arm (510);

- a liquid displacement device (540) arranged at the first end (521 ) of the second arm (520);

- a liquid displacement device (540) arranged at the second end (522) of the second arm (520);

- a liquid displacement device (540) arranged at the first end (531 ) of the third arm (530); and

- a liquid displacement device (540) arranged at the second end (532) of the third arm (530);

wherein the first, second and third arm (510, 520, 530) intersect at an intersection (535).

19. A remotely operated vehicle (500) according to claim 18 further comprising a attachment mechanism (550) for reliably attaching the remotely operated vehicle (500) to a sub-surface structure (300).

20. A remotely operated vehicle (500) according to claim 19, wherein a flange (551 ) is arranged in parallel with the first arm (510) form the attachment mechanism (550).

21. A remotely operated vehicle (500) according to any one of the claims 18- 20, comprising a robot arm system according to any one of the claims 1 -3.