GB2551123A

GB2551123A - System, method and computer program for determining ballast for an underwater vehicle

Info

Publication number: GB2551123A
Application number: GB1609676.0A
Authority: GB
Inventors: Wood Terry
Original assignee: Saab Seaeye Holdings Ltd
Current assignee: Saab Seaeye Holdings Ltd
Priority date: 2016-06-02
Filing date: 2016-06-02
Publication date: 2017-12-13
Anticipated expiration: 2036-06-02
Also published as: GB2551123B; GB201609676D0

Abstract

Determining ballast mass requirement for an unmanned underwater vehicle (100 fig 4) having at least one thruster (400, 402, 404, 406 fig 4) comprising determining a thrust requirement for the thrusters to provide the vehicle with an apparent weight in water S302 and in dependence on the thrust requirements determining the ballast mass required to provide the required weight in water S310. If there are two or more thrusters, the method may comprise determining the thrust requirement for each thruster to provide the vehicle with a required pitch and/or roll angles S306 and in dependence on the thrust requirement, determining the position (A-E fig 4) of the ballast mass required to maintain the required pitch and/or roll angles S312. These angles may be zero degrees to the horizontal. Predetermined positions that receive ballast mass may be analysed and positions may be selected to locate the ballast mass. The positions may also receive tooling equipment. The ballast mass may be determined in dependence on the discrete ballast masses within a set. The vertical thrust component of the thruster may be resolved. A method, system and computer program for determining ballast masses are disclosed.

Description

SYSTEM, METHOD AND COMPUTER PROGRAM FOR DETERMINING BALLAST FOR AN

UNDERWATER VEHICLE

The present invention relates to a method for determining the ballast mass required for an underwater vehicle to maintain substantially neutral buoyancy. In particular, the method is used to determine the ballast required for autonomous or remotely operated underwater vehicles. The invention also relates to a system and computer program for carrying out the method.

Remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs) require ballasting and/or trimming, in general, prior to each use due to changes in configuration of the vehicle, for example a change in tooling, and/or due to changes in the density of the water the vehicle is used in. It is well-known that the density of water changes with temperature and salinity. Furthermore, the ballasting and trimming requirements may vary depending on the task the vehicle is undertaking, legislative requirements, and/or contractual requirements.

Ballasting refers to the process of adding or removing weights so that the vehicle has an acceptable buoyancy while in use in water. If the vehicle is too positively or too negatively buoyant the vehicle can be difficult or impossible to manoeuvre, and at the very least will require additional power from the vehicle thrusters to maintain depth in the water.

Trimming refers to the process of adjusting the location of the ballast weights to maintain the pitch and/or roll of the vehicle while in use in water. Again, without such an adjustment the vehicle can be difficult or impossible to manoeuvre, and at the very least will require additional power from the thrusters to maintain the pitch and/or roll as required. In general, a vehicle is trimmed so that it is flat. As used herein, the term “flat” refers to the pitch and roll angles of the vehicle being 0 degrees to horizontal.

Presently, ROVs/AUVs are ballasted and trimmed using a trial-and-error process. The vehicle is launched into the water and the operator visually observes the characteristics of the vehicle such as sink rate and attitude in the water. The vehicle is then recovered and ballast weights are added, removed, and/or re-positioned. The vehicle is then re-launched into the water and the operator once again visually observes the characteristics of the vehicle. Such a process is iterative, and a substantial amount of time, and operator experience, is required to determine the ballast mass and locations to provide satisfactory results. In addition, the operational risk increases with each re-launch.

It is therefore an object of the present invention to provide a method and system for determining the ballast mass and/or location of the ballast mass for ROVs/AUVs in a more efficient manner. Such a method and system would therefore provide a more adaptable, accurate and safe means for ballasting and trimming a ROV or AUV.

According to a first aspect of the present invention, there is provided a method of determining ballast mass requirement for an unmanned underwater vehicle having at least one thruster. The method comprises the steps of: determining a thrust requirement for the or each thruster to provide the vehicle with a required apparent weight in water; and in dependence on the or each determined thrust requirement, determining the ballast mass required to provide the required weight in water.

Advantageously, such a method may significantly increase the efficiency of determining the required ballast mass. An underwater vehicle with correct ballast will be energy efficient and more easily manoeuvred through the water.

The apparent weight in water may be such that the vehicle is one of: substantially neutrally buoyant; positively buoyant; and negatively buoyant. To reduce power requirements of the vehicle, where the apparent weight in water is either such that the vehicle is positively or negatively buoyant, the required apparent weight is preferably less than 5% from the apparent weight required to provide the vehicle with substantially neutral buoyancy.

Certain unmanned underwater vehicles comprises more than one thruster having a vertical component of direction. Upon the vehicle having at least two such thrusters, the method further comprises: determining the thrust requirement for each thruster to provide the vehicle with at least one of: a required pitch angle; and a required roll angle; and in dependence on the thrust requirement for each thruster, determining the position of the ballast mass required to maintain the or each of the required pitch angle and the required roll angle.

Such further steps advantageously enable the unmanned underwater vehicle to be provided with both a required apparent weight in water, and a required attitude in the water, that is to say roll and pitch angles.

In a preferred embodiment, the method comprises submerging the vehicle in water, carrying out the steps as described above, and then recovering the vehicle to attach the determined ballast mass or masses. It will be appreciated that the ballast mass may be provided in more than one position on the unmanned underwater vehicle in order to both provide the required apparent mass in water and the required pitch and/or roll angles.

In this embodiment, the required pitch angle and the required roll angle may each be 0 degrees to the horizontal. Alternatively, the required pitch angle and the required roll angle may be any appropriate angle to the horizontal as required by the particular mission of the unmanned underwater vehicle.

As used herein, the term “ballast mass’’ refers to positive and negative mass of ballast in water. That is to say, the ballast mass may be either negatively buoyant, adding which increases the apparent weight in water, or the ballast mass may be positively buoyant, adding which decreases the apparent weight in water.

The step of determining the position of the ballast mass may further comprise, analysing a set of predetermined positions configured to receive ballast mass, and selecting at least one position as the determined position of the ballast mass.

At least one of said predetermined positions in the set of predetermined positions may be further configured to receive tooling equipment for the unmanned underwater vehicle, the step of determining the position of the ballast mass further comprising selecting at least one position as the determined position excluding the or each position having tooling equipment. In this way, advantageously the method enables the ballast mass to be determined even when the unmanned underwater vehicle has tooling equipment coupled to it in locations where ballast mass may ordinarily be provided.

The method may further comprise receiving, from a user, inputs corresponding to the location of tooling equipment, or corresponding to positions on the unmanned underwater vehicle that cannot for other reasons receive ballast mass.

Preferably, the ballast mass is determined in dependence on the discrete ballast masses within a set of ballast masses, the set of ballast masses comprising a plurality of masses. In this way, the operator is provided with a list of required ballast masses that must be added to the vehicle to provide the required weight in water.

Preferably, the step of determining the ballast mass required to provide the required weight in water further comprises resolving the vertical thrust component of the or each thruster. In this way, the ballast mass may be more accurately determined.

Preferably, the step of determining the thrust requirement for the or each thruster comprises calculating the thrust in dependence on the input voltage, input current, and propeller speed. This calculation is performed in dependence on the known operating characteristics of the thruster. The calculated thrust may be calculated based on a look-up table comprising known thrust forces for the or each thruster. Alternatively, the thrust may be calculated using an algorithm, the inputs to the algorithm being input voltage, input current and propeller speed.

According to a further aspect of the present invention, there is provided a system for determining ballast mass requirement for an unmanned underwater vehicle having at least one thruster. The system comprises: a thrust determining module configured to determine a thrust requirement for the or each thruster to provide the underwater vehicle with a required apparent weight in water; and a ballast mass determining module configured to, in dependence on the or each determined thrust requirement, determine the ballast mass required to provide the required weight in water.

Advantageously, such a system may significantly increase the efficiency of determining the required ballast mass. An underwater vehicle with correct ballast will be energy efficient and more easily manoeuvred through the water.

In a preferred embodiment of the system, upon the vehicle having at least two thrusters, the thrust determining module is further configured to determine the thrust requirement for each thruster to provide the vehicle with at least one of: a required pitch angle; and, a required roll angle. In this embodiment, the system further comprises: a ballast position determining module configured to, in dependence on the thrust requirement for each thruster, determine the position of the ballast mass required to maintain the or each of the required pitch angle and the required roll angle.

In the preferred embodiment, the system may further comprise a memory module for storing a set of predetermined positions configured to received ballast mass, wherein the ballast position determining module is further configured to analyse the set of predetermined positions configured to receive ballast mass, and select at least one position as the determined position of the ballast mass.

The system may further comprise a memory module for storing a list of discrete ballast masses, wherein the ballast mass determining module is further configured to determine the ballast mass in dependence on the discrete ballast masses within the list of ballast masses. Where the system comprises a memory module for storing a set of predetermined positions, the memory module for storing a list of discrete ballast masses may be the same module having different sectors for each data set, or different modules.

The system may further comprise a user interface module, configured to output the determined ballast mass to the user. Preferably the user interface is a graphical user interface, the output being a schematic representation of the unmanned underwater vehicle having the or each ballast mass overlaid thereon. Where the system determines the required positions of the ballast masses, the or each ballast mass is overlaid on the schematic representation of the unmanned underwater vehicle in the determined position.

The user interface may be further configured to receive inputs from the user, the inputs being at least one of: type of unmanned underwater vehicle; position of tooling equipment; required weight in water; required pitch angle; and required roll angle.

According to a yet further aspect of the present invention, there is also provided a computer program that when executed on a processor causes it to carry out the method of determining ballast mass requirement for an unmanned underwater vehicle having at least one thruster as described herein.

Where functional modules are referred to in apparatus embodiments for carrying out various steps of the described method(s) it will be understood that these modules may be implemented in hardware, in software, or a combination of the two. When implemented in hardware, the modules may be implemented as one or more hardware modules, such as one or more application specific integrated circuits. When implemented in software, the modules may be implemented as one or more computer programs that are executed on one or more processors.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

The invention will be further described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a representation of an unmanned underwater vehicle and a system for determining ballast mass requirements for that vehicle;

Figure 2 shows the degrees of freedom of the unmanned underwater vehicle;

Figure 3 shows a flow diagram of a method of determining ballast mass requirements for an unmanned underwater vehicle; and

Figure 4 shows a schematic representation of a plan view of an unmanned underwater vehicle.

Figure 1 shows an unmanned remotely operated underwater vehicle 100, connected by means of a cable carrying electric power and data 102 to a control system 102. It is noted that the present invention is equally applicable to an autonomous underwater vehicle, which would not be physically connected to a control system.

Such remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs) require ballasting and/or trimming, in general, prior to each use due to changes in configuration of the vehicle, for example a change in tooling, and/or due to changes in the density of the water the vehicle is used in. It is well-known that the density of water changes with temperature and salinity. Furthermore, the ballasting and trimming requirements may vary depending on the task the vehicle is undertaking, legislative requirements, and/or contractual requirements.

Trimming refers to the process of adjusting the location of the ballast weights to maintain the pitch and/or roll of the vehicle while in use in water. Again, without such an adjustment the vehicle can be difficult of impossible to manoeuvre, and at the very least will require additional power from the thrusters to maintain the pitch and/or roll as required. In general, a vehicle is trimmed so that it is flat. As used herein, the term “flat” refers to the pitch and roll angles of the vehicle being 0 degrees to horizontal.

As such, a system is provided which determines the ballast mass requirements for an unmanned underwater vehicle (UUV) without the need for an iterative process of adding mass, testing the buoyancy, and repeating if necessary. The system 102 comprises a processor 104, memory 106 and a graphical user interface 108. The system 102 may be part of the more general control system for the UUV which enables the operator to control the movement of the UUV and control any tools on the UUV. Alternatively, it may be provided as a stand-alone system.

As shown in Figure 2, a UUV 100 in water has six degrees of freedom, three translational degrees of freedom: surge in the x-direction; sway in the y-direction; and heave in the z-direction, and three rotational degrees of freedom: roll about the x-direction; pitch about the y-direction; and yaw about the z-direction. The present system seeks to provide the UUV with an apparent weight in water such that under zero-power input the thrusters on the UUV there is no movement in the z-direction, i.e. neutral buoyancy. Alternatively, the particular operation may require positive or negative buoyancy. In addition, the system may seek to provide the UUV with a specified pitch and/or roll angle relative to the horizontal. As is known, a body free to move in water will orient itself such that the centre of buoyancy (CoB) is directly above the centre of gravity (CoG). Therefore adjusting the centre of gravity and/or the centre of buoyancy enables the attitude of the UUV to be controlled.

The system 100 is provided with software stored on the memory 106 that when executed on the processor 104 carries out the method of determining the ballast mass requirement to provide an apparent weight in water and where applicable to determine the location of the ballast mass to provide the required pitch and/or roll angle. Figure 3 shows a flow diagram of the method. As can be seen, the operator launches the UUV (S300) into the water in which the UUV will operate. The system then operates the thrusters of the UUV to maintain the UUV at a certain depth in water, for example 1m. The thrust required to maintain that depth is then measured (S302). The thrust is determined by measuring the input voltage, input current and propeller speed of the thruster. The known specifications of the thruster are then used to determine the thrust output based on those measured parameters. The known specifications of the thruster may be obtained through testing to measure thrust versus input voltage, input current and propeller. Such specifications may then be stored in a look-up table or the like and stored in the memory 106.

If the UUV comprises multiple thrusters (S304), the thrust required to maintain a pitch and/or roll angle is also determined (S306).

In both cases, the system receives as in input from the operator, via the user interface 108, the required apparent weight in water, and where appropriate, the required pitch and roll angles. For example, the operator could request that the apparent weight in water is zero, or is slightly positively buoyant, or slightly negatively buoyant. Slightly positively buoyant means that the apparent weight in water of the UUV is negative, for example between about negative 0.5 kg and about negative 6 kg, dependent upon the weight of the vehicle. Slightly negatively buoyant means that the apparent weight in water of the UUV is positive, for example between about 0.5 kg and about 6 kg, again dependent on the weight of the vehicle.

The total ballast mass required to provide the apparent weight in water is then determined (S308, S310) in dependence on the total thrust force required to maintain the UUV at a certain depth in water. Where necessary, the vertical component of the thrust force for each thruster is resolved. This is required where the central axis of the thruster is not aligned with the z-direction. The total ballast mass required, for example to provide neutral buoyancy, is equal to the total thrust required to maintain the UUV depth in water.

Where multiple thrusters on the UUV enable pitch and/or roll control, the position of the ballast mass is determined (S312) to maintain the required pitch and/or roll angle. The determination is based on the moment of force acting on the UUV which can be derived from the varying thrust forces for each thruster, and the known distances of the thrusters from the CoB.

The total ballast mass required is then correlated to an inventory list of the available ballast masses. For example, the operating vessel may carry a number of different ballast masses having positive weight in water, such as 0.5 kg, 1 kg, 2 kg and 5 kg. In addition, the vessel may carry ballast masses having negative weight in water (i.e. buoyancy), such as negative 0.5 kg, negative 1 kg, negative 2 kg, and negative 5 kg.

The system may then determine (S316) the breakdown of the total ballast mass in terms of the available ballast masses. Where the required positions of the ballast mass are determined in step S312, the method may feedback the list of ballast masses required into the determination of the positions of the ballast masses. Such a feedback enables the system to determine the positions of the ballast masses more easily.

Once the positions are determined, the system outputs the positions and values of the required masses to the operator via the graphical user interface. Figure 4 shows an example of a graphical user interface, with a schematic representation of the UUV 100 in plan view. As can be seen, in this example the UUV has four thrusters 400, 402, 404, and 406 each having a vertical component of thrust. Overlaid into the UUV graphic are four positions, A, B, C, D and E each configured to receive ballast mass on the physical UUV in the corresponding positions. The system outputs the ballast masses required to be added to the UUV in each position. As such, to ballast and trim the UUV, the operator merely replicates the graphical representation of the ballast mass locations on the physical UUV.

Position A is provided at a point which corresponds to the CoB along the z-direction. In this way, adding ballast mass at position A will not affect the attitude of the UUV in water, merely the buoyancy of the UUV.

The distances of positions B, C, D, and E from the CoB, and the distances of the thrusters from the CoB along the x and y directions are also known. In this way, and as described above, the relative moments of force can be calculated to determine the required ballast masses at each position.

Claims

1. A method of determining ballast mass requirement for an unmanned underwater vehicle having at least one thruster, comprising: determining a thrust requirement for the or each thruster to provide the vehicle with a required apparent weight in water; and in dependence on the or each determined thrust requirement, determining the ballast mass required to provide the required weight in water.

2. A method according to Claim 1, wherein upon the vehicle having at least two thrusters, the method further comprises: determining the thrust requirement for each thruster to provide the vehicle with at least one of: a required pitch angle; and a required roll angle; and in dependence on the thrust requirement for each thruster, determining the position of the ballast mass required to maintain the or each of the required pitch angle and the required roll angle.

3. A method according to Claim 2, wherein the required pitch angle and the required roll angle are each 0 degrees to the horizontal.

4. A method according to Claim 2 or 3, wherein the step of determining the position of the ballast mass further comprises, analysing a set of predetermined positions configured to receive ballast mass, and selecting at least one position as the determined position of the ballast mass.

5. A method according to Claim 4, wherein at least one of said predetermined positions in the set of predetermined positions is further configured to receive tooling equipment for the unmanned underwater vehicle, the step of determining the position of the ballast mass further comprising selecting at least one position as the determined position excluding the or each position having tooling equipment.

6. A method according to any of the preceding claims, wherein the apparent weight in water is such that the vehicle is one of: substantially neutrally buoyant; positively buoyant; and negatively buoyant.

7. A method according any of the preceding claims, wherein the ballast mass is determined in dependence on the discrete ballast masses within a set of ballast masses, the set of ballast masses comprising a plurality of masses.

8. A method according to any of the preceding claims, wherein the step of determining the ballast mass required to provide the required weight in water further comprises resolving the vertical thrust component of the or each thruster.

9. A method according to any of the preceding claims, wherein the step of determining the thrust requirement for the or each thruster comprises calculating the thrust in dependence on the input voltage, input current, and propeller speed.

10. A method according to Claim 8, wherein the calculated thrust is calculated based on a look-up table comprising known thrust forces for the or each thruster.

11. A system for determining ballast mass requirement for an unmanned underwater vehicle having at least one thruster, comprising: a thrust determining module configured to determine a thrust requirement for the or each thruster to provide the underwater vehicle with a required apparent weight in water; and a ballast mass determining module configured to, in dependence on the or each determined thrust requirement, determine the ballast mass required to provide the required weight in water.

12. A system according to Claim 11, wherein, upon the vehicle having at least two thrusters, the thrust determining module is further configured to determine the thrust requirement for each thruster to provide the vehicle with at least one of: a required pitch angle; and, a required roll angle, the system further comprising: a ballast position determining module configured to, in dependence on the thrust requirement for each thruster, determine the position of the ballast mass required to maintain the or each of the required pitch angle and the required roll angle.

13. A system according to Claim 12, further comprising a memory module for storing a set of predetermined positions configured to received ballast mass, wherein the ballast position determining module is further configured to analyse the set of predetermined positions configured to receive ballast mass, and select at least one position as the determined position of the ballast mass.

14. A system according to Claim 11, 12 or 13, further comprising a memory module for storing a list of discrete ballast masses, wherein the ballast mass determining module is further configured to determine the ballast mass in dependence on the discrete ballast masses within the list of ballast masses.

15. A computer program that when executed on a processor causes it to carry out the method of any of claims 1 to 10.