Controlling buoyancy of an underwater vessel

Abstract

A buoyancy control system is disclosed. In an embodiment the buoyancy control system comprises a housing which sealably encloses an interior, a bore located within the interior, a piston fitted within the bore, and an actuator operatively associated with the piston. The bore has a first chamber in fluid communication with an opening in the housing and a second chamber in fluid communication with a void in the interior. The piston is configured to sealably separate the first chamber from the second chamber. The actuator is operatively associated with the piston and configured to control the position of the piston along the bore to vary the volume of the first chamber and the volume of the second chamber in indirect proportion. Also disclosed is an underwater vessel including a buoyancy control system and a method of controlling the buoyancy of the underwater vessel when operating in a surrounding water.

Description

CONTROLLING BUOYANCY OF AN UNDERWATER VESSELPRIORITY DOCUMENT

[0001] The present application claims priority from Australian Provisional Patent Application No. 2024902143 titled “UNDERWATER VESSEL” and fded on 11 July 2024, the content of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure broadly relates to underwater vessels. Various embodiments of the present disclosure relate to systems and methods for a controlling buoyancy of an underwater vessel in oceanographic or other applications including but not limited to autonomous floats, submersibles, submarines, and buoys.BACKGROUND

[0003] Underwater vessels can be used for various applications. Typical examples include ocean observation, military applications (such as a sonar buoy), object tracking, and others. Depending on the application, an underwater vessel may be completely or partially autonomous, or remotely controlled.

[0004] Underwater vessels referred to as “floats” ascend and descend in an ocean column by manipulating the overall buoyancy of the vessel. However, these traditional vessels typically change the overall buoyancy of the vessel by pumping fluid or gas in and out of external bladders. One example of a float which pumps an oil fluid between an internal reservoir and an external bladder is the “Alamo Float” manufactured by MRV Systems, LLC of San Diego, California.

[0005] Vessels which vary buoyancy by pumping fluid or gas between an internal reservoir and a bladder require large amounts of energy to operate and require complex controls, valves and the like.SUMMARY

[0006] In general terms, embodiments of the present disclosure involve a buoyancy control system which is operable to vary a volume of a fluid displaced by the system to thereby vary the buoyancy of an underwater vessel.

[0007] A first aspect of an embodiment of the disclosure provides a buoyancy control system comprising: a housing sealably enclosing an interior;a bore located within the interior, the bore having a first chamber in fluid communication with an opening in the housing and a second chamber in fluid communication with a void in the interior; a piston fitted within the bore, the piston configured to sealably separate the first chamber from the second chamber; and an actuator operatively associated with the piston, the actuator configured to control the position of the piston along the bore to vary the volume of the first chamber and the volume of the second chamber in indirect proportion.

[0008] In an embodiment, varying the volume of the first chamber regulates fluid communication into and out of the first chamber through the opening in the housing.

[0009] In an embodiment, the bore is an interior of a tube supported within the housing. The bore may comprise an open-ended tube having a first end which is contiguous with the opening in the housing and a second end having an opening to the void of the housing. The tube may be a rigid tube made of any suitable material.

[0010] In an embodiment, the actuator is a linear actuator. In one arrangement, the linear actuator is an electric linear actuator which is mechanically coupled to the piston and operable over a stroke length.

[0011] In an embodiment, the piston is fitted with a wiper seal for maintaining a sealed contact with an interior of the bore when the piston is stationary and in motion, such that when in motion the wiper cleans the interior wall.

[0012] In an embodiment, the piston is movable within the bore between an extended position and a retracted position. In an embodiment, when the piston is in a retracted position, the void is not positively pressurised when the buoyancy control system is submerged in surrounding water.

[0013] An embodiment of the buoyancy control system may further comprise a ballast attachment means for attaching a ballast to the housing to set a mass of the vessel.

[0014] Another aspect of an embodiment of the disclosure provides an underwater vessel capable of operating in a surrounding water, the vessel comprising: a buoyancy control system according to the above described aspect; a power supply; a processing module; wherein the actuator is configured to receive commands from the processing module to control the position of the piston along the length of the bore, wherein increasing the volume of the first chamber causes the first chamber to receivesurrounding water through the opening to decrease the buoyancy of the vessel and wherein decreasing the volume of the first chamber causes the first chamber to displace received surrounding water through the opening to increase the buoyancy of the vessel.

[0015] Yet another aspect of an embodiment of the present disclosure provides a method of controlling the buoyancy of an underwater vessel operating in a surrounding water, the method comprising: providing an underwater vessel incorporating a buoyancy control system according to the above described aspect; governing the piston to controllably vary the volume of the first chamber, wherein increasing the volume of the first chamber causes the first chamber to receive surrounding water through the opening to decrease the buoyancy of the vessel, and wherein decreasing the volume of the first chamber causes the first chamber to displace received surrounding water through the opening to increase the buoyancy of the vessel.

[0016] In an embodiment, the piston is governed using a timer which controls the actuator to vary the volume of the surrounding water contained in the chamber at pre-determined intervals.

[0017] In an embodiment, the piston is governed depending on sensed values acquired from a sensor payload of the vessel.BRIEF DESCRIPTION OF DRAWINGS

[0018] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0019] Figure 1 is a perspective view of an underwater vessel with a buoyancy control system in accordance with one or more embodiments of the present invention;

[0020] Figure 2 is a sectional view of the underwater vessel shown in Figure 1 ;

[0021] Figure 3 is a perspective view of the internal structure of the underwater vessel shown in Figure 1;

[0022] Figure 4 is a perspective end view of the internal structure of the underwater vessel shown in Figure 1;

[0023] Figure 5 is a perspective view of the internal structure of the underwater vessel shown in Figure 1 with an actuator shown in a partially installed position;

[0024] Figure 6 is a top perspective view of a piston of the underwater vessel shown in Figure 1;

[0025] Figure 7 is a side perspective view of the piston shown in Figure 6;

[0026] Figure 8 shows example descending and ascending operations for the underwater vessel shown in Figure 1;

[0027] Figure 9 shows a simplified functional block diagram of an underwater vessel according to an embodiment; and

[0028] Figure 10 shows a simplified system block diagram of an underwater vessel according to an embodiment.

[0029] In the following description, like reference characters designate like or corresponding parts throughout the figures.DESCRIPTION OF EMBODIMENTS

[0030] Referring now to Figures 1 and 2, there is shown an underwater vessel 10 according to an embodiment. The underwater vessel 10 shown here has a buoyancy control system 14 (ref. Figure 2) comprising a housing 12 sealably enclosing an interior 20, a bore 38 located within the interior 20, a piston 16 fitted within the bore 38, and an actuator 50 which is operatively associated with the piston 16.

[0031] In the present case, the underwater vessel 10 is illustrated as an autonomous float suitable for use in oceanographic or other underwater applications, such as to acquire subsurface data in an observation zone, such as an ocean environment. In such an application, the autonomous float may be configured to be neutrally buoyant and travel through the ocean environment in an ascending and descending pattern (ref. Figure 8) by using the buoyancy control system 14 to modify its buoyancy. Although the underwater vessel 10 is illustrated as an autonomous float, it is envisaged that the underwater vessel 10 could be another type of underwater vessel, such as an underwater glider, autonomous submersible, sonar buoy or the like.

[0032] In the present case, the bore 38 is an interior of a tube 39 which is supported in the housing 12 by suitable means. The bore 38 has a first chamber 24 in fluid communication with an opening 44 in the housing 12 and a second chamber 26 in fluid communication with a void 21 in the interior 20. As shown in Figure 2, the second chamber 26 has an opening 46 which allows fluid communication between the void 21 and the second chamber 26.

[0033] Actuator 50 controls the position of the piston 16 along the length of the bore 38 to thereby vary the volume of the first chamber 24 and the second chamber 26. In embodiments, the first chamber 24 has a volume which is controlled to regulate fluid communication into and out of the first chamber 24 through the opening 44 in the housing 12 to vary the buoyancy of the underwater vessel 10. As will be described in more detail below, the actuator 50 shown here is a linear electric actuator having a stroke length which governs the piston 16 to vary the volume of the first chamber 24 over a suitable range.

[0034] When the vessel 10 is in use in a surrounding water, varying the volume of the first chamber 24 varies, in indirect proportion, the volume of the second chamber 26. Varying the volume of the first chamber 24 and the second chamber 26 varies the total density of the vessel 10 for a given mass of the vessel 10. Varying the density in this way varies the buoyancy of the vessel 10.

[0035] In the embodiment shown in Figures 1 and 2, the housing 12 comprises a corrosion resistant cylindrical tube 6 having a suitable internal diameter and wall thickness. In the present case, the cylindrical tube 6 is manufactured from a composite material, such as a fibre reinforced thermoset epoxy matrix. The cylindrical tube 6 shown here has a length of about 620 mm, an internal diameter of about 180 mm and a wall thickness of about 3 to 4mm. It will of course be appreciated that different housing configurations may be used depending on the intended application and operating depth requirements.

[0036] End caps 18a, 18b, shown here as cowlings, are fitted to opposite ends of the cylindrical tube 6 to seal the interior 20 of the cylindrical tube 6 to resist moisture ingress thereinto in normal operating conditions. Braces 80 connect between the caps 18a, 18b and are configured to apply a tension between the caps 18a, 18b to secure the caps 18a, 18b to the respective ends of the cylindrical tube 6.

[0037] In the present case, end cap 18a is fitted with sensors 22, such as a temperature sensor 22a and / or a pressure sensor 22b, for monitoring the surrounding water. It will be appreciated that additional or different sensors or different sensor payloads may be used depending on the application.

[0038] There are securements 70 for attaching a ballast (not shown) to the underwater vessel 10. In the present case securement 70 comprises threaded holes 72 which are formed in the end cap 18b for coupling a ballast, such as a lead weight, thereto. In use, a ballast may be selected to surface ballast the underwater vessel 10 with a dry weight which makes the underwater vessel 10 neutrally buoyant for the deployment conditions. In this way, for example, target dry weights may be identified for the underwater vessel 10 to be neutrally buoyant in, for example, freshwater and seawater and / or for different ascent / de scent rates.

[0039] As shown in Figures 3 and 4, there is a structure 8 which mates with the housing 12 during assembly of the underwater vessel 10. The structure 8 shown here comprises stringers 28, bulkheads 34, and the tube 39.

[0040] In the present case, the stringers 28 comprise threaded rods 30 which each have a first end which is attached to end cap 18b. Spaced apart bulkheads 34 are held in place at spaced apart intervals along the threaded rods 30 by suitable securement means 32. In the present case, the securement means 32 are nuts which are fitted to the rods 32 and tightened against a respective bulkhead 34 to hold the bulkhead 34 in place. Bulkheads 34 are shaped to fit inside of the bore 38 of the cylindrical tube 6 so that the bulkheads 34 reinforce the cylindrical tube 6 along its length to resist inward deflection of the cylindrical tube 6 when the vessel 10 is operating in an underwater environment. When in place, an aperture 36 in each bulkhead is located in coaxial alignment with the apertures of the other bulkheads 34 so that the bulkheads 34 can be positioned lengthwise along the tube 39 (ref. Figure 3). Furthermore, as shown in Figures 1 and 9, when structure 8 is located within the housing 12, at least a part of the void 21 extends externally about and along the tube 39.

[0041] The tube 39 shown here is a rigid tube manufactured from a material capable of withstanding pressure caused by operating in an underwater environment at a desired operating depth. One example of a suitable material is a composite material such as a fibre reinforced thermoset epoxy matrix. In the present case, the bore 38 of the tube 39 has a 50 mm bore diameter and a length of about 310 mm. However, it will be appreciated that different tube configurations may be used. As will be explained below, the bore diameter may be selected according to depth requirements.

[0042] As shown in Figures 2 and 4, a first end (being the top end in Figure 2) of the tube 39 has the opening 46 for receiving an arm 48 and the piston 16 within the tube 39. When so received, there is a clearance about the opening 46 and the arm 48 which allows fluid in the void 21 to flow into and out of the second chamber 26 of the tube 39 through the opening 46. The opposite end 42 of the tube 39 is sealably fitted to an aperture 40 formed in the end cap 18b to provide the opening 44 which allows external fluid, which in this example is the surrounding water, to enter and exit the first chamber 24 inside the tube 39 through the opening 44.

[0043] As shown Figure 9, the buoyancy control system 12 also comprises a power supply 58, processing module 54 and a memory 56 storing a set of program instructions, in the form of an executable program code, which is executable by the processing module 54 to provide commands to the actuator 50 to vary the volume of the first chamber 24 to thereby control fluid communication between surrounding water and the first chamber 24, via the opening 44, to vary a volume of the surrounding water contained in the first chamber 24.

[0044] The power supply 58 shown here is a rechargeable lithium polymer battery, such as a 12V, 5000mAh rechargeable lithium polymer battery. However, non-rechargeable primary lithium or alkaline packs may be also used.

[0045] The processing module 54 may comprise one or more processors including multi-core CPUs operatively connected to one or more memories 56 which store instructions to configure the processor to perform embodiments of the method. In this context, the processing module 54 may include, for example, one or more processors (CPUs, GPUs), memories, storage, and input / output devices (e.g., monitor, keyboard, disk drive, network interface, Internet connection, etc.). One example of a suitable processing module 54 is an Arduino Uno 8-bit microcontroller board incorporating an ATmega328P processor.

[0046] The memory 56 is operatively coupled to the processing module 54 and may comprise RAM and ROM components. The memory 56 may be used to store the operating system and additional software modules or instructions.

[0047] The processing module 54 may be configured to load and execute the software modules or instructions stored in the memory. A computer program may be written, for example, in a general- purpose programming language (e.g., Pascal, C, C++, Java, Python, JSON, etc.) or some specialized application-specific language, and may utilise or call software libraries or packages.

[0048] As is shown in Figure 10, a vessel 10 according to an embodiment may also include a communications module 68 for communicating with an equivalent communications module in another device using a predefined communications protocol (e.g. Bluetooth, Zigbee, IEEE 802.15, IEEE 802.11, TCP / IP, UDP, etc). Communications module 68 could include a suitable satellite communications module for communicating sensed data to a ground station or for receiving command and control signals when the underwater vessel 10 has surfaced. One example of a suitable satellite communications module is an Iridium® modem. In some embodiments, a graphical processing unit (GPU) may also be included for providing a signal to a display (not shown).

[0049] With reference again now to Figures 2 and 8, the volume of the first chamber 24 is varied by controlling the actuator 50 to drive the piston 16 towards or away from the opening 44. By driving the piston 16 towards opening 44, the volume of the first chamber 24 (and thus the volume of fluid contained in the first chamber 24) reduces as fluid is displaced from the first chamber 24. On the other hand, by driving the piston 16 away from the opening 44 the volume of first chamber 24 is increased (and thus the volume of fluid contained in the first chamber 24 is also increased.

[0050] In the illustrated configuration, the volume of the second chamber 26 co-operates with the volume of the void 21. As described above, the volume of the second chamber 26 varies in indirectproportion to the volume of the first chamber 24 as the piston 16 is governed to thereby vary the overall density of the underwater vessel 10 when in use.

[0051] Turning now to Figure 8, when submerged in surrounding water 60, the surrounding water 60 contained in the first chamber 24, and thus the overall density of the underwater vessel 10, may be controllably varied, causing the underwater vessel 10 to ascend or descend in the surrounding water 60 responsive to the density variation caused by the actuator 50 governing the piston 16.

[0052] An advantage of the depicted arrangement is that it avoids the use of a flexible reservoir, such as a bladder, containing a fluid which is transferred to or from the flexible reservoir to vary the buoyancy of a vessel.

[0053] Continuing now with reference to Figures 2 and 5, and as discussed above, the actuator 50 shown here is a linear electric actuator having a stroke length which governs the piston 16 to vary the volume of the first chamber 24 over a suitable range.

[0054] The linear actuator 50 should have a maximum load rating which is capable of applying sufficient force to the piston 16, when the underwater vessel 10 is operating at a desired depth in a surrounding water with the first chamber 24 containing a volume of the surrounding water, to displace the contained surrounding water from the first chamber 24 to allow the underwater vessel 10 to ascend.

[0055] That is to say, the linear actuator 50 needs to be capable of overcoming the pressure exerted on the piston 16 by the surrounding water 60, when operating at a desired depth, to increase buoyancy by displacing water contained in the first chamber 24 to allow the underwater vessel 10 to surface.

[0056] The required maximum load rating of the actuator 50 will thus depend on the desired operating depth of the underwater vessel 10 and consequently the desired pressure rating (P) associated with operating at the desired depth. For example, a 6000N linear actuator could support operation at a deeper depth than, for example, a 1000N linear actuator.

[0057] The desired pressure rating (P) may be determined by selecting an appropriate linear actuator with force (F) to determine the cross-sectional area (A) or piston diameter for the vessel 10 in accordance with the relationship:P= F / A

[0058] A total available buoyancy volume of the second chamber 26 may be determined by the stroke length (L) of the selected actuator 50 using the equation:V= A * L

[0059] In view of the above, having set a value for P based on a maximum operating depth requirement, and knowing F for a selected linear actuator the diameter of the piston may be determined according to:A = F / P

[0060] The determined value for A may be reduced by a factor to allow for overshoot.

[0061] Using the determined value for A, the maximum volume of the second chamber 26 may be determined as:L = V / A

[0062] For example, for a desired 250m rated vessel 10 having a second chamber 26 requiring a displacement of 300mL (with a reserve safety factor) and which uses a 3kN linear actuator, a maximum piston diameter of 35mm could be used, which would require a 260mm stroke length to achieve the desired displacement.

[0063] On the other hand, for a 1000m rated vessel having a second chamber 26 requiring a displacement of 500mL and which uses a lOkN actuator, a maximum piston diameter of 32mm could be used, which would require a 625mm stroke length to achieve the desired displacement.

[0064] In one embodiment, the actuator 50 is a 12VDC linear actuator having a 250mm stroke length with a 10-minute duty cycle, although it will be appreciated that other types of actuators may be used. One example of a suitable linear actuator is the 6000N 12" Linear Actuator DC 12V Electric Motor from Vevor.

[0065] In some embodiments, it is possible that the actuator 50 is an inline linear actuator having suitable performance characteristics. Because an inline linear actuator provides a more compact arrangement, the use of an inline linear actuator may allow for a reduction in the diameter of the housing 12 and the use of less material for the end caps 18a, 18b to house the same piston 16 and tube 39 arrangement since less space is required to accommodate an inline linear actuator. An additional advantage of such a configuration is that the underwater vessel 10 may be more suitable for deploying from an aircraft.

[0066] Figures 6 and 7 illustrate an example configuration of a piston 16 suitable for use with an embodiment of the underwater vessel 10. The piston 16 has a head 62 and a body 64. In the present case, the body 64 has a diameter of 50 mm and the piston 16 has an overall length of about 62 mm. The piston16 illustrated here is CNC machined from of POM (Polyoxymethylene). However, other materials may be used.

[0067] In embodiments, the diameter of the body 64 of piston 16, and thus the diameter of the bore 38 of the tube 39, is determined based on the maximum diameter the linear actuator 50 is capable of moving when subjected to a pressure which is equivalent to its maximum load rating. For example, a 6000N linear actuator may be able to move a 50 mm piston towards the opening 44 at depths up to 320 metres but may not be able to move the piston 16 towards the opening 44 at depths beyond 350 metres.

[0068] In the present case, there are three circumferentially extending channels 66a, 66b, 66c formed in the body 64 of the piston 16. Channels 66a, 66b are sized to receive an O-ring (not shown) for forming a seal with the inner surface of the bore 38 of the tube 39, whereas channel 66c is sized to receive a wiper seal (not shown) for maintaining a sealed contact with the bore 38 of the tube 39 when the piston 16 is stationary and in use. In the present case, the bore 38 of the tube 39 has an inner surface of evenly dispersed low friction additives to allow the O-rings and the wiper seal to slidably cooperate with the inner surface with reduced resistance.

[0069] An advantage of using a wiper seal is that the wiper seal may provide a wiping type action as the piston 16 moves in the tube 39, This wiping action may at least partially clean that part of the bore 38 forming the chambers 24, 26 of the tube 39 when the piston 16 is in use, thus reducing the likelihood of fouling which may otherwise interfere with operation of the underwater device 10.

[0070] With reference to Figure 9, operation of the underwater vessel 10 may involve the processing module 54 implementing an open loop control technique using internal timers of the processing module 54 to control the actuator 50 to cause the underwater vessel 10 to ascend and / or descend at preprogrammed intervals by varying the volume of the surrounding water contained in the first chamber 24. In such an embodiment, during ascending and descending, sensed values may be acquired from sensors 22 (such as temperature sensors 22a and pressure sensor 22b shown in Figure 1) by the processing module 54 and logged in memory 56.

[0071] In an alternative embodiment, operation of the underwater vessel 10 may involve the processing module 54 implementing a closed loop control technique in which the actuator 50 is controlled to cause the underwater vessel 10 to ascend and / or descend depending on sensed values acquired from sensors 22 (such as temperature sensors 22a and pressure sensor 22b).

[0072] In relation to configuring the vessel 10 for use, during configuration of one embodiment the piston 16 should first be fully extended to help guide it into the bore 38 of the tube 39. To avoid positive pressure accumulation in the housing 10, end cap 18a is removed to equalise the interior 20 toatmospheric pressure as the piston 16 is fully retracted. Once the piston 16 is fully retracted the end cap 18a is then replaced to seal the interior 20. Using this approach means that when the vessel 10 is operating in a surrounding water, the internal air pressure is at or below the initial atmospheric pressure when the piston 16 is fully retracted. Configuring the vessel 10 in this way avoids introducing a positive air pressure inside the interior of the hull which reduces the potential for end cap separation in use.

[0073] Embodiments of the present disclosure are expected to provide a relatively low-cost and robust system for controlling buoyancy of an underwater vessel.

[0074] It will be understood that the terms “comprise” and “include” and any of their derivatives (e.g. comprises, comprising, includes, including) as used in this specification, and the claims that follow, is to be taken to be inclusive of features to which the term refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

[0075] In some cases, a single embodiment may, for succinctness and / or to assist in understanding the scope of the disclosure, combine multiple features. It is to be understood that in such a case, these multiple features may be provided separately (in separate embodiments), or in any other suitable combination. Alternatively, where separate features are described in separate embodiments, these separate features may be combined into a single embodiment unless otherwise stated or implied. This also applies to the claims which can be recombined in any combination. That is a claim may be amended to include a feature defined in any other claim. Further a phrase referring to “at least one of’ a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[0076] Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software or instructions, middleware, platforms, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

[0077] In view of the above, it will be appreciated that the steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by the processing module 54, or in a combination of the two. For a hardwareimplementation, processing module 54 may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, or other electronic units designed to perform the functions described herein, or a combination thereof. Various middleware and computing platforms may be used.

[0078] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application or applications described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and / or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.

Claims

CLAIMS1. A buoyancy control system comprising: a housing sealably enclosing an interior; a bore located within the interior, the bore having a first chamber in fluid communication with an opening in the housing and a second chamber in fluid communication with a void in the interior; a piston fitted within the bore, the piston configured to sealably separate the first chamber from the second chamber; and an actuator operatively associated with the piston, the actuator configured to control the position of the piston along the bore to vary the volume of the first chamber and the volume of the second chamber in indirect proportion.

2. A buoyancy control system according to claim 1 wherein varying the volume of the first chamber regulates fluid communication into and out of the first chamber through the opening in the housing.

3. A buoyancy control system according to claim 1 or 2 wherein the bore has a first end which is contiguous with the opening in the housing and a second end having a second opening which allows fluid in the void to flow into and out of the second chamber through the second opening.

4. A buoyancy control system according to any one of claims 1 to 3 wherein the bore is an interior of a tube supported in the housing.

5. A buoyancy control system according to any one of claims 1 to 4 wherein the actuator is a linear actuator.

6. A buoyancy control system according to claim 5 wherein the linear actuator is a linear electric actuator.

7. A buoyancy control system according to any one of claims 1 to 6 wherein the piston is fitted with a wiper seal for maintaining a sealed contact with an interior of the bore when the piston is stationary and in motion, such that when the piston is in motion the wiper cleans the interior of the bore.

8. A buoyancy control system according to claim 4 wherein at least part of the void extends externally about and along the tube.

9. A buoyancy control system according to any one of claims 1 to 8 further including a ballast attachment means for attaching a ballast to the housing to set a mass of the vessel.

10. An underwater vessel capable of operating in a surrounding water, the vessel comprising: a buoyancy control system according to any one of claims 1 to 9; a power supply; a processing module; wherein the actuator is configured to receive commands from the processing module to control the position of the piston along the length of the bore, and wherein increasing the volume of the first chamber causes the first chamber to receive surrounding water through the opening to decrease the buoyancy of the vessel, and wherein decreasing the volume of the first chamber causes the first chamber to displace received surrounding water through the opening to increase the buoyancy of the vessel.

11. A method of controlling the buoyancy of an underwater vessel operating in a surrounding water, the method comprising: providing an underwater vessel incorporating a buoyancy control system according to any one of claims 1 to 9; governing the piston to controllably vary the volume of the first chamber, wherein increasing the volume of the first chamber causes the first chamber to receive surrounding water through the opening to decrease the buoyancy of the vessel, and wherein decreasing the volume of the first chamber causes the first chamber to displace received surrounding water through the opening to increase the buoyancy of the vessel.

12. A method according to claim 11 wherein the piston is governed using a timer which controls the actuator to vary the volume of the surrounding water contained in the first chamber at pre-determined intervals.

13. A method according to claim 11 or 12 wherein the piston is governed depending on sensed values acquired from a sensor payload of the vessel.

14. A method according to claim 11 or 12 wherein the piston is governed to vary the operational depth of the underwater vessel in the surrounding water.

15. A buoyancy control system according to claim 1 further comprising: a sensor payload comprising one or more sensors for sensing one or more parameters of a surrounding water; a processing module; a memory storing a set of program instructions in the form of an executable program code which is executable by the processing module to cause the processing module to acquire a sensed signal from the one or more sensors and operate the actuator to control the position of the piston so as to vary avolume of the surrounding water contained in the first chamber depending on the sensed signal acquired from the one or more sensors.

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