Abstract:
A buoyancy control system for a submersible object submerged in an ambient fluid, comprising a piston housing, a piston member, a pump, control fluid, and working fluid. The piston housing is supported by the submersible object. The piston member defines a piston portion and a shaft portion. The piston member is supported within the piston housing such that the piston portion and the piston housing define a control chamber and an ambient chamber and the shaft portion and the piston housing define a working chamber. The pump is operatively connected to the working chamber. The control fluid is arranged within the control chamber. At least a portion of the working fluid is arranged within the working chamber. Operation of the pump displaces working fluid within the working chamber to displace the piston member to alter a volume of the control chamber.

Description:
RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application Ser. No. 61/009,364, which was filed on Dec. 27, 2007. The contents of the related specification listed above are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to systems and methods for controlling the buoyancy of waterborne objects and, more specifically, to buoyancy control systems and methods for controlling the buoyancy of devices and vehicles that are capable of being submersed. 
     BACKGROUND OF THE INVENTION 
     The ability to control the buoyancy of an object is desirable in many applications. For example, in the field of unmanned underwater vehicles (UUVs), it is often desirable to adjust the buoyancy of the vehicle to stabilize it in the water column (hover) or to make the vehicle rise or sink within the column. 
     Accordingly, many waterborne objects are provided with a buoyancy control mechanism, or “buoyancy engine”, that allows active control of the buoyancy of the object. Active buoyancy control allows the buoyancy of an object to be adjusted as necessary for a desired maneuver or to accommodate unknown or changing environmental conditions. For example, the buoyancy of the object may be adjusted to bring a submerged object to the surface so that it can communicate via radio, then return the object to a submerged condition. As another example, the buoyancy of an object might be adjusted to accommodate variations in density of the surrounding water due to changes in temperature and/or salinity. 
     The present application is generally applicable to any type of waterborne object for which buoyancy control is desirable. Examples of waterborne objects that employ or may employ buoyancy control include: floats, buoys, weaponry (torpedoes), and manned and unmanned powered submarines. The present invention is, however, of particular significance when applied to the class of UUV&#39;s referred to as “gliders”. A glider is propelled through the water completely by changes to the buoyancy of the vehicle. The present invention will be described in detail below in the context of a glider. 
     For UUVs that are powered by batteries or other fixed energy storage mechanisms, one design goal is to optimize the energy efficiency of all onboard systems. The buoyancy control engine can be a major consumer of stored energy, so an effective buoyancy control engine should be energy efficient. The buoyancy engine should also be reliable, low weight, and easily maintainable. 
     Conventional gliders have a buoyancy engine that effectively changes the volume of the glider. One class of conventional gliders (e.g., the “Seaglider” produced by the University of Washington and the “Spray” produced by Bluefin Robotics) uses hydraulic pumps to transfer hydraulic fluid from an internal bladder to an external bladder. Yet another class of gliders (e.g., the “Slocum Thermal” produced by Webb Research) harvests the thermal energy of the ocean to move a transfer fluid between an internal bladder and an external bladder. The buoyancy engines employed by these gliders will be referred to as “internal bladder/external bladder” buoyancy control engines. 
     Another class of gliders (e.g., the “Slocum Electric” produced by Webb Research) uses a motor to drive a ball screw. The ball screw in turn drives a piston inside a rolling diaphragm. The diaphragm/piston combination displaces water when extended and ingests water when retracted. This type of buoyancy engine will be referred to as “ball screw/piston” type buoyancy control engines. 
     A related class of UUVs includes floats or buoys (e.g., The “ALACE” (Autonomous Lagrangian Circulation Explorer) floats). In the case of floats or buoys, the purpose of the buoyancy control system is typically to maintain neutral buoyancy for a period of time at a predetermined depth and then adjust the buoyancy to cause the vessel to surface and communicate data. After the communication process is completed, the buoyancy of the vessel is again adjusted to cause the float or buoy to descend and then become neutrally buoyant at the predetermined depth. Such floats or buoys also use an “internal bladder/external bladder” configuration to control buoyancy. 
     One problem with the “internal bladder/external bladder” class of buoyancy engine is that a large amount of fluid is required to adjust the buoyancy of the device. Because the fluid is transferred into a bladder that directly displaces the water, there is a one to one ratio between required fluid and potential displacement (i.e. one liter of fluid is required to displace one liter of water). The ratio of required fluid to potential displacement limits the net buoyancy of the vehicle. In the context of gliders, this limitation on net buoyancy limits the speed of the glider and also the ability of the glider to adjust its buoyancy in response to changes in salinity and temperature. 
     Another disadvantage of the “internal bladder/external bladder” buoyancy engine is that the hydraulic pumps used in these designs are typically optimized for maximum efficiency at a significantly higher pressure than the operational pressure of the device. In particular, the hydraulic pump does not operate at maximum efficiency at the maximum operational depth of the vessel, and the hydraulic pump is even less efficient at shallower depths. 
     For example, the “Seaglider” glider developed by the University of Washington employs the Hydro LeDuc model PB32.5 pump. This pump has a maximum total efficiency (combined mechanical and volumetric efficiency) that peaks at approximately 34 MPa (˜5000 psi), while the pressure at the Seaglider&#39;s maximum operational depth of approximately 1,000 m yields a pressure of approximately 10 MPa (˜1500 psi). The efficiency of the buoyancy engine of the “Seaglider” glider is less than 15% at 200 m operation and only 40% at 1000 m operation. 
     The “ball screw/piston” type of buoyancy engine similarly suffers from low efficiency. Small DC motors are typically designed to run at high speeds (e.g. 5,000-10,000 rpm). While these motors can be highly efficient (typically 80-90%) at these relatively high operational speeds, the speed of such motors needs to be significantly reduced to drive a ball screw assembly of a “ball screw/piston” type buoyancy engine. A reduction gear is thus typically used to reduce the speed of the motor; a reduction gear is usually about 70% efficient, giving a combined efficiency in the range of 56-63%. In addition, the ball screw assembly itself typically operates at only about 95% efficiency, thereby reducing the maximum potential efficiency of this system to a range of 50-60%. The “Slocum Electric” device produced by Webb Research, which uses a ball screw/piston type buoyancy engine, has a published buoyancy engine efficiency of about 50%, which is at the low end of the theoretical range of efficiencies for the “ball screw/piston” type of buoyancy engine. 
     It is therefore an object of the current invention to provide buoyancy control systems and methods for a submersible vessel having improved efficiency over the entire operational depth range of the vessel. An additional object of the current invention is to provide buoyancy control systems and methods that are reliable and easy to manufacture and maintain. 
     SUMMARY 
     The present invention may be embodied as a buoyancy control system for a submersible object submerged in an ambient fluid, comprising a piston housing, a piston member, a pump, control fluid, and working fluid. The piston housing is supported by the submersible object. The piston member defines a piston portion and a shaft portion. The piston member is supported within the piston housing such that the piston portion and the piston housing define a control chamber and an ambient chamber and the shaft portion and the piston housing define a working chamber. The pump is operatively connected to the working chamber. The control fluid is arranged within the control chamber. At least a portion of the working fluid is arranged within the working chamber. Operation of the pump displaces working fluid within the working chamber to displace the piston member to alter a volume of the control chamber. 
     The present invention may also be embodied as a method of controlling the buoyancy of a submersible object submerged in an ambient fluid comprising the following steps. A piston housing is supported with respect to the submersible object. A piston member defining a piston portion and a shaft portion is supported within the piston housing such that the piston portion and the piston housing define a control chamber and an ambient chamber and the shaft portion and the piston housing define a working chamber. A pump is operatively connected to the working chamber. Control fluid is arranged within the control chamber. At least a portion of a working fluid is arranged within the working chamber. The pump is operated to displace working fluid within the working chamber, thereby displacing the piston member to alter a volume of the control chamber. 
     The present invention may also be embodied as a buoyancy controlled object to be submerged in an ambient fluid. In this form, the invention may comprise a hull assembly; a piston housing rigidly connected to the hull assembly; a piston member, a pump, an accumulator, a valve, control fluid, and working fluid. The piston member defines a piston portion and a shaft portion. The piston member is supported within the piston housing such that the piston portion and the piston housing define a control chamber and an ambient chamber, where the hull allows ambient fluid to enter and exit the ambient chamber, and the shaft portion and the piston housing define a working chamber. The pump is operatively connected to the working chamber. The accumulator is operatively connected to the working chamber. The valve is also operatively connected to the working chamber. The control fluid is arranged within the control chamber. At least a portion of the working fluid is arranged within the working chamber and at least a portion of the working fluid is arranged in the accumulator. Operation of the pump displaces working fluid within the working chamber to displace the piston member to alter a volume of the control chamber. Operation of the valve controls the flow of fluid into and out of the working chamber. 
     The present invention may be embodied in other configurations as will become apparent from the following discussion of examples of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of an example glider that may incorporate a buoyancy control system of the present invention; 
         FIGS. 2-4  are side elevation, partial schematic views illustrating the operation of the example glider of  FIG. 1 ; 
         FIG. 5  is a somewhat schematic view side elevation, cross-sectional view depicting details of the buoyancy control system of the example glider of  FIG. 1 ; and 
         FIG. 6  is a schematic block diagram illustrating an electrical portion of the example buoyancy control system of the glider depicted in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 1  of the drawing, depicted therein is an example waterborne vessel in the form of a glider  20 . The example glider  20  is generally conventional in that it comprises a hull assembly  22  and one or more fins and/or wings  24 .  FIGS. 2-3  illustrate that the example glider  20  further comprises a buoyancy control system  30  arranged within the hull assembly  22 . 
     The buoyancy control system  30  is depicted in further detail in  FIGS. 5-7  of the drawing. In particular,  FIG. 5  illustrates details of a mechanical portion  32  of the buoyancy control system  30 , while  FIG. 6  schematically illustrates both the mechanical portion  32  and a control portion  34  of the buoyancy control system  30 . 
     Referring initially to  FIG. 6  of the drawing, it can be seen that the example mechanical portion  32  comprises a piston assembly  40 , a pump assembly  42 , an accumulator assembly  44 , a valve assembly  46 , and a filter  48 .  FIG. 6  further shows that the example control portion  34  comprises a controller  50 , a position sensor  52 , and a depth sensor  54 . The piston assembly  40  defines a control chamber  60  containing a control fluid  62  and a working chamber  64  comprising a working fluid  66 . The control fluid  62  is compressible, while the working fluid  66  is incompressible. 
     In use, the controller  50  operates the pump assembly  42  and the valve assembly  46  to introduce the working fluid  66  into and withdraw working fluid  66  from the working chamber  64  to change a configuration of the piston assembly  40 . In particular, the controller  50  controls the pump assembly  42  and the valve assembly  46  to cause working fluid to flow into and out of the working chamber  64 . As the working fluid flows into and out of the working chamber  64 , the configuration of the piston assembly  40  is changed. 
     As the configuration of the piston assembly  40  changes, the volume of the control chamber  60  changes. Increasing the volume of the control chamber  60  increases the buoyancy of the buoyancy control system  30 . Decreasing the volume of the control chamber  60  decreases the buoyancy of the buoyancy control system  30 . Accordingly, as the configuration of the piston assembly  40  changes, the buoyancy of the buoyancy control system  30  changes. 
     Referring for a moment back to  FIGS. 2-4 , it can be seen that, as the buoyancy of the buoyancy control system  30  changes, the attitude of the glider  20  changes. Ideally, the buoyancy of the glider  20  (without the buoyancy control system  30  or with the buoyancy control system  30  in a neutral configuration) is substantially constant, at or near neutral, and distributed evenly so that the attitude of the glider  20  is substantially horizontal. Accordingly, when the buoyancy of the buoyancy control system  30  is substantially neutral, the attitude of the glider  20  is substantially horizontal ( FIG. 2 ). When the buoyancy of the buoyancy control system  30  is positive, the axis of the glider  20  is upwardly canted ( FIG. 3 ). And when the buoyancy of the buoyancy control system  30  is negative, the axis of the glider  20  is downwardly canted ( FIG. 3 ). 
     The buoyancy control system  30  thus allows the example glider  20  to be maneuvered through the water in the manner of a conventional glider. The buoyancy control system  30  may be used to control the buoyancy of any vessel that is designed to function underwater, whether designed to move without propulsion (e.g., a glider), designed to move with propulsion (e.g., a torpedo), or designed to move up and down within a substantially static water column (e.g., a float or buoy). 
     With the foregoing general understanding of the principles of the present invention in mind, the construction and operation of the example buoyancy control system  30  will now be described in further detail. 
     As shown in both  FIGS. 5 and 6  of the drawing, the example piston assembly  40  comprises a piston housing  70  and a piston member  72 . The piston member  72  comprises a piston portion  74  and a shaft portion  76 . The piston member  72  is arranged within the piston housing  70  to define the control chamber  60  and the working chamber  64 . 
     In particular, the piston housing  70  defines a low pressure cavity  80  and a high pressure cavity  82 . Further, a first seal member  84  is mounted on the piston portion  74  of the piston member  72 , and a second seal member  86  is mounted on the piston housing  70 . The piston portion  74  thus divides the low pressure cavity  80  into an ambient chamber  88  and the control chamber  60 . The shaft portion  76  lies within the high pressure cavity  82 , and the portion of the high pressure cavity  82  not occupied by the shaft portion  76  is the working chamber  64 . The first seal member  84  prevents fluid flow between the control chamber  60  and the ambient chamber  88 , while the second seal member  86  prevents fluid flow between the control chamber  60  and the working chamber  64 . 
     The piston portion  74  of the piston member defines a control surface  90  and an ambient surface  92 . The shaft portion  76  of the piston member  72  defines a working surface  94 . When the working fluid  66  is forced into the working chamber  64 , the working fluid  66  acts on the working surface  94  to displace the shaft portion  76  in a first direction. The shaft portion  76  is connected to the piston portion  74  such that, as the shaft portion  76  moves in the first direction, the piston portion  74  also moves in the first direction. As the piston portion  74  moves in the first direction, the volume of the control chamber  60  increases. 
     When the working fluid  66  is forced out of the working chamber  64 , the working fluid  66  acts on the working surface  94  to displace the shaft portion  76  in a second direction opposite the first direction. Because the shaft portion  76  is connected to the piston portion  74 , as the shaft portion  76  moves in the second direction, the piston portion  74  also moves in the second direction. As the piston portion  74  moves in the second direction, the volume of the control chamber  60  decreases. 
     When the volume of the working fluid  66  in the working chamber  64  is held constant, the shaft portion  76  does not move. Because the shaft portion  76  is connected to the piston portion  74 , if the shaft portion  76  does not move, the piston portion  74  also does not move. When the piston portion  74  is not moving, the volume of the control chamber  60  does not change. 
     Accordingly, by forcing working fluid  66  into the working chamber  64 , forcing working fluid  66  out of the working chamber  64 , and preventing the working fluid  66  from flowing into our out of the working chamber  64 , the volume of the control chamber  60  can be increased, decreased, or held constant. Controlling the volume of the control chamber  60  thus allows the buoyancy of the buoyancy control system  30  to be increased, decreased, or held constant. 
     Referring for a moment back to  FIG. 5 , it can be seen that holes  96  are formed in the glider hull assembly  22  to allow water to flow into and out of the ambient chamber  88 . The ambient chamber  88  is thus in fluid communication with the water surrounding the glider  20 . Accordingly, when the volume of the control chamber  60  increases, water is expelled from the glider  20 . Conversely, when the volume of the control chamber  60  decreases, water is drawn into the glider  20 . 
     The example controller  50  shown in  FIG. 6  generates a pump control signal for turning the pump assembly  42  on or off and a valve control signal for placing the valve assembly  46  in a closed configuration or an open configuration. By operating the pump assembly  42  with the valve assembly  46  in the closed configuration, the working fluid  66  is forced into the working chamber  64  to displace the piston member  72  in the first direction. By turning off the pump assembly  42  with the valve assembly  46  in the closed configuration, the volume of working fluid  66  within the working chamber  64  is held constant. 
     Accordingly, when the pump assembly  42  is off and the valve assembly  46  is in the open configuration, pressure on the ambient surface  92  of the piston portion  74  forces working fluid out of the working chamber  64  and into the accumulator assembly  44 . 
     Referring now more specifically to  FIG. 5 , it can be seen that the piston housing  70  comprises a bulkhead portion  120 , a low pressure portion  122 , and a high pressure portion  124 . 
     The example bulkhead portion  120  defines an annular surface  130  defining a stop flange  132  and a seal groove  134  that receives a seal member  136 .  FIG. 5  also shows that the hull assembly  22  of the glider  20  comprises a main portion  140  and a nose cone portion  142 . The main portion  140  is attached to the annular surface  130  to rigidly connect the main portion  140  to the bulkhead portion  120 . The seal member  136  forms a fluid tight seal at the juncture of the bulkhead portion  120  and the main portion  140 . The nose cone portion  142  is also attached to the annular surface  130  to rigidly connect the nose cone portion  142  to the bulkhead portion  120 . 
     The example low pressure portion  122  and high pressure portion  124  extend from the bulkhead portion  120  and define the low pressure cavity  80  and high pressure cavity  82 , respectively. The example low pressure cavity  80  is defined by a cylindrical inner surface  150  of the low pressure portion  122 , while the example high pressure cavity  82  is defined by a cylindrical inner surface  152  of the high pressure portion  124 . 
     The example controller  50  shown in  FIG. 6  is or may be a general purpose computing device running a software program. While the functions of the controller  50  can be implemented using dedicated electronics, the use of a general purpose computing device running a software program facilitates the changing of the logic carried out by the control system  34 . 
     As shown in  FIG. 6 , the controller  50  generates the pump control signal and the valve control signal based on one or more inputs. The controller  50  may function solely based on logic embodied in the software program, may function in response to external commands received through a communications system, or may function based on a combination of software program logic and external commands. The example system  30  operates based on a position sensor signal generated by the position sensor  52  and a depth signal generated by the depth sensor  54 . Alternative inputs include an attitude signal generated by an attitude sensor, a salinity signal generated by a salinity sensor, and a temperature signal generated by a thermometer. 
     The example accumulator assembly  44  will now be described in further detail with reference to  FIG. 5 . The accumulator assembly  44  comprises an accumulator housing assembly  160  and a pressure bag  162 . The accumulator housing assembly  160  comprises a main portion  164  and a cap portion  166 . A port  168  formed in the cap portion  166  is operatively connected to the pump assembly  42  and the valve assembly  46  as generally described above. 
     With the pump assembly  42  and the valve assembly  46  in a first set of configurations, pressurized working fluid  66  flows into the housing assembly  160  through the port  168  to collapse the pressure bag  162 . The pressure bag  162  thus allows working fluid  66  to flow into the accumulator  44  under pressure. The stored working fluid  66  is pressurized such that the working fluid  66  is forced out of the accumulator  44  when the pump assembly  42  and the valve assembly  46  are in a second set of configurations. 
     The accumulator  44  thus functions to store working fluid  66  under pressure for use by the buoyancy control system  30  as described above. The construction and operation of the example accumulator  44  is appropriate for use by the buoyancy control system  30 , but any accumulator that functions in a similar manner may be used by a buoyancy control system of the present invention. 
       FIG. 5  further illustrates that the example second seal member  86  is mounted on or within the piston housing  70  by a seal retaining member  170 . The second seal member  86  and the seal retaining member are disk-shaped members through which the shaft portion  76  of the piston member  72  extends. The example second seal member  86  helps to support the piston member  72  for movement as shown in  FIGS. 2-4 , establishes a fluid tight seal between the control chamber  60  and the working chamber  64 , and allows easy assembly and maintenance of the piston assembly  40 .