Abstract:
A buoyancy control system comprises a housing and first and second pistons movably supported by the housing. In a shallow mode, displacement of the first piston alters a buoyancy of the buoyancy control system. In a deep mode, displacement of the first and second pistons alters the buoyancy of the buoyancy control system.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of pending U.S. patent application Ser. No. 12/345,182, filed Dec. 28, 2008. 
     U.S. patent application Ser. No. 12/345,182 claims benefit of U.S. Provisional Application Ser. No. 61/009,364, filed on Dec. 26, 2007. The contents of the related applications listed in this paragraph 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 is 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 is 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 comprising a housing, a first piston, and a second piston. The first and second pistons are movably supported by the housing. In a shallow mode, displacement of the first piston alters a buoyancy of the buoyancy control system. In a deep mode, displacement of the first and second pistons alters the buoyancy of the buoyancy control system. 
     The present invention may also be embodied as a method of controlling buoyancy of a submersible object comprising the following steps. A housing is arranged within the submersible object. First and second pistons are movably supported relative to the housing. The first piston is displaced to alter a buoyancy of the submersible object in a first range of depths. The first and second pistons are displaced to alter a buoyancy of the submersible object in a second range of depths. 
     The present invention may also be embodied as a buoyancy control system for a submersible object comprising a housing and first second pistons. The housing is fixed relative to the submersible object. The first and second pistons are movably supported by the housing. In a shallow mode, displacement of the first piston alters a buoyancy of the buoyancy control system. In a deep mode, displacement of the first and second pistons alters the buoyancy of the buoyancy control system. 
     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 incorporating a first example 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 a first example buoyancy control system as mounted within the example glider of  FIG. 1 ; 
         FIG. 6  is a schematic block diagram illustrating an electrical control portion of the first example buoyancy control system; 
         FIG. 7  is a schematic side elevation, cross-sectional view of a second example buoyancy control system that may be used by the example glider of  FIG. 1  in place of the first example buoyancy control system described herein; 
         FIG. 8  is a side elevation, cross-sectional view of the second example buoyancy control system expelling ambient fluid in a shallow mode; 
         FIG. 9  is a side elevation, cross-sectional view of the second example buoyancy control system ingesting ambient fluid in the shallow mode; 
         FIG. 10  is a side elevation, cross-sectional view of the second example buoyancy control system expelling ambient fluid in a deep mode; 
         FIG. 11  is a side elevation, cross-sectional view of the second example buoyancy control system ingesting ambient fluid in the deep mode; 
         FIG. 12  is a side elevation, cross-sectional view of the second example buoyancy control system in a shallow mode; 
         FIG. 13  is a side elevation, cross-sectional view of the second example buoyancy control system in an intermediate mode; 
         FIG. 14  is a side elevation, cross-sectional view of the second example buoyancy control system in a deep mode. 
     
    
    
     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 operating fluid  66  into and withdraw operating 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 to 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 . 
     Referring now to  FIG. 7  of the drawing, depicted at  220  therein is a second example buoyancy control system of the present invention. Referring initially to  FIG. 7  of the drawing, it can be seen that the example buoyancy control system  220  comprises a piston assembly  222  comprises a piston housing  230 , a first piston  232 , and a second piston  234 ; the housing  230  and pistons  232  and  234  define a control chamber  240 , a first working chamber  242 , and a second working chamber  244 . The housing  230  and first piston  232  further define an ambient chamber  246 . 
     In a shallow mode as depicted in  FIGS. 8 and 9 , working fluid is introduced into the first working chamber  242  to displace the first piston  232  and thereby alter a volume of the control chamber  240 . In a deep mode as depicted in  FIGS. 10 and 11 , working fluid is introduced into both the first and the second working chambers  242  and  244  to displace the first and second pistons  232  and  234  to alter a volume of the control chamber  240 . In the deep mode, however, the second piston  234  occupies a portion of the control chamber and divides the control chamber  240  into a first portion  240   a  and a second portion  240   b.    
     By altering a volume of the control chamber  240  as generally described above, ambient fluid is drawn into or displaced from the ambient chamber  246 , and the buoyancy of the buoyancy control system  220  is altered. By altering the buoyancy of the buoyancy control system  220 , the buoyancy and attitude of a glider or other submersible object in which the buoyancy control system  220  is mounted may also be altered. Altering the buoyancy of a glider allows the glider to be controlled as generally described above with reference to  FIGS. 2-4 . 
     However, the buoyancy control system  220  is adaptable to allow the system  220  to operate more effectively at the different pressures associated with shallow and deep depths. In particular, the parameters of the buoyancy control system  220  are predetermined to provide optimal control within a first range of depths (e.g., 0-X feet) when operating in the shallow mode and also optimal control within a second range of depths (e.g., greater than X feet) in the deep mode. 
     In the deep mode, the use of both the first and the second pistons  232  and  234  increases the hydraulic pressure available to overcome the higher ambient pressures experience at greater depths. Additionally, the maximum volume of the control chamber  240  is effectively decreased in the deep mode, allowing finer control buoyancy changes at greater depths when the system  220  operates in the deep mode. 
     With the foregoing general understanding of the construction and operation of the second example buoyancy control system  220 , the details of the second example buoyancy control system  220  will now be described in further detail. 
     As shown in  FIG. 7 , the second example buoyancy control system  220  comprises a mechanical/hydraulic portion  250  and a control portion  252 . In addition to the piston assembly  222  generally described above, the example mechanical/hydraulic portion  250  comprises a pump assembly  260 , an accumulator assembly  262 , first and second valve assemblies  264  and  266 , first and second check valves  270  and  272 , and a filter  274 . The output of the pump assembly  260  is operatively connected through the first check valve  270  to the first working chamber  242  and through the filter  274  and first valve assembly  264  to the accumulator assembly  262 . The accumulator assembly  262  is operatively connected to the input of the pump assembly  260 . The output of the pump assembly  260  is also operatively connected through the first check valve  270  to the second working chamber through the second valve assembly  266 . The second check valve assembly  272  is connected in parallel with the second valve assembly  266 . 
       FIG. 7  further shows that the example control portion  252  comprises a controller  280 , first and second position sensors  282  and  284 , and a depth sensor  286 . The example controller  280  is electrically connected to the first and second valve assemblies  264  and  266 , the first and second position sensors  282  and  284 , and the depth sensor  286 . 
     The example controller  280  is a computer processor running software that causes the first and second valve assemblies  264  and  266  to open and close based on factors such as locations of the pistons  232  and  234  as detected by the position sensors  282  and  284  and the depth of the submersible object as detected by the depth sensor  286 . 
     In particular, depending upon the depth of the buoyancy control system  220  as detected by the depth sensor  286 , the controller  280  operates in either the shallow mode or the deep mode. The controller  280  places the second valve assembly  266  in its OFF configuration (prevents fluid flow) to allow the system  220  to operate in the shallow mode. In the shallow mode, placing the pump assembly  260  in its ON configuration and the first valve assembly  264  in its OFF configuration (prevents fluid flow) causes the buoyancy control system  220  to increase the volume of the control chamber  240  and expel ambient fluid by applying a force to extend the first piston  232  ( FIG. 8 ). When the system is in the shallow mode, placing the pump assembly  260  in its OFF configuration and the first valve assembly in its ON configuration causes the system  220  to decrease a volume of the control chamber  240  and intake ambient fluid. When the pump assembly  260  is OFF, the ambient fluid applies a force on the first piston  232  that causes the first piston  232  to retract ( FIG. 9 ). Placing the pump assembly  260  in its OFF configuration and the first valve assembly  264  in its OFF configuration allows a particular buoyancy configuration to be maintained. 
     Placing the second valve assembly  266  in its ON configuration allows the system  220  to operate in the deep mode. In the deep mode, placing the pump assembly  260  in its ON configuration and the first valve assembly  264  in its OFF configuration causes the buoyancy control system  220  to expel ambient fluid by applying a force to extend the first and second pistons  232  and  234  ( FIG. 10 ). When the system is in the deep mode, placing the pump assembly  260  in its OFF configuration and the first valve assembly in its ON configuration causes the system  220  to intake ambient fluid. Again, when the pump  260  is OFF, the ambient fluid applies a force on the first piston  232  that causes the first and second pistons  232  and  234  to retract ( FIG. 11 ). Again, placing the pump assembly  260  in its OFF configuration and the first valve assembly  264  in its OFF configuration allows a particular buoyancy configuration to be maintained. 
     The example housing  230  comprises a body  320  and a cap member  322 . A mounting flange  324  extends from the body  320  to facilitate connection of the housing  230  to the submersible device in which the buoyancy control system  220  is to be mounted. The body  320  is substantially symmetrical about a longitudinal axis A and defines a main cavity  330  comprising a first cylindrical portion  332 , a second cylindrical portion  334 , and an annular portion  336 . 
     The first piston  232  is an assembly comprising a plate member  340  and a rod member  342 . The plate member  340  resides in the first cavity portion  332 , while the rod member  342  resides partly within the first cavity portion  332  and partly within the second cavity portion  334 . The second piston  234  is or comprises an annular member  344  sized and dimensioned to fit within the annular cavity portion  336 . The housing body  320  further defines an ambient opening  350 . The cap member  324  defines a main port  352 , and the body  320  defines at least one secondary port  354 . 
     First and second seal members  360  and  362  are arranged between the plate member  340  and the body  320  to prevent fluid flow from the control chamber  240  and the ambient chamber  246 . A third seal member  364  is arranged between the rod member  342  and the body  320  to inhibit fluid flow between the first working chamber  242  and the control chamber  240 . A fourth seal member  366  is arranged between the annular member  344  and the body  320  to inhibit fluid flow between the second working chamber  244  and the first portion  240   a  of the control chamber  240 . A fifth seal member  370  is arranged between the plate member  340  and the rod member  342  to prevent fluid flow from the control chamber  240  to the ambient chamber  246 . Sixth and seventh seal members  372  and  374  are arranged between the body  320  and the cap  322  to prevent fluid flow from the first working chamber  242  to the exterior of the body  320 . 
     Referring now to  FIGS. 12-14 , a third example buoyancy control system  420  will now be described. The example buoyancy control system  420  is in many respects similar to the buoyancy control system  220  described above and will be described below only to the extent necessary for a complete understanding of the present invention. 
     The third example buoyancy control system  420  comprises a piston assembly  422  comprises a piston housing  430 , a first piston  432 , a second piston  434 , and a third piston  436 . The housing  430  and pistons  432 ,  434 , and  436  define a control chamber  440 , a first working chamber  442 , a second working chamber  444 , and a third working chamber  446 . The housing  430  and first piston  432  further define an ambient chamber  448 . 
     In a shallow mode as depicted in  FIG. 12 , working fluid is introduced into the first working chamber  442  to displace the first piston  432  and thereby alter a volume of the control chamber  440 . In an intermediate mode as depicted in  FIG. 13 , working fluid is introduced into the first and the second working chambers  442  and  444  to displace the first and second pistons  432  and  434  to alter a volume of the control chamber  440 . In a deep mode as depicted in  FIG. 14 , working fluid is introduced into the first, second, and third working chambers  442 ,  444 , and  446  to displace the first and second pistons  432  and  434  to alter a volume of the control chamber  440 . 
     By altering a volume of the control chamber  440  as generally described above, ambient fluid is drawn into or displaced from the ambient chamber  448 , and the buoyancy of the buoyancy control system  420  is altered. By altering the buoyancy of the buoyancy control system  420 , the buoyancy and attitude of a glider or other submersible object in which the buoyancy control system  420  is mounted may also be altered. Altering the buoyancy of a glider allows the glider to be controlled as generally described above with reference to  FIGS. 2-4 . 
     However, the buoyancy control system  420  is adaptable to allow the system  420  to operate more effectively at the different pressures associated with shallow, intermediate, and deep depths. In particular, in the intermediate mode, the second piston  434  occupies a portion of the control chamber  440 , while, in the deep mode, the second piston  434  and the third piston  434  occupy portions of the control chamber  440 . The parameters of the third buoyancy control system  420  may thus be predetermined to provide optimal control within a first range of depths (e.g., from 0 to X feet) when operating in the shallow mode, within a second range of depths (e.g., from X to Y feet) in the deep mode, and within a third range of depths (e.g., greater than Y feet) in the deep mode. 
     In the intermediate and deep modes, the use of the second and or third pistons  434  and  436  in addition to the first piston  432  increases the hydraulic to pressure available to overcome the ambient pressures experienced at different depths. Additionally, the maximum volume of the control chamber  440  is effectively decreased in the intermediate and deep modes, allowing finer control buoyancy changes at progressively greater depths when the system  420  operates in the intermediate and deep mode. 
     The third example buoyancy control system  420  comprises a mechanical/hydraulic portion  450  and a control portion (not shown). The control portion will be generally similar to the control portion  252  described above and will not be described in detail. 
     In addition to the piston assembly  422  generally described above, the example mechanical/hydraulic portion  450  comprises a pump assembly  460 , an accumulator assembly  462 , first, second, and third valves  464 ,  466 , and  468 , first, second, and third check valves  470 ,  472 , and  474  and a filter  476 . 
     The output of the pump assembly  460  is operatively connected through the first check valve  470  to the first working chamber  442  and through the filter  476  and first valve  464  to the accumulator assembly  462 . The accumulator assembly  462  is operatively connected to the input of the pump assembly  460 . The output of the pump assembly  460  is also operatively connected through the first check valve  470  to the second working chamber through the second valve  466 . The second check valve  472  is connected in parallel with the second valve  466 . The output of the pump assembly  460  is further operatively connected through the first check valve  470  to the third working chamber  446  through the third valve  468 . The third check valve  474  is connected in parallel with the third valve  468 . 
     The following table lists the status of the pump  460  and the first, second, and third valves  464 ,  466 , and  468  when the control portion controls the third example buoyancy control system  420  to intake and expel ambient fluid under the shallow, intermediate, and deep modes: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 First 
                 Second 
                 Third 
               
               
                 Mode 
                 Intake/Expel 
                 Pump 
                 Valve 
                 Valve 
                 Valve 
               
               
                   
               
             
             
               
                 shallow 
                 expel 
                 ON 
                 OFF 
                 OFF 
                 OFF 
               
               
                 shallow 
                 intake 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                 intermediate 
                 expel 
                 ON 
                 OFF 
                 ON 
                 OFF 
               
               
                 intermediate 
                 intake 
                 OFF 
                 ON 
                 ON 
                 OFF 
               
               
                 deep 
                 expel 
                 ON 
                 OFF 
                 ON 
                 ON 
               
               
                 deep 
                 intake 
                 OFF 
                 ON 
                 ON 
                 ON 
               
               
                   
               
             
          
         
       
     
     Given the foregoing, it should be apparent that the present invention may be embodied in forms other than those described above. For example, the present invention has been disclosed with one, two, and three pistons to operate in three modes, but additional pistons can be provided based on desired operating ranges and conditions to operate in more than three modes. The scope of the present invention should be determined by the claims appended hereto and not the following descriptions of examples of the invention. 
     Given the foregoing, it should be apparent that the present invention may be embodied in forms other than those described above. The scope of the present invention should be determined by the claims appended hereto and not the following descriptions of examples of the invention.