Patent Publication Number: US-2013239870-A1

Title: Underwater Vehicle Bouyancy System

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/038,373, filed Mar. 1, 2011, which claims the benefit of and priority from U.S. Provisional Patent Application Ser. No. 61/309,420, filed Mar. 1, 2010, and which is a continuation-in-part of U.S. patent application Ser. No. 12/890,584, filed Sep. 24, 2010, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The present teachings relate to a multiple stage buoyancy changing system for underwater vehicles. 
     BACKGROUND 
     Autonomous underwater vehicles that are propelled by changes in buoyancy have become commercial in recent years and demonstrated the ability to operate at sea for long periods. Such vehicles, known in the trade as underwater gliders, are in an early stage of deployment for oceanic research, coastline monitoring, and other applications. While such vehicles have shown viability for many desirable applications/missions, the existing designs are specialized to performing in limited ranges of depths that are optimized to the design of their “buoyancy engine” or buoyancy system. As a result, existing designs are typically optimized for shallow water (e.g., less than 200 meters), deep water (e.g., 200 meters to 1000 meters), or very deep water (e.g., 1000 to 6000 meters). This limits the operation of existing underwater gliders to a specific domain of underwater depth profiles that any specific vehicle can traverse. 
     Underwater gliders can work, for example, as described in U.S. Pat. No. 3,157,145 to Farris et al., the entire disclosure of which is incorporated herein by reference. A glider can comprise a main body, wings, and an adjustable portion such as an external bladder for changing the apparent displacement of the glider. The external bladder can initially be filled with a fluid such as oil to maximize the buoyancy of the glider when the glider is initially launched in the water. A valve can initially be set in a closed position to prohibit the fluid in the bladder from leaving the bladder. To begin the glider&#39;s descent, the valve can be opened, allowing fluid to escape the bladder (for storage in, for example, an internal storage reservoir). As fluid leaves the bladder, the apparent displacement of the glider decreases while the glider&#39;s mass stays the same, causing the glider to begin its descent into the water. 
     When the vehicle has reached the deepest point of its desired path, a pump system is used to move fluid from the internal reservoir back out to the external reservoir. As the glider descends, the wings of the glider cause it to move forward. Similarly, the wings cause the glider to move forward as it ascends through the water. To move forward, the glider must typically be ascending or descending in the water. The glider moves forward through its intended path by changing its buoyancy to move up and down through the water, propelling it forward. Because more vertical movement is possible in deeper waters, a greater horizontal distance can be traversed by a glider for a single descent and ascent in deeper waters. Thus, it may be possible to traverse 10 kilometers horizontally in a single dive in deeper water, whereas 10-20 dives can be required to traverse 10 kilometers in shallower water. If the same pump is used in both shallow and deep water, the 10-20 dives can use far more energy (e.g., pumping fluid into the bladder to cause the glider to ascend) than the single dive in deep water. Thus, smaller and more efficient devices such as pistons moving fluid in and out of the external bladder are typically used for gliders used in shallow water. 
     SUMMARY 
     The present teachings provide a multi-stage buoyancy changing system for an autonomous underwater vehicle comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a first segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of a second segment of the dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle. The first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile. 
     The present teachings also provide a method for employing a multi-stage buoyancy changing system for an autonomous underwater vehicle having an internal reservoir connected to an external bladder via one or more channels. The method comprises: in a first segment of a dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a first device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the first segment of the dive profile; and in a second segment of the dive profile, increasing an apparent displacement and buoyancy of the autonomous underwater vehicle by moving water from the internal reservoir to the external bladder using a second device configured to move fluid through a channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of the second segment of the dive profile. The first segment of the dive profile includes a different ambient pressure range than the second segment of the dive profile. 
     The present teachings further provide a multi-stage buoyancy changing system  400  for an autonomous underwater vehicle comprising: an internal reservoir configured to hold a fluid; an external bladder connected to the internal reservoir via one or more channels and configured to exchange fluid with the internal reservoir via the one or more channels; a pump motor in combination with a continuous variable transmission that can adapt to a torque-speed curve to obtain an optimal pressure/pumping rate needed for a current ambient pressure of the autonomous underwater vehicle, the pump motor and continuous variable transmission being configured to move fluid through a first channel from the internal reservoir to the external bladder at an optimized efficiency for an ambient pressure of more than one segment of a dive profile to increase an apparent displacement and a buoyancy of the autonomous underwater vehicle; and a third channel configured to allow fluid to move from the external reservoir to the internal reservoir, the third channel comprising a solenoid valve that can be selectively opened to allow water to pass from the external bladder to the internal reservoir. 
     Additional objects and advantages of the present teachings will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present teachings. The objects and advantages of the teachings will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     The present teachings provide a multiple stage buoyancy changing system, or variable buoyancy device, for an autonomous underwater vehicle. The system or device comprises: a pressure hull; a flexible sized internal reservoir configured to hold a fluid; and a flexible sized external reservoir, or bladder, connected to the internal reservoir via one or more channels, each channel having multiple valves and pumps. The one or more channels are configured to exchange fluid between the reservoirs as variable buoyancy device stages which are specifically optimized for energy efficiency at a range of ambient external pressures for multiple segments of a dive profile. 
     The first segment of the dive profile can be handled by a first stage of the variable buoyancy device, the second segment of the dive profile can be handled by the second stage of the variable buoyancy device, et cetera, up to an Nth stage corresponding to a maximum depth or pressure to which the vehicle is designed to dive. 
     The present teachings also provide a method for controlling a multiple stage buoyancy changing system, or variable buoyancy device, for an autonomous underwater vehicle having an external pressure sensor and a volume measurement system for either, or both, of an internal reservoir and an external reservoir. The external pressure sensor and the volume measurement system are connected to an internal processor and electronics that control the valves and pumps of the variable buoyancy device. The present teachings include logic for selecting which stage of the variable buoyancy device is used at any given depth. 
     The present teachings further provide a multiple stage variable buoyancy device for an autonomous underwater vehicle that includes a pump and motor combination with a continuous variable transmission that can electronically adapt to a torque-speed curve to rapidly obtain an optimal pumping rate for changing buoyancy. 
     Additional objects and advantages of the present teachings will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present teachings. The objects and advantages of the teachings will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and, together with the description, serve to explain the principles of the teachings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary path of an autonomous underwater vehicle such as a glider descending and ascending through multiple depth ranges. 
         FIG. 2  schematically illustrates an exemplary embodiment of a multiple stage fluid pump and valve system for an underwater vehicle in accordance with the present teachings, the system being optimized for one to “N” stages to be energy efficient at all ranges of pressure (or depth) underwater. 
         FIG. 3  illustrates an exemplary embodiment of a decision scheme used by an electronic control system for a multiple stage variable buoyancy device (VBD), wherein each stage is designed to pump at a particular output pressure range for maximum energy efficiency throughout an underwater dive profile. The illustrated decision scheme uses information from an external pressure sensor and an internal volume sensor to know if a target volume has been achieved. Other system of the underwater vehicle can determine whether a change in vehicle volume is needed. 
         FIG. 4  is a flow chart outlining the basic steps of an exemplary algorithm for implementing a multi-stage system to achieve efficiency at various depth profiles. 
         FIG. 5  is a schematic diagram illustrating an exemplary embodiment of a hydraulic multi-stage buoyancy system in accordance with the present teachings. 
         FIG. 6  is a schematic diagram illustrating another exemplary embodiment of a multi-stage buoyancy system in accordance with the present teachings. 
         FIG. 7  is an exemplary embodiment of an energy storage system onboard an autonomous underwater vehicle for powering fluid displacement mechanisms. 
         FIG. 8  is an exemplary embodiment of an energy storage system onboard an autonomous underwater vehicle for powering fluid displacement mechanisms. 
         FIG. 9  is a cross section of an exemplary embodiment of an energy storage system onboard an autonomous underwater vehicle for powering fluid displacement mechanisms. 
         FIG. 10A  is a top view of an exemplary embodiment of the autonomous underwater vehicle of the present invention. 
         FIG. 10B  is a perspective side view of the autonomous underwater vehicle of  FIG. 10A . 
         FIG. 11A  illustrates an autonomous underwater vehicle descending into a deep depth range of a universal glider range. 
         FIG. 11B  illustrates an exemplary dive profile having three distinct depth ranges for which multiple pump stages are utilized during ascent. 
         FIG. 12  is a chart illustrating pressure versus depth underwater. 
         FIG. 13  illustrates an exemplary underwater dive profile with time represented on the horizontal axis and depth represented on the vertical axis. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. In the drawings, the relative sizes of regions or features may be exaggerated for clarity. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     It will be understood that when an element is referred to as being “coupled” or “connected” to another element, it can be directly coupled or connected to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly coupled” or “directly connected” to another element, there are no intervening elements present. Like numbers refer to like elements throughout. 
     In addition, spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the expression “and/or” includes any and all combinations of one or more of the associated listed items. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     It is noted that any one or more aspects or features described with respect to one embodiment may be incorporated in a different embodiment although not specifically described relative thereto. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination. Applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to be able to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. These and other objects and/or aspects of the present invention are explained in detail in the specification set forth below. 
     Reference will now be made in detail to embodiments of the present teachings, examples of which are illustrated in the accompanying drawings. 
     In many implementations, an autonomous underwater vehicle (hereafter interchangeably called “AUV”)  100  uses most (e.g., about 75%) of its energy to pump fluid (e.g., hydraulic fluid, water, seawater, or other non-compressible fluids or fluids having low compressibility) into an external bladder  405  from an internal storage reservoir to increase the AUV&#39;s  100  apparent displacement and buoyancy to cause the AUV  100  to ascend to move forward and/or to reach the surface of the water for data receipt and transmission. The amount of pressure required to pump fluid into the external bladder typically varies by depth. For example, in shallow water (e.g., less than about 200 meters) the required pressure can have a magnitude of hundreds of psi, whereas in deep water (e.g., about 200 meters to about 1000 meters) the required pressure can have a magnitude of thousands of psi. 
     This difference in pressures required to pump fluid into the autonomous underwater vehicle&#39;s external bladder has created a design dilemma, because existing pumps that are powerful enough to create enough pressure to pump fluid into the external bladder in deeper water with high ambient pressure are typically inefficient for use in shallower waters with low ambient pressures and in certain dive profiles where the pump must cause the autonomous underwater vehicle to ascend more frequently to cover a given horizontal distance. The pressure that must be generated by existing deep water glider pumps makes those pumps less energy efficient. Low pressure pumps are more energy efficient but typically do not provide sufficient pumping force in deeper waters. This design dilemma causes existing autonomous underwater vehicles to be optimized for a limited range of depths. 
     As stated above, this pump design dilemma is imposed by the existence of increasing hydrostatic pressure with increasing depth, which is illustrated in  FIG. 12 . For example, as shown in  FIG. 11A , a system required to compensate for the hydrostatic pressures that are encountered in water that is typically considered shallow  200 , for example from the surface to a depth of 100 meters, must overcome ambient pressures ranging from 1 atmosphere (14.7 psi) to about 14 atmospheres (200 psi). By comparison, the pressure change that must be overcome by a deep diving vehicle (i.e. an AUV  100  deployed in a deep AUV range  250 ) can range from a surface pressure of about 1 atmosphere (14.7 psi) to nearly about 102 atmospheres (1500 psi).  FIG. 12  is a chart illustrating pressure versus depth underwater, where the vertical axis represents hydrostatic pressure in pounds per square inch (psi) and the horizontal axis represents depth in meters. A single pumping system that can overcome 1500 psi in the deep AUV range  250  will not be as energy efficient for pumping lower psi that occurs in shallower depths  200 . In contrast, a buoyancy system designed to handle a smaller range of pressure compensation will use significantly less energy to do so. Thus, as stated above, existing autonomous underwater vehicles are offered to be efficient in, generally, one of four ranges: 0 to 30 meters; 10 to 100 meters; 40 to 200 meters; and 200 to 1000 meters. 
     A pump capable of producing enough pressure to move fluid into an autonomous underwater vehicle&#39;s external bladder in deep water  250  is far less efficient than a pump that is capable of producing enough pressure to move fluid into the external bladder in shallow water  200 . A deep water pump can use, for example, nine times more energy than a shallow water pump. An example of a commercial pump used in deep diving gliders is the Hydro LeDuc model PB32.5 which can pump against 100 atmospheres and requires 14 ft lbs (20 nm) of energy to drive. By comparison a hydraulic pump, such as the MicroPump GB models, requires about 1.25 ft lbs energy to pump against the pressure at 100 meters, about 11 atmospheres. When the Hydro LeDuc is used to pump against the lower pressure (e.g., 11 atmospheres), it uses nearly the same amount of energy as it does when pumping against 100 atmospheres. 
     Turning to  FIG. 10A , the present teachings provide a universal or increased depth range autonomous underwater vehicle  100  comprising a multi-stage buoyancy system  400  and a control system  105  that can plan the travel of an autonomous underwater vehicle using a depth profile plan (See  FIG. 1 ) and depth sensors  110  (e.g., one or more pressure sensors and/or one or more acoustic altimeters). In one embodiment, the AUV  100  may determine a range and heading by, for example, an acoustic modem  107  USBL message to determine which portions of the multi-stage buoyancy system to use to achieve the profile plan while utilizing the least amount of onboard stored energy. By sensing the depth and/or the position of the underwater vehicle  100  in its dive profile (e.g., via sensors including depth/pressure sensors  110  and/or acoustic altimeters  112 ), a control system  105  and buoyancy system  400  in accordance with the present teachings allow a single autonomous underwater vehicle to produce efficient motion covering a broad range of depth, including shallow coastal waters  200  to deep ocean domains  250 . 
     As depicted in  FIGS. 2 and 6 , the present teachings provide a multiple stage buoyancy changing system  400 , or variable buoyancy device, that can make an autonomous underwater vehicle  100  energy efficient over a large range of depths  200 ,  250 . Multiple stages including a channel, a pump, a motor, and a valve can be optimized to each cover a portion of an external pressure range that the vehicle will encounter in a typical dive cycle. By sensing the depth or ambient pressure surrounding the underwater vehicle in a given dive profile, and engaging the correct stage for pumping fluid for that ambient pressure, a system in accordance with the present teachings allows a single autonomous underwater vehicle  100  to produce energy efficient vertical motion covering a broad range of depths, including shallow coastal waters  200  to deep ocean domains  250 . 
       FIGS. 2 ,  5  and  6  schematically illustrates an exemplary embodiment of a multiple stage buoyancy system  400  in accordance with the present teachings. The embodiment of  FIG. 5 , for example, illustrates a multiple stage buoyancy system  400  having a first channel  420  including a bypass channel  422 , a second channel  415 , and a third channel  425 . By utilizing multiple channels within the device, including bypass channels  422 , a multiple stage buoyancy system  400  of the present teachings achieves high efficiency in buoyancy changes without allowing one stage to compromise or restrict the performance of any other stage. Check valves  450  can be provided to prevent fluid from returning to previous pump stages. In the embodiment of  FIG. 2 , a filter is shown, which can protect the system  400  from contaminants in the fluid that would decrease the flow or clog the valves, but is not essential to operation. 
     An autonomous underwater vehicle  100  having a multi-stage buoyancy system  400  in accordance with the present teachings, an exemplary embodiment of which is illustrated schematically in  FIG. 11A , can traverse a dive profile ranging from shallow coastal water  200  to deep water  250  with a single vehicle  100 , without a significant compromise of energy consumption or reliability that might occur in a design optimized for a narrow range of depths. 
     The present teachings contemplate an underwater vehicle including as many stages as are deemed necessary and expedient to produce the best trade-off between energy use for pumping and (a) the mass of parts needed for each stage, (b) the volume occupied by each stage within the pressure hull, and (c) the complexity of controls and plumbing. 
       FIGS. 1 and 11B  illustrate an exemplary dive profile having one or more distinct depth (or pressure) ranges (hereafter interchangeably called depth “stages”) for which different variable buoyancy device pump stages are utilized. As indicated in the flow chart of  FIG. 3 , the choice  500  of which pump stage  505 ,  510 ,  515  to utilize can be made, for example, by the vehicle&#39;s on-board processor  105  with input from various sensors  110 ,  112  and command files. In accordance with certain embodiments, the autonomous underwater vehicle  100  can abort a dive when a problem (e.g., a system error) is detected. When a dive is aborted, the autonomous underwater vehicle  100  can, for example, pump as much fluid into the external reservoir  405  as possible to reach the surface  205  for retrieval, preferably using the most efficient stages of the variable buoyancy system. 
     The fluid displacement systems (e.g., the pump  460   a,    460   b,    460   n,  motor, and valve systems) of the present teachings need not be of the same type. Different dive stages  260   a,    260   b,    260   n  can comprise different components. Exemplary fluid displacement systems that can be used in accordance with the present teaching include, for example, a piston-driven pump, a systolic pump, a Stirling engine, and/or other suitable devices that can move fluid. 
     A multiple stage buoyancy system  400  in accordance with the present teachings can be implemented using a variety of approaches that embody the principle of depth/pressure dependent selection of the most efficient pump stage  505 ,  510 ,  515  corresponding to the dive stages  260   a,    260   b ,  260   n.  An example of an implementation and decision process  500  is illustrated in  FIG. 3 . By using a pressure sensor  112  that detects the surrounding water pressure at a given depth  260   a,    260   b,    260   n,  and a volume sensor  114  that detects the vehicle&#39;s  100  displacement volume, the control system  105  and/or electrical logic of the vehicle  100  can enable a pumping stage  505 ,  510 ,  515  that is most energy efficient for the detected environmental pressure if a change in external volume is needed. In principle, a system of many (“N”) stages can be employed, wherein two is the simplest case and may be adequate for many different vehicles. The present teachings illustrate in  FIG. 2 , however, that more than two stages can be utilized to achieve high efficiency across the entire depth of the ocean. 
     As depicted in  FIG. 5  as an optional element (dashed lines), another embodiment of the present teachings contemplates utilizing a pump  260  and motor  261   m  in combination with a continuous variable transmission  261   t  that can adapt to a torque and speed curve, resulting in different pumping rates at different depths to efficiently change the buoyancy of an autonomous underwater vehicle  100 . Continuously variable transmissions can provide an effective continuum of torque-speed ratios over a predetermined range, with slower speeds corresponding to higher torque output and higher speeds corresponding to lower torque output. 
     The illustrated exemplary embodiment of  FIG. 2  places the stages (or channels)  415 ,  420 , of the multiple stage buoyancy system in parallel with each other to eliminate a potential negative impact of serial placement. Serial placement can impede optimal performance by restricting the downstream pump&#39;s access to the internal reservoir. 
     Certain embodiments of the present teachings can combine two or more stages to increase the rate of pumping and thus change of buoyancy. 
     During an underwater vehicle&#39;s  100  descent, fluid can move from the external reservoir (or bladder)  405  to the internal reservoir  410  when a high-pressure return valve  452  between the external reservoir and the internal reservoir is opened. In one embodiment the external bladder  405  and/or the internal reservoir are expandable. Ambient pressure can be used to push fluid from the external reservoir  405  to the internal reservoir  410  by pressing on the external reservoir  405 . In addition, the autonomous underwater vehicle  100  can have a reduced internal pressure (e.g., a vacuum) that encourages fluid flow from the external reservoir  405  to the internal reservoir  410 . Certain embodiments of the present teachings also contemplate using one or more pumps to drive fluid from the external reservoir to the internal reservoir if more speed is required in that process. 
     In accordance with certain embodiments of the present teachings, a connection can exist from the output of one pump stage to the intake of another pump stage. This series-like plumbing can function as a safety path for any pump stage that needs priming. 
     An autonomous underwater vehicle  100  employing control and buoyancy systems  400  in accordance with the present teachings can travel long distances (e.g., thousands of kilometers) over durations of many months using buoyancy changes that combine algorithms and multiple stage buoyancy control to conserve onboard stored energy by utilizing an optimized fluid displacement strategy, selecting the most energy efficient fluid displacement mechanism(s) to traverse all desired diving profiles. 
       FIGS. 1 ,  11 B and  13  illustrate an exemplary dive profile of an embodiment of the AUV  100  having three distinct depth ranges (or stages) for which three pump stages are utilized during ascent, Pressure Range  1   260   a,  Pressure Range  2   260   b  and Pressure Range n  260   n.  Many depth (pressure) ranges may exist between Pressure Range  1   260   a  and Pressure Range n  260   n.  As shown, the dive profile includes a single deep dive having five segments: first SURFACE  265 ; DIVE  270 ; APOGEE  275 ; CLIMB  280 , and second SURFACE  285 . In a first SURFACE  265  segment, the autonomous underwater vehicle  100 , a position of which is indicated by various dots  290   a - 290   n,  starts a surface phase  265  and transmits information including, for example, vehicle health (e.g., all systems self-test and indicate that they are working normally), available onboard energy, a dive log, data from onboard instruments  113  (e.g., chemical compounds in water, optical backscatter, sound detection, salinity, predominant currents, images, other physical properties of the ocean, etc.), and receives information including a dive plan having waypoints (e.g., latitude, longitude, depth, descent rate, and ascent rate), instrument sampling rates, and other parameters associated with controlling instruments (turning them off or when to turn them off). After a dive plan is received, the vehicle  100  can calculate a correct rate and angle of descent based at least in part on the new dive plan. An initial GPS location is taken (GPS 1 ) with an onboard GPS sensor  114  when the vehicle  100  surfaces or is initially placed in the water and then, after data transmission and receipt of a new dive plan (which presently typically takes about 10-15 minutes (using, e.g., about 10 Watts of energy), another GPS location is taken (GPS 2 ) because the vehicle  100  may have moved during data transmission and receipt. Movement of the vehicle  100  from GPS 1  to GPS 2  can provide information regarding predominant currents affecting the vehicle  100 . A dive log can comprise data indicating how each dive profile step went. The dive log can also record errors and error mitigation attempts, and can collect instrument data. 
     Certain embodiments of the autonomous underwater vehicle  100  can remain surfaced without using an electro-motive force, while radio communications and electro-optics perform the above mentioned tasks. 
     During a second dive plan segment, labeled DIVE  270 , the autonomous underwater vehicle&#39;s  100  nose  155 , or front, is pointed downward and the vehicle  100  begins its dive phase by beginning a descent into a first depth range  260   a  (typically without using a pump but rather by letting fluid bleed out of the external bladder  405  to an internal reservoir  410 ). After descending through the first depth range  260   a,  the autonomous vehicle enters a second depth range  260   b  of the DIVE segment  270  that can be identified, for example, by external pressure sensor  112  readings indicating a depth of the vehicle  100  based on the ambient pressure. In accordance with certain embodiments, during descent, the underwater vehicle can change its angle of descent by changing its pitch angle as needed to follow the requested dive profile  255 . 
     After descending through the second depth range  260   b,  the autonomous vehicle  100  enters a third depth range  260   n  of the DIVE segment  270  that can be identified, for example, by external pressure sensor  112  readings indicating a depth of the vehicle  100  based on the ambient pressure. In certain embodiments of the present teachings in which a bathometric map has been stored, the autonomous underwater vehicle  100  can make sure it has reached a maximum depth set forth in the dive profile  255  and/or avoid collision with the bottom  210  (e.g., using acoustic pings to find the bottom) before beginning an APOGEE dive plan segment  275 . As the underwater vehicle  100  reaches the bottom of its dive, for example in the third depth range  260   n,  it enters an APOGEE dive plan segment  275 . The APOGEE dive plan segment  275  can include a transition from descent to ascent, wherein the autonomous underwater vehicle  100  levels out (becomes horizontal) and changes its inclination (by, e.g., turning its nose  115  upward for an ascent by shifting a mass  117  within the vehicle  100 ) before changing buoyancy by pumping fluid from the internal reservoir  410  to the external bladder  405  to begin its ascent and begin a CLIMB segment  280  of the dive plan  255 . 
     The CLIMB segment  280  of the illustrated dive plan  255  begins in the third depth range  260   n , where a Pump Stage (channel)  3   515  is utilized to pump fluid into the external bladder  405  of the autonomous underwater vehicle  100 , which requires a pumping force sufficient to overcome the ambient external pressure at the underwater vehicle&#39;s depth  260   n.  Pump Stage  3   515  can comprise one or more pumps  460   a,    460   b,    460   n,  optimized for the third depth range (i.e., deep water  250 ). As the external bladder  405  fills with fluid, the surface area and thus the buoyancy of the underwater vehicle  100  increase, causing the underwater vehicle to ascend. In accordance with certain embodiments, only a nominal amount of fluid is move to the external bladder  405 —just enough to get a desired rate of rise. As the autonomous underwater vehicle  100  begins to ascend, it may need to change the amount of fluid in the external bladder  405  because, for example, the density (e.g., the salinity) of the water may not be what was originally predicted. Thus, more fluid can be pumped into the external bladder  405  or some fluid can be allowed to bleed from the external bladder  405  to alter the rate of ascent. Adding and removing water from the external bladder  405  can be performed, for example, in a PID loop type of arrangement. In certain embodiments, the system  400  may not allow fluid to be bled from the external bladder  405  to slow the underwater vehicle&#39;s  100  ascent, because the vehicle  100  typically eventually hits an area of water in its ascent that slows the vehicle down and makes up for a too-rapid rise. Ocean water density tends to be more uniform near the ocean&#39;s bottom  210 . Toward the ocean&#39;s surface  205 , the density is more likely to vary, for example due to varying temperature or salinity. Salinity may vary due to, for example, fresh water sources such as rivers, streams, runoff, and rain water. 
     The underwater vehicle ascends through the third depth range  260   n  to the second depth range  260   b.  In the second depth range  260   b,  the depth and thus the ambient pressure decrease, and a Pump Stage  2   510  can be used to pump fluid into the external bladder  405  if needed to maintain a desired rate of ascent. Pump Stage  2   510  can comprise one or more pumps  460   a,    460   b,    460   n  optimized for the second depth range  260   b.  In accordance with certain embodiments, during ascent, the underwater vehicle  100  can change its angle of ascent by changing its pitch angle as needed to follow the requested dive profile. The underwater vehicle  100  ascends through the second depth range  260   b  to the first depth range  260   a.  In the first depth range  260   a,  the depth and thus the ambient pressure decrease, and a Pump Stage  1   505  can be used to pump fluid into the external bladder  405  if needed to maintain a desired rate of ascent. Pump stage  1   505  can comprise one or more pumps  460   a,    460   b,    260   n  optimized for the first depth range  260   a  (i.e., shallower water  200 ). Within the first depth range  260   a,  for example at about  10  meters or less, a second SURFACE segment  285  can begin as illustrated. At surfacing  285 , more fluid can be pumped into the external bladder  405  and the vehicle&#39;s mass  117  may be shifted to get the vehicle&#39;s tail (or rear)  120  up to allow an antenna  135  located at the tail  120  to rise for communication. 
     During the second SURFACE segment  285 , the autonomous underwater vehicle  100  can transmit information including, for example, vehicle health (e.g., all systems self-test and indicate that they are working normally), available onboard energy, a dive log, data from onboard instruments (e.g., chemical compounds in water, optical backscatter, sound detection, salinity, predominant currents, images, other physical properties of the ocean, etc.), and can receive information including a dive plan having waypoints (e.g., latitude, longitude, depth, descent rate, and ascent rate), instrument sampling rates, and other parameters associated with controlling instruments (turning them off or when to turn them off). After a dive plan  255  is received, the vehicle  100  can calculate a correct rate and angle of descent based at least in part on the new dive plan  255 . An initial GPS location is taken (GPS 1 ) with an on board GPS  114  when the vehicle  100  surfaces and then, after data transmission and receipt of a new dive plan  255  (which presently typically takes about 10-15 minutes (using, e.g., about 10 Watts of energy), another GPS location is taken (GPS 2 ) because the vehicle  100  may have moved during data transmission and receipt. Movement of the vehicle from GPS 1  to GPS 2  can provide information regarding predominant currents affecting the vehicle. A transmitted dive log can comprise data indicating how each dive profile step went. The dive log can also record errors and error mitigation attempts, and can collect instrument data. Certain embodiments of the underwater vehicle  100  can remain surfaced without using any energy. The underwater vehicle  100  can also be retrieved after a single dive. 
     In accordance with certain embodiments, the autonomous underwater vehicle  100  can abort a dive  255  when a problem (e.g., a system error) is detected. When a dive  255  is aborted, the autonomous underwater vehicle  100  can pump as much fluid into the external bladder  405  as possible to reach the surface for retrieval. 
     When the dive plan  255  requires the vehicle  100  to re-dive without surfacing, the vehicle  100  typically levels out and shifts a mass  117  within the vehicle to point its nose  115  downward before the vehicle  100  allows bleeding from the external bladder  405  to begin to dive again. 
     To provide an autonomous underwater vehicle  100  that can efficiently traverse a dive profile  255  ranging from shallow coastal water  200  to deep water  250 , the present teachings contemplate a multi-stage buoyancy system c 400  omprising, for example, a system employing multiple fluid displacement mechanisms (e.g., a multi-pump system or a system employing a combination of pumps and other fluid displacement systems) to provide efficient movement of fluid at a variety of depths. The fluid displacement systems need not all be the same type of fluid displacement system and can comprise, for example, a piston-driven pump, a systolic pump, a Stirling engine, and/or other suitable devices that can move fluid. 
     Various embodiments of the present teachings provide a system for changing the apparent displacement or incorporated mass of an autonomous underwater vehicle by displacing fluid within an underwater vehicle comprising two or more stages or subsystems of displacement mechanisms as set forth hereinabove, and a control system that determines an appropriate stage to utilize in the environment that is ambient to the underwater vehicle at any given segment of the underwater vehicle&#39;s dive profile. 
     As stated above, an autonomous underwater vehicle must descend and ascend in the water to move forward and traverse its intended path.  FIG. 13  illustrates an exemplary underwater dive profile with time represented on the horizontal axis and depth being represented on the vertical axis.  FIG. 3  shows that the ascent phase of the underwater vehicle&#39;s dive profile is where the multi-stage control of the present teachings is effective in allowing the underwater vehicle to employ more than one fluid displacement mechanism to move fluid to the external bladder with maximum efficiency while providing the pressure needed to fill the bladder based on the ambient pressure at the underwater vehicle&#39;s depth. 
     At the end of the ascent phase, the autonomous underwater vehicle can reach a surface level (or at least come close enough to the surface) where it can send data (e.g., via satellite transmission) regarding its preceding path and/or begin a new decent and ascent cycle. Upon surfacing, the underwater vehicle can reconcile its location by receiving its current GPS location and inputting that location into its dive profile. 
     A multi-stage buoyancy system in accordance with the present teachings can be implemented in a number of ways using a variety of approaches that embody the principle of depth-driven and pressure-driven selection of the most efficient stage. For example, by using a pressure sensor that detects the surrounding water pressure at a given depth, the control system or electrical logic of the vehicle can enable the pumping stage that is most efficient for the detected environmental pressure. In principle, a system of many stages can be employed, wherein two is the simplest case for use as an exemplary embodiment herein and may be adequate for many coastal to deep water oceanic missions for autonomous underwater vehicles. The present teachings contemplate, however, more than two stages being used to achieve high efficiency across the entire depth of the ocean from a few meters to 6000 meters or more. 
     The design principle driving selection of different pumps and pump drive motors for differing depth ranges can be such that the pumping energy for predefined depth ranges and associated pressure is minimized on the basis of balancing the rate of pumping against the torque and hence energy consumption required to resist and overcome the range of pressures within a depth range and move enough fluid to achieve a required buoyancy offset. For example, a depth range of from 0 to 100 meters typically has a corresponding pressure range of from about 1 atmosphere to about 11 atmospheres, and this would dictate that a pump and drive motor capable of most efficiently overcoming the 11 atmospheres maximum value would be selected for this depth range. For a range of 100 meters to 500 meters, having a pressure range of from 12 atmospheres to 50 atmospheres, a stronger pump/motor drive combination is needed, preferably having the best energy efficiency for that range. This design criteria can continue until a maximum depth demanded by the vehicle is serviced by a pump and motor drive stage that meets the maximum pressure demand, while using the minimum energy to achieve buoyancy change by volume of expelled fluid to overcome pressure at any given depth. 
     Using the above design approach for very large depth ranges can, in certain instances, produce a sub-optimal match of pumping stage to the encountered pressure at some depths, or can produce a design with an excessive number of stages and thus excessive complexity and a significant number of parts lending toward failure modes. Thus, another embodiment of the present teachings contemplates utilizing a pump motor in combination with a continuous variable transmission (CVT) that can adapt to a torque--speed curve resulting in an optimal pressure/pumping rate needed at any given depth of the autonomous underwater vehicle. CVTs can provide an effective continuum of torque-speed ratios over a predetermined range, with slower speeds corresponding with higher torque output. A continuous variable transmission would effectively allow a pump to work across an entire pressure range efficiently by virtue of operating at a faster rate (using a low gear ratio for shallower water, lower ambient pressure, lower torque requirements) or slower rate (using a high gear ratio for deeper water, higher ambient pressure, higher torque requirements) of fluid displacement as needed to minimize the torque and hence the energy required to change buoyancy as needed to allow the underwater vehicle to follow its dive profile. 
     A CVT-based implementation of the present teachings is practical for increased dive durations associated with increased dive depths. For example, dives to 50 meters will typically take from 15 to 20 minutes, whereas dives to 1000 meters can take up to 5 hours, affording a far longer time frame for the pumping system to move the fluid to achieve ascent velocity when ascending from a 1000 meter depth. In other words, a CVT-based embodiment would pump slowly, using less energy when at greater depths by employing a high gear ratio in the CVT, resulting in low pumping speed but high enough torque to overcome external pressure. Given the longer duration of deeper dives, this can produce an acceptable and optimized result with a single pump design. Where pressures are low in shallow dives, the amount of torque required by the pump to overcome the external water pressure is much lower, but rapid pumping to achieve rapid ascent is typically desirable, so the CVT would then be set to a low gear ratio between the drive motor and the output pump, achieving a higher pumping rate with the lower torque demand. The best effective gear ratio of the CVT for a given ambient pressure can be automatically selected by reading the pressure sensor, then applying an algorithm or other analog control scaling to cause the control arm or other mechanisms that determines the CVT&#39;s effective gear ratio to react proportionally or in steps to pressure changes, in a relationship that decreases the effective gear ratio as depth (and hence pressure) increases. 
     An exemplary embodiment of the present teachings that employs a CVT can utilize a NuVinci™. Model N360 continuously variable planetary drive train transmission or another continuously variable or step gearbox mechanism that can vary the pump-to-drive motor effective gear ratio based on a proportional algorithm that is keyed to pressure. As will be understood by those skilled in the art, low gear ratios can be used at shallower depths with lower external pressures, and high gear ratios can be used in deeper waters to produce the extra torque needed to push fluid into the external bladder and against the higher external pressure exerted on the external bladder. 
     A flow chart outlining the basic steps of an exemplary algorithm for implementing a multi-stage buoyancy system to achieve efficiency at various depth ranges is illustrated in  FIG. 4 . The flow chart of  FIG. 4  illustrates the basic concept of a two-stage system, one stage being for shallow dive segments of the autonomous underwater vehicle&#39;s dive profile and the other stage being for deep dive segments of the underwater vehicle&#39;s dive profile. The present teachings contemplate using either the profile sequence or the actual pressure to determine which of the fluid displacement mechanisms (e.g., which pump) is used for a given segment of a dive. In certain embodiments, two or more fluid displacement mechanisms (e.g., two pumps or two stages) can be used for a single segment. In certain embodiments, only the ascent phase of a dive, as illustrated in  FIG. 3 , uses the underwater vehicle&#39;s fluid displacement mechanisms to change the underwater vehicle&#39;s buoyancy. 
     As shown in  FIG. 4 , after a dive sequence begins, the autonomous underwater vehicle performs a next segment of the dive profile, which can be the initial dive profile segment. Each time a new segment of the dive profile begins (e.g., based on a reading of the depth, compass, attitude, or other sensors, singularly or in combination), the algorithm determines whether the profile is an ascent segment (in which one or more fluid displacement mechanisms may need to be employed to displace fluid into an external bladder). If the next segment of the dive profile is not an ascent segment, pressure can be bled from the external bladder to reduce buoyancy, as needed, and the underwater vehicle can begin a descent through the water until a next segment of the dive profile is reached. If the next segment of the dive profile is an ascent segment, the algorithm determines whether the depth of the next segment and the current ambient conditions are less than “Stage 2,” which means that the depth of the next segment and the current ambient conditions are less than a predetermined depth that is optimal for the pump/motor combination currently being utilized, and thus the ambient pressure is below a predetermined value. To determine the ambient pressure, the algorithm can utilize input from a pressure/depth sensor employed on the underwater vehicle. If the depth of the next segment and the current ambient conditions are less than Stage 2, a high pressure fluid displacement mechanism (referred to herein as a “Stage 1 pump”) can be disabled to improve efficiency of the overall system. Thereafter, the system can perform a buoyancy change with just the Stage 2 fluid displacement mechanism and then move on to perform a next segment of the dive profile. 
     If either the depth of the next segment or the current ambient conditions are greater than or equal to “Stage 2,” which means that the depth of the next segment or the current ambient conditions are greater than or equal to a predetermined depth that is optimal for the next range of pressure and thus the ambient pressure is above a predetermined value, the high pressure fluid displacement mechanism (the Stage 1 pump) can be enabled to provide the pressure needed to move fluid to the external bladder against higher ambient pressures. When the Stage 1 pump is enabled, it can be used alone (by disabling the stage 2 pump as shown), or in conjunction with the Stage 2 pump. The algorithm then performs a next segment of the dive profile. In certain embodiments, two fluid displacement mechanisms can be employed to create a three-stage buoyancy system when each fluid displacement mechanism can be used alone or the two mechanisms can be used together. 
     In certain embodiments, the control system for the autonomous underwater vehicle uses stored dive profile information, such as the profile illustrated in  FIG. 11B  or a profile including more than one dive segment such as the segments in  FIG. 13 , to determine at what time (or distance) an appropriate stage should be used—based on a profiled desired depth. Since this method depends on an accurate assessment of the vertical distance traversed by the underwater vehicle  100 , which can be significantly affected by currents and density structure of the dive environment, certain embodiments of the present teachings can employ a secondary method for selecting the buoyancy stage, such as by reading an external pressure sensor  112  or another method to determine actual depth (e.g., by an acoustic altimeter or by a range and heading determined by an acoustic modem USBL message). The depth and/or depth analog such as pressure can then be used to select the appropriate stage  505 ,  510 ,  515  to be used for the desired buoyancy changes in that segment of the dive profile  255 . The terms stages, pumps, and fluid displacement mechanisms are used interchangeably herein. 
       FIG. 5  is a schematic diagram illustrating an exemplary embodiment of a hydraulic multi-stage buoyancy system in accordance with the present teachings. The exemplary embodiment of  FIG. 5  includes, among other elements: a first stage (Stage 1) pump for high pressure depths; a second stage (Stage 2) pump for lower pressure depths; an internal reservoir for fluid (e.g., hydraulic fluid) used to change buoyancy; and a buoyancy chamber or external bladder mounted on an external surface of the underwater vehicle that changes in size and displacement when hydraulic fluid is pumped into it or expressed from it by ambient pressure as the underwater vehicle enters deeper water. The external bladder is preferably at least somewhat elastic. 
       FIG. 5  illustrates an exemplary embodiment of paths fluid can take between the internal reservoir  410  and the external bladder  405 . In the illustrated embodiment, three paths  415 ,  420 ,  425  exist between the accumulator/reservoir  410  and the external buoyancy chamber or bladder  405 , two of which contain a fluid displacement mechanism. One path  420  runs fluid through a low pressure “Second Stage” (Stage 2) pump  460   b  and through a check valve  450  such as the illustrated bypass (check) valve that prevents movement to fluid in an unwanted direction. Another path  415  runs fluid through a bypass (check) valve  450  and through a high pressure “First Stage” (Stage 1) pump  460   a.  The parallel channels having bypass check valves  450  can combine to eliminate unproductive loads on a stage that is currently operating, by providing a direct path to the internal reservoir  410 , without the fluid needing to be pushed or pulled through any non-operating elements (e.g., non-operating stages). The third path  425  allows fluid to return from the external bladder  405  to the internal reservoir  410  through a valve  452  such as, for example, a solenoid valve (e.g., a Skinner valve). 
     To cause the autonomous underwater vehicle  100  to descend, the Skinner valve  452  can be opened between the external bladder  405  and the internal reservoir  410 , allowing fluid to be driven by ambient pressure from the external bladder  405  to the internal reservoir  410 . In the illustrated embodiment of  FIG. 5 , a return valve such as an electronically actuated solenoid valve (e.g., a Skinner valve)  452  is located between the external bladder  405  and the internal reservoir  410 , although those skilled in the art will appreciate that other suitable types of valves can alternatively or additionally be used. The valve  452  between the external chamber  405  and the internal reservoir  410  should remain selectively closed while the external buoyancy chamber  405  is being filled to cause the underwater vehicle  100  to ascend. 
     In the illustrated embodiment, a check valve  450  is provided between the line returning fluid from the external bladder  405  to the internal reservoir  410  and the Stage 1 pump  460   a.  This check valve can prevent fluid returning to the internal reservoir  405  from being diverted to the Stage 1 pump  460   a . 
     While atmospheric pressure can be sufficient to drive fluid from the external bladder  405  to the internal reservoir  410 , certain embodiments of the present teachings also contemplate using one or more of the pumps  460   a,    460   b,    460   n  to drive fluid from the external bladder  405  to the internal reservoir  410 , for example if fluid is not moving therebetween or if fluid is not moving fast enough therebetween to achieve a desired rate of descent. 
     The illustrated exemplary embodiments of the present teachings eliminate the impact of serial placement of stages  415 ,  420 ,  425 , by placing the stages in parallel. Serial placement of the stages can impede an optimal performance of stages downstream or upstream in the system  400 . If, for example, a smaller pump was positioned between a larger pump and the reservoir  410 , the smaller pump could restrict the larger pump&#39;s access to the reservoir, making it less efficient and/or slower for the larger pump to move fluid from the reservoir  410  to the external bladder  405 . Pumps arranged in series between the reservoir  410  and the bladder  405 , rather than in parallel as illustrated in  FIG. 5 , would tend to add frictional and orifice (size) restrictions that can impede fluid flow. 
     Certain embodiments of the present teachings can combine two or more stages  415 ,  420 ,  425 , to achieve either greater total pressure output to overcome pressures at deeper depths  250  or to increase the rate of change of buoyancy by pumping more fluid into the bladder  405  to increase a rate of ascent. Certain embodiments of a control system  400  for an embodiment utilizing two stages to increase a rate of change in buoyancy can, for example, sense the rate at which buoyancy of the underwater vehicle  100  is being changed, which in some embodiments can be determined by the displacement of an internal plate inside the fluid reservoir  410 , and in other embodiments by, for example, measuring a reservoir pressure. When the rate of buoyancy change reaches or exceeds a level required to achieve the underwater vehicle ascent or descent rate desired in the dive profile, one of the stages can be halted to save energy. The stage to be halted can depend, for example, on the underwater vehicle&#39;s  100  depth. For example, if the underwater vehicle  100  is in deeper water  250  requiring use of a stage 1 pump  460   a,  the stage 2 pump  460   b  would be halted. Otherwise, if the underwater vehicle  100  is in shallower water  200  requiring use of a stage 2 pump  460   b,  the stage 1 pump would be halted. If less than all of the stages  415 ,  420 ,  425 , of the system  400  are being utilized and the rate of buoyancy change falls below a desired level, one or more additional stages can be switched on to provide additional buoyancy fluid flow. 
       FIG. 6  is a schematic diagram illustrating another exemplary embodiment of a multi-stage buoyancy system  400  in accordance with the present teachings. The exemplary embodiment comprises a main pump  462   a  and a boost pump  462   b  for pumping fluid from an internal reservoir  410  to an external bladder  405 . The main pump  462   a  can comprise, for example, a high pressure pump. The boost pump  462   b  can comprise, for example, a lower pressure pump. As shown, fluid can travel from the internal reservoir  410  to the external bladder  405  to cause ascent via a first path  415  and/or a second path  420 . The first path  415  includes the boost pump  462   b,  a filter  455 , and a check valve  450 , the check valve ensuring that fluid flows through this path only in the desired direction. A filter  455  is not essential, but can protect the system from contaminants in the fluid that would decrease the flow or clog the valves. The second path comprises the main pump  462   a  with a check valve  450  on either side thereof, each check valve ensuring that fluid flows through the second path  420  only in a desired direction. 
     During a descent, movement from the external bladder  405  to the internal reservoir  410  is called ‘bleeding’ and the ambient underwater pressure is used to push fluid from the external bladder  405  into the underwater vehicle&#39;s internal reservoir  410  by pressing on the external bladder  405 . In addition, the autonomous underwater vehicle  100  can have a negative internal pressure that assists the bleeding process and encourages fluid flow back to the internal reservoir  410  when a high pressure (h.p.) return valve  452  between the external bladder  405  and the internal reservoir  410  is opened. 
     In certain embodiments, the check valve  450   c  in the second path  420 , located outside the pressure hull  320 , is located within the external bladder  405  and can prevent fluid from flowing back into the pump(s)  462   a,    462   b.  Because the external bladder  405  is both elastic and exposed to the ambient pressure of the surrounding water, it will experience an internal pressure that tends to push fluid back toward the pump(s). The check valve  450   c  located outside the hull  320  (e.g., inside the external bladder  405 ) serves as a backflow preventer, making the return valve  452  the only outlet from the external bladder  405 . The return valve  452  is selectively openable and only opened when it is desirable to allow fluid to bleed from the external bladder  405 . 
     In certain embodiments of the present teachings, a flow-through connection can exist through an intake reservoir  463  of the main pump  462   a.  Pressure from the boost pump  462   b  can flow to the external bladder  405  until a predetermined ambient pressure of, for example,  200  psi exists. When the predetermined ambient pressure is reached, fluid from the boost pump  462   b  can be sent (circuitously but effectively) through the main pump&#39;s  462   a  intake reservoir  463  via a flow-through connection  464  and back to the internal reservoir  410 . The flow-through connection  464  thus can function as a safety path. 
     A  425  path exists for fluid to flow from the external bladder  405  to the internal bladder  410  to cause the underwater vehicle  100  to have a decreased apparent displacement and a decreased buoyancy, and therefore to descend, the path including a return valve  452  such as an electronically actuated solenoid valve (e.g., a Skinner valve)  452  as shown in the embodiment of  FIG. 5 . 
     The autonomous underwater vehicle  100  can comprise a variable buoyancy displacement chamber or variable volume enclosure that can be offset from the center of gravity CG of the underwater vehicle  100 , providing a means to change the displacement volume or the mass of the underwater vehicle  100  relative to its center of gravity CG, for example to tip the nose  115  of the underwater vehicle up or down. For example, such a mass distribution mechanism  117  can comprises a vehicle battery or another defined mass  117  within the underwater vehicle that can be adjusted within the underwater vehicle to tip the nose  115  of the underwater vehicle  100  up or down, or to roll the underwater vehicle  100  to its left or right. Movement of the mass distribution mechanism  117  can be controlled by the control system  105 , allowing the control system  105  to steer the underwater vehicle  100  as needed to cause the underwater vehicle  100  to descend to desired depths  200 ,  250 ,  260   a,    260   b ,  260   n,  ascend to the water surface  205 , roll/steer left or right, or keep station as might be determined by the buoyancy of the underwater vehicle  100  relative to the surrounding ambient water and the center of buoyancy CB of the underwater vehicle  100 . 
     The present teachings provide a multi-stage buoyancy engine or system  400  in which two or more stages  415 ,  420 ,  425 , can be combined to increase the rate of buoyancy change as determined by the control system  105  to maintain a desired rate of horizontal and/or vertical velocity for the vehicle  100  in accordance with a predetermined dive profile plan. A bypass system such as the bypass valves  450  disclosed above for the multiple stages  415 ,  420 ,  425 , of the buoyancy engine or system  400  enables use of one or more stages to obtain optimal energy consumption at a given depth, with no significant impedance or degradation of efficiency imposed by any other stage of the system  400 . 
     Various embodiments of the present teachings provide an arrangement of multiple stages  415 ,  420 ,  425 , such that they can be combined to provide a higher rate of buoyancy change or higher torque, whereby a bypass system allows the stages to be provided in parallel. Various embodiments can also comprise a mechanism to change the center of gravity of the autonomous underwater vehicle to cause the underwater vehicle to roll (rotation about the longitudinal axis of the vehicle) and pitch (rotation about the lateral axis of the vehicle), such that the attitude of the vehicle can be changed to provide a desired glide angle relative to forward motion. The external bladder  405  can be used to cause the underwater vehicle  100  to roll and pitch, and can change the center of gravity of the autonomous underwater vehicle  100 . 
     As depicted in  FIGS. 7-9 , one or more known energy storage systems  600   a,    600   b  onboard the autonomous underwater vehicle  100  can power the fluid displacement mechanisms  460   a,    460   b,    460   n , the sensors  110 ,  112 ,  114 , and the control system  105 . In certain embodiments, the energy storage systems  600   a,    600   b  can comprise one or more rechargeable (e.g., lithium) batteries. 
     As set forth above, various embodiments of the present teachings comprise a control system  400  for an autonomous underwater vehicle  100 , the control system  105  comprising a control computer, sensors to determine depth, heading angles, and rate of descent, and a buoyancy system  400  for changing the apparent displacement or mass of the underwater vehicle using fluid displacement mechanisms  460   a,    460   b,    460   n  to move fluid between an internal reservoir  405  and an external bladder  410 . 
     Certain embodiments of the present teachings provide an algorithm for determining the appropriate fluid displacement mechanism to use to achieve a desired change in buoyancy to maintain ascent or descent at a specified velocity through a specific range of depths. The fluid displacement mechanisms can comprise a hydraulic system configured with multiple pumping stages or alternate gearing ratios that can efficiently transfer work from one stage to another without significant impairment of a selected stage, and can work in concert or separately to produce changes in buoyancy with respect to the ambient pressure in effect at the time of execution of buoyancy change. 
     The present teachings provide a configuration of controls, sensors, and fluid displacement mechanisms that can include motors, pistons, or similar mechanisms that enable a change of buoyancy of the autonomous underwater vehicle in accordance with its environment, to minimize its expenditure of stored energy. An advanced method uses a continuously variable transmission to effectively obtain the benefits of a large number of physically separate stages by employing a single stage having continuously changeable torque, flow rate, and pressure outputs. 
     The present teachings also comprise a control algorithm for execution by the controller  105  that can store a desired path  255  of the autonomous underwater vehicle  100  including a depth profile and bathymetric information about the intended path of travel  255  of the vehicle  100 , such that appropriate buoyancy control actions can be programmed to use the most efficient employment of fluid displacement mechanisms to minimize utilization of onboard stored energy. 
     An autonomous underwater vehicle  100  employing control and buoyancy systems in accordance with the present teachings can travel across long distances (e.g., thousands of kilometers) over durations of many months using buoyancy changes that combine algorithms, controls, and multi-stage buoyancy control  400  to conserve onboard stored energy by utilizing an optimized fluid displacement strategy, selecting the most efficient fluid displacement mechanism(s) to traverse both shallow water  200  and deep water diving profiles  250 . 
     Other embodiments of the present teachings will be apparent to those skilled in the art from consideration of the specification and practice of the teachings disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims.