Abstract:
A shaft seal pressure compensation system for an underwater device such a remotely operated vehicle powered by motor driven thrusters where the shaft of the motor extends from a motor housing into the surrounding water and is sealed by a shaft seal by automatically supplying air to pressurize the thrusters to balance the internal pressure of the thrusters to match the ambient water pressure. The air is supplied by a variable volume container that responsive to a pressure differential between the external ambient pressure and an internal pressure within the variable volume gas container for adjusting the volume of the variable volume gas container so that the internal pressure equals the ambient pressure, and thus balancing the system.

Description:
FILED OF THE INVENTION 
     The present invention relates generally to shaft seals for underwater use, and more particularly, relating to shaft seal pressure compensation system for equalizing pressure across a thruster motor shaft seal of an underwater vehicle. 
     BACKGROUND OF THE INVENTION 
     Inspection class Remote Operated Vehicles (ROVs) are typically used to position a video camera underwater. The ROV usually contains electronics that are connected to a base station by a wire tether. Motor driven propellers called thrusters are used to move the ROV. 
     A problem with existing ROVs concerns the shaft seals inside the thrusters. In the past, inexpensive shaft seals were generally not used for thruster applications because of the pressure differential that occurs on either side of the seal. These seals may tolerate a pressure differential from the wet side to the dry side of 5 PSI. However, the pressure underwater increases approximately 1 PSI for every two feet of depth, so the low cost seal would leak at a depth of greater than 10 feet. ROVs may typically be used at depths of at least 25 feet. In order to solve this problem, more expensive seals are used which have a higher depth rating. The use of more expensive seals increases the production and maintenance costs of ROVs. Additionally, the traditional expensive seals are bulky and require ROVs using these seals to be constructed larger to accommodate the these seals, which also increases the production cost of the ROVs. The high costs associated with the manufacture and maintenance of traditionally built ROVs with expensive shaft seals reduces the ability for the consumer market to purchase ROVs. 
     Accordingly, there is a need to use less expensive shaft seals in such a way that the seals can be used at depths greater than 10 feet, which also reduces the costs of manufacturing and maintaining ROVs, thereby making ROVs more affordable to the consumer market. 
     SUMMARY OF THE INVENTION 
     The preferred embodiments of the present invention addresses this need by compensating the internal pressure acting on the inner side of the shaft seal according to the external pressure acting on the wet side of the shaft seal. Pressure compensation is accomplished by using a variable volume space containing air that is exposed to the external pressure. As the external pressure increases when submerging the ROV, the air contained by the variable volume space is compressed according to Boyle&#39;s Law, thereby increasing the internal pressure accordingly until an equilibrium is reached between the internal pressure and external pressure, and thus eliminating any appreciable pressure differential from occurring on opposing sides of the shaft seal. Likewise, as the external pressure decreases when raising the ROV to a shallower depth, pressure equalization across the shaft seal is maintained. 
     To achieve these and other advantages, in general, in one aspect, a shaft seal pressure compensation system for equalizing pressure across a shaft seal of an underwater device is provided. The system includes a variable volume gas container responsive to a pressure differential between an external ambient pressure and an internal pressure within the variable volume gas container for adjusting the volume of the variable volume gas container so that the internal pressure equals the ambient pressure, a motor housing defining an enclosed internal space, a motor within the enclosed internal space and having a shaft extending from the internal space outwardly of the motor housing and terminating at an external location of the motor housing, a shaft seal between the motor housing and the shaft for making sealing contact between the shaft and the motor housing, the variable volume gas container being pneumatically connected to the motor housing to allow the free exchange of air between the variable volume gas container and the motor housing for pressurizing the enclosed internal space of the motor housing so that it equals the ambient pressure, thereby equalizing the pressure across the shaft seal to within the operational differential pressure tolerances of the shaft seal. 
     In general, in another aspect, a remotely operated underwater vehicle having a shaft seal pressure compensation system for equalizing pressure across a shaft seal of each motor driven thruster is provided. The vehicle includes at least one motor driven thruster for providing a combination of propulsion and direction control to the vehicle; the at least one motor drive thruster including, a motor housing defining an enclosed internal space, a motor within the enclosed internal space and having a shaft extending from the internal space outwardly of the motor housing and terminating at an external location of the motor housing, a shaft seal between the motor housing and the shaft for making sealing contact between the shaft and the motor housing, a variable volume gas container responsive to a pressure differential between an external ambient pressure and an internal pressure within the variable volume gas container for adjusting the volume of the variable volume gas container so that the internal pressure equals the ambient pressure, the variable volume gas container being pneumatically connected to the motor housing to allow the free exchange of air between the variable volume gas container and the motor housing for pressurizing the enclosed internal space of the motor housing so that it equals the ambient pressure, thereby equalizing the pressure across the shaft seal to within the operational differential pressure tolerances of the shaft seal. 
     There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. 
     Numerous objects, features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon a reading of the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of descriptions and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the description serve to explain the principles of the invention, in which: 
         FIG. 1  is a diagrammatic perspective view of a remotely operated vehicle and control unit for use in connection with the shaft seal pressure compensation system for use with an underwater device in accordance with the principles of the present invention; 
         FIG. 2  is a diagrammatic perspective view of the remotely operated vehicle perspective view of  FIG. 1 ; 
         FIG. 3  is a diagrammatic view of the shaft seal pressure compensation system for use with an underwater device; 
         FIG. 4  is a diagrammatic cross-sectional view of a motor housing comprising the pressure compensated shaft seal; 
         FIG. 5A  is a diagrammatic cross-sectional view of a variable volume container in accordance with the principles of the present invention illustrating the variable volume container in low ambient pressure; 
         FIG. 5B  is the variable volume container of  FIG. 5A  illustrating the variable volume container at a higher ambient pressure; 
         FIG. 6A  is a diagrammatic cross-sectional view of an alternate variable volume container in accordance with the principles of the present invention illustrating the variable volume container in low ambient pressure; 
         FIG. 6B  is the variable volume container of  FIG. 6A  illustrating the variable volume container at a higher ambient pressure; 
         FIG. 7A  is a diagrammatic cross-sectional view of an alternate motor housing which comprises the variable volume container in accordance with the principles of the present invention illustrating the motor housing in low ambient pressure; and 
         FIG. 7B  is the motor housing of  FIG. 7A  illustrating the variable volume container at a higher ambient pressure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , there is shown a diagrammatic perspective view of an exemplarily remote operated vehicle ROV  10  that may used in connection with the pressure compensation across a shaft seal in accordance with the principals of the present invention. The ROV  10  is connected to a base station  12  by control tether  14 . The output of an ROV video camera  16  is displayed in real time on the screen of the display  18 , and controls  20 ,  22  and  24  are used to control the movement of the ROV  10 . Controls  20 ,  22 ,  24  are used to pilot the ROV  10  so as to position ROV  10  in order to display the desired information on display  18 . With reference to  FIG. 2 , an enlarged perspective view of the exemplarily remote vehicle is shown. The ROV  10  generally includes the camera  16 , horizontal thrusters  26  and  28 , vertical thruster  30  all mounted to a frame  32 , and a variable volume container  110 . 
       FIG. 3  illustrates the pressure compensation system  100  in accordance with the principals of the present invention. The pressure compensation system  100  includes a variable volume gas container  110  filled with air that is responsive to a pressure differential between an external ambient pressure and the internal pressure within the container for adjusting the volume of the container so that the pressure within container equals the external ambient pressure. The variable volume container  110  is pneumatically connected to each motor housing  118  of one or more of the thrusters  26 ,  28 ,  30  so as to allow the free exchange of air between the variable volume container and each motor housing. Flexible tubing  114  may provide the pneumatic connection between the variable container  110  and the motor housing  118 . While the variable volume container  110  is shown attached to each motor housing  118  it may be attached in any combination to the various motor housings present. 
     With reference to  FIG. 4 , which shows diagrammatic cross-section of the motor housing  118 . The motor housing  118  has an enclosed internal space  116  which contains a motor  130  mounted to a motor mount  132 . The motor  130  may be a low cost, brushed DC motor, although brushless motors may be used where weight is more important than cost. The motor  130  may include a gearbox (not shown). A shaft seal  134  is pressed into motor mount  132 , through which motor shaft  136  extends. A propeller (not shown) would connect to the end of motor shaft  136 . The shaft seal  134  may be a low cost radial shaft seal with a rubber coating and a short, flexibly suspended spring loaded sealing lip. 
     The variable volume container  110  may have a volume that is at least equal to the volume the dead space of each enclosed internal space  116  of each motor housing  118  to which the variable volume container is attached. Dead space is defined herein as the space within the enclosed internal space  116  of the motor housing  118  that is not occupied by the various components located within the enclosed internal space, such as the motor  130 , the motor mount  132 , the motor shaft  136 , etc. 
     In operation, as the device, such as the ROV  10 , containing the pressure compensation system  100  is submerged, the ambient pressure of the water on the variable volume container  110  causes the volume of the variable volume container to adjust so that the internal pressure of the variable volume container equals the ambient pressure according to the principals of Boyle&#39;s Law, which equally adjusts the pressure within in the enclosed internal space  116  of each motor housing  118  so that it equals the ambient pressure of the water, thereby equalizing pressure across the shaft seal  134  to within the operational differential pressure of the shaft seal. As the ROV  10  ascends, the ambient pressure is less than the pressure within the motor housings  118  and the variable volume container  110 . The higher pressure in the motor housings  118  and the variable volume container  110  forces the volume of the variable volume container to increase to equalize the internal pressure with the ambient pressure. The operational differential pressure of the shaft seal  134  may be 5 PSI. 
       FIGS. 5A and 5B  illustrates a cross-sectional view of the variable volume container  110 . Fitting  120  is a barbed fitting or luer fitting used to connect the tubing  114 , such as flexible tubing, to one or more fittings  124  on the thruster motor housing  118 . Membrane  122  creates a flexible diaphragm over an open end  124  of the pressure housing  110 .  FIG. 5A  shows diaphragm  122  when the pressure compensation system  100  is at low pressure such as when the device or ROV  10  is on land or at shallow depths.  FIG. 5B  shows diaphragm  122  when the pressure compensation system  100  is at increased depth, causing diaphragm to deform and compress air contained inside the pressure housing  110 . Increased pressure is conveyed to one or more thruster motor housings  118  through tubing  114  connected to fitting  120  on pressure housing and one or more fittings  124  on the thruster housings. Increased pressure in thruster motor housings  118  acts on internal side  126  of shaft seal  128 , as seen in  FIG. 5 , to offset increased external pressure on the shaft seal, thus allowing the shaft seal to maintain shape and resist deformation due to higher operating pressure. 
       FIGS. 6A and 6B  illustrate an alternative variable volume container  110 ′. For clarity, only the differences in the variable volume container  110 ′ will be described. The variable volume container  110 ′ includes an elongated cylindrical body  140  pneumatically connected to one or more motor housings at one end. The other end  142  of the body  140  is open to the ambient environment and a free piston  144  provides a movable gas tight seal within the body.  FIG. 6A  shows the position of the piston  144  when the pressure compensation system  100  is at low pressure such as when the device or ROV  10  is on land or at shallow depths.  FIG. 6B  shows the piston  144  when the pressure compensation system is at increased depth, causing the piston to move inwardly into the body  140  adjusting the volume of the variable volume container  110 ′, and compressing air contained therein. Increased pressure is conveyed to one or more thruster motor housings  118  through tubing  114 . 
     An issue that could arise with this method of pressure compensation is that an excessive change in ROV displacement could occur. For example, if each thruster motor housing  118  contained 10 cc of dead space, the membrane housing would have to change in displacement by 30 cc in order to double the internal pressure of the three thrusters. This may have the undesired effects of causing the displacement of the pressure housing  110 , and thus the ROV, to change greatly with depth. This problem can be largely overcome by minimizing the air volume in each thruster motor housings  118 . For example, if the thruster motor housing includes 60 cc of total space, but there is only 2 cc of dead space contained in each housing, then only 9 cc of total free space is required in the pressure housing  110  to provide a 2.5:1 ratio in pressure. Therefore, the thruster motor housing  118  can be designed to minimize dead space. This can be accomplished by the following: choosing an inside diameter for the cylindrical housing that closely matches the outside diameter of the motor  130 ; integrating the gearbox into the motor mount such the gearbox is solid except for the areas which contain the spur gears; and fitting a lightweight flywheel inside the motor so as to reduce the free space. 
     Additionally, the change in displacement of the ROV due to the pressure compensation may be offset by the vertical thruster  30 . The displacement can be offset by increasing the speed of vertical thruster  30 , either manually or automatically if for example a pressure sensor input is available to a microprocessor contained in the base station. Alternatively, additional circuitry in ROV  10  could use an on board pressure sensor input to locally adjust the speed of vertical thruster  30  so as to maintain a desired depth. 
     Another issue involved with dynamically changing the ROV displacement with depth is that depending on the location of the pressure housing  110 , the center of buoyancy of the device or ROV  10 , and thus the horizontal orientation of the device or ROV, can be effected. This could have the undesired effect of causing the ROV  10  to rotate as it descends. Although this effect could be partially compensated for by using a pan/tilt camera, this solution would require additional cost. A preferred solution is to design the variable volume container  110  such that is symmetrical both fore and aft and also side to side, and to locate the container approximately at the center of buoyancy of the device or ROV  10 . In this manner, the orientation of the camera can be kept relatively constant regardless of depth. 
     In an alternate embodiment, each motor housing  118  of can be individually compensated for pressure, and thus removing the need for a separated variable volume container. In this embodiment, the motor housing  118 ′ is modified slightly from motor housing  118  to comprises the variable volume container.  FIG. 7A  is a diagrammatic cross-section of a thruster illustrating one example of this embodiment. In this example, motor  130  is contained inside the thruster motor housing  118  which is constructed as a cylinder. The thruster motor housing is sealed at one end with a shaft seal  128 , and at the other end with movable plunger  138 .  FIG. 7A , shows the approximate location of plunger  138  when the thruster is at low pressure, for example at the surface. When the thruster is moved to a deeper location underwater, the external water pressure rises, causing plunger  138  to move inside the housing  118 , compressing the internal air and preventing shaft seal  128  from leaking as shown in  FIG. 7B . In an alternative configuration, the plunger could be replaced by a diaphragm or a flexible housing. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.