Variable Ballast System for Small Submersibles

Description

RELATED APPLICATIONS

This application claims the benefit of provisional application 63/646,578 filed on May 13, 2024. The entire provisional application is incorporated by reference herein.

STATEMENT OF GOVERNMENT INTEREST

Not applicable.

BACKGROUND OF THE INVENTION

A submersible vehicle must be able to develop states of positive, neutral, and negative buoyancy to control vehicle depth in the water.

A ballast system is often used to (1) submerge and return a vehicle to the surface, (2) dive in a controlled fashion to any commanded depth, and (3) operate the vehicle at commanded depths within the water column. It must operate in a natural column of water from the surface to the bottom of a body of water having differences in physical and chemical properties, including temperature and salinity at various depths.

Local temperature and salinity variations cause differences in water density, causing changes in vehicle buoyancy, applying forces that cause the vehicle to depart from its commanded depth. For reference the density of seawater is on average 1,025 kg/m³.

The equation for buoyancy is B=ρ×V×g where:

• • ρ—Density of the liquid the object is immersed
  • V—Volume of the displaced liquid
  • g—Gravitational acceleration; and
  • B—Buoyant force

As the density of the liquid around the submersible vehicle changes, the buoyant force changes, causing the vehicle to ascend or descend in the water column. Assuming the mass of the vehicle constant, changing the volume of the vehicle effectively changes the density of the vehicle. The buoyant force can be manipulated through changes in the volume of the vehicle, thereby controlling the depth of the vehicle.

Controlling motion in the vertical plane of motion is the purview of the ballast system. It achieves this by increasing or decreasing the overall volume of the system while keeping the mass constant.

Depth control is critical to the mission of most small submersibles which includes tasks like mapping of the sea floor, inspection of underwater infrastructure, examination of aquatic life, and locating and tracking objects at or below the surface.

There are a variety of manned and unmanned submersibles used for underwater surveying, exploration, and defense applications. They rely on primitive ballast systems or use constant forward motion so control surfaces such as diving planes and other trim surfaces control depth.

The latter method makes it very difficult, if not impossible, for a vehicle to stay on station or “hover” over a precise location. This has an adverse impact on the vehicle range and endurance.

An example of a small submersible specifications commonly in use:

Approximate Ballast System Size in Gallons as a function of System Size in lbs.

The submersibles defined so far run on battery power and therefore have limited range and endurance, two key performance metrics.

Variable ballast systems exist on large submarines that have fewer restrictions on space and available power. A nuclear submarine, for instance, with a virtually limitless power source and a large space uses compressed air to create positive buoyancy in ballast tanks. In this method, the ballast tank/compressed air system changes the overall mass of the submarine resulting in a change in buoyancy.

There are significant challenges to scaling down this type of system to fit a small submersible as small as eight feet long and less than 150 lbs.

The space constraints on these submersibles force designers to make difficult tradeoffs between battery size, vehicle power, control systems and payload.

It is possible to design a ballast system, using a variety of different pumps including centrifugal, piston, diaphragm, and screw pumps. But these pumps fall short either in their ability to create enough pressure and flow to drive an effective variable ballast system, or are too large and require too much power.

Conventional pumps are all driven by large brushless DC motors with power requirements too high to be suitable for small submersibles. They must be combined with auxiliary components such as accumulators, control valves and additional plumbing which take up more space and require more power.

In the absence of a variable ballast system a vehicle must use its propulsion and controls system constantly, thereby draining battery power and reducing range and endurance. Again, the latter also makes it nearly impossible for a vehicle to hover in a specific location, a critical requirement for many missions.

In the absence of a ballast system, small submersibles, especially those that rely on propulsion for depth control, run the risk of sinking in the event of a loss of power. It is important that this issue is addressed in any ballast system.

The ballast design must also be scalable to a small size and weight, be suitable for small submersibles, and manufactured at a lower cost.

SUMMARY OF THE INVENTION

An improved ballast control system is used which fits small submersible vehicles. The system varies the vehicle ballast in comparison to the surrounding water without the need for the propulsion system to regulate the depth.

A piezoelectric fluid pump controls the ballast system. The pump can satisfactorily move fluid between the internal reservoir tank and an external bladder by producing 500 psi at 0.25 LPM in a small package with low weight. This pressure is sufficient to operate at depths of up to 1,000 ft below the water surface, with enough flow to pump the system capacity in about 1 minute. The piezoelectric pump includes a transformer to obtain suitable motor voltage when connected to submersible battery voltages.

The ballast system can also be configured to trim the attitude of the vehicle along its longitudinal and lateral axes. The ballast control system provides these capabilities with minimal draw on the vehicle's battery system which improves the vehicle's payload capacity, range and endurance.

The ballast system includes a separate hydraulic circuit with a pressurized canister that inflates the bladder in the reservoir tank and moves fluid to the external bladder. The circuit provides submersible vehicle recovery when power is lost.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a schematic of the embodied ballast control system.

FIG. 2 shows a schematic of a hydraulic circuit that causes the small submersible to rise in the event of power failure.

DETAILED DESCRIPTION OF THE INVENTION

The numeric labels in the specification and in FIG. 1 are described as follows:

• • 1 Fluid Reservoir. The sealed fluid reservoir 1 houses an inert gas bladder 2.
  • 2 Internal Bladder. An internal bladder is an inert gas bladder 2 and is integrated into the fluid reservoir. It is filled with an inert, compressible gas such as nitrogen. It is filled with a known quantity of gas and sealed via a fitting accessible on the outside of the fluid reservoir 1. The bladder isolates the gas from the liquid, enabling the fully closed ballast system to move reservoir fluid into and out of the reservoir by increasing/decreasing the reservoir pressure.
  • 3 Pressure/Flow/Temp Sensor. The Pressure/Flow/Temp Sensor 3 senses the fluid pressure and temperature in the fluid reservoir 1. It sends this information to system control module 11.
  • 4 External Bladder. The external bladder 4 is constructed of durable rubber or other elastomer and is exposed directly or indirectly to the ambient ocean pressure, depending on the needs of overall vehicle design. It may range in volume from a fraction of a gallon to multiple gallons.
  • 5 Piezoelectric Pump. The piezoelectric fluid pump 5 provides pressure and flow to transfer fluid between the external bladder 4 and the fluid reservoir 1. The piezoelectric actuator may be equipped with a temperature sensor to prevent overheating and potential failure.
  • 6 Piezoelectric Driver. The piezoelectric driver 6 is powered from the vehicle battery(s) and converts it into an electrical signal that causes the piezoelectric pump actuator to expand and contract. This creates pressure and flow from the piezoelectric fluid pump 5.
  • 7 Four Way, Three Position Control Valve. The control valve 7 directs flow between the fluid reservoir 1, the piezoelectric fluid pump 5, and the external bladder 4. The control valve may be a single four-way valve, or it can be a combination of valves that achieves the same result.
  • 8 External Bladder Pressure/Flow Sensor. The external bladder pressure/flow/temp sensor 8 sends pressure, flow, and temperature information from the external bladder 4 to the ballast system control module 10.
  • 9 Depth Sensor. The depth sensor 9 is exposed directly to the ambient ocean water and determines the actual depth of the vehicle by processing temperature, salinity, and conductivity of the water. The sensor is powered by the vehicle batteries and transmits data directly to the ballast system control module 10.
  • 10 Ballast System Control Module. The ballast system control module 10 is a microprocessor that takes input from the depth sensor 9, the external bladder Pressure/Flow Sensor 9, and the Pressure/Temp Sensor 3 to determine overall system buoyancy. The system receives a depth setpoint signal from the vehicle navigation system 12 and sends commands to the piezoelectric driver 6 and the control valve 7 to transfer fluid between the fluid reservoir 1 and the external bladder 4. This creates the correct buoyancy to bring the vehicle to the commanded depth or to maintain the commanded depth.
  • 11 Vehicle Navigation System. The vehicle navigation system 11, carries a pre-programmed route or uses another method, such as a remote control, to command the vehicle propulsion and control systems to navigate the vehicle to the commanded location in three-dimensional space and time. The vehicle navigation system 11 sends depth commands to the ballast system control module 10.
  • 12a,b Piping Fluid direction into and out of the pump respectively.
  • 13 Pressure Boundary Wall. (Submarine Hull).
  • 14 Pump Reed Valves. These directional valves operate like rapid check valves.
  • 15 Pump Piston.
  • 21 Two-way solenoid valve. Used in an emergency backup system that brings the submersible vehicle to the surface when there is a power failure.
  • 22 Pressurized inert gas canister. Used in the emergency backup system.
  • 23 Shutoff arming valve. Used in the emergency backup system.

The embodied ballast system is designed to fit within the design constraints of a small submersible vehicle and to create a variable density differential between the vehicle and surrounding water. Typically, a submersible vehicle is unmanned and weighs less than 50 tons. The piezoelectric pump is less than 300 cubic inches in size, and weighs less than 10 pounds. This type of small pump is especially suitable for small, unmanned submersibles.

In contrast, a midget submarine is under 150 tons, operated by a crew of one to nine, without significant on-board living accommodations.

The ballast system can be adjusted for water temperature and salinity changes, enabling the vehicle to dive and ascend to desired depths in a controlled manner. It will maintain its commanded depth without use of the propulsion system or other controls surfaces. It will maintain a controlled depth without forward motion. Consequently, it does not require battery power, thereby extending the vehicle range and endurance.

The piezoelectric fluid pump is useable in a variable ballast system rated to 500 psi at 0.25 LPM, making it effective for depths up to 1,000 ft. It is approximately 4 inches in length and 2 inches in diameter.

In contrast, a comparable conventional pump ballast system will be over 20 times that size and require a large electromechanical motor running off 120V or 240V AC power, more than the small submersible would need to operate. Furthermore, the piezoelectric pump does not require auxiliary devices such as accumulators or multiple control valves.

The ballast system can be configured to trim the attitude of the vehicle along its longitudinal and/or lateral axes. The system provides these capabilities with minimal battery drain.

The ballast system is a closed loop hydraulic system. There is a defined amount of lighter-than-water fluid in the system. By transferring the fluid between the fluid reservoir 1 and the external bladder 4, the fluid volume of the system inside the submersible will increase or decrease while the submersible mass remains constant. This causes a change in buoyancy. The external bladder is pressurized by the water depth pressure around the submersible as shown in FIG. 1.

All components of the system are designed to run on vehicle power, often 12V or 24V DC.

The ballast system fluid can flow in two directions. In one direction it moves from the fluid reservoir 1 to the external bladder 4. The external bladder 4 expands as it fills with fluid, causing a decrease in submersible depth (rises). When fluid is moved from the external bladder 4 to the fluid reservoir 1, it causes an increase in submersible depth (falls).

Pressure is generated in the system through three means: a) by hydraulic power generated by the piezoelectric fluid pump 5, b) from the elastic forces of the external bladder 4, and c) the force of the water pressure exerted on the external bladder 4.

The fluid reservoir/bladder volumes can be a fraction of a gallon to multiple gallons to suit a wide range of submersible vehicles. The pump size can also be scaled to cover a range of pressures required to operate at different depths, and at different flow rates. It is capable of meeting the response time requirements of large and small systems.

The piezoelectric fluid pump 5 can be powered by a variety of actuator sizes that vary in length, diameter, and disc thickness. This will provide different amounts of pressure and flow based on the system needs. Preferably, one-way reed valves control the direction of fluid flow in the pump. They are advantageous because they default to the closed position when power is removed from the pump, preventing back flow. However, other embodiments may utilize other valve types, both active and passive.

Optionally, multiple pumps are linked in series to have a doubling effect on the pressure provided. Or, operating two pumps hydraulically placed in parallel will double the flow without compromising either the pressure or the instant on/off characteristics of the device.

As shown in FIG. 1, the ballast system can be fitted within the outer hull of a submersible. It is independently powered by the batteries that power the submersible electrical system.

The system may also have sensors to detect failures of any components that might result in the need to activate a back-up system, such as an emergency system that forces the submersible to the surface.

In FIG. 1, the system is controlled by the ballast system control module 10. The module includes a processor that monitors volume differentials, pressure, flow and temperature and the external environment.

There are two internal fluid sensors, the pressure/flow/temp sensor 3 housed in the fluid reservoir 1, and the external pressure/flow/temp 8 located at the inlet of the external bladder 4. Both sensors feed information to the ballast system control module 10 which uses them in calculating vehicle buoyancy.

Additionally, sensors may be incorporated into the system to improve accuracy and enable fault detection. For example, the piezoelectric driver 6 may also provide information on actuator movement to improve accuracy.

Sensor information is used in the overall vehicle buoyancy calculation to command the piezoelectric fluid pump 5 and control valve 7 to maintain the correct buoyancy. Buoyancy is controlled by the amount of fluid between the fluid reservoir 1 and external bladder 4 to achieve the correct system volume to ascend, descend, or maintain the commanded depth.

The ballast system control module 10 receives ambient water density information through the depth sensor 9 which uses a combination of water temperature and salinity readings to determine actual depth. In other embodiments the system may incorporate a sonar-based depth sounder, or other depth sensor, to improve overall system performance.

In a typical case, the vehicle navigation system 11 is preprogrammed with a mission profile that contains navigation information including commanded depths. It transmits the commanded depth to the ballast system control module 10 based on time and location.

The ballast system control module 10 obtains the depth sensor 9 output and the internal sensor output. It then calculates the differential between the two to determine the current state of buoyancy, positive, negative, or neutral. The ballast system control module 10 also takes the depth sensor 9 output and the depth commanded by the vehicle navigation system 11 to determine if there is a difference.

When preparing to launch the vehicle, the fluid reservoir 1 is partially filled with non-compressible, lighter-than-water fluid and the remainder of the volume is filled by an integrated bladder that is filled with an inert compressible gas, such as nitrogen. When the vehicle is at the surface, extra positive buoyancy may be generated to ease launch and recovery operations. Lighter-than-water fluid is a preferred embodiment, but is not a strict requirement.

Volume is changed by manipulating the position of the control valve 7 and activation of the piezoelectric fluid pump 5. The control valve 7 has three positions.

The System Operates the control valve 7 according to a depth algorithm:

To command an initial descent from the surface, the ballast system control module 10 sends a signal to the control valve 7, commanding it to Position III. This is the Powered Dive Position which directs flow from the external bladder 4 to the Piezoelectric Hydraulic Pump 5 and on to the fluid reservoir 1. It simultaneously turns on the piezoelectric pump to generate pressure, which forces fluid into the fluid reservoir 1, squeezing the internal bladder 2, creating a decrease in submarine air volume, causing the vehicle to descend.

As the vehicle passes approximately 10 feet of depth the ambient ocean pressure becomes enough to force fluid from the external bladder 4 into the fluid reservoir 1 unassisted. As the vehicle passes this point, the ballast system control module 10 continues with control valve Position III and turns the pump off.

As the vehicle approaches the commanded depth, the ballast system control module 10 sends a signal to the control valve 7 to switch to Position II. This is the descent arrest/ascent position where fluid is routed from the internal bladder through the control valve 7 to the piezoelectric pump 5 (on) which creates pressure, forcing fluid back into the external bladder 4. This creates a larger air volume in the submarine to hold the vehicle at the commanded depth.

In this configuration, the vehicle propulsion and control surfaces may be utilized to assist in arresting the descent and ensuring the vehicle levels at the commanded depth.

When the vehicle reaches the commanded depth, the ballast system control module 10 sends a signal to the control valve 7 commanding it to Position I (closed) and prevents any flow in the system. This holds both bladders constant and prevents depth changes. In Position I the piezoelectric pump 5 is turned off.

As the density of the ambient water changes as the vehicle moves between water of varying salinity and temperature the ballast system may be cycled between Position I, Position II, and Position III to maintain the commanded depth, with the pump on or off as needed.

When an ascent is required, the control valve 7 is switched to Position II, routing fluid from the internal bladder to the external bladder to allow ascent. The pump is turned on as needed to force fluid into the external bladder. As the vehicle reaches the commanded depth it will cycle to any position I, II, or III to maintain surface position. The vehicle propulsion and controls system may be used to assist the vehicle in leveling off.

In another embodiment a second piezoelectric fluid pump may be incorporated in place of, or in addition to the control valve 8.

The piezoelectric fluid pump 5 serves two functions, the first to fill and increase fluid pressure in the fluid reservoir. The second function is to block flow from the fluid reservoir 1 to the external bladder 4. The unique design of the piezoelectric fluid pump includes two reed valves 14a,b, which function as a check valve, simplifying the system and decreasing size.

The piezoelectric actuator is a high-frequency, short stroke piston which forces fluid through a set of internal valves that regulate the direction of flow. When an electric field is applied to the actuator it expands and contracts at a command frequency and amplitude, with no lag. When the electric field is removed, pump motion stops and the reed valve(s) return to the closed position. This immediately prevents back-flow from the higher pressure external bladder 4 or the higher pressure fluid reservoir 1, depending upon the control valve 7 position.

Normally, the control valve 7 also returns to the center position, additionally preventing back-flow. The pump can maintain this state without any electric power applied, reducing the load on the vehicle batteries. The piezoelectric actuator is preferably equipped with a temperature sensor to prevent overheating and potential pump failure.

The extremely short piston 15 stroke (microns in length) and high frequency of the pump 5 (hundreds of cycles/second) creates virtually instant maximum pressure, enabling very precise fluid control in the system, which translates to precise depth control.

The piezoelectric fluid pump 5 works most effectively with degassed fluid in a closed system. Any gas bubbles in the fluid will decrease in size when the submersible dives, and increase in size when the submersible rises. This causes undesirable control stability problems. For that reason, an inert gas bladder 2 is integrated inside the fluid reservoir 1. This configuration allows the fluid reservoir 1 volume to be varied for the required density differentials in the system, and allows the vehicle to navigate across a practical dept range. The maximum depth is determined by the pressure that can be generated by the piezoelectric fluid pump 5.

In FIG. 2, an emergency backup system is shown in the case of electrical failure. The goal is to provide an independent circuit for fluid transfer from the fluid reservoir 1 to the external bladder 4. A small canister 22 of high pressure inert gas is connected to the internal bladder 2 of the fluid reservoir 1.

An arming valve 23 is opened just before the submersible vehicle is about to launch.

The solenoid valve 21 is shown in the deactivated state. When the vehicle is powered, the pump controller 10 activates solenoid valve. Then no flow is allowed through the solenoid valve. If power is lost, the solenoid valve deactivates which allows fluid movement between the fluid reservoir 1 and the external bladder 4, and pressurized gas to the internal bladder. This lowers the fluid volume in the reservoir allowing the submersible vehicle to rise. Similarly, two solenoid valves could equally be used where the gas line and hydraulic lines have separate valves that open in case of power loss.

While various embodiments of the present invention have been described, the invention may be modified and adapted to various operational methods by those skilled in the art. Therefore, this invention is not limited to the description and figure shown herein, and includes all such embodiments, changes, and modifications that are encompassed by the scope of the claims.