Variable Ballast System for Small Submersibles

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.

Not applicable.

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/m.

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

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.

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.

The numeric labels in the specification and inare described as follows:

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 reservoirand the external bladder, 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.

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 reservoirto the external bladder. The external bladderexpands as it fills with fluid, causing a decrease in submersible depth (rises). When fluid is moved from the external bladderto the fluid reservoir, 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, b) from the elastic forces of the external bladder, and c) the force of the water pressure exerted on the external bladder.

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 pumpcan 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, 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, the system is controlled by the ballast system control module. 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 sensorhoused in the fluid reservoir, and the external pressure/flow/templocated at the inlet of the external bladder. Both sensors feed information to the ballast system control modulewhich 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 drivermay also provide information on actuator movement to improve accuracy.

Sensor information is used in the overall vehicle buoyancy calculation to command the piezoelectric fluid pumpand control valveto maintain the correct buoyancy. Buoyancy is controlled by the amount of fluid between the fluid reservoirand external bladderto achieve the correct system volume to ascend, descend, or maintain the commanded depth.

The ballast system control modulereceives ambient water density information through the depth sensorwhich 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 systemis preprogrammed with a mission profile that contains navigation information including commanded depths. It transmits the commanded depth to the ballast system control modulebased on time and location.