EP3728020B1 - Method for controlling the buoyancy of a submarine vehicle - Google Patents

Description

The field of the invention is that of underwater vehicles. These vehicles are capable of exhibiting negative buoyancy, that is to say capable of being completely submerged. It relates more particularly to unmanned underwater vehicles, also called UUVs with reference to the Anglo-Saxon expression "Unmanned Underwater Vehicles", which can be autonomous vehicles also called AUVs with reference to the Anglo-Saxon expression (" Autonomous Underwater Vehicle”) or non-autonomous vehicles also called ROV in reference to the Anglo-Saxon expression “remotely operated vehicle” or vehicles commonly called fish when they are devoid of thrusters.

Buoyancy is the force acting on the underwater vehicle resulting from the difference between Archimedes' thrust and the weight of the vehicle. This force can be directed from bottom to top (positive buoyancy, weight less than Archimedes' buoyancy) or from top to bottom (negative buoyancy, weight greater than Archimedes' buoyancy).

An underwater vehicle must conventionally have positive buoyancy when launched. Thus, the vehicle naturally returns to float on the surface of the water when it is only subjected to Archimedes' thrust and its weight. This allows its recovery from a surface platform when the vehicle no longer has energy for its propulsion. This also allows the underwater vehicle to communicate with a surface platform by radio communication means of the underwater vehicle which are then emerged. This also allows the underwater vehicle to be supplied with energy from a surface platform without it consuming energy for its propulsion. Once the underwater vehicle has been launched, it is conventionally intended to carry out underwater missions, for example inspection, so that it is brought to dive, by means of a thruster, to join positions at great depth before returning to the surface, for example, to refuel, to communicate with a high transmission rate, to be retrieved from a surface platform.

However, in order to dive under water to carry out its mission and maintain its submerged position, the underwater vehicle which has positive buoyancy, must overcome forces which are all the greater as its buoyancy is significant. He may thus find himself unable to dive underwater or have to consume, to dive, an energy all the more important as his buoyancy is important which has the effect of limiting his endurance and the duration of his mission. . When its positive buoyancy is too high, the vehicle may also have difficulty diving due to disturbances generated by the sea surface state which partially and randomly uncovers the craft's actuators, such as its thruster, becoming ineffective out of water. Adjusting the buoyancy of the vehicle (or balancing by adjusting its buoyancy) is therefore essential and this step must be carried out with great precision.

Underwater vehicles can also be balanced with negative buoyancy when launched. This makes it possible to be in a configuration where the underwater vehicle will naturally come to rest on the seabed and thus be invisible from the surface. During its mission, the underwater vehicle will be brought to the surface punctually or towards the surface. However, in order to rise to the surface the underwater vehicle which has negative buoyancy, must overcome forces which are all the greater as its negative buoyancy is low, that is to say has a high absolute value. It may thus find itself unable to ascend or having to consume, in order to ascend, an energy which is all the more important as its negative buoyancy is low, which has the effect of limiting its autonomy and the duration of its mission.

It therefore appears that to carry out its mission in an optimal manner, the underwater vehicle must be able to adjust its buoyancy according to the stages of the mission: positive buoyancy when launching, neutral during the diving phases and negative to be discreet.

One solution for adjusting the buoyancy of underwater vehicles is to carry out a static weighing before launching it so as to substantially balance the weight and the buoyancy of Archimedes. Flaws of this solution are that this setting is fixed for the duration of the mission (buoyancy cannot be controlled) and that this setting is not robust to local variations in water density (the vehicle initially weighed to be slightly buoyant in sea water can become sinking when arriving in fresh water) nor to variations in the mass of the underwater vehicle, for example when a shell or seaweed comes to be fixed on its hull, nor to the variations in volume of the body of the vehicle, for example, by compression of the hull under the effect of the pressure.

Another solution consists in equipping the underwater vehicle with means for varying its density of the underwater vehicle. A method for adjusting the buoyancy of an underwater vehicle when it is put in place conventionally consists in manually controlling the means for varying the density. The vehicle initially has a positive buoyancy which is not precisely known, an operator controls the means for varying the density to slowly increase the density of the underwater vehicle so that its buoyancy decreases continuously until that the vehicle reaches a lower positive target buoyancy. This solution is long, delicate and difficult to automate. For example, a small change in the mass of the underwater vehicle can cause the buoyancy of the underwater vehicle to vary significantly, so the adjustment of the buoyancy of the underwater vehicle must be carried out with great precision and very slowly. to prevent the underwater vehicle from sinking which may prevent the operator from recovering the vehicle. The documents US 4,266,500A , US2007/125289 A1 , JP H10 86894 A And US 2016/107735 A1 constitute the prior art.

An object of the invention is to limit at least one of the above drawbacks.

• à partir d'une flottabilité initiale du véhicule maintenant le véhicule au niveau de la première surface, une première étape de modification de la masse volumique du véhicule de façon qu'il se rapproche de la deuxième surface, la première étape étant mise en oeuvre jusqu'à une deuxième étape de détection du franchissement, par le véhicule, d'un seuil de distance non nul prédéterminé par rapport la première surface, selon l'axe vertical,
• puis une troisième étape de modification de la masse volumique du véhicule jusqu'à ce que le véhicule présente sensiblement la flottabilité cible.

To this end, the subject of the invention is a method for adjusting the buoyancy of an underwater vehicle so that it has substantially a predetermined target buoyancy when it is immersed in a volume of liquid delimited, along an axis vertical, by a first surface and a second surface, the method comprising:

• from an initial buoyancy of the vehicle maintaining the vehicle at the level of the first surface, a first step of modifying the density of the vehicle so that it approaches the second surface, the first step being implemented until 'in a second step of detecting the crossing, by the vehicle, of a predetermined non-zero distance threshold with respect to the first surface, along the vertical axis,
• then a third step of modifying the density of the vehicle until the vehicle has substantially the target buoyancy.

Advantageously, the variation in density of the vehicle during the third step is predetermined.

Advantageously, the first step is implemented so that the vehicle has a predetermined buoyancy when detecting the crossing of the threshold.

Advantageously, the target buoyancy has the same sign as the initial buoyancy and has an absolute value less than the absolute value of the initial buoyancy.

Advantageously, during the implementation of the method, the vehicle moves, along the vertical axis, solely under the effect of a variation in its buoyancy.

Advantageously, the first surface is the surface of the liquid, that is to say its upper surface, and the initial buoyancy is positive, the first step of modifying the density being a step of increasing the density of the vehicle , the distance threshold being an immersion threshold.

Advantageously, the first surface is a bottom of the volume of liquid, the initial buoyancy is negative, the first step of modifying the density being a step of reducing the density, the distance threshold being an altitude threshold with respect to on the first surface.

Advantageously, during the first step and during the third step, the mass of the vehicle is varied, at constant volume of the underwater vehicle, and/or the volume of the marine vehicle is varied at constant mass of the underwater vehicle .

Advantageously, the vehicle comprises a first tank with variable density, whose variation in density varies the density of the vehicle, arranged close to a first longitudinal end of the vehicle, and a second tank with variable density, whose density variation varies the density of the vehicle and which is disposed close to a second longitudinal end of the vehicle, in which, during the first step, the density of a single predetermined tank is varied, taken from among the first tank and the second tank.

Advantageously, the invention also relates to a balancing method comprising the method of adjusting the buoyancy followed by a step of adjusting the longitudinal attitude of the vehicle.

• à partir d'une flottabilité initiale maintenant le véhicule au niveau de la première surface ; une première étape de modification de la masse volumique du véhicule par les moyens pour faire varier la flottabilité, de façon que le véhicule se rapproche d'une deuxième surface délimitant également le volume de liquide selon la direction verticale, la première étape étant mise en oeuvre jusqu'à une deuxième étape de détection, par le capteur, du franchissement du seuil de distance par le véhicule,
• puis une troisième étape de modification de la masse volumique du véhicule, par les moyens pour faire varier la flottabilité, jusqu'à ce que le véhicule présente sensiblement la flottabilité cible.

The invention also relates to a device for adjusting the buoyancy making it possible to adjust the buoyancy of the underwater vehicle and comprising means for varying the density of the underwater vehicle and at least one sensor making it possible to detect the crossing, by the vehicle, of a predetermined non-zero distance threshold with respect to a first surface delimiting, in a vertical direction, the volume of a liquid in which the vehicle is immersed, the buoyancy adjustment device being configured to put implements a method comprising the following steps when a buoyancy adjustment condition is verified:

• from an initial buoyancy maintaining the vehicle at the level of the first surface; a first step of modifying the density of the vehicle by the means for varying the buoyancy, so that the vehicle approaches a second surface also delimiting the volume of liquid in the vertical direction, the first step being implemented up to a second step of detection, by the sensor, of the crossing of the distance threshold by the vehicle,
• then a third step of modifying the density of the vehicle, by the means for varying the buoyancy, until the vehicle substantially exhibits the target buoyancy.

Advantageously, the first step is implemented so that the vehicle has a predetermined buoyancy when detecting the crossing of the threshold.

Advantageously, the variation of the density during the third step is predetermined.

Advantageously, the target buoyancy has the same sign as the initial buoyancy and has an absolute value less than the absolute value of the initial buoyancy.

The invention also relates to a vehicle comprising the buoyancy adjustment variation device according to the invention.

• les figures 1a, 1b et 1c représentent différentes situations d'un engin sous-marin lors de la mise en oeuvre du procédé selon l'invention,
• la figure 2 représente schématiquement un exemple de moyens d'ajustement de la flottabilité d'engin marin selon l'invention.

The invention will be better understood on studying a few embodiments described by way of non-limiting examples, and illustrated by the appended drawings in which:

• THE figure 1a , 1b And 1 C represent different situations of an underwater vehicle during the implementation of the method according to the invention,
• there figure 2 schematically represents an example of means for adjusting the buoyancy of a marine vehicle according to the invention.

From one figure to another the same elements are designated by the same reference numerals.

In the present patent application, the vertical and horizontal directions are defined in a terrestrial frame of reference. The upper and lower positions being determined along a vertical axis z of the terrestrial reference.

The invention relates to a method for adjusting the buoyancy of the underwater vehicle 1 so that it has a final buoyancy substantially equal to a target buoyancy.

The underwater vehicle 1 initially has a buoyancy, called initial, such that it maintains the underwater vehicle 1 at the level of a surface S, called initial, delimiting the volume of liquid in which the underwater vehicle is immersed , for example water E , for example sea water or fresh water, in a vertical direction (defined in a terrestrial frame of reference). The vehicle 1 is maintained at the level of the initial surface S solely by its buoyancy.

This initial surface is, for example, the upper surface of the volume of the liquid, for example the surface of the water S. The initial buoyancy of the underwater vehicle 1 is then positive and the underwater vehicle initially floats on the surface of the water.

The volume of liquid in which the underwater vehicle is immersed is delimited, in the vertical direction z, by two surfaces spaced from each other in the vertical direction, including the upper surface and a lower surface. In the case of a vehicle immersed in sea water, this volume is delimited by the surface of the water S (upper surface) and by the seabed Fd (lower surface).

The initial surface may alternatively be the lower surface of the volume of the liquid, that is to say the bottom of the volume of the liquid, for example the seabed. The initial buoyancy of the vehicle is then negative and the vehicle is initially placed on the lower surface of the volume of the liquid, that is to say on the ground or the seabed.

On the picture 1a , there is shown an underwater vehicle 1 initially having a positive buoyancy. The underwater vehicle 1, in its initial situation represented on the picture 1a , floats on the surface of the water S. Alternatively, the underwater vehicle 1 initially has a negative buoyancy so as to be maintained against the seabed Fd.

Typically, the initial positive buoyancy of the vehicle is of the order of 5% of its weight. The purpose of the process is to give the underwater vehicle a final buoyancy closest to 0 while remaining positive. It is typically sought to achieve a setpoint buoyancy of between 0.05% and 0.01% of the weight of the vehicle. For a 1000 kg vehicle, this amounts to having to balance the vehicle with an accuracy of 100 g. When the underwater vehicle has very low positive buoyancy, its ascent to the surface, in the event of failure of its thruster, is ensured and its energy consumption for diving towards the seabed is minimal. The fact that the setpoint buoyancy is positive gives a buoyancy margin which makes it possible to guarantee that the final buoyancy of the underwater vehicle at the end of the implementation of the method is positive.

The method includes the steps shown in the figures 1a to 1c .

This method comprises a first step 100 of modifying the density of the vehicle 1 so that it moves away from the initial surface S while approaching the other surface delimiting the volume of liquid. Thus, the vehicle moves in the vertical direction. The variation in the density of the vehicle varies the buoyancy of the vehicle. In the example of figures 1a to 1c , this step 100 is a step of increasing the density of the underwater vehicle 1 so that the underwater vehicle 1 sinks, that is to say moves away from the surface of the water S or approaches the seabed Fd. In the case where the underwater vehicle has a negative initial buoyancy, this step is a step of decreasing the density of the underwater vehicle 1 so that the underwater vehicle 1 rises towards the surface of the water S , i.e. away from the seabed Fd.

The first step 100 of modifying the density is implemented up to a second step 200.

During the first step 100, the attitude of the vehicle can vary under the effect of the variation in the density, which does not impact the buoyancy adjustment method according to the invention.

This second step 200 is a step of detecting the crossing, by the underwater vehicle 1, of a predetermined non-zero distance threshold SD with respect to the initial surface S, along a vertical axis z. In the case where the initial surface S is the water surface, the distance threshold SD is an immersion threshold or distance threshold relative to the water surface. In the cases where the initial surface S is the seabed, the distance threshold SD is an altitude threshold, that is to say of distance relative to the seabed along the axis z.

The first step 100 is stopped as soon as the detection 200 of the crossing of the threshold SD by the vehicle 1.

The method then comprises a third step 300 of modifying the density of the underwater vehicle 1 until the buoyancy of the underwater vehicle 1 is substantially equal to the target buoyancy.

The density of the vehicle 1 advantageously varies in a single direction during the third step 300. In other words, the buoyancy of the vehicle varies only in one direction during the step 300.

When the underwater vehicle 1 crosses the distance threshold SD, it has a known or determinable buoyancy. When the vehicle crosses the predetermined distance threshold SD, it always has the same buoyancy, regardless of its initial buoyancy, if the buoyancy variation conditions, during the first step 100, are the same for the same initial longitudinal attitude. This buoyancy serves as a reference. Once this reference is available, it is possible to determine a density variation which will make it possible to reach the target buoyancy during step 300 whatever the initial buoyancy. It is then sufficient to evaluate this density variation only once.

For example, for a vehicle of approximately 1000 kg which is weighed down using a pump having a chosen flow rate, and an immersion trigger threshold at 1m, will cross the threshold with always the same buoyancy. An example of buoyancy when crossing the threshold of - 20N corresponds to approximately -20/(1000*10) = 0.2% of the weight of the vehicle for a newton constant rounded to 10 N m ² kg ^-2 . If the target buoyancy is 0.05% of the weight of the vehicle, then once the threshold is crossed, the vehicle must be lightened by (0.05+0.2)/100 * 1000 = 2.5 kg.

The method according to the invention requires a relative precision of adjustment of the buoyancy of the vehicle, which is easier to obtain than an absolute precision of the buoyancy of the vehicle. It is independent of the initial buoyancy of the vehicle and is therefore reproducible.

This method can be implemented at any time, that is to say, when launching the vehicle or during a mission or during a reconfiguration of the underwater vehicle (adding or removing sensors, for example).

This method can be easily automated because its steps are few and sequential, it therefore adapts well to UUV unmanned vehicles and does not require the intervention of an outside operator.

The method according to the invention is independent of the mass and/or volume of the underwater vehicle 1. It makes it possible to achieve the target buoyancy even if one of these two parameters varies, for example in the event of addition or removal voluntary or not of components or particles, in particular in the event of loss of a blade of a propeller thruster after its initial launch.

The method according to the invention is much faster than a series of weighings carried out by an operator in water having a certain density, making it possible to calculate the quantity of ballast to be added or removed from the vehicle.

This solution only requires an immersion or pressure sensor in order to detect the crossing of the threshold. This type of sensor is simple and inexpensive.

In summary, the proposed solution is inexpensive, simple to implement and makes it possible to balance the underwater vehicle in a reliable, repeatable and easily automatable manner (simple algorithm simply requiring an immersion sensor).

In the example of figures 1a to 1c , the initial buoyancy is positive, the target buoyancy is positive and lower than the initial buoyancy and the buoyancy of the vehicle when it crosses the threshold SD is negative. In other words, the vehicle is made slightly sinking for a short time, it then moves away from the surface of the water S, before making it float again. The buoyancy of the underwater vehicle increases during the third stage 300 until positive buoyancy, it then rises to the surface S of the water where it floats during the third stage 300. Alternatively, the final buoyancy is negative or zero.

In this embodiment, the buoyancy varies in one direction during step 100 and in the opposite direction during step 300 but alternatively, the variation of the density during these steps could be such that the buoyancy varies in the same meaning during these two stages.

As a variant, the initial buoyancy is negative, the buoyancy on crossing the threshold is positive and the target buoyancy is positive. Alternatively, the final buoyancy is negative or zero.

Advantageously, the target buoyancy has an absolute value lower than the absolute value of the initial buoyancy.

For example, the target buoyancy is negative and of absolute value less than the absolute value of the negative initial buoyancy and the buoyancy when crossing the threshold is positive. This makes it possible to limit the energy necessary for the vehicle to subsequently rise to the surface under the effect of its propulsion. This process is faster than a continuous variation of the vehicle and avoids an unexpected rise of the vehicle to the surface. This process is also more reliable than static weighing.

During steps 100 and 300, the density of the underwater vehicle 1 is varied by varying its mass at constant volume of the underwater vehicle and/or by modifying its volume at constant mass of the underwater vehicle.

The variation in density of the vehicle during step 300, that is to say the variation in mass or volume of the vehicle during step 300 depends on the buoyancy of the vehicle when crossing the threshold is detected 20 .

The variation of the density during step 300 depends on the distance threshold SD and on the target buoyancy Fc.

Advantageously, the method is implemented in such a way that the vehicle 1 has a predetermined buoyancy when the crossing of the threshold is detected, that is to say when the first step 100 stops.

This makes it possible to improve the precision of the adjustment of the final buoyancy of the underwater vehicle 1 and to simplify the process. It suffices to determine only once the variation of mass or volume then necessary to reach the target buoyancy during step 300. In other words, the variation of mass or volume required to reach the target buoyancy during step 300 is predetermined.

The change in density or mass or volume in step 300 is predetermined. It depends on the buoyancy of the vehicle when the crossing of the threshold is detected.

This variation during step 300 therefore depends on the initial conditions and the conditions for carrying out step 100. It is the same for the same initial conditions and the conditions for carrying out step 100.

The variation in density necessary to reach the target buoyancy during step 300 can be obtained beforehand iteratively or by trial and error. For example, the method according to the invention is implemented several times with predetermined initial conditions and predetermined density variation conditions during step 100 and, once the crossing of the threshold is detected and the step 100 stopped, the volume (or mass) of the vehicle is varied. During the different implementations of the method, the volume is varied by different values and the final buoyancy is compared each time with the target buoyancy. This comparison step can be carried out by measuring a magnitude representative of the final buoyancy and by comparing this value with the value that this magnitude should have for the target buoyancy. This is for example a distance, taken in the vertical direction, of the vehicle relative to a predetermined surface of the liquid. Alternatively, it is possible to determine the variation in volume necessary, during step 300, to reach the target buoyancy by varying the volume of the vehicle little by little until the target buoyancy is reached.

The buoyancy of the vehicle at the time of detection of the crossing of the distance threshold SD or at the stoppage of the first step depends on adjustable conditions of variations in the density of the vehicle during step 100 which have an influence on the buoyancy of the underwater vehicle 1 when the crossing of the threshold is detected. For example, if vehicle 1 has several tanks whose density can be varied independently to vary that of the vehicle, the choice of tank has an influence on the buoyancy of the vehicle when crossing the threshold, just like the speed of variation. the mass or volume of the vehicle (i.e. the rate of change in mass or volume of each tank). These parameters are predetermined so that the vehicle 1 has a predetermined buoyancy when detecting the crossing of the threshold, that is to say when the first step 100 is stopped.

Alternatively, variations in mass or volume to be applied to the vehicle during step 300 to reach the target buoyancy Fc or different target buoyancy can be determined prior to the implementation of the method for different values of these parameters and of the target buoyancy and listed in a table as explained above. The method advantageously comprises, prior to step 300, a step of determining the variation in mass or volume to be applied to the vehicle during step 300 to reach the target buoyancy Fc from the value of at least one parameter, for example by consulting a table.

The parameters may also include a volume density of the liquid. The change in mass or volume can be determined for several densities. The method can then comprise a step of determining a density of the liquid in which the vehicle is immersed, for example from a measurement of the salinity of the water obtained from a salinity sensor 35. Alternatively, the density is predetermined.

Advantageously, at least one initial condition for implementing step 100 having an influence on the buoyancy of the underwater vehicle at the time of detection of the threshold is predetermined, such as for example the initial longitudinal trim of the underwater vehicle. predetermined initial sailor, for example zero or of a different value.

The method can then comprise, prior to step 100, a step of adjusting the attitude of the vehicle so that the vehicle has a predetermined longitudinal attitude, if the attitude of the vehicle is different from the predetermined longitudinal attitude.

As a variant, the variation in mass or volume is determined independently of this initial condition.

Advantageously, during the implementation of the method, the speed of the underwater vehicle along the vertical axis is only induced by a variation of its buoyancy in calm sea conditions.

Advantageously, the vehicle 1 has a substantially zero speed, relative to the liquid in which it is immersed, in a horizontal plane.

This makes it possible to avoid the disturbances generated by the hydrodynamic lift on the buoyancy of the vehicle when crossing the threshold and therefore on the final buoyancy of the vehicle.

Advantageously, the initial speed of the vehicle relative to the liquid in which it is immersed is zero.

As can be seen in the figures, the underwater vehicle 1 may comprise a thruster 22 intended to propel the marine vehicle 1. Advantageously, the thruster 22 is stopped for the entire duration of the implementation of the buoyancy adjustment method. Alternatively, the vehicle has no propellant.

On the picture 2 , there are shown means for adjusting the buoyancy 10 of the vehicle 1 according to the invention being able to implement the method according to the invention. These means are advantageously configured to implement the steps of the method when a buoyancy adjustment condition is verified.

The vehicle 1 comprises means DET for detecting the crossing of the distance threshold SD making it possible to check whether the vehicle exceeds the distance threshold. These means comprise at least one sensor 2, also represented on the figures 1a to 1c , capable of measuring a quantity representative of the distance separating the vehicle from the initial surface along the z axis. This sensor is for example an immersion or pressure sensor. The detection means DET also comprise a comparator COMP making it possible to check whether a distance of the vehicle 1, with respect to the surface S, determined on the basis of this measurement is equal to the distance threshold SD. The sensor 2 is fixed relative to the body 3 of the vehicle 1.

The underwater vehicle 1 comprises a device REG for adjusting the buoyancy of the underwater vehicle 1 making it possible to adjust the buoyancy of the underwater vehicle 1.

The adjustment device REG comprises means VAR for varying the density of the vehicle 1 and a control member 26 making it possible to control these means so as to implement the method according to the invention. Advantageously, the control member is configured to control the VAR means to implement the method according to the invention.

The VAR means for varying the density comprise at least one reservoir 20 or 21 of variable density, that is to say of variable mass and fixed volume (as in the example of the figures 1a to 1c ) and/or at least one reservoir of variable volume and fixed mass, and means making it possible to vary this mass or this volume controllable by the control member 26.

The tanks 20, 21 are able to communicate with the environment in which the underwater vehicle is immersed so that the liquid in which the underwater vehicle 1 is immersed can circulate between these tanks and the marine environment so as to fill or empty the tanks of this liquid. This medium is for example the marine environment but can be any other liquid. In the rest of the text, reference will be made to the marine environment but the invention is of course applicable to any other liquid.

The tanks 20, 21 are able to communicate with the marine environment by respective hydraulic circuits 24, 25 which can be opened or closed by respective AV and AR valves 22, 23, the circulation of water from the marine environment to the tanks 20 , 21 (or vice versa) being caused by a pump 29 actuated by an actuator 30, for example, a motor. The actuator 30 and the AV and AR valves are controlled by the control member 26 to vary the masses of the reservoirs 20 and 21 by varying the volume of water contained in these reservoirs 20 and 21 (by rejection of the water contained in the tanks in the marine environment or vice versa) during steps 100 and 300. The control member 26 can also make it possible to control the actuator and the valves to vary the conditions of variation of the density (flow rate of the pump, distribution of the variations in mass between the reservoirs.) Alternatively, these conditions are fixed.

In this example, the mass of the tanks varying and the volume remaining fixed, the Archimedes thrust acting on the underwater vehicle is fixed during the implementation of the method (if it is considered that the portion of the vehicle situated outside water is negligible when the underwater vehicle floats) while its weight varies.

After detection of the crossing of the distance threshold SD, the variation in weight (in Newtons) necessary for the buoyancy of the underwater vehicle to reach the setpoint buoyancy Fc is constant and depends on the immersion threshold SD and the setpoint buoyancy Fc . For a setpoint buoyancy of 10 N and a buoyancy of the underwater vehicle 1 of -15 N on detection of the crossing of the threshold, the variation in weight that the underwater vehicle to reach the set buoyancy is 25 Newton.

The mass of the tanks must then be varied by Dm = DP/g = 25/9.81 = 2.548 kg, g being the acceleration due to gravity (9.81 ms ^-2 ). This represents a variation in water volume DV=Dm/d where d is the volumetric density of the liquid in which the underwater vehicle is immersed. In the case of seawater, d= 1.025 kg/L. So DV = 2.548/1.025 = 2.486 L.

This means that 2.486 L of seawater must be removed from the tanks to lighten the vehicle so as to obtain the set buoyancy.

On the non-limiting embodiment of the figures, the tanks 20 and 21 are spaced along an axis x of the underwater vehicle 1 which is, in the non-limiting example of the figures, a longitudinal axis along which the underwater vehicle extends longitudinally. The two tanks 20, 21 are then each placed close to one of the ends of the underwater vehicle 1. The tank 21 is placed close to the front front end and the tank 20 to the rear rear end of the vehicle under -marine. As a variant, the means for varying the buoyancy comprise a single reservoir or more than two reservoirs. The vehicle is intended to move mainly along the longitudinal axis in the direction of the rear rear end towards the front front end.

As a variant or in addition, the VAR means for varying the density comprise at least one tank, called an external tank of variable volume arranged so that a variation in the volume of the tank causes a modification of the volume of the underwater vehicle 1. This tank communicates for example with an internal tank disposed inside the body of the underwater vehicle via a valve so as to allow a fluid to pass from one of the tanks to the other or to block the passage of this fluid between the two tanks, a pump causing the circulation of the fluid via the valve. An actuator, for example a motor is provided to actuate the pump. The valve and the pump are controlled by a control member receiving measurements from an immersion sensor making it possible to measure immersion of the underwater vehicle and controlling the valve to vary the volume of the external tank so that the vehicle submarine has a set immersion received by the control unit. This solution causes less corrosion and reliability problems than the previous solution at the expense of the vehicle submarine. Two tanks may be provided, one at each longitudinal end of the underwater vehicle.

In this case, the weight of the underwater vehicle is constant but the buoyancy force varies during the implementation of the method. For a setpoint buoyancy of 10 N and a buoyancy of the underwater vehicle of −15 N at the start of step 200, the variation in Archimedes thrust DA that the underwater vehicle must undergo to reach the setpoint buoyancy is of 25 Newtons. It is therefore necessary to increase the volume of the underwater vehicle by DV = DA/(g*d) = 25/(9.81*1.025) = 2.486 L.

The control member 26 triggers the implementation of the method when the buoyancy adjustment condition is verified. The buoyancy adjustment condition can be verified when the control member receives a buoyancy adjustment setpoint C. As a variant, the method comprises a verification step consisting in verifying whether the buoyancy adjustment condition is verified, this step being implemented by the control member. This step can be performed from a measurement of the density of the liquid. The buoyancy adjustment setpoint is for example checked when the density of the water goes below or beyond a certain threshold or for example when a variation in the volume or the mass of the underwater vehicle exceeds a certain threshold (for example when shells have invested the hull of the underwater vehicle or during the installation of new equipment).

Advantageously, during step 100, the density of a single predetermined tank is modified, among the two tanks 20 and 21. This makes it possible to obtain a faster dive of the underwater vehicle 1 and therefore, the crossing of the immersion threshold takes place after adding a smaller quantity of water than when filling the two tanks simultaneously. The method is therefore faster (the quantity of water to be withdrawn from the reservoirs during step 200 is also less important) and requires less energy.

In the embodiment in which the volume of the underwater vehicle 1 is modified during step 100, it is possible to modify the volume of only one of the two variable-volume tanks located at one of the ends of the underwater vehicle so as to vary the volume of the underwater vehicle only near this end.

Advantageously, the tank whose density is varied during step 100 is the tank 20 located close to an end (here AR) opposite to another longitudinal end AV of the vehicle close to which a sensor or a vehicle radio wave transmitter for use when said sensor or detector is emerged for the vehicle to communicate with a sensor/detector external to the vehicle. This makes it possible to maintain communication between the vehicle and the outside for longer when it comes to dive.

As a variant, during step 100, the density of the reservoirs is modified according to a predetermined order of the reservoirs. For example, the tank 21 is first filled, then the tank 20 when the tank 21 is filled.

The invention also relates to a balancing method comprising the method for adjusting the buoyancy described above and a step of adjusting the longitudinal trim so that the vehicle has, at the end of the method, a set longitudinal trim .

To this end, the underwater vehicle 1 advantageously comprises means for adjusting the longitudinal trim of the body 11 of the underwater vehicle 10.

These means for adjusting the longitudinal trim of the body 11 comprise means for varying the longitudinal trim of the body of the underwater vehicle comprising, on the non-limiting example of the picture 2 , the two tanks 20, 21 spaced along the longitudinal axis x and placed respectively close to the rear rear end and the front front end of the body 11. The means for varying the longitudinal attitude of the body 10 comprise a hydraulic circuit 36 by which the reservoirs 21 communicate with each other so that the passage of a fluid from one to the other is possible via a valve 37 which can close the hydraulic circuit 36 or open it to allow or not this fluidic communication. A second pump 38 makes it possible to circulate the liquid between the two tanks via the valve 37 and a second associated actuator 39 making it possible to actuate the pump 38.

As a variant, the same pump can be used for the variation of the longitudinal trim and the buoyancy. A distributor or one or more additional valves are then provided to connect the pump to one of the two hydraulic circuits. The distributor or each valve is controlled by means of the control member.

The means for adjusting the longitudinal trim also comprise a control member making it possible to control the means making it possible to vary the longitudinal trim as a function of a set longitudinal trim and measurements of a trim sensor 40, allowing to measure the longitudinal attitude of the underwater vehicle, comprising for example immersion sensors arranged at the two respective longitudinal ends of the underwater vehicle or a gravity sensor measuring the verticality of the underwater vehicle or an inertial unit. This control member is the control member 26 of the buoyancy adjustment means on the picture 2 but may be another control device.

Adjusting the buoyancy and the longitudinal trim by the same tanks as shown in the figure makes it possible to pool these means, which limits the volume dedicated to these settings as well as the number of elements dedicated to these settings.

The means for adjusting the longitudinal trim are advantageously configured so that the step of adjusting the longitudinal trim of the underwater vehicle consists in transferring water (or other liquid) from a tank placed close to one end of the underwater vehicle, for example the tank 21, towards the other tank arranged close to the other longitudinal end of the underwater vehicle, for example the tank 20. Thus the buoyancy is not modified, only the plate varies.

Other types of controllable internal means can be used to vary the attitude of the underwater vehicle such as mobile masses in translations along the x axis, an example of which is described in the document UK 2,335,888 , but this system requires an additional and dedicated actuator.

As a variant, the reservoirs 20, 21 are replaced by variable volume reservoirs as described previously. Alternatively, the vehicle includes both types of tanks.

Each controller and the comparator may each include one or more dedicated electronic circuits or a general purpose circuit. Each electronic circuit can comprise a machine of reprogrammable calculation (a processor or a microcontroller for example) and/or a computer executing a program comprising a sequence of instructions and/or a dedicated calculation machine (for example a set of logic gates such as an FPGA, a DSP or an ASIC , or any other hardware module).

Claims (9)

1. A method for controlling the buoyancy of an underwater vehicle (1) so that it substantially has a predetermined target buoyancy Fc when it is immersed in a volume of liquid which is delimited by a first surface and a second surface, along a vertical axis (z), the method comprising:

   - from an initial buoyancy of the vehicle, which maintains the vehicle at the level of the first surface (S), a first step (100) of modifying the density of the vehicle (1) so that it approaches the second surface, the first step being implemented until a second step (200) of detecting that the vehicle (1) has exceeded a predetermined distance threshold (SD) which is not equal to zero and which is relative to the first surface (S), along the vertical axis, the first step being implemented in such a manner that the vehicle has a predetermined buoyancy when it is detected that the threshold has been exceeded,

   - then a third step (300) of modifying the density of the vehicle (1) until the vehicle substantially has the target buoyancy, the variation of the density during the third step being predetermined,

   the target buoyancy having the same sign as the initial buoyancy and an absolute value which is less than the absolute value of the initial buoyancy.
2. The method according to the preceding claim, wherein, during the implementation of the method, the vehicle moves along the vertical axis only under the action of a variation of the buoyancy thereof.
3. The method according to any one of the preceding claims, wherein the first surface is the surface of the liquid and the initial buoyancy is positive, the first step of modification of the density being a step of increasing the density of the vehicle, the distance threshold being an immersion threshold.
4. The method according to any one of claims 1 to 2, wherein the first surface is a base of the volume of liquid, the initial buoyancy is negative, the first step of modification of the density being a step of reducing the density, the distance threshold being an altitude threshold relative to the first surface.
5. The method according to any one of the preceding claims, wherein, during the first step (100) and during the third step (300), the mass of the vehicle is varied, with a constant volume of the underwater vehicle, and/or the volume of the marine vehicle is varied with a constant mass of the underwater vehicle.
6. The method according to any one of the preceding claims, wherein the vehicle (1) comprises a first tank which has variable density, the density variation of which varies the density of the vehicle (1) and which is arranged close to a first longitudinal end of the vehicle (1), and a second tank which has a variable density, the density variation of which varies the density of the vehicle (1) and which is arranged close to a second longitudinal end of the vehicle (1), wherein, during the first step (100), the density of a single predetermined tank taken from first tank and the second tank is varied.
7. A balancing method comprising the method for controlling the buoyancy according to any one of the preceding claims, followed by a step of controlling the longitudinal pitch of the vehicle (1).
8. A device for controlling the buoyancy of a submarine comprising means for variation of the buoyancy, which enable the density of the underwater vehicle to be varied, and at least one sensor, which enables detection that the vehicle has exceeded a predetermined distance threshold which is not equal to zero and which is relative to a first surface which delimits, in a vertical direction, the volume of a liquid in which the vehicle is immersed, the device for controlling the buoyancy being configured to implement a method comprising the following steps when a condition for controlling the buoyancy is verified:

   - from an initial buoyancy which maintains the vehicle at the level of the first surface; a first step of modification of the density of the vehicle by the means for variation of the buoyancy so that the vehicle approaches a second surface which delimits the volume of liquid in the vertical direction, the first step being implemented until a second step (200) of detection, by the sensor, that the distance threshold has been exceeded by the vehicle, the first step being implemented so that the vehicle has a predetermined buoyancy when it is detected that the threshold has been exceeded,

   - then a third step (300) of modification of the density of the vehicle (1) by the means for variation of the buoyancy until the vehicle has substantially the target buoyancy, the variation of the density during the third step being predetermined,

   - the target buoyancy having the same sign as the initial buoyancy and an absolute value which is less than the absolute value of the initial buoyancy.
9. An underwater vehicle comprising the device for controlling the buoyancy according to claim 8.

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