FR3101324A1 - UNDERWATER EXPLORATION SYSTEM INCLUDING A FLEET OF DRONES - Google Patents

Abstract

The invention relates to an underwater exploration system (1), comprising: a master underwater drone (2) arranged to move autonomously according to a predetermined flight plan (E) and comprising a communication module ( C) to send communications signals; a plurality of follower submarine drones (31, 32, 33, 34, 35, 36), each comprising at least one magnetic field detection system (D), each follower drone further comprising a communication module (C) to receive communication signals from the master drone; the master drone being arranged to send navigation instructions (I) to the follower drones and each follower drone being arranged to move autonomously according to said movement instruction so that its movement is slaved to the movement of the master drone . Figure for the abstract: Fig. 1

Description

UNDERWATER EXPLORATION SYSTEM INCLUDING A FLEET OF DRONES

The invention relates to the field of underwater exploration. More specifically, the invention relates to the exploration of the deep seabed using a fleet of underwater drones.

The deep seabed contains sites with high mining potential. Indeed, the hydrothermal activity at the level of the oceanic ridges has for example contributed to creating hydrothermal sites, such as polymetallic sulphide masses, rich in metals, and in particular in Zinc, Silica, Copper and Lead, and generally enriched in Gold and Silver.

Despite their potential, these sites are not yet exploited. On the one hand, the depth at which these sites are located, beyond 800m, or even more than 4000m, makes conventional underwater exploration techniques unsuitable. On the other hand, the small dimensions of these sites, 10 to 500 meters in radius, with regard to the considerably extended ocean surface in which they are likely to be found, make it difficult to explore this surface in order to identify them. Finally, not all sites are likely to be exploited. Indeed, some sites are said to be inactive, i.e. cold and with presumed low biodiversity, and other sites are said to be active, i.e. hot and presenting oases of life. It goes without saying that the active sites must be exempt from any exploitation, in order to preserve the marine ecological environment. However, current site discovery techniques are essentially based on the detection of hydrothermal plumes present in the water column which are only associated with active sites and are therefore ineffective for locating inactive sites.

There is therefore a need for a solution for exploring the deep seabed in order to identify sites with mining potential, which is efficient and fast and which makes it possible to detect active sites and inactive sites.

In the field of underwater exploration, a known solution consists in using several autonomous underwater drones, also called AUV (Autonomous Underwater Vehicle), equipped with sensors allowing the detection of sites with mining potential, the drones forming a fleet that covers the seabed. These drones, capable of moving autonomously according to a given flight plan, have the disadvantage of being extremely expensive, in particular because, on the one hand, of the precision of the navigation system that they use, and of on the other hand, due to the multiplicity of on-board sensors. However, in order to allow effective exploration of the seabed, it is necessary to use a multitude of drones, each covering part of the surface to be explored, so as to reduce the exploration time. These constraints thus make it impossible to explore the deep sea efficiently, quickly and at low cost. In addition, the sensors embedded in these drones do not make it possible to detect inactive sites.

The invention falls within this context and aims to overcome the problems identified, by proposing a solution for exploring the deep seabed meeting the needs for speed, efficiency and detection of active and inactive sites while maintaining a cost reasonable exploration.

1. un drone sous-marin maître agencé pour se déplacer de façon autonome selon un plan de vol prédéterminé et comportant un module de communication pour émettre des signaux de communication ;
2. une pluralité de drones sous-marins suiveurs, comportant chacun au moins un système de détection de champ magnétique, chaque drone suiveur comportant en outre un module de communication pour recevoir des signaux de communication du drone maître.

For these purposes, the subject of the invention is an underwater exploration system, comprising:

1. a master underwater drone arranged to move autonomously according to a predetermined flight plan and comprising a communication module for transmitting communication signals;
2. a plurality of follower underwater drones, each comprising at least one magnetic field detection system, each follower drone further comprising a communication module for receiving communication signals from the master drone.

According to the invention, the master drone is arranged to transmit navigation instructions to the follower drones and each follower drone is arranged to move autonomously according to said movement instruction so that its movement is slaved to the movement of the master drone.

It is thus understood that, on the one hand, only the master drone requires precise navigation equipment to move in accordance with its flight plan, the follower drones being limited to following the master drone in its movement. Therefore, the equipment of follower drones can be simplified, which reduces the cost. On the other hand, it has been found that hydrothermal sites with mining potential, such as sulphide piles, exhibit a significant magnetic signature with seabed magnetic anomalies whose peak-to-peak amplitude exceeds several hundreds to thousands of nanoTeslas. Moreover, this magnetic signature is persistent, even after the hydrothermal activity has ceased, as in the case of inactive sites. Because of this persistence, the use of magnetic field detection systems in follower drones thus makes it possible to detect magnetic anomalies that can be both active sites and inactive sites. The master drone can also embed other types of sensors making it possible, following the detection of a magnetic anomaly, to characterize the source of the magnetic anomaly and in particular to discriminate whether it is an active site or idle.

Underwater drone means any type of vehicle capable of moving autonomously under water.

Advantageously, the communication modules of the master and follower drones are acoustic communication modules. In other words, the data emitted by the communication module of each of the master and follower drones are for example coded by frequency modulation of an acoustic signal emitted by said communication module. If necessary, the master drone can be arranged to periodically transmit navigation instructions to all the follower drones, each follower drone being arranged to move in accordance with the last instruction received until the next instruction is received. The periodicity over a short period, for example 6 seconds, of the transmissions of the navigation instructions by the master drone makes it possible to ensure that the movement of the follower drones will be slaved to the movement of the master drone over a short distance, for example over a shorter distance at 15 meters, so as to avoid excessive drifts and thus increase the robustness of the system.

Advantageously, the master drone is arranged to periodically determine, from the flight plan, a course to follow and a formation geometry of the follower drones, each navigation instruction transmitted by the master drone to all the follower drones comprising said course to follow and said formation geometry. Where appropriate, each follower drone is arranged, upon receipt of each navigation instruction, to move in a direction conforming to said course to be followed, and to determine and adopt a relative position with respect to the master drone conforming to said formation geometry , until the next navigation instruction is received. For example, the predetermined flight plan can define a plan of navigation lines as well as a formation geometry, adapted to the exploration of a given zone according to conditions relating to the master and follower drones, the master drone being arranged to determining said course to steer from the navigation line plan and using its own position on said navigation line plan. By training geometry is meant a set of relative positions of the master and follower drones, and for example to understand, for each follower drone, its spatial position relative to the master drone, namely to the left, to the right, in front or behind, as well as the lateral distance separating this follower drone from the master drone.

According to one example, a formation geometry can consist of a formation of the master and follower drones in a comb centered on the master drone, the follower drones being distributed on either side of the master drone being regularly spaced 100 m apart. In this formation, it is understood that the master and follower drones move progressively and simultaneously, as navigation instructions are transmitted and received, along parallel lines as defined in the navigation lines plan of the flight plan, respecting the formation geometry defined by the master drone. Movements take place, for example, with a flight altitude of 100 m above the seabed and a navigation speed of 3 to 4 knots. This recognition strategy is a good compromise between the detection objectives and the need for efficiency and speed.

According to one example, the master drone and each follower drone comprise a memory, a plurality of training geometries being loaded into each of the memories and each training geometry being associated with a training code identifying this geometry. If applicable, the navigation instructions transmitted by the master drone contain the training code identifying the training geometry determined by the master drone and each follower drone is arranged to determine its relative position with respect to the master drone by selecting the training geometry loaded into its memory and associated with said transmitted training code.

In one embodiment of the invention, each follower drone comprises a navigation system arranged to determine a position of the follower drone at a given instant and to move from this determined position according to the navigation instruction received from the master drone . The follower drone can in particular be arranged to move, from this determined position, according to the heading to be followed and while maintaining its position relative to the master drone as defined in the formation geometry of the navigation instruction transmitted by the master drone . For example, the navigation system may comprise instruments for measuring the attitude of the follower drone at a given instant, and in particular a MEMS (Micro Electro Mechanical System) type attitude unit making it possible in particular to obtain information relating to the roll and the pitch of the follower drone and a magnetometer making it possible to obtain a magnetic heading of the follower drone. The position of the follower drone at the given instant can thus be estimated by the navigation system according to its attitude measured by said measuring instruments, the navigation instruction received and from its last estimated position. This type of system, performing so-called dead reckoning navigation, offers a satisfactory compromise between the accuracy of estimating the position of the follower drone and the cost of the navigation system.

Advantageously, the navigation system of each follower drone is arranged to determine an altitude of the follower drone relative to the seabed at a given instant, each follower drone being arranged to move while maintaining a constant altitude relative to the seabed. It is thus understood that the follower drone is autonomous as regards its altitude. Otherwise, a complete control of the movement of the follower drone, including with regard to its altitude, could lead to a collision with a local variation of the seabed which would not be taken into account by the flight plan at the level of the drone. master. The autonomy of the follower drone with respect to altitude thus makes it possible to take these local variations into account.

Preferably, the communication module of each follower drone is arranged to transmit to the master drone a signal comprising said determined position of the follower drone.

Advantageously, the communication module of each follower drone is arranged to transmit to the master drone a signal comprising said determined position of the follower drone and an operating state of the follower drone, the master drone being arranged to determine, from the state of operation of the follower drones, said course to follow and said formation geometry. For example, each follower drone is arranged to determine its operating state, namely a viable state or a failed state. If necessary, in the event of a failure, each follower drone can be arranged to leave the formation and to join a base. Advantageously, upon receipt of a signal comprising a follower drone's failure state, the master drone can be arranged to determine a new formation geometry suitable for follower drones whose state is viable.

Indeed, during a failure of a follower drone, the exploration of the zone to be explored by the formation of drones includes a recording gap corresponding to the zone now unexplored by the failed follower drone. In order to fill this gap, the master drone can determine a new flight plan, namely a new navigation lines plan and/or a new formation geometry. For example, in the event of a failure of a failed drone in a comb formation geometry, the master drone will be able to determine a new formation geometry in which a viable follower drone placed at the end of the comb takes the place of the failed follower drone , and a new navigation lines plan in which the navigation lines are tightened. The new flight plan could for example be selected, depending on the operating states of the follower drones, from among a plurality of flight plans stored in a memory of the master drone, each flight plan being for example associated with a given number of drones viable followers.

Advantageously, the communication module of each follower drone can be arranged to receive an echo of the signal emitted by the communication module from the follower drone to the master drone and reflected by the master drone and to determine, from said echo, a relative distance separating the follower drone of the master drone. Where applicable, the navigation system of the follower drone is arranged to determine said position of the follower drone at said given instant using the determined relative distance. The follower drone thus determines its distance relative to the master drone by echolocation of the master drone by means of the signal that the follower drone emits.

The navigation system of each follower drone can thus operate alternately in a so-called "dead reckoning" navigation mode, according to which only its attitude and the navigation instruction received are used to estimate its position at said given instant from its last estimated position, and in a so-called "contraction" mode, occurring each time a signal is transmitted by its communication module and according to which its distance relative to the master drone is integrated into the determination of its position at said given instant. This integration makes it possible to correct drifts that may occur during dead reckoning mode and that may cause a difference in the relative positioning of the follower drone to the master drone. For example, the distance relative to the master drone determined by echolocation could be used in determining the position at said given instant by comparing it to the relative distance fixed by the formation geometry determined by the master drone. In the event of drift, the follower drone can thus correct its movement so as to move in accordance with the heading to be followed while resuming its position relative to the master drone as defined in the formation geometry of the navigation instruction received from the master drone .

In one embodiment of the invention, the master drone comprises a navigation system arranged to determine a position of the master drone at a given instant and a memory in which the flight plan is loaded, the master drone being arranged so as to move from its determined position in accordance with the flight plan loaded in its memory. If applicable, the precision of the navigation system of the master drone is higher than the precision of the navigation systems of the follower drones. For example, the navigation system of the master drone may include an inertial unit of the FOG (Fiber Optic Gyroscope) type, receiving information from an acoustic positioning system such as a DVL (Doppler Velocity Log) and comprising a data processing module integrating for example a Kalman filter type algorithm.

Advantageously, the master drone comprises a plurality of seabed characterization systems of different types. Where applicable, each follower drone comprises a single system for characterizing the seabed, formed by said magnetic field detection system. The follower drones are thus said to be single-sensor and the master drone is said to be multi-sensor. For example, the master drone may include at least two seabed characterization systems chosen from the following non-exhaustive list of systems: a magnetic field detection system, a bathymetric mapping system (such as a multibeam bathymetric sounder, for example), an acoustic imaging system (such as for example a side scan sonar), an object detection system (such as a camera) and a system for measuring physical or chemical hydrodynamic parameters. In this way, in the event of detection of a magnetic anomaly using the magnetic field detection systems of the follower drones and possibly of the master drone, the master drone can make it possible to discriminate, using the characterization systems of the seabed it has, the type of source of the magnetic anomaly and in particular whether it is a hydrothermal site with active or inactive mining potential.

In an exemplary embodiment of the invention, the magnetic field detection system of each follower drone comprises a magnetic field sensor capable of measuring the magnetic field in the vicinity of the follower drone. Where appropriate, the system according to the invention may comprise a data processing module arranged to extract from the measured magnetic field a value independent of the values of the ambient magnetic field and of the magnetic field specific to the follower drone and to detect from said value a magnetic anomaly. If necessary, the magnetic field sensor can be a so-called 3-component vector magnetometer, comprising three magnetometers arranged at 90° from each other. This type of magnetometer partially overcomes the effects of parasitic magnetic fields generated by the follower drone.

Advantageously, the data processing module is arranged to correct the measured total magnetic field so as to extract said value, the correction being made as a function of the displacement attitude of the follower drone, namely its roll, its pitch and its heading , and depending on the ambient local geomagnetic field. It has indeed been observed that the magnetic field measured by the magnetic field sensor is the sum of the magnetic anomalies present locally at the level of the seabed, the ambient geomagnetic field and the parasitic magnetic fields linked to the follower drone, namely the magnetic fields induced and permanent. However, the induced and permanent magnetic fields generated by the follower drone depend on the displacement attitude of the follower drone. Since the follower drone is equipped with a navigation system, the data processing module of the magnetic field detection system can thus retrieve the data relating to the movement attitude of the follower drone at each instant and can determine from these given the induced and permanent magnetic fields, for example by determining the coefficients of the magnetic susceptibility tensor and the remanent magnetization vector of the follower drone. For example, the value could be retrieved using the following correction operation: , where A represents the extracted value, H _mes the magnetic field measured by the magnetic field sensor, H _i the induced magnetic field of the follower drone, H _p the permanent magnetic field of the follower drone and F the ambient local geomagnetic field.

If necessary, the data processing module of the magnetic field detection system can be arranged to detect a magnetic anomaly if said extracted value is greater than a predetermined threshold value.

If desired, the data processing module can be embedded in one, several, or even each follower drone. As a variant, the data processing module can be external to the drones of the system according to the invention.

According to one embodiment of the invention, the acoustic communication modules of the master and follower drones are arranged to transmit communication signals on the same communication channel. If necessary, the communication modules of the master and follower drones are arranged so that the communication signals transmitted by the master and follower drones on said communication channel are multiplexed. If desired, the communication modules of the master and follower drones can be arranged so that the communication signals transmitted by the master and follower drones on said communication channel are time multiplexed. If necessary, the communication modules of the master and follower drones can be arranged to transmit acoustic signals modulated on the same carrier frequency or on the same frequency band.

Preferably, the communication modules of the master and follower drones are arranged to periodically transmit together a data frame, each of these communication modules being arranged to transmit a communication signal in at least one time interval of this data frame which is assigned to him. In other words, the communication modules take turns accessing the communication channel using a technique of the time division multiple access type (also called TDMA from English Time Division Multiple Access).

Advantageously, the communication module of the master drone is arranged to send a navigation instruction to all the follower drones in the first time interval of the data frame. It is thus understood that the same instruction, namely a course to follow and a code relating to a formation geometry established from the flight plan of the master drone, is sent for all the follower drones. If desired, the communication module of the master drone is arranged to transmit several navigation instructions intended for all the follower drones in several disjoint time slots of the data frame, the communication modules of only part of the drones trackers being arranged to transmit a signal to the master drone in time slots of the data frame located between a first and a second successive time slots of the data frame allocated to the master drone. If necessary, the communication modules of another part of the follower drones are arranged to transmit a signal to the master drone in time slots of the data frame located between the second and a third successive time slot of the data frame assigned to the master drone. This characteristic makes it possible to accelerate the transmission of navigation instructions to the follower drones so as to reduce their drifts during their movement.

Advantageously, the communication module of each of the follower drones is arranged to transmit a signal intended for the master drone comprising information relating to its position in a time interval of the data frame which is allocated to it. Preferably, the communication module of each of the follower drones is arranged to transmit a signal intended for the master drone comprising information relating to its position and information relating to its operating state in the said time slot allocated to it.

Preferably, the communication modules of the master and follower drones are arranged so that a guard time interval separates two successive time intervals allocated to communication modules.

Advantageously again, the communication module of the master drone is arranged to send status information from the underwater exploration system to a remote control unit in the last time interval of the data frame.

According to an exemplary embodiment of the invention, the communication modules of the master and follower drones are arranged so that the duration of the time intervals allocated to these communication modules is identical for all the communication modules and so that the period of transmission of the navigation instructions by the master drone corresponds to a movement of each of the follower drones less than a predetermined threshold distance. For example, the duration of the navigation instruction transmission period is set so that the follower drones can move according to a heading given by the master drone over a maximum distance of 15 meters at a speed of 3.5 knots.

According to one embodiment of the invention, each follower drone comprises a body comprising a front compartment isolated from the rest of the body, and each follower drone comprises a propulsion system, a battery, and the magnetic field detection system is arranged in the front compartment and the propulsion system and the battery are arranged in the body outside the front compartment. This limits the impact of the induced and permanent magnetic fields of the propulsion system and the battery on the magnetic field detection system.

Advantageously, the body is formed of a rear compartment, a central compartment and the front compartment, each compartment being isolated from the others, and the propulsion system is arranged in the rear compartment and the battery is arranged in the central compartment.

If applicable, the acoustic communication module and/or the navigation system of the follower drone are arranged in the front compartment.

1. Sélection d’au moins un plan de vol prédéterminé par le drone maître ;
2. Déplacement autonome du drone maître en fonction du plan de vol sélectionné ;
3. Emission par le drone maître d’instructions de navigation, déterminées à partir du plan de vol sélectionné, à destination des drones suiveurs ;
4. Déplacement autonome de chacun des drones suiveurs selon l’instruction de navigation reçue.

The invention also relates to an underwater exploration method implemented by an underwater exploration system according to the invention. Advantageously, the method comprises the following steps:

1. Selection of at least one predetermined flight plan by the master drone;
2. Autonomous movement of the master drone according to the selected flight plan;
3. Emission by the master drone of navigation instructions, determined from the selected flight plan, to follower drones;
4. Autonomous movement of each of the follower drones according to the navigation instruction received.

If desired, the method may comprise a step of transmission by each of the follower drones of a signal intended for the master drone comprising at least information relating to its operating state and, in the case where the master drone receives a signal from a follower drone comprising information indicating a failure of this follower drone, a reconfiguration step in which the master drone selects a new flight plan as a function of the remaining viable drones.

Advantageously, the method can comprise a step of echolocation of the master drone operated by each of the follower drones using the transmitted signal comprising at least information relating to its operating state, and a step of correcting the movement of the follower drone at means of a distance relative to the master drone obtained during the echolocation step.

If desired, the method may comprise a step of measuring the local magnetic field implemented by each of the follower drones, a step of extracting from each measured magnetic field a value independent of the ambient magnetic field and of the field magnetic specific to the follower drone having measured this magnetic field, and a detection step based on said value extracted from a magnetic anomaly.

The present invention is now described using examples which are only illustrative and in no way limit the scope of the invention, and from the accompanying illustrations, in which:

represents, schematically and partially, a view of an underwater exploration system according to one embodiment of the invention;

represents an underwater exploration method implemented by the underwater exploration system of [FIG. 1];

represents a method of communication between the master and follower drones of the underwater exploration system of [FIG. 1];

represents movements of the master drone and of one of the follower drones of the underwater exploration system of [FIG. 1];

represents different steps of a method for detecting a magnetic anomaly implemented by the underwater exploration system of [FIG. 1]; And

shows a sectional view of a follower drone of the underwater exploration system of [Fig. 1].

In the following description, the identical elements, by structure or by function, appearing in different figures retain, unless otherwise specified, the same references.

We represented in a view of an underwater exploration system 1 according to one embodiment of the invention.

System 1 comprises a master underwater drone 2 and a plurality of follower underwater drones 31 to 36. Drones 2 and 31 to 36 are underwater drones capable of moving autonomously. Each of the master 2 and follower 31 to 36 drones is equipped with a communication module C for transmitting and receiving signals, with a magnetic field detection system D capable of measuring a magnetic field and with a navigation system N arranged to determine a position of the drone at a given instant. The master drone 2 also comprises, and contrary to the follower drones 31 to 36, a plurality of seabed characterization systems including a bathymetric mapping system in the form of a multibeam bathymetric sounder SBM and an acoustic imaging system in the form of an SBL side scan sonar. The master drone 2 is thus multi-sensor and the follower drones 31 to 36 are therefore single-sensor.

The navigation system N of each of the follower drones 31 to 36 comprises a MEMS type attitude unit able to determine the roll and the pitch of the follower drone. In addition, the magnetic field detection system D of each follower drone provides a magnetic heading to the navigation system N. Alternatively, provision could be made for each follower drone to be provided with a second magnetic field detection system to provide said magnetic heading, the magnetic field detection system D being in this case dedicated to the measurement of the magnetic field. The navigation system N of the master drone 2 comprises an inertial unit of the FOG type, receiving information supplied by an acoustic positioning system such as a DVL (from the English Doppler Velocity Log) and comprising a position data processing module integrating a Kalman filter type algorithm. The precision of the navigation system of the master drone 2 is thus much higher than that of the navigation system of each of the follower drones 31 to 36.

The communication modules C of the master 2 and follower drones 31 to 36 are acoustic communication modules, so that the drones can exchange data in the form of acoustic signals modulated and emitted by said communication modules on the same channel communication, namely on the same frequency band. Furthermore, the acoustic signals emitted by the master 2 and follower 31 to 36 drones on this communication channel are time multiplexed, so that the communication modules C access the communication channel in turn according to a technique of the type time division multiple access, each of the drones thus transmitting an acoustic signal in at least one time slot of a periodic data frame which is assigned to it.

We represented in , in top view, a method of underwater exploration of an exploration zone Z implemented by the exploration system 1, in [FIG. 3] a communication method between the master drone 2 and the follower drones 31 to 36 during exploration, as well as a data frame F which thus circulates in the communication channel formed between the drones and in [Fig. 4] an example of enslavement of a movement of one of the follower drones 31 to the movement of the master drone 2.

Prior to the exploration, a plurality of formation geometries is loaded into a memory of each of the master 2 and follower drones 31 to 36. Each formation geometry describes, for a given number of follower drones and for each of these follower drones , its spatial position relative to the master drone 2, namely whether it should be placed to the left, to the right, in front or behind the master drone 2, and a lateral distance separating this follower drone from the master drone 2. Always beforehand exploration, the master drone selects a flight plan E, namely a given formation geometry GF and a plane of navigation lines PL associated with said formation geometry GF and adapted to the exploration zone Z envisaged. Each line of the line plan PL thus defines the movement of a drone in the exploration zone Z (in solid lines for the master drone 2 and in dotted lines for the follower drones 31 to 36 on the ). As described in [Fig. 2], the formation geometry GF defined in the selected flight plan E corresponds to a comb formation centered on the master drone 2, the follower drones being distributed on either side of the master drone 2 being regularly spaced 100 meters.

The master drone 2 is arranged to move autonomously, using its navigation system N, in accordance with the flight plan E and its own line in the navigation line plan PL. During its movement along the flight plan E, the master drone 2 periodically transmits, for example every 6 seconds, via its communication module C, a navigation instruction I intended for all the follower drones 31 to 36 comprising a course to follow Cm, determined from the flight plan E as well as a code identifying the formation geometry GF defined in the selected flight plan E. Several navigation instructions I are thus transmitted to all the follower drones in several time slots of the data frame F.

Upon receipt of a navigation instruction I by its communication module C, each follower drone 31 to 36 determines its position relative to the master drone 2 by selecting the training geometry loaded in its memory and associated with said code contained in the instruction navigation I. Each follower drone 31 to 36 then moves autonomously, in accordance with the navigation instruction I received, depending on the heading to be followed and the estimated position of the follower drone by its navigation system N, while maintaining its relative position with respect to the master drone 2, until the reception of the next navigation instruction. More specifically, each follower drone 31 to 36 adopts a so-called dead reckoning navigation mode, in which it estimates its position P1 to P6 as a function of its displacement, determined by the magnetic heading measured by the magnetic field detection system and of its attitude measured by the attitude unit, from its last estimated position. The follower drone then moves to a new position determined by its estimated position and by the course to follow Cm contained in the navigation instruction I, and by keeping its position relative to the master drone 2 defined in the formation geometry GF.

As illustrated in , the dead reckoning mode introduces a drift in the movement of each follower drone (illustrated by the follower drone 31), due to the positioning uncertainty introduced by the estimation of the position P1 to P6. In other words, the positioning uncertainty (materialized by the circle IP in [FIG. 4]) increases over time, which can generate, on the one hand, a deviation of the follower drone 31 with respect to its navigation line in the plane of lines PL and, on the other hand, a difference between the real lateral distance d1 between the master drone 2 and the follower drone 31 and that fixed in the formation geometry GF.

Each follower drone 31 to 36 transmits a signal S1 to S6 to the master drone 2 containing its estimated position P1 to P6 as well as an operating state E1 to E6, namely a viable state or a failed state, in an interval of time of the data frame F assigned to it. In order to remedy the drifts of the movements of the follower drones described above, the communication module C of each follower drone receives an echo S1 'of the signal S1 reflected by the master drone 2 and determines, from said echo, the real relative distance d1 separating the follower drone from the master drone. The follower drone 31 then switches from the dead reckoning navigation mode to a so-called "contraction" navigation mode, in which this relative distance d1 determined by echolocation is integrated into the determination of the position of the follower drone by the navigation system N and in the movement of the follower drone. On the one hand, this determined relative distance d1 makes it possible to reduce the positioning uncertainty. On the other hand, the comparison between the real relative distance d1 and that fixed in the formation geometry GF allows the follower drone 31 to regain its line of navigation, and thus to respect this formation geometry GF.

In the example described, the navigation instructions I issued by the master drone 2 are devoid of instructions as to an altitude to be followed. In addition, the navigation system N of each follower drone 31 to 36 comprises an altimeter arranged to determine an altitude of the follower drone relative to the seabed at a given instant, each follower drone being arranged to move while maintaining a constant altitude by relative to the seabed.

In the example of the , the drone 32 suffers damage and thus sends, in its time interval, a signal S2 containing its position P2 and an operating state E2 indicating a failure. Following the sending of the signal S2, the drone 32 is no longer able to continue the exploration and leaves the exploration zone Z for example to join a base. Upon receipt of the signal S2 indicating the state of failure E2 of the follower drone 32, the master drone selects a new flight plan E defining a new formation geometry GF adapted to the remaining follower drones 31 and 33 to 36. In the example described, the follower drone 36 replaces the follower drone 32 and thus borrows its position. In order to maintain a homogeneous exploration of the exploration zone Z, the master drone 2 also determines a new plane of navigation lines PL. The master drone 2 thus carries out a reconfiguration of the fleet of drones.

As described in , a guard time interval G separates the successive transmission of signals by the master drones 2 and followers 31 to 36 in the data frame F. It will also be noted that the time intervals allocated to the follower drones 31 to 36 are distributed between the different time slots assigned to master drone 2.

Finally, the master drone 2 sends status information E of the underwater exploration system to a remote control unit, for example located in a boat on the surface of the exploration zone Z, in the last time interval of the data frame F.

The duration of the time intervals allocated to the communication modules C of the master 2 and follower drones 31 to 36 is identical for all these communication modules. In addition, the period of transmission of the navigation instructions I by the master drone is such that the movement of each of the follower drones 31 to 36 between two successive navigation instructions is substantially equal to 15 meters, at a speed of 3.5 knots. It is thus noted that the follower drones 31 to 36 move according to elementary displacements of 15 meters following the flight plan E, which makes it possible to obtain a precise servo-control of the trajectory of the follower drones 31 to 36 to that of the master drone 2, while guaranteeing robustness of the exploration system 1 in the event of drift or failure of one of the follower drones.

Each magnetic field detection system D of the master 2 and follower drones 31 to 36 comprises a magnetic field sensor, in the form of a so-called 3-component vector magnetometer, comprising three directional magnetometers arranged at 90° from each other, each being able to measure the magnetic field in the vicinity of the drone. We have thus represented in in the first graph G1 an example of measurements of the three components of the magnetic field obtained by each of the directional magnetometers of one of the follower drones 31 to 36 during the exploration of the zone Z.

The exploration system 1 comprises a data processing module (not shown), external to the drones 2 and 31 to 36, arranged to calculate a total magnetic field from the measurements of the three components of the magnetic field illustrated in the graph G1. This total magnetic field has been illustrated in the second graph G2.

The data processing module recovers, from the navigation system N during the exploration, the data relating to the movement of the follower drone during the exploration of the zone Z, and in particular its heading which has been represented in the third graph G3 and its roll and pitch which were illustrated in the fourth graph G4. In addition, the data processing module determines the magnetic susceptibility tensor of the follower drone as well as the remanent magnetization vector, for example by means of a so-called calibration magnetic data set. The data processing module corrects, from this set of data, the total magnetic field of the G2 graph, in order to eliminate the induced and permanent magnetic fields as well as the ambient magnetic field, in order to extract a value represented therefrom. in the G5 graph. It can be seen on this graph G5 that the extracted value presents a variation, corresponding to a magnetic anomaly A in the exploration zone. In addition, the data collected by the SBM and SBL seabed characterization systems of the master drone make it possible to characterize the source of the magnetic anomaly A and in particular to determine whether it is a hydrothermal site with inactive mining potential. .

Finally, we represented a sectional view of one of the follower drones 31, it being understood that the structure of all the follower drones 31 to 36 is identical. The drone 31 comprises a body 4 formed by a rear compartment 41, a central compartment 42 and a front compartment 43. Each compartment is isolated from the others.

The follower drone 31 comprises a propulsion system 5 arranged in the rear compartment 41 and a battery 6 arranged in the central compartment 42. The magnetic field detection system D, the acoustic communication module C and the navigation system N are arranged in the front compartment 43, so as to limit the impact of the induced and permanent magnetic fields of the propulsion system and the battery on the magnetic field detection system D.

The preceding description clearly explains how the invention makes it possible to achieve the objectives it has set itself, and in particular by proposing an underwater exploration system composed of a fleet of underwater drones comprising a master drone autonomous and follower drones whose movements are slaved to that of the master drone. The system that has been described thus allows rapid, efficient exploration of the deep seabed, making it possible to detect and discriminate between active sites and inactive sites and whose exploration cost is controlled.

In any event, the invention cannot be limited to the embodiments specifically described in this document, and extends in particular to all equivalent means and to any technically effective combination of these means. In particular, other types of training for master and follower drones can be considered. It is also possible to envisage the use of another type of communication method between the master and follower drones than a TDMA or even an internal structure of the follower drones different from that which has been described.

Claims (12)

System (1) for underwater exploration, comprising:

1. a master underwater drone (2) arranged to move autonomously according to a predetermined flight plan (E) and comprising a communication module (C) for transmitting communication signals;
2. a plurality of follower underwater drones (31, 32, 33, 34, 35, 36), each comprising at least one magnetic field detection system (D), each follower drone further comprising a communication module (C) to receive communication signals from the master drone;
3. the master drone being arranged to issue navigation instructions (I) to the follower drones and each follower drone being arranged to move autonomously according to said movement instruction so that its movement is slaved to the movement of the master drone .

System (1) according to the preceding claim, in which the communication modules (C) of the master (2) and follower (31, 32, 33, 34, 35, 36) drones are acoustic communication modules. System (1) according to one of the preceding claims, in which the master drone (2) is arranged to periodically transmit navigation instructions (I) to all the follower drones (31, 32, 33, 34, 35, 36), each follower drone being arranged to move in accordance with the last instruction received until the next instruction is received. System (1) according to the preceding claim, in which the master drone (2) is arranged to determine periodically, from the flight plan (E), a heading to be followed (Cm) and a formation geometry (GF) follower drones (31, 32, 33, 34, 35, 36), each navigation instruction (I) transmitted by the master drone to all the follower drones (31, 32, 33, 34, 35, 36) comprising said heading to follow and said formation geometry, and in which each follower drone is arranged, on receipt of each navigation instruction, to move in a direction conforming to said heading to follow, and to determine and adopt a relative position (d1 ) with respect to the master drone conforming to said training geometry, until the reception of the next navigation instruction. System (1) according to one of the preceding claims, in which each follower drone (31, 32, 33, 34, 35, 36) comprises a navigation system (N) arranged to determine a position (P1, P2, P3, P4, P5, P6) of the follower drone at a given instant and to move from this determined position according to the navigation instruction (I) received from the master drone (2). System (1) according to the preceding claim, in which the navigation system (N) of each follower drone (31, 32, 33, 34, 35, 36) is arranged to determine an altitude of the follower drone with respect to the seabed at a given moment, each follower drone being arranged to move while maintaining a constant altitude relative to the seabed. System (1) according to one of Claims 5 or 6, in which the communication module (C) of each follower drone (31, 32, 33, 34, 35, 36) is arranged to transmit to the master drone (2) a signal (S1, S2, S3, S4, S5, S6) comprising said determined position (P1, P2, P3, P4, P5, P6) of the follower drone. System (1) according to the preceding claim in combination with claim 4, in which the communication module (C) of each follower drone (31, 32, 33, 34, 35, 36) is arranged to transmit to the master drone (2 ) a signal (S1, S2, S3, S4, S5, S6) comprising said determined position (P1, P2, P3, P4, P5, P6) of the follower drone and an operating state (E1, E2, E3, E4, E5, E6) of the follower drone, the master drone being arranged to determine, from the operating state of the follower drones, said course to follow (Cm) and said formation geometry (GF). System (1) according to claim 7 or 8, in which the communication module (C) of each follower drone (31, 32, 33, 34, 35, 36) is arranged to receive an echo (S1') of the signal ( S1) transmitted by the communication module (C) of the follower drone to the master drone (2) and reflected by the master drone and to determine from said echo a relative distance (d1) separating the follower drone from the master drone; and in which the navigation system (N) of the follower drone is arranged to determine said position (P1, P2, P3, P4, P5, P6) of the follower drone at said given instant using said determined relative distance. System (1) according to one of the preceding claims, in which the master drone (2) comprises a plurality of seabed characterization systems of different types (SBM, SBL). System (1) according to one of the preceding claims, in which the magnetic field detection system (D) of each follower drone (31, 32, 33, 34, 35, 36) comprises a magnetic field sensor capable of measuring the magnetic field in the vicinity of the follower drone, the system (1) comprising a data processing module arranged to extract from the measured magnetic field a value independent of the values of the ambient magnetic field and of the magnetic field specific to the follower drone and to be detected from said value a magnetic anomaly (A). Underwater exploration method implemented by an underwater exploration system (1) according to one of Claims 1 to 11.