HYDRAULIC MULTI-ROTOR DRONE

Description

technical field

[0001] The present invention relates to a hydraulic drone. Previous technique

[0002] Electric-powered drones have seen significant development in recent years, with advances in batteries and onboard electronics.

[0003] It has been proposed, particularly for surveillance applications, to supply electrical power to the drone via a ground cable. The electric lift motors of drones experience significant current variations depending on stabilization requirements. Consequently, the power electronics associated with these types of motors are not designed for continuous operation over extended periods, necessitating frequent maintenance, and ultimately, the flight time is not as long as desired. EP 3450312 discloses a hydraulic drone according to the prior art.

[0004] US patent 3,345,016 describes a hydraulic drone with hydraulic motors. The rotational speed of each motor is controlled by varying the fluid flow rate through a "short circuit" system between the motor's supply and return lines. This circuit results in power loss due to energy dissipation in the short circuit, and precise motor speed control is impossible. Centralized control of the opening rates of the four "short circuits" fails to ensure proportionality, as the flow rate in each "short circuit" depends not only on the opening rate but also on the pressure. Pressure, in turn, reflects the propeller's resisting torque and therefore varies with the rotational speed and, consequently, with the flow rate.

[0005] Application WO 2019 / 074860 describes a hydraulic drone with variable-displacement hydraulic motors. The rotational speed of each motor is controlled by varying the motor's displacement, rather than by varying the fluid flow rate, which remains constant. The motor construction is complex due to the displacement control. Propeller speed control is also complex, as the speed is determined not directly by the flow rate but indirectly by the resisting torque and the displacement.

[0006] There is a need to address some of the known drawbacks of drones, mentioned above, in particular reducing their weight in order to increase their payload capacity, improving the accuracy of the regulation of the rotation speed of the hydraulic motors, and meeting the demand for reliability and responsiveness of this type of drone. Description of the invention

[0007] The invention aims to meet this need and relates to a hydraulic drone according to claim 1.

[0008] The invention allows users to benefit from the reliability of hydraulic motors, as well as their favorable power-to-weight ratio. The motor control channels, by ensuring adjustment from a given operating point corresponding to the valve's initial position, offer good control responsiveness and allow the use of lighter valves requiring less power for their operation.

[0009] The non-zero flow rate can then be obtained by a bypass in parallel with the pilot valve, which contributes at least partially, and in particular at least half, to the fluid flow rate received by the hydraulic motor when the pilot valve is in its first position. The pilot valve can be closed in its first open position, and the non-zero flow rate then received by the hydraulic motor can come entirely from the bypass. Preferably, this bypass has a calibrated orifice. Alternatively, the non-zero flow rate is obtained through a leakage flow from the valve when it is in its first position; in this case, the first position does not correspond to a complete closure of the valve. In this case, the non-zero flow rate received by the motor when the valve is in its first position can come partly from the bypass and partly from the leakage flow from the valve.The power supply system can be arranged so that the non-zero fluid flow received by at least one of the hydraulic motors when the controllable valve is in the first position is a predefined idle flow rate. The rotational speed of the hydraulic motor(s) receiving the predefined idle flow rate can produce sufficient thrust to provide lift for the drone.

[0010] When the valve is in the first position, the hydraulic motor speed can be set by a pressure limiter, which caps the fluid pressure at a predefined value.

[0011] The pump can operate at a fixed speed, or alternatively at a variable speed. Multi-rotor drone

[0012] A drone with 4 or more rotors is particularly suitable for hovering or near-hovering flight due to the distribution of lift in the horizontal plane.

[0013] Each rotor of the drone has a propeller driven by a hydraulic motor. This motor preferably has a fixed displacement. The drive is preferably direct. Alternatively, at least some of the motors may include a gear train to increase the rotational speed. Preferably, the propellers have a fixed pitch. They may be two-bladed, three-bladed, or even more complex.

[0014] The variation in flow rate in each motor due to the greater or lesser opening of the valve, placed in series with the motor, may constitute the only means of adjusting the orientation of the drone. Hydraulic drone

[0015] A "hydraulic drone" is defined as a drone with one or more rotors whose rotation is powered by one or more hydraulic motors. The rotational speed of each rotor is then preferably controlled by adjusting the flow rate of the hydraulic supply to the motor.

[0016] Compared to a drone whose propeller speed is controlled by electric motors (also called an "electric drone"), the use of hydraulic motors significantly improves reliability because the power modulation required to control the drone can be achieved hydraulically rather than electronically. This overcomes the weakness of the electronic components typically used in electric drones, which have a limited lifespan due to the current fluctuations caused by the instantaneous power variations required for drone control.

[0017] The drone can be configured to fly at a maximum altitude of 200 meters, preferably 150 meters, and even more preferably, at an altitude between 80 and 100 meters.

[0018] The drone without its propellers can fit into a square with sides between 0.75 and 1.25 m, the invention not being limited to particular dimensions.

[0019] When the drone is wired, its weight is preferably less than or equal to 25 kg.

[0020] When the drone is wireless, its weight is preferably less than or equal to 8 kg. Wired drone

[0021] The drone can be wired or wireless, but wired is preferable.

[0022] By "wired drone", we mean a drone connected to the ground by a cable (also called an umbilical cord) allowing in particular the transmission of energy such as electricity from the ground to the drone and / or the transmission of data from the ground to the drone and vice versa.

[0023] A tethered drone offers the advantage of extended battery life, allowing for long-duration flights of several days continuously. This is particularly desirable for observation and surveillance missions involving hovering or near-hovering flight. Furthermore, the drone's total battery life can be further increased by selecting energy-efficient components.

[0024] The cable can be between 2 and 250 m long and can be wound onto a reel placed on the ground or mounted on the drone. The reel can be used to keep the cable taut.

[0025] Preferably, the cable tension is regulated by a reel drive mechanism so that the pulling force exerted by the drone on the cable is constant, particularly during the drone's ascent or descent.

[0026] The cable may consist of an optical fiber, one or more twisted pairs, and / or a coaxial cable, to transmit data from the ground to the drone and vice versa. The cable can enable high-speed data transmission in real time, continuously or periodically.

[0027] The drone may include onboard electronics configured to generate a current suitable for powering the drone's electrical components, and in particular the hydraulic pump drive motor. This motor is preferably a brushless motor, powered by polyphase current, specifically three-phase, from a power inverter controlled by the flight controller. Onboard sensors

[0028] The drone can include any type of onboard sensor, useful for its piloting or its mission.

[0029] The drone, particularly its flight controller, may include and / or be connected to a 3-axis accelerometer, a 3-axis gyroscope, a 3-axis magnetometer, an atmospheric pressure sensor, a camera (including infrared), an altimeter, and / or a location data receiver, such as a GPS receiver. This allows the flight controller and / or a ground operator to know the drone's position, altitude, roll, pitch, and yaw angles, and / or speed.

[0030] The drone may include an onboard sensor for measuring the composition of the surrounding gaseous environment, a camera (including infrared), and a rangefinder, such as an acoustic, radar, and / or laser sensor like a lidar. This can enable surveillance missions and the detection of potential chemical, toxic, fire, or explosion risks, for example, above an industrial site.

[0031] The drone may include at least one temperature and pressure sensor for the fluid supplying the hydraulic motors.

[0032] The data generated by certain onboard sensors can be used for different functions. For example, a camera can be used to pilot the drone but can also be used to acquire images for surveillance and security missions.

[0033] The data generated by the onboard sensors can be transmitted to the ground via wireless communication and / or via cable when the drone is wired. Autonomous drone

[0034] The drone can be autonomous.

[0035] By "autonomous drone," we mean that the drone, particularly its flight controller, includes computer systems, such as a microcomputer, that allow it to perform a number of functions autonomously. For example, the drone is configured to autonomously carry out takeoff, landing, and / or hovering or near-hovering at a predetermined altitude. Hydraulic pump

[0036] Preferably, the hydraulic pump supplies pressurized fluid to the plurality of hydraulic motors.

[0037] The hydraulic pump is preferably of fixed displacement.

[0038] The hydraulic pump drive motor can be variable speed.

[0039] The motor can be of any type, including electric or thermal, preferably electric, especially brushless, as mentioned above.

[0040] The drone may include a cable configured to supply power to the pump's drive motor from the ground when the latter is electric, as described above. Alternatively, or additionally, the drone may include an onboard battery, including a rechargeable one, configured to power the pump's electric drive motor.

[0041] It can be advantageous for the drone to have multiple power sources to power the electric pump drive motor, such as a cable connecting the drone to the ground and an onboard battery, with one source able to take over if the other fails.

[0042] The drone may include a tank containing fuel to power the engine when the latter is thermal, or a tank powering a fuel cell providing the electrical energy needed for the pump drive motor.

[0043] The drone may include a system for producing an airflow to cool the pump drive motor, including one or more fans.

[0044] The pump can be coupled to several motors to ensure its operation. This allows one of the motors to take over if another fails. The motors can be of the same type or different types. Alternatively, the hydraulic pump can be coupled to a single drive motor.

[0045] The pump can be driven at a fixed speed during flight. The pump speed is chosen to obtain the pressure necessary for the proper operation of the hydraulic motors.

[0046] One or more pressure limiters can be placed at the outlet of the pump and upstream of the valves, as mentioned below.

[0047] The drone may contain a single hydraulic pump. Hydraulic fluid

[0048] The hydraulic fluid supplying hydraulic motors is preferably an oil.

[0049] This fluid can meet ISO 11158:2009, DIN 51524 and / or ASTM D6158 standards. The hydraulic fluid can be of ISO 32, ISO 46 or ISO 68 grade. Hydraulic fluid reservoir

[0050] The drone preferably includes an accumulator tank containing and / or intended to receive the hydraulic fluid and supplying the hydraulic pump with hydraulic fluid. This tank can compensate for variations in fluid volume, particularly its thermal expansion, related to the operation of the motors.

[0051] The fluid pressure within the tank can be atmospheric pressure.

[0052] The reservoir can have a volume between 0.75 and 1.25 L, the invention not being limited to a particular capacity.

[0053] Preferably, the reservoir is positioned so that the hydraulic pump operates under pressure. For example, the reservoir is placed above the hydraulic pump so that the fluid transfer from the reservoir to the pump occurs by gravity. This can eliminate the need for a suction system to transfer the fluid from the reservoir to the pump. Controllable valves

[0054] Preferably, controllable valves are electrically controlled.

[0055] The valves can be unidirectional. They are preferably proportional, but alternatively, they can operate on an on / off basis. The opening and closing of the valves can then occur at a high frequency.

[0056] The valves can be direct driven (DDV for "Direct Drive Valve").

[0057] Preferably, controllable valves are servovalves. These valves allow for flow control with high precision and a short response time.

[0058] Each valve can carry a fluid flow of less than or equal to 7 L / min and be subjected in operation to a fluid pressure of less than or equal to 400 bar.

[0059] Each motor can have a single controllable valve in series with it. Hydraulic motors

[0060] Each hydraulic motor is configured to drive a propeller in rotation, the rotational speed of the propeller being a function of the fluid supply flow rate to the hydraulic motor and the torque delivered by the hydraulic motor being a function of the fluid pressure at the inlet of the hydraulic motor.

[0061] Hydraulic motors are preferably high-speed motors.

[0062] Hydraulic motors are preferably configured to drive propellers in rotation at a rotational speed between 300 and 7000 rpm, preferably between 2100 and 4000 rpm.

[0063] The hydraulic motors can each be chosen to deliver a torque between 0.1 and 3.0 Nm

[0064] The fluid pressure at the hydraulic motor inlet can be between 50 and 400 bar, preferably between 250 and 300 bar. The fluid pressure at the hydraulic motor outlet can be atmospheric pressure.

[0065] Hydraulic motors can be of any type, including piston, vane, or gear-driven, but piston motors, particularly axial piston motors, are preferable. Such axial piston motors offer high mechanical power relative to their low weight.

[0066] Hydraulic motors can have a fixed displacement. Compared to variable displacement hydraulic motors, such fixed displacement hydraulic motors offer simplified operation and reduced power consumption.

[0067] Hydraulic motors can generally have a cylindrical external shape, for example with a length between 10 and 15 cm and a diameter between 2 and 4 cm.

[0068] Hydraulic motors can have two directions of rotation. Fluid cooling

[0069] The drone may include at least one heat exchanger to cool the hydraulic fluid, particularly at the outlet of the hydraulic motors and / or pressure relief valves. This allows for control of the fluid temperature, as it tends to heat up during drone operation.

[0070] Preferably, the drone incorporates several heat exchangers, including one per lane. These heat exchangers can be positioned on the drone's arms, near the propellers, so as to be located within the airflow generated by the propeller rotation. Preferably, in this case, the heat exchangers are positioned in the area where the airflow generated by the propeller rotation is at its maximum.

[0071] When the heat exchangers are integrated into the drone arms, they are advantageously used to help reinforce the rigidity of the arms and therefore of the overall structure of the drone.

[0072] The cooled hydraulic fluid exiting the heat exchangers can be directed to the fluid reservoir inlet.

[0073] The fluid pressure within the heat exchangers can be atmospheric pressure. This simplifies the manufacturing of the exchangers, which then do not have to withstand high pressure. Pressure limiter(s)

[0074] The drone may include at least one pressure limiter to limit the fluid pressure at the inlet of the controllable valve to a predefined limit value known as the limited pressure, in particular between 250 and 300 bars.

[0075] Preferably, the drone includes several pressure limiters, including one per channel, each limiter being positioned upstream of the controllable valve.

[0076] When the hydraulic fluid pressure at the outlet of the hydraulic pump exceeds the predefined limit value, the excess pressure can be diverted to the low pressure circuit, in particular to the inlet of the heat exchanger(s). High-pressure hydraulic circuit

[0077] The drone may include a high-pressure hydraulic circuit between the hydraulic pump outlet and the hydraulic motor inlet. For example, the hydraulic fluid pressure in the high-pressure hydraulic circuit is between 250 and 300 bar. Low-pressure hydraulic circuit

[0078] The drone may include a low-pressure hydraulic circuit between the outputs of the hydraulic motors and pressure limiters and the inlet of the reservoir.

[0079] For example, the hydraulic fluid pressure in the low-pressure hydraulic circuit is atmospheric pressure. Operating method

[0080] The invention discloses a method of operation for a multi-rotor hydraulic drone, in which: To take off, the speed of the pump drive motor is increased beyond a threshold which activates the pressure limiter, and the pump drive speed is then maintained at a constant value above this threshold, the pressure at the inlet of the controllable valve being then equal to a predefined value imposed by the pressure limiter. During the flight, each controllable valve is operated to change the orientation of the drone as needed, while the pressure limiter is active and maintains the pressure at the inlet of the valve at said predefined value. Brief description of the drawings

[0081] The invention will be better understood upon reading the detailed description that follows, the non-limiting examples of its implementation, and upon examination of the attached drawing, on which: [ Fig 1 ] there figure 1 schematically represents an example of a drone according to the invention, [ Fig 2 ] there figure 2is a view analogous to the figure 1 of a drone variant, [ Fig 3 ] there figure 3 represents schematically and partially, in perspective, a multi-rotor hydraulic drone according to an example of an implementation of the invention, [ Fig 4 ] there figure 4 represents the drone of the figure 3 from a different perspective, [ Fig 5 ] there figure 5 represents the drone of the figure 3 viewed from below, [ Fig 6 ] there figure 6 represents the drone of the figure 3 top view, [ Fig 7 ] there figure 7 represents an example of a hydraulic motor mount, [ Fig 8 ] there figure 8 represents, from below, the support of the figure 7 , [ Fig 9 ] there figure 9 represents a detail of the support's construction, [ Fig 10 ] there Figure 10 represents another detail in the construction of the support, [ Fig 11 ] there figure 11 represents a detail of the drone's construction, [ Fig 12 ] there figure 12schematically represents another example of a drone, [ Fig 13 ] there figure 13 is a diagram showing an example of the evolution of the hydraulic pressure at the inlet of the controllable valve and the pressure at the outlet of the hydraulic pump as a function of time during the operation of the drone according to the invention, and [ Fig 14 ] there figure 14 is a diagram showing the evolution of the pump drive motor speed as a function of time during the operation illustrated on the figure 13 . Detailed description

[0082] We represented at the figure 1 Schematically, the various constituent elements of a multi-rotor hydraulic drone 1 according to the invention are shown. In this figure, the solid lines correspond to the high-pressure part of the hydraulic circuit, while the dashed lines correspond to the low-pressure part of the hydraulic circuit.

[0083] In this example, drone 1 is a quadcopter and has four hydraulic motors 6, each directly driving a propeller 5. Alternatively, the drone could be a hexacopter or octocopter, among other possibilities.

[0084] Drone 1 includes a hydraulic pump 10 driven by an electric motor 11, which in this example is supplied with electrical energy from the ground by a cable (not shown).

[0085] The pump 10 is supplied at the inlet by a hydraulic fluid contained in an accumulator tank 13 and pressurizes this fluid so as to supply the motors 6.

[0086] The pressurized fluid exiting pump 10 is distributed via a flow divider 29 between several ports 35, 36, 37, 38 for adjusting the power of the hydraulic motors 6, each port supplying pressurized fluid to a corresponding motor 6. In the example illustrated in the figure 1, the pressurized fluid at the outlet of pump 10 is thus distributed between four channels so as to supply as many hydraulic motors 6.

[0087] Each channel includes a proportionally controllable valve 17, allowing the flow rate of pressurized fluid at the inlet of the corresponding motor 6 to be varied, and thus the rotational speed of the associated propeller 5. Preferably, the controllable valve 17 is a directly electrically actuated servovalve.

[0088] The drone 1 includes a flight controller 14 configured to control the speed of the drive motor 11 of the pump 10 and the opening of each valve 17 according to the flight regime and the orientation to be given to the drone in particular.

[0089] The valve 17 can move, in response to an electrical control signal from the flight controller 14, from a first position to a second position, so as to increase the flow of pressurized fluid passing through the valve 17.

[0090] Each channel includes a parallel branch of the valve 17, this branch having a calibrated orifice 31 chosen to deliver at the inlet of the corresponding motor 6, a substantially constant flow of pressurized fluid.

[0091] Each port includes a pressure relief valve 27 upstream of the valve 17 and the calibrated orifice 31. The pressure relief valve 27 limits the fluid pressure at the inlet of the valve 17 to a predefined and substantially constant value, regardless of the pressure upstream of the relief valve, provided that the latter is high enough to activate the relief valve. The operating speed of the pump 10 is sufficient to allow the pressure relief valve to be active regardless of the position of the valve 17.

[0092] Each motor 6 is associated with a heat exchanger 8 which receives at the inlet the fluid exiting the motor 6 and the pressure limiter 27 and delivers the cooled fluid to the accumulator tank.

[0093] When the drive motor 11 is electric, as in the illustrated example, the flight controller 14 controls a speed controller which is preferably a switching type (not shown) and allows the current used and therefore the torque imposed by the pump 10 to be limited.

[0094] The flight controller 14 advantageously includes a microcontroller or other computer (not shown) enabling it to perform a number of functions autonomously, such as maintaining the drone 1 in a stationary or quasi-stationary position based on measurement data from sensors on board the drone 1, such as wind speed sensors and roll, pitch and yaw angle sensors.

[0095] In the variant illustrated at the figure 2, drone 1 has a single pressure limiter 27 positioned between the outlet of the pump 10 and the inlet of the flow divider 29. Similarly, drone 1 may have a single heat exchanger 8, positioned between the outlet of the hydraulic motors 6 and the inlet of the reservoir 13.

[0096] In an example of drone 1 operation, the flow rate of pressurized fluid at the inlet of each motor 6 is such that: Q = Q 31 + Q 17 and Q 31 = r*Q with 0.5 ≤ r ≤ 1 for example, where Q: pressurized fluid supply flow rate at the inlet of each motor 6, Q 31: pressurized fluid flow rate at the outlet of element 31, and Q 17: pressurized fluid flow rate at the outlet of the pilot valve 17.

[0097] The pilot-operated valve 17 can be configured to be fully closed in the first position, so that the entire pressurized fluid supply to each motor 6 comes from the bypass with the calibrated orifice 31. When the pilot-operated valve 17 is open in the second position, the pressurized fluid supply to each motor 6 comes partly from the valve and partly from the bypass. For example, half the flow is supplied by the valve 17 and the other half by the bypass.

[0098] We illustrated to figures 3 to 6 an example of the arrangement of the constituent components of a multi-rotor hydraulic drone 1 according to the invention.

[0099] In this example, drone 1 is a quadcopter and has four arms 3, each carrying a rotor. Drone 1 can rest on the ground by two feet 4 connected to the arms 3.

[0100] The hydraulic motors 6 each have a rotor 2 driving a propeller 5, and are each supported by a corresponding arm 3.

[0101] A cover 9 houses the pump 10 and the electric drive motor 11 of the pump 10, the coupling between the pump 10 and its electric motor 11 being protected by a casing 12. The pump 10 and its electric motor 11 are preferably, as illustrated, positioned vertically in the center of the drone 1.

[0102] The arms 3 and feet 4 of drone 1 are formed by an assembly of tubes 7, for example made of carbon. These tubes can be used to conduct hydraulic fluid, where possible.

[0103] Each arm can have parallel tubes.

[0104] The heat exchangers 8 are each integrated within a corresponding arm 3, and the latter is stiffened by connecting the tubes 8.

[0105] As illustrated in figures 7 to 10Each arm 3 has at its end a support 15 in which the corresponding hydraulic motor 6 is fixed. The support 15 can be mounted, as illustrated, on the end of the tubes and connect them.

[0106] Each support 15 has on its outer side a collar 16 in which the hydraulic motor 6 is engaged and can support on its inner side the corresponding pilot valve 17.

[0107] As illustrated on the Figure 10 , each support 15 may have on its inner side, on the one hand holes 18 for fixing the corresponding valve 17, and on the other hand orifices 19, 20 and 21 respectively for leakage, outlet and inlet of fluid at the level of the latter.

[0108] On its lower side, each support 15 has ports 22, 23, 24 and 25 respectively for the intake of fluid from the pump 10, the sending of fluid to the corresponding heat exchanger 8, the outlet of fluid to the corresponding hydraulic motor 6 and the return of fluid from the corresponding hydraulic motor 6.

[0109] Ports 22 and 24 are connected to high-pressure hydraulic pipes (not shown). Ports 23 and 25 are connected to hydraulic pipes (not shown) that are not subjected to high pressure and may be less resistant.

[0110] Each support 15 includes a tube 26 receiving the corresponding pressure limiter 27.

[0111] As illustrated on the figure 11The cover 9 is fixed via a connecting element 28 to the flow divider 29, which can be made up as illustrated of a perforated block which accommodates the outlet of the pump 10. The connecting element 28 is for example in the shape of a cross.

[0112] Support plates 30 are fixed on one side to the perforated block 29 and on the other side to the arms 3 and allow the fluid tank 13 and the flight controller 14 to be held.

[0113] Drone 1 illustrated at the figure 12 It includes several hydraulic lift motors 6a and several hydraulic steering motors 6b. On the figure 12 , only two lift motors 6a and two orientation motors 6b have been shown to simplify understanding of the figure.

[0114] Each of the hydraulic lift motors 6a has a supply channel having a calibrated orifice 31 connected in series with the corresponding motor 6a so that each of the motors 6a receives a predefined constant supply flow, this supply flow being able to correspond to a flow enabling each of the lift motors 6a to rotate at a speed sufficient to produce a thrust ensuring just the lift of the drone 1.

[0115] Each of the 6b hydraulic slewing motors has a supply channel comprising a pilotable valve 17 connected in series with the corresponding 6b motor, the valve 17 being pilotable between a closed position and a maximum open position to regulate the supply flow to each of the 6b slewing motors.

[0116] Each of the lift motors 6a can receive a predefined constant power supply to ensure the drone 1 remains airborne while the orientation motors 6b are stopped, with the valves 17 in the closed position. By adjusting the degree of opening of the valves 17, it is possible to vary the rotational speed of the orientation motors 6b and thus control the drone's orientation and / or stabilize it, i.e., maintain it in a stationary or near-stationary position.

[0117] We will now describe with reference to Figures 13 and 14 the operation of a drone according to the first aspect of the invention, in which all the motors are connected in series with a controllable valve and receive a non-zero flow of fluid when the valve is in the first position, as is the case for drones whose hydraulic circuits are illustrated in the Figures 1 And 2 .

[0118] There figure 13represents the evolution 100 of the pressure p at the outlet of the hydraulic pump 10 as a function of time t and the evolution 200 of the pressure p at the inlet of the pilot valve 17 and the calibrated orifice 31 as a function of time t.

[0119] There figure 14 represents the evolution 300 of the speed v of the drive motor 11 of the hydraulic pump 10 as a function of time t.

[0120] In a first step A, the hydraulic motors 6 are started. To do this, the speed of the drive motor 11 of the hydraulic pump 10 is increased according to its torque capacity in order to increase the fluid pressure at the outlet of the hydraulic pump 10 and at the inlet of the pilot valve 17 and the calibrated orifice 31.

[0121] Next, in step B, drone 1 is waiting to take off. In this step B, the speed of the drive motor 11 of the hydraulic pump 10 remains constant. The rotational speed of the hydraulic motors 6 is then close to that required to ensure the takeoff of drone 1.

[0122] To make the drone take off, which corresponds to step C, the speed of the drive motor 11 of the hydraulic pump 10 is further increased according to its torque capabilities in order to increase the pressure of the fluid at the outlet of the hydraulic pump 10 until the latter reaches a predefined limit value allowed by the pressure limiters 27, called limited pressure p L.

[0123] Next, in step D, the speed of the hydraulic pump 10's drive motor 11 is further increased according to its torque capacity in order to increase the fluid pressure at the outlet of the hydraulic pump 10 to a value exceeding the limited pressure pL allowed by the pressure limiters 27. The pressure limiter 27 ensures that the limited pressure pL is present at the inlet of the controllable valve 17 and the bypass with the calibrated orifice 31. This limited pressure pL is transformed, through the calibrated orifice 31, into a constant idle flow rate which ensures a rotational speed of the hydraulic motors 6 sufficient to produce the thrust necessary to lift the drone 1.

[0124] During the next step E, the speed of the hydraulic pump 10's drive motor 11 is constant so that the fluid pressure at the outlet of the hydraulic pump 10 and at the inlet of the pilot valve 17 and the calibrated orifice 31 are constant. In this step E, the hydraulic pump 10 generates a pressure higher than the limited pressure pL allowed by the pressure relief valve 27. Thus, the fluid pressure at the outlet of the hydraulic pump 10 is higher than that at the inlet of the pilot valve 17 and the calibrated orifice 31.

[0125] In this step E, the speed of the hydraulic pump 10's drive motor 11, combined with the required torque, defines the power rating needed for the drive motor 11. This power rating is typically within a range of approximately 1.6 times the power required to lift the drone 1. Setting the power rating of the hydraulic pump 10's drive motor 11 at this value ensures that the drive motor 11 meets durability requirements without excessive stress, as it will not exceed this power rating regardless of the drone's operating mode. Furthermore, this range accommodates all the power variation requirements of the hydraulic motors 6, regardless of the flight conditions deemed acceptable for the drone 1, such as the wind gust speeds to which the drone 1 must be able to react and remain stable.

[0126] In this step E, the control of the valves 17 via the flight controller 14 ensures the stability of the drone 1. The valves 17 allow the supply flow rate of the hydraulic motors 6 to be modulated, thus ensuring a modification of the rotation speed according to the need defined by the flight controller 14, through control algorithms in particular.

[0127] To descend, which corresponds to step F, the rotation speed of the hydraulic motors 6 is modulated by acting on the valves 17 and by decreasing the speed of the drive motor 11 of the hydraulic pump 10 to ensure the landing of the drone 1.

[0128] Finally, in a step G, while the drone 1 is on the ground, the speed of the drive motor 11 of the hydraulic pump 10 is further reduced in order to ensure a gradual shutdown of the motors and the valves 17 are deactivated.

[0129] Of course, the invention is not limited to the examples just described. For example, the pump can be driven by a heat engine.

Claims

1. Multi-rotor hydraulic drone (1) comprising: - a plurality of hydraulic motors (6) each receiving a pressurized fluid, - propellers (5) driven by the hydraulic motors (6), - at least one hydraulic pump (10) driven by at least one motor (11) for pressurizing the fluid, - a system for supplying the hydraulic motors (6) with pressurized fluid, - a flight controller (14) for operating the supply system according to the desired rotation speed for the hydraulic motors (6), the supply system comprising a plurality of paths (35; 36; 37; 38) for adjusting the power of at least some of the hydraulic motors (6), each path (35; 36; 37; 38) comprising a proportionally controllable valve (17), making it possible to vary the supply flow rate of pressurized fluid at the inlet of the corresponding hydraulic motor (6), according to the speed required of this hydraulic motor (6), and thus the rotation speed of the associated propeller (5), the supply flow rate of the hydraulic motor (6) increasing when the controllable valve (17) passes from a first position to a second position of greater opening than the first, the fluid flow rate received by at least one of the hydraulic motors when the controllable valve (17) is in the first position being non-zero, the hydraulic motor (6) that receives a non-zero fluid flow rate when the controllable valve (17) is in the first position being connected in series with this controllable valve (17), characterized in that each path (35; 36; 37; 38) comprises a bypass in parallel with the valve 17, this bypass comprising a calibrated orifice (31) chosen to deliver a substantially constant flow rate of pressurized fluid to the inlet of the corresponding motor (6).

2. Drone according to Claim 1, comprising at least one pressure limiter (27), in particular one per path (35; 36; 37; 38), to limit the pressure of the fluid at the inlet of the controllable valve (17) to a predefined limit value, called a limited pressure.

3. Drone according to Claim 1 or 2, the controllable valve (17) being closed in the first open position and the non-zero flow rate then received by the hydraulic motor (6) coming entirely from the bypass.

4. Drone according to any one of the preceding claims, said drone being wired and being supplied with electrical power from the ground by a cable.

5. Drone according to any one of the preceding claims, the hydraulic motors (6) being fast axial piston motors with fixed displacement.

6. Drone according to any one of the preceding claims, comprising supports (15) each configured to allow the attachment of a hydraulic motor (6), of the corresponding controllable valve (17), and of a pressure limiter.

7. Drone according to any one of the preceding claims, comprising at least one heat exchanger (8), in particular one per path (35; 36; 37; 38), for cooling the fluid, in particular at the outlet of the hydraulic motors (6).

8. Drone according to any one of the preceding claims, comprising a tank (13) containing and / or intended to receive the fluid and allowing the hydraulic pump (10) to be supplied with fluid, the tank (13) preferably being arranged so that the hydraulic pump (10) operates under load.

9. Drone according to any one of the preceding claims, the hydraulic pump (10) supplying pressurized fluid to the plurality of hydraulic motors (6).