Patent Publication Number: US-2022236745-A1

Title: Drone and method for controlling the attitude thereof

Description:
TECHNICAL FIELD 
     The present description relates to a drone, in particular a multi-rotor drone with a winch for a suspended electric cable. The present description also relates to a method of controlling the attitude of the drone. 
     BACKGROUND OF THE DESCRIPTION 
     One or more drones can be connected to each other by electric cables in different possible configurations, in which at least one drone is connected to a base station capable of supplying electricity. Such drone networks may be used to perform different types of tasks in civil applications, such as monitoring via cameras. A drone can be equipped with a winch on which a suspended electric cable can be wound and unwound, in order to adjust its length. 
     For example, FR 3037448 A1, FR 3033256 A1, FR 3053259 A1 and US 2016/0083115 A1 describe drone networks equipped with a ground winch and voltage converters on the ground and on board the drones, to raise the level of electrical voltage on the cable and thus decrease the current transmitted at a same power, with a consequent reduction in the diameter and mass of the cable. These drones are also equipped with control systems that regulate the force on the cable and limit the maximum length of the cable being unwound, leaving free the possibility of rewinding. However, these known control systems have the relevant dwarback of constraining the drone&#39;s ability to maneuver, effectively limiting their movement to only the vertical direction with respect to the base station. This constraint is mainly due to the considerable risk that the suspended cable will get caught in obstacles and constitute an obstacle for people and things near the base station. To overcome this drawback, while maintaining the advantage of supplying power by cable, drone networks are known, connected by electric cables in different possible configurations so as to provide greater maneuvering capacity, keeping the formation geometry under control and consequently keeping under control the positioning of each suspended cable segment. 
     For example, US 2013/0233964 A1 describes drone networks equipped with winches to adjust the length of the electric cables, US 2016/0144958 A1 describes safety devices that act in the event of interruption of electrical connection in drone networks, for example because of a failure in one of the suspended cables, and the article “Systems of Tethered Multicopters” by L. Fagiano, published in the scientific journal IFAC-PapersOnLine, volume 50, issue 1, July 2017, pages 4610-4615, describes drone networks connected by suspended cables, where each drone can be equipped with a winch controlled by a system that regulates the length of the cable connected to the next drone. The article proposes a control system partly centralized and partly distributed to optimally adjust the length of the cables and the motion of drones, respecting operational constraints and pursuing a predetermined task. 
     A problem with such drone systems relates to the effect of mechanical forces applied by cables, which generally generate translation forces and moments on each drone. These forces must be balanced by a system that regulates the attitude and position of each drone through an additional effort of the rotors, with consequent potential problems for the stability of the motion of the drone. 
     A further problem relates to the additional mass and size of the voltage conversion system on board each drone, which also requires a cooling system to keep the converter temperature within acceptable limits. These masses and sizes must be minimized to reduce energy required by each drone and to make possible to use networks with a greater number of drones and longer cables. 
     WO2016121072, on the disclosure of which the preamble to each independent claim is based, describes a drone with a stable flight attitude, which allows to perform a given task and in which a sudden change in the load or the severing of a power cable does not cause the drone to fall. 
     US2017222594 discloses an intelligent power control system to drive motors of a drone, comprising: a temperature detection unit, a processing unit and an motor power control unit. The processing unit can be configured to compare if the read temperature exceeds a first temperature and check the control unit of the motors power to dynamically adjust the maximum allowed output power of the various motors according to the result of a comparison. 
     SUMMARY OF THE DESCRIPTION 
     A goal of the present description is thus to provide a drone free from such limitations. Said goal is achieved with a drone and a drone control method, whose main features are specified in the enclosed claims. 
     If provided with a particular winch integrated in the central structure, the drone according to the present description can bring the suspended cable at its center of mass to minimize the moments due to the forces on the cable, as well as to decrease the additional mass due to the winch. The control method may further impart an appropriate separate command action to the motors of the drone, so that the thrust difference between the different propellers generate a rotational torque that balances in an automatic way the torque caused by the motor that operate the winch. In this way, the impact of the winch movement on the drone&#39;s structure is minimized, also reducing the energy required to counterbalance the effects of the forces applied by the cables on the drone. 
     The drone may further comprise a particular energy transmission system in which the voltage conversion system for the propeller motors is split into several converters designed in an integrated way with respect to the motors, so as to reduce the power managed by each converter, to provide naturally each converter with a consistent flow of air through the propellers, and distribute the additional mass of the converters uniformly with respect to the center of mass of the drone, so as to further improve the stability of the drone. Since the converters are arranged under the propellers of the drone, the greater the power required by the propeller, the greater the flow of cooling air. The use of more than reduced mass converters, each dedicated to a motor and propeller system, allows to distribute in a homogeneous way the overall sizes and the masses, freeing also space in the central part of the drone for housing the winch and/or the payload of the drone, for example, a video collection device and images, thus creating a synergistic effect with the aforementioned method of controlling buoyancy in function of the rotation of the winch. 
     In addition, the use of multiple converters provides redundancy that increases the safety of the drone, since in the event of a converter failure it is possible to isolate the relative part of the electrical system and continue the flight with the remaining converters, alternatively by deactivating other motors selectively to balance the attitude of the drone. 
     The present description also relates to an automatic method for regulating the temperature of the converters on board each drone, which varies the working load of each motor so as to avoid excessive temperatures in the converters and in the motors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and characteristics of the drone and of the method according to the present description will become apparent to those skilled in the art from the following detailed and non-limiting description of an embodiment thereof, referring to the accompanying drawings in which: 
         FIG. 1  shows a schematic view of a drone network with a base station on the ground; 
         FIG. 2  shows a schematic view of a drone network with a suspended base station; 
         FIG. 3  shows a perspective view of the drone; 
         FIG. 4  shows a schematic transverse sectional view of the drone of  FIG. 3 ; 
         FIG. 5  shows a block diagram of a first control system of the drone of  FIG. 3 ; 
         FIG. 6  shows a schematic side view of the drone of  FIG. 3 ; 
         FIG. 7  shows a block diagram of a second drone control system of  FIG. 3 . 
     
    
    
     EXEMPLARY EMBODIMENTS 
     Referring to  FIG. 1 , it is seen that a drone network, in particular multi-rotor propeller electric drones, may comprise a power cable  1  which is electrically connected to a base winch  2 , which can be fixed to the ground. The power cable  1  can supply the base winch  2  with electricity from an electrical network, a generator or an accumulator of electricity. An auxiliary cable  4  is wound on the base winch  2  and is connected, at the opposite end, to a first drone  5 . The base winch  2  is equipped with a device, for example by means of sliding contact rings, designed to guarantee continuity of the transmission of energy between the power cable  1  and the auxiliary cable  4  even during the rotation of the drum of the base winch  2 . In this way, the auxiliary cable  4  can continuously supply electricity to the first drone  5 . A control system  3  comprises a control unit for controlling the operation of an electric motor of the drum of the base winch  2 . The first drone  5  can be connected via a suspended cable  6  to a second drone  7 . The first drone  5  is equipped with at least one winch  8 , on which the suspended cable  6  is wound. The suspended cable  6  can supply electrical energy and/or control signals to the second drone  7 , which in turn can be equipped with a winch  10 , on which a further suspended cable  11  is wound to supply electrical energy to a third drone  12 , which in the present example is the last of the series and thus preferably does not include an on board winch. Drone  5  can also be the only drone in the system and/or have the auxiliary cable  4  wound on the winch  8 . 
     Referring to  FIG. 2 , it can be seen that in a drone network similar to that of  FIG. 1  the base winch  2  can be mounted on a mobile support  13  which can translate and/or rotate with respect to a structure  14  which maintains the mobile support  13  suspended from the ground and for example constituted by one or more suspended cables, as shown in the figure, or by the frame of a structure, by the arm of a crane or by other equipment provided in turn with degrees of freedom of rotation or translation. 
     Referring to  FIGS. 3 and 4 , it can be seen that the drone  5  comprises a plurality of converters  15  capable of converting high voltage electric energy (for example about 1000V DC), received by the auxiliary cable  4 , into low voltage electric energy (e.g. 24V DC or 48V DC). The drone  5  further comprises a plurality of propellers  16  driven by motors  17  which are powered by such low voltage electricity and are supported by at least one structure  18 , in particular comprising at least one frame formed of a plurality of elements joined together. The converters  15  are arranged around the structure  18 , preferably fixed to the motors  17  and/or under the propellers  16 , i.e. under the vertical projection of the dimensions of the propellers  16 , in the horizontal flight position of the drone  5 . Preferably, each converter  15  supplies electrical energy to the motor  17  which drives the propeller  16  under which the converter  15  is arranged, so that the drone  5  comprises the same number of converters  15  and motors  17 . Alternative embodiments may include a smaller number of converters  15 , each of which feeds multiple motors  17 , for example four or two converters  15  for a drone with eight propellers  16 . At least two converters  15  can be arranged in opposite positions with respect to the structure  18 , preferably at substantially equal distances from the center of mass of the drone  5 , so that the center of mass of these at least two converters  15  falls into the drum of the winch  8  and/or substantially coincides with the center of mass of the drone  5 . 
     Preferably, the converters will be arranged so that the center of mass of the converters coincides to that of the drone and the drone is positioned so that the drone&#39;s inertia matrix is diagonal. Thanks to this arrangement, the control logic of the flight attitude is simplified and it is possible to use in an uniform manner the motors and the propellers during the life of the drone. 
     The structure  18  can comprise a central seat  19 , in particular defined by a portion of the frame having a substantially rectangular shape, in which the winch  8  is arranged which carries the suspended cable  6  and which can rotate around a shaft  20  arranged in the central seat  19 . The shaft  20  preferably extends outside the central seat  19  for connecting between their two motors  17  arranged in opposite positions with respect to the structure  18 . The center of mass of the drone  5  preferably falls into the drum of the winch  8 , in particular in a position substantially coinciding with the center of mass of the winch  8 . The winch drum  8  is preferably hollow for housing certain components of the drone  5 , in particular a central control unit  21  of the drone  5  and the motor  22  of the winch  8 , disposed between the shaft  20  and the drum, so as to optimize the use of space and to balance the drone  5 . 
       FIG. 5  shows a control system of the drone  5  for reducing the thermal stress of the converters  15  and of the motors  17 , maintaining the flight stability of the drone  5 , which system comprises temperature sensors  23  arranged in correspondence with the converters  15  and/or of motors  17  to transmit temperature data to a control unit of the temperature  24 , which analyzes the distribution of the temperatures measured by the temperature sensors  23  and calculates variation data of the rotation speed of the motors  17 , i.e. of their lift forces f1 . . . fn, in order to balance the temperatures, for example by raising the lower ones and lowering the higher ones, without changing the flight attitude of the drone  5 . This method can be performed when the drone  5  is equipped with more than four motors  17  and propellers  16 , so that there are a plurality of possible speed combinations of each propeller  16  that produce a same combination of lift forces of and of torques applied to the drone  5 , preferably including the torque caused by the winch  8 . The temperature control unit  24  sends calculated data of speed change to a flight control unit  25 , which varies the speed of the motors  17  accordingly. 
       FIG. 6  shows the effect on the attitude of the drone  5  of the torque t applied by the motor  22  to operate the winch  8 , for example when the suspended cable  6  must be unwound with a certain speed v. To carry out this operation, the motor  22  drives the winch  8  with a torque t which would cause an unwanted pitching of the drone  5 . To balance this pitch, the drone  5  varies the speed of the motors  17  to vary the lift forces f1 . . . fn of the propellers  16  and thus generates a pitch torque equal to and opposite to the torque t. In particular, one or more lift forces f1 . . . fn of the propellers  16  are increased or decreased to wind or unwind the suspended cable  6 . 
       FIG. 7  shows a control system of the attitude of the drone  5  to balance the rotation of the winch  8 , which system comprises a control unit of the attitude  26  which receives information as input data of the torque t transmitted by a control unit of the winch  27  and data of the speeds of the motors  17  transmitted by the flight control unit  25 . In the method according to the present embodiment, the control unit of the attitude  26  calculates a speed variation of the propellers  16  concerned, for example the two propellers  16  of  FIG. 6  placed along a the direction perpendicular to the axis of rotation of the winch  8 , so that the relative lift forces f1 and f2 balance the torque t, as described above, and sends the resulting varied speed data to the motors  17  of the two propellers  16 . If the direction perpendicular to the axis of rotation of the winch  8  does not coincide with the position of two propellers  16 , the control unit of the attitude  26  acts on a number superior of propellers  16 , so as to have anyway a torque at the center of mass of the drone  5  which balances the torque t. 
     The temperature control unit  24  and/or the control unit of the flight  25  and/or the control unit of the attitude  26  and/or the control unit of the winch  27  can be implemented in a known manner in at least a single electronic control unit, preferably arranged in the central control unit  21  of the drone  5 . 
     Any variants or additions may be made by skilled persons to the embodiment described and illustrated herein remaining within the scope of the following claims. In particular, further embodiments may include the technical characteristics of one of the following examples with the addition of one or more technical characteristics described in the text or illustrated in the drawings, taken individually or in any mutual combination. 
     EXAMPLES 
     1. Drone ( 5 ) which comprises a plurality of propellers ( 16 ) driven by motors ( 17 ) supported by at least one structure ( 18 ), characterized in that a plurality of converters ( 15 ) are arranged around the structure ( 18 ) to convert high voltage electrical energy into low voltage electrical energy. 
     2. Drone ( 5 ) according to the previous example, characterized in that the converters ( 15 ) are fixed to the motors ( 17 ). 
     3. Drone ( 5 ) according to one of the preceding examples, characterized in that the converters ( 15 ) are arranged under the propellers ( 16 ) in the horizontal flight position of the drone ( 5 ). 
     4. Drone ( 5 ) according to the previous example, characterized in that each converter ( 15 ) supplies electrical energy to the motor ( 17 ) which drives the propeller ( 16 ) under which the converter ( 15 ) is arranged. 
     5. Drone ( 5 ) according to one of the preceding examples, characterized in that at least two converters ( 15 ) are arranged in opposite positions with respect to the structure ( 18 ). 
     6. Drone ( 5 ) according to the previous example, characterized in that the center of mass of said at least two converters ( 15 ) substantially coincides with the center of mass of the drone ( 5 ). 
     7. Drone ( 5 ) according to one of the preceding examples, characterized in that the center of mass of all converters ( 15 ) substantially coincides with the center of mass of the drone ( 5 ). 
     8. Drone ( 5 ) according to the previous example, characterized in that the structure ( 18 ) comprises a central seat ( 19 ) in which a winch ( 8 ) is arranged and is provided with a drum which can rotate by means of a motor ( 22 ) to unwind or wind a suspended cable ( 6 ), wherein the center of mass of two or more converters ( 15 ) falls into the drum of the winch ( 8 ). 
     9. Drone ( 5 ) according to one of the preceding examples, characterized by comprising temperature sensors ( 23 ) which are arranged at the converters ( 15 ) and/or the motors ( 17 ) to transmit temperature data to a temperature control unit ( 24 ), which is suitable to calculate variation data of the rotation speed of the motors ( 17 ), which are sent to a flight control unit ( 25 ), according to the temperature data received from the temperature sensors ( 23 ). 
     10. Method for controlling the attitude of a drone ( 5 ) which comprises a plurality of propellers ( 16 ) driven by motors ( 17 ) supported by at least one structure ( 18 ), characterized in that it comprises the following operating steps:
         measuring temperatures at the motors ( 17 ) by means of temperature sensors ( 23 );   calculating variations of the lift forces (f1 . . . fn) of the propellers ( 16 ) according to said measured temperatures;   varying the rotation speed of the motors ( 17 ) according to said calculation, so as to vary the lift forces (f1 . . . fn) of the respective propellers ( 16 ).       

     11. Method according to the previous example, wherein the drone ( 5 ) also comprises a plurality of converters ( 15 ) which are suitable to convert high voltage electrical energy into low voltage electrical energy, characterized in that it comprises the following further operating steps:
         measuring temperatures at the converters ( 15 ) using temperature sensors ( 23 );   calculating variations of the lift forces (f1 . . . fn) of the propellers ( 16 ) according to said measured temperatures;   varying the rotation speed of the motors ( 17 ) according to said calculation, so as to vary the lift forces (f1 . . . fn) of the respective propellers ( 16 ).