A balloon enabled drone Field of the invention ^[0001] The present invention relates to a balloon enabled drone. More specifically, the present invention relates a balloon enabled drone with at least four propellers. Background [0002] Since unmanned aerial vehicles (UAVs or called drones) are becoming a very versatile and valuable platform for enabling various applications. Multi-rotor drones arguably represent the most widespread aerial robotic platform. Ever decreasing costs, combined with extreme agility – both indoors and outdoors – make them a very useful and important tool to bring sensing and networking where no other platform could reach, for example, in disaster zones, remote and icy mountains, and thick forest. Besides regulatory and safety issues, multi-rotor drones are also plagued by one major limitation, i.e., the operational time, since the power is mainly from the embedded batteries. ^[0003] Because of the physical dynamics of multi-rotor drones, their motors must produce ≈50% more thrust than the weight of the drone, which is independent of the physical movement in space, if any. Lifetime is dependent on the battery capacity and thus batteries represent a major weight factor; increasing battery capacity will in turn requires higher thrust and high-thrust motors, subsequently, adding more weight. Thus, the multi-rotor drones are inherently limited by how long they can operate – reaching some tens of minutes at best. [0004] There are alternatives to multi-rotor drones, yet with limited applicability. For example, tethered multi-rotor drones obtain energy from a static infrastructure; the necessary electrical connections limit their maneuverability and range. Fixed-wing drones, much like regular aircraft, are extremely difficult to control in constrained spaces, for example, due to their inability to retain a fixed position over time and also because of sophisticated take-off or launch requirements. They are typically employed only in long-range outdoor missions. Blimps, on the other hand, enjoy long operational times at the expense of limited maneuverability, especially in vertical motion. [0005] Therefore, there is a need for a novel drone design that can prolong the operation time and at the same maintain the maneuverability of the drone. Summary of the invention [0006] The present invention relates a balloon enabled drone. More specifically, the present invention relates a balloon enabled drone with at least four propellers. Additionally, in the present invention, a different and new aerodynamic phenomenon is used to enable the maneuverability of the balloon enabled drone. Brief description of the drawings ^[0007] The present invention will be discussed in more detail below, with reference to the attached drawings, in which: [0008] Fig.1 shows an example of a balloon enabled drone. ^[0009] Fig.2 shows an example of a balloon enabled drone. [0010] Fig.3 shows an example of a balloon enabled drone. [0011] Fig.4 shows an example of a balloon enabled drone. ^[0012] Fig.5 shows an example of a balloon enabled drone. ^[0013] Fig.6 shows an example of a controller. [0014] Fig.7 shows a method for controlling a balloon enabled drone. [0015] Fig.8 shows a fault control method. [0016] Fig.9 shows the logical modules of a balloon enabled drone. [0017] Fig.10 shows an example of a weight adjustment unit. Description of embodiments [0018] Embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. However, the embodiments of the present disclosure are not limited to the specific embodiments and should be construed as including all modifications, changes, equivalent devices and methods, and/or alternative embodiments of the present disclosure. ^[0019] The terms “have,” “may have,” “include,” and “may include” as used herein indicate the presence of corresponding features (for example, elements such as numerical values, functions, operations, or parts), and do not preclude the presence of additional features. [0020] The terms “A or B,” “at least one of A or/and B,” or “one or more of A or/and B” as used herein include all possible combinations of items enumerated with them. For example, “A or B,” “at least one of A and B,” or “at least one of A or B” means (1) including at least one A, (2) including at least one B, or (3) including both at least one A and at least one B. [0021] The terms such as “first” and “second” as used herein may modify various elements regardless of an order and/or importance of the corresponding elements, and do not limit the corresponding elements. These terms may be used for the purpose of distinguishing one element from another element. For example, a first printing form and a second printing form may indicate different printing forms regardless of the order or importance. For example, a first element may be referred to as a second element without departing from the scope the present invention, and similarly, a second element may be referred to as a first element. ^[0022] It will be understood that, when an element (for example, a first element) is “(operatively or communicatively) coupled with/to” or “connected to” another element (for example, a second element), the element may be directly coupled with/to another element, and there may be an intervening element (for example, a third element) between the element and another element. To the contrary, it will be understood that, when an element (for example, a first element) is “directly coupled with/to” or “directly connected to” another element (for example, a second element), there is no intervening element (for example, a third element) between the element and another element. [0023] The expression “configured to (or set to)” as used herein may be used interchangeably with “suitable for,” “having the capacity to,” “designed to,” “ adapted to,” “made to,” or “capable of” according to a context. The term “configured to (set to)” does not necessarily mean “specifically designed to” in a hardware level. Instead, the expression “apparatus configured to…” may mean that the apparatus is “capable of…” along with other devices or parts in a certain context. ^[0024] The terms used in describing the various embodiments of the present disclosure are for the purpose of describing particular embodiments and are not intended to limit the present disclosure. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. All of the terms used herein including technical or scientific terms have the same meanings as those generally understood by an ordinary skilled person in the related art unless they are defined otherwise. The terms defined in a generally used dictionary should be interpreted as having the same or similar meanings as the contextual meanings of the relevant technology and should not be interpreted as having ideal or exaggerated meanings unless they are clearly defined herein. According to circumstances, even the terms defined in this disclosure should not be interpreted as excluding the embodiments of the present disclosure. ^[0025] Before explaining the present invention, an orientation system is first defined based on at least one of: an earth plane being a plane/fat surface that is parallel to an earth surface (i.e., a small area around a point on the earth surface which can be seen by a skilled person as a flat surface in the context of the present invention); a horizontal plane being a plane/flat surface that is parallel to the earth surface; a vertical direction being perpendicular to the earth plane; a horizontal direction being any direction on the horizontal plane; and a yaw rotation being a rotation movement around a yaw axis of a drone, e.g., around the Y axis as shown in fig.1. The yaw axis is a vertical center line of the drone, i.e., a vertical line that may connect all the mass central points on the horizontal planes of the drone. [0026] Two main factors that affect the flying performance of drones are the battery time (also called flying time, airtime, maximal flying time, operation time, etc.) and the maneuverability (e.g., comprising lifting, descending, flying in different directions on the same horizontal plane, yaw rotation, etc.). The batteries have their own mass, thus, adding batteries to increase the flying time is not efficient because it will also further limit the maneuverability, e.g., when lifting or turning around the yaw direction. With balloon enabled aerial vehicles, the vertical movements of the vehicles are normally enabled by filling or removing gas from the balloon or by additional lifting/descending propellers, i.e., to provide a total lifting force to be higher or a total descending force lower than the gravity of a vehicle. However, the removing or filling gas may be slow and the additional lifting/descending propellers may add additional weight, which also use more battery power by driving the additional motors. [0027] In order to solve the above problem, a balloon enabled drone is presented, which is similar to blimps where both engines and a balloon with lighter than air gas are used, however, the maneuverability and operation time of the presented balloon enabled drone are much better. [0028] Fig.1 shows an example of a balloon enabled drone 100 (or called aerial vehicle 100). [0029] The balloon enabled drone 100 may comprise at least one balloon 101, which is configured to receive gas that is lighter than air (i.e., with a density that is lower than the air). In fig.1, an inflated state of the balloon 101 is shown, and when the drone 100 is not in the flying mode, the balloon 101 may be in a flat state. ^[0030] The lighter than air gas (or called lifting gas) may be a mix or one of hydrogen, helium, hot air, coal gas, ammonia, methane, and any other gas has a density lower than the air. In the present application, helium and hydrogen are preferred and helium is even more preferred since it is safer than hydrogen. ^[0031] The lifting force provided by the balloon 101 is more than, equal to, or less than the total gravity of the drone 100. [0032] The drone 100 may comprise at least one engine 103 (or called motor) and each of the engine 103 connects to (i.e., configured to drive) at least one propeller 102. In the example of fig.1, there are two engines 103 and each of them connects to a propeller 102. The engine and propeller behind the middle engine and propeller are not shown in fig. 1 since the view is blocked from the viewing direction. Preferably, there may be four or more engines and propellers. For example, the engines and propellers may be symmetrically/uniformly arranged around the balloon 101. [0033] The drone 100 may comprise four or more propellers 102. One propeller 102 may be formed by at least one propeller blades, preferably with two, three or even more blades, even more preferred with two blades. ^[0034] A propeller rotation plane of a propeller (i.e., the plane formed by a rotating propeller 102) is vertical to the horizontal plane, i.e., the propeller 102 is arranged on the horizontal plane as shown in fig. 1. Preferably, all the propellers are arranged on the horizontal plane. For example, a longitude direction of a propeller is a direction perpendicular to the propeller rotation plane. A central longitude line of a propeller is a line parallel to the longitude direction of the propeller and extends from the central turning point of the propeller (i.e., the central shaft line of the propellers). The central longitude line of all the propellers 102 may be parallel to the horizontal plane. [0035] The propellers 102 may be configured to be driven by the corresponding engines 103. When a propeller 102 is on (i.e., turning/rotating), the propeller 102 presses air inwards, i.e., on a direction towards the bottom of the balloon 100. [0036] The engines 103 may be reverse engines such that the corresponding propellers 102 may also be configured to be driven by the engines 103 to presses air outwards, i.e., away from the bottom of the balloon 100. [0037] The drone 100 may comprise a frame 104 to attach the engines 103 and balloon 101. Each propeller may be attached/fixed to the corresponding engine. The frame 104 may further includes at least one frame structure 1041 to stabilize the balloon 101, e.g., providing multiple holding points for the balloon 101. The balloon 101 may be fixed on the frame 104 and the frame structure 1041 may provide multiple points to support the balloon 101 in a standing state when inflated. [0038] The engines 103 may be horizontally fixed on the frame 104 as shown in fig. 2, or it may be fixed on the frame 104 by an additional vertical frame structure 1042 (e.g., being a part of the frame 104), e.g., as shown in fig.2. [0039] Alternatively, the engines 103 may be fixed directly on the frame structure 1041 as shown in fig.3. ^[0040] The propellers 102 may be evenly arranged around the bottom of the balloon 101 on the horizontal plane, and fixed by the frame 104. The propellers 102 may be arranged in a symmetric manner. [0041] The maneuverability of the drone 100 may be achieved by the controlling of the propellers 102. For example, when all the propellers 102 are on, air is pressed towards the bottom of the balloon 101, e.g., the air pressure under the balloon increases (e.g., the air cannot escape easily on the sides) such that the compressed air provides more lifting force to enable to drone 100 to lift (i.e., the drone 100 lifts when all the propellers 102). When one or more neighboring propellers 102 are on, since the air is pressured to a certain joined direction, the drone 100 moves on the horizontal plane. When two opposite propellers 102 are on, the air is pressed inwards to the bottom of the balloon 101, however, since the air can escape quickly on the sides, the air pressure drops such that the drone descends (i.e., when the opposite propellers 102 are on the drone 100 descends). [0042] If the engines 103 are reverse engines, the descending of the drone 100 may be achieved by turning on all the propellers to press air outwards from the bottom of the balloon 101 such that the air pressure below the balloon 101 is low. The moving on the horizontal plane may be achieved by blowing the air to an opposite direction when one or more propellers 103 are on. [0043] When the gravity of the drone 100 is equal to the lifting force of the balloon 101, the above maneuverability may be easily achieved. When the gravity of the drone is higher or lower than the lifting force from the balloon 101, the above maneuverability may be achieved by compensating the force difference between the lifting force from the balloon 101 and the gravity of the drone 100. [0044] For example, when the gravity of the drone 100 is lower than the lifting force from the balloon 101, the lifting of the drone 100 may be achieved by merely the force difference (i.e., lifting force difference from the lifting force from the balloon 101 minuses the gravity of the drone 100 ), or even a quicker lifting of the drone 100 may be achieved by both the force difference and turning on all the propellers 102. The descending of the drone 100 requires certain (stronger) power of the opposite propellers 102 such that force difference can be compensated and at the same time providing the additional descending force. When maneuvering on the horizontal plane, the force difference needs to be compensated as well with two opposite propellers always on and stronger than the other propellers. [0045] For example, when the gravity of the drone 100 is lower than the lifting force from the balloon 101, the descending of the drone 100 may be achieved by merely the force difference (i.e., descending force difference from the gravity of the drone 100 minuses the lifting force from the balloon 101), or even a quicker descending of the drone 100 may be achieved by both the force difference and turning on two opposite the propellers 102. The lifting of the drone 100 requires certain (stronger) power of all the propellers 102 such that force difference can be compensated and at the same time providing additional lifting force. When maneuvering on the horizontal plane, the force difference needs to be compensated as well with all the propellers always on. [0046] The frame 104 may include a receiving structure to receive weight such that the total weight/mass of the drone 100 may be adjusted. [0047] The drone 100 may comprise more components than the components shown in the figures, for example, memory, processor, communication unit, sensors, internal measurement units, batteries, camera(s), microphone(s), speaker(s) and any other components that may be fixed on or in the frame 104 and/or the balloon 101 as well. [0048] Fig. 4 shows a bottom-up view (or an up-bottom view) of the drone 100 (wherein the balloon 101 is not shown). [0049] The drone 100 may comprise four engines 1031, 1032, 1033 and 1034, each configured to drive one of propellers 1021, 1022, 1023 and 1024. The engines and propellers are arranged on the frame 104 in a symmetric manner. The frame 104 may include a plate that is open on the middle (as shown in fig.4) or closed in the middle (not shown). ^[0050] The maneuverability of the drone 100 may be controlled by the on an off of the engines 1031, 1032, 1033 and 1034, which are further configured to drive the four propellers 1021, 1022, 1023, and 1024. For example, when all the propellers are on, the drone 100 lifts; when two opposite propellers are on (e.g., 1031 and 1034, or 1032 and 1033), the drone 100 descends; when one or more neighboring propellers are on, the drone 100 moves on the horizontal plane. For example, when engine 1031 is on, the drone 100 moves towards the direction where propeller 1021 points (e.g., up-left in fig.4); when two engines 1031 and 1032 are on, the drone 100 moves towards the joint direction where propellers 1021 and 1022 point (e.g., left in fig.4); and when two engines 1031 and 1033 are on, the drone 100 moves towards the joint direction where propellers 1021 and 1023 point (e.g., down in fig.4). [0051] The drone 100 may comprise more than four engines and propellers. For example, as shown in fig.5, the drone 100 comprises six propellers 2021, 2022, 2023, 2024, 2025, and 2026 each configured to be driven by an engine. For example, when all the propellers are on, the drone 100 lifts; when two opposite propellers are on (e.g., 2021 and 2026, or 2022 and 2025, or 2023 and 2024), the drone 100 descends; when one or more neighboring propellers are on, the drone 100 moves on the horizontal plane. For example, when propeller 2021 is on, the drone 100 moves towards the direction where propeller 2021 points (e.g., up-left in fig.5); when two propellers 2021 and 2022 are on, the drone 100 moves towards the joint direction where propellers 1021 and 1022 point (e.g., up-left in fig.5); and when two propellers 2023 and 2026 are on, the drone 100 moves towards the joint direction where propellers 2023 and 2026 point (e.g., down infig.5). [0052] The drone 100 may comprise even more propellers, e.g., five, seven, eights or even more. [0053] Another maneuverability may be important for drone is the yaw rotation around the yaw line Y as shown in fig.1. In order the enable the yaw rotation, a torque on the horizontal plane needs to be generated by the propellers 102. [0054] The torque may be provided by the propellers in the following way. For example, a longitude direction of a propeller (e.g., 102, 1021, 2021, etc.) is a direction perpendicular to the propeller rotation plane (i.e., the plane is formed when the propeller is on, e.g., 1201, and the longitude direction is the shaft direction of the propeller). A central longitude line of a propeller is a line parallel to the longitude direction of the propeller and extends from the central turning point of the propeller (i.e., the central shaft line of a propeller). A torque around the yaw Y can be provided by a propeller when its central longitude line (i.e., central shaft line) has a distance larger than zero to the yaw axis (i.e., a vertical center line of the drone). [0055] For example, such torque can be provided by any of the four propellers in fig.4 and six propellers in fig.5. E.g., the torque provided by the propeller 1021 may be calculated according to the following ways. [0056] In fig.4, a first two neighbouring propellers 1021 and 1022 are arranged to be next to each other with a first angle α and a second two neighbouring propellers 1033 and 1034 are arranged to be next to each other with a second angle β, wherein the first angle and the second angle can be the same or different. ^[0057] Considering the torque provided by the propeller 1201 when it is on, the central longitude line of the propeller 1201 is perpendicular to the propeller rotation plane, wherein the central longitude line of the propeller 1201 does not meet the yaw axis Y of the drone 100, the yaw axis Y being vertical to the horizontal plane. Similar central longitude lines can be found for the other propellers 1022, 1023 and 1024. [0058] A meeting point of the central longitude lines of the first two neighbouring propellers 1021 and 1022 and a meeting point of the central longitude lines of the second two neighbouring propellers 1023 and 1024 may not overlap, and may have a distance d. Then the moment M1 provided by the thrust T1 from the propeller 1021 can be calculated as M1=T1*d*sin(α/2). Similarly, the moment M2 provided by the thrust T2 from the propeller 1022 can be calculated as M2=T2*d*sin(α/2); the moment M3 provided by the thrust T3 from the propeller 1023 can be calculated as M3=T3*d*sin(β /2); and the moment M4 provided by the thrust T4 from the propeller 1024 can be calculated as M4=T4*d*sin(β /2). [0059] According to the above equations, the movement of the drone 100 from fig. 4 on the horizontal plane may be controlled according to the thrust values provided in any one or more of the propellers. [0060] Similar calculations for the moments for rotation around the yaw axis Y provided by each of the propellers 2021, 2023, 2024 and 2026 may be performed. This applies to drones with other numbers of propellers as well. ^[0061] The drone 100 may further comprise a weight adjustment unit 400, for example as shown in fig. 10. The buoyancy of the drone 100 provided by the balloon 101 may only be adjusted from the amount of the lighter than air gas inside the balloon 101, however, it may be difficult to adjust the amount of lighter than air gas filled in the balloon 101 accurately. Thus, there may be a need for some other ways to adjust the weight (i.e., mass) of the total drone 100, in order to achieve intended buoyancy for the drone 100. The weight adjustment unit 400 may comprises at least one weight receiving parts, from example, the baskets 401 as shown in fig. 10. Especially, if there are multiple baskets 401, the center weight point (i.e., the center of mass) of the drone 100 may also be adjust by adding or removing weights in some or all the baskets 401. Some other alternatives for baskets may include at least one hock to hang weights, at least one magnet to attach magnetic weight (e.g., iron), etc. ^[0062] The middle of the weight adjustment unit 400 may be closed or open. It may be preferred that the middle of the weight adjustment unit 400 is open such that the air pressured by the propellers towards the bottom of the balloon can escape easily, for example when it lifts. [0063] The weight adjustment unit 400 may be detachable to the frame 104, or be a part of the frame 104. The weight adjustment unit 400 may be detachable to the frame 104 via at least one detachable means may be used, e.g., the weight adjustment unit 400 may comprise at least one magnet 402 as shown in fig.10 and the magnet may be neodymium N45 magnet, electric magnet or any other magnet. Alternatively, the weight adjustment unit 400 may be detachable to the bottom of the frame 104 via a hock, tape, Hook and Loop Tape, etc. [0064] The drone 100 may further comprise a controller 300. The controller 300 may be considered as a part of the drone 100, or a separate device and not a part of the drone 100. I.e., in the present invention, the term drone or aerial vehicle can only include the flying device (e.g., as shown in figs.1 to 5), or can only include the controller (e.g., as shown in fig.6), or can include both the flying device and the controller. This will not limit the scope of the invention. In order to simplify the description, hereafter, the drone is only about the flying device, and the controller is to control the flying device/drone, i.e., they are described as separate devices/apparatuses. However, it should be known that the present invention covers the option that both devices/apparatuses are comprised in the term “drone” and “aerial vehicle”. [0065] The controller 300 is configured to control the drone 100 and one example is shown in fig.6. The controller 300 may communicate with the drone 100, either with wired or wireless communications or both, to control the drone 100 to maneuver via the controlling of the engines (i.e., the propellers), e.g., in the vertical direction (e.g., lifting/descending) and/or on the horizontal plane (e.g., lateral moment and/or yaw rotation). For example, the engines may be turned on or off, and/or a specific thrust from a propeller may also be controlled/configured by the controller (e.g., via controlling the turning speed of the engine). [0066] The controller 300 may be further configured to control other components in the drone 100, e.g., controlling the cameras (e.g., on, off, directions, etc.), collecting sensing data from the sensors on the drone, updating the software/firmware in the drone 100, etc. [0067] An example of the controller 300 is shown in fig.6, wherein the components shown are optional. In this example, the controller 300 may comprise at least one input component, e.g., control buttons (and/or sticks) 301 to configure the movements of the drone and/or other components on the drone. The controller 300 may comprise at least one antenna 302 in order to communicate with drone 100 wirelessly (e.g., for receiving or sending data from/to the drone 100). Other components, e.g., a display 303 (e.g., to show the captured images by a camera on the drone) and/or a speaker 304 may be comprised in the controller. Some other components of the controller 300 may be inside, thus, not shown in fig. 6, e.g., battery, processor, memory, communication unit, and/or any other components. ^[0068] After the input is received in the controller 300, the configuration values of the components in the drone 100 may be calculated on the controller 300 or on the drone or calculated by both. For example, when a movement instruction to a certain direction is inputted via the input component 301, the controller 300 uses its internal processor (not shown in fig.6) to calculate the configurations for the engines (e.g., on, off, which input turning speed, which power, etc.) then sends the configurations to the drone 100, or the controller 300 directly may send the movement instruction to the drone 100 and the drone calculates the configurations by its internal processor. Alternatively, the controller 300 may calculate some of the configurations according to the instruction and the drone 100 may calculate other configurations. [0069] Fig.7 shows a method for controlling the drone/aerial vehicle 100. Some of the steps are optional, e.g., any of steps 701 to 705; and the order of the steps is not restricted to the steps shown in fig.7, e.g., steps 702 to 704 can be performed in any orders. [0070] In step 701, the state of the drone 100 may be estimated, e.g., via embed state sensors (for example altitude sensors, accelerate sensors, velocity sensors, etc.), received from an external device, received from the controller 300, or obtained in any other ways. This step may be optional. ^[0071] In step 702, a first thrust value for each propeller may be determined according to a targeted altitude. For example, a first thrust value from each propeller to drive the drone 100 from the current altitude to the targeted altitude. The first thrust value may be calculated for each propeller. The first thrust value may be a single value or a vector. When it is a vector, it may include different temporary thrust values corresponding to different time periods. For example, in order to rise from a first altitude to a second altitude, propeller 1201 needs to have a first temporary thrust value from time t0 to t1, a second temporary thrust value from time t1 to t2, …, etc. With the vector value for each propeller, the flying of the drone 100 may be controlled in a more accurate manner. When changing the altitude, all the propellers may be on for rising, and/or only the opposite propellers may be on for descending. The mapping between the altitude and the first thrust value may be from at least one look up table based on experiments or pre- calculations, or it may be dynamically calculated based on a model, which will be discussed later. [0072] In step 703, a second thrust value for each propeller may be determined according to a targeted yaw rotation angle. For example, a second thrust value from each propeller to drive the drone 100 to rotate a certain angle around the yaw axis. The second thrust value may be calculated for each propeller. The second thrust value may be a single value or a vector. When it is a vector, it may include different temporary thrust values corresponding to different time periods. For example, in order to turn a certain angle, propeller 1201 needs to have a first temporary thrust value from time t0 to t1, a second temporary thrust value from time t1 to t2, …, etc. With the vector value for each propeller, the rotation of the drone 100 may be controlled in a more accurate manner. When rotating around the yaw axis, one propeller may be on to enable the rotating, and the thrusts from other propellers may be zero. Or maybe a main propeller may be on to enable the rotate and after the drone 100 turns to the targeted angle, a second propeller may be on shortly to compensate the inertia from the turning action. The mapping between the rotation angle and the second thrust value may be from at least one look up table based on experiments or pre-calculations, or it may be dynamically calculated based on a model, which will be discussed later. Or it may be based on the previous disclosed equations based on the distance d and angle α (or β). ^[0073] In step 704, a third thrust value for each propeller may be determined according to a targeted movements on the horizontal plane, i.e., a lateral movement. For example, a third thrust value from each propeller to drive the drone 100 to move around on the same horizontal plane, which may include a certain distance and a certain direction. The third thrust value may be calculated for each propeller. The third thrust value may be a single value or a vector. When it is a vector, it may include different temporary thrust values corresponding to different time periods. For example, in order to move to a certain location, propeller 1201 needs to have a first temporary thrust value from time t0 to t1, a second temporary thrust value from time t1 to t2, …, etc. With the vector value for each propeller, the moving of the drone 100 may be controlled in a more accurate manner. When moving on the same horizontal plane, one or more propellers may be on to enable the moving towards to a certain direction. The mapping between the movements (e.g., moving direction/distance) and the third thrust value may be from at least one look up table based on experiments or pre-calculations, or it may be dynamically calculated based on a model, which will be discussed later. [0074] In step 705, a total thrust for each propeller may be determined in order for the drone 100 to combine all actions simultaneously, which may reflect the overall thrust at the same time that will enable the drone 100 to reach the target position. This step may be skipped. For example, the drone 100 may first change the altitude according to the first thrust configuration, then rotate according to the second thrust configuration, and finally move around according to the third thrust configuration, e.g., changing the position in separate steps, instead of one step as in step 706. [0075] In step 706, each engine may be configured according to the total thrust value for a corresponding propeller. The mapping between the required thrust of a propeller and the configuration parameters of the engine may be from at least one look up table based on experiments or pre-calculations, or they may be calculated dynamically. [0076] A thrust value calculation model is presented below, which may be based on the drone 100 in fig.4 with four propellers. The model can be presented as: ^̇ = ^ _^ , ^̇ = ^ _^ , ^̇ = ^ _^ ^̇ = ^ _^ , ^ _^̇ = ^ _^ = ^ _^ (^ _^ − ^ _^ ) ^[0077] where ^, ^, ^, ^^, ^^, ^^ denote the position and velocity and φ, ^^ denote the yaw angle and yaw angular rate. ^^ denotes the ^th motor/propeller thrust, ^1 := ^1 + ^3, ^2 := ^2 + ^4 and ^^, ^φ, ^^ denote the aerodynamic coefficients, and ^ denotes the mass. ^̇ is the time derivative of ^, which is similar to other symbols with a dot on top. The pitch and roll dynamics may be ignored due to their natural stability. ^, ^, ^ denote the movement in a three-dimensional system with three perpendicular axes, wherein ^ denotes the vertical direction (i.e., the altitude direction) and ^, ^ denote the two perpendicular horizontal direction. It is evident that the equations are nonlinear and therefore traditional control design may not be effective in stabilizing or steering the drone 100. [0078] One approach to control the drone 100 may be to invert this relation based on the requirement of accelerations. However, this may be computationally ineffective, moreover, the control bandwidth required for altitude and yaw may be higher than that of lateral steering, and the sensing modalities and consequently sensing rate and error are different as well. Therefore, a hierarchical approach may be taken such as the method in fig.7. ^[0079] First, a controlling may be designed for only altitude (702) and yaw (703). Denote (^ _^ , ^ _^ ) = ^ ₁ (^ ₁ , ^ ₂ ), where ^ ₁ is obtained from the above model. Let ^ _^ and φ^ denote the desired values. Then, ^1, ^2 may be assigned as follows, ^ _^ = ^ _^ (^ − ^ _^ ) − ^ _^ (^ _^ − ^ _^̇ ) − ^ _^ ^(^ − ^ _^ )^^, where F ₁ ^-1 is obtained via standard methods (Newton-Raphson iterative inversion). The terms ^ _∗ denote control gains. The control law so far, assumes that ^ ₁ = ^ ₃ , ^ ₂ = ^ ₄ . [0080] Given desired lateral position ^ _^ , ^ _^ , (^ ₃ −^ ₁ ) and (^ ₄ − ^ ₂ ) for step 703 may be varied while maintaining ^1, ^2 as derived above as follows, ^ _^ = ^ _^ (^ − ^ _^ ) − ^ _^ (^ _^ − ^ _^̇ ) − ^ _^ ^(^ − ^ _^ )^^, ^ _^ = ^ _^ ⁽ ^ − ^ _^ ⁾ − ^ _^ ^^ _^ − ^ _^̇ ^ − ^ _^ ^ ⁽ ^ − ^ _^ ⁾ ^^, [0081] Fig.8 shows a fault control method of the drone 100. [0082] In step 801, a fault on the drone 100 may be detected, e.g., a propeller is not working or not working properly as expected. For example, the signals from the sensors on the drone 100 are analyzed along with the commanded thrust inputs, and a deviation from the expected behaviour may be used as an indicator of fault. [0083] To distinguish from external disturbances such as wind gusts, the signals may be analyzed over a sufficient duration and compared against set thresholds rotor failure results in anomalous spin due to an imbalance in the yaw moment. Further, there may also be a resulting loss in altitude from a fault. ^[0084] The direction of spin may indicate which pair of motors are faulty. Also, the direction of lateral displacement may indicate the particular motor in the pair. In this way, the engine/propellers which are not working properly may be detected. ^[0085] In step 802, the first, second and thrust values for each working propeller/engine are determined according to a recovery location. For example, in case a failed rotor has been identified, then the control objective may be modified to descend and steer the drone 100 towards a point of recovery (e.g., a predetermined location in order for the drone to be fixed). [0086] To achieve this, the rotor adjacent to the failed one which may be switched off as well, and the drone 100 may be controlled in the lateral plane using only two adjacent rotors (like a typical blimp configuration). However, in the present invention the drone 100 may descend when only two adjacent rotors are active. Therefore, the Z-axis control objective may be automatically achieved while steering in the lateral plane. However, since the control is under-actuated, the desired lateral acceleration is determined and the desired yaw angle is derived from it, and not independently controlled as in normal operation. [0087] For example, in step 802 based on the drone 100 in fig. 4, if engine 1032/propeller 1022 is not working (stopped), the neighbouring engine 1034/propeller 1024 may be turned off; then the movement in the x and y axes (on the horizontal plane) and the yaw rotation may be controlled by adjust the thrust from engines 1031, 1033/propeller 1021, 1023 in order to arrive at the targeted location. ^[0088] Since not all the propellers are on, the drone 100 may already descending by itself, thus, there is no need for further descending control, however, the altitude may be keep monitored in order for reaching the recovery location on time. [0089] In step 803, each engine for each working propeller may be configured according to the re-determined thrust values, which may be similar to step 706. [0090] Fig. 9 shows the logical modules of drone 100 and the controller 300, wherein the controller 300 may be considered as a part of or separate from the drone 100. [0091] The drone 100 may comprise at least one of: at least one processor 1001, at least one Inertial Measurement Unit (IMU) 1002, at least one battery 1003, at least one memory 1004, at least one sensor 1005, at least one wireless or wired communication unit 1006 and multiple motors 1007 to 10010. [0092] The processor 1001 may be configured to control at least one of the other components, e.g., read the data from the IMU, control the battery supply to the motors, read/write the memory, read the data from the sensors, and/or control the communications with other devices via the communication unit. The processor 1001 may be configured to perform the methods in any of figs.7 and 8. [0093] The IMU 1002 may measure or infer the accelerations and/or angular velocities of the drone 100, e.g., a 6-axis inertial sensor. The data may be saved in the memory 1004 and/or directly sent to the controller 300 via the communication unit 1006. [0094] The battery 1003 may be configured to provide power supply to all the other components and especially the motors. The battery 1003 may be rechargeable or non-rechargeable. [0095] The memory 1004 may be configured to store all the data/information and firmware/software for the drone 100. The processor 1001 may read instructions from the memory 1004. ^[0096] The sensor 1005 may be configured to collect data either about the drone 100 (e.g., the altitude, lateral movement distance, etc.), or other information (e.g., the wind speed, temperature, etc.). For example, the sensor unit 1005 may comprise Time of Flight (ToF) sensor (e.g., an infrared laser-ranging sensor) mounted on the bottom of the frame 104 which can sense the height at which the platform is hovering from the object below (usually the ground). The sensor unit 1005 may comprise an optic flow sensor (e.g., in-built camera to detect the motion of surfaces) for estimating the translational velocity of the drone. Some other sensors may be amounted as well in the sensor unit 1005. [0097] The communication unit 1006 may be configured to send/receive data between the drone 100 and other devices, e.g., the controller 300 or a server, which can be wired or wireless or both. [0098] The motors 1007 to 10010 may be configured to provide motion to the drone 100. Each motor may comprise at least one of an engine and a propeller, and one motor may drive one or multiple propellers. [0099] The controller 300 may comprise at least one of: at least one processor 3001, at least one communication unit 3002, at least one I/O unit 3003, at least one memory 3004 and at least one battery 3005. [00100] The processor 3001 may be configured to control at least one of the other components, e.g., control the battery supply to other components, read/write the memory, read the data from sensors, receiving information or outing information via the I/O unit, and/or control the communications with other devices via the communication unit. The processor 3001 may be configured to perform the methods in any of figs.7 and 8. [00101] The communication unit 3002 may be configured to send/receive data between the controller and other devices, e.g., the drone 100 or a server, which can be wired or wireless or both. [00102] The input/output unit (I/O unit) 3003 may be configured to output or receive information. For example, when outputting data, it may output the sensor data from the IMU and sensors from the drone 100, outputting the parameters on all the motors, the battery information on the drone and/or the controller, etc. For example, when receiving input, it may receive user input on targeted positions of the drone 100, system configurations for the controller and/or the drone, etc. the I/O unit may comprise at least one of a display, a touch screen, a button, a direction control stick, power button, etc. [00103] The memory 3004 may be configured to store all the data/information and firmware/software for the controller 300 and/or the drone 100. The processor 3001 and/or the processor 1001 may read instructions from the memory 3004. ^[00104] The battery 3005 may be configured to provide power supply to at least one of the other components and especially the motors. The battery 3005 may be rechargeable or non-rechargeable. [00105] The advantages of the present invention include and are not limited to: [00106] Endurance. The present invention may include an inflated balloon with lighter-than-air gas, which can provide lift and enable hovering at a stable altitude with no additional used power. This removes the requirement for powering any actuator for maintaining its height at an equilibrium. [00107] Maneuverability. The present invention may have the freedom to move in all directions(omnidirectional) in the horizontal plane when hovering at a stable height. Further, it may be possible to correct its heading and altitude whenever needed. There may be a provision to move in all degrees of freedom as a traditional UAV. [00108] Optimized size/weight. The present invention may weigh as less as possible to maximize payload for a given balloon size. ^[00109] Resilience. The present invention may function despite failure in one or more rotors with the capacity to sense the failure and control the drone despite it. [00110] Simple design. The present invention may be accessible, easily replicated, and beneficial to the sensing and wireless community. This means no complex actuator or mechanical design. [00111] Stable and easy to control: The present invention gives it natural stability in the pitch and roll axes and also contributes to the dynamics. [00112] The aerodynamics based on the example drone 100 in fig. 4 are summarized below, which applies to the other drones based on the present invention as well, e.g., for the drone in fig.5: [00113] Single propeller is on. Then there are low-pressure regions formed both upstream and downstream around the rotating blade. The low-pressure region may be formed over the upstream of the propeller due to the formation of the wake (the region behind the body around which the fluid flows), while the one downstream is formed because of the air it displaces. There are also small, high-pressure pockets near the balloon. These may occur when the surrounding air is pushed into the mechanical structure. As a result of the low-pressure regions, there may be a tendency for the platform to be pushed downwards. However, the presence of regions of comparatively high pressure around the wake may cause this effect to be quite small. Dynamic buoyancy effects may also influence the total upward force acting on the system and therefore, as a whole, this may have a negligible effect on its motion. [00114] Two adjacent propellers are on. There may be mixing between the wakes which cause an increase in the local flow velocity near the rotating propeller blades. This effect of wake interaction may cause a downward movement of the system. But in this case, the effect of dynamic buoyancy may not counteract the effect of the wake. There may be a significant impact on the higher-pressure regions created by restricted air that try to push the system upwards. Therefore, it is observed that the entire system only moves downwards weakly and may even be neglected. [00115] Two opposite propellers are on. As expected, the interaction of the two wakes may non-existent, with the wakes from each propeller being restricted to around the periphery of the rotating blades. However, two very interesting flow phenomena may occur here: (a) the two opposite wakes create a circulation that, on one side, generates extremely high-pressure regions (near the body of the platform and balloon); (b) on the other side only low-pressure pockets create (near the motor and propeller). These pockets may have high velocities at the tips of the rotating propellers, and above and below them as well. This may imply that when there are no wake interactions between adjacent propellers, the impact of buoyancy seems to be lowered. Thus, the higher pressure regions and the dynamic buoyancy effect may have a lesser impact compared to the low-pressure wake regions. Therefore, the entire platform is pushed downwards. [00116] All four propellers are on. There may be very high pressure due to the interaction of all the wakes with the platform and balloon individually as well as due to their combined effect, thus, recirculating regions in between the platform and the balloon, as well as the presence of the additional regions above and below the propellers that have the highest velocity. This may be similar to what is observed when two opposite propellers are turned on. The impact of the mixing of adjacent wakes reduces the overall effect of the low-pressure wake region. This may be caused by the presence of two such wake regions (instead of one) and the presence of a mass of air that is essentially “trapped” in between the rotors and the platform. Therefore, the combined effect of this trapped mass of air along with the dynamic buoyancy causes the overall acceleration of the platform to be upwards. This is a very interesting phenomenon because while all the other cases make the overall system move downwards, the four propeller case makes it moves upwards. [00117] The presentation invention may be used but not limited in the below scenarios. [00118] People and Object Tracking: People and object tracking application is a combination of high endurance and high mobility of the present invention. ^[00119] City rooftop thermal imaging. Cities are gearing up to reduce energy in heating buildings. Estimating the heat escape from the rooftops is an important indicator. This application was requested by a local municipality that wanted to quantify the wastage of heat by monitoring rooftops. To aid this, a very low pixel thermal camera may be used to guarantee privacy. [00120] The present invention relates to an aerial vehicle, which may comprise at least one balloon configured to receive gas having a density lower than air; at least four engines, each coupled to a propeller; a structure for fixing the engines and the at least one balloon, wherein the propellers may be arranged in a horizontal plane parallel to an earth plane, wherein the propellers may be configured to generate air flow towards a bottom of the balloon alongside the horizontal plane. [00121] The aerial vehicle may be configured to lift when at least four of the propellers are turned on. ^[00122] The aerial vehicle may be configured to descend when two or more propellers on opposite directions are turned on, and the other propellers are off. [00123] The propellers may be evenly arranged around the bottom of the balloon on the horizontal plane. [00124] The aerial vehicle may be configured to perform a yaw rotation when one or more propellers are on and the other propellers are off. ^[00125] When the balloon is filled with the gas, the balloon may provide a lifting force to the aerial vehicle. [00126] When the aerial vehicle is configured to move on the horizontal plane, one propeller or two or more neighbouring propellers may be on and at least one propeller is off. ^[00127] At least one of the propellers may have a central longitude line and the central longitude line may have a distance larger than zero to a yaw axis of the aerial vehicle. [00128] First two neighbouring propellers that are next to each other may have a first angle and second two neighbouring propellers that are next to each other may have a second angle, wherein the first angle and the second angle may be the same or different, wherein the first two neighbouring propellers may be different from the second two neighbouring propellers. [00129] A meeting point of central longitude lines of the first two neighbouring propellers and a meeting point of central longitude lines of the second two neighbouring propellers may have a distance d that is larger than zero. [00130] The present invention may relates to a controlling method for the above aerial vehicle, which may comprise estimating a state of the aerial vehicle; determining a first thrust value for each propeller according to a targeted altitude of the aerial vehicle; determining a second thrust value for each propeller according to a targeted yaw rotation angle of the aerial vehicle; determining a third thrust value for each propeller according to a targeted movements on the horizontal plane; calculating a total thrust value for each propeller according to at least one of the first thrust value, the second thrust value and the third thrust value; and configuring each engine according to the total thrust values for a corresponding propeller. [00131] The controlling method may further comprise a fault control method, wherein the fault control method may comprise detecting a fault; re-determining the thrust value for each working propeller according to a recovery location; configuring a corresponding engine for each working propeller according to the re-determined thrust value for each working propeller. ^[00132] The present invention relates to a controller configured to control the above aerial vehicle. [00133] [00134] The controller may be further configured to perform the above controlling method. [00135] The above aerial vehicle may be further configured to perform the controlling method.