An electromagnetic-assisted recovery system and method for coaxial unmanned aerial vehicles (UAVs)

Abstract

This invention relates to an electromagnetic-assisted recovery system and method for a coaxial unmanned aerial vehicle (UAV). The system includes multiple stabilizing landing gears mounted on the bottom of the UAV fuselage, each landing gear having a corner magnet embedded at its bottom for passive attraction during landing. A landing platform is located at the bottom of the recovery compartment, with a visual identification marker in the center. Multiple electromagnetic device arrays are arranged below the surface of the landing platform and surrounding the visual identification marker, providing visual guidance and generating electromagnetic attraction. A flight control motherboard is mounted on the UAV and is connected to the UAV's satellite navigation module, downward-facing camera, laser rangefinder, and the recovery compartment's power drive module. Based on acquired satellite navigation data, visual identification marker images, laser rangefinder data, and dynamic data from the recovery compartment, the system performs phased recovery control to guide the UAV to approach from a distance, achieve visual alignment, and then descend continuously, generating electromagnetic attraction to lock the landing gears onto the landing platform.

Description

Technical Field

[0001] This invention relates to the field of unmanned aerial vehicles (UAVs) and photoelectric detection technology, and in particular to an electromagnetic-assisted recovery system and method for coaxial UAVs. Background Technology

[0002] In recent years, drone technology has developed rapidly and has been widely applied in many technical fields such as inspection and logistics. Among them, coaxial dual-rotor drones (hereinafter referred to as coaxial drones) have eliminated the traditional tail rotor structure, and have significant technical advantages such as compact fuselage structure, small footprint, high hovering efficiency and strong resistance to crosswinds. With the above physical characteristics, coaxial drones have significant advantages in take-off and landing operations in confined spaces, and are highly compatible with the routine deployment needs of vehicle-mounted, ship-mounted, and other cylindrical or cabin-type recovery devices.

[0003] However, the following significant technical shortcomings still exist in the fully automated landing and recovery phase of coaxial UAVs: First, the drone is susceptible to attitude deviation due to airflow disturbances during the terminal descent phase. As the drone approaches the bottom of the recovery bin, the rotor downwash is obstructed by the ground, easily generating a strong "ground effect" and turbulent disturbances. This disturbance can cause the drone to tilt or bounce within tens of centimeters of the landing end, making it difficult to land accurately in the pre-set physical locking position of the recovery device.

[0004] Secondly, traditional mechanical locking mechanisms have a low fault tolerance rate. Currently, most mainstream drone recovery bins use mechanical clamps or latches to secure the drone fuselage. This type of purely mechanical alignment method requires the drone to achieve millimeter-level alignment accuracy at the moment of landing. If the fuselage experiences a slight shift in a complex wind field environment, the mechanical latches may not only fail to lock properly but could also easily cause mechanical interference, thereby damaging the drone's landing gear or fuselage.

[0005] Third, single navigation guidance methods have perception blind spots during terminal landing. Traditional methods that rely solely on satellite positioning are prone to signal drift in complex, obstructed environments near the ground; if relying solely on visual guidance, when the drone is extremely close to the landing platform, the limitations of the camera's field of view or changes in ambient light often lead to the loss of landmark features, resulting in the drone losing closed-loop control within the terminal landing blind spot.

[0006] In summary, existing recovery devices and methods struggle to achieve both high-precision alignment during the descent phase and high-reliability locking at the moment of landing in complex environments and confined spaces. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide an electromagnetic-assisted recovery system and method for coaxial unmanned aerial vehicles (UAVs) to solve the technical problems of existing coaxial UAVs being susceptible to airflow disturbances when landing in narrow spaces, having low fault tolerance of mechanical locking mechanisms, and having stringent requirements for end-effector alignment accuracy.

[0008] The objective of this invention is mainly achieved through the following technical solutions: This invention provides an electromagnetic-assisted recovery system for coaxial unmanned aerial vehicles (UAVs), comprising the following steps: The coaxial drone has multiple stabilizing feet installed on the outer periphery of the bottom of the fuselage. Each stabilizing foot has a corner pad magnet at the bottom landing end to provide a passive adsorption end during landing. The recovery bin has a landing platform at its bottom. A high-contrast visual identification mark is set in the central area of ​​the landing platform. Multiple electromagnetic device arrays are arranged below the surface of the landing platform and around the visual identification mark to provide a visual guidance reference and generate electromagnetic attraction at the landing end. The flight control motherboard, mounted on the coaxial UAV, is connected to the satellite navigation module, downward-facing camera, laser rangefinder, and power drive module of the recovery bin, respectively. Based on the acquired satellite navigation data, visual recognition marker images, laser ranging data, and dynamic data of the recovery bin, it performs phased recovery control to guide the coaxial UAV to complete long-distance approach, visual alignment, and continuous descent in sequence, and triggers the electromagnetic device array to generate electromagnetic attraction force to assist in adsorbing and locking the stabilizing landing gear to the landing platform.

[0009] Furthermore, the phased recycling control includes: The first stage, guiding the coaxial UAV to approach the recovery bin from a distance, includes: based on satellite navigation data and dynamic data of the recovery bin, performing line-of-sight guidance with feedforward velocity compensation to guide the coaxial UAV to fly above the recovery bin; The second stage, guiding the coaxial UAV to visually align and descend continuously, includes: performing visual servo position fine-tuning based on the visual recognition mark image and laser ranging data collected by the downward-looking camera, so that the center of the coaxial UAV body is aligned with the recovery axis and descends continuously; In the third stage, the electromagnetic device array generates an electromagnetic attraction force to help attach and lock the stabilizing foot to the landing platform. This includes: when the relative height of the coaxial UAV is less than a height threshold and the total horizontal offset is less than a radius threshold, an electromagnetic activation trigger signal is output to attract the corner pad magnets at the bottom of the stabilizing foot of the coaxial UAV.

[0010] Furthermore, the recovery compartment is a vertical cuboid chamber with a square horizontal cross-section, used to cover the cylindrical boundary formed by the rotation of the coaxial UAV rotor.

[0011] Furthermore, the visual identification mark adopts a nested pattern of large-sized geometric blocks embedded with small-sized blocks, and uses a black and white matrix arrangement of high-contrast QR codes to provide a visual reference benchmark for the downward-looking camera.

[0012] Furthermore, the coaxial UAV has a cylindrical body, the downward-facing camera is mounted on a stabilization gimbal, and the stabilization gimbal is mounted at the center of the bottom surface of the cylindrical body, protruding downwards or embedded in the body; The stabilized gimbal also has a built-in inertial measurement unit, which is used to drive the stabilized gimbal into a vertically downward viewing state in the second stage, so that the optical axis of the camera is parallel to the direction of gravity and coincides with the geometric axis of the drone body.

[0013] Furthermore, the first stage includes: Based on the GNSS modules with integrated RTK positioning function deployed on the coaxial UAV and the recovery bin respectively, the real-time three-dimensional coordinates and velocity of the coaxial UAV and the electromagnetic landing platform of the recovery bin are acquired in real time. Based on the real-time three-dimensional coordinates of the coaxial UAV and the recovery bin, the relative position error vector and the linear Euclidean distance between the coaxial UAV and the recovery bin are calculated. Based on the relative position error vector and the linear Euclidean distance, an approximation velocity vector of the coaxial UAV pointing towards the recovery bin is generated using an exponential asymptotic convergence function. The velocity of the recovery bin is used as a feedforward compensation term and superimposed on the approximation velocity vector to obtain the desired velocity vector of the coaxial UAV. Based on the desired velocity vector, the flight control motherboard controls the coaxial UAV to fly above the recovery bin. If the absolute value of the position error vector between the coaxial UAV and the recovery bin in the horizontal direction is less than a preset cutting radius threshold within a preset period, and the relative velocity of the coaxial UAV relative to the recovery bin approaches zero, then the flight control system determines that the coaxial UAV has accurately hovered above the recovery bin.

[0014] Furthermore, the second stage includes: The flight control motherboard drives the downward-facing camera to a vertically downward-facing state. The downward-facing camera captures images of visual recognition markers on the electromagnetic landing platform and obtains the pixel coordinates of the center of the visual markers in the pixel coordinate system. Based on the relative altitude of the UAV to the electromagnetic landing platform, the equivalent focal length of the camera, the gimbal camera installation offset, and the steady-state attitude error of the gimbal, the pixel coordinates are converted into the actual physical horizontal offset of the coaxial UAV body relative to the center of the recovery bin. Based on the actual physical horizontal offset, a proportional-integral controller is used to generate lateral and longitudinal fine-tuning speed commands, while a constant low-speed vertical descent command is superimposed. Continuous closed-loop adjustments were made to align the center of the coaxial UAV fuselage with the line-of-sight guide axis of the recovery center and to lower it.

[0015] Furthermore, the third stage includes: The flight control motherboard monitors the relative altitude of the UAV to the electromagnetic landing platform in real time, as well as calculates the total horizontal offset of the fuselage in real time. When the relative height is less than a preset height threshold and the total horizontal offset is less than a preset effective magnetic capture radius threshold, the flight control motherboard outputs an electromagnetic activation trigger signal to the central control unit of the recovery bin. When the electromagnetic array inside the recovery bin is powered on, it generates an electromagnetic attraction force that attracts and locks the corner pad magnets at the bottom of the coaxial drone landing gear to the landing platform. The flight control motherboard outputs a rotor stop command to complete the recovery.

[0016] Furthermore, the recovery chamber is also equipped with a power drive module, which is connected to the flight control motherboard. After receiving the electromagnetic activation trigger signal, the power drive module outputs a large DC current to the electromagnetic device array to excite a strong magnetic field and generate electromagnetic attraction force.

[0017] This invention also discloses an electromagnetic-assisted recovery method for coaxial unmanned aerial vehicles, comprising the following steps: The three-dimensional coordinates and velocities of the coaxial drone and the recovery bin are obtained, and the line-of-sight guidance with feedforward compensation is used to control the coaxial drone to fly above the recovery bin. The system acquires visual recognition mark images, converts the center pixel coordinates of the visual recognition mark images into actual physical horizontal offset, uses a proportional-integral controller to generate horizontal fine-tuning speed commands, and superimposes constant low-speed vertical descent commands to align the center of the coaxial UAV fuselage with the line-of-sight guide axis of the recovery center and continue to descend. The system monitors the relative height of the drone with respect to the electromagnetic landing platform in real time, as well as calculates the total horizontal offset of the drone in real time. When the relative height and the total horizontal offset of the drone are less than the preset height threshold and the preset effective magnetic capture radius threshold, respectively, the electromagnetic device array is energized based on the electromagnetic activation trigger signal to generate electromagnetic attraction force, locking the coaxial drone to the landing platform, and the rotor stops.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. This invention effectively overcomes airflow disturbances at the end of landing and significantly improves the fault tolerance of mechanical locking. This invention uses corner magnets embedded in the bottom of the coaxial UAV landing gear and arranges an array of electromagnetic devices corresponding to the landing gear positions below the landing platform of the recovery capsule. At the end of landing, a downward electromagnetic attraction force is generated instantly, which can quickly counteract the fuselage bounce and turbulence caused by the ground effect. Simultaneously, relying on the magnetic field gradient effect at the edge of the magnetic poles, even if there is a horizontal physical error of several centimeters at the landing point, the strong magnetic field will passively pull the landing gear along the path of least magnetic resistance to the preset locking center. Compared to existing purely mechanical clamping or locking mechanisms that require millimeter-level alignment accuracy, this invention uses flexible magnetic correction to replace rigid locking, greatly reducing the risk of rigid interference and hardware damage at the moment of landing. 2. This invention employs a multi-source sensor fusion-based phased control strategy to improve the overall reliability of the recovery process. The invention utilizes a tiered approach combining long-range dynamic line-of-sight guidance, short-range visual servo alignment, and extremely short-range electromagnetic triggering locking. The first stage, based on satellite navigation and dynamic data from the recovery bin, uses line-of-sight guidance with feedforward velocity compensation, adapting to applications where the recovery bin is mounted on a mobile platform (such as a moving vehicle or a sailing ship). The second stage uses a downward-looking camera and a laser rangefinder to convert pixel coordinates into actual physical horizontal offsets, achieving centimeter-level visual servo fine-tuning. The third stage uses dual threshold judgments of altitude and horizontal offset to trigger electromagnetic locking only when safety conditions are met. This design fully leverages the physical advantages of satellite positioning (wide range, high visual alignment accuracy), and magnetic adsorption (ignoring near-ground optical blind spots), effectively compensating for the shortcomings of single navigation sensors that are prone to failure in complex obstructions or harsh environments. 3. This invention features a compact structure and low modification cost, requiring no alteration to the original aerodynamic shape of the drone. The hardware modification to the coaxial drone involves only fitting a corner magnet at the bottom landing end of the stabilizing foot, without adding any protruding moving mechanical parts. This provides a reliable physical adsorption end for landing locking while maintaining the drone's original aerodynamic shape. Simultaneously, the electromagnetic array of the recovery bin is concealed beneath the surface of the landing platform, occupying no additional space. Compared to existing technologies that require complex mechanical clamps, latches, or external locking mechanisms, this invention offers a more compact structure, lower modification cost, and is highly suitable for the routine deployment of cylindrical or cabin-type recovery devices in confined spaces such as vehicle-mounted or shipboard locations.

[0019] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0020] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0021] Figure 1 This is a schematic diagram of the coaxial UAV recovery system in an embodiment of the present invention; Figure 2 This is a flowchart of an electromagnetic-assisted recovery method for a coaxial UAV in an embodiment of the present invention. Detailed Implementation

[0022] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0023] To address the aforementioned deficiencies in existing technologies, there is an urgent need in this technical field for a coaxial UAV recovery system and method that possesses strong anti-interference capabilities, high fault tolerance, and the ability to balance flexible attitude correction with rigid safety locking. This invention provides a recovery system and method for coaxial UAVs. The invention aims to overcome technical challenges such as the susceptibility of coaxial UAVs to airflow disturbances during landing in confined spaces, the low fault tolerance of mechanical locking mechanisms, and stringent requirements for end-effector alignment accuracy. By combining a phased multi-source navigation guidance strategy with a bottom-mounted electromagnetic adsorption structure, it achieves seamless integration of long-distance orientation finding, close-range visual alignment, and passive end-effector correction locking, thereby significantly improving the success rate and system reliability of fully automated UAV recovery.

[0024] Example 1: One specific embodiment of the present invention discloses an electromagnetic-assisted recovery system for a coaxial unmanned aerial vehicle, such as... Figure 1 As shown, it includes: The coaxial drone has multiple stabilizing feet installed on the outer periphery of the bottom of the fuselage. Each stabilizing foot has a corner pad magnet at the bottom landing end to provide a passive adsorption end during landing. The recovery bin has a landing platform at its bottom. A high-contrast visual identification mark is set in the central area of ​​the landing platform. Multiple electromagnetic device arrays are arranged below the surface of the landing platform and around the visual identification mark to provide a visual guidance reference and generate electromagnetic attraction at the landing end. The flight control motherboard, mounted on the coaxial UAV, is connected to the satellite navigation module, downward-facing camera, laser rangefinder, and power drive module of the recovery bin, respectively. Based on the acquired satellite navigation data, visual recognition marker images, laser ranging data, and dynamic data of the recovery bin, it performs phased recovery control to guide the coaxial UAV to complete long-distance approach, visual alignment, and landing on the landing platform in sequence. It also triggers the electromagnetic device array to generate electromagnetic attraction force to help attach and lock the stabilizing landing gear to the landing platform.

[0025] The core of this invention's electromagnetic-assisted recovery system for coaxial UAVs lies in the innovative design of the overall system architecture and physical components. The entire system mainly consists of two parts: a specially modified coaxial UAV and a specially designed cuboid recovery bin.

[0026] The coaxial UAV has a cylindrical body, and the downward-looking camera is mounted on a stabilization gimbal. The stabilization gimbal is mounted at the center of the bottom surface of the cylindrical body, protruding downwards or embedded in the body. The stabilized gimbal also has a built-in inertial measurement unit, which is used to drive the stabilized gimbal into a vertically downward viewing state in the second stage, so that the optical axis of the camera is parallel to the direction of gravity and coincides with the geometric axis of the drone body.

[0027] A detailed analysis of the coaxial UAV's airframe structure is needed. The coaxial UAV employs a dual-rotor power layout with coaxial upper and lower rotors, completely eliminating the tail propeller, resulting in a highly compact cylindrical shape. Four stabilizing landing gears, providing support and shock absorption, extend downwards from the outer perimeter of the cylindrical fuselage. The key hardware modifications of this invention are concentrated here.

[0028] The bottom of each of the four stabilizing legs is securely fitted with a corner pad magnet assembly. Exemplarily, the corner pad magnet assembly is a high-strength permanent magnet or an alloy material with excellent magnetic permeability. In specific applications, the magnetic material can be changed according to specific requirements.

[0029] This modification did not add any outward-protruding moving mechanical parts to the coaxial drone, and provided a reliable physical adsorption end for subsequent landing locking while maintaining its original aerodynamic shape.

[0030] The recovery compartment is a vertical cuboid chamber with a square horizontal cross-section, used to cover the cylindrical boundary formed by the rotation of the coaxial UAV rotor.

[0031] Regarding the external shape and internal layout of the recovery bin, this system has specially designed a vertical rectangular cabin shell with a square horizontal cross section, i.e., the bottom surface, to ensure that when the UAV is viewed from above, the side length of the square can cover the projected boundary of the cylindrical rotor; the depth (height) is sufficient to ensure that the UAV can land completely inside the bin.

[0032] Since the overall outline of the coaxial drone is cylindrical, when the drone falls into the rectangular recovery compartment with a square cross-section, the inner wall of the square can perfectly cover the cylindrical boundary generated by the rotation of the drone's rotor.

[0033] At the very bottom of the cuboid recovery bay, a dedicated electromagnetic landing platform is installed for docking aircraft. This platform is specifically designed with visual guidance and magnetic locking as its two main functions.

[0034] First aspect: The design style of the visual identification markings on the electromagnetic landing platform. A marking pattern with extremely high color contrast will be printed in the very center of the platform.

[0035] The visual identification mark adopts a nested pattern of large geometric blocks containing smaller blocks, and uses a black and white matrix arrangement of high-contrast QR codes to provide a visual reference benchmark for the downward-facing camera.

[0036] The high-contrast nested array pattern of the visual identification markers provides an extremely clear visual reference for the downward-facing camera on the bottom of the drone.

[0037] With this, the flight control system can calculate in real time and quickly the specific distance between the UAV's center point and the center line-of-sight guidance axis of the electromagnetic landing platform, as well as the error value of the nose yaw angle.

[0038] The second aspect: an array of electromagnetic devices concealed inside the electromagnetic landing platform at the bottom of the recovery pod.

[0039] Beneath the surface of the electromagnetic landing platform, four arrays of electromagnetic devices are arranged around the central visual logo. The physical location of these four arrays of electromagnetic devices corresponds perfectly to the ideal landing points of the corner magnets on the four stabilizing landing gears at the bottom of the drone.

[0040] When the flight control system sensors detect that the coaxial drone has approached the surface of the recovery bin, the recovery bin control system instantly switches on the power to the electromagnetic array. The powered electromagnetic array then generates a powerful directional magnetic field.

[0041] Relying on the strong attraction of the directional magnetic field and the changes in the surrounding magnetic field gradient, the recovery pod control system can not only instantly pull the landing gear downwards to overcome airflow turbulence, but also slide the slightly tilted or offset landing gear along the magnetic field lines into the pre-set shallow pit. Finally, within the same second of landing, the fuselage is corrected in attitude and firmly attached to the electromagnetic landing platform.

[0042] The phased recycling control includes: The first stage, guiding the coaxial UAV to approach the recovery bin from a distance, includes: based on satellite navigation data and dynamic data of the recovery bin, performing line-of-sight guidance with feedforward velocity compensation to guide the coaxial UAV to fly above the recovery bin; The second stage, guiding the coaxial UAV to visually align and descend continuously, includes: performing visual servo position fine-tuning based on the visual recognition mark image and laser ranging data collected by the downward-looking camera, so that the center of the coaxial UAV body is aligned with the recovery axis and descends continuously; In the third stage, the electromagnetic device array generates an electromagnetic attraction force to help attach and lock the stabilizing foot to the landing platform. This includes: when the relative height of the coaxial UAV is less than a height threshold and the total horizontal offset is less than a radius threshold, an electromagnetic activation trigger signal is output to attract the corner pad magnets at the bottom of the stabilizing foot of the coaxial UAV.

[0043] This invention proposes a phased recovery control method based on multi-source sensor data fusion. This method operates on a flight control motherboard mounted on a coaxial UAV and divides the entire landing process into three steps: long-range dynamic coarse positioning guidance, close-range visual servo alignment, and near-endpoint trigger locking.

[0044] Flight control motherboard (hardware): This is a circuit board (PCB) that integrates physical electronic components such as a processor (CPU / MCU), inertial measurement unit (IMU, i.e., gyroscope + accelerometer), and interfaces. It is the hardware core and physical entity of the flight control system.

[0045] Flight control system (system): is a complete system. It consists of the flight control motherboard + peripheral sensors (such as GNSS / RTK modules, laser rangefinders, downward-facing cameras, airspeed indicators, etc.) + actuators (such as servo motors, electronic speed controllers) + software / algorithms running on the motherboard.

[0046] The first stage is the long-range dynamic coarse positioning and line-of-sight guidance stage.

[0047] The first stage includes: Based on the GNSS module with integrated RTK positioning function deployed on the coaxial UAV and the recovery bin, the real-time three-dimensional coordinates and velocity of the coaxial UAV and the electromagnetic landing platform of the recovery bin are acquired in real time. Based on the real-time three-dimensional coordinates of the coaxial UAV and the recovery bin, the relative position error vector and the linear Euclidean distance between the coaxial UAV and the recovery bin are calculated. Based on the relative position error vector and the linear Euclidean distance, an approximation velocity vector of the coaxial UAV pointing towards the recovery bin is generated using an exponential asymptotic convergence function. The velocity of the recovery bin is used as a feedforward compensation term and superimposed on the approximation velocity vector to obtain the desired velocity vector of the coaxial UAV. Based on the desired velocity vector, the flight control motherboard controls the coaxial UAV to fly above the recovery bin. If the absolute value of the position error vector between the coaxial UAV and the recovery bin in the horizontal direction is less than a preset cutting radius threshold within a preset period, and the relative velocity of the coaxial UAV relative to the recovery bin approaches zero, then the flight control system determines that the coaxial UAV has accurately hovered above the recovery bin.

[0048] The main objective of this phase is to guide the coaxial drone precisely from the work area to directly above the recovery bin, which may be stationary or in motion, mounted on a moving vehicle or a ship in motion.

[0049] The input data for the coaxial UAV flight control system at this stage comes from two sources.

[0050] On one hand, there is the airborne GNSS module with integrated RTK positioning function, which inputs the real-time three-dimensional coordinates of the coaxial UAV to the flight control motherboard. and the current flight velocity vector .

[0051] On the other hand, the flight control motherboard receives dynamic input data in real time from the recovery module via an onboard wireless data radio. This data includes the real-time three-dimensional coordinates of the center of the recovery module's landing platform. And the current spatial movement velocity vector of the recovery platform. .

[0052] Among them, the real-time three-dimensional coordinates of the center of the landing platform of the recovery module And the current spatial movement velocity vector of the recovery platform. The data is obtained in real time through a GNSS module with integrated RTK positioning function installed in the recycling bin.

[0053] The integrated GNSS (Global Navigation Satellite System) module with RTK (Real Time Kinematic) positioning provides centimeter-level positioning accuracy, ranging from 1 to 2 centimeters. This solution integrates a GNSS module with RTK positioning functionality, which is installed on the drone and recovery bin to receive the aforementioned satellite signals for positioning. Combined with RTK technology, it achieves the centimeter-level positioning accuracy required by this invention.

[0054] Since the recovery bin may be dynamically moving, traditional fixed-point closed-loop control is prone to tracking lag.

[0055] Therefore, this system employs a three-dimensional line-of-sight guidance algorithm with feedforward velocity compensation. The flight control system first calculates the relative position error vector of the coaxial UAV pointing towards the recovery bin based on the two sets of input position coordinates. And calculate the linear Euclidean distance between the coaxial UAV and the recovery bin. .

[0056] To ensure that the drone always approaches the target along the optimal line-of-sight trajectory, the flight control system generates an approach velocity vector pointing towards the recovery bin. Considering the need for rapid flight over long distances and smooth deceleration when approaching the recovery pod, the flight control system introduces an exponential asymptotic convergence function.

[0057] Let the maximum permissible level flight speed of the coaxial UAV be Approach speed adjustment coefficient is The approach velocity vector of the coaxial UAV pointing towards the recovery bin. The calculation is as follows: Formula (1) To completely eliminate tracking errors caused by the recovery bin's own movement, the flight control system will receive the real-time spatial velocity vector of the recovery bin. As a feedforward compensation term, it is superimposed on the UAV's approximation velocity vector. .

[0058] The desired spatial velocity vector of the UAV is obtained from the calculations of the flight control motherboard. As shown below: Formula (2) The flight control motherboard will calculate the desired velocity vector. The input is fed into the underlying attitude control calculation loop and converted into drive signals for the rotor drive motors. Under the combined action of the line-of-sight guidance law and feedforward compensation, the coaxial UAV smoothly tracks to the top of the recovery bin.

[0059] During the process of the coaxial UAV tracking the recovery bin, the absolute value of the horizontal position error vector between the coaxial UAV and the recovery bin is calculated in real time. ,as follows: Formula (3) when When the drone's relative speed approaches zero and the cut-in radius remains below the set threshold, the flight control system determines that the drone has accurately hovered above the recovery bin.

[0060] For example, the duration is 0.5 seconds; the cut-in radius threshold is 1 meter.

[0061] Relative speed of drones ,as follows: Formula (4) When the recovery bin is stationary, that is The relative velocity of a coaxial drone is the opposite direction of its own velocity. ; When the coaxial drone and the recovery bin move at the same speed, that is The relative speed is 0, meaning the drone is in a "hovering" state relative to the recovery bin.

[0062] At this point, the flight control motherboard outputs a phase switching command, entering the second phase.

[0063] The second stage is the near-range visual servoing and position fine-tuning stage.

[0064] The second stage includes: The flight control motherboard drives the downward-facing camera to a vertically downward-facing state. The downward-facing camera captures images of visual recognition markers on the electromagnetic landing platform and obtains the pixel coordinates of the center of the visual markers in the pixel coordinate system. Based on the relative altitude of the UAV to the electromagnetic landing platform, the equivalent focal length of the camera, the gimbal camera installation offset, and the steady-state attitude error of the gimbal, the pixel coordinates are converted into the actual physical horizontal offset of the coaxial UAV body relative to the center of the recovery bin. Based on the actual physical horizontal offset, a proportional-integral controller is used to generate lateral and longitudinal fine-tuning speed commands, while a constant low-speed vertical descent command is superimposed. Continuous closed-loop adjustments were made to align the center of the coaxial UAV fuselage with the line-of-sight guide axis of the recovery center and to lower it.

[0065] This phase aims to overcome the positional residuals left over from the coarse satellite positioning guidance in the first phase. Due to satellite navigation and dynamic guidance, the UAV can only be guided roughly above the recovery bin. There will inevitably be an initial horizontal offset ranging from tens of centimeters to several meters between the center of the UAV and the vertical line of the ideal recovery bin center, that is, the recovery axis.

[0066] Therefore, in this stage, the real-time offset is output through a visual recognition algorithm and converted into a continuous fine-tuning control quantity, enabling the UAV to eliminate the error and maintain its descent along the center line-of-sight guide axis.

[0067] The control strategy at this stage makes full use of the unique physical configuration of the coaxial UAV payload gimbal located at the very center of the bottom surface of the cylindrical body.

[0068] Upon entering this phase, the flight control motherboard first outputs gimbal attitude control commands, driving the stabilization gimbal into a vertically downward-facing state. Independent stabilization is achieved using the gimbal's built-in inertial measurement unit (IMU), forcing the optical axis of the downward-facing camera to be parallel to the direction of gravity.

[0069] The gimbal is located at the center of the bottom surface of the coaxial drone's cylindrical body. It can be extended downwards or embedded in the gimbal; the inertial measurement unit (IMU) and the downward-facing camera are built into the gimbal. The IMU is used to measure the coaxial drone's three-axis angular velocity and three-axis acceleration.

[0070] At this time, the optical center point of the downward-looking camera image In a physical sense, it is perfectly equivalent to the projection of the drone's geometric axis onto a horizontal plane.

[0071] The input data acquired by the flight control system at this stage mainly includes: Downward-facing camera: Real-time captured image frames containing visual identification markers of the electromagnetic landing platform; High-precision laser rangefinder sensor: transmitting drone altitude data relative to the platform ; Stabilized gimbal: Feedback on steady-state pitch angle error and steady-state roll angle error .

[0072] The image processing module initiates target recognition and dynamic tracking algorithms to extract features from continuously input image frames and outputs in real time the coordinates of the visual recognition marker of the electromagnetic landing platform of the recovery pod in the pixel coordinate system. This coordinate is the original visual offset input.

[0073] In order to accurately convert this visual offset into the actual control quantity of the UAV, the flight control motherboard must perform physical coordinate compensation for pixel deviations in conjunction with engineering errors.

[0074] The equivalent focal lengths of the downward-facing cameras are respectively and There is an inherent mounting offset between the mechanical mounting center of the camera on the gimbal and the ideal geometric axis of the camera body. and .

[0075] The flight control system calculates the actual physical horizontal offset of the UAV fuselage. and As shown below: Formula (5) Formula (6) in, and Let be the actual physical horizontal offset of the drone's fuselage axis relative to the center point of the recovery bin, and let be the target value to be solved. The altitude of the UAV relative to the electromagnetic landing platform is obtained by a laser rangefinder. The coordinates of the visually recognized logo's center in the pixel coordinate system are output by the image recognition algorithm. The image optical center (origin of the pixel coordinate system) is obtained from the intrinsic parameter calibration of the downward-looking camera; , The equivalent focal length of the downward-looking camera is obtained from the internal parameter calibration of the downward-looking camera. , The inherent installation offset between the camera mechanical mounting center and the geometric axis of the gimbal is a structural design parameter and a fixed value. , These are the steady-state pitch angle error and roll angle error of the gimbal, respectively, measured and fed back by the gimbal's built-in IMU.

[0076] The above formula combines the pixel displacement output by the visual recognition algorithm, the fixed bias of the mechanical installation, and the dynamic bias of the gimbal attitude to obtain the actual horizontal offset of the UAV axis.

[0077] In order to keep the drone consistently aligned with the center of the electromagnetic landing platform, the flight control motherboard adjusts the high-precision physical horizontal offset of the drone. and As feedback input, a proportional-integral controller is introduced to eliminate system steady-state error, and the output control quantity used for position fine-tuning in this stage is calculated in real time, namely the UAV's desired lateral velocity. Desired longitudinal velocity As shown below: Formula (7) Formula (8) in, and These are the proportional and integral gain parameters for visual servo control.

[0078] While outputting the aforementioned horizontal fine-tuning control values, the flight control mainboard also sends a constant low-speed vertical descent command to the power system. . It is a preset, constant low-speed vertical descent rate.

[0079] For example, a constant low-speed vertical descent command The preset speed is 0.3 m / s to ensure a smooth descent of the coaxial drone during the visual servoing phase; in practical applications, The value ranges from 0.2 m / s to 0.5 m / s and can be calibrated according to the UAV's dynamic response characteristics, landing gear damping performance, and electromagnetic lock trigger height.

[0080] Driven by this set of three-dimensional control commands in a high-frequency, continuous closed-loop manner, the system updates the latest offset in real time based on the recognition algorithm. and Continuously and dynamically adjust the lateral speed of the drone. Longitudinal velocity This allows the drone to quickly overcome wind disturbances and eliminate the initial errors left over from step one, always keeping the center of the drone aligned with the visual identification mark of the recovery bin, and descending smoothly and continuously along the visual guidance axis of the recovery center.

[0081] The third stage is the near-end triggering and passive magnetic locking stage.

[0082] The third stage includes: The flight control motherboard monitors the relative altitude of the UAV to the electromagnetic landing platform in real time, as well as calculates the total horizontal offset of the fuselage in real time. When the relative height is less than a preset height threshold and the total horizontal offset is less than a preset effective magnetic capture radius threshold, the flight control motherboard outputs an electromagnetic activation trigger signal to the central control unit of the recovery bin. When the electromagnetic array inside the recovery bin is powered on, it generates an electromagnetic attraction force that attracts and locks the corner pad magnets at the bottom of the coaxial drone landing gear to the landing platform. The flight control motherboard outputs a rotor stop command to complete the recovery.

[0083] The goal of this stage is to completely overcome the strong turbulent disturbances caused by the ground effect and to use physical means to achieve rigid correction and fixation at the last moment. In order to prevent the coaxial UAV from being abruptly powered on when its horizontal position deviates too much, which could lead to uneven force and cause it to tip over, the activation of the electromagnetic device must be strictly limited by the three-dimensional spatial constraint envelope.

[0084] At this stage, the input data for the system to trigger the judgment comes from two parts.

[0085] First, the laser rangefinder continuously transmits the relative height at extremely close distances. ; Second, the total horizontal offset of the fuselage calculated in real time by the visual servo module in step two. The total horizontal offset of the fuselage is calculated as follows: Formula (9) The control system pre-defines a trigger envelope region above the electromagnetic landing platform, resembling an invisible cylinder. Let the system's set critical trigger height threshold be... This altitude is typically set at the point where the drone has just entered the strong ground effect zone; for example, The effective magnetic attraction capture radius threshold is 10cm. The radius depends on the physical distribution size of the underlying electromagnetic array and the magnetic field coverage.

[0086] For example, The preset value is 5cm to accommodate horizontal deviations of several centimeters caused by ground effects or wind disturbances when a coaxial drone approaches an electromagnetic landing platform. In practical applications, The value ranges from 3 to 8 cm, depending on the physical distribution size of the electromagnetic transpose array, the magnetic field coverage, and the effective attraction area of ​​the corner pad magnets on the tripod.

[0087] The system determines that the real-time input data simultaneously satisfies both the height threshold and the horizontal center condition. as well as At that time, the flight control motherboard outputs an electromagnetic activation trigger signal to the central control unit of the recovery capsule, and the capsule hatch is opened simultaneously.

[0088] If the height is reached, but the horizontal deviation is too large, the system will temporarily suspend the trigger signal and maintain the second stage of fine-tuning alignment until the aircraft enters the effective magnetic capture radius.

[0089] The recovery chamber is also equipped with a power drive module, which is connected to the flight control motherboard. After receiving the electromagnetic activation trigger signal, the power drive module outputs a large DC current to the electromagnetic device array to excite a strong magnetic field and generate electromagnetic attraction force.

[0090] After receiving the trigger signal, the power drive module inside the recovery bin will calculate the required output excitation current based on the preset locking requirements.

[0091] To ensure that the electromagnetic attraction can completely overcome the upward aerodynamic lift disturbance generated by the rebound of the rotor downwash airflow upon impact with the ground. In addition to the supporting force of the drone's own weight, the system calculates the minimum excitation current required for a single set of electromagnets based on electromagnetic principles. .

[0092] Let the number of turns of the electromagnetic coil of a single electromagnet be... The effective cross-sectional area of ​​the electromagnet core is The vacuum permeability is The real-time air gap distance between the bottom of the tripod and the electromagnetic landing platform is .

[0093] The power control module calculates the minimum excitation current required for a single electromagnet. As shown below: Formula (10) in, A safety redundancy factor is set to ensure reliable adsorption.

[0094] Based on the above calculations, the power drive module of the recovery bin instantly outputs a large DC current to the four sets of electromagnet coils (electromagnetic device arrays) of the electromagnetic landing platform. The energized coils instantly generate a strong magnetic field, applying a vertically downward electromagnetic attraction to the magnetic components (corner pad magnets) at the bottom of the drone's landing gear.

[0095] At this point, the drone's descent trajectory was forcibly stabilized. Simultaneously, relying on the magnetic field gradient effect at the edge of the magnetic poles, even with a horizontal physical error of several centimeters at the landing point, the powerful magnetic field passively pulled the landing gear along the path of least magnetic resistance to the preset locking center. While outputting a large electromagnetic drive current, the flight control motherboard sent a rotor shutdown command to the drone's power motors. Thus, guided by the electromagnetic array, the drone successfully completed the automatic correction and rigid locking tasks.

[0096] Example 2: Another embodiment of the present invention, based on the electromagnetic-assisted recovery system for a coaxial UAV in Embodiment 1, discloses an electromagnetic-assisted recovery method for a coaxial UAV. The specific implementation of each step in the method is as described in the corresponding description in Embodiment 1. The method includes the following steps: Step S1: Obtain the three-dimensional coordinate position and velocity of the coaxial UAV and the recovery bin, and use line-of-sight guidance with feedforward compensation to control the coaxial UAV to fly above the recovery bin. Step S2: Acquire a visual recognition mark image, convert the center pixel coordinates of the visual recognition mark image into an actual physical horizontal offset, use a proportional-integral controller to generate a horizontal fine-tuning speed command, and superimpose a constant low-speed vertical descent command to make the center of the coaxial UAV body aligned with the line-of-sight guide axis of the recovery center and continue to descend. Step S3: Monitor the relative height of the UAV to the electromagnetic landing platform in real time, and calculate the total horizontal offset of the fuselage in real time; when the relative height and the total horizontal offset of the fuselage are less than the preset height threshold and the preset effective magnetic capture radius threshold, respectively, the electromagnetic device array is energized based on the electromagnetic activation trigger signal to generate electromagnetic attraction force, locking the coaxial UAV to the landing platform, and the rotor stops at the same time.

[0097] Since the electromagnetic-assisted recovery method for coaxial UAVs in this embodiment and the aforementioned electromagnetic-assisted recovery system for coaxial UAVs can be mutually referenced, and are therefore described repeatedly here, they will not be repeated. Because this method embodiment shares the same principle as the aforementioned system embodiment, it also possesses the corresponding technical effects of the aforementioned system embodiment.

[0098] In summary, the electromagnetic-assisted recovery system and method for coaxial unmanned aerial vehicles (UAVs) according to embodiments of the present invention have the following beneficial effects: 1. This invention effectively overcomes airflow disturbances at the end of landing and significantly improves the fault tolerance of mechanical locking. This invention uses corner magnets embedded in the bottom of the coaxial UAV landing gear and arranges an array of electromagnetic devices corresponding to the landing gear positions below the landing platform of the recovery capsule. At the end of landing, a downward electromagnetic attraction force is generated instantly, which can quickly counteract the fuselage bounce and turbulence caused by the ground effect. Simultaneously, relying on the magnetic field gradient effect at the edge of the magnetic poles, even if there is a horizontal physical error of several centimeters at the landing point, the strong magnetic field will passively pull the landing gear along the path of least magnetic resistance to the preset locking center. Compared to existing purely mechanical clamping or locking mechanisms that require millimeter-level alignment accuracy, this invention uses flexible magnetic correction to replace rigid locking, greatly reducing the risk of rigid interference and hardware damage at the moment of landing. 2. This invention employs a multi-source sensor fusion-based phased control strategy to improve the overall reliability of the recovery process. The invention utilizes a tiered approach combining long-range dynamic line-of-sight guidance, short-range visual servo alignment, and extremely short-range electromagnetic triggering locking. The first stage, based on satellite navigation and dynamic data from the recovery bin, uses line-of-sight guidance with feedforward velocity compensation, adapting to applications where the recovery bin is mounted on a mobile platform (such as a moving vehicle or a sailing ship). The second stage uses a downward-looking camera and a laser rangefinder to convert pixel coordinates into actual physical horizontal offsets, achieving centimeter-level visual servo fine-tuning. The third stage uses dual threshold judgments of altitude and horizontal offset to trigger electromagnetic locking only when safety conditions are met. This design fully leverages the physical advantages of satellite positioning (wide range, high visual alignment accuracy), and magnetic adsorption (ignoring near-ground optical blind spots), effectively compensating for the shortcomings of single navigation sensors that are prone to failure in complex obstructions or harsh environments. 3. This invention features a compact structure and low modification cost, requiring no alteration to the original aerodynamic shape of the drone. The hardware modification to the coaxial drone involves only fitting a corner magnet at the bottom landing end of the stabilizing foot, without adding any protruding moving mechanical parts. This provides a reliable physical adsorption end for landing locking while maintaining the drone's original aerodynamic shape. Simultaneously, the electromagnetic array of the recovery bin is concealed beneath the surface of the landing platform, occupying no additional space. Compared to existing technologies that require complex mechanical clamps, latches, or external locking mechanisms, this invention offers a more compact structure, lower modification cost, and is highly suitable for the routine deployment of cylindrical or cabin-type recovery devices in confined spaces such as vehicle-mounted or shipboard locations.

[0099] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0100] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. An electromagnetic-assisted recovery system for a coaxial unmanned aerial vehicle (UAV), characterized in that, include: The coaxial drone has multiple stabilizing feet installed on the outer periphery of the bottom of the fuselage. Each stabilizing foot has a corner pad magnet at the bottom landing end to provide a passive adsorption end during landing. The recovery bin has a landing platform at its bottom. A high-contrast visual identification mark is set in the central area of ​​the landing platform. Multiple electromagnetic device arrays are arranged below the surface of the landing platform and around the visual identification mark to provide a visual guidance reference and generate electromagnetic attraction at the landing end. The flight control motherboard, mounted on the coaxial UAV, is connected to the satellite navigation module, downward-facing camera, laser rangefinder, and power drive module of the recovery bin, respectively. Based on the acquired satellite navigation data, visual recognition marker images, laser ranging data, and dynamic data of the recovery bin, it performs phased recovery control to guide the coaxial UAV to complete long-distance approach, visual alignment, and continuous descent in sequence, and triggers the electromagnetic device array to generate electromagnetic attraction force to assist in adsorbing and locking the stabilizing landing gear to the landing platform.

2. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 1, characterized in that, The phased recycling control includes: The first stage, guiding the coaxial UAV to approach the recovery bin from a distance, includes: based on satellite navigation data and dynamic data of the recovery bin, performing line-of-sight guidance with feedforward velocity compensation to guide the coaxial UAV to fly above the recovery bin; The second stage, guiding the coaxial UAV to visually align and descend continuously, includes: performing visual servo position fine-tuning based on the visual recognition mark image and laser ranging data collected by the downward-looking camera, so that the center of the coaxial UAV body is aligned with the recovery axis and descends continuously; In the third stage, the electromagnetic device array generates an electromagnetic attraction force to help attach and lock the stabilizing foot to the landing platform. This includes: when the relative height of the coaxial UAV is less than a height threshold and the total horizontal offset is less than a radius threshold, an electromagnetic activation trigger signal is output to attract the corner pad magnets at the bottom of the stabilizing foot of the coaxial UAV.

3. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 1, characterized in that, The recovery compartment is a vertical cuboid chamber with a square horizontal cross-section, used to cover the cylindrical boundary formed by the rotation of the coaxial UAV rotor.

4. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 1, characterized in that, The visual identification mark adopts a nested pattern of large geometric blocks containing smaller blocks, and uses a black and white matrix arrangement of high-contrast QR codes to provide a visual reference benchmark for the downward-facing camera.

5. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 1, characterized in that, The coaxial UAV has a cylindrical body, and the downward-looking camera is mounted on a stabilization gimbal. The stabilization gimbal is mounted at the center of the bottom surface of the cylindrical body, protruding downwards or embedded in the body. The stabilized gimbal also has a built-in inertial measurement unit, which is used to drive the stabilized gimbal into a vertically downward viewing state in the second stage, so that the optical axis of the camera is parallel to the direction of gravity and coincides with the geometric axis of the drone body.

6. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 2, characterized in that, The first stage includes: Based on the GNSS modules with integrated RTK positioning function deployed on the coaxial UAV and the recovery bin respectively, the real-time three-dimensional coordinates and velocity of the coaxial UAV and the electromagnetic landing platform of the recovery bin are acquired in real time. Based on the real-time three-dimensional coordinates of the coaxial UAV and the recovery bin, the relative position error vector and the linear Euclidean distance between the coaxial UAV and the recovery bin are calculated. Based on the relative position error vector and the linear Euclidean distance, an approximation velocity vector of the coaxial UAV pointing towards the recovery bin is generated using an exponential asymptotic convergence function. The velocity of the recovery bin is used as a feedforward compensation term and superimposed on the approximation velocity vector to obtain the desired velocity vector of the coaxial UAV. Based on the desired velocity vector, the flight control motherboard controls the coaxial UAV to fly above the recovery bin. If the absolute value of the position error vector between the coaxial UAV and the recovery bin in the horizontal direction is less than a preset cutting radius threshold within a preset period, and the relative velocity of the coaxial UAV relative to the recovery bin approaches zero, then the flight control system determines that the coaxial UAV has accurately hovered above the recovery bin.

7. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 6, characterized in that, The second stage includes: The flight control motherboard drives the downward-facing camera to a vertically downward-facing state. The downward-facing camera captures images of visual recognition markers on the electromagnetic landing platform and obtains the pixel coordinates of the center of the visual markers in the pixel coordinate system. Based on the relative altitude of the UAV to the electromagnetic landing platform, the equivalent focal length of the camera, the gimbal camera installation offset, and the steady-state attitude error of the gimbal, the pixel coordinates are converted into the actual physical horizontal offset of the coaxial UAV body relative to the center of the recovery bin. Based on the actual physical horizontal offset, a proportional-integral controller is used to generate lateral and longitudinal fine-tuning speed commands, while a constant low-speed vertical descent command is superimposed. Continuous closed-loop adjustments were made to align the center of the coaxial UAV fuselage with the line-of-sight guide axis of the recovery center and to lower it.

8. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 7, characterized in that, The third stage includes: The flight control motherboard monitors the relative altitude of the UAV to the electromagnetic landing platform in real time, as well as calculates the total horizontal offset of the fuselage in real time. When the relative height is less than a preset height threshold and the total horizontal offset is less than a preset effective magnetic capture radius threshold, the flight control motherboard outputs an electromagnetic activation trigger signal to the central control unit of the recovery bin. When the electromagnetic array inside the recovery bin is powered on, it generates an electromagnetic attraction force that attracts and locks the corner pad magnets at the bottom of the coaxial drone landing gear to the landing platform. The flight control motherboard outputs a rotor stop command to complete the recovery.

9. The electromagnetic-assisted recovery system for a coaxial UAV according to claim 1, characterized in that, The recovery chamber is also equipped with a power drive module, which is connected to the flight control motherboard. After receiving the electromagnetic activation trigger signal, the power drive module outputs a large DC current to the electromagnetic device array to excite a strong magnetic field and generate electromagnetic attraction force.

10. An electromagnetically assisted recovery method for a coaxial unmanned aerial vehicle, characterized in that, Using the electromagnetic-assisted recovery system for coaxial UAVs according to any one of claims 1-9, the following steps are performed: The three-dimensional coordinates and velocities of the coaxial drone and the recovery bin are obtained, and the line-of-sight guidance with feedforward compensation is used to control the coaxial drone to fly above the recovery bin. The system acquires visual recognition mark images, converts the center pixel coordinates of the visual recognition mark images into actual physical horizontal offset, uses a proportional-integral controller to generate horizontal fine-tuning speed commands, and superimposes constant low-speed vertical descent commands to align the center of the coaxial UAV fuselage with the line-of-sight guide axis of the recovery center and continue to descend. The system monitors the relative height of the drone with respect to the electromagnetic landing platform in real time, as well as calculates the total horizontal offset of the drone in real time. When the relative height and the total horizontal offset of the drone are less than the preset height threshold and the preset effective magnetic capture radius threshold, respectively, the electromagnetic device array is energized based on the electromagnetic activation trigger signal to generate electromagnetic attraction force, locking the coaxial drone to the landing platform, and the rotor stops.

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