Though control algorithms for multirotor Unmanned Air Vehicle (UAV) are well understood, the configuration, parameter estimation, and tuning of flight control algorithms takes quite some time and resources. In previous work, we have shown that it is possible to identify the control effectiveness and motor dynamics of a multirotor fast enough for it to recover to a stable hover after being thrown 4 meters in the air. In this paper, we extend this to include estimation of the position of the Inertial Measurement Unit (IMU) relative to the Center of Gravity (CoG), estimation of the IMU rotation, the thrust direction of all motors and the optimal combined thrust direction. In order to guarantee a correct IMU position estimation, two prior throw-and-catches of the vehicle with spin around different axes are required. For these throws, a height as low as 1 meter is sufficient. Quadrotor flight experimentation confirms the efficacy of the approach, and a simulation shows its applicability to fullyactuated crafts with multiple possible hover orientations. @en

This paper presents the design of an Incremental Nonlinear Dynamic Inversion (INDI) controller for the novel, patent pending (NL 2031701) platform Variable Skew Quad Plane (VSQP). Part of the identified challenges is the development of a model for the actuator effectiveness and lift especially as a function of skew, the newly added degree of freedom. The models and assumptions are verified through static and dynamic wind tunnel tests at the Open Jet Facility (OJF) of TU Delft. Transition tests have been successfully performed thanks to an automatic skew controller derived from the proposed models and aimed to maximize control authority.

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Fixed-wing aircraft fly longer, faster, and further than rotorcraft, but cannot take off or land vertically. Hybrid drones combine VTOL with a wing for forward flight, but the hovering system generally makes them less efficient than a pure fixed-wing. We propose an alternative, in which a rotorcraft is used to assist the fixed-wing UAV with the VTOL portions of the flight. This paper takes the first steps towards this alternative by developing and testing an overactuated rotorcraft that can autonomously dock onto a target at fixed-wing velocities. The control system uses Incremental Non-Linear Dynamic Inversion Control (INDI) to achieve linear accelerations with lateral and longitudinal motors, enabling robust horizontal control independent of attitude. A relative guidance algorithm for the docking approach path is presented, along with a vision sensing approach using ArUco markers and IR LEDs. Successful docking and separation were achieved in the wind tunnel at speeds of up to 15m/s. @en

Unmanned Aerial Vehicles (UAVs) have the potential to perform many different missions, some of which may require a large aircraft for endurance and a small aircraft for maneuverability in wind gusts or cluttered environments such as buildings. This paper proposes a novel combination of a quadrotor and a hybrid biplane capable of joint hover, joint forward flight, and mid-Air separation followed by separate flight. We investigate cooperative control strategies during joint flight that do not require any communication between the quadcopter and the biplane. This means that the two aircraft have their own independent control strategy based on their own sensors. The biplane, which is the largest of the two with most control authority, leads the flight and the goal for the quadrotor is to help in producing thrust and increasing rotational stability. Three control strategies for the quadrotor are compared: A proportional angular rate damper, a proportional angular acceleration damper, and constant thrust without attitude control. Simulation and practical tests show that for desired attitude changes of the biplane, the quadrotor rate-and angular acceleration damper strategies lead to a small performance degradation. However, the angular rate damper strategy reduces the roll angle error in disturbance rejection experiments and requires the smallest input command. The in-flight release is successfully tested in joint hover up to a forward pitch angle of-18.

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Hybrid Unmanned Aerial Vehicles UAV are vehicles capable of take-off and landing vertically like helicopters while maintaining the long-range efficiency of fixed-wing aircraft. Unfortunately, due to their wing area, these vehicles are sensitive to wind gusts when hovering. One way to increase the hovering wind-rejection capabilities of hybrid UAV is through the addition of extra actuators capable of directing the thrust of the rotors. Nevertheless, the ability to control UAVs with many actuators is strictly related to how well the Control Allocation problem is solved. Generally, to reduce the problem complexity, conventional (CA) methods make use of linearized control effectiveness in order to optimize the inputs that achieve a certain control objective. We show that this simplification can lead to oscillations if it is applied to thrust vectoring vehicles, with pronounced non-linear actuator effectiveness. When large control objectives are requested or actuators saturate, the linearized effectiveness based CA methods tend to compute a solution far away from the initial actuator state, invalidating the linearization. A potential solution could be to impose limits on the solution domain of the linearized CA algorithm. However, this solution only reduces the oscillations at the expense of a lag in the vehicle acceleration response. To overcome this limitation, we present a fully nonlinear CA method, which uses an Sequential Quadratic Programming (SQP) algorithm to solve the CA problem. The method is tested and implemented on a single board computer that computes the actuator solution in real time onboard a dual axis tilting rotor quad-plane. Flight test experiments confirm the problem of severe oscillations in the linearized effectiveness CA algorithms and show how the only algorithm able to optimally solve the CA problem is the presented Nonlinear method.

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In vehicle control, control allocation is often used to abstract control variables from actuators, simplifying controller design and enhancing performance. Surveying available literature reveals that explicit solutions are restricted to strong assumptions on the actuators, or otherwise fail to exploit the capabilities of the actuator constellation. A remedy is to formulate hierarchical minimization problems that take into account the limits of the actuators at the expense of a longer computing time. In this paper, we compared the most common norms of the objective functions for linear or linearized plants, and show available numeric solver types. Such a comparison has not been found in the literature before and indicates that some combinations of linear and quadratic norms are not sufficiently researched. While the bulk of the review is restricted to control-affine plant models, some extensions to dynamic and nonlinear allocation problems are shown. For aerial vehicles, a trend toward linearized incremental control schemes is visible, which forms a compromise between real-time capabilities and the ability to resolve some nonlinearities common in these vehicles.@en

The authors regret to inform that an incorrect transfer function was included in Eq. (12). The correct transfer function is: [Formula Presented] The selected gains were [Formula Presented]. This leads to a real pole at 0.964 and two complex poles at 0.965 ± 0.0445i. The difference of the model compared to the measured step response has now reduced, the largest difference being 4.8% of the final step value at 0.14 s. The mistake does not influence any of the conclusions drawn in the paper. The authors would like to apologize for any inconvenience caused.

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This paper puts forward a novel design for an airspeed instrument aimed at small fixed-wing tail-sitter unmanned aerial vehicles. The working principle is to relate the power spectra of the wall-pressure fluctuations beneath the turbulent boundary layer present over the vehicle’s body in flight to its airspeed. The instrument consists of two microphones; one flush-mounted on the vehicle’s nose cone, which captures the pseudo-sound caused by the turbulent boundary layer, and a micro-controller that processes the signals and computes the airspeed. A feed-forward single-layer neural network is used to predict the airspeed based on the power spectra of the microphones’ signals. The neural network is trained using data obtained from wind tunnel and flight experiments. Several neural networks were trained and validated using only flight data, with the best one achieving a mean approximation error of 0.043 m/s and having a standard deviation of 1.039 m/s. The angle of attack has a significant impact on the measurement, but if the angle of attack is known, the airspeed could still be successfully predicted for a wide range of angles of attack.
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Tailsitter Micro Air Vehicles with two rotors are promising due to their simplicity and efficient forward flight, but actuator saturation due to ineffective pitch control at a high angle of attack flight is a challenge limiting the flight envelope. This paper proposes a novel tilt-rotor tailsitter design which features two tilting rotors as the only means for control moment generation. Incremental Nonlinear Dynamic Inversion (INDI) is applied to the attitude control problem of the tiltrotor tailsitter, whose attitude angle tracking performance is validated by indoor and outdoor flight tests. It is found that actuator saturation is largely avoided by using thrust vectoring which provides sufficient capability of pitch moment generation. However, it is also found that the proposed design with only leading-edge tilting motors excluding any aerodynamic control surfaces has limited roll control effectiveness in forward flight.

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In this paper, we derive a sensor-based nonlinear dynamic inversion (NDI) control law for a nonlinear system with first-order linear actuators, and compare it to incremental nonlinear dynamic inversion (INDI), which has gained popularity in recent years. It is shown that, for first-order actuator dynamics, INDI approximates the corresponding NDI control law arbitrarily well under the condition of sufficiently fast actuators. If the actuator bandwidth is low compared to changes in the states, the derived NDI control law has the following advantages compared to INDI: 1) compensation of state derivative terms, 2) well-defined error dynamics, and 3) exact tracking of a reference model, independent of error controller gains in nominal conditions. The comparison of the INDI control law with the well-established control design method NDI adds to the understanding of incremental control. It is additionally shown how to quantify the deficiency of the INDI control law with respect to the exact NDI law for actuators with finite bandwidth. The results are confirmed through simulation results of the roll motion of a fixed-wing aircraft.@en

This paper presents a modular autopilot framework for Unmanned Aerial Vehicles (UAVs) that addresses the limitations of modern flight controllers. The framework utilizes separate and external subsystems for actuator control and the resolution of the Control Allocation problem. The actuator subsystem, implemented on a Teensy 4.0 microcontroller, incorporates an Incremental Dynamic Inversion Control (INDI) RPM controller, enabling direct control of motor RPM and facilitating the implementation of dynamic inversion-based control laws. The primary flight computer, a Cube Orange, coordinates the system, while an OrangePi 5 singleboard computer serves as a companion computer. Real flight results demonstrate the effectiveness of the framework, highlighting its potential for robust and efficient UAV control. @en

Tailsitters have complex aerodynamics that make them hard to control throughout the entire flight envelope, especially at very high angle of attack (AoA) and reverse flow conditions. The development of controllers for these vehicles is hampered by the absence of publicly available data on forces and moments experienced in such conditions. In this paper, wind tunnel experiments are performed under different flap deflections and throttle settings at all possible AoA. The dataset is made open access. Our analysis of the data shows for the tested wing, flap deflections greatly affect the lift coefficient and stall occurs at (Formula presented.) AoA as well as (Formula presented.). Wing-propeller interaction is studied by analyzing the propeller induced force in the axis orthogonal to the thrust axis, which is dependent on AoA, airspeed, flap deflections and thrust in a nonlinear and coupled manner. The influence of inverse flow on the wing is also discussed: The data confirm that when the airflow over the wing is reversed, flap deflections will affect the pitch moment in an opposite way compared to the non-reversed case, but this opposite effect can be avoided by increasing the throttle setting. The data show the exact relationship between flap deflections and forces in this condition. Moreover, it is found that the flap control effectiveness for a wing with or without spinning propellers is usually higher around zero degrees AoA than at (Formula presented.) and it is more effective to change the flaps from (Formula presented.) to (Formula presented.) than from (Formula presented.) to the respective (Formula presented.).

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Unmanned Aerial Vehicles (UAVs) have the potential to perform many different missions, some of which may require a large aircraft for endurance and a small aircraft for manoeuvrability in a building. This paper proposes a novel combination of a quadrotor and a hybrid biplane capable of joint hover, joint forward flight, and mid-air disassembly followed by separate flight. We investigate cooperative control strategies during joint flight that do not require any communication between the quadcopter and the biplane. This means that the two aircraft have their own independent control strategy based on their own sensors. Secondly, to avoid communication the biplane leads the flight and the goal for the quadrotor is to help in producing thrust and increasing stability. Three control strategies for the quadrotor are compared: a proportional angular rate damper, a proportional angular acceleration damper, and constant thrust without attitude control. Simulation and practical tests show that for intentional attitude changes of the biplane, the quadrotor rate- and angular acceleration damper strategies lead to a small performance degradation. However, the angular rate damper strategy for disturbance rejection has the lowest roll angle error and requires the smallest input command. The in-flight release is successfully tested in joint hover up to a forward pitch angle of -18 [deg]. @en

In the last few decades, the UAV research has been focusing on hybrid vehicles with Vertical Takeoff and Landing (VTOL) capabilities. Opposed to copters, hybrid vehicles are highly influenced by wind disturbances. This paper presents a novel quad-plane design that uses four dual-axis tilting rotors to enhance the wind rejection capability of a conventional quad-plane vehicle. After the non-linear mathematical model derivation and the actuator identification, the performance of the vehicle is addressed and compared to a conventional quad-plane in simulation, showing a factor 3.4 improvement in linear acceleration reaction time and a reduction of the gust induced displacement of 80%. Free-flight wind tunnel experiments confirmed the simulation outcome and extended the vehicle wind rejection capabilities behavior also to the lateral gust scenario.

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Many Unmanned Air Vehicle (UAV) applications require vertical take-off and landing and very long-range capabilities. Fixed-wing aircraft need long runways to land, and electric energy is still a bottleneck for helicopters, which are not range efficient. In this paper, we introduce the NederDrone, a hybrid lift, hybrid energy hydrogen-powered UAV that can perform vertical take-off and landings using its 12 propellers while flying efficiently in forward flight thanks to its fixed wings. The energy is supplied from a combination of hydrogen-driven Polymer Electrolyte Membrane fuel-cells for endurance and lithium batteries for high-power situations. The hydrogen is stored in a pressurized cylinder around which the UAV is optimized. This work analyses the selection of the concept, the implemented safety elements, the electronics and flight control and shows flight data including a 3h38 flight at sea while starting and landing from a small moving ship.

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Rotorcraft, fixed wing and hybrid Unmanned Air Vehicles (UAV) each have applications in which they excel. Traditionally, dedicated autopilot control code is written to accommodate flight of each UAV type. This causes fragmentation of control code and may lead to performance differences or errors. In this paper, we propose to use the same INDI controller for rotorcraft, fixed wing and hybrid UAVs, with only parametric differences in control effective matrix definitions and roll, pitch and airspeed limits. The controller is based on earlier work, but relevant derivations are included in this paper. Successful test flights, performed with a Bebop2 quadrotor, a Disco fixed wing, and a Nederdrone tailsitter hybrid demonstrate the feasibility of this approach. @en

Hybrid unmanned aircraft can significantly increase the potential of micro air vehicles, because they combine hovering capability with a wing for fast and efficient forward flight. However, these vehicles are very difficult to control, because their aerodynamics are hard to model and they are susceptible to wind gusts. This often leads to composite and complex controllers, with different modes for hover, transition, and forward flight. This paper proposes incremental nonlinear dynamic inversion control for the attitude and position control. The result is a single, continuous controller that is able to track the desired acceleration of the vehicle across the flight envelope. The proposed controller is implemented on the Cyclone hybrid unmanned air vehicle. Multiple outdoor experiments are performed, showing that unmodeled forces and moments are effectively compensated by the incremental control structure. Finally, this paper provides a comprehensive procedure for the implementation of the controller on other types of hybrid unmanned air vehicles. @en

We propose a combined method for the collaborative transportation of a suspended payload by a team of rotorcraft. A recent distance-based formation-motion control algorithm based on assigning distance disagreements among robots generates the acceleration signals to be tracked by the vehicles. In particular, the proposed method does not need global positions nor tracking prescribed trajectories for the motion of the members of the team. The acceleration signals are followed accurately by an Incremental Nonlinear Dynamic Inversion controller designed for rotorcraft that measures and resists the tensions from the payload. Our approach allows us to analyze the involved accelerations and forces in the system so that we can calculate the worst case conditions explicitly to guarantee a nominal performance, provided that the payload starts at rest in the 2D centroid of the formation, and it is not under significant disturbances. For example, we can calculate the maximum safe deformation of the team with respect to its desired shape. We demonstrate our method with a team of four rotorcraft carrying a suspended object two times heavier than the maximum payload for an individual. Last but not least, our proposed algorithm is available for the community in the open-source autopilot Paparazzi.

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This paper presents the cascaded integration of Incremental Nonlinear Dynamic Inversion (INDI) for attitude control and INDI for position control of micro air vehicles. Significant improvements over a traditional Proportional Integral Derivative (PID) controller are demonstrated in an experiment where the quadrotor flies in and out of a 10 m/s windtunnel exhaust. The control method does not rely on frequent position updates, as is demonstrated in an outside experiment using a standard GPS module. Finally, we investigate the effect of using a linearization to calculate thrust vector increments, compared to a nonlinear calculation.

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Micro Air Vehicles (MAVs) can perform many useful tasks, such as mapping and delivery. For these tasks either rotorcraft are used, which can hover but are not very efficient, or fixed wing vehicles, which are efficient but can not hover. Hybrid MAVs combine the hovering of a rotorcraft with the efficiency of a fixed wing. The reason that these vehicles are not yet widely adopted is that they are very difficult to control. This thesis addresses the use of Incremental Nonlinear Dynamic Inversion (INDI) for the control of the attitude and velocity of hybrid MAVs. This control method had not been applied in a real world application prior to this thesis, which is why the thesis encompasses the application to a quadrotor at first, which is easier to control than a hybrid MAV. First, an INDI structure is proposed for the control of the angular accelerations of a quadrotor. I show that the delay that filtering of the angular acceleration produces should also be applied to the measurement of the actuator state. If this is done, the filtering does not appear in the transfer function from virtual control to angular acceleration, which turns out to be equal to the actuator dynamics. It is also shown that a disturbance, or unmodeled dynamics, is compensated with the transfer function of the actuator dynamics multiplied with the applied filter and a unit delay. Finally, it is shown how the effects of propeller inertia, which can be very significant in the yaw axis, can be dealt with and how the control effectiveness can be made adaptive. All these findings are validated with experiments on a Bebop quadrotor. Second, this thesis includes a Weighted Least Squares (WLS) control allocation algorithm with priority management into the INDI controller. This means that for vehicles with coupled control effectors, certain control objectives can be given pri- ority upon actuator saturation. This is very important for vehicles with controlled axes that are not very important for the stability of the vehicle, such as the yaw for a quadrotor. It is shown that for a quadrotor doing a 50 degree yaw change, the stability is greatly improved when the yaw axis is given very low priority. Third, this thesis introduces the control of linear accelerations in all three axes with INDI. The controller does not need a complex model, but instead relies on a measurement of the acceleration. It is shown through a wind tunnel experiment, that the disturbance rejection properties, that were shown for the inner loop, carry over to the control of linear accelerations. It is also shown that the method can be applied outdoors with an off-the-shelve GPS receiver. Finally, a nonlinear method of calculating the input increment is derived, which provides only a slight improvement in the tracking of aggressive acceleration commands. These three things are combined for the INDI control of hybrid MAVs. The result is a single, continuous INDI controller for the attitude, and a single, continuous INDI controller for the velocity of the vehicle. This is achieved by incorporating partial derivatives of the lift vector in the control effectiveness of the pitch and roll angles. Though no transition maneuver is explicitly defined, the transition follows implicitly from the increments in attitude and thrust that are calculated from desired acceleration changes. Further, as the control effectiveness of a hybrid MAV changes dramatically over the flight envelope, the control effectiveness is scheduled as a function of airspeed. When the airspeed is too low to measure, the pitch angle is used for this purpose. To prevent sideslip, a sideslip controller is included, where an estimate of the sideslip angle is obtained from the accelerometer. Test flights show that the INDI inner and outer loop controllers are indeed capa- ble of controlling the attitude and the linear acceleration of the vehicle throughout the flight envelope, within the physical limitations of the vehicle. It is shown that the tracking of accelerations can make the vehicle naturally transition to forward flight and back, and fly in the stall region as necessary. Because of the abstraction that the INDI acceleration control provides, it is straightforward to follow a velocity vector field, for example one that guides the aircraft along a line. The developed controller can be applied to different tailsitter MAVs with relative ease, as the model dependency is low. The algorithm may even be applied to different types of hybrid vehicles, such as quadplanes or tilt-wing aircraft, with minor adjustments. @en