Automatic control of a multirotor

Objective of this thesis is to describe the design and realisation phases of a multirotor to be used for low risk and cost aerial observation. Starting point of this activity was a wide literature study related to the technological evolution of multirotors design and to the state of the art. Firstly the most common multirotor configurations were defined and, according to a size and performance based evaluation, the most suitable one was chosen. A detailed computer aided design model was drawn as basis for the realisation of two prototypes. The realised multirotors were “X-shaped” octorotors with eight coaxially coupled motors. The mathematical model of the multirotor dynamics was studied. “Proportional Integral Derivative” and “Linear Quadratic” algorithms were chosen as techniques to regulate the attitude dynamics of the multirotor. These methods were tested with a nonlinear model simulation developed in the Matlab Simulink environment. In the meanwhile the Arduino board was selected as the best compromise between costs and performance and the above mentioned algorithms were implemented using this platform thanks to its main characteristic of being completely “open source”. Indeed the multirotor was conceived to be a serviceable tool for the public utility and, at the same time, to be an accessible device for research and studies. The behaviour of the physical multirotor was evaluated with a test bench designed to isolate the rotation about one single body axis at a time. The data of the experimental tests were gathered in real time using a custom Matlab code and several indoor tests allowed the “fine tuning” of the controllers gains. Afterwards a portable “ground station” was conceived and realised in adherence with the real scenarios users needs. Several outdoor experimental flights were executed with successful results and the data gathered during the outdoor tests were used to evaluate some key performance indicators as the endurance and the maximum allowable payload mass. Then the fault tolerance of the control system was evaluated simulating and experimenting the loss of one motor; even in this critical condition the system exhibited an acceptable behaviour. The reached project readiness allowed to meet some potential users as the “Turin Fire Department” and to cooperate with them in a simulated emergency. During this event the multirotor was used to gather and transmit real time aerial images for an improved “situation awareness”. Finally the study was extended to more innovative control techniques like the neural networks based ones. Simulations results demonstrated their effectiveness; nevertheless the inherent complexity and the unreliability outside the training ranges could have a catastrophic impact on the airworthiness. This is a factor that cannot be neglected especially in the applications related to flying platforms. Summarising, this research work was addressed mainly to the operating procedures for implementing automatic control algorithms to real platforms. All the design aspects, from the preliminary multirotor configuration choice to the tests in possible real scenarios, were covered obtaining performances comparable with other commercial of-the-shelf platforms

Survey on Aerial Multirotor Design: a Taxonomy Based on Input Allocation

This paper reviews the impact of multirotor aerial vehicles designs on their abilities in terms of tasks and system properties. We propose a general taxonomy to characterize and describe multirotor aerial vehicles and their design, which we apply exhaustively on the vast literature available. Thanks to the systematic characterization of the designs we exhibit groups of designs having the same abilities in terms of achievable tasks and system properties. In particular, we organize the literature review based on the number of atomic actuation units and we discuss global properties arising from their choice and spatial distribution in the designs. Finally, we provide a discussion on the common traits of the designs found in the literature and the main future open problems

Fault tolerant control of multi-rotor unmanned aerial vehicles using sliding mode based schemes

This thesis investigates fault-tolerant control (FTC) for the specific application of small multirotor unmanned aerial vehicles (Unmanned Aerial Vehicle (UAV)s). The fault-tolerant controllers in this thesis are based on the combination of sliding mode control with control allocation where the control signals are distributed based on motors' health level. This alleviates the need to reconfigure the overall structure of the controllers. The thesis considered both the over actuated (sufficient redundancy) and under-actuated UAVs. Three multirotor UAVs have been considered in this thesis which includes a quadrotor (4 rotors), an Octocopter (8 rotors) and a spherical UAV. The non-linear mathematical models for each of the UAVs are presented. One of the main contributions of this thesis is the hardware implementation of the sliding mode Fault Tolerant Control (FTC) scheme on an open-source autopilot microcontroller called Pixhawk for a quadrotor UAV. The controller was developed in Simulink and implemented on the microcontroller using the Matlab/Simulink support packages. A gimbal- based test rig was developed and built to offer a safe test bed for testing control designs. Actual flight tests were done which showed sound responses during fault-free and faulty scenarios. This work represents one of successful implementation work of sliding mode FTC in the literature. Another key contribution of this thesis is the development of the mathematical model of a unique spherical UAV with highly redundant control inputs. The use of novel 8 flaps and 2 rotors configuration of the spherical UAV considered in this thesis provides a unique fault tolerant capability, especially when combined with the sliding mode-based FTC scheme. A key development in the later chapters of the thesis considers fault-tolerant control strategy when no redundancy is available. Unlike many works which consider FTC on quadrotors in the literature (which can only handle faults), the proposed schemes in the later chapters also include cases when failures also occur converting the system to an under actuated system. In one chapter, a bespoke Linear Parameter Varying (LPV) based controller is developed for a reduced attitude dynamics system by exploiting non-standard equation of motions which relates to position acceleration and load factor dynamics. This is unique as compared to the typical Euler angle control (roll, pitch and yaw angle control). In the last chapter, a fault-tolerant control scheme which can handle both the over and under actuated system is presented. The scheme considers an octocopter and can be used in fault-free, faulty and failure conditions up to two remaining motors. The scheme exploits the differential flatness property, another unique property of multirotor UAVs. This allows both inner loop and outer loop controller to be designed using sliding mode (as opposed to many sliding mode FTC in the literature, which only considers sliding mode for the inner loop control)

Model-based Design Development and Control of a Wind Resistant Multirotor UAV

Multirotor UAVs have in recent years become a trend among academics, engineers and hobbyists alike due to their mechanical simplicity and availability. Commercial uses range from surveillance to recreational flight with plenty of research being conducted in regards to design and control. With applications towards search and rescue missions in mind, the main objective of this thesis work is the development of a mechanical design and control algorithm aimed at maximizing wind resistance. To these ends, an advanced multirotor simulator, based on helicopter theory, has been developed to give an accurate description of the flight dynamics. Controllers are then designed and tuned to stabilize the attitude and position of the UAV followed by a discussion regarding disturbance attenuation. In order to study the impact of different design setups, the UAV model is constructed so that physical properties can be scaled. Parameter influence is then investigated for a specified wind test using a Design of Experiments methodology. These results are combined with a concept generation process and evaluated with a control engineering approach. It was concluded that the proposed final design should incorporate a compact three-armed airframe with six rotors configured coaxially

Risk-sensitive motion planning for MAVs based on mission-related fault-tolerant analysis

Multirotor Aerial Vehicles may be fault-tolerant by design when rotor-failure is possible to measure or identify, especially when a large number of rotors are used. For instance, an octocopter can be capable to complete some missions even when a double-rotor fault occurs during the execution. In this paper, we study how a rotor-failure reduces the vehicle control admissible set and its importance with respect to the selected mission, i.e. we perform mission-related fault-tolerant analysis. Furthermore, we propose a risk-sensitive motion-planning algorithm capable to take into account the risks during the planning stage by means of mission-related fault-tolerant analysis. We show that the proposed approach is much less conservative in terms of selected performance measures than a conservative risk planner that assumes that the considered fault will certainly occur during the mission execution. As expected, the proposed risk-sensitive motion planner is also readier for accepting failures during the mission execution than the risk-insensitive approach that assumes no failure will occur

Geolocation of Diseased Leaves in Strawberry Orchards for a Custom-Designed Octorotor

In recent years, technological advances have shown a strive for more automated processes in agriculture, as seem with the use of unmanned aerial vehicles (UAVs) with onboard sensors in many applications, including disease detection and yield prediction. In this thesis, an octorotor UAV is presented that was designed, built, and flight tested, with features that are custom-designed for strawberry orchard disease detection. To further automate the disease scouting operation, geolocation, or the process of determining global position coordinates of identified diseased regions based on images taken, is investigated. A Kalman filter is designed, based on a linear measurement model derived from an orthographic projection method, to estimate the target position. Simulation, as well as an ad-hoc experiment using flight data, is performed to compare this filter to the extended Kalman filter (EKF), which is based on the commonly used perspective projection method. The filter is embedded onto a CPU board for real-time use aboard the octorotor UAV, and the algorithm structure for this process is presented. In the later part of the thesis, a probabilistic data association method is used, jointly with a proposed logic-based measurement-to-target correlation method, to analyze measurements of different target sources and is incorporated into the Kalman filter. A simulation and an ad-hoc experiment, using video and flight data acquired aboard the octorotor UAV with a gimballed camera in hover flight, are performed to demonstrate the effectiveness of the algorithm and UAV platform

Motion feedback in the teleoperation of Unmanned Aerial Vehicles

Teleoperation of unmanned vehicles is a valuable tool in scenarios where the operator can not or should not operate the vehicle from on-board. Applications range from hazardous environments where exposure needs to be avoided, control of Unmanned Aerial Vehicles (UAV) to retrieve overviews of inaccessible disaster areas, to deep sea exploration where on-board operation is simply not possible. However, limitations in sensor performance, noise and laten- cies introduced in the transmission, and ineffective display of the information to the operator can lead to a reduced amount of infor- mation, reduced performance, a loss of situation awareness, and in the worst case a loss of the remote vehicle. The spatial decoupling between the operator and the vehicle is one of the main challenges in teleoperation. Most setups include one or more control sticks to steer the ve- hicle, a monitor displaying the live video feed of the main vehicle camera, and a seat for the operator. This can be extended by display- ing additional state information using monitors or visual overlay, rendered on top of the main video stream [Tvaryanas, 2004; van Erp, 2000]. However, processing of multiple screens can increase mental workload. This can cause the operator to miss important information, leading to a loss of situation awareness and reduced performance or a crash of the vehicle. Instead of presenting information purely visually, other feedback modalities can be used to convey vehicle state or information about the task. The goal of this PhD thesis is to investigate the possibility of providing additional information using motion feedback. Here, motion feedback is defined as physically moving the operator using a motion simulator. In the work presented in this thesis a distinction between two motion feedback types is made. Vehicle-state motion feedback describes vehicle motion, while task-related motion feedback is the result of the combination of desired and actual vehicle motion. To investigate the effects of motion feedback in teleoperation several studies have been conducted. In the experiments presented participants either controlled a virtual quadrotor flying in a simu- lated environment or a real octorotor. Participants controlled the UAV from within the CyberMotion Simulator (CMS), an 8-DOF motion simulator located at the Max Planck Institute for Biological Cybernetics. The results show that providing motion feedback has a positive effect on performance in teleoperation of remote UAVs. If the remote vehicle is subject to external disturbances, e.g., wind gusts, vehicle- state feedback showed to improve disturbance rejection capabilities leading to increased performance. Furthermore, motion feedback can be shaped to include additional information about the task with positive effects on performance. This shows that the additional information included in the motion feedback can be used by the operator to improve performance and control behavior.Die Teleoperation eines unbemannten Gefährts ist ein wertvolles Werkzeug in Situationen, in denen der Pilot das Gefährt nicht von Bord aus steuern kann oder sollte. Beispiele hierfür reichen von, für den Piloten, toxischen Umgebungen, über Luftaufnahmen von Katastrophengebieten mithilfe von unbemannten Flugzeugen (engl. Unmanned Aerial Vehicle(UAV)), bis zur Erforschung der Tiefsee, bei der die Steuerung von Bord schlichtweg unmöglich wird. Allerdings führen Einschränkungen in der Sensorerfassung, Rau- schen und Latenzen in der Übertragung, sowie eine ineffiziente Darstellung der Informationen für den Piloten dann zu einem redu- zierten Informationsfluss, reduzierter Leistung, einem Verlust des Situationsbewusstseins und im schlimmsten Fall zu einem Verlust des Gefährts. Die räumliche Entkopplung zwischen dem Piloten und des Flugobjekts ist eine der wichtigsten Herausforderungen in der Teleoperation von UAVs. Die meisten Kontrollstationen beinhalten ein oder mehrere Steu- erknüppel um das Gefährt zu steuern, einen Monitor der eine di- rekte Videoübertragung der Hauptkamera anzeigt und ein Sitzplatz für den Piloten. Dies kann erweitert werden, in dem zusätzliche Statusinformationen mit weiteren Monitoren oder visuellen Über- lagerungen, die über die Hauptübertragung gezeichnet werden, angezeigt werden [Tvaryanas, 2004; van Erp, 2000]. Jedoch kann die Verarbeitung mehrerer Bildschirme die mentale Belastung erhö- hen. Dies kann dazu führen, dass der Pilot wichtige Informationen nicht aufnimmt, was zu einem Verlust des Situationsbewusstseins und einhergehender reduzierten Leistung oder einem Unfall des Gefährts führt. Anstatt Information rein visuell zu präsentieren, können ande- re Modalitäten genutzt werden Rückmeldungen über den Status des Gefährts oder Informationen über die Aufgabe zu präsentieren. Das Ziel dieser Doktorarbeit ist die Untersuchung der Modalität der Bewegung. Es soll untersucht werden, ob Bewegungen genutzt werden können, um dem Piloten zusätzliche Rückmeldungen über den Zustand des Gefährts bereit zu stellen. Bewegungsfeedback beschreibt hier die physikalische Bewegung des Piloten mit Hilfe eines Bewegungssimulators. In dieser Arbeit wird zwischen zwei Typen von Bewegungsfeedback unterschieden. Fahrzeugzustandsbe- wegungsfeedback beschreibt die Bewegung des Fahrzeugs, während Aufgabenabhängiges Bewegungsfeedback die Kombination aus tatsächli- chem und gewünschtem Fahrzeugzustand ist. Die Effekte von Bewegungsfeedback in der Teleoperation wurden in mehreren Studien untersucht. In den vorgestellten Experimenten kontrollierten Teilnehmer entweder einen virtuellen Quadrotor, der in einer simulierten Umgebung flog, oder einen echten Octorotor. Die Teilnehmer steuerten das UAV von der Kanzel des CyberMotion Simulators (CMS) aus, ein 8-DOF Bewegungssimulator, der sich am Max-Planck-Institut für biologische Kybernetik befindet. Die Ergebnisse zeigen, dass die Bereitstellung von Bewegungs- feedback positive Effekte auf die Leistung und das Verhalten des Piloten in der Steuerung des UAVs hat. Ist das UAV externen Stö- rungen ausgesetzt, wie z.B. Windstößen, zeigte sich, dass Fahr- zeugzustandsbewegungsfeedback die Fähigkeit der Störungsunter- drückung des Piloten verbessert, was zu Leistungsteigerungen führt. Außerdem zeigte sich, dass Bewegungsfeedback dahingehend ge- formt werden kann, zusätzliche Informationen über die Aufgabe bereitzustellen. Dies zeigt, dass die zusätzlichen Informationen vom Piloten genutzt werden können um Leistung und Kontrollverhalten zu verbessern

Real-Time UAV Trajectory Prediction for Safety Monitoring in Low-Altitude Airspace

The rising number of small unmanned aerial vehicles (UAVs) expected in the next decade will enable a new series of commercial, service, and military operations in low altitude airspace as well as above densely populated areas. These operations may include on-demand delivery, medical transportation services, law enforcement operations, traffic surveillance and many more. Such unprecedented scenarios create the need for robust, efficient ways to monitor the UAV state in time to guarantee safety and mitigate contingencies throughout the operations. This work proposes a generalized monitoring and prediction methodology that utilizes realtime measurements of an autonomous UAV following a series of way-points. Two different methods, based on sinusoidal acceleration profiles and high-order splines, are utilized to generate the predicted path. The monitoring approach includes dynamic trajectory re-planning in the event of unexpected detour or hovering of the UAV during flight. It can be further extended to different vehicle types, to quantify uncertainty affecting the state variables, e.g., aerodynamic and other environmental effects, and can also be implemented to prognosticate safety-critical metrics which depend on the estimated flight path and required thrust. The proposed framework is implemented on a simplified, scalable UAV modeling and control system traversing 3D trajectories. Results presented include examples of real-time predictions of the UAV trajectories during flight and a critical analysis of the proposed scenarios under uncertainty constraints

A Contribution to the Design of Highly Redundant Compliant Aerial Manipulation Systems

Es ist vorhersehbar, dass die Luftmanipulatoren in den nächsten Jahrzehnten für viele Aufgaben eingesetzt werden, die entweder zu gefährlich oder zu teuer sind, um sie mit herkömmlichen Methoden zu bewältigen. In dieser Arbeit wird eine neuartige Lösung für die Gesamtsteuerung von hochredundanten Luftmanipulationssystemen vorgestellt. Die Ergebnisse werden auf eine Referenzkonfiguration angewendet, die als universelle Plattform für die Durchführung verschiedener Luftmanipulationsaufgaben etabliert wird. Diese Plattform besteht aus einer omnidirektionalen Drohne und einem seriellen Manipulator. Um den modularen Regelungsentwurf zu gewährleisten, werden zwei rechnerisch effiziente Algorithmen untersucht, um den virtuellen Eingang den Aktuatorbefehlen zuzuordnen. Durch die Integration eines auf einem künstlichen neuronalen Netz basierenden Diagnosemoduls und der rekonfigurierbaren Steuerungszuordnung in den Regelkreis, wird die Fehlertoleranz für die Drohne erzielt. Außerdem wird die Motorsättigung durch Rekonfiguration der Geschwindigkeits- und Beschleunigungsprofile behandelt. Für die Beobachtung der externen Kräfte und Drehmomente werden zwei Filter vorgestellt. Dies ist notwendig, um ein nachgiebiges Verhalten des Endeffektors durch die achsenselektive Impedanzregelung zu erreichen. Unter Ausnutzung der Redundanz des vorgestellten Luftmanipulators wird ein Regler entworfen, der nicht nur die Referenz der Endeffektor-Bewegung verfolgt, sondern auch priorisierte sekundäre Aufgaben ausführt. Die Wirksamkeit der vorgestellten Lösungen wird durch umfangreiche Tests überprüft, und das vorgestellte Steuerungssystem wird als sehr vielseitig und effektiv bewertet.:1 Introduction 2 Fundamentals 3 System Design and Modeling 4 Reconfigurable Control Allocation 5 Fault Diagnostics For Free Flight 6 Force and Torque Observer 7 Trajectory Generation 8 Hybrid Task Priority Control 9 System Integration and Performance Evaluation 10 ConclusionIn the following decades, aerial manipulators are expected to be deployed in scenarios that are either too dangerous for human beings or too expensive to be accomplished by traditional methods. This thesis presents a novel solution for the overall control of highly redundant aerial manipulation systems. The results are applied to a reference configuration established as a universal platform for performing various aerial manipulation tasks. The platform consists of an omnidirectional multirotor UAV and a serial manipulator. To ensure modular control design, two computationally efficient algorithms are studied to allocate the virtual input to actuator commands. Fault tolerance of the aerial vehicle is achieved by integrating a diagnostic module based on an artificial neural network and the reconfigurable control allocation into the control loop. Besides, the risk of input saturation of individual rotors is minimized by predicting and reconfiguring the speed and acceleration responses. Two filter-based observers are presented to provide the knowledge of external forces and torques, which is necessary to achieve compliant behavior of the end-effector through an axis-selective impedance control in the outer loop. Exploiting the redundancy of the proposed aerial manipulator, the author has designed a control law to achieve the desired end-effector motion and execute secondary tasks in order of priority. The effectiveness of the proposed designs is verified with extensive tests generated by following Monte Carlo method, and the presented control scheme is proved to be versatile and effective.:1 Introduction 2 Fundamentals 3 System Design and Modeling 4 Reconfigurable Control Allocation 5 Fault Diagnostics For Free Flight 6 Force and Torque Observer 7 Trajectory Generation 8 Hybrid Task Priority Control 9 System Integration and Performance Evaluation 10 Conclusio