Chattering free tracking control of a fully actuated multirotor with passively tilted rotors

In this paper, a control allocation scheme is presented for a multirotor type of an Unmanned Aerial Vehicle (UAV). The control allocation scheme depends on the multirotor configuration and rotor system parameters, and it enables analysis of dynamics of different multirotor designs depending on the purpose and the task which the multirotor has to carry out. The analysis of force and moment distribution in space shows that the non-flat design with passively tilted rotors can overcome an inherent underactuated condition of flat multirotor configurations. By increasing the tilt angle, the multirotor is able to achieve full controllability over its six degrees of freedom (6 DOFs). A robust chattering-free sliding mode asymptotic tracking control design of a fully actuated multirotor is presented. The simulation results show satisfying tracking performance of the proposed controller

멀티로터 기반 다목적 비행 로봇 플랫폼을 위한 강건 제어 및 완전구동 비행 매커니즘

학위논문(박사)--서울대학교 대학원 :공과대학 기계항공공학부,2020. 2. 김현진.오늘날 멀티로터 무인항공기는 단순한 비행 및 공중 영상 촬영용 장비의 개념을 넘어 비행 매니퓰레이션, 공중 화물 운송 및 공중 센싱 등의 다양한 임무에 활용되고 있다. 이러한 추세에 맞추어 로보틱스 분야에서 멀티로터 무인항공기는 부과된 임무에 맞추어 원하는 장비 및 센서를 자유로이 탑재하고 비행할 수 있는 다목적 공중 로봇 플랫폼으로 인식되고 있다. 그러나 현재의 멀티로터 플랫폼은 돌풍 등의 외란에 다소 강건하지 못한 제어성능을 보인다. 또한, 병진운동의 제어를 위해 비행 중 지속적으로 동체의 자세를 변경해야 해 센서 등 동체에 부착된 탑재물의 자세 또한 지속적으로 변화한다는 단점을 가지고 있다. 위의 두 가지 문제들을 해결하고자 본 연구에서는 외란에 강건한 멀티로터 제어기법과, 병진운동과 자세운동을 독립적으로 제어할 수 있는 새로운 형태의 완전구동 멀티로터 비행 매커니즘을 소개한다. 강건 제어기법의 경우, 먼저 정확한 병진운동 제어를 위한 병진 힘 생성 기법을 소개하고 뒤이어 병진 힘 외란에 강건한 제어를 위한 외란관측기 기반 강건 제어 알고리즘의 설계 방안을 논의한다. 제어기의 피드백 루프 안정성은 mu 안정성 분석 기법을 통해 검증되며, mu 안정성 분석이 가지는 엄밀한 안정성 분석의 결과를 검증하기 위해 스몰게인 이론 (Small Gain Theorem) 기반의 안정성 분석 결과가 동시에 제시 및 비교된다. 최종적으로, 개발된 제어기를 도입한 멀티로터의 3차원 병진 가속도 제어 성능 및 힘 벡터의 형태로 인가되는 병진 운동 외란에 대한 극복 성능을 실험을 통해 검증하여, 제안된 제어기법의 효과적인 비행 지점 및 궤적 추종 능력을 확인한다. 완전 구동 멀티로터의 경우, 기존의 완전구동 멀티로터가 가진 과도한 중량 증가 및 저조한 에너지 효율을 극복하기 위한 새로운 매커니즘을 소개한다. 새로운 매커니즘은 기존 멀티로터와 최대한 유사한 형태를 가지되 완전구동을 위해 오직 두 개의 서보모터만을 포함하며, 이로 인해 기존 멀티로터와 비교해 최소한의 형태의 변형만을 가지도록 설계된다. 새로운 플랫폼의 동적 특성에 대한 분석과 함께 유도된 운동방정식을 기반으로 한 6자유도 비행 제어기법이 소개되며, 최종적으로 다양한 실험과 그 결과들을 통해 플랫폼의 완전구동 비행 능력을 검증한다. 추가적으로 본 논문에서는 완전구동 멀티로터가 가지는 여분의 제어입력(redundancy)를 활용한 쿼드콥터의 단일모터 고장 대비 비상 비행 기법을 소개한다. 비상 비행 전략에 대한 자세한 소개 및 실현 방법, 비상 비행 시의 동역학적 특성에 대한 분석 결과가 소개되며, 실험결과를 통해 제안된 기법의 타당성을 검증한다.Recently, multi-rotor unmanned aerial vehicles (UAVs) are used for a variety of missions beyond its basic flight, including aerial manipulation, aerial payload transportation, and aerial sensor platform. Following this trend, the multirotor UAV is recognized as a versatile aerial robotics platform that can freely mount and fly the necessary mission equipment and sensors to perform missions. However, the current multi-rotor platform has a relatively poor ability to maintain nominal flight performance against external disturbances such as wind or gust compared to other robotics platforms. Also, the multirotor suffers from maintaining a stable payload attitude, due to the fact that the attitude of the fuselage should continuously be changed for translational motion control. Particularly, unstabilized fuselage attitude can be a drawback for multirotor's mission performance in such cases as like visual odometry-based flight, since the fuselage-attached sensor should also be tilted during the flight and therefore causes poor sensor information acquisition. To overcome the above two problems, in this dissertation, we introduce a robust multirotor control method and a novel full-actuation mechanism which widens the usability of the multirotor. The goal of the proposed control method is to bring robustness to the translational motion control against various weather conditions. And the goal of the full actuation mechanism is to allow the multi-rotor to take arbitrary payload/fuselage attitude independently of the translational motion. For robust multirotor control, we first introduce a translational force generation technique for accurate translational motion control and then discuss the design method of disturbance observer (DOB)-based robust control algorithm. The stability of the proposed feedback controller is validated by the mu-stability analysis technique, and the results are compared to the small-gain theorem (SGT)-based stability analysis to validate the rigorousness of the analysis. Through the experiments, we validate the translational acceleration control performance of the developed controller and confirm the robustness against external disturbance forces. For a fully-actuated multirotor platform, we propose a new mechanism called a T3-Multirotor that can overcome the excessive weight increase and poor energy efficiency of the existing fully-actuated multirotor. The structure of the new platform is designed to be as close as possible to the existing multi-rotor and includes only two servo motors for full actuation. The dynamic characteristics of the new platform are analyzed and a six-degree-of-freedom (DOF) flight controller is designed based on the derived equations of motion. The full actuation of the proposed platform is then validated through various experiments. As a derivative study, this paper also introduces an emergency flight technique to prepare for a single motor failure scenario of a multi-rotor using the redundancy of the T3-Multirotor platform. The detailed introduction and implementation method of the emergency flight strategy with the analysis of the dynamic characteristics during the emergency flight is introduced, and the experimental results are provided to verify the validity of the proposed technique.1 Introduction 1 1.1 Motivation 1 1.2 Literature survey 3 1.2.1 Robust translational motion control 3 1.2.2 Fully-actuated multirotor platform 4 1.3 Research objectives and contributions 5 1.3.1 Goal #I: Robust multirotor motion control 5 1.3.2 Goal #II: A new fully actuated multirotor platform 6 1.3.3 Goal #II-A: T3-Multirotor-based fail-safe flight 7 1.4 Thesis organization 7 2 Multi-Rotor Unmanned Aerial Vehicle: Overview 9 2.1 Platform overview 9 2.2 Mathematical model of multi-rotor UAV 10 3 Robust Translational Motion Control 13 3.1 Introduction 14 3.2 Translational force/acceleration control 14 3.2.1 Relationship between \mathbf{r} and \tilde{\ddot{\mathbf{X}}} 15 3.2.2 Calculation of \mathbf{r}_d from \tilde{\ddot{\mathbf{X}}}_d considering dynamics 16 3.3 Disturbance observer 22 3.3.1 An overview of the disturbance-merged overall system 22 3.3.2 Disturbance observer 22 3.4 Stability analysis 26 3.4.1 Modeling of P(s) considering uncertainties 27 3.4.2 \tau-determination through \mu-analysis 30 3.5 Simulation and experimental result 34 3.5.1 Validation of acceleration tracking performance 34 3.5.2 Validation of DOB performance 34 4 Fully-Actuated Multirotor Mechanism 39 4.1 Introduction 39 4.2 Mechanism 40 4.3 Modeling 42 4.3.1 General equations of motion of TP and FP 42 4.3.2 Simplified equations of motion of TP and FP 46 4.4 Controller design 49 4.4.1 Controller overview 49 4.4.2 Independent roll and pitch attitude control of TP and FP 50 4.4.3 Heading angle control 54 4.4.4 Overall control scheme 54 4.5 Simulation result 56 4.5.1 Scenario 1: Changing FP attitude during hovering 58 4.5.2 Scenario 2: Fixing FP attitude during translation 58 4.6 Experimental result 60 4.6.1 Scenario 1: Changing FP attitude during hovering 60 4.6.2 Scenario 2: Fixing FP attitude during translation 60 4.7 Applications 63 4.7.1 Personal aerial vehicle 63 4.7.2 High MoI payload transportation platform - revisit of [1] 63 4.7.3 Take-off and landing on an oscillating landing pad 64 5 Derived Research: Fail-safe Flight in a Single Motor Failure Scenario 67 5.1 Introduction 67 5.1.1 Related works 68 5.1.2 Contributions 68 5.2 Mechanism and dynamics 69 5.2.1 Mechanism 69 5.2.2 Platform dynamics 70 5.3 Fail-safe flight strategy 75 5.3.1 Fail-safe flight method 75 5.3.2 Hardware condition for single motor fail-safe flight 80 5.4 Controller design 83 5.4.1 Faulty motor detection 83 5.4.2 Controller design 84 5.4.3 Attitude dynamics in fail-safe mode 86 5.5 Experiment result 90 5.5.1 Experimental settings 90 5.5.2 Stability and control performance review 92 5.5.3 Flight results 93 6 Conclusions 96 Abstract (in Korean) 107Docto

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

Design and control of next-generation uavs for effectively interacting with environments

In this dissertation, the design and control of a novel multirotor for aerial manipulation is studied, with the aim of endowing the aerial vehicle with more degrees of freedom of motion and stability when interacting with the environments. Firstly, it presents an energy-efficient adaptive robust tracking control method for a class of fully actuated, thrust vectoring unmanned aerial vehicles (UAVs) with parametric uncertainties including unknown moment of inertia, mass and center of mass, which would occur in aerial maneuvering and manipulation. The effectiveness of this method is demonstrated through simulation. Secondly, a humanoid robot arm is adopted to serve as a 6-degree-of-freedom (DOF) automated flight testing platform for emulating the free flight environment of UAVs while ensuring safety. Another novel multirotor in a tilt-rotor architecture is studied and tested for coping with parametric uncertainties in aerial maneuvering and manipulation. Two pairs of rotors are mounted on two independently-controlled tilting arms placed at two sides of the vehicle in a H configuration to enhance its maneuverability and stability through an adaptive robust control method. In addition, an impedance control algorithm is deployed in the out loop that modifies the trajectory to achieve a compliant behavior in the end-effector space for aerial drilling and screwing tasks

Force-Canceling Mixer Algorithm for Vehicles with Fully-Articulated Radially Symmetric Thruster Arrays

A new type of fully-holonomic aerial vehicle is identified and developed that can optionally utilize automatic cancellation of excessive thruster forces to maintain precise control despite little or no throttle authority. After defining the physical attributes of the new vehicle, a flight control mixer algorithm is defined and presented. This mixer is an input/output abstraction that grants a flight control system (or pilot) full authority of the vehicle\u27s position and orientation by means of an input translation vector and input torque vector. The mixer is shown to be general with respect to the number of thrusters in the system provided that they are distributed in a radially symmetric array. As the mixer is designed to operate independently of the chosen flight control system, it is completely agnostic to the type of control methodology implemented. Validation of both the vehicle\u27s holonomic capabilities and efficacy of the flight control mixing algorithm are provided by a custom MATLAB-based rigid body simulation environment

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

Rotor-Rotor Interactions in the Design of Unmanned Aerial Systems

This dissertation investigates the impact of rotor-rotor interactions on small Unmanned Aerial System (UAS) design. This work explores the aerodynamic effects of two rotor configurations, the first being non-coplanar overlapping rotors, tandem-rotors, and the second being the semi-coaxial rotor configuration, which is an adaptation of the traditional coaxial rotor configuration. This work is motivated by three UAS, two of which, the Tetracopter and the Dodecacopter, are designed and developed as a part of the work presented in this dissertation. The Tetracopter and Dodecacopter are multi-agent vehicles that implement multiple layers of non-coplanar overlapping rotors. The goal of these two vehicles is to implement a design where a multi-agent UAS can have the structural rigidity to withstand carrying payloads, whether the payload is carried above or below the vehicle while being as efficient as a multi-agent aircraft with coplanar rotors. The goal of the Y6sC is to show that the semi-coaxial rotor configuration allows a vehicle to be more efficient in hover than a traditional coaxial rotor configuration and that the semi-coaxial rotor configuration grants the vehicle more maneuverability than a traditional coaxial rotor configuration. This dissertation can be separated into two halves; the first half begins with the presentation of a thrust stand fabricated to collect data on both rotor configurations. This half also discusses the methods used to conduct these thrust stand experiments, the methods used to analyze the data, and discussions about the results and their comparison to established theories that predict the performance of these rotor configurations. A rotor configuration performance estimation method that is based on the empirical data collected is also presented, and the accuracy of this estimation method is validated. This estimation method is then used to estimate the optimal design of the Tetracopter and Dodecacopter, which accounts for the vehicle's weight and the performance of the vehicle's rotors which may be impacted by rotor-rotor interactions. The latter half of this dissertation discusses the design of the Dodecacopter along with the methods used to flight test the vehicle. The data produced from the flight tests are discussed, and estimations of the degradation in the vehicle's performance due to the rotor-rotor interactions are presented and discussed. The dissertation concludes with a brief discussion on the design implications derived from the results of the work presented.Ph.D