CONTROL STRATEGY OF MULTIROTOR PLATFORM UNDER NOMINAL AND FAULT CONDITIONS USING A DUAL-LOOP CONTROL SCHEME USED FOR EARTH-BASED SPACECRAFT CONTROL TESTING

Over the last decade, autonomous Unmanned Aerial Vehicles (UAVs) have seen increased usage in industrial, defense, research, and academic applications. Specific attention is given to multirotor platforms due to their high maneuverability, utility, and accessibility. As such, multirotors are often utilized in a variety of operating conditions such as populated areas, hazardous environments, inclement weather, etc. In this study, the effectiveness of multirotor platforms, specifically quadrotors, to behave as Earth-based satellite test platforms is discussed. Additionally, due to concerns over system operations under such circumstances, it becomes critical that multirotors are capable of operation despite experiencing undesired conditions and collisions which make the platform susceptible to on-board hardware faults. Without countermeasures to account for such faults, specifically actuator faults, a multirotors will experience catastrophic failure. In this thesis, a control strategy for a quadrotor under nominal and fault conditions is proposed. The process of defining the quadrotor dynamic model is discussed in detail. A dual-loop SMC/PID control scheme is proposed to control the attitude and position states of the nominal system. Actuator faults on-board the quadrotor are interpreted as motor performance losses, specifically loss in rotor speeds. To control a faulty system, an additive control scheme is implemented in conjunction with the nominal scheme. The quadrotor platform is developed via analysis of the various subcomponents. In addition, various physical parameters of the quadrotor are determined experimentally. Simulated and experimental testing showed promising results, and provide encouragement for further refinement in the future

Nonlinear robust control of tail-sitter aircrafts in flight mode transitions

© 2018 Elsevier Masson SAS In this paper, a nonlinear robust controller is proposed to deal with the flight mode transition control problem of tail-sitter aircrafts. During the mode transitions, the control problem is challenging due to the high nonlinearities and strong couplings. The tail-sitter aircraft model can be considered as a nominal part with uncertainties including nonlinear terms, parametric uncertainties, and external disturbances. The proposed controller consists of a nominal H∞controller and a nonlinear disturbance observer. The nominal H∞controller based on the nominal model is designed to achieve the desired trajectory tracking performance. The uncertainties are regarded as equivalent disturbances to restrain their influences by the nonlinear disturbance observer. Theoretical analysis and simulation results are given to show advantages of the proposed control method, compared with the standard H∞control approach

Bio-Inspired Hovering Control for an Aerial Robot Equipped with a Decoupled Eye and a Rate Gyro

International audienceThis work provides an hovering control strategy for a sighted robot, the eye of which being decoupled from the body and controlled by means of a tiny rotative piezo motor. The main purpose of this paper is to show the effectiveness and the efficiency of this fundamental bio-inspired mechanical decoupling. Indeed, it exhibits several benefits: * it enables to stabilize the robot's gaze on the basis of three bio-inspired oculomotor reflexes (ORs) : a visual fixation reflex (VFR), a translational and rotational vestibulo- ocular reflexes (tVOR and rVOR), * the eye can better, quickly and accurately compensate for sudden, untoward disturbances caused by the vagaries of the supporting head or body, * it yields a reference visual signal that can be used to unbias the rate gyro used to implement the VORs and to stabilize the hovering robot, * it increases the tracking accuracy with moving targets compared to without OR, This paper shows also that lateral disturbances are rejected 2 times faster with the decoupled eye robot, and roll perturbations induce a retinal error 20 times smaller. The occulomotor reflexes enables to cancel retinal error 6 times faster with 5 times lower retinal error picks. The conclusion of the paper is that decoupled eye must be considered as an efficient autonomous flight solution

Hybrid active force control for fixed based rotorcraft

Disturbances are considered major challenges faced in the deployment of rotorcraft unmanned aerial vehicle (UAV) systems. Among different types of rotorcraft systems, the twin-rotor helicopter and quadrotor models are considered the most versatile flying machines nowadays due to their range of applications in the civilian and military sectors. However, these systems are multivariate and highly non-linear, making them difficult to be accurately controlled. Their performance could be further compromised when they are operated in the presence of disturbances or uncertainties. This dissertation presents an innovative hybrid control scheme for rotorcraft systems to improve disturbance rejection capability while maintaining system stability, based on a technique called active force control (AFC) via simulation and experimental works. A detailed dynamic model of each aerial system was derived based on the Euler–Lagrange and Newton-Euler methods, taking into account various assumptions and conditions. As a result of the derived models, a proportional-integral-derivative (PID) controller was designed to achieve the required altitude and attitude motions. Due to the PID's inability to reject applied disturbances, the AFC strategy was incorporated with the designed PID controller, to be known as the PID-AFC scheme. To estimate control parameters automatically, a number of artificial intelligence algorithms were employed in this study, namely the iterative learning algorithm and fuzzy logic. Intelligent rules of these AI algorithms were designed and embedded into the AFC loop, identified as intelligent active force control (IAFC)-based methods. This involved, PID-iterative learning active force control (PID-ILAFC) and PID-fuzzy logic active force control (PID-FLAFC) schemes. To test the performance and robustness of these proposed hybrid control systems, several disturbance models were introduced, namely the sinusoidal wave, pulsating, and Dryden wind gust model disturbances. Integral square error was selected as the index performance to compare between the proposed control schemes. In this study, the effectiveness of the PID-ILAFC strategy in connection with the body jerk performance was investigated in the presence of applied disturbance. In terms of experimental work, hardware-in-the-loop (HIL) experimental tests were conducted for a fixed-base rotorcraft UAV system to investigate how effective are the proposed hybrid PID-ILAFC schemes in disturbance rejection. Simulated results, in time domains, reveal the efficacy of the proposed hybrid IAFC-based control methods in the cancellation of different applied disturbances, while preserving the stability of the rotorcraft system, as compared to the conventional PID controller. In most of the cases, the simulated results show a reduction of more than 55% in settling time. In terms of body jerk performance, it was improved by around 65%, for twin-rotor helicopter system, and by a 45%, for quadrotor system. To achieve the best possible performance, results recommend using the full output signal produced by the AFC strategy according to the sensitivity analysis. The HIL experimental tests results demonstrate that the PID-ILAFC method can improve the disturbance rejection capability when compared to other control systems and show good agreement with the simulated counterpart. However, the selection of the appropriate learning parameters and initial conditions is viewed as a crucial step toward this improved performance

Integrated fault-tolerant control for a 3-DOF helicopter with actuator faults and saturation

© The Institution of Engineering and Technology 2017. This study proposes a fault estimation (FE)-based fault-tolerant control (FTC) strategy to maintain system reliability and achieve desirable control performance for a 3-DOF helicopter system with both actuator drift and oscillation faults and saturation. The effects of the faults and saturation are combined into a composite non-differentiable actuator fault function, which is approximated by a differentiable function and estimated together with the system state using a non-linear unknown input observer. An adaptive sliding mode controller based on the estimates is developed to compensate the effects of the faults and saturation. Taking into account the bi-directional robustness interactions between the FE and FTC functions, an integrated design approach is proposed to obtain the observer and controller gains in a single step, so as to achieve robust overall FTC system performance. In fault-free cases, the proposed strategy can be considered as a new approach for anti-windup control to compensate the effect of input saturation. Comparative simulations are provided to verify the effectiveness of the proposed design under different actuator fault scenarios

A flexible mixed-optimization with H∞ control for coupled twin rotor MIMO system based on the method of inequality (MOI)- An Experimental Study

This article introduces a cutting-edge H∞ model-based control method for uncertain Multi Input Multi Output (MIMO) systems, specifically focusing on UAVs, through a flexible mixed-optimization framework using the Method of Inequality (MOI). The proposed approach adaptively addresses crucial challenges such as unmodeled dynamics, noise interference, and parameter variations. Central to the design is a two-step controller development process. The first step involves Nonlinear Dynamic Inversion (NDI) and system decoupling for simplification, while the second step integrates H∞ control with MOI for optimal response tuning. This strategy is distinguished by its adaptability and focus on balancing robust stability and performance, effectively managing the intricate cross-coupling dynamics in UAV systems. The effectiveness of the proposed approach is validated through simulations conducted in MATLAB/Simulink environment. Results demonstrated the efficiency of the proposed robust control approach as evidenced by reduced steady-state error, diminished overshoot, and faster system response times, thus significantly outperforming traditional control methods

Decoupling the Eye: A Key toward a Robust Hovering for Sighted Aerial Robots

International audienceInspired by natural visual systems where gaze stabilization is at a premium, we simulated an aerial robot with a decoupled eye to achieve more robust hovering above a ground target despite strong lateral and rotational disturbances. In this paper, two different robots are compared for the same disturbances and displacements. The first robot is equipped with a fixed eye featuring a large field-of-view (FOV) and the second robot is endowed with a decoupled eye featuring a small FOV (about ±5°). Even if this mechanical decoupling increases the mechanical complexity of the robot, this study demonstrates that disturbances are rejected faster and the computational complexity is clearly decreased. Thanks to bio-inspired visuo-motor reflexes, the decoupled eye robot is able to hold its gaze locked onto a distant target and to reject strong disturbances by profiting of the small inertia of the decoupled eye

Assisting dependent people at home through autonomous unmanned aerial vehicles

This work describes a proposal of autonomous unmanned aerial vehicles (AUAVs) for home assistance of dependent people. AUAVs will monitor and recognize human activities during flight to improve their quality of life. However, before bringing such AUAV assistance to real homes, several challenges must be faced to make them viable and practical. Some challenges are technical and some others are related to human factors. In particular, several technical aspects are described for AUAV assistance: (1) flight control, based on our active disturbance rejection control algorithm, (2) flight planning (navigation in obstacle environments), and, (3) processing signals, acquired both from flight-control and monitoring sensors. From the assisted person’s viewpoint, our research focuses on three cues: (1) the user’s perception about AUAV assistance, (2) the influence on human acceptance of AUAV appearance and behavior at home, and (3) the human-robot interaction between assistant AUAV and assisted person. Finally, virtual reality environments are proposed to carry out preliminary tests and user acceptance evaluations.This work has been partially supported by Spanish Ministerio de Ciencia, Innovación y Universidades, Agencia Estatal de Investigaci´on (AEI) / European Regional Development Fund (FEDER, UE) under DPI2016-80894-R grant, and by CIBERSAM of the Instituto de Salud Carlos III. Lidia M. Belmonte holds FPU014/05283 scholarship from Spanish Ministerio de Educaci´on y Formación Profesional

멀티로터형 비행체의 공력소음: 비행 제어 시스템과 공기역학적 상호작용의 영향

학위논문(박사) -- 서울대학교대학원 : 공과대학 항공우주공학과, 2022. 8. 이수갑.Multirotor configurations using a distributed electric propulsion (DEP) system have different aerodynamic and aeroacoustic characteristics from conventional rotorcrafts. Generally, DEP systems use electric motors to control the rotational speed (revolutions per minute, RPM) of individual rotors to perform flight control. Besides, aerodynamic interactions between multiple rotors occur significantly. The main objective of this study is prediction-based evaluations of RPM-controlled multirotor noise. Therefore, three numerical studies are conducted from the perspective of the flight control system and aerodynamic interactions. First, a comprehensive multirotor noise assessment (CONA) framework is developed for real-time noise prediction and psychoacoustic analyses of RPM-controlled multirotor configurations. The CONA framework utilizes flight control, aerodynamics, tonal and broadband noise prediction, and psychoacoustics modules. By this framework, it is possible to conduct the real-time noise assessment in actual flight environments considering the mission profile and gusty wind conditions. A high-resolution time-frequency analysis technique is introduced to analyze the frequency and amplitude modulation characteristics of rotor tonal noise. The Griffin-Lim algorithm is used for the phase reconstruction for time signal synthesis of predicted rotor broadband noise in the 1/3 octave band. Using the CONA framework, the noise of quadrotor configurations is analyzed in representative mission profiles, such as cruise, takeoff, and loitering flights. As flight parameters, flight speed, wind speed, and quadrotor flight type are selected, and the effects of each parameter on acoustic signatures are evaluated. Second, wake interaction effects of multirotor configurations are analyzed by developing the MultiPA framework based on the free-wake vortex lattice method. The aerodynamic and aeroacoustic performance of individual rotors is compared with that of a single rotor with RPM, forward velocity, and incidence angle as variables in two flight types of the quadrotor. Besides, induced circulation is introduced to analyze wake interactions quantitatively. Wake interaction effects are divided into wake-, rotor-, and motion-induced circulation. By circulation analyses, it is quantitatively confirmed that wake effects depend on the flight conditions and rotor topology. Finally, numerical techniques are developed to simulate the torque ripple in the hovering flight of multirotor configurations. In the MultiPA framework, a periodic RPM signal is applied to the numerical analysis. In the CONA framework, a statistical technique that introduces a periodic random RPM signal is used to implement uncertainties in torque ripple numerically. Based on the results of each framework, the effects of torque ripple are illustrated in aerodynamic and aeroacoustic characteristics. The implications can be derived that torque ripple should be considered in the noise assessment of RPM-controlled multirotor configurations using an electric motor. The frameworks developed in this study are specialized in analyzing the unique aerodynamic and aeroacoustic characteristics according to the flight control system and wake interaction effects of DEP systems. The entire process of the CONA framework can be utilized for various multirotor configurations to perform real-time noise prediction and noise impact assessment. The MultiPA framework and induced circulation concepts can be utilized to analyze the wake interaction effect and develop efficient wake models of multirotor configurations. This study illustrates the effects of the flight control system and wake interactions from various perspectives. It is expected that the research of low-noise and high-efficient urban air mobility will be possible through perception-based evaluations using the developed frameworks.멀티로터형 비행체는 분산 전기 추진(Distributed electric propulsion, DEP) 시스템을 활용하여 기존 회전익기와는 다른 공력 및 공력 소음 특성을 가진다. 일반적으로 DEP 시스템은 전기 모터를 이용하여 개별 로터의 회전속도(revolutions per minute, RPM)를 제어하여 비행 제어를 수행하며, 다수의 로터 사이에 공기역학적 상호작용이 뚜렷하게 발생한다. 본 연구의 주된 목적은 RPM 제어 멀티로터 소음의 해식 기반 평가이다. 따라서, 세 가지 수치적 연구가 비행 제어 시스템과 공기역학적 상호작용의 측면에서 수행되었다. 먼저, RPM 제어 멀티로터의 실시간 소음 예측과 심리음향학적 분석을 위한 CONA (Comprehensive multirotor noise assessment) 프레임워크를 개발하였다. CONA 프레임워크는 비행 제어, 공기역학, 로터 톤 및 광대역 소음 해석, 심리음향 해석 모듈을 활용하며, 멀티로터형 비행체의 임무 형상과 대기 바람 조건을 부여한 실제 비행 환경에서의 실시간 소음 해석이 가능하다. 로터 톤 소음의 주파수 및 진폭 변조 특성을 분석하기 위해 고해상도 시간-주파수 분석 기법을 도입하였고, 1/3 옥타브 밴드로 해석되는 로터 광대역 소음을 시간 신호로 변환하기 위하여 그리핀-림 알고리즘을 활용한 음원 합성을 수행하였다. 개발한 CONA 프레임워크로 대표적인 임무 형상인 순항, 수직 이륙, 선회 비행에서 쿼드로터의 소음 특성을 분석하였다. 비행 변수로 비행 속도, 바람 속도, 쿼드로터 비행 타입을 선정하여 각 변수의 공력 소음 특성에 대한 영향을 평가하였다. 두 번째로, 자유 후류 와류 격자 기법 기반의 MultiPA 프레임워크를 개발하여, 멀티로터형 비행체의 후류 상호작용 효과를 분석하였다. 쿼드로터의 두 가지 비행 타입에서 RPM, 전진 속도, 전진각을 변수로 하여 개별 로터의 공력 및 공력 소음 성능을 단일 로터와 비교하였다. 또한, 후류 상호작용 효과를 정량적으로 분석할 수 있는 유도 순환 지표를 도입하였다. 후류 상호작용 효과는 유도 순환 지표를 통해서 후류-유도 순환, 로터-유도 순환, 그리고 로터의 구동에 따른 순환으로 구분되며, 비행 조건과 로터 배치에 따라 후류 영향이 달라짐을 정량적으로 확인하였다. 마지막으로, 멀티로터형 비행체의 제자리 비행 시 발생하는 토크 리플의 수치적 모사 기법을 고안하였다. MultiPA 프레임워크에서는 주기적인 RPM 신호를 해석에 적용하였고, CONA 프레임워크에서는 불확실성이 강한 토크 리플을 수치적으로 구현할 수 있도록 주기적인 무작위 RPM 신호를 도입한 통계적 기법을 활용하였다. 각각의 해석 결과를 바탕으로 토크 리플에 의한 공력 및 공력 소음 특성을 분석하였고, 전기 모터를 활용한 RPM 제어 비행체의 소음 평가 시 토크 리플을 고려해야 한다는 시사점을 도출하였다. 본 연구에서 개발한 프레임워크들은 DEP 시스템의 비행 제어 시스템과 후류 상호작용 효과에 따른 독특한 공력 및 공력 소음 특성을 분석하는 데 특화되어 있다. CONA 프레임워크의 전체 해석 프로세스는 다양한 멀티로터형 비행체에 활용되어 실시간 소음 해석과 소음 영향 평가를 수행할 수 있다. MultiPA 프레임워크와 유도 순환 지표는 후류 상호작용의 효과를 분석하고, 멀티로터형 비행체의 효율적인 후류 모델을 개발하는 데 활용될 수 있다. 본 연구에서는 다양한 관점으로 비행 제어 시스템과 후류 상호작용 효과를 분석하였으며, 개발한 프레임워크를 활용해 인지-기반 평가를 통한 저소음 고효율 도심 항공 모빌리티의 연구를 수행할 수 있을 것으로 기대된다.1 Introduction 1 1.1 Background 1 1.1.1 Multirotor configurations 1 1.1.2 Noise assessment of novel aerial vehicles 3 1.2 Frequency-modulated multirotor noise 5 1.2.1 Frequency and amplitude modulation 5 1.2.2 Time-frequency analysis of the noise signal 7 1.2.3 Perception-influenced evaluation of aerial vehicles 8 1.3 Wake interactions in multirotor configurations 9 1.3.1 Wake interaction phenomena 9 1.3.2 Previous research on wake interactions 10 1.4 Research objectives and scope 12 1.4.1 Real-time noise prediction 13 1.4.2 Wake interactions in multirotor configurations 15 1.5 Dissertation organization 16 2 Real-time noise prediction framework 19 2.1 Flight control module 21 2.2 Aerodynamics module 22 2.2.1 HBEM with aerodynamic models 22 2.2.2 Beddoes wake models 24 2.2.3 Unsteady aerodynamic corrections 26 2.3 Time reconstruction module 28 2.4 Tonal noise prediction module 29 2.5 Time-frequency analysis module 31 2.6 Broadband noise prediction module 34 2.6.1 Semi-empirical model 34 2.6.2 Amiet's theory with wall-pressure spectrum models 35 2.7 Phase reconstruction module 36 2.7.1 Step 1: Narrowband spectrogram synthesis 36 2.7.2 Step 2: Time signal synthesis 37 2.8 Psychoacoustics module 38 3 Free-wake vortex lattice method solver 41 3.1 Aerodynamic and aeroacoustic solver 41 3.1.1 Free-wake vortex lattice method 41 3.1.2 Additional aerodynamic models 43 3.1.3 Acoustic analogy 44 3.2 Quantification factors for wake interaction 45 3.2.1 Wake interaction relations 45 3.2.2 Concepts of induced circulation 46 3.2.3 Quantification factors derived by induced circulation 47 4 Verification, validation, and numerical setup 49 4.1 CONA framework verification and validation: Flight control, aerodynamics, and tonal noise 49 4.1.1 UAV: Single rotor hovering flight 49 4.1.2 UAV: Single rotor forward flight 52 4.1.3 UAV: Quadrotor forward flight 57 4.1.4 UAM: Quadrotor forward flight 63 4.2 CONA framework verification and validation: Broadband noise and psychoacoustics 66 4.2.1 UAV: Airfoil self-noise 66 4.2.2 UAV: Single rotor hovering flight 69 4.2.3 UAV: Single rotor forward flight 73 4.2.4 UAV: Quadrotor forward flight 75 4.2.5 UAV: Quadrotor hovering flight 77 4.3 Free-wake vortex lattice method solver 79 4.3.1 Reference model for wake interaction analyses 79 4.3.2 Test matrix and target outputs 82 4.3.3 Solver validation 83 4.4 Torque ripple modeling 87 4.4.1 Sinusoidal RPM signal approach 89 4.4.2 Random periodic RPM signal approach 90 5 Frequency-modulated multirotor noise 93 5.1 Flight simulation for quadrotor configurations 93 5.1.1 Mission profile and numerical settings 93 5.1.2 Flight control results 96 5.2 High-resolution time-frequency analyses 101 5.2.1 Flyover noise 102 5.2.2 Takeoff noise 104 5.2.3 Loitering noise 107 5.3 Prediction-based psychoacoustic analyses 109 5.3.1 Auralization process 109 5.3.2 Flyover noise 114 5.3.3 Takeoff noise 119 5.3.4 Loitering noise 122 6 Wake interactions in multirotor configurations 125 6.1 Wake interactions in quadrotor hovering flight 125 6.2 Performance of quadrotor forward flight 129 6.2.1 Aerodynamic and aeroacoustic performance 129 6.2.2 Comparison of polynomial regression 133 6.3 Physics of quadrotor forward flight 140 6.3.1 Wake dynamics of quadrotor configurations 140 6.3.2 Distribution of induced circulation 146 6.3.3 Temporal characteristics of wake interaction 161 7 Torque ripple modeling 167 7.1 Sinusoidal RPM signal approach 167 7.1.1 Aerodynamic characteristics 167 7.1.2 Aeroacoustic characteristics 173 7.1.3 Effects of the angular frequency of RPM variations 176 7.2 Random periodic RPM signal approach 180 7.2.1 Effects on noise spectrum 180 7.2.2 Effects on noise directivity 182 8 Conclusions and recommendations 185 8.1 Conclusions 185 8.2 Recommendations for future work 187 8.2.1 Applications of the CONA framework 187 8.2.2 Applications of the MultiPA framework 189 Bibliography 191 Appendix A Control outputs of the CONA framework 205 국문초록 215박