Mathematical modelling, flight control system design and air flow control investigation for low speed UAVs

The demand for unmanned aerial vehicles (UAVs) has increased dramatically in the last decade from reconnaissance missions to attack roles. As their missions become more complex, advances in endurance and manoeuvrability become crucial. Due to the advances in material fabrication, wing morphing can be seen as an ideal solution for UAVs to provide improvements by overcoming the weight drawback. This thesis investigates the area of aircraft design and simulation for low speed UAVs looking at performance enhancements techniques for low speed UAVs, and their effects on the aerodynamic capabilities of the wing. The focus is on both suitable control design and wing morphing techniques based on current research findings. The low speed UAV X-RAE1 is used as the test bed for this investigation and is initially analytically presented as three dimensional body where the equations relate to the forces and moments acting on the UAV. A linearised model for straight flight at different velocities is implemented and validated against a non-linear model. Simulations showed the X-RAE1 to have acceptable stability properties over the design operating range. Control design techniques, linear quadratic regulators (LQR) and H-infinity optimisation with Loop Shaping Design Procedure (LSDP), are used to design simple control schemes for linearised longitudinal model of the X-RAE1 UAV at different velocities. The effectiveness and limitations of the two design methods show that both designs are very fast, with settling times 2-3 seconds in the height response and remarkably low variation of the results at different velocities. Computational fluid dynamics is then used to investigate and simulate the impact of introducing smart effector arrays on a UAV. The smart effector array produces a form of active flow control by providing localised flow field changes. These induced changes have direct impact on the aerodynamic forces and showed a substantial increase of lift at low angles of attack. There was also a significant increase to the lift to drag ratio at high angles of attack which resulted to a delay in stall

Analysis and control of self-sustained instabilities in a cavity using reduced order modelling

On considère un écoulement compressible bidimensionnel, autour d'une cavité ouverte. Des d'instabilité, auto-entretenues par l'effet de rétroaction de l'écrasement de la couche de cisaillement sur le bord aval de la cavité, génèrent des émissions acoustiques qu'il faut réduire. Des simulations numériques directes (DNS) permettent d'obtenir, avec ou sans actionnement, un modèle précis de l'écoulement. A partir des champs issus de la simulation, des décompositions orthogonales de modes propres (POD) sont proposées pour bâtir, par projection de Galerkin sur les équations isentropiques, des modèles d'ordre réduit non linéaires en prenant en compte l'actionnement (le contrôle). Pour éviter la divergence temporelle, les coefficients du système dynamique non forcé sont calibrés par diverses approches originales dont une basée sur la sensiblité modale. A partir du système dynamique forcé par un actionnement multifréquentiel (présent aussi dans les DNS), un contrôle en boucle fermée linéaire quadratique gaussien est proposé sur un système linéarisé. La reconstruction de l'état est basée sur une estimation stochastique linéaire sur 6 points de pression. Le contrôle optimal obtenu s'avère être périodique à la fréquence du second mode de Rossiter, qui est exactement celles des instabilits auto-entretenues dans la cavité. Par introduction de ce contrôle dans les simulations numériques directes, nous avons obtenu une réduction du bruit (faible) sur la fréquence du contrôle. ABSTRACT : We consider a two dimensional compressible flow around an open cavity. The Flow around a cavity is characterised by a self-sustained mechanism in which the shear layer impinges on the downstream edge of the cavity resulting in an acoustic feedback mechanism which must be reduced. Direct Numerical Simulations (DNS) of the flow at a representative Reynolds number has been carried to obtain pressure and velocity fields, both for the case of unactuated and multi frequency actuation. These fields are then used to extract energy ranked coherent structures also called as the Proper Orthogonal Decomposition (POD) modes. A Reduced Order Model is constructed by a Galerkin projections of the isentropic compressible equations. The model is then extended to include the effect of control. To avoid the divergence of the model while integrating in time various calibration techniques has been utillized. A new method of calibration which minimizes a linear functional of error, based on modal sensitivity is proposed. The calibrated low order model is used to design a feedback control of the Linear Quadratic Gaussian (LQG) type, coupled with an observer. For the experimental implementation of the controller, a state estimate based on the observed pressure measurements at 6 different locations, is obtained through a Linear Stochastic Estimation (LSE). The optimal control obtained is periodic with a frequency corresponding to the second Rossiter mode of the cavity. Finally the control obtained is introduced into the DNS to obtain a decrease in spectra of the cavity acoustic mode

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)

Advanced control for miniature helicopters : modelling, design and flight test

Unmanned aerial vehicles (UAV) have been receiving unprecedented development during the past two decades. Among different types of UAVs, unmanned helicopters exhibit promising features gained from vertical-takeoff-and-landing, which make them as a versatile platform for both military and civil applications. The work reported in this thesis aims to apply advanced control techniques, in particular model predictive control (MPC), to an autonomous helicopter in order to enhance its performance and capability. First, a rapid prototyping testbed is developed to enable indoor flight testing for miniature helicopters. This testbed is able to simultaneously observe the flight state, carry out complicated algorithms and realtime control of helicopters all in a Matlab/Simulink environment, which provides a streamline process from algorithm development, simulation to flight tests. Next, the modelling and system identification for small-scale helicopters are studied. A parametric model is developed and the unknown parameters are estimated through the designed identification process. After a mathematical model of the selected helicopter is available, three MPC based control algorithms are developed focusing on different aspects in the operation of autonomous helicopters. The first algorithm is a nonlinear MPC framework. A piecewise constant scheme is used in the MPC formulation to reduce the intensive computation load. A two-level framework is suggested where the nonlinear MPC is combined with a low-level linear controller to allow its application on the systems with fast dynamics. The second algorithm solves the local path planning and the successive tracking control by using nonlinear and linear MPC, respectively. The kinematics and obstacle information are incorporated in the path planning, and the linear dynamics are used to design a flight controller. A guidance compensator dynamically links the path planner and flight controller. The third algorithm focuses on the further reduction of computational load in a MPC scheme and the trajectory tracking control in the presence of uncertainties and disturbances. An explicit nonlinear MPC is developed for helicopters to avoid online optimisation, which is then integrated with a nonlinear disturbance observer to significantly improve its robustness and disturbance attenuation. All these algorithms have been verified by flight tests for autonomous helicopters in the dedicated rapid prototyping testbed developed in this thesis.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Hydrodynamic Control of a Submarine Close to the Sea Surface

To understand the behaviour of a submarine under the influence of surface waves at the early stages of design, the impact on whole boat design, from the perspective of the hydrodynamic shape of the hull, internal arrangements, performance requirements of ballast tanks and pumps and the requirements of control surfaces, a suitable design tool and analysis process is required. The thesis includes outcomes from different engineering disciplines; principally, naval architecture (particularly the specialised areas of submarine hydrodynamics and ocean engineering) and control engineering. The thesis particularly draws upon research from the area of ocean engineering, specifically in the methods of quantifying second order effects, to bring insights into control system design for the problem of submarine control under waves. This is achieved by providing a potential approach for developing control system specifications in reflection of the available assessment methods

Novel Blade Design Strategy to Control the Erosion Aggressiveness of Cavitation

With the reduction in size of turbomachinery systems, cavitation aggressiveness is intensified. Erosion, caused by the repeated collapse of gaseous bubbles in proximity to solid surfaces, occurs at rates that dramatically downgrade the life expectancy of rotating parts. As a result, the compacting strategy, meant to reduce cost and improve efficiency, fails for liquid flows. The research undertaken here proposes a novel design method aimed at controlling the erosion aggressiveness of cavitation. The underlying idea is that the cavity closure shock is a determining factor in the intensity of bubble collapse mechanisms: sharp and high amplitude shocks give rise to strong erosion, while low gradient and low amplitude recoveries reduce the erosive intensity. The working hypothesis is tested here, first, by developing a novel inverse design algorithm capable of handling cavitating flow. The code solves the inviscid Euler equations and models blade cavitation using the Tohoku-Ebara barotropic equation of state. Bespoke preconditioning and multigrid procedures are constructed to handle the large amplitudes in flow regime (from hypersonic in the cavity to very low Mach number in the liquid phase). The inverse solver is then used to produce a set of 2D cascade hydrofoil geometries with smoothed shock profiles at cavity closure. The blades are assessed numerically using both steady state and time-resolved approaches. Both hydrodynamic performance, given in terms of swirl, lift and drag, and cavitation dynamics are evaluated. Recently developed erosion prediction methodologies are implemented and demonstrate compelling correlations between the erosion patterns and shock profile. Finally, experimental testing is carried out using a purposefully developed observation platform. The erosive performance of two of the geometries is measured using the paint removal technique. Results reveal a significant improvement in erosive response for the shock smoothed design, thus confirming the numerical findings as well as the validity of the design hypothesis