471 research outputs found

    Study of TADF Emitters in OLEDs

    Get PDF
    Delayed fluorescence through thermally activated delayed fluorescence (TADF) has great potential for the creation of inexpensive and highly efficient white lighting applications, with superior colour rendering. Currently the highest external quantum efficiencies are achieved with small donor-acceptor-donor molecules utilising intramolecular charge transfer (ICT) states, and these molecules require a suitable host matrix to reside in. This thesis studies the effect of host material on the model molecule 2d, a proven efficient TADF emitter through diligent photophysical investigation. A combination of steady state and nanosecond time resolved spectroscopic studies confirm the importance of a high host triplet level to ensure that the ICT state is the lowest energy excited state to avoid high levels of quenching. More interestingly it is shown that the functional group combination of emitter and host is crucial in achieving efficient TADF in OLED devices. In particular combinations where both the host and dopant are carbazole-based should be avoided due to the formation of carbazole dimer. The effect of such dimerisation is to lower the host triplet level significantly, and further to deactivate the ability of the 2d dopant to produce the ICT state required for TADF by locking the 2d dopant in the ‘planar’ configuration. It is therefore clear that the chemical composition of the host is of critical importance for the design of future OLED devices. Experiment also suggests that there is a complex interplay between exciplex and ICT emission in 2d systems in the solid state, insofar as CT emission of any description has so far only been observed in conditions where exciplex can and does occur

    Invariance positive et observateur intervalles appliqués aux systèmes linéaires à retards sous contraintes

    Get PDF
    RÉSUMÉ Cette thèse porte sur l’étude du problème de la commande avec contraintes des systèmes linéaires continus à retards. Deux approches dans la littérature se sont données à développer des méthodes adéquates pour examiner la stabilité et contribuer à des procédures et outils de stabilisation. La première, considère l’effet de la saturation, tandis que la deuxième approche, basée sur la théorie d’invariance positive, repose principalement sur la conception d’une loi de contrôle non saturante ayant un comportement linéaire dans le domaine des contraintes. Des résultats concernant l’application du concept d’invariance positive à la stabilisation des systèmes à retards soumis à des contraintes ont été développés, mais restent restrictifs, de fait qu’ils sont indépendants du retard, un paramètre essentiel du système. On développe alors, dans la première partie de cette thèse des conditions nécessaires et suffisantes dépendantes du retard afin de garantir l’invariance positive de domaine des contraintes par rapport aux trajectoires de systèmes autonomes à retard. Ce résultat repose sur la transformation de premier ordre, basée sur la formule de Newton-Leibniz, de système original à retard discret, en un système à retard distribué. Une fonction de Lyapunov-Razumikhin associée au système à retard distribué garantissant la stabilité asymptotique dépendante de retard de système original est proposée. L’objectif principal visé dans la deuxième partie de cette thèse est d’appliquer le résultat du concept d’invariance positive dépendante du retard au problème de la commande sous contraintes, dissymétriques ainsi que symétriques, des systèmes à retards. Ainsi des conditions permettant la synthèse d’un régulateur par retour d’état stabilisant le système en boucle fermée en présence des contraintes, sont données. Ces conditions permettent de formuler un algorithme basé sur des schémas de Programmation Non Linéaire (NLP), ayant pour objectif la détermination du régulateur stabilisant le système en boucle fermée avec une borne maximale du retard. En effet la loi de retour d’état calculée assure, d’une part, la stabilité asymptotique de système sans retard, et d’autre part, la maintenir pour une valeur d’une borne maximale de retard, tout en respectant les contraintes : c’est la loi de commande sous contraint robuste vis à vis le retard. Les résultats obtenus sont intéressants et plus généraux que ceux développés dans la littérature. La troisième partie de cette thèse montre, pour la première fois à notre connaissance, que les observateurs intervalles, en appliquant le concept d’invariance positive, peuvent apporter des réponses intéressantes au problème de la commande sous contraintes des systèmes linéaires à retards, variable dans le temps.----------ABSTRACT In this thesis, the stabilization problem of linear continuous-time delay system with constrainted control is studied. There are two main approaches in the literature dealing with the problem of performance and stability of dynamical constrained control systems. The first one considers the effect of saturation while guaranteeing asymptotic stability. The second one, so-called positive invariance approach, is based on the design of the control law which works inside a region of linear behavior where saturations do not occur. Most of the works related to positive invariance concept have been developed for time delay systems with constrained control, but remain so restrictive, given that they are independent of delay, which is an essential parameter of the system. In the first part of this thesis, the necessary and sufficient algebraic conditions with delay dependence allowing to obtain the largest positively invariant set of delay system are given. The results can include information on the size of delay, and therefore, can be delay dependence positively invariant conditions. Based on the Newton-Leibniz formula, these results use a transformation form an original system with discrete delay to a system with distributed delay. A Lyapunov-Razumikhin function for system with distributed delay, in order to guarantee the asymptotic stability of the original system is proposed. The second part of this thesis, is to apply the concept of the delay dependent positive invariance to the robust regulator problem of continuous time delay system with symmetric and non-symmetric constraints. In fact the synthesis of state-feedback controllers is solved based on delay-dependent positively invariant set of system in closed-loop. We first obtain the necessary and sufficient algebraic conditions with delay dependence allowing to obtain the largest positively invariant set of delay systems, then we convert the constrained control problem into a Non-Linear Programming (NLP) problem with delay the objective function to be maximized. Indeed the control is firstly chosen in order to stabilize the closed loop system, free of delay, then to guarantee the asymptotic stability of the closed loop system with delay-dependence. To the best of our knowledge, it is the first time, that the output stabilization problem for time-varying delay systems with constrained control based on the interval observer technique by using the dependent delay positive invariance concept is studied. Hence, first both matrices observer gain, the lower and the upper, are obtained by solving a Sylvester’s matrix equation. Second, the interval observer is developed and guaranteed the positivity of the upper and lower observations errors

    Singular systems with time-varying delays

    Get PDF
    Preliminaries on time-delay singular systems -- Stability of time-delay singular systems -- State feedback controller for time-delay singular systems -- Static output feedback controller for time-delay singular systems with saturating actuators

    A review of convex approaches for control, observation and safety of linear parameter varying and Takagi-Sugeno systems

    Get PDF
    This paper provides a review about the concept of convex systems based on Takagi-Sugeno, linear parameter varying (LPV) and quasi-LPV modeling. These paradigms are capable of hiding the nonlinearities by means of an equivalent description which uses a set of linear models interpolated by appropriately defined weighing functions. Convex systems have become very popular since they allow applying extended linear techniques based on linear matrix inequalities (LMIs) to complex nonlinear systems. This survey aims at providing the reader with a significant overview of the existing LMI-based techniques for convex systems in the fields of control, observation and safety. Firstly, a detailed review of stability, feedback, tracking and model predictive control (MPC) convex controllers is considered. Secondly, the problem of state estimation is addressed through the design of proportional, proportional-integral, unknown input and descriptor observers. Finally, safety of convex systems is discussed by describing popular techniques for fault diagnosis and fault tolerant control (FTC).Peer ReviewedPostprint (published version

    Optimal path-tracking of virtual race-cars using gain-scheduled preview control

    No full text
    In the search for a more capable minimum-lap-time-prediction program, the presence of an alternative solution has been introduced, which requires the development of a high-quality path-tracking controller. Preview Discrete Linear Quadratic Regulator (DLQR) theory has been used to generate optimal tracking control gains for a given car model. The calculation of such gains are performed off-line, reducing the computational burden during simulated tracking trials. A simple car model was used to develop limit-tracking control strategies, first for an understeering and then for an oversteering car, travelling at a constant forward speed. Adaptation in the controller, with respect to front-/rear-lateral-slip ratio, facilitated superior tracking performance over the non-adaptive counterpart in a number of challenging tracking manoeuvres. Once complete, development work was focused on the control of a complex car model. Such a model required an extension to the preview DLQR theory, to allow variable speed, two-channel (x,y) optimal path tracking. Significant benefits were observed when using an adaptive control strategy, firstly scheduling with respect to forward ground speed and then including adaptation with respect to mean front-lateral-slip ratio. A variable weighting strategy was used to suppress oscillations in the tracking controller when operating near the limit of the car. Such a strategy places a higher cost on control effort expenditure, relative to tracking error, as the car approaches the limit of the front axle. Further oscillatory behaviour, due to the presence of lightly-damped eigenmodes, was suppressed by increasing the car’s suspension stiffness and damping parameters. The tracking controller, that has resulted from the work documented by this thesis, has demonstrated high-quality tracking when operating in a number of different scenarios, including lateral limit tracking. Variable speed limit tracking is suggested as the next development step, which will then allow the controller to be implemented in initial learning trials. Successful development of the speed and path optimisers in such trials will complete the development of a novel solution to the minimum lap-time problem

    5th EUROMECH nonlinear dynamics conference, August 7-12, 2005 Eindhoven : book of abstracts

    Get PDF

    5th EUROMECH nonlinear dynamics conference, August 7-12, 2005 Eindhoven : book of abstracts

    Get PDF

    Internally Sensed Optical Phased Arrays

    Get PDF
    The performance of existing ground-based space debris laser ranging systems can be improved by directing more light onto space debris by coherently combining multiple lasers using an optical phased array (OPA). If the power delivered to target is sufficiently high then these systems may also provide the capability to remotely manoeuvre space debris via photon radiation pressure and/or ablation. By stabilising the relative output phase of multiple lasers, OPAs form a coherent optical wave-front in the far field. Since the phase of each laser can be controlled independently, they also have the ability to dynamically manipulate the distribution of optical power in the far field, potentially enabling them to compensate for atmospheric turbulence. This beam-forming functionality, combined with their inherent scalability and high power handling capabilities make OPAs a promising technology for future space debris laser ranging and manoeuvring systems. In this thesis, we describe the iterative development of a high-power compatible internally sensed OPA, which---in contrast to externally sensed OPAs that sense the output phase of each laser externally using free-space optics---relies on the small fraction of light that is reflected back into the fibre at the output of the OPA to stabilise its relative output phase. This allows internally sensed OPAs to be implemented entirely within fibre without any dependence on free-space optics at the output, offering potential advantages over externally sensed techniques when operating in the presence of shock and vibration. A proof-of-concept experiment demonstrated the viability of internal sensing, but also highlighted a number of weaknesses that would affect its utility, specifically in supporting high optical powers greater than 100s of mW. An improved high-power compatible internally sensed OPA was designed to overcome these restrictions by isolating sensitive optical components from high optical powers using asymmetric fibre couplers. This concept was initially demonstrated experimentally using slave lasers offset phase-locked to a single master laser, and then again using fibre amplifiers in a master oscillator power amplifier configuration. The experimental demonstration of the fibre amplifier compatible OPA stabilised the relative output phase of three commercial 15 W fibre amplifiers, demonstrating a root-mean-squared output phase stability of λ/194\lambda/194, and the ability to steer the beam at up to 10 kHz. The internally sensed OPA presented here requires the simultaneous measurement, and control of the phase of each emitter in the OPA. This is accomplished using digitally enhanced heterodyne interferometry and digitally implemented phasemeters, both of which rely heavily on high-speed digital signal processing resources provided by field-programmable gate-arrays
    • …
    corecore