643 research outputs found
An improved delay-dependent stabilization criterion of linear time-varying delay systems: An iterative method
summary:This paper presents delay-dependent stabilization criteria for linear time-varying delay systems. A less conservative stabilization criterion is derived by invoking a new Lyapunov-Krasovskii functional and then, extended reciprocally convex inequality in combination with Wirtinger's inequality is exploited to obtain an improved stabilization criterion where a set of nonlinear matrix inequalities is solved by applying the cone complementarity algorithm. The proposed stabilization technique transforms a non-convex problem into a nonlinear trace minimization problem which is solved by an iterative approach. Numerical examples are considered to demonstrate the effectiveness of the proposed stabilization criteria and the presented iterative algorithm outperforms some existing results
robust stabilization using a sampled-data strategy of uncertain neutral state-delayed systems subject to input limitations
Producción CientíficaStabilization of neutral systems with state delay is considered in the presence of uncertainty and input limitations in magnitude. The proposed solution is based on simultaneously characterizing a set of stabilizing controllers and the associated admissible initial conditions through the use of a free weighting matrix approach. From this mathematical characterization, state feedback gains that ensure a large set of admissible initial conditions are calculated by solving an optimization problem with LMI constraints. Some examples are presented to compare the results with previous approaches in the literature.MICINnn DPI2014-54530-
Robust control strategies for unstable systems with input/output delays
Los sistemas con retardo temporal aparecen con frecuencia en el ámbito de la ingeniería, por ejemplo en transmisiones hidráulicas o mecánicas, procesos metalúrgicos o sistemas de control en red. Los retardos temporales han despertado el interés de los investigadores en el ámbito del control desde finales de los años 50. Se ha desarrollado una amplia gama de herramientas para el análisis de su estabilidad y prestaciones, especialmente durante las dos últimas décadas.
Esta tesis se centra en la estabilización de sistemas afectados por retardos temporales en la actuación y/o la medida. Concretamente, las contribuciones que aquí se incluyen tienen por objetivo mejorar las prestaciones de los controladores existentes en presencia de perturbaciones. Los retardos temporales degradan, inevitablemente, el desempeño de un bucle de control. No es de extrañar que el rechazo de perturbaciones haya sido motivo de estudio desde que emergieron los primeros controladores predictivos para sistemas con retardo. Las estrategias presentadas en esta tesis se basan en la combinación de controladores predictivos y observadores de perturbaciones. Estos últimos han sido aplicados con éxito para mejorar el rechazo de perturbaciones de controladores convencionales. Sin embargo, la aplicación de esta metodología a sistemas con retardo es poco frecuente en la literatura, la cual se investiga exhaustivamente en esta tesis.
Otro inconveniente de los controladores predictivos está relacionado con su implementación, que puede llevar a la inestabilidad si no se realiza cuidadosamente. Este fenómeno está relacionado con el hecho de que las leyes de control predictivas se expresan mediante una ecuación integral. En esta tesis se presenta una estructura de control alternativa que evita este problema, la cual utiliza un observador de dimensión infinita, gobernado por una ecuación en derivadas parciales de tipo hiperbólico.Time-delay systems are ubiquitous in many engineering applications, such as mechanical or fluid transmissions, metallurgical processes or networked control systems. Time-delay systems have attracted the interest of control researchers since the late 50's. A wide variety of tools for stability and performance analysis has been developed, specially over the past two decades.
This thesis is focused on the problem of stabilizing systems that are affected by delays on the actuator and/or sensing paths. More specifically, the contributions herein reported aim at improving the performance of existing controllers in the presence of external disturbances. Time delays unavoidably degrade the control loop performance. Disturbance rejection has been a matter of concern since the first predictive controllers for time-delay systems emerged. The key idea of the strategies presented in this thesis is the combination of predictive controllers and disturbance observers. The latter have been successfully applied to improve the disturbance rejection capabilities of conventional controllers. However, the application of this methodology to time-delay systems is rarely found in the literature. This combination is extensively investigated in this thesis.
Another handicap of predictive controllers has to do with their implementation, which can induce instability if not done carefully. This issue is related to the fact that predictive control laws take the form of integral equations. An alternative control structure that avoids this problem is also reported in this thesis, which employs an infinite-dimensional observer, governed by a hyperbolic partial differential equation.Sanz Díaz, R. (2018). Robust control strategies for unstable systems with input/output delays [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/111830TESI
Robust Adaptive Control and L
This paper deals with the robust stabilizability and L2 disturbance attenuation for a class of time-delay Hamiltonian control systems with uncertainties and external disturbances. Firstly, the robust stability of the given systems is studied, and delay-dependent criteria are established based on the dissipative structural properties of the Hamiltonian systems and the Lyapunov-Krasovskii (L-K) functional approach. Secondly, the problem of L2 disturbance attenuation is considered for the Hamiltonian systems subject to external disturbances. An adaptive control law is designed corresponding to the time-varying delay pattern involved in the systems. It is shown that the closed-loop systems under the feedback control law can guarantee the γ-dissipative inequalities be satisfied. Finally, two numerical examples are provided to illustrate the theoretical developments
Design and application of advanced disturbance rejection control for small fixed-wing UAVs
Small Unmanned Aerial Vehicles (UAVs) have seen continual growth in both research and
commercial applications. Attractive features such as their small size, light weight and
low cost are a strong driver of this growth. However, these factors also bring about some
drawbacks. The light weight and small size means that small UAVs are far more susceptible
to performance degradation from factors such as wind gusts. Due to the generally low
cost, available sensors are somewhat limited in both quality and available measurements.
For example, it is very unlikely that angle of attack is sensed by a small UAV. These
aircraft are usually constructed by the end user, so a tangible amount of variation will
exist between different aircraft of the same type. Depending on application, additional
variation between flights from factors such as battery placement or additional sensors may
exist. This makes the application of optimal model based control methods difficult.
Research literature on the topic of small UAV control is very rich in regard to high
level control, such as path planning in wind. A common assumption in such literature
is the existence of a low level control method which is able to track demanded aircraft
attitudes to complete a task. Design of such controllers in the presence of significant wind
or modelling errors (factors collectively addressed as lumped disturbances herein) is rarely
considered.
Disturbance Observer Based Control (DOBC) is a means of improving the robustness
of a baseline feedback control scheme in the presence of lumped disturbances. The method
allows for the rejection of the influence of unmeasurable disturbances much more quickly
than traditional integral control, while also enabling recovery of nominal feedback con-
trol performance. The separation principle of DOBC allows for the design of a nominal
feedback controller, which does not need to be robust against disturbances. A DOBC
augmentation can then be applied to ensure this nominal performance is maintained even
in the presence of disturbances. This method offers highly attractive properties for control
design, and has seen a large rise in popularity in recent years.
Current literature on this subject is very often conducted purely in simulation. Ad-
ditionally, very advanced versions of DOBC control are now being researched. To make
the method attractive to small UAV operators, it would be beneficial if a simple DOBC
design could be used to realise the benefits of this method, as it would be more accessible
and applicable by many.
This thesis investigates the application of a linear state space disturbance observer to
low level flight control of a small UAV, along with developments of the method needed
to achieve good performance in flight testing. Had this work been conducted purely in
simulation, it is likely many of the difficulties encountered would not have been addressed
or discovered.
This thesis presents four main contributions. An anti-windup method has been devel-
oped which is able to alleviate the effect of control saturation on the disturbance observer
dynamics. An observer is designed which explicitly considers actuator dynamics. This
development was shown to enable faster observer estimation dynamics, yielding better
disturbance rejection performance. During initial flight testing, a significant aeroelastic
oscillation mode was discovered. This issue was studied in detail theoretically, with a pro-
posed solution developed and applied. The solution was able to fully alleviate the effect in
flight. Finally, design and development of an over-actuated DOBC method is presented.
A method for design of DOBC for over actuated systems was developed and studied. The
majority of results in this thesis are demonstrated with flight test data
Survey on time-delay approach to networked control
This paper provides a survey on time-delay approach to networked control systems (NCSs). The survey begins from a brief summary on fundamental network-induced issues in NCSs and the main approaches to the modelling of NCSs. In particular, a comprehensive introduction to time-delay approach to sampled-data and networked control is provided. Then, recent results on time-delay approach to event-triggered control are recalled. The survey highlights time-delay approach developed to modelling, analysis and synthesis of NCSs, under communication constraints, with a particular focus on Round-Robin, Try-once-discard and stochastic protocols. The time-delay approach allows communication delays to be larger than the sampling intervals in the presence of scheduling protocols. Moreover, some results on networked control of distributed parameter systems are surveyed. Finally, conclusions and some future research directions are briefly addressed
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