Comparative Study on Decoupling Methods for Time-Delay Systems

Decoupling of a multivariable system is much pronounced control problem in many indus- trial process. This has received much importance since last 20 years to develop decoupling controllers as an alternative to multi-loop PI controllers in order to achieve satisfactory set- point responses when there exist multiple time delays, non-minimum phase zeros and large uncertainties. In this thesis, static and dynamic decoupling control strategies are discussed. Static decouplers are designed for control of multi-variable processes by RGA interaction analysis to determine the nature of interaction for static decouplers applied to multi-loop con- trollers and integral mode only at low and high frequencies. Here it is shown that static decouplers applied to integral modes perform better for processes in which non-diagonal ele- ments decrease faster as frequency increases. This approach is sensitive to process changes and require detailed models. In dynamic decoupling, internal model control and inverted decoupling methods are dis- cussed here. Internal model control approach comprises both open-loop decoupling and closed- loop decoupling techniques. Internal model control is a centralized controller derived from the desired diagonal system matrix defined taking in consideration of time-delay compensation, non-minimum phase zeros and H2 optimal performance objective; which effectively perform both decoupling and controlling functions. In open-loop IMC decoupling, the controller is de- signed from open-loop equation of process assuming that the model is exactly matching with the process. In closed-loop IMC decoupling, only model of the system is considered and con- troller is derived from closed-loop equation of the overall system. The presence of RHP zeros is elaborately discussed in different cases like no RHP zeros, finite RHP zeros, infinite RHP and LHP zeros and infinite RHP but finite LHP zeros. Inverted decoupling approach utilizes inverse transfer matrix of the multiva

Decoupling with internal stability for unity output feedback systems

Automatica282411-415ATCA

Active vibration control in linear time-invariant and nonlinear systems

Active vibration control techniques are widely used in linear time-invariant and nonlinear systems. However, there still exist many difficulties in the application of conventional active vibration control techniques, including the following: (1) In application, some of the degrees of freedom may not be physically accessible to actuation and sensing simultaneously; (2) large flexible structures are difficult in terms of isolating one substructure from the vibration of another; (3) the incomplete understanding of the effects of softening nonlinearity may put conventional active controllers at risk; and (4) global stability of under-actuated nonlinear aeroelastic systems, resulting from actuator failure or motivated by weight and cost constraints imposed on next-generation flight vehicles, is extremely challenging, especially in the case of uncertainty and external disturbances. These intellectual challenges are addressed in this research by linear and nonlinear active control techniques. A new theory for partial pole placement by the method of receptances in the presence of inaccessible degrees of freedom is proposed. By the application of a new double input control and orthogonality conditions on the input and feedback gain vectors, partial pole placement is achieved in a linear fashion while some chosen degrees of freedom are free from both actuation and sensing. A lower bound on the maximum number of degrees of freedom inaccessible to both actuation and sensing is established. A theoretical study is presented on the feasibility of applying active control for the purpose of simultaneous vibration isolation and suppression in large flexible structures by block diagonalisation of the system matrices and at the same time assigning eigenvalues to the chosen substructures separately. The methodology, based on eigenstructure assignment using the method of receptances, is found to work successfully when the open-loop system, with lumped or banded mass matrix, is controllable. A comprehensive study of the effects of softening structural nonlinearity in aeroelastic systems is carried out using the simple example of a pitch-flap wing, with softening cubic nonlinearity in the pitch stiffness. Complex dynamical behaviour, including stable and unstable limit cycles and chaos, is revealed using sinusoidal-input describing functions and numerical integration in the time domain. Bifurcation analysis is undertaken using numerical continuation methods to reveal Hopf, symmetry breaking, fold and period doubling bifurcations. The effects of initial conditions on the system stability and the destabilising effects of softening nonlinearity on aerodynamic responses are considered. The global stability of an under-actuated wing section with torsional nonlinearity, softening or hardening, is addressed using a robust passivity-based continuous sliding-mode control approach. The controller is shown to be capable of stabilising the system in the presence of large matched and mismatched uncertainties and large input disturbance. With known bounds on the input disturbance and nonlinearity uncertainty, the continuous control input is able to globally stabilise the overall system if the zero dynamics of the system are globally exponentially stable. The merits and performance of the proposed methods are exemplified in a series of numerical case studies

Sistemas de estructura variable : Aplicación al control con restricciones

En este trabajo de tesis se aborda el control de sistemas multivariables con restricciones. Uno de los principales objetivos es el desarrollo de nuevas estrategias de control que permitan reducir las interacciones cruzadas de los sistemas de múltiples entradas y múltiples salidas; es decir, mejorar su grado de desacoplamiento. Este problema se considera para diferentes limitaciones en la planta (restricciones a la entrada, restricciones a la salida, ceros de fase no-mínima) y distintas estructuras de control (controladores centralizados y descentralizados). Las estrategias propuestas combinan conceptos de la teoría de control por estructura variable y el acondicionamiento de la señal de referencia.Facultad de Ingenierí