On the strict monotonicity of spectral radii for classes of bounded positive linear operators

Strict monotonicity of the spectral radii of bounded, positive, ordered linear operators is investigated. It is well-known that under reasonable assumptions, the spectral radii of two ordered positive operators enjoy a non-strict inequality. It is also well-known that a “strict” inequality between operators does not imply strict monotonicity of the spectral radii in general—some additional structure is required. We present a number of sufficient conditions on both the cone and the operators for such a strict ordering to hold which generalise known results in the literature, and have utility in comparison arguments, ubiquitous in positive systems theory

Model Reduction by Balanced Truncation

Model reduction by balanced truncation for bounded real and positive real input-stateoutput systems, known as bounded real balanced truncation and positive real balanced truncation respectively, is addressed. Results for finite-dimensional systems were established in the mid to late 1980s and we consider two extensions of this work. Firstly, using a more behavioral framework we consider the notion of a finite-dimensional dissipative system, of which bounded real and positive real input-state-output systems are particular instances. Specifically, we work in a framework where we make no a priori distinction between inputs and outputs. We derive model reduction by dissipative balanced truncation, where a gap metric error bound is obtained, and demonstrate that the aforementioned bounded real and positive real balanced truncation can be seen as special cases. In the second part we generalise bounded real and positive real balanced truncation to classes of bounded real and positive real systems respectively that have non-rational transfer functions, so called infinite-dimensional systems. Here we work in the context of well-posed linear systems. We derive approximate transfer functions, which we prove are rational and preserve the relevant dissipativity property. We also obtain error bounds for the difference of the original transfer function and its reduced order transfer function, in the H-infinity norm and gap metric for the bounded real and positive real cases respectively. This extension to bounded real and positive real balanced truncation requires new results for Lyapunov balanced truncation in the infinite dimensional case, which we also describe. We conclude by highlighting possible future research.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The circle criterion for a class of sector-bounded dynamic nonlinearities

We present a circle criterion which is necessary and sufficient for absolute stability with respect to a natural class of sector-bounded nonlinear causal operators. This generalized circle criterion contains the classical result as a special case. Furthermore, we develop a version of the generalized criterion which guarantees input-to-state stability

Model reduction by balanced truncation for systems with nuclear Hankel operators

We prove the H-infinity error bounds for Lyapunov balanced truncation and for optimal Hankel norm approximation under the assumption that the Hankel operator is nuclear. This is an improvement of the result from Glover, Curtain, and Partington [SIAM J. Control Optim., 26(1998), pp. 863-898], where additional assumptions were made. The proof is based on convergence of the Schmidt pairs of the Hankel operator in a Sobolev space. We also give an application of this convergence theory to a numerical algorithm for model reduction by balanced truncation

A note on the eigenvectors of perturbed matrices with applications to linear positive systems

A result is presented describing the eigenvectors of a perturbed matrix, for a class of structured perturbations. One motivation for doing so is that positive eigenvectors of nonnegative, irreducible matrices are known to induce norms — acting much like Lyapunov functions — for linear positive systems, which mayhelp estimate or control transient dynamics. The results apply to both discrete- and continuous-time linear positive systems. The theory is illustrated with several examples

Small-gain stability theorems for positive Lur'e inclusions

Stability results are presented for a class of differential and difference inclusions, so-called positive Lur{\textquoteright}e inclusions which arise, for example, as the feedback interconnection of a linear positive system with a positive set-valued static nonlinearity. We formulate sufficient conditions in terms of weighted one-norms, reminiscent of the small-gain condition, which ensure that the zero equilibrium enjoys various global stability properties, including asymptotic and exponential stability. We also consider input-to-state stability, familiar from nonlinear control theory, in the context of forced positive Lur{\textquoteright}e inclusions. Typical for the study of positive systems, our analysis benefits from comparison arguments and linear Lyapunov functions. The theory is illustrated with examples

Low‐gain integral control for a class of discrete‐time Lur'e systems with applications to sampled‐data control

We study low-gain (P)roportional (I)ntegral control of multivariate discrete-time, forced Lur’e systems to solve the output-tracking problem for constant reference signals. We formulate an incremental sector condition which is sufficient for a usual linear low-gain PI controller to achieve exponential disturbance-to-state and disturbance-to-tracking-error stability in closed-loop, for all sufficiently small integrator gains. Output tracking is achieved in the absence of exogenous disturbance (noise) terms. Our line of argument invokes a recent circle criterion for exponential incremental input-to-state stability. The discrete-time theory facilitates a similar result for a continuous-time forced Lur’e system in feedback with sampled-data low-gain integral control. The theory is illustrated by two examples

Aizerman Conjectures for a class of multivariate positive systems

The Aizerman Conjecture predicts stability for a class of nonlinear control systems on the basis of linear system stability analysis. The conjecture is known to be false in general. Here, a number of Aizerman conjectures are shown to be true for a class of internally positive multivariate systems, under a natural generalisation of the classical sector condition and, moreover, guarantee positivity in closed loop. These results are stronger and/or more general than existing results. The paper relates the obtained results to other, diverse, results in the literature

Persistence and stability for a class of forced positive nonlinear delay-differential systems

Persistence and stability properties are considered for a class of forced positive non-linear delay-differential systems which arise in mathematical ecology and other applied contexts.The inclusion of forcing incorporates the effects of control actions (such as harvesting or breeding programmes in an ecological setting), disturbances induced by seasonal or environmental variation, or migration. We provide necessary and sufficient conditions under which the states of these models are semi-globally persistent, uniformly with respect to the initial conditions and forcing terms. Under mild assumptions, the model under consideration naturally admits two steady states (equilibria) when unforced: the origin and a unique non-zero steady state.We present sufficient conditions for the non-zero steady state to be stable in a sense which is reminiscent of input-to-state stability, a stability notion for forced systems developed in control theory. In the absence of forcing, our input-to-sate stability concept is identical to semi-global exponential stability

Positive state controllability of positive linear systems

Controllability of positive systems by positive inputs arises naturally in applications where both external and internal variables must remain positive for all time. In many applications, particularly in population biology, the need for positive inputs is often overly restrictive. Relaxing this requirement, the notion of positive state controllability of positive systems is introduced. A connection between positive state controllability and positive input controllability of a related system is established and used to obtain Kalman-like controllability criteria. In doing so we aim to encourage further study in this underdeveloped area