Thomas S. Brinsmead

Preliminary experiments in visual servo control for autonomous underwater vehicle

We consider a visually-guided autonomous underwater vehicle. We develop a position-based visual servo control of fixed and slow moving targets using visual position feedback and sensor-based orientation feedback. The visual position feedback is implemented on a stereo camera system. We use a compass and an inclinometer for orientation feedback. We also implement a computed torque controller, using Euler parameters to represent the orientation state, for the vehicle motion control. Using Euler parameters eliminates singularities in the model and the controller. Preliminary experimental results of visual servo control are reported.

The Vinnicombe metric for nonlinear operators

Describes an extension of the Vinnicombe metric on linear operators to a pseudometric on nonlinear operators. A metric for finite-dimensional time-varying operators is shown to be capable of guaranteeing stability and performance robustness and reduces to the standard Vinnicombe metric for the time-invariant operator case, which is known to be less conservative than the gap metric. The analysis exploits the time-varying operator equivalents of unstable poles and normalized coprime fractional descriptions. In addition, a time-varying operator equivalent of the winding number is defined.

Technical Communique: Cheap decoupled control

Requiring a controlled system to be decoupled may increase some performance costs, however, decoupling may be desirable from an applied perspective. This paper gives an explicit quantification of decoupling costs in terms of the average quadratic tracking error. The analysis exploits the parametrisation of all decoupling controllers, together with Wiener-Hopf frequency domain techniques.

Robust disturbance suppression for nonlinear systems based on H/sub /spl infin// control

The disturbance suppression problem for nonlinear systems is examined. We review the so-called nonstandard mixed sensitivity problem, which introduces an integrator to a selected weight, as well as the linear classical disturbance suppression problem and the linear H/sub /spl infin// disturbance suppression problem. We extend this H/sub /spl infin// problem to the nonlinear case, and present a method to reduce the order of the state feedback Hamilton-Jacobi partial differential equation for this nonlinear H/sub /spl infin// problem by extending the concept of comprehensive stability. Finally, we investigate the structure of the output feedback H/sub /spl infin// controller for disturbance suppression, and draw the conclusion that, as in the linear case, there must also be an integrator in the controller.

Cost of cheap decoupled control

Although decoupling a controlled system increases costs in terms of some performance measures, it is often desirable from an applied perspective point of view. Here, the average quadratic tracking error is used to explicitly quantify decoupling costs allowing the relative desirability of decoupling for particular applications to be determined. The Wiener-Hopf analysis and the parametrisation of all decoupling controllers are used.

Fundamental limits in sensitivity minimization: multiple-input-multiple-output (MIMO) plants

This paper provides a bridge between the H/sub /spl infin// literature and that of fundamental limits. By applying operator theoretic tools developed for the standard frequency domain, model-matching approach to sensitivity minimization, explicit closed form expressions are given for tight bounds on the H/sub /spl infin// norm of weighted sensitivity functions for output feedback control of linear multivariable systems.

Constant disturbance suppression for nonlinear systems design using singular perturbation theory

A relatively practical method of suppressing the effect of constant disturbances on nonlinear systems is presented. By adding an integrator to a stabilising controller, it is possible to achieve both a constant disturbance rejection and zero tracking error. Sufficient conditions for the rejection of a constant input disturbance are given. We give both local and global conditions such that the inclusion of an integrator in the closed loop maintains closed loop stability. The analysis is based on the singular perturbation theory. Furthermore, we extend these methods to deal with multiple-input multiple-output nonlinear systems. Finally, we implement our method in the control of a simulated helicopter model. The simulation results show that this method achieves satisfactory performance.

Constant disturbance rejection and zero steady state tracking error for nonlinear systems design

A relatively practical method of suppressing the effect of constant disturbances on nonlinear systems is presented. By adding an integrator to a stabilising controller, it is possible to achieve both constant disturbance rejection and zero tracking error. Sufficient conditions for the rejection of a constant input disturbance are given. We give both local and global conditions such that the inclusion of an integrator in the closed loop maintains closed loop stability. The analysis is based on singular perturbation theory. Furthermore, we also present some alternative locations for adding an integrator into the closed loop system and extend these methods to deal with Multiple-input Multiple-output nonlinear systems. Finally, we implement our method in the control of a simulated helicopter model. The simulation results show that this method achieves satisfactory performance.

Not all Vinnicombe metric neighbourhoods are homotopically connected

We prove by counterexample that even for two transfer functions which are close in the Nu-gap metric of Vinnicombe (1993, 1999), there does not necessarily exist a Vinnicombe metric homotopy from one transfer function to the other, such that intermediate transfer functions in the homotopy remain close to the transfer function at the beginning of the homotopy. This implies that the Vinnicombe metric neighbourhoods of some transfer functions in L-infinity space, are not connected.

Safe Switching in Multi-Controller Implementation

Abstract Multiple Model Adaptive Control involves a supervisor switching among one of a finite number of controllers as more is learnt about the plant. Safe Adaptive Control is concerned with ensuring that when the controller is changed in an adaptive control algorithm, the frozen plant-controller combination is never (closed loop) unstable. A controller scheme is proposed that combines these principles and involves a frequency-dependent performance measure based on the Vinnicombe metric. Safe switching is guaranteed to the extent which closed loop transfer function identification is accurate.