Dynamic programming solutions for decentralized state-feedback LQG problems with communication delays

This paper presents explicit solutions for a class of decentralized LQG problems in which players communicate their states with delays. A method for decomposing the Bellman equation into a hierarchy of independent subproblems is introduced. Using this decomposition, all of the gains for the optimal controller are computed from the solution of a single algebraic Riccati equation. © 2012 AACC American Automatic Control Council).

Robust approach to dynamic statistical process control

Statistical Process Control (SPC) technique are well established across a wide range of industries. In particular, the plotting of key steady state variables with their statistical limit against time (Shewart charting) is a common approach for monitoring the normality of production. This paper aims with extending Shewart charting techniques to the quality monitoring of variables driven by uncertain dynamic processes, which has particular application in the process industries where it is desirable to monitor process variables on-line as well as final product. The robust approach to dynamic SPC is based on previous work on guaranteed cost filtering for linear systems and is intended to provide a basis for both a wide application of SPC monitoring and also motivate unstructured fault detection.

Sequential decision making in general state space models

Many problems in control and signal processing can be formulated as sequential decision problems for general state space models. However, except for some simple models one cannot obtain analytical solutions and has to resort to approximation. In this thesis, we have investigated problems where Sequential Monte Carlo (SMC) methods can be combined with a gradient based search to provide solutions to online optimisation problems. We summarise the main contributions of the thesis as follows. Chapter 4 focuses on solving the sensor scheduling problem when cast as a controlled Hidden Markov Model. We consider the case in which the state, observation and action spaces are continuous. This general case is important as it is the natural framework for many applications. In sensor scheduling, our aim is to minimise the variance of the estimation error of the hidden state with respect to the action sequence. We present a novel SMC method that uses a stochastic gradient algorithm to find optimal actions. This is in contrast to existing works in the literature that only solve approximations to the original problem. In Chapter 5 we presented how an SMC can be used to solve a risk sensitive control problem. We adopt the use of the Feynman-Kac representation of a controlled Markov chain flow and exploit the properties of the logarithmic Lyapunov exponent, which lead to a policy gradient solution for the parameterised problem. The resulting SMC algorithm follows a similar structure with the Recursive Maximum Likelihood(RML) algorithm for online parameter estimation. In Chapters 6, 7 and 8, dynamic Graphical models were combined with with state space models for the purpose of online decentralised inference. We have concentrated more on the distributed parameter estimation problem using two Maximum Likelihood techniques, namely Recursive Maximum Likelihood (RML) and Expectation Maximization (EM). The resulting algorithms can be interpreted as an extension of the Belief Propagation (BP) algorithm to compute likelihood gradients. In order to design an SMC algorithm, in Chapter 8 uses a nonparametric approximations for Belief Propagation. The algorithms were successfully applied to solve the sensor localisation problem for sensor networks of small and medium size.

Dynamic properties of superconducting magnetic bearings

The potential use of YBa2j as an active component in a magnetic bearing is being investigated. Although the load bearing capacity is high and increases with the square of the magnetic field trapped, the stiffness is low. Both the stiffness and the lévitation height are a function of the loading history of the bearing. At Cambridge we have been investigating the effects of dynamic loading such as single large excursions from steady state loads and cyclically applied loads such as vibrations. Since a superconducting bearing has little inherent damping cyclic loads applied at or near its natural frequency can have catastrophic effects. The information being gathered at Cambridge will be used to enable these effects to be mitigated in the bearing design process. © 1997 IEEE.

Dynamic learning with the EM algorithm for neural networks

In this paper, we derive an EM algorithm for nonlinear state space models. We use it to estimate jointly the neural network weights, the model uncertainty and the noise in the data. In the E-step we apply a forwardbackward Rauch-Tung-Striebel smoother to compute the network weights. For the M-step, we derive expressions to compute the model uncertainty and the measurement noise. We find that the method is intrinsically very powerful, simple and stable.

A novel measurement of dynamic friction

A tribometer, based on a pin-on-disc machine, uses a PZT drive to produce small sinusoidal fluctuations of sliding speed. The frequency and amplitude of these fluctuations can be controlled, and the dynamic response measured. Preliminary test results show that the dynamic friction variation is influenced by the contact materials, normal force, oscillation frequency and steady sliding speed. The variation of friction force amplitude and phase with frequency gives clues about the underlying state variables determining the friction. Modelling studies illustrate the expected behaviour for idealized friction laws governed by, for example, sliding speed, contact temperature, and "rate-state" laws. © 2008 SAE International.

The frequency response of dynamic friction: Model comparisons

Any linearised theory of the initiation of friction-excited vibration via instability of the state of steady sliding requires information about the dynamic friction force in the form of a frequency response function for sliding friction. Recent measurements of this function for an interface consisting of a nylon pin against a glass disc are used to probe the underlying constitutive law. Results are compared to linearised predictions from the simplest ratestate model of friction, and a ratetemperature model. In both cases the observed variation with frequency is not compatible with the model predictions, although there are some significant points of similarity. The most striking result relates to variation of the normal load: any theory embodying the Coulomb relation F∝N would predict behaviour entirely at variance with the measurements, even though the steady friction force obtained during the same measurements does follow the Coulomb law. © 2011 Elsevier Ltd. All rights reserved.

Lowering the excitation threshold of a random laser using the dynamic scattering states of an organosiloxane smectic A liquid crystal

Smectic A liquid crystals, based upon molecular structures that consist of combined siloxane and mesogenic moieties, exhibit strong multiple scattering of light with and without the presence of an electric field. This paper demonstrates that when one adds a laser dye to these compounds it is possible to observe random laser emission under optical excitation, and that the output can be varied depending upon the scattering state that is induced by the electric field. Results are presented to show that the excitation threshold of a dynamic scattering state, consisting of chaotic motion due to electro-hydrodynamic instabilities, exhibits lower lasing excitation thresholds than the scattering states that exist in the absence of an applied electric field. However, the lowest threshold is observed for a dynamic scattering state that does not have the largest scattering strength but which occurs when there is optimization of the combined light absorption and scattering properties. © 2012 American Institute of Physics.

A novel measurement of dynamic friction

A tribometer, based on a pin-on-disc machine, uses a PZT drive to produce small sinusoidal fluctuations of sliding speed. The frequency and amplitude of these fluctuations can be controlled, and the dynamic response measured. Preliminary test results show that the dynamic friction variation is influenced by the contact materials, normal force, oscillation frequency and steady sliding speed. The variation of friction force amplitude and phase with frequency gives clues about the underlying state variables determining the friction. Modelling studies illustrate the expected behaviour for idealized friction laws governed by, for example, sliding speed, contact temperature, and "rate-state" laws. © 2008 SAE International.

Dynamic modelling of the brushless doubly fed machine

A coupled-circuit model for the brushless doubly fed machine (BDFM) has been developed. The transformation of the model into the d-q axis form, ultimately in a synchronous reference frame in which machine currents and voltages have constant values in the steady state, has been carried out. A model-reduction technique is presented, which gives a concise representation of the 'nested-loop' rotor design using a single d-q pair. These models have been experimentally verified and give a convenient and accurate way of calculating the dynamic behaviour of a BDFM. The ability to represent the BDFM with a single d-q pair considerably simplifies the design of suitable controllers. © The Institution of Engineering and Technology 2013.

Optimising the dynamic performance of an all-wide-bandgap cascode switch

A SPICE simulation model of a novel cascode switch that combines a high voltage normally-on silicon carbide (SiC) junction field effect transistor (JFET) with a low voltage enhancement-mode gallium nitride field effect transistor (eGaN FET) has been developed, with the aim of optimising cascode switching performance. The effect of gate resistance on stability and switching losses is investigated and optimum values chosen. The effects of stray inductance on cascode switching performance are considered and the benefits of low inductance packaging discussed. The use of a positive JFET gate bias in a cascode switch is shown to reduce switching losses as well as reducing on-state losses. The findings of the simulation are used to produce a list of priorities for the design and layout of wide-bandgap cascode switches, relevant to both SiC and GaN high voltage devices. © 2013 IEEE.