Global stabilization of feedforward systems with exponentially unstable Jacobian linearization

The global stabilization of a class of feedforward systems having an exponentially unstable Jacobian linearization is achieved by a high-gain feedback saturated at a low level. The control law forces the derivatives of the state variables to small values along the closed-loop trajectories. This "slow control" design is illustrated with a benchmark example and its limitations are emphasized. © 1999 Elsevier Science B.V. All rights reserved.

Distributed maximum likelihood for self-localization in sensor networks

We show that the sensor localization problem can be cast as a static parameter estimation problem for Hidden Markov Models and we develop fully decentralized versions of the Recursive Maximum Likelihood and the Expectation-Maximization algorithms to localize the network. For linear Gaussian models, our algorithms can be implemented exactly using a distributed version of the Kalman filter and a message passing algorithm to propagate the derivatives of the likelihood. In the non-linear case, a solution based on local linearization in the spirit of the Extended Kalman Filter is proposed. In numerical examples we show that the developed algorithms are able to learn the localization parameters well.

Modelling material transfer in metal rolling using plasticine

Plasticine strips are rolled between cylindrical rollers to model the phenomenon of material transfer in metal rolling. Strips of thin plastic film ('clingfilm') on the plasticine strip are used to model the oxide layer that covers the surface of aluminium. The effect of gaps opening up between the clingfilm strips is investigated. It is found that the percentage area of the exposed strip giving rise to transfer of material increases with the gap width. The evidence strongly suggests that plasticine particles transferred to the rolls are able to pick off plasticine from the strip on successive passes. Larger plasticine particles are more likely to show this behaviour and consequently grow in size. The results confirm the usefulness of plasticine as a suitable material to investigate transfer layer formation in metal rolling, and help inform development of experimental procedures to study the evolution of real metal transfer layers. © 2007 Elsevier B.V. All rights reserved.

Rhythm generation in monkey motor cortex explored using pyramidal tract stimulation.

We investigated whether stimulation of the pyramidal tract (PT) could reset the phase of 15-30 Hz beta oscillations observed in the macaque motor cortex. We recorded local field potentials (LFPs) and multiple single-unit activity from two conscious macaque monkeys performing a precision grip task. EMG activity was also recorded from the second animal. Single PT stimuli were delivered during the hold period of the task, when oscillations in the LFP were most prominent. Stimulus-triggered averaging of the LFP showed a phase-locked oscillatory response to PT stimulation. Frequency domain analysis revealed two components within the response: a 15-30 Hz component, which represented resetting of on-going beta rhythms, and a lower frequency 10 Hz response. Only the higher frequency could be observed in the EMG activity, at stronger stimulus intensities than were required for resetting the cortical rhythm. Stimulation of the PT during movement elicited a greatly reduced oscillatory response. Analysis of single-unit discharge confirmed that PT stimulation was capable of resetting periodic activity in motor cortex. The firing patterns of pyramidal tract neurones (PTNs) and unidentified neurones exhibited successive cycles of suppression and facilitation, time locked to the stimulus. We conclude that PTN activity directly influences the generation of the 15-30 Hz rhythm. These PTNs facilitate EMG activity in upper limb muscles, contributing to corticomuscular coherence at this same frequency. Since the earliest oscillatory effect observed following stimulation was a suppression of firing, we speculate that inhibitory feedback may be the key mechanism generating such oscillations in the motor cortex.

Mechanisms of erosion of unfilled elastomers by solid particle impact

The mechanisms of material removal were studied during the erosion of two unfilled elastomers (natural rubber and epoxidised natural rubber). The effects of impact velocity and of lubrication by silicone oil were investigated. The development of surface features due to single impacts and during the early stages of erosion was followed by scanning electron microscopy. The basic material removal mechanism at impact angles of both 30° and 90° involves the formation and growth of fine fatigue cracks under the tensile surface stresses caused by impact. No damage was observed after single impacts; it was found that many successive impacts are necessary for material removal. It was found that the erosion rate has a very strong dependance on impact velocity above about 50 ms-1.

Widening engineering horizons: Addressing the complexity of sustainable development

The complex, fragmented and diverse aspects of a sustainable development perspective are translated into an eight-point framework that defines a problem boundary larger than that traditionally adopted by civil engineers. This leads to practical questions intended to inform engineers who ask 'am I being sustainable?' during project implementation. The value of the questions is tested against a case history of a wastewater treatment project. This demonstrates the relevance of the questions to successive project delivery phases of defining the problem, choosing a solution and implementing that solution through design, construction and operation. The case history highlights that answers to several of the additional questions raised by considering this wider problem space are currently buried within government and clients' policies, regulations and standard practice; these answers may not be accessible to the professional engineer.

A comparison of methods for the coupled analysis of floating structures

The dynamic analysis of a deepwater floating platform and the associated mooring/riser system should ideally be fully coupled to ensure a reliable response prediction. It is generally held that a time domain analysis is the only means of capturing the various coupling and nonlinear effects accurately. However, in recent work it has been found that for an ultra-deepwater floating system (2000m water depth), the highly efficient frequency domain approach can provide highly accurate response predictions. One reason for this is the accuracy of the drag linearization procedure over both first and second order motions, another reason is the minimal geometric nonlinearity displayed by the mooring lines in deepwater. In this paper, the aim is to develop an efficient analysis method for intermediate water depths, where both mooring/vessel coupling and geometric nonlinearity are of importance. It is found that the standard frequency domain approach is not so accurate for this case and two alternative methods are investigated. In the first, an enhanced frequency domain approach is adopted, in which line nonlinearities are linearized in a systematic way. In the second, a hybrid approach is adopted in which the low frequency motion is solved in the time domain while the high frequency motion is solved in the frequency domain; the two analyses are coupled by the fact that (i) the low frequency motion affects the mooring line geometry for the high frequency motion, and (ii) the high frequency motion affects the drag forces which damp the low frequency motion. The accuracy and efficiency of each of the methods are systematically compared. Copyright © 2007 by ASME.

The time-resolved flow within an unsteady ejector

The flow field within an unsteady ejector has been investigated using experimental and computational techniques. The experimental results show a peak thrust augmentation of 1.4; numerical simulation gives a value of 1.37. It is shown that the vortex ring dominates the flow field. At optimal thrust augmentation the vortex ring acts like a fluid piston accelerating the fluid inside the ejector. A model is proposed for the operation of unsteady ejectors, based on the vortex ring acting like a fluid piston. Control volume analysis is presented showing that mass entrainment is responsible for thrust augmentation. It is proposed that the spacing of successive vortex rings determines the mass entrainment and therefore thrust augmentation. The efficiency of unsteady ejectors was found to vary between 28% and 32% depending on the L/D ratio of the unsteady jet source. Copyright © 2008 by J H Heffer.