Discrete two-time-scale systems

In this paper reduced-order modelling and control analysis of linear, discrete-time systems having dominant and non-dominant modes are presented. Decoupling of modes is achieved using an explicitly invertible linear transformation. A matrix norm condition is derived, the satisfaction of which enables approximate expressions for the block-diagonalizing matrices, eigenvalue distribution and state trajectories to be obtained. Design of stabilizing feedback controllers is developed and it is shown that two gain matrices are needed for separate assignment of dominant and non-dominant eigenvalues. The theoretical analysis is illustrated by control system examples.     

On building cyclic and shared structures in Haskell

We demonstrate how to build certain cyclic and other multi-linked structures in the lazy functional programming language Haskell. No explicit pointers are used in these constructions. Each task is accomplished by starting with a suitable specification and then calculating the required program.     

A divide-and-conquer approach for analysing overlaid data structures

We present a static program analysis for overlaid data structures such that a node in the structure includes links for multiple data structures and these links are intended to be used at the same time. These overlaid data structures are frequently used in systems code, in order to impose multiple types of indexing structures over the same set of nodes. Our analysis implements two main ideas. The first is to run multiple sub-analyses that track information about non-overlaid data structures, such as lists. The second idea is to control the communication among the sub-analyses using ghost states and ghost instructions. The purpose of this control is to achieve a high level of efficiency by allowing only necessary information to be transferred among sub-analyses and at as few program points as possible. Our analysis has been successfully applied to prove the memory safety of the Linux deadline IO scheduler and AFS server.     

Application of a stochastic modelling framework to characterize the influence of different oxide scales on the solid particle erosion behaviour of boiler grade steel

An earlier developed stochastic model has been applied to describe the relative rate of material loss from the steel surface subjected to simultaneous action of high temperature oxidation involving multiple oxides and mechanical erosion. Different oxide scale growths, namely, nickel, iron and chromium have been treated deterministically and erosion is described using a literature based probabilistic framework. Oxidation is described with a power law (parabolic) approach to quantify the rate of growth of all the three oxide scales. In consonance with the published model, erosion is treated using a probabilistic methodology as spatially random phenomena on the oxide surface. The concept of ‘erosion footprint’ has been incorporated in the present model to characterize the erosion-induced damage on the steel surface. The model has been applied to predict the relative material loss as a function of time resulted from erosion–oxidation interaction pertaining to nickel, iron and chromium oxides in dimensionless form. This investigation is expected to provide a quantitative understanding of relative material loss due to solid particle erosion for oxide scales, (composed of multiple oxides) formed on the steel components of coal-fired boilers.     

Gap-free fibre laser welding of Zn-coated steel on Al alloy for light-weight automotive applications

As a result of new policies related to global warming announced by the European Union, avoiding unnecessary energy waste and reducing environmental pollution levels are becoming a major issue in the automotive industry. Accordingly, the lap welding of Zn-coated steels process, which is commonly used for producing car doors, has been gradually developed to lap welding of Zn-coated steel to light materials, such as Al alloy, Mg alloy and composite materials, in order to effectively reduce the vehicle weight. In certain part of car manufacture, organic glues are used to temporally join the Zn-coated steels and Al alloys before permanent welding takes place. The stability of such temporary joining by glues needs improving. Laser “stitching” or low strength welding could be considered as an alternative. However, challenges exist in joining Zn-coated steel on Al alloy by laser welding, due to significant differences of material properties between the two welding materials. Porosity, spatter and intermetallic brittle phases are readily produced in the weld. In this study, the effects of welding speed, laser power, number of the welding passes and type of shielding gas in gap-free welding of Zn-coated steel on Al alloy were investigated using a 1 kW single mode continuous wave fibre laser. Results show that a weld with higher shear strengths in the laser stitching application and less intermetallic phases could be obtained when nitrogen gas was used as the shielding gas. The corrosion resistance and the surface finish of the weld could be improved in double pass welding, especially when argon gas was used as the shielding gas.     

Design rules for semi-transparent organic tandem solar cells for window integration

For window integration of semi-transparent solar cells in living and working areas, color neutral transparency perception and good color rendering are of pivotal importance. In order to tune the optical device properties, we simulate a parallel tandem configuration with two different absorber materials. Within a regime of convenient transparency perception, the transparency can be adjusted between 20% and 40% by choosing the right absorber layer thickness combination. From the optical field in the tandem devices we calculate the charge carrier generation profile and subsequently correlate the optical properties with the electrical device properties as derived from drift-diffusion modelling – altogether allowing for a comprehensive assessment of the transparency, the transparency perception and the device performance and their interdependencies.     

1064-nm DFB laser diode modules applicable to seeder for pulse-on-demand fiber laser systems

Semiconductor DFB (Distributed feedback) laser diodes with an operating wavelength of 1064 nm, which is suitable for pulse-on-demand fiber laser, have been developed. The stable performance of CW and nanosecond/picosecond pulsed operation is reviewed. By applying gain-switching operation with a simple direct modulation technique, 50-ps pulse generation with a stable spectral single-mode property was obtained. For the efficient amplification of the obtained 50-ps pulse, a monolithic semiconductor optical amplifier (SOA) was integrated into the DFB lasers. An improved peak power of 300 mW at 50-ps pulse was observed with limited optical noise injection when the synchronous modulation technique of the DFB and the SOA was employed. Short cavity lasers showed a high-frequency response compared to the original DFB lasers and achieved a short pulse width of 13 ps by standard gain-switched operation.     

Modelling of V–Hz and vector controlled ASDs in PSCAD/EMTDC for voltage sag studies

This paper deals with modelling and performance of adjustable speed drives (ASDs) subjected to voltage disturbances in electric supply. The aim of this study was to develop appropriate models of typical ASD and investigate their sensitivity to voltage disturbances under various practical modes of operation and control. Accordingly, scalar controlled open and closed loop volts–hertz (V–Hz) and vector controlled closed loop ASDs are modelled in PSCAD/EMTDC environment, and their performance in the presence of voltage disturbances is investigated under typical operating and loading conditions.