Transient performance study of a brushless doubly fed twin stator induction generator

This paper presents theoretical, simulation, and experimental study of the brushless doubly fed twin stator induction generator (BDFTSIG) dynamics under vector control based on the orientation on the power machine stator flux. A complex transfer function is derived which links the control current and power winding current space vectors in the field coordinates. Based on this result, the transient response of the BDFTSIG to step changes in the control current is examined theoretically. The oscillatory transients are explained in detail and linked to control flux transients triggered whenever operation point of the generator is changed. Furthermore, BDFTSIG operation with closed loop control of the power machine active and reactive powers is examined theoretically and experimentally. It is shown that in the closed loop operation, the system damping may be reduced so that the PI controller gains must be properly selected to achieve a good transient response.     

Invasive species detection in Hawaiian rainforests using airborne imaging spectroscopy and LiDAR

Remote sensing of invasive species is a critical component of conservation and management efforts, but reliable methods for the detection of invaders have not been widely established. In Hawaiian forests, we recently found that invasive trees often have hyperspectral signatures unique from that of native trees, but mapping based on spectral reflectance properties alone is confounded by issues of canopy senescence and mortality, intra- and inter-canopy gaps and shadowing, and terrain variability. We deployed a new hybrid airborne system combining the Carnegie Airborne Observatory (CAO) small-footprint light detection and ranging (LiDAR) system with the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) to map the three-dimensional spectral and structural properties of Hawaiian forests. The CAO-AVIRIS systems and data were fully integrated using in-flight and post-flight fusion techniques, facilitating an analysis of forest canopy properties to determine the presence and abundance of three highly invasive tree species in Hawaiian rainforests.

The LiDAR sub-system was used to model forest canopy height and top-of-canopy surfaces; these structural data allowed for automated masking of forest gaps, intra- and inter-canopy shadows, and minimum vegetation height in the AVIRIS images. The remaining sunlit canopy spectra were analyzed using spatially-constrained spectral mixture analysis. The results of the combined LiDAR-spectroscopic analysis highlighted the location and fractional abundance of each invasive tree species throughout the rainforest sites. Field validation studies demonstrated < 6.8% and < 18.6% error rates in the detection of invasive tree species at  7 m² and  2 m² minimum canopy cover thresholds. Our results show that full integration of imaging spectroscopy and LiDAR measurements provides enormous flexibility and analytical potential for studies of terrestrial ecosystems and the species contained within them.     

Self-assembly routes towards creating superconducting and magnetic arrays

Using self-assembly from colloidal suspensions of polystyrene latex spheres we prepared well-ordered templates. By electrochemical deposition of magnetic and superconducting metals in the pores of such templates highly ordered magnetic and superconducting anti-dot nano-structures with 3D architectures were created. Further developments of this template preparation method allow us to obtain dot arrays and even more complicated structures. In magnetic anti-dot arrays we observe a large increase in coercive field produced by nanoscale (50–1000nm) holes. We also find the coercive field to demonstrate an oscillatory dependence on film thickness. In magnetic dot arrays we have explored the genesis of 3D magnetic vortices and determined the critical dot size. Superconducting Pb anti-dot arrays show pronounced Little-Parks oscillations in T_c and matching effects in magnetization and magnetic susceptibility. The spherical shape of the holes results in significantly reduced pinning strength as compared to standard lithographic samples. Our results demonstrate that self-assembly template methods are emerging as a viable, low cost route to prepare sub-micron structures.     

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Abstract

A high-bandwidth optical fibre current sensor has been developed at Salford to monitor the waveform of the driving current pulse feeding into a discharge-pumped excimer laser. The sensor is a conventional intrinsic Faraday effect device but overcomes the normal bandwidth-sensitivity limits because the very large electrical current pulse used to drive the excimer laser allows detectable polarization rotation over a 60 cm length of fibre. The performance of the intrinsic sensor is compared with that of a current-viewing resistor, and of a Be₁₂GeO₂₀ crystal sensor.     

Problems with contracting out government services: lessons from orderly services at SCGH

This paper identifies and discusses the problems associatedwith contracting out labor-intensive government services. Itexamines in detail the contracting out of the orderly serviceat Sir Charles Gairdner Hospital (SCGH) in Western Australiaand its subsequent return to in-house delivery. When the orderlyservice was contracted out, the result was poor in terms ofcost, quality and externalities. Subsequent return of the serviceto in-house provision reduced costs and improved quality. Theproblems that led to failure of contracting out and subsequentsuccess of in-house provision can be explained in terms of transactioncost economics.     

Gain control and diffraction-managed solitons in metamaterials

The question of whether stable, active metamaterials can be created is addressed, both through a discussion of absolute instability and an analysis of a transmission line that produces dispersion analogous to that of the familiar split-ring resonator/wire-based metamaterial. The gain is introduced using negative conductance diodes and it is shown that the frequency bandwidth controls the stability window. The diodes are located as lumped elements in the unit cell. It is demonstrated that the production of an, active, negative phase (left-handed) frequency window, with spatial amplification is possible. The paper then goes on to assume that since loss can be controlled in this way special types of solitons can be created from phase and loss compensation and they could be the basis of future computational devices. The candidates selected are diffraction-managed spatial solitons and their basic properties are displayed numerically.