1,721,067 research outputs found
Duodenal erosion with bleeding from a non-functioning islet cell tumour. A case report
Full text linkCITATION: Shuttleworth, R. D. 1988. Duodenal erosion with bleeding from a non-functioning islet cell tumour. A case report. S Afr Med J, 73:546-547.The original publication is available at http://www.samj.org.zaA patient with a clinically non-functioning pancreatic islet cell tumour in the medial wall of the duodenum with erosion of the overlying mucosa presented with gastro-intestinal bleeding. At gastroduodenoscopy, the tumour with its apical ulcer was thought to be a leiomyoma. The lesion was excised locally and two other non-functioning APUDomas were shelled out of the body and tail of the pancreas.Publisher’s versio
Volt-ampere ratings in electronically tuned linear alternators for thermoacoustic engines
Full text linkLinear alternators (LAs) coupled to thermoacoustic engines (TAEs) provide a viable solution to extract energy from a heat source in a variety of applications such as waste heat, energy harvesting, solar thermal and biomass power generation. For the electrical power to be maximised, the acoustic impedances of LA and TAE have to match. This requirement cannot, in general, be met by relying only on the design of the LA, but can be achieved at the control level, by using a fraction of the LA inverter current to create 'electronic stiffness' which contributes to the overall stiffness tuning the resonance frequency. The same concept can, in principle, be used to replace part of the mechanical spring stiffness in order to overcome the limitations in the mechanical design, at the expense of an increase in LA and inverter ratings. The impact of electronic stiffness on LA power capability and ratings is analysed here. Two meaningful scenarios are considered in the analysis: The LA derating for resonance frequency tuning and the oversizing when springs are partially replaced by electronic stiffness. The study is supplemented with experiments on a small-scale LA test rig
Resonance tuning in linear alternator drives via direct-current amplitude modulation
No full textReciprocating Linear Alternators (LAs) are used in many energy harvesting technologies working with pressure waves, such as sea wave energy convertors, Stirling and Thermoacoustic engines. LAs usually incorporate mechanical springs realizing a resonant system which handles the alternating kinetic energy flow stored in the moving mass. In practice, however, parameter inaccuracies and drifts in the operating frequency result in off-resonance operation causing stroke and power drops. This paper presents an adaptive tuning strategy for the electronic stiffness in order to restore resonance and maximum power flow. The algorithm is based on a low-frequency amplitude perturbation of the current component in phase with the stroke. The response of the LA to this low-frequency parametric excitation allows the detection of out-of-resonance conditions and the correction of the current amplitude in phase with stroke in order to restore resonance. The paper discusses an approximated mathematical analysis of the control algorithm and presents validation via simulations
Current-Modulation-Based On-Line Resonance Tuning Strategy for Linear Generator Drives
Full text linkLinear generators (LGs) are frequently used for energy harvesting with free-piston Stirling engines, thermoacoustic engines, and wave energy converters. This article presents a control strategy to track and maintain LG resonance conditions in real time. The algorithm is based on the LG response to a low-frequency amplitude modulation of the current component in phase with the instantaneous position (d-axis). The averaged product of modulated air-gap power and modulation signal is fed into a controller to adjust the d-axis current and restore resonance. The use of air-gap power instead of dc power improves resonance tracking accuracy and eliminates steady-state low-frequency stroke oscillations. This article presents a full theoretical analysis providing accurate steady-state and small-signal models for control synthesis. The broad experimental validation included in the article proves that the control is able to restore resonance even when the force-source introduces significant additional mechanical impedance
Sensorless Control of a Linear Generator for Energy Harvesting Applications
No full textLinear generators are key components for enabling power conversion from reciprocating prime movers such as Stirling and Thermoacoustic engines and wave energy convertors. The control of a linear generator is usually synchronized with instantaneous speed which sets a reference for optimal current-usually in phase with speed. Measuring speed, however, is often challenging because the linear alternator is located in a sealed pressurized vessel or operates in a harsh environment. This paper proposes a simple sensorless control based on a linear observer which estimates instantaneous velocity of the plunger to be used as a phase reference to synchronise the control. The performance and sensitivity of the proposed observer is studied and verified by means of both simulation and experiment. For the particular example the estimated position (from estimated speed integration) is lagging 6.8° towards the measured position, and their amplitudes are different by less then 1%
Active Current Control of a Linear Generator for Energy Harvesting Applications
No full textThis paper presents an algorithm for automatic maximum power point tracking for linear generator drives used in Stirling or Thermoacoustic generators. The method adjusts the magnitude of the current command responsible for the active power set-point. The proposed approach combines a simple model-based feed-forward term with a slow but accurate correction based on the response of the output power to a lowfrequency modulation in the set-point current magnitude. The correction is derived from the phase of output power oscillations and used to adjust the power set-point and achieve activeload matching conditions. In the paper, the control principle is validated with simulations. As a basis for future experimental validation and to back part of the assumptions, the paper also includes experimental results on two-linear-machine test-rig showing the possibility to vary the internal equivalent damping of the prime-mover
Modulation and Feed-Forward Based Damping Matching Strategy for Linear Generators
Full text linkThis article presents an active damping matching control strategy to optimize the output power of linear generators in energy harvesting applications. The method adjusts the magnitude of the current command responsible for active power set-point. The proposed approach combines a simple model-based feed-forward term with a slow but accurate correction based on the response of output power to a low-frequency modulation in set-point current magnitude. The correction is derived from the phase of output power oscillations and used to adjust the power set-point to achieve active-load matching conditions. In order to remove the low-frequency oscillations in the power at steady state, the method automatically stops the current amplitude modulation when maximum output power is reached. In addition, the proposed control incorporates an overstroke protection, which increases the active current component thereby reducing the stroke rapidly to a safe limit. The overall control principle is validated experimentally
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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