Maximum voltage sag compensation using direct converter by modulating the carrier signal

The aim of this paper is to achieve maximum voltage sag compensation of 52% using direct converter based DVR. The DVR topology has only two switches. The DVR is designed to compensate the voltage sag in a phase by taking power from the same phase. A direct converter is connected between the series transformer and the line in which sag compensation is to be achieved. If the actual amplitude of the error signal is used and the amplitude of carrier signal is kept at 1 unit, it is possible to achieve only 22% of sag compensation. If the amplitude of the carrier signal is modulated according to the percentage of existing sag, 52% of the sag is compensated through ordinary PWM technique with the THD less than 5%. Matlab Simulation results are presented for the validating the analysis.

Voltage sag compensation using direct converter based DVR by modulating the error signal

<p class="Text"><span>The aim of this paper is to present a modulation technique to achieve highest voltage sag compensation using direct converter based dynamic voltage restorer (DVR). The DVR topology proposed in this paper, has a direct converter and a series transformer. The direct converter is fabricated using only two bi-directional switches. The DVR is designed to compensate the sag in a phase by taking power from the same phase. The direct converter is connected between the series transformer and the line in which sag compensation is to be achieved. Conventionally, the PWM pulses for the direct converters are produced by comparing the error signal with the carrier signal. The error signal is obtained by comparing the amplitude of voltage in the line with the amplitude of the reference voltage. If the amplitude of the carrier signal is kept constant and the actual amplitude of error signal is used for PWM generation, it is possible to achieve only 22% of voltage sag compensation. But if the error signal amplitude is modulated according to the amplitude of existing voltage sag in the line, 52% of the voltage sag can be compensated with the THD less than 5%. Simulation results are presented for validating the analysis. </span></p>

A comparison in university students of the amplitude of accommodation determined subjectively

Background: Historically, the push-up and the minus lens methods have been used for the measurements of the amplitude of accommodation, and the differences between the results of these methods are well known.Aim: The purpose of this study was to compare three methods for determining the monocular amplitude of accommodation and consider whether agreement exists between such methods.Setting: The study was conducted at the Optometry Clinic, University of Limpopo.Method: Thirty-four (N = 34) African optometry students participated in this study. There were 20 female and 14 male students. The age range of the participants was 20–34 years. Amplitude of accommodation was measured via the subjective push-up, push-down and minus lens methods only on the right eyes of the sample.Results: The highest average amplitude of accommodation was obtained with the push-up method (10.20 D ± 0.96 D), while the minus lens method produced the smallest mean amplitude of accommodation (9.66 D ± 0.75 D). A higher correlation was found between the push-up and push-down methods (r = 0.80, p = 0.06). The smallest correlation was observed between the push-up and the minus lens methods (r = 0.60, p = 0.062). There were no statistically significant differences between the amplitude of accommodation in male and female students for all three methods (p > 0.005).Conclusion: It seems easier to recognise the point where one can identify a target in pushdown amplitude than the point of first sustained blur in the push-up method. The push-up method tends to overestimate the actual amplitude of accommodation because of the effects of depth of focus. The less evaluated method in the literature is the push-away method; however, further research is necessary to answer the question of which (if any) method is more accurate.

Accuracy of Three-Dimensional Shape Measurement Using a Triangulation Method Sensor with Optical Skid

On-machine measurement is used in ultra-processing machining, but it is seldom used in precision machining. For on-machine measurement, it is necessary that the sensor not be easily affected by the moving error or environment vibration in the field of precision machining. A triangulation method sensor with the optical skid method made using a new design concept is proposed to remove the moving error and vibration. The optical skid method used two laser spots with different sizes: the small spot diameter is the stylus and the large spot diameter is the skid [1]. The difference between these two signals reflects the surface shape. The developed sensor comprises an optical source and two optical receiving systems. Each optical receiving system has an imaging lens and a detector. Instead of two laser spots of different sizes, two detectors with different sizes of receiving area serve as the optical skid. Results confirmed the possibility of reducing the influence of the vibration using the developed sensor. In on-machine measurements, measurement of the surface profile with long wavelength is often necessary. If the spot diameter of the skid is not much larger than the surface profile wavelength, then the smoothing effect of the skid is reduced. Therefore, the amplitude of the measured profile by the skid sensor is smaller than actual amplitude of the workpiece. This paper presents a method of reconstructing the surface profile from the measurement results and the obtained effects of the reconstruction method from simulations and experiments.

Difficulties in Predicting Cycles of Failure Using Finite Element Analysis of Acoustically Induced Vibration (AIV) Problems in Piping Systems

While finite element modeling analysis is becoming more frequent for analyzing AIV problems, in the absence of experimental data in large diameter pipe, there is no industry wide accepted methodology for representing the pressure excitation for the pipe so accurate cycles to failure may be predicted. The assumptions involved in determining the actual amplitude of the acoustic excitation, which modes may develop and how they couple with the structure all contribute to the overall uncertainty of the problem. Depending on the degree of correlation assumed between the structural and acoustical mode shapes the results vary dramatically. There are also variations based on the number of participating modes assumed. Relative strengths of a Weldolet®, an Insert Weldolet® that is a variation of Sweepolet® and a Reducing Tee connection were analyzed for a 24×6 inch Sch. 10S and STD connection assuming various degrees of correlation and mode participation. Wide fluctuations in the cycles to failure were observed based on the assumptions; however, the stress ratios between the connections are relatively stable. This suggests the use of an acoustic Stress Intensification Factor (SIF) in conjunction with Fatigue Strength Reduction Factors (FSRF) to determine suitability of connections in AIV service rather than an absolute value of cycles to failure. Further investigation of the trends in the value of SIF as the D/d (branch to header diameter) and D/t (diameter to thickness) ratios over a range of pipe diameter are required before these SIF’s could be put into use. Experimental data for a few controlled failure cases are required to ground the finite element prediction in reality. As the experiment is more likely to be conducted with air the possible pitfalls in extending the results from air to commonly used process fluid are also discussed.

Simulation Study on Amplitude of the Overhead Line Based on Simulink

Aeolian vibration often occurs in the high-voltage transmission overhead lines, and the amplitude of conductors has close relationship with conductors fretting wear. It is very difficult to measure actual amplitude of working conductor, so obtaining the amplitude of the conductor by the means of simulation is of practical significant to research quantitatively the relationship of the amplitude and the fretting wear. In this paper an imitation test-bed was built. The maximum amplitudes in the different location of a conductor in a test condition were obtained by Simulink software simulating, and the results were compared with the maximum amplitudes obtained by experimental test. The result of this comparison showed that between of them were of good similarity, and the locations of vibration nodes and maximum amplitudes were basically same. It was proved to be feasible that the aeolian vibration amplitude of the overhead conductor in kinds of conditions could be obtained by Simulink software simulating.

Partial Reconstruction of Wavelet Transforms: A Technique for Quantifying Amplitude Envelopes of Riser Response Modes

Knowledge of the frequencies and amplitudes and single-mode versus multi-mode behavior of deepwater riser responses is important for accurate fatigue analysis. The extraction of this information from measured riser responses is a significant challenge given the complexity of the phenomena involved with riser vibrations. An existing technique for obtaining modal amplitudes, wavelet ridge extraction (WRE), has been shown to return a smoothed version of the actual amplitude envelope, resulting in attenuated peak values. In this paper, we introduce a new wavelet-based technique, partial reconstruction of wavelet transforms, for accurately extracting amplitude envelopes of the modes from multi-mode signals. The partial reconstruction technique is proposed to circumvent problems presented by WRE by making use of information available from the inverse wavelet transform. It is demonstrated with synthesized test signals that the new technique provides much more accurate modal envelopes than does WRE. Finally, to illustrate the technique for practical riser response signals, the partial reconstruction technique is applied to representative acceleration fluctuations measured from a drilling riser in the Schiehallion field.

Laminar differences in the spatiotemporal structure of simple cell receptive fields in cat area 17

Previous studies of cat visual cortex have shown that the spatiotemporal (S-T) structure of simple cell receptive fields correlates with direction selectivity. However, great heterogeneity exists in the relationship and this has implications for models. Here we report a laminar basis for some of the heterogeneity. S-T structure and direction selectivity were measured in 101 cells using stationary counterphasing and drifting gratings, respectively. Two procedures were used to assess S-T structure and its relation to direction selectivity. In the first, the S-T orientations of receptive fields were quantified by fitting response temporal phase versus stimulus spatial phase data. In the second procedure, conventional linear predictions of direction selectivity were computed from the amplitudes and phases of responses to stationary gratings. Extracellular recording locations were reconstructed histologically. Among direction-selective cells, S-T orientation was greatest in layer 4B and it correlated well (r = 0.76) with direction selectivity. In layer 6, S-T orientation was uniformly low, overlapping little with layer 4B, and it was not correlated with directional tuning. Layer 4A was intermediate in S-T orientation and its relation (r = 0.46) to direction selectivity. The same laminar patterns were observed using conventional linear predictions. The patterns do not reflect laminar differences in direction selectivity since the layers were equivalent in directional tuning. We also evaluated a model of linear spatiotemporal summation followed by a static nonlinear amplification (exponent model) to account for direction selectivity. The values of the exponents were estimated from differences between linearly predicted and actual amplitude modulations to counterphasing gratings. Comparing these exponents with another exponent—that required to obtain perfect matches between linearly predicted and measured directional tuning—indicates that an exponent model largely accounts for direction selectivity in most cells in layer 4, particularly layer 4B, but not in layer 6. Dynamic nonlinearities seem essential for cells in layer 6. We suggest that these laminar differences may partly reflect the differential involvement of geniculocortical and intracortical mechanisms.

Heat transfer from a hot film in reversing shear flow

The two-dimensional thermal boundary layer over a finite hot film embedded in a plane insulating wall, with a shear flow over it which reverses its direction, is analysed approximately using methods similar to those previously developed for viscous boundary layers (Pedley 1976). The heat transfer from the film is calculated both for uniformly decelerated and for oscillatory wall shear, and application is made to predict the response of hot-film anemometers actually used to measure oscillatory velocities in water and blood. The results predict that the velocity amplitude measured on the assumption of a quasi-steady response will depart from the actual amplitude at values of the frequency parameter St greater than about 0·3 (St = ΩX0/U0, where Ω = frequency, U0 = mean velocity, X0 = distance of hot film from the leading edge of the probe). This is in good agreement with experiment. So too is the shape of the predicted anemometer output as a function of time throughout a complete cycle, for cases when the response is not quasi-steady. However, there is a significant phase lead between the predicted and the experimental outputs. Various possible reasons for this are discussed; no firm conclusions are reached, but the most probable cause lies in the three-dimensionality of the velocity and temperature fields, since the experimental hot films are only about 2·5 times as broad as they are long, and are mounted on a cylinder not a flat plate.