Frequency response

About: Frequency response is a research topic. Over the lifetime, 25705 publications have been published within this topic receiving 332249 citations.

Frequency characteristics of wavelets

Abstract: Wavelets detect and locate time of disturbances successfully, but for measurement of power/energy they also have to estimate and classify them accurately. This paper investigates the factors on choice of a certain wavelet function and qualitatively shows how the number of coefficients of the wavelets is an important number that affects output decomposition and energy distribution leakage. Wavelets provide an output in terms of the time-frequency scale. The frequency bandwidth characteristics of these individual wavelet levels provide better understanding of the wavelets. The sampling frequency and the number of data points are important parameters and must be carefully selected to avoid the frequency of interest falling into the end regions.

Primary frequency control by wind turbines

Abstract: The speed of modern converter controlled wind turbines is almost completely decoupled from the grid frequency. Accordingly, wind turbines do not possess a natural response to frequency excursions. In this paper three different control concepts that enable wind turbines to participate on grid frequency control are introduced. The first uses pitch control together with the provision of reserve capacity by operating the wind turbine in part load mode. The second and third strategies utilize the kinetic energy of wind turbines to provide frequency support for a limited time following a disturbance. One involves a control scheme which initiates the partial release of the kinetic energy immediately after the frequency drop is detected. Replenishing the stored energy and thus accelerating the wind turbine then follows during the frequency recovery phase. The third option calls for the wind turbine to accelerate first, and then decelerate by discharging energy during the phase of the disturbance in which the frequency is approaching its minimum, thus limiting the frequency drop more effectively. For all three concepts the control structures are presented, and the effectiveness of the suggested methods is demonstrated using simulation results.

On the multiple microphone method for measuring in-duct acoustic properties in the presence of mean flow

Abstract: Nowadays, the two-microphone method is accepted as the standard as specified in ASTM E1050-90 for measuring in-duct acoustic properties. However, research results on using the least square method with multiple measurement points and broadband excitation have been reported for enhancing the frequency response of the two-microphone method. In this paper, the effects of varying the relative measurement positions on errors in the estimation of the acoustic quantities is studied for the multiple microphone method. Both the theoretical and experimental results show that, among possible sensor positioning configurations, the equidistant positioning of sensors yields the smallest error within the effective measurement frequency range. In addition, it is noted that the measurement accuracy can be increased and the effective frequency range can be widened by increasing the number of equidistant sensors. Measurement examples are shown and the results support the findings.

Analog study of dead-beat posicast control

Abstract: A method is presented which eliminates the oscillations and overshoot in a lightly damped servomechanism in a time of considerably less than one cycle of the uncompensated oscillation. The frequency response of a resonant system compensated in this manner can be made flat up through and beyond the resonant frequency of the system. For dead-beat response in one-half period, the frequency response is down only 3 db at the resonant frequency. Systems designed to have dead-beat responses in much less than one-half period have correspondingly wider bandwidths. The posicast method involves splitting up the input excitation into several fragments. Each fragment is applied at such a time that the sum of all the transient terms is equal to zero after the last excitation. The method is general and can be applied to electrical, mechanical, and even pneumatic systems.

Microwave imaging for tissue assessment: initial evaluation in multitarget tissue-equivalent phantoms

Abstract: A prototype microwave imaging system is evaluated for its ability to recover two-dimensional (2-D) electrical property distributions under transverse magnetic (TM) illumination using multitarget tissue equivalent phantoms. Experiments conducted in a surrounding lossy saline tank, demonstrate that simultaneous recovery of both the real and imaginary components of the electrical property distribution is possible using absolute imaging procedures over a frequency range of 300-700 MHz. Further, image reconstructions of embedded tissue-equivalent targets are found to be quantitative not only with respect to geometrical factors such as object size and location but also electrical composition. Quantitative assessments based on full-width half-height criteria reveal that errors in diameter estimates of reconstructed targets are less than 10 mm in all cases, whereas, positioning errors are less than 1 mm in single object experiments but degrade to 4-10 mm when multiple targets are present. Recovery of actual electrical properties is found to be frequency dependent for the real and imaginary components with background values being typically within 10-20% of their correct size and embedded object having similar accuracies as a percentage of the electrical contrast, although errors as high as 50% can occur. The quantitative evaluation of imaging performance has revealed potential advantages in a two-tiered receiver antenna configuration whose measured field values are more sensitive to target region changes than the typical tomographic type of approach which uses reception sites around the full target region perimeter. This measurement strategy has important implications for both the image reconstruction algorithm where there is a premium on minimizing problem size without sacrificing image quality and the hardware system design which seeks to economize on the amount of measured data required for quantitative image reconstruction while maximizing its sensitivity to target perturbations.