Frequency response

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

Frequency dependence and frequency control of microbubble streaming flows

Abstract: Steady streaming from oscillating microbubbles is a powerful actuating mechanism in microfluidics, enjoying increased use due to its simplicity of manufacture, ease of integration, low heat generation, and unprecedented control over the flow field and particle transport. As the streaming flow patterns are caused by oscillations of microbubbles in contact with walls of the set-up, an understanding of the bubble dynamics is crucial. Here we experimentally characterize the oscillation modes and the frequency response spectrum of such cylindrical bubbles, driven by a pressure variation resulting from ultrasound in the range of 1 kHz ≲f≲ 100 kHz. We find that (i) the appearance of 2D streaming flow patterns is governed by the relative amplitudes of bubble azimuthal surface modes (normalized by the volume response), (ii) distinct, robust resonance patterns occur independent of details of the set-up, and (iii) the position and width of the resonance peaks can be understood using an asymptotic theory approach. Th...

A Suboptimal Power-Point-Tracking-Based Primary Frequency Response Strategy for DFIGs in Hybrid Remote Area Power Supply Systems

Abstract: Due to the presence of a power electronic converter, the doubly fed induction generator (DFIG)-based wind generators are isolated from grid frequency variations, which will impose significant burden on conventional generators to regulate frequency in a hybrid remote area power supply (RAPS) system. Thus, participation of wind generators in frequency control is increasingly demanded. In this paper, a primary frequency response strategy is proposed for the DFIG to regulate the RAPS system frequency. A droop control loop without conventionally used high-pass filter is integrated to generate the torque reference for the DFIG to provide primary frequency response, and a supplementary control loop is proposed to enhance the primary frequency response with torque feedback control. Furthermore, the suggested suboptimal power point tracking strategy is capable of reserving wind power to improve the frequency response. The proposed strategy enables DFIG to participate in RAPS system frequency regulation while alleviating high rate of change of power and thus stress on the diesel generators under highly variable load demand. The effectiveness of the proposed strategy is validated through simulations.

Modelling iron-powder inductors at high frequencies

Abstract: A high-frequency model of iron-powder core inductors is studied. The skin and proximity effects that cause the winding parasitic resistance to increase with the operating frequency are considered. The inductor self-resonance due to the parasitic capacitances is also taken into account. The frequency response of the inductor model is compared to that of an experimentally tested iron-powder core inductor. The first self-resonant frequency is determined from the plot of the measured reactance and allows for the calculation of the parasitic capacitance. Equations for the inductor parasitic resistance are derived in a closed form. Expressions giving the AC resistance as a function of the operating frequency are given. These expressions allow for an accurate prediction of the inductor power loss over a wide frequency range. The measured and calculated values of the inductor impedance magnitude end phase, the real and imaginary parts of the inductor impedance, the inductance, and the inductor quality factor are plotted versus frequency and compared. Theoretical results were in good agreement with those experimentally measured. Therefore, it is demonstrated that the discussed equivalent circuit has a frequency response matching that of the real inductor. Moreover, the circuit model is simple, it allows for an immediate understanding of iron-powder core inductor behavior and can be easily used in computer simulations of electronic circuits. >

Out-of-plane electret-based MEMS energy harvester with the combined nonlinear effect from electrostatic force and a mechanical elastic stopper

Abstract: This paper presents the fabrication, modeling and characterization of an out-of-plane electret-based vibrational energy harvester (e-VEH) that has both positive and negative charged electret plates integrated into a single seismic mass system. Strong electrostatic spring-softening effect is induced due to the electric field provided by the double-charged electret plates. An elastic stopper is introduced for reliability concern by limiting the motion of seismic mass and meanwhile serves as a functional element to realize spring-hardening effect. The device has an overall volume of about 0.14 cm3 and is fabricated based on MEMS compatible silicon micromachining technology. When subject to weak excitations, the device exhibits an approximately linear frequency response but changes to a significantly larger broadband when strongly excited due to the combined nonlinear effect from electrostatic force and a mechanical elastic stopper. At a high excitation level of 0.48 g, the experimental results show that the device has 3 dB bandwidths of 3.7 Hz for frequency-up sweep and 2.8 Hz for frequency-down sweep, respectively, which demonstrate a large enhancement compared to the linear response (1.3 Hz). An optimal output power of 0.95 μW is also achieved with a low resonance of 95 Hz. This corresponds to a normalized power density of 37.4 μW cm−3 g−2.

Loop shaping based robust control of a magnetic bearing

Abstract: H/sub infinity / robust control design of a magnetic bearing is considered. In particular, the design of a control system using the loop shaping design procedure (LSDP) is presented. After the introduction of the experimental machine and digital controller, model uncertainty and performance of the magnetic bearing are discussed. The performance and robust stability design objectives are specified as the requirements for particular closed loop transfer functions with the weighting functions. The frequency responses of these closed loop transfer functions are approximated by that of the open loop transfer function in a certain frequency band. Experimental results are presented to illustrate the robust stability and good performance of the designed control system. >