Showing papers on "Frequency response published in 2002"

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Abstract: Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Effect of cubic nonlinearity on auto-parametrically amplified resonant MEMS mass sensor

Abstract: Parametric resonance has been well established in many areas of science, including the stability of ships, the forced motion of a swing and Faraday surface wave patterns on water. We have previously investigated a linear parametrically driven torsional oscillator and along with other groups have mentioned applications including mass sensing, parametric amplification, and others. Here, we thoroughly investigate the design of a highly sensitive mass sensor. The device we use to carry out this study is an in-plane parametrically resonant oscillator. We show that in this configuration, the nonlinearities (electrostatic and mechanical) have a large impact on the dynamic response of the structure. This result is not unique to this oscillator—many MEMS oscillators display nonlinearities of equal importance (including the very common parallel plate actuator). We report the effects of nonlinearity on the behavior of parametric resonance of a micro-machined oscillator. A nonlinear Mathieu equation is used to model this problem. Analytical results show that nonlinearity significantly changes the stability characteristics of parametric resonance. Experimental frequency response around the first parametric resonance is well validated by theoretical analysis. Unlike parametric resonance in the linear case, the jumps (very critical for mass sensor application) from large response to zero happen at additional frequencies other than at the boundary of instability area. The instability area of the first parametric resonance is experimentally mapped. Some important parameters, such as damping co-efficient, cubic stiffness and linear electrostatic stiffness are extracted from the nonlinear response of parametric resonance and agree very well with normal methods.

Damage detection in composite materials using frequency response methods

Abstract: Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

A statistical path loss model for in-home UWB channels

Abstract: This paper describes a simple statistical model for evaluating the path loss in residential environments. It consists of detailed characterization of path loss model parameters of ultra-wideband band (UWB) signals having a nominal center frequency of 5 GHz. The proposed statistical path loss model is for the in-home channel and it is based on over 300,000 frequency response measurements. Probability distributions of the model parameters for different locations are presented. Also, time domain results such as RMS delay spread and percent of captured power are presented.

Synthesis of direct-coupled-resonators bandpass filters for WDM systems

Abstract: A simple technique for the exact synthesis of selective bandpass filters for wavelength division multiplexing (WDM) systems is presented. The filters consist of direct-coupled ring resonators or cascaded Bragg gratings. The proposed technique permits the calculation of the physical dimensions of the optical structures given the desired frequency response of the filter in terms of bandwidth, free spectral range, out-of-band rejection, and frequency characteristic. The technique, explained in detail for Butterworth and Chebyshev frequency responses, is exact, uses closed-form formulae, and is physically well based. The resulting devices are very compact, have a high out-of-band attenuation, and are suitable for high-density integrated optics. Several filters for applications such as channel add-drop, channel selection, demultiplexing, multichannel filtering, and interleavers are designed, analyzed, and discussed in the paper.

Dynamics and Control of Initialized Fractional-Order Systems

Abstract: Due to the importance of historical effects in fractional-order systems,this paper presents a general fractional-order system and control theorythat includes the time-varying initialization response. Previous studieshave not properly accounted for these historical effects. Theinitialization response, along with the forced response, forfractional-order systems is determined. The scalar fractional-orderimpulse response is determined, and is a generalization of theexponential function. Stability properties of fractional-order systemsare presented in the complex w-plane, which is a transformation of thes-plane. Time responses are discussed with respect to pole positions inthe complex w-plane and frequency response behavior is included. Afractional-order vector space representation, which is a generalizationof the state space concept, is presented including the initializationresponse. Control methods for vector representations of initializedfractional-order systems are shown. Finally, the fractional-orderdifferintegral is generalized to continuous order-distributions whichhave the possibility of including all fractional orders in a transferfunction.

Nonlinear dynamics of microcantilevers in tapping mode atomic force microscopy: A comparison between theory and experiment

Abstract: The nonlinear dynamic response of atomic force microscopy cantilevers tapping on a sample is discussed through theoretical, computational, and experimental analysis. Experimental measurements are presented for the frequency response of a specific microcantilever-sample system to demonstrate the nonlinear features, including multiple jump phenomena leading to reproducible hysteresis. We show that a comprehensive analysis using modern continuation tools for computational nonlinear dynamics and bifurcation problems reproduces all essential features of the data. A close connection is established between the features of the interaction potential well and the nonlinear forced tip response. In particular, the effects of the nonlinear van der Waals forces, the nanoscale contact nonlinearities, and microcantilever damping, as well as the effects of forced and parametric excitation on the bifurcations and instabilities of forced periodic motions of the microcantilever system, are discussed in detail. The results indicate that nonlinear system identification methods could be used as effective tools to extract detailed information about the tip‐surface interaction potential.

De-embedment of optical component characteristics and calibration of optical receivers using rayleigh backscatter

Abstract: Method and system are disclosed for de-embedding optical component characteristics from optical device measurements. In particular, the invention uses frequency domain averaging of the RBS on both sides of an optical component to determine one or more of its optical characteristics. Where the RBS has a slope (e.g., as in the case of a lossy fiber), a frequency domain least square fit can be used to determine the optical component characteristics. In addition, the invention uses a reference DUT to correct for variations in the frequency response of the photoreceiver. A reference interferometer is used in the invention to correct for sweep non-linearity of the TLS. The optical component characteristics are then de-embedded from optical device measurements to provide a more precise analysis of the optical device.

Some considerations on the accuracy and frequency response of some derivative filters applied to particle image velocimetry vector fields

Abstract: This paper concerns the computation of derivatives from particle image velocimetry (PIV) velocity fields with the goal of obtaining the vorticity component normal to the plane. A variety of derivative schemes are characterized by their transfer function, taking into account the truncation and noise amplification. The PIV measurement noise is supposed to be a white one in the Fourier space. A spectral approach is used in order to choose the best filter for turbulent flows. The derivative spectra are discussed. An application is presented on a real turbulent flow with two interrogation window sizes and different derivative schemes. The most significant schemes are also applied to a velocity field containing a single vortex. A comparison of the maximum of vorticity obtained with each scheme and through a least-square fit with an Oseen vortex, allows us to quantify the effect of the band pass filter and to select the best scheme.

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.

Frequency domain identification of dynamic friction model parameters

Abstract: This paper presents a frequency domain identification of dynamic model parameters for frictional presliding behavior. The identification procedure for the dynamic model parameters, i.e., (1) the stiffness and (2) the damping of the presliding phenomenon, is reduced from performing several dedicated experiments to one experiment where the system is excited with random noise and the frequency response function (FRF) of the phenomenon is measured. Time domain validation experiments on a servomechanism show accurate estimates of the dynamic model parameters for the linearized presliding behavior.

A Suggestion of Fractional-Order Controller for Flexible Spacecraft Attitude Control

Abstract: A controller design method for flexible spacecraft attitude control isproposed. The system is first described by partial differential equationwith internal damping. Then the frequency response is analyzed, and thethree basic characteristics of the flexible system, namely, averagefunction, lower bound and upper bound are defined. On this basis, afractional-order controller is proposed, which functions as phasestabilization control for lower frequency and smoothly enters toamplitude stabilization at higher frequency by proper amplitudeattenuation. It is shown that the equivalent damping ratio increases inproportion to the square of frequency.

Variable output power supply

Abstract: A variable output power supply for use in a highly efficient linear amplification system includes an envelope detector having at least one input for receiving an input signal to be amplified. The envelope detector generates, as an output, a control signal that is representative of an envelope of the input signal. The variable output power supply further includes a controllable source that is coupled to the envelope detector. The controllable source generates an output voltage and/or current that is responsive to the control signal, such that the output of the variable output power supply dynamically changes as a function of the envelope of the input signal. When employed in a linear amplification system, the variable output power supply provides a supply voltage to a linear amplifier which can be dynamically varied in response to the envelope of the input signal so as to provide a substantially constant voltage supply headroom for the amplifier. In this manner, an efficiency and frequency response of the amplification system is improved, and a reduction in overall power dissipation of the amplification system can be achieved.

Leak Detection in Pipes by Frequency Response Method Using a Step Excitation

Abstract: This paper presents a new procedure utilizing transient state pressures to detect leakage in piping systems. Transient flow, produced by opening or closing a valve, is analyzed in the time domain by the method of characteristics and the results are transformed into the frequency domain by the fast Fourier transform. This method is used to develop a frequency response diagram at the valve end. The frequency response diagram of a system with leaks has additional resonant pressure amplitude peaks (herein called the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks for the system if there were no leaks (herein called primary amplitude peaks). The location of a leak is determined from frequencies of the primary and secondary pressure amplitude peaks and the leak discharge is determined from the maximum and minimum discharge amplitudes. This method is applicable for practical values of the friction factor over the range 0.01 to 0.025 and can be used to detect leaks in...

Method and apparatus for reducing DC offsets in a communication system

Abstract: Methods and apparatuses for reducing DC offsets in a communication system are described. In a first aspect, a feedback loop circuit reduces DC offset in a wireless local area network (WLAN) receiver channel. The frequency response of the feedback loop circuit can be variable. In a second aspect, a circuit provides gain control in a WLAN receiver channel. First and second automatic gain control (AGC) amplifiers are coupled in respective portions of the receiver channel. Circuits for monitoring DC offset, and for providing control signals for controlling the frequency response of the DC offset reducing circuits are also provided.

On the design and implementation of FIR and IIR digital filters with variable frequency characteristics

Abstract: This paper studies the design and implementation of finite-impulse response (FIR) and infinite-impulse response (IIR) variable digital filters (VDFs), whose frequency characteristics can be controlled continuously by some control or tuning parameters. A least squares (LS) approach is proposed for the design of FIR VDFs by expressing the impulse response of the filter as a linear combination of basis functions. It is shown that the optimal LS solution can be obtained by solving a system of linear equations. By choosing the basis functions as piecewise polynomials, VDFs with larger tuning range than that of ordinary polynomial based approach results. The proposed VDF can be efficiently implemented using the familiar Farrow structure. Making use of the FIR VDF so obtained, an Eigensystem Realization Algorithm (ERA)-based model reduction technique is proposed to approximate the FIR VDF by a stable IIR VDF with lower system order. The advantages of the model reduction approach are: 1) it is computational simple which only requires the computation of the singular value decomposition of a Hankel matrix; 2) the IIR VDF obtained is guaranteed to be stable; and 3) the frequency response such as the phase response of the FIR prototype is well preserved. Apart from the above advantages, the proposed IIR VDF does not suffer from undesirable transient response during parameter tuning found in other approaches based on direct tuning of filter parameters. For frequency selective VDFs, about 40% of the multiplications can be saved using the IIR VDFs. The implementation of the proposed FIR VDF using sum-of-powers-of-two (SOPOT) coefficient and the multiplier block (MB) technique are also studied. Results show that about two-third of the additions in implementing the multiplication of the SOPOT coefficients can be saved using the multiplier block, which leads to significant savings in hardware complexity.

Full-polarization matched-illumination for target detection and identification

Abstract: This paper investigates the optimization of the full-polarization radar transmission waveform and the receiver response to maximize either target detection or identification performance. Application of such full-polarization matched-illumination techniques to simulated VHF-band frequency response data of mobile surface targets yields a significant performance improvement over that corresponding to chirped full-polarization transmission waveforms.

Small-signal dynamic modeling of HVDC systems

Abstract: This paper details the development of a linear continuous-time state model of the small-signal dynamics of a high-voltage direct current (HVDC) transmission system. The dynamics in the frequency range between 2 and 200 Hz on the DC side are of interest for the identification of possible stability problems and the design of the fastest level of converter controls. The state model is formed by representing the system as the collection of a number of interconnected subsystems. This approach is structured and flexible, and can be directly applied to other systems including FACTS. A small-signal state model of the CIGRE benchmark HVDC transmission system is formed, validated against PSCAD/EMTDC time-domain simulation, and used to determine the rectifier DC current control open-loop frequency response.

Exact analysis of dispersive SAW devices on ZnO/diamond/Si-layered structures

Abstract: In this paper, a formulation for calculating the effective permittivity of a piezoelectric layered SAW structure is given, and the exact frequency response of ZnO/diamond/Si-layered SAW is calculated. The effective permittivity and phase velocity dispersion of a ZnO/diamond/Si-layered half space are calculated and discussed. The frequency response of an unapodized SAW transducer is calculated, and the center frequency shift caused by the velocity dispersion is explained. In addition, the electromechanical coupling coefficients of the ZnO/diamond/Si-layered half space based on two different formulas are calculated and discussed. Finally, based on the results of the study, we propose an exact analysis for modeling the layered SAW device. The advantage of using the effective permittivity method is that, not only the null frequency bandwidth, but also the center frequency shift and insertion loss can be evaluated.

In-Cylinder Pressure Reconstruction Based on Instantaneous Engine Speed Signal

Abstract: This paper presents an original methodology for the instantaneous in-cylinder pressure waveform reconstruction in a spark-ignited internal combustion engine. The methodology is based on the existence of a linear correlation, characterized by frequency response functions, between in-cylinder pressure and engine speed signals. This correlation is experimentally verified and evaluated by simultaneous measurements of the above-mentioned quantities. The evaluation of different frequency response functions, one for each steady-state condition investigated, allows recovering the pressure waveform even under other engine running conditions (i.e., transients). In this way, during on-board operation, the pressure waveform could be recovered using only the engine speed signal, already present in current production electronic control units. In this paper the signal processing methodology and some experimental results, obtained during transient tests, are presented. The methodology could be interesting for the development of advanced engine control strategies aimed at the management of the torque generated by the engine. As an example, traction control in drive-by-wire systems could be a possible challenging application. The in-cylinder pressure reconstruction performed using the frequency response functions, in fact, allows the evaluation of the indicated torque. An important characteristic of this methodology is, furthermore, the diagnostic capability for the combustion process, that is guaranteed by the linear correlation between in-cylinder pressure and instantaneous engine speed waveforms. Also in presence of a misfiring cylinder, when the instantaneous engine speed waveform is strongly affected by the absence of combustion, the reconstructed in-cylinder pressure shows a good agreement with the measured one. The experimental tests have been conducted in a test cell using a four-cylinder production engine. It has to be noted, anyway, that the same methodology can be applied to engines with a higher number of cylinders.

Nonlinear dynamics of a noncontacting atomic force microscope cantilever actuated by a piezoelectric layer

Abstract: The nonlinear equations of motion for a silicon cantilever beam, covered by a piezoelectric lead–zirconate–titanate layer, subjected to a Lennard-Jones type boundary condition, are derived for voltage excitation. The Lagrangian of the system is obtained from the electric enthalpy density, including the virtual work of the Lennard-Jones potential, assuming the beam undergoes only small displacements. By application of Hamilton’s principle, the nonlinear equations of motion are consistently derived and truncated to third order for perturbation analysis. The evolution equations are obtained by the multiple scales method and periodic solutions to the equations of motion are determined and discussed with respect to different tip to sample distances. An analytically obtained frequency response function enables determination of the frequency shift of individually piezoactuated microbeams, which are proposed as fundamental elements of parallel atomic force microscopy, undergoing forced vibration in a dissipative environment.

Minimax design of stable IIR digital filter with prescribed magnitude and phase responses

Abstract: In this work, an iterative linear programming approach is presented to design stable IIR digital filters with prescribed magnitude and phase responses. At each iteration, the complex error of the frequency response is transformed into a linear form by treating the denominator polynomial obtained from the preceding iteration as a part of the weighting function, and the poles restricted inside the unit circle by using a set of linear constraints. After solving the standard linear programming problem at each iteration, the design algorithm converges to the minimax solution. Design examples demonstrate that our method provides better design results than the conventional linear programming method.

Frequency-domain identification algorithms for servo systems with friction

Abstract: Mechanical devices usually come with undesirable nonlinearities, such as friction, backlashes, and saturations. Under the assumption of linear systems, the commonly seen identification schemes utilize sinusoidal excitation signals for parameter identification. However, the data needed for identification are unavoidably distorted by the aforementioned nonlinearities and the identification result may not be satisfactory. In the paper, binary test signals are used to perform identification, thus simplifying the behavior of friction. An identification method based on the difference of binary multifrequency excitation signals is proposed. The modified identification algorithm does not suffer from the problem of nonlinear distortions in the signal shape and is able to determine the nonlinear friction such that an accurate servo system model can be derived. A high-precision ball-screw table with asymmetric friction is identified as a test plant for this approach. The results prove that the method can be used very successfully.

Pole-zero tracking frequency compensation for low dropout regulator

Abstract: Most low dropout regulators (LDRs) have a limited of load current operating range due to stability problems. This paper proposes a new frequency compensation scheme for LDR to optimize the regulator performance over a wide load current range. By introducing a tracking zero to cancel out the regulator output pole, the frequency response of the feedback loop becomes load current independent. The open-loop DC gain is boosted up by a low frequency dominant pole, which increases the regulator accuracy. To demonstrate the feasibility of the proposed scheme, a LDR utilizing the new frequency compensation scheme is designed and fabricated using the TSMC 0.35/spl mu/m digital CMOS process. Simulation results show that with output current from 0 /spl mu/A to 100 mA, the bandwidth variation is only 2.3 times and the minimum DC gain is 72 dB. Measurement of the dynamic response matches well with simulation.

Autoregressive modeling of an indoor UWB channel

Abstract: Based on frequency domain measurements in the 4.375-5.625 GHz band a channel model for the frequency response of the indoor radio channel is introduced. In particular. a second-order Autoregressive (AR) model is proposed for frequency response generation of the ultra wide band indoor channel. A complete characterization of the model parameters is described along with probability distributions and dependencies between parameters.

Fuzzy Finite Element Method for Frequency Response Function Analysis of Uncertain Structures

Abstract: A concept is presented for incorporating fuzzy uncertainties in dynamic e nite element analyses of uncertain structures. The objective is twofold. The e rst goal is to clarify and extend the classical fuzzy e nite element (FFE) method as it was introduced for static analyses. The shortcomings of the classical approach are described, and an extension to a generalized approach is proposed. This generalized approach is proven to be a more realistic and therefore more reliable concept for taking uncertainty into account. The second goal is to illustrate the applicability of the method for dynamic analyses. The classical and the generalized approach are compared using an eigenvalue analysis of a simple numerical example. The FFE method is also applied to the calculation of the total envelope frequency response function (FRF) using the modal superposition principle. This method requires safe approximations of the individual mode envelope frequency response functions. For this purpose a number of safe approximate optimization strategies are introduced. The numerical example shows that useful results are obtained using this FFE approach for FRF calculations.

Loudspeaker Voice-Coil Inductance Losses: Circuit Models, Parameter Estimation, and Effect on Frequency Response

Abstract: When the series resistance is separated and treated as a separate element, it is shown that losses in an inductor require the ratio of the flux to MMF in the core to be frequency dependent. For small-signal operation, this dependence leads to a circuit model composed of a lossless inductor and a resistor in parallel, both of which are frequency dependent. Mathematical expressions for these elements are derived under the assumption that the ratio of core flux to MMF varies as ω n-1 , where n is a constant. A linear regression technique is described for extracting the model parameters from measured data. Experimental data are presented to justify the model for the lossy inductance of a loudspeaker voice coil. A SPICE example is presented to illustrate the effects of voice-coil inductor losses on the frequency response of a typical driver.