Showing papers on "Frequency response published in 1994"

Applied system identification

Abstract: 1. Introduction. 2. Time-Domain Models. 3. Frequency-Domain Models. 4. Frequency Response Functions. 5. System Realization. 6. Observer Identification. 7. Frequency Domain System ID. 8. Observer/Controller ID. 9. Recursive Techniques. Appendix A: Fundamental Matrix Algebra. Appendix B: Random Variables and Kalman Filter. Appendix C: Data Acquisition.

Ultrasonic surgical handpiece and an energy initiator

Abstract: An ultrasonic oscillator drives a tool at a set frequency. An amplitude control runs the oscillator to set the vibration level. A frequency regulator joins the amplitude and the oscillator. A control feedback loop, in the frequency regulator, keeps handpiece linear dynamics. An operational transconductance amplifier, in the oscillator, governs gain of the loop. A circuit connects to the control to retard the rate of current application over time to the amplifier. The circuit has switching to either retard the rate or reset for start up. The amplifier is a current output device with current directly proportional to the bias current and input voltage with bias as gain change for the loop. The circuit limits the bias to the amplifier to modify frequency response and output current. A capacitor delays application of the bias to the amplifier. Replaceable tools of various lengths or shapes positioned along an axis vibrate for surgery at the frequency and a wave length. Tools longer than one wavelength and of configurations tuned to oscillate around the frequency resonate as a function of their material, length and configuration. A flue surrounds the tool and has a hollow elongate semi rigid central body about an axis with a funnel, at one end thereof and a nozzle, at the other to direct annular irrigant/coolant flow therethrough. The funnel and nozzle are resilient. Reinforcing ridges, inside the nozzle, act to maintain concentricity between the flue and nozzle tip and channel irrigant thereabout.

Variable gain active noise cancellation system with improved residual noise sensing

Abstract: An active noise cancellation system includes a series of features for more effective cancellation, greater reliability, and improved stability. A particular feature adapted for headset systems includes locating a residual microphone radially offset from the center of a sound generator to detect a signal more similar to that incident upon the eardrum of the user. In addition, an open back headset design includes perforations on the side of the headset instead of the back, so that the perforations are less susceptible to inadvertent blockage. The system also includes a mecchanism for detecting changes in the acoustic characteristics of the environment that may be caused, for example, by pressure exerted upon the earpieces, and that may destabilize the cancellation system. The system automatically responds to such changes, for example, by reducing the gain or the frequency response of the system to preserve stability. The system further includes other methods for detecting imminent instability and compensating, such as detecting the onset of signals within enhancement frequencies characteristic of the onset of instability, and adjusting the gain or frequency response of the system or suppressing the enhanced signals. The system further includes a mecchanism for conserving batteryl ife by turning the system off when sound levels are low, or adjusting the power supply to the system to correspond to the current power requirements of the system.

Mechanical sensors based on surface acoustic waves

Abstract: A survey is given of SAW sensor systems. Three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing systems. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance frequency, which are described quantitatively. To show the high resolution of active sensor systems, an electronic spirit level (based on SAW resonators) is described. New SAW chirp sensors allow the dispersive interdigital transducer or reflector geometry to be adjusted to the sensor system resolution and measurement range. In passive systems the sensor device is connected to an antenna. The SAW is excited by an interrogating radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensor's information. The time and frequency response of all sensor categories and the possibilities of temperature compensation are discussed.

Steady-state oscillation of hysteretic differential model. I: Response analysis

Abstract: This study considers the periodic forced vibration of nonlinear hysteretic system. Multiharmonic steady-state responses of a general hysteretic system adopting the Bouc-Wec differential model, subjected to arbitrary periodic excitation, are analyzed by the Galerkin/Levenberg-Marquardt method. The frequency/time domain alternation and the fast Fourier transformation (FFT) techniques are introduced in this frequency domain solution procedure. The frequency-controlled and amplitude-controlled algorithms are simultaneously formulated to obtain complete, probably multivalued, frequency response curves. This approach gives possible harmonic, superharmonic, and subharmonic responses of the Bouc-Wen hysteretic system. The response characteristics of softening, hardening, and quasilinear hysteretic systems are studied through numerical computations. The proposed procedure is also extended to analyze the periodic vibration of degrading hysteretic system with amplitude-dependent stiffness deterioration. In addition, a verification is given by comparing with the results obtained by a numerical integration procedure.

Method and apparatus for enhancing sharpness of a sequence of images subject to continuous zoom

Abstract: A bandpass enhancement filter (10) has a pass-band that tracks changes in spatial frequency due to the use of a zooming process upon a digital image signal The primary pass-band of the enhancement filter is derived from a combination of a plurality of secondary bandpass filter sections (30a-30d), each having a different frequency response and each responsive to a gain adjustment (34a-34d) A control signal (26) reflecting a particular zoom ratio is used in the adjustment of the gain applied to the filter sections, thereby proportioning the output of each filter section so that the combined output tracks the zooming process

Indoor propagation measurements at infrared frequencies for wireless local area networks applications

Abstract: In a combination tutorial and research paper, propagation aspects of transmission at infrared (IR) frequencies for wireless in-building communications are explored. The tutorial section of the paper presents basic principles of propagation at IR, a comparison with indoor radio propagation, and the derivation of the channel's baseband model. The research aspect of the paper reports on the results of recent frequency response measurements at eight different sites in a university building. A major result shows that the indoor wireless optical channel is very dynamic, with great variations in the channel's characteristics for data collected in different rooms, in different locations within the same room, and for different orientations of the optical receiver at the same location of the same room. Numerical values of the channel's relative path loss and 3 dB bandwidth, along with frequency response plots covering a wide range of conditions, are presented and discussed. Finally, on the basis of the results of measurements, schemes for improving the performance of future wireless in-building optical transceivers are proposed. >

Analysis of fibre transfer function and determination of receiver frequency response for dispersion supported transmission

Abstract: It is shown that a frequency modulated optical transmitter at 1.5 µm in conjunction with a standard singlemode fibre can have highpass transfer characteristics. Equalising this frequency response, the characteristics of the receiver lowpass filter can be determined that are required for the method of dispersion supported transmission.

Adaptive gain and filtering circuit for a sound reproduction system

Abstract: Adaptive compressive gain and level dependent spectral shaping circuitry for a hearing aid include a microphone to produce an input signal and a plurality of channels connected to a common circuit output. Each channel has a preset frequency response. Each channel includes a filter with a preset frequency response to receive the input signal and to produce a filtered signal, a channel amplifier to amplify the filtered signal to produce a channel output signal, a threshold register to establish a channel threshold level, and a gain circuit. The gain circuit increases the gain of the channel amplifier when the channel output signal falls below the channel threshold level and decreases the gain of the channel amplifier when the channel output signal rises above the channel threshold level. A transducer produces sound in response to the signal passed by the common circuit output.

Crack detection of a beam using the Wavelet Transform

Abstract: The dynamic behaviour of a structure changes when it is subject to damage such as cracking. Recently research has been directed at employing conventional Modal Analysis techniques to investigate this change in simple structures and to study the dependency on the location and size of a crack. The use of the modeshapes to characterise the dynamic behaviour of the structure would require several accelerometers spaced across the structure. As described in this article, a technique has been developed making use of the Wavelet Transform which can be applied to detect the presence of the crack using the measurement from a single accelerometer strategically located on the structure. The method has proved mow sensitive in detecting relatively small cracks than the techniques that attempt to use the shift in resonant frrquencies in the measrxed Frequency Response Function to identify the presence of damage. The study described has been undertaken by simulating a cracked cantilever beam using a simple finite element model to provide data for the analysis. In this case the stiffness matrix, which is affected by the presence of the crack, has been derived from the integration of stress intensit,y fact.ors. The model t,akes into account the effect of crack opening and closing.

Discrete frequency based learning control for precision motion control

Abstract: Concerns MIMO learning control design with well behaved transients during the learning process. The method allows dynamic and inverse dynamic control laws. The theory gives a unifying understanding of the stability boundary for convergence to zero tracking error, and of a stability condition obtained by using frequency response arguments. The former is easy to satisfy, making learning control converge with little knowledge of the system. The much more restrictive frequency response condition is interpreted as a robustness condition, representing the robustness relative to good transient behavior during learning. This ensures that the amplitudes of the frequency components of the error signal decay in a monotonic and geometric manner with each successive repetition. Noncausal zero phase filtering is used both to facilitate the generation of learning controllers having this convergence at important frequencies, and to ensure that the learning controllers maintain this property in the presence of unmodeled dynamics. The approach is in discrete time. Experiments are performed on a 7 degree-of-freedom robot, demonstrating the effectiveness of the design process for producing precision motion control. >

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. >

A robust H/sup /spl infin// power system stabilizer with no adverse effect on shaft torsional modes

Abstract: The H/sup /spl infin// optimal control theory has been used to design a robust power system stabilizer (PSS) to improve transient and dynamic stabilities of a turbogenerator connected to an infinite busbar. It is demonstrated that the effects of disturbances in the machine output can be minimized and sufficient closed-loop stability margins (robustness) can be obtained to tolerate variations in the loop transfer functions, such as those which might arise from unmodeled low-damped high-frequency modes of oscillations. The resulting controller would effectively enhance the synchronizing and damping torques of the machine without the risk of exciting the shaft torsional modes. This is in marked contrast with the unstable performance of linear quadratic (LQ) optimal controllers under similar conditions. The H/sup /spl infin// design methodology also ensures a satisfactory performance of the PSS under a wide range of system operating conditions. >

Effects of the sampling pulse width on the frequency characteristics of a sample-and-hold circuit

Abstract: Frequency characteristics, especially at high frequency, are important in applications such as sample-and-hold circuits for LCD driver ICs where the required signal bandwidth is much wider than the Nyquist rate. The frequency characteristics of a sample-and-hold circuit using analogue switches have hitherto been estimated from the frequency characteristics of a simple RC circuit comprising a storage capacitor and the on-resistance of the analogue switch. However, the effects of sampling pulse width and storage capacitor resetting are not taken into account. By giving an exact frequency-domain analysis of sample-and-hold circuits, the paper clarifies the limitations of conventional RC estimates. It also reveals the effects of storage capacitor resetting. A narrower pulse causes more ripples in the frequency characteristics. Storage capacitor resetting alleviates these ripples although the DC gain deteriorates for a narrow sampling pulse. This analysis was verified by simple experiments. A design for an LCD driver IC is also described as an application example.

Dual-mode dielectric or air-filled rectangular waveguide filters

Abstract: An accurate design technique of dual-mode microwave dielectric or air-filled rectangular waveguide filters is described. Dual-mode coupling is realized by a section of rectangular waveguide with square-corner-cut. The unequal couplings between dual-mode pairs of the cavities is provided by a section of evanescent mode waveguide. Rigorous analyses are applied to characterize the microwave junctions and to obtain the design parameters for the filter. The design procedure used to determine the dimensions of the waveguide junctions with a minimum of computational effort is described. The design is demonstrated by four-pole and six-pole dual-mode rectangular waveguide elliptic function filters. Excellent agreement between computer simulation and experimental results with absolutely no tuning justifies the design. >

Transient performance of substation grounding systems subjected to lightning and similar surge currents

Abstract: The frequency and time domain performance of a typical substation grounding system subjected to a lightning strike is described and discussed. The computed scalar potentials, electric fields and magnetic fields are presented graphically as a function of spatial coordinate, as a function of time and as a function of both. First, a double exponential lightning surge current is injected into the center of a square ground grid. This same surge current is then injected at one corner of the grid. The response of the grounding system to the frequency domain electromagnetic spectrum of this signal is computed by a frequency domain electromagnetic field analysis software package. Once all the frequency responses have been computed, the temporal and spatial distributions of the electromagnetic fields inside and near the substation are obtained by an inverse Fourier transformation of all these responses. The results are compared with some well known low frequency results and to published work in this area. These results indicate that the performance of the grounding system is significantly dependent on frequency and on the point of impact of the lightning strike. >

Nonlinear vibrations of a beam-spring-mass system

Abstract: The nonlinear response of a simply supported beam with an attached spring-mass system to a primary resonance is investigated, taking into account the effects of beam midplane stretching and damping. The spring-mass system has also a cubic nonlinearity. The response is found by using two different perturbation approaches. In the first approach, the method of multiple scales is applied directly to the nonlinear partial differential equations and boundary conditions. In the second approach, the Lagrangian is averaged over the fast time scale, and then the equations governing the modulation of the amplitude and phase are obtained as the Euler-Lagrange equations of the averaged Lagrangian. It is shown that the frequency-response and force-response curves depend on the midplane stretching and the parameters of the spring-mass system. The relative importance of these effects depends on the parameters and location of the spring-mass system.

A fast method for frequency and time domain simulation of high-speed VLSI interconnects

Abstract: Moment matching techniques have been proposed recently for the fast analysis of large linear lumped and distributed VLSI interconnect networks which can include lossy coupled transmission line models. Specifically, complex frequency hopping (CFH) has been applied to generate the actual dominant poles of a network up to any predefined highest frequency of interest. This paper introduces a method, within the context of CFH, which allows the incorporation of subnetworks characterized by measured data in a moment-matching simulation. For this purpose, a novel frequency analysis algorithm is proposed that enables the analysis of a network's frequency response without solving the network at every frequency point and without calculating the poles of the network. Instead, it relies on a technique of generating an approximating transfer function at a minimized number of frequency points. The proposed algorithm requires the solution of the network equations at fewer frequency points compared to a pole searching algorithm for the same accuracy level. The method enables a speedup of 20-100 times compared to conventional frequency analysis techniques of interconnect networks. Several examples are presented which demonstrate the accuracy and efficiency of the proposed technique. >

Estimating the time-delay and frequency decay parameter of scattering components using a modified MUSIC algorithm

Abstract: A superresolution technique to determine the time-delay and frequency response of individual components of electromagnetic scattering is presented. A modification of the MUltiple SIgnal Classification (MUSIC) algorithm is proposed for the data model which includes scattering components with frequency dependent amplitudes. The modified technique is based on pre-multiplying the scattering data by the inverse of an assumed frequency dependence parameterized by a variable /spl alpha/. Estimates of /spl alpha/ are then obtained by considering the roots of the root-MUSIC polynomial for various /spl alpha/. The algorithm is tested for simulated and experimental compact range data. For the example of measured scattering from a diagonal flat plate, different decay parameters corresponding to the single and double diffraction components are determined using the new algorithm. >

Optimization of weighted signal-to-noise ratio for a digital video encoder

Abstract: When processing video television information the low frequency band is inherently of primary interest due to the natural averaging properties of the human eye, combined with the limited frequency response of video display elements such as phosphorus and liquid crystal displays. Noise is generated and observed in the low frequency region of digitized analog video signals due to nonlinearities inherent in the digitization process. This invention reduces the noise measured in the low frequency region by shifting the noise upband and out of the frequencies of interest by adding a dither signal to the analog input signal and employing a 2X decimation digital filter to remove the unwanted dither and spurious intermodulation signals. The disclosed invention allows for a simple and inexpensive means for removal of the dither signal without having to resort to complex dither subtraction techniques employed in the prior art.

Frequency response of PVDF needle-type hydrophones.

Abstract: This paper examines the factors governing the frequency response of ultrasonic polyvinylidene flouride (PVDF) polymer needle-type hydrophones, in particular the sensitivity variations in the lower frequency range of 1–6 MHz. A theoretical model was used to analyze the influence of the hydrophone's diameter, the metal electrodes, thickness, PVDF material properties, the adhesive layer acoustical characteristics and the backing material, on the frequency response of the hydrophone. The results of the theoretical modelling differ by less than ± 0.5 dB from those obtained experimentally from the reciprocity calibration in the frequency range 1–20 MHz. It is shown that the needle hydrophone's diameter and backing material are the main reasons for the sensitivity variations observed in the frequency range below 6 MHz.

A surface motor-driven precise positioning system

Abstract: A novel precise positioning system incorporating a surface motor (SFM) drive, called the SFM positioning system, has been developed and its performance experimentally evaluated. The positioning stage is guided along a base plate by three air-bearing pads and driven in 3 of freedom ( x , y , θ z ) by coupled forces produced by three brushless-type linear DC motors. The positioning and guidance of the stage's planar motion is accomplished via the simultaneous operation of three identical force-controlled servo systems with a laser-interferometer position feedback loop. The system's advantageous design features and its fundamental configuration and operating principles are described. In addition, we discuss the design of the servo control system, being based on proportional-integral-derivative and decoupling compensations and providing dynamic accuracy as well as rapid response capabilities, including the control system configuration, which incorporates a high-performance parallel processor. Evaluations of experimental positioning tests indicate that a resultant positioning resolution of 10–20 nm, maximum speed of 150 mm/s, and frequency response bandwidth of 105 Hz are successfully achieved.

A new approach to phase cancellation in repetitive control

Abstract: Effective repetitive control requires that the q-filter, placed within the internal model loop for robustness, have zero phase shift and that the phase shift of the plant be cancelled in the feedforward loop. Previously reported methods of phase cancellation use a moving average q-filter and zero-phase error tracking techniques for plant phase cancellation and are memory and processor intensive. The theory and methods of using a new method to cancel plant and filter phase shift in a repetitive control system are described. This method uses a tunable time advance and enables an analog/digital repetitive control loop to be used. Comparison with previously reported methods shows that it provides good phase cancellation, reduces low frequency repetitive disturbances, is easily tunable based on the closed-loop frequency response of the system without repetitive control, is less memory intensive, and is more easily implemented. >

High speed, low power direct digital synthesizer

Abstract: A method and apparatus suitable for generating programmable digital sine waves which involves converting the output of a direct digital synthesizer or numerically controlled digital oscillator to a higher frequency with a multiplier-less structure that takes advantage of the properties of trigonometric identifies for sine and cosine. Sine waves are generated digitally using a phase accumulator which is clocked at one fundamental frequency. The phase accumulator input provides a control word which determines the intermediate frequency of the direct digital synthesizer output. Taking advantage of the periodicity of the phase accumulator operation, the outputs of the accumulator are utilized to address a read only memory ROM lookup table which produces in-phase and quadrature samples of the sine wave at the intermediate frequency. The in-phase and quadrature samples are then complemented (i.e. negated) to produce an additional set of in-phase and quadrature samples which are 180° out of phase from the original samples. Switching between these four possible outputs at a higher fundamental clock frequency results in the translation of the intermediate frequency output to a higher frequency.

Method and apparatus for measuring ground impedance

Abstract: A method and apparatus for measuring impedance of grounding system wherein a transient electric current is injected at the ground system under test while measuring the a transient ground potential difference, as well as the injected electric current. The ground potential difference is measured with respect to several points on the earth's surface at small distances from the test system. These measurements are processed by computer software which filters noise, corrects voltage and current transducer errors, and estimates the test system ground potential rise with respect to remote earth. Correction algorithms are used to remove irregularities in the frequency response of the voltage and current transducers and analog filters. The filtered and corrected measurements are used to estimate the ground impedance as a function of frequency. For this purpose, a parameter estimation procedure is used which takes into consideration the geometric arrangement of the voltage and current probes.

Dispersive models for the finite-difference time-domain method : design, analysis, and implementation

Abstract: Digital signal processing techniques are used to design, analyze, and implement discrete models of polarization dispersion in finite-difference time-domain (FDTD) simulations. These methods are warranted by the extreme importance of dispersion to the propagation behavior of femtosecond-duration optical pulses. Input-invariant and frequency-approximation methods of designing discrete analogs of continuous-time dispersive electromagnetic systems are presented. Our methods are shown to unify existing design techniques. The inherent accuracy of each dispersive design is quantified by truncation error and frequency response methods, and stability analysis of the total FDTD system is found through root locus techniques. Implementation of the design is accomplished by algebraic manipulation of the system function in the frequency domain, resulting in canonical, partial fraction, or cascade structures that minimize the number of stored variables and provide a trade-off between efficiency and sensitivity to finite precision.