Showing papers on "Impulse (physics) published in 1991"

Non-line-of-sight single-scatter propagation model

Abstract: A propagation model that describes the temporal characteristics of singly scattered radiation in a homogeneous scattering and absorbing medium is presented. The model generalizes previous results in the area and is used to analyze the angular spectrum of singly scattered energy as well as the impulse-response durations and path losses of short-range non-line-of-sight optical communication systems. It is shown that the angular response starts to drop off significantly at an off-axis angle equal to the receiver half-field of view. It is also shown that lower path losses correspond to longer impulse responses so that a lower available bandwidth is indicated. These results are based on numerical examples motivated by the operation of non-line-of-sight communications links in the middle-ultraviolet wave band.

Optimal infinite impulse response zero crossing based edge detectors

Abstract: We present formal optimality criteria and a complete design methodology for a family of zero crossing based, infinite impulse response (recursive) edge detection filters. In particular, we adapt the optimality criteria proposed by Canny (IEEE Trans. Pattern Anal. Mach. IntelligencePAMI-8, 1986, 679–714) to filters designed to respond with a zero crossing in the output at an edge location and additionally to impulse responses which are (allowed to be) infinite in extent. The spurious response criterion is captured directly by an appropriate measure of filter spatial extent for infinite responses. Infinite duration impulse responses may be implemented efficiently with recursive filtering techniques and so require constant computation time with respect to scale. As we will show, we can achieve both superior performance and increased speed by designing directly for an infinite impulse response than by any of the proposed finite duration approaches. We also show that the optimal filter which responds with a zero crossing in its output may not be implemented by designing the optimal peak responding filter (similar to Canny) and taking an additional derivative. It is necessary to formulate the criteria and design for a zero crossing response from the outset, else optimality is sacrificed. Filter parameters and performance criteria are presented for several designs, and experimental results are presented on a variety of images which demonstrate the behavior in the presence of very adverse noise, with respect to scale, and as compared to other “optimal” IIR filters which have been reported.

Impulsive excitation of ULF waves in the three‐dimensional dipole model: The initial results

Abstract: Impulsively excited ULF waves are studied in the three-dimensional dipole magnetosphere. The properties of coupled compressional and transverse wave fields are found to depend on the longitudinal size of the impulse which is assumed at the magnetopause. The distribution of energy density and the wave spectra are shown for different meridians. The authors study how compressional waves propagate in the three-dimensional space and excite the corresponding field line resonances, and how the impulse produces ULF waves with many azimuthal wave numbers m. The results suggest that, as the longitudinal width of the impulse becomes larger at the magnetopause, strong field line resonances are likely to be found with the lower m. On the contrary, an impulse which is narrow in longitude may excite the ULF waves with relatively high m. They examine the propagation of both compressional and transverse waves in the azimuthal direction. The numerical results indicate that transverse waves as well as compressional waves are inclined to have harmonic structures in the azimuthal direction, even though it is well known that the Alfven shear modes become one-dimensional modes along the field lines in the absence of coupling. The structures and coupling of global compressional and toroidal standing wavesmore » are also investigated and compared with the previous two-dimensional results.« less

A model for breaking wave impact pressures

Abstract: This paper discusses a mathematical model of the large, short-lived pressures brought about by waves breaking against coastal structures. The idea of pressure impulse, P (the integral of pressure with respect to time from the start to the finish of the impact) is used to simplify the equations of ideal incompressible fluid notion. P satisfies Laplace's equation in a domain which is the mean position of the wave during the very short time of impact. We solve analytically a two-dimensional boundary - value problem, which models an idealized wave striking a vertical wall. Expressions are derived for the impulse on the wall, the peak pressure distribution, and the change in fluid velocity due to impact. The results are insensitive to the shape of the wave far from the wall. The results agree with some experimental measurements, from the literature.

Estimating Orthogonal Impulse Responses via Vector Autoregressive Models

Abstract: Impulse response functions from time series models are standard tools for analyzing the relationship between economic variables. The asymptotic distribution of orthogonalized impulse responses is derived under the assumption that finite order vector autoregressive (VAR) models are fitted to time series generated by possibly infinite order processes. The resulting asymptotic distributions of forecast error variance decompositions are also given.

Surface charging and flashover of spacers in SF/sub 6/ under impulse voltages

Abstract: The authors describe a series of experiments to study prebreakdown spacer charging and subsequent flashover in a compressed gas insulated rod-spacer-plane system under positive impulse voltages. Prebreakdown corona is identified as the most likely source for spacer charging. Surface charges were measured in situ with capacitive probes after the applied lightning impulse voltage had decayed to zero. Numerical computation of resultant electric fields, using the surface charge simulation method, shows that the electric field due to surface charges significantly lowers the electric field near the rod electrode but enhances the overall electric field over the rest of the spacer surface. The flashover voltage with a spacer is higher than for a rod-plane gap. A reversal of the applied impulse polarity may lower the flashover voltage for a rod-spacer-plane gap. >

Mechanism of crystallization of multicomponent metallic coatings using the impulse plasma method

Abstract: The impulse plasma method was used to obtain complex metallic coatings. Taking into account the way of the central electrode of an impulse plasma accelerator is worn, and the features of the coatings, a mechanism for crystallization of metallic coatings from impulse plasma is proposed.

Quantum-mechanical impulse approximation for single ionization of hydrogenlike atoms by multicharged ions

Abstract: A quantum-mechanical approximation is developed for ionization of one-electron targets by charged-ion impact. The model is based on the nonrelativistic distorted-wave formalism valid for impact velocities larger than the electron orbital velocity in the initial state. The exact impulse wave function is used to describe the initial state, thus incorporating the projectile potential to all orders. The final state is represented by a product of continuum Coulomb wave functions around both centers, providing the correct asymptotic conditions and the projectile and target cusps. The theory is thought to be valid for large projectile charge, even larger than the ion velocity. The impulse approximation developed here is expensive in computing time, but it is probably one of the few models to deal with high projectile charges. Double-differential cross sections are computable in the forward and backward ejection angles. Comparisons with the experiments in different regions of interest are presented, including the binary sphere, capture to the continuum cusp, ridge electrons, and backward ejection angles. The theory proves to be quite successful, and it does not seem to deteriorate with increasing projectile charge.

Impulse compression using soliton effects in a monolithic GaAs circuit

Abstract: A monolithic GaAs impulse compressor circuit which utilizes soliton wave propagation effects in nonlinear transmission lines has been fabricated. The circuits compress a 20 dBm, 8 GHz sinusoid to a train of 3.9 V peak to peak, 5.5 ps full width at half maximum impulses.

Adjustable pressure dual piston impulse clutch

Abstract: A hydraulic torque impulse generator (10) uses a dual piston arrangement to provide impacts to a rotative anvil (20) and automatic shut-off and control (42, 44, 46, 48) for limiting the pressure without reversing the direction of the driving clutch cage (22), a pressure venting arrangement (34) permitting one impact per revolution.

A nonaliased integral method for dip moveout

Abstract: Integral (Kirchhoff-style) methods for dip moveout (DMO), while possessing several advantages over Fourier transform methods, are prone to problems of spatial aliasing. DMO methods with spatially aliased operators yield significant processing errors, even when applied to data that are not spatially aliased. In particular, such DMO methods may alter the amplitude and phase of horizontal reflections, to which DMO should do nothing.A simple test can be used to detect spatial aliasing in a DMO implementation. First, compute the responses of DMO to an impulse for several source-receiver offsets and recording times. Second, sum the traces in each of these impulse responses to obtain the corresponding horizontal-reflection responses. These horizontal-reflection responses should be identical to the original impulse for all offsets and times. Integral DMO methods with spatial aliasing problems fail this test.Integral DMO methods typically apply a sequence of time compressions to an input seismogram so that each input sample forms an elliptical impulse response. These methods typically yield horizontal-reflection responses with significant errors in amplitude and phase that vary with source-receiver offset and recording time. Perhaps surprisingly, the worst errors may occur for small offsets and late times, for which DMO action should be inconsequential.The integral DMO method proposed in this paper applies a sequence of time shifts to each input seismogram. These shifted input traces are mapped to output traces along curved trajectories that enable each sample to form the appropriate elliptical impulse response. The proposed method, by design, passes the test described above. Even the most approximate, highly efficient implementation of this method will improve the imaging of dipping reflections without altering horizontal reflections.

Applied-field MPD thruster geometry effects

Abstract: Eight MPD thruster configurations were used to study the effects of applied-field strength, propellant, and facility pressure on thruster performance. Vacuum facility background pressures higher than about 0.12 Pa were found to significantly influence thruster performance and electrode power deposition. Thrust efficiency and specific impulse increased monotonically with increasing applied field strength. Both cathode and anode radii fundamentally influenced the efficiency-specific impulse relationship, while their lengths influenced only the magnitude of the applied magnetic field required to reach a given performance level. At a given specific impulse, large electrode radii result in lower efficiencies for the operating conditions studied. For all test conditions, anode power deposition was the largest efficiency loss, and represented between 50 percent and 80 percent of the input power. The fraction of the input power deposited into the anode decreased with increasing applied field and anode radius. The highest performance measured, 20 percent efficiency at 3700 seconds specific impulse, was obtained using hydrogen propellant.

The Measurement of Nonlinear Damping in Single-Degree-of-Freedom Systems

Abstract: An efficient experimental method which provides for the measurement of both the linear and nonlinear damping for a single-degree-of-freedom system is proposed. The results from a numerical simulation study of a model with «drag» type quadratic damping are shown to give reliable estimates of parameters of the system when both random and impulse excitation techniques are used

A novel method for electret production using impulse voltages

Abstract: A novel method for producing electrets is proposed in which an electric discharge is induced by a negative impulse voltage in a small air gap. The dielectric foil to be charged is placed between two planar electrodes, being in contact with one of them and separated from the other by the air gap. The electric discharge in the gap causes electric charges to deposit onto the dielectric surface where they are trapped, thus forming the electret. Measurements carried out with Teflon FEP (fluoroethylene propylene) show a very uniform deposition, and that the sample surface potential increases linearly with increasing impulse peak voltage. In principle, this method compares with the corona discharge for the production of electrets with regard to high turnout and low cost. >

Modeling of the dynamic performance of transient recorders used for high voltage impulse tests

Abstract: Digital recorders are becoming more commonly used in the recording of steep-front HV impulses Although many digitizers exhibit a nonlinear deterioration in dynamic performance with increasing input signal steepness, there is no accepted method for predicting the magnitude of the resulting errors The authors describe the development and implementation of a computer modeling technique for establishing the magnitude of these errors The model is based on time-domain test data combined with a basic understanding of the digitizer's operating principles The model is general in nature and can be used for the prediction of errors generated by any recorder that has systematic errors The model was used to predict the maximum error which can occur when a particular EBS (electron-bombarded semiconductor) recorder is used to monitor HV impulse tests on power apparatus Its use allowed for the determination of error limits for records of steep-front impulse tests which will be standardized in the near future >

Active control of broadband structural vibration using the adaptive LMS algorithm.

Abstract: Adaptive feedforward control algorithms have been successfully implemented to attenuate structural vibration and its sound radiation when subjected to single and multiple sinusoidal excitations. Reduction of broadband noise in ducts and enclosures using both finite and infinite impulse response adaptive filters has been reported in the literature. However, feedforward control of structural response under broadband excitation has not been experimentally demonstrated. Here, an adaptive feedforward controller based on the filtered‐x LMS algorithm has been developed for the active control of broadband vibration in structures. The control signal is obtained by filtering the reference signal through an adaptive finite impulse filter (FIR). The filtered‐x LMS algorithm requires an estimate of the transfer function between the control input and the error output. This transfer function is represented by an infinite impulse response (IIR) filter. Two control configurations are then experimentally investigated on a simply supported beam. A power reduction of up to 15 dB is demonstrated. [Work supported by ONR/DARPA.]

Magnetospheric cavity modes: Some nonlinear effects

Abstract: Linear models of magnetospheric cavity modes are unlikely to be directly applicable to events generated by, for example, large solar wind pressure pulses. To determine basic nonlinear effects, we apply a two-dimensional, nonlinear hydromagnetic computer code to the problem of pure fast mode cavity resonances in the simplest possible box model of the magnetosphere, stimulated by a magnetopause compression and relaxation with maximum velocity ν0, normalized against the Alfven speed at the magnetopause boundary. The results show that significant nonlinear effects can occur for ν0 ∼ 0.2. The initial impulse develops into a relatively weak shock as it propagates into the magnetosphere. After cavity modes are set up, their temporal development is very different from the corresponding linear modes. In particular, the cavity mode frequencies are reduced, and the temporal structure of the modes is apparently not well represented by sinusoidal functions. Much of the change in behavior can be attributed to a large distortion of the background plasma mass density, which we consider to be an effect of the ponderomotive force (PMF). We develop expressions for the PMF as it applies to small but finite amplitude cavity modes in our box model and show that these expressions are consistent with the simulation results. For a cold plasma, extreme density enhancements occur at and near the magnetospheric equator. We consider such large enhancements to be unlikely, but enhancements of the order of 100% can occur in a warm plasma, accompanied by adiabatic heating within the enhancements, increasing the plasma temperature by a factor of about 2. The density enhancements are limited by the propagation of slow magnetosonic perturbations away from the positions of maximum density. This leads to quasi-cyclic density structures as the perturbations reflect from the ionospheric boundaries, the quasi-period depending on the sound speed and therefore the plasma parameter β. Complex density structures appear to evolve rapidly when β is of the order of 0.05. We consider that mass transport by the PMF is likely to occur in the magnetosphere when relatively large ULF wave fields exist, regardless of the exact driving mechanism.

Critical switching impulse strength of long air gaps: modelling of air density effects

Abstract: A physical modeling approach to investigate the effect of reduced air density on leader inception and sparkover of long air gaps under positive switching impulses with critical front time is presented. The model accounts for the effect of air density on continuous leader inception voltage, leader length, and sparkover voltage. The results of the model provides critical positive switching impulse air density correction factors for rod-plane, rod-rod, conductor-plane, conductor-rod, and conductor-tower window gaps over a wide range of gap distances and relative air densities. The model findings were extensively checked against previous experimental results with quite satisfactory agreement. >

Engineering mechanics : statics and dynamics

Abstract: 1. Fundamentals 2. Forces 3. Equilibrium of Particles 4. Moments of Forces 5. Equilibrium of Rigid Bodies 6. First Moments: Centroids and Centers of Gravity 7. Second Moments: Moments of Inertia 8. Structures 9. Friction 10. Virtual Work 11. Kinematics of Particles 12. Kinetics of Particles: Force and Acceleration 13. Kinetics of Particles: Work and Energy 14. Kinetics of Particles: Impulse and Monetum 15. Plane Kinematics of Rigid Bodies 16. Plane Kinetics of Rigid Bodies: Force and Acceleration 17. Plane Kinetics of Rigid Bodies: Work and Energy 18. Plane Kinetics of Rigid Bodies: Impulse and Momentum 19. Motion of Rigid Bodies in Three Dimensions 20. Vibrations Appendices A. Prior Basic Mathematics B. Review of Basic Mathematics C. Infinitesimal Angular Displacement D. Simultaneous Equations Solver E. Digital Root Finder F. Supplementary Software Answers to Selected Developmental Exercises and Problems Index 1. Fundamentals 2. Forces 3. Equilibrium of Particles 4. Moments of Forces 5. Equilibrium of Rigid Bodies 6. First Moments: Centroids and Centers of Gravity 7. Second Moments: Moments of Inertia 8. Structures 9. Friction 10. Virtual Work 11. Kinematics of Particles 12. Kinetics of Particles: Force and Acclleration 13. Kinetics of Particles: Work and Energy 14. Kinetics of Particles: Impulse and Momentum 15. Plane Kinematics of Rigid Bodies 16. Plane Kinetics: Force and Acceleration 17. Plane Kinetics: Work and Energy 18. Plane Kinetics: Impulse and Momentum 19. Motion of Rigid Bodies in Three Dimensions 20. Vibrations Appendices Prior Basic Mathematics Review of Basic Mathematics Infinitesimal Angular Displacement Simultaneous Equations Solver Digital Root Finder Supplementary Software Answers to Selected Developmental Exercises and Problems Index

Transient analysis of lossy multiconductor transmission lines in nonlinear circuits

Abstract: Using the frequency-dependent transmission line parameters, two time-domain models are developed for lossy multiconductor transmission lines. It is shown that the end-points of a lossy multiconductor line can be represented at each time step by discretized Thevenin or Norton equivalent circuits. Because these models contain only lumped elements, they can be easily implemented in a general circuit analysis program for simulating the transient responses of nonlinear circuits. The analysis procedure developed makes exclusive use of infinite line impulse responses in the formulation of the time-domain models. Because infinite lines are matched, there are no reflections in the impulse responses. The result is that these impulse responses are relatively short. Results are shown for the most general case of lossy multiconductor transmission lines with discontinuities and nonlinear terminations in which the modal transformation matrices are necessarily frequency dependent. >

An impulse-response based linear time-complexity algorithm for lossy interconnect simulation

Abstract: A linear time-complexity algorithm for lossy transmission line simulation within arbitrary nonlinear circuits is presented. The method operates by storing information about the state of the line at dynamically selected internal points and using an analytical formulation based on impulse responses to predict the line's future behavior accurately. Previous approaches using impulse responses possess quadratic-time complexity. The proposed method does not require rational or other approximations of transfer functions to achieve linear time-complexity, nor does it increase the size of the simulator's matrix by more than 2 for each transmission line. Experimental results on industrial circuits indicate that, for equivalent or superior accuracy, the state-based method can be faster for simulations of one or more block or data pulses, with speedups of more than 10 and 50 over the convolution and lumped-RLC methods for the longer simulations. >

Motion Detection and Adaptation in Crayfish Photoreceptors A Spatiotemporal Analysis of Linear Movement Sensitivity

Abstract: Impulse and sine wave responses of crayfish photoreceptors were examined to establish the limits and the parameters of linear behavior. These receptors exhibit simple low pass behavior which is well described by the transfer function of a linear resistor-capacitor cascade of three to five stages, each with the same time constant (tau). Additionally, variations in mean light intensity modify tau twofold and the contrast sensitivity by fourfold. The angular sensitivity profile is Gaussian and the acceptance angle (phi) increases 3.2-fold with dark adaptation. The responses to moving stripes of positive and negative contrast were measured over a 100-fold velocity range. The amplitude, phase, and waveform of these responses were predicted from the convolution of the receptor's impulse response and angular sensitivity profile. A theoretical calculation based on the convolution of a linear impulse response and a Gaussian sensitivity profile indicates that the sensitivity to variations in stimulus velocity is determined by the ratio phi/tau. These two parameters are sufficient to predict the velocity of the half-maximal response over a wide range of ambient illumination levels. Because phi and tau vary in parallel during light adaptation, it is inferred that many arthropods can maintain approximately constant velocity sensitivity during large shifts in mean illumination and receptor time constant. The results are discussed relative to other arthropod and vertebrate receptors and the strategies that have evolved for movement detection in varying ambient illumination.

Statistical study of impulse corona inception parameters on line conductors

Abstract: Laboratory tests on corona on line conductors of cylindrical cross-section have provided charge measurements and surface field data for negative corona resulting from double-exponential and oscillatory impulses. The charge measurements show that minimum corona charge is injected for impulse front times intermediate between the standard lightning and switching impulse shapes. This result is associated with the interdependence of the corona charge and the statistical time lag of corona inception and with the time required to clear the corona space charge. Time lag effects are simulated by applying critical volume theory to line conductor geometry. The corona space-charge field reduction is quantified by the electric field measurements using a cylindrical field filter.

Impulse photopyroelectric depth profiling of multilayers. Part II: Experiments with amplitude- and phase-modulated wideband spectrometry

Abstract: Impulse response photopyroelectric effect spectrometry (PPES) was implemented with the use of wideband excitation for the recovery of depth profiles of optical absorption in a series of multilayered thin-film polymer samples. The wideband PPES technique intensity modulates a cw laser beam with a waveform whose power spectrum is flat over the photothermal response bandwidth, enabling the recovery of the PPES impulse response (short pulse equivalent) by correlation and spectral analysis techniques. The impulse response measurement recovers a depth profile of subsurface optical absorption through the spatial dependence of the heat flux deposited in the sample by a short optical pulse. It shows excellent potential for use as a nondestructive evaluation tool in the analysis of polymers. The impulse response data recovered in this work could be accounted for by means of a theoretical model which neglected thermal reflections between the constituent sample layers. Absorption coefficient measurements on a single-layer sample were recovered through the time delay dependence of the PPES impulse response. The data were corrected for the effect of reflectivity at the thin-film pyroelectric detector surface, which significantly affects the time dependence of the impulse response. Several types of measurements were demonstrated on optically inhomogeneous samples. These included measurements of the thickness of transparent overlayers deposited on opaque substrates, depth-resolved spectroscopy, and techniques for improving multilayer contrast. The role of thermal wave reflections at the gas/solid interface was identified as a key factor in determining the depth profile contrast for subsurface spectral contributions, as well as the dependence of the impulse response on the thickness of nonabsorbing overlayers. Several approaches for optimizing photothermal depth profile contrast were demonstrated.

The Effect of Release Velocity and Geometry on Burning Times for Non-Premixed Fuel Gas Clouds

Abstract: Release velocity has a large effect on the burning time of a fuel gas cloud, changing it by a factor of up to four. Previous studies have usually ignored this aspect, as direct measurement of the velocity is difficult, due to the transient nature of the release. We measured the fuel gas release velocity from the impulse given to a ballistic pendulum. The mean velocity of the fuel gas is the impulse divided by the mass of fuel. Using this method, we studied fireballs with velocities up to 88m/sec, with fuel masses from IS to 13 grams. The release geometry was varied to simulate the flow from a ruptured fuel tank. The fireballs acted as scale models for much larger releases with velocities up to the sonic region. The results were correlated using dimensional analysis, which defines the conditions for results to be scaled to larger fuel masses. This gave an equation relating the burning time to the fuel mass and initial fuel release velocity. Burning times of other workers using pressurised LPG obey...