Showing papers on "Impulse (physics) published in 1989"

Simulation of the transient and steady‐state sound propagation in rooms using a new combined ray‐tracing/image‐source algorithm

Abstract: A new method for the calculation of room acoustical impulse responses is described, which is based on two well‐known computer algorithms, the ray‐tracing and the image‐source models. With the new method, the procedure of sieving the ‘‘visible’’ image sources out of the enormous quantity of possible sources is carried out by examination of the histories of sound particles. From the obtained list of visible image sources, the impulse response of the enclosure is easily constructed. The new method combines the advantages of the ray‐tracing process, namely, the relatively slow increase of computation time with the length of the impulse response, with the accuracy inherent to the image‐source model, which is even sufficient to calculate the Fourier transform, i.e., the steady‐state transmission function of the room, or to convolve the impulse response with sound signals.

Performance of Large Air Gaps under Lightning Overvoltages: Experimental Study and Analysis of Accuracy of Predetermination Methods

Abstract: The paper summarizes the results of the research recently carried out at CESI, with lightning overvoltages. Tests were performed on various gap configurations and with several impulse shapes of both polarities to get information about macroscopic parameters and basic processes associated with discharge development. The basic aim was to have a consistent amount of data to carry out a validation and comparison of the existing models, while providing the required information to support the refinement of those based on physical approach. Breakdown voltages versus time to breakdown characteristics, (Volt-time curves), were determined. The curves give an indication of the importance of the impulse-shape and configuration, thus underlying the necessity of calculations methods. Discharge parameters have been presented and discussed to put into evidence the relative importance of the impulseshape characteristics. Data about the discharge process have been compared with other available data and with evaluations according to methods suggested in the literature. A refinement of the available formulas with reference to leader velocity has also been proposed, as in the following: v1 = 170 * d * e(1.5* 10-3* U/d) * (U/x-Eo) where ?1 is the velocity (m/s), U is the applied voltage (kV), d the gap clearance (m), x the part of the gap unbridged by the leader (m) and Eo the average gradient in the gap of the configuration examined at 50% breakdown voltage with standard lightning impulse (kV/m). The equation is of general type, applicable for all the configurations and for impulses of different shape and polarity.

Estimation of multipath channel response in frequency selective channels

Abstract: A novel discrete-time method is proposed for estimating the impulse response of a frequency-selective digitally modulated communication channel. The received signal is first demodulated and sampled and then the fourth-order cumulants of the resulting discrete-time sequence are estimated. The method estimates the channel impulse response from the complex cepstrum of the aforementioned fourth-order cumulants (i.e. tricepstrum). The method depends only on the second- and fourth-order statistics of the transmitted sequence and is capable of reconstructing nonminimum-phase impulse responses. Monte Carlo simulation results demonstrate the effectiveness of the method, its low sensitivity to observation noise, and its improved performance in terms of probability of error or the reconstructed transmitted sequence. >

Systems governed by ordinary differential equations with continuous, switching and impulse controls

Abstract: Optimal control problem for systems governed by ordinary differential equations with continuous, switching and impulse controls are studied. It is proved that the value function of the problem is the unique viscosity solution of the corresponding Hamilton-Jacobi-Bellman system.

Impulse control of piecewise-deterministic processes

Abstract: In a recent paper we presented a numerical technique for solving the optimal stopping problem for a piecewise-deterministic process (P.D.P.) by discretization of the state space. In this paper we apply these results to the impulse control problem. In the first part of the paper we study the impulse control of P.D.P.s. under general conditions. We show that iteration of the single-jump-or-intervention operator generates a sequence of functions converging to the value function of the problem. In the second part of the paper we present a numerical technique for computing optimal impulse controls for P.D.P.s. This technique reduces the problem to a sequence of one-dimensional minimizations. We conclude by presenting some numerical examples.

η method of numerical integration

Abstract: In the finite element dynamic analysis, the governing partial differential equations are first discretized in space and then the resulting equations are integrated with respect to time. The time integration is an important aspect of the entire analysis since efficiency, economy and accuracy of the solution depends on it, to a large extent. In this paper, a new one step implicit algorithm known as ‘ν method’ suitable for wave propagation problems is introduced. The proposed algorithm includes a term defining an impulse load vector which permits the use of time increments that can be controlled solely by accuracy requirements. The stability and accuracy characteristics of the proposed method are compared with those of the other available methods.

Statistical properties of electrical breakdown in vacuum

Abstract: The breakdown probability distribution function was determined for DC and impulse voltages, for conditions of gap pressure of 10, 0.1, and 10/sup -4/ Pa and gap length of 0.1 mm. It was found that DC and impulse voltages are associated with different types of probability distribution function due to different initiation mechanisms. The statistical influence of the number of previous breakdowns on the probability distribution of the breakdown voltage was investigated. By applying the U test for analysis of measured data, it was found that at lower gap pressure, the breakdown voltage probability changes after a smaller number of breakdowns than it does at higher gap pressure. >

Initial state effects in deep inelastic neutron scattering.

Abstract: We develop a new approach to the impulse aproximation which was first used by Gunn et al [1]. We assume, as is consistent with the existence of a single particle momentum distribution, that the struck atom has initial and final state single particle wavefunctions ψi.(r) and ψf(r) respectively. This is clearly valid when the environment of the atom can be approximated by a single particle potential [2] as for example in the Born-Oppenheimer approximation and can also be formally justified in the impulse limit for a many body system such as He4.

Momentum Distributions in Liquid 4He

Abstract: We report high-resolution neutron scattering measurements at Q = 23 A−1 of 4He in the low-temperature superfluid and in the normal liquid phase at much higher momentum transfers than in previous measurements. When the impulse approximation prediction is broadened according to a recent theory for final-state effects, the data are in excellent agreement with ab initio Monte Carlo calculations of momentum distributions, which predict a Bose condensate fraction of 9.2% in the superfluid at T = 0 K.

A comparison of results from parameter estimations of impulse responses of the transient visual system

Abstract: Parameter estimations of a fourth-order linear model are applied to data from subthreshold measurements of impulse responses of the transient visual system. These impulse responses were obtained experimentally by several subjects, at two different background luminance levels and for different field sizes. The parameter estimations show consistent results over different subjects. For both different background levels and field sizes there are consequent changes in the estimated parameters. On the basis of these changes a proposal is made for a spatiotemporal model of the transient visual system.

Temporal impulse responses from flicker sensitivities: causality, linearity, and amplitude data do not determine phase.

Abstract: A recent paper [ J. Opt. Soc. Am. A4, 1130 ( 1987)] advances a method for the determination of an impulse response from amplitudes of a measured amplitude spectrum. The crucial step in this derivation is the assumption that if a transfer function is analytic and causal, then so is its logarithm. This assumption is tantamount to that of minimum phase and is not justified, both in principle and in practice. In the absence of additional information or assumptions, every amplitude spectrum is consistent with a multiplicity of phase spectra.

LVDT displacement sensor with signal noise reduction for use in weighing apparatus

Abstract: To weigh objects in quick succession, the objects are projected with a known horizontal velocity onto a ramp which has a surface which curves down through 90°. The ramp can move horizontally in response to the impulse corresponding to the change in horizontal momentum of the object. A transducer senses the movement of the ramp and the output horizontal velocity of the object is sensed, enabling the change in horizontal velocity to be calculated. The mass of the object is calculated from the horizontal movement of the ramp and the change in horizontal velocity of the object. In order to obtain accurate sensing of the movement of the ramp, the transducer is a linear variable differential transformer having a primary winding energized by a cyclical carrier signal and two matched secondary windings in series; the secondary winding output signal is rectified to a corresponding DC voltage having ripple components, and a sample is taken of the signal at the zero voltage of the principal ripple component.

Irreversible wall motion in inductive recording heads

Abstract: The effect of signal-induced irreversible domain wall motion on the isolated transition response of recording heads is investigated. An experimental method is presented allowing differentiation between the effects caused by irreversible wall motion and reversible wall motion. The method, utilizing the injection of a small DC current into the head coil, can also be used to determine whether the irreversible motion in a thin-film head occurs in the first pole tip, the second pole tip, or the backgap. The transients observed result from signal-induced irreversible wall motion in the second pole tip. The largest isolated impulse variations occur after saturation of the yoke. In addition, it is found that Barkhausen transitions can occur at or near the top of the isolated impulse (even on its leading edge) as well as after the impulse has decayed, and not only on the trailing edge of the impulse as reported previously. Finally, the authors observe two Barkhausen-noise-free zones, the one at the leading edge being the largest. >

Torque impulse power tool

Abstract: A torque impulse power tool primarily intended for tightening threaded joints comprises a housing (10), a rotation motor (11), a torque impulse generating mechanism (13), and a reduction gearing (12) coupling the impulse mechanism (13) to the motor (11). A torsion spring (21) is associated with the reduction gearing (12) to reduce substantially the difference in deceleration between the motor (11) and the impulse mechanism (13) by yielding elastically to the driving and inertia forces of the motor (11) and the gearing (12) during each impulse generation. The torsion spring (21) is connected to absorb the reaction torque peaks developed in the gearing (12) or the driving torque peaks between the gearing (12) and the impulse mechanism (13).

Electroimpulse deicing - Electrodynamic solution by discrete elements

Abstract: This paper describes a technique for analyzing the electrodynamic phenomena associated with electro-impulse deicing. The analysis is done in the time domain and utilizes a discrete element formulation concept expressed in state variable form. Calculated results include coil current, eddy currents in the target (aircraft leading edge skin), pressure distribution on the target, and total force and impulse on the target. Typical results are presented and described. Some comparisons are made between calculated and experimental results, and also between calculated values from other theoretical approaches. Application to the problem of a nonrigid target is treated briefly.

Active Control of Bending Wave Propagation At Acoustic Frequencies

Abstract: This paper investigates, analytically and experimentally, the possibility of actively blocking the propagation of bending waves along a uniform beam. The work takes the form of a case study; a thin brass plate-beam is used, it is excited with a short duration impulse, and the resulting disturbance spreads dispersively as it travels along the beam. A short portion of the beam is used as an active block. Strain gage sensors are used to drive thin piezoceramic bending moment actuators through a dynamic compensator. The compensation is designed in the frequency domain with reference to the beam equation, but independent of boundary conditions. This paper describes the analytical work, including the nominal design and its performance, the performance degradation due to modeling errors, and the performance degradation due to approximate implementation of the dynamic compensator. The laboratory implementation used analog electronics for the compensator and essentially verifies some of the sensitivity predictions.

Nanosecond Pulse Technique

Abstract: : Contents: Transient Processes in Transmission Lines; Transient Processes in Transmission Lines with the Discrete Heterogeneities. Transformation of Pulses; Impulse Shaping in Linear Distributed Circuits; Impulse Shaping from the Shock Electromagnetic Waves, Which are Propagated in the Lines of Transmission; Pulsing in RC-Circuits with Feedback; Pulsing in Circuits with Inductive Feedback. Recirculators; Pulsing in Solid-State Circuits with Negative Resistance; Other methods of Impulse Shaping; Amplification of Pulses; Oscillography of Pulses; and Measurements of Parameters of Pulses. Russian translations.

The global structure of the Bell-Szekeres solution

Abstract: It is shown that, apart from the impulse waves themselves, the Bell-Szekeres solution has no curvature singularities. Extensions are found in which the only singularity off the impulse wave is of a quasiregular covering-space type. The global structure found is thus quite different from that found by other authors for related solutions.

Prebreakdown currents of vacuum tubes with increased pressure stressed with AC voltage

Abstract: Prebreakdown currents at AC are mainly determined by emission currents, if the capacitive current component is compensated. Under certain conditions, which are dealt with in this study, HF (high-frequency) impulse currents are superimposed. The shape of the HF impulse current curve as a function of time was recorded with a high time resolution. The steepness of relevant current jumps has been evaluated. On the basis of the results the phenomenon of HF impulse currents is interpreted as a consequence of changing gas attachment to the metallic electrodes. >