Magnitude Of Attenuation Research Articles

Digital Filters for Gain Stabilization of Flexible Vehicle Dynamics

The necessity of having low-cost aerospace vehicles with short development times means that control designers need to work with simplified and approximate dynamic models. Aerospace vehicles typically being light and slender, exhibit body bending and flexibility effects at relatively lower frequencies. It may not be possible or practical in every case to carry out detailed test and analyses exercises to determine the structural dynamic characteristics of a vehicle. So, body bending shapes and slopes may not be precisely known; the mode frequencies can however be roughly estimated through simplified analysis. Here it will be assumed that the flexible mode frequencies are approximately known, and are sufficiently high so that gain stabilization is possible. This paper discusses different digital filters for gain stabilization of flexible vehicles, and elaborates their advantages and drawbacks. Various filters are compared; Butterworth, Bessel, Chebychev, Elliptical and simple quadratic filters of various orders are discussed. The filter selection is based on having desirable magnitude attenuation characteristics while at the same time leading to minimum phase lag near the closed-loop bandwidth. The filter design and selection process is illustrated by an example of a sounding rocket stabilization problem. Two flights of the vehicle have been conducted, the first with no consideration of the body-bending dynamics and hence no filters. Serious problems were observed, hence Elliptic filters were used to provide gain stabilization in the second flight. Flight test results are presented and discussed.

The Utility of Extended Longitudinal Profiles in Predicting Future Health Care Expenditures

Health care spending is highly concentrated. Prediction models that accurately identify the characteristics of individuals most likely to incur high levels of health expenditures in a subsequent year are important analytical and statistical tools. This study examined the capacity of alternative models to predict the likelihood of incurring high levels of medical expenditures in a subsequent year. This effort also evaluated the utility of an additional year of longitudinal information. A nationally representative sample from the Medical Expenditure Panel Survey (MEPS). The MEPS longitudinal data are used to examine the persistence of high expenditures during a 2-year period. With the unique linkage of the MEPS to the National Health Interview Survey, the utility of an additional year of data also was examined. Resultant models were evaluated in terms of sensitivity, specificity, and predictive capacity. Only modest marginal gains in discrimination capacity were realized from the use of extended longitudinal profiles from the National Health Interview Survey, relative to information on prior year characteristics. Our results highlight the continuing concentration of health care expenditures during the period 1996 to 2002 and reveal some attenuation in magnitude in the tail of this distribution over time. Further, our results provide evidence of the utility of probabilistic models as prediction tools to identify individuals likely to incur high levels of expenditures in future years. Predictive capacity does not suffer when restricted to a single year of prior information.

A consistent OFDM carrier frequency offset estimator based on distinctively spaced pilot tones

A pilot-tone-based maximum likelihood estimator (PBMLE) for carrier frequency offset (CFO) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. To obtain a consistent estimate of the CFO over a frequency-selective fading channel, the proposed method employs a preamble comprising distinctively spaced pilot tones. As a result of this preamble configuration, a large estimation range equal to the bandwidth of the OFDM signal can be achieved. Different from previous ad hoc pilot-tone-based CFO estimators, the PBMLE exploits the relationship between the CFO and the periodogram of the preamble. Analysis shows that the proposed PBMLE is asymptotically unbiased and efficient. To realize this PBMLE in practice, a suboptimal estimator is also introduced, in which a zero-padded fast Fourier transform is invoked and the CFO estimation is split into two phases: coarse and fine estimation. Coarse estimation is obtained through the correlation between the received preamble and its original pattern, whereas fine estimation is obtained by exploiting the magnitude attenuation in the vicinities of those CFO-shifted pilot tones. Both analytical investigations and computer simulations indicate that the accuracy of this simplified suboptimal estimator is proportional to the oversize ratio of zero padding. When the oversize ratio is sufficiently high, the performance of the suboptimal estimator approaches that of the proposed PBMLE.

Speed tuning of direction repulsion describes an inverted U-function

Direction repulsion describes the phenomenon in which observers typically overestimate the direction difference between two superimposed motions moving in different directions (Marshak & Sekuler, Science 205 (1979) 1399). Previous research has found that, when a relatively narrow range of distractor speeds is considered, direction repulsion of a target motion increases monotonically with increasing speed of the distractor motion. We sought to obtain a more complete measurement of this speed-tuning function by considering a wider range of distractor speeds than has previously been used. Our results show that, contrary to previous reports, direction repulsion as a function of distractor speed describes an inverted U-function. For a target of 2.5 deg/s, we demonstrate that the attenuation of repulsion magnitude with high-speed disractors can be largely explained in terms of the reduced apparent contrast of the distractor. However, when we reduce target motion speed, this no longer holds. When considered from the perspective of Edwards et al.’s (Edwards, Badcock, & Smith, Vision Research 38 (1998) 1573) two global-motion channels, our results suggest that direction repulsion is speed dependent when the distractor and target motions are processed by different global-motion channels, but is not speed dependent when both motions are processed by the same, high-speed channel. The implications of these results for models of direction repulsion are discussed.

Direct contact packed bed thermal gradient attenuators: Theoretical analysis and experimental observations

This article theoretically and experimentally characterizes direct contact, packed bed thermal gradient attenuators in which a fluid control stream, subject to broadband thermal disturbances, flows through a thermally attenuating packed bed medium. The theoretical model decomposes the device into three subsystems: an upper mixing volume, the packed bed, and a lower mixing volume. Analysis of the packed bed subsystem leads to a coupled system of equations that describe local heat transfer within the control stream and within individual spherical elements comprising the packed bed; the system of equations obtained has the same general form as that derived in an earlier investigation of tube-in-shell, noncontact thermal gradient attenuators [J. Heat Transfer 123, 796 (2001)]. Associated subsystem transfer functions are derived, and in analogy with observations reported in the earlier investigation, are shown to provide simple, unambiguous criteria for designing and optimizing packed bed attenuators. It is found that, for 52 different sets of experimental conditions, theoretical performance of the device matches experimental performance to within 2.5 dB root mean square, and that the device provides several orders of magnitude attenuation for flow rates on the order of 1 l/min and disturbance frequencies higher than approximately 10 mHz. The results of the present study and the earlier investigation together provide a useful basis for analyzing and designing large and small, contacting and noncontacting, thermal gradient attenuators.

Sensation seeking and startle modulation by physically threatening images

The potential moderating effect of sensation seeking on anxious reactivity to threatening experiences was assessed using the affective modulation of startle-blink paradigm. Startle blinks, as measured by electromyographic (EMG) activity in response to loud (100 dB) white-noise stimuli, were elicited during the presentation of positive, neutral, and threatening visual images. Unlike participants low in sensation seeking who showed blink potentiation during threatening versus neutral images, participants high in sensation seeking showed equal magnitudes of startle to neutral and threatening images. The results suggest that individuals high compared with low on sensation seeking are less anxiously reactive to physically threatening visual stimuli. No attenuation in startle magnitude was elicited by positive images among low or high sensation seekers suggesting that the positive images employed in the current study were not arousing enough to activate the appetitive arousal system.

The infant lung function model: a mechanical analogue to test infant lung function equipment.

To facilitate international multicentre studies and quality control of infant pulmonary function measurements, the European Respiratory Society-American Thoracic Society (ERS-ATS) working group for infant lung function testing aims to develop specifications for standardized infant lung function equipment and software. However, a standardized test device is also needed to test whether existing infant lung function equipment is able to meet these requirements. The authors have built a "mechanical model baby" consisting of a linear pump which can reproduce prerecorded tidal flow waveforms with a precision of 0.5% (full stroke), enabling the simulation of tidal and forced flow patterns. This linear pump can be connected to a series of copper lung volumes (range 50-300 mL) with known time constants, so that lung volumes can be reproduced with a precision of +/-1% at frequencies 10-120bpm. Five airflow resistors were built using sinter material. When assessed using flows 0-300 mL.s(-1) all resistors showed a quasilinear pressure/ flow relationship, with slopes 1.0-5.6 kPa.L(-1).s. These resistances could be reproduced with a precision of +/-2.5%. The infant lung model can also be used to assess frequency responses of infant lung function equipment, since the pump is capable of delivering low amplitude volumes up to 20 Hz in a pseudorandom noise manner. In summary, based on error estimations, this infant lung model is able to test whether or not infant lung function equipment meets the requirements suggested by the European Respiratory Society-American Thoracic Society standardization group, that is: flow measurements within +/-2.5%, volume and resistance measurements within +/-5%, frequency response: magnitude attenuation <+/-10% and phase shift <+/-3 degrees at 10 Hz.

Entrainment, instability, quasi-periodicity, and chaos in a compound neural oscillator.

We studied the dynamical behavior of a class of compound central pattern generator (CPG) models consisting of a simple neural network oscillator driven by both constant and periodic inputs of varying amplitudes, frequencies, and phases. We focused on a specific oscillator composed of two mutually inhibiting types of neuron (inspiratory and expiratory neurons) that may be considered as a minimal model of the mammalian respiratory rhythm generator. The simulation results demonstrated how a simple CPG model--with a minimum number of neurons and mild nonlinearities--may reproduce a host of complex dynamical behaviors under various periodic inputs. In particular, the network oscillated spontaneously only when both neurons received adequate and proportionate constant excitations. In the presence of a periodic source, the spontaneous rhythm was overridden by an entrained oscillation of varying forms depending on the nature of the source. Stable entrained oscillations were inducible by two types of inputs: (1) anti-phase periodic inputs with alternating agonist-antagonist drives to both neurons and (2) a single periodic drive to only one of the neurons. In-phase inputs, which exert periodic drives of similar magnitude and phase relationships to both neurons, resulted in varying disruptions of the entrained oscillations including magnitude attenuation, harmonic and phase distortions, and quasi-periodic interference. In the absence of significant phasic feedback, chaotic motion developed only when the CPG was driven by multiple periodic inputs. Apneic episodes with repetitive alternation of active (intrinsic oscillation) and inactive (cessation of oscillation) states developed when the network was driven by a moderate periodic input of low frequency. Similar results were demonstrated in other, more complex oscillator models (that is, half-center oscillator and three-phase respiratory network model). These theoretical results may have important implications in elucidating the mechanisms of rhythmogenesis in the mature and developing respiratory CPG as well as other compound CPGs in mammalian and invertebrate nervous systems.

Ten-year stability of cardiovascular responses to laboratory stressors.

In this study, we examined test-retest stability of cardiovascular stress responses over a decade of the life span. Participants were 55 male college undergraduates, 19 years of age at initial testing, and 29 years of age at follow-up testing. Stressors were a foot cold pressor and an aversive reaction time task. Cardiovascular measures included systolic and diastolic blood pressure, heart rate, and preejection period. For cold pressor, the magnitude and pattern of cardiovascular responses remained unchanged at the 10-year follow-up. For the reaction time task, the characteristic cardiovascular response patterns was preserved but with significant attenuation of magnitude. The present findings are consistent with previous observations of temporal stability but over a substantially longer test-retest interval. The long-term stability of stress responses is discussed in the context of stress test methodology, behavioral response demands, and maturation of the physiological systems involved in cardiovascular response expression.

Long-lasting conditioning of the human soleus H reflex following quadriceps tendon tap

Percussion of the quadriceps tendon was used to test the hypothesis that knee extensor muscle spindle discharge initiates down-regulation of the gain of the soleus H reflex. Seven subjects participated. Soleus H reflex magnitude was observed for up to 15 s, following conditioning tendon taps of 60 N or 80 N force and 10 ms or 100 ms duration, with the knee at 60° or 90° of flexion. The tap elicited quadriceps stretch reflexes in four subjects, with a mean latency of 42 ms. The major component of the conditioning of the soleus H reflex was significant attenuation of magnitude by 30–90% of controls, starting as early as 36 ms post-percussion and lasting as long as 3–8 s. The attenuation of reflex magnitude was evident, whichever combination of duration and force of tap was used. Preceding and/or following this inhibition, there was mild facilitation. Static stretch of quadriceps also significantly reduced soleus H reflex magnitude. These results support the spindle receptor origin for the gain attenuation seen during movement. The time course of the gain attenuation suggests a spinal route, by which the spindle discharge of the heteronymous extensor muscles initiates presynaptic inhibition of transmission through the reflex pathway.

B-scan finite-elements analysis of acoustical image formation model for three-dimensional surfaces

A B-scan acoustical image formation model based on finite-element analysis is introduced. The proposed model accurately predicts images of a three-dimensional (3-D) surface reflector under nonideal conditions. It is shown that the reflector may be located outside the uniform pressure zone of the focused transducer and the distance between the transducer and the surface as well as the angle of incidence may vary. In developing the model, essential elements in the B-scan acoustical image formations process are identified. It is shown that the phase, as well as the magnitude of the pressure wave, must be incorporated in the model. Phase interactions result in a strong sensitivity of the magnitude of the acoustical image to the angle of incidence. In the examples presented, it is shown that a 10-deg change in the angle of incidence results in a 50% magnitude attenuation. It is also shown that the temporal characteristics of the transducer can be neglected, and for angles of incidence less than 10 deg, the phase of the reflection coefficient can be ignored.

Effects of ryanodine and nifedipine on contractions induced by norepinephrine, acetylcholine, and potassium in the isolated vas deferens of the guinea pig.

In the present study we tried to determine the relative contributions of sarcoplasmic reticulum (SR) calcium release and gated calcium entry through voltage-dependent calcium-channels (VDC) to the contractions induced by norepinephrine (NE, 10 microM) and acetylcholine (ACh, 10 microM) in the vas deferens of the guinea pig. NE and ACh at 10 microM caused contractions composed of three phases: a prephasic, a phasic and a tonic component. Nifedipine strongly inhibited the contractions induced by potassium (K, 100 mM) and abolished phasic components of the contractions induced by NE or ACh. However, the tonic components of NE- or ACh-induced contractions were slightly attenuated by nifedipine. Pretreatment with ryanodine in combination with caffeine attenuated prephasic and tonic components of the contractions induced by NE and ACh in a dose-dependent manner. The inhibitory effects of ryanodine and caffeine on the phasic component of NE-induced contractions were divided into two types: 1) abolishment of the phasic contractions and 2) a delayed appearance of the phasic contractions without attenuation of magnitude. However, ryanodine with caffeine minimally affected phasic component by ACh- and K-induced contractions. Ryanodine pretreatment in combination with NE, ACh or K had no effect on the contractions induced by each agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of light in tissue in photodynamic therapy.

Abstract The dose rate in photodynamic therapy is proportional to the energy fluence rate and the concentration of the photosensitizer. Calculations of the energy fluence rate have been performed in slab, cylindrical and spherical geometries with the discrete ordinates transport method and diffusion theory. The attentuation of the energy fluence rate is least in slab geometry and greatest in spherical geometry. Violet (405 nm) light is attenuated much more rapidly than red (630 nm) light. Small tissue dimensions or narrow beam irradiation further decrease the energy fluence rate with radius and depth. Anisotropic scattering increases the energy fluence rate at large depths, but decreases it near the source. Measurements of the absolute energy fluence rate vs depth in a mouse tumor model exhibit an order of magnitude attenuation through the skin and a 3 mm thick tumor. Calculations of the energy fluence rate of the DHE fluorescence have been carried out to guide measurement of the concentration. Violet light excitation is much more efficient than red light excitation.

Idle-Channel Noise Characteristic of a Delta Modulator with Step Imbalance and Quantizer Hysteresis

In 1969 Iwersen [4] analyzed the quantizing noise (assuming no slope overload) associated with a delta modulator ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaM</tex> ) with step-size imbalance. In this concise paper we consider the effects of both step-size imbalance and quantizer threshold hysteresis on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaM</tex> quantizing noise. The analysis is restricted to the important case of zero input signal. For this case we extend Iwersen's work to show that quantizer threshold hysteresis can significantly change the spectral shape of the idle channel noise. Our approach is to first construct two locus functions, one for the samples (of the steady-state noise waveform) taken at arbitrarily defined even sampling times and one for those taken at the odd sampling times. The noise waveform is then obtained as a superposition of interleaved samples of the even and odd loci. It is shown that the noise spectrum has a discrete tonal struture whose frequencies are independent of hysteresis and are determined solely by step imbalance as in Iwersen's analysis. Hysteresis changes the magnitude and phase of the discrete tones. We characterize the tone magnitude changes due to hysteresis and show that hysteresis can amplify, attenuate, or completely eliminate a tone. We present curves which specify the tone magnitude attenuation as a function of step imbalance ε, average step size σ, and hysteresis <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</tex> . We also calculate P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> , the total noise power in the baseband up to half the sampling frequency. It is shown that for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0 \leq h \leq \epsilon</tex> , <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_{T} = \sigma^{2}/3</tex> independent of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</tex> and ε ;for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h &gt; \epsilon</tex> , P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> is an increasing function of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\epsilon/\sigma</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h/\epsilon</tex> .