Browsing by Author "Figueiredo, Rui J. P. de"

Now showing 1 - 12 of 12
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    A nonlinear adaptive equalizer
    (1980) Wendt, Richard Ernest; Figueiredo, Rui J. P. de; Clark, John W.; Johnson, Don H.
    The problem of removal of distortion caused by nonlinearities is investigated in light of a known technique of equalization. This inline, open loop, procedure, involving a contraction mapping, is well suited to compensating systems whose inputs are inaccessible. The present contribution is to add an adaptive capability to the scheme, extending its applicability to systems whose nonlinear parameters are unknown or drifting. Results of computer simulations are presented.
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    A ray model for head waves in a fluid-filled borehole
    (1982) Scheibner, David James; Parks, Thomas W.; Johnson, Don H.; Figueiredo, Rui J. P. de
    A model, suggested by tbe ray expansion of Roever et al., is constructed to rapidly generate the compressional and shear refracted arrivals, known as head waves, received from a point source on the axis of an ideal fluid-filled borehole. An impulse response is derived, and its frequency characteristics are investigated. The waves are compared to those obtained by the real axis integration method of Tsang and Rader, which results in a complete waveform, including the modes as well as the refracted arrivals. The ray model gives accurate results for the compressional head wave. The shear region of the complete waveform contains strong modal interference, making it difficult to evaluate the quality of the ray model shear wave. A useful filter results from insight provided by the basic structure of the model. This filter can be used to estimate the borehole diameter or formation compressional velocity. It can also remove the second and later compressional arrivals, thus providing an accurate estimate of the source pulse and and a relatively uncorrupted vie of the initial arrival in the shear region.
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    A transistor model based on spline functions
    (1974) Strietelmeier, Frederick Ernest; Figueiredo, Rui J. P. de
    The input and output characteristic curves of a transistor are such that they can be modeled by spline functions. The problems of such a representation are two-fold. First, the best representation for the spline function must be determined, and second, all the data needed to implement the spline functions must be obtainable from easily measured parameters of the transistor. In this work, the natural Hermite cubic spline has been found to provide the needed results. The data needed to implement the Hermite cubic spline include its knots, the values at the knots, and the slopes at the knots, each of which have been defined in terms of measurable transistor parameters. The ensuing spline model has been implemented on the computer and characteristic curves of different transistors computed by the spline model have been compared with the actual curves. Results of this comparison are shown as well as the results of a comparison of the spline model with the Ebers-Moll transistor model.
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    Computational methods in the design of linear control systems
    (1980) Kontos, Athanasios V.; Pearson, J. B.; Parks, Thomas W.; Figueiredo, Rui J. P. de
    This thesis considers the problem of computing controllers for multivariable systems. System representation is in terms of polynomial matrices and two algorithms are presented which are shown to be useful in the design of controllers for such systems. These algorithms are: i) Factorization of a polynomial matrix, and ii) Computation of a unimodular matrix U satisfying the relation [A B]U = I 1, where A and B are left coprime polynomial matrices. These algorithms do not involve numerically unsatisfactory Euclidean type operations. It is shown that the two algorithms can be used to compute solutions to the system stabilization problem and to the model matching problem. The Regulator Problem with Internal Stability (RPIS) is also discussed, and under certain assumptions it is shown how solutions can be computed.
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    Development of automatic sleep stage discrimination using Period Analyzed EEG
    (1977) Pruett, Roderick C.; Troelstra, Arne; Figueiredo, Rui J. P. de; Kazakos, T.; Burch, N. R.; Saltzberg, B.; Dossett, R.
    A sleep stage classification method has been developed which uses variables derived from the Period Analyzed EEG to discriminate seven stages of sleep with accuracies of 88-99%. Period Analysis is a method through which the vast EEG data base can be condensed to a relatively small number of highly informative values. These values are the numbers of zero-crossings per second of the signal and its first and second derivatives. Stepwise discriminant analysis was used to determine which of these variables to use, and to calculate coefficients for six classification functions, based on a learning set of visually classified EEG samples. To evaluate the performance of this method various learning and test sets from the same subject were used, obtaining agreements with the visual classifier of 96-99% on reclassifications of the night in the learning set, 9-94% on intra-night test sets, and 88-94% on test sets isolated from the learning set by a month. Stage REM was discriminated from the EEG without the use of EOG leads, along with Stages I-IV, Awake, and Artifact. These inter-night hit rates were much higher than others in the current literature, and even greater than published inter-judge or even intra-judge agreements when reclassifying the same samples -- indicating a sleep stage classification system which is both highly accurate and extremely precise.
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    Discrete nonlinear inverse filtering
    (1977) Rajala, Sarah A.; Figueiredo, Rui J. P. de
    A new mathematical derivation is developed for the solution of the problem of nonlinear inverse filtering. It is derived for systems with finite deterministic signals, i. e., the output y of the system is a finite set of data samples. It is assumed that the system generating this output can be represented by a discrete Volterra series, the kernels of this series being known. With knowledge of both the output and these nonlinear kernels# the model is derived for the nonlinear inverse filter. The output of this filter x
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    Estimation of repetition rate from signal and texture features
    (1983) Tagare, Hemant D.; Figueiredo, Rui J. P. de; Parks, Thomas W.; Dufour, Reginald J.
    This thesis develops relevant definitions and a theoretical basis for estimating the repetition rate of a random repetitive signal. The repetition rate is estimated by looking for repetition amongst local features of the signals. These features have to satisfy a uniqueness condition, and we have shown that the derivatives of a signal constitute a set of such features. The estimator has been shown to be asymptotically unbiased. The estimation algorithm can not only be tuned to the waveshape information of the signal (by a proper choice of features), but also to the extent of non-stationarity expected in the input signal class. A set of features has been obtained for applying this algorithm to repetitive textured images and voiced speech signals. Vith these features, it has been possible to extract the repetition rate in both the above classes of signals. In the case of voiced speech this rate corresponds to its pitch.
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    Potential fields from cardiac strands of cylindrical geometry
    (1984) Ganapathy, Nirmala; Clark, John W.; Glantz, Raymon M.; Figueiredo, Rui J. P. de
    This paper deals with the classical forward and inverse volume conductor field problems associated with the isolated active cardiac strand. The Purkinje fiber and the atrial trabeculum are chosen as specific examples of cardiac tissue that may be well modeled by cylindrical geometry. The electrical behavior of these strands is modeled in terms of the electrical activity of an equivalent single cell, with a representative membrane that separates an anisotropic intracellular medium from an isotropic extracellular medium. The isolated single atrial muscle fiber is also studied as an interesting special case. A Potential theory model is developed for the strand, that is based on a solution of Laplace's equation in the media of interest, subject to appropriate boundary conditions. The solution for potential at an arbitrary point in the extracellular medium is in the form of a Fourier integral; the equation is subsequently reformulated into a more convenient computational form using a discrete Fourier transform (DFT) method. Implementation of this method using a Fast Fourier Transform (FFT) technique, results in a fast and efficient numerical algorithm for the calculation of volume conductor potentials. A benefit of this approach is that the classical forward and inverse problems in electrophysiology may be viewed as equivalent filtering problems. Thus not only can volume conductor field potentials at varions distances from the strand be easily and rapidly computed, but given field potential data, good estimates of the action potential waveform can also be obtained provided the signal to noise ratio is adequate.
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    Surface reconstruction and display of left ventricular function for pet images
    (1984) Kehtarnavaz, Nasser; Figueiredo, Rui J. P. de; Clark, John W.; Zygouakis, Kyriacos
    A recently developed camera referred to by the name PET (Positron Emission Tomography) allows the biochemical activity associated with an organ inside the body to be viewed without surgical intervention. The organ considered for the present study is the heart and the goal of the thesis is to aid detecting the coronary artery diseases at an early stage. For this purpose, a three dimensional object of the left ventricle is formed from an available set of two-dimensional image slices generated by PET. An automatic boundary detection algorithm outlines the object surface and assigns appropriate intensity values to the surface points. The surface is then rotated with the long axis of the heart in a vertical position, allowing the myocardium to be viewed from different angles around the heart. Such a display of activity is shown to be an effective way of presenting the functional behavior of the myocardium.
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    Surface reconstruction from surface contours
    (1983) Simar, Laurence Ray; Figueiredo, Rui J. P. de; Clark, John W.; Dufour, Reginald J.
    Surface reconstruction from surface contours has become increasingly important in the medical field. Given a set of serial cross sections, it is shown that a structural technique of reconstruction can be developed. The motivation is to analyze the structure of the cross sections so as to represent the surface more faithfully than conventional techniques. The procedure involves syntactic and semantic techniques which allow the algorithm to analyze the cross sections at a high level. This information can then be used to recognize features on adjacent contours and then reproduce the surface which generated the cross sections.
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    Trajectory planning for robot arm positioning
    (1985) Datta, Subroto; Pearson, J. B.; Figueiredo, Rui J. P. de; Burrus, C. S.; Kohn, Wolf
    This thesis deals with the sub-optimal control of robots. Keeping in mind the current applications of robots in industry we propose a control scheme which minimizes a time-energy cost criterion. The control problem is solved for a piecewise linear model which is sufficiently close in a norm sense to the highly non-linear robot. A solution is obtained by splitting the control problem into two sub-problems. i) A trajectory planning problem which is solved off-line and is termed the NODES problem. The nodes problem primarily obtains the average force history. ii) A feedback controller problem which moves the robot between the successive nodes in real time and is termed the LOCAL problem. The nodes problem is solved using monotonicity ideas arising from dynamic programming and an iterative algorithm is presented for its solution. Numerical issues in the implementation of the algorithm are discussed. The local problem is solved by a modification of a recent paper on minimum time-fuel control.
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    Variable length chain coding with applications
    (1979) Pau, Charles King-Chiu; Figueiredo, Rui J. P. de; Johnson, Don H.; Sinclair, James B.
    A special scheme for coding a digitized picture is presented. This scheme encodes a picture by coding boundaries of areas in the picture as sets of line segments. It is shown that this scheme is superior to the conventional schemes in terms of the data compression factor and faithful reproduction of the picture. One specific application of this scheme is in template matching. Experimental results are presented which show that template matching using this scheme is faster than the conventional template matching technique by a factor of at least 1.