Showing papers on "Electrical impedance published in 1997"

Electromagnetic imaging and therapeutic (EMIT) systems

Abstract: This invention is a microwave tomographic device (10), including a single frequency three-dimensional microwave tomographic system in cooperation with a single frequency three-dimensional electrical impedance tomographic system, capable of imaging a full scale biological object. A cluster of antennas (16) and transceivers are used to provide MWT and EIT integrated in a single dimensional microwave tomographic system.

Noise Resistance Applied to Corrosion Measurements I. Theoretical Analysis

Abstract: The measurement of current and voltage fluctuations on the same electrochemical cell allows the evaluation of a quantity R{sub n} known as noise resistance, which has been proposed as an indication of the corrosion resistance of the material under study The theoretical basis for the relationship between R{sub n} and the electrode impedance Z is developed, taking into account the various measurement schemes currently in use Parameters such as cell geometry, solution resistance, and electrodes with different kinetics are considered It is shown that, in general, the modulus of the electrode impedance can be derived by measuring the power spectral densities (PSD) of the voltage and current noises The circumstances in which R{sub n} is equal to the polarization resistance of the electrode are also discussed

Method for producing a tomographic image of the body and electric impedance tomograph

Abstract: The invention is a method of obtaining tomographic images of the human body and the electrical impedance tomograph, in which a source of electric current is used to send electric current at levels undetectable by a human being to pairs of electrodes, between which at least two electrodes are placed. An algorithm of image reconstruction makes it possible to obtain the distribution of absolute conductivity of a body, characterizing the state of soft and bone tissues and blood vessels. The method is fast. It allows one to visualize the changes of conductivity during one cardiocycle and to observe blood filling the heart and vessels. It allows one to obtain time dependence of conductivity of internal areas of the heart, which is an impedance cardiogram, containing additional information about heart function. The use of visualization of conductivity of tissues allows one to observe the processes of internal hemorrhages, to reveal inflammations, to carry out studies of organs of digestion, to observe the state of various tumors, to carry out diagnosis of diseases of the mammary gland, to diagnose various lung diseases. The method allows one to monitor the variation of temperature of internal organs, raising the possibility of diagnosing diseases at early stages. The tomograph is a rather simple, compact device, convenient in operation, safe for a patient and attendants, operating with a standard personal computer. This device can be widely used in medical practice and clinical investigations.

Frequency Dependent and Transient Characteristics of Substation Grounding Systems

Abstract: In spite of the existence of a number of analytical models aimed for transient analysis of large grounding systems, more detailed analysis of the influence of different parameters on the transient performance of large ground grids subjected to lightning current impulse is not available. This paper presents analysis of the influence of soil conductivity, location of feed point, grid size, depth, conductor separation, ground rods, and shape of the lightning current impulse, on the transient performance of ground grids with sizes ranging from 10/spl times/10 m/sup 2/ to 120/spl times/120 m/sup 2/ and with 4 to 124 meshes. Maximal transient ground potential rise and frequency dependent impedance are analyzed in time and frequency domain, respectively. Computations are made with computer model based on the electromagnetic field theory approach, taking accurately into account frequency dependent characteristics of large ground grids. Instead of usual simple approximations of the lightning current impulse, recorded channel base currents from triggered lightning are used for the time domain analysis.

Calculation of self- and mutual impedances in planar sandwich inductors

Abstract: High-frequency planar magnetic components, employing thin film and thick film technology, have become important components in applications, such as filters and switching converters, due to their ease of manufacture and reliability. In a previous paper, the authors established a frequency dependent impedance formula for planar coils on a magnetic substrate that is infinitely thick. In this paper, two new impedance models are described: the first is for planar coils on a magnetic substrate of finite thickness, and the second represents a planar coil sandwiched between two substrates. The models include the electrical conductivity of the magnetic material so that the effects of eddy currents, particularly at high frequencies, are taken into account. The eddy currents reduce the inductance and increase the losses associated with the device. The new impedance formulas are derived from Maxwell's equations. Simulations were carried out on a typical device, using finite element analysis, and the results validate the new formulas. This paper establishes the frequency limitations of lossy magnetic substrates.

Negative incremental impedance and stability of fluorescent lamps

Abstract: Fluorescent lamps have a special V-I characteristic. Their incremental impedance is defined and studied in the frequency domain. It is found that the negative incremental impedance of a fluorescent lamp can be characterized by a right half plane zero. The existence of such a result is explained by the modified Francis equation. Two ballasting approaches, series impedance and close-loop operation, are proposed to stabilize lamp operation. Stability criteria based on the Nyquist criterion are developed for both approaches.

Method and apparatus for apical detection with complex impedance measurement

Abstract: An apical position detector (40) for use in dental endodontics includes an electronic controller (42) coupled to a conductive probe (46) and a lip electrode (48) The probe (46) is positioned in a root canal (60) and the lip electrode contacts another location on the patient's body The controller (46) produces a test signal as a combination of signals at selected frequencies The controller (46) then monitors a voltage of the probe (46) and extracts the amplitude and phase of each of the frequency components of the voltage at the probe (46) Test scores are generated from summations of the amplitudes and/or phases of the various frequency components to determine when the impedance between the probe (46) and the lip electrode (48) changes from a primarily reactive impedance to a primarily resistive impedance A first test score is displayed to indicate when the impedance changes from a reactive to a resistive impedance The second test score indicates when the measurement conditions are unsatisfactory The test scores are displayed in bar graph or similar form

An accurate equivalent circuit for the unloaded piezoelectric vibrator in the thickness mode

Abstract: An equivalent circuit model for the unloaded piezoelectric vibrator in the thickness mode is presented. The model contains two branches, the motional branch and the static branch, like the lossless resonator model, but the circuit elements are generalized by making each a complex constant. The mechanical, dielectric and piezoelectric losses associated with the vibrator are accounted for by the imaginary components of the circuit elements. The model produced impedance curves that closely matched the impedance calculated by using equations derived from vibration theory and the data measured for lead zirconate titanate and PVDF - TRFE co-polymer samples. The calculation of the circuit parameters from the complex elastic, dielectric and piezoelectric material constants is straightforward and the model accurately fits both the baseline dielectric behaviour and the piezoelectric resonance around and below the fundamental resonance. Conversely, when the complex circuit parameters are known, the complex material constants can be derived by straightforward calculations without any loss of information.

An Impedance Method for Dynamic Analysis of Active Material Systems

Abstract: This paper describes a new approach to analyzing the dynamic response of active material systems with integrated induced strain actuators, including piezoelectric, electrostrictive, and magnetostrictive actuators. This approach, referred to as the impedance method, has many advantages compared with the conventional static approach and the dynamic finite element approach, such as pin force models and consistent beam and plate models. The impedance approach is presented and described using a simple example, a PZT actuator-driven one-degree-of-freedom spring-massdamper system, to demonstrate its ability to capture the physics of adaptive material systems, which is the impedance match between various active components and host-structures, and its utility and importance by means of an experimental example and a numerical case study.The conventional static and dynamic finite element approaches are briefly summarized. The impedance methodology is then discussed in comparison with the static approach. The basic e...

Noise Resistance Applied to Corrosion Measurements II. Experimental Tests

Abstract: Electrochemical noise (EN) measurements have been carried out on several corroding systems, utilizing the technique of simultaneous recording of current and voltage fluctuations, and calculating the frequency dependent spectral noise impedance R{sub sn}(f) as the square root of the ratio of the power spectral densities (PSD) of the voltage to current noise. The results are compared with the theoretical predictions stemming from an analysis of the technique, which is presented in the preceding paper. It is shown that, over a quite large range of conditions, R{sub sn}, coincides with the impedance modulus of the electrodes under study. From the relationship between R{sub sn} and the noise resistance R{sub n}, which is usually obtained calculating the ratio of the standard deviations of the voltage and current fluctuations, it is shown that the latter quantity can be equal to the zero frequency limit of the impedance only if certain conditions are satisfied.

Electrical impedance tomography method and apparatus

Abstract: A method and apparatus for use in imaging a body uses electrical impedance tomography (EIT). A plurality of electrodes are provided in electrical contact with the body around the periphery of the body. A first electrical input signal is applied to at least one of the electrodes over a first time period, and a second electrical input signal, which is preferably in inverted form of the first, then applied to the at least one of the electrodes over a subsequent, second time period. The resulting electrical output signal is measured at one or more pairs of the remaining electrodes over the first and second time periods and the difference between the measured signal obtained during the first time period and that obtained during the second time period is calculated to provide a difference signal. The difference signal can stored and used for image reconstruction. The invention has particular application in imaging neurological function within the body, where the impedance changes associated with neuronal depolarizations are very small and the resulting electrical signals measured at the body periphery are even smaller. By use of this technique electrical signals corresponding to the dynamic components of impedance change can be reinforced whilst those corresponding to other unwanted components can be cancelled.

Sensitive Stress-impedance Micro Sensor Using Amorphous Magnetostrictive Wire

Abstract: A giant stress-impedance (GSI) effect with the strain-gauge factor more than 1200 was found in negative magnetostrictive amorphous CoSiB wires of 30-/spl mu/m diameter magnetized with a high frequency current or a sharp pulse current. A sensitive stress sensor is constructed using a CMOS IC multivibrator circuit in which the amorphous wire is magnetized with a sharp pulse train current. The amorphous wire GSI sensor will be applied for detection of such as pressure, tension, stream speed for liquid and gases and mechano-cardiogram with the sensitivity of 5-6 times higher than that of the semiconductor stress sensors utilizing the piezo-resistance effect showing a gauge factor of about 200.

Electrical impedance tomography with basis constraints

Abstract: In this paper, we consider the impedance tomography problem of estimating the conductivity distribution within the body from static current/voltage measurements on the body's surface. We present a new method of implementing prior information of the conductivities in the optimization algorithm. The method is based on the approximation of the prior covariance matrix by simulated samples of feasible conductivities. The reduction of the dimensionality of the optimization problem is performed by principal component analysis (PCA).

Equivalent circuit of dipole antenna of arbitrary length

Abstract: The purpose of this letter is to derive an equivalent circuit for a dipole of any length and to compare the input impedance versus frequency of the resulting equivalent circuit with published analytical and experimental data.

Piezoceramics for high-frequency (20 to 100 MHz) single-element imaging transducers

Abstract: The performance of transducers operating at high frequencies is greatly influenced by the properties of the piezoelectric materials used in their fabrication. Selection of an appropriate material for a transducer is based on many factors, including material properties, transducer area, and operating frequency. The properties of a number of piezoceramic materials have been experimentally determined by measuring the electrical impedance of air-loaded resonators whose thickness corresponds to resonance frequencies from 10 to 100 MHz. Materials measured include commercially available compositions of lead zirconate titanate (PZT) with relatively high dielectric constants and a modified lead titanate (PT) composition with a much lower dielectric constant. In addition, materials which have been designed or modified to result in improved properties at high frequencies are studied. Conclusions concerning the influence of the microstructure and composition on the frequency dependence of the material properties are made from the calculated properties and microstructural analysis of each material. Issues which affect transducer performance are discussed in relation to the properties. For transducers larger than about 1 mm in diameter, the use of a lower dielectric constant material is shown to result in a better electrical match between the transducer and a standard 50 /spl Omega/ termination. For transducers whose impedance is close to that of the connecting cables and electrical termination, equivalent circuit model simulations show improved performance without the need for electrical matching networks. Measurements of fabricated transducers show close agreement with the simulations, validating the measurements and showing the performance benefits of electrically matched transducers.

Wire antenna with stubs to optimize impedance for connecting to a circuit

Abstract: An antenna, used as a voltage and power source, is designed to operate with an arbitrary load, or front end. One or more stubs are added to one or more of the antenna elements. The stubs act as two conductor transmission line and are terminated either in a short-circuit or open-circuit. Where the transmission line is odd multiples of the guided wavelength in length, the short-circuit stubs act as lumped inductors and the open-circuit stubs act as lumped capacitors. The magnitude of these lumped capacitors and inductors (reactances) is affected by a stub length, a stub conductor width, and a stub spacing. Zero or more short-circuit stubs and zero or more open-circuit stubs are added to one or more of the antenna elements to change the reactive (imaginary) part of the antenna input impedance. In a preferred embodiment, the reactive part is changed to equal the negative magnitude of the reactive part of the front end input impedance.

A modified Wheeler cap method for measuring antenna efficiency

Abstract: An antenna efficiency measurement technique is presented which is based on the classic Wheeler Cap method. Wheeler's original method is regarded as one of the simplest and most accurate for measuring electrically small antennas, but the assumed RLC equivalent circuit model may not be appropriate for all antennas. In the present method we transform measured reflection coefficient data such that either a series or parallel RLC approximation is always valid in the vicinity of resonance. This procedure provides the capability to achieve significant improvements in accuracy over measuring the efficiency of VHF and UHF antennas in anechoic chambers.

Method of and apparatus for controlling reactive impedances of a matching network connected between an rf source and an rf plasma processor

Abstract: An r.f. field is supplied by a reactive impedance element to a plasma in a vacuum plasma processing chamber. The element and source are connected via a matching network including first and second variable reactances that control loading of the source and tuning a load, including the reactive impedance element and the plasma, to the source. The values of the first and second variable reactances are changed to determine the amount the first variable reactance is to change for each unit change of the second variable reactance to attain the best match between the impedances seen looking into and out of output terminals of the r.f. source. Then the values of the first and second variable reactances are varied simultaneously based on the determination until the best impedance match between the impedances seen looking into and out of output terminals of the r.f. source is attained.

High efficiency switching regulator with adaptive drive output circuit

Abstract: Switching regulator circuits and methods are provided in which the output circuit is adaptable to maintain high efficiency over various load current levels. The regulator circuits generate one or more control signals in response to the load current and selectively route a switch driver control signal to one or more switches in the output circuit. The switches differ in their size, such that the most efficient switch can be used at a particular load current level. At low load current levels, the driver control signal is routed to output circuitry with smaller switch devices, which incur smaller driver current losses for a given frequency of operation, thereby increasing the regulator efficiency. At high load current levels, the driver control signal is routed to large switch devices, which incur greater driver current losses for a given frequency of operation, but which have a lower impedance. The regulator thus maintains high efficiency over a wide range of load currents while operating at a constant frequency.

Position-based impedance control of an industrial hydraulic manipulator

Abstract: This article addresses the problem of impedance control in hydraulic manipulators. Whereas most impedance and hybrid force/position control formulations have focused on electrically driven robots with controllable actuator torques, torque control of hydraulic actuators is a difficult task. A position-based impedance controller (PBIC) is proposed and demonstrated on an existing industrial hydraulic robot (a Unimate MKII-2000). A nonlinear proportional-integral (NPI) controller is first developed to meet the accurate positioning requirements of this impedance control formulation. The NPI controller is shown to make the manipulator match a range of second-order target impedances. Various experiments in free space and in environmental contact, including a simple impedance modulation experiment, demonstrate the feasibility and the promise of the technique. Finally, explanation of an experimentally observed behaviour is offered, suggesting a basic limitation to the implementation of impedance control.

Method for determining frequency characteristics of battery power supply sources

Abstract: FIELD: electrical engineering. SUBSTANCE: nonsinusoidal voltages applied to battery and battery current are recorded and expanded into harmonic F-series, and by values of amplitudes and initial phases of voltage and current harmonics, complex impedance of battery for each harmonic are determined and also their resistive and reactive components. By frequency characteristics of chemical supply source one can judge serviceability of supply sources, their condition, state of charge and carry out automatic control of charging process. EFFECT: increased accuracy of checking of amplitude-frequency and phase-frequency characteristics of all types of storage batteries within wide frequency.

Comparison of methods for determining specific acoustic impedance

Abstract: An investigation was conducted to explore potential improvements provided by alternate implementations of two impedance-measurement methods, known as the two-microphone and multipoint methods. This investigation is part of a continuing technology development to find more efficient and convenient methods for impedance measurements in harsh environments at high frequencies. The two alternate methods are compared with other methods to determine relative merits. As expected, the “best method” depends upon trade-offs between convenience and accuracy. The single-microphone method eliminates one of the two microphones typically used in the two-microphone method. The single-microphone method should be useful where mechanical constraints allow only one microphone to be placed into the test environment. It is found to be quite accurate with a single discrete frequency source, but the most inaccurate with a random noise source. An alternate implementation of the multipoint method, achieved by replacing the single-tone source with a pseudo-random noise source, requires significantly less time with minimal loss of accuracy. Typical agreement between this implementation and the accepted standard is within 0.1 ρc units. However, slightly larger deviations at frequencies above 2.5 kHz indicate challenges in the proposed extension to high-frequency (up to 25-kHz) applications.

Semiconductor non-volatile latch device including embedded non-volatile elements

Abstract: A bistable non-volatile latch circuit adapted to store a non-volatile binary data state during a program operation, and to assume one of two stable states in response to a power up operation that correspond uniquely to the data state has first and second circuit sections. The first circuit section has a first non-volatile current path with means to set the impedance of the first current path in a non-volatile manner. A first end of the first current path is connected to provide a logic output signal, which represents a binary logic state depending on a voltage applied to the a first signal input node. The set/reset signal to the first current path varies between at least the power source voltage and a program voltage that is negative with respect to the power source voltage. A second circuit section generates an output voltage on a second output node that represents a binary logic state opposite from the output states of the first circuit section. Means are provided for connecting the first circuit section and the second circuit section into a bistable configuration.

The Influence of Laterally Inhomogeneous Contacts on the Impedance of Solid Materials: A Three-Dimensional Finite-Element Study

Abstract: The total impedance of samples with electrodes exhibiting only partial contact (porous electrodes) is investigated using the finite element method in three dimensions Emphasis is put on porous electrodes built up of arrays of small perfect contacts An equivalent circuit to analyze the impedance spectra is put on a firm basis enabling the reliable determination of bulk properties of “imperfectly” contacted samples Approximations are given to estimate the contact geometry impedance The results are also applicable to other imperfect contact problems as occurring at grain boundaries

Moisture sensor having low sensitivity to conductance changes

Abstract: A sensing device includes a pair of sensing electrodes disposed within a medium and a circuit connected to the sensing electrodes via impedance matching networks for producing an output signal which varies in response to a capacitive change in the medium. The circuit includes a first circuit portion including the sensing electrodes and a second circuit portion including the oscillator. The first and second circuit portions are tuned to match their impedance for more accurately measuring the capacitive changes.

Magnetic induction and plasma impedance in a cylindrical inductive discharge

Abstract: An inductive radio frequency discharge in the cylindrical configuration is modelled as a transformer with the inductive coil taken as the primary circuit and the plasma as the secondary circuit. The mutual inductance between the primary and secondary circuit, the self-inductance of the plasma and the impedance of the plasma are determined theoretically and related to the properties of the plasma evaluated using a global (volume-averaged) discharge model for argon plasma. The azimuthal electric field and the axial magnetic field profile within an inductive discharge in the cylindrical configuration are determined assuming a parabolic electron density profile.

High frequency impedance spectra of soft amorphous fibers

Abstract: Giant magnetoimpedance (GMI) spectra of soft amorphous magnetic fibers, measured in the 1 kHz–1.2 GHz frequency range, and GMI responses, measured in the field range of ±120 Oe, have reinforced the assumption that linear giant magnetoimpedance and ferromagnetic resonance (FMR) have the same physical origin. The samples, NiCo-rich, CoFe-rich, and Metglas-type fibers, 30–40 μm in diameter, were cast by melt extraction. Their impedance has been measured up to 13 MHz, in the presence of a magnetic field, using an impedance analyzer. These measurements have been extended up to 1.2 GHz by using a network analyzer. The reflection coefficient of a shorted coaxial line whose inner conductor was replaced by a magnetic fiber was measured, and the input impedance per unit length of this line was then calculated. The two impedances above are equivalent and their spectra show a behavior associated with FMR: the real part of the impedance peaks at a frequency where the imaginary part passes through zero.