Showing papers on "Electrical impedance published in 1991"

ACT3: a high-speed, high-precision electrical impedance tomograph

Abstract: Presents the design, implementation, and performance of Rensselaer's third-generation adaptive current tomograph, ACT3. This system uses 32 current sources and 32 phase-sensitive voltmeters to make a 32-electrode system that is capable of applying arbitrary spatial patterns of current. The instrumentation provides 16 b precision on both the current values and the real and reactive voltage readings and can collect the data for a single image in 133 ms. Additionally, the instrument is able to automatically calibrate its voltmeters and current sources and adjust the current source output impedance under computer control. The major system components are discussed in detail and performance results are given. Images obtained using stationary agar targets and a moving pendulum in a phantom as well as in vivo resistivity profiles showing human respiration are shown. >

Dual feedback control for a high-efficiency class-d power amplifier circuit

Abstract: A dual feedback control system for a Class-D power circuit maximizes efficiency by controlling the magnitude and phase angle of the resonant load circuit using two separate feedback loops. The first feedback loop changes the value of a first variable capacitor in response to difference in magnitude between the actual load impedance and the desired load impedance. The second feedback loop changes the value of a second variable capacitor in response to the phase difference between the actual phase angle and the desired phase angle. In this way, the real and imaginary components of the load impedance are controlled so that the Class-D circuit is in tune, and hence efficiency is maximized, over a wide range of load impedances. The dual feedback control of the present invention may be advantageously employed in the ballast of an electrodeless HID lamp system.

Automatic impedance matching apparatus and method

Abstract: An automatic impedance matching apparatus for matching an RF-signal generator to a load, such as a plasma etching chamber, is disclosed. The matching apparatus comprises a matching network having two variable impedance devices, a tune detector for detecting the condition of the impedance match between the RF-signal and the load, and a controller for modifying the values of the variable impedance components in response to the measured tune condition. The present invention disclosed improve reset and convergence unit and eliminates the need for the "dead-band" provided around the matching point found in prior art impedance matching controllers. Also disclosed is an improved adjustment unit for adjusting the variable impedance components which is faster and more stable than found in prior art controllers. Also disclosed is a normalization unit for normalizing the input detection siganls such that variations in turning performance due to variations in input power level from the source are substantially reduced.

Layer stripping: a direct numerical method for impedance imaging

Abstract: An impedance imaging problem is to find the electrical conductivity and permittivity distributions inside a body from measurements made on the boundary. The following experiment is considered: a set of electric currents are applied to the surface of the body and the resulting voltages are measured on that surface. The authors describe the performance of a direct numerical method for approximating the conductivity in the interior. The algorithm proceeds via two steps: first the conductivity is found near the bounding surface of the body from the data having the highest available spatial frequency; next the boundary data on an interior surface are synthesized using a nonlinear differential equation of Riccati type. The process is then repeated, and an estimate of the conductivity is found, layer by layer. They establish the theoretical basis for the algorithm and report on numerical tests.

Stacked dual dipole MMDS feed

Abstract: A multichannel, multipoint distribution services (MMDS) dipole antenna for receiving multiple channels in the S-band frequency range of 2000 and 3000 MHz is formed from a printed circuit board which is directly connected to a coaxial cable. On the printed circuit board are etched two stacked dipoles. Each of the dipoles has a first one-half element etched on the first side of the printed circuit board and the second one-half element etched on the second side of the printed circuit board. The first and second dipoles are oriented to be in phase with each other and are separated from each other at a wavelength spacing between 0.25 lambda and 0.40 lambda. The antenna of the present invention further uses a phase combining circuit and an impedance matching circuit etched on the printed circuit board for combining in phase the polarized signals, for canceling the non-polarized signals at 0° and 180°, from the two stacked dipoles and for matching the impedance from the two dipoles to the impedance of the coaxial cable.

A device for measurement of electrical impedance of organic and biological materials

Abstract: A device and method for noninvasive depth-selective detection and characterization of surface phenomena in organic and biological material such as tissues by surface measurement of the electrical impedance of the material. The device includes a probe with a plurality of measuring electrodes separated by a control electrode. Measuring equipment measures impedance in a desired frequency range. An adjustable amplifier maintains a chosen control signal derived from the potential of one of the measuring electrodes at the control electrode without loading the measuring electrode. Depth selectivity is achieved by controlling the extension of the electric field in the vicinity of the measuring electrodes by the control electrode actively driven with the same frequency as the measuring electrodes.

Robust hybrid impedance control of robot manipulators

Abstract: The objective of hybrid impedance control is defined, and a robust hybrid impedance control method is proposed. The task space is split into force-controlled and position-controlled subspaces based on the concept of hybrid control. Desired inertia and damping are introduced in the force control subspace to improve the dynamic behavior, and impedance control is used in the position-controlled subspace. In the proposed control scheme, the hybrid impedance control is equivalent to tracking a desired acceleration trajectory, which is generated in real-time. The computed torque technique and a PI control law are used to reduce the influence of model uncertainties. Experimental results obtained with a two-degree-of-freedom direct drive robot have shown the effectiveness of the proposed hybrid impedance control method. >

A de-embedding algorithm for electromagnetics

Abstract: A “double delay” de-embedding algorithm appropriate for electromagnetic analyses is described. This algorithm uses only two standards, a through and a double length through. By evaluating these standards, a special class of port discontinuities may be characterized and removed from the data calculated for a complete structure. Unlike related physical de-embedding algorithms, both the characteristic impedance and the velocity of propagation of the through lines are determined. The technique described here is difficult to implement in a physical de-embedding. The de-embedding theory also provides a new definition of characteristic impedance, “equivalent TEM impedance,” for inhomogeneous media, such as microstrip. This new impedance exhibits a nonmonotonic dispersion which has been measured experimentally but is not seen using previous impedance definitions.

Small signal equivalent circuit modeling of resonant converters

Abstract: A general analytical procedure is presented for the equivalent circuit modeling of resonant converters, using the series and parallel resonant converters as examples The switched tank elements of a resonant converter are modeled by a lumped parameter equivalent circuit The tank element circuit model consists, in general, of discrete energy states, but may be approximated by a low-frequency continuous time model These equivalent circuit models completely characterize the terminal behavior of the converters and are solvable for any transfer function or impedance of interest With the approximate model it is possible to predict the lumped parameter poles and zeros, and to quickly determine the relevant DC gains of the output impedance and the control to output transfer function Closed-form solutions are given for the equivalent circuit models of both converter examples Experimental verification is presented for the control-to-output transfer functions of both series and parallel resonant converters, and good agreement between theoretical prediction and experimental measurement is obtained >

On the Application of the Kramers‐Kronig Relations to Evaluate the Consistency of Electrochemical Impedance Data

Abstract: The use of the Kramers-Kronig (KK) relations to evaluate the consistency of impedance data has been limited by the fact that the experimental frequency domain is necessarily finite. Current algorithms do not distinguish between the residual errors caused by a frequency domain that is too narrow and discrepancies caused by a system which does not satisfy the constraints of the KK equations. A new technique is presented which circumvents the limitation of applying the KK relations to impedance data which truncate in the capacitive region. The proposed algorithm calculates impedance values below the lowest experimental frequency which "force" the data set to satisfy the KK equations. Internally consistent data sets yield low-frequency impedance values which are continuous at the lowest measured experimental frequency. A discontinuity between the calculated low-frequency values and the experimental data indicates inconsistency which cannot be attributed to the finite experimental frequency domain.

Moisture and salinity sensor and method of use

Abstract: This invention relates to moisture and salinity measurement and in particular to a sensor and its method of use which may provide values for the moisture/complex dielectric constant and/or salinity of a variety of mediums in which it is placed, which may includes soils, cotton, grain, coal dust and concrete A sensor apparatus (12) is arranged for indicating the complex dielectric constant and conductivity of a medium and comprises a tuned circuit (21, 22, 23 and 24) having a variable complex impedance and which is influenced by the variable complex dielectric constant of the medium in which the sensor apparatus is placed An oscillator (25) is connected to the tuned circuit so as to oscillate at a frequency dependant on the tuned circuit, such that the frequency of oscillation is representative of the complex dielectric constant of the medium and has a first mode of operation in which the oscillator frequency is representative of a first complex dielectric constant of the medium, and a second mode of operation, in which the tuned circuit is varied so that the oscillator frequency is representative of at least one second complex dielectric constant of the medium and the difference between an oscillator frequency of the first mode and an oscillator frequency of the second mode is representative of the conductivity of the medium

Input impedance of a probe-fed stacked circular microstrip antenna

Abstract: The input impedance of a microstrip antenna consisting of two circular microstrip disks in a stacked configuration driven by a coaxial probe is investigated. A rigorous analysis is performed using a dyadic Green's function formulation whereby the mixed boundary value problem is reduced to a set of coupled vector integral equations using the vector Hankel transform. Galerkin's method is used in the spectral domain, using two sets of disk current expansions. One set is based on the complete set of orthogonal modes of the magnetic cavity, and the other uses Chebyshev polynomials with the proper edge condition for the disk currents. An additional term is added to the disk current expansion to model the current properly in the vicinity of the probe/disk junction. The input impedance of the antenna, including the probe self-impedance, is calculated as a function of the layered parameters and the ratio of the two disk radii. Disk current distributions and radiation patterns are presented. The calculated results are shown to be in good agreement with experimental data. >

Nonlinear resistor control circuit and use in liquid crystal displays

Abstract: A control circuit comprises a combination of two or more nonlinear resistor elements having a common electrical junction and a nonlinear current/voltage characteristic, the impedance at the common electrical junction being controlled in accordance with switching voltages applied to the nonlinear resistor elements. These nonlinear resistor elements may be connected to one terminal of a load element, such as a liquid crystal element or a printing element. An array of such load elements, such as printing elements or liquid crystal elements of the microencapsulated type, combined with these nonlinear resistor elements form, respectively, a printing engine or display device. The nonlinear resistor elements are composed of semiconducting or conducting powder particles bonded together with an insulating or semiconducting binder.

Coupled inductance and reluctance models of magnetic components

Abstract: Different models for linear transformers are discussed relative to their use in circuit simulation. These models include: the reluctance model, the permeance model, the inductance matrix model, the T-model and the reduced T-model. Equations relating the parameter values of the models are derived for the two- and three-winding transformers. A method for direct measurement of the inductance matrix model parameters is described. Guidance is provided for deriving model parameters and using the models in circuit simulators. A model of an ideal transformer using controlled current and voltage sources useful for the T-model is derived. Several examples are discussed. >

A general analysis of propagation along multiple-layer superconducting stripline and microstrip transmission lines

Abstract: A rigorous spectral-domain formulation for a superconducting stripline or microstrip transmission line with a multiple-layer dielectric substrate is presented. The formulation models the strip conductor as a surface current with an equivalent surface impedance, where the surface impedance is approximated in closed form when the strip is either much thinner or much thicker than a penetration depth. In either case the surface impedance is related to the complex conductivity of the material, which is calculated from a two-fluid model. Results are presented to show the slow-wave propagation and attenuation along both microstrip and stripline packages in a realistic multiple-layer configuration, which accounts for the field penetration into the superconducting ground planes. >

[Measuring electrical impedance of organs--instrumental equipment for research and clinical use]

Abstract: An apparatus for measuring the impedance of intact biological organs or parts of organs in the frequency range of 10 Hz to 10 MHz is described. In this range impedance exhibits a large dispersion, which is dependent on tissue structures. The time course of alterations of electrical impedance such as occur during ischemia can be recorded with this equipment. Five specimens in five measuring chambers can be examined simultaneously at different temperatures. In the second part of the article, a portable impedance meter for measuring the modulus of impedance near 200 Hz, the phase of impedance at 5 kHz and the local temperature at the measuring point, is described. These parameters permit an intra-operative evaluation of the changing state of ischemic organs. Sterilizable probes with four surface electrodes and an integrated temperature sensor permit atraumatic measurements at the organ surface. The measurement itself is harmless to the tissue.

Detection and imaging of electric conductivity and permittivity at low frequency

Abstract: The authors discuss low-frequency electrical impedance imaging, the process of constructing images of the electrical impedance of a body's interior based on measurements of voltage and current made at the body's surface. The electrical impedance accounts for both resistivity and permittivity. It is shown how permittivity can be exploited to improve the performance of an electrical impedance imaging system. It is shown that explicit use of the independent information in the data due to the permittivity will enhance a system's ability to distinguish objects in the interior of a body. In addition. the results of experiments performed using the Rensselaer ACT 2 system on a saline bath containing various objects are reported. These objects include both living tissue and metal conductors with oxide layers. The system's ability to distinguish these objects is demonstrated. and gray scale images of both their resistivity and permittivity distributions are given. >

Current patterns for electrical impedance tomography

Abstract: A method in electrical impedance tomography uses an adaptive repeated series of calculations and measurements to determine the best current pattern for a body having an initially unknown internal conductivity, for producing the best electrical impedance image for the body. Initially, an arbitrary current pattern and conductivity is utilized to generate a voltage pattern which is measured and compared with the theoretical voltage pattern based on the arbitrary conductivity and current patterns. When both the current pattern and the conductivity yielded by the process closely match values derived by measurements, the voltage pattern is used to produce the image.

Device for inductive current limiting of an alternating current employing the superconductivity of a ceramic high-temperature superconductor

Abstract: A device for inductive current limiting of an alternating current consisting of an induction coil (2), which includes at least one winding and through which current flows, a body (3) made of a ceramic high-temperature superconductor arranged concentrically to the latter and having a centrosymmetrical form which is hollow in the interior, and having located in the interior of said body (3) a concentrically arranged core (4) made of a soft magnetic material of high permeability. In normal operation (rated current), the superconductivity of the body (3) is effective and impedance of the induction coil (2) is very low. With overcurrent (mains short-circuit) the superconductivity disappears and the impedance of the induction coil (2) reaches its maximum, current-limiting value.

A method for wide frequency range modeling of power transformers and rotating machines

Abstract: Wide frequency range representation of power system components such as power transformers, reactors and rotating machines is mainly needed for the analysis of electromagnetic transients in power systems. In this paper, a method is presented for state space formulation of machine windings. The described procedure is completely based on external terminal impedance measurements and data related to the internal structure is not required. First, the wide frequency behavior of machine windings is reviewed and the nonlinearity effects of the iron core are discussed. A numerical method is given for calculation of the state equation parameters directly from the measured terminal impedance characteristics. The presented modeling method is applicable to any transformer, reactor or rotating machine in service where sufficient information on the internal design is not available. Besides, the canonical structure of the model offers advantages in terms of computer memory and computation effort in time domain analyses of complex power systems. >

Multiply-tuned probe for magnetic resonance imaging or spectroscopy

Abstract: A multiply-tuned probe for magnetic resonance imaging or spectroscopy in which a driving inductor is separately coupled to two or more trap inductors. The impedance match at each frequency is adjusted independently of the impedance match at the other frequencies and independent of the tuning by using a novel coil geometry with no overcoupling. In a preferred embodiment, a triple-tuned probe has a first inductor L1 disposed in a first plane and tuned to a first resonant frequency and a second inductor L2 disposed in a second plane parallel to the first plane and tuned to a second resonant frequency. A driving and receiving primary inductor Lp is disposed in a third plane parallel to the first and second planes and placed with respect to the first and second inductors L1 and L2 such that mutual inductance between L1 and Lp in between L2 and Lp is adjustable in accordance with the positions of the inductors L1, L2 and Lp. A sample surface inductor Ls tuned to a third resonant frequency provides the third frequency. Mutual inductance between L1 and Lp in between L2 and Lp is changed by repositioning the inductors L1, L2 and Lp to vary the amount of overlap between the cross-sections of the primary inductor Lp and the circular coils L1, L2 and the distances between inductors L1 and Lp and between inductors L2 and Lp. Since each inductive coupling adds another variable to the probe circuit, more and easier ways are provided for adjusting the tuning and impedance matching.

Use of complex impedance spectroscopy in chemical sensor characterization

Abstract: The complex impedance method is a powerful tool in the characterization of gas adsorption in semiconductor structures used as gas sensors. Different tin oxide structures have been used. In tests of sensitivity to dry air, air + H2O, O2, etc, the operating temperature has increased to 175 °C. A variable frequency (10 μHz–32 MHz) a.c. voltage is applied to the sensor structure to perform the complex impedance spectrum measurements. The different parameters are a function of the type of gas in the atmosphere. A model has been proposed based on adsorption on the grain boundary.

Determination of rail internal impedance for electric railway traction system simulation

Abstract: Theoretical models for rail internal self-inductance and resistance are proposed which include material saturation and hysteresis. Model data are obtained using experimentally derived ferromagnetic material B-H loops. Analytical methods for deriving the impedance variations with current and frequency for large-signal AC and incremental AC superimposed on DC excitation are in agreement with experimental measurements.

Automatic electrode channel impedance measurement system for egg monitor

Abstract: An impedance measurement system for electrocardiographic recorders which allows measurement of the electrode channel impedance without the need for a separate piece of equipment. The impedance measurement system, which is an integral part of the recorder, applies complementary oscillating currents having short transition times to an electrode channel defined by a pair of electrode leads, a pair of electrodes and the patient tissues. The analog current signals flowing through the electrode channel create a voltage across the electrode channel which is amplified and applied to a dual range filtering stage. The low range section of the dual range filtering stage generates a time-dependent signal which causes the output of the low range filtering circuit to produce a signal applied to the timing input of a microprocessor. The microprocessor calculates the time elapsed between each transition of the oscillating current signals and the occurrence of a signal at the output of the low range filtering circuitry. From that elapsed time, the microprocessor calculates the impedance of the electrode channel being monitored. In addition, in response to the signal appearing at the output of the high range section of the dual range filtering stage, the microprocessor verifies contemporaneously the integrity of the electrode leads in order to validate the measured impedance value.

Load circuit commutation circuit

Abstract: Arrangement for arcless interruption of a-c load current in response to a current interruption command. Separable contact means serially connected with controlled impedance means, and parallel connected diversion means are intermediate the a-c source and the load. The controlled impedance comprises field effect transistors (FETs). Since FETs usually have only a single inherent junction, at least a pair of the FETs are oppositely poled so that load current can be cut off notwithstanding the direction of current flow when interruption is commanded. The pair of FETs may be connected back to back. An alternative embodiment has two branch circuits each comprising a FET and a separable contact means. Upon interruption load current is sequentially transferred from one to the other branch circuit and then to the diversion means.

Input impedance of hemispherical dielectric resonator antenna

Abstract: A coaxially fed hemispherical dielectric resonator antenna is investigated theoretically. The input impedance of the lowest order TE 111 mode that radiates efficiently in the broadside direction is calculated using the reaction formula. The result is compared with experimental data available in the literature. The erects of the probe length, feed position and dielectric constant on the input impedance are studied

Circuitry for interfacing telecommunications equipment to a communication channel

Abstract: Measuring the programming resistor in a data jack by placing the resistor in an AC voltage divider circuit via a transformer. An AC voltage (preferably generated by the modem's own transmitter circuitry) is applied to the voltage divider circuit, and an AC voltage measurement (preferably made by the modem's A/D converter) provides the resistance of the programming resistor. Matching the low-pass filter impedance characteristic of a local loop with a network having a matching low-pass filter characteristic, while also compensating the received signal with a compensation network having a high-pass filter characteristic, to reduce the frequency dependent gain effects caused by the matching network and/or the local loop.