Showing papers on "Nyquist plot published in 1996"
TL;DR: In this paper, the dissolution and passivation reactions of zinc have been studied in 1.0 M KOH solutions by electrochemical impedance spectroscopy, and a Tafel plot constructed from the charge-transfer resistances provides an exchange current of 0.11 A/cm{sup 2} and an α value of 0.36 for zinc oxidation.
Abstract: Electrochemical dissolution and passivation reactions of zinc have been studied in 1.0 M KOH solutions by electrochemical impedance spectroscopy. Equivalent circuits have been worked out by simulating the impedance data and using the results to model the dissolution and passivation reactions. A Tafel plot constructed from the charge-transfer resistances provides an exchange current of 0.11 A/cm{sup 2} and an {alpha} value of 0.36 for zinc oxidation. The maximum rate of zinc oxidation is observed at about {minus}1.30 V vs. the Hg/HgO reference electrode as judged from the charge-transfer resistance minimum obtained from impedance measurements. A negative polarization resistance with a reverse semicircle on the Nyquist plot illustrates the transition process from an active to passive potential region at {minus}1.10 V. At high anodic over-potentials, the zinc electrode behaved as a semiconductor electrode due to a compact ZnO passive film formed on the electrode surface.
101 citations
TL;DR: In this paper, the active dissolution mechanism of iron in acidic solutions containing halide ions was investigated by electrochemical impedance spectroscopy (EIS) with channel flow double electrode (CFDE).
36 citations
TL;DR: In this article, an inductive arc in Nyquist plots from the unwetted Pt/C electrode specimen was investigated in 1 M H2SO4 solution at room temperature by using a.c. impedance spectroscopy in combination with a current-decay transient technique.
25 citations
TL;DR: In this article, the authors introduce new bounds for the real structured singular value, based on absolute stability criteria with plant-dependent multipliers that exclude the Nyquist plot from fixed plane curve shapes containing the critical point − + jO. Unlike half-plane and circle-based bounds, the critical feature of the fixed curve bounds is their ability to differentiate between real and imaginary components of the uncertainty.
2 citations
TL;DR: In this article, the Nyquist plot of the equivalent system is shifted to satisfy the sufficient condition for stability, and a general compensator is proposed by incorporating into the controller a term involving the un-implementable control.
1 citations
11 Dec 1996
TL;DR: In this article, the critical direction theory is proposed as a new tool for analyzing the robustness of uncertain systems and the analysis is done using Nyquist arguments involving eigenvalues instead of singular values, and yields exact necessary and sufficient conditions for robust stability.
Abstract: The critical direction theory is proposed as a new tool for analyzing the robustness of uncertain systems. The analysis is done using Nyquist arguments involving eigenvalues instead of singular values, and yields exact necessary and sufficient conditions for robust stability. The concept of a critical line on the Nyquist plane is defined and used to calculate the Nyquist robust stability margin. The new approach gives alternatives to computing exact stability margins in some cases of highly directional uncertainty templates where other methods are not applicable. An overview of major robust stability results reported in the literature is given for the purpose of comparison with the perspective of the new paradigm.
TL;DR: In this paper, an enhanced automatic tuning procedure was developed for process control of PI and PID controllers, which addresses several potential problems present in current standard autotuners, such as frequency response at two specified phase lags on the Nyquist curve automatically.
Abstract: An enhanced automatic tuning procedure developed for process control of PI and PID controllers addresses several potential problems present in current standard autotuners. The proposed enhanced autotuner uses a novel technique based on relay feedback to estimate the process frequency response at two specified phase lags on the Nyquist curve automatically. An iterative procedure then uses these two points to obtain a transfer-function model of the process. Based on this model and a controller-selection scheme, an appropriate controller (PI or PID) is applied to the process automatically. The controller is tuned so that the Nyquist curve of the compensated system is appropriately shaped to satisfy a combined gain and phase-margin type of specification. The effectiveness of this enhanced autotuner is demonstrated both in simulations and in real-time experiments for level control of a coupled-tanks system.
TL;DR: In this paper, an attempt is made to clarify an argument related to the utilization of a.c. impedance spectroscopy for hydrating cement systems, which relates to which electrode configuration, 2-point, 3-point or 4-point measurement, is pertinent for impedance spectrum measurement.
Abstract: An attempt is made to clarify an argument related to the utilization of a.c. impedance spectroscopy for hydrating cement systems. The question relates to which electrode configuration, 2-point, 3-point or 4-point measurement, is pertinent for impedance spectrum measurement. Theoretical analysis and experiment indicate that these electrode configurations should in principle give the same results. The impedance spectra from 3- or 4-point measurement are, however, strongly influenced by the contact areas between the specimen and potential sensors. This influence is attributed to the potential sensor-specimen “contact capacitance effect”. The experiment indicates that when contact capacitance, or area, between the specimen and potential sensor is small enough the impedance spectrum in the Nyquist plot is characterized by an almost perfect semi-circle and negligible high frequency “offset” resistance. These are the typical characteristics of the spectra obtained from 3- and 4-point measurements with point contact between the potential sensors and specimen. The 2-point and 4-point measurements give the same spectra when contact capacitance approaches a sufficiently large value. It is apparent that the impedance spectra from 3- and 4-point measurements with point contact cannot reflect true information about hydrating cement systems. They are experimental artifacts. The 2-point measurements, however, can give more reliable results.
TL;DR: In this article, the performance of simulated grain-coarsened heat-affected zone (HAZ) of A516 carbon steel in an H2S containing acidic chloride solution was investigated by electrochemical impedance spectroscopy (EIS) measurement at open circuit potential at 25 °C.