Abstract: A family of Nyquist plots for 0.2 μm thick polypyrrole film was measured in a wide range of potentials in 0.25 M LiClO 4 /propylene carbonate (PC) solution. This film was galvanostatically deposited onto the surface of a Pt electrode, and the film was continuously cycled in 0.25 M LiClO 4 /PC solution until a stable cyclic voltammetric response was reached. No irreversible changes occurred in the film during impedance measurements. Two alternative finite-space diffusion models were used to determine useful kinetic and equilibrium characteristics: the total charge-transfer resistance; partial electron and ion interfacial transfers; the low-frequency resistance related to the capacitive region of the impedance spectra; the binary chemical diffusion coefficient D; and the low-frequency differential capacitance related to the charging of the film's bulk. All these quantities were determined as functions of potential and adequately interpreted. We also found that upon increasing the doping level, D increases from 1.0 X 10 -9 to 2.8 × 10 -8 cm 2 /s.

Abstract: The feedback relay test is usually used to identify one point on the Nyquist diagram of the plant model. It is shown that the derivatives of amplitude and phase of the plant model with respect to frequency at that point can be approximated by the Bode's integrals without any model of the plant. The precision of the approximation for typical industrial plant models is studied. The derivatives are used to design a PID controller for slope adjustment of the loop Nyquist diagram and improve the closed-loop performance. Simulation examples illustrate the effectiveness and the simplicity of the proposed method to design the PID controllers.

Abstract: Three different coating conditions influencing the quality of a silicon polyester galvanized steel system were investigated by means of EIS. The first aim was to investigate the influence of different types of pores or discontinuities on the corrosion resistance of the above-mentioned coating system. A second aim was to study the influence of mechanical deformation and damage of the coating on the corrosion behaviour of the coating system. Finally, degradation due to ultraviolet exposure was investigated. In each of these investigations it was possible to distinguish different time constants for different electrochemical events in the Bode phase diagram. Since the difference in magnitude of the values of the resistances was large, they could not be distinguished in the Nyquist plots. Therefore, this type of plot was not used in this work. From the build-up of the coating system and the number of time constants deduced from the EIS measurements, it was possible to model the system. From the model, the physical and electrochemical phenomena occurring during the service life of an imperfect organic coating system was explained.

Abstract: Three different coating conditions influencing the quality of a silicon polyester galvanized steel system were investigated by means of EIS. The first aim was to investigate the influence of different types of pores or discontinuities on the corrosion resistance of the above-mentioned coating system. A second aim was to study the influence of mechanical deformation and damage of the coating on the corrosion behaviour of the coating system. Finally, degradation due to ultraviolet exposure was investigated. In each of these investigations it was possible to distinguish different time constants for different electrochemical events in the Bode phase diagram. Since the difference in magnitude of the values of the resistances was large, they could not be distinguished in the Nyquist plots. Therefore, this type of plot was not used in this work. From the build-up of the coating system and the number of time constants deduced from the EIS measurements, it was possible to model the system. From the model, the physical and electrochemical phenomena occurring during the service life of an imperfect organic coating system was explained.

Abstract: In this paper, the response to the first harmonic component (2f) of the electrostatic force in single terminal driven electrostatic comb-drive and parallel-plate drive was used as a signal to extract device parameters, namely, the Q- factor and resonant frequency instead of the fundamental (1f) resonance response. It is shown that the difficulty in motional measurement due to electrical cross-talk (parasitics) using 1f measurement can be overcome with a higher signal-to-noise ratio of the 2f signal. Both atmospheric (low-Q) and reduced pressure environment were investigated using off-chip electronics and lock-in amplifier. The measurements were done on the electrostatic comb-drive and capacitive parallel plate sensing plates that form the two core modules of a yaw rate sensor (dual-axis resonator). The effects of AC and DC bias voltages on the measured response have been investigated. Experimental amplitude and phase response data have been analyzed using the Lorentzian curve-fit, Resonance Curve Area (RCA) method, the half-power bandwidth method (3dB) and the Nyquist plot for data fitting and determination of the Q-factor and resonance frequency.

Abstract: Electrochemical impedance spectroscopy measurements under a small alternating current density were performed to investigate electrochemical reactions on (110) single-crystalline nickel surfaces in nickel sulfamate electrolyte. The reaction impedance shows a power law of the ac modulation frequency, and two exponents for the reaction resistance and reaction capacitance are found to approximately agree with the values predicted by a homogeneous charge-transfer reaction model. In addition, the Nyquist diagrams exhibit the absence of inductive loops for the frequency range from 50 mHz to 100 kHz. A condition for the existence of inductive loops derived in this study suggests that the electrocrystallization of Ni2+ ions may take place in the slow reaction step Ni2+ + e- → and in the fast reaction step + e- → Ni.

Abstract: With proper attention to experimental design (i.e., electroding, cabling, stray apparatus imittances, etc.) impedance/dielectric spectroscopy is a powerful tool to study the electrical properties of nanoscale electroceramics. This study focuses on bulk non-ferroelectric materials (ZnO, CeO2, TiO2) and their frequency-dependent AC electrical properties, taken from a variety of literature sources. In particular, it is shown how to separate effective grain boundary and grain interior resistivities and also the effective capacitances associated with each region in the microstructure. This is possible even when Nyquist plots (-Z im vs. Z re ) without frequency markers are the only data supplied. A modified brick layer model (BLM) can be used to analyze the impedance/dielectric properties of nanoscale ceramics.

Abstract: This work deals with the equilibrium point and stability analysis of discrete linear systems under quantized feedback. The case of quantized state feedback based on quantized state measurements (QIQM) is treated here. Unlike the case of input quantization (QI) only, there is no closed-form solution for the equilibrium points. However, a computable condition for the origin to be the only equilibrium is given. The stability analysis requires the construction of an equivalent system and a stability theorem for systems with a sector nonlinearity that is multiplicatively perturbed by a bounded function of the state. The analysis reduces to a simple test in the frequency domain, namely, the closed-loop system is globally asymptotically stable about the origin if the Nyquist plot of a system transfer function lies to the right of a vertical line whose abscissa depends on the 1-norm of the feedback gain. A numerical example of the analysis technique and some guidelines for the synthesis of a stable feedback gain are also provided.

Abstract: A model of information transmission across a neuron is delineated in terms of source (stimulus)-encoder-channel-decoder-behaviour (response). From cybernetic analysis of experimental data, we perform frequency/time domain and stability analyses and obtain the Bode, Nichols and Nyquist plots, Root locus plane, transfer function and response equation, all confirmed by data. We consider a new paradigm of information theory based on non-equilibrium dynamics of fluc-tuation, organization and information (Nicolis-Prigogine), that is the counterpart of Shannon-Boltzmann approach to information-entropy based on equilibrial dyna-mics. The Prigogine theorem of minimum entropy production and Rosen's prin-ciple of optimum design were observed to characterize neural transmission in a particular test neuron operating near optimal sensitivity regime. Using Nyquist theorem and generalized temperature concept, we compute a non-equilibrial entropy production and neurodynamic temperature equivalent during neural information processing. A trans-information/temperature plot implies an order-disorder Bose transition and zero neurodynamic entropy (near 00N) as informational analog of third law of thermodynamics (near 00K). Neural applications are explored.

Abstract: The purpose of this paper is to study the problem of the absolute stability of nonlinear systems with variable plant parameters. New formulations of the circle and off axis circle criteria are developed which enable one to assess the stability of a control system containing a nonlinearity and plant with parametric uncertainties. In contrast to the classical approach which uses the Nyquist plot, the proposed method makes use of the narrowest possible Bode envelopes obtained from new results obtained by the authors where the plant transfer function is taken in factored form.

Abstract: The impedance value of the oil is measured using at least four different measurement frequencies and at various oil temperatures. The results are plotted as a Nyquist diagram and the geometric parameters of the resultant impedance-frequency arc for each temperature are extracted. The condition of the oil is judged by an expert system based on the variation in offset voltage at low measurement frequencies.

Abstract: The impedance spectra of non-sensitized and Ru(II)-sensitized Nb2O5 nanoporous coatings have been measured in the dark and under solar illumination using an electroactive electrolyte. All the Nyquist plots consist of a high and a low frequency depressed semicircle. The results have been modeled and fitted by an equivalent electric circuit consisting of a resistor Rs in the series with two parallel RC circuits containing both a constant phase element (CPE). The resistor Rs describes the total resistance of the electrolyte and conducting electrodes (SnO2:F). The high frequency semicircule (f<1kHz) describes the capacitance and resistance of the semiconducting materials (grain boundaries and interfaces). The low frequency cycle (f < 1 kHz) is related to the formation of a double charge layer capacitance at the nanoparticle/electrolyte interface and a charge transfer resistance. Both values are strongly dependant of the experimental conditions, in particular of the applied potential and the state of illumination. The evolution of the electric elements is presented and discussed. It is shown in particular that the measurements in the dark cannot be directly compared to those under illumination as in teh latter all the Nb2O5-film is accessed.

Abstract: Impedance spectroscopy (IS) is a non-destructive technique used for obtaining valuable information about bulk conductivities, dielectric constants, phase composition and grain boundaries of important ceramic materials, amongst many others. We obtained Nyquist plots for four different hot-pressed Si3N4 samples in two different frequency regimes: 1 Hz to 30 MHz and 1 kHz to 100 kHz. The information obtained was compared against results from scanning electron microscopy (SEM) and X-ray diffraction (XRD). The limitations of the frequency response techniques are also discussed.

Abstract: Nyquist stability criterion is largely used to determine the number of Right-Half Plane poles of feedback systems and circuits. However, visual inspection of open-loop gain polar plot is required, and automatic stability check within microwave CAD tools is not possible. In this paper, a procedure to check system and circuit linear stability by means of Nyquist criterion within CAD tools, is presented. The proposed method makes use of integral phase evaluation of a transfer function, and does not require visual inspection of Nyquist plots. The method has been successfully implemented in commercial microwave CAD tools.