Abstract: The robust stability of the Smith predictor scheme for time-delay compensation is analyzed with respect to uncertainty in the time-delay parameter. Appropriate definitions are introduced for the phase margin and complementary phase margin of nonminimum-phase systems, and are used to characterize the domains of stability for the closed loop using a Nyquist approach. Rigorous arguments are used to show that the stability domain is discontinuous when the nominal Nyquist function has more than one magnitude-crossover points. Robust stability results are given for the cases where the Nyquist plot has no magnitude-crossover points, and when it has a single or multiple crossover points. The results are illustrated through an example.

Abstract: Alternating current impedance data of alkaline Zn/MnO2 cells were analysed in view of identification of suitable parameters, which depend on the state-of-charge (SOC) of the cells. The impedance of a slightly discharged cell was found to possess impedance considerably lesser than that of an undischarged cell. The data in the form of Nyquist plot contained an inductance part at very high frequencies, a capacitive semicircle at high frequencies and a diffusion linear spike at low frequencies. The low frequency linear spike gradually transformed into a capacitive semicircle with the decrease of SOC of the cell, which was attributed to the nature of the reactions at the Zn anode. Of several impedance parameters that were examined, equivalent series capacitance (Cs) was found to have a strong dependence on SOC of the alkaline Zn/MnO2 cells. There was a continuous change in a partially discharged cell during its ageing, which was reflected by transformation of low frequency data into a clear semicircle.

Abstract: Stability conditions for discrete-time linear dynamical systems in two independent variables are considered. A new proof of a known condition is given, enabling a more transparent interpretation. It is shown that closed-loop stability of a feedback system can be determined from open-loop information by using an infinite family of Nyquist plots. On the basis of these results, the extension of classical design methods, using root loci and frequency response, is advocated.

Abstract: The effects of Cl- on the AC impedance features of carbon steel under anodic polarization conditions, and the critical pitting potential (Eb) at different temperature have been investigated respectively by EIS and potentiodynamic polarization in alkaline solutions containing H2S. The results indicate that the carbon steel gets into stage of passivity with increasing of polarization potential at the initial stage of anodic polarization. During the pitting initiation, the Nyquist diagram has a shrinkage of real part at low frequency and the polarization resistance sharply decreases. The Nyquist diagram is composed of two capacitive loops during the pits propagation.With increasing of polarization potential, the capacitive loops of high frequency disappears. With increasing of Cl- concentration at different temperature, the passivity trend of mild steel decreases. The relationship between Eb and Cl- concentration at different temperature follows the Eb=a+blnCa-.

Abstract: The method of ACIS (AC impedance spectroscopy) under DC polarization was applied to study the hydration process of cement. Various dynamic parameters of the hydration process were determined and its mechanism was discussed. It has been shown by Nyquist plot that during the hydration process there exist two different stages in which the microstructure of C S H is quite different.

Abstract: The frequency dependence of the electrical properties of the Bi12TiO20 single crystal was investigated by impedance spectroscopy The data were presented in the Nyquist diagram form, from which electrical resistivity was determined Values ranged from 193×105 and 107×103 Ω cm in the temperature range of 400–700 °C The electrical conductivity followed the Arrhenius law, with an activation energy of 099 eV The dielectric behavior was investigated from room temperature to 700 °C Permittivity was calculated based on the relaxation frequency, using an alternative approach based on the variation of the imaginary impedance component as a function of reciprocal angular frequency The permittivity values obtained by means of these two methods showed good agreement up to 600 °C The frequency dependence of real and imaginary permittivities from room temperature to 700 °C was also investigated