Sudeshna Dasgupta

Splay and bend elastic constants and rotational viscosity coefficient in a mixture of 4-4′-n-pentyl-cyanobiphenyl and 4-4′-n-decyl-cyanobiphenyl

Abstract Measurement of splay, bend elastic constants and rotational viscosity coefficient in mixtures of 4-4′- n -pentyl-cyanobiphenyl (5CB) and 4-4′- n -decyl-cyanobiphenyl (10CB) are reported. The mixture exhibits smectic A and nematic phases. The bend elastic constant and the rotational viscosity coefficient diverge as the smectic A phase is approached from the nematic side and we have determined the exponents for divergence of these two quantities. Both exponents seem to depend on the relative concentration of the constituents in the mixtures, and the values for the bend elastic constant show a jump from 0.7 to 1.0 when the mole fraction of 5CB increases from 0.25 to 0.30. This is consistent with the presence of a tricritical point for the phase transition. The possibility of the existence of a smecticA⇌nematic tricritical point is discussed in the framework of Landau theory.

Effect of a rigid, non-polar solute on the dielectric anisotropy, the splay and bend elastic constants, and on the rotational viscosity coefficient of 4-4'-n-heptylcyanobiphenyl

The effect of mixing a rigid, non-polar, non-mesogenic solute, biphenyl (C6H5-C6H5), in the nematic solvent 7CB (4,4′-n-heptylcyanobiphenyl) is investigated. The solute is found to reduce the nematic order and a two-phase region appears. We report measurements of the transition temperatures, dielectric anisotropy, and splay and bend elastic constants, as well as the rotational viscosity coefficient by the method of electric field-induced Fréedericksz transition for biphenyl concentrations up to 8.0%.

Internal model based v-norm decoupling control for Four Tank system

In this paper, an Internal Model Based VNorm Decoupling Control is proposed for a Four Tank system. The system is a 2×2 multivariable one with non-negligible time delays. For this purpose, an Internal Model Controller is designed using v-norm decoupling. Here, setpoint tracking, disturbance rejection, decoupling and robustness with respect to parametric uncertainty are demonstrated using simulations. Also, Gershgorin Bands are used for inspecting the degree of decoupling before and after compensation of the system.

Disturbance observer structure based internal model control for time delay systems

This paper presents a Disturbance Observer (DOB) like structure of Internal Model Control (IMC) for Single-Input-Single-Output (SISO) time delay systems. Here, a Conical Tank process which is modeled as a FOPDT (First Order Plus Delay Time) SISO system has been considered. A Modified Disturbance Observer (MDOB) concept has been used for the SISO time delay system. By using DOB like structure of IMC, set-point tracking, disturbance rejection and robustness with respect to parametric uncertainty have been evaluated using simulation.

Internal Model Control based controller design for a Stirred Water Tank

In this paper, we adapt a recently proposed Internal Model Control (IMC) based controller design approach for simultaneous decoupling and disturbance rejection for a practical Multiple Input Multiple Output (MIMO) system, namely, for designing a controller for a Stirred Water Tank System (Two Input Two Output). While retaining the central aspect of the reported method, namely using the off-diagonal elements of controller matrix for diagonalization whereas using the diagonal elements for satisfying other control objectives, the present method simplifies it for possible wider application. The proposed method is outlined, the controller is designed using this approach and its closed loop performance has been evaluated using simulation. Even though the plant has substantial open loop coupling and non-negligible delay, the closed loop system with the designed controller was found to exhibit good set-point tracking, good decoupling and adequate disturbance rejection properties.

Monte Carlo investigation of critical properties of the Landau point of a biaxial liquid-crystal system.

Extensive Monte Carlo simulations are performed to investigate the critical properties of a special singular point usually known as the Landau point. The singular behavior is studied in the case when the order parameter is a tensor of rank 2. Such an order parameter is associated with a nematic-liquid-crystal phase. A three-dimensional lattice dispersion model that exhibits a direct biaxial nematic-to-isotropic phase transition at the Landau point is thus chosen for the present study. Finite-size scaling and cumulant methods are used to obtain precise values of the critical exponent ν=0.713(4), the ratio γ/ν=1.85(1), and the fourth-order critical Binder cumulant U^{*}=0.6360(1). Estimated values of the exponents are in good agreement with renormalization-group predictions.

Performance comparison and robustness studies of robust bode and IMC based controllers for plants with large time delay

Controller design of a plant having large time delay using two different methods viz., Internal Model Control (IMC) and a controller based on the Robust Bode (RBode) method are reported. In particular, a boiler burning process is considered as plant which can be modeled as a First Order Plus Delay Time (FOPDT) system, where the time delay is fairly large and comparable to the plant time constant. Performance comparison and robustness studies are done considering nominal as well as off-nominal (using structured uncertainties) cases. Extension of the methods for higher order systems is conjectured.