Sarit K. Das

Decentralized Stabilization of Uncertain Systems With Interconnection and Feedback Delays: An LMI Approach

This note presents a broad LMI condition that can ascertain the stability of uncertain systems under decentralized feedback in the presence of interconnection and feedback delays. Based on the Lyapunovs direct approach with a four-term energy functional and a three-term quadratic formulation of the given state dynamics, this method has a larger search space than used so far. Numerical examples corroborate the superiority of this method vis-a-vis the existing ones for several subsets of the general problem.

The role of percolation and sheet dynamics during heat conduction in poly-dispersed graphene nanofluids

A thermal transport mechanism leading to the enhanced thermal conductivity of graphene nanofluids has been proposed. The graphene sheet size is postulated to be the key to the underlying mechanism. Based on a critical sheet size derived from Stokes-Einstein equation for the poly-dispersed nanofluid, sheet percolation and Brownian motion assisted sheet collisions are used to explain the heat conduction. A collision dependant dynamic conductivity considering Debye approximated volumetric specific heat due to phonon transport in graphene has been incorporated. The model has been found to be in good agreement with experimental data.

Periodic discrete-time systems: stability analysis and robust control using zero placement

A simple method (based on Floquet theory) for obtaining the characteristic equation (and hence stability) of periodic discrete-time systems is presented. Using this method it is shown that 2-periodic controllers can be used to relocate the zeros of SISO plants. Some examples are considered to illustrate the use of 2-periodic controllers for robust control of finite gain margin problems. >

Technical communique: Robust compensation of a Cart-Inverted Pendulum system using a periodic controller: Experimental results

Based on Das and Dey (2007), this paper designs and implements a periodic controller to achieve, via zero-placement, robustness of a physical Cart-Inverted Pendulum system with respect to differential gain variations in the output sensors. Experimental results that verify the superiority of this controller over linear time-invariant (LTI) ones are also presented.

A semianalytical model to study the effect of cortical tension on cell rolling.

Cell rolling on the vascular endothelium plays an important role in trafficking of leukocytes, stem cells, and cancer cells. We describe a semianalytical model of cell rolling that focuses on the microvillus as the unit of cell-substrate interaction and integrates microvillus mechanics, receptor clustering, force-dependent receptor-ligand kinetics, and cortical tension that enables incorporation of cell body deformation. Using parameters obtained from independent experiments, the model showed excellent agreement with experimental studies of neutrophil rolling on P-selectin and predicted different regimes of cell rolling, including spreading of the cells on the substrate under high shear. The cortical tension affected the cell-surface contact area and influenced the rolling velocity, and modulated the dependence of rolling velocity on microvillus stiffness. Moreover, at the same shear stress, microvilli of cells with higher cortical tension carried a greater load compared to those with lower cortical tension. We also used the model to obtain a scaling dependence of the contact radius and cell rolling velocity under different conditions of shear stress, cortical tension, and ligand density. This model advances theoretical understanding of cell rolling by incorporating cortical tension and microvillus extension into a versatile, semianalytical framework.

Realization of inverse saturable absorption by intracavity third-harmonic generation for efficient nonlinear mirror mode-locking

A technique of suppressing passive Q-switching by inverse saturable absorption in a cw nonlinear mirror mode-locked laser is presented. The nonlinear mirror saturable absorber consists of a second-harmonic generation crystal and a dichroic mirror while the inverse saturable loss is realized by intracavity third-harmonic generation. The inverse saturation reduces significantly the critical intracavity pulse energy for stable, self-starting, self-sustained, and power scalable cw mode-locking. Two LiB3O5 crystals are employed to realize the technique in a diode array pumped Nd:YVO4 oscillator providing a peak power of 918W, pulse width of 29ps, and repetition rate of 170MHz.

Periodic Compensation of Continuous-Time Plants

This note presents a periodic compensator which achieves robust stability for single-input-single-output (SISO), linear time invariant (LTI) plants having both right-half plane (RHP) poles and zeros, a job LTI controllers fail to do. In addition, for strictly proper plants this controller achieves model matching ensuring at the same time that the periodic oscillations present in the plant output are insignificant in magnitude. The design steps are straightforward and linear algebraic in nature

Open-Loop Decoupling of MIMO Plants

The complete set of decoupled forms of a MIMO plant P (which might be non-square and may have unstable pole-zero coincidences) achievable using open-loop precompensation is obtained in terms of certain multiplicities of the poles and zeros of P. This approach, which also yields the precompensator directly, is much simpler than the methods available.

Simultaneous stabilization of two discrete-time plants using a 2-periodic controller

A generic method for designing a 2-periodic controller for the simultaneous placement of the closed-loop poles of two single-input-single-output discrete shift-invariant plants at the origin is presented. The method consists of first recasting the simultaneous pole-placement problem as one of solving a coupled pair of linear polynomial equations involving three unknown polynomials, and then obtaining the controller parameters in terms of the coefficients of these polynomials. The isolated cases for which such pole placement is not possible have been listed. Simulation results show that the performances of systems thus compensated are superior to their performances when compensated using the higher periodicity controllers suggested in literature.

Simultaneous pole placement of M discrete-time plants using a M-periodic controller

A generic procedure for designing a M-periodic controller (sought in the controller canonical form) for the simultaneous placement of the closed-loop poles of M (=2,3,4,...) discrete, time-invariant plants is presented. The procedure is a two-stage one: first, a set of M simultaneous, linear, polynomial equations, arising out of the M given plants and the corresponding desired closed-loop pole locations, are solved via a generalized Sylvester matrix approach to obtain a set of (M+1) intermediate polynomials; and next, the controller parameters are obtained solving another set of simultaneous, linear polynomial equations that involve the above intermediate polynomials. Thus, both the computational steps are linear algebraic in nature. A list of the isolated plant configurations for which solutions do not exist is given. An example illustrates the procedure.