Jesus Gonzalez-Trejo

Minimax Control of Discrete-Time Stochastic Systems

This paper gives a unified, self-contained presentation of minimax control problems for discrete-time stochastic systems on Borel spaces, with possibly unbounded costs. The main results include conditions for the existence of minimax strategies for finite-horizon problems and infinite-horizon discounted and undiscounted (average) cost criteria. The results are specialized to control systems with unknown disturbance distributions---also known as games against nature. Two examples illustrate the theory, one of them on the mold level control problem, which is a key problem in the steelmaking industry.

Robust control of a class of uncertain first-order systems with least prior knowledge

In this work we deal with robust control of a class of first-order nonlinear systems which contain uncertainties. To cope with the uncertainties, an observer-based controller is proposed, and the uniform boundedness of all closed-loop signals and semiglobal practical convergence of the system state are guaranteed. In contrast with some previous attempts to relax the a priori knowledge on uncertainties bounds by using discontinuous control law, we propose a dynamic compensator which leads to continuous control laws. The applicability of the control scheme to regulation of temperature in stirred tanks and one-point control in distillation is shown by numerical simulations.

Robust control with uncertainty estimation for feedback linearizable systems: application to control of distillation columns

Abstract The goal of this paper is to describe a linearizing feedback adaptive control structure which leads to a high quality regulation of the output error in the presence of uncertainties and external disturbances. The controller consists of three elements: a nominal input–output linearizing compensator, a state observer and an uncertainty estimator, which provides the adaptive part of the control structure. In this way, the feedback controller, based on the disturbance observer, compensates for external disturbances and plant uncertainties. The effectiveness of the controller is demonstrated on a distillation column via numerical simulations. ©

Hydrodynamic Characterization of an Electrochemical Cell with Rotating Disc Electrode: A Three-Dimensional Biphasic Model

Electrochemical cells with a rotating disc electrode are the preferred devices to characterize electrochemical reactions because simple analytical expressions can be used to interpret the information obtained from physical experiments. These equations assume that the velocity field in the vicinity of the electrode active face is in accordance with the ideal behavior described by von Kármán. Experimental liquid velocity measurements inside the cell reported in recent works suggest that the actual liquid flow pattern is not fully in accordance with the assumed ideal behavior. In this work, the Computational Fluid Dynamics technique was employed to characterize numerically the flow pattern inside the electrochemical cell. By using a three-dimensional model, symmetric conditions were not imposed. A biphasic system was employed to evaluate the influence of liquid free surface over the flow pattern. Unsteady state numerical simulations were performed using the commercial software Fluent. Multiple electrode rotation speeds and several cell sizes were employed. Contrary to the assumed behavior, it was obtained that the flow pattern inside the electrochemical cell is not symmetric due to the synergetic effect of the cell walls, the submerged electrode side wall and the liquid free surface. This work states that the differences between actual and the ideal flow patterns can be minimized with plain electrode and cell geometrical modifications.

Fractal dimension algorithms and their application to time series associated with natural phenomena

Chaotic invariants like the fractal dimensions are used to characterize non-linear time series. The fractal dimension is an important characteristic of systems, because it contains information about their geometrical structure at multiple scales. In this work, three algorithms are applied to non-linear time series: spectral analysis, rescaled range analysis and Higuchis algorithm. The analyzed time series are associated with natural phenomena. The disturbance storm time (Dst) is a global indicator of the state of the Earths geomagnetic activity. The time series used in this work show a self-similar behavior, which depends on the time scale of measurements. It is also observed that fractal dimensions, D, calculated with Higuchis method may not be constant over-all time scales. This work shows that during 2001, D reaches its lowest values in March and November. The possibility that D recovers a change pattern arising from self-organized critical phenomena is also discussed.

Hydrodynamic Analysis of Electrochemical Cells

The systems in which an electrode immersed in a solution causes a chemical reaction have been studied for over a hundred years. It has long been known that the behavior of these systems is determined by two main factors: the rate with which the substance comes into contact with the electrode and the rate of the electrochemical reactions at the electrode. During the first four decades of the twentieth century, many works were devoted to this subject; however, most of them were experimental works. Those works found that the limiting current increases with increasing the rate of stirring (Bircumshaw & Riddiford, 1952). Usually, the results were expressed by means of a power relation of the form:

Bifurcated SEN with Fluid Flow Conditioners

This work evaluates the performance of a novel design for a bifurcated submerged entry nozzle (SEN) used for the continuous casting of steel slabs. The proposed design incorporates fluid flow conditioners attached on SEN external wall. The fluid flow conditioners impose a pseudosymmetric pattern in the upper zone of the mold by inhibiting the fluid exchange between the zones created by conditioners. The performance of the SEN with fluid flow conditioners is analyzed through numerical simulations using the CFD technique. Numerical results were validated by means of physical simulations conducted on a scaled cold water model. Numerical and physical simulations confirmed that the performance of the proposed SEN is superior to a traditional one. Fluid flow conditioners reduce the liquid free surface fluctuations and minimize the occurrence of vortexes at the free surface.

Physical and Numerical Analysis of Bioclimatic Strategies Implemented on Social Housing

This paper presents the results of aerodynamical performance research focused on maintaining the thermal comfort and increasing the energy efficiency of a typical social housing unit located in a high-density urban area. Bioclimatic design strategies are used to develop a sustainable and economic technology in existing housing clusters in Mexico City. A full-scale prototype, built on campus facilities, was used to study the flow conditions around the design. All scaled prototypes implement similar criterion. Furthermore, a scaled prototype is evaluated within a low speed wind tunnel installation. Additionally, numerical simulations were performed at transient state based on previous physical measurements and historical local climatic conditions to determine preferable modifications.

Three-Dimensional Flow Behavior Inside the Submerged Entry Nozzle

According to various authors, the surface quality of steel depends on the dynamic conditions that occur within the continuous casting mold’s upper region. The meniscus, found in that upper region, is where the solidification process begins. The liquid steel is distributed into the mold through a submerged entry nozzle (SEN). In this paper, the dynamic behavior inside the SEN is analyzed by means of physical experiments and numerical simulations. The particle imaging velocimetry technique was used to obtain the vector field in different planes and three-dimensional flow patterns inside the SEN volume. Moreover, large eddy simulation was performed, and the turbulence model results were used to understand the nonlinear flow pattern inside the SEN. Using scaled physical and numerical models, quasi-periodic behavior was observed due to the interaction of two three-dimensional vortices that move inside the SEN lower region located between the exit ports of the nozzle.

Hydrodynamic Study of a Submerged Entry Nozzle with Flow Modifiers

The fluid flow modifier technology for continuous casting process was evaluated through numerical simulations and physical experiments in this work. In the casting of steel into the mold, the process presents liquid surface instabilities which extend along the primary cooling stage. By the use of trapezoid elements installed on the external walls of the submerged nozzle, it was observed that it is possible to obtain symmetry conditions at the top of the mold and prevent high level fluctuations. The flow modifiers have equidistant holes in the submerged surface to reduce the velocity of the liquid surface by energy and mass transfer between the generated quadrants. A flow modifier drilled with a 25 pct of the submerged surface provides stability in the mold and structural stability of the proposal is guaranteed.