Eva-Henrietta Dulf

Robustness evaluation of fractional order control for varying time delay processes

In this paper, we investigate the robustness of a methodology to design fractional order PI controllers combined with Smith Predictors, for varying time delay processes. To overcome the drawback of possible instability associated with Smith Predictor control structures, mainly due to the changes in the time delay, the design focuses on ensuring robustness of the closed loop system against time delay uncertainties. The proposed method is based on time-domain performance specifications—more accessible to the process engineer, rather than the more abstract notions related to the frequency domain. A second advantage of the proposed method relies on additional robustness to plant uncertainties, achieved by maximizing open-loop gain margin. The convergence problems associated with optimization techniques, previously used in fractional order controller designs, are eliminated by an iterative procedure in computing the gain margin. The simulation example provided demonstrates the efficiency of the proposed method, in comparison to classical integer order PI controller.

Total Phenolic Contents, Antioxidant Activities, and Lipid Fractions from Berry Pomaces Obtained by Solid-State Fermentation of Two Sambucus Species with Aspergillus niger

The aim of this study was to investigate the effect of solid-state fermentation (SSF) by Aspergillus niger on phenolic contents and antioxidant activity in Sambucus nigra L. and Sambucus ebulus L. berry pomaces. The effect of fermentation time on the total fats and major lipid classes (neutral and polar) was also investigated. During the SSF, the extractable phenolics increased with 18.82% for S. ebulus L. and 11.11% for S. nigra L. The levels of antioxidant activity of methanolic extracts were also significantly enhanced. The HPLC-MS analysis indicated that the cyanidin 3-sambubioside-5-glucoside is the major phenolic compound in both fermented Sambucus fruit residues. In the early stages of fungal growth, the extracted oils (with TAGs as major lipid fraction) increased with 12% for S. nigra L. and 10.50% for S. ebulus L. The GC-MS analysis showed that the SSF resulted in a slight increase of the linoleic and oleic acids level.

Tuning algorithms for fractional order internal model controllers for time delay processes

ABSTRACT This paper presents two tuning algorithms for fractional-order internal model control (IMC) controllers for time delay processes. The two tuning algorithms are based on two specific closed-loop control configurations: the IMC control structure and the Smith predictor structure. In the latter, the equivalency between IMC and Smith predictor control structures is used to tune a fractional-order IMC controller as the primary controller of the Smith predictor structure. Fractional-order IMC controllers are designed in both cases in order to enhance the closed-loop performance and robustness of classical integer order IMC controllers. The tuning procedures are exemplified for both single-input-single-output as well as multivariable processes, described by first-order and second-order transfer functions with time delays. Different numerical examples are provided, including a general multivariable time delay process. Integer order IMC controllers are designed in each case, as well as fractional-order IMC controllers. The simulation results show that the proposed fractional-order IMC controller ensures an increased robustness to modelling uncertainties. Experimental results are also provided, for the design of a multivariable fractional-order IMC controller in a Smith predictor structure for a quadruple-tank system.

Fractional calculus in 13 C separation column control

Controller design for an isotope separation column is recognized as a difficult and challenging problem. The dynamics of the isotope separation process is difficult to model precisely using integer order transfer functions; thus, a fractional order approach is preferred. The objective of this work is to design two different PI controllers—a classical one and a fractional order one—and test their closed loop performance under nominal conditions as well as gain uncertainties. Since the process is represented by a fractional order mathematical model, the simplest approach to design both controllers is based on a frequency specification. For the fractional order of the PI controller and its parameters, the authors solve a system of equations that includes a robust performance specification to gain uncertainties. For the classical PI controller, a traditional tuning algorithm based on phase margin specification is implemented. The simulation results show that both controllers meet the design specifications, with the fractional order PI controller behaving more robustly to plant gain variations.

Fractional Model of the Cryogenic (13C) Isotope Separation Column

The column for cryogenic (13C) isotope separation is studied in this article, using equipment belonging to the National Institute of Research and Development for Isotopes and Molecular Technologies (INCDTIM), Cluj Napoca. The complete dynamics of the column is represented by nonlinear partial differential equations, the so called Cohens equations, which depend on the time as well as the spatial coordinates, but which are not appropriate for online applications and control needs. The proposed solution is to identify fractional order model from frequency response or from time response of the system. The model is validated by simulation, highlighting the performance of fractional order models in comparison with the integer model and experimental data of the column.

Systematic Modeling of the (13C) Isotope Cryogenic Distillation Process

Isotopic enrichment by low-temperature distillation of carbon monoxide is difficult to handle, the experiments providing with the necessary data are time-consuming and difficult to perform, so the mathematical modeling of the process is an appropriate approach to study and to further control the column. This paper develops a mathematical model for the (13C) isotope cryogenic distillation process using two subsystems of the plant: the hydrodynamic part and the isotope-separation part. The model is validated using experimental data from the pilot column from the National Institute for Research and Development of Isotopic and Molecular Technologies Cluj-Napoca. The proposed model is suitable for various cryogenic distillation columns, using the proper parameters.

Predictive control of the multivariable time delay processes in an isotope separation column

The secondary processes in a pilot plant carbon isotope cryogenic separation column are multivariable time delay processes. In general, for these types of processes, Smith Predictor control schemes are frequently used, however they exhibit poor robustness due to the explicit use of the process model in the compensation loop and require additional control techniques to enhance robustness against modeling uncertainties. An alternative solution used in this paper is a predictive controller, with intrinsic time delay compensation. The simulations presented show that the predictive controller is also robust to significant time delay estimation errors, without further need to design additional filters for increasing robustness.

Flooding process analysis by 13 C cryogenic separation column

The applications of stable isotopes from different chemical elements are well-known. Based on specific properties, a great diversity of isotope concentration (separation) methods have been developed, one of them being the cryogenic separation for (13C). After a short description of the separation equipment (separation column), the authors deal with the arising of the flooding process. Based on simplified equations, the evolution of an intended flooding is studied, divided in more stages: normal mode of operation, pre-flooding, ldquodischargerdquo period, flooding period and flooding damping. The next step of study will analyze the possibilities to avoid the flooding process.

Estimator based flooding predictor for ( 13 C) cryogenic separation column

The industrial distillation plants-like those for crude oil- and the cryogenic distillation column for (13C) isotope enrichment have in common the probability to appear an undesirable phenomenon: the flooding, when the plant efficiency decreases drastically. After a general description of the isotope separation problem, the paper describes the column mode of operation and gives the mathematical equations, which explain the column flooding process. The authors deal with the online prediction methods and propose a simple, low-cost method based on the supervision of the process variables: differential column pressure (top-bottom) and liquid level in boiler (variables, first and second derivatives). Using a test-version of the column, where the flooding can be observed directly, the method proposed by the authors proves its utility.

Observer based safe operation of the 13 C cryogenic separation column

The cryogenic 13C separation process is very slow, the concentration of this isotope reaches the desired value during weeks. A lot of variables, essential for the separation process must be controlled, so that any malfunction can compromise the whole production. The most important information about the system state is given by the carbon monoxide level sensor, with disastrous consequences by its failure (conductor breaking, power line failure, etc.). The authors try to solve this problem using an observer which generates the observed level information and compare, in normal mode of operation, the observed value with the real, measured variable. By sensor failure, the expected difference will increase drastically and the control system will switch to the observed level information.