The water level control in boiler drums is a crucial process in many process industries. In industries, water spillage or overheated tubes of the water boiler are the serious consequences of extremely high or low-level drum water level maintenance. The boiler drum is a MIMO system, consists of dead time nonlinearity and thereby there exists a transportation lag between the input and the system. Also, they possess high dynamic variations. Hence, the control of the boiler drum level is of great importance. Though, conventional PID controllers are employed in industries, due to the presence of nonlinearity, boiler drum performance can be affected when it is controlled by a PID controller. Moreover, the PID controller produces a larger settling time. Here, a robust controller for the boiler drum level control based on the H-infinity technique is designed. The first-principle mathematical model of the boiler drum is formulated. The uncertainties namely: structured uncertainty, unstructured uncertainty, and nonlinear uncertainty are modeled by incorporating the boiler drum dynamics including its inherent nonlinearity. The boiler drum level control is carried out using the H-infinity controller scheme with the uncertainties accounted for and the performance is compared with that of a conventional PID controller. The qualitative and quantitative comparison of performances of the above control schemes reveals that the H-infinity controller has a quick rise time and faster settling time.

A QFT-type frequency-domain controller design technique is presented for multiple input-multiple ouput (MIMO) nonlinear systems with significant structured (parametric) uncertainty. Hard time-domain constraints are imposed on the outputs and control inputs in response to step disturbances. The control design method is based on replacing the nonlinear MIMO system with an equivalent diagonal MIMO linear uncertain plant using compact and convex sets of output functions. Then, a diagonal controller is designed for the equivalent linear system by transferring the time-domain constraints into the frequency-domain to obtain a series of allowable regions for the frequency responses of the nominal loop transfer functions. Next, a loop-shaping procedure is used to shape the nominal loop transfer function and hence the controller for each loop. Finally, Schauder"s fixed point theorem is utilized to show that the same controller will also work for the nonlinear system. The proposed method is illustrated by an example.

Quantitative Feedback Theory, QFT, is frequency-domain-based robust control design methods that can conquest to structured uncertainty. In the last few years, its application to several control systems design such as flight control, hydraulic actuator control, etc., has been widely investigated but this theory has not been developed in the structural control field. The QFT method offers a direct design technique for satisfying a set of robust performance and stability objectives over a given range of plant parameter uncertainty based on Nichols Chart. In this paper firstly the QFT design methodology is presented through one illustrative example in order to make easy understanding of the basic principles and characteristics for single-input single-output system.Afterwards, feasibility and the efficiency of new structural control method are demonstrated by numerical simulation in multi degree of freedom structure and compared with the H∞ robust design approach. Simulation results show that the lower order of QFT controller is important feature of QFT controller in civil engineering.

In this paper, we introduce and study the structured condition pseudospectra and the structured essential condition pseudospectra of bounded linear operators on ultrametric Banach spaces. We establish a characterization of the structured condition pseudospectrum of continuous linear operators and we give a relationship between the structured condition pseudospectrum and the structured pseudospectrum of continuous linear operators on ultrametric Banach spaces. Many characterizations of the structured essential condition pseudospectrum of bounded linear operators and examples are given.

This study tried to examine EFL translators‟ uncertainty and uncertainty management strategies through employing think aloud procedures. The participants of this study were some MA andBA translators selected from several universities in Iran. To this aim, a proficiency test was firstly administered among the volunteers. Then, think aloud protocol and retrospective interview were used to collect data. Meanwhile, Angelone coding system was used to categorize the data. To identify the significance of differences, chi-square nonparametric test was utilized. The findings indicated that MA translators had greater tendency to show uncertainty at larger chunks of language such as collocation and sentence, while BA ones were more inclined to show uncertainty in textual level. At the same time, behavioral and locus options were compared and contrasted between MA andBA translators. It was also found out that look-up and rereading strategies were frequently used to manage uncertainty. The findings of this study can be helpful for translators to improve their translation ability by being more aware of what is happening inside their minds. Awareness of a stockpile of strategies helps them have fewer difficulties while translating a text.