1,721,056 research outputs found
An optimal checkpointing-strategy for real-time control systems under transient faults
No full textReal-time computer systems are often used in harsh environments, such as aerospace, and in industry. Such systems are subject to many transient faults while in operation. Checkpointing enables a reduction in the recovery time from a transient fault by saving intermediate states of a task in a reliable storage facility, and then, on detection of a fault, restoring from a previously stored state. The interval between checkpoints affects the execution time of the task. Whereas inserting more checkpoints and reducing the interval between them reduces the reprocessing time after faults, checkpoints have associated execution costs, and inserting extra checkpoints increases the overall task execution time. Thus, a trade-off between the reprocessing time and the checkpointing overhead leads to an optimal checkpoint placement strategy that optimizes certain performance measures. Real-time control systems are characterized by a timely, and correct, execution of iterative tasks within deadlines. The reliability is the probability that a system functions according to its specification over a period of time. This paper reports on the reliability of a checkpointed real-time control system, where any errors are detected at the checkpointing time. The reliability is used as a performance measure to find the optimal checkpointing strategy. For a single-task control system, the reliability equation over a mission time is derived using the Markov model. Detecting errors at the checkpointing time makes reliability jitter with the number of checkpoints. This forces the need to apply other search algorithms to find the optimal number of checkpoints. By considering the properties of the reliability jittering, a simple algorithm is provided to rind the optimal checkpoints effectively. Finally, the reliability model is extended to include multiple tasks by a task allocation algorithm
Design and stability analysis of single-input fuzzy logic controller
No full textIn existing fuzzy logic controllers (FLC's), input variables are mostly the error e and the change-of-error (e) over dot regardless of complexity of controlled plants. Either control input u or the change of control input Pu is commonly used as its output variable. A rule table is then constructed on a two-dimensional (2-D) space. This scheme naturally inherits from conventional proportional-derivative (PD) or proportional-integral (PI) controller. Observing that 1) rule tables of most FLC's have skew-symmetric property and 2) the absolute magnitude of the control input /u/ or /Delta u/ is proportional to the distance from its main diagonal line in the normalized input space, we derive a new variable called the signed distance, which is used as a sole fuzzy input variable in our simple FLC called single-input FLC (SFLC), The SFLC has many advantages: The total number of rules is greatly reduced compared to existing FLC's, and hence, generation and tuning of control rules are much easier. The proposed SFLC is proven to be absolutely stable using Popov criterion. Furthermore, the control performance is nearly the same as that of existing FLC's, which is revealed via computer simulations using two nonlinear plants.This work
was supported in part by the 1999 Taegu University Research Grant
Fuzzy logic-based tuning of the boundary layer thickness of the variable structure controller
No full textThe variable structure control (VSC) is a simple and powerful nonlinear controller, but it leads to a high frequency chattering on the control input. To decrease the chattering phenomenon of the VSC, a boundary layer is commonly introduced. Then its thickness requires a compromise between the steady state error and the chattering amplitude. In this paper, we propose a new VSC that tunes the boundary layer thickness using the fuzzy logic system. The tuning methods presented are two: One uses absolute error and its derivative as fuzzy input variables, which decreases the number of tuning rules as compared with using common fuzzy variables of error and change of error. The other uses only a single fuzzy variable of a distance. This variable is derived by the property of two-dimensional rule table which is composed of absolute error and its derivative. Since the second method uses a single variable for tuning the thickness, the number of tuning rules is greatly decreased. Furthermore, we obtain the good tracking performance which is almost the same as that of the first method. To ensure the control performance in both cases, we perform computer simulations using an inverted pendulum as a controlled plant
Design of a single-input fuzzy logic controller and its properties
No full textWe suggest a simple but powerful FLC (Fuzzy Logic Controller) design method using a single fuzzy input variable, which is equivalent to the pseudo sliding mode controller. Input variables of conventional FLCs are mostly the error e and the change-of-error (e) over dot regardless of the complexity of controlled plants. A rule table is then constructed in a two-dimensional input space. The output of fuzzy inference is applied to the plant as the control input u or the change of control input Delta u. This scheme came from concepts of linear PD (proportional-derivative) and PI (proportional-integral) controllers. We found that rule tables of most FLCs have skew-symmetry property, and the absolute magnitude of the control input \u\ or \Delta u\ is proportional to the distance from its main diagonal line in the normalized input space. Considering these facts, we derive a new variable called the signed distance, which is a sole fuzzy input variable in our simple FLC called single-input FLC (S-FLC). The S-FLC has many advantages: The total number of rules is greatly reduced compared to two-dimensional FLCs, and hence, generations and tuning of control rules are easy. Control performance is nearly the same as that of conventional FLCs. We also show that this S-FLC is equivalent to the pseudo SMC (sliding mode controller), and hence, the stability is guaranteed using the Lyapunov stability. The performance of S-FLC is revealed via computer simulations using a nonlinear plant. (C) 1999 Elsevier Science B.V. All rights reserved
A taxonomy of dirty data
No full textToday large corporations are constructing enterprise data warehouses from disparate data sources in order to run enterprise-wide data analysis applications, including decision support systems, multidimensional online analytical applications, data mining, and customer relationship management systems. A major problem that is only beginning to be recognized is that the data in data sources are often "dirty". Broadly, dirty data include missing data, wrong data, and non-standard representations of the same data. The results of analyzing a database/data warehouse of dirty data can be damaging and at best be unreliable. In this paper, a comprehensive classification of dirty data is developed for use as a framework for understanding how dirty data arise, manifest themselves, and may be cleansed to ensure proper construction of data warehouses and accurate data analysis. The impact of dirty data on data mining is also explored.This research was partially supported by Korea’s Brain Korea-21 grant.
This research was partially supported by Korea’s KISTEP grant
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
