Hurtado-Hernandez, Margarita
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Hurtado-Hernandez, Margarita
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Hurtado Hernández, Margarita María Elvira
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mhurtado
Hurtado Hernández, Margarita
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56163023500
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GBM-3031-2022

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The effects of supply variability on the performance of assembly systems

2022 , Romero-Silva, Rodrigo , Hurtado-Hernandez, Margarita

Assembly processes play a big role in the current business context as global supply chains depend on many subcomponents to produce a single finished product. Previous studies have shown contrasting results regarding the effect that supply variability (the variability of feeding stations) has on the performance of assembly systems, as opposed to the variability of the station matching and assembling the components. This paper aims to close this gap by studying the behaviour of simple assembly systems with differing degrees of variability allocation among the stations through an experimental simulation study. Results suggest that a reduction in feeding station variability results in higher throughput, even in systems where the variability of one of the feeding stations increases while the other decreases. Furthermore, in scenarios with high total variance, the highest throughput is reached by transferring both variance and work from one of the feeding stations to any other station, whereas in low variance systems symmetrical work transfer to the feeding stations results in the highest throughput, as previously shown. Finally, reducing feeding station variability decreased the time spent in the assembly station (waiting time for component matching plus time for the assembly operation) only in experiments with high total variance. © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

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A new paradigm in solid waste collection systems design and operation

2005-12-01 , Hurtado-Hernandez, Margarita , Debernardo, Héctor

A new solid waste collection model, called MST, has been developed. It is a result of combining System Thinking and Aggregation Theory and it takes into account real world constraints such as collection frequency, labor shifts and both preventive and corrective maintenance. MST is a paradigm shift in solid waste collection systems design and operation. It makes possible a more efficient utilization of resources (vehicles and labor) and it is robust against variability sources. MST is the result of having challenged and invalidated a deeply rooted assumption in all models developed up to date. Simulation and Design Of Experiments were used to compare MST against existing models. Experimental resultsshow significant reduction in the number of trips(Up to 33%), crews(Up to 49%) and vehicles (Up to 40%), which means dramatic operation cost and investment improvements. ©International Society for the Systems Sciences (ISSS).

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Estimation of Quantile Confidence Intervals for Queueing Systems Based on the Bootstrap Methodology

2017 , Romero-Silva, Rodrigo , Hurtado-Hernandez, Margarita

This paper presents a simple methodology for estimating confidence intervals of quantiles in queueing systems. The paper investigates the actual probability density function of quantile estimators resulting of independent replications. Furthermore, we present a methodology, based on the concepts of bootstrapping, i.e., re-sampling and sub-sampling, to calculate the variability of an estimator without running different independent replications. Contrary to what overlapping and non-overlapping batching procedures suggest, we propose to randomly select data points to form a sub-sample, instead of selecting time-consecutive data points. The results of this study suggest that this proposal reduces the correlation between sub-samples (or batches) and overcomes the issue of normality. © 2017, Springer International Publishing AG.

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Exploiting the characteristics of serial queues to reduce the mean and variance of flow time using combined priority rules

2018 , Romero-Silva, Rodrigo , Shaaban, Sabry , Marsillac, Erika , Hurtado-Hernandez, Margarita

This paper addresses the trade-off challenge from reducing either the mean or variance of flow time when using simple sequencing rules in balanced, multi-class, serial queues. Study results show that instead of the expected zero sum situation, a balance between the two objectives can be achieved by assigning different priority rules to different queues. The order of priority rule assignments in different queues is shown to be relevant because variability along the line creates unbalanced queue lengths for each station, depending on the characteristics of the line. Thus, it was found that a simple heuristic for reducing both the mean and the variance of flow time in non-heavy traffic environments is to assign the first queue a priority rule that reduces its mean queue length while assigning the other queues a priority rule that reduces the variance of flow time. Conversely, for very-high traffic environments, performance improvements are shown from assigning the first queue a priority rule that reduces the variance of flow time while assigning the other queues a priority rule that reduces queue length. © 2017 Elsevier B.V.

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Digital Twins in Supply Chain Management: A Brief Literature Review

2019 , Marmolejo Saucedo, José Antonio , Hurtado-Hernandez, Margarita , Suárez Valdés, Ricardo

The rapid interest in the continuous improvement of supply chain management systems has motivated the development of digital tools in the automation of business problems. Currently, companies must continually adapt to changing conditions with respect to the management of their supply chain. However, the lack of real-time data available and responsive planning systems make this adaptation difficult. The current situation of the technology of digital twins is to migrate to the digital. More and more companies will develop and introduce their own digital twins in their business processes. This manuscript presents a literature review of the current context of digital twins. A total of 4884 searches combining keywords with respect to digital twins were analyzed. The years analyzed in the databases were 2017–2019. © 2020, Springer Nature Switzerland AG.

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Reducing the variability of inter-departure times of a single-server queueing system : the effects of skewness

2019 , Romero-Silva, Rodrigo , Marsillac, Erika , Shaaban, Sabry , Hurtado-Hernandez, Margarita

A critical performance measure in serial production lines is the variability of inter-departure times of a single-server queue. Increasing upstream inter-departure time variability generates greater downstream variability, diminishing overall line performance. Theory suggests that the variability of inter-departure times of a single-server queue is reduced by decreasing the variance of inter-arrival and service times. This study investigates the effects of the skewness of inter-arrival and service time distributions on the variability of inter-departure times. Contrary to previous results suggesting that mean waiting times of a GI/G/1 queue can be reduced by increasing inter-arrival time skewness, this experimental study of a GI/G/1 queue with triangular inter-arrival and service times shows that the inter-departure time coefficient of variation is reduced through a combination of negative inter-arrival time skewness and positive service time skewness. These results also suggest that the absolute value of the negative autocorrelation between consecutive departures is reduced by the same combination of negative inter-arrival time skewness and positive service time skewness for low values of server's utilization, while positive skewness for both inter-arrival and service times reduces this value for high values of server's utilization. Finally, it was found that queue capacity constraints increase the coefficient of variation of inter-departure times, as has been previously suggested, as well as the skewness and the absolute correlation values of the inter-departure time distribution. © 2019 Elsevier Ltd

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Studying the effects of the skewness of inter-arrival and service times on the probability distribution of waiting times

2020 , Romero-Silva, Rodrigo , Shaaban, Sabry , Marsillac, Erika , Hurtado-Hernandez, Margarita

Previous studies have shown that the mean queue length of a GI/G/1 system is significantly influenced by the skewness of inter-arrival times, but not by the skewness of service times. These results are limited because all the distributions considered in previous studies were positively skewed. To address this limitation, this paper investigates the effects of the skewness of inter-arrival and service times on the probability distribution of waiting times, when a negatively skewed distribution is used to model inter-arrival and service times. Subsequent to a series of experiments on a GI/G/1 queue using discrete-event simulation, results have shown that the lowest mean waiting time and the lowest variance of waiting times can be attained with a combination of positive inter-arrival skewness and negative service skewness. Results also show an interesting effect of the skewness of service times in the probability of no-delay in environments with a higher utilization factor. © 2020 Brazilian Operations Research Society.

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Is the scheduling task context-dependent? A survey investigating the presence of constraints in different manufacturing contexts

2016 , Romero-Silva, Rodrigo , Hurtado-Hernandez, Margarita , Santos, Javier

Most studies regarding real scheduling constraints only consider the constraints related to the specific manufacturing environment studied, limiting the possibility of drawing general conclusions. A survey of 50 companies was conducted in order to discover which constraints were present and what their relationship was with the scheduling context. This paper investigates which practical scheduling constraints are present in the manufacturing industry and whether the scheduling task is context-dependent. Results of this study show that some practical production constraints are context-dependent. © 2016 Informa UK Limited, trading as Taylor & Francis Group.

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A framework for studying practical production scheduling

2014 , Romero-Silva, Rodrigo , Santos, Javier , Hurtado-Hernandez, Margarita

The aim of this paper is to present what we believe are the most relevant findings and results regarding practical scheduling in order to define practical production scheduling and create a framework that helps researchers to study the various topics that fall under the umbrella of practical production scheduling and to identify the current state of knowledge for each topic. Studies from different fields were analysed and included in this paper, contributing significant knowledge to build a definition of practical production scheduling. Finally, we discuss the applicability that scheduling, as a task, could have in real companies.

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Serial production line performance under random variation : dealing with the "law of variability"

2019 , Romero-Silva, Rodrigo , Marsillac, Erika , Shaaban, Sabry , Hurtado-Hernandez, Margarita

Many Queueing Theory and Production Management studies have investigated specific effects of variability on the performance of serial lines since variability has a significant impact on performance. To date, there has been no single summary source of the most relevant research results concerned with variability, particularly as they relate to the need to better understand the ‘Law of Variability’. This paper fills this gap and provides readers the foundational knowledge needed to develop intuition and insights on the complexities of stochastic simple serial lines, and serves as a guide to better understand and manage the effects of variability and design factors related to improving serial production line performance, i.e. throughput, inter-departure time and flow time, under random variation. © 2019 The Society of Manufacturing Engineers

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