Fernando Guerrero

Measuring the performance of nations at the Summer Olympics using data envelopment analysis

In this paper a well known tool for relative efficiency assessment, namely Data Envelopment Analysis (DEA), is used to measure the performance of the nations participating at the last five Summer Olympic games. The proposed approach considers two inputs (GNP and population) and three outputs (number of gold, silver and bronze medals won). To increase the consistency of the results, weight restrictions are included, guaranteeing a higher valuation for gold medals than for silver medals and higher for the latter than for bronze medals. Variable returns to scale are assumed. The results for the last five Summer Olympics are analysed. For each of them, a performance index as well as benchmarks are computed for each country. In addition, plotting the performance of a specific country for the different games can help identify trends as well as objective successes and disappointments.

Machine cell formation in generalized group technology

In this paper, we study the configuration of machine cells in the presence of alternative routings for part types. The objective function is the minimization of transportation costs. Limits on cell sizes as well as separation constraints (i.e. machines that are not allowed to be placed in the same cell) and co-location constraints (i.e. machines that must be placed in the same cell) may be imposed. An efficient Tabu Search (TS) algorithm is proposed to solve this problem. Extensive computational experiences with large-size problems show that this method outperforms some existing Simulated Annealing (SA) approaches.

Manufacturing cell formation using a new self-organizing neural network

Cellular manufacturing consists of grouping similar machines in cells and dedicating each of them to process a family of similar part types. In this paper, grouping parts into families and machines into cells is done in two steps: first, part families are formed and then machines are assigned. In phase one, weighted similarity coefficients are computed and parts are clustered using a new self-organizing neural network. In phase two, a linear network flow model is used to assign machines to families. To test the proposed approach, different problems from the literature have been solved. As benchmarks we have used a Maximum Spanning Tree heuristic.

Machine loading and part type selection in flexible manufacturing systems

This paper presents a new approach to the loading problem in flexible manufacturing systems. It focuses on the existence of alternatives routes for each part type. Also, the optimal number of copies of each tool type to be loaded into each tool magazine is directly determined. Thus, the decision variables are the routing mix and the tool allocation. The loading objective is to balance machine workloads. Constraints on the number of available tools and on tool magazine capacities can be imposed. The problem is modelled as a mixed-integer linear program. Also, an extension of the model is formulated that includes part type selection.

A flexible costing system for flexible manufacturing systems using activity based costing

Flexible manufacturing systems (FMSs) are designed to integrate the flexibility of job shops and the efficiency of mass production systems. Product costing methods have to adapt to this new technological environment. On one hand, the high production overhead cost of these systems requires a special attention to overhead allocation. On the other hand, the constantly changing setup configuration and production plans require a constant recalculation of overhead allocation and an a priori estimation of the expected production cost. This paper introduces the concept of flexible costing in FMSs, and proposes a method that modifies the overhead allocation based on the results of the production plan and on the simulated performance of the process. This approach is illustrated with some numerical examples.

Kohonen maps for solving a class of location-allocation problems

Location-Allocation problems occur whenever more than one facility need be located to serve a set of demand centers and it is not known or fixed a priori their allocation to the supply centers. This paper deals with a continuous space problem in which demand centers are independently served from a given number of independent, uncapacitated supply centers. Installation costs are assumed not to depend on neither the actual location nor the actual throughput of the supply centers. Transportation costs are considered to be proportional to the square Euclidean distance travelled and a minisum criterium is adopted. The problem is recognized as identical to certain Cluster Analysis and Vector Quantization problems. Such a relationship leads to applying Kohonen Maps, which are Artificial Neural Networks capable of extracting the main features, i.e. the structure, of the input data through a self-organizing process based on local adaptation rules. This approach has previously been applied to other combinatorial problems such as the Travelling Salesperson Problem.

Machine grouping using sequence-based similarity coefficients and neural networks

Abstract Most neural network approaches to the cell formation problem do not use information on the sequence of operations on part types. They only use as input the binary part-machine incidence matrix. In this paper we investigate two sequence-based neural network approaches for cell formation. The objective function considered is the minimization of transportation costs (including both intracellular and intercellular movements). Constraints on the minimum and maximum number of machines per cell can be imposed. The problem is formulated mathematically and shown to be equivalent to a quadratic programming integer program that uses symmetric, sequence-based similarity coefficients between each pair of machines. Of the two energy-based neural network approaches investigated, namely Hopfield model and Potts Mean Field Annealing, the latter seems to give better and faster solutions, although not as good as a Tabu Search algorithm used for benchmarking.

Cell design and loading in the presence of alternative routing

This paper deals with the design and loading of a cellular manufacturing system in the presence of alternative routing. The problem is decomposed into a cell design phase performed once and a cell loading phase performed on a recurrent basis. Two alternatives for the cell design problem are proposed: either the process plans of each part type are treated as if they were separate part types; or aggregate part types are considered. In either case, a conventional cell formation method is used to group machines. The cell loading problem is modelled as a multi-period LP formulation that determines the quantity of each part type that will follow each alternative route in each period of the planning horizon in order to minimize total transportation and holding costs while keeping both machine and cell utilizations approximately balanced.

A High Redox Potential Form of Cytochrome c550 in Photosystem II from Thermosynechococcus elongatus

Cytochrome c550 (cyt c550) is a component of photosystem II (PSII) from cyanobacteria, red algae, and some other eukaryotic algae. Its physiological role remains unclear. In the present work, measurements of the midpoint redox potential (Em) were performed using intact PSII core complexes preparations from a histidine-tagged PSII mutant strain of the thermophilic cyanobacterium Thermosynechococcus (T.) elongatus. When redox titrations were done in the absence of redox mediators, an Em value of +200 mV was obtained for cyt c550. This value is ∼300 mV more positive than that previously measured in the presence of mediators (Em = −80 mV). The shift from the high potential form (Em = +200 mV) to the low potential form (Em = −80 mV) of cyt c550 is attributed to conformational changes, triggered by the reduction of a component of PSII that is sequestered and out of equilibrium with the medium, most likely the Mn4Ca cluster. This reduction can occur when reduced low potential redox mediators are present or under highly reducing conditions even in the absence of mediators. Based on these observations, it is suggested that the Em of +200 mV obtained without mediators could be the physiological redox potential of the cyt c550 in PSII. This value opens the possibility of a redox function for cyt c550 in PSII.

Cell formation and scheduling of part families for reconfigurable cellular manufacturing systems using Tabu search

A reconfigurable cellular manufacturing system (RCMS) consists of multiple reconfigurable machining cells, each of which has one or more reconfigurable machine tools (RMTs), a setup station, and an automatic material handling and storage system. As part of the RCMS design process, similar parts must be grouped into part families and the RMTs must be arranged to form parallel cell configurations. A RCMS is designed at the outset for rapid changes in its components, allowing the production of multiple part families in each parallel cell. This paper proposes a new approach to simultaneously solve the cell formation and the scheduling of part families for an effective working of a RCMS. A new mixed integer linear programming model is used to represent both problems at the same time with the objective of minimizing production costs. Two types of production costs are considered: reconfiguration (i.e. setup) costs for changing from one family to the next one, and under-utilization costs for not using the RMT resources. A small size example is used to illustrate this integrated methodology. Computational experiments have been carried out adapting some larger instances from the literature on cellular manufacturing systems. Solving large instances optimally becomes prohibitive in terms of computational effort. That is why an approximate method, based on a Tabu search (TS) algorithm, has also been developed. Results show the ability of this algorithm to find good-quality production schedules of part families in a RCMS without requiring long computing times. It can be concluded that a RCMS can attain manufacturing flexibility without losing cost-effectiveness and that the approach proposed in this paper can efficiently solve real-world problems.