Variations in the efficiency of a mathematical programming solver according to the order of the constraints in the model

It is well-known that the efficiency of mixed integer linear mathematical programming depends on the model (formulation) used. With the same mathematical programming solver, a given problem can be solved in a brief calculation time using one model but requires a long calculation time using another. In this paper a new, unexpected feature to be taken into account is presented: the order of the constraints in the model can change the calculation time of the solver considerably. For a test problem, the Response Time Variability Problem (RTVP), it is shown that the ILOG CPLEX 9.0 optimizer returns a ratio of 17.47 between the maximum and the minimum calculations time necessary to solve optimally 20 instances of the RTVP, according to the order of the constraints in the model. It is shown that the efficiency of the mixed integer linear mathematical programming depends not only on the model (formulation) used, but also on how the information is introduced into the solverPeer Reviewe

Heuristic procedures for solving the General Assembly Line Balancing Problem with Setups (GALBPS)

The General Assembly Line Balancing Problem with Setups (GALBPS) was recently defined in the literature. It adds sequence-dependent setup time considerations to the classical Simple Assembly Line Balancing Problem (SALBP) as follows: whenever a task is assigned next to another at the same workstation, a setup time must be added to compute the global workstation time, thereby providing the task sequence inside each workstation. This paper proposes over 50 priority-rule-based heuristic procedures to solve GALBPS, many of which are an improvement upon heuristic procedures published to date

Planificación del tiempo de trabajo con jornada anualizada: clasificación de problemas.

Se propone una clasificación de los problemas de planificación del tiempo de trabajo con jornada anualizada y con una serie de restricciones que deben respetarse

La jornada anualizada como medio para hacer frente a las variaciones de la demanda.

Se presenta la anualización de la jornada laboral (distribución irregular de la jornada laboral a lo largo del año, sometida a diversas restricciones), como fuente de flexibilidad de los recursoshumanos, para hacer frente a las variaciones en la cantida

Evaluating urban freight transport policies within complex urban environments

Urban Freight Transport (UFT) entails significant advantages for the economic growth of cities, but can also hamper population quality of life, obstructing vehicles and people movements while exacerbating environmental problems. Many initiatives have been engaged by many city administrators in order to efficiently manage UFT, evaluating different policies at a global scale. From the perspective of operators, most works analyze a limited set of policies or only focus on the benefits of companies. In this work, a decisionmaking process is used to evaluate a large set of UFT policies, through different attributes representing the advantages and limitations of each policy over promoter companies and the society. To do so, an ex-ante procedure in five steps is proposed to classify the policies: (1) attributes definition, (2) attributes weighting, (3) policy-attribute assessment, (4) policy ranking, and (5) feasibility threshold satisfaction. The whole process is supported on consultations to 26 experts regarding shop supply and restocking activities within complex urban environments. Results show a classification of the analyzed policies, according to their suitability for implementation ; which could be extended (directly or with small adjustments) to other contexts, given the flexibility of the decision-making procedure developed.Peer ReviewedPostprint (author's final draft

ASALBP: The Alternative Subgraphs Assembly Line Balancing Problem

The classic assembly line balancing problem basically consist in assigning a set of tasks to a group of workstations while maintaining the task’ precedence relations. Habitually, such relations are represented by a predetermined precedence graph. However, a product’s assembly process may admit, for one or more of its parts, alternative precedence subgraph. This may be due to the fact that the processing time for some task are dependent on their processing sequence, or because there are alternatives for such a subassembly. In general, because of the great difficulty of the problem and the impossibility of representing alternative subgraphs in a precedence graph, the system designer will decide to select, a priori, one of such alternative subgraphs, which sometimes imposes additional constrains other than technological ones. In this paper we present, characterize and formulate a new general assembly line balancing problem: the Alternative Subgraphs Assembly Line Balancing Problem (ASALBP). Its novel characteristic is that it considers the possibility of having alternative assembly subgraphs, in which the processing times and/or the precedence relations of certain task are dependent on the assembly subgraph selected. Therefore, solving this problem implies simultaneously selecting an assembly subgraph for each part of the assembly that allows alternatives and balancing the line (i.e., assigning the task to the workstations). The potentially positive effects of this on the solution of the problem are shown here in a numerical example. Finally, the mathematical programming model developed is described and the results of a brief computational experiment are presented

Solving the Response Time Variability Problem by means of a psychoclonal approach

The Response Time Variability Problem (RTVP) is a combinatorial scheduling problem which has recently appeared in the literature. This problem has a wide range of reallife applications in, for example, manufacturing, hard real-time systems, operating systems and network environment. Originally, the RTVP occurs whenever products, clients or jobs need to be sequenced in such a way that the variability in the time between the instants at which they receive the necessary resources is minimized. Since RTVP is hard to solve, heuristic techniques are needed for solving it. In a previous study, three metaheuristic algorithms (a multi-start, a GRASP and a PSO algorithm) were proposed to solve the RTVP. These three metaheuristic algorithms have been the most efficient to date in solving non-small instances of the RTVP. We propose solving the RTVP by means of a psychoclonal algorithm based approach. The psychoclonal algorithm inherits its attributes from the need hierarchy theory proposed by Maslow and the artificial immune system (AIS) approach, specifically the clonal selection principle. In this paper we compare the proposed psychoclonal algorithm with the other three metaheuristic algorithms previously mentioned and show that, on average, the psychoclonal algorithm strongly improves the obtained results

Stabilizing work schedules in a call centre: expected and unexpected results

At call centres, work schedules change frequently and are often announced at the last minute, which causes absenteeism and turnover. We analyzed the call centre of a major Spanish electricity company. This centre requires a long initial training period and therefore turnover is especially damaging. New scheduling methods were adopted that limited the variability of individual timetables. Lower turnover and absenteeism were expected to compensate for the disadvantages of limitations to changes in timetables. We developed a software tool to assign timetables. An interval of time was assigned to each worker. The software calculated the coverage demand associated with these assignments. Next, the software establishes work on weekends and public holidays, weekday days off and, finally, individual timetables. This process is fully automatic, but every detail is displayed. Middle managers approved of the new policy and the tool. Nonetheless, turnover decreased less than expected and absenteeism, rather than decreasing, increased.Peer Reviewe

MILP model for the Visibility Windows Assembly Line Balancing Problem (VWALBP): the case of the Müller- Hannemann & Weihe problem

Preprin

The ASALB problem with processing alternatives involving different tasks: definition, formalization and resolution

The Alternative Subgraphs Assembly Line Balancing Problem (ASALBP) considers assembly alternatives that determine task processing times and/or precedence relations among the tasks. Capacho and Pastor formalized this problem and developed a mathematical programming model (MILP) in which the assembly alternatives are determined by combining all available processing alternatives of each existing sub-assembly. In this paper an extended definition of the ASALBP is presented in which assembly subprocesses involving different tasks are also considered. Additionally, a mathematical programming model is proposed to formalize and solve the extended version of the ASALBP, which also improves the performance of the former MILP model. Some computational results are included.Peer Reviewe