Rafael Pastor

Balancing and scheduling tasks in assembly lines with sequence-dependent setup times

The classical Simple Assembly Line Balancing Problem (SALBP) has been widely enriched over the past few years with many realistic approaches and much effort has been made to reduce the distance between the academic theory and the industrial reality. Despite this effort, the scheduling of the execution of tasks assigned to every workstation following the balancing of the assembly line has been scarcely reported in the scientific literature. This is supposed to be an operational concern that the worker should solve himself, but in several real environments, setups between tasks exist and optimal or near-optimal tasks schedules should be provided inside each workstation. The problem presented in this paper adds sequence-dependent setup time considerations to the classical SALBP in the following way: whenever a task is assigned next to another at the same workstation, a setup time must be added to compute the global workstation time. After formulating a mathematical model for this innovative problem and showing the high combinatorial nature of the problem, eight different heuristic rules and a GRASP algorithm are designed and tested for solving the problem in reasonable computational time.

An improved mathematical program to solve the simple assembly line balancing problem

The simple assembly line balancing problem (SALBP) has been extensively examined in the literature. Various mathematical programs have been developed to solve SALBP type-1 (minimising the number of workstations, m, for a given cycle time, ct) and SALBP type-2 (minimising ct given m). Usually, an initial pre-process is carried out to calculate the range of workstations to which a task i may be assigned, in order to reduce the number of variables of task–workstation assignment. This paper presents a more effective mathematical program than those released to date to solve SALBP-1 and SALBP-2. The key idea is to introduce additional constraints in the mathematical program, based on the fact that the range of workstations to which a task i may be assigned depends either on the upper bound on the number of workstations or on the upper bound on the cycle time (for SALBP-1 and SALBP-2, respectively). A computational experiment was carried out and the results reveal the superiority of the mathematical program proposed.

Tabu search algorithms for an industrial multi-product and multi-objective assembly line balancing problem, with reduction of the task dispersion

This paper presents a real-world industrial application of the multi-product and multi-objective assembly line balancing problem, for a company involved in the production of four models of a white goods product. The problem solved is a GALBP-2, with 10 workstations and multiple objectives (to maximize the production rate in order to deal with an increase of the demand forecasted, to reach an equal cycle time of all the models and an equal workload of the different workstations, and finally, to minimize the dispersion of worker tasks on each one of the different models—the common tasks of the different models at the same workstation). The paper presents an integrated approach based on four heuristics cited in the literature and: (1) an improvement procedure based on tabu search, with the objective of minimizing the cycle time; and, subsequently, (2) a second tabu search in order to increase the uniformity of the tasks performed at each workstation (the common tasks at the same workstation).

A MILP model to design hybrid wind–photovoltaic isolated rural electrification projects in developing countries

Electrification systems based on the use of renewable energy sources are a suitable option for providing electricity to isolated communities autonomously. Wind and hybrid wind–photovoltaic (PV) systems are increasingly getting attention. To electrify scattered communities, designs that combine individual systems and microgrids have recently proven advantageous. In this paper we present a mathematical programming model to optimize the design of hybrid wind–PV systems that solves the location of the wind–PV generators and the design of the microgrids, taking into account the demand of the consumption points and the energy potential. The criterion is the minimization of the initial investment cost required to meet the demand. The proposed hybrid model is tested with realistic size instances and results show the instances are efficiently solved. Moreover, the model is applied to real case studies in Peru; obtained results verify that the hybrid model efficiently finds solutions that significantly reduce costs.

Rotational allocation of tasks to multifunctional workers in a service industry

This paper deals with the assignment of tasks to the members of the multi-functional staff (each worker is able to perform a given subset of types of tasks) of a work centre, during each period (e.g. 1 h) into which the planning horizon (e.g. 1 shift or 1 week) can be divided. For each type of task to perform, all workers who can perform the task do so at equal worker efficiencies. There are constraints that, if possible, should be respected. The objective is that the percentage of working time dedicated by each worker to each type of task be as close as possible to reference values. The problem is modelled as a sequence of assignments, in which appropriate values for the cost matrix depend on the results of the previous assignments. The obtained results are satisfactory: the solutions meet the constraints, the scheduled percentages steadily approach reference values and the calculation times are very short. Therefore, the work presented constitutes a potential tool for assigning tasks to multi-functional workers in the service industry.

Heuristic procedures for solving the general assembly line balancing problem with setups

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 heuristic procedures, based on priority rules, for solving GALBPS, many of which are an improvement upon heuristic procedures published to date.

Using MILP to plan annualised working hours

Annualising working hours (AH) is a means to achieve flexibility in the use of human resources in order to face the seasonal nature of the demand. MILP models are proposed to solve the problem of planning the staffs working hours with an annual horizon. The computational experience with the models leads to the conclusion that MILP is an appropriate way to deal with the problem in many real cases.

An evaluation of constructive heuristic methods for solving the alternative subgraphs assembly line balancing problem

This paper evaluates a set of constructive heuristic methods developed to solve the novel Alternative Subgraphs Assembly Line Balancing Problem (ASALBP), which considers variants for different parts of a production or manufacturing process. Each variant is represented by a precedence subgraph that defines the tasks to be performed and their processing times. The proposed methods use priority rules and random choice to select the assembly subgraphs and to assign the tasks to the stations in order to minimize the number of required workstations. The methods are evaluated by a computational experiment based on medium- and large-scale benchmark problems.

Planning Annualised Hours with a Finite Set of Weekly Working Hours and Joint Holidays

The need to adjust productive capacity to seasonal demand and the increasing flexibility in the distribution of annual working hours has given rise to new problems with respect to the organisation of staff working time. This paper presents a problem of planning annual working hours, in which the number of weekly working hours for any worker must belong to a previously agreed finite set and holiday weeks are the same for all the staff members. The problem is modelled and solved as a mixed integer linear program.

ASALBP: the alternative subgraphs assembly line balancing problem

Assembly line balancing problems basically consist of assigning a set of tasks to a group of workstations while maintaining the tasks’ precedence relations, which are represented by a predetermined precedence graph. However, one or more parts of a products assembly process may admit alternative precedence subgraphs, which represent possible assembly variants. 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. This paper presents, characterizes and formulates a new general assembly line balancing problem with practical relevance: the alternative subgraphs assembly line balancing problem (ASALBP). Its novel characteristic is that it considers the possibility of having alternative assembly subgraphs, with the processing times and/or the precedence relations of certain tasks 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. The potentially positive effects of this on the solution of the problem are shown in a numerical example. Finally, a simple mathematical programming model is described and the results of a brief computational experiment are presented.