Cristóbal Miralles

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.

Branch and bound procedures for solving the Assembly Line Worker Assignment and Balancing Problem: Application to Sheltered Work centres for Disabled

In this paper a new problem called Assembly Line Worker Assignment and Balancing Problem (ALWABP) is introduced. This problem arises in those assembly lines where we have certain limited resources available (normally workers) in which the operation time for every task is different depending on who executes the task, and where there are also some task-worker incompatibilities defined. The problem consists of providing a simultaneous solution to a double assignment: (1) tasks to stations; and (2) available workers to stations. After defining the mathematical model for this problem, a basic Branch and Bound approach with three possible search strategies and different parameters is presented. We also propose the use of a Branch and Bound-based heuristic for large problems and analyse the behaviour of both exact and heuristic methods through experimental studies. Finally the implementation of these procedures in a Sheltered Work centre for Disabled-the real environment which has inspired this research-is described. In these centres the adoption of assembly lines provide many advantages, since the traditional division of work in single tasks may become a perfect tool for making certain worker disabilities invisible. Efficiently applying this configuration helps these centres to achieve their primary aim: growth in order to provide more jobs for more disabled people, but always considering the specific limitations that the disabled workers have. In this sense this paper shows one of the possible real applications where Operations Research can help not only to get economic and productive benefits but also certain social aims.

On solving the assembly line worker assignment and balancing problem via beam search

Certain types of manufacturing processes can be modelled by assembly line balancing problems. In this work we deal with a specific assembly line balancing problem that is known as the assembly line worker assignment and balancing problem (ALWABP). This problem appears in settings where tasks must be assigned to workers, and workers to work stations. Task processing times are worker specific, and workers might even be incompatible with certain tasks. The ALWABP was introduced to model assembly lines typical for sheltered work centers for the Disabled. In this paper we introduce an algorithm based on beam search for solving the ALWABP with the objective of minimizing the cycle time when given a fixed number of work stations, respectively, workers. This problem version is denoted as ALWABP-2. The experimental results show that our algorithm is currently a state-of-the-art method for the ALWABP-2. In comparison to results from the literature, our algorithm obtains better or equal results in all cases. Moreover, the algorithm is very robust for what concerns the application to problem instances of different characteristics.

Hybrid Metaheuristic for the Assembly Line Worker Assignment and Balancing Problem

The Assembly Line Worker Assignment and Balancing Problem (ALWABP) appears in real assembly lines which we have to assign given tasks to workers where there are some task-worker incompatibilities and considering that the operation time for each task is different depending upon who executes the task. This problem is typical for Sheltered Work Centers for the Disabled and it is well known to be NP-Hard. In this paper, the hybrid method Clustering Search (CS) is implemented to solve the ALWABP. The CS identifies promising regions of the search space by generating solutions with a metaheuristic, such as Iterated Local Search, and clustering them into clusters that are then explored further with local search heuristics. Computational results considering instances available in the literature are presented to demonstrate the efficacy of the CS.

Model and heuristics for the Assembly Line Worker Integration and Balancing Problem

We propose the Assembly Line Worker Integration and Balancing Problem (ALWIBP), a new assembly line balancing problem arising in lines with conventional and disabled workers. The goal of this problem is to maintain high productivity levels by minimizing the number of workstations needed to reach a given output, while integrating in the assembly line a number of disabled workers. Being able to efficiently manage a heterogeneous workforce is especially important in the current social context where companies are urged to integrate workers with different profiles. In this paper we present mathematical models and heuristic methodologies that can help assembly line managers to cope with this additional complexity. We demonstrate by means of a robust benchmark how this integration can be done with losses of productivity that are much lower than expected.

The use of employee participation in the USA and Spanish companies

Abstract This article presents a description of employee participation in Spain. The number of published papers in English about Spain regarding this subject is very low, and even lower by Spanish authors. In short, we can say that there is hardly any knowledge regarding employee participation in Spain in both the European and American settings. This paper summarized the results of a research project we carried out whose objective was to compare employee participation using samples from large American and Spanish companies following the works of Edward E. Lawler and his team. Our aim is to have professionals and researchers throughout the world better understand the actual situation in our country.

An exact calculation of the cycle service level in a generalized periodic review system

This paper is focused on the exact calculation of the cycle service level for a generalized periodic review system accepting any discrete demand with a known probability function and any cycle service target. Previously, it was necessary to modify the definition of the cycle service level metric (also known as non stock out probability) so that it could be applied to these cases. The purpose of the proposed (R, S) system is to handle simultaneously fast-moving items, slow-moving items and intermittent or sporadic demand items any of them with high, medium or low cycle service targets.

Flow shop scheduling with heterogeneous workers

We propose an extension to the flow shop scheduling problem named Heterogeneous Flow Shop Scheduling Problem (Het-FSSP), where two simultaneous issues have to be resolved: finding the best worker assignment to the workstations, and solving the corresponding scheduling problem. This problem is motivated by Sheltered Work centers for Disabled, whose main objective is the labor integration of persons with disabilities, an important aim not only for these centers but for any company desiring to overcome the traditional standardized vision of the workforce. In such a scenario the goal is to maintain high productivity levels by minimizing the maximum completion time, while respecting the diverse capabilities and paces of the heterogeneous workers, which increases the complexity of finding an optimal schedule. We present a mathematical model that extends a flow shop model to admit a heterogeneous worker assignment, and propose a heuristic based on scatter search and path relinking to solve the problem. Computational results show that this approach finds good solutions within a short time, providing the production managers with practical approaches for this combined assignment and scheduling problem.

The assembly line worker assignment and balancing problem with stochastic worker availability

Assembly lines can be employed successfully in sheltered work centres to better include persons with disabilities in the labour market as well as to improve production efficiency. The optimal assignment of a heterogeneous workforce is known as the assembly line worker assignment and balancing problem (ALWABP). These assembly lines are characterised not only by a heterogeneous workforce, but also by high levels of absenteeism, which makes it more difficult to obtain stable and efficient line balancing solutions. In this paper, an extension of the ALWABP to minimise the expected cycle time under uncertain worker availability is proposed. We model this problem as a two-stage mixed integer program, and propose local search heuristics for solving it. Computational experiments show that stochastic modelling can help to improve the line’s efficiency and that the proposed heuristics produce good results for instances of practical size.

Universal design of workplaces through the use of Poka-Yokes: Case study and implications

Purpose: Employment plays an important part in many people’s lives beyond merely providing income, since continued participation in work can have many therapeutic benefits for workers defined as disabled. However, disabled workers face a range of barriers to employment, despite legislation intended to improve workplace accessibility emphasizing adaptations to the workplace, which many employers often find difficult and expensive. The Poka-Yoke approach was developed in the manufacturing industry as a way of improving productivity by reducing errors using often very simple adaptations. This paper argues that, as Poka-Yokes are designed to make life easier and improve the performance of workers without impairments, they are closer to the philosophy of Universal Design than to Accessible Design, and offer an easy and inclusive way of making work more accessible for all kind of workers. Design/methodology/approach: This paper provides a case study demonstrating the use of the Poka-Yoke approach in a sheltered work centre for disabled; highlighting how they served to improve accessibility to work by fulfilling Universal Design principles. Findings: Our research allows us to demonstrate the great potential of Poka-yokes for gaining accessibility to the workplace. The real application of this approach, Journal of Industrial Engineering and Management http://dx.doi.org/10.3926/jiem.2011.v4n3.p436-452 437 both in sheltered work centres and ordinary companies, can contribute to improve the high unemployment rates of disabled people. Research limitations/implications: The proposal is innovative and was applied in one specific company. Thus, a range of customized Poka-yokes would be desirable for different industrial sectors. Practical implications: Managers of sheltered work centres, and also of ordinary companies, can realize about the great potential of Poka-Yokes as an easy means of gaining flexibility and accessibility. Originality/value: There are very few papers relating lean manufacturing tools and disability. Our approach analyzes the benefits of this approach for the labour integration of disabled people.