Discrete Optimization
Incorporating ergonomic risks into assembly line balancing

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Abstract

In manufacturing, control of ergonomic risks at manual workplaces is a necessity commanded by legislation, care for health of workers and economic considerations. Methods for estimating ergonomic risks of workplaces are integrated into production routines at most firms that use the assembly-type of production. Assembly line re-balancing, i.e., re-assignment of tasks to workers, is an effective and, in case that no additional workstations are required, inexpensive method to reduce ergonomic risks. In our article, we show that even though most ergonomic risk estimation methods involve nonlinear functions, they can be integrated into assembly line balancing techniques at low additional computational cost. Our computational experiments indicate that re-balancing often leads to a substantial mitigation of ergonomic risks.

Introduction

The problem of unfavorable working conditions, or poor workplace ergonomics, is an acute topic today. Ergonomic risks at the workplace cause a lot of damage on health and quality of life of workers, deteriorate economic results of employers and of the economy as a whole. In 2008, along 315,000 cases of work-related musculoskeletal disorders (MSDs, often referred to as ergonomic injuries), requiring a median of 10 days away from work, were reported in the US (Bureau of Labor Statistics, 2009). Annual compensation cost for MSDs paid by employers in the US amount to 15 to 20 billion US dollars. Moreover, occupational diseases of workers indirectly cause further cost on firms: via loss of production capacity due to absenteeism of workers, lower worker productivity and higher defect rates in work. This can be illustrated by the example of Peugeot, whose ergonomics program reduced the cycle time for the final vehicle assembly line together with a simultaneous decrease by 30% in new cases of musculoskeletal disorders (Moreau, 2003).

Workplace ergonomics is becoming even more important following recent developments in legislation (EU Machinery directive, 2006/42/EC, 89/391/EEC, Occupational Safety and Health act of 1970 among others) and an on-going ageing of the workforce in most of the developed countries.

Already today in assembly line production, especially in final assembly, where the share of manual labor is high, a special attention is paid to ergonomics. Most renowned companies incorporate methods for ergonomic risk estimation of working places in their production routine (Toyota Verification of Assembly Line at Toyota, GM-UAW at General Motors, AP-Ergo at Volkswagen to name a few). If ergonomic risks are detected, re-balancing of the assembly line is recommended as an effective method in the short-run (Hilla, 2006).

Ergonomic aspects have been barely considered in assembly line balancing literature, though they are becoming increasingly important in practice. Few articles on this topic are those of Miralles et al. (2008) and Costa and Miralles (2009), who introduce and analyze a problem of assigning workloads to stations and to workers with different (dis-) abilities. Another article, written by Carnahan et al. (2001), examines an assignment of a certain class of tasks – gripping tasks – and their influence on fatigue and recovery dynamics of workers. However, to our best knowledge, no attempt has been made yet to incorporate ergonomic risk estimation methods used in practice into assembly line balancing models, though they are considered important by manufacturers.

To close this gap, we address this important question in the present study. We provide an overview of some methods for ergonomic risks estimation, which are recommended and utilized in practice. Most of those methods are based on nonlinear functions such that incorporating them into state-of-the-art line balancing models and (exact) solution procedures is not straightforward.

We propose different ways to model ergonomic aspects and a two-stage heuristic approach, based on the well-known exact balancing procedure SALOME and the heuristic meta-strategy simulated annealing. By means of this heuristic approach, we can achieve a significant reduction in ergonomic risks of workplaces at low computational cost even without increasing manufacturing capacity, i.e., number of workstations (and workers). The proposed two-stage heuristic approach, furthermore, allows for a controllable increase in manufacturing capacity considering the trade-off between increased costs from adding stations on the one hand and reduced ergonomic risks on the other hand.

We precede with an overview of ergonomics tools in Section 2. A line balancing problem incorporating ergonomic risk factors, ErgoSALBP, is described and modeled in Section 3. In Section 4, we propose a two-stage heuristic, which is tested in comprehensive computational experiments in Section 5. A discussion in Section 6 concludes the paper.

Section snippets

Methods for estimating ergonomic risks

In the mandatory Appendix D.1 to §1910.900 of “Final Ergonomics Program Standard”, the Occupational Safety and Health Administration (2000; OSHA for short) provides a list of methods recommended for the estimation of ergonomic risks of workplaces. In this section, we provide a brief description of selected methods recommended by OSHA for application in assembly line production – the revised NIOSH (the National Institute for Occupational Safety and Health) equation and the job strain index; the

Assembly line balancing considering ergonomic risks

Task assignment to stations can substantially influence the level of ergonomic risks at the workplaces, even keeping such profitability parameters as number of stations and cycle time unchanged. Moreover, since better ergonomics may also decrease the defect rate (Eklund, 1995; González et al., 2003) and the number of days of sick leave, incorporation of ergonomics into assembly line balancing could even improve profitability of production.

Colombini and Occhipinti (2006) report results of a

Two-stage heuristic procedure for ErgoSALBP

The different versions of ErgoSALBP are NP-hard optimization problems, because they are generalizations of the NP-hard SALBP-1 (cf. Wee and Magazine, 1982; Scholl, 1999; Chapter 2.2.1.5). Moreover ErgoSALBP cannot be solved by available assembly line balancing methods as, in general, it contains nonlinear ergonomic constraints or a nonlinear objective function.

Therefore, we propose a two-stage heuristic based on a well-known procedure for SALBP-1 (stage 1) and the simulated annealing technique

Computational experiments

We conduct two sets of experiments. First, we would like to find out, how large the improvement in ergonomics is that could be achieved without increasing the number of workstations and how large the computational costs of it are. Secondly, we examine how many additional stations we need to bring ergonomic risks under the acceptable maximal level in order to figure out the trade-off between capacity and ergonomics.

Summary and Conclusions

In manufacturing, control of ergonomic risks at manual workplaces is a necessity commanded by legislation, care for health of workers and economic considerations. Methods for estimating ergonomic risks of workplaces are integrated into production routines at most firms using the assembly-type of production. Assembly line re-balancing, i.e., re-assignment of tasks to workers, is an effective and, in case that no additional workstations are required, inexpensive method to reduce ergonomic risks.

Acknowledgements

This article was supported by the Federal Program “ProExzellenz” of the Free State of Thuringia.

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