Productivity and ergonomic risk in human based production system: a job-rotation scheduling model

The competitiveness of modern manufacturing systems is based on a high production rate and a high level of flexibility. Despite the high level of automation achieved in production systems, flexibility is often provided by human dexterity and the cognitive capabilities of the workforce, as in assembly lines. In the case of repetitive manual tasks, workers are exposed to the risk of musculoskeletal disorders (MSDs). In these contexts, a high production rate leads to high physical workload, and job rotation is adopted in order to reduce the ergonomic risk. Traditionally, ergonomics and human performance issues have been investigated separately. However, in the design and scheduling of human-based manufacturing systems, a reliable description of human components is required in order to jointly evaluate production system performance and assess workers risk of MSDs In this paper, the authors propose a model which aims to find optimal job rotation schedules in work environments characterized by low load manual tasks with a high frequency of repetition (e.g. assembly lines). The model is a mixed integer programming model allowing for the maximization of production rate jointly reducing and balancing human workloads and ergonomic risk within acceptable limits. Risk and its acceptability are evaluated using the OCRA (OCcupational Repetitive Actions) method (ISO 11228-3:2007), widely recognized as an effective tool for the risk assessment of Upper Limb Work related MSDs (UL-WMSDs). Moreover, the different workers performance due to their respective training levels and skills is considered in the problem formulation. The model is applied to an industrial case study. Results show the models capacity to identify optimal job rotation schedules jointly achieving productivity and ergonomic risk goals. Performances of the solutions obtained improve as workforce flexibility increases.