A RULA-Based Optimization Model for Workers’ Assignment to an Assembly Line

Performance of assembly lines require meeting conflictual goals: a high production rate and a high level of flexibility. Flexibility is often provided by human dexterity and the cognitive capabilities of the workforce. 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. The problem is of particular interest in the view of the workforce aging, a social European phenomenon which is also affecting production systems performance. Designing and scheduling of human-based assembly systems require a joint evaluation of production system performance and a good balancing of MSDs risk among workers. The authors propose a mixed integer non linear programming model allowing for the balancing of MSDs risk while meeting production rate of an assembly line. Risk and its acceptability are evaluated using the RULA method (Rapid Upper Limb Assessment), widely recognized as an effective tool for the risk assessment of Upper Limb Work related MSDs (UL-WMSDs). Different workers' performance due to their respective training levels / skills and age is considered in the problem formulation. Results show the model's capacity to identify optimal job rotation schedules jointly achieving productivity and ergonomic risk goals. Performances of the solutions obtained improve as workforce flexibility increases