A model for rest allowance estimation to improve tasks assignment to operators

Manual activities are still present in production systems, such as order picking in warehouses, material handling in production systems, loading and unloading of work centres. Many scientific contributions have widely demonstrated production system efficiency is strictly linked to the operator wellbeing. In fact, activities at different pace and duration imply different productivity but also different effects on the fatigue accumulation acquired by the operators, with consequently required resting period. It is necessary to integrate operator fatigue and recovery analysis into traditional decision support models for the design and management of production systems. This paper aims to present an analytical model for setting the time necessary for operators to recover from the performed activity. According to previous research, the exponential trend of fatigue accumulation and recovery alleviation is studied. The energy expenditure rate, predicted with heart rate monitoring, is used to model the fatigue/recovery level, varying the physiological factors of the operators and the characteristics of the analysed manual activities. The model is limited to the activities where the whole body is used rather than a specific part stressed continuously. Finally, it is applied to optimise the scheduling of activities among operators in a manual order picking system. It is demonstrated that its use improves performance in terms of productivity.