Nowadays, there are many production systems in which the manufacture of all or part of the production takes place in assembly lines. In addition, the current demand of the market, makes necessary that companies provide a wide range of products with different options. This situation can easily be found in the automobile sector, where different product types, despite belonging to the same family, have different characteristics and require, therefore, different component consumptions and resource use. Indeed, not all vehicles carry the same type of engine, and not all vehicles are equipped with the same components, both indoors and outdoors.
A clear example of this type of assembly lines with mixed products (MMAL, Mixed-Model Assembly Lines), is found in the engine lines or in assembly lines, where different components (seats, steering wheel and pedals) are incorporated into the body of the vehicle. This variety of the product range, leads to the need for the current production or assembly lines are flexible and , therefore, the lines can adapt to the diversity of product types that are manufactured in them, without incurring excessive costs.
Thus, with the aim of making flexible and reducing costs by labor, handling and storage, the mixed product lines have two basic problems: (1) the balancing assembly line and (2) the sequencing of units of mixed-products.
Among the latter problems, we find the study object of this thesis, known as MMSP (Mixed-Model Sequencing Problem) in the literature. This problem consist of establishing a manufacturing order of the products with the aim of: minimising the product and component stock levels; (2) minimising the work overload or the uncompleted work; or (3) minimising the sub-sequence number with special options.
Specifically, in this thesis we study the mixed-model sequencing problem, in assembly lines, with the minimisation of the uncompleted work or work overload (MMSP-W: Mixed-Model Sequencing Problem with Workload Minimisation). Indeed, with the focus of addressing the literature problem, not only to the improvement of productivity, but also to the improvement of the working conditions of the operators of the line, we study four variants, in which we incorporate aspects of the real-life situations that occur in the current production systems.
In the first studied variant, in addition to consider workstations arranged in series and, therefore, interlinked, we consider that in a same workstation may coincide different homogeneous processors, as well as the possibility that all stations can hold all product units a time longer than the cycle time, in order to complete the required work. This variant will result in two equivalent mathematical models, whose objectives will be based on the optimization of the work overload or the completed work and which will serve as the starting point of the following studied variants.
The second variant, incorporates concepts from the management ideology JIT (Just In Time), since, in addition to minimise the work overload or maximise the completed work, this extension considers the convenience of obtaining product sequences that distribute evenly over time the required work, the completed work or the work overload corresponding to all work stations in a workday. This study will give rise to new multi and mono-objectives mathematical models, whose purpose will be to minimize the workload avoiding undesirable excess efforts for human resources.
In the third variant, are considered variable processing times of operations according to the rhythm of activity of operators throughout their workday. Thus, based on the idea that the activity of operators is not maintained constant along the time, different profiles for the factor of activity or work pace are defined. These profiles will force an increase of the working speed of the operators, at certain times of the workday, thus completing more required work and, therefore, reducing the overall work overload.
At last, taking into account, also, the presence of human resources on workstations, we consider the working conditions agreed between the company and trade unions with respect to saturation or level of employment of the workers of the line. Thus, we formulate new mathematical models for the MMSP-W, which, in addition to minimise the amount of lost work, respect the maximum values, laid down by collective agreements, in terms of average and maximum saturation of workstation processors.
Finally, note that all studied extensions for the MMSP-W are evaluated through a case study linked to the plant of engines from Nissan in Barcelona. In this way, we can compare the results obtained with the reference models, with those obtained with the proposed models throughout this thesis, from a computational, economic, social and legal point of view.