Simultaneous balancing and buffer allocation decisions for the design of mixed-model assembly lines with parallel workstations and stochastic task times (original) (raw)

The buffer allocation problem (BAP) and the assembly line balancing problem (ALBP) are amongst the most studied problems in the literature on production systems. However they have been so far approached separately, although they are closely interrelated. This paper for the first time considers these two problems simultaneously. An innovative approach, consisting in coupling the most recent advances of simulation techniques with a genetic algorithm approach, is presented to solve a very complex problem: the Mixed Model Assembly Line Balancing Problem (MALBP) with stochastic task times, parallel workstations, and buffers between workstations. An opportune chromosomal representation allows the solutions space to be explored very efficiently, varying simultaneously task assignments and buffer capacities among workstations. A parametric simulator has been used to calculate the objective function of each individual, evaluating at the same time the effect of task assignment and buffer allocation decisions on the line throughput. The results of extensive experimentation demonstrate that using buffers can improve line efficiency. Even when considering a cost per unit buffer space, it is often possible to find solutions that provide higher throughput than for the case without buffers, and at the same time have a lower design cost. Web-site: www.impianti.dii.unipg.it/tiacci -3 -provides single-model, paced line with fixed cycle time and deterministic task times. The SALBP can be classified with respect to the objective type: in 'type-1' problem one tries to minimize the number of stations for a given cycle time, while in 'type-2' problem one tries to minimize the cycle time for a given number of workstations. So usually the two basic objectives (performances and costs) of the problem are treated in a separate way, that is fixing one of the two as a constraint, and trying to optimize the other one. If both, number of stations and the cycle time, can be altered, the problem is of 'type E', i.e. the line efficiency E can be used to determine the quality of a balance. It is possible to maximize the line efficiency by simultaneously minimizing the cycle time and the number of workstations, for example through a unique objective function by applying objective-specific weighting factors. Real problems are much more complicated than the SALBP, and research has recently evolved towards formulating and solving generalized problems (namely Generalized Assembly Line Balancing Problem, GALBP) with different additional characteristics, such as cost functions, equipment selection, paralleling, stochastic task times and others. For a comprehensive classification of the possible features of the GALBP see and . The Mixed-model Assembly Line Balancing Problem (MALBP) can be seen as a particular case of the GALBP. Here a set of similar models, that are variations of the same base product and only differ in specific customizable product attributes, can be assembled simultaneously. Set-up times between models can be reduced sufficiently enough to be ignored . Studies published in the last years utilize different approaches to solve it, such as: