SEQUENCING MIXED-MODEL ASSEMBLY LINES TO LEVEL PARTS USAGE AND MINIMIZE LINE LENGTH

The problem of sequencing units on a mixed-model assembly line can be viewed with several objectives in mind. Past research has focused main ly on two separate performance measures: (1) minimizing the length of the line (which is equivalent to minimizing the risk of stopping the c onveyor when system variability is present and the station lengths are fixed); or (2) maintaining a rate of assembly equal to the demand rat e for each model type in the production schedule. The latter is the mo re appropriate in a just-in-time environment. We present a bicriteria formulation of the problem that can be used to examine the tradeoffs b etween line length and parts usage. The resultant model takes the form of a mixed integer nonlinear program and is solved with a combination of heuristics and branch and bound. Results are reported for a wide r ange of problem sizes, as defined by the number of stations on the lin e, the number of different model types, and the total number of units to be assembled. In almost all cases, at least one of the heuristics f ound either the optimum or the best available solution. Computation ti mes were quite reasonable for the heuristics, but grew exponentially f or branch and bound. In general, it was only possible to verify optima lity on problems with less-than-or-equal-to 20 units.