Constrained scheduling of jobs with different due dates in a flowshop

In the paper an attempt to combine the potential of object oriented simulation tools with the efficiency of the heuristic algorithms in detecting optimal line configurations has been done to solve the flowshop sequencing problem when the mean tardiness of jobs is assumed as objective to be minimized. Both the optimal scheduled sequence and the optimal resources configuration are contemporary determined. Two procedures have been implemented to cope with this problem. A hierarchical procedure which consists of two successive steps; rant a simplified model of the line is optimized through a properly developed simulated annealing algorithm, which selects the optimal schedule; then, the optimal sequence is forced into the simulator which reproduces the dynamics of the real manufacturing line, i.e. all the manufacturing resource constraints are taken into account; in this way, the procedure optimizes the line resources trying to respect the required delivery times. Another solution procedure of the problem is developed by considering the contemporary optimization of both scheduled sequences and resources capacities. With this approach, a heuristic, which selects different process scenarios, is developed and interfaced with the simulation environment, where these scenarios are simulated to evaluate the line dynamics and the satisfaction of the selected objectives. A set of test cases is proposed to study the problem and to verify the efficiency of the procedures.