Robust and Flexible Scheduling with Evolutionary Computation

Mikkel T. Jensen

November 2001


Over the last ten years, there have been numerous applications of evolutionary algorithms to a variety of scheduling problems. Like most other research on heuristic scheduling, the primary aim of the research has been on deterministic formulations of the problems. This is in contrast to real world scheduling problems which are usually not deterministic. Usually at the time the schedule is made some information about the problem and processing environment is available, but this information is uncertain and likely to change during schedule execution. Changes frequently encountered in scheduling environments include machine breakdowns, uncertain processing times, workers getting sick, materials being delayed and the appearance of new jobs. These possible environmental changes mean that a schedule which was optimal for the information available at the time of scheduling can end up being highly suboptimal when it is implemented and subjected to the uncertainty of the real world. For this reason it is very important to find methods capable of creating robust schedules (schedules expected to perform well after a minimal amount of modification when the environment changes) or flexible schedules (schedules expected to perform well after some degree of modification when the environment changes).

This thesis presents two fundamentally different approaches for scheduling job shops facing machine breakdowns. The first method is called neighbourhood based robustness and is based on an idea of minimising the cost of a neighbourhood of schedules. The scheduling algorithm attempts to find a small set of schedules with an acceptable level of performance. The approach is demonstrated to significantly improve the robustness and flexibility of the schedules while at the same time producing schedules with a low implementation cost if no breakdown occurs. The method is compared to a state of the art method for stochastic scheduling and concluded to have the same level of performance, but a wider area of applicability. The current implementation of the method is based on an evolutionary algorithm, but since the real contribution of the method is a new performance measure, other implementations could be based on tabu search, simulated annealing or other powerful ``blind'' optimisation heuristics.

The other method for stochastic scheduling uses the idea of coevolution to create schedules with a guaranteed worst case performance for a known set of scenarios. The method is demonstrated to improve worst case performance of the schedules when compared to ordinary scheduling; it substantially reduces program running times when compared to a more standard approach explicitly considering all scenarios. Schedules based on worst case performance measures often have suboptimal performance if no disruption happens, so the coevolutionary algorithm is combined with a multi-objective algorithm which optimises worst case performance as well as performance without disruptions.

The coevolutionary worst case algorithm is also combined with another algorithm to create schedules with a guaranteed level of worst deviation performance. In worst deviation performance the objective is to minimise the highest possible performance difference from the schedule optimal for the scenario that actually takes place. Minimising this kind of performance measure involves solving a large number of related scheduling problems in one run, so a new evolutionary algorithm for this kind of problem is suggested.

Other contributions of the thesis include a new coevolutionary algorithm for minimax problems. The new algorithm is capable of solving problems with an asymmetric property that causes previously published algorithms to fail. Also, a new algorithm to solve the economic lot and delivery scheduling problem is presented. The new algorithm is guaranteed to solve the problem to optimality in polynomial time, something previously published algorithms have not been able to do

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