| Scheduling theory first appears in the mid 1950s. Since then the problems
addressed become closer to industrial applications, thus increasing in complexity.
The layout of the shops taken into account are closer and closer to
those met in practice: we encounter shops where the machines are found in
different multiple copies, shops where an operation may require several resources
simultaneously, or with multipurpose machines, etc. At the same time
the embedded constraints are more and more concrete: many authors take
into account release dates, the preemption of the jobs, the resource availabilities,
etc.
Paradoxically, the literature shows that in the majority of the problems addressed,
schedules are only evaluated by a single criterion. During the diflFerent
phases of planning different criteria can be considered. At a strategic level,
at the long term planning phase with several years in view, the objectives
concern minimising the costs related to the investment plans for materials,
finance, or personel, related to the choice of new directions, or the launching
of publicity campaigns. For tactical planning at the medium term phase with
several months in view, the objectives always focus on minimising the costs:
stock costs (supply or interruption of stocks), costs of getting supplies, costs
of modifying production capacity, launching costs, costs of modifying production
systems and certain commercial costs ([Merce, 1987], [Giard, 1988]).
At the short term planning phase (with the order of a week in view), or
scheduling phase, several objectives require the attention of the production
executive: above all he must consider the delays that satisfy the customer,
next, he must minimise the work-in-process costs in the shop, and finally he
must minimise the manufacturing costs related to the time spent to set up
the machines or idle periods of the machines. Therefore, a scheduling problem
involves multiple criteria. |
