A new load adjustment approach for job-shops

This chapter presents a middle-term new load adjustment approach by overlapping for a set of jobs in a job-shop context, guaranteeing the existence of a limited capacity schedule without scheduling under the assumption of pre-emptive tasks. The proposed approach is based on the exploitation of the tasks scheduling time segments overlapping and on the distribution of the job’s margins between tasks in a just in time context. First, we present a literature review concerning load adjustment approaches. Second, we introduce the overlapping load adjustment approach. Third, we present an original heuristic to use this approach in the case of job-shops organized firms. After that, we present the associated scheduling approach. Finally, we will discuss a more general use of this approach and the possible extensions. Generally, the production planning is made in a hierarchical way in two planning and scheduling decision levels. In the first step, we decide which products to supply, in which quantities and delays, in the second step, we adjust load to the capacity and schedule the tasks on the machines. There are three main and classical load adjustment approaches. First of all, we have the placement. This approach consists in calculating a detailed tasks schedule. A new task is integrated in the planning if we find a hole in the planning which is bigger than the duration of this task. This approach estimates only one schedule which can be destroyed by any disturbance. The second approach is the periodic and cumulative approach. It consists in calculating the cumulative load and capacity for a latest loading and for each period. This approach does not guarantee the existence of a scheduling solution because it does not take into consideration the ready dates constraints. The third approach is the periodic and noncumulative approach. It consists in assigning tasks to periods and comparing period by period the available and the required capacities. This method estimates only the solutions in which the tasks are fixed in a specific period. The problem of sequencing decisions in production planning and scheduling was studied by (Dauzere-Peres & Laserre, 1999). They think that it is better in some cases to integrate the scheduling decision level in the lot sizing decision level and propose an iterative approach for planning and scheduling production. Some researchers integrated the scheduling and capacity constraints in their lot sizing model (see for example, Fleishmann & Meyr, 1997). O pe n A cc es s D at ab as e w w w .ite ch on lin e. co m