For my graduation product, I am modelling a production planning system in the context of a rolling horizon. The model can allocated products to plant A and plant B, based on the expected demand for the upcoming periods. Plant A has a production lead-time of 1 week and plant B has a production lead-time of 2 weeks.
Note that for some products, it is cheaper to produce at plant A, whereas for other products, it is cheaper to produce at plant B.
I formulated the following constraint in AIMMS to update the inventory:
In which p refers to the set of products, h to the horizon, and r to the set of plants. The LeadTime(r) is modeled as a parameter and takes value 1 for plant A and value 2 for plant B.
However, with this constraint, AIMMS always allocates all products to plant A.
When modelling the inventory constraint by:
the model does allocate some products to plant A and some to plant B (when production is cheaper at plant A, the product are allocated to plant A and otherwise to B.
Additionally, the first-formulated constraint (unnecessary) counts Lost sales, while the second constraint does not count them (even if the products are allocated to the same plant).
- Why does the model behave less optimal by included the first constraint compared to the second constraint?
- How to correctly include a plant-dependent lead time in the horizon?