Concepts as items, locations, customers, demands, …don’t need a much of
The modelling concepts of operations, material and capacity consumption
however need some introduction.
The key modelling element is an operation, which defines an activity.
There are different operation types:
- fixed time: the duration is identical regardless of the planned quantity.
Eg, a transport operation
- time-per: the duration increases linearly with the planned quantity.
Eg, a manufacturing operation
- routing: this operation type represent a sequence of sub-operations.
- alternate: an alternate operation models the choice among multiple choices.
Operation “Make product X” takes 10 minutes per unit.
Items are stocked in buffers. A buffer, often called SKU -stock keeping unit-,
is a (physical or logical) inventory point.
There are different buffer types:
- default: a buffer that is replenished with a producing operation
- procure: a buffer that is replenished with a procurement operation
- infinite supply: a buffer without replenishing operation
Buffer “Part A” models inventory of the item “part A” in location “my factory”.
Buffer “Part B” models inventory of the item “part B” in location “my factory”.
Buffer “Bulk product” models inventory of the item “bulk product” in location “my factory”.
Operations consume and produce material through flows.
A flow defines an association between the operation and a buffer. Flows thus
represent the Bills of Material (aka BOM) and Bills of Distribution (aka BOD)
in your model.
There are 2 types of flows:
- start: Flows happening at the start of the operation. These are normally consuming
material and have a negative quantity.
- end: Flows happening at the end of the operation. These are normally producing
material and have a postive quantity.
Operation “Make product X” has 3 flows.
A first flow to consume 1 units of a buffer “Part A” at the start of the
A second flow to consume 2 units of a buffer “Part B” at the start of the
And finally a third flow to model the production of 1 unit of “product X”
when the operation finishes.
Operations also require capacity, which is modelled through loads. Capacity is
available in resources and a load defines an association between the operation
and a resource.
The resource “production line” is required to perform the operation.
An operation only statically defines the activity, and doesn’t specify any
planned dates or quantities. Concrete activities are then instantiated in
To satisfy a customer demand we plan to run “Assemble product X” for 12
units from tomorrow 8am till 10am.
Another instance of “Make product X” is planned the next day from 3pm to 4pm
for 6 units to meet another customer demand.
Another instance of “Make product X” for 20 units is planned today to
replenish a buffer storing the product X to its safety stock level.
Putting it all together
Combining all of the above modelling objects we can construct a network
representing the flow of material and usage of capacity in your plant.
The picture below shows a simple network with 3 levels.
Drawing this type of schematic network of your environment is extremely useful
before you start entering data in frePPLe. Each shape and line in such a
diagram will be defined as an input record in the frePPLe data model.