Capacity Constraint

[Jindu]

Hello Everyone.

I'm trying to model capacity constraint in the attached model. Is there not a function in Vensim that stops the inflow once a stock reaches a particular value?
Would really appreciate your help.
Kind regards.

[Administrator]

No there is not a function to limit inflow.

What level in the model are you trying to limit?

[Jindu]

Hi.
I want to settle (limit) capacity constraint on Work in Process Inventory to about 1000 and Inventory to about 400 if that is what you mean.

[tomfid]

Something like this:

WIP = INTEG( production start rate - completion rate, initial WIP )
completion rate = WIP/completion time
max WIP = 1000
production start rate = MIN( desired production start rate, max production start rate )
max production start rate = MAX(0,max WIP-WIP)/TIME STEP

[Jindu]

Thanks for your reply. I'm finding it hard to model and input these equations in the structure.

[tomfid]

Nice to see balanced units in your model.

The basic idea, I think, is that you need to limit production starts, so that the inventory and WIP constraints are not exceeded. This means you need an additional term in the production starts equation, creating a negative feedback loop that slows or stops the inflow when the stocks are full.

The equation I suggested enforces only one of the two constraints (the WIP level), in a very simple way. The real logic could be more complex. For example, what happens if inventory and WIP nearly full, and sales stop? I could imagine two possibilities. The simple one is that the Start Rate goes to 0, but production from WIP continues, potentially overfilling inventory temporarily. A more complex situation would require production to slow or stop, so that the inventory constraint is always respected.

[Jindu]

Thanks again for your reply. I will try to work out how to implement your suggestion in my model.