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.
Capacity Constraint
Capacity Constraint
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Re: Capacity Constraint
No there is not a function to limit inflow.
What level in the model are you trying to limit?
What level in the model are you trying to limit?
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Re: Capacity Constraint
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.
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.
Re: Capacity Constraint
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
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
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Re: Capacity Constraint
Thanks for your reply. I'm finding it hard to model and input these equations in the structure.
Re: Capacity Constraint
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.
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.
/*
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Blog: http://blog.metasd.com
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*/
Advice to posters (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391
Blog: http://blog.metasd.com
Model library: http://models.metasd.com
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*/
Re: Capacity Constraint
Thanks again for your reply. I will try to work out how to implement your suggestion in my model.