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problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”
Posted: Sun May 20, 2018 5:32 am
by anjomshoae
Hello,
I am facing a problem in initializing my model in equilibrium. I receive the following error:
“Simultaneous initial value equation involving…”
Can anyone help to resolve this problem? thank you.
Model is attached to this post.
The parametric value of the “InTansit” stock for equilibrium is “60”.
Best regards,
Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”
Posted: Sun May 20, 2018 4:18 pm
by tomfid
The circularity is listed in the error report:
Code: Select all
ERROR: Simultaneous initial value equations involving: InTansit.
: Inventory position.
: Order decision.
: Demand2.
: Shipment requirement2.
: Shipment2.
: InTansit.
In general, a stock initial value expression can't refer to itself, however indirectly. I'm not sure why the two structures are connected, so it's hard to suggest a fix. One way to handle this would be to set it up so that the inventory is in equilibrium, i.e. arrival = shipment which implies that in transit = shipment*lead time.
Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”
Posted: Tue May 22, 2018 3:01 am
by anjomshoae
Hi,
I simplified my model to only one layer of a supply chain. the model has only 4 stocks. as suggested the formulation for the initial value of "in transit = shipment*lead time" works. But there are other stocks that not sure how what could be the initial value formulation. specifically the stock "Local inventory". the rate out of this stock include a "min" function: Shipment= MIN( Local inventory, Shipment requirement). the rate in is: InTansit/Transit leadtime.
so analytically I need to solve this formulation for local inventory: MIN( Local inventory, Shipment requirement)=InTansit/Transit leadtime
would you please help for this?
Model is attached to this post.
regards,
Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”
Posted: Tue May 22, 2018 3:33 am
by tomfid
Generally the thing to do in nonlinear cases like this is to assume that the inventory constraint is nonbinding, i.e. shipment = shipment requirement.
Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”
Posted: Tue May 22, 2018 3:35 am
by tomfid
Notice also that MIN( Local inventory, Shipment requirement) is dimensionally inconsistent - MIN( widgets, widgets/time ) is not valid. I'd encourage you to define units for the whole model before you get much further.