problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

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anjomshoae
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problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

Post 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,
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SC Model.mdl
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tomfid
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Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

Post 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.
anjomshoae
Member
Posts: 46
Joined: Thu Oct 23, 2014 1:08 pm
Vensim version: DSS

Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

Post 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,
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SC Model simplified.mdl
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tomfid
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Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

Post 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.
tomfid
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Re: problem in initializing my model in equilibrium “Simultaneous initial value equation involving…”

Post 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.
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