Modeling a system with advance demand lead-time information

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juanos
Posts: 4
Joined: Wednesday 17 December, 2014 - 15:28

Modeling a system with advance demand lead-time information

Post by juanos »

Hi,

I am trying to figure out how to model a make-to-stock supplier operating under an order-base-stock policy.
An arrival triggers a replenishment order as well as a customer order. (See attached picture of what I am trying to model)

This is what I was able to model:
A splitter splits products from a source into a replenishment order and a customer order, the upper part of my model is the "in-process replenishment orders" part, and the lower part is the "In-process customer orders" part.
In the replenishment part, the product is delayed by a server that needs to produce the order, which then puts it into a queue named: inventory.
In the customer order part, the order waits a predetermined (and assigned as a lable in the source) LeadTime, and then enters the queue named: backorder.
The assembler puts a customer order and a replenishment order back together, and puts them in a sink.

If there is a product in the inventory queue then there is no stay time for a product in the backorder queue and vice-versa.
By recording the average stay time in both these queues i want to determine the optimal behavior of the system under different conditions.

I have two problems with my model:
1. The server in the bottom part can only delay one product at a time, so products are waiting in the queue before the server which is not right. A product needs to start his leadtime delay the moment it is split and enter the backorder queue when the leadtime is finished. So, I guess this is not the right way to model this but I can't figure out how to do it. (I tried using the funcion DelayEvents but could not find a way to make it work.)

2. The(Inventory) Queue atom starts (off course) with an initial inventory level of 0, but in order to get the base stock policy right I want to vary my initial inventory level. I also can not figure out how to do this.

Attached you can find my ED model, all help is appreciated!
Attachments
ModelALTDI.mod
(20.83 KiB) Downloaded 269 times
ModelALTDI.jpg
ModelALTDI.jpg (9.95 KiB) Viewed 5421 times
juanos
Posts: 4
Joined: Wednesday 17 December, 2014 - 15:28

Re: Modeling a system with advance demand lead-time informat

Post by juanos »

Ok, I made some progress. I am using a multi-server now at the bottom part, with >10000 channels to process the lead-time delay. This seems to work fine. For the inventory queue I use an extra source that produces a maximum of, i.e. 10 products, right at the beginning with 0 interarrival time, and this also works fine.

The problem that I have left now, is that the assembler consumes directly 1 item from one of the queues (Inventory/Backorder) if there is one available, while there is no item available in the other queue. This is the wrong behavior because I want that items stay in the queue until there is one available in the other queue. Does anyone know how to solve this?
juanos
Posts: 4
Joined: Wednesday 17 December, 2014 - 15:28

Re: Modeling a system with advance demand lead-time informat

Post by juanos »

I added a condition control atom, which controls the input channels of the assembler atom.
on condition that one of the queues (inventory/backorder) is empty:

or(content(in(1,c))=0,content(in(2,c))=0)

it closes input and allows output.

However this still doesn't work. I assume that the assembler uses a kind of round robin system, because it now can happen that it consumes an item from the inventory queue, and directly closes the input channels, so an item gets locked up in the assembler while there are items available in the backorder queue. It works fine for the case where the backorder queue becomes empty.

I again uploaded my model, help would be greatly appreciated!
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MAMS4.mod
(29.36 KiB) Downloaded 279 times
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HarryBunnik
Posts: 362
Joined: Monday 07 February, 2011 - 11:22

Re: Modeling a system with advance demand lead-time information

Post by HarryBunnik »

Ha Juanos,

I made some changes to your model to try to get to what I think is what you want:
MAMS4.1.mod
(29.23 KiB) Downloaded 325 times


First, I change the color of the order after the splitting. That way I can see what is going on.

Secondly I removed the conditional control. (I saw a different solution ;) )

Then I place on each of the both queues code that when an order enters the queue (OnEntry triggers), the queue is closed. Then the queue checks if the other queue already contains an order. If so, both queues are opened simultaneously, otherwise they both stay closed.
On the assembler I then close the output of the queue's again (OnEntry triggers). Once an assembled order leaves the assembler, I again check if both incoming queues are filled. If they are, I open both simultaneously, and otherwise I wait for the missing order. That is about it.

Hopefully this helps you further,

Cheers,

Harry
juanos
Posts: 4
Joined: Wednesday 17 December, 2014 - 15:28

Re: Modeling a system with advance demand lead-time information

Post by juanos »

Hi Harry,

Thanks for your solution! This is indeed how I wanted it to work.

Cheers.
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