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Felix H avatar image
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Felix H asked Matt Long commented

Problems using both pull from list and port connections on a queue

Hi guys,

I am building a model for distribution of pallets and im having a problem with a queue sending items with both port connections and lists. The queue is for storing empty pallets and has an onEntry trigger pushing all pallets to an itemlist. There is one TE pulling from the queue using the list, and there is one combiner pulling from the queue using A-connections. When the combiner pulls from the queue it does not pull from the list, so the item still remains on the itemlist for empty pallets. I've tried using an onExit trigger on the queue, pulling from the itemlist, but then when the TE arrives, items gets pulled twice. I've also tried using an event-triggered source waiting for SendToPort, followed by a pull form list, but then it seems like it pulls everything.

So is there a way to solve this problem? The TE-pull from list is important, so it would be great if the solution fixes the combiner-part of the problem.

Thank you in advance!

Model_test2.fsm


FlexSim 18.0.10
list pullflexsim 18.0.10port connections
model-test2.fsm (71.9 KiB)
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1 Answer

Matt Long avatar image
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Matt Long answered Felix H commented

Once you push something to a List it remains on that List until it gets pulled or the list entry is manually deleted. Those are the only two ways of removing an entry. This means you could either pull the item from the list in the On Entry of the combiner, or when you push items to the list, store the List.Entry reference on a label on the item and then using

 item.ListEntry.as(List.Entry).remove();

To remove the entry.

However, neither of these are very clean solutions. My recommendation is to use Process Flow to control your Combiner's logic. I've attached a small model showing how you might accomplish this.

The combiner has two inputs, one from a source that just feeds it pallets, the other is from a dummy queue that does nothing but give the combiner another input connection. Queue1 pushes items to a list.

I then created a Task Executer Process Flow (so I could use this same flow on any number of combiners) and connected it to the combiner.

The flow has a List shared asset Item List that points to ItemList1. This is just a pass through to allow a Pull from List activity to pull items from that list. The first activity is an Event-Triggered Source which is listening for the On Entry event of the combiner. When a pallet moves onto the combiner this activity will create a new token.

Next there is a breathe (delay activity with time = 0) to allow the combiner to finish getting the pallet into the combiner. Then the Pallet Arrived Decide activity checks if it was a pallet arriving or if this token arrived from a box entering the combiner. If a pallet arrived, then the token pulls an item off of the list and moves it into the combiner.

When items are moved into the combiner, the On Entry activity creates a token and this time Pallet Arrived sends the token to the Pallet Full? activity. If the pallet is full, the token is destroyed. Otherwise the token pulls an additional item from the list.

Hopefully this helps!

CombinerExample.fsm


1590325417429.png (19.5 KiB)
combinerexample.fsm (36.5 KiB)
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Matt Long avatar image Matt Long commented ·

@Felix H I updated this answer with an actual solution since, as you stated, the combiner can't pull from a list. This model was built in FlexSim 2020. You can download the express version of 2020 through your account on our website https://www.flexsim.com to open the model. The logic will still be exactly same when building in FlexSim 2018.

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Felix H avatar image Felix H Matt Long commented ·

Perfect. Thank you!

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Felix H avatar image Felix H commented ·

Ok, i see. Well there is no reason for the combiner not using pull from list other than that I didnt think it was possible? The important aspect is that the pallet has to arrive on the combiner before the items or else the items will be defined as the container of the pallet.

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Joerg Vogel avatar image Joerg Vogel commented ·

I would pull the value of the list, when the item gets pulled by the combiner. The problem with an On Entry trigger is a delay by transport. When a combiner pulls an item, the operator is not involved at this moment. If the item gets pulled, the next logic step creates a tasksequence for an operator to transport this pulled item. At this step or before the return code line in Pull Requirement gets executed, the value of the list must vanish.

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Felix H avatar image Felix H Joerg Vogel commented ·

I dont use any operators for transport tasks, actually there is not any transport tasks at all from the queue to the combiner. The pallet arrives at the combiner immediately after it gets pulled from the queue through the A-Connection. However, the problem with the onEntry trigger is that the target quantity for one pallet is 10 items. So instead of only pulling 1 item for each pallet, it pulls 1 + 10 items. Therefore the list will always be empty.

It doesnt seem like there is any default option for pull strategy on the combiner, so i can get it to pull from the list. I can only choose between different pull requirements. I hoped there would exsist any other events on the combiner that could trigger a pull from list that pulls only the 1 pallet that arrives at the combiner.

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