question

Fiorenzo A avatar image
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Fiorenzo A asked Phil BoBo commented

Conveyor Transfer Type issue

test_svincolo_1.fsm

Hi,

I make a model with a conveyor crossing. The second and the third horizontal conveyors send items or right or at first horizontal conveyor. The first one from above instead sends items or right or to the third horizontal conveyor. All transfers are made through the central conveyor.

The problem is that the central conveyor has the default upward speed. This is good for the transfers from second and third conveyor but when I must send items from first to third conveyor I must change speed. The speed is changed when items arrive at Control points of start conveyors.

Where is the problem? looking at the picture below the red item is entering in its exit conveyor and it is crossing the Transfer Type. The item circled in black is coming at control point and it will send to third conveyor. In this moment in the central conveyor there isn't items as indicate the current property.

When the item arrives to the control point (the item color changes in green) the target speed of central conveyor reverses becouse in this way the item can reach the third conveyor. Suddenly the red item, despite being out from central conveyor, feels the change speed and it wrongly goes to the third conveyor too.

Why it happens? Could it be a problem due to the transfer type that in someway still link the red item to the central conveyor?

In any case in this model before sending the item of first conveyor, it is stopped and with the process dalay (highlighted in yellow in the picture above) it will wait a time equal to the translation time of the red item on the transfer type.

It might be a good temporary solution.

Do you have any idea how to solve this issue?



I attach the model to the question.

Thanks in advance for the support.

FlexSim 21.0.2
conveyorsflexsim 21.0.2transfer
1613660813688.png (774.9 KiB)
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1613661842822.png (359.8 KiB)
test-svincolo-1.fsm (198.9 KiB)
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tannerp avatar image tannerp commented ·

@Fiorenzo A,

This might be a timing issue or a bug. I ran the Event Log over the time it happened, and the Conveyor is actually sending the box back to the central conveyor, as shown here.

There's not a clear trigger for this behavior, but the FlexSim Engine is what's handling it. Like I said, it might be a bug, or it might just be something that can be fixed using a breathe event, of which I noticed you have a few implemented. @anthony.johnson may have more insight into this.

Would it be possible to add a delay time of 2 seconds somewhere to restrict changes and entries for the central conveyor until the items have fully crossed the Transfer? I think this might be better than waiting for the conveyor's content to be 0 because, as you pointed out, the content changes before this behavior actually occurs. Just an idea I had. Sorry I don't have a clear-cut solution.

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1 Answer

anthony.johnson avatar image
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anthony.johnson answered Phil BoBo commented

From the user's manual:

When a conveyor's direction is reversed the following things happen.

  • All stopped items on the conveyor will be fully resumed.
  • Each item on the conveyor will be set to a moving state.
  • Each item's target destination will be erased. For example, if the item previously had been sent to a point in the system using the Conveyor.sendItem() method, that destination will be forgotten.
  • Each item will regenerate its list of traversal points, e.g. photo eyes, decision points, etc. as if the item is entering the conveyor at the point where it is when the conveyor is reversed.

... [Reversing conveyor speed] was designed with a very specific scenario in mind, which is to give modelers the ability to simulate accumulation buffers, where items can be pulled off a main conveyor line when the line gets backed up, and then moved back onto the line when it clears out. The functionality has been tested only for this scenario. Using reversible conveyors for other scenarios has not been tested and thus may not work perfectly.

When an item is transferring off of a conveyor, it is still "on" the conveyor, i.e. the conveyor still manages the data for that item, even though it does not show up in the conveyor's content stat (you can still get its data with Conveyor.itemData). Thus, when you reverse the conveyor, all actions mentioned in the user manual also happen to the item transferring off of the conveyor.

In the end, you are using the conveyor in a way that it was not intended to be used. Kudos for getting it to work in this way, but you're technically outside the scope of the conveyor's design.

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Fiorenzo A avatar image Fiorenzo A commented ·

Hi,

Thank you for the answer. I have only a doubt, why the exit trigger event starts before all the item has passed the conveyor transfert, if the conveyorItem stats are still linked to the previous conveyor?

In the end is there any development planned for this kind of applicstion with conveyors? Many customers have cases like this and it might be useful.

In this regard I also inserted an idea on the forum https://answers.flexsim.com/idea/97958/bidirectional-conveyors.html

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Phil BoBo avatar image Phil BoBo ♦♦ Fiorenzo A commented ·

The exit trigger event happens when the item is physically moved in the tree hierarchy from one conveyor to the next. As Anthony explained, this might happen sooner than when its itemData is no longer accessible on the previous conveyor.

As you noted, that's not necessarily when the item's back edge is physically past the transfer point.

The conveyor system is designed to work as a whole system instead of just a set of individual conveyors. You should use decision points or photo eyes, which both execute based on item edges, for logic rather than caring about the entry/exit of items through individual conveyors.

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