Gap optimizing merge controller merge gap below target

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Devdatta D asked Jul 1, '22 Jeanette F commented Sep 10, '22

Gap optimizing merge controller merge gap below target

T2.fsmGap optimizing merge controller keeps throwing 'merge gap us below target' message. How to debug this? does having a curved section between slug building lane and take away conveyor have any impact? I am still running into issues, if anybody can please help.

Attached an example file.

On further digging I found following:

Seleceted item should reach DP at 98.64 secs, which it does.

Selected item reaches DP at 103.12 Secs. The time difference the two trailing items to reach DP is 4.48 Secs. However, including slug gap of 16 the distance between two trailing items in 108.61 inches which gives time gap of 4.52 secs ( speed of 24 inch/secs). Second slug should reach DP at 103.16 Secs.

Software Version:

FlexSim 22.1.1

conveyors gap optimizing merge controller merge control

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t2.fsm (89.3 KiB)

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Jeanette F ♦♦ commented · Jul 03, 2022 at 01:10 AM

Hello @Devdatta D,

This exception has been mentioned on answers before. Does the information provided in the following posts help answer your question.

https://answers.flexsim.com/questions/87065/error-when-using-fixed-gap-merge-controller-proces.html

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Felix Möhlmann commented · Jul 04, 2022 at 08:55 AM

If Jeanette's link doesn't provide an answer it would be helpful if you could attach your model or at least provide more information.

Does the error appear in specific situations (like when more than x conveyors have pending slugs?

How large is the difference between target gap and actual gap in the messages?

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Devdatta D Felix Möhlmann commented · Jul 05, 2022 at 03:45 PM

I saw the post mentioned but that doesn't help. Can you please point to how OnSlugStart and OnSlugReady event added to gap controller logic?

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Felix Möhlmann Devdatta D commented · Jul 05, 2022 at 04:14 PM

These events are used in 3 points in the gap-optimizing process flow.

The looped part of the 'Lane Management' section just writes some time stamps to the tokens. These could be used to influence the release order but are not used in the current logic.

The activity on the left holds the token until another attempt to release a lane should be make (either when a new slug is ready, or a slug has cleared the decision point)

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Jeanette F ♦♦ commented · Sep 10, 2022 at 02:17 PM

Hi @Devdatta D, was one of Logan Gold's or Felix Möhlmann's answers helpful? If so, please click the "Accept" button at the bottom of the one that best answers your question. Or if you still have questions, add a comment and we'll continue the conversation.

If we haven't heard back from you within 3 business days we'll auto-accept an answer, but you can always unaccept and comment back to reopen your question.

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Answer 1 · 2022-09-02T23:30:48Z

Logan Gold answered Sep 2, '22 Logan Gold edited Sep 2, '22

@Devdatta D, I apologize for taking so long in getting back with some more answers for you. I was able to get a developer to take a look at this and there seems to be a bug. I haven't been able to track down if there is an issue with the logic in the process flow, or a bug deeper in the conveyor logic, but there is a workaround.

The problem is that more than the max of 3 items is being released as part of a slug, which is messing up the logic. Apparently, the logic isn't properly stopping the 4th item because there is no gap between it and the one in front of it.

For the workaround, you need to enforce a small gap (0.01 inches is sufficient, possibly smaller could work too) between the items using their stop, move, and entry spacing. I believe this would be similar to a real system, since you would need to enforce some gap between items so that photo eyes can recognize the items leaving. However, let me know if I am mistaken.

Answer 2 · 2022-07-06T08:44:46Z

Felix Möhlmann answered Jul 6, '22 Devdatta D commented Jul 15, '22

I made some adjustments to the gap-optimizing flow to get it to work with a non-accumulating merge conveyor.

In the 'Calculate Release Time' activity, I check if there are currently slugs on the merge conveyor by looking a how many children the main token has. If there are, instead of taking the current estimate of when that slug will clear the decision point, I update that time by looking at the last item in the slug and estimating how long it would take to reach the DP from its current position. This way, if the slug was delayed, the clear time will be updated to represent this.

Due to the possibility of a slug stopping while being released, I also had to add a small loop that listens for extra items that might be added to the slug after the release starts. If an item bump is detected, the 'tailItem' label on the statistics token will be updated to the new item. Otherwise, adding items after the fact would mess up the calculationg and gap checking again.

To copy these adjustments into your model, you should be able to just overwrite the process flow node in the tree with a copy of the one from the attached model. You will (probably) have to link the merge controller to the flow again.

Nonaccum_MergeController.fsm

What this will not fix, is if the merge conveyor is accumulating but the slugs get stopped due to backlog from downstream conveyors. (Not sure if this is the case in your model). In that case you could just deactivate the error message if the behaviour is ok otherwise. To deactivate it, delete or comment out line 16-24 of the code in 'Record Stats and Error Check'.

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nonaccum-mergecontroller.fsm (78.5 KiB)

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Devdatta D commented · Jul 15, 2022 at 01:50 PM

I am still running into an issue where gap is below target. I have attched the model. I tried to debug the logic, i couldn't see any issue. T2.fsm

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t2.fsm (89.3 KiB)

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