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Old February 13th, 2009, 01:08 PM   #1
bara_hence
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Nice way to track pallets on a conveyer?

Programming my first conveyer and what I have to work with is a conveyer with one motor and four sensors one sensor where the pallet gets loaded one sensor in the middle of the conveyer and one before the end station and one at the end station.. Actually the conveyer changes direction at the end station but this isnt the problem..

What I want to do is to track if there are any pallets on the conveyer in case of an emergency stop or something like that the pallets could have stop in between the sensors and this is what I want to avoid..

What would the nicest way to track the palletts be.. I have been thinking of up and down counters and other logic such as set reset on positive and negative flanges on the sensors but I believe there must be a nicer way..

Any suggestions??
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Old February 13th, 2009, 04:42 PM   #2
Ned_Flanders
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Most conveyors I have come across for pallets use set and reset commands.
They use two different pieces of logic per roller section (Motor). One is "Pallet infeeding" and one "Pallet discharging". The conveyors runs until the pallet makes the sensor (infeed). As long as the next conveyor is clear, conveyor one will discharge (Outfeed) onto conveyor two. And so on.

You could program some timers, so if the pallet fails to arrive within a set time a "Pallet stuck" alarm can be raised.

Make sure you include some manual control as well!! Pallets are critical to robots/conveyors but forklift drivers put any old broken rubbish onto them!!!

Is this what you were after?
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Old February 13th, 2009, 04:56 PM   #3
OkiePC
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I like to do product tracking like this in inches or feet.

If the conveyors run at a fixed speed, you can find out what those speeds are and accumulate a position register for each pallet in the system.

I do this with cartons on my sorting conveyors and pallets too.

It is then easy to tell when there is excessive slippage or when a photocell fails, etc.

It could be overkill in some situations where you are only dealing with a very simple conveyor.

Paul
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Old February 13th, 2009, 06:07 PM   #4
bara_hence
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markie :
The trouble is that one conveyor section (motor) can hold several pallets and I must se to buffer som paletts because the machinery acts fast filling/emptying these.. So the trouble is holding in memory that Ive buffered a pallett in between two sensors..

If I had sections with sensors so i could detect each pallet im certain your idea would work great..


OkiePC: If one only had the time.. But for this machine maybe to much overkill..
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Old February 13th, 2009, 09:16 PM   #5
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Sounds like a WIP thing to me. I don't know what PLC you are using. I use a bit shift left (AB) to load each pallet into the WIP (work in process)then it is removed from the WIP when it leaves the end.
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Old February 14th, 2009, 03:31 PM   #6
Terry Woods
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From your description it looks like you want to know if you have any pallets at all on the conveyor... is that right?

The hardware you already have in place will allow you to not only know if you have any pallets but also how many and where they are located.

You said you had 4 sensors, right?
So that makes for 3 sections between sensors; Sec-A, Sec-B & Sec-C

Depending on the PLC, I would use either three Up/Down Counters or three Add/Sub pairs. In some PLC's an Up/Down Counter comes at somewhat of a premium. If this is the case then save the counters and use the Add/Sub pairs.

Note: In the following...
Inc = Increment Up/Down Counter total
Dec = Decrement Up/Down Counter total
Add = Add 1 to specified section total
Sub = Subtract 1 from specified section total

As a pallet comes in Inc or Add the Sec-A total.

When a pallet trips the second sensor, Dec or Sub Sec-A total, and Inc or Add the Sec-B total.

When a pallet trips the third sensor, Dec or Sub the Sec-B total, and Inc or Add the Sec-C total.

When a pallet trips the fourth sensor, Dec or Sub the Sec-C total.


At this point the program knows how many pallets there are and where they are located.

The question for you now is... what is it that you really want to know when an E-Stop occurs?

Do you simply want to know if there are any pallets at all?
Do you want to know how many pallets?
Do you want to know where the pallets are located?

Any at all...
If using the Up/Down Counters then simply look for Not-zero in each counter.
If using the Add/Sub pairs, then add the three section totals together and look for Not-zero.
In either case, if a Not-zero is produced then you have at least one pallet, somewhere on the conveyor.

How many...
If using the Up/Down Counters then add the three Up/Down counts.
If using the Add/Sub pairs, then add the three section totals.

Where...
If using the Up/Down Counters then U/D Counter-A shows the current count in Section-A. Likewise for Counter-B/Section-B and Counter-C/Section-C.
If using the Add/Sub pairs then the value in variable Section-A is the current count in Section-A. Likewise for Section-B and Section-C.

Things to be aware of...
Whether you use counters or Add/Sub, it's possible that the total in any section might go negative unless something prevents it from doing so.
It's possible that a pallet might enter a section just as one is leaving. Be sure that the Add or Inc is done before the Sub or Dec.
During an E-Stop there is no telling what the operators are doing in terms of the pallets. They might remove pallets or relocate them from one section to another.
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Old February 15th, 2009, 11:13 AM   #7
PeterW
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Aways advise against using count up/count down at photo eye methods for this sort of thing as they can drift in error over time.

The best option is to track the material, using a tacho, proximity sensor on a toothed wheel or even using an internal pulse (depends on the accuracy you need for tracking really).

By doing it this way, you can build in the counts for materials on conveyors and they automatically correct themselves if items are removed or ghost items are created.
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Old February 15th, 2009, 11:19 AM   #8
bara_hence
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Terry Woods:

Yes were closing in to my problem..

I have been thinking my self about these up/down counters to but as you pointed out a problem can be that the operator can remove palletts when the machine is stopped.. Maybe I should add a button for inc/dec on the panel to say there are this many palletts in between sensor a and b..

Maybe a bit overkill becausse the only thing i really want to now is if there are any pallets in between sensor a-d..
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Old February 15th, 2009, 01:22 PM   #9
Clay B.
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Quote:
Originally Posted by bara_hence View Post
Terry Woods:

Yes were closing in to my problem..

I have been thinking my self about these up/down counters to but as you pointed out a problem can be that the operator can remove palletts when the machine is stopped.. Maybe I should add a button for inc/dec on the panel to say there are this many palletts in between sensor a and b..

Maybe a bit overkill becausse the only thing i really want to now is if there are any pallets in between sensor a-d..
One way I would get around adding a switch is to compare.
You know you should never have a negative count or a count in a zone higher than that zone can handle. If either of these conditions becomes true then reset your count with a fill count. IE all zones zero out and you start over filling the zones again. You can set a bit that says pallets present but unknown count. Then when your lead pallet of your reset count get to the last sensor you can clear this bit.

Like others have said, lots of ways to get this done. Myself I prefer add, subtract functions with limit compares. That way I am always counting and always checking the integerty of the count.

Not sure how your conveyor is configured but one system I worked on we had operators removing pallets while the conveyor was in auto. This threw our count system. So we devised the compare functions to account for this.

Just because an operator is NOT supposed to do something does not guarentee they won't.

Another note: put a time delay in your count trigger. If someone flashes your sensor with there hand you count goes out the window as well.
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Old February 15th, 2009, 04:52 PM   #10
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All of the side effects you guys have mentioned with regard to manually manipulating products is what led me to develop my "tracking by position" logic.

If they remove a pallet, the system will not get the identity of the others wrong, it will simply not have photocell(s) blocked when it expects them to be. No need to mess with FIFOs or counters and all the what-if scenarios.

Paul
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Old February 15th, 2009, 05:27 PM   #11
bara_hence
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You have a good point OkiePC but for this project i feel there is no time but when Im not so stuffed with work Ill make a FunctionBlock for conveyors with maybe the functionality you describe.. So its nice getting all of these ideas..

For this project I think the add/dec compare and have to do there are no more time left to fiddling with the conveyors..

But I will listen to more smart ideas about conveyer tracking..
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Old February 15th, 2009, 06:12 PM   #12
Clay B.
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OT a bit

Quote:
Originally Posted by OkiePC View Post
All of the side effects you guys have mentioned with regard to manually manipulating products is what led me to develop my "tracking by position" logic.

If they remove a pallet, the system will not get the identity of the others wrong, it will simply not have photocell(s) blocked when it expects them to be. No need to mess with FIFOs or counters and all the what-if scenarios.

Paul
Your system is best if indexing is consistant. What do you do when it is a tendency style system. Tendency Systems always give me the biggest fits when trying to track parts on.
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Old February 15th, 2009, 06:52 PM   #13
Dan Mays
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According to the physical layout of your conveyor maybe use a
long range transmitter/receiver type of eye.

The eye could look down the entire length of the conveyor to
see if any pallets are on the conveyor.
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Old February 16th, 2009, 10:44 AM   #14
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There's a lot good good ideas in this thread. Let me add a couple of things.

1. Filter your photoeye inputs so that "jittery" inputs do not cause you to count one pallet as multiple pallets. A time delay off filter will prevent a lot of pain.

2. Tracking can be thrown off by an operator accidentally blocking a photoeye. If the pallets are NEVER going to be moved when the cconveyor is not running, you might make the conveyor running an additional condition for a photoeye to see a pallet.
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Old February 17th, 2009, 02:53 PM   #15
bara_hence
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My few conveyors is building up.. PeterW Ill take your suggestion and use a internal pulse..

Is this how you usually do it?

(if motor running xxseconds then pulse cnt down section a)??
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