Tracking and sorting multiple logs on conveyor.

Jasondelane

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Mar 2015
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Florence sc
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I've got a task to be completed within 1 month. The task is to assign categories to different diameter, length, and defect logs; track them along a 300 foot long trio of conveyors, and kick them into one of three bins based on category.

PLC is Controllogix with RSLogix version 20.

Right now my plan is to assign a DINT to each log and use FFL/FFU to track each log's position on the conveyor. There's already an incremental encoder installed on the first conveyor.

The main problem I'm trying to solve is how to program will handle log slippage and hang ups along the conveyor that will cause the track in the program to reach a bin before the actual log gets there. I'm going to use photocells to control the actual kick, but I'm afraid that if a log is far enough out of position on the conveyor the kick will be completely missed and the log will end up at the final bin where it will be to large for the equipment there.

Any ideas?
 
Hey,

Seems you'll need multiple ways to track position, maybe some kind of array sensor looking down at the conveyor surface from above? Assuming it wont get ripped off after about 30 seconds due to a log jam. If there's a chance more than one log could go through at a time, you may also run into trouble here.

If it were me I think I would probably assign each log a byte, where each bit represents a condition. From what you posted it sounds like you're sorting for 3 conditions, I guess my next question would be what happens if one log fails multiple of those conditions? Does it matter at that point which holding bin it goes to?
 
this is my observation and concerns.

how do you classify a defective log?
what is the maximum length a log can be?
if its 50 ft and you have a 55 ft log, what will happen?
can multiple logs enter the system at the same time?
if yes, how to tell when one log ends and another starts.
how much gap will be between the logs?
the incremental encoder will also be an issue.
you must figure out how to handle the roll over when the encoder goes from 1024 for example to zero and starts over. what happens if this happens multiple times?
as you said, log slippage is an issue.
you have 3 conveyors.
this can work if 1 log is on the conveyor at a time.
#1 entry conveyor
Conveyor tracking position
keep track of the conveyor movement and use a bit shift for the incremental
movement.
Log tracking
do the same for the log entering and exiting.
position verification.
have a sensor 10 ft let's say from the exit of the first conveyor to confirm when the log gets there. if the log is late, adjust the position tracking information.

repeat for conveyor's 2 and 3.

for the eject work, use a similar method.

this is a quick write up, so there are several details to be worked out.

you must also take into consideration that the conveyor chain may and probably stretch and eventually have to be shortened.
regards,
james
 
Last edited:
I used to track by bit and word shift, but now I track based on position. It's very easy in Logix where you can create UDTs. I have attached a word doc showing some example code. I basically loop through and update positions if there is something there and if I saw a pulse. If you look there is a multiplier, so each pulse is defined as a distance. It can be anything 1", 0.23" , 4mm, etc.

Each UDT element is a part/log, etc. So, you need to size the UDT based on the max number of logs you could have at one time, then add some buffer. So, if the smallest log is 8ft then figure out how many logs could be butt to butt, if that number is 25, then add a fluff and make the UDT size 30.

I can update positions both forward and backwards. So to your question, if I have a photoeye at a known position and the log isn't there, subtract position until you see it. This accounts for slippage.

It's very easy to kick based on position as well. I don't have a lot of time, so if you need more info and are interested let me know. Hopefully what I posted was self explanatory. I didn't show code on how to enter data, but I can later if you need me to.
 

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