New to PLC's, simple question about sequencing

quarter_20

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Dec 2006
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A really simple question for those who know ladder logic.
I'm using a Keyence KV PLC. I have 3 devices the plc will control the loading of. The problem is they can only be loaded one at a time, and the load request from these 3 devices can come in any order, or all at once.
I need to alternate the loading of these 3 devices so one does not load more often than the others.

I have played with latches, trying to toggle the load sequence, but seam to be confusing myself.
Any suggestions of a simple way of doing this, or where I can see a piece of ladder that performs something similar?
ANY help would be appreciated.
Jeff
 
I've done something similar - product feeding two lanes into three, aim was to keep the three lanes balanced with uneven feed.

Possible method for your program
1) basic method was a sequencer with 4 states
  • State 1 decision step - wait for a load signal and then select which bin to load
  • State 2 load bin 1
  • State 3 load bin 2
  • State 4 load bin 3
2) The smarts was a bin priority based on which bin had the least qty
So when both 'feed me' requests arrived the ladder logic loaded the bin with the least quantity

eg Maintain a int for each bin representing the number of times loaded
  • Add 1 for each bin load
  • When all int are > 1 then sub 1 from all bins (keeps the numbers small when all bins are working)
  • If any bin is > 10 (number depends on your process) then Sub 1 from any bin > 0 (this is to keep the numbers sensible if one bin goes offline for a long time)
Simple test logic for bin selection in step 1
eg if "feed 1 request" and "feed 2 request" and Bin1 < bin2 then feed bin 1
 
Jeff:

When you say "doesn't laod more often than the others" could imply that you actually must run them in a certain sequence. Never varying from say, 1-2-3, 1-2-3, etc. Is that the case?

In any event, a counter should help you solve this problem. Let's say you want to load one machine every one second (you can modify the timer to suit your actual speed needs). Add a timer with a preset of 3 and time base of 1 second. Then, the logic that causes the machines to load would examine whether the machine has requested a load, and examine the value of the timer. If both are true "Machine #2 Ready" is true AND the timer = 2, output to load machine #2 goes true. This way, even if all the requests arrive at the same scan of the program, the timer acts as the "traffic cop".

Will that work for you?

Bill
 
I need to look into the timer/counter idea.
The sequence will vary depending on how fast each device uses material.
The goal is to keep all three as close to full as posible, even if one device is using material at 4 times the rate of the other two.

The complication is that they can only load one at a time as they share a pump.

Thanks
 
Jeff:

In my first sentence I said "counter" where I meant "timer". You can set the timer for any period and preset that works for you, and make your fill process work perfectly.

I use that same principal to prevent large pumps from starting simultaneously. If the timer accumulator value is not at the proper value, the pump can't start. Just be sure to latch the rung in the "energized" condition or it will go false as soon as the timer advances.

Bill
 

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