Please advise on this application

Hi all,
Anyone please advise on the below application.

I have 9 unit of water filter system, each filter system having two operating cycle, which is filtration and back washing. The system will start the filtration cycle for 10minute, and back washing 1 minute after filtration.
The 9 unit system is then control by one master Plc, which is allow the back washing cycle. This 9 system is operate randomly.
My question is , when the filtration of each system is done, it will request master Plc for a back washing cycle, And the master Plc need to arrange turn by turn giving a back wash cycle. Example: system 1 is in back washing, then system 5 request a back washing cycle follow by system 3 and so on randomly. So the master need to arrange by system 1 , 5 , 3 and so on! Any idea what function can I apply on this arrangement ?

I'm using Siemens S7-1200 V11 TIA pro.
Can any expert please help and advise on this?

I'm not sure if you have a similar function in the Siemens PLCs, but I would probably do this with a FIFO buffer. In AB PLCs we have the FFL and FFU functions. See this thread on this forum:

http://www.plctalk.net/qanda/showthread.php?t=45715

If the S7 doesn't have similar instructions (and i'd be surprised if it didn't) then you could probably roll your own using a chain of copy instructions and a pointer, but to be honest that's a lot more debugging to do.

Thanks Saffa.

Is there any other advise from other ??
is waiting for u all respone .

When an unit call for a wash, store the time. Compare time stamps, the first t.stamp (least number of (milli)seconds) <> 0 is due.

Kalle

Kalle,

Can u please explain in more detail ?

I don't know the TIA11.

My purpose was to sow an idea of comparing values (timestamps) instead of using a FIFO.

What is it you don't understand?

Kalle

Thanks Kalle,

i tot you mentian a function in TIA V11 which i cant found in it !
Anyway, thx for your advise!

Im looking forward and try on it!

Thanks.

I used a word shift register instruction in Step7 v 5.3 , I think it was FC90 WSR in the standard library, but then when I updated to 5.4 or 5.5 Siemens had used fc90 for some other function. (That really bugs me.)

I looked all over but I could not find WSR anywhere.

I eventually went back to my old project and copied the old FC into my new project and then into my personal library.

If you can find it, WSR works just like a FIFO.

FIFO would be one way to do it.

You could make your own FIFO with a DB or just use an array of words and shift them along manually.



Or each tank sets a ready bit when it is done and needs a backwash.

And then the system looks at each tank in sequence 1,2,3,4,5 if the bit is set, do backwash, then go to the next tank.

If the count is incremented on each scan, you wont miss anything. Scan 1-n tanks, if tank needs a backwash give it one, then continue the increment

reset increment back to zero when count is > n

Simple easy to follow.

Works in simple PLC. However, the bit method works through the tanks in series. It would be possible to have all the tanks done at the same time and then each one would have to wait for the previous tank to backwash. And it would eventually encourage the tanks to work in order of number.

If the system can only backwash one tank at a time it doesn't matter.

The same bit that requests the backwash can also be used to condition the filtration cycle.

Make sure you reset the bit after the backwash is done.