Need help on a plc program

[Jamie Andrews]

Hello. I am working on a PLC program that needs to check input 1 after timer two is complete to see if a valve can close or not. Below is the flow chart that I'm following. Any help would be great

 

[parky]

Welcome to the forum, the information is scarce for example what platform i.e. Rockwell (type of PLC), Omron etc.
Have you got an I/O list i.e. input 0 is level sensor ? what about limits on the valve, if motorised may have feedback limits to indicate if it is open or closed (some do not just disconnect the power to motor on reaching limit mechanically.
It looks like this is homework is that correct ?.
The flowchart looks a bit iffy to me, not the way I would do it for example have you got two 2 hour timers ?, only need one.

 

[Jamie Andrews]

I am using a Siemens Logo PLC. The timers are for a valve and I need to make a program that opens the valve 12 after the pond fills and close 2 hours after it opens. But i need to have another timer for when the valve or sensor is still wet so it can stay open for another 2 hours.

 

[parky]

It looks like this is a pond refresh, i.e. after the pond if full, 12 hours later it starts to empty it perhaps a detailed explanation of the whole process will help, for example what fills the pond, is it continuous etc.
is there a level to say it is full ?, just seems a little odd.
Also why start a timer again after 2 hours, if the probe is wet it will not close the valve & go to the end there seems to be no need for the 3 timers you show just run a time for 12 hours, open the valve, when the probe has been dry for 2 hours close the valve ?
Well that's what it looks like to me.
for example:
Start.
Is probe wet yes start 12 hour timer
is timer up YES then open valve
wait until probe goes dry, start 2 hour timer
2 hour timer up close valve.
END

 

[drbitboy]

yah, here is the magic question: for the boxes that say "set X hour timer (wait X hour)," does that mean the logic flow does not leave that timer box (e.g. and check wet/dry again) until that timer expires? Or does it continually cycle back and restart the X hours timer from zero e.g. if the level probe is wet?


If the former, which is probably the case, then I think separate timers are needed (or some other chicanery).

If the latter, then a TOF may do the trick.

Either way, I think the simplest, easiest to program, debug and maintain approach is to use steps:

• Step 0
  • If dry, do nothing (stay in step 0)
    • Or assign Step = 1000 (END)
  • If wet, assign Step = 10
• Step 10
  • Start 12h timer
  • Assign Step = 20
• Step 20
  • If 12h timer expired, assign Step = 30
  • Else do nobbut (stay in step 20)
• Step 30
  • Open valve
  • ...
• Step ...
  • ...
• Step 1000 (END)
  • Assign Step = 0 (start over)

 

[plcnoop]

It looks like this is a pond refresh, i.e. after the pond if full, 12 hours later it starts to empty it perhaps a detailed explanation of the whole process will help, for example what fills the pond, is it continuous etc.
is there a level to say it is full ?, just seems a little odd.
Also why start a timer again after 2 hours, if the probe is wet it will not close the valve & go to the end there seems to be no need for the 3 timers you show just run a time for 12 hours, open the valve, when the probe has been dry for 2 hours close the valve ?
Well that's what it looks like to me.
for example:
Start.
Is probe wet yes start 12 hour timer
is timer up YES then open valve
wait until probe goes dry, start 2 hour timer
2 hour timer up close valve.
END

This is a good point. I'm not sure why they would need to continually check the sensor on that step of the program.

 

[parky]

This often happens when people design flow charts, it sounds logical but in reality not necessary, also the OP has not really given enough information.
How is the sequence initiated ?
Is it a case of something fills the pond, how is it filled, it appears this is not part of the process as such. What is the requirement for the 12 hour timer is it just for perhaps a delay before refresh/drain of the pond.
Why 2 hours to check if the probe is dry ?.
It appears that the process will keep the valve open until the probe is dry & 2 hours has elapsed but that will vary for example after the 12 hour is up if the probe is still wet then it will take 2 hours before the valve is closed (if by then the probe is dry), if after 2 hours the probe is not dry then will it start another 2 hours ? then you have another 2 hour time delay after the probe is dry.
Usually you would keep the valve open until the probe is dry, then if required have a delay timer if it needs to drain the pool for either a time to allow complete drain or at least a level below the probe.

 

[Puddle]

Any help would be great

From the flowchart, this LOGO program should do it.

 

[plcnoop]

This often happens when people design flow charts, it sounds logical but in reality not necessary, also the OP has not really given enough information.
How is the sequence initiated ?

The sequence is initiated by the level probe/float switch. As soon as it detects water filling the pond it will signal the PLC.

What is the requirement for the 12 hour timer is it just for perhaps a delay before refresh/drain of the pond.

Essentially yes. It's environmental standard from the state. They want to let the detention system fill, then drain. There will be other aspects of the overall system that take place while it's filling/draining that aren't relevant to this particular program (checking flowrate, sedimentation, etc etc)

Why 2 hours to check if the probe is dry ?.
It appears that the process will keep the valve open until the probe is dry & 2 hours has elapsed but that will vary for example after the 12 hour is up if the probe is still wet then it will take 2 hours before the valve is closed (if by then the probe is dry), if after 2 hours the probe is not dry then will it start another 2 hours ? then you have another 2 hour time delay after the probe is dry.
Usually you would keep the valve open until the probe is dry, then if required have a delay timer if it needs to drain the pool for either a time to allow complete drain or at least a level below the probe.

This is the part that's confusing me now. As someone else brought up, I don't see why the loop off to the right is necessary at all.

 

[Firejo]

Is this homework?