Pressure Switch?

I have a double acting solenoid, and a PLC controlled Pressure Regulator.

At this point i have the regulator controlled and i'm able to read the output signal for feedback.

What i want to do is simple, if the pressure is greater than or equal to XPSI, Turn on the Solenoid.

The problem is that it is switching back and forth wildly, even though i'm using a Latch Output.

Any hints?

Might need to redo your question. Do you have a ressure regulator or a pressure transducer? A pressure regulator controls pressure a pressure transducer measures pressure.

If you are latching the solenoid output, what are you unlatching it with?

Take a look at this thread. Maybe the same problem?

http://www.plctalk.net/qanda/showthread.php?t=56527

Clay B. said:

Might need to redo your question. Do you have a ressure regulator or a pressure transducer? A pressure regulator controls pressure a pressure transducer measures pressure.

If you are latching the solenoid output, what are you unlatching it with?

Click to expand...

This is a regulator that has both input and output signaling.

I am unlatching when the pressure is below the limit.

You need to program a deadband. For example latch at 10# unlatch at 8#.

What brand of PLC are you using?

Post your code if you can. Zip it first.

cornbread said:

Take a look at this thread. Maybe the same problem?

http://www.plctalk.net/qanda/showthread.php?t=56527

Click to expand...

I'm not having a bounce issue, i can increase the pressure well beyond the limit and still have the same problem. I'm using comparitors, GEQ and LES, both Source A's are integer and reading correctly, Source B's are the limit. The GEQ, if true latches the output, the LES, if true unlatches the output.

This is difficult for me since i'm new to it and more of a labview person.

Any help would be great, i'm sure it's something stupid...

Sounds like you need some hysteresis, or "dead band". This means the the turn on point is slightly different than the turn off point to avoid the chatter you are seeing when the PV (process variable) hovers at the setpoint. There are several ways to do this, quick and easy is to use a timer to "debounce" the input signal so that it needs to stay at a level for a certain period of time before switching. More complex would be to move a different value into your setpoint once the device switches on and move the original back once the device switches off. Hope this helps.

Mickey said:

You need to program a deadband. For example latch at 10# unlatch at 8#.

What brand of PLC are you using?

Post your code if you can. Zip it first.

Click to expand...

RSLOGIX 5000, CompactLogix PLC This is all AB stuff

Flemingtb said:

I'm not having a bounce issue, i can increase the pressure well beyond the limit and still have the same problem. I'm using comparitors, GEQ and LES, both Source A's are integer and reading correctly, Source B's are the limit. The GEQ, if true latches the output, the LES, if true unlatches the output.

This is difficult for me since i'm new to it and more of a labview person.

Any help would be great, i'm sure it's something stupid...

Click to expand...

Again post your code and someone will take a look.

View attachment PLC_TEST.zip

I hope this works

I'm not a RSLogix5000 user, so cannot open the file. Hopefully those that can will jump in a take a look.

Flemingtb said:

View attachment 18964

I hope this works

Click to expand...

Ok I see your program. I see where you latch the output labeled Sol 1. I do not see where you unlatch this solenoid.

I see where you are sending an anlog signal out ofter some math conditioning.

I see where you are reading a signal and doing some math conditioning.

Now looking at this what I expect to happen is this:

You enter your pressure value at ST_PT_PSI.

This is converted into a signal for your pressure regulator's Analog Input. (Assume all the math is for scaling)

You then read the same presure regulator's output analog signal and convert that. (not sure the reasoning behind this other than to get some scaling units)

Then you use a GRT to latch you output once your scaled feed back from the pressure regulator reaches a value greater than 12.

Now I am going to step off the reservation and make some guesses.

First guess from what you said is your unlatch is based off pressure as well (guessing it was deleted from the program you posted)

So what I believe you are seeing is cavitation from the air cylinder you are firing with the pressure regulator and this is causing you to bounce on and off (latch to unlatch) due to dramatic pressure swings.

So if I am right your problem is mechanical not controls. More than likely you have a massive pressure drop whenever that valve opens up.

Now based on my guess work here are my suggestions:

A) put in a timing filter. Basically your GRT and LES run timers that once complete turn your valve on or off.

B) put an accumlator tank after the pressure regulator that can hold a larger volume of air than the cylinder can consume.
You will still see a pressure drop but it will be slower and lower

C)Put a flow regulator on the outlet of the pressure regulator so it can not dump the air as fast thus reducing your drop in pressure.

D) Get drunk and forget about the whole thing. Take up a new hobby. (this is my favorite option)

Then again I could be in left field but with what you have said this is my best guess.

Your going ON and OFF because your solenoid latch is on the same bit of memory as some logic you have in your flip flop routine. So your output is quite literally Flip Flopping

As a side note, there are much easier ways to create a toggle (aka flip flop). Is your flip flop just for toggling a button on your touchscreen or somehting?

You have 2 different tags aliased to the same physical output:

SOL_1 and LIGHT both point to Local:3:O.Data.0

<Damian beat me to it. I had to install Logix 18>