Measuring flow rates of Two control valves using a single flow meter

[Mickey]

Thank Mickey! Now I'm getting an idea of how split-range control is implemented.. I'm trying to write my own program and will post it once I finished it. Also I was unable to open the TwoControl_split.zip file. Can you please post .pdf version of the program.

Thank you,

Ding

Sure it's only two rungs.

 

[ddeshi2]

Mickey.. can you please take a look at the attached file and let me know if I am on the right path.

 

[Mickey]

Mickey.. can you please take a look at the attached file and let me know if I am on the right path.

It's a little hard to read and the last SCP is cut-off, I can only see part of it.
Can you use RSLogix and then post the .rss FILE (ZIP IT FIRST)

I see you are trying to do the split range in the PLC, is that correct? If so, when you scale the CV for the output word it needs to be 0-8192 = 0-16383 for the low air and 8192-16383 = 0-16383 for the Large air.

I would use the PD file type for your PID instead of an integer.

I'm using my grandson'scomputer right now, so no RSLogix. Later tonite I
will do the logix and post a PDF. Do you have RSLogix500 to raed a .RSS file?

 

[ddeshi2]

Mickey, I'm using a 4-20 ma Analog output..so is it 0-16383 or 6242-31208? And I don't have RSLOGIX 500 on my laptop..so PDF FILE would be great?

Thank you,
Ding

 

[Mickey]

Mickey, I'm using a 4-20 ma Analog output..so is it 0-16383 or 6242-31208?

Thank you,
Ding

That depends on how you setup your output card. I prefer "Scaled for PID" ( 0-16383). That's what the PID instruction requires for it's PV input and gives you on it's CV output.
So don't have to use extra "SCP" instructions.

See pdf below for normal operating range data formats.

Also see pdf for sample program based on some of the addresses you provided.

 

[Mickey]

See revised sample program. Changed limit checking for output word.
This has not been tested so if you use it let us know how it goes.

 

[ddeshi2]

Mickey, Thanks for the ladder diagram. You have been a great help. As you know I'm new to this PLC programming and PID loops I have asked the same question to some of my friends and one of my friends replied like this
"Use one PID for smaller valve at startup to get needed flow keep larger valve PID at manual and 0% at this point. When you get ignition and flame is detected set smaller valve PID to manual and 100% (dont know how flame handles this, so might need ramp) and enable larger valve PID"

I'm not sure whether he was right or not...but if I had to write a ladder logic for what he said how would it be..I'm confused at PID manual/auto mode and the CV%. And Would it have two set points (one for low air valve and one for large air valve) for two PID loops or only one set point like 2 SCFM(low flow rate to get ignition) and ramp the setpoint to desired flow rate while opening the large air valve ..can you please explain.

Thank you,

Ding

 

[Mickey]

There are several ways to do this, no right or wrong just different. Does it work is what counts.

Get your friend to write his for you. Pick one and try it in the real world.

You have three to go with now your friends, split range and the inching control. Try them all pick the one that works for you.

There are others of-course but that may just confuse you more. Maybe someone else will jump in and give you more options.

Bottom line, pick one ,try it and come back with questions about why it does or doesn't work.

 

[ddeshi2]

Mickey, I have asked my friend about the PLC program and he said he's busy this whole week and couldn't help me.. so I have written the ladder logic for what he said. Can you please take a look at it and let me know whether it is right or wrong

 

[Mickey]

Mickey, I have asked my friend about the PLC program and he said he's busy this whole week and couldn't help me.. so I have written the ladder logic for what he said. Can you please take a look at it and let me know whether it is right or wrong

It's difficult to follow the hand written pdf. Write your logic with your software and repost.

You cannot write a value to a bit level location the auto/manual bit is bit level. Reread the PID instruction.
The PID "CV" is scaled 0-16383 by the PID instruction, no need to rescale it for your output card.

 

[ddeshi2]

Mickey, what should I do if I want to change the PID from AUTO to Manual mode in the middle of the process? Are you saying that I don't need ladder logic to change the mode and any HMI software will do that by pointing it to the right bit? And I don't have the software on my laptop as it's running Windows Vista and I cannot install RSLOGIX on Vista. My boss has the software on his computer but he won't let me use it.. I'll try my best to post the ladder diagram written using software.

Thank you,

Ding

 

[Mickey]

See picture below for auto/manual control. To control from your HMI write to B3:0/3 ( or whatever address you give it)

 

[ddeshi2]

Can you please explain the term "scaled for PID" ...or give me the link to an article or thread where it is explained. In all the PID examples I have seen they used the SCP instruction.

Thank you,
Ding

 

[Mickey]

Scaled for PID is 0-16383.

The PID instruction for SLC's and microligix controllers require the PV input to be scaled "Scaled for PID" ( 0-16383).
It's confidant to scale your input card data format for this if you are going to use the data in a PID instruction. The PID instruction will also scale it's CV to 0-16383.
Now input and output crads can be setup for different data formats, if they are then you can use the "SCP" instruction to rescale the data for the PID instruction and for your output cards.