Pid

ianavina · Feb 28, 2014

hello
im havinf a difficult time tryong to figure out how to make a plc tell a vfd how fast or slow to run a pump based on level of a tank.

I have a micrologix 1100 ser b with 2 analog output and inputs
a 4-20ma level sensor
and im using rslogix500

so far all ive been able to do is setup up my SCP
to read my tank level

from there I want to set a specific level as a setpoint to have my pump maintain the level there either by speeding up or slowing down.

I am open to other simpler solution as well

thanks

dginbuffalo · Feb 28, 2014

Have you wrote any PID logic yet? You need to use a PID block , bring in your level as the PV (process variable) and have the PID CV (control variable) write to the analog output that will go to the VFD. Everything needs to be scaled properly as well.

ianavina · Feb 28, 2014

no ive never done a PID actually im a complete beginner to plcs in general

ianavina · Feb 28, 2014

would I input the level as the pv as the raw date coming from the sensor or the scaled number from my scp

widelto · Feb 28, 2014

example

Ianavina:

Please download and read manual 1763-rm001 from www.ab.com.
Go to chapter 19, look for PD Data File, Input Parameters, Output parameters.
Attached you will find an example I wrote some years ago, it's not for a pump/tank but can serve you as a guide.

dginbuffalo · Feb 28, 2014

I would use scaled and then the SP will correspond that to the scaling of the PV (input from tank).u can then scale the CV to match the range of your output card. I haven't used RSLogix 500 in a long time so I'm not 100% sure what functions are available in that PID block.

Note- Setting up a PID is not really where a beginner wants to start learning PLC programming...

ianavina · Feb 28, 2014

I know < I keep reading that pid can be a pain. I read something about a proportional band approach that may be simpler?

widelto · Feb 28, 2014

Proportional band is part of PID equation.

ianavina · Feb 28, 2014

so its the same thing?

widelto · Feb 28, 2014

Maybe you want to control level without PID, by using a "band".
Let's say that your band is 40%-50%, when level is higher than 50% you turn on the pump, when level is lower than 40% you turn off the pump, I'm assuming that you're pumping out of the tank.

ianavina · Feb 28, 2014

yes im pumping out of the tanks. yes a "band" might work. the program I have in place now turns the pump on at a certain level and turns it off at a lower level but we still have to manually speed up or slow the pump down. the goal is to have the pump do it on its own

dginbuffalo · Feb 28, 2014

you could do a band too. you could either use limit tests, or GRT and LES around your setpoint. You would then move a certain output to the analog output module based on how far away from setpoint it was. there's a lot of ways to skin the cat.

widelto · Feb 28, 2014

ianavina said:
yes im pumping out of the tanks. yes a "band" might work. the program I have in place now turns the pump on at a certain level and turns it off at a lower level but we still have to manually speed up or slow the pump down. the goal is to have the pump do it on its own

Though think it again, you can change your VFD speed depending on your level, in other words when level is much higher than hi limit of the band you can increase your speed to a certain value and you can lower that value as you approach lower limit of your band, it's worth a try.

Otherwise, as Mickey said, I did it too. You have to read chapter 19 of 1763-rm001 manual. not once but twice until you understand the whole thing, also look at the example I posted.

ianavina · Feb 28, 2014

I have something like that now. im using scp geq and leq to to tell when to turn the pump on and off. so the pump runs in that "band" however how do I make the pump speed up the higher the level or slow the lower the level? I might be missing something

widelto · Feb 28, 2014

You have to connect your analog output module to your VFD. Have you done that yet.
Example, use comparator when level is 60-70 speed is 80%, level 59-50 speed is 70%, when level is 49-40 speed is 50% for example. You have to play with it.

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