PID block

we are trying to control the level in a tank using a pnumatic actuator, we can cause the actuator to open allowing the flow to enter the tank using a 4-20m amp signal from a loop calibrator which is connected to the analog input of the allen bradley Control logix5000 PLC.
I was wondering how would you send a 4-20m amp signal into the PID block without using a loop calibrator but instead using logix5000.

RS logix5000 Ladder logic language does not support PID instruction. you may need RS logix5000 FSD package

can this proess be completed using a function block diagram.

tutu said:

RS logix5000 Ladder logic language does not support PID instruction. you may need RS logix5000 FSD package

Click to expand...

RSLogix 5000 DOES indeed support a ladder logic PID instruction.

It does not, however, support the PIDE instruction.

To actually answer the asked question though, generally, you feed your analog PV signal into an analog input module, scale it appropriately, and use that value for the PV input to the PID(E) instruction.

Either scale the signal right at the analog module (on supported platforms/modules), or scale it internally with some basic y=mx+b computations.

If you need to find out how to scale an analog signal, just search here for "Analog Scaling".

RS logix5000 Ladder logic language does not support PID instruction. you may need RS logix5000 FSD package

Click to expand...

I think you're mistaking the PIDE with PID ...

the RSLogix5000 DOES support PID in its standard ladder logic instruction set ... the PIDE ("E" for "enhanced") requires the FBD editor ...

Gunther ... are you sure that you even NEED to use PID for this? ... in many cases, the only reason to maintain tank level with PID is to have "just as much coming in" as you have "going out" ... for most tank applications, that's not a real requirement ... usually we just let the tank drain until it reaches a certain "low" level - and then we crank the inlet valve wide open and start filling the tank - until it reaches a "high" level ... then we completely close the inlet valve ...

I'm not sure that I fully understand your original question ... are you asking what type of sensor to use for finding the level of material in the tank? ...

obviously my distinguished colleague rdrast types faster than I do ...

just trying to move the original question along ... is this along the lines of what you're asking for?

Thank you for your responses and the example.

We are using a pressure transmitter to measure the level in the tank.

But we are using PID because it is an instructors request for us to use it.

Any advice how to edit the INT data type for local 3:0 in the controller tags?

For PID, the control variable must be REAL or it will be forced 0.

not sure I fully understand your question - but here's my best guesses based on what I think you're asking ...



guess 1 ... depending on the software version and on how "deep you've waded in" with using the tagname in your application, you'll probably have to go offline before you can make the change ... you might also have to delete the tagname and start over ... if it won't let you delete it, then rename the tag (example: rename MYTAG to MYTAG2) ... now make a brand new MYTAG and set it up the way you want it ... then edit the program (offline is best) and change all of the references for MYTAG2 (old) to use MYTAG (new) instead ... then delete the old MYTAG2 from the tag listing ...



so ... are we having fun yet? ...



guess 2 ... are you referring to an ANALOG input from an analog module? ... if so, you might have to reconfigure the module to use “floating point” format instead of integer format ... again, you’ll probably have to go offline - and possibly delete the module and then start again ... a lot depends on what you’ve already done to use the tagnames assigned to that module ...



survival tip: when you post on the forum give a LOT of information and do NOT abbreviate ... most of the “big dogs” around here won’t even respond to your posts unless all of the information is provided ... we like to help, but we usually don’t have time to guess at what you’re talking about ... posting the .ACD project file is often a good idea ...



hope this helps ... good luck with your project ...