Buzzen
Member
is it necessary to scale a process variable before the pid loop. I will be using a flow meter with a 4-20ma output to control a vfd.
scp for pid
- Thread starter Buzzen
- Start date
Generally, if you want, you don't have to. I do, because it make it easier to keep track of everything. Your SPs for instance will be to be scaled to something the PID understands.Ken Roach
Lifetime Supporting Member + Moderator
What controller are you using ? Because you mentioned the "SCP" instruction it sounds like an Allen-Bradley SLC-500 or MicroLogix controller.
Many A-B analog input modules have the option to represent the full range of the channel as a value between 0 and 16383. Usually this is called "Scaled for PID" mode because that's the numeric span used by the PID equation for the Setpoint and the Process Variable.
So the second question is "what input module are you using" ?
Many A-B analog input modules have the option to represent the full range of the channel as a value between 0 and 16383. Usually this is called "Scaled for PID" mode because that's the numeric span used by the PID equation for the Setpoint and the Process Variable.
So the second question is "what input module are you using" ?
If you get an Out-of-Range errror, most likely your values are too large for the PID instruction (larger number than that instruction can handle).
Which option did you choose for your Analog Input module configuration, PID or Engineering units?
Here is an example to set up an analog input module for PID Scaling. Notice that even though this is an Input module, you "configure" it by writing to an Output word for the Slot that the module is plugged into.
Which option did you choose for your Analog Input module configuration, PID or Engineering units?
Here is an example to set up an analog input module for PID Scaling. Notice that even though this is an Input module, you "configure" it by writing to an Output word for the Slot that the module is plugged into.
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????
I do not understand the question.
The analog module configuration 16-bit word value is determined by the options that you choose (see manual).
The PID Auto or Manual mode is set by a bit value in one of the PID Setup words, controlled by your program.
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Okay,
I was wondering if the processor adds the logic to the program itself like the first pass bit. I have a rung that has a copy file addressed to Source #N9:0
Dest. #O:11.0 that is associated with the input card initialization. I didn't clarify very well my apologies. My question is when you pick options under the IO configuration of the program click on slot 11, click on configure, click the channel you want. When you do this will it add the necessary logic to the program? Like if I were to choose scale for PID>
I was wondering if the processor adds the logic to the program itself like the first pass bit. I have a rung that has a copy file addressed to Source #N9:0
Dest. #O:11.0 that is associated with the input card initialization. I didn't clarify very well my apologies. My question is when you pick options under the IO configuration of the program click on slot 11, click on configure, click the channel you want. When you do this will it add the necessary logic to the program? Like if I were to choose scale for PID>
Will RSLogix add "necessary logic"? Well, I have never been able to get it to add anything. I always have to do it the hard way, adding logic as needed to do the job. Maybe someone knows a short cut.
Here is an example of scaling a temperature input (I:16.1) to the range of 0-16383 for use in a PID block Process Variable (N9:51). I also had to scale the Control Variable N9:52 from 0-100% to 0-16383, on Rung 19 right above the picture.
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Wow awesome,
thanks for the professional courtesy there I really appreciate it that will help with my scaling for my HMI I am programming also. The problem that I had was the light kept blinking. What was going on I had only one ma signal going to only one channel. I had three other channels enabled that were associated with that light. I disabled the other channels and now no blinking light.....embarrassing!
thanks for the professional courtesy there I really appreciate it that will help with my scaling for my HMI I am programming also. The problem that I had was the light kept blinking. What was going on I had only one ma signal going to only one channel. I had three other channels enabled that were associated with that light. I disabled the other channels and now no blinking light.....embarrassing!
Here is an example of scaling a PID output so that it is the proper range for the analog output module. O:13.0 is a 4-20 mA output to a VFD motor drive.
The CV from a PID is in the range of 0 to 16383 (represents 0 to 100%). You can change it, in the PID Setup Screen, to be anything from 0 up to 16383. The problem is that an analog output module uses a different range, so the PID output must be scaled.
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