Help In PID Control

[Joel201]

Good Day,

I have been programming for a while and a complete newbie in using a PID Control block. Now i am studying it using PC worx and with the help of the internet about the control philosophy of a PID block but i can't still figure how it affect the output value so i am here if anyone can further more help me in understanding how to use the PID block of a PLC, much better if phoenix contact,. As for my simulation i am playing with the values of KP TD TR i have now idea how to control a VFD using the real number i am getting do i convert it to an int and then scale to the range of the analog output module am i on the right track? and can someone enlighten me so that i am not just putting random values at the KP TR and TD. Thanks in advance.

Regards,
Joel

:EDIT:
just another thing my application is flow control using a VFD and Flow Transmitter.

 

[nehpets]

A very simplistic view on PID's

Kp - Proportional - Is the setpoint to which you need to achieve, depending on the process this could take a long time to be reached (technically if you want to be pedantic it is never reached). This is where the integral comes in.
Tr - Integral - Is a time constant put in to for an output boost, If the proportional is taking too long to achieve the setpoint value (the value set in Tr) then the output is boosted to get there. If this time is too long then oscillation about the setpoint could be introduced. This is where the derivative helps.
Td - Derivative, This value opposes the Integral, in effect reducing the rate of change (this control is not always required depending on the process).

As I said this is a very simplistic view of PID that I used to try to explain it to a mechanic.

Steve

 

[Joel201]

nehpets

Ok Thanks for the reply. i have just one question. In your statement am i right that the KP is the value that is needed to be reached by the process variable if so why is there a SP value in the PID block. Thx i understood the Integral and Derivative just the KP because i can't seem to get what it does.

Regards,
Joel

 

[Velociraptor]

Wikipedia has a very good primer on PID controls at http://en.wikipedia.org/wiki/PID_controller

 

[Lancie1]

I have just one question. In your statement am I right that the KP is the value that is needed to be reached by the process variable? If so why is there a SP value in the PID block?

No, Kp is NOT the target value of the Process Variable. Kp is a constant-times the-error-term or the proportional term for the Process Variable. The SP or Setpoint is indeed the target that you want the Process Variable to achieve using the combined three terms of the PID equation.

 

[nehpets]

Quite right, my apologies, I had just come off a nightshift and did'nt really absorb the question, thought it was only the three terms that were in question

Steve

 

[Joel201]

Thanks for the reply everyone. Just another question regarding the KP how do i get the value of KP you said it was a constant x error or PV how do i know that constant or do i just play with the parameter till it gives a smooth reading? thanks again

Regards,
Joel

 

[Joel201]

I have another question regarding the PID block of PC Worx. I a somewhat lost in the output being produced by the PID Block is there a range like in other PLC which has for example -32000 to 32000? when i simulate PID block in PC worx i if the initial value of the Xout in the block is 0 and the PV is < SP it just increment so little from 0.00 to a specific real number when the PV = SP. I am asking cause the Out will be the one i am scaling for the 4-20mA output of the PLC to the VFD am i right on this? Thanks again guys

 

[Lancie1]

do I just play with the parameter till it gives a smooth reading?

Yes, but I would call it "change-and-test" instead of playing. Playing implies that there is some fun involved. There are some rule-of-thumb methods for zooming in on the best Proportional value.

I am asking cause the Out will be the one i am scaling for the 4-20mA output of the PLC to the VFD am i right on this?

I don't know anything about the PC Worx brand, but the PID Output (Control Variable or CV) is the parameter that you want to scale and send to your 4-20 mA analog Output (that connects to your VFD analog input).

 

[Velociraptor]

It may take a little time to fully tune a PID loop. Before you choose your initial constants, try to understand the relative significance of a numeric change and rate of change in the process variable versus the amount of change a numeric adjustment to the output is likely to produce. If you think it through, you should have a reasonable ballpark for your initial set of control constants.

Your best tool in this process is your brain. Before every adjustment, think through the change in performance you are trying to acheive.

 

[Lancie1]

Here is a link to a PID Loop Tuning Tips Pocket Guide:

http://www.controlglobal.com/whitepapers/2005/36/

 

[Peter Nachtwey]

What kind of system are you trying to control? You don't tune a PID, you tune the closed loop system.

 

[Joel201]

Thanks everyone i have a better understanding now of how a PID works and how to tune the loop properly...

@Peter - i am using a PID for Flow control where a flow sensor will control the VFD.

I am now just confused regarding how the PID block output works cause in PC worx when i simulate the PID block ,i don't on other softwares, when my PV is not still equal the SP it just ramps up and up and never stop and it has no limit cause i get a real number equivalent to 100,000 doesn't it need to have a range for example -32,000 to 32,000 or other ranges or maybe i need to adjust the logic to limit it to a certain range thanks again guys.

Regards,
Joel