PID Troubles on a Flow controller???

[shanky1023]

Hi GenU1Chevy

Sorry had been out for long..

No i don have the PID VI s .. I am using basic codes to do the flow control feedback. right now i am just doing a proportional control from the error.. Isnt it possible to do a rudimentary PI po PD control ?? just using basic mathematical co-relations. COuld you help me on that

[Ben]

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shanky1023 wrote:

Hi GenU1Chevy

Sorry had been out for long..

No i don have the PID VI s .. I am using basic codes to do the flow control feedback. right now i am just doing a proportional control from the error.. Isnt it possible to do a rudimentary PI po PD control ?? just using basic mathematical co-relations. COuld you help me on that

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Yes LV can do that once you decide what that is.

Hint: The PID toolkit user manual documents the expresions it uses.

Ben

[shanky1023]

Hi Ben.. 

I shall look into it..

i am new to LV so culd you tell me how to start about it.. can it be programmed like a C programmeand integrated into the current VI i have ??

Message Edited by shanky1023 on 01-27-2010 03:16 PM

[shanky1023]

Hi

I finally managed to install the PID toolkit and trying to run it on my flow loop which has a  frequency output for flow and a electro-pneumatic flow control valve. as i dunno the gains in the PID i used the autotuning option and i am seeing these major problems.

1. The code does not give me an output for PID gains. it just outputs the gains i start with . this way the system doesnt actually reach steady state at all.

2. the valve keeps opening between maximum and minimum position and does not get to the desired frequency i set it to.

I am new to the PID tool kit so could anyone suggest me how i could make this loop work bettr and suggest changes in the PID ??

[Ben]

The "PID Control Toolset User Manual" explains how it works and which version do what. look for the "Auto-Tune flavor. It has an auto-tune mode that will shuttle between two set points and will guess at some PID values provided the system is linear. If the Auto-tune is still giving you trouble try a "PI" instead of a PID since noise can cause problemsn with the "D" term in real world apps.

Ben

[shanky1023]

Hi Ben

I tried doing that too.. Just the PI.. i also set the range within the maximum and minimum of the current O/P to the valve but it keeps oscillating and  even after a long time it doesnt not reach a a steady value. I am sure i followed all the instructions but the PID gains just dont update. 

[shanky1023]

Hi Mr Ben..

I realized a few mistakes after reading a bit more in detail.. I did the corrections but still the autotuning never gets completed . it keeps oscillating between the maximum and minimum.... And the PID gains output doesnt update at all..

I altered all parameters i could but no use.

Also i tried just th PI and doesnt seem to work too..

Could you please have a look at the code.. 

[Ben]

1) Change the boolean constants for "reinitialize" and "Autotune" to controls.

• Reinit when true clears the history so the PID turns into a "P" not a "PID".
• Autotune will give you numbers but you have to manually transfer them since you PID settings are a constant on the diagram. Make that a control so you can set the gains.

2) Toss the "dt" input and let the PID do that. If you pass a value it will use what you pass.

Ben

[shanky1023]

Hello Mr Ben..

I tried doing what you said. The system definitely responds to changes in initial PID constants. But it still keeps oscillating between the Maximum and minimum. The Autotuning wizrd suggests to reduce the Relay Amplitude. I reduced it to a valuse as low as 0.005 and went upto even 10. The output control just keeps oscillating between the maximum and minimum. There seems no way out.. 

you can have a look at the code. i am pretty sure i am not doing anything wrong.. Just cant to get it working..

[shanky1023]

Sorry this is the actual code..