topic Re: PID Troubles on a Flow controller??? in LabVIEW

PID Troubles on a Flow controller???

GenU1Chevy — Tue, 13 May 2008 18:56:54 GMT

Hello,

I'm having trouble using PID control on an flow valve that requires a 0-12VDC power source. My setup includes an Agilent 34970A with a mulitfunctional module to output power to a Kepco power supply which ouputs power to the flow controller. The only way I can get PID to work is if I use a very small Gain such as (0.2), an Integral Time of (-.49) and a Derivative Time of (.005). It seems to dial in the flow meter within 2-3 minutes but it doesn’t seem right to put a negative value in for the Integral Time though?? I'm wondering if it has something to do with the hysteresis of the flow meter. Have you guys heard of anything like this happening with PID on a control valve? Please let me know.

Thanks, Jeff

Flow controller manf: Teknocraft~Standard proportional control valve.

http://www.teknocraft.com/pdfs/proporvalve1.pdf

Re: PID Troubles on a Flow controller???

Nate.H — Wed, 14 May 2008 23:31:46 GMT

Dear Jeff,

I agree that a negative value for the integral time does not make much sense. I don't think this would result from the hysteresis of the flow meter, but then again, I don't have much experience with flow meters. Does your system behave as expected when just performing proportional control? I would get the system tuned using proportional control only, then add in the integral gain and finally the derivative gain. You could also follow a tuning method such as the Ziegler-Nichols Method to see if that would give you valid and more sensible gains for the system.

Please let me know how it goes in trying the above suggestions.

Best Regards,

~Nate

Re: PID Troubles on a Flow controller???

siva0182 — Fri, 08 Aug 2008 07:23:54 GMT

HI i would also like to use labview to control a solenoid valve using a HP data acquistion system (HP 34970A)+ multifunctional module.

Is it possible to do temperature measurement also using multifunctional module?

Are the drivers common for all modules?

Is it possible to read data from one module and set the output of a multifunctional module accordingly?

Re: PID Troubles on a Flow controller???

siva0182 — Fri, 08 Aug 2008 08:18:04 GMT

Alternatively Is it possible to send an output signal from labview to a digital to analog card/cable and then suitably control a solenoid valve.?

Re: PID Troubles on a Flow controller???

Mathan — Fri, 08 Aug 2008 08:25:22 GMT

Re: PID Troubles on a Flow controller???

siva0182 — Fri, 08 Aug 2008 08:46:49 GMT

Thanks for that link.I did search thro the results ..but most of them use some kind of DAQ card or so.
I am more interested in using a multifunction module or a digital to analog cable..

regards
Sivanand Somasundaram

Re: PID Troubles on a Flow controller???

muks — Fri, 08 Aug 2008 12:34:21 GMT

Hi sivanand somasundaram,
Have you checked this

Re: PID Troubles on a Flow controller???

RT4CY — Mon, 11 Aug 2008 14:14:42 GMT

Hello,

The driver for the HP 34970A can be found in our Instrument Driver Network at www.ni.com/idnet. Here is the link for the driver. In addition to the forum post linked by muks, check out the following Developer Zone.

NI Developer Zone Tutorial: Complete resources for a PID control system with NI M Series devices

FYI, you will communicate with the HP 34970A via GPIB or Serial communication. Thus, the underlying driver will be either 488.2 or VISA (respectively). If you decide to go with a NI M-Series device, the driver used will be NI-DAQmx.

Re: PID Troubles on a Flow controller???

shanky1023 — Mon, 18 Jan 2010 19:01:42 GMT

I am trying to write a simple code on Having a Loop control on a flowmeter. I wrote 2 codes one involving a P control and one Involving a Differnce in the set Point to the Actual output.

The idea is to control a flowmeter based on the pressure of flow . The Pressure measurement gives a Voltage output and the valve need a Current input so i need to control flow using desired Voltage output. But my code with the P control doesn not function as desired. ANd the code which uses just a differnce in a loop seems to work fine but for the fact that it never touches the desired value . it keeps oscilatting around the desired set point.

Could anyone help me set right the P Control ?

Re: PID Troubles on a Flow controller???

GenU1Chevy — Mon, 18 Jan 2010 19:48:32 GMT

Hi shanky1023 ,

Do you have the PID toolkit? It might make it easier for you to write the code. I don't have your subvi's or DAQmx vi's but the PID toolkit has all the PID subvi's to control a flowmeter. I would look into that. Let me know if you have any questions.

Re: PID Troubles on a Flow controller???

shanky1023 — Wed, 27 Jan 2010 18:08:38 GMT

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

Re: PID Troubles on a Flow controller???

Ben — Wed, 27 Jan 2010 21:00:32 GMT

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

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

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

Ben

Re: PID Troubles on a Flow controller???

shanky1023 — Wed, 27 Jan 2010 21:16:02 GMT

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

Re: PID Troubles on a Flow controller???

shanky1023 — Wed, 24 Mar 2010 18:00:33 GMT

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 ??

Re: PID Troubles on a Flow controller???

Ben — Wed, 24 Mar 2010 18:09:57 GMT

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

Re: PID Troubles on a Flow controller???

shanky1023 — Wed, 24 Mar 2010 18:37:35 GMT

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.

Re: PID Troubles on a Flow controller???

shanky1023 — Wed, 24 Mar 2010 22:24:50 GMT

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..

Re: PID Troubles on a Flow controller???

Ben — Thu, 25 Mar 2010 12:21:58 GMT

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

Re: PID Troubles on a Flow controller???

shanky1023 — Fri, 26 Mar 2010 21:45:03 GMT

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..

Re: PID Troubles on a Flow controller???

shanky1023 — Sat, 27 Mar 2010 00:07:10 GMT

Sorry this is the actual code..