PID Troubles on a Flow controller???

Question

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
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Answer 1

Hi
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 ?
Attachments:
PID_Air_actual.vi ‏49 KB
PID_Air_Diffenrce.vi ‏54 KB