Noise in PID control output at 100% even when constants used as inputs

[loves2]

I am using the PID toolkit to control valve movement in a system.  We are having an issue with large output fluctuations (~4%) when the valve is controlled completely open, PID output 100%.  This does not occur when the valve is controlled completely closed, PID output 0%.  We thought it might be caused by noise in the SP or PV signal but it still occurs even when constants are used for these variables.  The output range for the PID.vi is set to 0-100%.  Has anyone seen this before and if so how do I prevent it from occurring?  Thanks,

[Brian_Beal]

Are you saying that the PID output is going between 96% and 100% when you think it should be 100%?  Are you saying the PID output is at 100%, but some other output(DAQ?) oscillates?  What hardware is in your system?

[loves2]

The PID output is going between 96% and 100% when I think it should be 100%.  For example if my setpoint is 5gpm and the process variable is 3gpm I would expect the the valve to open to allow more flow.  If I set the SP as a constant 5gpm and the PV as a constant 3gpm, I would expect the PID to move to 100% and stay there.  But it fluctuates rather than remains steady.  We are using a cRIO to output 4-20mA signal to several control valves. 

I know that my other post is mostly a repeat.  Our internet timed out and I thought I lost my original message.  I have not seen a way to remove the duplicate.  If you know, please tell me.  Thanks,

[Brian_Beal]

It sounds just like a matter of tuning.  If you are getting oscillations, then reduce the proportional gain.

Don't know of a way to delete a post, but you can edit and just delete the body.

[nathand]

Can you attach your code, or a simplified version that demonstrates the behavior you're seeing including tuning parameters?