PID motor control and error over hours running

[jimmyinCT]

 Hello.So with the help of NI, was able to make a really powerful motion control program for cyclical application of load control. The program seems to work great except that, over the course of many hours to days, there starts to be over shoot of the set point. Im only using the P term of the PID advanced VI, while I and D are 0.   The overshoot that occurs looks similar to what occurs during manual tuning when proportional gain is increased as  little over shoots of the set point start to happen except that these get larger over time.  If I shut off the program and restart it, the error in the loading goes back to normal.    If I change the I or D, to a level of say .1 or 1,10 or 100, the program starts to go wild.  Although the program has for the most part been working, I would like to get this under control.  Im not sure how the PID algorithm is really working. I have looked through the manual and im not really sure what types of error would build up over the course of say 1 cycle to 10 cycles never mind minutes or hours or day of repeated cycling.thanks

Message Edited by jimmyinCT on 05-21-2010 10:59 AM

[The_Chancenator]

Hi Jimmy,

Could you give me a little more detail about what hardware you are using?  Did you ever see this issue in Measurement and Automation Explorer when configuring those settings?  When you use just the "regular" PID.vi, do you also see this performance?  Also, would you mind posting a portion of your code so that we could take a look at it?  What we may have to do is just adjust those gains for the I and D and see if this resolves the issue.

Just to clarify, you see this when running the program, and then the overshoot error returns to normal after stopping the VI, and restarting?  

Cheers,