Strange output of the PID module

[iyearning]

Dear all, 

I meet a problem when using the PID module. I am only set P and use the PID as a linear gain. I found in some of my VIs , the output of the PID module is the opposite value, while in other VIs, the output is normal. In these case, I used to set the linearity of the PID as -1. Even though the VI can run normally in this way, I want to figure out what shall be the wrong about the PID module. Could you help me explain about the strange behavior about the PID module?

Please see the attached file, I add some probes like 24 and 25 to show the values that represent this problem.

Thanks

best

[AndyClegg]

Your set point is set to zero, your process variable appears to be 8.181 i.e. too high and error (SP-PV) is negative, so the output of your controller should be negative (-8.181 x Kp). The controller output is clamped to be between 0.5 and -0.5, and it is -0.5V, which is what you would expect if your Kp is the 0.1 one (I can't see which set of gains it is).

So it appears to be correct ?

Note: linearity should be between 0 and 1 (read the manual). Keep this set to 1 until you know things are working and only change if you need nonlinearity with error.

[iyearning]

Dear AndyClegg,

Yes, you are right! I mix the relationship between the set point value and the process variable. It is now clear to me.

Thank you very much!

best