hi,

i am stuck, so hitting out everywhere.

what is the effect of increasing D term in tuning practically? i just can't distinguish the p action to the D action. when to use it?

 

I'm guessing this is also related to your previous post about flow control with a limited valve opening?

Derivative action reacts to the RATE of Change of error. (P action reacts only to the new error since the last sample). Typically D action is much more aggressive than P action and needs to be filtered separately.

If correctly set, the D Term can decrease the time taken to recover from an error or reach a new setpoint.

If incorrectly set, the D Term will increase the PID sensitivity to noise, make the loop less stable and <b>move the valve around much more</b>.

Since tuning by hand means that you're basically entering an inappropriate D Value 95% of the time, you're making things worse 95% of the time. And if you're also adjusting P & I settings at the same time things are probably not going to go well for you.

For this reason, the standard advice you'll get from most people is "D means Do Not Use" if you are tuning by hand.

And now I'm going to refer you to my previous answer

Rob
www[.]lymac.co.nz