Controller PID function T1 time constant definition?

[ProtoGuy]

According to the (very minimal) documentation:
"T1 is the time constant for the exponential filter on the output of the derivative component."
Can anyone explain this better? Is this a low-pass filter? What are the units?

I like using GUIDE, but the documentation should be more in depth, especially for the one and only PID function - it's pretty important. I notice that the Component documentation includes examples, but the Function Block Library seems to be lacking examples.

[Richard]

Hello Protoguy,

I haven't checked recent documentation for this but from my memory of years back I remember being told that the value of T1 on exponentional filters is the time in mS for the output to reach 60% of the input change.  i.e if the input to the filter (which in this case is the output from the D only of the PID) goes from 0 to 2000 in an instant and T1 was set to 500mS then 500mS later the out put would be at 1200.

If your controller only uses P and I then I believe this has no effect anyway.

Richard

[ProtoGuy]

Ok, so the filter in question is only filtering the D output to prevent instability from noise in the feedback signal, for example, right?

It's very interesting that it is so short on documentation, considering the universal application of PID and the potential for trouble. I certainly would not have wanted to filter the entire PID output like that.

Thanks!

[Richard]

That's right, and any other filtering on inputs and outputs you do independantly on the PID controller.  There is some explanation inside the PID page too.  I agree with you that some examples wouldn't go amiss in many cases.