Now that is a cool routine! I am looking forward to experimenting with it.

I have seen filtering applied to the PID output signal before. I'm not quite sure why except for maybe any minimal noise present in the error signal gets amplified in the PID gain block becoming a significant noise issue.

What I usually see is filtering on the Process Variable to remove noise signals that might interfere with the Derivative action. This type of noise is electrical in nature and can be caused by RFI, EMI, PWS noise, etc. It is picked up by long lead lengths and poor shielding between the sensor transmitter and controller.

And, now that I think about it, an analog filter would be preferred to target just the electrical noise thereby minimizing any filter delay that might impair control performance.

So now, I think that both the digital filter on the Controller Output and the analog filter on the Sensor Output is great way to harden the PID Controller against noise when used in harsh or noisy environments.

Thank you for the extraordinary research and help in jump-starting my brain cells!

Jacob