Disturbance Rejection?

Greetings from Michigan, USA!

I’m using a RMC150 to cycle a valve on a part-under-test. We’ve been doing this for some time and it works good, however I want to know what I can do for better disturbance rejection. I can tune the loop manually and get the pressure trace right where I want it, cycling the valve at ~2 Hz. However, over time (usually a couple hours of cycling at ~2 Hz) as the part-under-test begins to “break in”, the trace starts drifting and usually is over-shooting on the ramp up. So then of course I have to de-tune, which typically I just bring the integral gain down some until the trace is back where I want it. After some time when the part-under-test is fully broken in and I’ve tuned the control loop to the fully broken in part, its fine and it can run like clock work for days at 2 Hz cycle rate (ramp up, ramp down, etc). What can I do to mitigate or eliminate having to adjust the tuning periodically until the part-under-test is fully broken in?? Is it a simple matter of better PID tuning? Will that take care of the “problem” or is adjusting the tuning parameters inevitable for some scenarios such as mine? Thank you.

Maybe you could use the technique in this post: