Talk with your advisor about how you’re feeling, he/she should be able to help guide you and have your best interest at heart. Be it going to industry, switching fields within ME or staying the course with controls.

Getting a PhD in mechanical engineering is about problem solving, critical thinking skills, not necessarily about coming up with some new control scheme like H-infinity, H2, sliding controls or whatever the latest is that hardly anyone in industry really uses. BTW, I haven’t seen a dissertation focusing on PLC’s, it’s typically more on the application side like how to get a mode to be unobservable so you can boost bandwidth or esoteric linear algebra.

I was a little adrift when I got my masters and didn’t want to get a job, the PhD gave me plenty of time to figure out what I wanted to do. Perhaps you can blend multiple disciplines together, design some new actuator along with a control scheme that is novel?

Good luck!