[Thorondor]

In my experience, there are a lot of applications that are "trivial" enough that PID works fine. I'm reasonably comfortable with state space modeling for systems with more variables. But OK, I'll bite - always open to picking up something new. What resources would you suggest for someone who wants to learn more about nonlinear controls?

[crispisulcans]

Without knowing any specifics I would say the most universally useful tool to have in your nonlinear controls belt is Lyapunov control design and its extensions (if you're familiar with Lyapunov equations in linear systems that's where the connection starts). It leads to useful methods that are applicable to many systems, and allows you to handle some particularly tricky situations. Wikipedia's articles on these topics are surprisingly decent: https://en.wikipedia.org/wiki/Control-Lyapunov_function https://en.wikipedia.org/wiki/Backstepping as well as Stanford's lecture notes: https://web.stanford.edu/class/ee363/lectures/lyap.pdf Check out Robust Nonlinear Control Design by Freeman and Kokotovic for more on this.

Compared to linear control theory, nonlinear controls is much more fractured and domain-specific. No concept is as widely applicable as ones from linear controls. Khalil's Nonlinear Systems book is generally considered a top reference, but it does start to get into the PhD-level stuff and I don't recommend it to a non-specialist unless you are really into this stuff. I can give more specific recommendations if you have a particular industry or application in mind.