Tim Westcott is a great author and this is a solid article. I'll add, I've been meaning to write up my own "realities of PID", one day I'll get around to it.

Tim Westcott is a great author and this is a solid article. I'll add, I've been meaning to write up my own "realities of PID", one day I'll get around to it. What is NOT readily apparent when you look all these treatments of PIDs is what happens with the poles-zeros of your control-loop (PID) and the interplay with the system or plant. I have consistenly found all these "it's so simple, integrator removes long-term error, differentiation improves response time" ignores the fact that if you don't pay attentions to the poles-zeros of your overall response you get some very puzzling results. In a nutshell, understanding the s-domain (Bode) view of your plant and control-loop means you can usually use PID to sovle control-loop problems. But most people don't have a model of their plant (like some of the examples DannyBee has listed) which means you can't easily auto-tune a PID or most control-loops. Examining the step-response is probably the first clue to your plant, assuming you won't physically break something with that stimulus...