By the way, we’ve covered a PID library for Arduino before. While this post talks about temperature, PID control is used for everything from flight control to levitation.

Mruthunjaya U. Dhariwal, H. C. Dhariwal, Investigation of Control of Speed of Diesel Engines Using PID Governor, SAE Transactions, Vol. 109, SECTION 3: JOURNAL OF ENGINES (2000), pp. 222-237 ...