A small niobium magnet attached (tape works) to the turbine/generator shaft creates a narrow pulse at pin 2 of the Arduino for each revolution. The Arduino program checks the time between pulses, calculates the RPM, and makes the AC motor that operates the butterfly valve run in a direction so as to keep the RPM constant. There are some fancy PID (Proportional Integral Differential) loops programmed in to keep things stable. Here is the diagram.
I could not upload a more readable PDF here but I can email it to you if you drop me a line.
I think I posted the Arduino program elsewhere on this blog. This link will get you to a simplified schematic of the hall sensor / Arduino hookup. http://schematics.com/editor/hall-sens-rpm-control-16718/ I'm still trying to use their schematic editor on a Mac but it is frustratingly windows centric, not intuitive -- yet, but I'll keep at it, so expect more clarity.