period measurement for calculating acceleration - high speed counter resolution

[Peter Nachtwey]

Peter, wouldn't that define the phase of the signals and not the frequency deviation?

Yes, if you want speed control that would be much better than simply updating every time a zero crossing is detected. One can still do the zero crossing calculation too. The frequency due to zero crossing updates only every 16.666ms. That may be fast enough for this application but one must seriously look at all the other delays.

Or is the idea that you assume you can stay within a given phase range and the control will ultimately settle on an output that stabilizes phase error?
Keith

Yes, and therefore control the speed.