What are your problems with tachos?
(fellow wannabie ex engineer wanting to help)
There are 2 types
3 phase with squirrel cage and drag cup.
And inductance tachos which generate a voltage from a probe near a spinning disk.
The inductance tacho are the ones used mostly these days because they develope a voltage and not a current.
Because of this they don't suffer from signal drop over distance and they also make it easy to put into a a/d converter.
When i sat Inst there were 4-5 questions on this subject so it seems to be fairly common.
MJ