How to make tachnometer

I don't understand some of what you're asking but I know of
two ways to make a "tachometer". These are chopper, and
armature voltage.

A chopper wheel with an sensor will give you pulses to count
against a timebase. Optical, magnetic are popular, choices
come from application environment (e.g. optical not good in
a grimy environment).

You can get "pretty good" speed information from sensed
armature voltage minus (winding resistance times current).
For absolute accuracy those would have to be known, and
for -real- stability the temperature as well, for winding
resistance tempco. We used to use this for speed control on
heart-lung perfusion pumps, so it's "good enough" for a lot
of things (though probably impractical to get sub-1% without
a lot of hand measurements). Quad op amp level of complexity
to get a voltage-mode speed quantity worth closing the loop
around.