How to get output voltage from the tachometer output

[fadli]

I have an assignment to find out the transfer function for the dc motor. In order
to get the transfer function for the dc motor, I need to get the dc motor graph, that means output voltage versus time when we applied step input to the dc motor. The problem is, the output of dc motor is in RPM. so I need to use the tachometer to get the speed of dc motor. So, what types of tachometer that can connected to oscilloscope in order to show the output voltage which represent the RPM value. other way, how to get the output voltage for dc motor.

 

[FredGarvin]

We use simple magnetic pick ups a lot of times. We put a small wheel with interruptions or use gear teeth as the triggers. When the triggers pass the pick up it gives you a pulse and thus a speed based frequency.