The way of measuring motor speed is to use an encoder to convert rotatory movement into a pulse train. The frequency of the pulse train will allow us to know the motor speed once the number of pulses/revolution of the encoder is known.
The best way to measure the pulse train frequency is to use a counter. You can use a multifunction data acquisition board (NI 60xx->2 counters per board) or an specific counter board (NI 66XX->4/8 counters per board).
The basic idea is to continuously count the number of rising or falling edges of a know frequency timebase for a single pulse of the signal comming from the encoder.
Have a look at the following URL.
http://zone.ni.com/devzone/conceptd.nsf/webmain/CD2C8AAED8FA59A486256C230078FE75?opendocument
It has some info th
at could help you.
If you search on NI support web page (http://www.ni.com/support/) with keywords such as "measure frequency train" you will find several examples and documents that will be useful to you as well.
Hope it helps.
César Verdejo
Training and Certification | National Instruments