What are the basic principles to measure dc motor speed ?

[Albert.Geven]

Normally people mount an encoder to the motor and measure the pulses sometimes referred to as s0-s90 (for quadrature systems)

count the upd/down pulses and voila you have a distance, the first derivative is the speed.

[JMA_eng]

Alternately, you can use a feedback potentiometer (assuming motor attached to a lever arm or similar).  A feedback pot is much cheaper and easier to deal with than an encoder (only need excitation source and AD).  If you are going to measure motor speed, you might also want to monitor motor current and or back EMF.

Cheers,

JMA 

[Henrik_Volkers]

1  something that provide n>=1 puls per turn, count the pulses during a fixed time or the time between two or more pulses

2 stroboscope (a nice manual way to do it)  

3 AC part of the DC current

4 accelerometer, microphone

3,4 tricky to select the right frequency that represents (a known multiple of) the speed

[Henrik_Volkers]

5 dynamometer