Velocity transducers are based on transforming the physical velocity (or speed) of a rotating shaft into an electrical signal. The shaft velocity v is however the derivative function of its angular position α :
It follows therefore that it is possible to measure velocity by using a position transducer and calculating the rate of change of its output variable. This leads to a classification of velocity transducers into two distinct types as shown in Table l.
The latter category, being derived from position transducers, is also based on the use of encoder disks to produce either optical, capacitive or magnetic (Hall effect) incremental transducers. The use of the potentiometer as a velocity transducer would require interfacing to a differentiator circuit to provide the position derivative; but these circuits are difficult to stabilize and their use is not recommended. The angular resolution of capacitive and magnetic encoders is poor compared with optical ones, so they do not find wide application in the field of robotics.
The optical incremental encoder and, arguably, the tachogenerator therefore remain the most popular velocity transducers in the field of machine control and robotics.