Velocity SensorsA most important device that is used to provide velocity feedback is thetachometer. It is also known asrpm gauge, andrevolution counter. A tachometer is employed in a motor to calculate the rotational speed of a shaft. The output is displayed as RPM (revolution per minute) in an analog device. Nowadays, the use of digital displays has also been increased to a larger extent.The two common types of a tachometer are: Permanent magnet AC tachometer DC tachometerPermanent magnet AC tachometer:The construction of an AC tachometer is almost same as the brushless DC motor and stepper motor. It possesses primary and secondary stators with fixed windings, and a rotor withpermanent magnet. If the rotor is stationary, a constant output voltage will be obtained. If the rotor is moving,proportional to the speedof a rotor is induced.This type of tachometer cannot provide information of direction with only one output winding. However, it can be done by having additional winding in the quadrature.DC tachometer:DC tachometer is themost commonlyused instrument in the robotics. It is aDC generatorimplemented to provide an output voltage that is proportional to the angular velocity of the armature. In this mechanism, the rotor and rotational part will be attached directly. It has a stationary device called ascommutator, which is connected with the split slip rings. It is used for picking the induced output signal from the rotating coil.Sometimes, the use of commutator will lead to the production ofsmall ripplesin the output voltage. Most importantly, it is impossible to filter out as well. In this condition, theAC tachometerwill be the best one because it does not have such problems.