Tachometer generators and tachogenerators are small AC or DC generators that output a voltage in proportion to the rotational speed of a shaft. They are capable of measuring speed and direction of rotation, but not position. They convert a rotational speed into an isolated analog voltage signal that is suitable for remote indication and control applications.  Tachometer generators and tachogenerators are also used in servo systems to supply velocity or damping signals and may be mounted on or in the same housing as a servo motor. There are two basic types of tachometer generators and tachogenerators: alternating current (AC) generators and direct current (DC) generators. Both types of products develop an output voltage that is proportional to the rotational speed. In the case of DC generators, the phase or polarity depends upon the direction of rotation.