In some applications it is the rotational speed of a shaft and not its position that must be controlled. A Rate Servo is shown in the diagram . The input demand signal is used to control the angular velocity of the output shaft and not its position. To make the speed of the driving motor exactly proportional to the input demand a servomechanism is essential. If a servomechanism were not used the speed of the output motor would vary with changes in the supply voltage or any changes of the friction in the motor or its load.
Note that there is no position feedback.
Movement of the speed control potentiometer produces a voltage proportional to the demanded speed. The tacho-generator provides a voltage proportional to the angular velocity of the output shaft. If there is a difference between these two signals an error voltage will be fed to the amplifier. The output of the amplifier will accelerate or decelerate the motor until the output of the tacho-generator produces a voltage exactly equal to the input demand voltage and the motor will run at the demanded speed.
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