In applications requiring absolute accuracy in the speed control of dc servo motors, there's no substitute for the traditional tachometer-based feedback loop. But for somewhat less demanding situations, adequate accuracy often can be achieved without the complication and expense of a tach. This can be done by taking advantage of the built-in electromechanical constants of the motor itself. For example, the fact that every permanent-magnet dc motor exhibits a stable relationship between rpm and armature back-EMF (electromotive force) implies that a reasonable job of constant-speed operation can be accomplished merely by driving the motor from a well-regulated voltage supply.
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