the drive shaft of a motor can be coupled to an output shaft by an electromagnetically activated friction clutch . the output shaft can be braked by an electromagnetically activated friction brake . the motor drive shaft rotates at constant speed . the rotary speed of the output shaft is regulated by negative - feedback action to maintain a selected value , by activating the clutch and brake alternately as necessary . the activating signals for the clutch and brake are pulse - width - modulated activating signals . the output shaft can be stopped in at least one predetermined angular position when commanded to do so . the movable parts of the brake and clutch are of minimal mass , and the speed - regulating system includes a speed feedback transducer system of very high resolution .