Stepper motors are DC voltage motion actuators that move in discrete steps. Multiple sets of coils organized in groups called "phases" determine the motors armature position. Energizing each phase in sequence causes armature rotation one step at a time. With computer or microcontroller managed stepping, precise positioning and/or speed control is achieved. Steppers are selected by torque, steps per revolution, step angle, NEMA frame size, coil resistance, polarity and shaft features.