Stepper motor is converted to a pulse signal angular or linear displacement. 
First, the good overload. Its rotational speed independent of the load size, unlike ordinary motor, when the load will increase when the speed to be lowered, while the use of the stepper motor speed and position, there are stringent requirements. 
The second is easy to control. Is a stepper motor "steps" rotate as a unit, a digital signature obvious. 
Third, the whole structure is simple. Conventional mechanical speed and position control structure is complicated, difficult to adjust, after the stepping motor, so that the whole structure becomes simple and compact. The rotation speed of the motor speed is converted into a voltage, and is transmitted as a feedback signal to the input terminal. An auxiliary motor for the speed motors, the motor speed is mounted at the end of an ordinary DC motor, the voltage generated by the DC power back to the motor speed, DC motor speed control to achieve the purpose.