We’ve all experimented with small “hobby motors”, or free-spinning DC motors. Have you ever tried to position something accurately with one? It can be pretty difficult. Even if you get the timing just right for starting and stopping the motor, the armature does not stop immediately. DC motors have a very gradual acceleration and deceleration curves; stabilization is slow. Adding gearing to the motor will help to reduce this problem, but overshoot is still present and will throw off the anticipated stop position. The only way to effectively use a DC motor for precise positioning is to use a servo. Servos usually implement a small DC motor, a feedback mechanism (usually a potentiometer with attached to the shaft by gearing or other means), and a control circuit which compares the position of the motor with the desired position, and moves the motor accordingly. This can get fairly complex and expensive for most hobby applications.
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