Stepper motor is controlled by DC pulse , generally its power is small , it suits to be used in open-loop automatic control system , in which the accuracy is not requested so strict. One of its weaknesses is out of step easily. Servo motors are divided into two categories AC and DC, which both bigger in power and higher in precision. The main difference between the two is to see that if there is an encoder at the end of motor. Servo motors return position signals by photoelectric encoder. Incidentally, closed-loop control is be split into semi closed loop and whole closed loop, but semi-closed loop device is generally used, only the very precise equipment will use the whole closed loop. |