Stepper Motor

AC Servo Motor

Resolution and Accuracy

Since the stepper motor is open loop, it does not necessarily achieve the desired location, especially under load.  Particularly poor positional accuracy can result when using micro stepping, which is mostly useful for smoothness of motion. 

Servo Motor resolution depends upon the encoder used. Since servos are close loop, they can and do achieve the resolution available and they are able to maintain positional accuracy.

High Speed and Power

Steppers have very poor torque characteristics at higher speeds.  This condition is improved only slightly by micro stepping, however,  unless the motor is used in a close loop mode, it does not usually perform as well as a servo. Once the stepper is used in a closed loop mode, it usually becomes more expensive than the servo system of comparable size.

Servos can produce speeds and powers two to four times that of similarly sized steppers. This improvement is a direct result of the closed loop (i.e., constant position feedback), which allows for higher speed and greater reliability.  The closed loop nature of the servo also allows such a system to better utilize peak torque capabilities.

Open loop compare to close loop

Steppers can be made to do 3D carving applications but because of the drop in torque at high speeds, they usually have to move slower than servo motors to make sure the motor does not stall or miss steps.

Servos can perform high-speed continuous motion much more reliably, making them much better than steppers in three-dimensional contouring applications.  We have found time reductions of up to 80% on some applications.  The continuous motion also results in better finish quality.  In addition, the servo's reliable high-speed continuous motion can reduce the possibility of scorching and melting when working with woods and plastics.