It is an engineering truism that there is no perfect solution, just the best solution for the problem at hand. That holds particularly for servo motors and stepper motors. Both are broadly used in industry. Neither is a universal solution. When properly applied, however, both stepper motor and servo motor can provide effective, reliable power for a highly successful system. The decision tree for choosing between the two has many branches but the most important are speed, acceleration, and price target.
Stepper motors consist of a rotor with permanent magnets and a stationary stator that carries the windings. When current runs through the stator windings, it generates a magnetic flux distribution that interacts with the magnetic field distribution of the rotor to apply a turning force.
Stepper motors generally are not available in frame sizes larger than NEMA 34, with most applications falling in the NEMA 17 or NEMA 23 motor sizes. As a result, it is unusual to find stepper motors capable of producing more than 1000 to 2000 ounce inches of torque.
Like stepper motors, servo motors have many implementations. Let’s consider the most common design, which incorporates a rotor with permanent magnets and a stationary stator with the windings. Here too, the current creates a magnetic field distribution that acts on the rotor to develop torque. Servo motors have significantly lower pole counts than stepper motors, however. As a result, they must be run closed-loop stepper motor.
Servo motors offer an undeniable performance advantage. In terms of repeatability, however, stepper motors can be quite competitive. This point brings up a common misconception about stepper motors, which is the myth of lost motion. As we discussed previously, the mass-spring nature of a stepper motor may result in a few lost steps. Because the drive is commanding the stepper to move to an angular location, lost steps are not carried over from rotation to rotation, however. Rotation to rotation, stepper motors are highly repeatable. Look for a more detailed discussion of this topic in a future blog post.