Ser vos or Steppers?
closed-loop motors and contain built-in encoders, and posi-
tional data is frequently fed to the controller. This feedback
gives the controller more control over the motor. The control-
ler can make adjustments to operations, in varying degrees,
if something is not running the way it should be. This type
of crucial information is a benefit open-loop stepper motors
cannot offer.
Stepper motors also operate on commands sent to the
motor’s drive to dictate distance moved and velocity. Typically,
this signal is a step-and-direction command. However, open-
loop steppers cannot provide feedback to operators, so their
controls cannot properly assess a situation and make adjust-
ments to improve the motor’s operation.
For example, if a motor’s torque isn’t enough to handle the
load, the motor can stall or miss certain steps. When this hap-
pens, the target position will not be hit. With the open-loop
characteristics of the stepper motor in mind, this inaccurate
positioning will not be adequately relayed back to the control-
ler so it can make adjustments.
The servo motor seems to have clear advantages in terms of
efficiency and performance, so why would someone choose a
stepper motor? There are a couple of reasons. The most com-
mon one is price; operational budgets are important consid-
erations in making any design decision. As budgets tighten,
decisions must be made to cut unnecessary costs. Not only
does this refer to the cost of the motor itself, but routine and
emergency maintenance tends to be less expensive for step-
per motors as opposed to servos. So, if the benefits of a servo
motor will not justify its costs, a standard stepper motor may
be sufficient.
From a purely operational standpoint, stepper motors are
notably easier to use than standard servo motors. Operating a
A servo motor usually has high torque in a small package and seg-
mented laminations.MAY 2019 MACHINE DESIGN