Ser vos or Steppers?stepper motor is much simpler to under-
stand and easier to configure. Most per-
sonnel would agree that if there is no rea-
son to over-complicate operations, keep
things simple.
The advantages offered by the two
different motor types are very differ-
ent. Servo motors are ideal if you need
a motor with speeds over 3,000 rpm and
high torque. However, for an applica-
tion that requires only speeds of a few
hundred rpm or less, a servo motor is
not always the best choice. Servo motors
may be overkill for low-speed applica-
tions.
Low-speed applications are where
stepper motors shine as the best pos-
sible solution. Stepper motors are not
only repeatable when it comes to stop-
ping, but are also designed to run at a
low speed while providing high torque.
By the very nature of this design, stepper
motors can be controlled and run up to
their velocity limits. The velocity limit of
typical stepper motors is usually under
1,000 rpm, whereas servo motors can
have rated speeds up to 3,000 rpm and
higher—sometimes even in excess of
7,000 rpm.
If a stepper is sized correctly, it can
be the perfect choice. However, when a
stepper motor is running on an open-
loop configuration and something goes
wrong, operators may not get all of the
data they need to fix the problem.SOLVING THE OPEN-LOOP
PROBLEM
Over the past few decades, there
have been several different approaches
offered to solve the traditional problems
with open-loop steppers. Homing the
motor to a sensor on power-up, or even
several times during an application,
was one method. Although simple, this
slows operations and doesn’t capture
problems that arise during the normal
operating processes.Adding feedback to detect if the motor
is stalling or out of position is another
approach. Engineers at motion control
companies created “stall detection” and
“position maintenance” features. There
have even been a few approaches going
even further that treat stepper motors
much like servos, or at least mimic them
with fancy algorithms.
In the grand spectrum of motors—in
between servos and open-loop stepper
motors—lies a somewhat new technol-
ogy known as the closed-loop stepper
motor. It is the best and most cost-con-
scious way to solve the problem of appli-
cations that require positional accuracy
and low speeds. By applying high-reso-
lution feedback devices to close the loop,
engineers can enjoy the “best of both
w o r l d s .”
Closed-loop stepper motors offer all
the advantages of stepper motors: ease
of use, simplicity, and the ability to run
consistently in low speeds with accu-ONLY AMETEK’S 958 SERIES LDT RESISTS 1,000 G’S.
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50 MAY 2019 MACHINE DESIGN