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Figure 1. The
mechanical structure of
a hybrid stepping motor
Figure 2. PRO2 Servo motor from
Servotronix
Figure 3. Position feedback in a closed-loop, servo system
controlled motion application, the
control system assumes that the
motor is always moving correctly.
However, when a problem is
encountered, such as jammed part
that causes the motor to stall, the
controller does not know the actual
location of the machine, causing
it to lose position. The servo’s
inherent closed-loop system holds
an advantage: should the machine
snag on an object, it will be sensed
immediately. The machine will stop
operating and never lose position.
Speed and Torque
Performance differences between
steppers and servos derive from
their dissimilar motor designs.
Stepper motors have a lot more
poles than servo motors, thus one
complete rotation of a stepper
motor requires many more current
exchanges through the windings,
causing its torque to fall off
dramatically as speed increases.
Furthermore, steppers can lose
their step synchronization if the
maximum torque is exceeded. For
these reasons, servos are preferred
for most high-speed applications.
Conversely, the stepper’s high pole
count has a beneficial effect at lower
speeds giving the stepper motor a
torque advantage over the same
size servo motor.
Heat and Energy Consumption
Open-loop stepper motors operate
with constant current and give off a
significant amount of heat. Closed-
loop control avoids the heat problem
by supplying only the current that is
demanded by the velocity loop.
Summary of Steppers versus Servos
Servo control systems are best suited
to high-speed applications that
involve dynamic load changes like
robot arms. Stepper control systems
are preferred for applications that
require low-to-medium acceleration
and high holding torque such
as 3D printers, conveyors, and
accessory axes. Because they are
less expensive, steppers are favored
as they can lower the cost of
automation systems whenever they
can be used. Motion-control systems
that require the properties of servos
must justify the higher cost of these
motors.
ALTERING PERCEPTIONS
What if the advantages of closed-
loop servo technology could
be adapted to steppers? Could
we realize the cost benefits of
steppers while achieving servo-like
performance? By adopting closed-
loop technology, steppers are able
to deliver the combined benefits
of servos and steppers in a low-
cost stepper package. Because of
their significant performance and
energy-efficiency improvements,
closed-loop steppers can replace
more expensive servos in a growing
variety of demanding applications.
24 l New-Tech Magazine Europe