Tech Briefs Magazine - November 2021 - 51

Mechanical & Fluid Systems
DC Drives as Replacements for AC Servos
Low-voltage DC stepper and servo drives offer space-saving options for applications
traditionally handled by AC servo drives.
Siemens Industry, Norcross, Georgia
lternating current (AC)-driven servo
systems are no longer the only reliable
option for industrial motion control,
with advances in direct current (DC) stepper
and servo motors and drives providing
robust alternatives. While many traditional
low-voltage DC drives cannot handle the
rigor of hostile plant environments, modern
high-performance drives are changing
this situation for the better.
A
Before comparing AC to DC systems,
there are several important differences be -
tween steppers and servos. The application
gap is closing quickly between stepper
and servo motors and drives, as advances
in stepper drive accuracy are greatly in -
creasing their usability. However, key performance
differences between the two still
rule out the use of steppers in the most
demanding applications.
Most notably, steppers provide high
torque at low speed but this decreases substantially
as speed increases. Servos, on the
other hand, maintain consistent torque
across the motor-speed spectrum, making
them excellent for executing safety functions
such as safety-limited torque (SLT)
and speed (SLS) with active braking.
Steppers provide means for safe-torque
off (STO) safety control and they can typically
be run open-loop, meaning no
encoder or resolver is required. Servos
deliver higher speed, acceleration, and
accuracy than steppers but they cost
more and require an encoder for precise
positioning feedback.
Though technically discrete due to
their incremental movements with each
consistent pulse, the 50 to 100 poles per
revolution of modern steppers approach
the performance of analog continuous
positioning and speed. This makes steppers
a viable alternative to servos in
many applications.
While medium- to high-voltage AC variable
frequency drives and servo drives fulfill
many needs on a typical plant floor,
AC power is sometimes unavailable or
unacceptable for certain applications.
Most notably, this includes battery-driven
and human-contact applications, such as
Tech Briefs, November 2021
Cov
Figure 1. A magnetic resonance imaging facility deployed a low-voltage F-TM StepDrive and an S7 PLC for
motion control to ensure patient safety throughout the imaging process. (Photo courtesy of Siemens)
magnetic resonance imaging (Figure 1),
using robots or cobots.
Additionally, modern low-voltage DC
drives (Figure 2) can enhance other applications
because they are more compact
and efficient than traditional AC systems,
provide fast motor acceleration and deceleration,
and are typically less complicated
to configure. Unlike consumer-grade
drives, they are industrially hardened to
withstand extreme temperatures, vibration,
and electrical noise.
When used in safety systems, engineers
can take advantage of low-voltage stepper
drives' hardwired STO functionality to
directly cut torque- or force-producing
energy from being supplied to the motor.
DC servo drives are safety-ready with configurable
SLT and SLS. These drives provide
a motor data collection interface,
enabling compatible controllers to
process and act upon motor status as
configured in the programming environment.
A PLC communicating with the
drive controls motor motion through software-embedded
technology objects for
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ToC
Figure 2. The Siemens F-TM StepDrive offers configurable
control and can be safely deployed in
hostile plant environments, human safety
applications, and other scenarios. (Photo courtesy
of Siemens)
51

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Tech Briefs Magazine - November 2021

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