Tech Briefs Magazine - August 2022 - MD-2

Encoder Selection:
Finding the Right Fit
Key considerations to keep in mind while selecting the best encoder for your application.
E
ncoders are a diverse product category
used in applications requiring
position and/or speed feedback.
Most applications can be satisfied by a
few key selection criteria, such as measurement
range, resolution, and mounting
requirements.
For more challenging applications,
many specialty encoder technologies are
available. If your design project requires
an encoder, you'll need to choose one
that meets your specific needs. Here are
some key considerations to keep in mind
to ensure the right encoder selection.
Incremental versus absolute feedback
is the first criterion to be addressed when
selecting a rotary encoder. Incremental
encoders produce a pulsed signal as their
position changes, resulting in fast and
precise feedback. Absolute encoders are
used to produce a unique position signal
that describes the specific location of a
system within its range of motion.
For systems where only speed feedback
is necessary, incremental encoders are
2
the easy choice as they are less costly, have
a broader selection of designs, and are
simpler to implement. When position
feedback is necessary, it requires a deeper
look at the pros and cons of incremental
vs. absolute technologies.
Incremental vs. Absolute
There are five key features differentiating
incremental and absolute technologies
that must be considered, including
range, resolution, calibration, availability
and interchangeability, and cost.
Range: Incremental encoders can measure
practically infinite movement ranges
as they pulse per a fixed position change;
the maximum pulse count is dictated not
by the encoder but by the controller performing
the counting. Absolute encoders
have a fixed output
range; when exceeded,
they " roll over " to the start of a
new signal level. Single-turn encoders roll
over their output with each revolution,
and multiturn encoders store a revolution
count and include this data in addition
to the single-turn data. Some absolute
encoders require a battery to store
this multiturn data through power loss.
However, battery-less options rely on
multi-staged gear sets, each with its own
feedback tracks to retain multiturn data.
Resolution: Incremental encoders come
in a variety of pulse per revolution (PPR)
choices, with common choices ranging
from single PPR (coarse resolution) to
more than 16,000 ppr (fine). One of the
most common PPRs used in industrial
applications would be a 1024 ppr (10bit)
quadrature encoder (meaning two
pulse tracks, A and B), which gives an
approximate accuracy of +/- 0.1 degrees.
Absolute encoders commonly come
with single-turn resolutions higher than
what is seen in incremental encoders.
Common resolutions are in the 13+ bit
range, often as high as 20 or 24 bit.
This gives the ability to measure position
changes of less than 0.0001 degrees.
Calibration: Incremental encoders can
only provide pulses, so it is up to the conMotion
Design, August 2022
pisitpong2017/Shutterstock.com
Tech Briefs Magazine - August 2022

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