Tech Briefs Magazine - August 2022 - MD-5

A Comparison of Encoder
Technologies
Encoder selection consideration for
your application requires a thorough understanding
of the most common encoder
technologies. These technologies
have their own strengths and weaknesses
(Figure 1).
Optical Encoders: Multiple categories
of optical encoders are available, with
both incremental and absolute options.
However, they tend to share a few key
characteristics. They measure position
changes by detecting the existence and
absence of light from an emitter (typically
an LED or laser) as seen refracted
through or reflected from a rotating
disc with a predefined pattern of reflective
or transparent lines. There can be
multiple pattern tracks, each with its
own sensor (such as with an incremental
quadrature encoder that will have
both A and B tracks) that are out of
phase with each other by a half pulse,
creating four times the resolution and
allowing the interpretation of direction
from the feedback.
For absolute options, a unique track
with a non-repeating pattern can be
referenced from a lookup table to determine
where the shaft is located
within a revolution. Once the initial
lookup is completed after power-up,
the encoder commonly uses the incremental
track(s) to maintain faster update
rates. Note that the absolute position
is only available once movement
has begun, so these are commonly referred
to as pseudo-absolute.
Strengths: Low cost, high resolution,
small sizes.
Weaknesses: Vulnerability to contamination,
vibration, and condensation.
Magnetic Encoders: Magnetic encoders
work via a sensor or sensors, such as the
hall-effect type that passes over a series of
magnets placed along a disc. These can
have incremental and absolute varieties.
The sensors detect oscillating magnetic
field strength and interpret this into position
changes. By implementing multiple
tracks or " keyed " magnetic patterns,
absolute position can be determined by
the combination of sensor signals read.
As the magnetic field can be read without
motion, these can be truly absolute
with no motion necessary before reporting
the shaft position.
Strengths: Low cost, immunity to nonmetal
contaminants, small sizes available.
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Weaknesses: Low resolution, weak magnetic
immunity, temperature variance that skews
measurements.
Resolvers: Resolvers are a rotary feedback
technology based upon an induced
magnetic field generated by an alternating
current in one conductor. The conductor
then creates varying-strength alternating
currents in secondary conductor
windings that can be measured to
interpret the rotor position. They are
naturally
absolute
feedback
devices;
however, it's important to note they are
analog devices, so their accuracy will depend
on the analog amplifier circuit
they are connected to.
Strengths: Robust, reliable, no embedded
electronics.
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Tech Briefs Magazine - August 2022

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