Tech Briefs Magazine - June 2021 - MD-9
accuracy needed
can range from
rough to moderate
to precise. A CNC
in practice. Too
much of an apparently
machine tool needs precise rotor information,
an automobile power-window
controller can accept approximate data,
and a clothes washer or dryer will be satisfied
with only coarse information.
Closing the Loop
For sensing the rotor position or
motion, the most common options are
resolvers, optical or capacitive encoders,
and Hall-effect devices, in roughly
descending order of precision, resolution,
and cost. These sensors are very different
in their physical design, implementation,
and electrical interface, so
users must understand what is needed,
what the best choice is in a given application,
and how they will interface the sensor
to the circuitry of the controller.
Incremental encoders - used when
only relative position is needed or cost is an
issue - are typically used with AC induction
motors. In contrast, absolute encoders
- which give a different binary output at
Motion Design, June 2021
Cov
ToC
each
position, so
shaft position is ab -
solutely determined - are
often paired with permanent-magnet
brushless motors in servo applications. The
application, of course, is the primary factor
that determines whether incremental or
absolute information is needed.
Although most motor control is now
done via a digital control loop, the sensor
signal itself is either all-analog and needs
to be digitized or is a digital signal but
with voltages and other attributes that
make it incompatible with standard digital
circuitry. Although some of the feedback
sensors are offered with raw outputs
that can be tailored as preferred, many
also have conditioned, ready-to-interface
outputs that are compatible with standard
I/O ports, formats, and protocols.
Although more resolution might
seem like a good idea, it might not be so
good thing - resolution - can slow
a system by requiring extra processing of
information that is not needed or useful,
so limiting resolution to the minimum
needed is a good idea.
Resolvers
Resolvers are extremely accurate,
rugged, absolute transducers of position.
They are based on fundamental transformer
principles, with one primary
winding plus two secondary windings,
which are oriented in quadrature (90°)
with respect to each other (Figure 2).
The effective turns ratio and polarity
between the primary and secondary
windings varies, depending on the angle
of the shaft. The primary is excited with
a reference AC waveform at constant frequency,
which can range from 50 Hz/60
Hz to several hundred kHz, and the outputs
of the secondary windings will be
out of phase because of their physical
placement. The peak voltages of the secondaries
will vary as the shaft rotates and
will be proportional to the shaft angle. By
9
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Tech Briefs Magazine - June 2021
Table of Contents for the Digital Edition of Tech Briefs Magazine - June 2021
Tech Briefs Magazine - June 2021 - Intro
Tech Briefs Magazine - June 2021 - Sponsor
Tech Briefs Magazine - June 2021 - Band1
Tech Briefs Magazine - June 2021 - Band2
Tech Briefs Magazine - June 2021 - Cov1
Tech Briefs Magazine - June 2021 - Cov2
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