Product Feature
Introduction
These comparative features become even more critical
at resolutions above 1,200 CPR, since, in order to
achieve these higher resolutions, nearly every manufacturer
uses a glass disk. Many of the readers of this Bulletin
may have heard other encoder manufacturers criticize
glass disks, calling them unreliable, fragile, etc.
While this may be true for their products, it is certainly
not true for EPC’s, even in applications with
higher levels of shock and vibration. As you will
see, EPC has pioneered a number of innovations that
solve this problem in all its products, but especially
in its Model 725I. This comparison will also highlight
those features.
The Other
Guys
The typical construction for a Size 25 encoder is
shown below. Note the narrow air gap, especially at
higher resolutions, and the size and thinness of the
disk. In an encoder, the disk is fastened to the rotating
hub, while the sensor is mounted to a fixed printed
circuit board (PCB). The disk rotating past the sensor
produces the signals that become the square wave output.
The air gap is adjusted to “calibrate”
the encoder and produce the optimal square wave.
As you can see,
because the air gap is narrow, and due to the fact
that there is a single set of bearings, it does not
take a lot of side loading to move the shaft in such
a way that the attached disk will make contact with
the sensor or some other portion of the PCB. This
side loading can be in the form of a force generated
by installation requirements, excess vibration, shock,
or other conditions. The result is damage to the disk
or sensor, affecting reliability, or, in extreme cases,
breakage of the disk.
Also note that
the thin (0.030" thick) disk is only supported over
15% of its total surface area. These factors combine
to increase the likelihood of fracture even if the
contact is only momentary, as might happen with a
sudden shock.
Better
- The 725N
The 725N was designed to address many of these problems,
without affecting the price of the encoder. The first
step was a simple one – make the glass thicker
– in fact, more than twice as thick, dramatically
reducing the chance of breakage. And our proprietary
method of disk production reduces the stress and chips
in the glass increasing long term reliability. The
next step was to make the disk smaller, minimizing
the amount of deflection that can occur in the presence
of side loading. Also, the smaller disk is supported
over 30% of its surface, making it much more stable
in the presence of shock and vibration. The last step
was to increase the air gap, which means that the
edge of the disk has to travel much further before
it even has a chance of contacting the sensor. The
combination of these factors makes the 725N much more
reliable than competitive designs.
Best - The 725I
The 725I (the “I” stands for industrial)
uses the 725N as a foundation to build an encoder that
is as robust as possible within its price category.
Using EPC’s innovative “encoder-within-an-encoder”
design, the 725I adds two extra, heavy-duty bearings
to the two contained within the internal encoder for
a total of four bearings! These two extra bearing sets
are separated in such a way that side load stresses
become isolated between the two bearing sets and never
reach the encoder. In addition, the internal encoder
is mounted to the 725I’s housing using EPC’s
pioneering flex mount, further isolating the internal
optics and electronics from damaging shock, vibration,
or shaft loading.
By examining all the design features, and looking at
them as a whole, it is obvious why the 725I is the industrial
encoder of choice.
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