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The encoder disc interrupts the light as the encoder
shaft is rotated, and it is the code pattern etched on the disc
which is primarily responsible for the accuracy of the electrical
signal generated by the encoder. Should the disc pattern be inaccurate,
the resulting signal will reflect that inaccuracy.
BEI has been a world leader in the development
of sophisticated, accurate divided circle machines. These machines
are capable of accuracies in the sub arc second range. Originally
intended for the military and aerospace industries, this quality
is automatically incorporated into the industrial products.
The shaft
and bearings
maintain accurate rotation of the disc and help to eliminate such
errors as wobble and eccentricity which would be translated into
position errors. The encoder disc must be carefully mounted to avoid
eccentricity as the pattern is read. Such eccentricity can cause
inaccuracies in the encoder output that will not be apparent to
the user during electrical testing but will cause false position
information.
In order to eliminate eccentricity errors, BEI
has developed electronic centering fixtures capable of centering
accuracies up to 40 millionths of an inch.
When selecting an optical encoder for the industrial
environment, the following areas may be considered:
Heavy Loads
In applications utilizing gears or drive belts, excessive radial
(side) loading on the shaft can shorten bearing life. Therefore,
encoders should be specified in accordance with the anticipated
side loading. Typical maximum loads for industrial encoders are
5, 40, and 100 lbs. Ultra heavy duty encoders are available to withstand
heavier loads as well as shocks of up to 200g’s.
Corrosive or Washdown
Aluminum encoder housings with a chemical film coating (ex.: Iridite
or Alodine) finish are sufficient for most applications. However,
if the encoder is intended for operation in a corrosive environment,
a hard anodize finish with a dichromate seal should be considered.
For food or medical grade applications where a washdown may occur,
an electroless nickel coating may be required.
Temperature Extremes
The temperature specification of the selected encoder must be consistent
with the application. 0-70 degrees Celsius is the standard operating
temperature on BEI’s industrial encoders. Extended temperature
testing from -55 to +105 degrees Celsius is available.
(see Note 8)
Hazardous
Environments
Your application may require a special certification, such as explosion
proof. Testing for this certification determines that if certain
flammable gases infiltrate the encoder housing and are ignited by
the internal electronics, the resulting flame or explosion is not
able to escape from the housing and ignite the surrounding atmosphere.
Specially designed encoders are available that meet the appropriate
specification (NEMA Class 1, Group D, Division 1, and NEMA Class
2, Group E, F, & G, Division 1). Intrinsically safe encoders
are also available.
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Industrial
environments can really test the integrity of a mechanical
design. The encoders shown here have just undergone
a leak test in order to ensure that they are properly
sealed against wet environments. |
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Wet or Dirty Environments
If your application requires operation in a liquid or dusty
environment, the encoder must be selected accordingly. Adequate
sealing is a “must” to ensure against contamination
from liquids or dust, particularly through the shaft/bearing
assembly. Contaminants that infiltrate the shaft bearing can
rapidly degrade encoder performance; they may also work their
way to the encoder interior where they can disrupt the optical
components or damage the circuit board. A shaft seal is recommended
in general, and must be used in applications where liquids
are present. If liquid exposure is expected, the user should
request a leak test. |
Electrically
Noisy Environments
The increasing use of controllers and microprocessors has resulted
in industrial environments that are rich in a variety of electrical
signals that can create Electromagnetic Interference (EMI).
Some protection can be afforded by the use of shielded cable,
especially in conjunction with the use of twisted pair conductors.
When this type of cable is used with an encoder and its complements
and a differential line receiver, a significant improvement
in noise immunity can be realized. |
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