Cut Sheet

V8-T12-32 Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Contrast
Contrast measures the
ability of a photoelectric
sensor to detect an object.
The contrast of a sensor is a
ratio of the excess gain under
light conditions to the excess
gain under dark conditions. A
ratio of 10:1 is desired.
Contrast is important when
a sensor has to detect
semi-transparent objects or
extremely small objects.
Each mode handles contrast
differently.
Thru-Beam and Reflex
These modes are affected by:
●
Light transmissivity of an
object or surface
●
Size of an object in relation
to the beam size
Diffuse
This mode is affected by:
●
Distance of the object or
surface from the sensor
●
Color or material of the
object or surface
●
Size of the object or
surface
The ideal application provides
infinite contrast ratio of the
detection event. This is the
case when 100% of the
beam is blocked in reflex or
thru-beam modes. For diffuse
sensing, this occurs when
nothing is present.
Understanding the contrast
ratio is critical when this
situation does not exist, such
as when detecting
semitransparent objects. In
some cases, it might be
necessary to use a special
low-contrast sensor designed
for these applications, like a
clear object detector version.
In the Workplace
A thru-beam pair is positioned ten inches apart to detect a
semi-transparent plastic bottle moving through the sensing
zone. But the sensor is not picking up the bottle.
The Right Sensor Type Makes the Difference Between
Reliable Sensing and No Sensing at All
Given that the excess gain at that range is 100, and the bottle
blocks only 5% of the light energy, the contrast ratio is close
to 1 (100/95). This does not meet the advised 10:1 ratio. The
thru-beam pair is just too powerful.
Using a focused sensor positioned three to four inches from
the bottle changes things. In this detection zone, the excess
gain is between 20 and 100. (See Effective Detection Area
Graph on Page V8-T12-31.)
Environment
The list below ranks the level
of contamination in a range of
typical application
environments.
As you work your way down
the list, the excess gain
needed to overcome what’s
hanging in the air gets higher.
To further complicate
matters, with the reflex and
thru-beam modes, the source
and reflector can be in
different locations with
different levels of
contamination.
For outdoor use, the
environment can range from
lightly dirty to extremely dirty.
Level of Contamination Ranking
Review 7
Answer the following questions without referring to the material
just presented. Begin the next section when you are confident
that you understand what you’ve already read.
1. Excess gain is a measurement of how much sensing power
a photoelectric sensor has available beyond the
________________________
________________________
2. Name the three factors that can affect excess gain.
________________________
________________________
________________________
3. Nearly every diffuse reflective sensor has a unique excess
gain curve because nearly every sensor has a unique
combination of lenses and beam angles. TRUE FALSE
4. Only corner cube style reflectors should be used with
polarized reflex sensors. TRUE FALSE
5. Contrast is not important when sensing semi-transparent
targets. TRUE FALSE
Answers to Review 7 are on Page V8-T12-41.
Ranking Description
Minimum
Excess Gain
Required
Relatively
clean
No dirt buildup on lenses or reflectors 1.5 X
Slightly
dirty
Slight buildup of dust, dirt, oil, moisture, and so on, on lenses
or reflectors. Lenses should be cleaned on a regular schedule.
5 X
Moderately
dirty
Obvious contamination of lenses or reflectors. Lenses are
cleared occasionally or when necessary.
10 X
Very dirty Heavy contamination of lenses. Heavy fog, mist, dust,
smoke or oil film. Minimal cleaning of lenses takes place.
50 X