Cut Sheet

V8-T12-20 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
Environment
The sensor’s environment
can affect its performance
dramatically. Let’s take a look
at some of these
environmental factors.
Debris can accumulate on
the sensing cap, changing
the range of the sensing
field. In an application where
metal chips are created, the
sensor should be mounted to
prevent those chips from
building up on the sensor
face. If this is not possible,
then coolant fluid should be
used to wash the chips off
the face. An individual chip
generally doesn’t have
enough surface area to cause
the sensor to turn on, but
several of them could extend
the sensing range and
interfere with the accuracy of
the sensor.
Magnetic fields caused by
electrical wiring located in
the vicinity may affect
sensor operation. If the field
around the wires reaches an
intensity that would saturate
the ferrite or the coil, the
sensor will not operate.
Sensors used in areas with
high frequency welders can
also be affected. To
compensate for a welder,
weld field immune sensors
can be installed. Or, if the
sensor is used with a PLC, a
time delay can be
programmed to ignore the
signal from the sensor for the
time period that the welder is
operating.
Radio transceivers (such as
a walkie-talkie) can
produce a signal with the
same frequency as the
oscillator circuit of the
sensor. This is called radio
frequency interference (RFI).
Most manufacturers have
taken steps to provide the
maximum protection against
RFI and false operation of the
sensor.
Electrical interference from
nearby motors, solenoids,
relays and the like could have
an affect on sensor operation
as well. An induced line or
current spike (called a
showering arc or EFT) can
cause a false operation of
the sensor. This spike can be
produced by the electrical arc
created when an electrical/
mechanical switch or a
contactor closes. If the lines
connecting the sensor and
these devices are adjacent
and parallel to one another,
the spike will affect the
sensor. Most codes and
specifications call for a
separation of control and
power leads so this is not
often a problem.
The ambient temperature
can affect sensing range.
The effect is referred to as
temperature drift. The
sensing range can change by
as much as ±10%.
Because sensing ranges can
vary due to component,
circuit and temperature
variations, along with the
effects of normal machine
wear, sensors should be
selected based on sensing
the target at 75%, and
releasing at 125% of the
rated sensing distance.
Sensing Distance Tolerances
In the Workplace
On the automated processing line at Harris House Paints, a can
would occasionally come through the packaging process without
a lid. Lids entered the line through a gravity feed and
occasionally a lid would get momentarily hung up.
An Inductive Proximity Sensor Keeps a Lid on Things
By mounting an inductive proximity sensor over the passing
cans, the line could reject a can with a missing lid.
Review 4
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. Inductive proximity sensors work best with
_____________ metals.
2. The target size rule of thumb is: the size of the sensor’s
diameter, or three times the sensor’s sensing range,
whichever is greater. TRUE FALSE
3. A target with a rough surface has no impact on the sensing
range. TRUE FALSE
4. A slide-by approach to the sensor is called a lateral
approach. TRUE FALSE
5. A straight on approach is called an axial approach.
TRUE FALSE
6. When two sensors are to be mounted side-by-side, the
use of an alternate frequency head on one of the sensors
will not prevent the sensors’ sensing fields from interacting.
TRUE FALSE
Answers to Review 4 are on Page V8-T12-41.
125%
75%
A
CB
A = Rated Sensing Range
B = Maximum Usable Sensing Range
C = Maximum Reset / Release Range
Target
Sensor