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In
the world of industrial sensing, repeatability is king.
It dictates the overall reliability of a sensor and often
can make or break an automated process. Although the diffuse
photoelectric sensor proves itself a boon in many applications,
issues tend to arise in repeatability when backgrounds
start to interfere. To tackle these vexing problems, Baumer
Electric developed a unique operating principle known as
true background suppression by triangulation, available
in Baumer’s MINOS, 10,
12, 14 and 16 Series laser and photoelectric sensors. Unlike
fixed-field and adjustable-field photoelectric sensors,
which rely on electrical adjustment of receiver sensitivity,
true background suppression sensors operate on the theory
of triangulation making them insensitive to fluctuations
in the color and reflectivity of the intended target.
Diffuse
photoelectric sensors use the target as the “reflector” such
that detection occurs upon reflection of the light off
the target back onto the receiver. The emitter sends out
a beam of light (infrared, visible red or laser beam) which
is reflected by the target when it enters the detectable
area. The beam is diffused off of the target in all directions
and detection occurs when sufficient light is returns to
the receiver.
Due to the operating principle of using the target as the reflector,
diffuse photoelectrics are often at the mercy of target material
and surface properties; a non-reflective target such as matte-black
paper will have a significantly decreased sensing range as compared
to a bright white target. This sensitivity to color and reflectivity
can also cause false triggers when targets are presented against
reflective or light colored backgrounds. Basically, in a reflective
background situation, sufficient light is received to “fool” the
sensor into triggering on background reflection as opposed to
an actual target. See Diagram 1.
Deviations
of sensing distances and false triggers led to the development
of other diffuse sensors, which can “see” an
object while simultaneously ignoring any objects behind
it. In the simplest of terms, the sensor is focused on
a specific point in the foreground and ignoring anything
beyond that point. There are two ways in which this function
is achieved, the first and most common is using fixed-field
technology, in which the emitter sends out a beam of light
like a standard diffuse photoelectric sensor. In turn,
the light returns to two receivers: one receiver is focused
on the “sweet spot” or desired sensing location
and the other on the background. If the long-range receiver
is detecting a higher intensity of reflected light, than
the amount on the target receiver, the output will not
fire. Only when the intensity of light on the target receiver
is above the long-range receiver will an output occur.
In some versions, the sensitivity can be electrically adjusted
via a potentiometer. Fixed-field and adjustable-field photoelectric
sensors operate optimally at their preset “sweet
spot”. However, target material surface qualities,
such a high gloss, can produce various results. And, like
standard diffuse sensors, reflective or light colored objects
outside of the sensing area tend to send enough light back
to the receivers for the output to trigger.
To combat these limitations, Baumer Electric developed true background
suppression by triangulation. True background suppression sensors
emit a beam of light like a standard diffuse, but unlike fixed-field
sensors, which rely on light intensity, true background suppression
units rely completely on the angle at which the beam returns
to the sensor. Baumer Electric employs two unique methods for
accomplishing this.
Mechanically Adjustable Triangulation-Based
Background Suppression
The fist generation of Baumer Electric background suppression
sensors employ two receivers accompanied by a focusing lens.
The receivers remain in a fixed position, while the lens is mechanically
adjusted to change the angle of received light.
As exhibited in DIA II, receiver 1 is focused, through lenses,
on the background. Any light returning along that focal plane
is “ignored”. Receiver 2 is focused on the target,
and any light returning at that angle will provide an output.
Known as mechanically adjustable background suppression, this
configuration allows for an extremely steep cutoff between target
and background, sometimes smaller than 0.1 mm. This technology
provides extremely stable and repeatable performance, even in
applications with wide color ranges and extremely reflective
backgrounds.
Diode Array Based Adjustable
Background Suppression
Found exclusively in Baumer Electric’s new MINOS line
of ultraminiature photoelectric sensors, diode array based adjustable
background suppression by triangulation is the latest development
in photoelectric sensors.
The diode array technology is based largely upon Baumer Electric’s
expertise in distance/displacement measuring sensors.
Originally designed for Baumer Electric’s OADM 12 miniature
laser displacement sensor, the high density receiver array was
a response to the expense, slow response times and extreme color
sensitivity of PSD (position sensitive diode) technology. PSDs
are commonly used in photoelectric displacement sensors and have
been applied in background suppression sensors with little success.
By replacing the PSD with a diode array, a fast, color-insensitive
sensor is realized.
Baumer Electric applies the diode array to background suppression
using principles similar mechanical background suppression. The MINOS line
replaces the two receivers and lens, with the high density receiver
array. As Illustrated in DIA III, A beam of light is emitted
and reflected back to the sensor. The actual range of the reflection
point affects the beam’s angle of reflection and thereby,
the point upon which the beam is projected onto the receiver
array.
The array’s threshold point is electronically programmed,
via an on board “Teach-In” button or remotely. In
essence, the user sets which receivers are detecting the beam
reflected from the target, and which are detecting background
reflection. Light hitting the “target” receivers
will trigger an output. Light hitting “background” receivers
will not. Color and reflectivity does not affect operation.
This cutting edge technology allows sensors to provide the benefits
of true background suppression by triangulation in miniature
housing previously impossible. Baumer Electric’s MINOS sensors
measure only 8 x 10.8 x 16.2 mm (the size of playing dice), and
retain the full adjustability, repeatability and color insensitivity
of their mechanically adjustable counterparts.
User Benefits
To sum up the real benefits of true background suppression by
triangulation, there are a few simple points to remember:
Triangulation-based Background suppression is the superior, stable
method when reflective backgrounds are present, or large target
color variations are an issue: reflectivity and color affect
the intensity of reflected light, not the angles of refraction
used by triangulation-based background suppression photos.
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