Banner M12 Series—Visible Red LED - Manual (Page 3)

P/N 129721 rev. E
3
Banner Engineering Corp. • Minneapolis, MN U.S.A
www.bannerengineering.com • Tel: 763.544.3164
M12 Series Metal Barrel Sensors
Background Reflectivity and Placement
Avoid mirror-like backgrounds that produce specular reflections. False sensor response will
occur if a background surface reflects the sensor’s light more strongly to the near detector,
or “sensing” detector (R1) than to the far detector, or “cutoff” detector (R2). The result is a
false ON condition (Figure 4). Use of a diffusely-reflective (matte) background will cure this
problem. Other possible solutions are to angle the sensor or angle the background (in any
plane) so the background does not reflect light back to the sensor (see Figure 5). Position the
background as far beyond the cutoff distance as possible.
An object beyond the cutoff distance, either stationary (and when positioned as shown in
Figure 6), or if it moves past the face of the sensor in a direction perpendicular to the sensing
axis, can cause unwanted triggering of the sensor if it reflects more light to the near detector
than to the far detector. The problem is easily remedied by rotating the sensor 90° (Figure 7).
The object then reflects the R1 and R2 fields equally, resulting in no false triggering. A better
solution, if possible, may be to reposition the object or the sensor.
Color Sensitivity
The effects of object reflectivity on cutoff distance, though small, may be important for some
applications. It is expected that at any given cutoff setting, the actual cutoff distance for lower
reflectance targets will be slightly shorter than for higher reflectance targets. This behavior is
known as color sensitivity.
The excess gain curves on page 5 were generated using a white test card of 90% reflectance.
Objects with reflectivity of less than 90% reflect less light back to the sensor, and thus
require proportionately more excess gain in order to be sensed with the same reliability as
more reflective objects. When sensing an object of very low reflectivity, it may be especially
important to sense it at or near the distance of maximum excess gain.
E = Emitter
R2 = Far Detector
R1 = Near Detector
Core of
Emitted
Beam
Cutoff
Distance
Reflective
Background
Strong
Direct
Reflection
to R1
Fixed
Sensing
Field
M12..FF..
R1
R2
E
Fixed
Sensing
Field
Cutoff
Distance
Reflective
Background
or
Moving Object
E = Emitter
R2 = Far Detector
R1 = Near Detector
M12..FF..
R1
R2
E
Figure 4. Reflective background – problem
Figure 5. Reflective background – solution
Figure 6. Object beyond cutoff – problem Figure 7. Object beyond cutoff – solution
M12..FF..
Fixed
Sensing
Field
E = Emitter
R2 = Far Detector
R1 = Near Detector
Cutoff
Distance
E, R2, R1
Reflective
Background
or
Moving Object
M12..FF..
E = Emitter
R2 = Far Detector
R1 = Near Detector
Core of
Emitted
Beam
Reflective
Background
Fixed Sensing
Field
Strong
Direct
Reflection
Away
From Sensor
Cutoff
Distance
R2
R1
E
A reflective background object in this position or
moving across the sensor face in this axis and
direction may cause false sensor response.
A reflective background object in this position or
moving across the sensor face in this axis will be
ignored.