Banner T18 Series - Manual (Page 3)

P/N 121526 rev. A
3
Banner Engineering Corp. • Minneapolis, MN U.S.A
www.bannerengineering.com • Tel: 763.544.3164
T18 Sensors — dc-Voltage Series
E
R2
R1
T18FF
R1 = Near Detector
R2 = Far Detector
E = Emitter
Core of
Emitted
Beam
Cutoff
Distance
Reflective
Background
Fixed Sensing
Field
Strong
Direct
Reflection
Away
From Sensor
Fixed
Sensing
Field Reflective
Surface
or
Moving Object
Cutoff
Distance
R1 = Near Detector
R2 = Far Detector
E = Emitter
T18FF
R1
E
R2
Figure 3. Reflective background – problem Figure 4. Reflective background – solution
Figure 5. Object beyond cutoff – problem Figure 6. Object beyond cutoff – solution
Fixed
Sensing
Field
R1 = Near Detector
R2 = Far Detector
E = Emitter
T18FF
Cutoff
Distance
Reflective
Surface
or
Moving Object
R1, R2, E
T18FF
E
R2
R1
R1 = Near Detector
R2 = Far Detector
E = Emitter
Core of
Emitted
Beam
Cutoff
Distance
Reflective
Background
Strong
Direct
Reflection
to R1
Fixed
Sensing
Field
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.
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 (see Figure-of-
Merit information on page 5). This behavior is known as color sensitivity.
For example, an excess gain of 1 (see page 5) for an object that reflects 1/10 as much light as
the 90% white card is represented by the horizontal graph line at excess gain = 10. An object
of this reflectivity results in a far limit cutoff of approximately 20 mm (0.8"), for the 25 mm (1")
cutoff model for example; thus 20 mm represents the cutoff for this sensor and target.
These excess gain curves 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.