Banner T18 Series - Manual (Page 2)

Banner Engineering Corp. • Minneapolis, MN U.S.A
www.bannerengineering.com • Tel: 763.544.3164 2
P/N 121526 rev. A
T18 Sensors — dc-Voltage Series
Sensing
Axis
R1
R2
E
As a general rule, the most reliable sensing
of an object approaching from the side
occurs when the line of approach is parallel
to the sensing axis.
Figure 2. Fixed-field sensing axis
R1
R2
Lenses
Object
A
Object B
or
Background
Sensing
Range
Cutoff
Distance
E
Receiver
Elements
Near
Detector
Far
Detector
Emitter
Object is sensed if amount of light at R1
is greater than the amount of light at R2
Figure 1. Fixed-field concept
Fixed-Field Mode Overview
T18 Series self-contained fixed-field sensors are small, powerful, infrared diffuse mode
sensors with far-limit cutoff (a type of background suppression). Their high excess gain and
fixed-field technology allow them to detect objects of low reflectivity, while ignoring background
surfaces.
The cutoff distance is fixed. Backgrounds and background objects must always be placed
beyond the cutoff distance.
Fixed-Field Sensing – Theory of Operation
The T18FF compares the reflections of its emitted light beam (E) from an object back to
the sensor’s two differently aimed detectors, R1 and R2 (see Figure 1). If the near detector
(R1) light signal is stronger than the far detector (R2) light signal (see object A, closer than
the cutoff distance), the sensor responds to the object. If the far detector (R2) light signal is
stronger than the near detector (R1) light signal (see object B, beyond the cutoff distance), the
sensor ignores the object.
The cutoff distance for model T18FF sensors is fixed at 25, 50 or 100 millimeters (1", 2",
or 4"). Objects lying beyond the cutoff distance usually are ignored, even if they are highly
reflective. However, it is possible to falsely detect a background object, under certain
conditions (see Background Reflectivity and Placement).
In the drawings and discussion on these pages, the letters E, R1, and R2 identify how the
sensor’s three optical elements (Emitter “E”, Near Detector “R1”, and Far Detector “R2”) line
up across the face of the sensor. The location of these elements defines the sensing axis (see
Figure 2). The sensing axis becomes important in certain situations, such as those illustrated
in Figures 5 and 6.
Sensor Setup
Sensing Reliability
For highest sensitivity, position the target object for sensing at or near the point of maximum
excess gain. Excess gain curves for these products are shown on page 5. They show excess
gain vs. sensing distance for sensors with 25 mm, 50 mm, and 100 mm (1", 2", and 4")
cutoffs. Maximum excess gain for the 25 mm models occurs at a lens-to-object distance of
about 7 mm; for the 50 mm models, at about 10 mm; and for the 100 mm models, at about
20 mm. Sensing at or near this distance will make maximum use of each sensor’s available
sensing power. The background must be placed beyond the cutoff distance. (Note that the
reflectivity of the background surface also may affect the cutoff distance.) Following these two
guidelines will improve sensing reliability.
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 3). To cure this problem, use a diffusely reflective (matte) background,
or angle either the sensor or the background (in any plane) so the background does not reflect
light back to the sensor (see Figure 4). 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 5), or moving past the face of the sensor in a direction perpendicular to the sensing
axis, can cause unwanted sensor triggering if more light is reflected to the near detector than
to the far detector. The problem is easily remedied by rotating the sensor 90° (Figure 6).