Banner QMT42 Series Fixed-Field Sensors - Manual (Page 2)

QMT42 Series Fixed-Field Sensors
page 2
Banner Engineering Corp. • Minneapolis, U.S.A.
Website: http://www.baneng.com • Tel: 612.544.3164
Fixed-Field Sensing, Theory of Operation
A fixed-field sensor compares the reflections of its emitted light
beam (E) from a target 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 (object A is 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
(object B is beyond the cutoff distance), the sensor ignores the
object. At the cutoff distance (dashed line), the signals from the
two detectors are equal. Objects lying beyond the cutoff distance
are ignored, even if they are highly reflective, (see below).
However, it is possible to falsely detect a background object, if
it is positioned as shown in Figure 3, or if it moves past the
face of the sensor in a direction perpendicular to the sensing
axis (and especially if it has a reflective surface; see Figures 2
and 3). To solve this problem, rotate the sensor 90º, as shown
in Figure 4.
Lenses
Object
A
Object
B
Background
Sensing Field
Cutoff
Distance
E
Near
Detector
Far
Detector
Emitter
R2
R1
Figure 1. Fixed-field sensing, theory of operation
Object is
sensed if the
amount of
light at R1 is
greater than
the amount
of light at
R2.
Receiver
Element
Emitter
Sensing Axis
Figure 2. Sensing axis
As a general rule when an object
approaches from the side, the
most reliable sensing occurs
when the line of approach is
parallel to the sensing axis.
Figure 3. Object beyond cutoff distance (problem)
Cutoff Distance
Sensing Field
Sensor
(side view)
An object in this
position or moving
across the sensor
face in this axis and
direction may cause
false sensor
response.
Figure 4. Object beyond cutoff distance (solution)
Cutoff Distance
Sensing Field
Sensor (top view)
An object in this
position or moving
across the sensor
face in this axis
will be ignored.