Banner QMT42 Series 400 mm Adjustable Field Sensors - Manual (Page 2)

QMT42 Series 400 mm Adjustable Field Sensors
page 2 Banner Engineering Corporation
Adjustable Field Sensing - Theory of Operation
The receiver element of an adjustable field sensor produces two
currents: I1 and I2. The
ratio of these two currents changes as the
received light signal moves along the length of the receiver element
(see Figure 1). The sensing cutoff distance relates directly to this
ratio, which can be adjusted using the sensor’s multi-turn
potentiometer.
The cutoff distance for model QMT42...AFV400 sensors is
adjustable from 125 to 400 millimeters (5 to 16 inches). Objects
lying beyond the cutoff distance are ignored, even if they are highly-
reflective.
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 which is perpendicular to the sensing axis
(Figures 2 and 3). To solve this problem, rotate the sensor 90
degrees, as shown in Figure 4.
R
Lenses
Object
A
Object
B
Background
Adjustable
Sensing Field
Cutoff
Distance
E
Receiver
Element
Minimum
Sensing
Distance
I
1
I
2
Receiver
Element
Emitter
Sensing Axis
Figure 1: Adjustable Field Sensing Concept
Figure 2: Sensing Axis
Figure 3: Object Beyond Cutoff Distance - Problem
Figure 4: Object Beyond Cutoff Distance - Solution
Cutoff Distance
Sensing Field
Sensor
(side view)
Cutoff Distance
Sensing Field
Sensor (top view)
As a general rule, the most
reliable sensing of an object
which approaches from the
side occurs when the line of
approach is parallel to the
sensing axis.
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.