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QMT42 Series Fixed-Field Sensors

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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

Background

Sensing Field

Cutoff

Distance

Near

Detector

Far

Detector

Emitter

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.

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