Fixed-Field Sensing – Theory of Operation

The QMT...FF... 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 on page 2. If the near detector's (R1) light signal is stronger than the far

detector's (R2) light signal (see object A in the Figure below, closer than the cutoff distance), the sensor responds to the

object. If the far detector's (R2) light signal is stronger than the near detector's (R1) light signal (see object B in the

Figure below, beyond the cutoff distance), the sensor ignores the object.

The cutoff distance for model QMT...FF... sensors is fixed at 500, 750, 1000, 1500, or 2000 millimeters (20, 30, 39, 59, or

79 inches Objects lying beyond the cutoff distance are ignored, even if they are highly reflective. However, under certain

conditions, it is possible to falsely detect a background object (see Placement on page 2).

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

Receiver

Element

Emitter

Sensing Axis

Figure 2. Fixed-Field Sensing Axis

In the drawings and information provided in this document, 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 on page 2. The sensing axis becomes important in certain

situations, such as those illustrated in Figure 3 on page 3 and Figure 4 on page 3.

Placement

An object beyond the cutoff distance, either stationary (and when positioned as shown in Figure 3 on page 3), or

moving past the face of the sensor in a direction perpendicular to the sensing axis, may cause unwanted triggering of the

sensor if more light is reflected to the near detector than to the far detector. Correct the problem by rotating the sensor

90° (Figure 4 on page 3). 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.

QMT42 Series Fixed-Field Sensors

2

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