Fixed-Field Sensing – Theory of Operation

The Q40FF 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 Q40FF sensors is fixed at 200, 400 or 600 millimeters (8 inches, 16 inches, or 24 inches) Objects lying

beyond the cutoff distance are usually ignored, even if they are highly reflective. However, under certain conditions, it is possible to

falsely detect a background object (see Background Reflectivity and 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

Sensing

Axis

R2

R1

E

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 5

on page 3 and Figure 6 on page 3.

Installation

Sensing Reliability

For highest sensitivity, position the target for sensing at or near the point of maximum excess gain. Maximum excess gain for all models

occurs at a lens-to-object distance of about 40 millimeters (1.5 inches). Sensing at or near this distance makes the 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 guidelines improves sensing reliability.

Background Reflectivity and Placement

Avoid mirror-like backgrounds that produce specular reflections. A false sensor response occurs if a background surface reflects the

sensor's light more to the near detector (R1) than to the far detector (R2). The result is a false ON condition (Figure 3 on page 3).

Correct this problem by using a diffusely reflective (matte) background, or angling either the sensor or the background (in any plane) so

the background does not reflect light back to the sensor (Figure 4 on page 3). 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 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 6 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.

Q40 Sensors: dc-Voltage Series

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