Page 187 - Sensors and Control Systems in Manufacturing

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Cha p te r
T h ree

FIGURE 3.25 Polarization.

3.3.3 Contrast
Contrast measures the ability of a photoelectric control to detect an
object; it is the ratio of the excess gain under illumination to the excess
gain in the dark. All other things being equal, the sensor that pro-
vides the greatest contrast ratio should be selected. For reliable opera-
tion, a ratio 10:1 is recommended.

3.3.4 Polarization
Polarization is used in reflection sensors in applications where shiny
surfaces, such as metal or shrink-wrapped boxes, may trigger the con-
trol falsely. The polarizer passes light along only one plane (Fig. 3.25),
and the corner-cube reflectors depolarize the light as it passes through
the plastic face of the retroreflector (Fig. 3.10). Only light that has
been rotated by the corner-cube retroreflector can pass through the
polarizer, whereas light that bounces off other shiny objects cannot.
Like regular reflex photoelectric sensors, polarized sensors have a
high light/dark contrast ratio and are simple to install and align.
However, the polarizers do limit the sensor’s operating range because
light is lost passing through them.

3.4 Inductive Proximity Sensors—Noncontact
Metal Detection
Inductive proximity sensors are another common choice for position
sensing. An inductive proximity sensor consists of four basic elements:

• The sensor, which comprises a coil and ferrous core
• An oscillator circuit
• A detector circuit
• A solid-state output