Page 185 - Sensors and Control Systems in Manufacturing

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FIGURE 3.21 Excess gain curves.

Excess gain is the single most important consideration in choosing a
photoelectric control in manufacturing. It is the extra punch that the
sensor has available within its detecting region.
By definition, excess gain is the ratio of the amount of light the
detector sees to the minimum amount of light required to trip the
sensor. This ratio is depicted graphically for all photoelectric sensors.
In Fig. 3.21, excess gain is plotted along the vertical logarithmic axis,
starting at 1, the minimum amount of light required to trigger the
detector. Every point above 1 represents the amount of light required
to trigger the photoelectric control—the excess gain.
Often, the standard of comparison for choosing between different
photoelectric sensors is range. Actually, more important to most
applications is the excess gain. For a typical application, the higher
the excess gain within the sensing region, the more likely the applica-
tion will work. It is the extra margin that will determine whether the
photoelectric control will continue to operate despite the buildup of
dirt on the lens or the presence of contamination in the air.

Example. An application requires detecting boxes on a conveyer in a filthy
industrial environment (Fig. 3.22). The boxes will pass about 2 to 5 mm from

Sensing distance between
box and sensor is
Proximity 2-5 inches Target
Sensor Object

25 inches

Background
Object

FIGURE 3.22 Box detection.