Page 180 - Sensors and Control Systems in Manufacturing

Fiber Optics in Sensors and Contr ol Systems

FIGURE 3.13 Application of diffuse refl ective photoelectric sensor in automated 141
guided vehicle system.

The sensing distance of the far output is adjustable up to 3 m maxi-
mum. The sensing distance of the near output is adjustable from 30 to
80 percent of the far output. Indicators include a red LED that glows
with the near output ON, and a yellow LED that glows with the far
output ON.
An ideal application for this family of sensors is the automated
guided vehicle (AGV), which requires both slow-down and stop con-
trols to avoid collisions when obstacles enter its path (Fig. 3.13).
A modulated infrared light source provides immunity to random
operation caused by ambient light. Additionally, unwanted sensor
operation caused by adjacent sensor interference (crosstalk) is also
eliminated through the use of multiple-position modulated frequency
adjustments.

3.2 Fiber Optics
Fiber optics has greatly expanded the applications of photoelectric
sensors. Fiber optics uses bundles of thin plastic or glass fibers that
operate on a principle discovered in 1854 by John Tyndahl. When
Tyndahl shined a beam of light through a stream of water, instead of
emerging straight from the stream of water as might be expected, the
light tended to bend with the water as it arced towards the floor. Tyn-
dahl discovered that the light was transmitted along the stream of
water. The light rays inside the water bounced off the internal walls
of the water and were thereby contained (Fig. 3.14). This principle has
come to be known as total internal reflection.
Industry has since discovered that the principle of total internal
reflection also applies to small-diameter glass and plastic fibers, and
this has lead to rapid growth of applications throughout the industry.
Because optical fibers are small in diameter and flexible, they can
bend and twist in confined places. Also, because they contain no