Page 94 - Sensors and Control Systems in Manufacturing

FIGURE 2.18 Garage door control Classification and Types of Sensors 55
with photoelectric control. FIGURE 2.19 Web loop control.

A photoelectric sensor that generates a “curtain of light” detects the
length of a loop on a web drive system by measuring the amount of
light returned from an array of retro-reflectors. With this information,
the analog control unit instructs a motor controller to speed up or slow
down the web drive (Fig. 2.19).
Small objects moving through a curtain of light are counted by a
change in reflected light. A low-contrast logic module inside the pho-
toelectric sensor unit responds to slight but abrupt signal variations
while ignoring slow changes such as those caused by dust buildup
(Fig. 2.20).
A pair of through-beam photoelectric sensors scan over and under
multiple strands of thread. If a thread breaks and passes through one
of the beams, the low-contrast logic module detects the sudden
changes in signal strength and energizes the output. As this logic
module does not react to slow changes in signal strength, it can oper-
ate in a dusty environment with little maintenance (Fig. 2.21).

FIGURE 2.20 Small parts
detection. FIGURE 2.21 Broken-thread detection.