Page 194 - Sensors and Control Systems in Manufacturing

Fiber Optics in Sensors and Contr ol Systems
precise than the optical convergent-beam sensing mode, this approach 155
to reflective sensing uses light more efficiently than diffuse sensing and
gives a greater depth of field than true optical convergence.
Mechanical convergence may be customized for an application
by mounting the emitter and the receiver to converge at the desired
distance. Depth of field is controlled by adjusting the angle between
the emitter and the receiver.

3.9 Fiber Optics in Industrial Communication
and Control
The application of fiber optics to industrial information transfer is a
natural extension of the current commercial uses of this technology in
high-data-rate communications. While the primary advantage of
fiber optics in traditional application areas has been extremely reli-
able communication at high rates, exceeding 1 Gbit/s over distances
exceeding 100 km, other intrinsic features of the technology are more
important than data rate and distance capability in industrial uses.
The physical mechanism of light propagating through a glass
fiber has significant advantages that enable sensors to carry data and
plant communications successfully and in a timely manner—a
fundamental condition that must be constantly maintained in a
computer-integrated manufacturing environment:
• The light signal is completely undisturbed by electrical noise.
This means that the fiber-optic cables can be laid wherever
convenient without special shielding. Fiber-optic cables and
sensors are unaffected by electrical noise when placed near
arc welders, rotating machinery, electrical generators, and so
on, whereas in similar wired applications, even the best con-
ventional shielding methods are often inadequate.
• Fiber-optic communication is devoid of any electrical arcing
or sparking, and thus can be used successfully in hazardous
areas without danger of causing an explosion.
• The use of a fiber link provides total electrical isolation
between terminal points on the link. Over long plant dis-
tances, this can avoid troublesome voltage or ground differ-
entials and ground loops.
• A fiber-optic system can be flexibly configured to provide
additional utility in existing hardware.

3.10 Principles of Fiber Optics in Communications
An optical fiber (Fig. 3.30) is a thin strand composed of two layers: an
inner core and an outer cladding. The core is usually constructed of