Page 184 - Sensors and Control Systems in Manufacturing

Fiber Optics in Sensors and Contr ol Systems
Opposed sensing is the most efficient photoelectric sensing mode 145
and offers the highest level of optical energy to overcome lens contami-
nation, sensor misalignment, and long scanning ranges. It is also often
referred to as direct scanning and sometimes called the beam break mode.
The addition of fiber optics to photoelectric sensing has greatly
expanded the application of photoelectric devices. Because they are
small in diameter and flexible, optical fibers can bend and twist into
tiny places formerly inaccessible to bulky electronic devices.
Optical fibers operate in the same sensing modes as standard
photoelectric controls—through-beam, proximity, and reflex. The
sizes and shapes of sensing tips have been developed to accommo-
date many applications.
Optical fibers have a few drawbacks:

• Limited sensing distance. Typical sensing distance in the
proximity mode is 80 mm; 380 mm for the through-beam
mode.
• Typically more expensive than other photoelectric sensing
controls.
• Easily fooled by a small drop of water or dirt over the sensing
surface.

Optical fibers’ advantages:
• Sensing in confined places.
• Ability to bend around corners.
• No electronics at sensing point.
• Operation at high temperatures (glass).
• Total immunity from electrical noise and interference.
• Easily cut to desired lengths (plastic).

3.3 Optical Fiber Parameters
The most important parameters affecting optical-fiber performance
are:
• Excess gain
• Background suppression
• Contrast
• Polarization

3.3.1 Excess Gain
Excess gain is the measure of energy available between the source and
the detector to overcome signal loss due to dirt or contamination.