Page 208 - Sensors and Control Systems in Manufacturing

FIGURE 3.47 Fiber Optics in Sensors and Contr ol Systems 169
Pair of fi bers.

telecommunication fibers. Although its ability to accept light is less
than that of a bundle, a 200- or 400-μm core diameter plastic clad sil-
ica (PCS) fiber provides the ability to place sensing points hundreds
of meters away from corresponding electronics. The fiber and the
cable construction shown in Fig. 3.47 lend themselves particularly
well to conduit pulling, vibratory environments, and general physi-
cal abuse. These fibers are typically proof-tested for tensile strength
2
to levels in excess of 50,000 lb/in . A pair of fibers (Fig. 3.44) is used
much like a bundle, where one fiber is used to send light to the sens-
ing point and the other to return light to the detector. The perfor-
mance limitation of a fiber pair compared to a bundle is reduced scan
range; however, lenses may be used to extend the range. A fiber pair
may be used in one of two configurations: (1) a single continuous
probe (i.e., an unbroken length of cable from electronics to sensing
point), or (2) a fiber-optic extension cord to which a standard probe in
either a bundle or a fiber pair is coupled mechanically. This allows the
economical replacement, if necessary, of the standard probe, leaving
the extension cord intact.
The typical application for a fiber pair is object detection in explo-
sive or highly corrosive environments (e.g., ammunition plants). In such
cases, electronics must be remote by necessity. Fiber pairs also allow the
construction of very small probes for use in such areas as robotics, small
object detection, thread break detection, and small target rotation.

3.13.4 Fiber-Optic Liquid Level Sensing
Another technique for interfacing with fiber-optic probes involves
the use of a prism tip for liquid sensing (Fig. 3.48). Light traveling
down one leg of the probe is totally internally reflected at the prism-
air interface. The index of refraction of air is 1. Air acts as a cladding
material around the prism. When the prism contacts the surface of a
liquid, light is stripped from the prism, resulting in a loss of energy at
the detector. A properly configured system can discriminate between
liquid types, such as gasoline and water, by the amount of light lost
from the system, a function of the index of refraction of the liquid.
This type of sensor is ideal for set point use in explosive liquids,
in areas where electronics must be remote from the liquid by tens or
hundreds of meters, and where foam or liquid turbulence make other
level-sensing techniques unusable.