Page 200 - Sensors and Control Systems in Manufacturing

Fiber Optics in Sensors and Contr ol Systems
Active Handshake 161
Carrier

Fiber Optic Fiber Optic
Processor Terminal
Line Driver Line Driver

Fiber Optic Cable

Fiber Optic Terminal
Line Driver

Fiber Optic Terminal
Line Driver

FIGURE 3.35 System employing optical carrier.

3.12.4 Analog Links
It is well-recognized that, in motion control and process measure-
ment and control, transmitting analog information without distortion
is important. Analog information can be dealt with in several ways
using fiber optics.
Analog data cannot easily be transmitted through light intensity
variation. A number of external factors—such as light source varia-
tion, bending losses in cable, and connector expansion with tempera-
ture—can affect the amount of raw light energy reaching the detector.
It is not practical to compensate for all such factors and deliver accu-
rate analog data. A viable method of transmitting data is to use an
unmodulated carrier whose frequency depends on the analog signal
level. A more advanced means is to convert the analog data to digital
data, where accuracy also is determined by the number of bits used,
multiplex the digital bits into one stream, and use the pulsed digital
link approach.
Figure 3.36 illustrates a link in which this last approach is used to
produce both digital and analog forms of the data at the output.

3.12.5 Video Links
Long-distance video transmission in industrial situations is easily
disrupted by radiated noise and lighting. Repeaters and large-
diameter coaxial cables are often used for particularly long runs. The
use of fiber optics as a substitute for coaxial cable allows propagation
of noise-free video over long distances. Either an intensity- or