Page 374 - Sensors and Control Systems in Manufacturing

Industrial Sensors and Contr ol
In the transmission-type sensor, the emitted light from the input 329
fiber is interrupted by the object, resulting in no received light in the
output fiber located at the opposite side. Typical obstacle interrupters
employ low-cost large-core plastic fibers because of the short trans-
mission distance. The minimum detectable size of the object is typi-
cally limited to 1 mm by the fiber core diameter and the optical beam.
The operating temperature range of commercially available sensors
is typically –40 to +70°C. Optical-fiber sensors have been utilized in
industry in many ways, such as:

• Detection of lot number and expiration dates—for example,
in the pharmaceutical and food industries
• Color difference recognition—for instance, colored objects on
a conveyer
• Defect detection, such as with missing wire leads in electronic
components
• Counting discrete components—for example, bottles or cans
• Detecting absence or presence of labels—for instance,
packaging in the pharmaceutical and food industries

Fiber-optic sensors for monitoring process variables such as tem-
perature, pressure, flow, and liquid level are also classified into two
types: (1) the normally OFF type in which the shutter is inserted
between the fibers in the unactivated state. Thus, this type of sensor
provides high and low levels as the light output corresponds to ON
and OFF states, respectively, and (2) the normally ON type where the
shutter is retracted from the gap in the inactivated state.
In both types, the shutter is adjusted so it does not intercept the
light beam completely but allows a small amount of light to be trans-
mitted, even when fully closed. This transmitted light is used to mon-
itor the cable (fiber) continuity for faults and provides an intermediate
state. Commercially available sensors employ fibers of 200-μm core
diameter. The typical differential attenuation, which determines the
ON-OFF contrast ratio, is about 20 dB. According to manufacturers’
specifications, these sensors operate well over the temperature range
–40 to +80°C with 2-dB variation in light output.

7.3 Temperature Sensors in Process Control
Temperature is one of the most important parameters to be controlled
in almost all industrial plants, since it directly affects material proper-
ties and thus product quality. During the past few years, several
temperature sensors have been developed for use in electrically or
chemically hostile environments. Among these, the practical temper-
ature sensors, which are now commercially available, are classified