Page 373 - Sensors and Control Systems in Manufacturing

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Se v e n
Cha p te r

In many instances today, sensors are analog (they involve a con-
tinuously changing output property), and control loops make use of
digital computers. Therefore, an analog-to-digital converter between
the pre-processor and the digital control loop is often required.
The sensor may operate either passively or actively. In the passive
case, the physical stimulus is available in the environment and does
not have to be provided. For an active case, the particular physical
stimulus must be provided. Machine vision and color identification
sensors are an active means of sensing, because visible light must be
used to illuminate the object before a physical stimulus can be received
by the sensor. Laser sensors are also active-type sensors. Passive sen-
sors include infrared devices (the physical stimulus being generated
from infrared radiation associated with the temperature of a body)
and sensors to measure pressure, flow, temperature, displacement,
proximity, humidity, and other physical parameters.

7.2 Sensors in Manufacturing
Many types of sensors have been developed during the past several
years, especially those for industrial process control, military uses,
medicine, automotive applications, and avionics. Several types of
sensors are already being manufactured by commercial companies.
Process control sensors in manufacturing will play a significant
role in improving productivity, qualitatively and quantitatively,
throughout the coming decades. The main parameters to be mea-
sured and controlled in industrial plants are temperature, displace-
ment, force, pressure, fluid level, and flow. In addition, detectors for
leakage of explosives or combustible gases and oils are important for
accident prevention.
Optical-fiber sensors may be conveniently divided into two
groups: (1) intrinsic sensors and (2) extrinsic sensors.
Although intrinsic sensors have, in many cases, an advantage of
higher sensitivity, almost all sensors used in process control at pres-
ent belong to the extrinsic type. Extrinsic-type sensors employ light
sources such as LEDs, which have higher reliability, longer life, and
lower cost than semiconductor lasers. They also are compatible with
multimode fibers, which provide higher efficiency when coupled to
light sources and are less sensitive to external mechanical and ther-
mal disturbances.
As described in Chapter 3, objects can be detected by interrupting
the sensor beam. Optical-fiber interrupters are sensors for which the
principal function is the detection of moving objects. They may be
classified into two types: reflection and transmission.
In the reflection-type sensor, the light beam emitted from the fiber
is reflected back into the same fiber if the object is situated in front of
the sensor.