Page 338 - The Mechatronics Handbook

Infrared type sensors use the radiation heat to sense the temperature from a distance. These noncontact
sensors can also be used to sense a field of vision to generate a thermal map of a surface.
Proximity Sensors
They are used to sense the proximity of an object relative to another object. They usually provide a on
or off signal indicating the presence or absence of an object. Inductance, capacitance, photoelectric, and
hall effect types are widely used as proximity sensors. Inductance proximity sensors consist of a coil wound
around a soft iron core. The inductance of the sensor changes when a ferrous object is in its proximity.
This change is converted to a voltage-triggered switch. Capacitance types are similar to inductance except
the proximity of an object changes the gap and affects the capacitance. Photoelectric sensors are normally
aligned with an infrared light source. The proximity of a moving object interrupts the light beam causing
the voltage level to change. Hall effect voltage is produced when a current-carrying conductor is exposed
to a transverse magnetic field. The voltage is proportional to transverse distance between the hall effect
sensor and an object in its proximity.
Light Sensors
Light intensity and full field vision are two important measurements used in many control applications.
Phototransistors, photoresistors, and photodiodes are some of the more common type of light intensity
sensors. A common photoresistor is made of cadmium sulphide whose resistance is maximum when the
sensor is in dark. When the photoresistor is exposed to light, its resistance drops in proportion to the
intensity of light. When interfaced with a circuit as shown in Fig. 16.5 and balanced, the change in light
intensity will show up as change in voltage. These sensors are simple, reliable, and cheap, used widely
for measuring light intensity.
Smart Material Sensors
There are many new smart materials that are gaining more applications as sensors, especially in distributed
sensing circumstances. Of these, optic fibers, piezoelectric, and magnetostrictive materials have found appli-
cations. Within these, optic fibers are most used.
Optic fibers can be used to sense strain, liquid level, force, and temperature with very high resolution.
Since they are economical for use as in situ distributed sensors on large areas, they have found numerous
applications in smart structure applications such as damage sensors, vibration sensors, and cure-monitoring
sensors. These sensors use the inherent material (glass and silica) property of optical fiber to sense the
environment. Figure 16.6 illustrates the basic principle of operation of an embedded optic fiber used
to sense displacement, force, or temperature. The relative change in the transmitted intensity or spectrum
is proportional to the change in the sensed parameter.

POTENTIOMETER
5V
v OUT
PHOTO
RESISTOR LIGHT
FIGURE 16.5 Light sensing with photoresistors.

Host material Optical fiber

Relative change
Known source
of light in Intensity or
Spectrum or Phase
e.g., deflection, or temperature,
Environmental disturbance,
or force

FIGURE 16.6 Principle of operation of optic fiber sensing.

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