Page 417 - Sensors and Control Systems in Manufacturing

Industrial Sensors and Contr ol

FIGURE 7.45 Setup for beam steering of a laser-generated ultrasound by 371
fi ber-optic phased array.

In this way, the generated ultrasonic beam can be focused and directed
to a particular inspection point below the surface of an object (Fig. 7.45).
This system has been optimized for the detection of fatigue cracks at
rivet holes in aircraft structures.
The combination of laser-generated ultrasound and an optical-fiber
interferometer for the detection of the resultant surface displacement
has led to a technique that is useful for a wide variety of inspection
tasks in manufacturing, including high-temperature objects and areas
that are difficult to access, as well as more routine inspection and
quality control in various industrial environments. Such a system can
be applied to the measurement of thickness, velocity, flaws, defects,
and grain size in a production process.

7.28 Process Control Sensors for Acceleration
The principle of operation of the process control acceleration sensor
is illustrated in Fig. 7.46. The sensor element, consisting of a small
cantilever and a photoluminescent material, is attached to the end of
a single multimode fiber. The input light of wavelength λ is transmit-
s
ted along the fiber from a near-infrared LED source to the sensor ele-
ment. The sensor element returns light at two different wavelengths,
one of which serves as a signal light and the other as a reference light,
into the same fiber. The signal light at wavelength λ is generated by
s
reflection from a small cantilever. Since the relative angle of the
reflected light is changed by the acceleration, the returned light is
intensity-modulated. The reference light of wavelength λ is generated
r