Page 451 - Sensors and Control Systems in Manufacturing

Sensors in Flexible Manufacturing Systems

FIGURE 8.24 Small surfaces are either a small portion of a large object or simply 405
the surface of a small object.

For small surfaces, the sound waves diffract or wrap around the
surface rather than diverge from it as in a large surface. Similarly,
ocean waves diffract around a rock that is smaller than the distance
between crests (Fig. 8.25). Because of diffraction, the sound-vision
recognition system is sensitive to the volume of the shape change on
a small surface (Fig. 8.26).
Volume changes can be positive or negative. Small objects or pro-
trusion from a surface represent positive volume changes, while holes
or cavities represent negative volume changes (Fig. 8.27).
Measurement accuracy for small surfaces depends on the change
in volume of the surface being measured. Listed in Table 8.3 are the
smallest volume change that can be detected and the approximate size
of a cube that has that volume for representative acoustic frequencies.
8.9.5 Positioning
The sound-vision sensor array system compares a particular par with
a reference or standard part and detects the difference between the

FIGURE 8.25 Ocean waves diffract around a rock that is smaller than the distance
between crests.