Page 415 - Sensors and Control Systems in Manufacturing

Industrial Sensors and Contr ol
7.26 Control of the Input/Output Speed of 369
Continuous Web Fabrication Using Laser
Doppler Velocity Sensor
A laser Doppler velocimeter (LDV) can be configured to measure
any desired component velocity, perpendicular or parallel to the
direction of the optical axis. An LDV system has been constructed
with a semiconductor laser and optical fibers and couplers to conduct
the optical power. Frequency modulation of the semiconductor laser
(or, alternatively, an external fiber-optic frequency modulator) is used
to introduce an offset frequency. Some commercial laser Doppler
velocimeters are available with optical-fiber leads and small sensing
heads. However, these commercial systems still use bulk optical com-
ponents such as acoustooptic modulators or rotating gratings to
introduce the offset frequency.
With an LDV system, the velocity can be measured with high
precision in a short period of time. This means that the method can
be applied for real-time measurements to monitor and control the
velocity of objects as well as measure their vibration. Because the
laser light can be focused to a very small spot, the velocity of very
small objects can be measured, or if scanning techniques are applied,
high spatial resolution can be achieved. This method is used for var-
ious applications in manufacturing, medicine, and research. The
demands on system performance with respect to sensitivity, measur-
ing range, and temporal resolution are different for each of these
applications.
In manufacturing processes, for example, LDV systems are used
to control continuous roll milling of metal (Fig. 7.43), control the roll-
ing speed of paper and films, and monitor fluid velocity and turbu-
lence in mixing processes. Another industrial application is vibration
analysis. With a noncontact vibrometer, vibration of machines,
machine tools, and other structures can be analyzed without disturb-
ing the vibrational behavior of the structure.

FIGURE 7.43 A fi ber-optic laser Doppler velocimeter at a rolling mill controls
pressure by measuring input speeds.