Page 147 - Sensors and Control Systems in Manufacturing

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Cha p te r
T w o

the circuitry must be able to resolve 167 ps. This may pose a signifi-
cant problem.
The alternative method to resolve a target at a short range involves
changing the frequency of a continuous oscillator. This method is
better-suited to industrial applications. An oscillator starting at
10.525 GHz and sweeping at 50 MHz in 10 ms in the 6 ns mentioned
earlier will have changed its frequency by:
(6 ns × 50 MHz/0.01 s) = 30 Hz

The returning wave will still be 10.525 GHz. The output from the
mixer diode as the sweep continues will be the 30-Hz difference. If
this frequency is averaged over time, it is not difficult to resolve a
range to 0.001 in.
The preceding calculation indicates that the frequency is high for
faraway objects and low for targets that are close. This leads to two
conclusions:

• The closer the object is, the lower the frequency, and therefore
the longer the measurement will take.
• The signal processing amplifier should have a gain that
increases with frequency.
Problems can arise when the target is moving or there are mul-
tiple targets in the area of interest. Movement can be detected by
comparing consecutive readings and can be used as a discrimina-
tion technique. Multiple targets can be defined by narrow-beam
antennas to reduce the width of the area of interest. Gain adjust-
ments are also required to eliminate all but the largest target. Audio-
bandwidth filters may be used to divide the range into sectors for
greater accuracy.
Other sensor types, such as photoelectric and inductive sen-
sors, may be utilized to measure distances. Inductive sensors are
used in tank level measurements. They must be coupled with a
moving component that floats on the surface of the substance to be
measured.
Photoelectric sensors measure position by focusing a beam on a
point in space and measuring the reflection on a linear array. This can
give very precise measurements over a limited range but is subject
to adverse environmental conditions. Photoelectric sensors can focus
a camera on a target for a better picture. Ultrasonic sensors may per-
form the same function, but their range is limited and they can be
defeated by a hostile environment.
Microwave sensors for measuring range have an impressive array
of applications, including measurement of the level of liquid or solid
in a tank, sophisticated intrusion alarms, autonomous guided vehicle
industrial systems, and noncontact limit switching. In tank level