Page 386 - Sensors and Control Systems in Manufacturing

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reflective surface, with a series of concentric grooves at predeter-
mined spacing. This zone plate works as a spherical concave mirror
whose effective radius of curvature is inversely proportional to the
wavelength. At the focal point of the concave mirror, a second fiber is
placed which transmits the returned light to two photodiodes with
different wavelength sensitivities. When broadband light is emitted
from the first fiber to the zone plate, and the zone plate moves back
and forth relative to the optical fibers in response to the applied pres-
sure, the wavelength of the light received by the second fiber is varied,
causing a change in the ratio of outputs from the two photodiodes.
The ratio is then converted into an electrical signal, which is relatively
unaffected by any variations in parasitic losses.

7.6 Nano-Positioning Capacitive Metrology Sensors
The nano-positioning capacitive sensor (Fig. 7.15) provides fast
response, and precise trajectory control. It is capable of providing
digital controllers with a fast fiber-link interface and an ID-chip for
automatic calibration functions. It provides a resolution of 0.1 nm.

7.6.1 Nano-Capacitive Positioning Sensors
Single-electrode capacitive sensors are direct metrology devices. They
use an electric field to measure the change in capacitance between
the probe and a conductive target surface, without any physical
contact. This makes them free of friction, and hysteresis, and provides

FIGURE 7.15 A nano-positioning capacitive sensor.