handbook of modern sensors

7.5 Optical Sensors 275
Therefore, electric pulses injected into the waveguide’s coaxial conductor produce
mechanical twist pulses which propagate along the waveguide with the speed of sound
specific for its material. When the pulse arrives at the excitation head of the sensor,
the moment of its arrival is precisely measured. One way to detect that pulse is to
use a detector that can convert an ultrasonic twitch into electric output. This can be
accomplished by piezoelectric sensors or, as it is shown in Fig. 7.24, by the magnetic
reluctance sensor. The sensor consists of two tiny coils positioned near two permanent
magnets. The coils are physically coupled to the waveguide and can jerk whenever
the waveguide experiences the twitch. This sets up short electric pulses across the
coils. The time delay of these pulses from the corresponding excitation pulses in the
coaxial conductor is the exact measure of the ring magnet position. An appropriate
electronic circuit converts the time delay into a digital code representative of a position
of the permanent magnet on the waveguide. The advantage of this sensor is in its high
linearity (on the order of 0.05% of full scale), good repeatability (on the order of
3 µm), and long-term stability. The sensor can withstand aggressive environments,
such as high pressure, high temperature, and strong radiation. Another advantage of
this sensor is its low-temperature sensitivity which by careful design can be achieved
on the order of 20 ppm/ ◦ C.
Applications of this sensor include hydraulic cylinders, injection-molding machines
(to measure linear displacement for mold clamp position, injection of molding
material, and ejection of the molded part), mining (for detection of rocks movements
as small as 25 µm), rolling mills, presses, forges, elevators, and other devices where
fine resolution along large dimensions is a requirement.
7.5 Optical Sensors
After mechanical contact and potentionometric sensors, optical sensors are probably
the most popular for measuring position and displacement. Their main advantages are
simplicity, the absence of the loading effect, and relatively long operating distances.
They are insensitive to stray magnetic fields and electrostatic interferences, which
makes them quite suitable for many sensitive applications. An optical position sensor
usually requires at least three essential components: a light source, a photodetector,
and light guidance devices, which may include lenses, mirrors, optical fibers, and so
forth. An example of single- and dual-mode fiber-optic proximity sensors are shown
in Figs. 4.17 and 4.18 of Chapter 4. Similar arrangements are often implemented
without optical fibers when light is guided toward a target by focusing lenses and is
diverted back to detectors by the reflectors. Currently, this basic technology has been
substantially improved. Some more complex and sophisticated products have evolved.
The improvements are aimed to better selectivity, noise immunity, and reliability of
the optical sensors.
7.5.1 Optical Bridge
The concept of a bridge circuit, like a classical Wheatstone bridge, is employed in
many sensors and the optical sensor is a good example of that. One such use shown in