handbook of modern sensors

282 7 Position, Displacement, and Level
(A)
(B)
Fig. 7.33. Optical displacement sensor with grating light modulator: (A) schematic; (B) transfer
function.
sectors), the sensitivity is low, but, the full-scale displacement is large. For the higher
sensitivity, the grating pitch can be made very small, so that the minute movements
of the grating will result in a large output signal. This type of modulator was used in a
sensitive hydrophone [11] to sense displacements of a diaphragm. The grating pitch
was 10 µm, which means that the full-scale displacement was 5 µm. The light source
was a 2-mW He–Ne laser whose light was coupled to the grating through an optical
fiber. The tests of the hydrophone have demonstrated that the device is sensitive with
a dynamic range of 125 dB of pressure as referenced to 1 µPa, with a frequency
response up to 1 kHz.
A grating principle of light modulation is employed in very popular rotating or
linear encoders, where a moving mask (usually fabricated in the form of a disk) has
transparent and opaque sections (Fig. 7.34).
The encoding disk functions as an interrupter of light beams within an optocoupler;
that is, when the opaque section of the disk breaks the light beam, the detector is
turned off (indicating digital ZERO), and when the light passes through a transparent
section, the detector is on (indicating digital ONE). The optical encoders typically
employ infrared emitters and detectors operating in the spectral range from 820 to 940
nm. The disks are made from laminated plastic and the opaque lines are produced by a
photographic process. These disks are light, have low inertia and low cost and exhibit
excellent resistance to shock and vibration. However, they have a limited operating
temperature range. Disks for a broader temperature range are fabricated of etched
metal.
There are two types of encoding disk: the incremental, which produces a transient
whenever it is rotated for a pitch angle, and the absolute, whose angular position
is encoded in a combination of opaque and transparent areas along the radius. The
encoding can be based on any convenient digital code. The most common are the
gray code, the binary, and the BCD (binary coded decimals).
The incremental encoding systems are more commonly used than the absolute
systems, because of their lower cost and complexity, especially in applications where
count is desirable instead of a position. When employing the incremental encoding
disks, the basic sensing of movement can be made with a single optical channel (an