How Do Glass Scales Work?
Updated September 26, 2017
A light emitting diode (LED) is housed in the photo-electronic reading head (a photosensitive sensor) of the glass scale. The photo-electronic reading head moves through the length of the glass scale. While in motion, the photo-electronic head illuminates chrome lines (more than a thousand lines per inch) and spaces that have been photo-lithographed onto the glass scale. Photodetectors detect the pattern of chrome lines, and then transmit an electronic signal. The photodetectors in the glass scale read infrared light. For visible light applications, the sensor has a grating on it with a silicon sensor chip, which has a tiny slit. The distance in nanometers between these parts gives the scale the ability to read precise measurements. A concave grating (with a very short-range focal) is printed on to a convex lens, yielding excellent light diffraction to the glass scale’s silicon-made sensor (the sensor is on the opposite side of the grating). The glass scale encoder will examine its own position against the scale (thousands of times per inch of travel along the glass scale--machine scales measure only around 200 marks per inch of travel).
The glass scale is made almost entirely of a weather resistant glass. Because glass maintains its proportions and dimensions through temperature changes, it is the preferred material for photosensitive scales. The markings on the glass are made precise through a lithographing process. These markings do not change because of the stability and temperature resistance of the glass medium. The glass scale will never change its density, size or form due to extreme temperature changes (which means the measured markings remain precise).
A metal housing with end caps encapsulates the glass scale and sensor. This metallic fortification permits the use of glass scales in industrial and manufacturing environments where the glass scale is more likely to be exposed to extreme heat/ cold temperatures and debris contamination. A lip seal is fixed at the base of the housing to protect the sensor and the scale. Another kind of housing for glass scales is the sealed linear encoder. The housing is constructed of aluminum. An elastic seal keeps the glass scale protected from fluids and debris. Within the housing, the glass scale is carried on a scanner carriage, which moves along a mechanical guide. The low friction of the mechanical guide, as well as the aluminum housing, protects the glass scale from vibrations that occur in industrial machinery.
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