Glass scale

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A glass ruler is a very finely divided ruler made of cut glass, sometimes made of plastic similar to glass. The advantage of glass and glass ceramic is the very low thermal expansion, so that temperature fluctuations have hardly any influence on dimensional accuracy.

In the case of hand-held devices, the reading is usually carried out directly or with a magnifying glass, in the case of built-in devices or closed structures, it is usually electronic-digital.

Rulers with visual reading

For good eyes, a glass scale allows measuring accuracies of 0.1 to 0.2 mm, with a suitable magnifying glass even a few hundredths of a millimeter. With a dial gauge at high magnification, even more precise readings are possible if the reading optics are attached directly to the rule.

The graduations of the scales have depending on the design and intended use distances of 0.5 mm or 0.1 mm by, in optical instruments with a micrometer (machine tools, stereocomparators etc.) also 0.01 mm.

The scope of application of the rulers ranges from measurement drawings, diagrams, graphics, repro and cartography to template technology for electronics, molded parts and optics. For precision leveling, there are vertical glass rulers that can be attached directly to a fixed point at the end of a measuring line .

Standards with digital reading

Scheme of a scanning unit: 1 light source (LED), 2 condenser , 3 scanning plate, 4 glass scale, 5 photo diodes ; The tick marks are shown greatly enlarged.

Glass scales with photoelectric reading are produced for installation in turning , milling and other machine tools . In many cases, they are also produced as individual devices in a closed design or with an extension measuring rail and connected data cable. The measuring length can be up to about 4 m. Steel tape measures are used for longer distances up to about 30 m.

Due to the harsh workshop conditions, the scale and scanning unit are protected from chips, dust and liquids by a stable hollow profile. With special sealing lips , vibration protection, or sealing air , even higher protection is possible.

Values ​​between 0.004 and 0.1 mm (4 to 100  µm ) are commonly used as the distance between the graduation lines . The tick marks can be copied from an original using the Diadur process .

With the incremental glass scale, the electrical signals of the 3 photo elements are transmitted directly or amplified via the lines and counted in the evaluation unit. (see incremental encoder # signal evaluation )

The absolute glass scale either has many tracks (see absolute encoder ) or, in addition to the incremental track, a coded scale division from which the absolute position is determined directly. The position values ​​are transmitted via a serial interface, e.g. B. SSI or fieldbus .

Individual evidence

  1. Ernst, Alfons: Digital length and angle measurement technology - position measurement systems for mechanical engineering and the electronics industry (=  The library of technology . Volume 165 ). 4th edition. Modern industry publishing house, 2001, ISBN 3-478-93264-5 .