Application-oriented mounting tolerances Very small signal periods usually come with very narrow mounting tolerances for the gap between the scanning head and scale tape. This is the result of diffraction caused by the grating structures. It can lead to a signal attenuation of 50% with a gap change of only ±0.1 mm. Thanks to the interferential scanning principle and innovative index gratings in encoders with the imaging scanning principle it has become possible to provide ample mounting tolerances in spite of the small signal periods.The mounting tolerances of exposed linear encoders from HEIDENHAIN have only a slight infl uence on the output signals. In particular the specifi ed gap tolerance between the scale and scanning head (scanning gap) causes only negligible change in the signal amplitude. This behavior is substantially responsible for the high reliability of exposed linear encoders from HEIDENHAIN. The two diagrams illustrate the correlation between the scanning gap and signal amplitude for the encoders of the LIDA 400 and LIF 400 series. Refl ective layerTransparent layerRefl ective primary layer Durable measuring standards SUPRADUR By the nature of their design, the measuring standards of exposed linear encoders are less protected from their environment. HEIDENHAIN therefore always uses tough gratings manufactured in special processes.In the DIADUR process, hard chrome structures are applied to a glass or steel carrier. The AURODUR process applies gold to a steel strip to produce a scale tape with a hard gold graduation.In the SUPRADUR process, a transparent layer is applied fi rst over the refl ective primary layer. An extremely thin, hard chrome layer is applied to produce an optically three-dimensional phase grating. Graduations that use the imaging scanning principle are produced according to the METALLUR procedure, and have a very similar structure. A refl ective gold layer is covered with a thin layer of glass. On this layer are lines of chromium only several nanometers thick, which are semitransparent and act like absorbers. Scales with SUPRADUR or METALLUR graduations have proven to be particularly robust and insensitive to contamination because the low height of the structure leaves practically no surface for dust, dirt or water particles to accumulate.

Substrate

METALLURTransparent layer Refl ective primary layer Semitransparent layer180°
Mounting tolerance LIDA 400
Signal amplitude [%]Signal amplitude [%] 1) = Scale tape2) = Scale-tape carrier Scale-to-reticle gap [mm] Scale-to-reticle gap [mm]
Mounting tolerance LIF 400

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