Photoelectric Scanning

To put it simply, the imaging scanning principle functions by means of projected- light signal generation: two scale gratings with equal or similar grating periods are moved relative to each other—the scale and the scanning reticle. The carrier material of the scanning reticle is transparent, whereas the graduation on the measuring standard may be applied to a transparent or refl ective surface.When parallel light passes through a grating, light and dark surfaces are projected at a certain distance, where there is an index grating. When the two gratings move relative to each other, the incident light is modulated. If the gaps in the gratings are aligned, light passes through. If the lines of one grating coincide with the gaps of the other, no light passes through. An array of photovoltaic cells converts these variations in light intensity into electrical signals. The specially structured grating of the scanning reticle fi lters the light current to generate nearly sinusoidal output signals.The smaller the period of the grating structure is, the closer and more tightly toleranced the gap must be between the scanning reticle and scale.The LC, LS and LB linear encoders operate according to the imaging scanning principle.Imaging scanning principle Most HEIDENHAIN encoders operate using the principle of photoelectric scanning. The photoelectric scanning of a measuring standard is contact-free, and therefore without wear. This method detects even very fi ne lines, no more than a few microns wide, and generates output signals with very small signal periods.The fi ner the grating period of a measuring standard is, the greater the effect of diffraction on photoelectric scanning. HEIDENHAIN uses two scanning principles with linear encoders:The imaging scanning principle interferential scanning principle for grating periods from 20 µm and 40 µm The for very fi ne graduations with grating periods of 8 µm and smaller.••
LED light source Measuring standardCondenser lensScanning reticlePhotovoltaic cell array

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