>> P.19
MeasurementprincipleOpticaltransmission-typelinearencodersThegageheadsmainlyuseopticaltransmission-typelinearencoders,theprincipleofwhichisshownbelow.Inthistype,thelightsource(LED)andthedetectorelement(photodiode)faceeachotherwiththemainscaleandindexscale(20µmpitch)positionedbetweenthem.Asthescalemoveswithrespecttothedetector,theintensityofthelightpassingthroughthewindowintheindexscalevariesconstantly.Atthistime,twosynchronizedsine-wavesignalshavingarelative90-degreephasedifferenceareoutput.Thesesignalsarethenamplifiedandsplitelectrically(withadditionalwaveformsinserted)andoutputas0.1µm,0.5mor1µmsquare-wavesignals.Photo-diodeMainscaleIndexscaleCondenserlensLEDPhase-APhase-B20µm5µmPhase-PAPhase-PB20µmMeasuringpitch1-2
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