The sensor consists of scale grating, indicator grating, optical path system and measuring system (see figure). When the scale grating moves relative to the indicator grating, overlapping grating stripes with alternating light and dark are formed, which are roughly distributed according to sinusoidal law. These stripes move at the relative speed of the grating, directly irradiate the photoelectric element, and get a series of electric pulses at its output, which are amplified, shaped, direction-differentiated and counted to generate digital signals, and directly display the measured displacement.
The optical path of the sensor has two forms: one is transmission grating, whose grating lines are engraved on transparent materials (such as industrial white glass and optical glass); The other is reflective grating, whose grating lines are engraved on highly reflective metal (stainless steel) or glass plated metal film (aluminum film). The advantages of this sensor are large measuring range and high precision. Grating sensor is used in program control and CNC machine tools and coordinate measuring machines, which can measure static and dynamic linear displacement and fillet displacement. It is also used in the field of mechanical vibration measurement and deformation measurement.
Moire fringe
Taking the transmission grating as an example, when the line pattern on the indicator grating forms a small angle θ with the line pattern on the scale grating, and the scale facets of the two gratings are placed relatively parallel, they are located on the almost vertical grid pattern under the illumination of the light source, forming alternating light and dark stripes. This kind of stripe is called "Moire stripe"? . Strictly speaking, the arrangement direction of moire fringes is perpendicular to the bisector of the included angle between two grating lines. The distance between two bright lines or two dark lines in moire fringe is called the width of moire fringe, which is expressed by w.
Moire fringe
W=ω /2* sin(θ /2)=ω /θ.[ 1]?
Moire fringe has the following characteristics:
(1) The variation law of moire fringe: two gratings move by one grating distance, and moire fringe moves by one fringe distance. Due to the diffraction and interference of light, the change law of moire fringe is similar to sine (cosine) function, and the number of change periods is synchronous with the grating pitch number of relative displacement of grating.
(2) Amplification: When the included angle between two gratings is small, the moire fringe width w has the following relationship with grating spacing ω and grating angle θ. Where θ is in rad.