working principle of linear scale！

working principle of linear scale
Moire
In a transmission grating, for example, when lined lined between the ruler and the index grating on the grating formed on a small angle θ, and the relative placement of two parallel grating facets when, under the irradiation of the light source, located almost vertical the gate pattern, forming light and dark stripes. This streak is called "moire" (shown at right). Strictly speaking, the direction of the moire arrangement is lined with two grating angle perpendicular bisector phase. Moire patterns or the distance between two light Obscure between the two is called the width of the moire in W represents.
Moire
Moire
W = ω / 2 * sin (θ / 2) = ω / θ. [1]
Moire fringes having the following characteristics:
(1) Variation of moire
Two gratings moves by one period, striped moire moves over a distance. Since the diffraction and interference of light variation moire approximately normal (cosine) function, the number of cycles and the grating pitch change relative displacement synchronization.
(2) amplification
In the gate line angle between two gratings is small, there is the following relationship between the width W and moire grating pitch ω, the gate line angle θ. Where, θ in units of rad, W is in mm. Since the inclination is small, sinθ is small,
W = ω / θ
If ω = 0.01mm, θ = 0.01rad, then the above equation can be obtained W = 1, ie, grating magnified 100 times.
(3) homogenizing effect of the error
Moire is shared by a number of grating formed, for example, 100 lines per mm grating, 10mm width 1000 there moire lined, so pitch between adjacent error was averaged, and eliminating the pitch error caused by uneven fracture.

Previous：Linear scale detection and data processing
Next：What is linear encoder？