The basic principle of the grating sensor is to use the moire fringe phenomenon of the grating.

As shown in the figure below, grating 1 is similar to a long grating, called the ruler grating (also called the main grating), which can move; grating 2 only takes a small piece, called the indicating grating, which is fixed. Conversely, the indicator grating can also be moved. Both the grating pitch and the width of the grating line are the same, so that the grating surface is superimposed relative to each other and a gap d is left in the middle to form a grating pair.

It is placed in the optical path of the parallel beam formed by the light source and the lens, if there is a small angle θ between the two grating lines, the direction approximately perpendicular to the grating lines is much wider than the grating pitch W. The light and dark fringes, these fringes are called “Moire fringes”, the signal light intensity distribution is shown in the curve in the figure, the middle is the bright band, and the upper and lower two are dark bands. When the scale grating is along the x direction perpendicular to the grating line When moving a grating pitch W, the moiré fringes move approximately one fringe pitch along the grating line direction. After receiving the moire fringe signal with the photoelectric element and processing it by the circuit, the distance that the scale grating moves can be obtained with the counter.

In actual use, the light source, indicating grating and photoelectric elements are generally combined together, which is called a reading head. There are many structural forms of the reading head, and its optical path can be divided into the following types.

(1) Spectroscopic reading head

Its working principle is shown in the figure below. The light emitted by the light source Q is transformed into parallel light by the lens L1 and irradiated on the gratings G1 and G2 to form moire fringes; the lens L2 focuses the moire fringes; On the focal plane, the photoelectric element P receives the moire fringe signal. This optical system is the basic type of Moiré optical system. Because the grating spacing is relatively small, and the gap between the two gratings is also small, it is not possible to stick a glass protective layer on the surface of the grating.

In actual use, as shown in the figure below, the equal-magnification projection lenses L3 and L4 are installed between the gratings G and G, so that the image of G1 is projected on G2 with the same size image to form moire fringes, so G 1 is elongated distance from G2. This reading head is mainly used in high-precision coordinate control systems and precision measuring instruments.

(2) Vertical incidence reading head

This readhead is mainly used for glass transmission grating systems with 25 to 125 lines per mm. The following figure is a schematic diagram of the structure of the vertical incidence reading head. The light source 1 turns into parallel light through the lens 2 and illuminates the scale grating 3 and the indicator grating 4 to form moire fringes, and then the photocell 5 receives the signal. 8 in the figure is the rolling bearing, which guarantees a constant gap between the scale grating and the indicating grating. The scale grating and the reading head are respectively mounted on the fixed part and the moving part. The scale grating is clamped with the pressure plate 7 and the reading head is fixed with the screw 6. Its accuracy can reach 0.001mm/1000mm.

(3) Reflective reading head

This reading head is mainly used for reflection grating systems below 25 to 50 lines per millimeter as shown in the figure below. β in the figure is the incident angle (usually 30°, the reflected beam passes through the indicator grating G2 to form moire fringes, and then passes through the lens Lz and receives the signal from the photoelectric element P.

The grating can only be used for incremental measurement. At present, some grating reading heads are equipped with an absolute zero point, so that when the wrong number is remembered due to power failure or other reasons, the zero can be reset. It has a small section of grating on the scale grating and a corresponding small section of grating on the indicator grating. When the two small sections of grating overlap, a zero-position signal is issued and displayed on the digital display.
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