The operation and use of commonly used fiber optic sensors are all based on the basic principle of the generation of Moire fringes in physics to carry out their work tasks. Figure 4-9 shows the working principle of its task. When placing two grating rulers at the same angle of view as the lines on the marker fiber optic sensor and the line on the scale fiber optic sensor, it will inevitably cause the lines on the two grating rulers to intersect with each other. Under direct light from the light source, the small area near the junction point has the lowest total shading area and the weakest shading effect due to the overlapping black and gray lines. The accumulation of light promotes the appearance of bright bands in this area. On the other hand, in areas far from the intersection point, due to the overlap of the partially opaque black gray lines of the two grating rulers, the total area of the partially transparent area gradually increases, that is, the total shading area gradually increases, promoting better shading effect. Only a small number of light sources can use this area to penetrate the fiber optic sensor, resulting in dark bands in this area. The lines of this type of fiber optic sensor are basically vertical, and the bright and dark bands appearing in two colors are called Moir é fringes. Moire stripes have the following characteristics:

(1) If a parallel beam is used to directly beam a fiber optic sensor, the intensity of the light penetrating the Moire fringes is distributed approximately as a cosine function.

(2) If W is used to express the total width of Moir é fringes and d is used to express the gate distance of the fiber optic sensor, θ Expressing the intersection angle of two grating lines, the geometric relationship between them is W=d/sin. When the angle is very small, the above equation can be similar to writing W=d/ θ

If d=one thousandth of a millimeter, θ= One thousandth of rad, then W=1 millimeter can be obtained from the above equation. This indicates that without the need for complex diffraction gratings and electronic control systems, the diffraction phenomenon of light refraction can be transformed into the total width of Moir é fringes that increase by a hundred times in the grating distance of the fiber optic sensor. This type of amplification effect is a key feature of fiber optic sensors.

(3) Because Moire fringes are generated by the interference of several fiber optic sensor lines, they have a mean effect on the deviation of the grid spacing between certain lines in the fiber optic sensor, and can eliminate the harm caused by uneven grid spacing in the fiber optic sensor.

(4) The movement of Moir é fringes corresponds to the relative movement between the two grating rulers. When the two grating rulers move one grid distance d relative to each other, the total width of the Moir é fringes is shifted by one Moir é fringes relative to each other. Its orientation is perpendicular to the relative movement direction of the two grating rulers, and when the relative movement direction of the two grating rulers changes, the direction of the Moir é fringes movement changes accordingly.