The grating ruler, also called the grating ruler displacement sensor (the grating ruler sensor), is a measurement feedback device that uses the optical principle of the grating. Grating rulers are often used in closed-loop servo systems of CNC machine tools, and can be used to detect linear or angular displacement. The measured output signal is a digital pulse, which has the characteristics of large detection range, high detection accuracy and fast response speed. For example, it is often used in CNC machine tools to detect the coordinates of the tool and the workpiece, to observe and track the error of the tool, in order to play a role in compensating for the error of the tool.

Take a transmission grating as an example. When a small angle θ is formed between the line pattern on the indicator grating and the line pattern on the scale grating, and the two grating scale facets are placed relatively parallel, under the illumination of the light source, they are almost On the vertical grid pattern, light and dark stripes are formed. This kind of fringe is called "Moiré fringe". Strictly speaking, the direction of the moiré fringe arrangement is perpendicular to the bisecting line of the angle between the two grating lines. The distance between two bright lines or two dark lines in the moiré fringe is called the width of the moiré fringe, which is represented by W. W=ω /2* sin(θ /2)=ω /θ.