The characteristics and application of grating scale displacement sensor

Photocoupler

The scale displacement sensor (referred to as the grating scale) is a measurement feedback device that works by the optical principle of the grating. The scale displacement sensor is often used in machine tools and current machining centers and measuring instruments, and can be used for linear displacement or angular displacement detection. The signal output by the measurement is a digital pulse, which has the characteristics of large detection range, high detection precision and fast response speed. For example, in CNC machine tools, it is often used to detect the coordinates of the tool and the workpiece to observe and track the error of the tool to play a role in compensating the motion error of the tool.

Overview

The contribution of grating scales in modern industry is also very large, not only to further improve the current machining accuracy, but more importantly to improve the efficiency of the current processing. At present, China's processing industry and manufacturing industry are becoming more and more mature. When the precision of processing is getting higher and higher, gratings can be installed on various machine tools, such as milling machines, grinding machines, lathes, wire cutting, and electric sparks. Ruler, the working environment requirements are relatively not very harsh, and it is very simple for the operator's use.

It should be noted that the grating ruler is only a reaction device, which can feed back the displacement amount and the displacement direction through the signal output, but it cannot be directly displayed. It also needs a display device, which is referred to as a digital display display box. , also known as digital display. Only when the scale and the digital display are connected together can the value be normally reflected to each operator. Therefore, we need to know more about the use of the scale. If it is not a professional person, it needs Knowing some professional knowledge, you can use the grating ruler as a feedback device alone.

Device classification

The scale displacement sensor is divided into a transmission grating and a reflection grating according to different manufacturing methods and optical principles.

The light source and the receiving device of the transmission grating are respectively placed on both sides of the grating scale, and the position change is reflected by receiving the diffracted light change transmitted by the grating scale. More common is the glass grating. .

The reflective grating refers to the light source and the receiving device mounted on the same side of the grating scale, and the positional change is reflected by the change of the diffracted light reflected by the receiving grating scale. More common are steel strip gratings and glass gratings. .

Device structure

The scale displacement sensor is composed of a scale grating and a grating reading head. The scale grating is typically attached to the moving part of the machine. The raster readhead is mounted on the fixed part of the machine and the indicator is mounted in the raster readhead. The figure on the right shows the structure of the scale displacement sensor.

Grating detection device

Raster detection device structure: The key part of the grating detection device is the grating read head, which is composed of a light source, a condenser lens, an indicator grating, a photoelectric element and an adjustment mechanism. The grating readhead has many forms of structure. It is divided into a direct receiving readhead according to the structure and use of the readhead (or silicon photocell readhead, mirrored readhead, spectroscopic readhead, metal grating reflective readhead).

Grating detection device

working principle

The working principle of common gratings is based on the formation principle of physical moiré fringes. When the two lines are placed at an angle to the line on the scale grating and the line on the scale grating, the lines on the two scales are inevitably crossed. Under the illumination of the light source, due to the overlapping of the black lines in the small area near the intersection, the light-shielding area is the smallest, the light blocking effect is the weakest, and the accumulation of light causes a bright band in this area. On the contrary, in the region farther from the intersection, the overlapping portion of the black lines which are opaque from the two grating scales becomes less and less, and the area of ​​the opaque region gradually becomes larger, that is, the light-shielding area gradually becomes larger, so that the light blocking effect becomes stronger. Only a small amount of light can pass through the grating through this area, causing dark bands in this area, thus forming the moiré stripes we have seen.

Moire fringe

Taking the transmission grating as an example, when a small angle θ is formed between the line on the indicating grating and the line on the scale grating, and the two grating facets are placed in parallel, the illumination is almost vertical under the illumination of the light source. On the grid, light and dark stripes are formed. This stripe is called "moire fringe" (shown on the right). Strictly speaking, the direction in which the moiré stripes are arranged is perpendicular to the bisector of the angle between the two grating lines. The distance between two bright lines or two dark lines in a moiré is called the width of the moiré, and is expressed in W.

Moire fringe

W=ω /2* sin(θ/2)=ω /θ.