The contribution of grating rulers in modern industry is also very huge. It not only further improves the current processing accuracy, but more importantly, it improves the work efficiency during processing. At present, when China's processing and manufacturing industries are becoming more and more mature and the processing accuracy is getting higher and higher, grating rulers can be installed on various machine tools, such as milling machines, grinders, lathes, wire cutting, EDM and other machine tools. The working environment requirements are relatively not very harsh, and it is also very simple for operators to use.

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

Device classification

Grating ruler displacement sensors are divided into transmission gratings and reflection gratings according to different manufacturing methods and optical principles.

Transmission grating refers to glass grating.

Reflection grating refers to steel strip grating.

Device structure

The grating scale displacement sensor consists of two parts: scale grating and grating reading head. The scale grating is generally fixed on the movable part of the machine tool, the grating reading head is installed on the fixed part of the machine tool, and the indicating grating is installed in the grating reading head. The right figure shows the structure of the grating scale displacement sensor.

Grating detection device

The key part of the grating detection device structure is the grating reading head, which is composed of a light source, a converging lens, an indicating grating, a photoelectric element and an adjustment mechanism. There are many structural forms of grating reading heads. According to the structural characteristics of the reading head and the occasion of use, it is divided into direct receiving reading heads (also called silicon photocell reading heads, mirror reading heads, spectroscope reading heads, and metal grating reflection reading heads).

Working principle

The working principle of common gratings is based on the physical principle of the formation of moiré fringes. When the lines on the indicator grating and the lines on the scale grating are placed at an angle, the lines on the two grating scales will inevitably cross each other. Under the illumination of the light source, the small area near the intersection has the smallest light-shielding area and the weakest light-shielding effect due to the overlap of the black lines. The accumulated effect of light causes a bright band to appear in this area. On the contrary, in the area farther from the intersection, the overlapping part of the opaque black lines of the two grating scales becomes less and less, and the area of ​​the opaque area gradually increases, that is, the light-shielding area gradually increases, making the light-shielding effect stronger. Only less light can pass through this area through the grating, causing a dark band to appear in this area.

Moiré fringes

Take the transmission grating as an example. When a small angle θ is formed between the lines on the indicator grating and the lines on the scale grating, and the two grating scale facets are placed relatively parallel, under the illumination of the light source, they are located on the almost vertical grating lines, forming light and dark stripes. This kind of stripe is called "Moiré fringes" (as shown in the right figure). Strictly speaking, the direction of the moiré fringes is perpendicular to the bisector of the angle between the two grating lines. The distance between two bright lines or two dark lines in the moiré fringes is called the width of the moiré fringes, expressed as W.

Moiré fringes

W=ω /2* sin(θ/2)=ω /θ .

Moiré fringes have the following characteristics:

(1) The changing law of moiré fringes

When the two gratings move relative to each other by one grating pitch, the moiré fringes move by one fringe distance. Due to the diffraction and interference of light, the changing law of moiré fringes is approximately a sine (cosine) function, and the number of changing cycles is synchronized with the number of grating pitches of the relative displacement of the gratings.

(2) Amplification effect

When the angle between the two grating lines is small, the moiré fringes width ω and the grating pitch W and the grating line angle θ have the following relationship. In the formula, the unit of θ is rad and the unit of W is mm. Since the inclination angle is very small, sinθ is very small, then

W=ω/θ

If ω=0.01mm, θ=0.01rad, then the above formula can be obtained that W=1, that is, the grating is magnified 100 times.

Moiré fringe calculation

(3) Equalization error effect

Moiré fringes are formed by a number of grating fringes. For example, a grating with 100 lines per millimeter has 1000 lines of 10mm wide moiré fringes. In this way, the adjacent errors between the grating pitches are averaged, eliminating the errors caused by uneven grating pitches, breaks, etc.

Detection and data processing

The essence of grating displacement measurement is to use the grating pitch as a standard ruler for measurement. High-resolution grating rulers are generally expensive and difficult to manufacture. In order to improve the system resolution, the moiré fringes need to be subdivided. At present (2006), grating ruler displacement sensor systems mostly use electronic subdivision methods. When two gratings overlap at a slight angle, moiré fringes will be generated in a direction roughly perpendicular to the grating lines. As the grating moves, the moiré fringes also move up and down. In this way, the measurement of the grating pitch is converted into the measurement of the number of moiré fringes.

Within the width of a moiré fringe, placing 4 photoelectric devices at a certain interval can realize the electronic subdivision and direction determination function. For example, a grating ruler with a grating line of 50 line pairs/mm has a grating pitch of 0.02mm. If it is subdivided into four parts, a counting pulse with a resolution of 5μm can be obtained, which has achieved a very high accuracy in ordinary industrial measurement and control. Since displacement is a vector, both its size and direction must be detected, so at least two photoelectric signals with different phases are required. In order to eliminate common-mode interference, DC components and even harmonics, a differential amplifier composed of low-drift op amps is usually used. The four photoelectric signals obtained by the four photosensitive devices are sent to the input ends of two differential amplifiers respectively. The phase difference of the two signals output from the differential amplifier is π/2. In order to obtain the direction determination and counting pulses, the two signals need to be shaped. First, they are shaped into square waves with a duty cycle of 1:1. Then, by comparing the phase of the square wave, the moving direction of the grating ruler can be obtained. By counting the square wave pulses, the displacement and speed of the grating ruler can be obtained.

Installation Guide

The installation of the grating ruler displacement sensor is relatively flexible and can be installed in different parts of the machine tool.

Generally, the main scale is installed on the workbench (slide) of the machine tool, and moves with the cutting of the machine tool. The reading head is fixed on the bed, and the reading head is installed below the main scale as much as possible. The choice of its installation method must pay attention to the splashing direction of chips, cutting fluid and oil. If the reading head must be installed upward due to the limitation of the installation position, an auxiliary sealing device must be added. In addition, in general, the reading head should be installed on a stationary part relative to the machine tool as much as possible. At this time, the output wire does not move and is easy to fix, while the ruler body should be installed on a part that moves relative to the machine tool (such as a slide).

Installation base

When installing a grating ruler displacement sensor, the sensor cannot be directly installed on a rough and uneven machine tool body, let alone a machine tool body that is primed and painted. The grating main scale and the reading head are installed on two parts of the machine tool that move relative to each other. Use a micrometer to check the parallelism between the main scale installation surface of the machine tool table and the direction of guide rail movement. The micrometer is fixed on the bed and the table is moved. The parallelism is required to be within 0.1mm/1000mm. If this requirement cannot be met, a grating ruler base needs to be designed and processed.

The base requirements are as follows: (1) A base with the same length as the grating ruler body should be added (it is best that the base is about 50mm longer than the grating ruler). (2) The base is processed by milling and grinding processes to ensure that its plane parallelism is within 0.1mm/1000mm. In addition, a reading head base with the same height as the ruler body base needs to be processed. The total error between the reading head base and the ruler body base shall not exceed ±0.2mm. During installation, adjust the reading head position to achieve a parallelism of about 0.1mm between the reading head and the grating ruler body, and a spacing of about 1~1.5mm between the reading head and the grating ruler body.

Main scale installation

Use M4 screws to fix the grating main scale on the mounting surface of the workbench installed on the machine tool, but do not tighten it. Fix the micrometer on the bed and move the workbench (the main scale and the workbench move at the same time). Use a micrometer to measure the parallelism of the main scale plane and the direction of movement of the machine tool guide rail, adjust the position of the main scale M4 screw, and when the main scale parallelism meets 0.1mm/1000mm, tighten the M2 screw thoroughly.

When installing the grating main scale, pay attention to the following three points:

(1) When installing the main scale, if you install a grating longer than 1.5M, you cannot install only the two ends like a bridge type, but you still need to have support in the entire main scale body. (2) After installation with a base, it is best to use a clip to clamp the midpoint (or several points) of the scale body. (3) When the clip cannot be installed, it is best to use glass glue to stick the grating scale body to fix the base scale and the main scale.

Reading head installation

When installing the reading head, if you find that the installation conditions are very limited, you can consider using accessories, such as angle aluminum and straight plate. First, ensure that the base surface of the reading head meets the installation requirements, and then install the reading head. The installation method is similar to the main scale. Finally, adjust the reading head to ensure that the parallelism between the reading head and the grating main scale is within 0.1mm, and the gap between the reading head and the main scale is controlled within 1~1.5mm. After installation, you can use your thumb to touch the surface of the degree head and the grating scale body to see if they are smooth and flat.

Limit device

After all the grating line displacement sensors are installed, limit devices must be installed on the machine tool rails to prevent the reading head from colliding with the two ends of the main scale when the machine tool processing products move, thereby damaging the grating scale. In addition, when purchasing grating line displacement sensors, users should try to choose grating scales that exceed the machine tool processing size by about 100mm to leave a margin.

Sensor inspection

After the grating line displacement sensor is installed, the digital display can be connected, the workbench can be moved, and the digital display count can be observed to see if it is normal.

Select a reference position on the machine tool and move the working point back and forth to the selected position. The readings of the digital display should be the same (or return to zero). In addition, a micrometer (or percentage indicator) can be used to adjust the micrometer and the digital display to zero at the same time (or memorize the starting data), and return to the initial position after multiple round trips to observe whether the data of the digital display and the micrometer are consistent.

Through the above work, the installation of the grating scale line displacement sensor is completed. However, for general machine tool processing environments, there are more iron filings, cutting fluids and oil stains. Therefore, the sensor should be equipped with a protective cover. The design of the protective cover is determined by enlarging the cross-section of the sensor and leaving a certain space size. The protective cover usually adopts rubber sealing to make it waterproof and oil-proof.