Development trend of measuring system for hottest

2022-08-19
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Development trend of machine tool measurement system

with the popularization of numerical control machine tools in the field of machine tool manufacturing, modern machine tools have also been greatly improved in processing speed, processing accuracy and reliability, which to a large extent is suitable for grating measuring elements used in machine tools

single field scanning technology and absolute measurement technology are the two most important directions in the development of angle and length measurement technology, and single field scanning technology and absolute measurement technology can also be combined and applied. At the same time, the single field scanning absolute measurement equipment using these two technologies is far better than the traditional measurement equipment in terms of signal quality, anti pollution ability, measurement speed and reliability. The rapid promotion of such products in the market also confirms this point

single field scanning technology

in the four field imaging scanning method adopted by the traditional angle and length measurement equipment, the grating ruler moves relative to the scanning mask with similar or the same grating structure. The intensity of light passing through the scale and mask changes regularly according to the different relative positions of the scale and mask: when the gap between the scale and the mask coincides, the light can pass through; When the grid line coincides with the gap, no light passes through. The photosensitive element converts the change of light intensity into an electronic signal. There are four scanning areas on the scanning mask, and the gratings in each scanning area are staggered by 1/4 grid spacing. The photosensitive elements corresponding to these four scanning areas generate four sinusoidal signals with a phase difference of 90 °. These four scanning signals do not take the zero line as the middle line, so it is necessary to subtract the four signals in pairs to obtain two output signals L1 and L2 with 90 ° phase difference and the neutral line as the zero line

in the new single field scanning technology, the scanning mask is equipped with a large-size grating, and its grid spacing is slightly different from that of the grating in which the anti-oxidation and anti-corrosion agents in the lubricant are beneficial to the inhibition of corrosion and wear (Fig. 1). Therefore, the light and dark alternation phenomenon will occur in the length of the scanning mask grating: in some places, the grid lines overlap with the grid lines, and the light can pass through; In some places, the grid line overlaps with the gap, and the light cannot pass through; Between the two, the gap is partially blocked, which plays the role of optical filtering, making it possible to produce a uniform high sinusoidal signal. The special grating sensor replaces the independent sensor and generates four scanning signals with a phase difference of 90 °

Figure 1 principle of four field imaging photoelectric scanning method

single field scanning optical scanning system plays a decisive role in improving the performance of angle and length measurement equipment. Its large scanning area and special optical filtering can produce a stable quality scanning signal in the whole travel of the measuring equipment, which is the premise of the following points: the position error in the signal cycle is small; High grating running speed; When using direct drive, the control quality is high

this advantage of single field scanning can be clearly seen in the XY display mode of oscilloscope (Fig. 2): the output signal of single field scanning grating ruler has better roundness and smaller signal noise, which means higher positioning accuracy and better control quality. For linear motors, the speed control can be smoother when equipped with a single field scanning grating ruler

Figure 2 principle of single field imaging photoelectric scanning method

the large-scale scanning surface covering the full width of the grating scale and the alternating and repeated strip scanning area make the measuring equipment using the single field scanning principle particularly insensitive to the interference of pollution, which can be confirmed by the anti pollution test: even in the case of large-scale pollution interference, the measuring equipment can still provide high-quality measurement signals (Figure 3), The position error is far lower than the error value corresponding to the calibration accuracy level of the measuring equipment

Figure 3 Comparison of anti pollution ability between four field and single field scanning

compared with four field scanning, using single field scanning can avoid the complete absence of signal output of the equipment in some pollution and interference situations

absolute measurement technology

the so-called "absolute measurement (Figure 4)" is relative to incremental measurement. The incremental measuring equipment has been used by more than 20 technicians who have just joined the job since today to count the grating lines scanned by the grating probe to obtain the distance data of relative motion. In order to obtain the absolute position, the incremental measuring equipment must perform the action of passing the reference point after starting up, and the absolute measuring equipment directly produces the absolute position data in the form of coding with grid lines of different widths and different spacing into the measuring equipment, which can provide the absolute position information after starting up. Generally, the absolute measuring equipment is equipped with an incremental track in addition to the absolute track to further improve the accuracy and resolution of the measuring equipment

Figure 4 Comparison of antifouling performance of single field/four field scanning technology (green: single field; red: four fields)

in recent years, absolute grating has been more and more widely used because of its irreplaceable advantages. These advantages are mainly reflected in the following aspects:

1 Reduce the non production time of the machine tool

the machine tool is often forced to shut down and restart due to faults or other reasons. For machine tools equipped with only incremental gratings, it is necessary to pass through the reference point for each axis after startup. Different from passing through the reference point after normal startup, the tool of the machine tool is usually still in the processing position at this time, in direct contact with the workpiece, and sometimes even in the interior of the workpiece (such as drilling, tapping, etc.). In order to pass through the reference point safely, the tool must first be moved out of the processing position manually. This is more difficult for multi axis machine tools with inclined tools, which often takes a lot of time and manpower, because it is more difficult to manually remove the tool (after failure) when the tool is pointed out not parallel to the X, y, z axes

for the automatic production line composed of multiple processing equipment, when one of the equipment fails and needs to be restarted, it may have to pass through the reference point for all the equipment in the production line, and it is necessary to carry out special treatment for the unfinished workpiece. The machine tool or production line equipped with absolute grating immediately regains the absolute position of each axis and the spatial direction of the tool after restart, so it can immediately continue the original processing program from the interruption, which greatly improves the effective processing time of the machine tool

Figure 5 Schematic diagram of absolute measurement

2 Improve the reliability of machine tools

the reliability of machine tools directly determines the efficiency and economy of parts production. The failure time of machine tools per minute means an increase in cost for users. Therefore, a trend of machine tool design is to monitor the status of important components in real time. Heidenhain's absolute computer software should not use interconnected gratings to support such real-time monitoring functions through its two-way communication EnDat data interface. In addition to the absolute position value, the EnDat data interface also transmits the working conditions of the grating to the CNC system, such as whether the light source adjustment allowance of the reading head is close to the limit, whether the maximum allowable moving speed of the reading head exceeds the limit, and whether the working temperature exceeds the allowable range. When the above situation occurs, EnDat can send an early warning signal to the CNC system. At this time, the grating can still work normally, but the user can take measures in advance according to the early warning signal

when the grating has serious failure, EnDat will also send an alarm signal to the CNC system, which can decide whether to stop processing based on this signal to protect the machine tool and workpiece. When troubleshooting, the maintenance personnel can find the root cause of the fault in the shortest time according to the information provided by EnDat, such as the grating power supply is too low, the reading head is not working normally, the output signal amplitude is too low, etc

by sending two levels of fault signals of early warning and alarm, the absolute grating can greatly improve the efficiency of user maintenance, reduce the probability of shutdown caused by grating failure, and thus improve the reliability of machine tools

3. Improve the safety of the machine tool

in order to protect the mechanical movement part of the machine tool, the travel switch can usually be used to ensure that the movement of each part of the machine tool does not exceed the allowable range. The travel switch is in a bad environment for a long time (coolant, metal chips, etc.), which is very easy to damage. In addition, the travel switch is rarely used during normal operation of the machine tool, so it is impossible to determine whether its function is still normal at any time. In order to ensure work safety, users have to regularly check the status of the travel switch. The machine tool with absolute grating can determine the position of the moving parts of the machine tool at any time. By making corresponding settings in the numerical control system, the travel switch can be omitted, which not only saves the equipment maintenance time of the machine tool user, but also avoids the safety problems that may be caused by the damage of the travel switch, and improves the safety of the machine tool during use

originally published in AI auto manufacturing magazine (end)

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