China Machine Tool Network | Ollie () 06-June-2011 Grating Moire fringe technology is an old and modern measurement technology. The study of Moire fringes can be traced back to the end of the 19th century. Since the 1950s, it has been applied to practical measurement, and has gradually carried out extensive research on the formation mechanism of moiré fringes, and has formed three major The theory: (1) Based on the principle of shadow imaging: It is believed that the new trajectory consisting of stripes can represent the intensity distribution of moiré fringes; (2) Based on the principle of diffraction interference: the new light intensity distribution consisting of fringes can be diffracted The interference results between the waves are described; (3) Based on the Fourier transform principle: The formed moire fringes are composed of lower frequency terms than gratings. All three theories can explain the Moire fringe phenomenon more satisfactorily. In general, the third theory is a broad interpretation. The grating gratings are more sparse and can be directly explained by shading shadow principle, and it is more appropriate to interpret grating gratings with denser diffraction interference principle.
In the 1960s and 1970s, due to the improvement of the grating manufacturing process and the development of electronic technology, it was possible to provide inexpensive grating products in batches, and electronic subdivision technology emerged, which enables the resolution and accuracy of gratings to meet the requirements of modern metrology. Moire fringe technology has been rapidly applied, and many new grating moiré fringe measurement techniques have emerged.
The traditional four-field scanning grating system (the grating sensor developed by Chengdu Tool Research Institute belongs to this kind of system) is gradually affected by the contamination, and has been gradually adopted by the quasi-single-field scanning and single-field scanning systems. The indication grating of the quasi-single-field scanning system is composed of two gratings with different phases. After the scale grating (main grating) is reflected, it is received by four photovoltaic cells, and four moiré signals with a phase difference of 90° are obtained; the single-field scanning system adopts A large grating with a slightly different grating pitch from the main grating forms the indicator grating and receives signals with a grid-like optoelectronic device. In both of these structures, since one scanning field is used, the local pollution on the grating has approximately the same effect on the light intensity of each group of signals, which greatly reduces the measurement error caused by the pollution. Both of these scanning systems belong to the imaging scanning principle and are currently widely used as grating systems.
In 1987, Haidenhain Company introduced an interferential scanning system. In this system, both the scale grating and the indicator grating use a phase grating. The level and phase of the diffraction are controlled by rationally designing the height direction of the grating line, and the moire fringe is controlled by The input light is twice diffracted and interference light is formed. The interferometric scanning system is a high-precision, high-resolution grating system, such as Haidenhain's LIP382, with a measurement length of 270 mm, a resolution of 1 nm, and an accuracy of 0.1 μm.
In order to ensure the formation of moire fringe quality and avoid the coexistence of shadow and diffraction, the grating pitch adopted by the moiré fringe system develops towards both ends (dense or dense). In the imaging scanning system, it is considered that the light is completely rectilinear, conforming to the principle of geometric imaging, and the grating pitch used is generally larger than 20 μm. In the interferometric scanning system, the system is completely in a state of diffraction and interference, and the used grating pitch is generally smaller than 8μm. Since the interferometric scanning system uses telecentric imaging and the Fraunhofer diffraction system, the grating pair can be operated in a large pitch state (even close to 1mm), and the variation of the pitch has little effect on the amplitude of the signal. When the system is adjusted, it does not have to be like the past. This looks for the Frensnel focal plane, which is extremely advantageous for actual measurements.
In order to meet the needs of CNC machine tools, absolute gratings are becoming a trend. Absolute grating is to set the absolute track on the incremental grating, and design a series of zero-position grating encoded on different distances on the absolute track. When using, the absolute position can be determined by detecting the distance of the adjacent zero-position grating, and it can be matched with the absolute track. The EnDat bi-directional data interface is used to determine the absolute position. In addition, the encoder can send an alarm signal to the CNC machine tool in case of a fault in the raster to ensure the processing safety.
The position accuracy of the grating lines and the quality of the lines in one signal cycle are the main factors affecting the accuracy of the grating. Through the strict control of the grating manufacturing environment and the use of a special two-dimensional etching process and the average effect of a single field scan, the current grating The accuracy is up to ±0.1μm/m and the error is less than 1% in one fringe period. The excellent Moiré fringe signal is the premise of high-power subdivision, and the highest subdivision of Haidenhain grating products is up to 4096.
In the 1990s, two-dimensional gratings emerged. Their line patterns are grid-like and can measure in both directions simultaneously. It is a new type of grating system. If the accuracy of the PP271R, PP281R grating is ±0.1μm, the system can directly use the X-Y table as the coordinate system, and it can also be used to detect the interpolation error of the CNC machine tool (KGM182).
Due to the continuous development of grating moiré technology, grating sensors have become a measuring instrument comparable to laser interferometers. At present, except for the measurement accuracy which is lower than the laser interferometer, the measurement cost is low and the measurement speed is high (480m/min). The large measuring range (above 100m) and the small influence on the environment are all beyond the scope of the laser interferometer. It can be foreseen that with the continuous advancement of technology, the application of grating Moire fringe technology will surely become increasingly widespread.
In the 1960s and 1970s, due to the improvement of the grating manufacturing process and the development of electronic technology, it was possible to provide inexpensive grating products in batches, and electronic subdivision technology emerged, which enables the resolution and accuracy of gratings to meet the requirements of modern metrology. Moire fringe technology has been rapidly applied, and many new grating moiré fringe measurement techniques have emerged.
The traditional four-field scanning grating system (the grating sensor developed by Chengdu Tool Research Institute belongs to this kind of system) is gradually affected by the contamination, and has been gradually adopted by the quasi-single-field scanning and single-field scanning systems. The indication grating of the quasi-single-field scanning system is composed of two gratings with different phases. After the scale grating (main grating) is reflected, it is received by four photovoltaic cells, and four moiré signals with a phase difference of 90° are obtained; the single-field scanning system adopts A large grating with a slightly different grating pitch from the main grating forms the indicator grating and receives signals with a grid-like optoelectronic device. In both of these structures, since one scanning field is used, the local pollution on the grating has approximately the same effect on the light intensity of each group of signals, which greatly reduces the measurement error caused by the pollution. Both of these scanning systems belong to the imaging scanning principle and are currently widely used as grating systems.
In 1987, Haidenhain Company introduced an interferential scanning system. In this system, both the scale grating and the indicator grating use a phase grating. The level and phase of the diffraction are controlled by rationally designing the height direction of the grating line, and the moire fringe is controlled by The input light is twice diffracted and interference light is formed. The interferometric scanning system is a high-precision, high-resolution grating system, such as Haidenhain's LIP382, with a measurement length of 270 mm, a resolution of 1 nm, and an accuracy of 0.1 μm.
In order to ensure the formation of moire fringe quality and avoid the coexistence of shadow and diffraction, the grating pitch adopted by the moiré fringe system develops towards both ends (dense or dense). In the imaging scanning system, it is considered that the light is completely rectilinear, conforming to the principle of geometric imaging, and the grating pitch used is generally larger than 20 μm. In the interferometric scanning system, the system is completely in a state of diffraction and interference, and the used grating pitch is generally smaller than 8μm. Since the interferometric scanning system uses telecentric imaging and the Fraunhofer diffraction system, the grating pair can be operated in a large pitch state (even close to 1mm), and the variation of the pitch has little effect on the amplitude of the signal. When the system is adjusted, it does not have to be like the past. This looks for the Frensnel focal plane, which is extremely advantageous for actual measurements.
In order to meet the needs of CNC machine tools, absolute gratings are becoming a trend. Absolute grating is to set the absolute track on the incremental grating, and design a series of zero-position grating encoded on different distances on the absolute track. When using, the absolute position can be determined by detecting the distance of the adjacent zero-position grating, and it can be matched with the absolute track. The EnDat bi-directional data interface is used to determine the absolute position. In addition, the encoder can send an alarm signal to the CNC machine tool in case of a fault in the raster to ensure the processing safety.
The position accuracy of the grating lines and the quality of the lines in one signal cycle are the main factors affecting the accuracy of the grating. Through the strict control of the grating manufacturing environment and the use of a special two-dimensional etching process and the average effect of a single field scan, the current grating The accuracy is up to ±0.1μm/m and the error is less than 1% in one fringe period. The excellent Moiré fringe signal is the premise of high-power subdivision, and the highest subdivision of Haidenhain grating products is up to 4096.
In the 1990s, two-dimensional gratings emerged. Their line patterns are grid-like and can measure in both directions simultaneously. It is a new type of grating system. If the accuracy of the PP271R, PP281R grating is ±0.1μm, the system can directly use the X-Y table as the coordinate system, and it can also be used to detect the interpolation error of the CNC machine tool (KGM182).
Due to the continuous development of grating moiré technology, grating sensors have become a measuring instrument comparable to laser interferometers. At present, except for the measurement accuracy which is lower than the laser interferometer, the measurement cost is low and the measurement speed is high (480m/min). The large measuring range (above 100m) and the small influence on the environment are all beyond the scope of the laser interferometer. It can be foreseen that with the continuous advancement of technology, the application of grating Moire fringe technology will surely become increasingly widespread.
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