The grating moire fringe technique is an old and modern measurement technique. The research on Moire fringe can be traced back to the end of the 19th century. It was applied to actual measurement after the 1950s, and extensive research on the formation mechanism of Moire fringe has been carried out. Three main types have been formed so far. The theory: (1) based on the principle of shadow imaging: the new trajectory composed of stripes can be said to represent the intensity distribution of moiré fringes; (2) based on the principle of diffraction interference: the new intensity distribution composed of fringes can be diffracted The results of the interference between the waves are described; (3) Based on the Fourier transform principle: the formed moire fringes are considered to be composed of lower than grating frequency terms. All three theories can explain the moire phenomenon more satisfactorily. In general, the third theory is a broad interpretation. The grating stripe can be directly explained by the principle of shading shading, and the grating fringe is more densely explained by the principle of diffraction interference.
In the 1960s and 1970s, due to the improvement of the grating manufacturing process and the development of electronic technology, it was able to provide inexpensive grating products in batches, and electronic subdivision technology appeared, so that the resolution and precision of the grating can meet the requirements of modern metrology. Moiré stripe technology has been rapidly popularized and applied, and many new grating moire 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) has been gradually taken into the single-field scanning and single-field scanning system due to the large influence of pollution. The indicating grating of the quasi-single-field scanning system is composed of two gratings with different phases. The grating (main grating) is reflected by four photocells to obtain four moiré signals with a phase difference of 90°. The single-field scanning system adopts A large grating having a slightly different pitch from the main grating constitutes an indicating grating, and the signal is received by the grid-shaped photovoltaic device. In both structures, since one scanning field is used, the local pollution on the grating has substantially the same influence on the light intensity of each group of signals, and the measurement error caused by the pollution is greatly reduced. Both of these scanning systems belong to the imaging scanning principle and are widely used grating systems.
In 1987, Haidenhain introduced an interferometric scanning system in which both the scale grating and the indicator grating use a phase grating to control the order and phase of the diffraction by rationally designing the shape of the height of the grating line. The interference light after the input light is diffracted twice is formed. The interferometric scanning system is a high-precision, high-resolution grating system, such as Haidenhain's LIP382, measuring 270mm in length, 1nm resolution, and 0.1μm accuracy.
In order to ensure the quality of the moire fringes and avoid the coexistence of shadows and diffraction, the grating pitch of the Moiré fringe system is obviously developed toward both ends (sparse or dense). In the imaging scanning system, the light is considered to be completely linear, conforming to the principle of geometric imaging, and the grating pitch is generally larger than 20 μm. In the interference scanning system, the system is completely in the state of diffraction and interference, and the grating pitch used is generally smaller than 8 μm. Because the interferometric scanning system uses telecentric imaging and Fraunhofer diffraction system, the grating pair can work in a large pitch state (even close to 1mm), and the variation of the pitch has little effect on the signal amplitude. When adjusting the system, it is not necessary to adjust the system. Looking for the Frensnel focal plane in that way is extremely beneficial for actual measurements.
In order to meet the needs of CNC machine tools, absolute gratings are becoming a development trend. The absolute grating is to set the absolute rail on the incremental grating, and a series of zero-position gratings coded by different distances are designed on the absolute rail. In use, the absolute position is determined by detecting the distance of the adjacent zero-grating grating, and the absolute rail is matched with the absolute rail. In addition to the absolute position, the EnDat bidirectional data interface can immediately send an alarm signal to the CNC machine when the grating fails, to ensure the safety of the processing.
The positional accuracy of the grating line and the quality of the line in a signal period are the main factors affecting the accuracy of the grating. Through the strict control of the grating manufacturing environment and the special two-dimensional etching process and the average effect of single-field scanning, the current grating The accuracy has reached ±0.1μm/m, and the error is less than 1% in one stripe period. The excellent Moiré fringe signal is the premise of high-order subdivision, and the maximum number of subdivisions of Haidenhain grating products can reach 4096.
In the 1990s, two-dimensional gratings appeared, and their lines were grid-like, which can measure in two directions at the same time. It is a new type of grating system. For example, the accuracy of the PP271R ​​and PP281R gratings is ±0.1μm. The system can directly use the X-Y table as a coordinate system, and can also be used to detect the interpolation error of the CNC machine tool (KGM182).
Due to the continuous development of grating moiré fringe technology, grating sensor has become a measuring instrument comparable to laser interferometer. At present, the measurement cost is low and the measuring speed is high (480m/min), except for the measurement accuracy. The measurement range is large (100m or more), and the environmental impact is small, which is beyond the reach of the laser interferometer. It is foreseeable that with the continuous advancement of technology, the application of grating moire technology will become increasingly widespread.
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