Development of Grating Moiré Fringe Technology

Grating moiré fringe technology is an ancient and modern measurement technology. The study of moiré fringe can be traced back to the late 19th century. It has been applied to actual measurement since the 1950s, and the formation mechanism of moiré fringe has been widely studied. So far, three main theories have been formed: (1) Based on the shadow imaging principle: it is believed that the new trajectory formed by the fringes can represent the light intensity distribution of the moiré fringe; (2) Based on the diffraction interference principle: it is believed that the new light intensity distribution formed by the fringes can be described by the interference result between the diffraction waves; (3) Based on the Fourier transform principle: it is believed that the formed moiré fringe is composed of terms lower than the grating frequency. All three theories can fully explain the moiré fringe phenomenon. Generally speaking, the third theory is a generalized explanation. Grating fringes with sparse grating fringes can be directly explained by the shading shadow principle, while grating fringes with dense grating fringes can be explained more appropriately by the diffraction interference principle.

In the 1960s and 1970s, due to the improvement of grating manufacturing technology and the development of electronic technology, cheap grating products could be provided in batches, and electronic subdivision technology emerged, so that the resolution and accuracy of gratings could meet the requirements of modern measurement. Moiré fringe technology was rapidly promoted and applied, and many new grating moiré fringe measurement technologies emerged. The traditional four-field scanning grating system (the grating

sensor developed by Chengdu Tool Research Institute belongs to this system) has been gradually replaced by quasi-single-field scanning and single-field scanning systems due to the greater impact of pollution. The indicator grating of the quasi-single-field scanning system consists of two gratings with different phases. The scale grating (main grating) is reflected and received by four photocells to obtain four moiré fringe signals with a phase difference of 90°; the single-field scanning system uses a large grating with a grating pitch slightly different from the main grating to form the indicator grating, and uses a grating photoelectric device to receive the signal. In these two structures, since one scanning field is used, the local pollution on the grating has roughly the same effect on the light intensity of each group of signals, which greatly reduces the measurement error caused by pollution. Both of these scanning systems belong to the imaging scanning principle and are currently widely used grating systems. In 1987, Haidenhain introduced an interference scanning system, in which both the scale grating and the indicator grating use phase gratings. The order and phase of diffraction are controlled by rationally designing the shape of the grating line height direction. The moiré fringes are formed by the interference light after the input light is diffracted twice. The interference scanning system is a high-precision, high-resolution grating system, such as Haidenhain's LIP382, with a measuring length of 270mm, a resolution of 1nm, and an accuracy of 0.1μm. In order to ensure the quality of the formation of moiré fringes and avoid the coexistence of shadows and diffraction, the grating pitch used in the moiré fringe system is obviously developed towards both ends (sparse or dense). In the imaging scanning system, it is believed that the light is completely propagated in a straight line, which conforms to the principle of geometric imaging. The grating pitch used is generally greater than 20μm; while in the interference scanning system, the system is completely in a diffraction and interference state, and the grating pitch used is generally less than 8μm. Since the interferometric scanning system uses telecentric imaging and Fraunhofer diffraction system, the grating pair can work in a large spacing state (even close to 1mm), and the change of spacing has little effect on the signal amplitude. When adjusting the system, there is no need to find the Frensnel focal plane as in the past, which is extremely beneficial for actual measurement. In order to meet the needs of CNC machine tools , absolute gratings are becoming a development trend. Absolute gratings are absolute rails set on incremental gratings. A series of zero-position gratings with different distance encodings are designed on the absolute rails. When used, the absolute position is determined by detecting the distance between adjacent zero-position gratings. The EnDat bidirectional data interface used in conjunction with the absolute rail can not only determine the absolute position, but also send an alarm signal to the CNC machine tool immediately when the grating fails to ensure the safety of processing. The position accuracy of the grating lines and the quality of the lines within a signal cycle are the main factors affecting the accuracy of the grating. Through strict control of the grating manufacturing environment and the use of special two-dimensional etching processes and the average effect of single-field scanning, the accuracy of the grating can now reach ±0.1μm/m, and the error is less than 1% within a fringe cycle. Excellent moiré fringe signals are the prerequisite for high-multiple subdivision. The maximum subdivision number of Haidenhain grating products can reach 4096. In the 1990s, two-dimensional gratings appeared. Its lines are grid-shaped and can be measured in two directions at the same time. It is a new type of grating system. For example, the accuracy of PP271R ​​and PP281R gratings is ±0.1μm. The system can directly use the X-Y workbench as a coordinate system, and can also be used to detect the interpolation error of CNC machine tools (KGM182). Due to the continuous development of grating moiré fringe technology, grating sensors have become a measuring instrument comparable to laser interferometers. At present, except for the measurement accuracy that is not as good as laser interferometers, they are inferior to laser interferometers in terms of low measurement cost, high measurement speed (480m/min), large measurement range (more than 100m), and little environmental impact. It can be predicted that with the continuous advancement of technology, the application of grating moiré fringe technology will become increasingly widespread.