rus
Issue #7-8/2022
B.G.Turukhano, I.A.Turukhano, Yu.M.Lavrov, O.G.Ermolenko, S.N.Khanov
NANOLENGTH HOLOGRAPHIC ENCODER
NANOLENGTH HOLOGRAPHIC ENCODER
INTRODUCTION
The prior art knows the optical gauge [1] designed for measuring linear dimensions of the parts which contains the main measuring rod which one tip contacts a side of the part to be measured while the other tip is kinematically linked to a twisted spring.
The operating principle of this device is based on elastic properties of the twisted spring tape. The optical gauges have a graduation mark of 0.1; 0.2 and 0.5 μm, with a measuring range of 24 to 100 μm. Due to the design and properties of the twisted tape it is not possible to measure large linear dimensions of the order of a millimetre or more with this device, nor it is possible to measure with nano accuracy.
Also known is the "TUBOR" micrometer head device [2] which is a holographic length encoder. The device contains a measuring holographic grating provided with line marks on a substrate which is rigidly connected to the measuring rod located in a groove of the measuring cylindrical rod parallel to the plane of the cut. A readout head consisting of an indicator grating with line marks on a substrate, an illuminator, a lens and photodetectors is rigidly connected to the housing.
The line marks of the measuring grating are arranged on the base surface of the substrate perpendicular to its longitudinal axis, the line marks of the indicator grating are arranged on the base surface of its substrate parallel to the line marks of the measuring grating and within their aperture.
The length encoder bearings are spaced along the length of the measuring bar and are designed to move the rod with the measuring grating parallel to the indicator grating with constant clearance between the grating base surfaces.
OPERATING PRINCIPLE OF THE LENGTH ENCODER
The length encoder functions as follows: the beam of radiation generated by the illuminator is collimated and falls onto diffraction gratings. The field of interference fringes generated behind the gratings converts the intensity distribution of the interference fringes into electrical signals in the installed matrix of photodetectors. When the measuring rod with the tip is shifted during measuring of the linear size of the object, the measuring grating, which is rigidly connected to the rod, moves relative to the indicator grating, and variable electrical signals, shifted by 90° in phase, are generated at the outputs of the photodetectors matrix. Then, these signals are fed into the electronic unit where a comparator is used to generate counting pulses that determine the linear size of the object.
The bearings ensure parallel movement of the measuring grating relative to the indicator grating with constant gap between them, which is necessary to maintain constancy of the interference fringe period throughout the whole process of measuring linear size of the object, due to the accuracy of measurements is ensured.
Fig.1 shows the micrometer head "TUBOR" with the measurement length L = 30 mm, and Table 1 shows characteristics of a model range of micrometer heads "TUBOR" with the measurement lengths L = 30/100/200 mm. Such length encoders have been examined in GOSSTANDART of Russia and in the Federal Agency of Russia where they have been approved as "types of measuring instruments".
The flat metal longitudinal section of the measuring rod serving as a guide is machined and is characterized roughness and low flatness. Microvibrations arising from the movement of the rod with the measuring grating along inaccurate guides lead to violations of the parallel movement of the grating substrate base surfaces and, accordingly, to undesirable changes in the angle between the line marks of their grating. A change in the angle between the line marks of the diffraction gratings results in a change in the period of the interference fringes and the phase shift of 90° between them, which reduces the accuracy of determining the linear size of the object, the accuracy decreasing with increasing length measured. Hence, using this device to measure linear dimensions of objects allows of getting measurements with a resolution of no more than 0.01 μm, but for lengths over 200 mm and especially for lengths close to 500 mm and more, the accuracy of the results decreases and does not allow of measuring large linear dimensions with high accuracy and even less with nano accuracy, which limits the scope of the device.
Therefore, in order to extend the measurement range of the linear dimensions of an object to 500 mm length or more while maintaining nano accuracy throughout the entire measuring range, which extends the scope of the device, the vertical nanolength holographic encoder (NHE), shown in Fig.2, has been designed and manufactured.
The holographic nanolength encoder contains housing 1 rigidly connected to measuring diffraction grating 2 with lines on a glass substrate [3÷7], and readout head 3 (Fig.2a and Fig.2b) consisting of indicator diffraction grating 8 with line marks on its substrate. Readout head 3 is rigidly connected to the end of measuring rod 4 with tip 5 and movement drive 6, which is made in the form of a flexible thread. In addition, the NHE includes a self-contained glass guide with a base and back surface with high precision flatness of the base surface, which is rigidly connected to the end of the measuring grating substrate. The glass substrate of measuring grating 2 also contains base and reverse 7 surfaces (Fig.2b), which are also characterized by high flatness. Readout head 3 contains indicator grating 8. In order to maintain a constant gap between the measuring and indicator gratings, the NHE is provided with two bearing assemblies designed to move the rod with the measuring grating in parallel to the indicator grating.
The bearings of one of the support assembly provide for movement of readout head 3 along measuring grating 2, and the bearings of the other support assembly allow of movement of readout head 3 along the self-contained glass guide, and all bearings are connected to the readout head. The stroke density of two holographic gratings (measuring and display) in this example embodiment is 1,000 lines/mm.
Photodetectors matrixes 9 and 10 are electrically connected to the electronic control unit [8].
The device operates as follows: during the measurement process, thread 6 connected to reading head 3 and measuring rod 4 is moved in the direction of the reference gauge placed under the tip. As reading head 3 with measuring rod 4 and tip 5 moves, indicator grid 8 is shifted relative to measuring grating 2. The radiation beam generated by radiation source 11 and rigidly connected to readout head 3 is collimated by collimator 12 and passes through diffraction gratings 8 and 2. In the field of interference fringes generated behind gratings 8 and 2, the intensity distribution of the interference fringes is converted by photodetector matrixes 9 and 10 into electrical signals phase-shifted by 90 degrees. These signals are then electronically transmitted from the matrix of photodetectors to the electronic control unit where counting pulses are generated using a comparator to determine the linear size of the object. At the same time, when the bearings move on the glass substrates of measuring grating 2 and the glass autonomous guide rigidly connected to it, which has high flatness of the base surfaces, the microvibrations and displacements that are caused by movement on rough and uneven surfaces are eliminated.
During measurement, movement of readout head 3 with indicator grating 8, which is smaller than the substrate with measuring grating 2, subjects the entire system to fewer stresses and constraints, such as the condition of keeping the movement line in parallel to the measurement axis and reducing vibrations, while measuring grating 2 can be any size and is rigidly fixed in housing 1 (Fig.2a).
The structural design of the device, in particular mutual positioning of diffraction gratings 2 and 8 with their supports and measuring rod 4 in housing 1, made it possible to reduce the distance between the diffraction gratings, which reduces the Abbe errors and, in turn, makes it possible to improve the measurement accuracy. Also, the use of guides to move reading head 3 over the substrates of the measuring grating and a self-contained guide ensures that nanolength encoders with measuring gratings of large dimensions, up to a metre or more, can be manufactured with minimized Abbe errors. The NHE uses ‘float-processed’ glass, which has high flatness characteristics that are retained at long lengths without the need for machining, as with the holographic length encoders (Fig.1).
Thus, in order to achieve high measurement accuracy, particularly in the nano range, and to maintain this accuracy over the entire range of motion of the readout head, the lines of two gratings 2 and 8 should maintain their inclination relative to each other, thus maintaining a constant difference in the electrical signal phases.
During NHE measurements, strict uniformity and straightness of movement is ensured, the specified angle between the lines of diffraction gratings 2 and 8 is maintained thus preserving the period and slope of the interference fringes formed behind the gratings and resulting in the improved accuracy up to nanoscale, as experimentally verified by a series of measurements of reference gauges at 100 mm, 200 mm and 500 mm (Table 2).
The high accuracy of the holographic nanolength encoder depends not only on the mechanical accuracy of the nanolength encoder itself but also on precision of the linear holographic diffraction gratings (LHDGs) themselves.
CONCLUSIONS
The authors have managed to record and replicate high-frequency holographic diffraction gratings of up to and over one metre in length and 1,000 lines/mm, unparalleled in length and accuracy, on special devices for LHDG synthesis [3–7].
These gratings were studied and certified at the D.I. Mendeleev All-Russian Institute for Metrology (VNIIM). The authors have produced a domestic record high-frequency LHDG with length of 1,200 mm and density of 1,000 lines/mm (Fig.4). On the basis of such LHDG it is possible to develop NHE-1200 with a measurement range of the linear object dimensions up to 1,200 mm and to achieve nanoscale accuracy of these measurements in the whole measured range, which extends the field of application of this device.
Fig.5.1, Fig.5.2 and Fig.5.3 show the NHEs having a possibility to measure objects with dimensions L = 100 mm; L = 200 mm and L = 500 mm, and Table 2 and Table 3 show their characteristics. The presented holographic nanolength encoders can be used for nanometric research and measurements, for verification of high-precision end-length gaugess, in systems for sorting precise parts by size, in micro- and nanolithography and in other cases where nanoprecision of measurements is necessary. Their use is also relevant in mechanical engineering, opto-mechanical, aerospace and other industries for measuring linear dimensions of objects in the nanoscale.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The prior art knows the optical gauge [1] designed for measuring linear dimensions of the parts which contains the main measuring rod which one tip contacts a side of the part to be measured while the other tip is kinematically linked to a twisted spring.
The operating principle of this device is based on elastic properties of the twisted spring tape. The optical gauges have a graduation mark of 0.1; 0.2 and 0.5 μm, with a measuring range of 24 to 100 μm. Due to the design and properties of the twisted tape it is not possible to measure large linear dimensions of the order of a millimetre or more with this device, nor it is possible to measure with nano accuracy.
Also known is the "TUBOR" micrometer head device [2] which is a holographic length encoder. The device contains a measuring holographic grating provided with line marks on a substrate which is rigidly connected to the measuring rod located in a groove of the measuring cylindrical rod parallel to the plane of the cut. A readout head consisting of an indicator grating with line marks on a substrate, an illuminator, a lens and photodetectors is rigidly connected to the housing.
The line marks of the measuring grating are arranged on the base surface of the substrate perpendicular to its longitudinal axis, the line marks of the indicator grating are arranged on the base surface of its substrate parallel to the line marks of the measuring grating and within their aperture.
The length encoder bearings are spaced along the length of the measuring bar and are designed to move the rod with the measuring grating parallel to the indicator grating with constant clearance between the grating base surfaces.
OPERATING PRINCIPLE OF THE LENGTH ENCODER
The length encoder functions as follows: the beam of radiation generated by the illuminator is collimated and falls onto diffraction gratings. The field of interference fringes generated behind the gratings converts the intensity distribution of the interference fringes into electrical signals in the installed matrix of photodetectors. When the measuring rod with the tip is shifted during measuring of the linear size of the object, the measuring grating, which is rigidly connected to the rod, moves relative to the indicator grating, and variable electrical signals, shifted by 90° in phase, are generated at the outputs of the photodetectors matrix. Then, these signals are fed into the electronic unit where a comparator is used to generate counting pulses that determine the linear size of the object.
The bearings ensure parallel movement of the measuring grating relative to the indicator grating with constant gap between them, which is necessary to maintain constancy of the interference fringe period throughout the whole process of measuring linear size of the object, due to the accuracy of measurements is ensured.
Fig.1 shows the micrometer head "TUBOR" with the measurement length L = 30 mm, and Table 1 shows characteristics of a model range of micrometer heads "TUBOR" with the measurement lengths L = 30/100/200 mm. Such length encoders have been examined in GOSSTANDART of Russia and in the Federal Agency of Russia where they have been approved as "types of measuring instruments".
The flat metal longitudinal section of the measuring rod serving as a guide is machined and is characterized roughness and low flatness. Microvibrations arising from the movement of the rod with the measuring grating along inaccurate guides lead to violations of the parallel movement of the grating substrate base surfaces and, accordingly, to undesirable changes in the angle between the line marks of their grating. A change in the angle between the line marks of the diffraction gratings results in a change in the period of the interference fringes and the phase shift of 90° between them, which reduces the accuracy of determining the linear size of the object, the accuracy decreasing with increasing length measured. Hence, using this device to measure linear dimensions of objects allows of getting measurements with a resolution of no more than 0.01 μm, but for lengths over 200 mm and especially for lengths close to 500 mm and more, the accuracy of the results decreases and does not allow of measuring large linear dimensions with high accuracy and even less with nano accuracy, which limits the scope of the device.
Therefore, in order to extend the measurement range of the linear dimensions of an object to 500 mm length or more while maintaining nano accuracy throughout the entire measuring range, which extends the scope of the device, the vertical nanolength holographic encoder (NHE), shown in Fig.2, has been designed and manufactured.
The holographic nanolength encoder contains housing 1 rigidly connected to measuring diffraction grating 2 with lines on a glass substrate [3÷7], and readout head 3 (Fig.2a and Fig.2b) consisting of indicator diffraction grating 8 with line marks on its substrate. Readout head 3 is rigidly connected to the end of measuring rod 4 with tip 5 and movement drive 6, which is made in the form of a flexible thread. In addition, the NHE includes a self-contained glass guide with a base and back surface with high precision flatness of the base surface, which is rigidly connected to the end of the measuring grating substrate. The glass substrate of measuring grating 2 also contains base and reverse 7 surfaces (Fig.2b), which are also characterized by high flatness. Readout head 3 contains indicator grating 8. In order to maintain a constant gap between the measuring and indicator gratings, the NHE is provided with two bearing assemblies designed to move the rod with the measuring grating in parallel to the indicator grating.
The bearings of one of the support assembly provide for movement of readout head 3 along measuring grating 2, and the bearings of the other support assembly allow of movement of readout head 3 along the self-contained glass guide, and all bearings are connected to the readout head. The stroke density of two holographic gratings (measuring and display) in this example embodiment is 1,000 lines/mm.
Photodetectors matrixes 9 and 10 are electrically connected to the electronic control unit [8].
The device operates as follows: during the measurement process, thread 6 connected to reading head 3 and measuring rod 4 is moved in the direction of the reference gauge placed under the tip. As reading head 3 with measuring rod 4 and tip 5 moves, indicator grid 8 is shifted relative to measuring grating 2. The radiation beam generated by radiation source 11 and rigidly connected to readout head 3 is collimated by collimator 12 and passes through diffraction gratings 8 and 2. In the field of interference fringes generated behind gratings 8 and 2, the intensity distribution of the interference fringes is converted by photodetector matrixes 9 and 10 into electrical signals phase-shifted by 90 degrees. These signals are then electronically transmitted from the matrix of photodetectors to the electronic control unit where counting pulses are generated using a comparator to determine the linear size of the object. At the same time, when the bearings move on the glass substrates of measuring grating 2 and the glass autonomous guide rigidly connected to it, which has high flatness of the base surfaces, the microvibrations and displacements that are caused by movement on rough and uneven surfaces are eliminated.
During measurement, movement of readout head 3 with indicator grating 8, which is smaller than the substrate with measuring grating 2, subjects the entire system to fewer stresses and constraints, such as the condition of keeping the movement line in parallel to the measurement axis and reducing vibrations, while measuring grating 2 can be any size and is rigidly fixed in housing 1 (Fig.2a).
The structural design of the device, in particular mutual positioning of diffraction gratings 2 and 8 with their supports and measuring rod 4 in housing 1, made it possible to reduce the distance between the diffraction gratings, which reduces the Abbe errors and, in turn, makes it possible to improve the measurement accuracy. Also, the use of guides to move reading head 3 over the substrates of the measuring grating and a self-contained guide ensures that nanolength encoders with measuring gratings of large dimensions, up to a metre or more, can be manufactured with minimized Abbe errors. The NHE uses ‘float-processed’ glass, which has high flatness characteristics that are retained at long lengths without the need for machining, as with the holographic length encoders (Fig.1).
Thus, in order to achieve high measurement accuracy, particularly in the nano range, and to maintain this accuracy over the entire range of motion of the readout head, the lines of two gratings 2 and 8 should maintain their inclination relative to each other, thus maintaining a constant difference in the electrical signal phases.
During NHE measurements, strict uniformity and straightness of movement is ensured, the specified angle between the lines of diffraction gratings 2 and 8 is maintained thus preserving the period and slope of the interference fringes formed behind the gratings and resulting in the improved accuracy up to nanoscale, as experimentally verified by a series of measurements of reference gauges at 100 mm, 200 mm and 500 mm (Table 2).
The high accuracy of the holographic nanolength encoder depends not only on the mechanical accuracy of the nanolength encoder itself but also on precision of the linear holographic diffraction gratings (LHDGs) themselves.
CONCLUSIONS
The authors have managed to record and replicate high-frequency holographic diffraction gratings of up to and over one metre in length and 1,000 lines/mm, unparalleled in length and accuracy, on special devices for LHDG synthesis [3–7].
These gratings were studied and certified at the D.I. Mendeleev All-Russian Institute for Metrology (VNIIM). The authors have produced a domestic record high-frequency LHDG with length of 1,200 mm and density of 1,000 lines/mm (Fig.4). On the basis of such LHDG it is possible to develop NHE-1200 with a measurement range of the linear object dimensions up to 1,200 mm and to achieve nanoscale accuracy of these measurements in the whole measured range, which extends the field of application of this device.
Fig.5.1, Fig.5.2 and Fig.5.3 show the NHEs having a possibility to measure objects with dimensions L = 100 mm; L = 200 mm and L = 500 mm, and Table 2 and Table 3 show their characteristics. The presented holographic nanolength encoders can be used for nanometric research and measurements, for verification of high-precision end-length gaugess, in systems for sorting precise parts by size, in micro- and nanolithography and in other cases where nanoprecision of measurements is necessary. Their use is also relevant in mechanical engineering, opto-mechanical, aerospace and other industries for measuring linear dimensions of objects in the nanoscale.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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