In order to solve a series of application problems caused by the difficulty of adjusting prism collimation and the short working distance of small period gratings in current collimation solutions, this paper proposes a six-degree-of-freedom measurement scheme that uses a combination of large period gratings and long focus plano-convex lenses. This solution can achieve six degrees of freedom measurement at a working distance of more than 80mm. The main principle is that the displacement is calculated using the phase information formed by the interference of the four diffracted lights of the two-dimensional grating. The angle is based on the principle of autocollimation. According to the propagation direction of the diffracted light caused by the change of the grating diffracted light with the grating angle changes, thereby changing the angle obtained based on the location changes in the detected diffraction spot, ultimately forming a measurement of six degrees of freedom.
In order to solve the problem of the mutual limitations of large measurement range and high precision of absolute grating encoders, this paper employs absolute codes to individually number each spliced grating section on the substrate, leveraging the asymmetric code arrangement. With the light splitting ability of beam splitter, the light source is divided into dual beams and inputted into the encoder to form a dual beam reading head. At least one beam of light can always generate a stable signal within the travel range, and then the serial number of this grating area is determined through absolute codes. The signals generated by gratings with different numbers can be calibrated using calibration parameters. With the help of the data splicing principle and the structure of dual-beam, a nanoscale positioning accuracy scheme with a meter-level measurement range is proposed.
High-precision position and orientation measurement plays a crucial role in modern engineering and scientific fields, finding applications in areas such as advanced manufacturing, aerospace, and medical technology. Grating encoders are stable and highly accurate position and orientation measurement solutions, particularly well-suited for multi-degree-of-freedom high-precision measurement scenarios.
This paper addresses the modeling and compensation of crosstalk errors resulting from grating angle variations in a previously designed six-degree-of-freedom grating encoder for position and orientation measurement. The six-degree-offreedom grating encoder consists of components such as a light source module, reference and measurement gratings, an angle measurement module, and a displacement measurement module. The deflection of the grating affects the interference signal for displacement measurement. The emitted light beam from the light source undergoes diffraction by the measurement grating, using for measuring three degrees of freedom (angles). The interference between the diffracted light from the measurement grating and the reference grating is detected by the displacement measurement module, using for measuring three degrees of freedom (displacements). The traditional grating displacement measurement does not consider the influence of rotation on the laser phase. This paper innovatively models the influence of rotation on the laser phase, which can compensate the displacement settlement, and the ultra-high precision displacement measurement can be obtained.
In this work, a solution based on sub-sampling technology for heterodyne signals is proposed. While achieving higher measurement resolution and measurement speed, the performance requirements for the analog-to-digital converter and microprocessor are greatly reduced. The heterodyne signal is a sparse signal with a single frequency at each moment, only its phase offset contains displacement information. We use the pulse counting method to obtain the periods of the signal, and a sampler with a sampling rate well below the frequency of the heterodyne signal. The phase of the sampling point can be restored through sub-sampling technology and extended Kalman filtering. In the experiment, we used 16-bit ADCs with a 600 Ksas sampling rate to sample the heterodyne signals with a center frequency of 10 MHz and the dynamic range from 1 MHz to 19 MHz. The simulation results indicate that our method can effectively calculate the phase information of the interference signal.
The accuracy of the absolute position and orientation of sub-mirrors in large-aperture optical systems significantly impacts system performance. The precision of sub-mirror alignment directly affects the surface shape error of the main mirror, which, in turn, affects imaging quality and overall system performance. Two main methods for position and orientation measurement are electromagnetic displacement measurement and optical displacement measurement. While electromagnetic methods achieve high precision, their complex structures and susceptibility to environmental factors pose challenges. Optical displacement measurement using grating encoders, which rely on grating pitch as a reference, offers high stability and broad applicability.
To address sub-mirror alignment accuracy in large-aperture telescopes, we propose an absolute six-degree-of-freedom grating encoder based on spot position monitoring. This encoder achieves four degrees of freedom (θX, θY, θZ, Z) absolute position and orientation detection using gratings. Additionally, we employ right-angle prisms for absolute position and orientation detection in the X and Y directions, enabling six-degree-of-freedom absolute position and orientation monitoring for sub-mirrors. The monitoring results serve as feedback for sub-mirror pose correction. To mitigate the impact of grating motion on X and Y displacement calculations, we introduce a displacement calculation algorithm based on ray tracing for error compensation, enhancing the accuracy of X and Y displacement calculations and achieving high-precision six-degree-of-freedom measurement and computation.
This paper presents a study on the spot location method and system based on QPDs. We construct a mathematical model of the relationship between spot position variations and detector responses, systematically analyzing the impact of the spot size and detector parameters on spot location accuracy. We propose an ultra-precision Gaussian spot location algorithm based on QPDs, along with a common-path laser light source fluctuation error compensation structure and method, and validated the method through simulation experiments. The experimental results show that this scheme can achieve submicron level spot positioning accuracy. In the measurement range of 1 mm, the measurement error after compensation is reduced by 97% compared to before compensation. Additionally, the repeatability and stability demonstrate excellent performance. This study provides a laser light source fluctuation error compensation method and an ultra-precision Gaussian spot location algorithm based on QPDs for laser measurement technology, significantly improving measurement accuracy and environmental interference resistance.
Absolute measurement has always been one of the important development directions of precision measurement, there are problems that the diffraction and mask working parameters are not considered in the positioning pulse analysis of the absolute code mask at present. Therefore, in order to solve the coupling optimal performance problem of absolute positioning code in actual work, an absolute code should be designed for working in the best parameter-model. In this paper, a multi-parameter model of absolute code working status is established, and the influence of working parameters on its positioning performance is analyzed respectively. The analysis shows that the distance and the angle between the mask and the grating, and the width of the unit code will affect the positioning accuracy. The three parameters restrict each other, and there is a coupling optimal solution. The optimal working state can be obtained through parameter analysis, so as to provide the design and installation parameter guidance of mask. The proposed research can help the practical application of absolute positioning measurement.
Absolute testing for metrology has always been one of the important development directions of precision measurement. The mask with binary code is an important structure for forming absolute positioning pulses in grating encoders. The increase in the number of codes is beneficial to the resolution of positioning, but design of codes has always existed the problem that the optimal design cannot be obtained when the number of codes increases. This paper proposes a design method of binary code based on the genetic algorithm, which can get the required binary code more quickly when the number of codes is greater than 150 or even higher. The specific method can randomly generate binary codes with their fitness factors, and the binary codes enter the algorithm as the parents based on the mutation, crossover, and selection. Then the reproduce binary codes will have higher and higher fitness factor. This method can quickly generate satisfactory binary codes with specified performance, thus providing high resolution at the nanometer level for absolute positioning measurement. This work provides help and reference for future absolute positioning measurements.
Pixelated micro-polarizer array is an attractive polarization imaging device because of its real-time fully detecting capability to Stokes parameters and high integration level. However, this micro-device is commonly fabricated by electron beam lithography, thus is often high-cost. Since periodic unit of this device is a 2×2 array of four types of one-dimensional (1D) gratings with different orientations (0°, 45°, 90°, 135°), a low-cost grating fabrication technique, interference lithography (IL), is possible to be used to fabricate this device. In this research, a four-steps exposure patterning method of micro-polarizer array based on single Lloyd’s mirror interferometer IL is developed. This interferometer is composed by two perpendicular parts: a mirror and a photoresist holder, and a 1D grating parallel with mirror can be fabricated in the exposure region on photoresist through lithography. A Cr-based mask, whose transparent pattern is a square array of square windows with 15×15 μm2 window size and 17 μm window spacing, is introduced and clamped onto the photoresist to cut exposure region. In four exposure steps, each 15×15 μm2 quadrant area of a polarizer array with 2 μm quadrant spacing will be exposed on photoresist, respectively. Between two exposure steps, mask and photoresist substrate need to be simultaneously horizontal rotated, to adjust the orientation of grating fabricated in next exposed quadrant on photoresist. Based on this method, we fabricated the single-state patterned micro-polarizer array, which effectively controls the cost and fabrication cycle. The experimental results show that the photoresist grating within a single window has stable structure, uniform period and good directivity.
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