Dual-frequency Laser Interferometer Research Articles

Efficient identification of geometric errors in CNC machine tools based on dual-frequency laser interferometry

In contemporary production, the computer numerical control machine tool is an essential processing apparatus. However, its geometric errors can often impact processing accuracy and stability. Therefore, an innovative geometric error identification method for CNC machine tools is proposed, which uses the polarization information in the dual-frequency laser interferometer to improve the measurement accuracy. By optimizing the polarization state of the laser system, the ability to identify the geometric error of the machine tool is improved. The findings of this study indicated that the measurement accuracy, measurement range, ease of operation, reliability, cost and applicability of the dual-frequency laser interferometer-based geometric error identification method for computer numerical control machine tools were 0.91, 0.87, 0.93, 0.77, 0.94, 0.85 and 0.97, respectively, which were better than the comprehensive performance of other methods. The study's suggested method offers a solid foundation for raising the precision and machining quality of machine tools by effectively and precisely identifying the geometric faults of CNC tools. The study's findings also establish the groundwork for future widespread use of dual-frequency laser interferometers in the detection of geometric faults in computer numerical control machine tools by offering theoretical justification for real-world uses.

Real-Time Direction Judgment System for Dual-Frequency Laser Interferometer.

Current real-time direction judgment systems are inaccurate and insensitive, as well as limited by the sampling rate of analog-to-digital converters. To address this problem, we propose a dynamic real-time direction judgment system based on an integral dual-frequency laser interferometer and field-programmable gate array technology. The optoelectronic signals resulting from the introduction of a phase subdivision method based on the amplitude resolution of the laser interferometer when measuring displacement are analyzed. The proposed system integrates the optoelectronic signals to increase the accuracy of its direction judgments and ensures these direction judgments are made in real time by dynamically controlling the integration time. Several experiments were conducted to verify the performance of the proposed system. The results show that, compared with current real-time direction judgment systems, the proposed system makes accurate judgements during low-speed motions and can update directions within 0.125 cycles of the phase difference change at different speeds. Moreover, a sweep frequency experiment confirmed the system's ability to effectively judge dynamic directions. The proposed system is capable of accurate and real-time directional judgment during low-speed movements of a table in motion.

Research on hydrostatic leveling system to provide elevation constraints for control network adjustment

As the precise sensor system for monitoring the relative altitude changes among multiple points, the capacity hydrostatic leveling system (HLS) is widely used in particle accelerators. To expand its application in providing the elevation constraint for the control network adjustment, the research on the issue of the HLS for altitude difference measurement between multiple points is carried out. Based on the working principle of the HLS sensor, a comparison system composed of dual-frequency laser interferometer, high-precision Z stage, HLS sensors and others is designed and manufactured. The system is used to control multiple sensors to observe the same liquid level in the same coordinate system. The zero-position difference among sensors can be obtained by comparison. Then the altitude difference measurement can be realized, and it is verified that the measurement accuracy is better than 5 μm. In addition, a simulation experiment for 3D control network measurement is run, in which the HLS system provides the elevation constraint for the adjustment processing. The results show that for the 100m linear tunnel, the errors accumulation in the elevation direction is significantly improved compared to the classics adjustment.

Design and Analysis of a Long-Stroke and High-Precision Positioning System for Scanning Beam Interference Lithography

A macro–micro dual-drive positioning system was developed for Scanning Beam Interference Lithography (SBIL) which uses a dual-frequency laser interferometer as the position reference and exhibits the characteristics of long travel, heavy load, and high accuracy. The macro-motion system adopts a friction-driven structure and a feedforward PID control algorithm, and the stroke can reach 1800 mm. The micro-motion system adopts a flexible hinge–plus-PZT driving method and a PID control algorithm based on neural networks, which achieves sufficient positioning accuracy of this system at the nanometer level. An optical-path-sealing system was used to reduce the measurement noise of the dual-frequency laser interferometer. The static stability of the positioning system, the stepping capacity of the macro-motion system, the stepping capacity of the micro-motion system, and the positioning accuracy of the system were tested and analyzed. Additionally, the sources and effects of errors during the motion process were assessed in detail. Finally, the experimental results show that the workbench can locate at the nanoscale within the full range of travel, which can satisfy the SBIL exposure requirement.

Mechanical Property Test of Grass Carp Skin Material Based on the Digital Image Correlation Method.

Fish is a common and widely distributed creature. Its skin has a unique physiological structure and plays an important role in many fields. Fish skin also has important potential value for bionics research. This study aims to provide a method and a reliable data for the study of bionics. A method of measuring the mechanical properties of fish skin samples using a binocular stereo digital image correlation (DIC) system combined with a synchronous tensile testing machine was proposed. The mechanical properties (e.g., elastic modulus E and strain) of grass fish skin samples (GFSA) were tested in hydrophilic and dry states. A dual-frequency laser interferometer was used to calibrate the tensile testing machine synchronously, and the feasibility and strain accuracy of DIC in GFSA measurement were verified by finite element method (FEM). The results show differences in the mechanical properties of GFSA between different individuals, different parts, and different states. Under the same stress, the head was easy to deform, and the strain was the largest, and E was the smallest. The tail result was the opposite of the head result.

Environmental compensation of laser interferometer based on particle swarm algorithm.

A dual-frequency laser interferometer is an important instrument that can provide a meter-level measurement range with nanometer-level measurement accuracy at several meters per second measurement speed. The measurement accuracy of the dual-frequency laser interferometer plays a vital role based on laser wavelength, which is easily affected by environmental factors, so the air refractive index needs to be corrected. In this paper, an improved Edlen empirical formula air refractive index compensation method is introduced, in which a particle swarm optimization algorithm is used to optimize the Edlen formula by adjusting the compensation model online according to the compensation effect to adapt to different environmental conditions. The experimental results show that the root mean square error of the interferometer compensated for by the unoptimized Edlen formula can be reduced by 79.3% and 72.1% under different optical path lengths, and by 90.7% and 91.9% after compensation by the optimized Edlen formula, which means that the average compensation effect is improved by about 20%.

Error Analysis and Fault Diagnosis of CNC Machine Tools under Artificial Intelligence Technology

With the development of technology and the arrival of the era of artificial intelligence, the relationship between machine and human is becoming closer and closer. This paper mainly studies the error analysis and fault diagnosis of CNC machine tools based on artificial intelligence technology. The detection of machine tool error is mainly completed by laser interferometer. In this paper, a dual frequency laser interferometer is used. In order to ensure the efficiency of the program, the display image is refreshed in 20ms. According to the structure analysis, the error branches of CNC machine tools are determined, and the position feature relationship and motion feature relationship between adjacent bodies in each error branch are analyzed. In this paper, the fault tree analysis method is used to establish the fault tree model of computerized numerical control machine tool, and the fault reasoning method based on fault tree is given. On this basis, the deep neural network model is used to classify and identify the difficult to identify features such as fault degree, so as to achieve the purpose of deep fault diagnosis. The data show that the average recognition rate of BP network before feature reduction is 82%, and the average recognition rate of BP network after feature reduction is 86%. The results show that artificial intelligence technology can improve the accuracy of error analysis of CNC machine tools.

Microchip Nd:YAG dual-frequency laser interferometer for displacement measurement.

The dual-frequency laser interferometer is an indispensable instrument to measure length, position, deformation and other parameters with high precision and long measurement distance in the advanced manufacturing industry and scientific research. In general, the light source of interferometer is the dual-frequency He-Ne laser. The disadvantages of He-Ne laser include generally large size, more heat radiation to the environment, and limited service life, which limits its application. In this paper, we study a microchip Nd:YAG dual-frequency laser interferometer with a 17.4 MHz frequency difference, which is formed by the stress-induced birefringence in the microchip itself. The down-conversion mixer is designed to decrease the beat frequency to about 5 MHz in heterodyne signal preprocessing modular to meet the bandwidth of phase meter. The experimental results show that the microchip Nd:YAG dual-frequency laser interferometer has a displacement resolution of 10 nm and a measuring range of 500 mm. Due to the advantages of the microchip dual-frequency laser, such as small size (40×40×35 mm), good portability, less power consumption and almost infinite service life, the microchip Nd:YAG dual-frequency laser interferometer has broad application prospects.

基于振动节点优化提高双频激光干涉仪的测量精度

基于振动节点优化提高双频激光干涉仪的测量精度

Resolution verification sensing device for a stereo digital image correlation system based on dual-frequency laser interference.

Digital image correlation (DIC) is widely used in materials mechanics, nondestructive testing, and other fields due to its advantages of noncontact, full-field measurement, and simple experimental setup. As an optical measurement method to measure the three-dimensional shape and deformation of an object, measurement accuracy is one of the most important technical indicators of DIC. If DIC is made into a measuring instrument, the resolution is an important parameter that must be provided. In general, the higher the measurement accuracy of the instrument, the higher the resolution of the instrument. At present, the research on DIC focuses on the analysis of factors affecting measurement accuracy, the noise reduction of measurement results, and the improvement of correlation algorithms. There are few reports on the verification of the resolution of DIC measurement instruments. However, accuracy analysis and resolution verification of the measurement instrument is a vital technical task to ensure the credibility of the measurement data. In this paper, a high-precision dual-frequency laser interferometer principle is used to design a sensing device to verify the measurement resolution of the DIC instrument. The accuracy and resolution of the self-made stereo DIC instrument were tested and evaluated using this sensing device.

Multivariate orthogonal polynomial-based positioning error modeling and active compensation of dual-driven feed system

The nonsynchronous error of the dual-driven feed system seriously affects the positioning and machining accuracy of CNC machine tools. Improving the positioning error is an extremely important task as it determines the geometric accuracy of the manufactured parts. In this paper, a Multivariate Orthogonal Polynomial Regression model has been developed to predict the positioning error in terms of motion parameters such as position and speed of the X1 and X2 axes of the dual-driven worktable. Orthogonal Experimental Design has been engaged in conducting experiments. The positioning errors are measured by a dual-frequency laser interferometer. In addition, a real-time error compensation system is developed based on the proposed active compensation control strategy in the dual-driven feed system controlled by Beckhoff motion control card. Experimental results show that the proposed positioning error model and error compensation strategy can be utilized as an effective manner to improve the accuracy of dual-driven CNC machine tools.

Intelligentization of machine tools: comprehensive thermal error compensation of machine-workpiece system

The real-time compensation of thermal errors is an important part of the intelligent functions of machine tools. In order to promote the integrated application of thermal error compensation technique in machine tools, extensive research is still lacking. Thermal error compensation for machine tools and workpiece systems is not widely used, and in-depth research on physically based modeling methods is limited. The thermal behavior of the machine-workpiece system was analyzed, and a thermal error prediction model, based on the heat transfer theory, was established. The effects of screw discretization, as well as single- or multi-heat sources on the physically based prediction model, were analyzed using simulation data. The effect of comprehensive thermal error compensation of the machine-workpiece system was verified in three ways. These ways were measuring with a dual-frequency laser interferometer, machining features visible to the naked eye, and measuring with CMM (coordinate measuring machine). The results proved the high accuracy and strong robustness of the proposed compensation method. This research work will greatly promote the application of thermal error compensation technology in machine tools.

Nonlinear error analysis and experimental measurement of Birefringence-Zeeman dual-frequency laser interferometer

Abstract After a long-term research, the birefringence-Zeeman dual-frequency (BZDF) laser has entered the practical stage, and the BZDF laser interferometer has more powerful displacement and size measurement functions, especially the near-zero nonlinearity. This paper theoretically analyzes the mechanism of the near-zero nonlinear error, including theoretical simulation and experimental measurements. The measurement results show that the nonlinear error of the BZDF laser interferometer is only 0.3 nm, which is one order of magnitude smaller than that of traditional Zeeman dual-frequency (ZDF) laser interferometer. It has broad application prospects in the field of ultra-precision metrology.

Simple and compact grating-based heterodyne interferometer with the Littrow configuration for high-accuracy and long-range measurement of two-dimensional displacement.

We propose a simple and compact reading head with the Littrow configuration that will increase measurement range and reduce the complexity of a two-dimensional grating-based interferometer. The reading head contains only a beam splitter, two polarizing beam splitter modules, and two mirrors. The theoretical resolutions in two directions are 0.27nm and 0.18nm, respectively. In comparison with a dual-frequency laser interferometer, the proposed interferometer can measure displacement from 3nm to 10mm with high accuracy. The 3σ values in two directions for the difference are 1.67nm and 1.35nm for a displacement of 9nm. Repeatability for a displacement of 1μm is better than 2nm.

Alignment Method for Linear-Scale Projection Lithography Based on CCD Image Analysis.

This paper presents a method to improve the alignment accuracy of a mask in linear scale projection lithography, in which the adjacent pixel gray square variance method is applied to a charge-coupled device (CCD) image to obtain the best position of the focal length of the motherboard and then realize the alignment of the focal plane. Two image positions in the focal plane of the CCD are compared with the traits overlap according to the image splicing principle, and four typical errors are corrected on the basis of the total grating errors. Simultaneously, the rotation error of the mask is used to summarize the grayscale variation function of the CCD image. Threshold functions are employed to express the factors including the wave crests of the amplitude, period error, and phase error, which govern the rotation accuracy and weight alignment accuracy expression of the established four error factors. Finally, in the experiment, the slope of the mask is corrected and adjusted to the same direction as the slide plate with the assistance of a dual-frequency laser interferometer. The effect of the alignment error on the lithography accuracy is discussed and verified in the static case, and it is found that the CCD maximum resolution pixel is 0.1 μm and accuracy of the scale is 0.79 μm in only a 200-mm-measurement range.

Kinematic errors prediction for multi-axis machine tools’ guideways based on tolerance

In this paper, a systematic approach on how to predict kinematic errors based on tolerance of machine tools’ guideways is introduced. Firstly, the truncated Fourier series function is applied to fit curve of guideways surface. Since geometric profile errors are regarded as a bridge between tolerance and kinematic errors of machine tools’ guideways, the mapping relationship between tolerance and geometric profile errors of machine tools’ guideways is formulated, and the mapping relationship between geometric profile errors and kinematic errors of guideways is established. Then, kinematic errors prediction model based on tolerance of guideways is subsequently proposed. Finally, simulation verification is conducted with this method. Simulation results show the range of the predicted kinematic errors (positioning error, y direction and z direction straightness error, roll error, pitch error, and yaw error) is 17.12 μm, 56.57 μm, 70.71 μm, 28.28 μrad, 141.42 μrad, and 113.14 μrad, respectively. In order to verify the feasibility and effectiveness of the presented method, a measuring experiment is carried out on guideways of a gantry-type five-axis milling machine tools by using a dual-frequency laser interferometer. The measured and identified discrete data can be fitted precisely by Fourier curve fitting method. The fitting results show the range of the measured kinematic errors is 16.96 μm, 59.43 μm, 68.63 μm, 28.65 μrad, 135.40 μrad, and 111.58 μrad, respectively. The maximum residual errors between the predicted and measured values of kinematic errors are 1.67 μm, 5.19 μm, 5.50 μm, 1.87 μrad, 9.81 μrad, and 7.07μrad, respectively. Comparing with the measured results of kinematic errors, residual errors are considerably small and can be neglected. Therefore, there is no doubt that this method is effective enough for predicting kinematic errors and can be used to replace the measurement of kinematic errors. In the design stage of machine tools, this approach is convenient for engineers to derive the distribution of kinematic errors. And its basic idea can be applied to other type of machine tools’ guideways.

A Large Range Linear Displacement Calibration System Based on Coordinate Measurement and Laser Interference

The linear displacement sensor is widely used in industrial measurement. The linear displacement sensor with measuring range less than 200mm is usually measured on a universal length measuring instrument. There is no good method to calibrate linear displacement sensor with measuring range larger than 200mm. The calibration system of linear displacement based on coordinate measurement and dual frequency laser interference is developed. The guide rail and measuring positioning system of coordinate measuring machine is used. The sampling signal of dual frequency laser interferometer is connected with the pulse signal triggered by coordinate measuring machine. The accurate position information of the linear displacement sensor is obtained and processed by industrial computer. According to JJF 1305-2011 Calibration Specification for Linear Displacement Sensor, a large range of linear displacement sensors is calibrated. The accuracy and reliability of linear displacement calibration system based on coordinate measurement and laser interference is verified.

Analysis and compensation of synchronous measurement error for multi-channel laser interferometer

Dual-frequency laser interferometer has been widely used in precision motion system as a displacement sensor, to achieve nanoscale positioning or synchronization accuracy. In a multi-channel laser interferometer synchronous measurement system, signal delays are different in the different channels, which will cause asynchronous measurement, and then lead to measurement error, synchronous measurement error (SME). Based on signal delay analysis of the measurement system, this paper presents a multi-channel SME framework for synchronous measurement, and establishes the model between SME and motion velocity. Further, a real-time compensation method for SME is proposed. This method has been verified in a self-developed laser interferometer signal processing board (SPB). The experiment result showed that, using this compensation method, at a motion velocity 0.89 m s−1, the max SME between two measuring channels in the SPB is 1.1 nm. This method is more easily implemented and applied to engineering than the method of directly testing smaller signal delay.

Design and testing of micro-tribometer with macro applied load

With the development of superlubricity, the requirement for the accuracy of measuring super low friction force becomes more and more high. In this study, a novel micro-tribometer has been designed. The resolution and accuracy of friction force are 0.01 mN by using the dual frequency laser interferometer. Experiments were performed to investigate the ability of measuring friction force from different aspects. The interference signal mixed in the measured friction force curve was analyzed and can be removed by a designed filter. The results of experiment show that the tribometer is capable of measuring a super low friction force in the order of magnitude of 0.01 mN with an applied load up to 1 N.

3D robust digital image correlation for vibration measurement.

Discrepancies of speckle images under dynamic measurement due to the different viewing angles will deteriorate the correspondence in 3D digital image correlation (3D-DIC) for vibration measurement. Facing this kind of bottleneck, this paper presents two types of robust 3D-DIC methods for vibration measurement, SSD-robust and SWD-robust, which use a sum of square difference (SSD) estimator plus a Geman-McClure regulating term and a Welch estimator plus a Geman-McClure regulating term, respectively. Because the regulating term with an adaptive rejecting bound can lessen the influence of the abnormal pixel data in the dynamical measuring process, the robustness of the algorithm is enhanced. The robustness and precision evaluation experiments using a dual-frequency laser interferometer are implemented. The experimental results indicate that the two presented robust estimators can suppress the effects of the abnormality in the speckle images and, meanwhile, keep higher precision in vibration measurement in contrast with the traditional SSD method; thus, the SWD-robust and SSD-robust methods are suitable for weak image noise and strong image noise, respectively.