Interferometric polarization compensation based on one single polarization-maintaining fiber.

In an interferometry system based on one single polarization-maintaining fiber (PMF), defects like the laser's ellipticity, the alignment error between the PMF and the laser source, and the PMF's internal stress will cause the emitted light from the PMF to be incompletely linearly polarized, resulting in nonlinear errors that cannot be ignored. This paper proposes a novel method that can realize polarization compensation for heterodyne interferometry, reduce the ellipticity of the emitted light, and thereby reduce the nonlinear error of the system. When using a PMF with an Extinction Ratio (ER) of 22 dB, the experimental results show that this method can reduce the polarization and increase the ER to 33.95 dB. After polarization compensation, the nonlinear error is reduced from 7.22 nm to 2.02 nm. The proportion of the nonlinear error reduction reaches to 71.99%, which greatly improves the accuracy of the system.

Analysis and experimental verification of the polarization characteristics of cube-corner reflectors.

The polarization effect of cube-corner reflectors (CCRs), which influences the performance of optical systems, requires comprehensive analysis. This study developed a model for the polarization state of uncoated solid and hollow CCRs using the Jones matrix derivation and Zemax software simulations. The accuracies of theoretical analyses and simulations were verified using an experimental setup. Theoretical analysis, simulation, and experimental results revealed that hollow CCRs are insensitive to the polarization state of the incident light, exhibiting average variations of 0.8° and 0.7° in the polarization direction and ellipticity, respectively. Contrastingly, the high sensitivity of solid CCRs to the polarization state of the incident light varied across different incident regions. The propagation paths 2-1-3 and 3-1-2 with minor polarization effects involved light that entered from one side of the CCR, traversed the bottom, and emitted from the other side. In these regions, the average variations in the polarization direction and ellipticity were 10.7° and 6.6°, respectively, whereas more affected regions exhibited corresponding values of 44.8° and 20.0°. These findings guide the enhancement and optimization of the performance of optical systems using CCRs.

Method and system for simultaneously measuring six degrees of freedom motion errors of a rotary axis based on a semiconductor laser.

The rotary axis is the basis of rotational motion. The motion errors of a rotary axis have an extremely important impact on the accuracy of precision machining measuring equipment such as CNC machines, robot manipulators, and laser trackers. It is a difficult problem to realise the fast and precision simultaneous measurement of multi-degree-of-freedom motion errors of a rotary axis. Therefore, a novel method for the simultaneous measurement of six-degree-of-freedom motion errors of a rotary axis by a single-mode fiber coupled semiconductor laser is proposed in this paper. The corresponding system is developed, which has the advantages of high measurement efficiency, simple structure and low cost. A phase-solving method taking the advantages of both the eight-subdivision and the Cordic algorithm is proposed to solve the phase of interference signal, cannot only realize the high-resolution solving of the current signal phase but also quickly obtain high-precision interferometric results. A series of experiments were carried out on the developed system. An experimental system was built and a series of experiments were performed. The experimental results show that the standard deviation of stability for 1 hour of the six-degree-of-freedom measurement is 0.03 µm, 0.02 µm, 0.03 µm, 0.10 ' ' , 0.05 ' ' and 0.03 ' ' , respectively. The repeatability deviation of measuring a rotary axis is ±0.16 µm, ± 0.29 µm, ± 0.25 µm, ± 0.65 ' ' , ± 0.62 ' ' and ±13.42 ' ' , respectively. The maximum deviation of comparison with standard instruments is 0.46 µm, 1.00 µm, 0.49 µm, 1.06 ' ' , 1.53 ' ' and 0.74 ' ' , respectively. It provides a low-cost and high-precision measurement method for simultaneous measurement of six-degree-of-freedom motion errors of rotary axis of precision machining and measuring equipment.

High-precision method for simultaneously measuring the six-degree-of-freedom relative position and pose deformation of satellites.

The high-precision measurement of the six degrees-of-freedom (6DoF) relative position and pose deformation of satellites on the ground in vacuum and high-/low-temperature environments plays a critical role in ensuring the on-orbit mapping accuracy of satellites. To meet the strict measurement requirements for a satellite of a high accuracy, high stability, and a miniaturized measurement system, this paper proposes a laser measurement method for simultaneously measuring 6DoF relative position and attitude. In particular, a miniaturized measurement system was developed and a measurement model was established. The problem of error crosstalk between the 6DoF relative position and pose measurements was solved by conducting a theoretical analysis and OpticStudio software simulation, and the measurement accuracy was improved. Laboratory experiments and field tests were then conducted. The experimental results revealed that the measurement accuracy of the developed system for the relative position and relative attitude reached 0.2 µm and 0.4", within the measurement ranges of 500 mm along the X axis, ±100 µm along Y and Z axes, and ±100", and the 24-h measurement stabilities were superior to 0.5 µm and 0.5", respectively, which meets the ground measurement requirements for the satellite. The developed system was successfully applied on site, and the 6Dof relative position and pose deformation of the satellite were obtained via a thermal load test. This novel measurement method and system provides an experimental means for satellite development, in addition to a method for the high-precision measurement of the relative 6DoF position and pose between two points.

Photoelectric Measurement and Sensing: New Technology and Applications.

Laser-based measurement and sensing technology has been paid more and more attention by academia and industry because of its incomparable advantages, such as high sensitivity, fast response, and no contact [...].

Speckle Measurement for Small In-Plane Vibration Using GaAs.

In this study, the measurement characteristics of speckles based on the photoinduced electromotive force (photo-emf) effect for high-frequency, small-amplitude, and in-plane vibration were theoretically and experimentally studied. The relevant theoretical models were utilized. A GaAs crystal was used as the photo-emf detector for experimental research, as well as to study the influence of the amplitude and frequency of the vibration, the imaging magnification of the measuring system, and the average speckle size of the measuring light on the first harmonic of the induced photocurrent in the experiments. The correctness of the supplemented theoretical model was verified, and a theoretical and experimental basis was provided for the feasibility of using GaAs to measure in-plane vibrations with nanoscale amplitudes.

Recent Advances in Wayside Railway Wheel Flat Detection Techniques: A Review.

Wheel flats are amongst the most common local surface defect in railway wheels, which can result in repetitive high wheel-rail contact forces and thus lead to rapid deterioration and possible failure of wheels and rails if not detected at an early stage. The timely and accurate detection of wheel flats is of great significance to ensure the safety of train operation and reduce maintenance costs. In recent years, with the increase of train speed and load capacity, wheel flat detection is facing greater challenges. This paper focuses on the review of wheel flat detection techniques and flat signal processing methods based on wayside deployment in recent years. Commonly used wheel flat detection methods, including sound-based methods, image-based methods, and stress-based methods are introduced and summarized. The advantages and disadvantages of these methods are discussed and concluded. In addition, the flat signal processing methods corresponding to different wheel flat detection techniques are also summarized and discussed. According to the review, we believe that the development direction of the wheel flat detection system is gradually moving towards device simplification, multi-sensor fusion, high algorithm accuracy, and operational intelligence. With continuous development of machine learning algorithms and constant perfection of railway databases, wheel flat detection based on machine learning algorithms will be the development trend in the future.

Error Analysis of Heterodyne Interferometry Based on One Single-Mode Polarization-Maintaining Fiber.

Using polarization-maintaining fiber (PMF) in dual-frequency heterodyne interferometry has the advantages of reducing the laser's own drift, obtaining high-quality light spots, and improving thermal stability. Using only one single-mode PMF to achieve the transmission of dual-frequency orthogonal, linearly polarized beam requires angular alignment only once to realize the transmission of dual-frequency orthogonal, linearly polarized light, avoiding coupling inconsistency errors, so that it has the advantages of high efficiency and low cost. However, there are still many nonlinear influencing factors in this method, such as the ellipticity and non-orthogonality of the dual-frequency laser, the angular misalignment error of the PMF, and the influence of temperature on the output beam of the PMF. This paper uses the Jones matrix to innovatively construct an error analysis model for the heterodyne interferometry using one single-mode PMF, to realize the quantitative analysis of various nonlinear error influencing factors, and clarify that the main error source is the angular misalignment error of the PMF. For the first time, the simulation provides a goal for the optimization of the alignment scheme of the PMF and the improvement of the accuracy to the sub-nanometer level. In actual measurement, the angular misalignment error of the PMF needs to be smaller than 2.87° to achieve sub-nanometer interference accuracy, and smaller than 0.25° to make the influence smaller than ten picometers. It provides theoretical guidance and an effective means for improving the design of heterodyne interferometry instruments based on PMF and further reducing measurement errors.

Simultaneous measurement of 5DOF spindle error motions in CNC machine tools.

We present a method for the simultaneous measurement of 5 degree-of-freedom (DOF) spindle error motions in computer numerical control (CNC) machine tools and develop a measurement system. The measurement system uses polarization-maintaining fiber coupled with a dual-frequency laser as the light source. The axial error motion of the spindle is measured by heterodyne interferometry, and other 4DOF error motions are obtained by collimation measurement. Based on the measurement system, a comprehensive measurement model of spindle error motion was established. The influence of system error on the measurement accuracy of the spindle tilt error motion and radial error motion was analyzed. According to the measurement model, a corresponding data-processing method is proposed, which removes some systematic errors. A series of experiments was conducted to verify the feasibility and effectiveness of the proposed measurement system and the corresponding measurement model.

On-Site Calibration Method for Line-Structured Light Sensor-Based Railway Wheel Size Measurement System.

Line-structured light has been widely used in the field of railway measurement, owing to its high capability of anti-interference, fast scanning speed and high accuracy. Traditional calibration methods of line-structured light sensors have the disadvantages of long calibration time and complicated calibration process, which is not suitable for railway field application. In this paper, a fast calibration method based on a self-developed calibration device was proposed. Compared with traditional methods, the calibration process is simplified and the calibration time is greatly shortened. This method does not need to extract light strips; thus, the influence of ambient light on the measurement is reduced. In addition, the calibration error resulting from the misalignment was corrected by epipolar constraint, and the calibration accuracy was improved. Calibration experiments in laboratory and field tests were conducted to verify the effectiveness of this method, and the results showed that the proposed method can achieve a better calibration accuracy compared to a traditional calibration method based on Zhang's method.

In-Service Detection and Quantification of Railway Wheel Flat by the Reflective Optical Position Sensor.

Railway wheel tread flat is one of the main faults of railway wheels, which brings great harm to the safety of vehicle operation. In order to detect wheel flats dynamically and quantitatively when trains are running at high speed, a new wheel flat detection system based on the self-developed reflective optical position sensor is demonstrated in this paper. In this system, two sensors were mounted along each rail to measure the wheel-rail impact force of the entire circumference by detecting the displacement of the collimated laser spot. In order to establish a quantitative relationship between the sensor signal and the wheel flat length, a vehicle-track coupling dynamics analysis model was developed using the finite element method and multi-body dynamics method. The effects of train speed, load, wheel flat lengths, as well as the impact positions on impact forces were simulated and evaluated, and the measured data can be normalized according to the simulation results. The system was assessed through simulation and laboratory investigation, and real field tests were conducted to certify its validity and correctness. The system can determine the position of the flat wheel and can realize the quantification of the detected wheel flat, which has extensive application prospects.

Simultaneous Measurement of 6DOF Motion Errors of Linear Guides of CNC Machine Tools Using Different Modes.

Based on the prior work on the six degrees of freedom (6DOF) motion errors measurement system for linear axes, and for the different types of machine tools and different installation methods, this study used a ray tracing idea to establish the measurement models for two different measurement modes: (1) the measurement head is fixed and the target mirror moves and (2) the target mirror is fixed and the measurement head moves. Several experiments were performed on the same linear guide using two different measurement modes. The comparative experiments show that the two measurement modes and their corresponding measurement models are correct and effective. In the actual measurement process, it is therefore possible to select the corresponding measurement model according to the measurement mode. Furthermore, the correct motion error evaluation results can be obtained.

A Rail Fastener Tightness Detection Approach Using Multi-source Visual Sensor.

At present, the method of two-dimensional image recognition is mainly used to detect the abnormal fastener in the rail-track inspection system. However, the too-tight-or-too-loose fastener condition may cause the clip of the fastener to break or loose due to the high frequency vibration shock, which is difficult to detect from the two-dimensional image. In this practical application background, 3D visual detection technology provides a feasible solution. In this paper, we propose a fundamental multi-source visual data detection method, as well as an accurate and robust fastener location and nut or bolt segmentation algorithm. By combining two-dimensional intensity information and three-dimensional depth information generated by the projection of line structural light, the locating of nut or bolt position and accurate perception of height information can be realized in the dynamic running environment of railway. The experimental results show that the static measurement accuracy in the vertical direction using the structural light vision sensor is 0.1 mm under the laboratory condition, and the dynamic measurement accuracy is 0.5 mm under the dynamic train running environment. We use dynamic template matching algorithm to locate fasteners from 2D intensity map, which achieves 99.4% accuracy, then use the watershed algorithm to segment the nut and bolt from the corresponding depth image of located fastener. Finally, the 3D shape of the nut and bolt is analyzed to determine whether the nut or bolt height meets the local statistical threshold requirements, so as to detect the hidden danger of railway transportation caused by too loose or too tight fasteners.

Railway Wheel Flat Detection System Based on a Parallelogram Mechanism.

Wheel flats are a key fault in railway systems, which can bring great harm to vehicle operation safety. At present, most wheel flat detection methods use qualitative detection and do not meet practical demands. In this paper, we used a railway wheel flat measurement method based on a parallelogram mechanism to detect wheel flats dynamically and quantitatively. Based on our experiments, we found that system performance was influenced by the train speed. When the train speed was higher than a certain threshold, the wheel impact force would cause vibration of the measuring mechanism and affect the detection accuracy. Since the measuring system was installed at the on-site entrance of the train garage, to meet the speed requirement, a three-dimensional simulation model was established, which was based on the rigid-flexible coupled multibody dynamics theory. The speed threshold of the measuring mechanism increased by the reasonable selection of the damping coefficients of the hydraulic damper, the measuring positions, and the downward displacements of the measuring ruler. Finally, we applied the selected model parameters to the parallelogram mechanism, where field measurements showed that the experimental results were consistent with the simulation results.

Long-Term Stable Online Acetylene Detection by a CEAS System with Suppression of Cavity Length Drift.

A trace acetylene (C2H2) detection system was demonstrated using the cavity-enhanced absorption spectroscopy (CEAS) technique and a near-infrared distributed feedback (NIR-DFB) laser. A Fabry⁻Perot (F⁻P) cavity with an effective optical path length of 49.7 m was sealed and employed as a gas absorption cell. Co-axis cavity alignment geometry was adopted to acquire a larger transmitted light intensity and a higher sensitivity compared with off-axis geometry. The laser frequency was locked to the cavity fundamental mode (TEM00 mode) by using the Pound⁻Drever⁻Hall (PDH) technique continuously. By introducing a cavity length-locking loop, the drift of the cavity length was suppressed, and the stability of the system was enhanced. To demonstrate the efficacy of the system, a C2H2 absorption spectrum near 6534.36 cm-1 was acquired by tuning the laser operation temperature. Measurements of C2H2 samples with different concentrations were carried out, and a good linear relationship between C2H2 concentration and the cavity-transmitted signal voltage was observed. The measurement results showed the system could work stably for more than 2 h without major fluctuations. The Allan variance analysis results demonstrated a detection limit of 9 parts-per-billion (ppb) with an averaging time of 11 s corresponding to a minimum detectable absorption coefficient of 1.1 × 10-8 cm-1.

A High-Precision Method for Dynamically Measuring Train Wheel Diameter Using Three Laser Displacement Transducers.

Wheel diameter is a significant geometric parameter related to the safe operation of trains, and needs to be measured dynamically. To the best of the authors' knowledge, most existing dynamic measurement methods and systems do not meet the requirement that the wheel diameter measurement error for the high-speed vehicle is less than 0.3 mm. In this paper, a simple method for dynamically and precisely measuring train wheel diameter using three one-dimensional laser displacement transducers (1D-LDTs) is proposed for the first time, and a corresponding measurement system which was developed is described. The factors that affect the measurement accuracy were analyzed. As a main factor, rail deformation caused by the wheel-rail interaction force at low (20 km/h) and high (300 km/h) speeds was determined based on the combination of multi-body dynamics and finite element methods, and the effect of rail deformation on measurement accuracy is greatly reduced by a comparative measurement. Field experiments were performed to verify the performance of the developed measurement system, and the results of the repeatability error and measurement error of the system were both less than 0.3 mm, which meets the requirement of wheel diameter measurements for high-speed vehicles.

A Method for Simultaneously Measuring 6DOF Geometric Motion Errors of Linear and Rotary Axes Using Lasers.

A novel method for simultaneously directly measuring six-degrees-of-freedom (6DOF) geometric motion errors of CNC machine tools was proposed, and a corresponding measurement system was developed. This method can not only be applied for measuring a linear axis, but also for a rotary axis. A single-mode fiber was used to separate the measuring unit from the laser source in order to ensure system thermal stability and measurement accuracy. The method has the advantages of high efficiency and good accuracy, and requires no complicated decoupling calculation. The positioning error of the linear axis and radial motion error of the rotary axis are measured by laser interferometry and other 5DOF geometric motion errors by laser collimation. A series of experiments were performed to verify the feasibility and effectiveness of the developed measurement system.

Method for simultaneous measurement of five DOF motion errors of a rotary axis using a single-mode fiber-coupled laser.

The rotary axis is the basis for rotational motion. Its motion errors have critical effects on the accuracy of the related equipment, such as a five-axis computer numerical control machine tool. There are several difficult problems in the implementation of high-precision and fast measurement of the multi-degree-of-freedom motion errors of a rotary axis. In this paper, a novel method for the simultaneous measurement of five-degree-of-freedom motion errors of a rotary axis is proposed, which uses a single-mode fiber-coupled laser with a full-circle measuring range. It has the advantages of high efficiency, low cost, and it requires no decoupling calculation. An experimental system was built and a series of experiments were performed. The standard deviation of stability for 60 min of the five-degree-of-freedom measurement is 0.05 arcsec, 0.06 arcsec, 0.04 µm, 0.03 µm, and 0.19 arcsec, respectively. The repeatability deviation of measuring an indexing table is ± 3.4 arcsec, ± 4.6 arcsec, ± 2.6 µm, ± 2.4 µm, and ± 3.2 arcsec. The maximum deviation of comparison is 3.9 arcsec and 3.2 arcsec. These results demonstrate the effectiveness of the proposed method; thus, a new approach of simultaneous measurement of the multi-degree-of-freedom motion errors of a rotary axis is provided.

Measurement system and model for simultaneously measuring 6DOF geometric errors.

A measurement system to simultaneously measure six degree-of-freedom (6DOF) geometric errors is proposed. The measurement method is based on a combination of mono-frequency laser interferometry and laser fiber collimation. A simpler and more integrated optical configuration is designed. To compensate for the measurement errors introduced by error crosstalk, element fabrication error, laser beam drift, and nonparallelism of two measurement beam, a unified measurement model, which can improve the measurement accuracy, is deduced and established using the ray-tracing method. A numerical simulation using the optical design software Zemax is conducted, and the results verify the correctness of the model. Several experiments are performed to demonstrate the feasibility and effectiveness of the proposed system and measurement model.

System for simultaneously measuring 6DOF geometric motion errors using a polarization maintaining fiber-coupled dual-frequency laser.

A novel method for simultaneously measuring six degree-of-freedom (6DOF) geometric motion errors is proposed in this paper, and the corresponding measurement instrument is developed. Simultaneous measurement of 6DOF geometric motion errors using a polarization maintaining fiber-coupled dual-frequency laser is accomplished for the first time to the best of the authors' knowledge. Dual-frequency laser beams that are orthogonally linear polarized were adopted as the measuring datum. Positioning error measurement was achieved by heterodyne interferometry, and other 5DOF geometric motion errors were obtained by fiber collimation measurement. A series of experiments was performed to verify the effectiveness of the developed instrument. The experimental results showed that the stability and accuracy of the positioning error measurement are 31.1 nm and 0.5 µm, respectively. For the straightness error measurements, the stability and resolution are 60 and 40 nm, respectively, and the maximum deviation of repeatability is ± 0.15 µm in the x direction and ± 0.1 µm in the y direction. For pitch and yaw measurements, the stabilities are 0.03″ and 0.04″, the maximum deviations of repeatability are ± 0.18″ and ± 0.24″, and the accuracies are 0.4″ and 0.35″, respectively. The stability and resolution of roll measurement are 0.29″ and 0.2″, respectively, and the accuracy is 0.6″.