Shape Measurement Method Research Articles

An Efficient Polarization Code-Multiplexed 3-D Shape Measurement Method for High-Dynamic Range Objects

An Efficient Polarization Code-Multiplexed 3-D Shape Measurement Method for High-Dynamic Range Objects

Three-dimensional shape measurement method based on composite cyclic phase coding.

Phase coding is widely used in 3D measurement due to its good anti-interference and robustness. However, the measurement accuracy is affected by the limitation of the number of codewords. To solve this problem, we propose a 3D shape measurement method based on composite cyclic phase coding. The traditional phase coding is quantized cyclic without adding extra patterns, further adopting composite coding, using the composite cyclic phase coding grayscale values to distinguish the same cyclic codewords, and finally integrating them into a new fringe order sequentially for phase unwrapping to achieve effective expansion of codewords. The related experimental results show that the proposed method stably achieves high accuracy 3D reconstruction, which overcomes the misjudgment of codewords caused by traditional phase coding under high-frequency fringes due to system nonlinearity and noise. Meanwhile, compared with the improved phase coding method of temporal domain combined with spatial domain information, such as the method of quantized phase coding and connected region labeling, it can effectively avoid the phenomenon of error propagation, with high robustness and low algorithm complexity.

Cylindrical surface shape measurement method and measurement comparison

Optical elements are used in various fields, in which it is required to measure spherical, aspherical, and free-form surface shapes. We use a nanoprofiler previously developed by our research group to measure a cylindrical surface shape. Unlike spherical or aspherical shapes, the cylindrical surface shape of free-form surface is rotationally asymmetrical and thus difficult to determine the measured position. Therefore, we processed fiducial marks on the measured sample and specified the measurement position by fiducial marks. Then we performed comparative measurements with a Fizeau interferometer using a computer generated hologram (CGH) as a reference plane. The nanoprofiler and interferometer measured to accuracies of 43.7 nm peak-to-valley (PV) and 60.9 nm PV, respectively; the difference between the two was 17.2 nm PV. Furthermore, a spherical shape and an aspherical shape were similarly measured by both devices. The spherical shape had a difference of 8.4 nm PV and that of the aspherical shape was 9.7 nm PV; thus the difference between the measurements of the cylindrical surface shape was larger. Considering that the guaranteed accuracy of CGH for measuring a cylindrical surface shape is ∼30 nm PV, we concluded that this is a reasonable result and fiducial marks are effective to determine the position.

Omnidirectional 3D shape measurement using image outlines reconstructed from Gabor digital holography

We propose an omnidirectional 3D shape measurement method using the contour information of an object from Gabor holograms captured in omnidirectional measurements. To obtain the correct contour information from Gabor holograms, an edge-enhanced image reconstruction method is developed. The 3D shape can be restored by the rigid-body transformation of the contour information acquired from all directions. Omnidirectional 3D shape measurements are conducted using an aspheric lens and a spiral-shaped wire. The proposed method can successfully restore the complex 3D shapes of objects.

A structured-light 3D sensor-based shape measurement method for casting allowance evaluation

The evaluation of casting allowance currently relies on manual operation, which is time-consuming and unstable. The structured-light three-dimensional (3D) sensor-based 3D shape measurement technology is characterized by non-contact, high accuracy, and fast measuring speed, which provides the complete 3D shape of casting for accurate allowance evaluation. However, measuring a complex shape casting still requires numerous manual operations, including attaching marks, planning the measurement process, and data processing. To solve these problems, a robot-driven structured-light 3D sensor-based measurement method is proposed, with a six-degree of freedom (6-DOF) industrial robot and a turntable to control the structured-light 3D measurement sensor, to transform the 3D results from different measuring viewpoints into a coherent coordinate system, and to form a complete 3D shape of the casting. Then, the mechanical processing allowance will be calculated by comparing the 3D measurement result and the standard CAD model automatically. The experiment results show that the proposed method is accurate and efficient, and the casting allowance evaluation time is about ten times faster than that of manual operation. Moreover, the proposed method would provide valuable experience for other industrial applications.

Calibration method for a structured light system based on DIC.

Structured light is a non-contact three-dimensional shape measurement method. The structured light system based on diffractive optical elements (DOEs) is widely used due to its low cost and compact structure. However, compared with a time sequence coded structured light system based on a digital projector, its projector cannot change the content, so it cannot be calibrated with a phase-shift-based method. This paper proposes a calibration method based on the pseudo-camera method and digital image correlation (DIC). It is suitable not only for the calibration of systems based on time series coding but also for the calibration of a speckle structured light system based on DOEs. Validation experiments were conducted in which our method achieved a reprojection error of 0.68 pixels in calibration and a radius error of less than 1% in a measurement of a cylinder in a 20cm×10cm field. Compared with existing calibration methods, this method does not need to set a datum plane, takes fewer photos, operates simply, and has higher calibration efficiency.

High-efficiency and robust binary fringe optimization for superfast 3D shape measurement.

By utilizing 1-bit binary fringe patterns instead of conventional 8-bit sinusoidal patterns, binary defocusing techniques have been successfully applied for high-speed 3D shape measurement. However, simultaneously achieving high accuracy and high speed remains challenging. To overcome this limitation, we propose a high-efficiency and robust binary fringe optimization method for superfast 3D shape measurement, which consists of 1D optimization and 2D modulation. Specifically, for 1D optimization, the three-level OPWM technique is introduced for high-order harmonics elimination, and an optimization framework is presented for generating the 'best' three-level OPWM pattern especially for large fringe periods. For 2D modulation, a single-pattern three-level OPWM strategy is proposed by utilizing all the dimensions for intensity modulation to decrease the required projection patterns. Thus, the proposed method essentially belongs to the 2D modulation technique, yet iterative optimization is carried out along one dimension, which drastically improves the computational efficiency while ensuring high accuracy. With only one set of optimized patterns, both simulations and experiments demonstrate that high-quality phase maps can be consistently generated for a wide range of fringe periods (e.g., from 18 to 1140 pixels) and different amounts of defocusing, and it can achieve superfast and high-accuracy 3D shape measurement.

Verification of Xiphoid Point as a New Measuring Point for Chest Shape Measurement.

The measurement point of chest shape based on non-rigorous ergonomics is the key problem affecting the accuracy of measurement results. Xiphoid point, as a representative human skeleton point, can provide a theoretical basis for the development of new chest shape measurement methods and technology. In order to verify the stability of xiphoid point, 30 subjects were selected to measure the chest shape based on upper limb movement in the transverse plane of human body. The displacement of mastoid and xiphoid points in the sagittal plane, coronal plane, and axial plane was compared, and the factors influencing the displacement of mastoid and xiphoid points were analyzed. The results showed that the displacement of the mammary point was the largest in the coronal plane during the upper limb movement. The displacement of xiphoid point is far less than that of mastoid point. The displacement of the breast point is closely related to the chest circumference when the upper limb is in the horizontal adduction. There was a positive correlation between the displacement of xiphoid point and the amplitude of upper limb movement. Compared with mammary point, xiphoid point is a fixed and stable measurement point of chest shape.

Phase shifting profilometry based on Hilbert transform: An efficient phase unwrapping algorithm

Digital fringe projection profilometry based on phase-shifting technology is a reliable method for complex shape measurement, and the phase is one of the most important factors affecting measurement accuracy. The calculation of the absolute phase depends on the calculation of the wrapped phase and encoding technology. In this paper, a technique of obtaining the absolute phase of multi-frequency heterodyne fringe images using the Hilbert transform is presented. Since the wrapped phase can be calculated from only one fringe image of each frequency, the method does not need phase-shifting. The absolute phase can be obtained from the wrapped phase by applying the heterodyne method. The measurement time and computational complexity are dramatically reduced, the measurement efficiency is greatly improved, and this benefit from the number of images is greatly reduced. The experimental results show that the method presented in this paper performs well in the application, and the accuracy is no different from that of the phase-shifting method while the efficiency is greatly improved.

A Shape Reconstruction and Measurement Method for Spherical Hedges Using Binocular Vision.

The center coordinate and radius of the spherical hedges are the basic phenotypic features for automatic pruning. A binocular vision-based shape reconstruction and measurement system for front-end vision information gaining are built in this paper. Parallel binocular cameras are used as the detectors. The 2D coordinate sequence of target spherical hedges is obtained by region segmentation and object extraction process. Then, a stereo correcting algorithm is conducted to keep two cameras to be parallel. Also, an improved semi-global block matching (SGBM) algorithm is studied to get a disparity map. According to the disparity map and parallel structure of the binocular vision system, the 3D point cloud of the target is obtained. Based on this, the center coordinate and radius of the spherical hedges can be measured. Laboratory and outdoor tests on shape reconstruction and measurement are conducted. In the detection range of 2,000–2,600 mm, laboratory test shows that the average error and average relative error of standard spherical hedges radius are 1.58 mm and 0.53%, respectively; the average location deviation of the center coordinate of spherical hedges is 15.92 mm. The outdoor test shows that the average error and average relative error of spherical hedges radius by the proposed system are 4.02 mm and 0.44%, respectively; the average location deviation of the center coordinate of spherical hedges is 18.29 mm. This study provides important technical support for phenotypic feature detection in the study of automatic trimming.

Research on Towed Linear Array Shape Measurement Method Based on Biorthogonal Signal

Systematic error and random error are main factors affecting the positioning of biorthogonal signal underwater positioning system. Based on the operation theory of USBL, this paper quantitatively analyzes a theoretical analysis and simulation calculations on influence of different error sources on the positioning accuracy and qualitatively analyzes the effectiveness of the towed linear array shape measurement method based on biorthogonal signal through practical experiments. By comparing the measurement effect after correction by using the measurement array with the measurement effect based on the linear array, the effectiveness of the method is verified.

Structural diversity in the series of lanthanide complexes [Ln(Ph3PO)n(NCS)3] (n = 3, 4)

Two unique series of lanthanide complexes [Ln(Ph3PO)4(NCS)3] (Ln = La-Tb, except Pm) and [Ln(Ph3PO)3(NCS)3] (Ln = Sm-Lu, Y) have been structurally characterised. The isomorphous series [Ln(Ph3PO)4(NCS)3] crystallises in the monoclinic centrosymmetric space group P21/c, and the metals display a capped octahedral coordination geometry. In contrast, the isomorphous series [Ln(Ph3PO)3(NCS)3] (Ln = Sm-Lu, Y) crystallise in the non-centrosymmetric space group R3, with the metal displaying an octahedral coordination geometry; the fac-isomer being observed in all cases. Intriguingly, for the metals Sm-Tb both the seven and six coordinate complexes can be isolated from the respective synthetic routes suggesting a structural equilibrium in solution. An analysis of the distortions of the ligand polyhedron in the two series using the continuous shape measurement (CShMs) method suggests trends across the series relating to the fit of the ligands with the size of the lanthanide ions.

Augmenting a single-point laser Doppler vibrometer to perform scanning measurements

Laser Doppler vibrometers (LDV) are used for non-contact vibration measurements of various structures and are frequently used for stringed instrument measurements. Single-point LDVs can be used with the roving hammer or LDV method for mode shape measurements, but this is time-consuming and requires constant attention. Scanning LDVs exist but are expensive and often out of reach of musical acoustics researchers. An inexpensive apparatus to modify a common single-point LDV such that it can perform automated scanning measurements is presented. The augmentation consists of a mirror galvanometer, impact hammer controller, and 3D printed mounting hardware. The scanning system is controlled by a microprocessor and can be easily automated. The total cost of the system, excluding the LDV and impact hammer, is under two hundred dollars. Measurements of guitars are presented to validate the scanning system and discuss any shortcomings.

A 3D shape measurement method for high-reflective surface based on accurate adaptive fringe projection

Fringe projection profilometry (FPP) is a popular three-dimensional (3D) shape measurement technique with advantages of high-precision and non-contact. However, measuring high-reflective surface is still a challenging task for conventional FPP because image saturation leads to absolute phase errors and reconstruction errors. In this paper, a new adaptive fringe projection (AFP) method is proposed. Compared with existing AFP methods, we simplify the experimental process and get more accurate coordinate mapping. First, we built the projection intensity model for each projector pixel rather than camera pixel. Then, a uniform gray pattern was projected and two images were captured under high and low exposures respectively to calculate the quantitative low projection intensity. In coordinate mapping, sub-pixel coordinates were mapped from camera image onto projector image. They were filtered with absolute phase to deal with resolution difference between camera and projector and get accurate coordinate correspondence. Finally, adaptive fringe patterns were generated. Besides, we proposed a novel quantitative criterion to evaluate the effectiveness of AFP methods called point cloud integrality (PCI), which is calculated based on the number of 3D points of high-reflective areas. The experiments demonstrate that the proposed method increases both the measurement accuracy and PCI compared with conventional FPP and some existing AFP methods.

Advances and Prospects of Vision-Based 3D Shape Measurement Methods

Vision-based three-dimensional (3D) shape measurement techniques have been widely applied over the past decades in numerous applications due to their characteristics of high precision, high efficiency and non-contact. Recently, great advances in computing devices and artificial intelligence have facilitated the development of vision-based measurement technology. This paper mainly focuses on state-of-the-art vision-based methods that can perform 3D shape measurement with high precision and high resolution. Specifically, the basic principles and typical techniques of triangulation-based measurement methods as well as their advantages and limitations are elaborated, and the learning-based techniques used for 3D vision measurement are enumerated. Finally, the advances of, and the prospects for, further improvement of vision-based 3D shape measurement techniques are proposed.

Improved dual-frequency phase-coding fringe projection method for 3D shape measurement

The phase-coding method is useful in 3D shape measurement due to its low sensitivity to surface contrast and camera noise. However, as the number of fringe periods increases, the wrapped phase recovered from the sinusoidal phase-shifting patterns is always misaligned with the fringe order calculated by the stair-coding patterns. To solve this, an improved dual-frequency phase-coding fringe projection method is designed to realize absolute phase retrieval. Specifically, the auxiliary fringe order determined by phase-shifting is used to get the absolute phase, which can eliminate fringe-order errors. Moreover, to increase speed and the number of codewords, this paper presents a scheme that only requires seven patterns, including a uniform gray image, two high-frequency sinusoidal fringe patterns, two low-frequency sinusoidal fringe patterns and two phase-coding fringe patterns. Therefore, this method requires fewer patterns to realize high-resolution measurement than existing methods. The success of the proposed method can be proved by experiments.

55 Years of Holographic Non-Destructive Testing and Experimental Stress Analysis: Is there still Progress to be expected?

Holographic methods for non-destructive testing, shape measurement, and experimental stress analysis have shown to be versatile tools for the solution of many inspection problems. Their main advantages are the non-contact nature, the non-destructive and areal working principle, the fast response, high sensitivity, resolution and precision. In contrast to conventional optical techniques such as classical interferometry, the holographic principle of wavefront storage and reconstruction makes it possible to investigate objects with rough surfaces. Consequently, the response of various classes of products on operational or artificial load can be examined very elegantly. The paper looks back to the history of holographic metrology, honors the inventors of the main principles, discusses criteria for the selection of a proper inspection method, and shows exemplary applications. However, the main focus is on modern developments that are inspired by the rapid technological process in sensing technology and digitization, on current applications and future challenges.

High-speed three-dimensional shape measurement with inner shifting-phase fringe projection profilometry

Fringe projection profilometry (FPP) has been extensively studied in the field of three-dimensional (3D) measurement. Although FPP always uses high-frequency fringes to ensure high measurement accuracy, too many patterns are projected to unwrap the phase, which affects the speed of 3D reconstruction. We propose a high-speed 3D shape measurement method using only three high-frequency inner shifting-phase patterns (70 periods), which satisfies both high precision and high measuring speed requirements. Besides, our proposed method obtains the wrapped phase and the fringe order simultaneously without any other information and constraints. The proposed method has successfully reconstructed moving objects with high speed at the camera’s full frame rate (1700 frames per second).

Dynamic Three-Dimensional Shape Measurement Method Based on Misaligned Gray Code

Dynamic Three-Dimensional Shape Measurement Method Based on Misaligned Gray Code

Basic study of body shape measurement method for estimating body length and weight of a farmed fish under water

Basic study of body shape measurement method for estimating body length and weight of a farmed fish under water