Abstract
In order to improve the accuracy of the mechanical key component model constructed by 3D laser scanning and reduce the economic loss to the marine port, the factors affecting the accuracy of 3D laser scanning were studied. By studying the error of the instrument, errors related to the target object, errors caused by external environmental conditions, errors caused by human operations, and errors caused by different measurement methods were discussed. In view of the errors caused by different measurement methods and different measurement methods for different parts, the scanning experiment was carried out and the obtained data was analyzed. A method of improving the scanning accuracy was proposed, which made the three-dimensional scanning more accurate. This method was applied to the design of key part structures for marine port machinery. The results show that the research in this paper can help designers accurately and completely obtain 3D point cloud data of mechanical key parts, improve scanning accuracy and reconstruct 3D model. Therefore, according to the application prospect of 3D scanning technology and the status quo in China, it is of great practical significance to study the accuracy of 3D laser scanning technology.
Introduction
Three-dimensional laser scanning technology is a high-tech, high-precision and fully automatic stereo scanning technology, which is also known as the scene reproduction technology. It is mainly used for 3D modeling and reconstruction of reverse engineering. According to statistics, there are very few positive designs in actual work, and most of them use reverse engineering techniques. Therefore, the application of three-dimensional laser scanning technology is becoming more and more frequent and important. However, 3D scanning is not a perfect field in China. Traditional measurement techniques such as three-coordinate measuring instruments are mainly based on single-point measurement, while three-dimensional scanning technology can obtain three-dimensional data on the surface of objects [1]. Three-dimensional laser scanning technology can directly scan the three-dimensional point cloud data of large, complex and irregular objects into the computer. Then, based on these three-dimensional data, a three-dimensional model of the object is constructed [2].
However, during the scanning process, due to machine or human factors, inevitable errors will occur, which will have a great impact on modeling. If the errors are ignored, the final object model constructed will be greatly different from the actual object model due to the existence of errors, and may even cause losses. Therefore, it is very important to study and analyze the causes of various errors and improve the scanning accuracy [3]. With the development of optical and electronic technologies, image processing, artificial intelligence, and pattern recognition have made important breakthroughs [21–28]. Three-dimensional non-contact scanning based on industrial cameras, projectors and grating technology, image acquisition systems and computers has been regarded as the focus of research in China and abroad [4].
Literature review
At present, many scholars study 3D laser scanners and data processing. In 2015, Lei et al. proposed a three-dimensional laser scanning technology, which mainly includes modern optical, mechanical, computer, electronic and other technologies [5]. In 2015, Chai et al. proposed the use of light strips to extract three-dimensional information on the surface of objects [6]. In 2015, Conde et al. used laser or white light as a light source to form a projection of the grating. The camera takes light that is reflected from the surface of the object. Through the triangulation method, the three-dimensional shape of the object is obtained [7]. In 2016, Zeraatkar et al. proposed that the processing technology of 3D scanning point cloud data mainly includes point cloud data acquisition, noise removal, data registration and feature extraction [8]. In 2015, Chao proposed that the application of 3D reconstruction technology in reverse engineering has a very strong professionalism. The selection of a suitable modeling software system for products has a very practical meaning [9]. In 2016, Ustinov and Bolodurin proposed that the method of non-contact three-dimensional scanning is very fast, so it is the most practical method to scan irregular objects in reverse engineering [10]. In 2017, Eyre et al. introduced the principle of 3D laser scanning technology and the method of stitching point clouds [11]. The coordinate transformation combined with the 3D laser scanning technique and the control measurement is given to obtain the local coordinates of the scanning target. In addition, the accuracy is analyzed.
The three-dimensional measurement method does not cause any damage to the measuring object and has a high resolution. It has been applied in geological exploration, industrial product testing, biomedical, stereo printing, aerospace and other fields. In 2017, Ma analyzed the unique advantages of French MENSI 3D laser scanning technology. According to the actual needs of the front end of the enterprise, a new 3D laser scanning measurement system was developed [12]. In 2015, Weinkauff et al. achieved micron-scale measurement accuracy in the study of laser differential confocal lens center thickness measurement system [13]. In 2016, Xu et al. studied a three-dimensional contour measurement method with high-precision and multi-angle fusion using aeroengine engine turbine blades as the main measurement object [14]. In 2017, Staso et al. used three-dimensional scanning technology to establish a digital model of cultural relics through computer, and the physical dimensions, texture and surface color of the cultural relics were obtained. This will not only avoid damage to the original artifacts during the research process, but also preserve the cultural relics information for a long time [15]. In 2016, Hao et al. proposed a method based on the idea of octree to use 3D cubes for data reduction [16]. In 2016, Yu et al. proposed the main idea of data processing for laser scanning. The original scattered point cloud is transformed into a grid model, and then filtered by the filtering method of the grid model [17]. In 2017, Lyu et al. considered that the laplace filtering and the second laplace filtering method of laser scanning can effectively smooth the model of the point cloud. However, the characteristics of the point cloud cannot be effectively maintained, which causes distortion of the edges [18]. In 2018, Yang et al. proposed that the mean curvature rule of laser scanning is mainly dependent on the curvature estimation of the workpiece, and the effect is not obvious when there is too much noise [19]. In 2018, Li et al. proposed that the dimensions of the parts were obtained by a three-dimensional scanner. Inevitably, foreign data points are introduced, especially the nearby patches are offset from the original surface. At the same time, due to the shape of the parts and the limitation of the scanning method, some measurement gaps and blind spots are often generated during data scanning, which has an impact on the subsequent modeling of the components [20].
Methodology
The laser trigonometry is the most widely used measurement method. The basic principle of laser trigonometry is to use a laser beam or an analog probe. Laser beams or analog probes have regular geometric shapes and are continuously scanned along the surface of the object under test. A light spot or a light band that forms a diffuse reflection on the surface of the object to be measured is imaged on an image sensor in the light path. According to the triangle principle, the three-dimensional space coordinates of the measured point can be measured. Triangulation is divided into direct and oblique. The direct method is characterized by small structure, small measurement dead zone, and displacement of measurement results. It is more suitable for measuring complex free-form surfaces. The measurement principle of the 3D laser measurement scanner was analyzed.
The principle of triangulation is shown in Fig. 1. The 3D laser measuring scanner consists of five parts: laser, convergent lens, measured surface, receiving lens and photodetector. The light from the laser is focused by the condenser lens and is directed perpendicularly to the surface of the object being measured. The scattered light at the incident spot is imprinted on the sensitive surface of the photoelectric position detector and is received on the lens. When the object displacement changes or the surface height changes, the incident light spot moves along the incident optical axis. At this time, the image on the sensitive surface will also have corresponding displacement. According to the triangular geometric relationship and the imaging relationship, the formula (1) can be obtained:
Schematic diagram of triangulation.
The formula (1) is transformed to obtain the height value of the measured point A. According to the geometric relationship and the imaging relationship, the formula (2) is obtained:
When α= 90°, the formula (2) can be transformed into:
Formula (3) is the expression for calculating the height of points on the surface of the measured object in direct triangulation. In order to ensure accurate imaging on the sensitive surface of the detector at the point of the measurement range, the following relationship must be satisfied for the angle:
In the formula, K is the coefficient of transverse amplification.
At the time of measurement, there is an angle between the laser beam emitted by the laser source and the main axis of the receiving optical system, which is the triangular imaging angle. When the position of the laser spot or laser line is changed, the detector will reflect the change in depth. According to the photoelectric conversion and the principle of geometric optics, the coordinate information of the surface of the object such as the displacement of the laser beam direction can be calculated.
It can be seen from Table 1 that although the contact type coordinate measuring instrument has high precision, it is not frequently used because of its small scanning range, slow speed and high cost. However, although the accuracy of the non-contact scanner is not as high as that of the contact scanner, it is widely used because of its low cost, large scanning range, and high scanning speed. The laser triangulation scanner has higher scanning precision and wider scanning range than the projection grating scanner, so it is widely used. Coordinate information of the surface of the object such as displacement is recorded. The error of 3D laser scanning is mainly analyzed.
Comparison of several scanning methods
Influence of instrument error on structural design of mechanical parts
The instrument error is due to the measurement error produced by the 3D laser measuring instrument itself. In general, it includes the error of the laser measurement distance, the error caused by the scanning angle, the resolution of the measuring instrument, the link error with the scanner, the error caused by laser beam deflection, the influence error caused by camera calibration and focusing ability. Among these errors, the error caused by the laser ranging error and the scanning angle are the two most important errors.
The error of the scanning angle includes the horizontal scanning angle measurement error and the vertical scanning angle measurement error. The error of the measurement angle includes the combined effects of the uneven rotation control error of the scanning motor, the mirror plane angle error of the scanning mirror, and the fine vibration when the scanning mirror is rotated. In addition, the errors of measurement angle will also be caused by the rotation between the axes. As shown in Fig. 2.
Laser spot causes ranging error.
At any given time, the reflection characteristics of an object will affect the strength of the reflected signal. Errors associated with the target object include surface roughness, material, surface color, and surface tilt errors. The material used in some objects, even due to the reflection characteristics, may be several times higher than the standard deviation of normal laser ranging. Because of the multi-value characteristics of the three-dimensional laser echo signal, some three-dimensional laser scanning systems can only process the last reflected echo signal, and some three-dimensional laser scanning systems can comprehensively process the first and last reflected echo signal.
The laser is irradiated onto the surface of the workpiece and absorbed and reflected to varying degrees. Therefore, laser ranging mainly depends on the ability of laser reflection of the workpiece. In general, the laser reflectivity is affected by the roughness, surface color and material of the workpiece. The surface color is determined by the wavelength of the electromagnetic wave reflected by the object, which reflects the characteristics of the material. Table 2 shows the reflectance of different materials at different wavelengths.
Reflection characteristics of common materials at different wavelengths
Reflection characteristics of common materials at different wavelengths
Laser scanning produces small systematic root mean square errors due to differences in surface reflectance of various workpieces. The white surface has higher measurement accuracy than the black and dark grey surface, and the influence error is small. In order to improve the accuracy of the measurement, the surface of the workpiece to be tested can be sprayed before the test, so that the surface of the workpiece to be tested exhibits good diffuse reflection. Defects such as poor quality of scanning data caused by reflective, transparent, or various colors and materials of the workpiece under test are effectively improved. It is easier to scan and obtain high-quality point cloud data.
The accuracy of a three-dimensional laser scanner is related to the sampling distance and the reflectivity of the object. For the same workpiece, the data measured at different locations is different, so the sampling distance determines the result of the accuracy. At the same time, workpieces with different reflectivity also have an influence on the measured root mean square error at the same position. A reasonable setting of the reference type, number and position can effectively improve the accuracy of data scanning. In theory, the position of the workpiece must have at least four common targets, and an appropriate increase in the number of targets can effectively improve the stitching accuracy. At the same time, because the paper black and white target is unstable in viscosity and prone to bulging, in order to improve the stitching precision, the spherical target or the spherical target is generally used in combination with the magnetic target. In addition, the location of the target should avoid excessive positioning, linear alignment or regular distribution of the positioning points. The distance between any two points is guaranteed to be between 20 mm and 100 mm. As shown in Fig. 3.
Location distribution.
In addition, there are some setting parameters in the scanning process that affect the scanning results. For example, the polar error parameter affects the software’s matching of corresponding marker points in the left and right cameras. Gain is used to adjust the sharpness and contrast of the image. The setting of scanning range parameters is related to the accuracy of scanning results. The scanning accuracy can be improved by re-calibration.
The impact of the external environment mainly includes temperature, lighting conditions, wind, air pressure, air quality and other effects. The influence of temperature and air pressure on three-dimensional scanning is mainly that the temperature change affects the relationship of precise mechanical structure, the vibration of the wind during scanning, the direction of laser propagation in the air and the speed of light propagation. The atmospheric refraction effect will cause the measurement errors of target position and actual position. When passing through the atmosphere, the optical path increases due to the refractive index of different light, and the propagation path is curved, which must be corrected. Temperature is the most influential factor in the external environment for measurement accuracy. It is also one of the best controlling factors. When the temperature exceeds the critical range, the power supply performance of the equipment, precision mechanical components and scanner measurement speed are affected. Therefore, when using industrial-grade three-dimensional laser scanners and handheld laser scanners, the normal operating temperature range of the instrument is given, which is generally 0–45°C in a non-condensing environment. Therefore, before using the instrument, the appropriate external environment is selected first, thereby improving the accuracy of the three-dimensional scanning.
Conclusion
Factors affecting the accuracy of three-dimensional scanning are analyzed. The factors affecting the accuracy of three-dimensional scanning mainly include the error of the instrument itself, the error related to the measured object, the error caused by external environmental conditions, the error caused by human operation, and the error caused by different measurement methods. The following conclusions are drawn: The factors affecting the accuracy of scanning data can be roughly divided into five categories: instrument error, error related to the object under test, operation method, external environment and post-processing error of point cloud data. The error of angle measurement and distance measurement is the main factor of instrument error. In addition, due to the boundary effect, the focal length and wavelength of the instrument should be reduced in order to reduce the error. The reflection characteristics and tilting factors of the workpiece have an effect on the accuracy. The reduction of the surface roughness of the workpiece, the increase of the inclination angle of the surface, and the improvement of the reflectivity of the workpiece can improve the scanning accuracy, which is also considered in the design stage. In general, rough is better than smooth. If the surface of the workpiece to be tested is very smooth, different degrees of specular reflection will occur, causing large errors in measurement data or incomplete cloud data.
Three-dimensional laser scanner has the advantage of fast scanning speed. At the same time, it is difficult to manually measure the size of some large and complex parts of the marine port machinery. At this time, the laser 3D scanner can be used for measurement. Nowadays, with the development of technology, 3D scanning technology is becoming more and more important, which requires high scanning accuracy. However, due to various factors during the scanning process, errors may occur, and errors have a great influence on modeling. If the error is ignored, the final constructed object model will be very different from the actual object model due to the error. Therefore, the improvement of scanning accuracy is of great significance for the design of key parts structure of marine port machinery in 3D scanning.