Analysis of factors affecting the measurement accu

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Analysis of influencing factors and Countermeasures of optical scanning measurement accuracy

1 introduction

Reverse Engineering (or reverse engineering) is a rapidly developing new technology. Physical reverse engineering technology is used for product redesign based on existing physical products, or digital conversion of some product models (such as sludge model) that cannot be expressed and designed directly by digital means, but can only be expressed in physical form, so as to implement the digital design and manufacturing of these products. For example, in the aerodynamic design of aircraft, missiles and other aircraft, the physical model based on experimental optimization can not be directly modeled and expressed by the existing cad/cam system. Therefore, in order to realize the digital design and manufacturing of these aircraft, we must use the point cloud data acquisition technology in physical reverse engineering to convert the physical model into CAD model. At present, physical reverse engineering technology has been widely used in new product design, product maintenance, product testing and so on

in the reverse engineering of physical objects, in order to realize the digitization of objects, we must use the corresponding measurement or scanning equipment to measure or scan the three-dimensional physical model of products, so as to obtain the spatial topology discrete point data cloud of the physical model. Therefore, point cloud data acquisition is the first work that must be completed in the reverse engineering of physical objects. Among various physical measurement technologies, the optical scanning point cloud data acquisition technology that has emerged in recent years has many advantages, such as high measurement efficiency, good data integrity, wide application range, wide data acquisition range (from several millimeters to tens of meters), and so on

in recent years, many domestic universities and scientific research institutions have carried out a lot of research and development on the reverse engineering technology of physical objects, and made breakthroughs in some key technologies. Through independent development and technical cooperation, the development and application of contact measurement technology and equipment have become increasingly mature in China. However, for the optical scanning point cloud data acquisition technology of non-contact measurement, there are few public reports of mature products developed in China and their successful applications in automotive, mold and other industries. In order to promote the research and development of optical scanning point cloud data acquisition and processing technology, and provide external technical services, our school has introduced mobile optical scanning equipment produced by a foreign company. In the process of using the equipment, the measurement error is often too large due to misoperation, improper adjustment and other reasons. In order to thoroughly digest the foreign introduced technology, the author studied and analyzed it in combination with work practice, and put forward the corresponding solutions

2 the main form of measurement error is

(1) the collected data is missing or the data density cannot meet the requirements. Using this incomplete data for point cloud fitting, the error is large, and it is difficult to achieve the required measurement accuracy

(2) the data acquisition results of the same surface are multi-layer point clouds. This situation often occurs when the measured object is a large workpiece or the workpiece is a transparent object

(3) the inaccuracy of single collected data affects the overall measurement accuracy

(4) the cumulative error is too large, resulting in obvious deviation of the measurement results

(5) point cloud splicing error, resulting in large measurement error

(6) there are too many coarse points (noise) data in the measurement results

3 error cause analysis and Countermeasures to improve accuracy

the author combines practical work experience and through test analysis, summarizes the main reasons for large measurement errors as follows: improper calibration and improper use of ruler; Improper selection of probe lens combination; Improper measurement sequence; Improper selection of measurement strategy; The mark points on the workpiece surface are improperly placed; Improper operation during measurement; Improper pretreatment of the measured surface of the workpiece; Improper post-treatment; Improper selection of measurement environment is analyzed as follows:

(1 failure is inevitable) improper calibration, improper use of ruler

the scanning measuring head (probe) is composed of light source, CCD camera and corresponding lens group. Before the point cloud data collection, the probe needs to be initialized first. The main contents include:

① choose different lens combinations according to the size of the measured object, the number of surface features and their complexity

② determine the light intensity of the main light source according to the measurement site conditions, the surface morphology and surface treatment of the measured object

③ calibrate the selected lens combination according to the system standard workflow, so that the calibration accuracy value is ≤ 0.020

if the above work is not carried out before the measurement, but the previously calibrated probe is directly used for the measurement, the measurement accuracy may not be guaranteed because the lens combination, light source intensity and calibration accuracy do not meet the measurement requirements, resulting in large errors

in the measurement, if the probe is impacted or collided due to misoperation, the probe should be checked in time. If it is damaged, it should be repaired; If there is no damage, the probe must also be recalibrated; Even if there is no operation error in the measurement, if the measurement time is long, it is also necessary to calibrate the probe regularly and check the accuracy of the probe

the ruler is a necessary tool to use the digital camera to locate the identification points on the whole workpiece when collecting data on large workpieces. The standard size on the ruler used should be consistent with the size value displayed when processing with photos

(2) improper selection of probe lens combination

when collecting point cloud data on the surface of large workpieces, the lens combination with a large measurement range should be selected to realize the rapid collection of overall data; For some areas with more and smaller features, it is best to select the lens combination with smaller measurement range and then carry out the prominent measurement of local small features to obtain better measurement results

for large workpieces, if the lens combination with small measurement range is selected for measurement, more identification points for point cloud splicing are required on the surface of the workpieces, which will prolong the pretreatment time of the workpieces and increase the measurement time span. The error caused by the change of ambient temperature with time will be reflected in the measurement results and affect the whole measurement efficiency. If the digital camera is used to locate the identification points on the whole workpiece and the automatic splicing is carried out during measurement, the probability of the same relationship between the identification points will increase due to the large number of identification points, which is prone to splicing errors; If you do not use a digital camera to locate the identification points on the whole workpiece, and use common identification points to splice between adjacent single point clouds, due to the large number of splicing times, the cumulative splicing error will also be too large

on the contrary, for small workpieces, if the lens combination with a large measurement range is selected for measurement, the small features on the workpiece cannot be accurately reflected, so that the measurement results cannot meet the required accuracy. It is necessary to replace the appropriate lens combination, calibrate and measure again

(3) improper measurement sequence

measurement sequence refers to the superposition sequence between adjacent single measurement results during measurement. Taking measuring a slender workpiece as an example, the rectangular area shown in Figure 1,2,3,4 and 5 is the measurement range of the current calibration probe. When measuring the workpieces according to the emission arrangement shown in the figure, first measure the middle position of the workpieces, and then measure the second one after completing the measurement of the first one in the middle, and then use three common mark points to combine the second one with the first one. At this time, a splicing error will occur. Similarly, a splicing error will also occur when the third and second frames are spliced. Assuming that all splicing errors are the same, the cumulative error between 1,2,3 is 28, and the cumulative error between 1,4,5 is also 28. The measurement results show that the cumulative errors between 1,2,3 and 1,4,5 do not form a superposition relationship, so the total cumulative error is still 28. If the workpieces are measured in the order shown in Figure 2, the maximum cumulative error is 48. Therefore, the measurement sequence of "taking the center as the benchmark and arranging in emission" should be adopted as far as possible to reduce the cumulative error

(4) improper selection of measurement strategy

when measuring, the measured workpieces should be classified according to large workpieces, medium-sized workpieces, small-size multi feature workpieces, inner cavity workpieces, etc., and different measurement strategies should be adopted for each type of workpieces

when measuring large workpieces, you can first use a digital camera to locate the identification points, and then select a lens combination with a large single measurement range for measurement; If the size of the workpiece is too large, it can be measured twice, and then assembled with a common reference point; If there are many local small features in large-size workpieces, a group of lens combinations with small measurement range can be selected for local measurement after the measurement is basically completed. In order to facilitate the automatic splicing of small-scale measurement, the density of reference points should be increased when preprocessing this part

when measuring medium and small workpieces, pay attention to the correct measurement sequence to reduce the cumulative error. In fact, the measurement strategy for large-size workpieces can also be used for medium and small workpieces, but it must be equipped with a digital camera and related software for the overall positioning of the reference point

when measuring the surface of the inner cavity of the workpiece, in order to overcome the depth of field limitation of optical scanning measurement by raising two children alone, some technical means can be taken to convert the measurement of the inner cavity into the measurement of the appearance, such as injecting silica gel into the inner cavity of the workpiece, taking it out after it solidifies, and measuring its appearance

(5) improper placement of marking points on the surface of workpieces

the placement of marking points on the surface of workpieces will be encountered in the measurement of large workpieces, medium and small workpieces

the data acquisition of large workpieces generally requires the use of a ruler and a digital camera, and the measurement can be carried out in two steps: the first step is to use large digital points to construct the overall structure of the mark points used for the combination of a single measurement point cloud. In order to ensure that the mark point cloud used for a single measurement can be obtained through normal operation, the arrangement rules shown in Figure 1 must be followed; The second step is to take the marked point cloud as the reference system, and the system will compare the measured marked points in each single point cloud with those in the existing reference point cloud. If the two match, they will be automatically spliced, and there is no need for overlap between two adjacent single point clouds. It can also be measured, but the premise is that each single point cloud must contain at least three marker points. At this time, the marker points should be pasted appropriately on the measured area, otherwise it will cause difficulties in measurement or reduce the measurement accuracy

the marking point pasting of medium-sized workpieces is different from that of large workpieces. Because the automatic (or manual) splicing of two adjacent point clouds needs to be completed according to the common marking points of adjacent single point clouds, the marking point pasting density of medium-sized workpieces should be greater than that of large workpieces, otherwise it is difficult to realize the splicing of two adjacent point clouds

since the placement of small workpiece marker points will mask the features on the workpiece to varying degrees, the workpiece surface should be pasted or not pasted with marker points as little as possible to obtain more complete scanning data

in addition, generally, the mark points should be pasted at a relatively flat position on the workpiece to reduce the difficulty of filling the gap of the point cloud at the mark points and the corresponding measurement error

(6) improper operation in the measurement process

in the measurement process, pay attention to the following operating points:

① adjust the position of the probe so that the measured part is within the measurement range of two probes at the same time

② adjust the light intensity of the main light source, and adjust the definition of the identification point and the workpiece surface respectively, so that the identification point of the measuring part and the workpiece table