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为了验证文中所提出的基于视觉测量的回转轴线标定方法的正确性和有效性,首先按照上文所述的标定方法和步骤,对回转工作台的轴线位置进行了标定。在标定过程中,应用千分表对标定块的方位进行机械调整的过程如图9所示;而控制工业影像测头对焦于标定块表面并采集其锋利棱边图像的过程如图10所示;最终采集到的锋利棱边图像及边缘提取结果如图11所示。
Figure 11. Images of the sharp edges and edge extraction results: (a) Left edge; (b) Right edge; (c) Upper edge
根据公式(1)~(6),即可计算出回转工作台的轴线原点OR在机器坐标系O-XYZ中的三维空间坐标为(X0,Y0,Z0)。为了减小随机误差对标定精度的影响,文中采用多次标定实验的平均值作为最终的回转轴线位置标定结果,即(204.8 328,278.4 686,–10.2 804)。
为了对该标定结果进行验证,文中选用了一个标称尺寸为80 mm的标准量块作为被测物体,应用所搭建的四轴视觉坐标测量系统分别对标准量块的两个被测面进行测量,并基于回转轴线位置的标定结果来获得两个被测面之间的距离,而后再与标称尺寸进行比对,通过测量误差的大小来检验标定方法及标定结果的正确性和有效性。实验现场如图12所示,连续对标准量块进行10次重复性测量,测量结果如表1所示。
No. Measuring results/mm Measuring errors/mm 1 80.006 0.006 2 80.008 0.008 3 79.995 −0.005 4 80.006 0.006 5 79.992 −0.008 6 79.993 −0.007 7 79.996 −0.004 8 80.009 0.009 9 80.005 0.005 10 80.006 0.006 Table 1. Measuring results
从表1可以看出,应用回转轴线位置的标定结果,在对该标准量块的10次测量实验中,所测长度尺寸的平均值为80.001 6 mm,标准差为0.006 4 mm,并且各次测量结果与真实值之间的误差均小于±0.01 mm。影响标准量块测量精度的因素很多,必须进行详细分析,以确保测量结果的准确与可靠。在上述实验过程中,对合成标准不确定度影响显著的因素主要有:(1)测量重复性引起的不确定度分量;(2)三坐标测量机示值误差引起的不确定度分量;(3)对焦评价函数的判别误差引起的不确定度分量。
首先,对标准量块进行了10次重复性测量,则测量重复性引起的不确定度分量u1为:
其次,PEARL 555型三坐标测量机的示值误差为(2.5+3×L/1 000)μm(L为测量长度),在标准量块的测量过程中,三坐标测量机的最大移动范围为30 mm,则分布区间半宽a为:
设示值误差服从均匀分布,取包含因子k=
$\sqrt 3 $ ,则三坐标测量机示值误差引起的不确定度分量u2为:再次,文中应用基于Laplacian算子的对焦评价函数来使工业影像测头正确对焦于标准量块的被测表面。经过实验验证,该函数的对焦位置判别误差小于0.005 mm,则分布区间半宽为2.5 μm,设该误差服从均匀分布,取包含因子k=
$\sqrt 3 $ ,则对焦评价函数的判别误差引起的不确定度分量u3为:由于不确定度分量u1、u2和u3相互独立,因而标准量块测量结果的合成标准不确定度uc为:
实验结果验证了文中所提出的标定方法的正确性和有效性,可以满足了高压涡轮叶片上的气膜孔特征的形位参数检测需求。因此,文中所提出的标定方法,能够准确确定出四轴视觉坐标测量系统中的回转轴线的空间方位,从而为后续不同回转工作台角度位置下的测量数据的坐标转换奠定了基础,使其转换到同一坐标系下,以便于后续的数据处理;同时,回转轴线位置的标定精度也使系统能够满足其他回转体类零件的多轴视觉测量,从而提供了一种四轴视觉坐标测量系统下的回转轴线位置标定方法,解决了回转体零件的多轴视觉测量中的关键问题。
Study on calibration method of rotary axis based on vision measurement
doi: 10.3788/IRLA202049.0413004
- Received Date: 2020-01-05
- Rev Recd Date: 2020-02-25
- Publish Date: 2020-04-24
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Key words:
- vision measurement /
- coordinate measuring machine /
- rotary table /
- film cooling hole
Abstract: For the purpose of inspecting the film cooling holes on high-pressure turbine blade in a rapid and accurate manner, a non-contact four-axis vision coordinate measuring system was designed and established in the paper. With regard to the determination difficulty of the spatial location of the rotary axis, a calibration method based on vision measurement was proposed in the paper, which fully considered the imaging characteristics of industrial CCD used in the system. In the calibrating procedure, a specially designed cuboid mental block was applied as the target. Through the coordination between the turntable and the coordinate measuring machine, the industrial CCD focused on the block surface and then captured the sharp edges of the block. And then, the spatial orientation of the rotary axis in the machine coordinate system could be determined by edge extraction, pixel distance calculation, physical distance conversion and algebraic operation etc. Finally, a gauge block was selected to be inspected by the measuring system to verify the calibration method and results, whose nominal size was 80 mm. As the experimental results showed, the measuring errors were all smaller than ±0.01 mm. Therefore, the calibration method of rotary axis proposed in the paper showed higher calibration precision and repeatability, which could meet the inspecting requirements of the shape and position parameters of film cooling holes.