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Deployment and evaluation of a dual-sensor autofocusing method for on-machine measurement of patterns of small holes on freeform surfaces

Chen, Xiaomei, Longstaff, Andrew P., Fletcher, Simon and Myers, Alan (2014) Deployment and evaluation of a dual-sensor autofocusing method for on-machine measurement of patterns of small holes on freeform surfaces. Applied Optics, 53 (10). pp. 2246-2255. ISSN 1559-128X

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This paper presents and evaluates an active dual-sensor autofocusing system that combines an optical vision sensor and a tactile probe for autofocusing on arrays of small holes on freeform surfaces. The system has been tested on a two-axis test rig and then integrated onto a three-axis CNC milling machine, where the aim is to rapidly and controllably measure the hole position errors while the part is still on the machine. The principle of operation is for the tactile probe to locate the nominal positions of holes and the optical vision sensor follows to focus and capture the images of the holes. The images are then processed to provide hole position measurement. In this paper, the autofocusing deviations are analyzed. First, the deviations caused by the geometric errors of the axes on which the dual-sensor unit is deployed. are estimated to be 11 m when deployed on test rig and 7 m on the CNC machine tool. Subsequently, the autofocusing deviations caused by the interaction of the tactile probe, surface and small hole are mathematically analyzed and evaluated. The deviations are a result of the tactile probe radius, the curvatures at the positions where small holes are drilled on the freeform surface and the effect of the position error of the hole on focusing. An example case study is provided for the measurement of a pattern of small holes on an elliptical cylinder on the two machines. The absolute sum of the autofocusing deviations is 118 m on the test rig and 144 m on the machine tool. This is much less than the 500 m depth of field (DOF) of the optical microscope. Therefore, the method is capable of capturing a group of clear images of the mall holes on this workpiece for either implementation.

Item Type: Article
AuthorLongstaff, Andrew
Uncontrolled Keywords: Instrumentation, measurement, and metrology; Imaging systems; Machine vision.
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TK Electrical engineering. Electronics Nuclear engineering
T Technology > TS Manufactures
Schools: School of Computing and Engineering
School of Computing and Engineering > Centre for Precision Technologies > Engineering Control and Machine Performance Research Group
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Depositing User: Xiaomei Chen
Date Deposited: 01 Apr 2014 14:03
Last Modified: 28 Aug 2021 19:17


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