Monocular-vision-based method for online measurement of
pose parameters of weld stud
Bin Wu
a
, Fang Zhang
a
, Ting Xue
b,
⇑
a
State Key Laboratory of Precision Measurement Technology and Instruments, Tianjin University, Tianjin 300072, China
b
College of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, China
article info
Article history:
Received 2 July 2014
Received in revised form 4 October 2014
Accepted 28 October 2014
Available online 11 November 2014
Keywords:
Pose measurement
Computer vision
Image processing
Camera calibration
Weld stud
abstract
With the rapid development of stud welding technology, weld studs are extensively used
in automobile industry. A weld stud differs from a regular stud in that it has special exter-
nal features. This study develops a novel non-contact, fast, and high-precision method
based on monocular vision for measuring the position and attitude parameters of a weld
stud. Under this method, the measuring principle for the weld stud’s position and attitude
parameters is derived and an accurate mathematical model is set up. Based on this
mathematical model, a precise calibration method for the projective transformation’s cor-
responding relationship parameters is developed and an optimal observation condition is
then introduced as the constraint into the measurement process so as to enhance the loca-
tion precision. The experiment results show that the proposed method is fast and accurate,
and it satisfies the requirement of online, flexible, fast, and high-precision measurement of
weld studs’ pose parameters in automobile and other manufacturing industries.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Stud welding is a process in which a metal stud or other
metal parts such as bolts and screws are welded to a work-
piece. In this case, the stud to be welded is called weld
stud. In addition to the upward helical thread, weld stud
is also different from regular stud in that it has spiral
groove, as shown in Fig. 1.
In the automobile industry, weld studs are extensively
used to connect and fasten components, instruments, and
interior decorations as every vehicle body needs up to hun-
dreds of weld studs [1–4]. Therefore, it is essential for their
position and attitude to be accurate for assembling the
remaining components. Deviations of the positions or atti-
tudes from the standard reference pose could cause assem-
bly difficult or even assembling failures, in turn harming
the production efficiency and reliability of vehicles.
Presently, the position and attitude parameters of a weld
stud are mainly measured by artificial means using con-
ventional instruments such as a step gauge, dial indicator,
steel rulers, and wedge plug ruler. Non-automatic calibra-
tions have low efficiency and precision and do not satisfy
the requirements of online, continuous, and high precision
measurement in modern manufacturing industries.
In recent years, the emerging industrial computer vision
has a wide range of applications from automobile produc-
tion to aircraft manufacturing and other fields due to its
non-contact, online, high-efficiency, moderate accuracy
and other characteristics [5–9]. Perceptron Inc. (United
States) studied a weld stud’s position and attitude param-
eters through online measurement based on a multi-line
structured light vision sensor. In the measuring process,
the sensor is driven by an industrial robot to scan the weld
stud to be measured, and the stud’s position and attitude
parameters are evaluated by morphology reconstruction.
http://dx.doi.org/10.1016/j.measurement.2014.10.041
0263-2241/Ó 2014 Elsevier Ltd. All rights reserved.
⇑
Corresponding author.
E-mail address: xueting@tju.edu.cn (T. Xue).
Measurement 61 (2015) 263–269
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Measurement
journal homepage: www.elsevier.com/locate/measurement