Journal of Robotics, Networking and Artificial Life, Vol. 3, No. 2 (September 2016), 124-127

Geometric Parameters Measurement of Wheel Tread Based on Line Structured LightJiwu Wang1

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, China
Zhijing Jian1, Yaodong Li1

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, China
Sugisaka Masanori2

Alife Robotics Corporation Ltd, Japan and Open University, United Kingdom
E-mail: jwwang@bjtu.edu.cn; ms@alife-robotics.co.jp
www.bjtu.edu.cn

Abstract
In order to develop the geometric parameters measurement technology of wheel tread, it is important to ensure thepositioning precision firstly before measurement. In this paper, a new method with no-contact measurement basedon line structured light is given to solve this problem. Here the traditional mechanical locating method is used as arough reference. Then, according to the feature points (particularly, the key points that have not been worn), theprecision positioning is adjusted dynamically based on the calculated results of non-contact measurement. With thedeveloped digital image processing techniques, the line structured light on the wheel tread is extracted effectively.
Thus the pixels of the extracted line structured light can be transformed to the real geometric dimensions. In thispaper, the experiments show that it improves the accuracy of positioning and reduces the factors of measurement
errors with the proposed method, and it is possible to implement non-contact measurement of the wheel tread,
wheel flange thickness and flange width.
Keywords: non-contact measurement, line structured light, wheelset tread, image segmentation

1. Introduction
With the rapid development of rail traffic, the issues oftrain safety are becoming prominent more and more. Atrain wheelset, which is the most important component
in each train, bears all the static and dynamic loads fromthe vehicle. Since it directly contacts with the rail by
gradually and inescapably frictions, the wheelset isworn down by the interactions between wheel and rail.
It will bring the undermining of critical dimensions. In along run, the safety and operation quality are seriouslyaffected.1 In practical applications, it is necessary tomeasure geometric parameters of train wheelset tread

periodically. At present, the wheelset detection method
can be roughly classified into the contact and noncontact measurement. The traditional methods still more
depend on the special mechanic tools like artificial
calipers.2 Although it is easy to be implemented, there
are the problem of high labor intensity, low efficiency
and low accuracy. With the development of image
processing and camera technology, non-contact
measurement gets more and more attention and is
widely applied in practical dynamic measurement ofvarious parts.
In this paper, based on line structure light, theexperimental model is established and the method withPublished by Atlantis Press
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Jiwu Wang, Zhijing Jian, Yaodong Li, Masanori Sugisaka

no-contact measurement is developed for the geometricparameters measurement of the train wheel tread.
Moreover, the precision of positioning between laser
measurement device and wheelset, is studied carefully.
The geometric parameters, such as flange height, flange
width and wheel diameter etc. are measured using
captured images with CCD, processed them with C++
and OpenCV.
Besides the precision of positioning, the accuracy ofmeasurement is also influenced by the extraction
precision of line structured light. Hence in the paper, themethod of extraction of the feature points and lines isstudied too.

Flange height H is the length between flange base lineand flange top. Rim width L is the distance between the
medial surface and the lateral surface of wheelset.
Moreover, as shown in Fig. 2, the rolling circle is also
defined with the circle passing the base point on thetread. Wheel diameter D is the average diameter ofrolling circle.
From the definitions, the base point is the key point for
the whole measurement. That is, the accurate
positioning is the key step to obtain effective
measurement results. In this paper, the key parameters
H, W and D, are mainly studied to be measured with
non-contact measurement method.

2. Experimental principle
Compared with other simple parts measurement, thewheel tread measurement is more complex. This isbecause multiple dimensions are relative.2 If there iserror in some dimension, the dimensions in other parts
are also influenced. In order to explain clearly, here thedefinition of wheelset’s parameters and experimental
principle are illustrated firstly.
2.1. Definition of wheelset’s parameters
Wheel tread is the contacting surface between wheelset
and railway. For the measurement of wheel tread, thefollowing wheelset’s parameters are necessary toobtained.3 As shown in Fig. 1, the left side is the medialsurface of wheel rim (ostensibly, it is a line in theimage).

Fig. 2. The rolling circle illustration of one wheel

2.2. Experimental system and principle
The structure of the measurement is shown in Fig. 3.
There are three cameras and line structure light in thesystem. The camera A and the camera C, in both ends ofthe system, are used to accurate positioning. Camera B
is used to grab images for tread measurement. EachLaser line and camera are installed with fixed
dimensions and calibrated beforehand.

Fig. 1. The critical parameters of wheel tread

Based on this surface, in the horizontal direction, thedistance being 70mm to the medial surface of wheel rim,
there is a point on the tread, and it is customarily called
the base point. According to the base point, flange baseline is defined with the horizontal line 12mm above thebase point. Flange thickness W is the horizontal
thickness along flange base line intersected with rim.

Fig. 3. The illustration of experimental system structure

The non-contact measurement method is shown in
Fig.4. Here, we assume the camera view angle in width

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Geometric parameters measurement ofdirection is λx, λy is in the height direction, the length
between the lens center and laser center is L, thecoordinates of the laser center in image plane is P(x,y),θ is the angle between the lens center line and thevertical line from camera center to the laser center line,
the image dimension is 2w×2h. Thus we can get theangle β, which is the angle between the line fromcamera center to P(x,y) and the optical axis of thecamera.

 tan(λ y ) × ( y − h) β = arctan h(1)Then the distance between the camera and the object
point will be:

HY =L × tan(θ − β )(2)After the distance is calculated, the other dimensions
can be calculated easily with the pin hole theory.

Fig.5. Illustration of accurate positioning

The accurate positioning method is shown in Fig.5.
With the non-contact method, the distance between thetop point of axis and the optical center of camera can beobtained when the light is perpendicular to the axis. If
the distance AC is equal to BD, and line AC and BD in
the same plane, it means two axes are parallel. At thesame time, the condition of the minimum of AC or BD
is considered too. By this way, the accurate positioning
can be controlled.
Using the same non-contact measurement method, thetread geometric dimension can be measured. Then thekey point and base line can be determined. Parameter H,
W can be calculated. As shown in Fig.5, the distanceh1, between the optical center of camera and the centerof wheel axis, is fixed by the hardware. When the basepoint distance h2 is measured, the wheel diameter D, as
shown in Fig.2, can be calculated by Eq. (3):

=D 2( h1 − h2 )(3)

3. Image processing of the laser line
Fig.4. Triangulation measurement principle

The traditional method based on mechanism is difficult
to realize accurate positioning directly. The geometricparameters of wheel tread are measured with laser line
projected on the surface of work piece. From theprevious analysis, it is important to determine the key
point accurately, and the laser line must be on thecorrect region to get accurate measurement. The
accurate positioning is required by all of those factors.

Using the developed system, the image with laser line
on tread is captured. As shown in Fig.6, in that image
there are lots of non-target objects besides the red laserline. Then the image segmentation is used to extract thelaser line. In this paper, the separation algorithm isbased on the variation of intensity between the target
pixels and the background pixels by a comparison ofeach pixel intensity value with respect to a threshold. At
the same time, a bilateral filtering is used to smooth theextracted target edges. The bilateral filtering formula isgiven as Eq. (4) and Eq. (5).

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Jiwu Wang, Zhijing Jian, Yaodong Li, Masanori Sugisaka~f ( x) = η −1 ∫ w( x, y )f ( f ( x), f ( y )) f ( y )dy (4)whereη = ∫ w( x, y )f ( f ( x), f ( y ) ) dy(5)In the formula, 𝑓(𝑥) and 𝑓(𝑦) is the intensity of eachpixel, 𝑤(𝑥, 𝑦) measures the geometric proximity
between the pixel of target 𝑥 and neighbor pixel 𝑦. Itsrole is to localize the averaging to a neighborhood of
pixel x. Edge-preserving characteristic of bilateralfiltering is mainly achieved through combination of
spatial function and range of kernel function in theconvolution process. The typical kernel function isGauss distribution function.4

4. Conclusions
The wheelset is important parts for a train. Because its
geometric parameters are related, its measure process ismore complex than other parts’ measurement. In thispaper, the non-contact measurement method with laserline is given. In order to improve its measurement
accuracy, two steps positioning method is developed:
1)
Rough positioning, positioning with tradition
method; 2) Accuracy positioning, paralleling themeasurement frame to the wheel axis. With this method,
the laser line can be projected on the correct section ofwheel tread. Moreover, the bilateral filter algorithm isdeveloped to extract the target edge accurately. Thesealgorithms are verified with a wheelset measurementexperiment.
References

Fig.6. The captured wheel tread image with laser line

With the above processing, the target extract result isshown in the Fig.7. The target character can becontrolled effectively.

Fig.7. Laser line extraction with bilateral filtering

After target laser line is extracted, the real three
dimension coordinates are calculated for each pixel inthe laser line. Then with the definition of wheelset’sparameters, the coordinates of point a, b, c and d can beobtained. 5Thus, as shown in Fig.8, flange thickness W and flangeheight H can be calculated by Eq. (6) and Eq. (7)W= xb − xd

(6)

H=(7)

ya − ycFig.8. The calculation process of wheel tread parameters

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