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ONLINE MEASURING METHOD AND SYSTEM FOR DIAMETER PARAMETERS OF WHEEL SET WU Kaihua, ZHU Feng, ZHUANG Fei, YAN Kuang School of Automation, Hangzhou Dianzi University, Hangzhou 310018, P.R.China, Email: Keywords: Wheel set diameter, online measurement, optoelectronic measurement, automatic measuring system. Abstract The measuring of diameter parameters of wheel set is an important step for the safety of train vehicle running. The wear degree of wheel increases with the speedup of train. The paper introduced an online measuring method of diameter parameters of running wheel set based on optoelectronic detecting technique while the train is running in low-speed within 5-10km/h. The method used precision laser displace-ment sensor, high speed and high resolution CCD and digital image processing technology to realize the non-contact automatic measuring of wheel set parameters. These parameters include the diameter, roundness and diameter difference. The main influence factors for measuring accuracy, including movement, vibration and trigger error were analyzed. The automatic measuring system was designed. The system included wheel detector, laser displacement sensor, semiconductor laser source, CCD, online image acquisition and processing circuits. Theoretic and experimental results showed that the measuring accuracy of diameter parameters was within 1.0mm. The accuracy meets the demand of online diameter measurement. 1 Introduction In order to ensure the safety of a running train, its very important to detect the status of the wheel set automatically and regularly. The wearing degree of the wheel set is one of the main factors that influence the safety and stability of running train. In China, the measurement of wheel set wear is still static and by handwork mostly which limits the accuracy and reliability. The difference between different operators is often too big. The detecting efficiency is also very low. In recent years, the running speed has increased to 160-200km/h. The wear degree and speed are much quicker than before. The static and handwork measuring method cannot meet the development of high-speed train. Researching the automatic and online method is an urgent need of transportation and maintaining departments. The wear parameters of wheel set include geometrical parameters and surface defects. The main geometrical parameters include flange thickness, flange height, wheel diameter, roundness and diameter difference etc. The surface defects include scotching and flaking on wheel tread. These parameters are the main factors that influence the safety and stability of vehicle. The paper mainly introduced the online measuring method and system of diameter parameters. Up to now, most of the maintaining factories were still using special mechanic tools to measure the geometrical parameters by manual work, especially in China. These tools, including special vernier caliper, wheel diameter ruler and rim inside distance ruler, will wear down. The status of these tools will influence the measuring accuracy. The manual work manner had strong labor intensity and low efficiency. The man-made factors produced by different operators perhaps bring different and variable error. The measuring accuracy, reliability and repeatability cannot meet the demand of wheel set maintaining departments. The automatic measuring methods and systems had been researched in recent years1,2,3,4. Yet these methods were only suitable for maintaining factory and the vehicle must be disassembled. Several countries, such as Germany5, Japan, Italy and the United States, have developed online automatic detecting systems based on different principles. These systems are very expensive and have not been widely used. In China, there are also research on this field6, but no practical equipment had been developed. Regular measurement and repair cannot fill the requirements of maintenance. The online measuring of profile parameters is important to the distinguishing and forecast of failures while the train is running in low-speed. The preventive repair will reduce the maintenance cost greatly. The paper introduced a new non-contact online measuring method and system of diameter parameters. 2 Measuring method 2.1 Definition of diameter parameters Train vehicle wheel set is a rolling parts produced by pressing two wheels into an axle. Fig. 1 is the profile of a cargo wheel section. Tread is the contacting part with the rail. The right plane is rim inside plane. The plane wont contact with the rail and has no wear. The part between rim inside plane and tread is called flange. The taping point is a base point on the tread and has 70mm distance to rim inside plane7. The rolling circle is a circle passing through the taping point on the tread . The average diameter of rolling circle equals wheel diameter. The roundness can be derived from diameters at different positions on the rolling circle. The diameter difference is the difference between left and right wheel. These parameters belong to geometric parameters. 2.2 Measuring principle The measuring principle of diameter parameters is based on the projection imaging of structured light beam. Fig.3 is the schematic diagram. According to the definition of diameter parameters, the diameter can be deduced if the rolling circle was obtained. The left-right movement was unavoidable while the vehicle was running. So the position of rolling circle was uncertain. A laser displacement sensor was used to acquire the position of rim inside plane in real time. So the position of rolling circle was also obtained. Two laser lines from semiconductor source illuminated on the surface of tread along the direction of rolling circle plane. The rolling circle was between the two lines. The space between two lines was designed to ensure the rolling circle was always between them. The incident optical plane of light source paralleled with the rolling circle plane and two bright light strips were formed on the tread surface. The narrow strips recorded the diameter information of wheel. If a CCD sensor captured the strips image at a certain angle relative to the incident optical plane, the profile information was recorded in the image . Then the geometric parameters can be calculated by digital image processing method. 2.3 Position the rim inside plane The position of taping point should be obtained before measuring the diameter. It means that the position of rim inside plane should be determined firstly according to Fig.1. The position of rim inside plane was varied because different wheel set had different rim inside distance. The measuring accuracy of diameter was directly influenced by the position accuracy of rim inside plane. A precision laser displacement sensor was used to acquire the position of rim inside plane accurately based on the triangulation principle. The sensor had color compensation function to adapt the change of surface color. The position accuracy was 0.03mm and response time was less than 1ms. The characteristic can meet the online measuring need. 2.4 Image acquisition and processing For image acquiring, the key problem is how to obtain the clear tread profile at certain position. So the relative position of light source, CCD and wheel detecting sensor is very important. Because the movement of the train will results in overlap of consecutive two field images, the CCD cannot work in mode of frame. Only the single field image will be acquired or the CCD supports the progressive scan mode. In order to obtain clear image, the exposure time must very small and the CCD and image acquisition card must support asynchronous reset function. The moment of acquiring is controlled by wheel detecting sensor. The output of wheel detector triggers the acquisition of image in real time. The images are acquired by high-speed image acquisition card with DSP function. Tread FlangeFig. 1: the profile of wheel section. 70 mmRim inside planeTaping point Left wheel Right wheel Wheel set Axle Rolling circle Rolling circleRim inside plane the wheel set. The natural light had influence on the captured original image. In order to improve the original image quality and obtain high image SNR, a set of narrow band-pass optical filters which wavelength matched with laser source were added in front of CCD lens. Besides, the electronic exposure time of CCD was controlled to avoid the image blur. The resolution of CCD was 1024768. The central wavelength of laser source was 650nm. The band width was 15nm. The electronic exposure time was set to 1/10000s. The original profile curve image was diffused because the laser line had 1.0mm width. The one pixel width central line of the profile curve was obtained by applying (1)geometric correction, (2)window adjacent average smoothing, (3)threshold segmentation, (4) thinning, (5)edge tracing, and (6)curve fitting algorithm. Fig.4 showed the digital image processing results (photography condition: speed: 5.6km/h, power of laser source: 20mw, resolution of CCD: 1024x768, shutter time: 1/10000s, light wavelength: 650nm, band width: 15nm, distance between 2 lines: 5mm, width of line:1.0mm). 2.5 Measurement of wheel diameter Based on Fig.4 and the position of rim inside plane decided by laser displacement sensor, a rolling circle was fitted according to the relative position between two lines and base point. The object-image relation and magnifying ratio were determined by the optical imaging system and will be used in the calculation process. Diameter of a circle was certain if 3 points coordinates were known on an arc. A serial of diameters at different positions could be deduced based on Fig.5. If image was acquired while vehicle rolled to different position, many other diameter values could be concluded, then wheel diameter was the average of these diameters. The roundness could also be deduced according to the distribution of these diameters. The diameter difference is the maximum difference between left and right wheel diameters. 3 Measuring system The schematic diagram of measuring system based on the above detecting principle showed in Fig. The system consisted of wheel detector, precision laser displacement sensor, laser line source, CCD and lens, optical narrow-band filter, signal processing and isolated amplifying circuits, image acquisition circuits, interface circuits, control circuits and industrial controlled computer. The arrangement of measuring units was symmetrical. Two to three measuring units every side were necessary in order to ensure the precision and reliability. All of the initial data and measured parameters were transferred to database and could be transmitted to central control computer by Internet for further processing and analyzing. In the detecting process, the laser source, CCD and laser displacement sensor didnt contact with the wheel set. This method had high position accuracy and simplified the image acquisition equipment. The non-contact measuring manner was easy to maintaining and had characteristics of high reliability, high accuracy and repeatability. 4 Error analysis The measuring precision was influenced by many factors, such as lens aberrance, CCD resolution, trigger error produced by wheel detector, movement error, location error of rim inside plane, error produced by the vibration, image acquisition and processing error etc. The measuring units were fixed and vibrated with the railway and wheel, so the vibration error can be ignored. In these factors, the movement and trigger error were the main error source. 4.1 Movement error If the running speed was 10km/h and the exposure time was set to 1/10000 second, the movement error e will be 0.278mm. 4.2 Trigger error The trigger moment may delay or ahead of the designed time and position. So the image acquisition was also different to the scheduled time. If the response speed of wheel detector was 100 kHz, the position trigger error will be less than 0.1 mm. 4.3 General error The position accuracy of laser displacement sensor was 0.03mm. The algorithm error of image processing was one element. Movement and trigger error were 0.278 mm and 0.1mm relatively. Experimental results showed the general error of diameter measuring was within 1.0 mm. 5 Conclusion A new method for wheel set diameter measuring was introduced in running condition. The method combined optoelectronic measuring and digital image processing technology together and realized the non-contact automatic measuring of diameter parameters. The dynamic measuring system was designed. The measuring accuracy of diameter parameters was less than 1.0mm. The repeatability and accuracy of the system can meet the demand of online wheel set maintaining. Acknowledgements The paper was supported by (1) Zhejiang Provincial Natural Science Foundation of China (Y104578), (2) Education Department of Zhejiang Province of China (20040446) and (3) Science and Technology Department of Zhejiang Province of China (2005C31064). References 1 Zuo Jianyong, Zhou Wenxiang, Zeng Jing, et al, “Experimental research on measuring wheel rim profile using laser sensor”, Railway Vehicle, 40(2): 11-13, 2002. 2 Zheng Fenfang, Liu Ji, Fan Peixin, “Measuring wheel set tread profile using digital camera”, Railway Vehicle, 40(1): 19-22, 2002. 3 Wu Kaihua, Yan Kuang, “Research on the method of measuring defects of wheel set tread using optoelectronic technique”, Optical Technique, 31(3): 465-467, 2005. 4 Wu Kaihua, Zhang Jianhua, Yan Kuang, Jiang Peng, “Optoelectronic Automatic Measuring System for Wheel Set Parameters”, Chinese Journal of Scientific Instrument, 27(3): 298-301, 306, 2006. 5 Wang Hao, Wang Li, Gao Xiaorong, “Application of electro-magnetic and ultrasonic technology in the detection of wheel and the processing of detection signals”, Locomotive & Rolling Stock Technology, 6, 34-36, 2004. 6 Yan Kuang, Wu Kaihua, Wang Ruirong, Jiang Peng, “Theoretical research on the measuring method of the running wheel set tread defects based on optoelectronic technique”, Proc. of SPIE, 6150, 61502w-1-61502w-7, 2006. 7 Train Vehicle 1998 No.2, “Assemble and Regulation of Cargo Wheel set and Rolling Axle”, Beijing: China Railing Press, 1998. Fig.7: measurement flow chart. I/O control Image acquisition Data acquisition Industrial Computer Database Image processing measuring unitTrigger control Data processing对轮副直径参数的联机测量办法与测量系统WU Kaihua, ZHU Feng, ZHUANG Fei, YAN Kuang杭州电子大学自动化学院 中国杭州 310018 Email: 关键词：轮辐直径，联机测量，光电测量，自动测量系统摘要：轮辐直径参数的测量是确保车辆系安全运行的一个重要环节。车轮的磨损程度与列车增加的速度成正比。这篇论文介绍了当列车在以5-10千米/时的低速运行时，运用光电探测技术的一种对运行中的齿轮的直径参数联机测量的方法。此法利用精密激光位移传感器，高速高分解CCD和数字图象处理技术实现轮辐参数的无接触自动测量。这些参数主要包括直径，圆度及直径偏差。测量的精确度的主要影响方面是对机械装置，震动装置和启动装置的误差的分析。自动测量系统因此而生。此系统包含车轮探测器，激光位移传感器，半导体激光器来源，CCD，联机图象搜索器处理电路。理论与实践的结果表明，直径参数的测量精确度在1.0毫米内。此精确度与联机直径测量的要求正好符合。1 绪论为了确保运行中的列车的安全，自动地规律地探测轮辐的状况很重要。轮辐的磨损度是影响列车运行的安全性，稳定性的几个主要方面之一。在中国，轮辐磨损度的测量方法仍是静止的，人工的，而这限制了它的准确性与可靠性。不同的操作人员得出的结果往往大不相同，探测效率也很低。近年来，其运行速度已达到160-200千米/时。但其磨损的程度与速度比以前快了很多。静止的人工的测量方法已不能适应高速列车的发展。探索自动联机的方法是运输与维修部门的紧急需要。轮辐的磨损参数包括几何参数与表面缺陷。主要的几何参数包括边缘厚度，边缘高度，车轮直径，圆度，直径差异等等。表面缺陷包括车轮支撑面上的擦伤，发裂。这些参数是影响车辆安全性，稳定性的主要方面。这篇论文主要介绍联机直径参数的测量方法与测量系统。至今为止，尤其在中国，大多数维修工厂仍通过手动使用专门的机械工具来测量几何参数。这些工具包括专业游标弯脚规，车轮直径尺，内边缘距离尺，会磨损坏掉。这些工具的使用程度会影响测量效果。手动人工的工作方式带来的是高强度劳动量和低工作效率。不同操作员引起的人为因素可能会产生各种各样的不同的误差，其测量精密度，可靠性，可重复性不符合轮辐维修部门的要求。自动测量方法与系统已被研究多年1,2,3,4。而这些方法仅仅适合于维修工厂，而且车辆要被分解。一些国家，例如德国5，日本，意大利和美国，已不同原则地发展了联机检测系统。这些系统非常昂贵，还没有被广泛使用。在中国，这个领域也有研究6，但没有实际的设备发展。正常的测量与维修不能满足维修的要求。轮廓参数的联机测量在低速运行的列车对于区别与预测失误很重要。预防的维修会大大降低维修成本。这篇论文介绍一种新的无接触直径参数联机测量方法与系统。2 测量方法2.1 直径参数的定义车辆系的轮辐是通过挤压两个车轮形成一个轮轴的卷型部分。图.1是载物轮区域的轮廓（PROFILE）。滑动面（TREAD）是指与铁轨接触的部分。右平面是指内边缘平面。平面与铁轨不接触，没有磨损。内边缘平面与滑动面（TREAD）之间的部分叫轨底。绝缘绕阻点是指在滑动面上的一个基本点，距离内边缘平面70毫米7。齿轮的滚动圆是指过滑动面上绝缘绕阻点的一个圆圈。滚动圆的平均直径等于车轮直径。圆度得自不同位置滚动圆的直径。直径差异是指左右车轮的差异。这些参数属于几何参数。2.2测量原则直径参数的测量原则是基于由光束组成的图象设计。根据直径参数的定义，如果可以获得滚动圆，直径可以推断出来。当车辆运行时，左右机械装置不可避免的，所以滚动圆的位置是无法确定的。激光位移传感器被用来获取内边缘平面的位置，这样滚动圆的位置就可以确定了。两道来自半导体激光器的光束沿着滚动圆的滚动方向照亮滑动面表面。滚动圆在两道光束之间。两道光束间的空隙的设计是为了确保滚动圆总在这里。外来的视觉光源平面并列滚动圆平面,两道亮光带在滑动面上形成。较窄的光带记录车轮的直径信息。若CCD感应器在与外来视觉平面相关的某一角度获取了图像带，轮廓信息图像中就可以记录轮廓信息。然后几何参数就能够通过数字图像处理方法记录下来。2.3内边缘平面的位置在测量直径之前绝缘绕阻点的位置应该确定下来。这表明内边缘平面的位置应该由图.1先确定下来。内边缘平面的位置是多种多样的，因为不同的轮辐有不同的内边距。内边缘平面的位置的精确度直接影响直径测量的效果。用三角测量原则，用精密激光位移传感器精确获取内边缘平面的位置。感应器拥有色彩补偿功能，适应表面色彩的改变。位置的精确度是0.03毫米，反应时间低于1秒钟。这个特点能够满足联机测量的需要。2.4图象的获取与处理要获得图象，关键问题是如何获得某一位置清晰的滑动面轮廓。所以光源的相关位置，CCD和车轮检测器都非常重要，因为列车的运动会使两个局域图象的连续交搭，CCD不能在结构方式中工作。只有单一局域的图象能被获得，而CCD支持先进的浏览方式。为了获得清晰的图象，曝光时间一定要短，CCD和图象获得卡必须支持异步复位方式。取得的一瞬要由车轮探测感应器控制。用高速图象获取卡通过DSP方式能够获得图象。图2：轮辐自然光对于获取的原始图象是有影响的。为了提高原始图象的质量，获得高质量的图象SNR，我们要在CCD透镜前加上一个和激光源匹配波长的窄带视觉过滤器。除此以外，CCD的电子曝光时间要控制住，以免图象模糊。CCD的分辨率是1024768。激光光源的中央波长是650纳米，波宽是15纳