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The technical characteristics, advantages and applications of an automated optoelectronic measuring system for inspection of internal and external thread geometry parameters, designed by "Optel" company Ltd of Ufa State Aviation Technical University is presented in this paper. The measuring apparatus can be applied for research and in industry. Its main advantages are non-contact non-destructive inspection and high productivity.
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At present there is an important problem of high accuracy thread measurements. This problem is especially actual for oil industry where oil tubes and clutches, which require high quality inspection of thread, are used.
Most of existing methods for external thread inspection of caliber use more accurate calibers, which are screwed on manually, or manual inspection with the help of instrumental microscope
The internal thread is usually inspected with the help of calibers. Besides, the second methods of inspection with the help of thread models, which are made manually by impressing the material into thread, are used. Then those thread models are measured with the help of instrumental microscope. Such Рјanual visual methods of thread inspection widely used in industry. But these methods have several serious disadvantages: it is very prolonged and labor-intensive, but the main point is that it has low accuracy and high subjectivity; it is difficult to obtain detailed information about most important parameters of calibers thread.
In spite of its relevance, this problem doesn't have a satisfactory solving, because most of existing methods of thread quality inspection are based of contact technique.
One of systems designed by "OPTEL" company is computer-aided laser system3, which are intended to carry out non-contact measurements of geometrical parameters of caliber thread to inspect tube and clutch caliber. These automated systems carry out caliber thread geometry inspection with the help of laser scanning, profile geometry measurement, and thread parameters calculation and saving of measuring results.
Systems can work both autonomously and as a part of process control system. They can output obtained measuring results and service signal in digital form and define if the thread corresponds to specified one and if the article is fitted.
The application field of systems includes non-contact inspection of caliber thread for high-precision and rapid measurement of geometrical parameters, process control in industry, computer-aided design and manufacturing (CAD-CAM).
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"OPTEL" system provides non-contact measurements of thread profile of smooth and thread caliber. The measuring principle is based on scanning of thread profile by laser beam. Scanning is carried out with help of displacement of laser head of optic mechanical unit in relation to measured object.
To carry out measurements of thread geometry the corresponding modes of scanning of object thread and calculation of its specific parameters are built in.
The profile of caliber can be measured on various scan angles. This gives the full adequate information about real profile of object thread, including calculated parameters, which are saved in computer memory, transferred to process control system and can be displayed or printed.
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Figure 1 - Shadow method of measurement
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The operation of measuring system is based on shadow and triangulation measuring methods. These methods can be implemented with the help of semiconductor lasers.
Shadow method (Figure 1) is used for measuring of external thread profile of object. Shadow projection of thread is made when object is illuminated with narrow laser beam in the form of strip, which is directed from laser illuminator to photo detector unit. The width of analyze strip is 0.01…0.02 mm. According to crossing position of object edge by laser beam the video pulse of video signal is generated, the time position of which is transferred into computer in the form of codes.
Simultaneous positioning of edge and current coordinate of scanning head according to table position allow obtaining of thread profile. The CMOS array or CCD is used as a photo detector.
The triangulation method is used for measurement of internal caliber surface profile, including end edge. Apart from this the thickness of caliber wall can be measured with allowance for shadow projection of its external surface.
Triangulation method consists in following: the object is illuminated with narrow laser beam, which is emitted from laser illuminator to measured surface. The image of laser spot is projected onto photo detector unit from another angular direction. The position of spot projection corresponds to current profile of object. The video impulse of video signal is generated and its time position is inputted into computer in the form of code. Simultaneous positioning of edge and current coordinate of scanning head according to table position allow recording of profile of measured internal surface of object.
The electronic unit of system consists of several specialized cards and is included in industrial computer. It provides respective processing of photo detector units signals in required dynamic range of intensity change to select information about simultaneous position of two edges (shadow method), two centers of narrow laser beam (triangulation method) and position of coordinate table. Codes, which correspond to thread profile and internal object surface, are inputted into computer. Due to geometrical limitations "OPTEL-THREAD" system is able to carry out triangulation measurements of internal surface of object on a depth, which is equal to several thread laps.
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Optoelectronic "OPTEL-R" system consists of mechanical measuring unit and industrial computer. Laser heads of mechanical unit move automatically in respect to object under control and allow obtaining measured sections for any angular position of object. This gives the full information, necessary for making a decision about caliber workability as well as for possible recovery of caliber profile.
Automated measurements of thread geometry are carried out by industrial computer, which operates under control of specialized original software. This software also makes calculations of thread parameters, displays and saves the values of real thread profile for every type and size of caliber.
Measurements of thread sections are carried out non-contactly and with high productivity and precision. Measuring resolution of linear parameters of thread (coordinates and profile) is 0.5 mm, the measuring error of geometrical parameters of thread under confidence probability 0,95 is less than 0,3 maximal tolerance for thread parameters.
Measuring results are displayed in visual form (text and graphic protocol), saved and can be stored on data carrier for unlimited period. They also can be printed on paper (figure 3 and 4).
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Figure 3. Examples of graphic protocols of measurement of thread profile.
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Figure 4. Calculated thread parameters
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Software can calculate the following thread parameters:
В В - step
В В - height
В В - average diameter
В В - thread cone angle
В В - right and left side angle of a lap profile
В В - radiuses of picks and pits
В В and others
Calculation of all of the thread parameters is carried out by software in real time and lasts for several seconds. Each thread parameter is calculated in according with unique algorithm. For example, the average lap diameter is carried out as follows: the circumference with specified diameter is adjuncted to pit of lap (according to API (American Petroleum Institute) standard). The point of average diameter is calculated as a midpoint of distance between two tangency points. The distance between two opposite points of average diameter is the middle diameter of given lap.
The system can work both autonomously and as a part of process control system (slave subsystem of PCS technological line). To communicate with PCS the special communications protocol is developed. Information interchange can be divided into three main parts:
- interchange of commands and data;
- interchange of bit commands;
- interchange of measuring results.
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1.V. Bastl, G. Bendit, P. Berveger and others, Measurements in industry, Reference book, P. Profos, ed., Metallurgiya, Moscow, USSR, 1990.
2. C. P. Parfenchuk, "Device for inspection of average diameter of thread" in Metal-cutting and monitoring tool (Vol. 5), p. 38, NIIMash, Moscow, 1981.
3. Rav. M. Galiulin, B.G. Ilyasov, Rish.M. Galiulin, and others, Optoelectronic computer-aided systems for three-dimensional inspection of complex objects in Machine Vision and Three-Dimensional Imaging Systems for Inspection and Metrology, Kevin G. Harding, John W. V. Miller, Bruce G. Batchelor, Editors, Proceedings of SPIE Vol. 4189, Bellingham, USA, 2001, pp. 268-275
4. Russian national standard (GOST) 12690-67. Conical thread calibers. Methods and means of calibration.
5. Russian national standard (GOST) 8.128-74. Cylindrical thread calibers. Methods and means of calibration.
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Russian