JP5133210B2 - Method and apparatus for manufacturing tubular parts - Google Patents

Description

  The present invention supplies a blank made of a thermoplastic material, in particular quartz glass, to a heating zone, and softens each region within the heating zone, and from the softened region, a tubular strand having a longitudinal axis extends in the direction of the longitudinal axis. A method for producing a tubular part by forming a blank by drawing out and obtaining a tubular part by shortening the length of the tubular strand, and the actual value of the outer diameter of the tubular strand is continuously calculated. And the calculated actual value relates to the method used for diameter control.

  Furthermore, the present invention is an apparatus for carrying out this method, a measuring apparatus for continuously calculating an actual value of the outer diameter of a tubular strand having a longitudinal axis, and a diameter for controlling the outer diameter using the actual value. It is related with an apparatus provided with a control device.

  By such a method, a hollow cylinder made of a thermoplastic material is formed into a tube having an arbitrary cross-sectional profile. This shaping increases or decreases the radial tube size relative to the radial dimension of the hollow cylinder, or changes the cross-sectional profile of the hollow cylinder. In order to ensure sufficient dimensional stability of the drawn tubular strand, it is inevitable that the radial dimension of the tubular strand, such as outer diameter, inner diameter, or wall thickness, is continuously recorded and controlled. . As operating variables for such control, the internal overpressure in the hollow bore of the hollow cylinder, the feed rate of the hollow cylinder to the heating zone, the withdrawal rate of the tubular strand, and the temperature in the heating zone are commonly used. In order to reduce the dead time and the compensation time of the control process, the radial tubular strand dimensions (control operating variables) are recorded as soon as possible, i.e. immediately after the forming zone of the hollow cylinder. In this region, the high temperature and low viscosity of the tubular strand surface requires non-contact recording of the relevant radial dimensions.

  Thus, German Patent 41 21 611 describes a method for producing a quartz glass tube, in which the wall thickness of the drawn quartz glass tubular strand is controlled. Here, the quartz glass hollow cylinder is continuously pushed and rotated while passing through the heating furnace, and inside the heating furnace, a water-cooled graphite plate is disposed at a radial distance from the longitudinal axis of the tube. Has been placed. Due to the excess pressure in the hollow cylinder, the soft hollow cylinder is sprayed towards the graphite plate, so that the radial distance of the graphite plate from the longitudinal axis of the tube roughly approximates the resulting outer diameter of the tube. It will be prescribed. Here, when viewed in the supply direction of the blank to the furnace, the soft quartz glass is stagnating in front of the graphite plate and forms a peripheral bead around the outer wall of the blank. Optically record the size of this bead using a camera and use the surrounding bead size for process control by using the deviation from a predetermined target bead size for process control. Has been proposed. For this purpose, the image of the bead size generated by the camera is evaluated using an image processing apparatus, and the deviation of the actual value from the target value of the bead size calculated empirically is calculated. As an operating variable for control, the excess pressure in the inner bore of the hollow cylinder is selected. The fluctuations in the inner diameter of the tube and thus the wall thickness of the tube can thus be minimized.

  It is clear that adjusting the outer diameter of the tubular strands by inflating the hollow cylinder towards the graphite molding jaws is no longer sufficient as the demand for dimensional stability of the parts tends to increase. is there. This is because the diameter further changes according to the process conditions after the tubular strand is pulled away from the forming jaw. In particular, the optical recording and evaluation of bead size, which is applied to wall thickness control in a known manner, is not suitable for diameter control.

  Also known are camera-supported measurement and control methods in which an image of an object to be measured is directly generated using a camera and this image is evaluated by an image processing device in terms of controlled radial dimensions. This method creates problems when the recorded radial dimensions are very large. This requires a long distance from the object to be measured, or the use of a so-called telecentric objective, but the former involves significant measurement errors, and in the latter case, an undistorted image is obtained with sufficient resolution. For this, it is necessary that the distance does not exceed 50 cm when the camera is oriented vertically with respect to the measurement object. However, this required proximity of the measuring device to the measuring object means that the current control records the actual value of the diameter where the distance to the forming zone is as short as possible, ie in the region of the maximum process temperature. Is inconvenient. High temperatures can damage the camera and lead to erroneous measurements.

  This disadvantage is avoided by laser measurement methods in which diameter measurement is performed based on the silhouette of the laser band. Commercially available measuring devices are designed for measuring objects having an outer diameter of up to 300 mm. For the measurement of measuring objects with a larger profile, a so-called dual-head measuring system (Doppelkopfmesssystem: dual-head measuring system) must be used, which involves a lot of effort in the construction. While this non-contact laser measurement method can certainly be used in high temperature environments, at the cost of special and complex measures to protect and cool the measurement device.

  Complex coordinate measuring equipment is also used for two-dimensional or three-dimensional measurements of objects. This usually consists of three displacement axes, which are arranged in an overlapping manner to form a Cartesian coordinate system. The position of the individual displacement axis is usually determined by the respective length measuring system. In the coordinate measuring instrument described in DE 39 411 44, a plurality of pivotable video cameras record the geometry of the part to be processed.

  Such a coordinate measuring instrument requires that the measuring object is stationary and is therefore not suitable for measuring tubular strands during the withdrawal process for the purpose of diameter control.

  The object of the present invention is therefore to present a method which is suitable for the production of tubular parts made of thermoplastic materials, in particular parts having a large outer diameter, which have a high dimensional stability.

  Furthermore, the present invention is based on the problem of providing an apparatus for carrying out the method, which is inexpensive.

  With regard to this method, this task is based on a method of the type mentioned at the outset, according to the invention, a first optical image showing a first longitudinal edge of the outer contour of the tubular strand, and a second outer contour. The second optical image indicating the vertical edge is generated, and the position data of the relative position of each vertical edge in the first image and the second image is calculated by the image processing device, and these positions are calculated. This is solved by calculating the actual value of the outer diameter of the tubular strand from the data, taking into account the predetermined spatial relationship between the first image and the second image.

  In order to better explain the method according to the invention simply, in the following, the “first” longitudinal edge of the outer contour of the tubular strand and the “first” image of this longitudinal edge are associated with the “left” side. Conversely, the “second” longitudinal edge and the “second” image of the outer contour of the tubular strand are associated with the “right” side. The outer profile of the tubular strand to be measured is predefined in the two-dimensional projection by the optical system of the device that generates the image and is defined by the longitudinal axis of the tubular strand and the axis extending perpendicular to this longitudinal axis. Appears in the object plane to be formed. The left and right vertical edges of the outer contour extend parallel to the longitudinal axis of the tubular strand in the object plane. The method according to the invention contemplates at least two images, a left image in which a part of the left vertical edge is optically recognizable and a right image in which a part of the right vertical edge is optically recognizable. ing.

  The relative positions of the left and right vertical edges in each image are determined by the image processing apparatus with respect to a coordinate system defined in advance by each image. Here, the straight edges of the tubular strand contour can be easily detected by the image processing device and evaluated in the “image coordinate system” with respect to the position of those edges.

  The first image and the second image are in a predetermined spatial relationship with respect to each other. Therefore, it is considered that the recording positions of the left image and the right image in the coordinate system of the object plane are known. In particular, the interval between the image recording positions in the object plane in the direction perpendicular to the longitudinal axis of the tubular strand is important. This interval is also referred to as “image interval” or “image interval” in the following.

  From the known "image spacing" and the position data of each longitudinal edge of the tubular strand calculated from these images, the actual spacing between the right and left longitudinal edges can be calculated, This corresponds to the outer diameter of the tubular strand. If an image is obtained immediately after the area of the forming zone between the blank and the tubular strand, the actual value of the outer diameter of the measured tubular strand can be measured.

  For the purpose of controlling the outer diameter, the left and right vertical edge images are generated and processed continuously, i.e. continuously or occasionally, as described above. The actual value calculated here of the outer diameter of the tubular strand is provided to a normal outer diameter control device.

  Each image must show only a (small) part of the longitudinal edges of the outer contour of the tubular strand (and must not show both longitudinal edges at once), so the images are at a magnification that is advantageous for evaluation. It can also be generated when the recording distance from the measurement location is long.

  Therefore, the method according to the invention makes it possible to accurately record the position of the longitudinal edges in the area immediately after the end of the molding process and also in the heating zone, without fear of damaging the optical measuring device or measurement results due to high temperatures. And thus it is possible to record the outer diameter accurately and continuously. The measurement accuracy achieved depends on the quality and evaluability of the obtained image, but does not depend on the diameter of the tubular strand to be measured. Thus, recording at a wide angle and a significant reduction in magnification, as in known methods for measuring large diameters, are not necessary in the measuring method according to the invention. In this way, the large diameter of the tubular strand (greater than 100 mm) is also successfully calculated and controlled by optical calculations with high accuracy and better reproducibility than 1/10 mm.

  For the generation of the first image and the second image, a first camera having a first optical axis extending in the first measurement plane, with respect to the first measurement plane and the longitudinal axis of the tubular strand Using a measuring device with a second camera having a second optical axis extending in a second measuring plane extending in parallel with each other has an advantageous effect on the image quality.

  Here, there are provided at least two cameras facing the object plane and each longitudinal edge of the tubular strand, and a left image having at least a part of the left longitudinal edge is generated using the first left camera, A second right camera is used to generate a right image having at least a portion of the right longitudinal edge of the tubular strand.

  The optical axes of both cameras extend parallel to each other and extend parallel to (and without angle to) the longitudinal edges of the tubular strands at the measurement plane. Therefore, an image with particularly small distortion at each vertical edge is possible. Since the optical axes do not form an angle with each other, the interval between the optical axes corresponds to the “image interval” in the object plane when viewed in a direction perpendicular to the longitudinal axis of the tubular strand. The optical axis interval can be equated with the camera interval. The cameras can be movably arranged on a common guide rail, and the distance between the cameras relative to each other can be accurately recorded, for example by a high resolution displacement sensor.

  The optical axis of the first camera extends as much as possible in the tangential plane in contact with the first longitudinal axis of the outer contour of the tubular strand, and the optical axis of the second camera is possible in the tangential plane. As long as it extends as far as the second longitudinal axis of the outer contour of the tubular strand, a particularly suitable embodiment for reducing image distortion results.

  Here, the cameras are positioned and aligned so that, in the ideal case, their optical axes are exactly towards each longitudinal edge of the outer contour of the tubular strand (the “measurement plane” is tangential) Corresponding). Here, each vertical edge appears in the center of the image in an ideal case. Deviations from this ideal case where each vertical edge extends within the tolerance of +/− 1 mm from the optical axis of the camera can be tolerated without significantly impairing measurement accuracy and reproducibility. Regardless of whether the optical axis of the camera extends exactly in the tangential plane, i.e. whether it faces the vertical axis, according to the present invention, the relative position of the vertical edges in the image is determined by image evaluation. As a result.

  In this connection, it has been found that diameter control is particularly suitable to include position tracking of the first camera and the second camera.

  If the deviation between the longitudinal edge of the tubular strand and the optical axis of the camera exceeds the predetermined tolerance of 1 mm, again the camera optical axis is within the tolerance, preferably around the relevant tangent plane It is possible to control the tracking of the camera so as to extend in this tangential plane. The controlled tracking of this camera (camera optical axis) is the corresponding tracking of the camera optical axis when the outer diameter changes or the tubular strand moves laterally, so that the image is always Ensure that it is obtained with the highest possible optical resolution and with the least possible optical distortion. As a result, the distance between the right camera and the left camera, and the accompanying optical axis distance of these cameras, and the distance between the right image and the left image relative to each other are continuously adjusted. As described above, the interval value is taken into consideration when calculating the outer diameter of the tubular strand.

  In particular, for applications where the molding zone between the blank and the tubular strand is surrounded by a heating device or the like, which makes it difficult or obstructed to look down at this molding area at a right angle, the first light It has been found that an embodiment of the method according to the invention is suitable, in which the axis and the second optical axis form an inclination angle in the range of 10 to 70 degrees with respect to the longitudinal axis of the tubular strand.

  The inclination of the optical axis with respect to the vertical axis makes it possible to record the outer diameter near the molding zone even if the molding zone is surrounded by a cover.

  Preferably, the first camera and the second camera are arranged at least 1.5 m away from the forming zone between the blank and the tubular strand.

  By increasing the distance, it is avoided that the measurement result is impaired or the measuring device is damaged by the high temperature in the molding zone.

  Furthermore, it has proved advantageous to adjust the apertures and objectives of the first camera and the second camera so as to obtain a depth of field of at least 20 mm, preferably at least 30 mm.

  If the depth of field is large, the image will always be clear and accurate without complicated readjustment, even if the vertical edges move out of the object plane predefined by the camera optics. Shows the evaluable vertical edge, which facilitates the detection and evaluation of the vertical edge by the image processing apparatus.

  With respect to the device, the problem presented above is that according to the invention based on the device having the features mentioned at the outset, the measuring device has a predetermined spatial relationship with the first camera and the first camera. A second camera and an image processing and evaluation unit, the first camera facing the first longitudinal edge of the outer contour of the tubular strand and the first longitudinal edge first Generating an optical image, wherein the second camera is directed towards the second longitudinal edge of the outer contour of the tubular strand and produces a second optical image of the second longitudinal edge, an image processing unit Is suitable for calculating the position data of the relative positions of the respective vertical edges in the first image and the second image, and the evaluation unit uses the first camera and the first image from these position data. Taking into account the spatial relationship of the two cameras, It is solved by suitable to calculate the actual value of.

  To better illustrate, in the following, the “first” camera, the “first” longitudinal edge of the outer contour of the tubular strand, and the “first” image are associated with the “left” side, Conversely, the “second” camera, the “second” longitudinal edge of the outer contour of the tubular strand, and the “second” image are associated with the “right” side.

  The outer contour of the tubular strand to be measured is predefined in the two-dimensional projection by the camera optics and is in the object plane formed by the longitudinal axis of the tubular strand and the axis extending perpendicular to this longitudinal axis. Appear in The left and right vertical edges of the outer contour extend parallel to the longitudinal axis of the tubular strand in the object plane.

  The left and right vertical edges of the outer contour extend parallel to the longitudinal axis of the tubular strand in the object plane. In the device according to the invention, at least two cameras are intended, a left camera and a right camera. These cameras are suitable for generating images in which a part of the left vertical edge and a part of the right vertical edge are each optically recognizable.

  The image processing apparatus is suitable for obtaining the relative positions of the left and right vertical edges in each image with respect to a coordinate system defined in advance by each image.

  The first camera and the second camera are in a predetermined spatial relationship with respect to each other. These cameras can be movably arranged on a common guide rail, for example, and the distance between these cameras relative to each other can be accurately recorded by a high-resolution displacement sensor or the like. In particular, the spacing between both recording positions for each image in the object plane in the direction perpendicular to the longitudinal axis of the tubular strand is important. This interval is defined in advance by the position of the camera, the direction of each optical axis, and the interval with respect to the object plane. In the simplest case (when the optical axes are parallel), this spacing corresponds to the camera spacing and is also referred to below as “camera spacing”.

  The known "camera spacing", the direction of the optical axis of the camera, the resulting position of the left and right images in the object plane, and the position of each longitudinal edge of the tubular strand calculated from these images From the data, the actual spacing between the right and left vertical edges can be calculated, which corresponds to the outer diameter of the tubular strand. The actual value calculated here of the outer diameter of the tubular strand is provided to the outer diameter control device.

  Each image must show only a (small) part of the longitudinal edges of the outer contour of the tubular strand (and must not show both longitudinal edges at once), so the images are at a magnification that is advantageous for evaluation. It can also be generated when the recording distance from the measurement location is long.

  Therefore, the apparatus according to the present invention comprises a wide-angle objective lens or a telecentric objective lens that is susceptible to distortion without fear of damaging the optical measurement apparatus or measurement results due to high temperatures in the region immediately after the end of the molding process This makes it possible to accurately record the position of the longitudinal edge without requiring a complicated measuring device, and thus to accurately and continuously record the outer diameter of the tubular strand. Using this apparatus, large diameters of tubular strands (greater than 100 mm) are also successfully calculated and controlled with high accuracy and reproducibility better than 1/10 mm by optical calculations.

  Preferably, the first camera comprises a first optical axis extending at the first measurement plane, and the second camera extends parallel to the first measurement plane and the longitudinal axis of the tubular strand. And a second optical axis extending on the second measurement surface.

  Here, the optical axes of both cameras extend parallel to each other and parallel to the longitudinal edges of the tubular strands in the measurement plane (and without an angle to these longitudinal edges). Extend. This enables an image with less distortion at each vertical edge. Since each optical axis extends parallel to each other, the optical axis spacing is the camera spacing, and further the recording position spacing in the object plane (i.e., “image” when viewed in the direction perpendicular to the longitudinal axis of the tubular strand. Corresponds to the “interval of”). Here, the optical axis interval can be regarded as the same as the camera interval.

  Further advantageous embodiments of the device according to the invention emerge from the dependent claims. Insofar as the embodiment of the apparatus presented in the dependent claims mimics the scheme set forth in the dependent claims relating to the method according to the invention, the corresponding explanations of the above embodiments correspond. See method claims.

  In the following, the present invention will be described in more detail based on examples and drawings.

  FIG. 1 shows a thin walled hollow cylinder 1 made of quartz glass having an outer diameter of 200 mm and having a thick wall to produce a thin walled tubular strand 2 having an outer diameter of 340 mm, Figure 2 shows an apparatus for cutting to obtain a plurality of tubes having a desired length.

  The hollow cylinder 1 is continuously pushed into the resistance furnace 4 while being rotated about its longitudinal axis 3 at a feeding speed of 4 cm / min by a feeding device. The resistance furnace 4 surrounds the hollow cylinder 1 in an annular shape, and heats each region at a temperature of about 2100 ° C. inside.

  In order to take out, a pulling device is used in which the annular strand 2 is rotated around its longitudinal axis 3 and pulled in the direction of the longitudinal axis 3 at a withdrawal speed of about 12 cm / min. Here, the free front surface of the annular strand 2 is provided with a pull-out bar made of quartz glass, and is fused with the pull-out bar in an airtight manner.

  The quartz glass hollow cylinder 1 is sealed by an airtight rotary joint at its free front face. Protruding into the furnace 4 is a molding tool with two water-cooled molding jaws 5 covered by Graphitzungen (graphitzungen: graphite tongues) (FIG. 1 shows these moldings for illustration reasons). Only one of the jaws is shown schematically, the position of the shaped jaw 5 can be better recognized from the plan view of FIG. The hollow cylinder 1 expands towards the forming jaw 5. For this purpose, an oxygen stream is introduced into the rotating quartz glass hollow cylinder 1 through a rotary joint and the internal overpressure is initially set to 10 mbar. Due to this internal excess pressure, most of the low viscosity of the quartz glass hollow cylinder 1 is sprayed toward the fixed molding jaw 5 with a target diameter of 340 mm as a guide. Here, in front of the forming jaw 5, a peripheral bead 6 is formed around the outer periphery of the tubular strand 2. Each of these shaped jaws can be moved by a motor drive in a direction perpendicular to the longitudinal axis 3 using a shaped jaw tracking device 14 controlled by a computer.

  A measurement / control device is provided to measure and control the outside diameter actually adjusted, and this device includes two high-resolution CCD cameras 7 and 8. The optical chip has 1620 pixels in the horizontal axis and 1220 pixels in the vertical axis (2 megapixels). The cameras 7 and 8 are arranged 1.5 m away from the heating zone 4 on the left and right above the tubular strand 2, and are inclined with respect to the longitudinal axis 3 of the tubular strand in this arrangement. The angle formed by 9 is about 30 degrees.

  Cameras 7 and 8 record the measurement field of the left camera 7 with a part of the left circumference of the outer sheath, and the measurement field of the right camera 8 records a part of the right circumference of the outer sheath of the tubular strand, These are each directed to the tubular strand 2 and focused so as to be reproduced at a magnification of 15 times as the left longitudinal edge 10 and the right longitudinal edge 11 of the tubular strand outer contour. These reproductions are transferred as digital information to an evaluation unit represented by the computer 13 and displayed on the monitor 12 as an image. An image data processing program is installed in the computer 13, and the relative axial positions of the vertical edges 10 and 11 in each image are calculated using this program.

  From FIG. 2 it can be seen that the cameras 7 and 8 are mounted on a carriage 16 which is movable on a common guide rail 15. The distance “A” between the optical axis 9 of the left camera 7 and the optical axis 9 of the right camera 8 is up to 0.01 mm using a high-resolution displacement sensor (the output signal of this sensor is sent to the computer 13). It can be determined accurately. The cameras 7 and 8 are oriented such that their optical axes 9 extend parallel to each other and in parallel to the longitudinal axis 3 in the plane. Thus, the camera spacing “A” is defined as the outside of the tubular strand when the optical axis 9 is in contact with the longitudinal edge 11 of the right tubular strand and the longitudinal edge 10 of the left tubular strand, as shown in FIG. Corresponds to the diameter.

  Control of the outer diameter of the tubular strand 2 will be described in more detail below with reference to FIG. On the x-axis of the illustrated coordinate plane, the axial positions of the cameras 7 and 8 on the guide rail 15 are plotted. These positions, and hence the camera interval “A” (interval of the optical axis 9), are also constantly monitored by the high-resolution displacement sensor and are thus set and known. The images “L” and “R” shown above the x-axis show the measurement field of each camera 7, 8 in the object plane or are generated by the camera 7, 8 and displayed on the monitor 12 The recorded image is shown. Each measurement field includes a longitudinal edge 10 and 11 of the outer contour of the tubular strand 2, respectively.

  Using the image data processing program, the relative positions of the vertical edges 10 and 11 with respect to each optical axis 9 are calculated. The deviation is represented by X1 and X2 in FIG. From these position data, the computer 13 calculates the actual value of the outer diameter of the tubular strand based on A + X2-X1. The actual value is used for outer diameter control whose operating variable is the spacing of the forming jaw 5. If the actual value deviates from the predetermined target value, the computer 13 provides a corresponding signal to the motor driven shaping jaw tracking device 14 for the shaping jaw 5.

  For optimal image evaluation, each vertical edge 10, 11 in the left image L and right image R extends exactly to the center of the image in the ideal case. If the evaluation of the recorded image of the camera results in a vertical edge position shift X1, X2 that is greater than a predetermined threshold of 15 pixels (0.3 mm), the associated cameras 7, 8 on the guide rail 15 will have this threshold. Move until it falls below again.

  Based on the control technology record of the forming process under the calculation of the control variable (outer diameter of the tubular strand) by optical calculation, it is achieved that the outer diameter of the tubular strand is sufficiently constant. . The reproduction accuracy of the measured value is better than 0.05 mm (when the lower limit value of the optical resolution is 0.01 mm).

1 is a schematic view of an embodiment of an apparatus for carrying out a method of manufacturing a tube according to the present invention, in side view and partially in cross section. FIG. 2 is a schematic plan view of a part of the device according to FIG. 1. It is the schematic explaining calculation of the outer diameter of the pipe | tube in the method by this invention.

Claims (10)

A blank (1) made of quartz glass is supplied to the heating zone (4), and is softened for each region in the heating zone (4). From the softened region, a tubular strand (2) having a longitudinal axis (3) In the direction of the longitudinal axis (3), and obtaining the tubular part by shortening the length of the tubular strand (2), thereby producing the tubular part by molding the blank (1) The actual value of the outer diameter of the tubular strand is continuously calculated, and the calculated actual value is used for diameter control,
A first optical image (L) showing a first vertical edge (10) of the outer contour of the tubular strand (2) and a second optical image (second) showing a second vertical edge (11) of the outer contour (2). R) and position data of relative positions of the vertical edges (10; 11) in the first image (L) and the second image (R) are calculated by the image processing device. From the position data, the actual value of the outer diameter of the tubular strand is calculated in consideration of a predetermined spatial relationship (A) between the first image (L) and the second image (R). ,
A first camera (7) having a first optical axis (9) extending in a first measurement plane to generate the first image (L) and the second image (R); A second camera (8) having a second optical axis (9) extending in the first measuring surface and a second measuring surface extending parallel to the longitudinal axis (3) of the tubular strand. ) Is used, and
The first optical axis (9) and the second optical axis (9) form an inclination angle within a range of 10 degrees to 70 degrees with respect to the longitudinal axis (3) of the tube. ,Method. The optical axis (9) of the first camera (7) extends in contact with the first longitudinal axis (10) of the outer contour of the tubular strand as accurately as possible in the tangential plane, The optical axis (9) of the second camera (8) extends as close as possible to the second longitudinal axis (11) of the outer contour of the tubular strand in the tangential plane. The method according to claim 1 . The method according to claim 1 or 2, characterized in that the diameter control comprises a position tracking of the first camera (7) and the second camera (8). The first camera (7) and the second camera (8) are arranged at least 1.5 m away from the forming zone between the blank (1) and the tubular strand (2). to the method according to any one of claims 1 to 3. An objective lens and the aperture of the first camera (7) and said second camera (8) is characterized in that it is adjustable to the depth of field of at least 20mm is obtained, according to claim 1 5. The method according to any one of 4 . An apparatus for performing the method according to any one of claims 1 to 5 ,
A measuring device for continuously calculating the actual value of the outer diameter of the tubular strand (2) having a vertical axis;
A diameter control device (13) for controlling the outer diameter using the actual value;
With
The measurement apparatus includes a first camera (7), a second camera (8) having a predetermined spatial relationship with the first camera (7), and an image processing / evaluation unit (13). The first camera (7) is directed towards the first longitudinal edge (10) of the outer contour of the tubular strand (2), the first longitudinal edge (10) first An optical image (L) is generated, wherein the second camera (8) is directed towards the second longitudinal edge (11) of the outer contour of the tubular strand (2), the second longitudinal generating a second optical image of the edge (11) (R),
The image processing unit (13) is suitable for calculating position data of relative positions of the vertical edges (10; 11) in the first image (L) and the second image (R). and, as well as the evaluation unit (13), from said position data, taking into account the spatial relationship of the first camera (7) and said second camera (8), the outer diameter of the tubular strand Suitable for calculating actual values,
The first camera (7) has a first optical axis (9) extending in a first measurement plane, and the second camera (8) includes the first measurement plane and the tubular. Having a second optical axis (9) extending in a second measuring plane extending parallel to the longitudinal axis (3) of the strand;
The first optical axis (9) and the second optical axis (9) form an inclination angle within a range of 10 degrees to 70 degrees with respect to the longitudinal axis (3) of the tube. ,apparatus. The optical axis (9) of the first camera (7) extends in contact with the first longitudinal axis (10) of the outer contour of the tubular strand as accurately as possible in the tangential plane, The optical axis (9) of the second camera (8) extends as close as possible to the second longitudinal axis (11) of the outer contour of the tubular strand in the tangential plane. The apparatus according to claim 6 . Device according to claim 6 or 7 , characterized in that diameter control comprises position tracking of the first camera (7) and the second camera (8). The first camera (7) and the second camera (8) are arranged at least 1.5 m away from the forming zone between the blank (1) and the tubular strand (2). The device according to any one of claims 6 to 8 . It said first camera (7) and openings and the objective lens of the second camera (8) is characterized in that it is adjustable to the depth of field of at least 20mm is obtained claims 6 to The apparatus according to any one of 9 .

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