Method for measuring the roundness of round profiles

A method for measuring the roundness profiles moved forward in longitudinal direction inside a rolling mill, using two laser scanners, respectively provided with a light-sensitive sensor and a laser. At least three shadow edges that fit against the round profile to be measured and enclose the round profile to form a polygon are generated and measured and the corresponding tangents are computed. The method includes: a) determining a center (Z0) in the measuring field prior to the measuring operation; b) determining perpendicular lines from the center (Z0) to the tangents and measuring the distance from the center (Z0) to the tangents; c) determining a contour by computing the corner points of the polygon enclosing the round profile; d) positioning a reference circle relative to the contour so that: i) the square shape deviation of the contour relative to this reference circle reaches a minimum; ii) the reference circle represents the smallest possible circle that can fit around the contour, iii) the reference circle represents the largest possible circle that can fit inside the contour; or iv) the reference circle together with a different circle, arranged concentric to the reference circle, encloses the contour with a minimum radial difference; e) computing the diameter (Dref) of the reference circle and determining from the position in space of the reference center (Zp), which represents the center point of the reference circle, and f) computing at least two vectors extending from the reference center (Zp) to the contour and determining the out-of-roundness from the obtained data.

The invention relates to a method for measuring the roundness or the shape deviation of round profiles, moved forward in longitudinal direction inside rolling mills, for which at least three shadow edges placed against the round profile to be measured so as to enclose it and form a polygon are generated and measured in a manner known per se with the aid of a measuring instrument provided with at least two laser scanners, each comprising at least one light-sensitive sensor and one laser, and that the respective tangents are computed from this.

In the steel-producing industry, so-called long products are rolled in specialized rolling mills to obtain the desired end products. If these long products are to be rolled into round rods, they are for the most part rolled in a 3-roll finishing stand provided with several roll blocks (in most cases a 3-roll block) to the final dimensions and are then moved to a cooling bed for the cooling down. Normally, four roll blocks with respectively three cylindrical roll discs are used, wherein the center planes of the cylindrical roll discs are rotated by respectively 60° from one roll block to the next roll block. This type of steel rod oftentimes exhibits polygonal shape deviations from the circular shape, most often in the “three-wave” or “six-wave” form.

If the diameter of such a polygonal steel rod is measured across its circumference with a mechanical caliper gauge or optically, all determined diameters can have the same value. In reality, however, the product is not round but is out-of-round/wavy. A product of this type is also referred to as “curve with constant width.”

To determine so-called constant width curve errors, caliper gauges with support prisms were used in the past and are still used to date. Depending on the waviness of the circumferential profile, different support angles are recommended for this.

The mechanical measuring and computing operations for determining the roundness have been explained and described for decades with the aid of the relevant DIN standards, for example the German Standard DIN ISO 4291 “Method for Determining the Deviation from the Roundness,” DIN ISO 6318 “Roundness Measurement, Terms and Parameters for the Roundness” and DIN ISO 4292, “Method for Measuring Roundness Deviations, Two-Point and Three-Point Measuring Methods.”

The mechanical measuring of long products of the type as discussed herein with the aid of mechanical roundness measuring instruments must be realized offline. For this measuring operation, a sample must be clamped into a precision turning mechanism. A tracer then measures the radial deviations of the profile during the rotary movement, resulting in a diagram that shows the circumferential profile with the radii as they relate to the respective angle degree. The evaluation of these circumferential profiles is described in detail in the aforementioned relevant standards.

During the mechanical measuring inside a laboratory, an infinite number of circumferential points can be determined during the rotation of the sample. However, all desired tangents must be measured simultaneously during the measuring operation along the production line where the product is transported in longitudinal direction, so as to be able to determine the profile of a local cross section. A mechanical online measuring is therefore not possible.

One important starting parameter for determining and evaluating the out-of-roundness is the so-called reference circle and its center, which form the reference for all further steps of the measuring operation. Four different methods for determining this are described in the aforementioned standards.

In addition to the mechanical measuring instruments, contactless measuring instruments have also been known for decades (e.g. as disclosed in the documents DE 39 16 715 and the DE 40 37 383 A1). A profile measuring method is furthermore described in the JP 56-117107 A, which uses laser beams to measure and/or scan the long product to be measured. For example, this reference describes that a precise profile measurement can be obtained even with a constant width by placing a first, a second and a third tangent against the outside circumference of an object for which the profile is to be measured and that the profile is measured by determining the difference between a circle determined by these tangents and the profile of the long product to be measured. These tangents are positioned with the aid of laser beams and/or projection beams.

A method for measuring the out-of-roundness of round products and/or round profiles as discussed herein is also known from the document DE 100 23 172 A. This method utilizes a measuring instrument consisting of three or more laser scanners, which are respectively provided with a light-sensitive sensor and a laser. The round product is illuminated by the laser beam of each laser scanner in such a way that the round product projects one or two shadow edges onto the associated sensor. A straight line that extends parallel to the laser beam is computed for each of the shadow edges. A circle against which these straight lines are placed in the form of tangents is furthermore computed from respectively three determined degrees. The computing of the circle is repeated and the out-of-roundness determined as the difference between the largest and the smallest diameter for the circles.

This out-of-roundness determination has the disadvantage that the measuring values are strongly distorted with the smallest of angle errors, in particular if the tangents do not come to rest precisely on the maximum or minimum of the circumferential profile. In addition, the center location in space is not specified. As a result, e.g. with asymmetrical shape deviations, the determined profile can have a symmetry that is periodic to the tangent number and arrangements and does not reflect the true profile character.

It is the object of the present invention to provide a method of the aforementioned type, which uses a contactless measuring instrument for measuring with the highest possible precision the profile and the out-of-roundness along a production line.

This object is solved with the teaching as disclosed in the claims.

For the method according to the invention, at least three shadow edges placed against the round profile to be measured are generated with the aid of a measuring instrument provided with at least two laser scanners. These laser scanners comprise respectively one light-sensitive sensor and one laser.

FIG. 1shows a cross section through a round profile to be measured, for which the outside contour is shown with a continuous, bold line. A total of twelve shadow edges are fitted against this round profile with the aid of six laser scanners, wherein these shadow edges lead to the tangents T1, T2, T3, T4, T5and T6as well as T1′, T2′, T3′, T4′, T5′and T6′and wherein each tangent pair T1, T1′; T2, T2′; T3, T3′; T4, T4′; T5, T5′and T6, T6′respectively belongs to one laser scanner. A total of six laser scanners are therefore used, wherein the individual round profile to be measured is always located completely inside the measuring field for these laser scanners.

The center Z0of the measuring field for the measuring instrument was otherwise determined more precisely and calibrated prior to placing the shadow edges, respectively the tangents.

Even if a total of 12 tangents are placed against the round profile according to the present example, the number of tangents T can be optional. However, a minimum of at least three tangents are required to form a polygon that encloses the round profile, wherein the tangents are positioned at a known angle, relative to each other.

Following the determination of the tangents, the perpendicular lines r1, r2, r3, r4, r5, r6, r1′, r2′, r3′, r4′, r5′and r6′are determined and thus the perpendicular distance from Z0to the respective tangents.

FIG. 2shows that the tangents obtained in the manner as described in the above form a polygon with the corners A to L that encloses the round profile. InFIG. 2, the polygon is shown with a continuous, bold line while the round profile is shown with a dotted line.

FIG. 3shows the form of the polygon according toFIG. 2(indicated with a dotted line inFIG. 3) following the smoothing with the aid of an adapted spline interpolation. A simulated contour is generated as a result (continuous, bold line), which for the most part corresponds to the real round profile (continuous thin line). In this way, usable data are obtained along the complete curve. In order words, data can also be determined for locations outside of the real values determined with the aid of the shadow edges or tangents.

The diagram inFIG. 4shows how a reference circle (dash-dot line) is placed inside the simulated contour (continuous, bold line), such that the square shape deviations of the simulated contour to this reference circle (dash-dot line) are at a minimum, wherein the diameter Drefis computed for this reference circle. The reference center Zpis determined from the position of the reference circle in space.

FIG. 5shows how two vectors are determined, starting from the thus determined center Zp, namely the minimum distance Rminand the maximum distance Rmaxfrom the reference center Zpto the simulated contour and how the out-of-roundness is determined from these values. The determined extreme values can be located anywhere along the simulated contour, thus also at angle positions that are located in contour sections between the original measuring points.

An alternative calculation method is explained with the aid of the diagram shown inFIG. 6. The vectors VGT1, VGT2and VGT3on the one hand and VDT1, VDT2and VDT3on the other hand are computed for this in step f) of the method according to the invention, wherein these vectors extend from the reference center Zpin the direction of the cylindrical roll discs of the two last 3-roll stands of a rolling mill. It is assumed that the center plane for the cylindrical roll discs of the next to the last roll stand is at 0°, 120° and 240° and that the plane for the last roll stand is at 60°, 180° and 300°. The 0° and/or 180° plane according toFIG. 6is that plane, which extends perpendicular to the paper plane and through VGT1and VDT3and is indicated inFIG. 6with the perpendicular dash-dot line. In other words, the vectors VGT1, VGT2and VGT3point toward the gap between the cylindrical roll discs of the last roll stand while the vectors VDT1, VDT2and VDT3point toward the center of pressure and/or the center of the rolls for the last roll stand. This center of pressure otherwise is located where the roll gap for the rolls of the next to the last roll stand is normally located.

Using the vectors VGT1, VGT2and VGT3, it is furthermore possible with the aid of simple mathematical calculations to compute the GT value of interest for the adjustment of roll stands, which is a length measure. The same is true for computing the desired DT value, also a length measure, from the vectors VDT1, VDT2and VDT3.

These values are critical—as previously explained—for optimizing the adjustment of the individual 3-roll blocks and are turned relative to each other by a fixed angle of 60°.

The measuring instrument required for the method according to the invention frequently cannot be arranged directly behind the last roll stand (for example for space reasons), but only at a distance thereto in downstream direction, resulting in the problem that the completely rolled round profile is rotated during the distance traveled from the last roll stand to the measuring plane for the measuring instrument. The angle at which the round profile is rotated around the longitudinal axis over this distance is generally known for the individual rolling mills.

The diagram shown inFIG. 7illustrates how the desired GT and DT values can be computed despite the rotation of the round profile. Since the angle of rotation with reference a inFIG. 7is known, the above-described vectors are not determined in angle direction 0°, 120° and 240° (applicable for VGT1, VGT2and VGT3) and/or 60°, 180° and 300° (applicable for VDT1, VDT2and VDT3) from Zpto the simulated contour, as shown inFIG. 6. Rather, these vectors are also rotated by the angle of rotation α. The vectors VGT1, VGT2and VGT3and/or VDT1, VDT2and VDT3are therefore computed rotated by the angle α from Zpto the simulated contour (bold, continuous line). The vectors VGT1, VGT2and VGT3are shown inFIG. 7respectively by an arrow with a continuous line while the vectors VDT1, VDT2and VDT3are shown by an arrow with a dashed line.

Thus, the typical values for GT and DT can also be determined at the location of the last roll stand, starting with the same reference center Zpand measured for optional angle positions α, taking into consideration all three vectors VGT1, VGT2and VGT3and/or VDT1, VDT2and VDT3, even if the measurement is realized following a specific distance after leaving the last roll stand.

Since each vector of the vectors VGT1, VGT2and VGT3and/or VDT1, VDT2and VDT3can be determined individually, the absolute feed distance for the individual rolls on the respective roll stands can be determined with the method according to the invention. For example, if the center of pressure of a roll in a 3-roll block is moved in radial direction further toward the inside than the center of pressure of the other two rolls, this can be determined according to the invention. In that case, the radial position of only one roll must be corrected.