Title of Invention

APPARATUS FOR MEASURING THE TENSILE STRESS DISTRIBUTION IN A METAL STRIP

Abstract

Method for measuring the tensile stress distribution over the width of a metal strip, in order to measure the tensile stress in lthe metal strip between two roll stands or between a roll stand and a coiler lor a guide roll, a force being exerted on the metal strip by means of a suction device and effecting a deflection of the metal strip, the deflection of the metal strip being measured and used for calculating the tensile stress distribution over the width of the metal strip.

Full Text

Description Apparatus for measuring the tensile stress distribution in a metal strip The invention relates to a method and an appara tus for measuring the tensile stress distribution in a metal strip between two roll stands or between a roll stand and a coiler. In addition, the invention can be used in conjunction with S rolls and blooming-mill stands. During the rolling of metal strips, in particular during cold rolling, the tensile stress distribution in the metal strip, along the metal width, has to be deter- mined on line, that is to say it usually has to be measured, since the tensile stress distribution is the decisive variable for controlling the flatness of the metal strip. For example, it is known to guide the metal strip over a so-called measuring roll, that is to say a seg- mented guide roll, which has piezoelectric pressure sensors at intervals of about 2-5 cm. The force acting on the sensors in this case is a measure of the tensile stress distribution. The method involves contact - and can therefore leave behind impressions in the metal - and, in addition, involves wear and is thus maintenance- intensive. Furthermore, DE-A-26 17 958 discloses an apparatus for measuring the tensile stress of a metal strip, oscillatory waves being generated in the metal strip by means of electric hammers and their propagation being measured. This method is also subject to the disadvantages mentioned above. Furthermore, DE-C-31 30 572 discloses the opera- tion of measuring the tensile stress of a metal strip by means of ultrasound. To this end, the metal strip is made to oscillate by means of ultrasound, and the propagation of this oscillation is measured. However, it has been shown that this method is suitable only in exceptional cases, in particular in the case of particularly thin metal strips, because of the low energy transmission by means of ultrasound. The object of the invention is to specify an apparatus for avoiding the disadvantages described above. According to the invention, the object is achieved by an apparatus according to (Claim l, The apparatus according to the invention for measuring the tensile stress distribution in a metal strip between two roll stands, between a roll stand and a coiler, in a blooming-mill stand or upstream or downstream of a guide roll has a deflection device (arranged upstream or down- stream of a roll) for deflecting the metal strip, a measuring device for measuring the deflection of the metal strip and a computing device for calculating the tensile stress distribution as a function of the deflec- tion of the metal strip, the deflection device being designed as a suction device to apply suction to the metal strip. This apparatus, on the one hand, makes it possible to measure the tensile stress or tensile stress distribution without damaging the metal strip and, on the other hand, supplies a particularly strong and precise measurement signal, which permits particularly precise determination of the tensile stress or tensile stress distribution in the metal strip. The apparatus according to the invention is also considerably more straight- forward and cost-effective than a measuring roll. Further advantages and details emerge from the following description of exemplary embodiments and from the subclaims. Specifically: FIG 1 shows an exemplary configuration of a suction device, FIG 2 shows an alternative configuration of a suction device, FIG 3 shows a further alternative configuration of a suction device, FIG 4 shows an advantageous configuration of plates of a suction device, FIG 5 shows an arrangement of sensors, PIG 6 shows an alternative arrangement of sensors, and FIG 7 shows a comparison between ascertained tensile stress and actual tensile stress. FIG 1 shows an exemplary configuration of an apparatus according to the invention for measuring the tensile stress distribution in a metal strip 1. The metal strip 1 runs out of a roll stand 3 and is guided in the direction of the arrow 5 by means of a guide roll 4. A suction device 6 is arranged at a distance a, advantageously 10 to 30 cm, from the guide roll 4. By selecting the distance a to be 10 to 30 cm, particularly good spatial resolution of the tensile stress over the strip width is achieved. Plates 9 and 10 are arranged at the upper end of the suction device 6. The air stream that is taken in by the suction device effects a deflec- tion of the metal strip 1. This deflection is indicated by the dashed line 2. Integrated into the plates 9 and 10 of the suction device are deflection sensors 7 and 8, which measure the deflection 2 of the metal strip 1. The deflection sensors 7 and 8 are designed as eddy-current sensors, capacitive distance sensors, optical distance sensors or ultrasonic distance sensors. The design as eddy-curzrent sensors, in particular time-synchronized eddy-current sensors, is particularly advantageous. The distance sensors are particularly advantageously designed as rows of sensors over the width of the metal strip 1. The suction device 6 has a rotary throttle 11, by means of which the air stream which has been taken in is modulated.. The air stream is advantageously modulated at a frequency of 5 to 40 Hz, in particular at a frequency of 5 to 20 Hz. In this way, without additional damping, a virtually quasi-static deflection is achieved, so that the result, even without additional damping measures, is a deflection that is essentially proportional to the exciting force and inversely proportional to the tensile stress distribution. The tensile stress distribution may be measured particularly precisely in this way. It has proven to be particularly advantageous to excite the metal strip 1 at a frequency which lies below its reson- ant frequency. Surprisingly, a more precise measurement of the tensile stress distribution may be achieved in this frequency range. The plates 9 and 10 are arranged at a distance b of advantageously 1 to 10 mm, in particular 5 mm, from the metal strip 1. A gap of width c is provided between the metal plates 9 and 10. This gap is advantageously 0.5 to 5 cm wide. A gap width c = 2 cm is particularly advantageous. between 5 and 30 cm, in particular The plates 9 and 10 are about 20 cm wide, with the result that the vacuum produced by the suction device 6 acts on a finite area of the metal strip 1. In order to evaluate the measurement signals supplied by the distance sensors 7 and 8, provision is made for an evaluation device (not illustrated). The suction device 6 is dimensioned such that a vacuum between 5 and 50 mbar is produced between the plates 9 and 10 and the metal strip 1. FIG 2 shows an alternative configuration of a suction device 21. In FIGS 2 and 3, the arrows identified by reference symbol 20 indicate the movement of the air (an air stream pulsed at a frequency between 5 and 20 Hz) , and the arrows designated by 30 indicate the forces acting on the metal strip 1 on account of the air movement. By means of the suction device 21, air is blown towards the metal strip 1 and led away laterally parallel, and counter, to the strip running direction 5. The velocity of the air and the distance b between plates 22 and 23, which are arranged on the suction device 21, and the metal strip 1 are in such a relationship that the hydrodynamic paradox is in effect. This means that, although air is blown towards the metal strip 1, the metal strip 1 is attracted by the suction device 21 because of the high air velocity. This principle also forms the basis of the configuration of a suction device 24 according to FIG 3, FIG 3 illustrating a particularly advantageous configuration of the basic principle described in FIG 2. By means of the suction device 24, air is blown towards the metal strip 1. The pressure of this air is so high that the flow velocity of the air allows the hydrodynamic paradox to take effect: the metal strip 1 is attracted by the suction device 24. In addition, provision may be made for a suction duct 25, by means of which air is addi- tionally taken in. The configuration according to PIG 3 allows high flow velocities of the air between metal strip 1 and suction device 24, which in turn leads to the metal strip being attracted to a pronounced extent. FIG 4 shows a cross section through a metal strip 1 and a plate 26. The plate 26 is a particularly advan- tageous configuration for the plates 9, 10, 22 and 23 and for that surface of the suction device 24 which faces the metal strip 1. Milled into the plate 26 are channels 27 that extend in the direction in which the strip runs. The flow direction of the air under the metal strip 1 can be influenced in this way. In addition, provision may be made to control the flow velocity of the air in the channels 27 individually, and thus to define measurement zones which may be subjected to a specific force. In the case of hot-rolling stands, there is the possibility, instead of using air, of spraying a liquid (water, emulsion or the like) in a pulse manner at high pressure onto the material being rolled, in order to produce a periodic force in this way. This emulsion simultaneously effects cooling of the deflection sensors, which do not withstand the high temperatures during hot rolling (up to 1000°C) without cooling. As a result of various interfering influences during the deflection of the metal strip 1, the distance sensors 7 and 8 supply very noisy measurement signals. The measurement signals are therefore filtered. The signal filtering is advantageously effected by means of a digital fit algorithm or an FFT analysis. The metal strip 1 is deflected sinusoidally. The time profile of the measurement signal for each period is therefore ideally a sine wave. However, interference signals, in particular the resonant oscillations of the metal strip 1, are superimposed on this sine-wave signal. After each complete period, therefore, a sine curve is fitted to the measurement signal (minimization of the squares of the errors): of the arrors Since the phase and frequency are known, the signal offset (basic distance between the sensors) and the amplitude are the only fit parameters. The amplitude value AF ascertained is then converted into the tensile stress. A further distinct improvement in the filtering is achieved by using two distance sensors 7 and 8 in accordance with the exemplary embodiment from FIG 1 and FIG 5 and evaluating the expression A" is a measure of the curvature of the metal strip 1 at the location where the force is introduced. The curvature is low for long-wave resonant oscillations. These are therefore effectively filtered out. The forced deflec- tion, on the other hand, produces a "kink" in the metal strip 1 at the location where force is introduced. In the expression A", therefore, the information content in relation to the metal-strip deflection that is forced by the suction device is considerably greater than in the measurement signals A1 and A2 which are supplied by the sensors 8 and 7. FIG 6 shows the arrangement of three distance sensors, which are designed in particular as rows of sensors. In this case, a third distance sensor 50 is provided to measure the distance A3. In the case of using three distance sensors, the value for A" is advantageously corrected in accordance with In this way, errors as a result of non-round rolls or sagging of a roll may be compensated for. By means of an FFT or a fit algorithm, an amplitude value A"F is formed from the values A", this amplitude value A"F corresponding in principle to the amplitude value Ap but being formed from the values A" rather than from direct measurement values. The amplitude distribution A"F(x) into a tensile stress distribution s(x) is effected in accordance with s(x) = C(x) F(x) [1/A"F(x) - 1/AO] (3). Here, x designates the location coordinate along the width of the metal strip 1. A"F(x) is the measured amplitude distribution of the deflection, which is calculated in accordance with Equation 1 or Equation 2 and by means of the digital fit algorithm. F(x) is the force distribution with which the metal strip 1 is deflected and has to be determined once experimentally. C (x) is a proportionality factor, which includes the elastic constants of the material of the metal strip 1. Because of the transverse contraction, given a homogeneous tensile stress distribution, the amplitude at the edge of the metal sheet is greater than in the centre. Therefore, C(x) is a function of x. C(x) can be determined from finite-element calculations. 1/AO is a term that takes into account the flexural rigidity of the metal sheet. Ao is the amplitude that is measured when the tensile stress is zero. Because of the flexural rigidity of the metal strip 1, Ao is finite. For thin metal strips, that is for metal strips that are thinner than 1 mm, 1/AO can be set to be equal to zero. FIG 7 shows a curve 40, determined within the context of a finite-element simulation by means of the method according to the invention, for tensile stress s in N/mm2 in a metal strip, plotted against the position x in mm on the metal strip in the transverse direction, in comparison with a curve 41 for tensile stress a in N/mm2 to which the metal strip is subjected. The curve illus- trates that, by means of the method according to the invention, it is possible to ascertain the tensile stress s in a metal strip particularly precisely. WE CLAIM: 1. Apparatus for measuring the tensile stress distribution in a metal strip) (1) between two roll stands (3) between a roll stand and a coiler, in a blooming—mill stand or upstream or down- stream of a guide roll, characterized in that a deflection device (6,21,24) is provided for deflecting the metal strip, having a measuring device (7,8) for measuring the deflection of the metal strip and having a computing device for calculating the tensile stress distribution as a function of the deflection of the metal strip, the deflection device being designed as a suction device to apply suction to the metal strip. 2. Apparatus as claimed in claim 1 wherenin the suction device (6,21,24) is designed to generate a periodic air stream for the stream for the periodic deflection of the metal strip, that is to say for generating a flexure wave, in particular a flexure wave in the longitudinal direction of the metal strip. 3. Apparatus as claimed in claim 2 wherein the suction device .has a modulation device (11) for generating a periodic air stream at a frequency between 5 and 40 Hz, in particular at a frequency between 5 and 20 Hz. 4. Apparatus as claimed in claim 1, 2 or 3, wherein in that the suction device is designed to generate a vacuum between 5 and 50 mbar. 5. Apparatus as claimed in claims 1, 2, 3 or 4 wherein the suction device has air-feed plates (9, 10; 22, 23) which, with the metal strip, farm an air duct. 6. The apparatus as claimed in any one of the preceding claims, wherein said air—feed plates of the suction device are arranged at a distance of 1 to 10 mm, advantageously 5 mm, from the metal strip. 7. Apparatus as claimed in claim 6 wherein the air-feed plates are between 5 and 30 cm, in particular 20 cm, wide. S. Apparatus as claimed in one of the preceding claims, wherein it has a guide roll (4) for guiding the metal strip. 9. Apparatus as claimed in claim 8 wherein the suction device is arranged 10 to 30 cm away from the guide roll. 10. Apparatus as claimed in one of the preceding claims, wherein the suction device is designed as a compressed-air device for blowing compressed air towards the metal strip at a velocity that lies above the velocity from which the hydrodynamic paradox takes effect. 11. Apparatus as claimed in one of the preceding claims, wherein the measuring device has at least one deflection sensor C 7, S, 50) which is designed as an eddy-current sensor, capacitive distance senso", optical distance sensor or ultrasonic distance sensor,. 12. Apparatus a.s claimed m one of the preceding claims, wherein the measuring device has at least two deflection sensors, in particular designed as raws of sensors. 13. Apparatus as claimed in claim 12, wherein the measuring device has at least three deflection sensors, in particular designed as rows of sensors. Method for measuring the tensile stress distribu- tion over the width of a metal strip, in order to measure the tensile stress in the metal strip between two roll stands or between a roll stand and a coiler or a guide roll, a force being exerted on the metal strip by means of a suction device and effecting a deflection of the metal strip, the deflection of the metal strip being measured and used for calculating the tensile stress distribution over the width of the metal strip.

Documents:

00279-cal-1998-abstract.pdf

00279-cal-1998-claims.pdf

00279-cal-1998-correspondence others.pdf

00279-cal-1998-description complete.pdf

00279-cal-1998-drawings.pdf

00279-cal-1998-form 1.pdf

00279-cal-1998-form 2.pdf

00279-cal-1998-form 3.pdf

00279-cal-1998-form 5.pdf

00279-cal-1998-gpa.pdf

00279-cal-1998-letter patent.pdf

279-CAL-1998-(10-10-2012)-FORM-27.pdf

279-CAL-1998-FORM-27.pdf

279-cal-1998-granted-abstract.pdf

279-cal-1998-granted-claims.pdf

279-cal-1998-granted-correspondence.pdf

279-cal-1998-granted-description (complete).pdf

279-cal-1998-granted-drawings.pdf

279-cal-1998-granted-examination report.pdf

279-cal-1998-granted-form 1.pdf

279-cal-1998-granted-form 2.pdf

279-cal-1998-granted-form 3.pdf

279-cal-1998-granted-form 6.pdf

279-cal-1998-granted-gpa.pdf

279-cal-1998-granted-letter patent.pdf

279-cal-1998-granted-priority document.pdf

279-cal-1998-granted-reply to examination report.pdf

279-cal-1998-granted-specification.pdf

279-cal-1998-granted-translated copy of priority document.pdf