Patent Application
20110139392
The invention pertains to a method for measuring the temperature in a mold by means of a fiber-optic measuring method and to a correspondingly designed mold. For this purpose, light waveguides, through which laser light is conducted, are provided on the outside surface of a mold. The invention serves to improve the local resolution of the temperature detection in a mold in comparison with the known temperature detection systems and makes it possible in particular to improve the detection of longitudinal cracks and fractures.
Temperature detection in a mold is a critical problem, which is becoming even more important in the case of casting machines operating at high speed. In most cases, temperatures in the mold are detected primarily by thermocouples, which are either guided through bores in the copper plate of the mold or welded onto the copper plates of the mold. Such measurement methods are based on the evaluation of thermal stresses. The number and size of these thermocouples is limited. In many cases, the only way to avoid the great expense of reconstructing the entire mold is to install the thermocouples only where the necked-down bolts are located. Increasing the number of thermocouples, furthermore, leads to a very large amount of cabling work. These sensors are also susceptible to the electromagnetic fields caused by electromagnetic brakes or stirrers, for example. To protect the thermocouples, including their cabling, it is also necessary to provide complicated protective devices. When the copper plates of a mold are to be replaced—a task which must be done at regular intervals—the thermocouples must be disconnected and then reconnected to their cables, which not only requires a great deal of work but also involves the danger of making wrong connections.
WO 2004/082869 describes a method for determining the temperature in a continuous casting mold by the use of thermocouples, which are arranged on a copper plate outside the mold and which project into the mold through bores.
DE 3436331 describes a similar method for measuring temperatures in metallurgical vessels, especially in continuous casting molds, in which large numbers of thermocouples are arranged in transverse bores.
These two methods suffer from the disadvantages cited above. In addition, producing a large number of bores is expensive and time-consuming. A very large number of thermocouples installed in this way results unfortunately in an extremely large amount of cabling work.
From JP 09047855 is known a method for predicting breakout in continuous casting, in which an optical fiber is arranged on the hot side of the mold in an embedded or exposed manner in a serpentine fashion. This arrangement is found in the direction of the width on the hot side of the mold.
DE 102 36 033, which pertains to the monitoring of refractory linings of melting furnaces, especially induction furnaces, describes a temperature measuring method-using optical fibers, wherein optical fibers are attached to lining material behind several layers of insulation and used for fiber-optic backscatter measurement. In this form, however, such systems are unsuitable for measuring temperatures in a mold and are not designed for the exact detection of local temperatures in a casting mold.
The technical problem which therefore arises is to find an improved method for measuring temperatures in a mold, especially for measuring them with greater local resolution, namely, a method which requires the least possible amount of installation work and which improves, among other things, the detection of longitudinal cracks and/or through-fractures in the mold.
The technical problem explained above is solved by the invention disclosed below. In particular, the invention provides a method for measuring temperatures in a mold of a casting machine, wherein sensors for measuring the temperature in at least one copper plate of the mold are used, these sensors being connected to a temperature detection system, characterized in that at least one light waveguide fiber, through which laser light is conducted, is used as a sensor, wherein grooves, in which the at least one light waveguide fiber is arranged, are formed in the outside surface of the copper mold plate.
Temperature detection by means of optical fibers makes it possible to achieve a significant reduction in the amount of cabling work in comparison with the use of thermocouples in the mold. In addition, much less work and much lower cost are required to install the fibers in the copper plate of the mold. The use of light waveguides according to the above method also makes it possible to achieve much greater local resolution than temperature measurement by the previously described systems based on the use of thermocouples in bores. One glass fiber line, for example, can replace more than a hundred thermocouples with their cabling. Nor is there any need for complicated devices to protect the thermocouples and their cabling.
In another preferred form, the method comprises at least one light waveguide fiber, which is arranged in meander fashion in the grooves on the outside surface of the mold's copper plate.
In another preferred embodiment, the method comprises at least two light waveguide fibers, longitudinally offset from each other, each of which is arranged in a groove. The local resolution of the temperature measurement can be improved even more by this means.
In another preferred embodiment, the method comprises grooves between cooling channels, which are arranged on the outside surface of the mold's copper plate.
In another preferred embodiment, the method comprises light waveguide fibers, which are arranged in the fixed side, in the loose side, and preferably in both of the two narrow sides of the mold.
In another preferred embodiment, the light waveguide of each individual side is connected to the temperature detection system by a coupler and by an additional, separate light waveguide.
In another preferred embodiment, the light waveguides of the individual sides are connected to each other in series by couplers and are connected to the temperature detection system by another coupler.
In another preferred embodiment of the method, the laser light is guided to the mold by at least one coupler, through which the channels of several light waveguide fibers are transmitted simultaneously.
In another preferred embodiment of the method, the couplers are lens couplers.
In another preferred embodiment of the method, the data of the temperature detection system are transmitted to a process computer, which processes these data and controls the casting operation accordingly.
The invention also consists of a mold for the casting of metal, which comprises at least one copper plate and which is characterized in that grooves, in which light waveguide fibers for temperature measurement are arranged, are provided on the outside surface of the mold's copper plate.
In another preferred embodiment of the mold, the light waveguide fibers are arranged in meander fashion in the grooves.
In another preferred embodiment of the mold, at least two light waveguide fibers, which are longitudinally offset from each other, are provided, and each of which is arranged in its own groove.
In another preferred embodiment of the mold, the grooves are arranged between cooling channels located on the outside surface of the mold's copper plate.
Independently of this exemplary embodiment, it is possible, for example, to embed the light waveguide fibers 2 in the grooves 4 by means of a casting resin, but they could also be held in place in the grooves 4 by some other conventional method.
It is also possible for the light waveguide fibers 2 to have a high-grade steel jacket to protect them more effectively from external influences. In general, several of these optical fibers 2 can be arranged inside a high-grade steel jacket or high-grade steel sheath, so that, if one of the fibers 2 should prove defective—which happens rarely—another fiber 2, which is already present in the sheath, can take over. It is also conceivable that several fibers 2 arranged inside a sheath could be used for measurement purposes simultaneously, as a result of which the measurements acquire even greater accuracy, because now the measuring sites 3 can be arranged as close together as desired.
The light waveguide fibers 2 can preferably have a diameter of 0.1-0.2 mm or some other conventional diameter. The diameter of a sheath of high-grade steel, for example, can vary in the range of 0.5-6 mm. The diameter of the grooves 4 can be preferably in the range of 1-10 mm or can even be as large as several cm, depending on the application.
For the purpose of improving the local resolution, it is also possible to arrange several light waveguide fibers within a single groove 4. As a result, the number of measuring sites 3 can be significantly increased. Thus the resolution in the direction of the cooling channels 6, that is, in the casting direction, can increased by any desired factor in comparison with that shown in the figure; for example, it can be doubled or quadrupled.
In general, it is possible to replace 60-120 thermocouples together with their cabling through the use of one or two glass fiber lines or light waveguide fibers. The number of measurement sites is limited in principle only by the computing capacity of the selected temperature detection system 10. It is therefore possible, with a corresponding temperature detection system 10, to increase the number of measuring sites significantly, so that more than 500 measuring sites can be realized per optical fiber 2. As a result of this much denser measurement site number, the local resolution can be multiplied even more.
The temperature detection system 10 is connected to a process computer 20. The laser light which is fed into the light waveguide 2 is generated by this temperature detection system 10 or optionally also with the help of an additional external system. The data collected by the light waveguide fibers 2 are converted into temperatures by the temperature detection system and assigned to the various locations on the mold. The evaluation can be accomplished by means of, for example, the known fiber-Bragg-grating method (FBG method). In this method, suitable light waveguides, into which measurement sites are inscribed by periodic variation of the index of refraction, and/or gratings with such variations are used. This periodic variation of the index of refraction leads to the ability of the light waveguide to act, as a function of the periodicity, as a dielectric mirror for certain wavelengths at the measurement site. A change in the temperature at a certain point on the mold has the effect of changing the Bragg wavelength, wherein precisely the light of this wavelength is reflected. Light which does not fulfill the Bragg condition is not significantly affected by the Bragg grating. The various signals of the different measurement sites can then be differentiated from each other on the basis of the differences in their transit times. The detailed design of such fiber Bragg gratings and the corresponding evaluation systems are generally known. The accuracy of the local resolution is determined by the number of inscribed measurement sites. The size of a measurement site can be in the range of, for example, 1-5 mm.
Alternative methods which can be used to measure the temperatures include “optical frequency domain reflectometry” (OFDR) and “optical time domain reflectometry” (OTDR). These two methods are based on the principle of fiber-optic Raman backscattering, wherein the phenomenon that a temperature change at a certain point of an optical fiber results in a change in the Raman backscattering of the light waveguide material is exploited. With the help of an evaluation unit such as a Raman reflectometer, it is then possible to determine the locally resolved temperature values along a fiber, wherein, in this method, a mean value over a certain length of the conductor is determined. This length is currently a few centimeters. The various measurement sites are again distinguished from each other on the basis of the differences in their transit times. The design of such evaluation systems according to the previously mentioned methods is generally known, as is the design of the lasers required to generate the laser light sent through the fibers 2.
The locally resolved temperature data acquired by the temperature detection unit 10 are then sent on preferably to a process computer 20, which can control the casting parameters such as the casting speed or the cooling and/or other standard parameters as a function of the temperature distribution in the mold.
Either the FGB method, the OTDR method, or the OFDR method can be used for evaluation, as in the case of
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 032 341.1 | Jul 2008 | DE | national |
| 10 2008 060 507.7 | Dec 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/004901 | 7/7/2009 | WO | 00 | 2/23/2011 |
