Abstract:
The invention relates to a method and arrangement for the contactless determination of conductivity-influencing properties and their spatial distribution over the entire cross section of an electrically conductive substance moving in a primary magnetic field (B). The substance may be a liquid or a solid. A simultaneous measurement of a number of mechanical state parameters of the magnetic system is performed (three-dimensional components of the force and the torque), said parameters being variable by the effect of a secondary field on the magnetic system, the secondary field being produced on the basis of eddy currents induced in the substance by the primary field (B). To determine the spatial distribution of the property that is sought, the primary field is changed in intensity or form a number of times and a measurement of the state parameters is carried out for each change. The determination of the properties sought or their distribution takes place by solving an inverse problem using the method of least squares.

Description:
FIELD OF THE INVENTION 
     The invention concerns a method and arrangement for the contactless determination of properties of electrically conductive moving substances, which can be solid or liquid. In the preferred case of the investigation of liquid substances, these are viewed as a moving substance during flow through a pipe, channel, or similar guiding device. The invention also concerns a method and arrangement for the contactless determination of the spatial distribution of the properties of such substances, such as the flow velocity and electrical conductivity of the substance. The invention can be used in particular for measuring flow and flow volume in metallurgy, but it is also suitable for tasks in nondestructive materials inspection. The arrangement according to the invention shall also be called hereafter a Lorentz force anemometer. 
     BACKGROUND OF THE INVENTION 
     The precise measurement of flow velocities, flow volumes, and other properties which affect the electrical conductivity is of great importance, for example, for metallurgical processes, for the growing of semiconductor monocrystals, and for the production of glass. These substances are hot and corrosive during the melt process, so that contactless electromagnetic methods are especially suited for such measuring chores. 
     Contactless electromagnetic flow measurement methods are known from the publications DE 33 47 190 A1, DE 43 16 344 A1, DE 199 22 311 C2, DE 100 26 052 B4 and the publication “The use of magnetohydrodynamic effects to investigate fluid flow in electrically conducting melts” (J. Baumgartl, A. Hubert, G. Müller, Physics of Fluids, Vol. A 5, Series 1993, pages 3280-3289), in which a magnetic field (the so-called primary field) is coupled into the substance and the magnetic field disturbance induced by eddy currents (the so-called secondary field) serves as a measure of the flow velocity. These methods can be used to determine both mean flow velocities and spatial distributions of the flow velocity, in the latter case making use of the method of least squares of the error to determine the flow distribution. 
     However, these already known measurement methods have three serious drawbacks. First, due to their limited sensitivity, they do not allow measuring flow velocities in very slowly flowing or very weak electrically conductive substances, such as glass melts. Second, the measurement accuracy of the systems is greatly limited in an electromagnetically perturbed environment, since the magnetic field sensors will be affected by even the slightest parasitic fluctuations of the magnetic field, due to their small spatial dimension. Third, the measurement sensitivity of the method, being characterized by the ratio between secondary field and primary field, cannot be increased by heightening the primary field. 
     The mentioned drawbacks are only partly remedied by the methods as described in the publications JP 571 999 17 A, U.S. Pat. No. 6,538,433 B1 and JP 071 811 95 A. These are characterized in that, rather than make a direct measurement of the secondary field, one performs a measurement of the force exerted by the secondary field on the system producing the magnetic field. Despite their advantages over the first mentioned methods, the force-measuring systems are of little or no use for a number of important special problems, including flow measurement. 
     In the layout portrayed in JP 571 999 17 A, the primary magnetic field is generated by coils through which currents flow, entirely surrounding the tubular flow being measured. Such a system is very heavy, it requires an elaborate current supply, and can only be transported to a different place of use after costly disassembly of the measurement system. 
     The systems presented in the documents U.S. Pat. No. 6,538,433 B1 and JP 071 811 95 A involve local sensors, which can only measure the flow velocity in their immediate surroundings. These local sensors are not suited to determining the mass flow or the volume flow, because the magnetic field generated by them only penetrates part of the cross section through which the conductive substance is flowing. Furthermore, the measurement sensitivity of the local sensors declines with the third power (or even the fourth in the case of greater distances) of the distance from the conductive substance and is therefore not adequate for many application instances. 
     SUMMARY OF THE INVENTION 
     Thus, the basic problem of the present invention is to furnish a method and arrangement for the contactless measurement of attributes influencing the electrical conductivity, such as flow velocity and volume flow for electrically conducting substances, which on the one hand allow very high measurement precision and sensitivity. Furthermore, it should be easy to implement such a method. The corresponding arrangement should be economical in construction and easily convertible. Furthermore, it should be possible to determine the spatial distribution of the various properties, such as flow velocity and conductivity in the substance, and also to determine the contour of the gutter (or the like) where the flow is occurring. 
     The present invention should furthermore enable an identification of inhomogeneities of the electrical conductivity that are localized in time and space, such as slag particles in a melt or cracks in a moving metallic component. 
     The entirety of the measurement problems whose goal is in particular to discover the distributions of flow velocities and electrical conductivity in an electrically conductive substance shall be summarized hereafter by the term, inspection. 
     The mentioned problems are solved, according to the invention, by the features of the independent claims  1  and  13 , while the subclaims reveal advantageous modifications of the invention. 
     The advantages of the invention are to be seen, in particular, in that the system producing the magnetic field is designed to be so flexible that it can at any time be taken down, rearranged, and used for other measurement problems. As compared to the existing stationary sensors which only scan the substance locally, an improved utilization of the magnetic field lines in the invention, along with minimal use of material to generate the magnetic field, is an advantage. 
     If an electrically conductive substance is moving relative to a magnetic field, the primary magnetic field, the eddy currents induced in the substance will cause a disturbance of the magnetic field, the secondary magnetic field. The structure of the secondary magnetic field depends on the distribution of the primary magnetic field, as well as the distributions of the flow velocity (i.e., the velocity with which the substance is moving, if only partially, in the primary field) and the electrical conductance of the substance. 
     The secondary magnetic field exerts forces and torques on a magnetic system producing the primary field, hereinafter known as Lorentz forces, regardless of the physical details of their creation. 
     Now, because of these Lorentz forces on the magnetic system, according to the invention, it is possible to obtain information as to the flow velocity and the electrical conductance of the substance. 
     Due to the central role of the Lorentz force, the proposed arrangement of the invention shall be called a Lorentz force anemometer. 
     The Lorentz force anemometer comprises a magnetic system with at least two magnetic poles (NORTH and SOUTH) to create the primary field. The poles are preferably arranged at basically opposite sides of the cross section of a substance being inspected. The substance is thus arranged in the air gap of the magnetic system. This arrangement ensures that the primary field or the lines of force running between NORTH and SOUTH in the air gap pass through the entire cross section of the substance and thus create the precondition for a complete determination of the spatial distribution of flow velocity and electrical conductance. 
     In an especially advantageous embodiment, the poles of the magnetic system can be removed from the Lorentz force anemometer. This feature enables a flexible use of the arrangement. On the one hand, the Lorentz force anemometer can easily be mounted on various layouts being measured. and on the other hand this enables an easy packaging of the Lorentz force anemometer for different conditions of use. 
     An actuator system coupled to the magnetic system allows a specific setting of at least one parameter of the primary magnetic field. 
     The variation of at least one parameter of the primary magnetic field, hereinafter known as the actuating variable, serves the purpose of concentrating the lines of the magnetic field on certain partial regions of the substance being inspected so as to scan the distribution of flow velocity and electrical conductance. 
     The primary magnetic field produced by the magnetic system can be configured, according to the invention, as either a direct field or an alternating field. 
     In the configuration of the invention, the magnetic system can consist of permanent magnets, of normally conducting or superconducting coils, or of a combination of these elements. Moreover, the magnetic system can contain components of magnetically conducting material as well as ferrofluids, making possible a specific shaping of the primary magnetic field. 
     The actuating variable can be, for example, the distance between the magnetic system and the substance being inspected, the speed of rotation of the magnetic system, the strength of the magnetic field or the aperture angle of an elliptical cylinder. Elliptical cylinders are special arrangements of permanent magnets that are characterized by an especially high intensity and homogeneity of tie magnetic field prevailing in them. A more detailed explanation will be given in the description of the figures. 
     The magnetic system is characterized by so-called state parameters, at least one of which varies under the influence of the Lorentz forces with the actuating variable held constant. That is, under the influence of the Lorentz forces, various forces and torques act on the magnetic system, bringing about a movement and/or deformation of the magnetic system, which can then be measured. 
     One state parameter can be, e.g., a component of a three-dimensional displacement vector, which describes the translation of the magnetic system as a whole under the influence of the Lorentz force. 
     Another state parameter can be a component of a three-dimensional rotation vector, which describes the rotation of the magnetic system as a whole by the torques produced by the Lorentz forces. 
     Moreover, the state parameters can be internal stresses, as well as elastic deformations of the magnetic system. 
     The inspection of the electrically conductive substance occurs by measuring and evaluating one or more state parameters. In the method of the invention, one distinguishes between a monitoring mode and an exploratory mode. 
     In the monitoring mode, the actuating variables are held constant and at least one, preferably several, of the state parameters are measured as a function of time. This makes it possible to determine individual properties of the substance being inspected, such as flow rate, mean electrical conductance or level of filling in a flow channel, pipe, gutter, or the like, as a function of time. The monitoring mode is the basic function of the Lorentz force anemometer. 
     The exploratory mode is an expansion of the invented method, making it possible not only to determine individual properties, but also to fully ascertain a spatial distribution of the properties, such as flow velocity and electrical conductance in a substance being inspected. In the exploratory mode, preferably with the help of an actuator system, at least one actuating variable is varied in a defined way and at the same time at least one state parameter is measured. 
     If one knows a cross sectional shape of the substance being inspected, one can calculate the relation between measured and sought variables in customary fashion, known to the practitioner, starting from the fundamental equation of magnetohydrodynamics. 
     In practice, the shape of the actual flow cross section is often not known with precision, since it changes over the course of time due to erosion processes or build-up of slag in the pipe. Therefore, a switching from the monitoring mode to the exploratory mode takes place in the Lorentz force anemometer of the invention. 
     It is also conceivable to continually measure at least two state parameters in the monitoring mode and infer changes in certain properties by solving an inverse problem. 
     By solving the inverse problem, from the measured state parameters one determines field variables, such as a velocity profile of a flow, a contour of a flow gutter, or the position and shape of a crack in a solid. 
     The inverse problem is solved by the method of least error squares. The function to be minimized is the mean square deviation of the state parameter, as calculated by the assumed distribution of the properties such as electrical conductance and flow velocity, from the respective measured state parameters. 
    
    
     
       BFIEF DESCRIPTION OF THE DRAWING 
       Further details and benefits of the invention will be seen from the following sample embodiments, explained with the help of the figures, in which 
         FIG. 1  shows a first embodiment of a Lorentz force anemometer with a static and homogeneous primary field; 
         FIG. 2  shows a second embodiment of a Lorentz force anemometer with a magnetic system for generating a static and inhomogeneous primary field; 
         FIG. 3  shows a magnetic system of a third embodiment with a static and inhomogeneous primary field in the exploratory mode; 
         FIG. 4  shows a fourth embodiment of the Lorentz force anemometer with a rotating primary field; 
         FIG. 5  shows a fifth embodiment of the Lorentz force anemometer with a static primary field and a two-part magnetic system; 
         FIG. 6  shows a sixth embodiment of the Lorentz force anemometer with a magnetic system in the form of a fold-out elliptical cylinder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a first preferred embodiment of the invented Lorentz force anemometer for contactless inspection of a substance  01 , which is flowing in a channel  02  (hereinafter also called a gutter). The flow velocity of the substance  01  is indicated by the vector v. The Lorentz force anemometer comprises a magnetic system  03 , a measurement system  04 , an evaluation unit  05  and an actuator system  06 . Magnetic system  03 , evaluation unit  05  and actuator system  06  are arranged in a common holder  07 . 
     The magnetic system  03  comprises at least two permanent magnets  08 ,  09 , arranged opposite each other, outside of the gutter  02 , and a yoke  11  of a magnetically conducting material. It serves to generate a primary field B, which is represented by its field lines  12 . 
     Integrated in the holder  07  is the measurement system  04 , preferably containing several strain gage strips, which measures the state parameters of the substance  01 . The state parameters are preferably components of the force F and the torque M acting on the magnetic system  03  and the holder  07  due to the Lorentz forces. 
     The mode of operation of such a measurement system  04  corresponds to that of wind tunnel scale, which already finds application in aerodynamics, for example. The measurement system  04  can thus be designed and built in a manner familiar to the practitioner. 
     In the monitoring mode, the magnetic system  03  is arranged stationary to the gutter  02 . Then from the preferably six measured state parameters, one calculates in the evaluation unit  05  at most six properties of the substance  01  being inspected, such as flow rate, level of filling, flow cross section, shear rate, inclination, and mean electrical conductance, as functions of time. 
     Of course, it is within the scope of the invention, if need be, to measure and evaluate a subset of the six state parameters, such as only the force acting in the principal direction of flow. 
     In the embodiment depicted, the magnetic system  03  can be positioned at different places by the actuator system  06 , preferably outfitted with step motors. The exploratory mode can be implemented in this way. In the exploratory mode, the magnetic system  03  is positioned at various different points one after the other and/or the primary field is varied in a defined manner. At each of these points or upon each such variation, a measurement of the state parameters (six force and torque components) takes place. 
     By solving an inverse problem, one calculates in the evaluation unit  05  both the profile of the desired property, preferably the flow velocity, and the profile of the electrical conductance of the substance  01 . From the latter, one gets the shape of the gutter  02  and the shape of the free surface of the substance  01 . 
     The invented Lorentz force anemometer with exploratory mode thus allows one to carry out a flow measurement even when the flow geometry is not precisely known. Furthermore, the exploratory mode makes it unnecessary to have a precise, predetermined positioning of the Lorentz force anemometer with respect to the substance  01  being inspected and thus increases the flexibility of the system and makes it unnecessary to use costly positioning systems. 
     Instead of using strain gage strips in the measurement system  04 , it may be advantageous to perform the force and torque measurement by means of optical methods or by means of a compensation method. 
     It should be pointed out once again, for clarity, that the substance can also be a solid component which is moved in the primary field. The component can be guided in traditional manner by bearings or the like, which take over the function of the gutter. 
       FIG. 2  shows a second embodiment of a Lorentz force anemometer. In this embodiment, the magnetic system  03  is asymmetrical and so the primary field B has an inhomogeneous configuration. The oppositely placed permanent magnets  08 ,  09  have different sizes and/or different magnetization strengths. The inhomogeneous design of the magnetic field B has the advantage that all components of the force F and the torque M acting on the magnetic system  03  (i.e., all state parameters) are different from zero, thereby achieving maximum information about the substance  01 . 
     This circumstance is illustrated in part a of  FIG. 2  by representing the directional vector of the torque M acting on the magnetic system  03 . In contrast to this, the vertical force component in the system shown in  FIG. 1 , for example, is equal to zero because of the left/right symmetry and does not contribute to the information obtained. 
     Part b of  FIG. 2  shows a plan view of the Lorentz force anemometer of the second embodiment. One recognizes the force components F 1  and F 2  of different magnitude, as well as the velocity vector v of the substance  01 . 
       FIG. 3  shows, in a third embodiment of the invented Lorentz force anemometer, a magnetic system  03  with inhomogeneous primary magnetic field B in exploratory mode. Parts a and b of the figure show different positions of the magnetic system  03  relative to the substance  01 , part c shows a plan view of part a. The magnetic system  03  has two poles, consisting of permanent magnets  08 ,  09 , which are arranged with a horizontal offset relative to each other and magnetically joined together by the yoke  11 . This produces an inhomogeneous primary field B, which is shown by the field lines  12  drawn. The overall magnetic system  03  can be moved vertically by means of the actuator system  06 , not shown. The yoke  11  is at least partly enclosed by a coil  13 . The strength of the primary field B created by the permanent magnets  08 ,  09  and the coil  13  is varied by the size of the current flow in the coil  13 . 
     The magnetic system  03  shown in  FIG. 3  has two actuating variables for the exploratory mode, namely, the vertical position L, which can be changed by translation, and the strength of the primary magnetic field B, which can be changed by varying the current strength in the coil  13 . 
     If the magnetic system  03  is in the upper position, shown in part a and c of  FIG. 3 , a greater Lorentz force F 1  will act on the permanent magnet  09  situated at bottom right in part a of the figure than on the permanent magnet  0  situated at top left (force F 2 ). The torque M acting on the overall magnetic system  03  in this case points upper right, as is shown by the vector arrow M in  FIG. 3   a.    
     If the magnetic system  03 , as shown in  FIG. 3   b , is in the bottom position, a greater Lorentz force will act on the permanent magnet  08  situated at upper left in this figure than on the permanent magnet  09  situated at bottom right and the direction of the torque M will change accordingly. 
     In the exploratory mode, the strain gage strips integrated in the measuring system  04  measure the state parameters (force and torque components) acting on the magnetic system  03 , and this is performed at a plurality of vertical positions L of the magnetic system  03  and for a plurality of current strengths of the current flowing through the coil  13 . If it is also desired to ascertain a distribution of the properties along a given length of the channel or gutter  02 , this can be done by an appropriate positioning of the magnetic system at a second or several points of the channel  02 . 
     From the measured state parameters, the evaluation unit (not shown) determines the profile of the flow velocity v and the electrical conductance of the substance  01 , once again by solving an inverse problem. Since the electrical conductance has a discontinuity at the boundary between the substance  01  being inspected and the wall of the gutter, one also gets the shape of the gutter  02  in this way. 
       FIG. 4  shows a fourth embodiment of the Lorentz force anemometer with a rotating primary field B. The magnetic system  03  in this case comprises two oppositely placed disks  14  of magnetically conductive material, joined together, which form the yoke, in which a plurality of permanent magnets  08 ,  09  with alternating polarity are embedded or glued on ( FIG. 4   b  and  c ). 
     This disks  14  are arranged opposite each other so that each time one magnetic north pole of a permanent magnet  08  lies opposite a magnetic south pole of a permanent magnet  09 . 
     In the exploratory mode, the magnetic system  03  is placed in rotation by the Lorentz force generated by the moving substance  01  and the rotary speed ω is measured continually by means of a tachometer  15 . From the measured speed ω, the evaluation unit  05  calculates the sought property of the substance  01 , such as the flow rate. 
     It also within the scope of the invention to install a motor with integrated torque measuring system, instead of the tachometer  15 . In the monitoring mode, then, the motor will rotate with a defined rotary speed. At the same time, the torque produced by it will be measured and from this the sought property of the substance  01 , such as the flow rate, will be calculated in the evaluation unit  05 . 
     In the exploratory mode, the speed ω of the magnetic system  03  shown in  FIG. 4  constitutes the actuating variable, while the torque M embodies the state parameter. If the speed ω is low, the lines of the magnetic field  12  entirely penetrate the substance being inspected  01  ( FIG. 4   d ). 
     But if the speed ω is great ( FIG. 4   e ), the lines of the magnetic field  12  are displaced from the substance  01  by virtue of the skin effect and only move through the regions of the substance  01  near the walls. Thus, by varying the speed ω, one can deliberately influence the distribution of the primary field B at the site of the substance being inspected  01 . 
     In the exploratory mode, the measuring system integrated in the motor measures the torque M acting on the magnetic system  03  for a plurality of speeds ω. From this, the evaluation unit  05  calculates the profile of the flow velocity and the electrical conductance of the substance  01 , again by solving an inverse problem. 
       FIG. 5  shows a fifth embodiment of the Lorentz force anemometer. The magnetic system  03  is two-part and comprises an active component  03   a  and a passive component  03   b . The passive component  03   b  of the magnetic system  03  is arranged stationary beneath the substance being inspected  01 , while the active component  03   a  of the magnetic system  03  is positioned above the substance  01 . The substance  01  is covered with a layer of slag. 
     In the monitoring mode, the active component  03   a  is also stationary. By measuring or plotting one or more state parameters (force and torque components) acting on the active component  03   a  of the magnetic system  03  by means of the measurement system  04  as a function of time, the sought property of the substance, such as the flow rate, is calculated in the evaluation unit  05 . 
     In the exploratory mode, the position of the active component  03   a  relative to the substance  01  and to the passive component  03   b  is changed in a defined maimer by means of the actuator system  06  and at the same time one measures the force and torque components acting on the active component  03   a  of the magnetic system  03  by means of the measurement system  04 . By solving an inverse problem, the profiles of velocity v and electrical conductance of the substance are calculated from this in the evaluation unit  05 . 
       FIG. 6  shows in a sixth embodiment of a Lorentz force anemometer a magnetic system  03 , which is designed as a fold-open elliptical cylinder. In this embodiment, the permanent magnets  08 ,  09  are fastened to a housing consisting of two half shells  16 . The half shells  16  are joined together by means of a hinge  17  and can be opened and closed by means of the actuator system  06 . 
     In the open state ( FIG. 6   a ), the magnetic system  03  can be removed and it generates an inhomogeneous magnetic field B in the substance being inspected  1 . In the closed state ( FIG. 6   b ), the magnetic field B is homogeneous and possesses a maximum value. 
     Of course, it also falls within the scope of the invention to vary the aperture angle of the elliptical cylinder continuously in the exploratory mode. 
     In the monitoring mode, the aperture angle of the elliptical cylinder is held constant and once again the force and/or torque components acting on it are ascertained as state parameters as a function of time by the measurement methods described in the preceding sample embodiments. 
     In the exploratory mode, the force and torque components acting on the magnetic system  03  are measured for a plurality of aperture angles of the elliptical cylinder and once again used in the evaluation unit (not shown here) to solve an inverse problem and calculate the profile of flow velocity and electrical conductance of the substance being inspected. 
     LIST OF REFERENCE NUMBERS 
     
         
           01  electrically conductive substance 
           02  channel/gutter 
           03  magnetic system
         03   a  active component of the magnetic system     03   b  passive component of the magnetic system   
     
           04  measurement system 
           05  evaluation unit 
           06  actuator system 
           07  holder 
           08  permanent magnet 
           09  permanent magnet 
           10  - 
           11  yoke 
           12  field lines 
           13  coil 
           14  disk 
           15  tachometer 
           16  half shell 
           17  hinge 
         v flow velocity 
         B magnetic primary field 
         F force 
         M torque 
         ω rotary speed