Patent Application
20130221950
This application claims priority under 35 U.S.C. §119 to Switzerland Application No. 0237/12 filed on Feb. 23, 2012, the entire content of which is hereby incorporated by reference in its entirety.
The disclosure relates to methods for measuring mechanical stresses produced by torsional, shearing, bending, thrusting, tensile and/or compressive forces in ferromagnetic workpieces.
For investigating, characterizing and monitoring the stress states of ferromagnetic workpieces, for example, steel, magnetoelastic measuring methods are used, which can use the dependencies between mechanical and magnetic properties of ferromagnetic materials. In the case of the mechanical stressing of a ferromagnetic workpiece, as occurs, for example, when acted upon by torsional, shearing, bending, thrusting, tensile and/or compressive forces, and its forced geometric deformation, its magnetic properties and magnetic parameters can be altered.
When acted upon by external forces on steel, the characteristic values of the magnetism reversal characteristics can be altered. With increasing tensile stress, for example, the hysteresis loop becomes flatter; the slope of the hysteresis loop thus decreases, and the contributions of the magnetic characteristics drop. The magneto-elastic transformatory method of the DYNAMG Company makes use of these properties to monitor remotely existing stresses and forces in prestressed concrete parts or in steel-reinforced bridge elements. The method is based on the fact that steel that is in thermal equilibrium and that does not have any macroscopic magnetization reacts in an operating external magnetic field with a rotation of its Weiss zones in the direction of the external magnetic field vector. This domain rotation can be affected by the stress state, however, that prevails in the monitored workpiece. The method is associated with a relatively high expense, and a calibration on the respective steel grade to be examined is needed. This expense can be justifiable in the monitoring of bridge components or prestressed concrete parts, for example in power plant engineering; however, the method may not be suitable for mass-industrial applications, such as, for example, the automotive field.
An electromagnetic test method for stresses in ferromagnetic workpieces, which is known under the designation 3MA method, was developed by the German Fraunhofer Institute for Non-Destructive Test Methods. This electromagnetic method is based on the interaction of the magnetic structure that consists of domains, their Bloch wall movements, the microstructure, and the mechanical stress fields of the material. The 3MA method (Micromagnetic Multiparameter Microstructure and Stress Analysis) uses the simultaneous measurement and superposition of various magnetic effects (Barkhausen noise signal, superposition permeability, harmonic wave properties of frequency-dependent eddy current measurement values) in order to determine stretching and tensile strength properties and boundary values of workpieces. The 3MA method is calibrated on samples with known properties. Hardness, hardness depth and internal stresses can thus also be characterized with the corresponding calibration. The 3MA method thus represents a combination of several electrical and magnetic testing methods, from which their different measured values, which react differently to stress and structure influences, can be determined. The method can be relatively expensive because of the design, the analysis and the interpretation and may not be suitable for use en masse.
Another known method uses the phenomenon of magnetoconstriction, a change in length of a ferromagnetic material under the influence of a magnetic field. Conversely, a change in length produced by mechanical stresses because of external forces produces a change in the magnetic parameters of the material. The altered magnetic permeability allows a return to the inner mechanical stress state of a ferromagnetic workpiece. The imprinting of the magnetic field and the measurement of permeability can be done using two concentrically-arranged coils, which are arranged extremely close or around the ferromagnetic workpiece. The measuring principle is based on a type of eddy current measurement and analysis of the determined electrical impedance of the measuring coil. The method is used for monitoring mechanical stresses in bridge elements as well as for monitoring mechanical stresses, for example in atomic power plants. A pre-magnetization of the measured steel workpiece, which can affect the sensor operating point may be required. The method is associated with relatively high equipment cost and may not be designed for mass-industrial applications.
The above-described methods were developed primarily for the testing and monitoring of stress states and stress load limits in prestressed concrete parts or in steel-reinforced bridge elements, in which the expense and the costs of the measuring method may play a less important role.
Based on the automation of the measuring technology, however, the desire exists, even in mass-technical applications, such as, for example, in the automotive field, or, for example, in the generation of energy by means of windmills, to monitor the mechanical stress loads produced by torsional, shearing, bending, thrusting, tensile and/or compressive forces in workpieces, in order to match and to optimize the operating parameters based on the measured stress values.
One method for determining stress states of a workpiece uses wire-strain gauges, which are bonded to the test piece. Using the wire-strain gauge, for example, a stress-induced deformation of the workpiece can be converted into an electrical voltage. The wire-strain gauge can be bonded to the workpiece with as little slip as possible so that it can follow any expansion of the surface. Bonded compounds can fail because of mechanical influences, under the action of moisture, contamination and temperature changes. Wire-strain gauges can also have relatively slight sensitivity, which is manifested as a small signal-noise ratio and can limit the use of wire-strain gauges to relatively large expansions.
For monitoring stresses due to torque on rotating shafts, it is known to impress opposite orientations of the magnetic field on areas adjacent to the shaft. The changes in the directions of the magnetic fields in the individual zones can then be scanned with a direction-dependent magnetic field sensor and analyzed with respect to mechanical stresses. The changes in torque that occur with this method can be easy to determine. However, the effort involved in impressing opposite directions of the magnetic field onto areas on the shaft can be very costly and labor-intensive.
A method is disclosed for measuring mechanical stresses in a ferromagnetic workpiece, comprising: arranging at least two exciters of a magnetic field along a surface of a ferromagnetic workpiece, wherein a section of the workpiece is located between the at least two exciters of the magnetic field; arranging a direction-dependent magnetic field sensor at a position along the surface of the workpiece between the at least two exciters; determining a change in position and/or a direction of a dividing line between north and south poles of the magnetic field with the direction-dependent magnetic field sensor; and analyzing the change in the position and/or direction of the north and south poles of the magnetic field to measure mechanical stress in the workpiece.
A measurement arrangement is disclosed for measuring mechanical stress in a ferromagnetic workpiece, comprising: at least two exciters of a magnetic field for arrangement along a surface of a ferromagnetic workpiece, such that a section of the workpiece will be located between the at least two exciters of the magnetic field; and a direction-dependent magnetic field sensor positioned relative to the at least two exciters for arrangement at a position along a surface of the workpiece once positioned between the at least two exciters.
In the following, the disclosure will be described in greater detail by exemplary embodiments with reference to the attached drawings, in which:
a-7c show exemplary embodiments of measurement arrangements for implementing the method according to the disclosure; and
a-8b show exemplary embodiments of measurement arrangements for implementing the method according to the disclosure.
A method and a measurement arrangement is disclosed for measuring mechanical stresses produced by torsional, shearing, bending, thrusting, tensile and/or compressive forces in ferromagnetic workpieces are to be provided, which can allow small deformations to be reliably detected. For example, the method and the measurement arrangement can be suitable for mass-industrial applications, such as, for example, the automotive field. Moreover, the method can be simply and economically performable, and the measurement arrangement can have a simple and economical design.
In accordance with an exemplary embodiment, a method for measuring mechanical stresses in ferromagnetic workpieces is disclosed, in which a ferromagnetic workpiece impresses a magnetic field and a magnetic field value can be measured and analyzed with respect to the mechanical stress, wherein at least two exciters of the magnetic field can be arranged along the surface of the workpiece in such a way that a section of the workpiece is located between the two exciters of the magnetic field, a direction-dependent magnetic field sensor is arranged at a position along the surface of the workpiece, for example, approximately at half the distance between the two exciters of the magnetic field, and with the direction-dependent magnetic field sensor, the change in position and/or the direction of a dividing line between the north and south poles of the magnetic field is determined and analyzed (e.g., via a processor).
A method for measuring mechanical stresses in ferromagnetic workpieces is disclosed, in which method a ferromagnetic workpiece impresses a magnetic field and a magnetic field value is measured and analyzed with respect to the mechanical stress. In accordance with an exemplary embodiment, at least two exciters of the magnetic field can be arranged along the surface of the workpiece in such a way that a section of the workpiece can be located between the two exciters of the magnetic field. A direction-dependent magnetic field sensor is arranged at a position along the surface of the workpiece, which can be approximately at half the distance (or mid-point) between the two exciters of the magnetic field. With the direction-dependent magnetic field sensor, the change in position and/or the direction of a dividing line between the north and south poles of the magnetic field can be determined and analyzed.
For an exemplary implementation, the method according to the disclosure can include two exciters for a magnetic field and a direction-dependent magnetic field sensor including a connected analysis unit, which can be integrated into the magnetic field sensor. In accordance with an exemplary embodiment, the method uses the dependencies between mechanical and magnetic properties of ferromagnetic materials. In the case of the mechanical stressing of the ferromagnetic workpiece, as occurs, for example, when acted upon by torsional, shearing, bending, thrusting, tensile and/or compressive forces, and its forced geometric deformation, its magnetic properties and magnetic parameters can be altered. The direction-dependent magnetic field sensor can detect the shifting of the boundary between the north and south poles of the impressed magnetic field or the change in direction of the magnetic field vector. The measured values can be analyzed in order to determine therefrom the mechanical stress in the workpiece. In accordance with an exemplary embodiment, the type of material, and the type of alloy, can be taken into consideration by comparison with empirically determined values, and the determined stress value can be correspondingly corrected.
The arrangement of the exciters for the magnetic field can be suitably carried out in such a way that they are spaced apart in the longitudinal direction of the ferromagnetic workpiece. In addition, the exciters for the magnetic field can also be spaced apart angularly.
In accordance with an exemplary embodiment, the method can be implemented with only two exciters for the magnetic field and with a direction-dependent magnetic field sensor that is arranged in between the two exciters. In accordance with an exemplary embodiment, the assessment on the stresses prevailing in the workpiece can include n exciters for the magnetic field to be arranged and/or spaced apart along the longitudinal extension of the workpiece in such a way that sections with reversed orientation of the impressed magnetic field can be created on the workpiece. For example, a number of n−1 direction-dependent magnetic field sensors can be arranged in each case approximately at half the distance between two successive exciters of the magnetic field. The signals measured by the n−1 direction-dependent magnetic field sensors can be linked to one another for determining the stress state of the workpiece.
An exemplary embodiment of the disclosure can include permanent magnets to be used as exciters for the magnetic field. In accordance with an exemplary embodiment, permanent magnets can be economical and, despite a small design, can generate a relatively high magnetic field strength when using corresponding materials. For example, as permanent magnets, magnets that consist of SmxCoy, ferrite, NdFeB or plastic-bonded magnets can be used.
In an exemplary embodiment of the method according to the disclosure, the exciters for the magnetic field, for example the permanent magnets, can be rigidly connected to the workpiece. For example, the exemplary embodiment can be used for static or stationary workpieces, for example, in the case of steering links of motor vehicles, in prestressing steels in the construction industry, or in steel-reinforced concrete parts. The connection of the permanent magnets with the workpiece can be done, for example, integrally, for example, by gluing, soldering, or bonding. In the case of permanent magnets connected rigidly to the surface of the workpiece, the method according to the disclosure can be implemented on non-ferromagnetic workpieces.
An exemplary embodiment of the disclosure can include for the n exciters for the magnetic field and the n−1 sensors to be arranged in a common housing, which can be arranged in the immediate vicinity of the ferromagnetic workpiece for contact-free measurement of the mechanical stresses. In accordance with an exemplary embodiment, the method according to the disclosure can be implemented by means of a measuring device whose geometric design can be matched to the respective special application. For example, components of the housing can be exchanged, or the arrangement of the exciters for the magnetic field and the direction-dependent magnetic field sensors can be altered within the housing in order to match the measuring device to the special specifications.
In accordance with an exemplary embodiment, instead of the permanent magnets, a source for an electromagnetic DC (direct current) field can also be used as an exciter for the magnetic field. For example, DC carrying conductors or conductor loops, which can be arranged next to the workpiece.
An exemplary embodiment of the disclosure can include an electromagnetic AC (alternating current) field that can be superimposed on the electromagnetic DC fields. For example, to determine the mechanical stresses in the ferromagnetic material, in addition to the change in direction of the dividing line between the north and south poles of the magnetic field, its permeability and/or its differential permeability and/or its superposition permeability and/or the change in amplitude in the magnetic field and/or the oscillation behavior of the change in the magnetic field can be measured. The superposition of the magnetic DC field and the magnetic AC field can broaden the spectrum of the detectable measurement values from which material parameters and the respective mechanical stress state can be derived.
In accordance with an exemplary embodiment, the method according to the disclosure with the measuring structure according to the disclosure can be implemented with widely varying types of magnetic field sensors in order to precisely determine different material parameters from the measurement values. For example, sensors from the group that consists of flux-gate magnetometers, coils with ferrite cores, Hall sensors and magnetoresistive sensors can be used as direction-dependent magnetic field sensors for determining mechanical stresses produced by torsional, shearing, bending, thrusting, tensile and/or compressive forces in ferromagnetic workpieces.
In accordance with an exemplary embodiment, because of the structure according to the disclosure, for example, the method can be suitable for applications in the automotive field. For example, the method for continuous steering-wheel monitoring can be used to supply the desired force feedback to the connecting rod by means of a corresponding input into the servo support of the steering. The method can also be used for continuous fine adjustment of the travelling-gear setting of a motor vehicle, by, for example, the stresses in the steering links or stabilizers of a motor vehicle being monitored and being matched to the shock-absorber properties depending on the measured stress values. For example, the chassis frame properties can be matched to the respective existing specifications. In accordance with an exemplary embodiment, the method according to the disclosure in the automotive field consists in the continuous monitoring of the drive chain in order to optimize the torque that occurs in the transmission.
The method according to the disclosure with the measurement arrangement according to the disclosure can also be used for the monitoring of mechanical stresses in a rotating shaft. For example, the use of magnetically-coded shafts replaces the arrangement according to the disclosure of the exciters for the magnetic field. In accordance with an exemplary embodiment, the method can be used for the monitoring of the torque of drilling equipment or of screwing devices. For example, the torque can be optimized, and in the case of screwing devices, the method according to the disclosure can be used to tighten a screw with minimal torque variations.
In accordance with an exemplary embodiment, the method according to the disclosure can be used, for example, for monitoring mechanical stresses in components of windmills. Other applications relate to the monitoring of stress states in pre-stressed steel structures, for example bridges or buildings, in reinforced concrete components.
The diagrammatic representation in
The measurement arrangement (permanent magnets 2, 3 and direction-dependent magnetic field sensor 5) can, as indicated in
The measurement arrangement is not limited to the numbers of permanent magnets and direction-dependent magnetic field sensors depicted, for example, in
a and
In an exemplary measurement arrangement, which is indicated in
a and 8b show two views of an exemplary embodiment of a measurement arrangement for implementing the method according to the disclosure. In addition, spaced apart along the longitudinal extension of the ferromagnetic workpiece provided in turn with the reference number 1, the two permanent magnets 2 and 3 also can be spaced apart angularly. The magnetization directions of the two permanent magnets 2, 3 are indicated with the reference numbers I, J. The direction-dependent magnetic field sensor 5 can be arranged, for example, approximately halfway between the two permanent magnets 2, 3.
In the measurement arrangements depicted in
The method according to the disclosure can be implemented with a relatively simply-designed measurement arrangement and can be used in the most varied applications. Examples include the automotive field, the monitoring of stresses in prestressed elements of bridges and buildings, the optimization and control of torque in rotating components in engines, drilling equipment and screwing devices.
Thus, it will be appreciated by those skilled in the art that the present disclosure can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0237/12 | Feb 2012 | CH | national |
