Method, device and arrangement for load measurement on a test object

Abstract

To reduce a hysteresis error, the invention provides a load measurement method (12) for measuring a load in a test object (14), comprising: a) generating a magnetic field in the test object (14) by means of at least one magnetic field generating coil (Lg) to which a periodically alternating current is applied; b) detecting a magnetic field parameter which changes on the basis of a load in the test object (14), using at least one magnetic field detecting device, in order to generate a magnetic field parameter signal (51) which changes periodically according to the periodically generated magnetic field, characterized by: c) detecting the hysteresis-to-signal ratio of the magnetic field parameter signal (51) over time within one period; and d) disregarding magnetic field parameter signal values from at least one predetermined timespan within each period in which a maximum hysteresis-to-signal ratio occurs.

Claims

1. A load measurement method for measuring a load in a test object, comprising: a) generating a magnetic field in the test object by means of at least one magnetic field generating coil (Lg) to which a periodically alternating current is applied; b) detecting a magnetic field parameter which changes on the basis of a load in the test object, using at least one magnetic field detecting device, in order to generate a magnetic field parameter signal which changes periodically according to the magnetic field; c) detecting a hysteresis-to-signal ratio of the magnetic field parameter signal over time within one period; d) disregarding values in the magnetic field parameter signal from at least one predetermined time span within each period in which a maximum hysteresis-to-signal ratio occurs; and determining the load in the test object based on the magnetic field parameter signal with the values in which the maximum hysteresis-to-signal ratio occurs excluded.

2. The load measurement method according to claim 1, characterized by: e) recording magnetic field parameter signal values in at least one predetermined time span within each period in which a minimum hysteresis-to-signal ratio occurs.

3. The load measurement method according to claim 1, characterized in that step b) comprises capturing values of the magnetic field parameter signal at predetermined times within the period, step c) comprises detecting the hysteresis-to-signal ratio at the predetermined times to determine those predetermined times with more or with less hysteresis-to-signal ratio, and step d) comprises disregarding values in the magnetic field parameter signal at those of the predetermined times per period at which the maximum hysteresis-to-signal ratios occur.

4. The load measurement method according to claim 2, characterized in that step e) comprises recording values in the magnetic field parameter signal at those of the predetermined times per period in which the minimum hysteresis-to-signal ratios occur.

5. The load measurement method according to claim 1, further characterized by: f) determining a B/H characteristic from a time curve of current and/or voltage at the at least one magnetic field generating coil (Lg) and a time curve of the magnetic field parameter signal (Lg), and g) determining at least one measuring signal from the B/H characteristic.

6. The load measurement method according to claim 5, characterized in that step g) comprises at least one or more of the following steps: g1) determining a relative permeability from a change in gradient of the B/H characteristic as the measuring signal; g2) determining a change in coercive field strength from the B/H characteristic as the measuring signal; g3) determining a change in remanence from the B/H characteristic as the measuring signal; g4) considering one or more sub-regions of the B/H characteristic for determining the measuring signal; or g5) determining the gradient of the B/H characteristic at particular points.

7. A load measurement device for measuring a load in a test object, comprising: a magnetic field generating device for generating a magnetic field in the test object, the magnetic field generating device comprising at least one magnetic field generating coil (Lg) and a current source for supplying the magnetic field generating coil (Lg) with a periodically alternating current; a magnetic field detecting device for detecting a magnetic field parameter in the test object which changes on the basis of a load in the test object and for generating a magnetic field parameter signal which changes periodically according to the magnetic field; and an evaluation device for generating a measuring signal from the magnetic field parameter signal, characterized in that the evaluation device is configured to: obtain information on a hysteresis-to-signal ratio of the magnetic field parameter signal over time within one period; disregard values in the magnetic field parameter signal from at least one predetermined time span within each period in which a maximum hysteresis-to-signal ratio occurs; and determine the load in the test object based on the magnetic field parameter signal with the values in which the maximum hysteresis-to-signal ratio occurs excluded.

8. The load measurement device according to claim 7, characterized in that an evaluating means is provided which records the hysteresis-to-signal ratio of the magnetic field parameter signal over time within one period; and/or a memory is provided in which information on the hysteresis-to-signal ratio over time within one period is stored.

9. The load measurement device according to claim 7, characterized in that the evaluation device is configured to capture magnetic field parameter signal values in at least one time span within each period in which a minimum hysteresis-to-signal ratio occurs and to generate a measuring signal from it.

10. The load measurement device according to claim 7, characterized in that the magnetic field detecting device is configured to capture magnetic field parameter signal values at predetermined times within one period, and the evaluation device is configured to disregard magnetic field parameter signal values at those of the predetermined times per period at which the maximum hysteresis-to-signal ratios occur for the generation of the measuring signal.

11. The load measurement device according to claim 10, characterized in that the evaluation device is configured to use values in the magnetic field parameter signal at those of the predetermined times per period at which a minimum hysteresis-to-signal ratios occur for the generation of the measuring signal.

12. The load measurement device according to claim 7, characterized in that the evaluation device is configured to determine a B/H characteristic from a time curve of the current and/or voltage at the at least one magnetic field generating coil (Lg) and the time curve of the magnetic field parameter signal and to generate the at least one measuring signal from the B/H characteristic.

13. The load measurement device according to claim 12, characterized in that the evaluation device is configured to: 13.1 determine a relative permeability from a change in gradient of the B/H characteristic and to generate the measuring signal from it, and/or 13.2 determine a change in coercive field strength from the B/H characteristic and to generate the measuring signal from it, and/or 13.3 determine a change in remanence from the B/H characteristic and to generate the measuring signal from it, and/or 13.4 determine the gradient of the B/H characteristic at predetermined points and to generate the measuring signal from it and/or 13.5 use only certain sub-regions of the B/H characteristic for the generation of the measuring signal.

14. A load measurement arrangement, comprising a test object and the load measurement device according to claim 7 for contactless load measurement in the test object.

Description

(1) Embodiments of the invention will be described in more detail below with reference to the attached drawings wherein it is shown by:

(2) FIG. 1 a first preferred embodiment of a sensor head of a load measurement device for measuring a mechanical load, such as especially force, tension or torque in a test object;

(3) FIG. 2 a second preferred embodiment of the sensor head;

(4) FIG. 3 a lateral view of the sensor head of FIG. 1 along with the test object;

(5) FIG. 4 a view similar to FIG. 3 of another embodiment of the sensor head;

(6) FIG. 5 a view similar to FIG. 3 of still another embodiment of the sensor head;

(7) FIG. 6 a graph for a waveform of a magnetic field parameter signal measured by means of a magnetic field detecting device of one of the sensor heads of the FIGS. 1 to 5 during excitation of a magnetic field generating device of the sensor head by a square wave, wherein different support points (measuring times) are shown;

(8) FIG. 7 a graph of a measuring signal of a torque measurement using a magnetically inductive sensor head of the type shown in the FIGS. 1 to 5 when considering all support points from FIG. 6;

(9) FIG. 8 a graph similar to FIG. 7, wherein only some support points from the measuring curve of FIG. 6 have been considered;

(10) FIG. 9 a comparison of a measurement of a linear torque increase, wherein the measurement is shown only at the measuring points 1, 6 and 10 from the measuring period of FIG. 6;

(11) FIG. 10 a graph of a B/H characteristic obtained from a direct detection of a magnetic field parameter signal from the magnetic field detecting device and a current consumption of the magnetic field generating device over time and the subsequent computation.

(12) In the FIGS. 1 to 5, various embodiments of sensor heads 10 for a load measurement device 12 are shown. The load measurement device 12 serves for measuring loads, such as especially torques, forces or tensions, in an at least partially magnetizable test object 14, such as a shaft, a transmission part, a wheel hub, a chain wheel or the like, which can rotate about a rotation axis. In other designs, the test object 14 can also be stationary, such as a support or a strut in a support structure where loads or forces must be measured. In a different design, the test object can be a membrane of a pressure sensor as further described in German patent application 10 2017 104 547.3 to which reference is explicitly made for further details. The test object 14 is at least partially formed from a ferromagnetic material, at least in a measuring region. The test object 14 and the load measurement device 12 together form a load measurement arrangement 16.

(13) The load measurement device 12 comprises a magnetic field generating device 18 and several magnetic field detecting devices 20, 22.

(14) The load measurement device additionally comprises an evaluation device 42. The evaluation device 42 is coupled to the magnetic field detecting devices 20, 22. The evaluation device 42 is particularly designed for compensating hysteresis effects, which will be described in more detail below. For this purpose, the evaluation device 42 comprises a measuring value recording device 44 for recording measuring values at predetermined times of a measuring period, an evaluating means 46 for evaluating the measuring values recorded at different times of a measuring period, and a memory 48 for storing information on the evaluation.

(15) The magnetic field generating device 18 includes a magnetic field generating coil Lg and a driver circuit 50 for driving the magnetic field generating coil Lg. The driver circuit 50 supplies the magnetic field generating coil Lg (also referred to as primary coil) with a periodically alternating current, e.g. with a rectangular current, sinusoidal current, sawtooth current or the like, having a predetermined frequency f and thus a predetermined period T=1/f. The frequency is, for example, within a range of 1 kHz to 200 kHz.

(16) The magnetic field detecting devices 20, 22, include magnetic field sensors 26 in the form of detector coils A1, A2, B1, B2 (also referred to as measuring coils or secondary coils) or in the form of solid state magnetic field sensors 27, and the evaluation device 42 for evaluating the signals from the magnetic field sensors 26.

(17) The embodiment shown in FIG. 1, looking at the front side of the sensor head 10 to be directed to the test object 14, is shown from the side in FIG. 3. This embodiment comprises two first magnetic field sensors 26-1 configured as first detector coils A1, A2 and two second magnetic field sensors 26-2 configured as second detector coils B1, B2. The detector coils A1, A2, B1, B2 are provided in a cross arrangement or X-arrangement 28 on a common flux concentrator 30 from ferromagnetic material. In this case, the magnetic field generating coil Lg is arranged in the center—here also on a corresponding projection of the flux concentrator 30—with the first detector coils A1 and A2 being opposite each other and the second detector coils B1 and B2 being opposite each other.

(18) FIG. 2 shows another embodiment of the sensor head 10 in a V-arrangement 32 where only one first magnetic field sensor 26-1—e.g. the first detector coil A1—and only one second magnetic field sensor 26-1 are arranged at an angle to each other, with the magnetic field generating coil Lg at the tip of the angle shape.

(19) As shown in FIG. 4, the detector coils can also be replaced by solid state magnetic field sensors 27 as first and second magnetic field sensors 26-1, 26-2.

(20) FIG. 5 shows one embodiment of the sensor head 10 in which the coils—detector coils A1, A2, B1, B2 and the magnetic field generating coil Lg—are provided as planar coils 34 in a circuit board element 36—designed as PCB boards for instance.

(21) As described in the different literature D1-D7, the magnetic field sensors 26-1, 26-2 deliver a magnetic field parameter signal that depends on the load in the test object. For example, in the X-arrangement 28, the magnetic field parameter signal among the signals from the detector coils A1, A2, B1 and B2 is processed as described in D7 in order to determine the difference of the coil pair A-B as a magnetic field parameter signal.

(22) The magnetic field parameter signal follows the periodical change in the excitation current applied to the magnetic field generating coil Lg with a phase shift.

(23) FIG. 6 shows one example of a magnetic field parameter signal that is produced by the magnetic field sensors 26-1, 26-2 when a rectangular current is applied to the magnetic field generating coil Lg. The output of the magnetic field sensors 26-1, 26-2 is connected to the evaluation device 42. This comprises an analog-digital converter (not shown), which is part of the measuring value recording device 44. The measuring value recording device 44 records the respective magnetic field parameter signal value for different support points (vectors) that are shown by numbers on the x axis of FIG. 6. FIG. 6 represents one period of the secondary measuring curve—magnetic field parameter signal—in case of excitation with a square wave signal on the primary side. The magnetic field parameter signal shown here on the secondary side follows the excitation on the primary side with a phase shift, i.e. with a corresponding period as on the primary side. In this case, this period is sampled with 25 support points. Each support point is located at a predetermined point in time within the period.

(24) In the evaluating means 46, each of these support points is evaluated for its suitability for accurate measurement of loads.

(25) For example, after an initial installation of the load measurement arrangement 16 or after an initial or repeated start of the load measurement device 12, a calibration or learning process can be carried out in which an evaluation of each of these support points is performed.

(26) In this process, the test object 14 is subject to a predetermined load. In the example shown in more detail below, for this purpose, the test object is subject to a linearly increasing load and to a corresponding linearly decreasing load and is read out several times corresponding to FIG. 6 during the increase or decrease of the magnetic field parameter signal, and each support point 1 to 25 is evaluated.

(27) For example, a torque is measured in a shaft as a test object 14. To this end, a uniformly linearly increasing torque is first applied to the shaft as the test object 14 up to a maximum value in one direction and is then uniformly decreased again; thereafter an increasing torque is applied in the other direction also up to a maximum value and is then decreased again to zero.

(28) The x-axis in FIG. 7 shows the applied torque and the y-axis shows the measuring signal. The ideal measuring curve is indicated at pos. 52, and the real measuring curve is indicated at pos. 54. Because of the hysteresis, the real measuring curve 54 deviates from the ideal measuring curve 52.

(29) On the other hand, FIG. 8 shows a corresponding curve similar to FIG. 7. In this curve, however, not all but only some support points from the secondary measurement curve shown in FIG. 6 were considered. It can be seen in FIG. 8 that a significant reduction of the hysteresis error can be achieved.

(30) FIG. 6 accordingly shows the wave form on the secondary side with 25 support points that were captured by the analog-digital converter on the primary side when a rectangular signal was used for excitation.

(31) The FIGS. 7 and 8 show two measuring curves, both of which have been extracted from the same data set. FIG. 7 shows a measuring curve in which all 25 support points were used for calculation. On the other hand, in the measuring curve of FIG. 8, only certain support points were considered. It can be seen that a reduction of hysteresis from about 6% in FIG. 7 to less than 0.5% in FIG. 8 can be achieved.

(32) In the following, FIG. 9 is used to explain the way in which support points are determined that are to be considered for the measuring curve. FIG. 9 is an exemplary representation of the way in which the exemplary support points 1, 6 and 10 of the 26 support points of a secondary period behave over a linearly changing torque load. The different support points 1, 6 and 10 have a different sensitivity, but also have different hystereses, a different temperature behavior, behavior on RRN and other measuring effects.

(33) By applying an appropriate calibration load with linear torque increase and torque decrease and evaluating the data sets obtained at each of the support points, the evaluating means 46 determines the hysteresis-to-signal ratio for each of the support points. The corresponding information can be stored in the memory.

(34) In subsequent measurements, only those support points with the best, i.e. the smallest hysteresis-to-signal ratio are considered by the evaluation device 42. The measuring values of support points with the highest hysteresis-to-signal ratio are disregarded for obtaining the measuring signal.

(35) For example, if the learning process reveals that the support points 4-9 and 12-16 have the smallest hysteresis-to-signal ratio, subsequent measurements by the measuring value recording device 44 will only collect measuring values at these support points 4 through 9 and 12 through 16. Those time spans of the period which are outside the time spans including the support points which are considered, remain unconsidered, i.e. no measuring values are recorded at all at these support points.

(36) It is thus possible to achieve the measuring curve shown in FIG. 8, in which the hysteresis error is significantly reduced. Overall, a measurement with a significantly reduced hysteresis error can be achieved in this way.

(37) In the embodiments shown in the FIGS. 6 to 9, only the magnetic field parameter signals from the magnetic field sensors 26-1, 26-2 have been considered.

(38) However, it is also possible to consider parameters at the magnetic field generating device 18, such as the current applied to the magnetic field generating coil Lg. In this way a B/H characteristic can be obtained from which measuring signals can also be generated.

(39) FIG. 10 shows a typical B/H characteristic obtained with a primary coil—magnetic field generating coil Lg—and a secondary coil—for instance one of the detector coils A1, A2, B1 and B2. An alternating field has been used for excitation of the primary coil. For the secondary coil, the current consumption was measured via a shunt.

(40) In the embodiments of the FIGS. 6 to 9, only the B-share—for instance the induction voltage in the secondary side—was used. On the other hand, when B and H are measured at the same time, the characteristic curve shown in FIG. 10 can be calculated, and from this characteristic curve a measuring signal can be calculated which behaves linearly to the torque.

(41) As can be seen in FIG. 10, the hysteresis in the B/H characteristic is particularly high in a central part of the graph of FIG. 10.

(42) Analogous to the above-described procedure, in which only some support points from the measuring curve of FIG. 6 were used for generating the measuring signal, it is also possible to consider only a part of this characteristic curve when the B/H characteristic is used.

(43) For example, only the outlined areas of the B/H characteristic of FIG. 10 are used for generating the measuring signal.

(44) For example, one or more of the following measuring signals can be generated from the B/H characteristic: e.g. the change in the gradient of the characteristic dB/dH for calculating the relative permeability the change in the coercive field strength—H.sub.c the change in remanence—M.sub.r the gradient of the characteristic at predetermined points the generation of a measuring signal under consideration of dedicated areas, for instance the outlined areas in FIG. 10

(45) In FIG. 10, the curve of the magnetic field strength H is plotted at pos. 56. This corresponds to the current consumption of the magnetic field generating coil Lg. At pos. 58, the magnetic flux density B is plotted which can be obtained via the current consumption at one of the detector coils A1, A2, B1 and B2. To record the characteristic curve of FIG. 10, the current of the magnetic field generating coil Lg can, for example, be increased and decreased by the driver circuit 50 in a linearly increasing and linearly decreasing manner and then, on the one hand, when increasing and, on the other hand, when decreasing, the corresponding value B on the secondary side can be assigned to each value H on the primary side. This can also be done over the measurement periods with corresponding excitation with the frequency f.

LIST OF REFERENCE SIGNS

(46) 10 sensor head 11 measuring region 12 load measurement device 14 test object 16 load measurement arrangement 18 magnetic field generating device 20 first magnetic field detecting device 22 second magnetic field detecting device 26 magnetic field sensor 26-1 first magnetic field sensor 26-2 second magnetic field sensor 27 solid state magnetic field sensor 28 X-arrangement 30 flux concentrator 32 V-arrangement 34 planar coil 36 circuit board element 42 evaluation device 44 measuring value recording device 46 evaluating means 48 memory 50 driver circuit 51 magnetic field parameter signal 52 ideal measuring curve 54 real measuring curve 56 magnetic field strength H 58 magnetic flux density B A1 first detector coil A2 second detector coil B1 third detector coil B2 fourth detector coil Lg magnetic field generating coil