SE1451454A1

SE1451454A1 - A method of calibrating an evaluation arrangement by magnetic Barkhausen noise

Info

Publication number: SE1451454A1
Application number: SE1451454A
Authority: SE
Inventors: Fahlkrans Johan; Hammerström Lars
Original assignee: Scania Cv Ab
Priority date: 2014-11-28
Filing date: 2014-11-28
Publication date: 2016-05-29
Also published as: EP3224610A1; EP3224610A4; WO2016085382A1; SE538669C2

Description

15 20 25 30 WO 2004/021024 discloses an evaluation arrangement for evaluating a test object on the basis of magnetic Barkhausen noise. The evaluation arrangement comprises a magnetization coil for creating a varying magnetic field in the test object, a sensor in the form of a sensing coil for sensing magnetic Barkhausen noise originating from the test object, which is caused by the varying magnetic field, and an apparatus for evaluating the test object based on signals from the sensing coil.

By means of the evaluation arrangement, the hardening depth of the test object is determined. ln order to calibrate the evaluation arrangement, several calibration samples are used.

GB2495292 discloses a method of calibrating an evaluation arrangement for evaluating a test object on the basis of magnetic Barkhausen noise. According to the disclosed method, several calibration samples with known and gradually increasing thickness of surface hardened layers are used.

US488103O discloses evaluation arrangement for evaluating a test object on the basis another method of calibrating an of magnetic Barkhausen noise. ln the calibration process, several calibration samples of a predetermined material with known hardness and known internal stresses are used.

Thus, samples with known properties are used in all above mentioned calibration methods, several to calibrate the evaluation arrangements. However, calibration samples used in such calibration methods age over time and may get damaged during use, which affect their magnetic properties and thereby 10 15 20 25 30 also the quality of the calibration. lt is therefore desirable to find an alternative method of calibration, which does not rely on the use of calibration samples with known properties.

SUMMARY OF THE INVENTION lt is an object of the present invention to provide a solution by means of which an evaluation arrangement for non-destructive evaluation of test objects on the basis of magnetic Barkhausen noise can be calibrated without the use of calibration samples with known properties.

According to the invention, this object is achieved by means of the method initially defined, which is characterized in that the method comprises the steps: creating a varying magnetic field of an expected magnitude, measuring the magnitude of the created varying magnetic field and determining a potential deviation from the expected magnitude, and adjusting the evaluation arrangement to compensate for said deviation, wherein the evaluation arrangement is used in one of the steps of creating and measuring the varying magnetic field.

Thus, the method according to the invention does not rely on measurements of Barkhausen noise from a sample of known properties, but rather on the creation and measurement of a varying magnetic field of an expected magnitude. This method is stable over time, since it does not rely on properties of a 10 15 20 25 30 calibration sample which may age and become damaged during use. The method is also exact, since the magnitude of a varying magnetic field may be accurately determined and controlled using electrical signals which can be very precisely controlled.

Furthermore, the handling of physical calibration samples can be phased out, Which increases the time efficiency of the entire evaluation procedure.

According to an embodiment of the invention, the calibration is carried out using a calibration arrangement comprising at least one calibration coil which is used in one of said steps of creating and measuring the varying magnetic field. ln this embodiment, is used to either create or to field, and the arrangement is used in the other of said step. Using a calibration the calibration arrangement measure a varying magnetic evaluation coil, it is possible to very accurately create or sense a varying magnetic field by means of induction. Different calibration coils may preferably be used for the steps of creating and measuring the varying magnetic field, since the frequencies of the magnetic field which is to be generated by the magnetization coil of the evaluation arrangement for non-destructive testing and the magnetic field which is to be measured by the sensor differ from each other. of the calibration coil is arranged so that a varying magnetic field According to a further embodiment invention, the created by the calibration coil induces a signal from the sensor.

This can be achieved for example by accurate positioning of the calibration coil with respect to the sensor of the evaluation arrangement. The method can in this embodiment be used to 10 15 20 25 30 compensate for errors occurring in the sensing and measurement of a magnetic field using the sensor.

According to another embodiment of the invention, the calibration arrangement is used in the step of creating a varying magnetic field. Thus, the sensor of the evaluation arrangement is used to sense the created magnetic field. Since the magnitude of the created varying magnetic field can be very precisely defined, a deviation in the measured magnitude indicates that the sensing coil of the evaluation arrangement does not function accurately.

The method according to this embodiment is therefore preferably used to discover and compensate for errors due to wear of the SGÛSOF.

According to a further embodiment of the invention, the step of adjusting the evaluation arrangement includes manipulating the strength of a signal from the sensor. For example, the signal from the sensor may be amplified if the strength of the sensed magnetic field is lower than expected. This is a straight-forward way of compensating for a detected deviation.

According to another embodiment of the invention, the calibration coil is arranged so that a varying magnetic field created by the magnetization coil induces a current in the calibration coil. This can be achieved for example by accurate positioning of the calibration coil with respect to the magnetization coil of the evaluation arrangement. The method can in this embodiment be used to compensate for errors occurring in the in the creation of a magnetic field using the magnetization coil. 10 15 20 25 30 According to another embodiment of the invention, the calibration arrangement is used in the step of measuring the created varying magnetic field and determining its magnitude. ln this case, the magnetization coil of the evaluation arrangement is used in the step of creating a varying magnetic field. lf the magnitude of the created varying magnetic field deviates from an expected value, Thus, the method according to this embodiment is preferably used to discover and the deviation can be compensated for. compensate for errors due to wear of the magnetization coil.

According to a further embodiment of the invention, the step of adjusting the evaluation arrangement includes manipulating a current used to create the varying magnetic field. lf the strength of the sensed magnetic field is lower than expected, an amplifier may be used to amplify the current and thereby also the magnitude of the created magnetic field.

According to another embodiment of the invention, the method further comprises the step: comparing the determined deviation to a pre-defined acceptance value, wherein the step of adjusting the evaluation arrangement is carried out only if said acceptance value is exceeded. This allows compensating for deviations only if said deviations are deemed to be of importance.

According to another embodiment of the invention, a frequency of the varying magnetic field is varied over a calibration interval.

This enables the detection of deviations for certain frequencies or frequency intervals and compensation for such errors. The 10 15 20 25 30 frequency of the varying magnetic field can easily be varied by varying the frequency of an alternating current used to generate the field.

According to another embodiment of the invention, the method is carried out using a calibration arrangement comprising at least two calibration coils. The use of at least two calibration coils may increase the accuracy of the method of calibration, since it will be easier to position the sensor correctly in the more homogeneous magnetic field around the coils. The calibration coils are preferably identical and can be coupled in series or in parallel.

According to another embodiment of the invention, at least a first one of the calibration coils is configured to be used in the step of creating a varying magnetic field, and at least a second one of the calibration coils is configured to be used in the step of measuring the varying magnetic field. The same calibration arrangement can thus be used both for calibrating the sensor and coil of the evaluation for calibrating the magnetization arrangement. The calibration arrangement may of course comprise more than two calibration coils, e.g. two calibration coils to be used for creation of a magnetic field and two calibration coils to be used for measuring a magnetic field.

According to another embodiment of the invention, at least two of the calibration coils are cumulatively coupled in series. The two calibration coils which are coupled in this way are preferably configured to be used for either creation or measurement of a varying magnetic field. ln the case when two calibration coils are 10 15 20 25 30 configured for creation of a varying magnetic field, a calibration coil can be placed on each side of the sensor of the evaluation arrangement. The magnetic field sensed by the sensor will be the added field of the calibration coils at the location of the sensor.

Further embodiments and advantages of the invention will be apparent from the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, a specific description of preferred embodiments of the invention cited as examples follows below. ln the drawings: Fig. 1 is a schematic view of an evaluation arrangement and a calibration arrangement, Fig. 2 is a flow chart illustrating a method according to an embodiment of the invention, and Fig. 3 is a flow chart illustrating a method according to a different embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig. 1 schematically shows an evaluation arrangement 1 for non- destructive evaluation of a test object on the basis of magnetic Barkhausen noise. The evaluation arrangement 1 comprises a magnetization coil 2 for creating a varying magnetic field in a test object. The magnetization coil 2 is wound around a magnetizing yoke 3, made from e.g. a ferromagnetic or a ferrimagnetic 10 15 20 25 30 material. The evaluation arrangement 1 further comprises a sensor 4, placed between contact surfaces 5, 6 of the magnetizing yoke 3. The sensor 4 is configured to sense magnetic Barkhausen noise originating from the test object, which is caused by the varying magnetic field. An apparatus 7 is used to evaluate the test object based on signals from the sensor 4. The sensor 4 is here in the form of a sensing coil, but may be any kind of magnetic field sensor.

When used to evaluate a test object (not shown), an alternating (AC) current is fed through the magnetization coil 2. The contact surfaces 5, 6 of the magnetizing yoke 3 is placed in contact with the test object, so that a varying magnetic field is induced in the test object. The AC current should be of sufficient magnitude to periodically cause magnetic saturation of the test object. The test object, when subjected to the varying magnetic field, undergoes stepwise magnetization. This gives rise to Barkhausen noise, which is specific for the material that undergoes magnetization.

The Barkhausen noise is measured by means of the sensor 4 and analyzed using the apparatus 7.

Fig. 1 further schematically shows a calibration arrangement 8 used to calibrate the evaluation arrangement using a method according to the invention. ln the shown embodiment, the calibration arrangement 8 comprises a single calibration coil 9 and a control unit 10 for controlling the calibration coil 9.

Calibration of the evaluation arrangement 1 according to a first embodiment of the present invention is shown in the flow chart of fig. 2. ln a first step A1, the calibration coil 9 is placed in the 10 15 20 25 30 10 absolute vicinity of the evaluation arrangement 1, so that a magnetic field created by the calibration coil 9 induces a signal from the sensor 4. The control unit 10 is used to produce an AC current which is fed through the calibration coil 9, so that a varying magnetic field of a well-defined magnitude is created. ln a second step A2, the sensor 4 of the evaluation arrangement 1 is used to measure the magnitude of the created magnetic field. ln the case when the sensor 4 is a sensing coil, this is simply achieved by measuring the amplitude of a current induced in the sensing coil. ln a step A3, the result of the measurement is evaluated using the apparatus 7 and a possible deviation in the measured magnitude from the well-defined expected magnitude of the created magnetic field is determined. ln a step A4, the evaluation arrangement 1 is adjusted to compensate for the found deviation, e.g. by manipulating the strength of the signal from the sensor 4.

By means of the steps A1-A4, the sensor 4 is calibrated and errors arising from the sensing of Barkhausen noise can be compensated for. However, also the magnetization coil 2 of the evaluation arrangement might need to be calibrated. A second embodiment of the invention, in which this is done, is shown in the flow chart in fig. 3. ln a step B1, the calibration coil 9 is placed in the absolute vicinity of the evaluation arrangement 1, so that a magnetic field created by the magnetization coil 2 induces a current in the calibration coil 9. An AC current is fed through the magnetization coil 9, so that a varying magnetic field of an expected magnitude is created. ln a second step B2, the calibration coil 9 of the calibration arrangement 8 is used to measure the magnitude of the created magnetic field by 10 15 20 25 30 11 measuring the amplitude of a current induced in the calibration coil 9. ln a step B3, the result of the measurement is evaluated using the control unit 10 and a possible deviation in the measured magnitude from the expected magnitude of the created magnetic field is determined. ln a step B4, the evaluation arrangement 1 is adjusted to compensate for the found deviation, e.g. by manipulating the current fed to the magnetization coil 2.

Of course, it is possible to combine the method steps A1-A4 of the first embodiment with the method steps B1-B4 of the second embodiment to achieve calibration of both the sensor 4 and the magnetization coil 2. This may be done either by first calibrating the sensor 4 using the steps A1-A4 and thereafter calibrating the magnetization coil 2 using the steps B1-B4, or the other way around. ln both described embodiments of the invention, the method may also include a step in which a determined deviation in measured amplitude of the created magnetic field is compared to a pre- defined acceptance value. The step A4, B4 of adjusting the evaluation arrangement 1 is in this case carried out only if said acceptance value is exceeded. Adjustment of the evaluation arrangement 1 may, in addition to manipulation of signals and currents, include e.g. reconditioning of the contact surfaces 5, 6 of the magnetizing yoke 3, or of the sensor. ln the described embodiments, a frequency of the created varying magnetic field may be varied over a calibration interval by varying the frequency of the AC current used to create the magnetic field. Thus, deviations occurring in certain frequency 10 15 20 25 12 intervals may be discovered and compensated for, both for the sensor 4 and for the magnetization coil 2.

The calibration arrangement 8 may be configured with one or more calibration coils depending on the design of the evaluation arrangement 1. calibration Preferably, the arrangement comprises at least two calibration coils, of which one is configured for creating a magnetic field which is to be sensed by the sensor 4, and one is configured for measuring a magnetic field created by the magnetization coil 2. lf the calibration arrangement has e.g. two calibration coils for creating a magnetic field, these may be arranged so that they can be located one on each side of the sensor 4 of the evaluation arrangement 1. By coupling the calibration coils used to create the magnetic field cumulatively in series, the magnetic field created by the calibration coils will vary in the same way in both coils. The cumulative coupling ensures that the created magnetic fields aid instead of cancel each other.

The invention is of course not in any way restricted to the em- bodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordi- nary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

10 15 20 25 13 CLAIMS

1. A method of calibrating an evaluation arrangement (1) for non-destructive evaluation of test objects on the basis of magnetic Barkhausen noise, the evaluation arrangement (1) comprising: a magnetization coil (2) for creating a varying magnetic field in a test object, a sensor (4) for sensing magnetic Barkhausen noise originating from the test object, which is caused by the varying magnetic field, and an apparatus (7) for evaluating the test object based on signals from the sensor (4), characterized in that the method comprises the steps: creating a varying magnetic field of an expected magnitude, measuring the magnitude of the created varying magnetic field and determining a deviation from the expected magnitude, and adjusting the evaluation arrangement (1) to compensate for said deviation, wherein the evaluation arrangement is used in one of the steps of creating and measuring the varying magnetic field.

2. The method according to claim 1, wherein the calibration is carried out using a calibration arrangement (8) comprising at least one calibration coil (9) which is used in one of said steps of creating and measuring the varying magnetic field. 10 15 20 25 30 14

3. The method according to claim 2, wherein the calibration coil (9) is arranged so that a varying magnetic field created by the calibration coil (9) induces a signal from the sensor (4).

4. The method according to claim 3, wherein the calibration arrangement (8) is used in the step of creating a varying magnetic field.

5. The method according to claim 4, wherein the step of adjusting the evaluation arrangement (1) includes manipulating the strength of a signal from the sensor (4).

6. The method according to claim 2, wherein the calibration coil (9) is arranged so that a varying magnetic field created by the magnetization coil (2) induces a current in the calibration coil (9).

7. The method according to claim 6, wherein the calibration arrangement (8) is used in the step of measuring the created varying magnetic field and determining its magnitude.

8. The method according to claim 7, wherein the step of adjusting the evaluation arrangement (1) includes manipulating a current used to create the varying magnetic field.

9. The method according to any of the preceding claims, further comprising the step: comparing the determined deviation to a pre-defined acceptance value, wherein the step of adjusting the evaluation arrangement (1) is carried out only if said acceptance value is exceeded. 10 15 15

10. The method according to any of the preceding claims, wherein a frequency of the varying magnetic field is varied over a calibration interval.

11. The method according to any of the preceding claims, wherein the method is carried out using a calibration arrangement (8) comprising at least two calibration coils.

12. The method according to claim 11, wherein at least a first one of the calibration coils is configured to be used in the step of creating a varying magnetic field, and at least a second one of the calibration coils is configured to be used in the step of measuring the varying magnetic field.

13. The method according to claim 11 or 12, wherein at least two of the calibration coils are cumulatively coupled in series.