EP3224610A1

EP3224610A1 - A method of calibrating an evaluation arrangement for sensing magnetic barkhausen noise.

Info

Publication number: EP3224610A1
Application number: EP15862959.2A
Authority: EP
Inventors: Johan FAHLKRANS; Lars HAMMERSTRÖM
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2014-11-28
Filing date: 2015-11-04
Publication date: 2017-10-04
Also published as: SE538669C2; SE1451454A1; WO2016085382A1; EP3224610A4

Abstract

A method of calibrating an evaluation arrangement for non- destructive evaluation of test objects on the basis of magnetic Barkhausen noise. The evaluation arrangement comprises a magnetization coil, a sensor for sensing magnetic Barkhausen noise, and an apparatus for evaluating the test object based on signals from the sensor. The method comprises 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 to compensate for said deviation. The evaluation arrangement is used in one of the steps of creating and measuring the varying magnetic field.

Description

A METHOD OF CALIBRATING AN EVALUATION

ARRANGEMENT FOR SENSING MAGNETIC BARKHAUSEN NOISE.

TECH N ICAL FI ELD OF TH E I NVENTION The i nvention relates to a method of calibrating an evaluation arrangement for non-destructive evaluation of test objects on the basis of magnetic Barkhausen noise according to the preamble of claim 1 . BACKG ROU N D AN D PRIOR ART

In the manufacturing of components made from ferromagnetic materials such as steel, a hardening process is often carried out to harden the surface material of the component i n order to increase the wear resistance of the component. I n the hardening process, martensite is formed and accou nts for the increased hardness of the surface material. After hardening, the components may be subject to a g ri nding process which increases the temperature in the su rface layers at the risk of tempering the martensite structure. The tempering reduces the hardness of the heated zone and also affects the magnetic properties of the material. A non-destructive evaluation of the properties of the su rface of a test object can therefore be based on measu rement of the magnetic properties of the test object, and more specifically on measu rement of mag netic Barkhausen noise arising in a test object upon subjecting it to a varying mag netic field. Evaluation arrangements relying on measurement of mag netic Barkhausen noise are commonly used e.g. in quality control i n the manufactu ri ng of components such as crankshafts and camshafts. WO 2004/021 024 discloses an evaluation arrangement for evaluating a test object on the basis of mag netic Barkhausen noise. The evaluation arrangement comprises a magnetization coil for creati ng 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 mag netic 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 . I n 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 mag netic Barkhausen noise. According to the disclosed method , several calibration samples with known and g radually increasing thickness of surface hardened layers are used . US4881 030 discloses another method of calibrating an evaluation arrangement for evaluating a test object on the basis of magnetic Barkhausen noise. I n the calibration process, several calibration samples of a predetermined material with known hardness and known internal stresses are used .

Thus, in all above mentioned calibration methods, several samples with known properties are used 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 also the quality of the calibration. It 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 TH E I NVENTION

It 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 mag netic 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 i n that the method comprises the steps :

creating a varying magnetic field of an expected mag nitude, measu ring the mag nitude of the created varying mag netic field and determining a potential deviation from the expected mag nitude, and

adjusting the evaluation arrangement to compensate for said deviation,

wherein the evaluation arrangement is used i n one of the steps of creating and measuring the varying mag netic 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 calibration sample which may age and become damaged du ring use. The method is also exact, since the mag nitude of a varying mag netic field may be accu rately determi ned and controlled using electrical sig nals which can be very precisely controlled. Fu rthermore, the handli ng of physical calibration samples can be phased out, which i ncreases the time efficiency of the entire evaluation procedure.

According to the i nvention , the calibration is carried out usi ng a separate calibration arrangement comprising at least one calibration coil which is used in one of said steps of creating and measuring the varying magnetic field. The calibration arrangement is used to either create or to measure a varying mag netic field, and the evaluation arrangement is used in the other of said step. Using a calibration coil , it is possible to very accu rately create or sense a varying mag netic field by means of induction. Different calibration coils may preferably be used for the steps of creating and measu ring the varying mag netic 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 measu red by the sensor differ from each other.

According to the i nvention, a frequency of the varying mag netic 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 frequency of the varying magnetic field can easily be varied by varying the frequency of an alternating cu rrent used to generate the field . According to a further embodiment of the invention , the calibration coil is arranged so that a varying magnetic field created by the calibration coil induces a sig nal from the sensor. This can be achieved for example by accu rate positioning of the calibration coil with respect to the sensor of the evaluation arrangement. The method can in this embodiment be used to compensate for errors occu rring 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 mag netic field . Thus, the sensor of the evaluation arrangement is used to sense the created magnetic field . Since the mag nitude 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 accordi ng to this embodiment is therefore preferably used to discover and compensate for errors due to wear of the sensor.

According to a fu rther 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 mag netic 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 varyi ng mag netic field created by the mag netization coil induces a cu rrent in the calibration coil. This can be achieved for example by accu rate positioning of the calibration coil with respect to the magnetization coil of the evaluation arrangement. The method can in this embodi ment be used to compensate for errors occurring i n the in the creation of a mag netic field using the magnetization coil.

According to another embodiment of the invention , the calibration arrangement is used i n the step of measuring the created varying mag netic field and determining its mag nitude. I n this case, the mag netization coil of the evaluation arrangement is used in the step of creating a varying mag netic field . If the magnitude of the created varying mag netic field deviates from an expected value, the deviation can be compensated for. Thus, the method according to this embodiment is preferably used to discover and compensate for errors due to wear of the magnetization coil.

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

According to another embodiment of the invention, the method fu rther 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 i nvention , 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 arou nd the coils. The calibration coils are preferably identical and can be coupled i n 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 mag netic field, and at least a second one of the calibration coils is configu red to be used in the step of measuring the varyi ng magnetic field . The same calibration arrangement can thus be used both for calibrating the sensor and for calibrating the magnetization coil of the evaluation arrangement. The calibration arrangement may of cou rse 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 measu ri ng a mag netic field. According to another embodiment of the invention, at least two of the calibration coils are cu mulatively coupled in series. The two calibration coils which are coupled in this way are preferably configu red to be used for either creation or measurement of a varying mag netic field. I n the case when two calibration coils are configu red 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. In 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 material. The evaluation arrangement 1 further comprises a sensor 4, placed between contact surfaces 5, 6 of the mag netizing yoke 3. The sensor 4 is configured to sense mag netic 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 f rom 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 th rough the magnetization coil 2. The contact surfaces 5, 6 of the magnetizing yoke 3 is placed i n contact with the test object, so that a varying magnetic field is induced i n the test object. The AC cu rrent should be of sufficient magnitude to periodically cause mag netic satu ration of the test object. The test object, when subjected to the varying magnetic field, u ndergoes stepwise magnetization. This gives rise to Barkhausen noise, which is specific for the material that u ndergoes magnetization. The Barkhausen noise is measured by means of the sensor 4 and analyzed using the apparatus 7.

Fig . 1 further schematically shows a separate calibration arrangement 8 used to calibrate the evaluation arrangement using a method according to the invention. I n the shown embodiment, the calibration arrangement 8 comprises a si ngle calibration coil 9 and a control unit 1 0 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. I n a fi rst step A1 , the calibration coil 9 is placed in the absolute vicinity of the evaluation arrangement 1 , so that a mag netic field created by the calibration coil 9 induces a signal from the sensor 4. The control unit 1 0 is used to produce an AC current which is fed th rough the calibration coil 9, so that a varying mag netic field of a well-defined mag nitude is created. I n a second step A2, the sensor 4 of the evaluation arrangement 1 is used to measure the magnitude of the created magnetic field . In the case when the sensor 4 is a sensing coil , this is simply achieved by measuring the amplitude of a cu rrent i nduced in the sensing coil. In a step A3, the result of the measu rement 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 . I n a step A4, the evaluation arrangement 1 is adjusted to compensate for the found deviation, e.g . by manipulating the strength of the sig nal 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 i nvention, in which this is done, is shown in the flow chart in fig. 3. I n a step B1 , the calibration coil 9 is placed in the absolute vicinity of the evaluation arrangement 1 , so that a mag netic field created by the mag netization coil 2 induces a cu rrent i n the calibration coil 9. An AC current is fed th rough the mag netization coil 9, so that a varying magnetic field of an expected magnitude is created. I n 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 measuring the amplitude of a cu rrent induced in the calibration coil 9. I n a step B3, the result of the measu rement is evaluated using the control u nit 1 0 and a possible deviation in the measured magnitude from the expected magnitude of the created mag netic field is determined. I n a step B4, the evaluation arrangement 1 is adjusted to compensate for the fou nd deviation, e.g. by manipulati ng the current fed to the mag netization coil 2.

Of cou rse, it is possible to combi ne 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 mag netization coil 2. This may be done either by fi rst calibrating the sensor 4 using the steps A1 -A4 and thereafter calibrating the mag netization coil 2 using the steps B1 -B4, or the other way around.

In both described embodiments of the invention , the method may also i nclude 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 mag netizi ng yoke 3, or of the sensor.

In 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 cu rrent used to create the mag netic field. Thus, deviations occu rring in certain frequency intervals may be discovered and compensated for, both for the sensor 4 and for the magnetization coil 2.

The calibration arrangement 8 may be configu red with one or more calibration coils depending on the design of the evaluation arrangement 1 . Preferably, the calibration arrangement comprises at least two calibration coils, of which one is configu red for creating a magnetic field which is to be sensed by the sensor 4, and one is configu red for measuring a mag netic field created by the magnetization coil 2. If the calibration arrangement has e.g. two calibration coils for creating a mag netic 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 mag netic field cu mulatively in series, the magnetic field created by the calibration coils will vary in the same way i n both coils. The cumulative coupling ensu res that the created mag netic fields aid instead of cancel each other. The invention is of cou rse not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

1 . A method of calibrating an evaluation arrangement (1 ) for non-destructive evaluation of test objects on the basis of mag netic Barkhausen noise, the evaluation arrangement (1 ) comprising :

a mag netization coil (2) for creating a varying magnetic field in a test object,

a sensor (4) for sensi ng magnetic Barkhausen noise originating from the test object, which is caused by the varying mag netic field , and

an apparatus (7) for evaluating the test object based on signals from the sensor (4) ,

wherein the method comprises the steps :

creating a varying magnetic field of an expected magnitude, measuring the mag nitude of the created varying mag netic field and determining a deviation from the expected mag nitude, and

adjusting the evaluation arrangement (1 ) to compensate for said deviation,

wherein the evaluation arrangement is used i n one of the steps of creating and measuring the varying mag netic field,

characterized in

that 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 measu ri ng the varying magnetic field , and that a frequency of the varying magnetic field is varied over a calibration interval.

2. The method according to claim 1 , wherein the calibration coil (9) is arranged so that a varyi ng mag netic field created by the calibration coil (9) induces a sig nal from the sensor (4) .

3. The method according to claim 2, wherein the calibration arrangement (8) is used in the step of creating a varying mag netic field .

4. The method according to clai m 3, wherein the step of adjusting the evaluation arrangement (1 ) includes manipulating the strength of a signal from the sensor (4) .

5. The method according to claim 1 , wherein the calibration coil (9) is arranged so that a varyi ng mag netic field created by the mag netization coil (2) induces a current in the calibration coil (9).

6. The method according to claim 5, wherein the calibration arrangement (8) is used i n the step of measu ring the created varying magnetic field and determi ning its mag nitude.

7. The method according to clai m 6, wherein the step of adjusting the evaluation arrangement (1 ) includes manipulating a current used to create the varying magnetic field.

8. The method according to any of the preceding claims, fu rther 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.

9. 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.

1 0. The method according to claim 9, wherein at least a first one of the calibration coils is configu red to be used i n the step of creating a varying mag netic field, and at least a second one of the calibration coils is configu red to be used in the step of measuring the varyi ng magnetic field.

1 1 . The method according to claim 9 or 1 0, wherei n at least two of the calibration coils are cumulatively coupled in series.