WO2003052439A2

WO2003052439A2 - Detecting magnetic rotational non-uniformity

Info

Publication number: WO2003052439A2
Application number: PCT/EP2002/014520
Authority: WO
Inventors: Lutz Axel May
Original assignee: Fast Technology Ag
Priority date: 2001-12-14
Filing date: 2002-12-16
Publication date: 2003-06-26
Also published as: GB0129982D0; AU2002366373A8; WO2003052439A3; AU2002366373A1

Abstract

A method and apparatus is disclosed to characterise a part with respect to an axis through the part.The part is exemplified as a ferromagnetic shaft (10) rotatable about a longitudinal axis (A-A). A source of magnetisation (22), a permanent magnet or an electromagnet, is located adjacent the shaft (10) to magnetise the local surface-adjacent zone (12) of the shaft (10). The shaft is rotated so that the local magnetisation carries to a magnetic field detector (26) axially aligned with, but angularly offset from, source (22). The output from the detector (26) varies with angle in accordance with some magnetic property of the shaft (10). The shaft (10) may be given an axial translation by means (16) to enable the surface of the shaft (10) to be successively scanned. An alternative arrangement uses an in-line source and detector. A particular application is to investigating the uniformity of case-hardening of steel shafts.

Description

Title: Detecting Magnetic Rotational Non-Uniformity

FIELD OF THE INVENTION

This invention is concerned with the measurement of a physical characteristic of a part, which physical characteristic affects a magnetic property of the part. The invention relates to a method and apparatus for determining the distribution of a magnetic property about an axis through a part. The invention also relates to apparatus for use in determining the distribution of a magnetic property in a part.

A particular, though not exclusive, application of the invention is in assessing the case-hardening of shafts or other parts. Steel shafts are commonly case-hardened to impart higher wear resistance to the surface of the shaft. The case hardening is found in an annular zone extending inwardly from the surface of the shaft. With a shaft of circular cross-section the annular hardened zone should have uniform depth and metallurgical properties around the annulus. Alternatively it may be required to have a shaft in which the depth is not uniform about the axis but varies in a prescribed fashion.

BACKGROUND TO THE INVENTION

As the result of investigation into the magnetisation of ferromagnetic shafts, we have found that the magnetic property of the case-hardened annular outer zone of a ferromagnetic shaft differs from that of the underlying inner material of the shaft. If the case hardening is not uniform about the longitudinal axis of the shaft, this can pose problems in establishing a magnetic transducer element in an integral portion of the shaft where the element stores a surface-adjacent annular region of permanent magnetisation. Such transducer elements find utility in torque transducers for measuring torque in the shaft. Examples of annular magnetisations of this kind are disclosed in published PCT applications WO01/13081 and WO01/79801 , the disclosure of which is incorporated herein by reference. These documents also disclose method and apparatus for creating the desired remanent magnetisation in the transducer region. Non-uniformity about the shaft axis of the magnetic property of the material is liable to lead to a stored magnetisation which emanates an external magnetic field which is non-uniform as a function of angle about the shaft axis. This effect may be referred to as non-rotational uniformity (NRU) or, alternatively, as rotational non-uniformity (RNU). The latter term will be used hereinafter. The emanated field, which is torque sensitive, is detected by a non-contacting magnetic field sensor arrangement with which the parameter rotational signal uniformity (RSU) is associated. The quality of the RSU parameter is thus affected by the RNU of the transducer region with which the sensor arrangement co-operates.

An alternative situation which can arise with case-hardened shafts is that they are further treated in a way which results along at least a portion of the shaft in an RNU which varies in a known manner with angle about the axis of the treated shaft. This known variation can be used to form the transducer region of a magnetic-based angle transducer as is described in PCT application PCT/EP01/13698 filed 21^st November 2001 and published as WO02/42713, and the disclosure of which is incorporated hereby reference. It is considered, therefore, useful to devise a method and apparatus for investigating the RNU of case-hardened shafts and more generally for investigating magnetic inhomogeneities, irregularities in the material or structure of a part rotatable about axis, or investigating how some magnetic property due to a desired material or structural characteristic, varies in a desired manner with angle about an axis. SUMMARY OF THE INVENTION

The present invention has stemmed in one aspect from the concept of magnetising a local surface-adjacent region of a shaft as it rotates relative to a magnetising source to induce at least a temporary stored magnetisation in the region and sensing the magnetic field emanated by the region, the magnitude of which is a function of the degree of magnetisation of the region, and which is in turn dependent on a magnetic property of the magnetised material.

In another aspect, the present invention has stemmed from the concept of applying a local magnetic field to a surface-adjacent region of a shaft and concurrently detecting a magnetic field external of the surface which is dependent on a magnetic property of the surface-adjacent region to which the local field is applied.

These procedures can be used to sense the uniformity of case hardening about the axis of a hardened shaft. It may be applied more generally to shafts to which some surface treatment has been applied which gives rise to a distinctive magnetic property or to checking the homogeneity of material of a shaft or other part or body about an axis. The procedure can be extended to include an axial translation of the magnetising source and sensor with respect to the shaft to provide a profile of rotational uniformity along a length of the shaft.

The uniformity of case-hardening in ferromagnetic shafts may be investigated in terms of inhomogeneities or irregularities, or in terms of the depth of the case-hardened zone. As indicated above, a uniform depth may be desired or a depth varying in some prescribed fashion. The depth is a parameter affecting the magnetic response of the shaft.

Aspects and features of this invention for which protection is presently sought are set forth in the Claims following this description. The invention and its practice will be further described with reference to the accompanying drawings, in which

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows diagrammaticaliy an apparatus embodying the invention; and

Figs. 2a and 2b are graphical representations of signal output having no RNU and a signal output exhibiting RNU respectively; and

Fig. 3 shows an alternative magnetic source and magnetic field detector for performing the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows an apparatus for scanning a ferromagnetic shaft 10 of circular cross-section for non-rotational uniformity in respect of a magnetic property. In the embodiment illustrated the shaft is case-hardened to have an annular hardened zone or region 10a about a longitudinal axis A-A of the shaft. As already pointed out the hardened zone 10a has a different magnetic property to the underlying inner material of the shaft. However, the invention can be applied to investigating a shaft which is of nominally homogeneous material throughout.

The shaft 10 is mounted for rotation about its longitudinal axis A-A. The means for mounting and rotating the shaft is diagrammaticaliy indicated at 16. The apparatus comprises a carriage 20 which is movable in the direction of axis A-A. The carriage 20 carries a magnetising source 22 mounted closely adjacent the surface of shaft 10 to induce a magnetisation into the adjacent portion 12 of the shaft. For the moment consider the carriage 20 to be axially stationary. Mounted at an angle to source 22 about axis A-A is a magnetic field detector 24 comprising a sensor device 26 (or more than one device) connected to a signal processing circuit 28. The device 26 is also located closely adjacent the surface of shaft 10 to detect the field 14 emanated by portion 12, now at position 12', as the shaft rotates. The sensor device 26 and source 22 may be positioned 180° apart, that is diametrically opposite one another with respect to the shaft. The sensor device 26 and source 22 are aligned axially, that is essentially in the same axial plane or planar slice allowing for any axial motion of the magnetised portion 12 in carrying from the source to the sensor. The source and sensor may be angularly offset or separated to any desired extent. The signal Vo from circuit 28 representing the field sensed by sensor device 26 is applied to signal analyser unit 30 and a graphical display unit 32. The rotation of the shaft scans the source 22 over the shaft surface to produce a local magnetisation which varies with angle as a function of the RNU of the shaft. The cause of the RNU is discussed below. The rotational scan may be combined with an axial scan as described below.

The magnetising source may be a pair of axially spaced magnets 34 and 36 with a low reluctance member or flux enhancer 38 to generate a flux in the adjacent portion 12 of the shaft which penetrates at least the surface- adjacent zone 10a of the shaft with an essentially axially-directed magnetisation. The exact contours of the field (which is a D.C. field) within the shaft will depend on the nature of the material of the shaft. For example, if the shaft is case-hardened as has been discussed above, the hardened surface zone of the shaft has a magnetic property different from the more interior portion of the shaft. Sufficient remanent magnetisation is induced in the shaft to be carried round as the shaft rotates, the resultant magnetisation now emanating a magnetic field 14 detectable by the detector 24 as the relevant portion of the shaft rotates round to face sensor device 26. The sensor device is oriented to detect an axially-directed component of the emanated flux. The induced magnetisation need only be a temporarily stored magnetisation provided it carries in a reliable fashion from the magnetic source position to the magnetic field detector. The radial depth of penetration of the shaft by the magnetic field should be sufficient to penetrate into or through the zone 40 so that the field 14 emanated from the shaft is primarily dependent on the surface zone 40 and gives an adequate signal for detection. On the other hand, the depth of penetration of the applied field should not be so great that the emanated field 14 due to the surface zone 10a is diluted or masked by a component due to the underlying material of the shaft. Thus the strength of field from source 22, the depth of the surface zone 40 and the magnetic property of the underlying material have to be taken into account in optimising operation. These factors are discussed further below.

Reverting to the operation of the apparatus of Fig. 1 , what is of primary interest is the variation of the output signal Vo in each revolution of the shaft. The ideal for an entirely rotationally uniform shaft would be a constant value of Vo. Magnetic inhomogeneities in the shaft 10, and particularly zone 10a, as it rotates pass source 22 will be reflected as fluctuations in Vo as the shaft rotates through 360°.

The apparatus of Fig. 1 also provides for the carriage 20 to be axially movable in the direction A'-A' with the axis of carriage movement aligned parallel to the axis of rotation A-A of shaft 10. The axial translation of the carriage can be performed step-wise, e.g. a step movement following each rotation of the shaft; or by a continuous linear translation (which may be done in a stepwise fashion) combined with continuous rotation (which may be done in a stepwise fashion) to thereby scan a spiral about the shaft, e.g. with overlapping scan areas bearing in mind the width of the magnetised region and the lateral resolution of the sensor device. By this means a complete length of shaft can be scanned to obtain a magnetic profile in both angle and axial dimensions . Referring now to Fig. 2a, if the shaft is entirely rotationally uniform magnetically about its axis along its scanned length, a display of output voltage Vo with time (t) as the axis scan progresses will be a constant as shown.

On the other hand, if the shaft is not rotationally uniform and a inhomogeneity occurs at the same angle about the shaft axis along the scanned length the resulting output might have an appearance such as illustrated in Fig. 2b, where the notations 1 , 2, 3 on the time axis indicate complete revolutions of the shaft. It will be appreciated that the invention is of wide utility in investigating inhomogeneities or irregularities in a shaft which lead to a non-uniform magnetic property. An indexed motion in both axial translation and in rotation is of value in relating a detected irregularity etc. with a known position on the surface of the shaft. The teaching can be extended to structural faults or irregularities or even to scanning desired structural features which lead to a change of the magnetic characterisation of the shaft.

While a shaft of circular cross-section is the easiest shape to scan, scanning of other cross-sections is contemplated. The detector 24 and magnetising source 22 can be mounted at an angle about axis A-A other than 180° apart, and, for example, the detector could be mounted to the carriage 20 by means providing for an adjustable angle. In general it will be seen that the detector 24, and particularly sensor device 26, and the magnetising source 22 act in a common plane normal to axis A-A, both detector and source facing the common axis. In addition, the source 22 and/or detector 24 could be made to track the local surface of the shaft at a selected distance. If both are fixed to the carriage 20, a radially-acting tracking arrangement for source 22, e.g. an optical laser interferrometric arrangement, could be coupled to means for adjusting the position of carriage 20 with respect to axis A-A. The detector could be provided with a separate radially-acting arrangement. The converse equally applies as regards the source 22. The magnetic source 22 has been described as using a permanent magnet source. The source could use an electromagnet.

The sensor device may be a sensor arrangement of more than one device. In the configuration shown it is oriented to detect an axially-directed component of the emanated field. The sensor device may be a Hall effect device, magnetoresistive device or a saturating core inductor device. The latter type of device can be incorporated with a signal conditioning circuit to act as circuit 28, the circuit being of the kind described in published PCT application WO98/52063.

While the invention has been described in relation to investigating a magnetic property or characteristic associated with a ferromagnetic shaft, it will be understood that the above teaching in accordance with the invention may be applied with other parts or structures in which a magnetic-based characterisation is desired about an axis through the part or structure. The characterisation may be performed through less than a full rotation if desired.

The techniques described above can be applied to the measurement of the depth (i.e. radial depth) of case-hardening of a ferromagnetic shaft or to measuring the depth of some other surface-adjacent zone of a shaft which has a different magnetic property to the underlying material of the shaft. Such measurement could also be applied to measure the depth of a magnetic coating or layer applied to the exterior surface of a shaft.

Referring again to Fig. 1 , mention has already been made of the extent of penetration of the excitation field generated by source 22 into the shaft with respect to the depth of the surface-adjacent zone 10a such as a case- hardened zone. Typically for a 15-18 mm diameter shaft of high performance steel, the depth of case-hardening is about 1 mm. The depth of field penetration should be sufficient to magnetise, at least temporarily, that depth of material. The magnetisation may extend deeper, say 1.5 mm. The foregoing description has referred to detecting irregularities etc. in what would be an otherwise uniform surface zone 10a. One aspect of uniformity of a case-hardened shaft is the depth of hardening, the hardened zone having a different magnetic property to the underlying material. The apparatus described above may be employed to scan the shaft to produce an output signal that represents the depth of hardening. To this end the locally magnetised portion of the shaft should extend through the depth of the hardened zone, but not excessively so, in order to produce an output Vo which reflects the depth of the hardened zone. Thus variation seen in the output curve, as in Fig. 2b, can be correlated with depth variations in signal analyser unit 30. A constant output Vo would represent a constant depth of hardening around the shaft (and the absence of any irregularity etc.) and the output voltage Vo is a measure of that depth.

It has been found that the depth of the case-hardened annular zone can be investigated with a magnetic source 22 which is intended to create an annulus of stored or permanent magnetisation in a region of shaft 10, the region then acting as a transducer element emanating a torque-dependent magnetic field. A magnetisation procedure to this end is described in WO01/79801 mentioned above. Particular reference may be made to Figs. 5 to 6c which illustrates a procedure in which a powerful magnetising source (bearing reference numeral 30 in WO01/79801) is brought from a distance to a position closely adjacent the shaft surface, at which position the annulus of remanent magnetisation is induced in the shaft, and then withdrawn from the shaft. It has been found that if an axially-oriented sensor, such as 24 in the present Fig. 1 , is placed on the opposite side of the shaft 10, then as the magnetic source approaches the shaft several millimetres away, the sensor begins to register a magnetic field and at a closer distance of a few millimetres the sensor will provide an output which is a function of depth of case-hardening. Another method of detecting the depth of case hardening is to employ a modification of the apparatus and its method of use disclosed in PCT application PCT/EP02/00784 filed 24^th January, 2002 and published under the number WO02/059555, the disclosure of which is incorporated herein by reference. Fig. 3 corresponds to Fig. 1 of WO02/059555.

In Fig. 3 a magnetic source 40 comprises a magnetic structure 42 of a generally U-shaped configuration having a pair of spaced poles 44a, 44b which are disposed to produce a longitudinal (axially-directed) field along a surface zone portion 32 of a shaft 10' rotatable about axis A-A. A component of the field external to the shaft is detected by magnetic field sensor 60 located in line between the poles (see also Fig. 2 of WO02/059555). While the unit in WO02/059555 is particularly described as a portable or hand-held unit for measuring the torque in a rotating shaft, for the purposes of the practice of the present invention, the unit can be fixedly mounted in the manner of source 22 described above. Also the ensurance of magnetic contact between the source 40 and the shaft is eased if the shaft is rotating relatively slowly.

In Fig. 3, the magnetic source 40 may be magnetically energised by a permanent magnet included within structure 42 to provide a direct (D.C. type) magnetic field or it may be energised by a current-carrying coil 46 enabling a direct or alternating magnetic field to be employed dependent on the energising current I being D.C. or A.C. respectively.

In WO02/059555, the sensor 60 has a sensing device oriented normally to the longitudinal field in a tangential or circumferential direction. This device provides a torque-dependent output. Also described is the provision of a separate longitudinally or axially-oriented sensing device in sensor unit 60 to provide a reference output representing the longitudinal field. For the present purposes the shaft 10' is taken to be case-hardened (the hardened zone is not indicated). The sensor unit 60 comprises an axially-oriented sensing device. It is found that the longitudinal field will be a function of the depth of hardening of the shaft 10', and consequently the output of the sensor device detecting the axially-directed flux component will be a function of the depth of hardening. Thus the shaft can be scanned as previously described by rotating the shaft and the unit 40 axially-scanned along the shaft. It will be recognised that irregularities or inhomogeneities may also be detected by this means, including in non-hardened shafts.

|f the longitudinal field between poles 44a and 44b is of the direct type, it is desirable to de-gauss the shaft 10' beforehand. Degaussing procedures, also referred to as magnetic cleansing are also described in the aforementioned WO01/79801. If the applied field is alternating, prior degaussing will probably not be required. A.C. energisation provides another variable for determining the depth of penetration of the field into the shaft. The depth of penetration varies inversely with the A.C. frequency. For satisfactory penetration into steel shafts, a frequency of say 20 or even 10 Hz may be desirable.

It will be understood that while the foregoing procedures have been described in relation to measuring the depth of case-hardening of a shaft, they are applicable to measuring a surface-adjacent annular zone of a shaft or other part which has a magnetic property or characteristic different to that of the underlying material.

Claims

1. A method for determining the distribution of a magnetic property about an axis through a part, comprising

effecting relative rotation about said axis between said part and a combination of a magnetising source and magnetic field sensor device,

disposing said magnetising source adjacent said part to magnetise a local surface adjacent zone of said part as relative rotation progresses,

the local surface adjacent zone being magnetised to emanate a magnetic field externally of said part at a locafion angularly displaced from the magnetising source, and

disposing said magnetic field sensor device adjacent said part at said location to provide a signal dependent on the magnetic field detected by said sensor device.

2. A method as claimed in Claim 1 further comprising effecting a relative movement in the direction of said axis between said part and said combination.

3. A method as claimed in Claim 2 in which said relative movement in the direction of said axis is effected in a step-wise fashion.

4. A method as claimed in Claim 3 in which a rotation of at least 360° of the part relative to said combination is performed for each step of relative axial movement.

5. A method as claimed in Claim 2 in which said relative movement in the direction of said axis is continuous during said relative rotation.

6. A method as claimed in any one of Claims 1 to 5 in which said magnetising source and said sensor device are disposed 180° apart about said axis.

7. A method as claimed in any one of Claims 1 to 6 further comprising sensing the distance between at least one of said magnetising source and said sensor device and the surface of said part adjacent thereto, and maintaining the sensed distance at a predetermined value.

8. A method as claimed in any preceding claim in which said part comprises a surface-adjacent portion annular about said axis and having a magnetic-property different to that of the underlying material of said part, the depth of magnetisation being selected to provide said signal as a function of the depth of said surface adjacent portion.

9. Apparatus for determining the distribution of a magnetic property about an axis through a part, comprising:

a magnetising source,

a magnetic field sensor device,

first means for mounting the combination of said magnetising source and said sensor device

second means for mounting the part

said first and second means enabling a relative rotation to be effected between said part and said combination about an axis through said part

said magnetising source being mounted to said first means for disposition adjacent a local surface zone of said part to magnetise same as relative rotation progresses, said sensor device being mounted to said first means for disposition adjacent a local surface zone of said part to detect a magnetic field emanated thereby and provide a signal dependent on the detected field.

10. Apparatus as claimed in Claim 9 in which said first and second means are relatively movable in the direction of said axis to effect a relative movement in the axial direction between said part and said combination.

11. Apparatus as claimed in Claim 10 in which said first means is movable axially and said second means is operable to rotate said part.

12. Apparatus as claimed in Claim 9, 10 or 11 in which distance sensing means is mounted to said first part to sense the distance between at least one of said magnetising source and said sensor device and the local surface of said part.

13. Apparatus as claimed in Claim 9, 10, 11 or 12 in which at least one of said magnetising source and said sensor device is mounted to said first means so as to be radially adjustable with respect to said axis.

14. Apparatus for use in determining the distribution of a magnetic property in a part, comprising:

a carriage controllably movable in a predetermined direction,

a magnetising source and a magnetic field sensor mounted to the carriage,

said magnetising source and sensor being mounted to act in a common plane normal to said predetermined direction and each facing a common axis lying in said predetermined direction for providing a magnetising field to act on and for sensing a magnetic field emanated by, respectively, different portions of a part disposed to have said common axis pass therethrough.

15. A method for determining a magnetic property of a surface-adjacent zone of a part, which zone is annular about an axis and which has a magnetic property different to that of the underlying material of the part, the method comprising:

placing a magnetic structure having two magnetic poles adjacent a portion of the surface of said part, said two magnetic poles being spaced apart,

magnetically energising said structure to give said two magnetic poles opposite polarity so as to establish a magnetic field therebetween,

disposing a magnetic field sensor device between said poles to respond to a component of field extending externally to the part surface in the direction between said two magnetic poles; and

selecting the strength of the magnetic field between said poles such that the sensor device provides an output which is a function of the local magnetic property of said surface-adjacent zone between said two magnetic poles.

16. A method as claimed in Claim 15 in which said magnetic structure is placed with said two magnetic poles aligned in an axial-direction.

17. A method as claimed in Claim 16 in which said magnetic structure is U- shaped.

18. A method as claimed in Claim 15, 16 or 17 in which said part is rotated relative to said poles to scan at least part of said surface-adjacent zone angularly about said axis.

19. A method as claimed in Claim 15, 16, 17 or 18 in which said part is moved axially relative to said poles over at least part of said surface-adjacent zone in an axial direction.

20. A method as claimed in any one of Claims 15 to 19 in which said magnetic structure is magnetically energised by a permanent magnet.

21. A method as claimed in any one of Claims 15 to 19 in which said magnetic structure is magnetically energised by a current-carrying coil.

22. A method as claimed in Claim 21 in which said coil is energised by an alternating current.

23. A method as claimed in any one of Claims 15 to 22 in which said magnetic property relates to the depth of a surface adjacent zone of the shaft.

24. The use of the method as claimed in any one of Claims 15 to 23 to determine the depth of the case-hardened surface-adjacent zone of a case- hardened ferromagnetic shaft.