WO1996031767A1

WO1996031767A1 - Apparatus for automatic magnetic particle inspection

Info

Publication number: WO1996031767A1
Application number: PCT/SE1996/000444
Authority: WO
Inventors: Karl-Gunnar Bergstrand
Original assignee: Contest Marketing Ab
Priority date: 1995-04-05
Filing date: 1996-04-04
Publication date: 1996-10-10
Also published as: SE9501243L; AU5294196A; SE9501243D0; EP0819245A1

Abstract

A device for automatic magnetic powder testing of test samples (2) on which a ferromagnetic powder is applied after magnetization. If fissures are present, the powder will cumulate around the edges of said fissures. The device comprises a light source (16) having a light path (12) directed towards the sample, thereby producing a light spot on its surface, which light spot is moved in accordance with a scanning procedure. A light-detection unit (18) picks up the light which is reflected when the light spot strikes the surface of the sample. This reflected light follows the same path as the ray of light towards the sample. A light-reflecting element (14) is provided with a light opening (15) which, by adaptation of its size, lets through concentrated light which is reflected directly from the surface of the sample, whereas light which is generated due to the fact that the ray of light incides upon an accumulation of magnetic powder is spread outside the light opening and reaches the light-detection device due to deflection by means of the reflecting element.

Description

TITLE:

Apparatus for automatic magnetic particle inspection

TECHNICAL FIELD:

The invention relates to a device for automatic testing of magnetic powder and including a device for automatic inspection of test samples to which magnetic powder has been applied.

BACKGROUND OF THE INVENTION: Magnetic powder testing is a known method for examining whether fissures are present in test samples; in particular, fissures may occur in cast, hammered or rolled blanks and workpieces . Necessary is only that the sample is of ferromagnetic material. The sample is exposed to a magnetic field which is suitable for the test conditions, after which a fine iron powder is applied on its surface. If any fissure is present in the surface or close thereto, the powder is attracted by the magnetic field which forms lines of force at the edges of the fissure, which attract the powder. In order to discover the magnetic powder lines more easily, it is common to treat the powder with a substance which makes the powder fluorescent when illuminated with ultraviolet light.

Previously, it has been common practice that the samples treated in said manner were inspected by an operator who could mark, or in some other way distinguish, the samples which were proven to have any fissure indicated by means of the powder. In this regard, the operator works in a dark room with ultraviolet illumination of the samples. Work of this kind is very monotonuous and strenuous, and experience has shown that there is a risk that the operator does not discover some faults, in particular during long working hours .

For this reason, attempts have been made to provide devices for automatic inspection. For example, one such device is known from US-A-5003831 (Link) . In this device, a video camera and a digital image analysis processor is used. In order to distinguish signal parts from the image signal, which signal parts represent collections of magnetic powder inside the contour of the workpiece, a pattern is projected on its surface. This provides a regular frequency of the image signal. If this frequency is disturbed by any impulse from a collection of magnetic powder breaking the pattern, this disturbance can be picked up relatively easily by the processor.

From US, A, 4 428 672 (Allard et al) a device is known by means of which the surface of the workpiece is scanned by a concentrated light ray. The light which is reflected and scattered by the workpiece is picked up by means of a number of photoelectric sensors and the detected signal is analyzed. In this regard, for each section of the surface of the workpiece the signal is compared with a corresponding signal for surrounding surface sections. If differences occur in the signal from a particular section in relation to the average number of the signals from surrounding sections, this is taken as an indication that a fissure is present and causes a deviation from the average level of the reflected light.

However, it has been found that both these methods for performing automatic inspection have implied a great risk of incorrect readings and a risk that not all fissures are discovered. Furthermore, these methods for inspection of workpieces cannot be applied in workpieces having a more complicated form and, in particular, if cavities and holes are present.

A significant difficulty during automatic inspection in magnetic powder testing is to avoid incorrect indications due to varying surface properties of the sample. For example, smooth portions may occur as well as sections having a rough surface. In certain samples, smooth and machined surfaces, for example in the form of polishing marks after removal of cast material may occur. Such distinctions create a varying character of the reflected light. However, this should not lead to any detection of errors since discrimination of the samples due to varying surface properties should not occur. Such discrimination should only occur when fissures and fissure-like formations are present.

SUMMARY OF THE INVENTION:

The object of the invention is to provide a device for automatic magnetic powder testing, including inspection of the workpiece, by means of which testing can be performed automatically, even for samples having a complicated shape.

In the device, scanning of the surfaces of the sample by means of a concentrated ray of light is utilized. The light which is reflected from the surface of the sample is picked up in the same path as the path of the light ray inciding on the surface. In this regard, parallax errors can be avoided. Furthermore, irrespective of how the section of the surface is located and is positioned in relation to surrounding sections, a light indication will be obtained.

The indication corresponds in a totally adequate manner to the state of the illuminated section, thus irrespective of whether any magnetic powder is applied to the surface or not. An object of the invention is also to obtain as high safety as possible in the automatic inspection and to detect only fissures and similar formations as errors in the sample, whereas surface differences which are not supposed to give rise to any special treatment of the sample are not detected.

This is accomplished in accordance with the invention due to the fact that only the light which strikes a collection of magnetic powder ..and consequently is given a scattered reflection is allowed to reach the light-sensitive means which detects the light conditions which should lead to an error indication. To this end, a mirror provided with a light opening having specially adapted dimensions is used.

BRIEF DESCRIPTION OF THE DRAWINGS:

An embodiment of the invention will now be described with reference to the annexed drawing.

PREFERRED EMBODIMENT:

The device according to the invention is shown schematically as a central cross-section in the drawing. According to the drawing, the device comprises a support 1 for test samples 2. The support is provided with means (not shown) for holding the samples which are intended to be treated in the device. The support forms part of an arrangement with several stations and is moved together with the sample between these stations. The shown station is the one in which the invention is applied, i.e. the inspection station.

This station comprises a light-proof cover 3 in which the support 1 can be inserted. The inside of the cover 3 is provided with a black surface. The cover is provided with a jacket 4 and a base 5. A central hole 6 is formed in the base, around which hole a tubular support 7 is arranged. Opposite the hole 6 and inside the cover 3, a mirror 8 is arranged at an angle of 45°, which mirror faces a mirror 9 on the jacket 4 adjacent to the base 5. The latter mirror is also arranged at an angle of 45°. Another mirror 10 is mounted on the inside of the jacket at a distance from the base 5. This mirror is also arranged at an angle of 45° and the mirrors are so arranged that a ray of light passing through the opening 6 towards the mirror 8 is directed towards the mirror 9, and from said mirror 9 and the mirror 10 towards the sample 2 which is centrally positioned inside the jacket 4. The path which the ray of light follows is indicated by a chain line 12.

Outside the cover 3, facing the hole 6 and centrally arranged with respect to the line 12, yet another mirror 14 is arranged, which mirror is arranged at an angle of 45° in relation to the line 12 and with a light opening 15 in the centre of the mirror, centered in relation to the path 12 of the ray. A light source 16 is arranged in the path 12 outside the mirror 14, which light source is arranged so as to transmit a concentrated ray of light in the direction of the path 12 and through the opening 15 of the mirror 14.

Due to reflection in the mirrors 8, 9 and 10, this ray of light follows the path 12 and thus incides upon the sample 2. The mirrors may be arranged at angles other than 45°, but must provide a light path from the light source to the sample.

The light source 16 should emit ultraviolet light. Preferably, an ultraviolet laser is used, since its monochromatic and coherent light cluster is very suitable for this purpose.

Due to the fact that the mirror 14 is inclined in relation to the light path 12, it may direct the flow of light which is scattered outside che opening 15 at an angle in relation to the path 12. A photoelectric unit 18 is arranged so as to face this flow of light which is deflected in this manner. The flow of light is indicated with a dotted line.

The light opening 15 is accurately adapted as regards it shape and size so as to provide a more concentrated cluster of light, of the kind which is obtained by direct reflection of the ray of light along the path 12 from surfaces on the sample which are so smooth that they provide direct reflection, which does not reach outside the edges of the light opening. The light which is reflected from the collections of magnetic powder due to their contents of fluorescent material generates a more scattered light, which to some extent reaches outside the edges of the light opening 15 which is adapted as regards its size. In this manner, direct, reflected and concentrated light from the ray of light reflected by the mirror 14 and further to the photoelectric unit 18 can be avoided. Where magnetic powder collections occur and the light is reflected in a scattered manner, it will be reflected by the mirror 14 from its reflecting surface outside the edges of the light opening 15.

The cover 3 is arranged in a support portion 20 via its tubular bearing piece 7. The support portion 20 is arranged with a drive device which is adapted to provide rotation of the cover 3. The support is also arranged on a guide and can be provided with a reciprocating movement parallel to the longitudinal axis of the sample 2 by means of another drive device.

These two motions imply that the mirror 10 can be rotated around the sample and can also move along the sample. The ray of light which originates from the light source 16 along the path 12 forms a concentrated light spot on the surface of the sample. Due to said motion, this light spot can be made to scan the surface of the sample according to a helix. Each point along this helix is represented by a predetermined angular position as regards the rotational movement of the cover and a predetermined coordinate as regards the longitudinal movement of the cover.

The light source 16 is connected to a current supply. The photoelectric unit 18 is connected to a detection device which detects when light incides towards the unit 18. Preferably, the detection device is also adapted to detect the instantaneous angular position as regards the rotation of the cover and its instantaneous length coordinate, by means of sensors which are connected to the detection device.

As already mentioned, the arrangement of which the above- described and shown apparatus forms a part consists of a number of stations between which the samples are moved. The main stations for performing the operating steps of the magnetic powder testing are: a station for magnetization of the respective ferromagnetic samples, a station for applying ferromagnetic powder to which is added a component which is fluorescent in ultraviolet light, a station for rinsing the sample so as to discard magnetic powder which is not held due to the the magnetic force, after which a station with the above-mentioned inspection apparatus is arranged. Service stations, for example for initial cleaning and final washing of magnetic powder, may also be present. Furthermore, a loading and removal station for the samples, preferably provided with robots must be provided.

When the respective sample 2 , on the support 1 in the inspection station, has been inserted in the cover 3, the sample is scanned by means of the light spot from the light source 16. This is accomplished by the rotational and longitudinal movement of the cover. As long as the surface of the sample is clean, the photoelectric element 18 does not react. This is partly due to the fact that it has limited sensitivity for ultraviolet light and partly due to the fact that the light from the light spot on the clean surface of the sample is not scattered so that any substantial portion reflected light reaches the mirror 14 outside the opening 15. However, if a collection of magnetic powder occurs on the surface, its fluorescent component converts the ultraviolet light to visible light and also provides a light scattering. Due to reflection in the mirrors 10, 9, 8, this light will incide on the mirror 14, also outside of the opening 15, and it reflects the light to the photoelectric unit 18. Since a collection of magnetic powder indicates the presence of a fissure in the material of the sample, the detection device will detect that a fault is present. As mentioned above, the detection device may at the same time be arranged to detect the position on the surface of the sample where the light spot is located. By combining these two detections, a value indicating the position on the sample where a fault occurs can be stored.

The treatment of the sample if a fault has been detected depends on the further treatment intended. Certain types of samples must be discarded if fissures are discovered. In such cases, the detection device is adapted to control the removal station so that defective samples are sorted out. In cases of complete rejection, there is no reason to detect the position of the fault on the sample. However, in other cases there may be reason to post-examine the sample so as to determine if it can be adjusted so as to eliminate faults which occur, for example by removing the material where the fissure occurs or by suitable repairing. In this regard, it is important to obtain a protocol as to where the error is located. This the apparatus can thus be adapted for by means of said positional detection. Furthermore, the detection device can be arranged for selection of incoming detected data. During magnetic powder testing, lump-like collections of magnetic powder may thus also occur where fissures are not present. In order to avoid an error indication at such positions, the detection device may be adapted to distinguish the shape of magnetic powder collections so that, for example, point-shaped collections do not lead to any error indication, whereas longish collections, which are typical for fissure formations, leads to an error indication.

An important feature of the device according to the invention is that the path of the ray of light which is directed towards the sample is the same as the path which the reflected light follows from the sample until this light is deflected towards the photoelectric unit. This is accomplished by means of the mirror 14 which, due to its opening 15, is centered on said path but at the same time directed with the mirror's surface so as to deflect light outside the hole, with an angle in relation to the path, so as to convey it to the photoelectric unit. Such a deflection can also be obtained by optical means other than an inclined mirror provided with a light opening. Alternatively, a prism may for example be used in this application, as well as in other situations where the light is to be reflected. The light opening may be a hole through the mirror or the prism, or a non-reflecting area, such as an area without any mirror coating.

The essential principle of the invention is not the choice of technical components but instead the fact that the light which is reflected from the sample follows the same path as the light which forms the light spot on the surface of the sample. Due to this arrangement, it can be assured that neither parallax errors between the illumination and the light detection nor shadow formations occur. In all positions where the ray of light may reach the surface of the sample so as to form a light spot, the thus illuminated area may also provide an indication to the light-sensitive detection unit. The drawing shows the sample 2 with two flanges and a groove therebetween. If detection of the outgoing flow of light should occur at a certain angle in relation to the light flow of the illumination, the light spot between the flanges could be shadowed in certain positions of the light spot. In this manner, an error may remain unnoticed. It is particularly important to avoid this situation when using samples having a complicated shape, for example samples including holes or protruding portions.

In the arrangement for obtaining this function, there is a risk that light from the ray of light which incides towards the sample is reflected back along the same path if the sample, in any part, has a surface portion with high reflectivity. Consequently, the other described function in said arrangement is also important, i.e. the fact that such directly reflected light is separated from the light which is scattered due to occuring magnetic powder collections, which is the only light which is intended to provide an indication of an error.

Claims

CLAIMS :

1. Device for automatic magnetic powder testing of samples (2) of ferromagnetic material on which a ferromagnetic powder is applied after magnetization, said powder cumulating around the edges of fissures, if present, the device being adapted for automatic detection of whether such collections occur and the device comprising at least one light source (16) adapted to emit a concentrated ray of light towards the sample (2), and at least one light- detection unit (18) and a device (3, 20) for moving the sample in relation to the path of the ray of light in such a manner that the light spot from the concentrated ray of light formed on the surface of the sample is moved according to a scanning procedure along the surface of the sample which is intended for detection of the occurance of magnetic powder collections, c h a r a c t e r i z e d i n that the light-detection unit (18) is adapted and positioned to pick up light which is reflected when the light spot illuminates a section of the sample's (2) surface presenting any magnetic powder collection and thus the light which follows the same path (12) as the ray of light, but in the direction from the sample towards the light source (16), during deflection of the reflected light to the light-detection unit (18) at a position along the extension of the light path, so that the light reflected back to the light-detection unit and exiting from the illumination spot on the surface of the sample initially follows the same path as the ray of light, a light- reflecting means (14) being arranged for said deflection of the reflected light to the light-detection unit (18), said means (14) being provided with a light-permeable opening (15) through which the ray of light exiting from the light source (16) passes and which is adapted as regards its size and shape to let through, with essentially no deflection, a ray of light directly from the surface of the sample, where it has reflecting properties, which ray of light is reflected and thus essentially concentrated, whereas light which is reflected from occuring magnetic powder collection and which is scattered reaches outside the light opening (15) and, due to reflection on the reflecting surface of said means (14), is deflected to the light-detection unit (18).

2. Device according to claim 1, c h a r a c t e r i z e d i n that it comprises a support (1) for the sample, a cover (3) arranged for essentially light-proof enclosure of the sample, at least one light-reflecting means (10) arranged in the cover and connected to a transport device (20) which is adapted to move said means (10) in relation to the sample (2) in a path of movement for obtaining the scanning procedure, a light source, preferably in the form of an ultraviolet laser, having a light path (12) preferably directed towards said means (10) via further light-reflecting means (8, 9) so as to generate said light spot on the surface of the sample, and also comprising said light-detection unit (18) and said light-reflecting means (14) provided with a light opening (15) and directed towards said light-detecting unit (18).

3. A device according to claim 2, c h a r a c t e r i z e d i n that the transport device (20) is arranged to rotatingly convey said means (10) around the sample (2) during axial displacement, so that the light spot, during said scanning procedure, is conveyed in a helix along the surface of the sample, in that a further light-reflecting arrangement (8, 9) is adapted to deflect said light path (12) so as to follow the axis of rotation of the rotating movement of said means (10), so that when the light source (16) and the light-detecting unit (18) are fixed in relation to the axis of rotation, the light path (12) is maintained in a linked relationship thereto irrespective of the instantaneous angle of rotation and axial position.

AMENDED CLAIMS

[received by the International Bureau on 4 September 1996 (04.09.96); original claims 1-3 replaced by amended claims 1-4 ( 2 pages )]

(15) through which the ray of light exiting from the light source (16) passes whereas light which is reflected from occuring magnetic powder collection and which is scattered reaches outside the light opening (15) and, due to reflection on the reflecting surface of said means (14), is deflected to the light-detection unit (18).

2. Device according to claim 1, c h a r a c t e r i z e d i n that said light-permeable opening (15) of said light- reflecting means (14) as regards its size and shape is adapted to let through, with essentially no deflection, a ray of light directly from the surface of the sample, where it has reflecting properties, which ray of light is reflected and thus essentially concentrated, so that said ray of light does substantially not reach the light- detection unit (18) .

3. Device according to claim 1 or 2, c h a r a c t e r i z e d i n that it comprises a support (1) for the sample, a cover (3) arranged for essentially light-proof enclosure of the sample, at least one light- reflecting means (10) arranged in the cover and connected to a transport device (20) which is adapted to move said means (10) in relation to the sample (2) in a path of movement for obtaining the scanning procedure, a light source, preferably in the form of an ultraviolet laser, having a light path (12) preferably directed towards said means (10) via further light-reflecting means (8, 9) so as to generate said light spot on the surface of the sample, and also comprising said light-detection unit (18) and said light-reflecting means (14) provided with a light opening (15) and directed towards said light-detecting unit (18).

4. A device according to claim 3, c h a r a c t e r i z e d i n that the transport device (20) is arranged to rotatingly convey said means (10) around the sample (2) during axial displacement, so that the light spot, during said scanning procedure, is conveyed in a helix along the surface of the sample, in that a further light-reflecting arrangement (8, 9) is adapted to deflect said light path (12) so as to follow the axis of rotation of the rotating movement of said means (10), so that when the light source (16) and the light-detecting unit (18) are fixed in relation to the axis of rotation, the light path (12) is maintained in a linked relationship thereto irrespective of the instantaneous angle of rotation and axial position.

Statement under Article 19

Amendments under Article 18(1) and Rule 46 are hereby filed in the form of replacement sheets 12 and 13 which shall replace the previous sheets 12 and 13.

In the replacement sheet 12 major part of lines 2-6 is deleted from the replaced sheet 12 and forms substantially basis for a new claim 2.

The accompanying subclaims and their reference to previous claims have been renumbered.