A METHOD AND AN APPARATUS FOR DETECTING MAGNETIC DISCONTINUITIES IN A TEST SAMPLE OF MAGNETIC MATERIAL

Abstract: A method of detecting magnetic discontinuities in a test sample of magnetic material includes the steps of generating a magnetic field in said test sample, placing one or more magnetic sensors adjacent said test sample for the detection of magnetic leakage flux, if any, adjacent the surface of the test sample, and generating electric signals representing the leakage flux, if any, said sensors being moved across the surface of the test sample. Each sensor (7) carries out a scanning movement substantially transverse to the advancing direction during the advancing movement. In this manner it is sufficient to observe whether the output signal of the sensor discloses sudden changes during a scanning. As a result, the associated electronics can be simplified.

Description: Title: A method and an apparatus for detecting magnetic discontinuities in a test sample of magnetic material Technical Field The invention relates to a method of detecting magnetic discontinuities in a test sample, whereby a magnetic field is generated in said test sample and one or more magnetic sensors are placed adjacent said test sample for detection of a magnetic leakage flux, if any, and generation of an electric signal representing the leakage flux, if any, said sensors being moved across the surface of the test sample. Background Art US-PS No. 4,814,705 discloses an apparatus for detection of magnetic discontinuities in an iron sheet. The apparatus comprises a vehicle to be moved across the iron sheet. The vehicle supports an elongated horse¬ shoe magnet the pole shoe of which faces the iron sheet. Without dis- continuities in the iron sheet, almost the entire magnetic field is transmitted through the sheet. A number of magnetic field sensors are arranged between the pole shoes, said sensors being able to detect a leakage flux caused by discontinuities in the iron plate, said leakage flux being transmitted beside the iron sheet as a consequence of the discon- tinuity. The signal of each sensor is subtracted from the mean value of a following and a succeeding sensor. If the difference exceeds a prede¬ termined threshold value, that indicates the presence of a leakage flux deriving from a discontinuity, which, however, would require a special calculating equipment. In addition, the detection perpendicular to the advancing direction of the apparatus can only be performed discon- tinuously. Description of the Invention The object of the invention is to provide a method of the above type, whereby the processing of the signal has been simplified and does not depend on the advancing speed of the sensors. In addition, the method allows an improved scanning transverse to the advancing direction. The method according to the invention is characterised in that each sensor carries out a scanning movement substantially transverse to the advancing direction during said advancing movement. In this manner it is sufficient to observe whether the output signal of the sensor discloses sudden changes during a scanning. Accordingly, the associated elec- tronic equipment can be considerably simplified. In addition, the measur¬ ing does not depend on the advancing speed, as well as a continuous scanning transverse to the advancing direction is rendered possible, i.e. a highly improved scanning. Initially an area of the test sample without errors can be scanned and in order to generate a calibration curve for a correction of succeeding measurements. As a result, data is not only provided concerning the presence of discontinuities in the test sample therebelow, but also con¬ cerning the size of the discontinuity. Furthermore according to the invention each sensor may carry out a difference measurement or a differential measurement, whereby the constant portion of the field has no influence on the measurement. As a result it is possible to achieve a very high sensitivity. According to the invention, a sensor may be used for carrying out the difference measurement, said sensor comprising two juxtaposed mag- netic field detectors, the difference between the signals from the two detectors being used. In addition according to the invention, the scanning movement of each sensor may be provided by mounting said sensor on a body placed on a rotatable shaft with a helical groove, a ratchet extending from the body and downwards into said groove. The resulting scanning movement is carried out in a particularly simple manner. Furthermore according to the invention the scanning movement of each sensor may be provided by the rotatable shaft being provided with helical grooves in both directions in such a manner that a reciprocating movement can be carried out by the body on which the sensor is mounted. The scanning movement may according to the invention alternatively be provided by the sensor being mounted on a chain with a substantially transverse run, said chain being driven by means of one or more sprockets. According to the invention, the chain may comprise an upper run and a lower run, the scanning being performed at the lower run. The invention relates furthermore to an apparatus for detecting magnetic discontinuities in a test sample of magnetic material, said apparatus being moved across said test sample and thereby generating a magnetic field in said test sample, where the apparatus comprises a number of sensors for the detection of magnetic leakage flux adjacent the surface of the test sample and for generating electric signals representing said leakage flux. The apparatus is according to the invention characterised by each sensor being displaceable substantially transverse to the ad¬ vancing direction of the apparatus. The resulting apparatus is more simple than previously known apparatuses, and in addition it is possible to carry out a continuous scanning transverse to the advancing direction of the apparatus. Moreover according to the invention, each sensor may be mounted on a body placed on a rotatable shaft with a helical groove, means being provided for ensuring that said body does not rotate about the shaft, and a ratchet may extend from the body and downwards into the groove, whereby the scanning movement can be provided in a particu¬ larly simple manner. Each sensor may according to the invention alternatively be mounted on a chain driven by means of one or more sprockets, said chain preferably comprising an upper run and a lower run, the scanning being performed at the lower run. In this manner a very fast scanning is allowed. Brief Description of the Drawing The invention is described in greater detail below with reference to the accompanying drawings, in which Fig. 1 illustrates an apparatus according to the invention with a scanning sensor for the detection of magnetic discontinuities in a test sample of magnetic material, Fig. 2 illustrates an apparatus with an alternative embodiment of a scanning sensor. Fig. 3 is a side view of the apparatus of Fig. 2, Fig. 4 illustrates an alternative embodiment of the apparatus whereby the sensors are permanently arranged and activated one by one, and Fig. 5 illustrates an arrangement of several interconnected apparatuses. Best Mode for Carrying Out the Invention The apparatus of Fig. 1 for the detection of magnetic discontinuities in a test sample therebelow of magnetic material comprises an elongated horseshoe electromagnet generating a magnetic field in the test sample. The elongated pole shoes 1 of the electromagnet are maintained at a predetermined distance from the test sample by means of four wheels 104, cf. Fig. 3. A plurality of permanent magnets 1 10 are arranged between the pole shoes 1. One or more coils can alternatively be wound on the portion of the core connecting the poles shoes 1 , said coils being fed with an AC or a DC current. One or more magnetic field sensors in form of Hall units are arranged between the elongated pole shoes 1 and are advanced in the direction of the arrow. During the advancing move¬ ment of the pole shoes 1 in the direction of the arrow, the Hall units carry out a scanning movement transverse to the advancing direction. The scanning movement is provided by each sensor being mounted on a body 2 placed on a rotatable shaft 3, which is caused to rotate by means of an electromotor 4. The rotatable shaft 3 is provided with a helical groove 5, a ratchet extending from the body 2 and downwards into said groove 5. Some guide rails 6 are arranged parallel to the shaft 3 and abut the body 2. These guide rails 6 ensure that the body 2 does not follow the rotating movement of the shaft 3. During the rotation of the shaft 3, the body 2 is displaced from one end of the shaft 3 to the opposite end. As the helical groove 5 at the end of the shaft 3 continues into an oppositely directed helical groove and as the ratchet extending into the groove is pivotally arranged in the body 2, said body can carry out a reciprocating movement during the rotation of the shaft 3. The sensor mounted on the body 2 can detect changes, if any, in the scanned magnetic field during the above reciprocating movement. Such changes can for instance be caused by leakage flux deriving from dis¬ continuities in the test sample therebelow. The scanning speed is in this embodiment limited to approximately 10 scannings/sec, which, how¬ ever, can be increased by arranging several similar scanning devices successively in the advancing direction of the apparatus. It is also poss- ible to arrange several juxtaposed scanning devices transverse to the advancing direction of the apparatus. An alternative embodiment is shown in Fig. 2, and here the scanning movement of each sensor is provided by one or more sensors 7 being mounted on a chain 8 of a sprocket chain. The sprocket chain is pro¬ vided by means of two sprockets 9, 10 and comprises an upper run and a lower run. When the sensor 7 is positioned in the lower run, it is in direct contact with the test sample to be examined. After having been suitably amplified, the signal from the sensor 7 is transmitted to an oscilloscope indicating changes, if any, in the received signal during a scanning. In the latter case, the scanning can be performed very fast. When two sensors 7 are mounted on the same chain 8, cf. Fig. 2, said two sensors 7 can perform a scanning alternately, which means that the scanning speed is doubled. Furthermore, areas are no longer missed during the advancing of the apparatus in the direction of the arrow. Fig. 3 is a side view of the apparatus with the sprockets 104 and the perma¬ nent magnets 110 interconnecting the pole shoes 1. As illustrated in this Figure, it is possible to successively arrange several scanning devices in the direction of the arrow with the result that an even faster scanning speed is rendered .possible. The sprockets at one end can be connected to a common driving shaft. The sensors 7 may alternatively scan at varying speeds. Like the embodiment of Fig. 1 , the signal from each sensor 7 is amplified and transmitted to an oscilloscope indicating changes, if any, in the received signal. The associated electronic equip- ment is therefore relatively simple. An additional advantage of this em¬ bodiment is that the total apparatus can even be static during a mea¬ surement. The above chains can then be replaced by wire cables. According to an alternative embodiment of the apparatus according to the invention, the scanning sensor has been replaced by a number of stationary sensors activated one by one, cf. Fig. 4. Each sensor is formed by a U-shaped electromagnet 12, the pole shoes of which extend downwards towards the test sample to be examined. The yoke of the U-shaped electromagnet 12 is provided with a notch 13. The notch 13 implies that the sensor is not active. The notch 13 can, how- ever, be filled with a body 14 of magnetically conductive material, said body 14 extending from one sensor to the next sensor by means of a wire cable 15 of non-magnetic material. The wire cable 15 at the end of the run extends about a roller 16 and 17, respectively, the body 14 secured to the wire cable 15 being returned via a return run after having passed said roller 16. Each apparatus 100 is preferably supported by four sprockets 104. Several apparatuses can, however, be coupled in an arrangement as illustrated in Fig. 5. This arrangement comprises a vehicle with some driving wheels 103 and a supporting wheel simultaneously driving an encoder providing information on the position. A modular frame system 106 on both sides of the vehicle ensures that unimpeded by the rest of the arrangement each apparatus can maintain its distance to the surface of the test sample. The distance to the surface of the test sample is determined by the position of the sprockets 104 on each apparatus. This arrangement renders it possible to scan a relatively wide path of the test sample, even when the surface of said test sample is not completely plane. A boom 107 can be secured to the rearmost portion of the arrangement, a number of colour-spraying units 108 being arranged on said boom. A computer ensures that dye is released proportional to and time-delayed relative to the found maximum or minimum exceedings of a material thickness of the test sample encoded in advance. The apparatus according to the invention can, if desired, be lowered in a tank filled with liquid. Then the apparatus must be completely encap- sulated. The position of the apparatus is then indicated relative to the lowering location. The advancing direction is maintained by means of a built-in gyroscope, whereby it is not necessary to empty the tank before a measuring.

Claims: Claims 1. A method of detecting magnetic discontinuities in a test sample, whereby a magnetic field is generated in said test sample and one or more magnetic sensors are placed adjacent said test sample for detec- tion of a magnetic leakage flux, if any, and generation of an electric signal representing the leakage flux, if any, said sensors being moved across the surface of the test sample, c h a racte ris ed in that each sensor (7) carries out a scanning movement substantially trans¬ verse to the advancing direction during said advancing movement. 2. A method as claimed in claim 1, c h a racte r i s e d in that each sensor (7) initially scans an area of the test sample without errors and generates a calibration curve for a correction of succeeding meas¬ urements. 3. A method as claimed in claim 1, c h a ra ct e r i s e d in that each sensor (7) carries out a difference measurement or a differential measurement. 4. A method as claimed in claim 3, cha racte rised by using a sensor for carrying out the difference measurement, said sensor com¬ prising two juxtaposed magnetic field detectors, the difference between the signals from the two detectors being used. 5. A method as claimed in claim 1, characterised by provid¬ ing the scanning movement of each sensor (7) by mounting said sensor on a body (2) placed on a rotatable shaft (3) with a helical groove (5), a ratchet extending from the body (2) and downwards into said groove (5). 6. A method as claimed in claim 5, characterised by provid- ing the scanning movement of each sensor (7) by the rotatable shaft (3) being provided with helical grooves (5) in both directions in such a man¬ ner that a reciprocating movement can be carried out by the body (2) on which the sensor (7) is mounted. 7. A method as claimed in claim 1, characterised by provid¬ ing the scanning movement of each sensor (7) by said sensor (7) being mounted on a chain (8) with a substantially transverse run, said chain (8) being driven by means of one or more sprockets (9, 10). 8. A method as claimed in claim 7, characterised in that the chain (8) comprises an upper run and a lower run, the scanning being performed at the lower run. 9. A method as claimed in claim 7 or 8, ch a ra cte ris e d in that several sensors are mounted on the same chain (8) in such a man¬ ner that one of said sensors (7) is always positioned at the lower run. 10. A method as claimed in claim 1 , characterised by replac¬ ing each scanning sensor by a number of stationary sensors (12) being activated one by one (Fig.4). 11. A method as claimed in claim 0, characterised by each sensor (12) in the number of stationary sensors being formed by a yoke of magnetically conductive material with a notch (13), which can be filled by means of a body (14) of magnetically conductive material, said body (14) being dϊsplaceable from sensor to sensor (12). 12. An apparatus for detecting magnetic discontinuities in a test sample of magnetic material, said apparatus being moved across said test sample and thereby generating a magnetic field in said test sample, where the apparatus comprises a number of sensors for the detection of magnetic leakage flux adjacent the surface of the test sample and for generating electric signals representing said leakage flux, cha r¬ a c t e r i s e d by each sensor (7) being displaceable substantially transverse to the advancing direction of the apparatus. 13. An apparatus as claimed in claim 12, c h a ra ct e ri s e d in that each sensor (7) is mounted on a body (2) placed on a rotatable shaft (3) with a helical groove (5), means being provided for ensuring that said body does not rotate about the shaft, and that a ratchet extends from the body (2) and downwards into the groove (5). 14. An apparatus as claimed in claim 12, c h a ra cte ris e d by each sensor (7) being mounted on a chain (8) driven by means of one or more sprockets (9, 10). 15. An apparatus as claimed in claim 14, c h a ra ct e r i s e d in that the chain (8) comprises an upper run and a lower run, the scanning being performed at the lower run. AMENDED CLAIMS [received by the International Bureau on 27 July 1993 (27.07.93); original claims 2-4 cancelled; original claim 1 amended; claims 5-15 renumbered as claims 2-12 (3 pages)] 1. A method of detecting magnetic discontinuities in a test sample, whereby a magnetic field is generated in said test sample and one or more magnetic sensors are placed adjacent said test sample for detec- tion of a magnetic leakage flux, if any, and generation of an electric signal representing the leakage flux, if any, said sensors being moved across the surface of the test sample, each sensor (7) carrying out a scanning movement, characterised in that the scanning move¬ ment is performed substantially along a line substantially transverse to the advancing direction during said advancing movement. 2. A method as claimed in claim 1, characterised by provid¬ ing the scanning movement of each sensor (7) by mounting said sensor on a body (2) placed on a rotatable shaft (3) with a helical groove (5), a ratchet extending from the body (2) and downwards into said groove (5). 3. A method as claimed in claim 2, characterised by provid¬ ing the scanning movement of each sensor (7) by the rotatable shaft (3) being provided with helical grooves (5) in both directions in such a man¬ ner that a reciprocating movement can be carried out by the body (2) on which the sensor (7) is mounted. 4. A method as claimed in claim 1, characterised by provid¬ ing the scanning movement of each sensor (7) by said sensor (7) being mounted on a chain (8) with a substantially transverse run, said chain (8) being driven by means of one or more sprockets (9, 10). 5. A method as claimed in claim 4, characterised in that the chain (8) comprises an upper run and a lower run, the scanning being performed at the lower run. 6. A method as claimed in claim 4 or 5, characterised in that several sensors are mounted on the same chain (8) in such a man¬ ner that one of said sensors (7) is always positioned at the lower run. 7. A method as claimed in claim 1, characterised by replac- ing each scanning sensor by a number of stationary sensors (12) being activated one by one (Fig.4). 8. A method as claimed in claim 7, characterised by each sensor (12) in the number of stationary sensors being formed by a yoke of magnetically conductive material with a notch (13), which can be filled by means of a body (14) of magnetically conductive material, said body (14) being displaceable from sensor to sensor (12). 9. An apparatus for detecting magnetic discontinuities in a test sample of magnetic material, said apparatus being moved across said test sample and thereby generating a magnetic field in said test sample, where the apparatus comprises a number of sensors for the detection of magnetic leakage flux adjacent the surface of the test sample and for generating electric signals representing said leakage flux, char¬ acterised by each sensor (7) being displaceable along a line sub¬ stantially transverse to the advancing direction of the apparatus. 10. An apparatus as claimed in claim 9, characterised in that each sensor (7) is mounted on a body (2) placed on a rotatable shaft (3) with a helical groove (5), means being provided for ensuring that said body does not rotate about the shaft, and that a ratchet extends from the body (2) and downwards into the groove (5). 11. An apparatus as claimed in claim 9, cha racte rised by each sensor (7) being mounted on a chain (8) driven by means of one or more sprockets (9, 10). 12. An apparatus as claimed in claim 11, c h a racte ri s ed in that the chain (8) comprises an upper run and a lower run, the scanning being performed at the lower run.

Inventors: NORTHEVED ALLAN, OLSEN HANS OLE, NORTHEVED, ALLAN, OLSEN, HANS, OLE
Applicants: FORCE INST, FORCE INSTITUTTERNE
Application year: 1993
Main classification: G01N27
IPC codes: G01N27