DK169956B1

DK169956B1 - Method and apparatus for detecting magnetic discontinuities in a sample of magnetic material

Info

Publication number: DK169956B1
Application number: DK37692A
Authority: DK
Inventors: Allan Northeved; Hans Ole Olsen
Original assignee: Force Inst
Priority date: 1992-03-23
Filing date: 1992-03-23
Publication date: 1995-04-10
Also published as: AU3889193A; DK37692A; WO1993019362A1; DK37692D0; EP0632891A1

Description

DK 169956 B1DK 169956 B1

Opfindelsen angår en fremgangsmåde og et apparat til detektion af magnetiske diskontinuiteter i et prøveemne, ved hvilken fremgangsmåde der tilvejebringes et magnetfelt i prøveemnet og anbringes en eller flere magnetfølere i nærheden af prøveemnet 5 for detektion af en eventuel lækagefluks og tilvejebringelse af et elektrisk signal repræsenterende den eventuelle lækage-fluks, hvilke følere føres hen over overfladen af prøveemnet.The invention relates to a method and apparatus for detecting magnetic discontinuities in a sample blank, wherein a method provides a magnetic field in the sample blank and places one or more magnetic sensors in the vicinity of sample blank 5 for detecting any leakage flux and providing an electrical signal representing the any leakage flux, which sensors are passed across the surface of the specimen.

Fra US patentskrift nr. 4.814.705 kendes et apparat til detek-10 tion af magnetiske diskontinuiteter i en jernplade. Apparatet består af en vogn, der føres hen over jernpladen. Vognen understøtter en aflang hesteskoformet magnet, hvis polsko vender mod jernpladen. Hvis der ikke er diskontinuiteter i jernpladen, ledes næsten hele magnetfeltet gennem denne. Mellem pol-15 skoene sidder der en række af magnetfeltdetektorer, der i tilfælde af diskontinuiteter i jernpladen vil kunne detektere en eventuel lækagefluks, der som følge af diskontinuiteten ledes uden om jernpladen. Signalet fra den enkelte føler subtraheres fra middelværdien af signalerne fra en foran liggende og en 20 efterfølgende føler. Hvis denne differens overstiger en bestemt tærskelværdi, indikerer dette, at der er tale om en lækagef luks hidrørende fra en diskontinuitet. Dette kræver imidlertid et særligt beregningsudstyr. Dertil kommer, at aftastningen på tværs af apparatets fremføringsretning kun kan ske 25 diskontinuert.US Patent No. 4,814,705 discloses an apparatus for detecting magnetic discontinuities in an iron plate. The apparatus consists of a trolley which is passed over the iron plate. The trolley supports an elongated horseshoe-shaped magnet with pole shoes facing the iron plate. If there are no discontinuities in the iron plate, almost the entire magnetic field is passed through it. Between the pole shoes are a series of magnetic field detectors which, in the event of discontinuities in the iron plate, will be able to detect any leakage flux which, due to the discontinuity, is conducted outside the iron plate. The signal from the individual sensor is subtracted from the mean value of the signals from a preceding sensor and a subsequent sensor. If this difference exceeds a certain threshold, this indicates that this is a leakage lux resulting from a discontinuity. However, this requires a special calculation equipment. In addition, the scanning across the feed direction of the apparatus can be done only discontinuously.

Formålet med opfindelsen er at anvise en fremgangsmåde af den indledningsvis nævnte art, hvor signalbehandlingen er enklere og uafhængig af følernes fremføringshastighed. Endvidere skal 30 det være muligt at foretage en finere aftastning på tværs af fremføringsretningen.The object of the invention is to provide a method of the kind mentioned in the introduction, in which the signal processing is simpler and independent of the speed of feeding of the sensors. Furthermore, it should be possible to make a finer scan across the feed direction.

Fremgangsmåden er ifølge opfindelsen ejendommelig ved, at den enkelte føler under fremføringen udfører en skanderende bevæ-35 gelse i hovedsagen på tværs af fremføringsretningen. Derved skal man blot holde øje med, om følerens udgangssignal udviser pludselige ændringer under en skandering. Den dertil hørende DK 169956 Bl 2 elektronik bliver derved væsentlig enklere. Dertil kommer, at målingen nu er uafhængig af fremføringshastigheden. Endvidere muliggøres en kontinuerlig aftastning på tværs af fremførings-retningen, dvs. en meget finere aftastning.The method according to the invention is peculiar in that the individual sensor during the conveyance performs a scanning movement substantially transversely of the conveying direction. In doing so, you just have to keep in mind if the sensor's output signal shows sudden changes during a scan. The associated DK 169956 Bl 2 electronics thereby become considerably simpler. In addition, the measurement is now independent of the feed rate. Furthermore, continuous scanning is possible across the feed direction, ie. a much finer scan.

55

Ifølge opfindelsen kan man starte med at skandere et fejlfrit område af prøveemnet til tilvejebringelse af en kalibreringskurve for korrektion af efterfølgende målinger. Derved får man ikke kun oplysninger om, hvorvidt der er diskontinuiteter i 10 det underliggende prøveemne, men også oplysninger om diskonti-nuiteternes størrelse.According to the invention, one can start by scanning an error-free area of the sample blank to provide a calibration curve for correcting subsequent measurements. This not only gives information about whether there are discontinuities in the underlying sample, but also information about the size of the discontinuities.

Endvidere kan ifølge opfindelsen den enkelte føler indrettes til at foretage en differensmåling eller en differentialmå-15 ling. Den del af feltet, der er konstant, vil derved ikke indvirke på målingen, og derigennem opnås en væsentlig højere følsomhed.Furthermore, according to the invention, the individual sensor can be arranged to make a differential measurement or a differential measurement. The portion of the field which is constant will thereby not affect the measurement, and thereby a significantly higher sensitivity is obtained.

Ifølge opfindelsen kan der til udførelse af differensmålingen 20 anvendes en føler bestående af to ved siden af hinanden anbragte magnetfeltdetektorer, idet man tager differensen imellem signalerne fra de to detektorer.According to the invention, a sensor consisting of two adjacent magnetic field detectors can be used to perform the difference measurement 20, taking the difference between the signals from the two detectors.

Desuden kan ifølge opfindelsen den skanderende bevægelse af 25 den enkelte føler tilvejebringes ved, at føleren monteres på et legeme anbragt på en roterbar aksel med et skruelinieformet spor, idet der fra legemet er ført en pal ned i sporet. Derved tilvejebringes den skanderende bevægelse på en særlig enkel måde.In addition, according to the invention, the scanning movement of the individual sensor can be provided by mounting the sensor on a body mounted on a rotatable shaft with a helical groove, a body being led down from the body into the groove. This provides the scanning movement in a particularly simple manner.

3030

Endvidere kan ifølge opfindelsen den skanderende bevægelse af den enkelte føler tilvejebringes ved, at den roterbare aksel udstyres med skruelinieformede spor i begge retninger således, at der kan udføres en frem- og tilbagegående bevægelse af le-35 gemet, hvorpå føleren er monteret.Furthermore, according to the invention, the scanning movement of the individual sensor can be provided by providing the rotatable shaft with helical grooves in both directions so that a reciprocating movement of the body on which the sensor is mounted can be performed.

Alternativt kan ifølge opfindelsen den skanderende bevægelse 3 DK 169956 B1 af den enkelte føler tilvejebringes ved, at føleren monteres på en kæde med et i hovedsagen tværgående løb, hvilken kæde drives ved hjælp af et kædehjul.Alternatively, according to the invention, the scanning movement of the individual sensor can be provided by mounting the sensor on a chain with a generally transverse barrel, which chain is driven by means of a sprocket.

5 Ifølge opfindelsen kan kæden have et over- og et underløb, idet skanderingen foretages ved underløbet.According to the invention, the chain may have an overflow and an underflow, the scanning being performed at the underflow.

Opfindelsen angår også et apparat til detektion af magnetiske diskontinuiteter i et prøveemne af magnetisk materiale, hvil-10 ket apparat kan føres hen over prøveemnet og kan tilvejebringe et magnetfelt i prøveemnet, i hvilket apparat der er indrettet en række af følere til detektion af en eventuel magnetisk lækagef luks nær overfladen af prøveemnet og til at tilvejebringe elektriske signaler repræsenterende lækagefluksen. Apparatet 15 er ifølge opfindelsen ejendommeligt ved, at den enkelte føler kan forskydes i hovedsagen på tværs af apparatets fremførings-retning. Derved opnås et apparat, der er enklere end hidtil kendt samtidig med, at der muliggøres en kontinuerlig aftast-ning på tværs af apparatets fremføringsretning.The invention also relates to an apparatus for detecting magnetic discontinuities in a specimen of magnetic material, which apparatus can be passed over the specimen and can provide a magnetic field in the specimen, in which an array of sensors is provided for detecting any magnetic leakage leak near the surface of the specimen and to provide electrical signals representing the leakage flux. The apparatus 15 according to the invention is peculiar in that the individual sensor can be displaced substantially transversely of the feeding direction of the apparatus. Thereby an apparatus is obtained which is simpler than hitherto known, while at the same time permitting continuous scanning across the direction of the feed forward.

2020

Fremdeles kan ifølge opfindelsen den enkelte føler være monteret på et legeme anbragt på en roterbar aksel med et skrueli-nieformet spor, idet der fra legemet er ført en pal ned i sporet. Derved kan den skanderende bevægelse tilvejebringes på en 25 særlig enkel måde.Still, according to the invention, the individual sensor may be mounted on a body disposed on a rotatable shaft with a helical groove, with a body being led down from the body into the groove. Thereby, the scanning movement can be provided in a particularly simple manner.

Alternativt kan ifølge opfindelsen den enkelte føler være monteret på en kæde, der drives ved hjælp af et kædehjul. Kæden har fortrinsvis et over- og et underløb, idet skanderingen fo-30 retages ved underløbet. Derved muliggøres en hurtigere skandering.Alternatively, according to the invention, the individual sensor may be mounted on a chain driven by a sprocket. The chain preferably has an overflow and an underflow, the scanning being performed at the underflow. This enables faster scanning.

Opfindelsen skal nærmere forklares i det følgende under henvisning til tegningen, hvor 35 fig. 1 viser et apparat ifølge opfindelsen med en skanderende føler til detektion af magnetiske diskontinuiteter i et prøve- 4 DK 169956 B1 emne af magnetisk materiale, fig. 2 et apparat med en skanderende føler i en alternativ udførelse, 5 fig. 3 det i fig. 2 viste apparat set fra siden, fig. 4 apparatet i en alternativ udførelsesform, hvor følerne er fastsiddende og aktiveres en efter en, og 10 fig. 5 en konstruktion bestående af flere sammenkoblede apparaterThe invention will be explained in more detail below with reference to the drawing, in which 1 shows an apparatus according to the invention with a scanning sensor for detecting magnetic discontinuities in a sample of magnetic material; FIG. 2 shows an apparatus with a scanning sensor in an alternative embodiment; FIG. 3 shows the embodiment of FIG. 2 is a side view of FIG. 4 shows the apparatus in an alternative embodiment where the sensors are fixed and actuated one by one, and FIG. 5 shows a structure consisting of several interconnecting devices

Det i fig. 1 viste apparat til detektion af magnetiske diskon-15 tinuiteter i et underliggende prøveemne af magnetisk materiale omfatter en aflang hesteskoformet elektromagnet, der genererer et magnetfelt i prøveemnet. I figuren ses polskoene 1, som holdes i en defineret afstand fra prøveemnet ved hjælp af fire hjul 104, som er vist i fig. 3. Imellem den del af polskoene 20 1, som vender væk fra prøveemnet, er anbragt et antal perma nentmagneter 110. Alternativt kan der omkring den del af kernen, som forbinder polskoene 1, være viklet en eller flere spoler, der får tilført enten vekselstrøm eller jævnstrøm. Imellem de aflange polsko 1, der føres frem i pilens retning, 25 er der indrettet en eller flere magnetfeltfølere i form af hallelementer, der under polskoenes 1 fremføring i pilens retning udfører en skanderende bevægelse på tværs af fremføringsretningen. Denne skanderende bevægelse er tilvejebragt ved, at den enkelte føler er monteret på et legeme 2 anbragt 30 på en roterbar aksel 3, der roteres ved hjælp af en elektromotor 4. I den roterbare aksel 3 er der et skruelinieformet spor 5, idet der fra legemet 2 er ført en pal ned i sporet 5. Nogle styreskinner 6, der ligger parallelt med akslen 3 og støder an mod legemet 2, sikrer, at legemet 2 ikke drejer med rundt, når 35 akselen 3 roterer. Legemet 2 bliver derved under akslens 3 ro- * tation forskudt fra den ene ende af akselen 3 til den anden.The FIG. 1, for detecting magnetic discontinuities in an underlying magnetic material specimen, an elongated horseshoe shaped electromagnet generating a magnetic field in the specimen. In the figure, the pole shoes 1, which are kept at a defined distance from the test piece, are seen by means of four wheels 104 shown in FIG. 3. Between the portion of the pole shoes 20 1 facing away from the specimen, a plurality of permanent magnets 110 are disposed. Alternatively, around the portion of the core connecting the pole shoes 1 may be wound one or more coils which are supplied with either alternating current or direct current. Between the elongated pole shoes 1 advanced in the direction of the arrow 25, one or more magnetic field sensors are arranged in the form of hall elements which, during the movement of the pole shoes 1 in the direction of the arrow, perform a scanning movement across the feeding direction. This scanning movement is provided by the individual sensor being mounted on a body 2 disposed on a rotatable shaft 3 which is rotated by an electric motor 4. In the rotatable shaft 3 there is a helical groove 5, which from the body 2, a pawl is led down into the groove 5. Some guide rails 6, which lie parallel to the shaft 3 and abut the body 2, ensure that the body 2 does not rotate as the shaft 3 rotates. The body 2 is thereby displaced from one end of the shaft 3 to the other during the rotation of the shaft 3.

Som følge af, at det skruelinief ormede spor 5 for enden af ak- 5 DK 169956 B1 selen 3 løber tilbage i et modsat rettet skruelinieformet spor, og som følge af at den i sporet indførte pal er drejeligt monteret i legemet 2, vil sidstnævnte kunne udføre en frem- og tilbagegående bevægelse under akselens 3 rotation.As a result of the helically wound groove 5 at the end of the shaft 3 running back in an oppositely directed helical groove, and because the pallet inserted in the groove is pivotally mounted in the body 2, the latter will be able to perform a reciprocating motion during rotation of shaft 3.

5 Den på legemet 2 monterede føler vil under den frem- og tilbagegående bevægelse kunne detektere eventuelle ændringer i det aftastede magnetfelt. Sådanne ændringer kan f.eks. være forårsaget af lækagefluks som følge af diskontinuiteter i det underliggende prøveemne. Skanderingshastigheden er i denne ud-10 formning begrænset til ca. 10 skanderinger per sek. Dette kan imidlertid afhjælpes ved, at der anbringes flere lignende skanderingsindretninger efter hinanden i apparatets fremføringsretning. Også på tværs af apparatets fremføringsretning vil der kun anbringes flere skanderingsindretninger ved siden 15 af hinanden.5 The sensor mounted on the body 2 will be able to detect any changes in the sensed magnetic field during the reciprocating movement. Such changes may e.g. be caused by leakage flux due to discontinuities in the underlying specimen. In this embodiment, the scanning speed is limited to approx. 10 scans per sec. However, this can be remedied by placing several similar scanning devices one after the other in the direction of delivery of the apparatus. Also across the feed direction of the apparatus, only several scanning devices will be placed next to each other.

I en alternativ udførelsesform, der er vist i fig. 2, er den skanderende bevægelse af den enkelte føler tilvejebragt ved at montere en eller flere følere 7 på en kæde 8 af en kædehjuls-20 føring. Kædehjulsføringen tilvejebringes ved hjælp af to tandhjul 9, 10 og har et overløb og et underløb. Når føleren 7 befinder sig i underløbet, er den i direkte kontakt med det prøveemne, der skal undersøges. Signalet fra føleren 7 føres efter en passende forstærkning til et oscilloskop, der viser 25 eventuelle ændringer i det modtagne signal under en skandering. Skanderingen kan i dette tilfælde foretages væsentlig hurtigere. Hvis der, som vist i fig. 2, er monteret to følere 7 på den samme kæde 8, vil de to følere 7 kunne skandere skiftevis. Dette svarer til en fordobling af skanderingshastighe-30 den. Endvidere undgår man at springe områder over under fremføringen af hele apparatet i pilens retning. Fig. 3 viser ap-paratet set fra siden med hjulene 104 og permanentmagneterne 110, der forbinder polskoene 1. Som vist i denne figur kan der være indrettet flere skanderingsindretninger efter hinanden i 35 pilens retning. Derved muliggøres en endnu større skanderings-hastighed. Tandhjulene i den ene ende kan i givet fald have en fælles drivaksel. Der er imidlertid intet til hinder for, at 6 DK 169956 B1 de forskellige følere 7 skanderer med forskellige hastigheder. Ligesom i den i fig. 1 viste konstruktion tilføres signalet .In an alternative embodiment shown in FIG. 2, the scanning movement of the individual sensor is provided by mounting one or more sensors 7 on a chain 8 of a sprocket guide. The sprocket guide is provided by means of two gears 9, 10 and has an overflow and an underflow. When the sensor 7 is in the underflow, it is in direct contact with the specimen to be examined. The signal from the sensor 7 is fed to an oscilloscope after appropriate amplification, showing 25 possible changes in the received signal during a scan. In this case, the scanning can be done significantly faster. If, as shown in FIG. 2, if two sensors 7 are mounted on the same chain 8, the two sensors 7 will be able to scan alternately. This corresponds to a doubling of the scanning speed. Furthermore, avoiding skipping areas while advancing the entire apparatus in the direction of the arrow. FIG. 3 is a side view of the apparatus with the wheels 104 and the permanent magnets 110 connecting the pole shoes 1. As shown in this figure, several scanning devices may be arranged one after the other in the direction of the arrow. This allows an even higher scanning speed. The gears at one end may have a common drive shaft if required. However, there is nothing to prevent the various sensors 7 from scanning at different speeds. As in the FIG. 1, the signal is applied to the signal.

fra den enkelte føler 7 efter en passende forstærkning til et oscilloskop, der viser eventuelle ændringer i det modtagne 5 signal. Den tilhørende elektronik er derfor forholdsvis enkel.from the individual sensor 7 after an appropriate gain to an oscilloscope showing any changes in the received 5 signal. The associated electronics are therefore relatively simple.

En yderligere fordel ved denne konstruktion er, at det samlede apparat endog kan være stillestående under en måling. De nævnte kæder kan i givet fald erstattes af trådkabler.A further advantage of this construction is that the entire apparatus may even be stationary during a measurement. The chains mentioned may be replaced by wire cables if necessary.

10 I en alternativ udførelsesform for apparatet ifølge opfindelsen er den skanderende føler erstattet af en række af stationære følere, der aktiveres en efter en - se fig. 4. Den enkelte føler udgøres af en U-formet elektromagnet 12, hvis polsko er ført ned mod det prøveemne, der skal undersøges. I åget 15 af den U-formede elektromagnet 12 er der en udskæring 13. Udskæringen 13 bevirker, at føleren ikke er aktiv. Udskæringen 13 kan imidlertid fyldes ud med et legeme 14 af magnetisk ledende materiale, hvilket legeme 14 føres fra den ene føler til den næste ved hjælp af et trådkabel 15 af umagnetisk materia-20 le, hvilket trådkabel 15 for enden af løbet er ført rundt om en valse 16 henholdsvis 17, idet det til trådkablet 15 fastgjorte legeme 14 efter at have passeret valsen 16 tilbageføres via et returløb.In an alternative embodiment of the apparatus according to the invention, the scanning sensor is replaced by a series of stationary sensors which are activated one by one - see FIG. 4. Each sensor is constituted by a U-shaped electromagnet 12, the pole shoes of which are brought down towards the specimen to be examined. In the yoke 15 of the U-shaped electromagnet 12, there is a cutout 13. The cutout 13 causes the sensor to be inactive. However, the cut-out 13 can be filled with a body 14 of magnetic conductive material, which body 14 is passed from one sensor to the next by means of a wire cable 15 of non-magnetic material, which wire cable 15 is passed around the end of the barrel. a roller 16 and 17, respectively, the body 14 attached to the wire cable 15 after passing the roller 16 is returned via a return passage.

25 Det enkelte apparat 100 er fortrinsvis understøttet af fire hjul 104. Flere apparater kan imidlertid være sammenkoblet som vist i fig. 5. Denne konstruktion omfatter en vogn med nogle drivhjul 103 og et støttehjul, der samtidigt driver en indkoder, som afgiver information om positionen. På begge sider af 30 vognen sidder et modulopbygget rammesystem 106, der sikrer, at det enkelte apparat uhindret af den øvrige del af konstruktionen kan holde den afstand til overfladen af det underliggende prøveemne, som er bestemt af hjulenes 104 placering på det enkelte apparat. Ved hjælp af denne sammensatte konstruktion er 35 det muligt at af taste en forholdsvis bred bane af prøveemnet/ * også selv om overfladen af prøveemnet ikke er helt plan.The individual apparatus 100 is preferably supported by four wheels 104. However, several apparatus may be interconnected as shown in FIG. 5. This construction comprises a trolley with some drive wheels 103 and a support wheel which simultaneously drives an encoder which provides position information. On both sides of the trolley is a modular frame system 106 which ensures that the individual apparatus can keep unobstructed by the rest of the structure the distance to the surface of the underlying specimen determined by the placement of the wheels 104 on the individual apparatus. By means of this composite construction, it is possible to type a relatively wide web of the sample / * even though the surface of the sample is not completely flat.

Claims

A method for detecting magnetic discontinuities in a sample blank, wherein method provides a magnetic field in the sample blank and placing one or more magnetic field sensors (7) near the sample blank for detecting any magnetic leakage flux and providing an electrical signal. representing the possible leakage flux which sensors (7) are passed across the surface of the specimen, characterized in that each sensor (7) performs, during the feeding, a scanning movement substantially transversely of the feeding direction.

Method according to claim 1, characterized in that the individual sensor (7) first scans an error-free area of the sample and provides a calibration curve for correction of subsequent measurements.

Method according to claim 1, characterized in that the individual sensor (7) is arranged to be able to make a difference measurement or a differential measurement. DK 169956 B1 s c

Method according to claim 3, characterized in that a sensor consisting of two adjacent magnetic field detectors is used to perform the difference measurement, taking the difference between the signals from the two detectors.

Method according to claim 1, characterized in that the scanning movement of the individual sensor (7) is provided by mounting the sensor on a body (2) arranged on a rotatable shaft (3) with a helical groove (5), from the body (2) a pawl is led into the groove (5).

Method according to claim 5, characterized in that the scanning movement of the individual sensor (7) is provided by providing the rotatable shaft (3) with helical grooves (5) in both directions such that For example, a reciprocating movement of the body (2) on which the sensor (7) is mounted can be performed.

Method according to claim 1, characterized in that the scanning movement of the individual sensor (7) is provided by mounting the sensor (7) on a chain (8) having a substantially transverse bar, which chain (8) is driven by using one or more sprockets (9, 10). 25

7 DK 169956 B1 To the rear of this structure, a boom 107 may be affixed to which are mounted a plurality of color spray units 108 which, by means of a built-in computer, deliver dye proportional to and time delayed relative to the maximum 5 found. or minimal excesses of a pre-encoded material thickness for the specimen. The apparatus according to the invention may optionally be immersed in a liquid-filled tank. In this case, the device must be fully encapsulated. The position of the appliance is then stated in relation to the place of immersion. The direction of travel is maintained by means of a built-in gyroscope. This means that you do not need to empty the tank before a measurement.

15 Patent claims.

Method according to claim 7, characterized in that the chain (8) has an overflow and an underflow and that the scanning is performed at the underflow.

Method according to claim 7 or 8, characterized in that several sensors are mounted on the same chain (8) such that one of the sensors (7) is constantly at the underflow.

Method according to claim 1, characterized in that the individual scanning sensor is replaced by a series of stationary sensors (12) which are activated one by one (Fig. 4). 9 DK 169956 B1

Method according to claim 10, characterized in that the individual sensor (12) in the series of stationary sensors is constituted by a yoke of magnetically conductive material with a notch (13) which can be filled out by means of a body (14) of magnetic conductive material, which body (14) can be displaced from sensor to sensor (12).

Apparatus for detecting magnetic discontinuities in a sample of magnetic material, which apparatus may be passed over the specimen and may provide a magnetic field in the specimen, in which apparatus a plurality of sensors (7) are provided for detecting magnetic leakage flux near the surface of the sample blank and to provide electrical signals representing leakage flux one, characterized in that the individual sensor (7) can be displaced substantially transverse to the direction of delivery of the apparatus.

Apparatus according to claim 12, characterized in that the individual sensor (7) is mounted on a body (2) arranged on a rotatable shaft (3) with a helical groove (5), with means for securing , that the body does not rotate about the shaft and from the body (2) a pawl is led into the groove (5).

Apparatus according to claim 12, characterized in that the individual sensor (7) is mounted on a chain (8) driven by one or more sprockets (9, 10).

Apparatus according to claim 14, characterized in that the chain (8) has an overflow and an underflow, the scanning being possible at the underflow.