FR2530013A1 - MAGNETIC DETECTOR - Google Patents

Abstract

IT IS DESCRIBED A MAGNETIC DETECTOR REACTING TO LOW-RELUCTANCE OBJECTS AND CONTAINING, ARRANGED BETWEEN FOUR MAGNETIC POLAR FACES 11, 12, 21 AND 22 LOCATED IN A WORK AREA SO THAT THEIR POLARITIES ALTERNATE A BISTABLE MAGNETIC ELEMENT, IN PARTICULAR ONE WIEGAND 9 WIRE, AND A DETECTOR WINDING 8 ASSOCIATED WITH THE LATTER AND IN WHICH A PULSE IS PRODUCED BY MAGNETIC INDUCTION WHEN AN OBJECT OF SAID KIND PASSES BEFORE THE DETECTOR.

Description

Magnetic detector

  the present invention relates to a magnetic detector

  that reacting to low reluctance objects and constituted by a bistable magnetic element (EMB), an electric detector winding associated with E 1 B and magnets which define with polar faces of different polarities a substantially flat working area of the detector behind which the EF and the detector winding are mounted A detector of this kind is described in European patent 34 821 It comprises, arranged on a support, two bars

  magnetized, preferably magnets of high intensity of ai-

  mantation, in cobalt-samarium, oriented anti-parallel.

  The two magnets have planar polar faces ali-

  hindered between them Two polar faces aligned between them of this kind define a work area whose objects

  to which the detector must react are brought closer

  dear Laterally next to the magnets and behind this work area is a bistable magnetic element, hereinafter also abbreviated as "OE", surrounded

by a detector winding.

  As bistable magnetic elements, also called bistable magnetic switching cores, it is advisable to use in particular wires of the so-called Wiegand type, the constitution and manufacture of which are described in the published German Patent Application No. 2 143 326 and which are also used in the known detector which is the subject of European patent 34 821 Wiegand wires are,

  as for their composition, homogeneous ferromagnetic wires

  nes (e.g. an alloy of iron and nickel, pre-

  ference 48% iron and 52% nickel, or an alloy of iron and cobalt, or an alloy of iron with cobalt and nickel, or even an alloy of cobalt with iron and vanadium, preferably 52% cobalt, 38% iron and% vanadium), which by following a special mechanical and thermal treatment have a soft magnetic core and a hard magnetic envelope, i.e. the envelope

  has a coercive force greater than that of the nucleus.

  Wiegand wires typically have a length of 10 to 50 mm, preferably 20 to 30 mm. If a Wiegand wire, in which the direction of magnetization of the soft magnetic core corresponds to the direction of magnetization of the hard magnetic envelope, is placed in an external magnetic field whose direction corresponds to the direction of the axis of the wire but

  the direction of which is opposite to the direction of magnetization of the wire Wie-

  gand, then the direction of magnetization of the soft core of the Wie- wire

  gand is inverted if a field strength of about 16 A / cm is exceeded This inversion is also

  called resetting In the event of a new in-

  version of the direction of the external magnetic field the direction of ai-

  mantation of the nucleus reverses again as soon as the intensity of the external magnetic field exceeds a critical value (which is called the intensity of the priming field), so that the nucleus and the envelope are magnetized again

  at the same time This reversal of the direction of magnetization works

  performs very quickly and is accompanied by a strong corresponding variation in the magnetic flux per unit of time (Wiegand effect) This variation in the magnetic flux can induce a voltage pulse in an induction coil, which is called a detector winding. (Wiegand pulse)

  short and very strong (depending on the number of spi-

  res and load resistance of the induction coil

  reach up to around 12 volts).

  when returning the kernel to its initial state an im-

  pulse is also produced in the detector winding but this pulse presents, compared to the case of the transition from the antiparallel direction of magnetization to that parallel,

  significantly lower amplitude and the opposite sign.

  If the Wiegand wire is in a magnetic field, the direction reverses from time to time and which is strong enough to be able to reverse first the magnetization of the nucleus and then that of the envelope and bring them chadun to the state of magnetic saturation, then it occurs, by

  following the change in the direction of magnetization of the magneti-

  that soft, Wiegand pulses having alternately a positive polarity and a negative polarity and we can then speak of symmetrical excitation of the Wiegand wire For this we need field intensities of about - (80 to 120 A / cm) and + ( 80 to 120 A / cm) The inversion of the magnetization of the envelope also occurs suddenly and also leads to a pulse in the detector winding, but this pulse is much weaker than that induced

  upon reversing the magnetization of the nucleus.

  If, on the other hand, one chooses as external magnetic field a field capable of changing only the direction of magnetization of the soft core and not that of the hard envelope, then the strong Wiegand pulses do not occur

  that with the same polarity and we speak in this case of exài-

  asymmetrical tation of the Wiegand wire For this you need in

  one direction a field strength of at least 16 A / cm (for

  lead the Wiegand wire to the initial state) and in the direction

  pours a field intensity of about 80 to 120 A / cm.

  It is characteristic of the Wiegand effect that the

  impulses produced by this effect are, as for their am-

  plitude and width, to a large extent independent of the speed of variation of the external magnetic field and have a high signal-to-noise ratio In the context of the invention can also be

  used bistable magnetic elements designed differently

  remment, provided that these have two regions magnetically coupled together and having one

  compared to the other a magnetic hardness (force coerci-

  tive) different and may, similarly to-

  Wiegand wires, used to generate pulses by reverse-

  fast, induced sion of the magnetization of the magnetic region

  soft tick Thus, for example, in German patent No. 2 514 131, a bistable magnetic switching core is described, presented in the form of a wire which consists of a hard magnetic core (for example made of nickel-cobalt), of an electrically conductive intermediate layer (for example copper) deposited on the core and a soft magnetic layer (for example nickel-iron) deposited on the * intermediate layer Another variant further comprises a core formed of an internal metallic conductor devoid of permeance (for example beryllium-copper) on which is then deposited the hard magnetic layer which receives the intermediate layer, which is finally covered with the soft magnetic layer This magnetic switching core

  known bistable, however, generates switching pulses

  tion lower than those generated by a Wiegand wire.

  In the case of the detector described in the European patent

  péen 34 821 the Wieàand wire is inside the

  dispersion field of the two magnets in an intense area

  relatively small field strength However, when an ob-

  low reluctance jet, such as an iron bar, is brought to approach the two polar faces located

  in the work area, this object acts as a per-

  dependent on the flow through which, depending on the degree

  which it approaches the polar faces, a more or less important part of the magnetic flux The magnetic field initially determined by the two magnets is strongly deformed and when the object is sufficiently close to the anterior polar faces located in the work area the flux of magnetic dispersion leaving the posterior polar faces is deflected towards the object and influences, in a reinforced way, also the Wiegand wire The deflection of the magnetic field causes at the level of the wire ïWiegand a change of direction of the magnetic flux, thus a Wiegand impulse to occur When the object with low reluctance moves away from the detector again, the magnetic field returns to its primitive state, which is accompanied by a new change of direction of the magnetic flux

at the Wiegand wire.

  The disadvantage of this known detector is that

  a considerable deformation of the magnetic field to ob-

  hold the field strength rise necessary for the

  pulse generation W Jiegand To allow this deformation

  the magnetic field, the object to be detected must be brought to less than 1 mm distance, simultaneously, from each of the two polar faces located in the area of

  work It also means that the length of these ob-

  jets must not be less than the spacing of the pole faces located in the work area, i.e. it must be at least equal to the length of a wire

  Typical Wiegand (20 to 30 mm) These conditions severely limit

  the possibilities of use of the detector.

  The object of the invention is to create a detector of the kind mentioned above which is more suitable for monitoring the movement of toothed wheels, racks and the like and has better ability.

  to react so that it can be used more widely.

  This object is achieved, for a magnetic detector reacting to low reluctance objects and consisting of a bistable magnetic element (EMB), a winding

  electric detector associated with EYIB and magnets which

  end with polar faces of different polarities

  a substantially flat working area of the detector behind

  which are the E 1 M and the detector winding, by the fact that the EMB is arranged near the work area between four magnetic pole faces located

  in the work area so that their polarities alternate

  Other advantageous features of the invention

are defined below.

  The magnetic detector according to the invention reacts to low reluctance objects, mainly iron objects, which move transversely to the axes of the magnets and transversely to the longitudinal axis of the E Lf B in such a way that the objects first approach one of the pairs of blades located in the work area of the detector, then the ELB located between the pairs of magnetic poles and finally the other pair of magnetic poles

  located in the work area and pass appreciably

  parallel to the work area on top of it to

move away from it.

  Objects which thus pass in front of the detector

  first embrace the magnetic flux between the ma-

  of the first pair of p 8 s on which they pass and, at a later time, the magnetic flux between the magnetic poles of the other pair Therefore, when an object made of iron or similar material is scrolled

  l E 1 MB is subject first to an increasing flow then decreases

  health of the magnetic dispersion field of the second pair of magnetic poles and then to an increasing then decreasing flux of the magnetic dispersion field of the first

  pair of magnetic poles Since the fields ma-

  of the two pairs of magnetic poles have meanings

  opposite, the 51 M is alternately subjected to magnetic flux

  opposite ticks which cause its magnetization to reverse suddenly, thus inducing in an known manner an electric pulse in the associated detector winding

at the ELS.

  The fact that four magnetic poles arranged at the corners of a quadrilateral are influenced alternately

  by pears allows to produce, at the level of E 1 v 3 located in

  be the magnetic poles, variations in intensity of

  significantly larger field than in the case of

  guard described in European patent 34 821 For ob-

  hold electrical impulses in the detected winding

  tor associated with the EMB the objects to which the detector is intended to react need not be brought as close to the latter as hitherto is necessary for the known detector of the same kind In addition, unlike of the known detector described in European patent 34 821, the EMB does not need to be permanently the subject of a

  bias voltage from the detector magnets.

  On the contrary, the EMB can be placed in the magnetic zone.

  only neutral (i.e. the area with vanishing magnetic field strength) between the magnetic poles or in the area with minimum magnetic field strength between

  p 8 the magnetic ones, so that the variation in intensity

  field tee at the EMB level when detecting a

  object with low reluctance can be particularly strong.

  It is also advantageous that a large magnetic flux can be obtained in both directions at the level of the EMB.

  so that it can be caused to generate-impulses

  Let us say both by symmetrical excitation and by asymmetric excitation. With a view to obtaining field strengths as high as possible, cobalt magnets associated with a metal from the rare earth group are advantageously used, in particular cobalt-samarium magnets.

  electromagnets instead of permanent magnets would be possible

  ble but, due to the necessary current supply and

  the increased cost of construction, it is not preferable.

  In order to avoid losses by dispersion, the magnets are preferably used in the form of a U, the ELB then being placed between two U-shaped magnets of this kind near the working area which is defined by the polar faces.

  of the two U-shaped magnets In most cases a construction

  symmetrical detection is preferred In the case of objects which pass in front of the detector in very close succession, for example the teeth of a toothed wheel with reduced pitch, the geometric resolving power of the detector

  however, may no longer be sufficient for the value

  riation of field strength required for generation

  an impulse is still produced by two successive teeth

  sives In such a case it can be remedied by resorting to

an asymmetric type detector.

  Other characteristics of the invention defined below have the common aim of favorably influencing

  the detector's ability to react and on the level of

generated by him.

  Two exemplary embodiments of detectors according to the invention are shown diagrammatically in the drawings

  annexed Identical or corresponding constituent elements

  between them in the two exemplary embodiments are

  designated by the same reference numbers.

  Figure 1 shows a top view of its side

  before, a symmetrically constructed magnetic detector

  Figure 2 shows the same detector in section along line IIII of Figure 1; Figure 3 shows the detector seen along arrow III of Figure 1; and Figure 4 is a view similar to Figure 3 of a

  detector built asymmetrically.

  The detector represents in Figures 1 to 3 com-

  takes two permanent magnets in U 1 and 2 of the same dimensions

  sions and of the same intensity which are arranged antiparallel-

  lie with each other and define with their

  four planar polar faces 11, 12, 21 and 22, which are so-

  killed in a common plane, the work area 3 (front side) of the detector Each of the U-shaped magnets 1, 2 consists of two magnetic bars 4 and 5 arranged antiparallel

  ment, preferably in cobalt-samarium, another bar

  mantis 6, preferably also in cobalt-samarium, which

  binds like a vault the two polar faces of

  magnetic bars 4, 5 which are distant from the work area

  vail 3, as well as two pieces of iron 7 permeable to flow located at the ends of the magnetic bar 6 and which form

  the two angles of the respective U-shaped magnet 1, 2.

  In the median longitudinal plane between the two ai-

  U-shaped mantles 1 and 2 is arranged a Wiegand 9 wire surrounded by

  a detector winding 8, the Wiegand wire being sufficient

  close to work area 3 so that it rolls up

  detector 8 is tangent to work area 3.

  The Wiegand 9 wire is located in a region which is substantially neutral from a magnetic point of view between the U-shaped magnets 1 and 2; in case of exact adjustment of the Wiegand 9 wire the dispersion fluxes of the two magnets 1 and 2 compensate each other at the level of the Wiegand 9 wire the balance is disturbed when the magnet

  1 and magnet 2 are practically short one after the other

  magnetically circuited by a tooth 10

  a tooth is directly above the magnet 1

  the Wiegand 9 thread is only influenced by the dispersal flow

  sion of the other magnet 2 When tooth 10 advances along arrow 13 until it is above the other magnet 2, it is short-circuited and the Wiegand wire 9 is

  then under the influence of the dispersion field of the first ai-

  man t 1 Between these two tooth positions the Wiegand wire 9

  "sees" a zero crossing of field strength.

  the variation in field strength linked to a zero crossing causes in the Wiegand wire, as a result of the Wiegand effect, a sudden change in the direction of magnetization and the triggering of a voltage pulse in the detector winding, which pulse can be brought to a circuit

  connected to the detector.

  Following a proven example the magnets in U 1 and 2 in

  cobalt-samarium have a thickness of 3 mm, a long

  7 mm width (measured in the direction of the magnetic bars

  tees 4, 5) and a width of 14.5 mm (measured in the direction

  tion of the magnetic bar 6); their rectangular polar faces 11, 12, 21 and 22 measure 3 x 4.5 mm The I Viegand wire 9 is 11 mm long and the detector winding 8 surrounding this wire is 9 mm long and has 2,000 turns. detector

  can react to the teeth 10 of a rack or a toothed wheel

  iron tees which move along arrow 13 (or in direction

  opposite) along working area 3 of the detector.

  The use of this detector in the presence of a

  iron mesh with a trapezoidal tooth profile, a module of 2.5 mm and a tooth width b = 14 mm makes it possible to produce Wiegand pulses of 0.8 to 1.2 volts in the detector winding 8 when the teeth 10 pass on

  work area 3 at a distance of 1.5 to 2 mim from it

  ci As a result of the symmetrical construction these impulses

  are induced in both directions It has furthermore been found

  t the detector still reacted to teeth 10 present

  both a width of 6.8 mm only; there is no upper limit of the tooth width The greatest distance (d) between the teeth 10 and the working area 3 for which a safe priming of the pulses still occurs

  Wiiegand has been found to be in the case of this in-

  about 2.3 mm; this value is more than double that

  obtainable with a detector according to the eu patent

European 34,821.

  For the detector to work properly the

  placement of the two magnets 1 and 2 and the module of the creation

  mesh or cogwheel must obviously be adapted

  tees to each other in such a way that the two magnets

  1 and 2 are not simultaneously short-circuited magneti-

  only by two derits 10 but that a trough is found

  above one of the magnets when a tooth is placed over it

on the other magnet.

  In the case of a weak gear pitch the magnets 1 and 2 must on the other hand not be placed so close

  from each other only by a single tooth 10, located in re-

  gard of one of the magnets 1, 2, that is to say at the same time

  ment significantly weakened the dispersion field of the other magnet 2, 1 because the variation of the field strength then risks under certain conditions not to

  sufficient to initiate Wiegand impulses.

  This can be remedied by choosing a construction

  asymmetric detector function - like the one shown in figure 4 The detector has two permanent magnets

  1; and 2 of different magnetization intensities between the-

  which the Wiegand wire 9 surrounded by a detector winding 8 is mounted, as in the first example, in an area

  substantially neutral magnetic point between the ai-

  mants 1 and 2 Due to the difference in air intensity

  mantation between magnets 1 and 2 the neutral zone is not located centrally between the two magnets 1 and 2 but

  closer to the magnet with lower magnetization intensity 2.

  The gear pitch and spacing of the magnets 1, 2 are

  adapted to each other in such a way that one of the ai-

  mants is exactly opposite a tooth 10 when the other magnet is exactly opposite the second

trough following 14.

Claims (9)

  1 Magnetic detector reacting to low reluctance objects and consisting of a bistable magnetic element (EMB), an electric winding detector associated with the EMB, and magnets which define with po-   areas of different polarities a sensitive work area   flatly of the detector behind which the EMB and the detector winding are mounted, characterized in that the EMB (9) is arranged near the working area (3) between four magnetic pole faces (11, 12, 21 and 22)   located in the work area (3) so that their pola- rites alternate.   2 detector according to claim 1, characterized in that the EMB is arranged between the four pole faces (11, 12, 21 and 22) so that the magnetic flux   at E 1 B (9) is minimal as long as the magnetic field   tick is not disturbed from the outside.   3 Detector according to claim 1 or 2, charac-   terized in that the EMB (9) is located in or near the magnetically neutral zone between the four faces polar (11, 12, 21 and 22).   4 Detector according to any one of the claims.   1 to 3, characterized in that the four polar faces   res (11 i, 12, 21 and 22) are located at the corners of a rec- tangle.   Detector according to any one of the claims   1 to 4, characterized in that the four polar faces   res (11, 12, 21 and 22) belong to two magnets in form   u me (1, 2) arranged parallel to each other and   preferably having the same dimensions.   6 Detector according to claim 5, characterized in that the EY 3 (9) is arranged parallel to the area of   work (3) and U-shaped magnets (1, 2) between them.   7 detector according to claim 6, characterized in that the two U-shaped magnets (1, 2) are identical with regard to the intensity of their poles and in that the EMB (9) is disposed centrally between the magnets ( 1, 2) 8 Detector according to claim 6, characterized   in that the two U-shaped magnets (1, 2) have   voltages of different poles and in that the EFB (9), if-   killed closer to the magnet (2) of lower intensity of magneti-   tation than the magnet (1) with greater intensity of ai-   mantation, is placed in the area where the magnetic flux is minimal.   9 Detector according to any one of the claims   1 to 8, characterized in that the EMB (9) is a Wiegand wire. Detector according to claim 9, characterized in that the spacing of the pole faces (11, 12, 21 and 22) with respect to each other and the length of the Wiegand wire (9) are adapted to each other by so that the Wiegand wire (9) does not extend outside the region of the work area (3) jointly bounded by the pole faces (11 il, 12, 21 and 22).   11 Detector according to claims 5 and 10, ca-   characterized in that the length of the Wiegand wire (9) corresponds to   lays approximately at the distance between the two polar faces (I 41, 12; 21, 22) of a magnet (1; 2).   12 Detector according to any one of the claims.   1 to 11, characterized in that the detector winding (8) surrounds the EMB (9).