GB2126348A - Magnetic sensor - Google Patents

Abstract

A magnetic sensor responsive to objects with low magnetic resistance 10 has a bistable magnetic element, preferably a Wiegand wire 9, disposed between four magnet pole faces 11,12, 21,22 of alternating polarity in a working surface 3 of the sensor, and a sensor winding 8 associated therewith, in which winding 8 a pulse is produced by magnetic induction when such an object 10 moves past the sensor. The arrangement is such that the bistable magnetic element 9 is aligned with a neutral magnetic axis, of which the neutrality is disturbed by the approach of an object 10 in a direction parallel to the working surface 3 and perpendicular to the axis. <IMAGE>

Description

SPECIFICATION Magnetic sensor The present invention relates to a magnetic sensor responsive to objects of low magnetic resistance.

A sensor of the abovementioned type is known from EP-A-134821. It has two bar magnets, preferably high-power magnets of cobalt/samarium, mounted on a carrier in anti parallel arrangement.

The two magnets have flat pole surfaces in alignment with each other. Two such pole surfaces in alignment with each other define a working surface into proximity with which objects to which the sensor is desired to respond are brought. Laterally adjacent to the magnets and behind this working surface is a bistable magnetic element (hereinafter referred to briefly as "BME"), surrounded by a sensor winding.

As bistable magnetic elements, also referred to as bistable magnetic switch cores, it is recommended in particular that so-called Wiegand wires be employed, whose structure and manufacture are described in DE-OS 2,143,326 and which are also employed in the sensor described in EP-A-134821.

Wiegand wires as used in the present invention are homogeneous ferromagnetic wires (e.g. of an iron-nickel alloy, preferably 48% iron and 52% nickel, or of an iron and cobalt alloy, or of an iron, cobalt and nickel alloy, or of a cobalt, iron and vanadium alloy, preferably 52% cobalt, 38% from and 10% vanadium), which, as a result of special mechanical and thermal treatment, possess a soft magnetic core and a hard magnetic shell, i.e. the shell possesses a higher coercive force than the core. Wiegand wires normally have a length of 10mm - 50mm, preferably 20mm - 30mm.If a Wiegand wire, in which the magnetisation direction of the soft magnetic core is the same as that of the hard shell, is introduced into an external magnetic field the direction of which corresponds to that of the axis of the wire but is disposed opposite to the magnetisation direction of the Wiegand wire, on exceeding a field strength of appx. 1 6A/cm, the magnetisation direction of the soft core of the Wiegand wire is reversed - the resulting state is conveniently referred to as non-parallel magnetisation. This reversal is also referred to as resetting.On further direction-reversal of the outer magnetic field, on exceeding a critical field strength of the external magnetic field (which is referred to as ignition field strength) the magnetisation direction of the core is again reversed whereby the core and shell are once again magnetised in the same direction - this state is conveniently referred to as a parallel magnetisatisation. This reversal of the direction of magnetisation occurs very quickly and proceeds with a correspondingly sharp change in magnetisation flow per unit of time (Wiegand effect).

This change in magnetic flow can induce in an induction winding (referred to as a sensor winding) a short, very high voltage pulse (according to the number of turns and load resistance of the induction coil, up to appx 12 volts) known as a Wiegand pulse.

Also, when the core is reset, a pulse is triggered in the sensor winding, in this case however of a much lower amplitude and with reverse sign to that in the case of the reversal from anti-parallel to parallel magnetisation direction. If the Wiegand wire lies in an external magnetic field whose direction reverses from time to time and which is so strong that it can reverse the magnetisation firstly of the core and then also of the shell and in each case bring these to magnetic saturation, as a result of the reversal of magnetisation direction of the soft magnetic core Wiegand pulses are produced alternately of positive and negative polarity and this is referred to as symmetrical excitation of the Wiegand wire. For this purpose field strengths of approximately - (80 120A/cm) to + (80 - 120A/cm) are required.Reverse magnetisation of the shell also occurs at high speed and also produces a pulse in the sensor winding but this pulse is much smaller than that produced by magnetisation direction reversal of the core.

If however the external magnetic field chosen is one which is capable of reversing the magnetisation direction of only the soft core and not the hard shell, the high amplitude Wiegand pulses maintain constant polarity which is termed unsymmetrical excitation oftheWiegandwire. For this purpose afield strength is required in one direction of at least 16A/cm (for resetting the Wiegand wire) and in the opposite direction a field strength of approximately 80-120Alcm.

It is characteristic of the Wiegand effect that the pulses produced thereby are largely independent in amplitude and width of the speed of alteration of the outer magnetic field and show a high signal-to-noise ratio.

Bistable magnetic elements of different construction are also suited to the invention provided that these have two zones of differing 'hardness' (coercive force) which are magnetically coupled together and which can be employed in a manner similar to Wiegand wires for producing pulses by induced rapidly occurring reversal of the soft magnetic zone.

Thus, in DE-PS 2514131, for example, a bistable magnetic switch core is described, in the form of a wire comprising a hard magnetic core (e.g. of nickel-cobalt), an electrically conductive intermediate layer (for example of copper) deposited thereon, and a soft magnetic layer (e.g. of nickel-iron) deposited thereon. Another variant employs in addition a core of a magnetically non-conductive metal inner conductor (e.g. of beryllium-copper) whereon are successively deposited the hard magnetic layer, the intermediate layer, and then the soft magnetic layer.

This known bistable magnetic switch core does however produce weaker switch pulses than does a Wiegand wire.

In the case of the magnetic sensor described in EP-A-134821 the Wiegand wire lies in the stray field of the two magnets in an area of relatively low field strength. If however an object with low magnetic resistance e.g. an iron rod, is brought into proximity to the two pole surfaces lying in the working surface, the rod acts as a magnetic field concentrating member through which, according to the degree of proximity, a greater or lesser portion of the magnetic flux passes.

The originally present magnetic field of the two magnets is considerably distorted and, when the object is sufficiently close to the front pole surfaces lying in the working surface, the magnetic stray flux emanating from the rear pole surfaces is deflected towards the object and also strongly influences the Wiegand wire. The deflection of the magnetic field produces at the position of the Wiegand wire a reversal of the magnetic flux by means of which a Wiegand pulse is triggered. If the object with low magnetic resistance is withdrawn from the sensor, the magnetic field returns to its original condition whereupon the direction of the magnetic flux is again reversed at the position of the Wiegand wire.

A disadvantage of the known sensor is that a strong distortion or perturbation of the magnetic field is necessary to create the rise in field strength required to produce Wiegand pulses. In order to obtain this distortion of the magnetic field, it is necessary to move the object to be sensed to a distance which is simultaneously less than 1 mm from each of the two pole surfaces lying in the working surface. This also means that the length of the object must not be less than the separation between the pole surfaces lying in the working surface, i.e. it must be at least as long as a typical Wiegand wire (20 mm- 30 mm). These conditions represent a severe limitation on the usefulness of this sensor.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages.

The present invention provides a magnetic sensor responsive to objects of low magnetic resistance and comprising a bistable magnetic element (BME), an electrical sensor winding associated with the BME, and magnets with pole faces of different polarities in an essentially flat working surface of the sensor, behind which surface the BME and the sensor winding are located, wherein the BME is located close to the working surface between four magnetic pole faces of alternating polarity disposed in the working surface.

A magnetic sensor of the present invention can respond to objects having low magnetic resistance, especially objects of iron which traverse at rightangles the magnets and the longitudinal axis of the BME whereby the objects move firstly towards one pair of magnetic poles lying in the working surface of the sensor, then towards the BME lying between the pairs of magnet poles, and finally towards and then beyond the other pair of magnetic poles lying in the working surface and move generally parallel to the working surface.

The objects which are conveyed in this way past the working surface of the sensor firstly link the magnetic flux between the first pair of magnetic poles which they sweep past and subsequently they link the magnetic flux between the other pair of poles. Consequently, as an article of iron or similar material is conveyed past, the BME is subjected firstly to an increasing and then decreasing flux of the magnetic stray field of the second pair of magnetic poles and is then subjected to an increasing and then decreasing flux of the magnetic stray field of the first pair of poles. Since the magnetic fields of the two pairs of magnet poles are disposed opposite to each other, the BME receives alternately magnetic fluxes of different directions which produce in it abrupt magnetic reorientation and, in the known manner, this induces an electrical pulse in the associated sensor winding.

By the alternate influencing, in pairs, of four magnetic poles arranged at the corners of a rectangle, much greater changes in field strengh are produced at the position of the BME located between the magnet poles than in the case of the known sensor as disclosed in EP-A-134821. In order to obtain electrical pulses in the associated sensor winding, the objects to which the sensor is intended to respond do not require to be brought into such close proximity as heretofore with the known sensor of this kind. Moreover, in contrast to the case of the sensor of EP-A-134821, no permanent pretensioning of the BME by the magnets of the sensor is required.Indeed, the BME can be located in the magnetically neutral zone (i.e. the zone of disappearing magnetic field strength) between the magnet poles, or as the case may be in the zone of minimum magnetic field strength between the magnet poles, so that the change in field strength at the position of the BME during sensing of an object of low magnetic resistance can be of a very high value. Afurther advantage is that a considerable magnetic flux can be obtained in both directions at the position of the BME, whereby the BME can be caused to produce pulses not only by symmetrical but also by asymmetrical excitation.

In order to obtain the greatest possible field strengths, it is preferable to employ magnets of cobalt with a metal from the rare earths group, especially cobalt-samarium magnets. The employment of electromagnets would be possible but is not recommended owing to the current supply required and the higher cost of construction.

In order to mimimize stray losses, the magnets are preferably of generally U-shape construction in which case the BME can be located between two such U-shaped magnets close to the working surface defined by the pole faces of the two U-shaped magnets. In most cases a symmetrical form of construction is preferred for the sensor. In the case where a particularly closely spaced series of objects is moved past the sensor, e.g. the fine teeth of a gear-wheel, the spatial resolution capability of the sensor may however be insufficient to provide the change of field strength required for pulse production during traversing from one tooth to the next. In such cases recourse may be had to a sensor of asymmetric construction with U-shaped magnets of unequal pole strength, the BME being disposed closer to the weaker magnet than to the stronger one.

In order to further improve sensitivity of response and signal efficiency, the BME is in the form of a Wiegand wire which preferably has a length so that it does not project beyond the area between the pole faces and most preferably is approximately equal to the spacing between the two pole faces of the U-shaped magnet used. Conveniently the sensor winding extends around the BME.

Further preferred features and advantages of the invention will appear from the following detailed description given by way of example of two preferred embodiments illustrated with reference to the accompanying drawings in which like parts are denoted by like reference numbers and in which: Figure 1 is a plan view of a symmetrical construction magnetic sensor of the invention; Figure 2 is a vertical longitudinal section through the sensor at the line II - II, Figure 1 between two U-shaped magnets thereof; and Figure 3 is an end elevation of the sensor of Figures 1 and 2 viewed in the direction of the arrows Ill; and Figure 4 is a view of an asymmetric construction sensor corresponding to the view of Figure 3.

The sensor shown in Figures 1 to 3 comprises generally U-shaped permanent magnets 1 and 2 of equal size and strength, which are placed in antiparallel arrangement (i.e. with opposite sense poles opposed) with four flat pole surfaces 11, 12, 21, 22 thereof, disposed in the same plance and define a working surface 3 of the sensor. Each of the U-shaped magnets 1, 2 comprises (a) two bar magnets 4, 5 in anti-parallel arrangement, preferably of cobalt/samarium, (b) another bar magnet 6 which connects in the manner of a yoke the two pole faces of the bar magnets 4,5 disposed away from the working surface 3, which magnet 6 is also preferably of cobalt/samarium, and (c) two magnetic field concentrating pieces 7 of iron at the extremities of the bar magnet 6 which form the two corner angles of the U-shaped magnet 1 or 2.

In the central longitudinal plane between the two U-shaped magnets 1 and 2 a Wiegand wire 9 surrounded by a sensor winding 8 is located and is so close to the working surface 3 that the sensor winding 8 is substantially tangential to said surface 3.

The Wiegand wire 9 lies in a mostly neutral magnetic zone between the U-shaped magnets 1, 2.

With exact adjustment of the Wiegand wire 9 the stray fluxes of the two magnets 1 and 2 are balanced at the position of the Wiegand wire 9. This balance is disturbed when, first, one magnet 1 and then the other magnet 2 are successively magnetically practically short-circuited by a gear wheel tooth 10 or other object to be sensed. When a tooth appears directly above the magnet 1, the Wiegand wire 9 is influenced only by the stray flux of the other magnet 2. If the tooth 10 continues to move in direction of the arrow 13 to a point above the other magnet 2, this magnet 2 is short-circuited and the Wiegand wire 9 comes under the influence of the stray field of the first magnet 1. Between, the Wiegand wire 9, experiences or "sees" a zero field strength transition.

Due to the Wiegand effect, the change in field strength connected with a zero transition leads to a sudden change in direction of magentisation in the Wiegand wire and to the triggering of a voltage pulse in the sensor winding which can be conveyed to an evaluator circuit connected thereto.

In one practical test there were used U-shaped magnets 1,2 of cobalt/samarium which were 3 mm thick, 7 mm long (measured in the direction of the side arm bar magnets 4, 5) and 14.5 mm broad (measured in direction of the base member bar magnet 6); and their rectangular pole faces 11, 12, 21,22 were 3 mm x 4.5 mm. Wiegand wire 9 was 11 mm long and the sensor winding 8 surrounding it was had an axial length of 9 mm and had 2000 turns.

This sensor is responsive to the teeth 10 of an iron spur rack or gear moving transversely of the Ushaped magnets 11,21 and BME 9 therebetween in the direction of the arrows 13 (or in the opposite direction) past the working surface 3 of the sensor.

When the above described sensor was used on an iron spur rack with a trapezoidal tooth profile, with a pitch of 2.5 mm and a tooth width b = 14 mm, Wiegand pulses of 0.8vto 1.2vwere produced in the sensor winding 8 when the teeth 10 were conveyed past the working surface 3 at a separation therefrom of between 1.5 mm and 2 mm. Due to the symmetrical construction these pulses were induced in both directions. It was also observed that the sensor still responded to teeth 10 having a width as low as 6.8 mm. Limitation of tooth width in the upward direction does not exist.The maximum separation between the teeth 10 and the working surface 3 at which reliable firing of the Wiegand pulses is possible, amounts in the case of the particular sensor described above to approximately d = 2.3 mm; that is more than twice as large as is possible with a sensor according to EP-A-134821.

For reliable functioning of the sensor it is manifest that the distance between the two magnets 1 and 2 and the pitch of the spur rack or gear wheel, must be so correlated that both magnets 1 and 2 are not magnetically short-circuited simultaneously by two teeth 10 but so that there is a tooth gap above on magnet when there is a tooth above the other.

With a small tooth pitch on the other hand, the magnets 1 and 2 may not be placed so closely together that one single tooth 10 located opposite one of the magnets 1,2 can at the same time considerably weaken the stray field of the other magnet 2, 1 because then the change in field strengh can in certain circumstances no longer be sufficiently strong to trigger Wieg and pulses.

This problem can however be solved with an asymmetric construction as illustrated in Figure 4 wherein the sensor contains two U-shaped permanent magnets 1 and 2 of differing strengths, between which the Wiegand wire 9 and sensor winding 8 are positioned as in the first example in a magnetically mostly neutral zone between the magnets 1 and 2.

Due to the different strengths of the magnets 1 and 2, the neutral zone does not lie in the centre between the two magnets 1,2 but is closer to the weaker magnet 2. The gear tooth pitch and the separation between the magnets 1 and 2 are so correlated that one magnet is positioned exactly opposite a tooth 10 when the other is positioned exactly opposite the next tooth-gap 14 but one.

Claims (13)

1. A magnetic sensor responsive to objects of low magnetic resistance and comprising a bistable magnetic element (BME), an electrical sensor wind ing associated with the BME, and magnets, with pole faces of different polarities, in an essentially flat working surface of the sensor, behind which surface the BME and the sensor winding are located, wherein the BME is located close to the working surface between four magnetic pole faces of alternating polarity disposed in the working surface.

2. A sensor according to claim 1, wherein the BME is so arranged between said four pole faces that when the magnetic field is free from external perturbation, the magnetic flux at the position of the BME is minimal.

3. Asensor according to claim 1 or claim 2 wherein the BME lies in or in proximity to a magnetically neutral zone between the four pole faces.

4. A sensor according to any of of the preceding claims, wherein the four pole faces are disposed substantially at the corners of a rectangle.

5. A sensor according to any one of the preceding claims, wheren the four pole faces belong to two generally U-shaped magnets which are generally parallel to each other.

6. A sensor according to claim 5, wherein the BME is disposed parallel to the working surface and to the U-shaped magnets between which it lies.

7. A sensor according to claim 5 or claim 6, wherein the two U-shaped magnets are of equal pole strength and the BME is located centrally between the magnets.

8. A sensor according to claim 5 or claim 6, wherein the two U-shaped magnets have different pole strengths and the BME is positioned in the zone of minimum magnet flux in closer proximity to the weaker magnet than to the stronger magnet.

9. A sensor according to any one of the preceding claims, wherein the BME is a Wiegand wire.

10. A sensor according to claim 9, wherein the distance between the various pole faces and the length of the Wiegand wire are so correlated that the Wiegand wire does not extend out beyond the area of the working surface commonly limited by the pole faces.

11. A sensor according to claim 10 when dependent on claim 5 wherein the length of the Wiegand wire is approximately equal to the central spacing between the two pole faces of a said U-shaped magnet.

12. A sensor according to any one of the preceding claims, wherein the sensor winding surrounds the BME.

13. A magnetic sensor responsive to objects of low magnetic resistance substantially as described hereinbefore with particular reference to Figures 1 to 3 or Figure 4 of the accompanying drawings.