GB2071335A - Magnetic Proximity Switch - Google Patents

Abstract

A magnetic proximity switch is described which produces a non- transient electrical output signal, i.e. an output signal which is maintained as long as a position is held within a predetermined critical distance. The proximity switch employs a bistable magnetic element (1) e.g. a Wiegand wire which is coupled with an electrical winding (3) supplied with an alternating voltage or pulsating direct current, and which produces at the position of the bistable magnetic element (1) a magnetic alternating field which is sufficiently strong to change periodically the magnetisation of the bistable magnetic element (1). The change of magnetisation occurs abruptly and induces in a sensor winding (2) a train of electrical voltage pulses. A magnetic direct field, e.g. the field of a displaceable bar-magnet (4), is superimposed upon the magnetic alternating field. By altering the position of the bar-magnet (4), the resulting magnetic field can be so changed that it is no longer capable of changing the magnetisation of the bistable magnetic element. Thus, there are positions, e.g. of the bar- magnet (4), in which pulses occur in the sensor winding (2) and other positions in which the pulses do not occur in the sensor winding. <IMAGE>

Description

SPECIFICATION Magnetic Proximity Switch The invention relates to a magnetic proximity switch of the type incorporating a bistable magnetic element which is influenced in accordance with the distance between an object being monitored and the proximity switch.

It has been previously proposed to provide a proximity switch in which a Wiegand wire is employed as bistable magnetic element (hereinafter referred to as BME).

As bistable magnetic elements, also referred to as bistable magnetic switch cores, it is recommended in particular that so-called Wiegand wires be employed, the structure and manufacture of which are described in DE-OS 2,143,326. Wiegand wires are homogeneous, ferromagnetic wires (e.g. of an alloy of iron and nickel, preferably 48% iron and 52% nickel; or of an alloy of iron and cobalt; or of an alloy of iron with cobalt and nickel; or of an alloy of cobalt with iron and vanadium, preferably 52% cobalt, 38% iron and 10% vanadium) which, due to special mechanical and heat treatment, possess a soft magnetic core and a hard magnetic outer surface, i.e. the surface possesses higher coercive force than the core. Typical Wiegand wires have a length of 5mm to 50mm, preferably 20mm to 30mm.If a Wiegand wire, in which the direction of magnetisation of the soft magnetic core coincides with that of the hard magnetic surface, is introduced into an external magnetic field whose direction coincides with that of the axis of the wire but is opposed to the direction of magnetisation of the Wiegand wire, on exceeding a field strength of approximately 1 6 A/cm, the direction of magnetisation of the soft core of the Wiegand wire is reversed. This reversal is also referred to as resetting On further reversing of the direction of the external magnetic field, and on the external magnetic field exceeding a critical field strength, the direction of magnetisation of the core is again reversed, so that the core and the surface are again of parallel magnetisation.

This reversal of the direction of magnetisation occurs very abruptly and is accompanied by a correspondingly notable change in magnetic flux per unit of time (Wiegand effect). In an induction coil, this alteration of magnetic flux may induce a short and very high voltage pulse (according to the number of turns and to the load resistance of the coil, up to approximately 1 2v.) known as a Wiegand pulse.

Also on resetting of the core, a pulse is produced in an induction coil, which is however of much lower amplitude and of a different sign from the case of the reversal from the anti-parallel to the parallel direction of magnetisation.

If, as external magnetic field, an alternating field is selected, which is capable of reversing magnetisation firstly of the core and then also of the surface layer and of bringing these to magnetic saturation, Wiegand pulses occur, due to the reversal of the direction of magnetisation of the soft magnetic core, alternately of positive and negative polarity, which is termed symmetrical excitation of the Wiegand wire. For this purpose, field strengths of approximately(80 to 120 A/cm) to +(80 to 120 A/cm) are required. The change of magnetisation of the surface also occurs abruptly and also produces a pulse in the induction coil, which is however much smaller than the pulse induced in the reversal of the core and is generally not evaluated.

If however an external magnetic field is selected which is capable of reversing only the soft core but not the hard surface layer in direction of magnetisation, the high Wiegand pulses occur only with unchanged polarity which is referred to as asymmetrical excitation of the Wiegand wire. For this purpose, a field strength is required in one direction of at least 1 6 A/cm (for the resetting of the Wiegand wire) and in the opposite direction a field strength of approximately 80 to 120 A/cm.

It is characteristic of the Wiegand effect that the amplitude and width of the pulses it produces are largely independent of the speed of change of the external magnetic field and that they possess a high signal-to-noise ratio.

Also suitable for the purpose of the invention are differently constructed bistable magnetic elements which possess two zones of differing magnetic hardness (coercive force) magnetically coupled to each other, and may be employed in the same manner as Wiegand wires for producing pulses by an induced, abrupt reversal of the soft magnetic zone. Thus, a bistable magnetic switch core in the form of a wire is proposed, for example, in DE-PS 2,514,1 31, which comprises a hard magnetic core (e.g. of nickel-cobalt), an electrically conductive intermediate layer (e.g. of copper) deposited thereon, and a soft magnetic layer (e.g.

of nickel-iron) deposited thereon. Another variant additionally employs a core of a magnetic, nonconductive metal inner conductor (e.g. of beryllium-copper), on to which the hard magnetic layer is deposited, then on this the intermediate layer, and on this the soft magnetic layer. This bistable magnetic switch core does, however, produce smaller switch pulses than a Wiegand wire.

In a previously proposed proximity switch, the Wiegand wire is enclosed in an electrical winding which is intended to receive and to indicate the magnetic signal associated with a reversal of the direction of magnetisation in the Wiegand wire (Wiegand pulse), whereby it is referred to as a sensor winding. In the previously proposed proximity switch, the Wiegand wire is normally in a state in which the directions of magnetisation of the hard magnetic shell and of the soft magnetic core are anti-parallel.Now, if a permanent magnet is moved inwardly towards the Wiegand wire, the field of said magnet at the position of the Wiegand wire being opposite to the magnetisation direction of the core of the Wiegand wire, when a point is reached within the predetermined distance between the permanent magnet and the Wiegand wire, the strength of the magnetic field of the permanent magnet (provided the magnet is sufficiently strong) at the position of the Wiegand wire is such that the soft magnetic core of the Wiegand wire is caused to reverse its direction of magnetisation. This change of magnetic polarity of the Wiegand wire from anti-paralRel (relative to the magnetisation direction of the shell of the Wiegand wire) to parallel orientations, occurs abruptly and produces in the sensor winding the characteristic Wiegand pulse.This principle is utilised in the case of previously proposed proximity switches in that a permanent magnet is moved towards a Wiegand wire provided with a sensor winding, and, when a point is reached within the predetermined critical minimum distance, a Wiegand pulse is produced in the sensor winding, said voltage pulse being further conveyed to an evaluation circuit.

Previously proposed proximity switches have a disadvantage that the output signal is transient, that is to say, on reaching a point within the critical minimum distance, it is released only once and does not persist as long as this critical position is maintained. Moreover, on each occasion that a point is reached within the critical distance, the magnetisation of the Wiegand wire must again be changed to the condition with antiparallel directions of magnetisation, in order that a further Wiegand pulse may be released when the wire is once more approached.

Proximity switches have been proposed which produce a persistant signal, as long as a position is assumed within a predetermined critical distance, e.g. inductive or capacitive proximity switches or proximity switches with Hall generator, or with semiconductors of magnetically variable resistance, but these have serious disadvantages. As far as semiconductors which are sensitive to magnetic fields are concerned, these can be employed only under strictly limited temperature conditions, whereas proximity switches with Wiegand wire are completely unaffected by ambient influences.

Other switches have a disadvantage that a relatively expensive active electronic circuit is required at the measurement point An object of the present invention is to provide a robust proximity switch, employing a BME which is unaffected by ambient influences, said switch producing a non-transient electrical output signal, ie. an output signal which is maintained as long as a position is held within a predetermined critical distance.

According to the present invention there is provided a magnetic proximity switch comprising a bistable magnetic element (hereinafter referred to as BME); means for the production of a magnetic direct field, the degree of influence of which upon the BME is a function of the distance between an object to be monitored and the proximity switch; an electrical sensor winding coupled magnetically with the BME, in which winding, on passing a predetermined critical distance between the object to be monitored and the proximity switch, an electrical pulse is induced as a result of an abrupt change of magnetisation direction in the BME, characterised in that there is coupled magnetically with the BME an electrical exciter winding which is supplied with a periodic electrical signal, the polarity and strength of which are of such a magnitude in each cycle that the field strength of the magnetic field produced by the periodic electrical signal in the exciter winding is capable of changing the direction of magnetisation of the BME at the position of said BME.

In the present invention, when, on passing the critical distance threshold, the resulting change of the action of the magnetic direct field upon the BME changes the direction of magnetisation of said BME, this change is periodically reversed by the magnetic field periodically produced by the exciter winding and acting in opposite direction.

Thus, two signal states can be distinguished. On one side of the critical distance threshold, no pulses are received in the sensor winding, which indicate a change of magnetisation direction in the BME, because no magnetic field of such direction and strength is present at the position of the BME which might produce such a change. On the other side of the critical distance threshold, a suitably high field strength is present at the position of the BME to reverse the magnetisation of the BME and thereby produce in the sensor winding an associated voltage pulse.However, on this other side of the critical distance threshold, due to superimposing of the periodic magnetic field upon the magnetic direct field, the resulting reversal of the magnetisation direction of the BME is periodically reversed, so that a periodic train of pulses is produced in the sensor winding as long as the object to be monitored is located on this other side of the critical distance threshold.

Thus, the alternative arrangements are, that the train of pulses appears when a point is reached within the critical distance threshold, or that they appear when a point is reached beyond the critical distance threshold. In the first case, the BME and a permanent magnet may, for example, be fixed, and a ferromagnetic eiement, e.g. a sheet-metal plate connected to the object to be monitored, which plate, when moved towards the permanent magnet, weakens the field of said magnet at the position of the BME.

When the plate is removed at some distance, the field of the permanent magnet at the position of the BME dominates the magnetic field periodically produced by the exciter winding, whereby at the position of the BME a periodically fluctuating direct field occurs which is unable to alter the direction of magnetisation of the BME.

As the ferromagnetic plate approaches more and more closely, the field of the permanent magnet is increasingly weakened at the position of the BME and finally the periodical magnetic field dominates the field of the permanent magnet. On reaching a point within the critical distance threshold, the periodic field of the exciter winding dominates the opposed field of the permanent magnet periodically so strongly that the magnetisation direction of the BME is changed periodically.

In the second case, a permanent magnet, for example, can be fitted to the object to be monitored and, on being moved into proximity to the BME, increases the magnetic field strength thereof at the position of the BME; or, as before, a stationary permanent magnet is employed and the object to be monitored is ferromagnetic or is provided with a ferromagnetic plate which, on being moved towards the proximity switch, strengthens the magnetic field of the permanent magnet at the position of the BME. When the object is removed at a distance, the magnetic field of the exciter winding is dominant periodically at the position of the BME and a periodic train of pulses is produced in the sensor winding.If, however, the object is moved to a critical distance from the proximity switch, or reaches a point within this distance, the field of the permanent magnet becomes so strong in relation to the oppositely-directed periodic magnetic field of the exciter winding, that the periodic magnetic field can no longer make retrogressive the reversal of magnetisation direction of the BME induced by the permanent magnet, so that, on reaching a point within the threshold distance, the impulsetrain in the sensor winding ceases.

In both cases, therefore, the appearance or disappearance of a train of pulses in the sensor winding, is an indication that a point has been reached either within or beyond a predetermined distance threshold.

In one embodiment of the present invention, a pulsating magnetic direct field is produced by the sensor winding. This can be realised by supplying to the sensor winding a periodic train of electrical pulses, e.g. pulses of saw-tooth or of square waveform, but preferably an alternating current rectified in the way of full-wave rectification. At the position of the BME, the magnetic direct field, for example, of a permanent magnet, is superimposed upon the pulsating magnetic direct field, which magnetic direct field is of opposite direction to that of the pulsating magnetic field.

Thus, two basic variants of the switch are possible. In the case of the first variant, when the object is removed to a considerable distance, the influence of the magnetic direct field dependent upon the distance of this object to be monitored is negligible at the position of the BME and is strengthened with increasing distance of the object from the proximity switch. Thus, at a great distance, the pulsating magnetic field predominates considerably and is so strong that it brings the BME periodically to magnetic saturation, at which the hard and soft magnetic portions of the BME have parallel magnetisation.

Admittedly, in the area of minimal values of the pulsating magnetic field, the resulting magnetic field changes its sign periodically due to the influence of the oppositely-directed direct field, but in this area with reversed sign, with the object at a great distance from the proximity switch, the resulting magnetic field is still too weak to reverse the direction of magnetisation of the soft magnetic portion of the BME, so that it is directed opposite to the magnetisation of the hard magnetic portion. (This change from parallel to anti-parallel magnetisation is referred to hereinafter as "resetting" of the BME). When the object is moved, towards the proximity switch, at a predetermined critical distance, the resulting magnetic alternating field attains, in the area of the minimal values of the pulsating field, sufficient strength to reset the BME.Upon the next change of sign of the resulting magnetic field, the field strength in opposite direction increases to such an extent that the magnetisation of the soft magnetic portion again abruptly changes its sign, accompanied by the production of a characteristic pulse in the sensor winding, and the BME is again brought to the area of magnetic saturation. As long as the point reached is within the critical distance, the BME is therefore excited asymmetrically, and in each cycle a high characteristic pulse is produced. (With resetting of the BME, a pulse is also produced, which is however of much smaller amplitude).

In the case of the second variant of the switch, the influence at the position of the BME of the magnetic field coupled with the object to be monitored, is not strengthened (this differs from the first variant) with increasing distance of the object from the proximity switch, but is weakened. It was already explained much earlier how such an occurrence is possible. When the object is at a considerable distance, the magnetic field dependent upon the distance of the object should dominate the pulsating magnetic direct field produced by the exciter winding to such an extent that the resulting magnetic field is a pulsating direct field of reversed polarity. In this state, no characteristic train of pulses is produced in the sensor winding.As the distance decreases, the influence of the non-pulsating magnetic field at the position of the BME becomes weaker, and the resulting magnetic field is converted from a pulsating direct field to a pulsating alternating field, which, as from a critical distance, is capable of resetting the BME and, after the next sign change and within the same cycle, of returning it to magnetic saturation, again accompanied by an abrupt change in the direction of magnetisation of the soft magnetic portion of the BME, so that, as in the case of the first variant, a pulse-train is produced in the sensor winding by asymmetrical excitation.

In the case of both variants, it is possible, of course, by a suitable magnetic field arrangement, for the characteristic pulses to be produced in the sensor winding, not within the predetermined critical distance threshold, but beyond it, i.e. with the object removed to a considerable distance, the two magnetic fields are superimposed to form a magnetic alternating field which permits of asymmetrical excitation of the BME. On the other hand, as the object approaches more and more closely to the proximity switch, the resulting magnetic alternating field, due to a change in its direct field component, approaches more and more closely the state of a magnetic direct field, the ability of which to reset the BME ceases as at a certain critical distance, whereby the puise-train in the sensor winding is cut-off.

In the case of a further method of execution of the invention, the characteristic pulses are produced in the sensor winding, not by asymmetrical excitation, but by symmetrical excitation. The dimensions and arrangement of the exciter winding and the dimensions of the exciter alternating current, preferably an alternating current of sinusoidal waveform, are selected so that, in the absence of a further magnetic field, the magnetic alternating field emanating from the exciter winding excites the BME symmetrically, whereby, during each halfwave, a characteristic voltage pulse is produced in the sensor winding, the pulses possessing alternately changing polarity.With symmetrical excitation, during each half-wave, firstly the magnetic polarity of the soft magnetic portion of the BME is reversed, which leads to a high pulse in the sensor winding, and as the field strength continues to increase in the same half-wave, the magnetic polarity of the hard magnetic portion is also reversed, producing a much smaller pulse in the sensor winding, which is not usually evaluated.

The proximity switch operating with symmetrical excitation of the BME can function basically in two ways. In the first case, the magnetic direct field, whose strength at the position of the BME is dependent upon the distance of the object to be monitored, is selected so that its strength at the BME increases as the distance between the object and the proximity switch decreases. Increasingly then, a magnetic direct field is superimposed upon the magnetic alternating field. When a point is reached within a first critical distance, the amplitude of the resulting magnetic field in the one direction which is opposite to that of the direct field, is no longer sufficient to change the magnetisation of the hard magnetic portion of the BME in this direction.The result of this is that the symmetrical excitation of the BME is converted to an asymmetrical excitation and the pulses of one polarity no longer occur. As the object approaches more closely to the proximity switch, at a point within a second (smaller) critical distance between the object and the proximity switch, the resulting magnetic field in one direction, which is opposite to that of the direct field, becomes so weak that it is no longer strong enough to reset the BME, whereby the asymmetrically excited pulses in the sensor winding are also absent.

In the second case, the magnetic direct field, whose strength at the position of the BME is dependent upon the distance of the object to be monitored from the proximity switch, is selected so that, as the distance between the object and the proximity switch decreases, it becomes weaker at the position of the BME. When the object is at a considerable distance, the magnetic field emanating from the exciter winding is superimposed so strongly upon it* with a direct field component that the resulting magnetic field at the position of the BME is a pulsating direct field. As the object approaches more closely to the proximity switch, the magnetic direct field component is weakened at the position of the BME.A point having been reached within a first critical distance, the BME passes from the state of non-excitation firstly, to the state of asymmetrical excitation, in which firstly, in each cycle a pulse of constant polarity is produced, and, at a point within a second critical distance, it passes finally to the state of symmetrical excitation, in which pulses of alternating polarity are produced in the sensor winding, that is to say, two pulses per cycle (one pulse per half-wave).

In both cases, the two-stage signal transition from symmetrical to asymmetrical excitation, and transition from asymmetrical excitation to non-excitation (no pulses)may be employed with advantage for releasing two different, consecutive operations. For example, on reaching the first (greater) critical distance, the speed of a machine element can be reduced (creep gear, slow speed) and on reaching a point within the second (lesser) critical distance, the power can be completely switched off.

A further advantage of symmetrical excitation of the switch according to the invention lies in the possibility of distinguishing, from which of two directions an object is approaching the proximity switch, or which of two different objects is approaching the proximity switch.

This can be arranged by constructing for the two directions, or for the two objects, magnetic direct fields of opposite polarity, which are superimposed upon the magnetic alternating field of the exciter coil. This may, for example, appear as follows: two objects approaching the proximity switch each carry a permanent magnet, which magnets create at the position of the BME fields of opposite polarity. If one object approaches unduly closely to the proximity switch, pulses of one polarity appear in the sensor winding in the transition zone of asymmetrical excitation, whilst, if the other object approaches, pulses of opposite polarity appear, whereby the polarity of the pulses curing the period of asymmetrical excitation can be evaluated to distinguish between the two objects.

A phase detector may be provided with advantage, which is connected to the sensor winding and which determines the phase position of the pulses produced in the sensor winding in relation to the phase of the periodic exciter current in the exciter winding. Assuming, for example, the case of symmetrical excitation of the BME, if, as the object to be monitored approaches the critical distance threshold, a magnetic direct field is imposed with increasing strength upon the alternating field, then, when the alternating field is in its phase in which it is opposite to the direct field, the pulses produced are displaced increasingly towards the peak of the exciter current in this phase region, until finally, at the peak, only the change of magnetisation of the hard magnetic portion to the appropriate direction can occur; as the object approaches more closely to the proximity switch, the BME can now only be excited asymmetrically, and the pulses of one polarity do not occur in the sensor winding. From the change in phase position of the pulses, it is possible therefore to deduce, how far the object to be monitored is still removed from its critical distance threshold. By this arrangement, suitable measures can be employed to decelerate a machine element in good time, even before reaching this critical distance threshold. As means for the production of the magnetic direct fields, permanent magnets, or arrangements or permanent magnets, can be employed, although basically, electromagnets may also be employed.

The BME is preferably stationary, whilst the permanent magnets, or ferromagnetic components influencing said permanent magnets, are displaceable and are coupled with the object to be checked for distance. Basically, however, the reverse arrangement is also possible, in which it is the BME which is displaceable. Because of the high signal yield, a Wiegand wire is preferably employed as BME, the sensor winding and exciter winding being placed around the BME.

Specific embodiments of the present invention will now be described with reference to the accompanying, diagrammatic, drawings, in which: Fig. 1 shows a proximity switch employing a Wiegand wire; Fig. 2 is a diagram illustrating the manner of operation of the switch of Fig. 1, with symmetrical excitation and showing one switching state; Fig. 3 is a diagram, similar to Fig. 2, showing a second switching state; Fig. 4 is a diagram, similar to Fig. 2, showing a third switching state; Fig. 5 shows the switch of Fig. 1 for operation with asymmetrical excitation; Fig. 6 is a diagram for the switch of Fig. 5, showing one switching state; and Fig. 7 is a diagram, similar to Fig. 6, for the other switching state of the switch of Fig. 5.

Referring to Fig. 1, a sensor winding 2 and an exciter winding 3 are wound upon a Wiegand wire 1. A bar magnet 4 is coupled mechanically with an object to be monitored. It is disposed parallel to the Wiegand wire 1 and is displacable parallel to itself, so that it can be moved towards the Wiegand wire 1, whereby the direct field emanating therefrom is strengthened at the position of the Wiegand wire 1. The exciter winding 3 is supplied with an alternating current of sinusoidal waveform 10 with the angular frequency o: le=lO sin cat (t=time, lo= amplitude of the exciter current).

Thereby, a magnetic alternating field is produced at the position of the Wiegand wire 1 which also changes sinusoidally: He=HO-sin cat (H=magnetic field strength).

As long as the bar-magnet 4 is maintained at a distance, practically no direct field is superimposed upon the alternating field He (Fig.

2). On reaching the field strengthens Ha and -H8, with symmetrical excitation, in each case a strong Wiegand pulse 5 is produced in the sensor winding 2. The Wiegand pulses 5 have alternately differing signs (Fig. 2); these result from the fact that the magnetisation of the soft magnetic core of the Wiegand wire 1 is reversed to a direction which is anti-parallel in relation to that of the hard magnetic shell. At an even higher field strength H9 or -H8, the polarity of the shell is also changed and is orientated parallel to the core of the Wiegand wire. With this is associated a small pulse 6 in the sensor winding 2, which is not normally utilised.

As the bar-magnet 4 is moved closer to the Wiegand wire 1, a magnetic direct field Hm is superimposed upon the alternating field He. If H exceeds the value H0-H9 at the position of the Wiegand wire 1, the field strength H=H0-H is no longer capable of reversing the magnetisation of the hard magnetic shell of the Wiegand Wire, and the symmetrical excitation is converted into asymmetrical excitation, whereby only Wiegand pulses 5 of one polarity remain (Fig. 3).

If Hm exceeds the value HoHR at the position of the Wiegand wire 1, HA being the field strength required to return the core of the Wiegand wire 1 from its parallel direction of magnetisation (relative to that of the shell) to anti-parallel magnetisation (approximately 1 6 A/cm), then this resetting of the Wiegand wire 1 also fails to occur, and no Wiegand pulses are produced (Fig.

4).

If, instead of alternating current, the Wiegand wire 1 is excited by a pulsating direct current l (Fig. 5), for this purpose a bridge rectifier 7 may be connected to the exciter winding 3, which rectifier is supplied with a source of alternating current 8 and rectifies the alternating current by way of full-wave rectification.

If the bar-magnet 4 is removed at a distance from the Wiegand wire 1 such that its field at the position of the Wiegand wire 1 is negligibly small, only the pulsating direct field Heg acts upon the Wiegand wire 1 (claim 6). Since no sign-change occurs in the magnetic field Heg, no Wiegand pulses are produced either in the sensor winding 2. If, however, the magnet 4 is moved closer to the Wiegand wire 1, the magnetic field Hm of the bar-magnet 4 is superimposed upon the pulsating direct field He, to which it is opposed. The resulting magnetic field H,,--H, is an alternating field. As soon as the bar-magnet 4 is close enough to the Wiegand wire 1 for Hm to exceed the value HA at the position of the Wiegand wire 1, he being the field strength required for the magnetic resetting of the Wiegand wire, i.e.

approximately 16 A/cm (see above, description relating to Fig. 4), the Wiegand wire 1 can now be periodically reset, whereby Wiegand pulses 5 are also produced periodically in the sensor winding 2 (Fig. 7).

In both embodiments, an evaluation circuit is connected to the terminals 9 and 10 of the sensor winding 2. This may also be provided with a phase detector which determines the phase position of the Wiegand pulses 5 in relation to the phase of the exciter alternating current 1e, or direct current leg.

Claims (12)

Claims

1. A magnetic proximity switch comprising a bistable magnetic element (hereinafter referred to as BME); means for the production of a magnetic direct field, the degree of influence of which upon the BME is a function of the distance between an object to be monitored and the proximity switch; an electrical sensor winding coupled magnetically with the BME, in which winding, on passing a predetermined critical distance between the object to be monitored and the proximity switch, an electrical pulse is induced as a result of an abrupt change of magnetisation direction in the BME, characterised in that there is coupled magnetically with the BME an electrical exciter winding which is supplied with a periodic electrical signal, the polarity and strength of which are of such a magnitude in each cycle that the field strength of the magnetic field produced by the periodic electrical signal in the exciter winding is capable of changing the direction of magnetisation of the BME at the position of said BME.

2. A proximity switch according to claim 1, characterised in that the exciter winding is connected to a current source producing a pulsating direct current, the pulsating magnetic field of which is oppositely disposed, with at least one magnetic field component, to the distancevariable magnetic field at the position of the BME.

3. A proximity switch according to claim 2, characterised in that said current source is a fullwave rectifier supplied with alternating current.

4. A proximity switch according to claim 1, characterised in that the exciter winding is connected to a source of alternating current, whereby the strength of the magnetic alternating field emanating from the exciter winding is so calculated that, in the absence of a further magnetic field, it is capable of exciting the BME symmetrically in order to reverse the direction of magnetisation of the BME.

5. A proximity switch according to claim 4, characterised in that, for the purpose of differentiating between two objects alternately approaching the proximity switch, said objects have means for producing different magnetic direct fields, whereby said direct fields are disposed opposite to each other at the position of the BME, and at this location both become either stronger or weaker as the distance between the particular object and the proximity switch increases.

6. A proximity switch according to claim 4, characterised in that, for the purpose of differentiating between two directions from which objects may approach the proximity switch, said objects have means for producing different magnetic direct fields, whereby, as objects approach from one direction, these direct fields are oppositely directed, at the location of the BME, to the direct fields when objects approach from the other direction, and both become either weaker or stronger at this point as the distance of the objects from the proximity switch increases.

7. A proximity switch according to any preceding claim, characterised in that a phase detector is connected to the sensor winding, which detector determines the position of the voltage pulses produced in the sensor winding as a result of the reversal of the direction of magnetisation in the BME, in relation to the phase of the electrical, periodic signal supplied to the exciter winding.

8. A proximity switch according to any preceding claim, characterised in that a permanent magnet, or an arrangement of permanent magnets, is provided for producing the magnetic direct field.

9. A proximity switch according to claim 8, characterised in that the permanent magnet or magnets are stationary and are influenced by ferromagnetic components, the movement of which is coupled with the movement of the object(s) to be monitored.

10. A proximity switch according to any preceding claims, characterised in that the BME is a Wiegand wire.

11. A proximity switch according to any preceding claim, characterised in that the sensor winding is located on the BME.

12. A proximity switch according to any preceding claim, characterised in that the exciter winding is located on the BME.

1 3. A proximity switch substantially as hereinbefore described with reference to Figs. 1 to 4 or Figs. 5 to 7 of the accompanying drawings.