GB2126351A - Electro-magnetic saturation and possible re-setting of Wiegand effect sensors - Google Patents

Abstract

A Wiegand sensor 4, which is triggered by a rotating permanent magnet 2, is saturated electromagnetically by means of a circuit 8 triggered by the Wiegand pulses, which then supplies for a short period an electrical current to the sensor induction winding 7 of the Wiegand sensor 4 in order to build up a saturating magnetic field. This is stated to be preferable to the conventional use of a permanent magnet to saturate the sensor. The re-setting permanent magnet 5 can also be replaced by a further winding which is energised between the periods when electromagnetic saturation is taking place. <IMAGE>

Description

SPECIFICATION Method and apparatus for the magnetic excitation of Wiegand sensors The present invention relates to a method and apparatus for the magnetic excitation of the Wiegand sensors. The operation of Wiegand sensors by permanent magnets is characteristic of the majority of uses of these sensors, e.g. for monitoring the position of moving machine parts, in proximity switches, in revolution counters, position encoders, angle encoders and the like.

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% iron 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 10 mm - 50 mm, preferably 20 mm - 30 mm.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. 16A/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 parallel magnetisation. 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 1 20A/cm) to + (80 - 1 20A/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 whch 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 of the Wiegand wire. For this purpose a field strength is required in one direction of at least 1 6A/cm (for resetting the Wiegand wire) and in the opposite direction a field strength of approximately 80- 120A/cm.

An arrangement of a Wiegand wire and an electrical winding influencing it and generally enclosing it, which is the optimal coupling method, is referred to hereinafter as a Wiegand sensor. With symmetrical excitation Wiegand sensors produce Wiegand pulses with high, stable amplitudes when the hysteresis loop of the Wiegand wire is continued as far as possible in both directions into the area of saturation. In the case of asymmetrical excitation, in practice the more ususal situation, Wiegand sensors produce particularly high, stable Wiegand pulses when the hysteresis loop of the Wiegand wire is continued as far as possible in one field direction into the area of saturation, whereas, in the opposite field direction, a field strength is simply required which reliably resets the Wiegand wire magnetically (i.e. at least 16 A/cm, preferably ca. 20 A/cm).The resetting field stength should preferably, however, not exceed ca. 25 A/cm because, above this field strength, reverse magnetisation of the hard magnetic shell of the Wiegand wire may already begin to occur in places - in other words, one is already reaching the transition zone between asymmetrical and symmetrical excitation in which, compared with purely asymmetrical excitation, Wiegand pulses occur with lower, and more particularly, fluctuating amplitudes.

In technical applications of Wiegand sensors, the object is generally to obtain excitation of the Wiegand sensors by means of permanent magnets, whose position relative to the Wiegand sensors can be varied whose magnetic field at the Wiegand sensors can be varied e.g. by relative movement of ferromagnetic elements, with time, whereby, the Wiegand sensor is subjected to a field strength change corresponding to that required for its excitation.

Due however to the inevitable air gap between the poles of the permanent magnets and the Wiegand wire, the requisite saturation field strengths often cannot be achieved though with the result that the quality of the Wiegand pulses is impaired.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages and in particular to improve the level of amplitude of the Wiegand pulses and their uniformity whilst retaining the principle of firing the Wiegand pulses by means of permanent magnets.

The present invention provides a method for the magnetic excitation of a Wiegand sensor comprising a Wiegand wire and an associated induction winding in which sensor the Wiegand pulses occurring in the sensor induction winding are triggered by magnetic interaction between the Wiegand wire and at least one permanent magnet, wherein each Wiegand pulse is used to trigger an electrical circuit formed and arranged to supply to the sensor induction winding for a predetermined period of time, an electrical current of predetermined polarity and sufficient magnitude so as to magnetically saturate the Wiegand wire.

In order to saturate the Wiegand wire the invention no longer uses a permanent magnet but produces the saturating magnetic field instead, electromagnetically directly in the sensor induction winding already provided which basically can be located close to the Wiegand wire, if at the same time care is taken to ensure close magnetic coupling between the two which should however as far as possible enclose the Wiegand wire. In order to ensure that, after triggering or firing of a Wiegand pulse, the Wiegand sensor becomes once again capable of triggering as quickly as possible, the current pulse which produces the saturation magentic field is preferably switched on by the Wiegand pulse itself, most preferably by its leading side. The length and amplitude of the current pulse are so selected that the Wiegand wire is reliably magnetically saturated.

If the leading side or edge of the Wiegand pulse is used immediately for releasing the current pulse, this produces a pulse in the sensor winding, the leading side of which has firstly the characteristic form of a Wiegand pulse. From the moment of triggering however the height and length of the pulse is determined essentially by the electrical circuit triggered by the Wiegand pulse. Thus, the essential characteristic of the invention is the manner in which the Wiegand pulses produced are transformed and strengthened electronically in a predetermined manner and are so employed for the creation of a predetermined magnetic saturation field in the sensor winding. It would also be possible to have a less advantageous form of the invention wherein the saturation magnetic field is produced with the aid of a separate second winding.

A significant advantage of the invention is that, in order to produce the necessary saturation field strength, the, hitherto conventional, strong permanent magnets which necessitated the provision of narrow air gaps, thus creating problems of accommodation and application, are no longer required. In contrast, by employing electrical means the required saturation field strengths can be produced without any problem. No special problems of accommodation arise since, in order to supply the current to produce the magnetic saturation field, the two leads to the sensor winding which are already required for conveying the Wiegand pulse to a receiver circuit, are used. Positioning of the sensor winding on the Wiegand wire offers the great advantage that the Wiegand wire is saturated in a strictly homogeneous magnetic field parallel to its axis.As a result the magnetic structure of the Wiegand wire is favourably influenced and consequently also the height and form of the Wiegand pulses. Such a favourable effect cannot be obtained with the use of permanent magnets.

In typical practical applications of the Wiegand wire with excitation by permanent magnets the Wiegand wire is exposed to different magnetic fields by moving the magnets themselves relative to the Wiegand wire or by moving ferromagnetic objects (e.g. machine parts) relative to the permanent magnets to modify the magnetic fields thereof. In this way the duration of action of the magnetic saturation field is governed by relative speed and, in addition, its amplitude in certain circumstances varies. Both factors can adversely influence the form and amplitude of the Wiegand pulse. In accordance with the present invention though, both problems are substantially avoided.

The most important mode of operation of Wiegand sensors is that with asymmetrical excitation. Here, the invention has the advantage that the high saturation field strength (preferably ca. 100 A/cm) no longer needs to be produced by permanent magnets. For magnetic resetting and triggering of the Wiegand wire a field strength increase of no more than + 20 A/cm is required at the Wiegand wire. This can be achieved without any serious problems with permanent magnets. With symmetrical excitation care must be taken to ensure that saturation occurs first in one and then in the other direction of the Wiegand wire and correspondingly the current passes through the sensor induction winding alternately in one direction and then in the other in order to buiid up the magnetic saturation field.This can be effected by a simple change-over circuit, e.g. by means of a bistable circuit controlled by the Wiegand pulses. For triggering of a Wiegand pulse in the Wiegand wire, which is still necessary with symmetrical excitation, a field strength increase of max. + 20 A/cm is required at the Wiegand wire, which can be achieved without difficulty with permanent magnets.

An advantageous improvement of the invention with regard to a symmetrically excited Wiegand sensors is characterised in that not only the saturation magnetic field but also the oppositely directed resetting magnetic field can be produced by periodical injection of a suitable electrical current, preferably a constant current, into the sensor induction winding. The resetting magnetic field is built up between the time periods during which the magnetic saturation field exists, to be precise during the whole of the intervening period between two successive time periods of magnetic field saturation, in order to avoid unintentional faulty triggerings as a result of the low ignition field strength, generally of a value clearly less than 10 A/cm. In the case of the improved form the triggering field strength is obtained by superimposing on the resetting magnetic field a stronger oppositely directed magnetic field of a permanent magnet, e.g. in an arrangement wherein the resetting magnetic field constantly assumes at the Wiegand wire a strength of 20 A/cm, whilst the permanent magnet, on its closest approach to the Wiegand wire produces thereat a magnetic field of the strength of + 40 Alcm for triggering the Wiegand wire, so that, on superimposing the two fields a resulting field strength of + 20 A/cm remains, which is generally sufficient for triggering.

The advantage of this further development of the invention is that only one permanent magnet, that is to say one permanent magnetic field of unchanging polarity, is additionally required to operate the Wiegand wire. The strength of said field must not exceed 40 A/cm, which is therefore attainable without serious difficulty. A further advantage resides in the fact that the value of the resetting field strength at the position of the Wiegand wire is adjustable and reproducible much more accurately by electrical means than with permanent magnets. It is therefore possible to ensure that the resetting field strength remains within a relatively narrow range appx.

between 18 A/cm and 25 A/cm. Below 18 A/cm resetting of the Wiegand wire is generally unsatisfactory and results in weaker Wiegand pulses. Above 25 A/cm magnetic reorientations are already setting in in some parts of the hard magnetic shell of the Wiegand wire, i.e. we are already entering a transition zone between asymmetrical and symmetrical excitation, which also results in weaker and less suitably formed Wiegand pulses.By the further development of the invention such disadvantages can be obviated. Afurtherfavourable influence on the form of the Wiegand pulses is that, as in the case of the electrically produced magnetic saturation field and also in that of the resetting magnetic field produced in the sensor induction winding, the Wiegand wire is positioned in a strictiy homogeneous magnetic field directed parallel to its longitudinal axis.

Technically, with regard to circuitry, the invention is in general, best put into practice with a, preferably monostable, gate circuit which is triggered by the Wiegand pulses and which opens for a predetermined period of time, thereby connecting the sensor induction winding to an electrical current source which supplies the current for the magnetic saturation field. During the time periods in which the gate circuit is closed, it is possible to pass a constant current of opposite polarity into the sensor winding employing the same current source, or using a separate current source, in order to produce a magnetic resetting field.

Further preferred features and advantages of the invention will appear from the following detailed description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a Wiegand sensor for monitoring a rotating component: Figure 2 is a side elvation of the device of Figure 1; Figure 3 is a block circuit diagram of the electrical circuit employed in Figure 1; Figure 4 is a more detailed form of the circuit diagram of Figure 3; Figures 5a - care graphs illustrating the various stages in the mode of operation of the device of Figures 1 to 4; Figure 6 is a block circuit diagram of an extended circuit including means for electrically producing the resetting magnetic field; and Figure 7 is a graph illustrating variation with time of the magnetic fields influencing the Wiegand wire when using the circuit of Figure 6.

Figures 1 and 2 show a rotating disc 1 near the edge of which is secured a bar magnet 2 which is disposed parallel to the rotational axis of the disc 1.

In proximity to the disc 1 is a fixed carrier 3 on which a Wiegand sensor 4 and a second, fixed, bar magnet 5 are mounted, both of which have their axes disposed parallel to the rotational axis of the disc.

The Wiegand sensor 4 comprising a Wiegand wire 6 and a sensor induction winding 7 which encloses it, is disposed between the disc 1 and the second bar magnet 5. The sensor induction winding 7 is connected to an electrical circuit 8. The two magnets 2 and 5 have opposite directions of magnetisation.

The second bar magnet 5 influencing the Wiegand sensor 4 and having a fixed position relative thereto is intended to magnetically reset the Wiegand wire 6 and produces at the Wiegand wire a magnetic field having a strength of approximately - 20 A/cm and is disposed predominantly parallel to said Wiegand wire 6. The bar magnet 2 following the motion of the disc 1 is intended to trigger the Wiegand pulse as it approaches close to the Wiegand sensor 4. At its point of closest approach (as shown in Figures 1 and 2) the bar magnet 2 produces at the Wiegand wire 6 a magnetic field having a strength of approximately + 40 A/cm and disposed predominantly parallel to the axis of said wire whereby the resulting magnetic field provides at the Wiegand wire 6 a field strength of approximately + 20 A/cm which is generally sufficient for firing.Due to the influence of the two bar magnets 2 and 5, the Wiegand wire 6 is subjected during one revolution of the disc 1 to an increase in field strength of ca. - 20 A/cm to + 20 A/cm. which is sufficient for triggering and resetting of the Wiegand sensor 4 but not for its saturation which must necessarily take place after firing. This magnetic field required for saturation is produced according to the invention by the electrical circuit 8 in the sensor induction winding 7. The block diagram of the circuit 8 in Figure 3 shows a comparator means 9, to one input of which a reference voltage UR iS supplied and whose other input is connected to the sensor induction winding 7.When, and as long as, the pulse voltage produced by a Wiegand pulse and delivered by the sensor winding is higher than the reference voltage Us, there is released from a pulse circuit 10 downstream of the comparator circuit 9 a control pulse which, for the duration of this pulse, closes a switch 11 which connects the sensor induction winding 7 to a constant current source 12 which supplies the sensor induction winding 7 with a current sufficiently strong to saturate the Wiegand wire 6.

Figure 4 shows one embodiment of a monostable circuit as in Figure 3. The sensor induction winding 7 is connected via a differential member C1-R1-R2 to one input of a comparator circuit 22, the other input of which receives a constant reference voltage which is derived from the voltage +Ug of a constant voltage source by a potentiometer comprising the resistances R3 on the one hand and R4 and R2 on the other. The output of the comparator circuit 22 is connected to UB via two series connected resistances R5 and Re forming a potentiometer or potential divider.Between Rg and R6 the control voltage for a transistor T1 is tapped, the emitter of which is connected to U5 and the commutator of which is connected via a pre-resistance Rev to the sensor induction winding 7, the other end of which goes to earth. The comparator circuit 22 is so constructed that, when at rest, i.e. when there is no Wiegand pulse, its output lies at the potential +U5 so that the transistor T1 is blocked. When a Wiegand pulse is fired in the Wiegand sensor 4, this is conveyed to one input of the comparator circuit 22 via the differential means C1-R1-R2 whose time constant is determined basically by C1 and R1.When the reference voltage lying at the other comparison input is exceeded by the Wiegand pulse voltage, the output of the comparator circuit is switched to potential "O". As a result, the base of the transistor T1 receives another potential, the transistor T1 becomes conductive, and it supplies to the sensor induction winding 7 a constant current, the level of which can be adjusted by selecting the suitably variable preresistance Rv. The constant current is supplied to the sensor winding until the voltage at the input of the comparator circuit 22 connected to the sensor winding 7 has fallen below the reference voltage at the other comparator input since then the output of the comparator circuit 22 is again switched to the potential +Ug SO that the transistor T1 is again blocked.The comparator circuit 22 is so designed that only the potentials "0" and "Us" obtain alternately at its output. The time during which the transistor T1 is conductive is determined by the time constant of the differential means C1-R1-R2.

Figures 5a, b and care parallel representations of the variation with time (t) or the following properties: field strength (A/cm) produced at the Wiegand wire by the bar magnets 2 and 5 (Figure 4 a); the potential (U) at the output of the comparator circuit 22 (Figure 5b); and the voltage (U) at the extremities of the sensor induction winding (Figure 5c).

The latter shows, after triggering of the Wiegand pulse (corresponding to the point Z in Figure 5a), firstly the typical rise of a Wiegand pulse. On reaching the reference voltage UR (see Figure 5b) however, the transistor T1 becomes conductive and the subsequent behaviour of the voltage in the sensor induction winding is now determined by the electrical circuit 8 connected thereto. The period of revolution Tof the disc 1 and the period of time tp during which the transistor T1 is conductive, which is determined by the time constant of the member Ci R1 - R2, are shown.The saturation field strength in the sensor induction winding 7 which is superimposed on the permanent magnetic field shown in Figure 5 a and which is produced by the electrical circuit for the time tp is shown in dashed outline and only partially.

Figure 6 is a block circuit diagram of an extended form of the electrical circuit 8 and differs from that shown in the block diagram of Figure 3 only in that, in addition to the constant current source 12, a constant current source 15 of opposite polarity is provided which, during the periods of time in which the constant current source 12 is not connected to the sensor winding 7, is itself connected to the sensor induction winding 7 and supplies it with a current which produces within the sensor induction winding 7 a magnetic field with a strength of ca, 20 A/cm, i.e. sufficientto reset the Wiegand wire 6. For this purpose the on-off switch 11 of Figure 3 is replaced by a change-over switch 14. The bar magnet 5 in Figures 1 and 2 is replaced by the constant current source 15.The only permanent mag net still required is that for triggering the Wiegand pulse (e.g. bar magnet 2 in Figures 1 and 2).

Figure 7 is a graphical representation similar to that of Figure 5 a and showing the variation with time of the various magnetic fields influencing the Wiegand wire 6 where a circuit similar to that in Figure 6 is used. In Figure 7 the field strength of the resetting field (ca. -20A/cm), shown by a broken line, is maintained up to the moment of triggering of the Wiegand pulse (indicated by the pointZ) and is again built up when the saturation magnetic field (ca.

+100A/cm) fades. The magnetic field produced temporarily at the Wiegand wire 6 by the permanent magnet (e.g. bar magnet 2 in Figures 1 and 2) is shown in dashed outline. It compensates the resetting field periodically and, at a resulting field strength of ca. +8 A/cm, produces triggering of the Wiegand pulse. With the occurrence of triggering at Zthe supply of current from the constant current source 12 to the sensor winding winding is initiated and lasts for a short period of time tp during which the constant current source 15 is also separated from the sensor winding. The overall resulting field strength at the Wiegand wire 6 is shown by the solid-line curve.

Claims (12)

1. A method for the magnetic excitation of a Wiegand sensor comprising a Wiegand wire and an associated induction winding in which sensor the Wiegand pulses occurring in the sensor induction winding are triggered by magnetic interaction between the Wiegand wire and at least one permanent magnet, wherein each Wiegand pulse is used to trigger an electrical circuitformed and arranged to supply to the sensor induction winding for a predetermined period of time, an electrical current of predetermined polarity and sufficient magnitude so as to magnetically saturate the Wiegand wire.

2. A method according to claim 1, wherein the leading side of the Wiegand pulse is used to trigger the electrical circuit.

3. A method according to claim 1 or claim 2, wherein the electrical circuit is formed and arranged to supply said current to the sensor induction winding substantially immediately after triggering of a Wiegand pulse.

4. A method according to any one of the preceding claims for the asymmetrical excitation of a Wiegand sensor, wherein is provided means arranged for supplying to the sensor induction winding in between said periods of electrical current of predetermined polarity an electrical current of opposite polarity to that of said current of predetermined polarity and having a strength sufficient for magnetic resetting of the Wiegand wire, and wherein said at least one permanent magnet is used substantially exclusively for triggering the Wiegand pulses.

5. A method according to claim, 4 wherein the electrical current for the magnetic resetting of the Wiegand wire is supplied during the whole of the intervening period between successive periods of time in which an electrical current is supplied to the sensor induction winding for saturation of the Wiegand wire.

6. A device for use in the magnetic excitation of a Wiegand sensor comprising a Wiegand wire and an associated induction winding, in which sensor the Wiegand pulses occurring in the sensor induction winding are triggered by magnetic interaction between the Wiegand wire and at least one permanent magnet, wherein the sensor induction winding is connected to a gate circuit formed and arranged to be triggered by the Wiegand pulses and is opened for a predetermined period of time, and wherein the sensor winding is connected via the gate circuit to an electrical current source which is formed and arranged to supply to the sensor winding, when the gate circuit is opened, a current of predetermined polarity and sufficient magnitude so as to magnetically saturate the Wiegand wire.

7. A device according to claim 6, wherein the gate circuit is a monostable circuit.

8. A device according to claim 6 or claim 7 for the symmetrical excitation of a Wiegand sensor, wherein is provided, between the sensor winding on the one hand and the gate circuit and electrical current source on the other hand, a change-over switch formed and arranged to be controlled by the Wiegand pulses and after each saturation current pulse to reverse the polarity of the sensor induction winding.

9. A device according to claim 6 or claim 7, for the asymmetrical excitation of a Wiegand sensor, wherein is provided for producing a resetting field for the Wiegand wire, an electrical current supply circuit connected to the sensor winding so as to supply a current of opposite polarity to the sensor winding during periods of time in which the gate circuit is closed.

10. A device according to claim 9, wherein the switching in of the electrical current supply circuit for resetting of the Wiegand wire is controlled by the gate circuit.

11. A method for the magnetic excitation of a Wiegand sensor according to claim 1 substantially as described herein before with particular reference to Figures 1 to 3, Figures 1 to 5, or Figures 6 and 7 of the accompanying drawings.

12. A device for use in the magnetic excitation of a Wiegand sensor according to claim 1 substantially as described hereinbefore with particular reference to Figures 1 to 3, Figures 1 to 5, or Figures 6 and 7 of the accompanying drawings.