Technical Field

The present invention relates to an improved method and an improved device for measuring the distance between two mutually opposing surfaces by means of the reluc- tance method. To this end, there is used a position indicator whose measuring device is mounted on an asso¬ ciated core which is mounted on one of the two surfaces, wherein the air gap separating the surfaces forms part of the measuring circuit. The position indicator in- eludes a cylinder which is connected to the core and which, similar to said core, is comprised of a magnetic, highly permeable material.

Background of the Invention

A method and a device of this kind are known from SE-C 7904903-7 (416 844). The position indicator of the known device functions to detect a magnetic field which is closed over the air gap formed between two opposing and mutually spaced surfaces where the distance therebe¬ tween is to determined, through the intermediary of the magnetically conductive material in the opposing sur¬ face. The material of the opposing surface is thus a passive circuit element in the magnetic circuit incσrpo- rated in the position indicator. The flux paths generat¬ ed over the air gap have a pronounced curvature or spread, which results in a weak field that makes measur¬ ing of air gaps of large widths difficult or impossible to accomplish, i.e. distance measuring when applying the known method and the known device is limited to solely relatively small distance values.

When .using the known method and the known device to measure the distance between the grinding discs of a refiner or like apparatus, the measuring result is highly dependent on the extent of wear on the surfaces of the grinding discs. This wear accentuates the path curvature or path spreading of the magnetic flux across the gap between the grinding discs. Similar measuring methods and devices are also known from SE-C 7503505-5 (401 896) and SE-C 7611326-5 (400 379). In these cases, the measuring device has the form of an inductive posi¬ tion indicator which is placed in one of the grinding discs while the other of said grinding discs forms a passively active ferromagnetic body.

Disclosure of the Invention

One object of the inventive method and the inventive device is to provide better possibility of determining large distances or large gap widths between two mutually opposing surfaces, irrespective of whether the material in these surfaces is ferromagnetic in itself or not.

A further object of the invention is to provide a method and a device for measuring the distance between grinding discs in refiners and like apparatus which remain unaf¬ fected by surface wear on the grinding discs to the greatest possible extent.

These two objects are fulfilled with a method and a device according to the present invention, in that the surface located opposite the surface which incorporates the position indicator is magnetized so as to be mag¬ netically active. This is effected by incorporating a magnetic device in this surface.

Thus., when practicing the inventive method and using the inventive device, the magnetic device on said opposing surface will actively influence the measuring device of the position indicator. This will extend the range of the measuring device, i.e. enable larger gaps or gap widths to be measured. Because the flux paths emanating from the magnetic device in said opposing other surface are essentially straight, there is less dependency on the wear of the two mutually coacting surfaces in the case of refiner grinding discs for instance.

The magnetic device may be either a permanent magnet or an electromagnet.

Description of Preferred Embodiments

The invention will now be described in more detail with reference to exemplifying embodiments thereof and with reference to the accompanying drawings which illustrate the principles of these embodiments, in which drawings

Figure 1 illustrates a first embodiment of an inventive measuring device intended for measuring the distance between mutually opposing and relatively rotating sur- faces;

Figure 2 illustrates a second embodiment of the inven¬ tive measuring device, wherein the distance is deter¬ mined by measuring a static magnetic field; Figure 3 illustrates a further development of the e - bodiment illustrated in Figure 2; and

Figure 4 illustrates a further embodiment of the inven¬ tive measuring device while using an electromagnet to magnetize the second, opposing surface.

All of the Figures illustrate schematically two surfaces 1, 2 with an intermediate gap 3 whose width B is to be

determined. The gap 3 may be an air gap between two grinding discs 1, 2 of a refiner, although they may egually as well be any gap between two surfaces that can be moved relative to one another, as indicated by the double arrow P. Thus, the gap need not necessarily be an air gap, but can be an interspace filled with some other medium or media, either gaseous or liquid.

A measuring device 4 is provided in the one surface 1, for instance mounted in a cavity inwardly of said sur¬ face. The measuring device 4 is preferably of a known kind, for instance of the kind described in SE-C 7904903-1 (416 844). Thus, the device may include a core 6 of highly permeable magnetic material and a cylinder 8 comprised of a similar material. The core 6 and the cylinder 8 are mutually connected at the inner part 5 of the measuring device 4, as seen from the gap 3. The space between the core 6 and the walls of the cylinder 8 is filled with material, preferably a magnetically inactive material.

The core 6 carries a measuring device, which in the embodiment of Figure 1 is comprised of a winding Nl. The magnetic field detected by the winding Nl gives rise to a signal on the electric outputs 7 of the winding. This signal can be used in known manner (not shown in detail) to indicate the distance between the surfaces defining the gap 3 or the width B of said gap. The possibility of determining the distance B with the aid of this simple winding or coil Nl is afforded because the opposing, second surface 2 is magnetically active, i.e. because it is provided with a magnetic device M. In the case of the illustrated embodiment, this magnetic device has the form of a permanent rod-like magnet 10 which is placed generally perpendicular to the gap 3 and in line with the core 6 incorporated in the opposing one surface 1.

A further condition for determining the distance B with the aid of the magnetic-field-detecting winding Nl is that the surfaces 1, 2 move relative to one another as indicated by the arrow R. In the case of refiners, the relative movement R is the relative rotational movement of respective grinding discs 1, 2. This relative move¬ ment will induce an electromotive force, emf, in the winding Nl, which then produces an electric signal on the output 7. The amplitude of the signal constitutes a measurement of the width B of the gap 3 and can thus be indicated directly in terms of the gap width B with the aid of a suitably graduated instrument (not shown) connected to the output 7.

Figure 2 illustrates a second embodiment of the inven¬ tive device. In the case of this embodiment, the measur¬ ing device comprises conventionally a Hall-element 9 which is fitted on the core 6 of the measuring device 4 mounted on the one surface 1. The other, opposing sur- face 2, carries the magnetic device M, which also comprises a permanent magnet 11, although in this case the magnet has a cylindrical shape corresponding to the shape of the measuring device 4. The cylindrical casing 12 of the permanent magnet 11 lies in line with the cylinder 8 of the measuring device 4. A leg 13 on the permanent magnet 11 is placed in line with the core 6 of the measuring device 4. The one pole S of the permanent magnet 11 is formed by the casing 12, while the other pole N is formed by the leg 13.

The measuring device illustrated in Figure 2 enables the gap width B to be determined even when conditions be¬ tween the surfaces 1, 2 are static. In this case, the Hall-element 9 measures the static magnetic field 01 which is generated by the permanent magnet 11 in the measuring device 4. The strength of the static field 01

constitutes a measurement of the distance B between the surfaces 1, 2, and a corresponding electric voltage signal can be taken out from the output 7 of the Hall- element 9.

Figure 3 illustrates a further development of the mea¬ suring device shown in Figure 2. In this case, the static magnetic field 01 measured with the Hall-element 9 is balanced out by a coil or winding N2 which gener- ates a counter-directed magnetic field 02, so that the total magnetic field 01+(-02) will endeavour "to assume a zero value. This is achieved by using the signal on the output 7 of the Hall-element 9 to produce an input signal on the input 17 of the winding N2, so that the winding N2 will generate the desired counter-directed magnetic field 02. The current value of the electric input signal will therewith constitute a measurement of the distance B between the surfaces 1, 2.

When the surfaces 1, 2 move relatively to one another, it is advantageous to generate an alternating current, for instance as described in SE-C 7904903-7 (416 844). In this case, two magnetic devices whose poles are reversed in relation to one another are preferably used as alternating field generators.

The magnetic device M may also have the form of an electromagnet 14, as illustrated in Figure 4. The elec¬ tromagnet 14 has a coil or winding N3, which may option- ally be connected electrically in series with an auxili¬ ary coil or winding N2' on the core 6 of the measuring device 4. This auxiliary coil N2' generates a magnetic field which is added to the counter-directed field 02 in dependence on the size and direction of the supply current delivered to the coil N3 of the electromagnet 14 via the input 27. When a constant current is supplied,

the aforedescribed method is preferably alternated.

The present invention places no particular demands on magnetic conductance of the materials from which the surfaces 1 ; 2 are formed, and this material can be fully adapted to other properties required of the apparatus or machine in which these surfaces 1 , 2 are included.

Although the invention has been described with reference to preferred, exemplified embodiments thereof illustra¬ ted in the drawings, it will be understood by those skilled in this art that different variations and modi¬ fications can be made without departing from the inven¬ tive concept. Consequently, the invention should not be considered restricted to the illustrated exemplifying embodiments thereof, since other embodiments included in the scope of the following Claims are also conceivable.