GB2165648A

GB2165648A - Electro-magnetic measuring probe

Info

Publication number: GB2165648A
Application number: GB08425662A
Authority: GB
Inventors: Helmut Fischer
Original assignee: Fischer & Co Inst fur Ele GmbH
Current assignee: Fischer & Co Inst fur Ele GmbH
Priority date: 1984-10-11
Filing date: 1984-10-11
Publication date: 1986-04-16
Also published as: HK1589A; GB2165648B; GB8425662D0

Abstract

An electro-magnetic measuring probe comprises a pot core 11 having a central bore 18, a pot chamber 39 coaxial to the central bore 18 accommodating a winding 33, 36, and a pot rim 43; and a pole- piece 21, 48 fixed into the central bore 18 at one of its ends and having a coaxial recess 51 machined into its free front-face such that an outer shell 48 remains around the recess, a hard-metal pin 47 being seated in the recess, the front-face 52 of which forms a crowned contact surface, whereby the contact surface stands a little proud of the pot rim 43. A coaxial annular disk 42 of highly permeable magnetic material, has an outer edge 41 in contact with the pot rim 43, and an intrados surface diameter 53 greater than that of the pole-piece 48 but significantly smaller than that of the pot chamber 39, and is close to the front-face 49 of the pole-piece 48, whereby the magnetic resistance between the intrados surface 53 and the shell 48 is significantly greater than that-with the probe set down-between the free front-face 49 and the intrados surface 53. <IMAGE>

Description

SPECIFICATION Electro-magnetic measuring probe The present invention relates to an electromagnetic measuring probe as per the pre-characterizing clause of the main patent application.

This probe is to be developed further without sacrificing the characteristics described therein. Thus, it will still be possible to measure lineally, above all in the region of thin coatings, avoiding the non-linearity problems which result from the magnetisation curve as well as the geometry of the crowned contact surface. The probe will still be suitable for measuring small specimens. It is desired, however, to achieve the following additional benefits: a) The indication is to be more exact in the region of very thin coatings. For instance, if the said probe is set down on an uncoated substrate, then, despite of the absence of a coating, a coating thickness is indicated.

b) It is desired that a larger output signal is obtained for the same coating thickness which, of course, aids a better indication.

This object is achieved, according to the invention, by means of the features which are evident from the characterizing clause of the main patent claim. The invention has, among others, the following advantages: 1. The surface curvature plays an even less significant role.

2. Better measurements can be made on smaller specimens.

3. Almost the same or, in some cases, identical characteristics are obtained for markedly different substrates.

4. The power consumption is so small that the probe can be used in a battery instrument without an external power supply.

5. Despite the improved characteristics, the space for the windings must not be enlarged, rather the same space for the windings is sufficient.

6. The probe is even less sensitive to tilting.

The features of claim 2 allow the effect of the annular disk as a short-circuit winding to be reduced.

The features of claim 3 allow the annular disk to have practically no short-circuit winding effect.

The features of claim 4 allow, in the same manner, any damping effect of the pot core to be avoided and, when the two slots are mutually aligned, allow the disadvantages of a possible bridging which could result in the appearance of damping anyway, to be avoided.

The features of claim 5 allow the reduction in damping to be optimal for such cases in which the internal stress of the material allows the radial slot to be continued all the way through.

The features of claim 6 allow an optimization of the shunt on one hand and the bundelling of the field on the other. Factors which are too small bring with them a shunt. Factors which are too large cause bundelling.

A further improvement within the scope of the object of the invention.

The features of claim 8 allow an axial-symmetric constriction which is easy to manufacture.

The features of claim 9 allow measurements to be made in recesses such as grooves etc..

The features of claim 10 allow the field to be better bundelled without approaching saturation. It is important that the field is bundelled as tightly together as possible without, however, becoming saturated, because saturation would falsify the result of the measurement.

The features of claim 11 allow the area which is embossed downwards to be obtained in a particularly easy manner during the manufacturing process.

The features of claim 12 allow field-line concentrations which would radiate from the points of the stamp-fin if this were serrated to be avoided.

The features of claim 13 allow those effects which the highly permeable disk 37 and the highly permeable cylinder 41 of claim P 33 31 407 have to be compensated, since these components are no longer to be found in the apparatus according to the invention. In the aforementioned claim a low permeability hard metal of roughly the composition 48% WC; 15% Co; 2% TiC/TaC/NbS was used; this has now been replaced by a material of high permeability.

Optimum values with regard to the precision of the coating thickness indication and the optimally large signal are allowed by the features of claim 15.

The features of claim 16 allow the optimum of claim 15 to be particularly well adjusted.

The features of claim 1 7 allow the fine adjustment to be particularly easily effected.

Naturally, the coils-as usual-are fixed and the pole-piece moves relatively to them in one direction, namely outwards, when the grub screw is screwed in.

The invention is now described using preferred illustrative embodiments. Because of the exact dimensions, millimeter scales are specified. The drawings show: Figure 1 a radical section through a setdown measuring probe of a first design example, Figure 2 a similar section through a second design example, Figure 3 a similar section through a third design example, Figure 4 a curve which shows in comparison to the state-of-the-art how much better the indication for an uncoated substrate of one of the design examples is, also as a function of the diameter of the substrate which is in the form of a disk.

Figure 5 a graph showing the improvement of the measuring signal in comparison to the state-of-the-art, Figure 6 the bottom view of an annular disk for a modification of Fig. 1.

A pot core 11 of soft iron is rotationallysymmetric about its geometric longitudinal axis 12. It has a base 13 and a circular-cylindrical wall 14.

Near the base 13 in its lower half is a horizontal threaded through-bore 16. A grub screw 17 is screwed into said bore. Coaxial to the geometrical longitudinal axis 12, the base 13 has a through-bore 18 which, however, has a diameter substantially smaller than that of the internal face 19 of the base 13. A circular-cylindrical core 21 made of material of high magnetic permeability is located in the through-bore 18 as per the upper half of Fig.

1. Its top side 23 which has a bevel 22 is at the same height as a second threaded through-bore 24 which lies above and parallel to the threaded through-bore 16. A grub screw 26, the shaft of which is shaped as the frustum of a cone 27 and penetrates into the through-bore 18, is screwed into this. The angle of inclination of the bevel 22 corresponds to that of the taper of the frustum of the cone 27 so that, when the grub screw 26 is screwed inwards, the effect of its pitch is added to that of the pitch of the frustum 27 allowing the core 21 to be pushed downwards out of the bore with a correspondingly fine adjustment. The crowned leading face 28 of the grub screw 17 contacts under pressure the circumference of the core 21 supplying the friction which must be overcome when the grub screw 26 is screwed inwards.A plastic washer 29 which has a coaxial, central hole 31 of the same size is seated on the pot core 11.

Two terminals 32 for a exciter winding 33 are provided on the plastic washer 29. Two other terminals 34 on the plastic washer 29 are used for connecting the induction winding 36. The exciter winding 33 and the induction winding 36 are wound on formers which are fixed relative to the pot core 11. The core 31 can move through the winding.

In order to allow free passage for the wires running down from the terminals 32, 34 a vertical slot 37 is provided which lies radially, connected with the throughbore 18, however, which does not quite reach the outside, so that as per Fig. 1, a wall 38 remains left of the slot 37. Below the base face 19 coaxial to the geometrical longitudinal axis 12 is a circular-cylindrical winding space 39. The volume above the constriction described later is taken up by the exciter winding 33 and the induction winding 36. These are, naturally, not drawn to scale but only represented symbolically and have the usual configuration for this technique.

Under the circular-cylindrical wall 14 is an annular pot rim 41 which is of uniform thickness throughout and is polished so that the outer edge 43 of an annular disk 42 lies congruently and for the magnetic field lines practically lostless on it. The annular disk 42 is stuck onto the pot rim 41 with a very thin layer of adhesive so that there is practically no gap. A constriction 44, which is practically radial, on the core 21 starts within the space for the windings. The constriction 44 is located underneath the windings and underneath the top face 46 of a pin 47. Below the constriction 44 is a single monolithic circular-cylindrical shell 48, which is coaxial to the geometrical longitudinal axis 12. The lower lip of the shell 48 is formed as a crowned front face 49 forming part of a sphere, the center of which lies on the geometrical longitudinal axis.A circular-cylindrical, coaxial blind hole 51 is bored into the lower face of the core 21. This extends almost as far as the windings and beyond the constriction 44. The exact dimensions in this area are to be taken from the drawing. The bottom end of the pin 47 also has a crowned front-face 52 which also forms part of a sphere and represents a direct continuation of the crowned front-face 49. The intrados-radius surface 53 lies at the indicated distance from the outer surface of the shell 48. It is coaxial to the longitudinal axis 12. The annular disk 42 has a permeability greater than 100 z0, typically 1000 80. The crowned front-face 52 is positioned on the top surface of a coating 55 which is to be measured, which, for its part, is deposited on a substrate 56.

After the axial position of the core 21 has been adjusted, the space for the winding 39 is filled with cast resin. The pin 47 is made of a sintered hard metal as before and has a higher permeability than the pin 27 described in patent application P 33 31 407, namely in the range from 50 to 200 ILo.

The design example as per Fig. 2 differs from the design example as per Fig. 1 in three points: Firstly, the slot 37 is continued outwards by a through-slot 57 which extends parallel to the longitudinal axis 12 through the entire height of the pot core 11.

Secondly, a coaxial tapered face 58, lying on a circular cone, starts at the position where the constriction 44 is to be found in the first design example. Instead of the sudden reduction in cross-sectional area, therefore, this example has a gradual reduction in cross-sectional area. The tapered face 58 continues into a frustum of a cone 59 of the pin 47. The crowned front-face 52 is still present but has a smaller external radius. This design example is particularly suitable for measurements in grooves or other recesses.

Thirdly, the annular disk 52 is embossed outwards 61 in the form of a flat frustum of a cone. The pot rim 41 lies correspondingly higher so that the probe can no longer be tilted without the outside lower edge being set down on the surface of the coating 54.

The intrados-radius surface 53 lies at such a position that the tapered face 58 and not the frustum of a cone 59 is opposite.

The design example as per Fig. 3 is similar to that in Fig. 1, but also in some features to that of the design example in Fig. 2. In this case, a co-axial, circular-cylindrical hole 62 is stamped during manufacture. The stamping produces a stamping fin 63 which points downwards, the protruding front-face 64 of which is ground so that the field lines can exit uniformly from this point.

The annular disk 65 shown in Fig. 6 differs from the other annular disks in that it has a full through radial slot 66. If the annular disk 65 is used and the slot 57 is present for this design example, then, the radial slot 65 is aligned with the slot 57.

A probe as per the invention was set down on uncoated substrate disks of different diameters, the dimensions are shown in millimeters in Fig. 4. The first thing to note is that measurements can still be made, for example, on disks of 5 mm diameter. The coating thickness in zm indicated by the instrument in accordance with the earlier patent application, shown by curve 67, is plotted to the right. In theory, an uncoated substrate must give a coating thickness of 0. The measurement error can be clearly seen. Moreover, it is also clear that the effect of limited area (edge effect) as the disk diameter becomes smaller is substantially smaller than for the probe to be improved.

The apparatus as per the invention has a curve as per curve 68. It can be seen that the apparatus as per the invention shows 5 m as opposed to 19 iim for measurements on a disk of 10 mm diameter, an improvement by a factor of 3.8. This improvement is quite substantial. It can be seen from the trend that the improvement becomes greater as the disk diameter becomes smaller. A substantial improvement is also seen for disks of larger diameter, for example, for a 15 mm disk the factor is 2.7 and for a 20 mm disk the improvement of 2.4 lies well beyond that which is usual and can be expected for the technique.

The apparatus as per the invention does more than indicate more exactly. The logarithmic coating thickness in,um is plotted on the Y-axis in Fig. 5. The normalized output voltage at the induction winding 36 is plotted on the X-axis. Curve 69 represents the results obtained from a probe in accordance with the earlier patent application, whereas curve 71 shows the results for the present invention. If a coating thickness of 10 m is taken, for example, then, a normalized voltage of 0.065 at point 72 would have been found earlier.

Due to the invention, a normalized voltage of 0.18 at 73 is now found, so that a roughly 300% improvement is obtained, which is quite definitely far beyond the usual improvements which could be expected and which (if at all) would normally be of only a few percent.

Claims

1. Electro-magnetic measuring probe having a pot core which has a central bore, a pot chamber coaxial to the central bore accommodating a winding, and a pot rim, a pole-piece, which is fixed into the central bore at one of its ends, and which has a coaxial recess machined into its free front-face such that an outer shell remains around the recess, as well as a hard-metal pin which is seated in the recess, the front-face of which forms a crowned contact surface, whereby the contact surface stands a little proud of the pot rim, the characterizing features wherein-a coaxial annular disk is provided, made of highly permeable magnetic material, the outer edge of which is in contact with the pot rim, the intrados-radius surface of which has a diameter greater than that of the pole-piece but significantly smaller than that of the pot chamber, and is to be found close to the front-face of the pole-piece, whereby the magnetic resistance between the intrados-radius surface and the shell is significantly greater than that-with the probe set down-between the free frontface and the intrados-radius surface.

2. Probe as claimed in claim 1, wherein the annular disk has a radial slot.

3. Probe as claimed in claim 2, wherein the radial slot passes through the disk.

4. Probe as claimed in claim 2 or 3, wherein the radial slot of the annular disk continues as a radial slot of the pot core.

5. Probe as claimed in claim 4, wherein the radial slot of the pot core continues through to its central bore.

6. Probe as claimed in claim 1, wherein the diameter of the intrados' radius surface is approximately 1.5 to 2 times the diameter of the part of the pole-piece immediately opposite.

7. Probe as claimed in claim 1, wherein the outer shell is constricted in the area of the pin.

8. Claim as claimed in claim 7, wherein the construction is circular cylindrical.

9. Probe as claimed in claim 7, wherein the constriction is conical.

10. Probe as claimed in claim 1, wherein the annular disk lies lower in the area of its intrados-radius surface than in the area of the pot rim.

11. Probe as claimed in claim 10, wherein the annular disk is embossed outwards in the form of the frustum of a cone.

12. Probe as claimed in claim 10, wherein the annular disk has a stamp fin, the bottom surface of which is ground flat.

13. Probe as claimed in claim 1, wherein the pin has a magnetic permeability of 40-80 ILo, preferably in the range of 60 ,uO.

14. Probe as claimed in claim 13, wherein the pin is made of an alloy of the type 10.5% Co; 6.5% Va; 7% Mo.

15. Probe as claimed in claim 1, wherein the outer front-face of the hard-metal pin stands proud of the annular disk by about 5-50%, preferably by 30%, of the diameter of the intrados-radius surface.

16. Probe as claimed in claim 1, wherein a fine adjustment device is provided at the pot core for adjusting the pole-piece during assembly.

17. Probe as claimed in claim 16, wherein the back face of the pole-piece has a bevel which is in contact with the conical face of a grub screw screwed into the pot core and a second grub screw presses on the pole-piece with its crowned front-face.

18. An electro-magnetic measuring probe substantially as described with reference to Figs. 1, 2, 3 or 6 of the accompanying drawings.