GB1564727A - Bore measring instruments - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO BORE MEASURING INSTRUMENTS (71) I, THE SECRETARY OF STATE FOR INDUSTRY, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to gauges for the measurement of internal diameters, and more particularly to capacitative probes for the determination of such diameters, by electrical measurements.

It is a basic principle of electrical theory that the capacitance between two conductors is inversely dependent upon the distance between them. Using conventionl AC bridge techniques, this capacitance may be measured to obtain a measure of the separation between the conductors. As a development of this principle, it was discovered empirically that the capacitance between two electrodes separated by an earth plane is modified in a quasi-linear manner by the approach of an earth planar conductor. Proximity gauges have been designed which employ this principle, typical arrangements employing a set of such coplanar electrodes to sense separation from a parallel plane surface.

However, use of this prior art technique has hitherto been limited solely to detecting distances from planar surfaces, and no further applications have been envisaged.

The present invention provides an apparatus and a method for detecting the dimensions of bores and holes by the use of a capacitance measuring technique.

According to the present invention, a probe for measuring the diameter of a hole or bore in an electrically conducting member comprises two ring-shaped, coaxial, axially spaced apart electrodes, a respective electrical connection to each electrode to permit the determination of inter-electrode capacitance as a measure of bore diameter, an electrically conducting screen which extends between the electrodes so as to restrict lines of electrical force to the region outside the electrodes and insulating means to insulate the electrodes from the screen.

The ring-shaped electrodes preferably have equal cross-sections and equal mean radii.

The electrodes, the screen and the insulating means each conveniently have cylindrical outer surfaces forming in combination a continuous cylindrical surface to facilitate centering of the gauge within a hole or bore.

The electrical connections are preferably screened from one another to restrict stray capacitance between them.

The probe preferably includes two guard rings connected to the screen, the guard rings being disposed coaxially with and on either side of the two spaced apart electrodes, the outer periphery of each guard ring being equal to that of each electrode.

The probe may conveniently be in the form of a metal tube having formed thereon two annular recesses, each electrode being located within a respective recess and being insulated therefrom by the said insulation means. The insulation means and the electrodes are also conveniently annular.

The present invention further comprises a method of detecting the bore diameter of a hole in an electrically conducting member, the method employing a probe according to any one of the last six preceding paragraphs wherein the said method comprises the steps of:- - (I) Inserting the probe within a hole in a conducting member, (2) Adjusting the axis of the. electrodes to be parallel with that of the hole (3) Arranging a meter to display a reading proportional to the inter-electrode capacitance, and (4) Moving the probe along two successive orthogonal directions each perpendicular to the axis of the hole until the measured capacitance exhibits either a maximum or a minimum value thereby centralising the probe in the hole and obtaining a measure of bore diameter from the said maximum or minimum value.

A further embodiment of the invention is described in which, a method of detecting the bore diameter of a hole in a metal member according to the last preceding paragraph includes the additional step of comparing the measured maximum or minimum capacitance either with a previously calibrated scale or with the capacitance ratio measured with the aid of a second probe of the invention inserted in a second hole in the or another metal member.

In order that the invention may be more fully understood, an embodiment thereof will now be described with reference to the drawings accompanying the provisional specification, in which: Figure 1 is a side elevation of a capacitance probe of the invention.

Figure 2 is an enlarged sectional view of the probe shown in Figure 1.

Figures 3 and 4 illustrate assembly of the probe shown in Figures 1 and 2.

Figure 5 is a calibration curve comparing the capacitance of two probes inserted in respective tubes.

With reference to Figures 1 and 2 the probe I comprises two annular electrodes 2 of equal dimensions. Each electrode 2 is inset into a respective annular plastics insulator 3 itself set in the surface of a tubular metal support 4. The tubular metal support 4 provides an inter-electrode screen between the electrodes 2.

The tubular metal support 4 has formed thereon two annular recesses 11. The recesses 11 have shoulders formed by annular radial projections of the tubular support 4, these being end annular projections 5 and a common central annular projection 6.

The outer surface of the electrodes 2, the insulators 3 and the projections 5 and 6 are cylindrical, and in combination these surfaces form a cylindrical surface 7. The wall 8 of the tubular support 4 is provided with holes 9 to permit coaxial cables 10 to be connected to the electrodes 2 through the insulators 3.

The probe 1 shown in Figures 1 and 2 requires a non-obvious series of assembly operations. This is because there is no immediately apparent means for locating the electrodes 2 and insulators 3 within the recesses 11 without shrinkage. Figures 3 and 4 illustrate intermediate stages in the assembly of the probe 1.

With reference to Figure 3, a metal tube 31 is prepared by machining annular recesses 32 therein. A closely fitting outer metal tube 33 is then slid over the tube 31 as shown to cover the recesses 32. Referring now also to Figure 4, two pilot holes 34 are drilled through the tubes 33 and 31. The holes 34 are just sufficiently wide to accommodate the core of a coaxial cable, and each hole 34 is located centrally and radially with respect to a respective annular recess 32. The tube 33 is removed from the tube 31 to uncover the section of each hole 34 within the wall of the tube 31: these sections are then widened by drilling to remove the cross-hatched portions 35 allowing each section to accommodate the insulation and screen of a respective coaxial cable (not shown).

Following the removal of the portions 35, the outer tube 33 is slid back over the tube 33, and the annular recesses 32 are filled with epoxy resin cement (not shown).

Before this cement has set, a respective coaxial cable (not shown) is inserted into each hole 34 so that each cable core fills the respective hole 34 in the tube 33, and also each cable screen and insulator fills the respective portions 35 of the tube 31. The cable cores are each soldered within a respective hole 34 in the tube 33.

When the epoxy resin cement has set, the assembly of Figure 4 is mounted in a lathe to machine away the cross-hatched regions 36 and 37 in the tubes 31 and 33 respectively.

The end and central portions 38 of the outer tube 33 are soldered to the adjoining surfaces of the tube 31, and the spaces left by machining away the regions 36 are filled with epoxy resin cement. When this second cement filler has set, the assembly of Figure 4 is placed in a lathe and the outer surface of the outer tube 33 is faced off to remove any unwanted epoxy resin and solder.

Comparing Figure 4 with Figures 1 and 2, it is seen.that the insulators 3 are formed by the epoxy resin in the annular recesses 32 and the spaces formerly occupied by the regions 36 of the tube 31. Furthermore, the end and central annular projections 5 and 6 (see Figures 1 and 2) of the tubular metal support 4 consist partly of the shoulders of the annular recesses 32 and partly of the regions 38 of the outer tube 33.

The construction of the probe 1, illustrated in Figures 1 to 4, is in accordance with the following principles.

Consider the electrodes 2 in the absence of the tube 4 and annular projections 5 and 6. When a potential difference is applied between the electrodes 2, inter-electrode lines of electrical force are set up. The lines of force occupy three regions, the region outside the outer cylindrical surfaces of the electrodes, ie part of the surface 7, the direct gap region between the electrodes 2 (otherwise partially occupied by the central annular projection 6) and the space region within the rings (otherwise occupied by the tube 4). These three regions give rise to three corresponding contributions to the inter-electrode capacitance.An earthed plane approaching the part of the surface 7 constituted by the electrode outer surfaces would affect significantly only those lines of force in the region outside that part of the cylindrical surface 7 and therefore also only the corresponding contribution to the inter-electrode capacitance. To increase the proportional sensitivity of the interelectrode capacitance to the approach of an earthed surface, lines of force between the electrodes which would be substantially unaffected by the earthed surface are reduced by screening. The screening is achieved by the central projection 6 and the portion of the tube 4 adjoining the projection 6.

With regard to the outer projections 5, these merely act as guard rings operative when the probe 1 is inserted within a bore.

The end projections 5 then restrict interelectrode lines of force to the regions within the bore, thus avoiding edge-effect inaccuracies in the capacitance measurement.

The probe 1 is operated by inserting it within a bore or hole the internal diameter of which is required to be determined. The cables 10 are connected to a conventional AC capacitance bridge (not shown) sensitive to 10-6pF. The cables 10 are shielded from one another by a metal sheet (not shown) to avoid cross-capacitance, and this sheet and the cable screens are earthed.

The metal support 4 is also earthed to restrict inter-electrode capacitance to that due to lines of force external to the probe 1.

The approximate dimensions of the probe 1 shown in Figures 1 and 2 are as follows: Probe diameter 0.875 in, tube internal diameter 0.438 in, end annular projections 0.060 in thick, electrode annular cross-section 0.375 in by 0.040 in, and the insulation thickness is 0.050 in. These dimensions give an inter-electrode capacitance in the region of 0.1 pF. A capacitance measuring sensitivity of the order of at least 10-6pF is therefore required to measure the inter-electrode capacitance to one part in 105.

There are two basic methods of operating the probe shown in Figures 1 and 2. The first method consists of inserting the probe within a hole in a metal object, or within the bore of a metal tube; the capacitance between the electrodes 2 is then measured, this capacitance being modified by the metal surface of the tube or object which surrounds the electrodes and which is maintained at earth potential. The axis of the probe and the hole are aligned to within 1", and the probe is then moved smoothly along a chord of the hole. A display of the inter-electrode capacitance is obtained on the meter of a conventional AC capacitance measuring bridge. As the probe passes through the centre of the chord the capacitance displayed passes through either a maximum or a minimum value.Having identified the centre of the chord, this procedure is repeated along the diameter orthogonal to the chord to centralise the probe. The maximum or minimum capacitance thus obtained at the hole centre is directly related to hole size which can be obtained from comparison with a previously calibrated scale. The scale may be obtained by, for example, measuring the interelectrode capacitance of the probe when inserted in accurately standardised holes.

The foregoing technique suffers from the disadvantage that the measured capacitance value is that of an air dielectric capacitor.

Accordingly, the capacitance value measured varies with the relative humidity and Co2 content of the atmosphere. The accuracy of measurement of a hole diameter therefore depends on the degree of conformity between the ambient conditions under which the calibrated scale was obtained and those under which the measurement was taken of probe capacitance within that hole. The effect of ambient conditions varying entails a correction to the measured capacitance which is usually considered to be small. A typical capacitance change would be one part in 104 for a change in the relative humidity between 20 /" and 40%. However, such a change can be significant when high sensitivity measurements are being made.

Accordingly, a second method is preferred, this being a comparison technique in which two probes are employed.

The second method employs two identical probes of the kind described above, one probe being accurately centred in a carefully machined hole or tube.

The centering is performed by the maximum/minimum capacitance technique previously outlined. The second probe is similarly centred in the hole to be measured, and the ratio of the inter-electrode capacitances of the two probes is determined using a commercial capacitance ratiometer. Since the relative humidity affects the dielectric constant equally in both measured capacitances, the ratio of the capacitance is virtually unaffected by humidity variations. The capacitance ratio obtained is then compared with a calibrated scale to obtain the hole size.

Figure 5 shows a calibration graph in which bore diameter is plotted against relative capacitance or capacitance ratio.

The calibration graph was obtained using two identical probes, of the type shown in Figures 1 and 2, each having an outer diameter of 0.875 in. One probe, the reference probe, was located centrally in a reference bore with a bore diameter of 1.183 in, and was connected electrically in one arm of the capacitance bridge. The second or measuring probe was located in successive standard gauge rings having a range of accurately-known bore diameters, and was in the other arm of the capacitance bridge. The ratio of the inter-electrode capacitances (reference probe value/measuring probe value) was then read directly from the bridge to yield the plot of Relative Capacitance against Bore Diameter shown in Figure 5, which shows a quasi-linear relationship.

In Figure 5, the relative capacitance goes to zero at point A on the curve when the unknown bore diameter is equal to the measuring probe diameter. The relative capacitance is unity at point B on the curve when the reference and unknown bores are equal with a diameter of 1.183 in. The curve goes through a point of inflexion at the point C which denotes a relative capacitance of 0.66 and an unknown bore diameter of 1.060 in. The significance of the point C is that, when a probe is being centred by the technique hereinbefore described, the indicated capacitance goes through a maximum or minimum value depending on which side of the point C corresponds to the unknown bore. For a bore size corresponding to a point at or near the point C the reading is insensitive to the probe position.

The value of relative capacitance ratio at which the point of inflexion C occurs is a function of the diameter of the reference bore. A reference bore of 1.060t would move C to unity on the relative capacitance scale. The value of bore size at which the point of inflexion occurs depends on the electrode spacing on each probe, and also on the width of the central earthed projection. By adjustment of these probe dimensions the point C may be moved to a convenient point on the calibration curve.

Accordingly, if bore diameters close to a given value are to be determined on a repetitive basis, the probe dimensions can be chosen to put the point of inflection at that bore diameter value. The measured capacitance is then much less, if at all, sensitive to probe position in the bore.

The technique and apparatus described are capable of high measuring accuracy.

Using commercially available AC bridges.

the ratio of capacitance of two 0. pF capacitors can be determined to the order of one part in 105, providing an extremely accurate means of measuring appropriate dimensions. A most important application of this technique is to the measurement of many nominally equal bore diameters. Such a device would be used as follows. A standard part is measured first and the capacitance ratiometer scale zeroed at the measured value using the ratio technique outlined above. By adjusting the gain of the ratiometer, full scale deflection may be set to any convenient value, such as the typical maximum variation in nominally equal bore sizes. A typical full scale deflection might correspond to a difference in bore size of 0.001 in.The probe and reference bore dimensions are chosen as described above to make the point of inflexion C coincide with the nominal bore size, so that the capacitance value is fairly insensitive to probe position in a bore. Each piece to be measured is then placed with its bore approximately centred about the probe, and the bore size difference is directly read off.

If a probe of appropriate dimensions is not available, the probe must be centred somewhat more accurately in each successive bore by the technique hereinbefore described. A typical centering accuracy required in this worse case to achieve a measuring tolerance of 0.0001 in comprises bore and probe axes less than 0.003 in apart and aligned to within 1".

The present invention performs a similar function to bore-measuring pneumatic gauges. The latter employ an air blast when centred within a bore, the bore being measured with reference to the size of the back pressure set up by the blast when escaping between the probe and the walls of the bore. The range of a pneumatic gauge of this type is about 0.005 in. In contrast, a 0.875 in diameter probe of the present invention covers a range of at least 0.375 in, some 75 times greater. Furthermore, this 0.375 in range can be extended, although with sensitivity reducing with increasing range.

Applicant's invention provides a noncontact measuring technique of high intrinsic sensitivity, and achieves a consider able improvement in performance over tfle prior art pneumatic gauge. A cylindrical probe was envisaged in the description given. It will be apparent to those skilled in the art of electrical measurements that many variations on, for example, inter-electrode screen geometry and electrode or insulation cross-sections, are possible within the scope of the invention.

WHAT I CLAIM IS: I. A probe for measuring the diameter of a hole or bore in an electrically conducting member comprising two ring-shaped, coaxial, axially spaced apart electrodes a respective electrical connection to each electrode to permit the determination of inter-electrode capacitance as a measure of bore diameter, an electrically conducting

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   reference probe, was located centrally in a reference bore with a bore diameter of 1.183 in, and was connected electrically in one arm of the capacitance bridge. The second or measuring probe was located in successive standard gauge rings having a range of accurately-known bore diameters, and was in the other arm of the capacitance bridge. The ratio of the inter-electrode capacitances (reference probe value/measuring probe value) was then read directly from the bridge to yield the plot of Relative Capacitance against Bore Diameter shown in Figure 5, which shows a quasi-linear relationship.

   In Figure 5, the relative capacitance goes to zero at point A on the curve when the unknown bore diameter is equal to the measuring probe diameter. The relative capacitance is unity at point B on the curve when the reference and unknown bores are equal with a diameter of 1.183 in. The curve goes through a point of inflexion at the point C which denotes a relative capacitance of 0.66 and an unknown bore diameter of 1.060 in. The significance of the point C is that, when a probe is being centred by the technique hereinbefore described, the indicated capacitance goes through a maximum or minimum value depending on which side of the point C corresponds to the unknown bore. For a bore size corresponding to a point at or near the point C the reading is insensitive to the probe position.

   The value of relative capacitance ratio at which the point of inflexion C occurs is a function of the diameter of the reference bore. A reference bore of 1.060t would move C to unity on the relative capacitance scale. The value of bore size at which the point of inflexion occurs depends on the electrode spacing on each probe, and also on the width of the central earthed projection. By adjustment of these probe dimensions the point C may be moved to a convenient point on the calibration curve.

   Accordingly, if bore diameters close to a given value are to be determined on a repetitive basis, the probe dimensions can be chosen to put the point of inflection at that bore diameter value. The measured capacitance is then much less, if at all, sensitive to probe position in the bore.

   The technique and apparatus described are capable of high measuring accuracy.

   Using commercially available AC bridges.

   the ratio of capacitance of two 0. pF capacitors can be determined to the order of one part in 105, providing an extremely accurate means of measuring appropriate dimensions. A most important application of this technique is to the measurement of many nominally equal bore diameters. Such a device would be used as follows. A standard part is measured first and the capacitance ratiometer scale zeroed at the measured value using the ratio technique outlined above. By adjusting the gain of the ratiometer, full scale deflection may be set to any convenient value, such as the typical maximum variation in nominally equal bore sizes. A typical full scale deflection might correspond to a difference in bore size of 0.001 in.The probe and reference bore dimensions are chosen as described above to make the point of inflexion C coincide with the nominal bore size, so that the capacitance value is fairly insensitive to probe position in a bore. Each piece to be measured is then placed with its bore approximately centred about the probe, and the bore size difference is directly read off.

   If a probe of appropriate dimensions is not available, the probe must be centred somewhat more accurately in each successive bore by the technique hereinbefore described. A typical centering accuracy required in this worse case to achieve a measuring tolerance of 0.0001 in comprises bore and probe axes less than 0.003 in apart and aligned to within 1".

   The present invention performs a similar function to bore-measuring pneumatic gauges. The latter employ an air blast when centred within a bore, the bore being measured with reference to the size of the back pressure set up by the blast when escaping between the probe and the walls of the bore. The range of a pneumatic gauge of this type is about 0.005 in. In contrast, a 0.875 in diameter probe of the present invention covers a range of at least 0.375 in, some 75 times greater. Furthermore, this 0.375 in range can be extended, although with sensitivity reducing with increasing range.

   Applicant's invention provides a noncontact measuring technique of high intrinsic sensitivity, and achieves a consider able improvement in performance over tfle prior art pneumatic gauge. A cylindrical probe was envisaged in the description given. It will be apparent to those skilled in the art of electrical measurements that many variations on, for example, inter-electrode screen geometry and electrode or insulation cross-sections, are possible within the scope of the invention.

   WHAT I CLAIM IS: I. A probe for measuring the diameter of a hole or bore in an electrically conducting member comprising two ring-shaped, coaxial, axially spaced apart electrodes a respective electrical connection to each electrode to permit the determination of inter-electrode capacitance as a measure of bore diameter, an electrically conducting

   screen which extends between the electrodes so as to restrict lines of electrical force to the region outside the electrodes and insulating means to insulate the electrodes from the screen.
2. 2. A probe according to claim I wherein the electrodes have equal mean radii.
3. 3. A probe according to claim 1 or 2 wherein the electrodes have equal crosssections.
4. 4. A probe according to claim 1, 2 or 3 wherein the electrodes, the screen and the insulating means have cylindrical outer surfaces, the surfaces being coaxial, of equal radii and forming a continuous cylindrincal surface.
5. 5. A probe according to any preceding claim including two guard rings connected to the screen and disposed either side of the two ring-shaped electrodes.
6. 6. A probe according to claim 5 when dependent on claim 4 wherein the guard rings have cylindrical outer surfaces continuous with the outer surfaces of the electrodes, the screen and the insulating means.
7. 7. A probe according to any preceding claim wherein the probe includes a metal tube having three spaced apart annular projections defining therebetween two recesses, each recess accomodating a respective electrode and insulating means to insulate the electrode from the tube and its projections.
8. 8. A method of measuring the bore diameter of a hole in an electrically conducting member, the method employing a probe according to any one of claims 1--7 and comprising the steps of: (1) Inserting the probe within the hole in the conducting member, (2) Adjusting the axis of the electrodes to be substantailly parallel with that of the hole, (3) Arranging a meter to display a reading proportional to the inter-electrode capacitance, and (4) Moving the probe in two successive orthogonal directions each perpendicular to the axis of the hole until the measured capacitance exhibits either a maximum or minimum value, thereby centering the probe in the hole and obtaining a measure of bore diameter from the said maximum or minimum value.
9. 9. A probe for measuring the diameter of a hole or bore in an electrically conducting member substantially as herein described with reference to, and as illustrated in Figures 1 and 2 of the drawings accompanying the provisional specification.