GB2131176A

GB2131176A - Method of manufacturing a capacitance distance measuring probe

Info

Publication number: GB2131176A
Application number: GB08228727A
Authority: GB
Inventors: John William Hender Chivers
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-10-07
Filing date: 1982-10-07
Publication date: 1984-06-13
Also published as: GB2131176B

Abstract

A capacitance distance measuring probe (23) is manufactured by adhesively bonding together first and second sheets (10, 13), each comprising a thermoplastic polymer film having a copper coating deposited on one side thereof, so that the copper coated face (12) of one sheet (10) is bonded to the uncoated face of the other sheet (13). The exposed copper coating (15) is divided into a first area (16) which constitutes a sensing electrode and a second area (17) which at least partially surrounds the sensing electrode (16). The second area (17) and the copper coating (12) on the first sheet (10) are electrically interconnected to define a guard electrode for the sensing electrode (16). <IMAGE>

Description

SPECIFICATION Method of manufacturing a capacitance distance measuring probe This invention relates to a method of manufacturing a capacitance distance measuring probe.

It is known to measure distances between surfaces, at least one of which is electrically conductive, by the use of a capacitance probe. The probe is positioned on one of the surfaces so that it is opposite the electrically conductive surface. Hence the probe, the opposite surface and the air gap (or other medium) between them together define a capacitor. If it is assumed that the nature of the air gap (or other medium) between the surfaces does not alter, then the value of the defined capacitor is proportional to the distance between the surfaces. Consequently monitoring of the value of the defined capacitor will, after suitable calibration, provide an indication of the distance between the surfaces.

Conventional capacitance probes are frequently quite bulky devices which can be difficult to install in certain distance measuring applications. Moreover they can also be expensive to produce. It is an object of the present invention therefore to provide a method of manufacturing a capacitance distance measuring probe which is cheap and results in a very compact probe.

According to the present invention, a method of manufacturing a capacitance distance measuring probe comprises the steps of adhesively bonding together first and second sheets, each of which is an electrically insulating polymer film having an electrically conductive coating deposited on one side thereof, said sheets being bonded together in such a manner that they are in superposed relationship and the polymer film of the second of said sheets is interposed between the conductive coatings on said sheets, a portion of the exposed conductive coating on said second sheet having been removed so that the conductive coating remaining on said second sheet comprises a first area which constitutes a sensing electrode and a second area which at least partially surrounds but is electrically isolated from said first area, and electrically interconnecting said second area with the electrically conductive coating on said first sheet so that together they define a guard electrode for said sensing electrode.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a plan view of a first sheet of an electrically insulating polymer film having an electrically conductive coating on one side thereof for use in the method of the present invention.

Figure 2 is a plan view of a second sheet of an electrically insulating polymer film having an electrically conductive coating on one side thereof for use in the method of the present invention.

Figure 3 is a sectional side view of a capacitance probe manufactured in accordance with the method of the present invention.

Figure 4 is a diagram of a circuit for use with a capacitance probe manufactured in accordance with the method of the present invention.

With reference to Figure 1, a first sheet 10 which is of generally T-shaped configuration, is formed from an electrically insulating polyimide polymer film 11 having an electrically conductive copper coating 12 electrodeposited on one side thereof. The polyimide polymer film 11 is 0.004 inches thick and the copper coating 120.001 inches thick.

A second sheet 13, which is of the same generally T-shaped configuration as the first sheet 10, is formed from the same copper coated polyimide film as that used to form the first sheet 10. Thus the second sheet 13, which can be seen in Figure 2, comprises a polyimide polymer film 14 having a copper coating 15. The second sheet 13 differs from the first sheet in that a portion of the copper coating 15 has been etched away by conventional printed circuit board manufacturing techniques to leave two areas 16 and 17 of the copper coating which are electrically isolated from each other. The first area 16 is of generally T-shaped configuration and the second area 17 surrounds all but a portion 18 at the tail of the T-shaped configuration area 16. Thus a generally T-shaped portion 19 of the polymer film 11 is exposed on the copper coated surface of the sheet 13.

The side of the first sheet 10 which is not copper coated is grit-blasted in order to roughen its surface. The first sheet 10 is then bonded using an adhesive layer 20 to a first surface 21 as can be seen in Figure 3 such that its copper coated surface 12 is uppermost.

A small notch 22 is then cut out of the second copper coated area 17 of the second sheet 13 whereupon the side of the second sheet 13 which is not copper coated is grit-blasted. The second sheet 13 is then bonded to the copper coating 12 of the first sheet 10 using an adhesive layer 22 so that the two sheets 10 and 13 are in aligned superposed relationship with the copper coating 15 on the second sheet 13 uppermost. Thus as can be seen from Figure 3, the polymer film 14 of the second sheet 13 is interposed between the conductive copper coatings 12 and 15 on the sheets 10 and 13.

The second copper coated area 17 on the second sheet 13 is then electrically interconnected with copper coating 12 on the first sheet 10 by soldering a fine, tinned copper wire to the copper coated area 17 and that portion of the copper coating 12 on the first sheet 10 which is exposed by the notch 22.

The first and second sheets 10 and 13 bonded together in this manner define a capacitance probe 23. The generally T-shaped copper coated areas 16 on the second sheet 13 constitutes the sensing electrode of the probe 23 whilst the electrically interconnected copper coating 12 of the first sheet 10 and the second copper coated area 17 on the second sheet together constitute a guard electrode for that sensing electrode 16.

The capacitive probe 23 is utilised to measure the distance d between the probe surface and on electrically conductive surface 24. To this end, the first area 16 which constitutes the sensing electrode and the electrically conductive surface 24 are incorporated into the circuit 25 shown in Figure 4. Thus the capacitor Cu in the circuit 25 is constituted by the sensing electrode 16, the electrically conductive surface 24 and the air gap d between them.

A source potential V1 is applied to the circuit 25. A capacitor Ck and resistor R of known values are interposed between the source potential V1 and an amplifier 29 of gain A. The current passed by the capacitor Ck is designated i, and the input voltage to the amplifier 29 designated e. The capacitor Cu is connected in parallel with the amplifier 29 and the current which it feeds to the amplifier 29 is designated i2.

The amplifier output is designated VO The amplifier output V0 = Ae = A(i1 - i2) R(1 80C phase difference between ii and i2).

Now Cu is comparable with Ck and the amplifier gain A is so high that < VOT i.e. eo 0 ii il - i2- 0 O for finite values of R i.e.

ii = i2 and e = 0 in the limit.

Vc VO = i2Xu and V1 and i1X8 in the limit.

i.e.

Vow1 = (i2/i1) (XuIX8) and Vo = (V1IX8)Xu = (V11X8) (1/WCu) i.e.

V0 = K(1/CU) where K is a constant = V1/wX8 But Cu = Ald (dielectric air) where A = sensing electrode 16 cross-sectional area d = separation between sensing electrode 16 and the surface 24 .-. VO = K(d/A) i.e.

V0 = K1d [Where K1 is a constant = K/A = V1(wX8A)J Thus the output voltage from the amplifier A is proportional to the distance d between the sensing electrode 16 and the surface 24. Hence if the thickness of the probe 23 is added to the distance d, then the total distance between the surfaces 21 and 24 is obtained.

The guard electrode constituted by copper coating 12 of the first sheet 10 and the second copper coated area 17 on the second sheet is earthed so as to provide effective screening of the sensing electrode 16.

It will be seen therefore that the present invention provides a method of manufacturing an extremely thin capacitive probe 23 which is very cheap. Indeed the total thickness of the probe 23 is in order of 0.014 inches using the materials described above. It will be appreciated however that alternative materials may be used in the construction of the probe 23. For instance, the probe 23 described above is based on a polyimide polymer film which has good thermal resistance properties. However if the probe 23 is not intended to operate in a high temperature environment, other polymers which have less thermal resistance could be employed. Moreover conductive coatings other than those formed from electrodeposited copper may be utilised if desired.

Claims

1. A method of manufacturing a capacitance distance measuring probe comprising the steps of adhesively bonding together first and second sheets, each of which is an electrically insulating polymer film having an electrically conductive coating deposited on one side thereof, said sheets being bonded together, in such a manner that they are in superposed relationship and the polymer film of the second of said sheets is interposed between the conductive coatings on said sheets, a portion of the exposed conductive coating on said second sheet having been removed so that the conductive coating remaining on said second sheet comprises a first area which constitutes a sensing electrode and a second area which at least partially surrounds but is electrically isolated from said first area, and electrically interconnecting said second area with the electrically conductive coating on said first sheet so that together they define a guard electrode for said sensing electrode.

2. A method of manufacturing a capacitance distance measuring probe as claimed in claim 1 wherein said first sheet is bonded to a support surface prior to the bonding of the second sheet to said first sheet.

3. A method of manufacturing a capacitance distance measuring probe as claimed in any one preceding claim wherein said polymerfilm is formed from a polyimide polymer.

4. A method of manufacturing a capacitance distance measuring probe as claimed in any one preceding claim wherein said electrically conductive coating is formed from copper.

5. A method of manufacturing a capacitance distance measuring probe as claimed in any one preceding claim wherein said first and second sheets are bonded together by an adhesive.

6. A method of manufacturing a capacitance distance measuring probe substantially as hereinbefore described with reference to Figures 1-3 of the drawings.

7. A capacitance distance measuring probe manufactured in accordance with the method of any one preceding claim.