GB2225855A

GB2225855A - Capacitance probe

Info

Publication number: GB2225855A
Application number: GB8825961A
Authority: GB
Inventors: David Richard Foulds
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1988-11-05
Filing date: 1988-11-05
Publication date: 1990-06-13
Anticipated expiration: 2008-11-05
Also published as: GB2225855B; GB8825961D0

Abstract

A variable capacitance electrical transducer 1 of the type used as a distance measuring probe for measuring, eg., working clearances in machinery comprises two concentric ceramic components 4, 6 forming a ceramic substrate for a plated metal coating which not only forms sensor and guard electrodes 58, 60 of the probe but also facilitates electrical connections from the conductors 20, 22 of a coaxial cable 9 to the electrodes. The central ceramic component 6 forms the substrate for the sensor electrode 58 and may partially underlie the guard electrode 60. The outer ceramic component 4 forms the substrate for the rest of the guard electrode 60. At the mutual interface between the two ceramic components 4, 6 their confronting surfaces have the metal coating and are brazed together, the metal coating providing an annular electrical connection from the screen conductor 22 to the guard electrode 60, thereby also providing electrical shielding for the connection to the sensor electrode 58. <IMAGE>

Description

CAPACITANCE PROBE This invention relates to a variable capacitance electrical transducer of the type used as a distance measuring probe.

It is known to measure distance between confronting surfaces by means of a capacitive electrical transducer device which has an electrode in or near the plane of one of the surfaces. For convenience, such capacitive electrical devices may be termed capacitance probes.

Assuming that the opposing surface whose distance is to be measured is inherently electrically conductive, or is effectively rendered conductive in some way, then that surface, the electrode surface and the air gap (or other insulating medium) between the surfaces together define a variable capacitor. It is conventional for the capacitor so defined to be connected into the overall feedback loop of a high gain amplifier, whose input is a high frequency signal from -an internal oscillator. In this way, the amplifier output is proportional to the feedback impedance and so directly proportional to the distance between the electrode surface and the opposing surface. Provided that the permittivity of the air gap does not alter, monitoring of the amplifier's output will, with suitable calibration, provide an indication of the distance between the confronting surfaces.

Properly designed capacitance probes used in appropriate situations give a sensitive, accurate method of measuring small variations in distance. For this reason they have been used to measure working clearances in machinery.

A typical known type of capacitance probe capable of such use comprises a sensing electrode and a guard electrode of sheet metal or metal foil construction, with ceramic insulation between the electrodes and also between the guard electrode and other conductors at ground potential. However, such probes are not suitable for use at very high temperatures, and/or under very high 'g' forces and vibrations, such as are found in the cylinders of internal combustion engines, or in the turbines of gas turbine engines, and also tend to be too bulky and massive to incorporate in some of the smaller moving components of such engines. Furthermore, there is a need for high temperature capacitance probes having a simpler and cheaper construction than heretofore available.

Accordingly, the present invention provides a capacitance probe comprising ceramic substrate means and metallic coating means thereon, the metallic coating means forming electrode means and forming or facilitating at least one electrical connection to the electrode means.

Such a construction can provide a simple probe having a high degree of thermal and mechanical integrity when suitably implemented.

High-purity alumina, silicon nitride or other ceramic mouldings with similar properties provide suitable ceramic substrate means and the metallic coating means preferably comprises a metal deposited on the ceramic by a physical plating process, e.g. a first layer of molybdenummanganese and a second layer of nickel, each layer being deposited on the ceramic by means of painting onto the ceramic surfaces a liquid suspension of the powdered metal(s) in a fugitive vehicle binder and firing the resulting coating in a furnace to drive off the binder and fuse the metallic particles to obtain the finished coating. Alternatively, the metallic coating may comprise a layer of melted braze material of a type capable of wetting the ceramic surface.

It is preferred that the ceramic substrate means comprises a plurality of (preferably two) ceramic components bonded together at interface means defined by confronting surfaces of the components. Conveniently, the metallic coating means comprises at least one of the confronting surfaces of the interface means and provides an electrical connection to the electrode means, and preferably the electrode means comprises a sensor electrode and a guard electrode, the electrical connection being to the guard electrode.

In more detail, an embodiment of the invention has an arrangement in which the sensor electrode comprises a circular area of metallic coating means and the guard electrode comprises an annular area of metallic coating means concentric with the sensor electrode and separated therefore by an insulating gap in the coating means. In addition, one of the ceramic components comprises a central component forming the substrate for the sensor electrode and another ceramic component surrounds the central component and forms the substrate for at least part of the guard electrode. Preferably, the central component is substantially cylindrical and the other component is also substantially cylindrical with an aperture therein for the central component.

In this embodiment, the ceramic components are adapted to accommodate a coaxial electrical cable, a central conductor of the coaxial cable being bonded to the central ceramic component and electrically connected to the sensor electrode and a screening conductor of the coaxial cable being bonded to the other ceramic component and electrically connected to the guard electrode. In order to provide good electrical shielding within the ceramic substrate means, the metallic coating means providing the electrical connection to the guard electrode circumscribes the electrical connection to the sensor electrode.

It should be understood that the term "coaxial cable" includes cables with a central conductor and one or more concentric screen or shield conductors.

It is further specified that the ceramic components are bonded together and to the conductors of the coaxial cable by means of brazing, such brazing being facilitated by the provision of the metallic coating means on the relevant surfaces of the ceramic components.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a greatly enlarged cross-section of a capacitance probe in accordance with the invention, and Figure 2 is a plan view of the capacitance probe showing its electrode area.

The present invention arose from consideration of a specific problem, namely, how to measure the clearances between the piston crown and both intake and exhaust valves of a small diesel engine between + 10 degrees crank angle relative to top dead centre on the exhaust stroke, whilst the engine was running.

In view of the obvious access difficulties involved in instrumenting a piston crown, it was decide that clearance measurement should be made by transducers installed in the heads of the valves. Various design constraints has to be taken into account, such as the very limited space available to house the transducers, the high temperatures, high accelerations and high frequency of oscillation of the valves, and requirements for high reliability and relatively simple, inexpensive construction. It was concluded that the novel design of capacitance probe disclosed herein would best satisfy these constraints.

Referring now to the accompanying drawings, there is illustrated a capacitance probe 1 situated in the head 2 of the exhaust valve in the diesel engine (not shown).

The probe 1 is a composite component comprising an outer ceramic cup 4 and an inner ceramic plug 6 received in the central aperture or recess 8 of the cup 4. These ceramic parts are moulded in the shapes shown from a high purity alumina, although silicon nitride is a possible alternative. "Sintox" (Trade Mark) alumina parts, as manufactured by Lodge Ceramics Limited, of Rugby, England, are suitable. Both parts are metallised on their outer and inner surfaces utilising techniques already well known for metallising ceramic parts for other purposes. They are metallised to the extent shown by the cross-hatching in Fig. 1, the metallisation being an underlayer of molybdenum-manganese alloy and an overlayer of nickel.

Both layers are obtained by applying to the indicated areas of the ceramic parts metallic paints containing particles of the appropriate metals in a fugitive vehicle binder and firing the layers separately in furnaces to drive off the binder and fuse the metallic particles to obtain the finished coating. The underlayer must be fired in a vacuum furnace to prevent oxidation.

The metallisation serves two purposes; namely, it forms the electrode surfaces of the probe 1, as detailed later, and it also enables brazing of the ceramic parts to each other, to a metal-sheathed cable 9, and to the valve head 2. The probe 1 is shown in the as-assembled condition within the valve head 2, before brazing has occurred.

Looking in more detail at the shapes of the component parts of the finished assembly, and at the assembly and manufacturing procedure involved, it will first be noted that the valve head 2, together with the stem of the valve, has been through - drilled or electro-discharge machined to create the circular-section hole 10, through which the cable 9 extends from the other end of the valve stem (not shown). A larger diameter hole 12, on the same axis as hole 10, has then been drilled in the top of the valve head 2 to accommodate the probe 1. In order to allow brazing wire 14 to be inserted around the outside diameter of the cable 9, thereby enabling brazing of the cable 9 into the valve stem as indicated by the x-symbols, the upper end of hole 10 has been widened to create a small recess 16 at the bottom of hole 12.

It should be mentioned here that cable 9 is of the well known mineral-insulated triaxial minature type of about 2 mm diameter as supplied, e.g., by Smiths Industries plc of Cheltenham, England, and comprises a centre wire 20, an inner annular screen wire 22 and an outer metallic sheath 24 which are all made of stainless steel or Inconel (Trade Mark) or similar alloy and are insulated from each other by a compacted cohesive magnesium oxide or silica powder fill. It will be seen that the outer sheath 24 has been stripped from an end portion of the cable 9 to reveal a short length of inner screening 22, part of which itself has been stripped to reveal the centre wire 20.The annular spaces between the centre wire 20, screen wire 22 and outer sheath 24 are sealed by means of a silicone varnish sealant to prevent ingress of moisture into the powder fill before assembly and final manufacture of the probe 1. However, this varnish sublimes during the subsequent brazing operations, leaving no undesirable contaminants.

Having inserted the cable 9 into the valve head 2 as shown, with brazing filler wire 14 in place, it is brazed into position using vacuum brazing techniques.

Thereafter, the probe 1 can be assembled within hole 12 in the valve head 2, as will now be described.

Firstly, brazing filler wire 28 is inserted at the bottom of hole 12 as shown. Then, the cylindrical cup 4, which has a bevelled bottom edge 30 to accommodate brazing filler wire 28 and a central hole 32 to accommodate the screen wire 22 of cable 9, is placed in position over the screen wire 22. In order to allow brazing filler wire 34 to be inserted around the screen wire 22, thereby enabling brazing of the screen wire 22 to the cup 4, the bottom of the cylindrical recess 8 is provided with a smaller recess 36 which forms a wider diameter portion of the hole 32.

Having positioned the ceramic cup 4 correctly within the hole 12, further brazing filler wire 38 is inserted at the bottom of the recess 8. The cylindrical plug 6 has a bevelled bottom edge 40 to accommodate brazing filler wire 38, but before insertion of plug 6 into recess 8, further brazing filler wire 42 is inserted into an annular groove 44 which runs circumferentially around the outside of the plug. Of course, the plug 6 also has a central hole 44 to accommodate the centre wire 20 of cable 9. Again, a small recess 46, this time of conical shape, is provided in the central top surface 48 of the plug 6 in order that after the plug's insertion into the recess 8, brazing filler wire 50 can be placed around the centre wire 20 of the cable 9.

It will be apparent from the above that the stripping of cable 9 and its positioning within the valve is such that, when the parts are assembled together as described above, the end face 52 of the centre wire 20 is not raised above the top surface 48 of the probe 1. However, the ends of the centre wire 20, the screen wire 22 and the sheath 24 must project into their respective recesses 46,36 and 16 sufficiently far to both achieve adequate contact with the melted brazing filler wires and, in the case of brazing wire 34,14 in recesses 36,16, prevent contact of the melted braze with inner conductors 22 and 24 respectively.

Having assembled the component parts of the probe 1 into the valve head 2 as aforesaid, vacuum brazing techniques are then used to secure the component parts to each other, to the valve head and to the cable 9. In this connection it should be noted that the ceramic cup 4 and ceramic plug 6 should just be sliding fits in and on their respective mating components such that the confronting surfaces to be brazed together have very small clearances between them to allow the melted braze to run between them by capillary action and thereby secure the assembly very firmly. However, the braze is only drawn along the surfaces of the ceramic components as far as the metallisation extends, since the ceramic surfaces are non-wetable by the metallic braze materials used.

For brazing the cable to the valve head, braze NK8 or similar is recommended.

For brazing the metallised ceramic components together and to the valve head, Orobraze 950 (Trade Mark) or similar is recommended.

In order to make the assembled and brazed probe 1 into a functioning instrument, it is necessary to create a sensing electrode and a guard electrode on the top face 48 of the ceramic components. The first stage of this process is to separate the metallic coating on the face 48 from the valve body by machining or abrading a circular groove 54 through the thickness of the metallic coating around the periphery of the face 48, concentric with the wire 20, as shown in plan view in figure 2 (but not in cross-section of figure 1). The second stage is to define separate sensing and guard electrodes 58 and 60 respectively by machining a second circular groove 56 through the metallised layer on the top face of the ceramic plug component 6 of the probe 1, the groove 56 also being concentric with wire 20.

As a result, the sensing electrode 58 is electrically connected only to the centre wire 20 of cable 9 and the guard electrode 60 is electrically connected only to the screen wire 22 via the metallised coatings and braze associated with the recesses 8 and 36, while the valve body itself is electrically insulated from both and is electrically connected to the metallic sheath 24 of the cable 9 in order to shield both the centre wire 20 and the screen wire 22 (and the metallised surfaces to which they are connected) from external electrical interference.

As an alternative to the above-described type of metallisation to provide the desired electrode surfaces and electrical connections, as well as a wettable surface for the subsequent brazing processes, it may be mentioned here that it would be possible to achieve metallisation of the ceramic by utilising a brazing composition which upon heating to brazing temperature wetted the surfaces of the ceramic parts and covered them with a suitable metallic coating except for areas to which a so-called "stop-off" (maskant) coating had been applied. Again this would provide the desired electrode surfaces and electrical connections, as well as a surface adapted for the subsequent brazing processes during assembly of the probe 1.

Although brazing is the preferred method of bonding the metallised ceramic parts to each other and to the valve head 2, it would be possible to utilise other bonding methods. For instance, by utilising parts which are sized and toleranced to give appropriate fits between confronting metallised and metallic surfaces upon assembly, it would be possible to utilise diffusion bonding or activated diffusion bonding techniques to bond the surfaces together. For a capacitance probe suitable for low to medium temperature environments, non-conducting epoxy resin could be utilised to bond the surfaces to each other, while for higher temperatures, a ceramic cement could be utilised.

One general problem with use of the above-described capacitance probe is that the capacitance or capacitance change to be measured is extremely small - of the order of 0.03 picofarads. Consequently, any "stray" capacitance produced in the sensing electrode 58 must be much less than that. Since significant capacitance changes cannot be induced in the electrode 58 from directions below or to the side of it, due to the shielding effected by the valve head and those metallised areas of the ceramic components 4 and 6 which are not connected to the sensing electrode, it is really only vulnerable to the effect of stray capacitance produced by interaction with conductive objects adjacent to the object whose distance is to be sensed - in this case, the cylinder walls surrounding the piston.Such stray capacitance or "fringing" can be minimised by the known technique of using a so-called "driven guard". In the present case, the guard electrode 60 is the driven shield and is electronically maintained at the same electrical potential and phase as the sensing electrode. This establishes an electrical field between the guard electrode and any objects close to the front of the probe which tends to exclude the field associated with the sensing electrode from interaction with any objects except that immediately in front of it, i.e. the electrical flux lines in the field are maintained mutually parallel.

Plainly, if the guard electrode is too small relative to the sensor electrode, the probe 1 may be unreliable as a distance sensor due to the above-mentioned fringing effect. Consequently, in order to attain a desired sensing range for the probe, the relative surface areas of the electrodes must be carefully chosen.

It should be understood that the sheath 24 of cable 9, and hence the valve body, are maintained at ground ptential.

It e be appreciated that the mode of construction Of the probe allows the diameter of the sensing electrode to be chosen to suit, within limits, the desired measurement range. We believe that for the purpose for which the above-described embodiment of the probe was devised, a suitable diameter for the sensing electrode 58 is about 3 mm and a suitable diameter for the guard electrode 60 is about 7 mm, the grooves 54 and 56 being between 0.25 and 0.5 mm wide.

Of course, although electrodes of circular shape have been specified above for the probe 1, square or rectangular electrodes could also be utilised, with, if necessary, matching shapes for the underlying ceramic components.

The embodiment of Figures 1 and 2 was devised for duty in a high-temperature, corrosive environment as already explained, and in order to protect the metallised top surface 48 of the probe 1 from corrosion and oxidation it may be preferable to additionally plate surface 48 with a suitable noble metal, such as gold or platinum.

Referring to the problem of differential thermal expansion between the probe 1 and the valve head 2, a thin annulus of nickel-iron alloy (e.g. Nilo-K, trademark) may be brazed in place between the base of the ceramic cup 4 and the bottom of the hole 12 in the valve head 2 in order to provide stress isolation between the valve head and the probe.

It should be understood that besides being suitable for the type of duty specifically described above, transducers in accordance with the invention, with appropriate modifications as required, are also suitable for other types of duty. For example, they could be used in gas turbine aeroengines for the purpose of measuring the clearances between the tips of turbine blades and their surrounding casings, or they could be used to monitor operating clearances in many other types of machinery.

Although the specific embodiment shows a probe 1 composed of only two ceramic components 4 and 6 bonded together, it would of course be possible to utilise three or even more ceramic components. For instance, a third ceramic component, coaxial with the other two, could take the place of the portion of the metallic valve head 2 shown in Figure 1, being, as regards internal shape, a larger version of the component 4 and having similarly metallised inner surfaces, connected to the sheath 24 of the cable 9 in the same way as the component 4 is connected to the shield wire 22, and also brazed to the outer metallised surface of component 4.

This third, outer, ceramic component could in fact be a ceramic valve head, which would ease the situation regarding differential thermal expansion between the valve head and the probe 1. An arrangement involving more than three ceramic components is deemed unlikely since it adds unwanted complexity and manufacturing costs to the probe.

Claims

Claims:

1. A capacitance probe comprising ceramic substrate means and metallic coating means thereon, the metallic coating means forming electrode means and forming or facilitating at least one electrical connection to the electrode means.

2. A capacitance probe according to claim 1 in which the ceramic substrate means comprises a plurality of ceramic components bonded together at interface means defined by confronting surfaces of the components.

3. A capacitance probe according to claim 2 in which the ceramic substrate means comprises two ceramic components.

4. A capacitance probe according to claim 2 or claim 3 in which the metallic coating means comprises at least one of the confronting surfaces of the interface means and provides an electrical connection to the electrode means.

5. A capacitance probe according to claim 4 in which the electrode means comprises a sensor electrode and a guard electrode, the electrical connection being to the guard electrode.

6. A capacitance probe according to claim 5 in which the sensor electrode comprises a circular area of metallic coating means and the guard electrode comprises an annular area of metallic coating means concentric with the sensor electrode and separated therefrom by an insulating gap in the coating means.

7. A capacitance probe according to claim 6 in which one of the ceramic components comprises a central component forming the substrate for the sensor electrode and another ceramic component surrounds the central component and forms the substrate for at least part of the guard electrode.

8. A capacitance probe according to claim 7 in which the central component is substantially cylindrical and the other component is also substantially cylindrical with an aperture therein for the central component.

9. A capacitance probe according to claim 7 or claim 8 in which the ceramic components are adapted to accommodate a coaxial electrical cable, a central conductor of the coaxial cable being bonded to the central ceramic component and electrically connected to the sensor electrode and a screening conductor of the coaxial cable being bonded to the other ceramic component and electrically connected to the guard electrode.

10. A capacitance probe according to any one of claims 7 to 9 in which the metallic coating means which provides the electrical connection to the guard electrode circumscribes the electrical connection to the sensor electrode thereby providing electrical shielding within the ceramic substrate means.

11. A capacitance probe according to claim 9 or claim 10 in which surfaces of the ceramic components to be bonded to the conductors of the coaxial cable comprise metallic coating means, the conductors being bonded to the ceramic components by means of braze material.

12. A capacitance probe substantially as described in this specification with reference to and as illustrated by the accompanying drawings.

12. A capacitance probe according to any one of claims 2 to 11 in which all confronting surfaces of the ceramic components have metallic coating means and the components are bonded together by means of braze material between the confronting surfaces.

13. A capacitance probe according to any one of claims 1 to 12 in which the ceramic substrate means comprises high-purity alumina.

14. A capacitance probe according to any one of claims 1 to 13 in which the metallic coating means comprises metal deposited on the ceramic by a physical plating process.

15. A capacitance probe according to any one of claims 1 to 13 in which the metallic coating means comprises an under layer of molbdenum-manganese alloy and an overlayer of nickel.

16. A capacitance probe according to any one of claims 1 to 13 in which the metallic coating means comprises a layer of braze.

17. A capacitance probe substantially as described in this specification with reference to and as illustrated by the accompanying drawings.

Amendments to the claims have been filed as follows 1. A capacitance probe comprising a ceramic substrate underlying a central sensor electrode area and a peripheral guard electrode area, the ceramic substrate comprising concentric ceramic components, the central ceramic component having an electrical connection extending therethrough to the sensor electrode, the ceramic components being bonded together at confronting surfaces thereof and at least one of the confronting surfaces having a metallic coating thereon to provide the guard electrode with an electrical connection thereto which surrounds the electrical connection to the sensor electrode, thereby providing electrical shielding within the ceramic substrate, the sensor and guard electrode areas comprising a metallic coating on the ceramic substrate, the central ceramic component being the substrate underlying the sensor electrode and the ceramic component surrounding the central ceramic component being at least part of the substrate underlying the guard electrode.

2. A capacitance probe according to claim 1 in which the ceramic substrate means comprises two ceramic components.

3. A capacitance probe according to claim 1 or claim 2 in which the sensor electrode comprises a circular area of metallic coating and the guard electrode comprises an annular area of metallic coating concentric with the sensor electrode and separated therefrom by an insulating gap in the coating.

4. A capacitance probe according to any one of the preceding claims in which the central ceramic component is substantially cylindrical and the surrounding ceramic component is also substantially cylindrical with an' aperture therein for the central component.

5. A capacitance probe according to any one of the preceding claims in which the ceramic components are adapted to accommodate a coaxial electrical cable, a central conductor of the coaxial cable being bonded to the central ceramic component and electrically connected to the sensor electrode and a screening conductor of the coaxial cable being bonded to the surrounding ceramic component and electrically connected to the guard electrode through the electrical shielding within the ceramic substrate.

6. A capacitance probe according to claim 5 in which surfaces of the ceramic components to be bonded to the conductors of the coaxial cable have a metallic coating thereon, the conductors being bonded to the ceramic components by means of braze material.

7. A capacitance probe according to any one of the preceding claims in which components are bonded together by means of braze material between the confronting surfaces.

8. A capacitance probe according to any one of the preceding claims in which the ceramic substrate means comprises high-purity alumina.

9. A capacitance probe according to any one of the preceding claims in which the metallic coatings comprise metal deposited on the ceramic by a physical plating process.

10. A capacitance probe according to any one of the preceding claims in which the metallic coatings comprise an under layer of molybdenum-manganese alloy and an overlayer of nickel.

11. A capacitance probe according to any one of claims 1 to 8 in which the metallic coatings comprise a layer of braze material.