CAPACITANCE TRANSDUCER APPARATUS AND CABLES

Abstract: Capacitance transducer apparatus, for example for measuring blade tip clearance in a jet engine of gas turbine, comprises an oscillator (14) connected by a cable (6) to a capacitance means such as a probe (4). The cable is tri-axial comprising an inner conductor (22), an outer protective layer (24) and an intermediate layer (26) between the outer layer and inner conductor, the inner conductor, intermediate layer and outer layer all being electrical conductors. The intermediate layer (26) consists of a material that is different to that forming the outer layer and is of relatively high electrical conductivity. The resistance of the intermediate layer is preferably less than 1 Ohm metre<-1>, and especially les than 0.1 Ohm metre<-1>, which may be achieved by forming the intermediate layer from a nickel based material, and especially from pure nickel. The reduction in the intermediate layer resistance can lead to higher sensivity of the apparatus by enabling any guard driver amplifier (165) in the apparatus to drive the capacitance over the full length of the cable.

Description: CAPACITANCETRANSDUCERAPPARATUSANDCABLES The present invention relates to capacitance transducer apparatus, to cables and in particular, though not exclusively, to cables for use in transmitting signals to/from a frequency modulation capacitance detector and to such detectors. Frequency modulation capacitance transducers are used in the form of capacitance detectors measuring blade tip clearance in, for instance, jet engines and gas turbines. An arrangement suitable for such an application is shown in figure 1 of the drawings that follow. In figure 1 there is shown, schematically, a target 2 which could be a jet engine blade and which is electrically connected to earth 3. Spaced from the target 2 is a capacitance probe 4. In this configuration, the target 2 acts as an earthed plate of a parallel plate capacitor and the probe 4 as the other plate. A tri-axial cable 6 electrically connects the probe 4 to an oscillator circuit 8. The circuit 8 comprises a power supply 10, a buffer 12 electrically connected to an output of power supply 10 and an oscillator 14 electrically connected to an output of power supply 10 and the output of buffer 12. A unity gain amplifier 16 is electrically connected to an output of power supply 10 and to the output of the oscillator 14. The buffer 12, oscillator 14 and unity gain amplifier 16 are each electrically connected to earth at 18. A 50 ohm co-axial cable 20 is electrically connected to the power supply 10 and buffer 12 to connect the oscillator circuit 8 to a detection circuit (not shown). . 1 . In this known arrangement the tri-axial cable 6 comprises a central stainless steel conductor 22 electrically connected to the output of oscillator 14. The cable 6 has a stainless steel outer layer 24 electrically connected to earth 18 and a stainless steel intermediate layer 26 electrically connected to the output of amplifier 16. The outer layer 24 acts as a screen against the high frequency radiation that would otherwise occur. Between the conductor 22 and intermediate layer 26, and between the intermediate layer 26 and outer layer 24 is provided a mineral insulation material 28 such as silica or aluminium oxide. Stainless steel has been used until now for the intermediate and outer layers 26 and 24 respectively because it imparts the required strength, durability and heat resistance to the cable. In particular, in gas turbine and jet engine environments high temperatures of approximately 1000° are encountered as a result of which the cable must not be degraded or its performance affected significantly. However, as engine technology has advanced, the length of cable 6 between the probe 4 and oscillator circuit 8 has had to be increased. It is important for the oscillator circuit 8 to be located away from the high temperature body of the engine. As engine size and complexity increases, so by necessity does the length of cable required. It has been found that the sensitivity of the frequency modulated distance measuring devices using such cabling has decreased as the length of such cable has increased. The performance of known detectors has become unsatisfactory as the cable length has approached about 3 metres. It is an aim of preferred embodiments of the present invention to obviate or overcome disadvantages encountered in the prior art, whether referred to herein or otherwise. In accordance with a first aspect of the present invention, there is provided a capacitance transducer apparatus comprising an oscillator connected by a cable υ a capacitance means, which cable is tri-axial comprising an inner conductor, an outer protective layer and an intermediate layer between the outer layer and inner conductor, the inner conductor, intermediate layer and outer layer all being electrical conductors, in which the intermediate layer consists of a material that is different to that forming the outer layer and is of relatively high electrical conductivity. In this specification, the term "tri-axial" used in relation to a cable includes cables having at least three layers. That is, a four-layer cable would still be tri-axial. By substantially reducing, as compared with the prior art, the resistivity of the intermediate layer, increased sensitivity is achieved. Although it had not been appreciated before the present invention, it appears that the relatively high resistivity of the stainless steel intermediate layer in the prior art has caused the impedance to be high as a result of which the phase shift and amplitude are not maintained along the length of the cable. Reducing the total impedance (reactance) of the cable by increasing the electrical conductivity of the intermediate layer enhances the sensitivity of the apparatus considerably and makes accurate distance measurements possible using cable lengths of about 5 metres and more, typically up to about 10 metres (although this is not intended to be an upper limit for use of the present invention). Put another way, the guard driver amplifier (165) needs to be able to drive the capacitance over the full length of the cable, which it cannot do if the intermediate layer has a high resistivity. The present invention is not intended to be limited by the explanation in this paragraph. The apparatus may be a frequency modulated capacitance transducer. The apparatus may, for example constitute a capacitance distance detector. The capacitance means will normally comprise a distance probe. Normally the intermediate layer will consist of a different material from that in the outer layer and/or the central conductor, since the materials employed for the different elements are chosen taking into account different properties required of the elements. The outer layer may comprise a drawn metallic material which is preferably capable of withstanding temperatures over about 1000°C, and which resists oxidation at such temperatures. The preferred material for the outer layer is stainless steel. The intermediate layer will also need to withstand high temperatures, and will preferably have a melting point above 1 100°C, especially above 1200°C, but the intermediate layer is also required to exhibit a relatively low resistance rather than (as the outer layer is required to) oxidation resistance at elevated temperatures. Thus the conductivity of the intermediate layer is preferably substantially higher than that of stainless steel. A nickel based material may be employed for the intermediate layer because it can withstand the temperatures required for annealing the other stainless steel layers, while at the same time exhibiting a sufficiently high conductivity. Preferably, the intermediate layer is a drawn metallic material. Examples of nickel based alloys that may be used for the intermediate layer include alloys of nickel with copper, e.g. having between 25% and 75% nickel (all percentages given herein being by weight) such as cupronickel (approx. 70% Cu 30% Ni) and Monel (approx. 70% Ni 30% Cu), and other alloys comprising nickel with Chromium and/ or with cobalt, e.g. Ni Cr Fe Co alloys. Examples of such alloys include those sold under the trademarks Inconel and Incoloy (e.g. Incoloy 800 which comprises 21% Ni, 0.1% C, 0.5% Cu remainder Fe). Alternatively, pure nickel may be employed. Non-nickel based alloys may be used for example the intermediate layer may be formed from cobalt and cobalt based alloys or carbon steel. Alternatively composite materials may be employed which have the necessary conductivity and high temperature performance. For example an intermediate layer of copper sheathed in stainless steel would have the appropriate conductivity by virtue of the copper and high temperature strength by virtue of the stainless steel. Preferably the resistance of the intermediate layer is less than 1 Ohm/metre and preferably less than 0.1 Ohm/metre. Preferably also, the resistance of the cable (that is to say, the end-to-end d.c. resistance) is less than 2 Ohms, more preferable, less than 1 Ohm, especially less than 0.5 Ohms and most especially less than 0.1 Ohm. The central conductor, however, is preferably formed from a material that is relatively tough such as stainless steel. This is required in order to reduce or minimise the degree of powder damage to the central conductor as the cable is drawn down during manufacture. Suitably, a mineral insulant is provided between the inner conductor and intermediate layer and/or between the intermediate layer and outer layer. The mineral insulant preferably has a relatively low dielectric constant, for example not more than 5 and more preferably not more than 4, in order reduce the attenuation of the signal transmitted by the cable. Thus, the mineral insulant preferably is preferably silica or aluminium oxide. However, depending on the length of the cable, it is possible to employ higher dielectric constant insulants such as magnesium oxide. The apparatus may be a frequency modulated capacitance transducer. According to a second aspect, the present invention provides a tri-axial cable suitable for use as part of capacitance transducer apparatus, the cable comprising an inner conductor, an outer protective layer and an intermediate layer between the outer layer and central conductor, in which the intermediate layer consists of a different material from the outer layer and is of relatively high electrical conductivity. The cable is preferably as described above with reference to the apparatus. The present invention will now be described, by way of example only with reference to the drawings that follow; in which: Figure 1 is a schematic illustration of a prior art arrangement to which reference is also made below to explain an embodiment of the invention. Figure 2 is an enlarged cross-section through a cable peφendicular to the longitudinal axis through a cable according to the present invention. Referring to Figure 2 of the drawings that follow there is shown a cross-section through a cable 106 according to the present invention. The cable 106 comprises a central stainless steel conductor 122, a stainless steel outer layer 124 and a nickel intermediate layer 126. Between the central conductor 122 and the intermediate layer 126, and between the intermediate layer 126 and outer layer 124 is a mineral insulation material 128. In its configuration and mode of operation the cable 106 is substantially similar to that shown in and described in relation to Figure 1. However, in the case of the cable 106 according to the present invention, the substantially lower resistivity of the nickel layer 126 ensures that the phase shift and amplitude is maintained along the whole length of the cable thereby to improve greatly the sensitivity of the device as compared to that using a stainless steel intermediate layer as in the prior art. Performance over the full range of cable lengths up to 5 metres is expected to be substantially improved and substantially indistinguishable from performance at shorter lengths. Performance for all lengths of cable should be improved. Manufacture of the cable described herein is within the ambit of skilled addressee of the present specification for example the cable 106 may be manufactured by first drawing the central conductor 122 in a known manner. The intermediate nickel layer and outer stainless steel layers 126, 124 respectively are then drawn and disposed concentrically about the central conductor 122. The layers are then annealed. The nickel layer is advantageous at this stage compared with, say, copper because it can withstand without significant degradation the temperatures required to anneal the stainless steel. The cable 106 is filled with mineral insulant such as silica or aluminium oxide as is well known. While the cable has been described in relation to its applicability and usefulness as part of a frequency modulated capacitance transducer, it can also be used advantageously as part of an amplitude modulation based capacitance system.

Claims: Claims: 1. Capacitance transducer apparatus comprising an oscillator connected by a cable to a capacitance means, which cable is tri-axial comprising an inner conductor, an outer protective layer and an intermediate layer between the outer layer and inner conductor, the inner conductor, intermediate layer and outer layer all being electrical conductors, in which the intermediate layer consists of a material that is different to that forming the outer layer and is of relatively high electrical conductivity. 2. Apparatus as claimed in claim 1, wherein the intermediate layer has a resistance less than 2 Ohm metre "1 . 3. Apparatus as claimed in claim 2, wherein the intermediate layer has a resistance less than 0.1 Ohm metre ' . 4. Apparatus as claimed in any one of claims 1 to 3, wherein the cable has a resistance of less than 2 Ohms. 5. Apparatus as claimed in claim 4, wherein the cable has a resistance less than 0.5 Ohm. 6. Apparatus as claimed in any one of 1 to 5, wherein the intermediate layer comprises a nickel based material. 7. Apparatus as claimed in claim 6, wherein the intermediate layer comprises an alloy of nickel with copper, or with chromium and/or cobalt. 8. Apparatus as claimed in claim 6, wherein the intermediate layer comprises pure nickel. 9. Apparatus as claimed in any one of claims 1 to 5, wherein the intermediate layer comprises cobalt (e.g. a cobalt based alloy) or carbon steel, or a composite material. 10. Apparatus as claimed in any one of claims 1 to 9, wherein the cable includes mineral insulation between the central conductor and intermediate layer and between the intermediate layer and the outer layer. 1 1. Apparatus as claimed in claim 10, wherein the mineral insulation has a dielectric constant of not more than 5. 12. Apparatus as claimed in claim 10 or claim 11, wherein the mineral insulation is silica or aluminium oxide. 13. Apparatus as claimed in any one of claims 1 to 12, which measures blade tip clearance in a jet engine or gas turbine. 14. A tri-axial cable suitable for use as part of capacitance transducer apparatus, the cable comprising an inner conductor, an outer protective layer and an intermediate layer between the outer layer and central conductor, in which the intermediate layer consists of a different material from the outer layer and is of relatively high electrical conductivity. *******

Inventors: KNOX BRYAN, STRINGFELLOW JOHN FRANCIS, WAYMAN LESLIE, KNOX, BRYAN, STRINGFELLOW, JOHN, FRANCIS, WAYMAN, LESLIE
Applicants: BICC PLC, FYLDE ELECTRONIC LAB LTD, KNOX BRYAN, STRINGFELLOW JOHN FRANCIS, WAYMAN LESLIE, BICC PUBLIC LIMITED COMPANY, FYLDE ELECTRONIC LABORATORIES LTD., KNOX, BRYAN, STRINGFELLOW, JOHN, FRANCIS, WAYMAN, LESLIE
Application year: 1997
Main classification: G01B7
IPC codes: G01B7, H01B11