GB2245709A - Dual readout temperature and strain sensor combinations - Google Patents

Abstract

A combined strain and temperature sensor system includes a strain gauge (1) and the hot (7, 8) and cold (31, 32) junctions of a thermocouple. A common electrical connection is provided for both the strain gauge (1) and the hot junction (7, 8) of the thermocouple by means of thermocouple cable (11, 17) comprising thermoelectrically dissimilar conductors (9, 10; 15, 16). The thermocouple cable (11, 17) is connected to a switch (42) for switching the sensor system between temperature and strain measurement modes through the thermocouple cable (11, 17). The location of the hot junction may be at the same position as the gauge grid (2). Alternatively the temperature measurement may be taken at a remote location by using a further thermocouple cable connected into cable (11) and choice of wires to eliminate hot junctions (7, 8). A three channel system is described. <IMAGE>

Description

DUAL READOUT TEMPERATURE AND STRAIN SENSOR COMBINATIONS The present invention relates to sensor systems combining the functions of both thermocouples and dynamic strain gauges and capable of giving an instrument readout for both temperature and strain.

Thermocouples and dynamic strain gauges, commonly used in the aerospace and other industries for measuring temperature and dynamic strain on the surfaces of test components, are typically two-terminal sensors, which therefore usually have two wires leading to each sensor, strain measurement being performed by measuring current or voltage changes caused by changes in gauge resistance due to elongation of the gauge elements and temperature measurement being performed by measuring the e.m.f.

produced in a circuit comprising two or more dissimilar metals such as nickel-chromium and nickel-aluminium alloys.

There is a practical limit to the number of different sensors which can be fixed to the surface of a component, or wired out of a component test rig or test assembly such as a turbofan aeroengine. Consequently, it is often necessary to have two separate builds and tests in order to obtain measurements for both strain and temperature.

An object of the present invention is to facilitate reductions in the number of discrete sensors and/or the number of different wires needed to instrument such rigs and assemblies.

Expressed at its simplest, the present invention provides a combined strain and temperature sensor system including strain gauge sensor means and thermocouple sensor means comprising first and second junction means between dissimilar conductors, wherein thermocouple cable means comprising thermoelectrically dissimilar conductors provides a common electrical lead for both the strain gauge sensor means and the first junction means of the thermocouple sensor means.

Conveniently, the sensor system includes switch means for switching the sensor system between temperature and strain measurement modes through the thermocouple cable means.

Also according to the invention, a combined strain and temperature sensor system includes: strain gauge sensor means; thermocouple sensor means comprising first junction means between dissimilar conductors at a location whose temperature is to be determined and second junction means between dissimilar conductors at a temperature reference location; switch means for switching the sensor system between temperature and strain measurement modes; and electrical conductor means connecting the switch means to the strain gauge sensor means through the first and second thermocouple junction means, wherein the electrical conductor means between the first and second thermocouple junction means comprises thermocouple cable means comprising thermoelectrically dissimilar conductors, thereby enabling both temperature and strain measurements to be obtained through the thermocouple cable means.

In our disclosed embodiments of the invention, there is a junction between the thermocouple cable means and a non-thermocouple cable means, which junction comprises the second thermocouple junction means of the thermocouple, the non-thermocouple cable means comprising thermoelectrically similar conductors connecting the thermocouple cable means to the switch means.

For situations where the sensor means are subject to high temperatures, the thermocouple cable means can conveniently comprise a high temperature resistant cable, such as a mineral-insulated metal-clad type, at least in the region nearest the sensor means.

In one embodiment of the invention, the thermocouple cable means includes an extension thereof having an extremity distant from the strain gauge means, said extension comprising thermoelectrically dissimilar conductors which are joined at said extremity thereby to constitute the first junction means of the thermocouple sensor means.

In a different embodiment, dissimilar conductors of the thermocouple cable means are connected directly to the strain gauge sensor means. Alternatively, dissimilar conductors in the thermocouple cable means are connected to the strain gauge sensor means through electrical leads, the leads being thermoelectrically similar with respect to the thermocouple cable conductors to which they are connected. Both these arrangements are such that the first junction means of the thermocouple sensor means is at the same location as the strain gauge sensor means.

In a further embodiment of the invention, dissimilar conductors in the thermocouple cable means are again connected to the strain gauge sensor means through electrical leads, but at least one of the leads is thermoelectrically dissimilar with respect to the thermocouple cable conductor to which it is connected, such that the first junction means of the thermocouple sensor means is at the connection between said lead and said thermocouple cable conductor.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a diagram of a thermocouple and strain gauge combination suitable for use with the invention; Figure 2 shows the sensor combination of Figure 1 as part of a simple system for measuring dynamic strain and temperature on stationary components; Figure 3 is similar to Figure 2 but showing a multichannel system; Figure 4 is similar to Figure 1 but showing an alternative arrangement for a sensor combination; Figure 5 is a pictorial view of a sensor combination installed in a component; and Figure 6 is a schematic sectional side elevation of part of the compressor of a turbofan aeroengine instrumented with sensor combinations according to the invention.

In Figure 1 and Figure 2 a sensor combination 1 for dynamic strain and high temperatures includes a strain gauge grid 2 of e.g. Karma (Trade Mark) alloy wire. This is basically a nickel-chromium alloy. The grid wire is connected at both ends by, e.g. resistance spot welds 3,4 to short lead wires 5,6 having a similar composition to the grid wire. These in turn are connected at 7,8 to the ends of respective wires 9,10 which form the twin conductors of an mineral-insulated metal-clad (MIMC) cable 11.

The cable 11 is a type often used for thermocouple lead-wires in high temperature locations such as gas turbine engines. Besides thermoelectrically dissimilar twin core conductors 9,10 it also comprises an outer metallic sheath 12, the conductors and sheath being insulated from each other by a compacted mineral powder filling. One of the core conductors 9 consists of a special-purpose,nickel-chromium alloy and the other core conductor 10 consists of a special-purpose nickel-aluminium alloy. Hence, it can be arranged that junctions 7,8 can be the "hot junction" of a thermocouple in order to sense temperatures near the strain gauge grid.

The temperature sensed is in fact the mean temperature of junctions 7 and 8, but since these are close together they would be at substantially the same temperature anyway.

It is unlikely that grid wire 2 would be made of exactly the same alloy as lead wires 5,6 or core conductor 9 because, as will be apparent to the skilled person, their exact compositions are chosen to give them different characteristics according to their different functions.

However, this is not in practice a problem with respect to the thermoelectric characteristics of the circuit.

Alternative alloys can of course be used for the various wires of the above arrangement, provided that lead-wires 5,6 both have the same composition and provided that wires 9 and 10 are sufficiently thermoelectrically dissimilar.

For instance, a platinum-tungsten strain gauge grid can be substituted for the nickel-chromium strain-gauge grid 2, and assuming junctions 3 and 4 are both at substantially the same temperature there is no need to match the composition of the short lead wires 5,6 to the strain gauge grid because the junctions 3 and 4 are identical. It is in any case preferred that lead wires 5,6 are nickel-chromium.

An alternative type of MIMC thermocouple cable 11 which could be used is the so-called "Type N", in which one conductor 9 is nickel-chrome-silicon alloy and the other conductor 10 is a nickel-silicon alloy. These are supplied under, e.g. the "Nicrosil" and "Nisil" trademarks. The consequent change in hot junction characteristics, due to junctions 7 and 8 also being junctions between thermoelectrically dissimilar alloys, is not a problem and can be allowed for during calibration of the system. This assumes of course that junctions 7 and 8 are at substantially the same temperature because they are close together.

The length of high-temperature thermocouple cable 11 which is used will depend upon the temperatures that the cable experiences at increasing distance from the sensor 1. At some stage it is usually possible to change from the high-temperature MIMC thermocouple cable 11 to a medium temperature thermocouple lead-out cable 17 of the well-known flexible type. This again has twin conductors 15,16 which are joined at 13,14 to conductors 9 and 10 of cable 11 by means of brazing or resistance spot-welding techniques. It will of course be convenient for electrical calibration of the system if the conductors 15,16 in cable 17 are the same respective alloy compositions as conductors 9,10 in cable 11, thus avoiding the creation of a junction with a further thermoelectric effect.

Following the electrical path further to the right in Figure 2, it will be seen that cable 17 is terminated in a cooler location at a junction box 30, where the conductors 15,16 of cable 17 are joined at 31,32 to low-resistance insulated copper conductors 33,34 included in the core of a cable 36, which at its other end terminates at a junction and switch box 38. The junctions 31 and 32 can be considered to be the "cold" junction of the thermocouple, the junction box 30 therefore being the thermal reference plane for measurement of the thermoelectric voltage produced at the junctions 7,8.

In thermocouple instrumentation systems for engineering test beds and the like it is usually impractical or too costly to maintain the thermal reference plane at a constant relatively cool temperature and so the junction box 30 includes a resistance thermometer 40, which, when calibrated, gives an absolute measure of the temperature of the thermal reference plane.

Thus, even though the temperature of "cold" junction 31,32 may vary considerably, an accurate measure of the hot junction temperature can still be obtained by applying a correction factor which, after calibration of the whole system, allows for the effect of temperature variation in the thermal reference plane. As shown, the leads of the resistance thermometer form part of the core of copper cable 36 and are connected through junction and switch box 38 to resistance measuring circuitry (not shown, but well known, such as a Wheatstone bridge type of instrument).

Since measurement of strain variation using strain gauges depends upon changes of resistance in the gauges and measurement of temperature changes using thermocouples depends upon changes in the thermoelectric emf produced by the junctions of dissimilar metals, different means of measuring these changes are required. Hence, in box 38 the two copper lead wires 33,34 are connected to the poles of a double ganged switch 42 which is used to switch their connection between the leads 44,46 of the resistancemeasuring circuitry (not shown) for the dynamic strain gauge 2 and the leads 48,50 of the voltage-measuring circuitry (not shown) for the thermocouple junctions 7,8 and 31,32. The voltage-measuring circuitry can be merely a digital voltmeter.

Turning now to Figure 3, there is schematically shown a three-channel system according to the invention. Those parts which are identical to Figure 2 are given the same reference numbers and will not be further described. The junction box 30' differs from the box 30 in Figure 2 only in the extra terminals required to handle the two extra data channels, whose sensors, and their wires, shown in dashed lines, are identical to the first. A switch box 38' differs from the box 38 in Figure 2 in having a threechannel switching arrangement comprising a first double-ganged switch 52 in series with a second doubleganged switch 54, switch 52 being switchable between three separate pairs of terminals, one pair for each channel, and switch 54 acting similarly to switch 42 in Figure 2 for taking separate dynamic strain and temperature measurements for each sensor channel.

While simple mechanical switches are shown in Figure 3, it would of course be possible to utilise a patchboard or an electronic multiplexer instead.

It will be noted from the above description and Figure 1 that the hot and cold junctions 7,8 and 31,32 of the thermocouple are electrically in series with the gauge grid 2.

If desired, the location of the hot junction can be at the same position as the gauge grid 2 simply by eliminating thermoelectrically dissimilar junctions 7,8 and either making lead-wires 5,6 of the same material as wires 9,10, or continuing wires 9,10 up to junctions .3,4, which then become the hot junction. In fact, by choosing where to terminate the high temperature thermocouple cable 11, a dissimilar metal "hot" junction can be formed wherever it may be required to give temperature measurements.

Turning now to Figure 4, the parts which are the same as in Figure 1 are given identical reference numbers and need no further description. Looking at the differences, the aim of the embodiment of Figure 4 is to facilitate strain measurement at one location and temperature measurement at another entirely different location, but without departing from the principle of using standard thermocouple cable 11,17 as a common electrical lead to both the strain gauge and the hot junction of the thermocouple. To this end, the nickel-chromium wire 9 and the nickel-aluminium wire 10 (or the previously discussed alternatives) of cable 11 are terminated at junctions 18,19 respectively, junction 18 being with a wire 20 of like material to wire 9, wire 20 being part of MIMC cable 22 whose second core wire 21 is also of the same material as wire 9.Wires 20 and 21 of cable 22 are terminated at junctions 7' and 8' where they are joined to the nickel-chromium lead wires 5,6 of gauge grid 2.

At junction 19, nickel-aluminium wire 10 of cable 11 is joined to a further nickel-aluminium wire 23 in a similar high temperature thermocouple cable 25, the other wire 24 in the cable 25 being nickel-chromium and joined to nickel-chromium wire 21 of cable 22 at junction 26 near junctions 18 and 19. At the far end of thermocouple cable 25 the two wires 23,24 are joined together to form the hot junction 27 of the thermocouple. As in Figures 1 and 2 the thermocouple's cold junction 31/32 and hot junction 27 are in electrical series with the strain gauge grid 2 through its lead-wires, but the insertion of the extra length of high temperature thermocouple cable 25 in series with wire 21 of the non-thermocouple cable 22 and wire 10 of high-temperature thermocouple cable 11 enables the hot junction 27 of the thermocouple to be remote from both the strain gauge and the cold junction.

Use of thermocouple cables 11 and 17 to provide lead-wires which are common to both the thermocouple hot junction and the strain gauge enables temperature and strain measurement from the same installation using only one set of wires leading to the read-out instruments.

This can significantly reduce the number of separate builds and tests necessary to obtain dynamic strain and temperature measurements from highly instrumented test rigs and assemblies.

Moreover, in relation to Figure 1, or more particularly to an arrangement in which the hot junction of the thermocouple is junction 3,4, the invention facilitates accurate knowledge of the temperature of the strain gauge grid 2, thus enabling accurate corrections to be applied for gauge resistance, strain sensitivity and substrate material modulus changes.

Figure 5 illustrates in more detail the application of a sensor arrangement like that of Figure 1 to an actual component. In this case the component is a compressor stator vane 60 in a gas turbine engine, part of the multi-stage axial compressor 62 in which the stator vane 60 is situated as shown in Figure 6. In Figure 5 part of the vane's outer platform 64 has been machined away to form a compressor air bleed slot 66 and this bleed slot 66 forms a convenient access for the lead-wires 5',6' to the strain gauge grid 2', which is attached to the surface 68 of one flank of the vane's aerofoil 70. At junctions 7',8', the lead wires 5',6' are joined to the respective conductors 9'10' of a high temperature thermocouple cable 11', which is secured to a mounting lug 72 of the vane 60 by a cable clip 74.As in Figure 1, high temperature MIMC thermocouple cable 11' transitions to the medium temperature flexible thermocouple cable 17' at a suitable convenient point. As emphasised previously, the dissimilar thermocouple alloy conductors can be terminated at any of points A,B or C, or any intermediate position, to give the hot junction of the thermocouple at that point.

A process for installing the combined strain gauge and thermocouple in the vane 60 can be summarised as follows.

The area to which the strain gauge grid 2' and its lead wires 5',6' are to be attached is mechanically cleaned and an insulating layer of aluminium oxide-based ceramic cement is painted on and heat cured. Grid wire 2' and lead wires 5',6' are laid onto the cement and temporarily held in position with adhesive tape while more ceramic cement is painted over the wires to insulate them and bond them in place. Lead wires 5' and 6' are then brazed or spot-welded to the respective conductors 9' and 10' of the high temperature thermocouple cable 11', which has been fixed in position by the clip 74, this having been spot-welded to the lug 72.

Turning now to Figure 6, the part of the compressor 62 which is schematically illustrated is instrumented in accordance with the invention as follows.

The stator vane 60 is instrumented with a combined strain gauge 2' and hot thermocouple junction 7',8' as already discussed in relation to Figure 5.

Three compressor rotor blade stages 75,77 and 79 are also instrumented with respective strain gauges 81,83,85.

The rotor blade stages are fitted to a compressor drum 86 having internal discs 88,90,92 with respective hub portions 94,96 and 98 to resist centrifugal stress in the drum 86. These all rotate as a unit together with driven shaft 99.

In rotor blade stage 75 the connections to the strain gauge grid 81 on the blade flank comprise short lead wires 87 connected to a longer MIMC cable 89 leading down through drillings in the blade root and the compressor drum 86 to a position on the stage's rotor disc 88 where the cable 89 is connected to a medium temperature flexible leadout cable 93. Thereafter the cable 93 passes to a slipring or telemetry unit 95 via the surface of the disc's hub 94 and the interior of a rotating shaft 99 to which the disc is attached.

On this stage, it is desired to measure the temperature of the mid-point of the compressor disc 88 and so the hot junction 101 of the thermocouple is formed there by choosing cable 93 to be of the thermocouple type, with dissimilar alloy conductors, and choosing cable 89 to be of the non-thermocouple type, where the conductors are of the same alloy as each other, preferably of the same type as the short lead wires 87 to the strain gauge grid 81.

In rotor blade stage 77 the short lead wires 103 to the strain gauge grid 83 are the same type as in rotor blade stage 75, but are attached to the conductors of a high temperature thermocouple MIMC cable 105 to form the hot junction 107 of a thermocouple in the blade root 109.

Cable 105 passes through drillings in the compressor drum 86 and compressor disc 88 and is attached to structure (not shown) rotating with compressor drum 86 for support on its way to the unit 95.

Finally, in rotor blade stage 79 the arrangement for grid lead wires 111 and associated MIMC cables 113,115 and 117 is similar to that shown in Figure 4, with cables 115 and 117 being of the high temperature thermocouple type and cable 113 being a high temperature non-thermocouple type. The twin conductors of cable 115 are joined to form a thermocouple hot junction 119 on the face of compressor disc 92 near hub portion 98. Cable 113 passes through a drilling in the compressor drum 86 and is joined to cables 115 and 117 on the drum's inside surface. Cable 117 passes through another drilling in the radially outer part of disc 90 and then takes a parallel route to that of cable 105.

In the above-described instrumentation of the compressor 62, all the cables must of course be securely attached by clips (not shown) and high temperature-resistant cements to the rotating surfaces which they traverse.

which they traverse.

Whereas the above-described embodiments relate specifically to temperature and stress sensors in high temperature locations, the invention is of course equally applicable to such sensors adapted for use in lower temperature locations. Under such circumstances it is possible to use flexible plastic-insulated cable, such as cable 17 in Figures 1 to 5, to replace MIMC cables 11,22 and 25. Also, wire strain-gauge grids 2 can be replaced by metal foil grids fixed to the component surface by epoxy resin cement or the like.

Claims (13)

CLAIMS:

1. A combined strain and temperature sensor system including strain gauge sensor means and thermocouple sensor means comprising first and second junction means between dissimilar conductors, wherein thermocouple cable means comprising thermoelectrically dissimilar conductors provides a common electrical lead for both the strain gauge sensor means and the first junction means of the thermocouple sensor means.

2. A sensor system according to claim 1 including switch means for switching the sensor system between temperature and strain measurement modes through the thermocouple cable means.

3. A combined strain and temperature sensor system including: strain gauge sensor means; thermocouple sensor means comprising first junction means between dissimilar conductors at a location whose temperature is to be determined and second junction means between dissimilar conductors at a temperature reference location; switch means for switching the sensor system between temperature and strain measurement modes; and electrical conductor means connecting the switch means to the strain gauge sensor means through the first and second thermocouple junction means, wherein the electrical conductor means between the first and second thermocouple junction means comprises thermocouple cable means comprising thermoelectrically dissimilar conductors, thereby enabling both temperature and strain measurements to be obtained through the thermocouple cable means.

4. A sensor system according to claim 2 or claim 3 in which there is a junction between the thermocouple cable means and a non-thermocouple cable means, which junction comprises the second thermocouple junction means of the thermocouple, the non-thermocouple cable means comprising thermoelectrically similar conductors connecting the thermocouple cable means to the switch means.

5. A sensor system according to any one of claims 1 to 4 in which the sensor means are subject to high temperatures, the thermocouple cable means comprising, at least in the region nearest the sensor means, a mineral-insulated metal-clad type.

6. A sensor system according to any one of claims 1 to 5 in which the thermocouple cable means includes an extension thereof having an extremity distant from the strain gauge means, said extension comprising thermoelectrically dissimilar conductors which are joined at said extremity to constitute the first junction means of the thermocouple sensor means.

7. A sensor -system according to any one of claims 1 to 5 in which dissimilar conductors in the thermocouple cable means are connected directly to the strain gauge sensor means, such that the first junction means of the thermocouple sensor means is at the same location as the strain gauge sensor means.

8. A sensor system according to any one of claims 1 to 5 in which dissimilar conductors in the thermocouple cable means are connected to the strain gauge sensor means through electrical leads, the leads being thermoelectrically similar with respect to the thermocouple cable conductors to which they are connected, such that the first junction means of the thermocouple sensor means is at the same location as the strain gauge sensor means.

9. A sensor system according to any one of claims 1 to 5 in which dissimilar conductors in the thermocouple cable means are connected to the strain gauge sensor means through electrical leads, at least one of the leads being thermoelectrically dissimilar with respect to the thermocouple cable conductor to which it is connected, such that the first junction means of the thermocouple sensor means is at the connection between said lead and said thermocouple cable conductor.

10. A combined strain and temperature sensor system substantially as hereinbefore described with reference to and as illustrated by Figures 1 and 2 of the acompanying drawings.

11. A combined strain and temperature sensor system substantially as hereinbefore described with reference to and as illustrated by Figure 3 of the acompanying drawings.

12. A combined strain and temperature sensor system substantially as hereinbefore described with reference to and as illustrated by Figure 4 of the acompanying drawings.

13. A combined strain and temperature sensor system substantially as hereinbefore described with reference to and as illustrated by Figures 5 and 6 of the acompanying drawings.