GB2219664A

GB2219664A - Position transducer

Info

Publication number: GB2219664A
Application number: GB8913367A
Authority: GB
Inventors: Frank David Charlesworth; William Duncan Dover; Kenneth Alan Taylor
Original assignee: Technical Software Consultants Ltd
Current assignee: TSC Subsea Ltd
Priority date: 1988-06-09
Filing date: 1989-06-09
Publication date: 1989-12-13
Anticipated expiration: 2009-06-09
Also published as: GB8913367D0; GB8813632D0; GB2219664B

Abstract

A position transducer utilizes components made of non-ferromagnetic materials, rather than the ferromagnetic materials conventionally used for such components, to avoid spurious effects caused by the temperature dependency of ferromagnetic properties. In a linear variable differential transformer embodiment 12, a non-ferromagnetic core 22 moves within a non-ferromagnetic tube 13. This movement changes the distribution of an alternating magnetic field induced at the surface 14 of the tube 13 by a coil 24, and hence gives rise to a net e.m.f, indicative of core position, in secondary windings 25, 26. Alternatively the non-ferromagnetic components may be plates (P1, P2, figure 1a) and the field may be generated by passing an alternating electric current through one plate (P2, figure 1a). <IMAGE>

Description

Position Transducer This invention relates to a position transducer and, particularly but not exclusively to a position transducer incorporating a core of non-ferromagnetic material.

At present a whole class of position transducers rely for their operation upon the movement of a ferromagnetic core which alters the magnetic coupling between primary and secondary windings. The primary winding of such a transducer is excited with an alternating current and an alternating voltage is sensed on the output from the secondary windings.

Whilst the efficiency (ie the coupling of an a.c signal from the primary to secondary windings) of a transducer having a ferromagnetic core is relatively good, it is well known that above about 7000C ferromagnetic materials lose their ferromagnetism and hence transducers using such a core material cannot operate in their normal manner above this temperature. Further, changes in the magnetic properties of ferromagnetic materials with temperature can affect the sensitivity of transducers which have cores made from this material. In other words, there is often some sensitivity to temperature changes and gradients even below the Curie point.

According to the present invention, there is provided a position transducer comprising a first component of nonferromagnetic material, a second component of nonferromagnetic material adjacent the first component, means for generating an alternating magnetic field adjacent a surface of the first component, separation of the components being such that field distribution relative to the first component is influenced by presence of the second component, and detection means for detecting variation in the magnetic field at the said surface, such variation being indicative of relative movement of the detection means and/or at least one of the components.

It will be appreciated that the alternating magnetic field can be produced in a number of ways, for example by direct injection of an alternating electric current into the first component or, preferably, by indirect means for example surrounding the first component by a solenoid carrying the alternating electric current and producing the magnetic field by induction.

Since the magnitude of the variation in magnetic field falls with the frequency of that field, a preferred lower limit for the frequency is 0.5 KHz. Conversely, at higher frequencies the associated instrumentation may be susceptible to noise. The preferred upper limit of the frequency range is, therefore, 50 KHz.

The detection means may be a contacting probe, or, more preferably-, a non-contacting probe.

Following is a description, by way of example only and with reference to the accompanying drawings, of various embodiments of the invention.

In the drawings: Figure la is a aiagrammatic representation of one embodiment of the invention comprising two plates of non-ferromagnetic material a first plate of which overlaps a second plate and is injected with an alternating electric current to provide a uniform magnetic field on a top surface thereof and the second plate of which is movable relative to the first plate and the detection means being a probe for contacting the said surface of the first plate for detecting the field; Figure ib is a graph indicting decrease of voltage detected by the probe when moved relative to the said surface in a direction towards the second plate;; Figure ic is a graph similar to Figure ib indicating decrease of voltage detected by the probe when the second plate is moved relative to the first plate in a direction towards the probe; Figure id is a diagrammatic representation of another embodiment of the invention and is similar to that shown in Figure la except that the probe is not in physical contact with the said surface.This non-contacting probe senses the alternating magnetic field present adjacent the surface; Figure ie is a diagrammatic represention of a further embodiment of the invention and is similar to that shown in Figure la except that the alternating magnetic field is produced by induction; Figure if is a diagrammatic represention of another embodiment of the invention and is similar to that shown in Figure le except that the probe is not in physical contact with the said surface; Figure 2 is a diagrammatic cross section of a linear variable differential transformer (LVDT) embodying the present invention; Figure 3 is a circuit diagram of the embodiment shown in Figure 2 together with a block diagram of the detection means;; Figure 4 is a graph indicating the relationship between movement of a core of the LVDT shown in Figure 2 and voltage output; Figure 5a is an end elevation of another LVDT embodying the present invention; Figure 5b is a diagrammatic cross section of the embodiment shown in Figure 5a; Figure 6a is an end elevation of a further LVDT embodying the present invention; Figure 6b is a side elevation of the embodiment shown in Figure 6a; and Figure 7 is a diagrammatic perspective view of a further embodiment of a transducer in accordance with the present inventiion capable of sensing movement in two directions.

Referring now to the drawings, a first embodiment of a position transducer according to the invention is shown schematically in Figure 1. The first component is represented by a rectangular plate P2 of non-ferrromagnetic material and the second component by another rectangular plate P1 of non-ferromagnetic material and of smaller area than the plate P2. The plate P2 is located above the plate P1 in spaced relation thereto such that the upper plate P2 overlaps the lower plate P1. The upper plate P2 has opposite facing surfaces 3 and 4 and an upper surface 5.

Each of the opposite facing surfaces 3 and 4 has connected thereto a corresponding one of a pair of leads 6 and 7 which are connected to means (not shown) for supplying alternating electric current. The upper surface S of the upper plate P2 is contacted by a probe 8 having a lead 9 extending therefrom for connecting the probe 8 to means (not shown) for measuring a.c. field.

Since the efficiency of a transducer manufactured from nonferromagnetic materials is low, it is necessary to provide a relatively large alternating current to the first component and to detect the very small changes which are produced by relative movement of the components. One suitable instrument for this purpose is disclosed in U.K. patent application No. 2012965 wherein a pair of current input leads are engaged on the workpiece on respective sides of a crack. An alternating current is passed through the leads and through the workpiece and potential differences detected between electrodes of a probe located such that the electrodes also are on respective sides of the crack.

The arrangement is such that the lower plate P1 is movable below the upper plate P2 in direction y, an upper edge 10 of a surface of the lower plate P1 extending substantially parallel to the surface 4 of the upper plate P2 comprising a 'leading edge' of lower plate P1, and/or the probe 8 is movable in a direction x on the upper surface 5 of the upper plate P2 from an area B thereof of a portion of the upper plate P2 which extends beyond the lower plate P1 to an area A of the upper surface 5 of a portion of the upper plate P2 which extends above the lower plate P1 and movable in a reverse direction from A to area B.

Referring now to Figure ib of the drawings, there is shown a graph indicating voltage signal measured at the probe 8 during movement of the probe from the area B to area A of the upper plate P2. The graph indicates that the presence of the lower plate P1 causes the surface potential distribution of the magnetic field to be different (i.e.

smaller) in area A compared with area B and movement of the probe from area B to area A is indicated by the graph and identifies the leading edge 10 of the lower plate P1.

Similarly, movement of the lower plate P1 in the direction y of Figure la results in the field measured by the probe 8 decreasing in magnitude when the leading edge 10 of the lower plate P1 is below and moves beyond the probe 8, as indicated in the graph shown in Figure ic.

Referring now to Figure id of the drawings, there is shown an arrangement similar to that shown in Figure la except that in Figure id, the probe 8 is located above and does not physically contact the upper surface 5 of the upper plate P2.

In Figure le, there is shown an arrangement similar to that shown in Figure la except that the leads 6 and 7 are connected in series with four spaced parallel wires 11 which are located on and insulated from the upper surface 5 of the upper plate P2. When the leads 6 and 7 are connected to an a.c. source, the passage of the current through the wires 11 induces the magnetic field on the upper surface 5 of the upper plate P2 and the presence of several wires ensures that the field is substantially uniform.

Referring now to Figure if of the drawings, there is shown an arrangement similar to the arrangement shown in Figure le except that the probe 8 is not in physical contact with the upper surface 5 of the upper plate P2.

When an alternating current flows in a conducting material, it often flows in only a thin 'skin' below the outer surface. For a plate whose thickness is large compared to this 'skin' depth, the depth of the skin is given by the formula:

Where d = skin depth of current flow defined as the point at which the voltage falls to a value lZe-(i.e.0.368) of the voltage at the surface of the plate.

f = frequency of alternating current.

= = relative magnetic permeability of material of non-ferromagnetic plate P2 (approximately equal to 1.0).

b = conductivityof material of non- ferromagnetic plate P2.

0 = 4 x 10-7 # = 3.1416 For a situation where the thicknesses of the plates for P1 and P2 are much less than the skin thicknesses for the given material and frequency, the measured voltage would approximately double when moving the probe 8 from area A to area B.If the length of the probe 8 is made much greater than the skin depth d the measured voltage with the probe spanning the leading edge 10 is a simple proportion of the two values of voltage as follows:

Where x = distance moved by probe 1 = probe 'length Vm = measured voltage on P2 V2 = steady voltage on P2 when plate is under the probe Referring now to Figure 2 of the drawings, there is shown a position transducer according to the invention in the form of z linear variable differential transformer (LVDT) 12, which represents a practical version of the schematic embodiment of Figure 1 f. The LVDT 12 comprises an elongate cylinder 13 of non-ferromagnetic material having an outer circumferential surface 1 4 and an inner circumferential surface 1 5. Opposite end portions of the cyliner 1 3 are plugged with bushes 16,17 each having a central bore 18,19.

The bores 1 8 , 1 9 each receive a corresponding one of a pair of shafts 20,21 extending in opposite directions from a cylindrical core 22 of non-ferromagnetic material, the central longitudinal axes of the shafts 20,21 being in line and being coaxial with a central longitudinal axis of the core 22. The core 22 has an outer circumferential surface 23 the diameter of which is slightly less than the diameter of the inner circumferential surface 15 of the cylinder 13 and the longitudinal dimension of which is considerably less than the longitudinal dimension of the cylinder 13. The arrangement is such that the shafts 20,21 are slidable in the corresponding bores 18,19 of the bushes 16,17 whereby application of a force axially on the shaft 20 or 21 effects movement of the core 22 longitudinally of the cylinder 13.

The outer surface 14 of the cylinder 13 is provided with an insulated wire wound thereon to provide a primary winding 24 extending longitudinally on the cylinder 13 symmetrically about a central transverse axis thereof. The outer surface 14 of the cylinder 13 also is provided with a second insulated wire wound thereon in two portions, a first portion 25 thereof being located on one side of the primary winding 24 and a second portion 26 thereof being wound on an opposite side of the primary winding 24 whereby the portions 25 and 26 each comprise a half secondary winding connected in series-opposition.

Comparing Figure 2 with Figure 1f, the core 22 corresponds with the plate P1, the cylinder 13 corresponds with the plate P2, the primary winding 24 corresponds with the wires 11 and the secondary winding 25,26 corresponds with the probe 8.

Referring now to Figure 3 of the drawings, there is shown a circuit diagram of the LVDT 12 shown in Figure 2 wherein the primary winding 24 is connected to a power supply 27 for supplying constant alternating current and the series connected portions 25,26 of the secondary winding are connected to a high gain amplifier 28 which supplies a phase sensitive detector 30 via a band pass filter 29 whereby a signal indicative of longitudinal displacement of the core 22 relative to the cylinder 13 is received by the phase sensitive detector 30 and compared with a reference signal received from the power supply 27 to provide an analogue output which is supplied to a digital voltmeter 31.

It will be appreciated that the core 22 comprises a shorted turn and an e.m.f. is induced in it when current flows through the primary winding 24 producing a flux which opposes the original flux generated in the primary winding 24. When the core is central, it has equal effect on flux linking both secondary windings 25,26. However, when offset to one side, the core 22 reduces the flux linking the secondary windings 25,26 on that side allowing it to increase on the other side producing a net output e.m.f.

from the secondary windings 25,26, unlike a ferromagnetic core which has the opposite effect.

The cylinder 1 3 has an effect such as to linearise the operation of the device by masking ('smoothing-out', 'swamping') small irregularites in the windings. The thickness of the wall of the cylinder 13 should be less than the depth d of the skin of the field caused by flow of current longitudinally of the cylinder 13.

Output of the LVDT 12 as a function of movement of the core 22 relative to the cylinder 13 is shown in Figure 4 which demonstrates the linear relationship between these two parameters.

It will be appreciated that, instead of moving the core by external means, relative movement could be provided if the core and cylinder were made of different non-ferromagnetic materials with different rates of thermal expansion. In this variation, the transducer could be used to measure temperature.

In Figures 5a and 5b there is shown another embodiment of a position transducer incorporating an LVDT 32 which is similar to the LVDT 12 shown in Figures 2 and 3. However, in the LVDT 32, a cylinder 33 is provided with bushes 34,35 each having a central bore 36,37 in which is received a shaft 38 extending through the cylinder 33. A central portion of the shaft 38 is provided with an external screw thread 39. Located within the cylinder 33 is a core 40 of non-ferromagnetic material, the core having a circumferential outer surface 41 and central longitudinal bore provided with an internal screw thread 42 complementary to the screw thread 39 on the shaft 38.The outer surface 41 of the core 40 is provided with a longitudinally extending rib 43 which is located in a complementary groove 44 extending longitudinally of an inner surface of the cylinder 33.

The arrangement is such that, when the shaft 38 is turned on a central longitudinal axis thereof, the core 40 moves longitudinally of the shaft 38 due to the reaction of the thread 39 on the shaft 38 and the thread 42 on the core 40, rotation of the core 40 on the shaft 38 being prevented by engagement of the rib 43 in the groove 44. Primary and secondary windings of the LVDT 32 would be connected in a circuit similar to that shown in Figure 3 so that longitudinal movement of the core 40 relative to the cylinder 33 would result in signals being directed to a digital volt meter.

Referring now to Figures 6a and 6b of the drawings, there is shown a further embodiment of a position transducer according to the invention in the form of an LVDT 45 comprising a cylinder 46 of non-ferromagnetic material having bushes 47,48 each provided with a central bore 49,50 in which is received a shaft 51 extending through the cylinder 46 and on which there is secured a rectangular nonferromagnetic core 52. The arrangement is such that, when the shaft 51 is rotated on a central longitudinal axis thereof, the core 52 turns with the shaft on the same axis.

Unlike the previous embodiments, primary and secondary windings 53,54 are wound in planes extending parallel to planes containing a longitudinal axis of the shaft 51 rather than in planes transverse to the axis, as in the LVDT's 12 and 32. The arrangement is such that angular movement of the core 52 on a central longitudinal axis of the shaft 51 generates signals in the secondary windings 54 which are recorded by a digital voltmeter of a circuit similar to that shown in Figure 3.

Referring now to Figure 7 of the drawings, there is shown a diagrammatic perspective view of a further embodiment of a transducer 55 in accordance with the present invention.

The transducer 55 comprises an upper rectangular plate 56 of non-ferromagnetic material and a lower rectangular plate 57 also of non-ferromagnetic material and. of reduced area compared with the upper plate 56. An upper surface 58 of the upper plate 56 has mounted thereon four groups of windings 59,60,61 and 62 each comprising two elongate loops being a primary winding and a secondary winding, the seondary winding being located within the primary winding.

The groups 59 to 62 are arranged such that central longitudinal axes of two of the windings 59,60 are in line and central longitudinal axes of the other two winding 61,62 also are in line and the axis of one pair of windings 59,60 extends at right angles to the axis of the other pair of windings 61,62 and each axis extends parallel to a corresponding one of two adjacent edges of the upper plate 56.

The arrangement is such that, by comparing signals received from secondary windings of each of the windings 59 to 62, movement of the lower plate 57 relative to the upper plate 56 in either or both of two directions extending parallel to the two adjacent edges of the upper plate 56 can be determined.

From the foregoing description, it can be seen that a position transducer according to the invention has the following advantages compared with a conventional ferromagnetic position transducer: (a) Operation at temperatures above the Curie temperature at which ferromagnetic materials lose their magnetism (i.e. typically above 7000C), (b) The change in sensitivity of non-ferromagnetic position transducers with temperature can be minimised by suitable choice of the non ferromagnetic material used in its construction with regard to thermal expansion coefficient and thermal coeffecient of resistivity. This avoids the need for any measurement of the position transducer temperature and subsequent correction of the sensitivity.Also, the non ferromagnetic position transducer can be used in a situation where a temperature gradient exists with little effect on its accuracy. This applies equally to all temperatures.

(c) The choice of the non-ferromagnetic material to be used can be made with reference to its particular properties and the environment in which the position transducer is to operate.

Accordingly, position transducers according to the present invention may be used in the mechanical testing of materials at high temperatures and/or in corrosive environments.

One suitable material for general high temperature use is the non-ferromagnetic stainless steel AISI 303. For very high temperature operation, a non-ferromagnetic nickel alloy (up to about 14000C) or tungsten (up to about 20000C) could be used. If the position transducer is to be operated for long periods at high temperature, the creep resistance of the non-ferromagnetic material is also of importance.

(d) As non-ferromagnetic materials exhibit little non-linearity or hysteresis in their properties, these factors will not significantly feature in the output of the position transducer.

(e) It should be much easier theoretically to predict the change in output voltage of a non ferromagnetic position transducer with core movement. This should simplify the design process and enable the temperature sensitivity of the position transducer to be predicted.

Further, this theoretical modelling should remove the need for empirical calibration of the position transducer.

Claims

1. A position transducer comprising a first component of non-ferromagnetic material, a second component of nonferromagnetic material adjacent the first component, means for generating an alternating magnetic field adjacent a surface of the first component, separation of the components being such that field distribution relative to the first component is influenced by the presence of the second component, and detection means for detecting variation in the magnetic field at the said surface, variation being indicative of relative movement of the detection means and/or at least one of the components.

2. A position transducer according to Claim 1, wherein the detection means is in physical contact with the said surface.

3. A position transducer according to Claim 1, wherein the detection means does not physically contact the said surface.

4. A position transducer according to any preceding claim, wherein the alternating magnetic field is generated by application of an alternating electric current to the first component.

5. A position transducer as claimed in Claim 4, wherein the alternating magnetic field is generated by induction.

6. A position transducer according to any preceding claim, wherein the frequency of the alternating magnetic field lies in the range from 0.5 to 50 KHz.

7. A positon transducer according to any preceding claim, wherein the first and second components are made of nonferromagnetic stainless steel AISI 303.

8. A position transducer according to Claim 5,6 or 7, wherein the first component is a hollow cylinder of nonferromagnetic material and the second component is a cylindrical core of non-ferromagnetic material supported inside the cylinder for longitudinal movement therein, the generating means is a primary winding about the cylinder and the detection means is a secondary winding about the cylinder in two portions one either side of the primary winding.

9. A position transducer according to Claim 8, wherein the core is supported in the cylinder by a shaft extending parallel to the longitudinal axis of the cylinder.

10. A position transducer according to Claim 8 wherein the core has a central longitudinal bore provided with an internal screw thread for rotational engagement with a complementary external screw thread on the shaft and the core is additionally provided with a longitudinally extending rib for engagement in a complementary groove on the inner surface of the cylinder.

11. A position transducer according to Claim 5,6 or 7 wherein the first component is a hollow cylinder of nonferromagnetic material and the second component is a cylindrical core of non-ferromagnetic material supported inside the cylinder for rotational movement therein, the generating means is a primary winding in planes extending parallel to planes containing the axis of the cylinder and the detection means is a secondary winding extending in said planes and in two portions one either side of the primary winding.

12. A position transducer as claimed in Claim 5,6 or 7, wherein the first component is a rectangular plate of nonferromagnetic material and the second component is a rectangular plate of non-ferromagnetic material of lesser area than the first plate and wherein the first plate carries on its surface remote from the second plate four groups of windings arranged at right angles to each other and such that the central longitudinal axis of each group is parallel to an edge of the plate, each winding comprising two elongate loops being an outer primary winding and an inner secondary winding, the primary windings forming the generating means and the secondary windings forming the detection means.

13. A position transducer, substantially as hereinbefore described with reference to, and as shown in, any one of Figures 1 to 7 of the accompanying drawings.