GB2202331A

GB2202331A - Electromagnetic transducer

Info

Publication number: GB2202331A
Application number: GB08706594A
Authority: GB
Inventors: Graham Harbottle; William Ellison Jeffers
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1987-03-19
Filing date: 1987-03-19
Publication date: 1988-09-21
Anticipated expiration: 2007-03-19
Also published as: GB8706594D0; GB2202331B

Abstract

A coil 10 has superimposed wraps of turns all wound in the same sense and each containing at least one layer of turns. The difference in length "d" between adjacent wraps is equal to or less than the length of a body 12 which may be a non-ferro-magnetic "float" of a variable-area flow rate measuring apparatus for fluid passing through a table 14. The coil 10 is energised with AC and the effective inductance of the coil 10 decreases substantially linearly as the float 12 rises. If the float 12 is ferro-magnetic the inductance increases linearly as the float rises. A circuit containing the coil 10 as an inductance may include a tuned amplifier section and a detector, the tuned frequency changing with the coil inductance. Alternatively, the coil has a continously tapered form in which the number of turns per unit length or the coil diameter changes along the length; or two coils one inside the other can be used, the inductance of the one increasing as that of the other decreases. Other measurement applications are envisaged e.g. liquid level, liquid pressure using a manometer with a float, viscosity, or speed and acceleration of small projectiles. <IMAGE>

Description

ELECTROMAGNETIC TRANSDUCER The invention relates to electromagnetic transducers.

Electromagnetic inductive transducers are known in two forms. In the first, a ferro-magnetic core moves relatively to a coil and the change in self-inductance in the coil is used as a measure of the change in the length of the core overlapping the coil. In the second, a variable differential transformer has a ferro-magnetic core movable relatively to two opposed coils connected in series. Separate AC excitation of the core is provided. The change in position of the core is measured as a change in the mutual induction between the coils.

It has been proposed in German patent specification OLS 2946399 to use a coil of tapered form in a transducer of the first form mentioned above.

An electromagnetic inductive transducer, according to the invention, comprises a body of material which is electrically conductive or ferro-magnetic or which has both such properties, a coil and a source providing a cyclically varying supply to the coil, the body and the magnetic field produced by the coil being relatively movable in a range of movement in which the body within the field and the length of the body in the direction of relative movement being relatively small compared with the range, the magnetic field being non-uniform in the path of relative movement so that inductance in the circuit varies in a predetermined relationship with the relative position of the body and the field and the circuit produces an output dependent on the inductance.

The coil preferably comprises turns in a stepped or in a tapered arrangement.

In one form of the transducer the turns are in superimposed wraps of progressively decreasing length.

It is preferred in any stepped arrangement that the length of each step is equal to or less than the length of the body in the direction of the length of the coil.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic vertical section through a first embodiment showing the transducer as part of a variable-area fluid-flow indicating apparatus; Figure 2, 3 & 4 are schematic longitudinal sections through different forms of coil usable in the transducer; Figures 5 to 8 are schematic diagrams of different forms of circuit usable in the transducer; Figure 9 is a schematic longitudinal section through another form of coil arrangement using two coils one around the other; Figure 10 is a schematic diagram of another form of circuit containing the two coils shown in Figure 8.

Figure 11 is a plot of voltage against frequency to illustrate how circuit values are determined; and Figure 12 is a diagrammatic vertical section through a coil showing the number of turns and dimensions in a typical example.

Figure 1 shows the main parts of a transducer namely a coil 10 and a body 12 forming part of a variable-area flowmeter measuring the rate of flow of a fluid, such as air or other gas or liquid, for example. The flowmeter also comprises a tube 14 around which the coil 10 is arranged; upper and lower fittings 16, 18, respectively, secured to a support 20; upper and lower tubular inserts 22, 24, respectively, of synthetic plastics material which support the tube 14 between the fittings 16, 18; and a needle valve assembly 26 carried by the lower fitting 18.

The fluid enters the flowmeter, as shown by the lower arrow, via the lower fitting 18, passes the valve assembly 26, passes upwardly through the tube 14 and leaves the flowmeter via the upper fitting 16, as shown by the upper arrow.

The tube 14 is of glass and its internal and external longitudinal surfaces taper slightly towards the lower end. The body 12 is typically of aluminium alloy, for example, and has a conical lower face and an external annular array of turbine blades 28 adjacent its upper end.

As the fluid flows upwardly between the body 12 and the tube 14, the blades 28 cause the body 12 to spin about its vertical central axis.

This spin motion and the conical lower face stabilise the body 12 so that it always remains in a central position with respect to the tube 14 and does not touch the tube wall. There is a pressure difference across the body 12 which, together with drag causes by fluid motion, result in an upward force which depends directly on the rate of flow of the fluid and inversely on the clearance between the body 12 and the tube wall.

Accordingly, as the flow rate increases the body 12 rises to a position at which it remains, in which position the upward force equals the weight of the body 12.

The needle valve can be adjusted by a knob 30 to adjust the flow rate for zero-ing.

In a conventional flowmeter the position of the body 12 is read off against a vertical scale (not shown) calibrated in units of fluid flow rate.

In the embodiment shown in Figure 1 the body 12 forms the body of the transducer which produces an electrical output useful, for example, in enabling remote monitoring of the fluid flow rate.

The transducer includes the coil 10 which has an external stepped form.

In this embodiment there are nine superimposed wraps of turns in the coil. Each wrap preferably comprises a single layer of turns, but more than one layer can be used if preferred. The winding in each wrap is uniform and identical with the winding in the other wraps so far as turns per unit length of the coil are concerned. However, the wraps are of progressively shorter lengths from the tube 14 outwards.

The difference in length "d" of the wraps is dictated by the length of the body 12 in the direction of the length of the tube 14, as indicated in Figure 1. The difference in length "d" of the wraps is preferably equal to or less than the length of the body 12.

The coil 10 shown in Figure 1 was wound on a former preferably conforming closely to the outer surface of the tube 14. The innermost wrap was formed in one layer by starting winding at one end of the former corresponding to the upper end of the coil shown in Figure 1. The wrap was continued to its end, corresponding to the lower end of the coil.

Then, the wire was wound in the same sense in no more than one turn back through a length "d" towards the other end of the coil. Then, close winding as before in the same sense was continued to form the second wrap in one layer only superimposed on the first wrap.

The second wrap was completed at the start end of the coil corresponding to the upper end in Figure 1. The third wrap was formed in a single layer superimposed on the second wrap by continuing close winding towards the end of the coil remote from the start end.

At the end of the third wrap the wire was wound in the same sense in less than one turn through a length "d" to the start of the fourth wrap, which was completed by close winding in the same sense as for the second wrap.

The fifth to ninth wraps were completed in similar manner to the procedure just described, winding continuing throughout in the same sense.

The linearity of the characteristic of the coil can be improved by providing in the lowermost and upper steps of the coil a number of turns different from the number provided by winding the turns as described above. For example, in the last three steps (i.e. the exposed parts of the inner wraps at the lower end and in the outermost wrap and two exposed parts of the next succeeding wraps at the upper end) the number of turns per unit length may be increased compared with the turns per unit length elsewhere in the wrap. This increase may involve a second layer of turns in the exposed part of the wrap. This variation can be used to compensate for 'end effects' at the coil ends.

Figure 2 shows part of a coil 40 which, for example, could be part of the lower zone of the coil 10 shown in Figure 1. Suppose a small body 44 of ferro-magnetic material of length "1" (less than the difference in length "d" of the wraps of the coil) is moved in the direction of the arrow into the coil. The effective inductance of the coil, energised from an AC source, varies as shown at 42 along the length of the coil, because the magnetic field produced by the coil is non-uniform along the path of the body 44.

The characteristic 42 has pronounced steps each corresponding to an end of a wrap of the coil 40. For transducer purposes, the steps must not be separated by too great a distance i.e. the length "d" must not be excessive. It is preferred that the length "d" is equal to or less than the length "l" of the body, so that the steps are reduced or eliminated and the inductance varies in a substantially linear relationship with the position of the body. An example of such a preferred arrangement is indicated with reference to the body 45 shown in ghost outline. Using p such a body, made of ferro-magnetic material, the inductance characteristic of the coil 40 is changed to that indicated at 46. The length "d" is less than the length "l" and the effect is to smooth out the steps.The inductance is shown somewhat increased throughout because the body 45 is larger than the body 44.

The body 45 is preferably of non-ferro-magnetic material, such as aluminium alloy, for example. In that case the inductance decreases as the body 45 moves towards the left in Figure 2 because the increasing number of effective turns in the coil causes increasing generation of eddy currents in the body 45 causing corresponding reduction in inductance. The corresponding characteristic is shown at 47.

Figure 3 shows a modified coil 48 in which the difference in length of successive wraps is disappearingly small. In this case the inductance characteristic of the coil 48 with a body 50 of ferro-magnetic material is as shown at 52. There are no steps in the characteristic. The difference in length "d" between successive wraps cannot be shown but is very much less than the length "1" of the body 50.

When the body 50 is of non-ferro-magnetic, conductive material, such as aluminium alloy for example, eddy-currents are generated in the body 50 and losses are greatly increased. The inductance of the coil 48 (or of a coil such as 40 or 10) is considerably reduced as shown by the characteristic 54 in Figure 3.

A coil of the kind shown in Figure 3 can be regarded as of stepped form p similar to the coil of the kind shown in Figures 1 and 2 but having very small steps. For example, the eoil may have wraps, each comprising a single layer of turns, with each wrap being only one or only one or two turns shorter than the wrap below.

Figure 4 shows yet another form of coil which can be used in transducers according to the invention. The coil 56 was wound on a former 57 having an internal bore to suit the flowmeter tube 14, for example, and having a tapered external surface 58 to receive the coil 56. The coil 56 consists of one or more layers of turns all in the same sense and preferably wound at a uniform number of turns per unit length of the coil from one end of the former to the other. In this case the coil is preferably not of stepped form but is of uniform thickness of one or more layers. The coil is thus preferably non-uniform along its length only so far as uniformly changing diameter of the turns is concerned.

The characteristic of the coil 56 is similar to that described in relation to the coil 48 shown in Figure 3.

As an alternative form of stepped coil construction for example, the coil can be wound on a former of stepped shape, the coil being preferably wound with one layer of turns all in the same sense from one end of the former to the other, the turns on each step per unit length being constant; or more than one such layer can be wound; or the number of turns can be varied along the length of the former. With such a stepped construction the length of each step is preferably equal to or less the length of body, such as the body 12 for example.

Figure 5 shows one example of electric circuit of the transducer. The circuit includes the coil 10 or equally the coil 40 (Figure 2) or the coil 48 (Figure 3) or other coil variant, the coil being represented by a variable inductance 60. The coil is energised from a source of varying output such as an oscillator 62 providing an AC output via a resistor 64 and a fixed value capacitor 66 forms with the inductance 60 a tuned circuit in order to amplify the voltage developed across the coil.

The voltage passes to a detector 68.

The value of the inductance 60 corresponding to zero fluid flow is backed off by means of a potentiometer and an operational amplifier 72. The final voltage output Eo represents the fluid flow rate.

The output from the coil 10 (or the coils 40, 48 or other variant) has a substantially linear relationship with the fluid flow rate, owing to the linear inductance characteristic discussed above. Accordingly, the final output Eo also has a linear relationship with the flow rate.

The change in inductance as the body 12 moves is relatively small compared with the basic value. The circuit shown in Figure 6 using a bridge or the circuit of Figure 7 using an operational amplifier to linearise the bridge characteristic can be used to reduce or eliminate drift which might occur to an extent which cannot be tolerated in some applications.

The circuit shown in Figure 8 using a colpitts oscillator 74 is a further variation which provides a frequency output fo appropriate to digital instrumentation.

The substantially linear characteristic of the coil can be further p improved where desired using the two coils shown in Figure 9 which, for example, can be energised using the circuit shown in Figure 10.

The inner coil 80 shown in Figure 9 is made as described in relation to Figure 3 i.e. is similar to the coil 48. Alternatively, the inner coil 80 may be similar to the coil 10 or the coil 40. The outer coil 82 shown in Figure 9 is wound on the inner coil or is fitted onto it having been wound separately on a suitable former. The outer coil 82 is complementary to the inner coil so far as turns diameter is concerned, or lengths of wraps in the case of a stepped coil.

The coils 80, 82 are connected in a bridge 84 as shown in Figure 10, the coils being represented as variable inductances 86, 88, respectively.

The inductance in one coil increases as the inductance in the other decreases. The characteristic of the bridge 84 is linearised using the two coils 80, 82 and its output is greater than it would have been using only one coil.

The body 83 which moves relatively to the coil arrangement is of either ferro-magnetic or non-ferro-magnetic material.

Although the transducer has been described above by way of example as applied to flowmeter apparatus, the invention is not limited to transducers suitable for such an application. The transducer according to the invention can be applied quite generally to monitoring, indicating and measuring, for example, whether as part of variable-area p flowmeter apparatus or otherwise. Some examples of other applications are: liquid level measurement using a small float; pressure measurement at a manometer using a small float; position, speed and acceleration of small projectiles e.g. bullets; viscosity measurement using a ball and a spring ball forming the body movable relatively to the coil.

As already mentioned above, the body corresponding to the body 12, 45, 50 or 83, for example, is movable either inside or outside the coil. For some applications the coil is curved instead of being straight, the body being mounted guided or otherwise constrained to move along a correspondingly curved path.

The power levels used to energise the coil must not be so high as to cause excessive heating of the body by eddy currents generated in the body. Where the body is of ferro-magnetic material it is preferred that the body be laminated, as indicated, for example, at the body 44 (Figure 2), particularly where the AC source is of audio frequency. For higher frequencies, a ferro-magnetic body preferably comprises ferrite material.

The following example describes the conversion of a conventional variable-area flowmeter to give an electrical output. Figures 11 and 12 should be referred to here. The steps taken are as follows:1. Measure the external dimension of the glass metering tube (i.e. tube 14 of Figure 1); 2. Select a coil former of the same length as the glass metering tube p with internal dimensions such that the former is a clearance fit over the metering tube. Typical wall thickness of the former would be in the order of 2mm. One end cheek may be required to be fitted to the large end of the coil depending on the number of steps employed in the construction, as shown in Figure 12; 3. Select the size of wire, for this example varnished copper wire, 0.6mm diameter, in order to give some amount of mechanical strength; 4.Measure the length of the metering float excluding the tapered bottom end cone; 5. Determine the size of step required for the coil windings i.e. the gradient in field strength. This is dictated by the length of the float and should preferably not be greater than the float length.

Ideal step size would be in the order of lmm less than the float's effective length. One other consideration is that the scale of the metering tube or indeed the metering tube may not be linear, so the field gradient may require adjustment at that position by varying the number of turns, i.e. by reducing the step size the gradient would increase. The first and last step may need to be decreased to half-steps to overcome end-effects. With very short coils the metal tube retaining inserts may have to be replaced with non-metallic inserts if a problem is incurred owing to end-effects: wind the coil as described previously and as shown in Figure 12; 6. Measure the inductance of the coil, this can also be calculated as p the coil will approximate to a number of seperate coils at different diameters; 7.Select the oscillator frequency to be of the order of 100 KHz, a square wave is preferable. If the frequency is increased, so the eddy current effect will increase and produce a greater change. The change must be kept within a usable range i.e. for the electronics that are employed. Eddy current loss is proportioned to: (FREQUENCY x FLUX)2 Depending upon float material, saturation due to eddy currents may occur at higher frequencies; 8. Calculate a suitable value of capacitance: for this example a series LC circuit was used, this is not to say that a parallel circuit could not be used. The value of capacitance is given by: w (4n2fo2L)-l 9. Plot the resonant tuned circuit characteristic i.e. capacitor or inductor voltage versus frequency; 10.Select the usable range, both from the limitations of the electronics and the linearity of that part of the characteristic as shown in Figure 12 the range can be on either side of fo; 11. Determine suitable values of fu' and fu" from the characteristic p and recalculate the capacitance value.

12. Use a suitable diode detector and low pass filter to detect the change in voltage that is developed across the inductance and remove the ripple, so giving a DC voltage that is a function of float position in the metering tube. Typically, for example, the coil inductance is 3.809 milli-henries and the capacitance in the series LC circuit is 570 pico-farads. Typically, for example, fo is 100 KHz in Figure 11 and fu', fu" are 80 KHz and 120 KHz, respectively.

Although the body, such as 12 for example, has been described as movable, it is possible in modifications (not shown) for the magnetic field to move instead of, or in addition to, the movement of the body. In general, the body and the field are relatively movable, the body being within the field.

In a modified embodiment (not shown) at least one member of ferromagnetic material such as soft iron, for example, is positioned in relation to the coil so that the magnetic field traverses the member. The member or members conduct the field to the path of relative movement of the body.

The member or each of at least some of the members is structured so that the field is non-uniform in the path of the body. For example each such member is tapered or of stepped form.

The effect of the non-uniformity of the field is similar to that in the embodiments shown in the drawings or gives some other predetermined relationship between the variation in inductance in the electric circuit due to the coil and the relative position of the body in the magnetic field.

Although it is preferred that the non-uniformity of the magnetic field (whether produced by a stepped, tapered or other coil structure or by structured ferromagnetic members) is such as to produce a linear relationship between the inductance in the circuit and the position of the body, a different relationship may be required for some purposes and can be provided for by suitable choice of structure for the coil or the members, or both.

In a conventional variable-area flowmeter, the change in the position of the float with flow is commonly non-linear. The invention can be used with such meters to produce an electrical output which has an accurately linear relationship with flow. This can be done quite readily by suitable choice of turns in the wraps or in parts of the wraps of the stepped coil, for example.

It is preferred that the inductance of the coil with the body present shall not be high enough to cause radio interference. The frequency is high enough to generate eddy currents especially in the case of a body of non-ferromagnetic material.

The embodiment shown in Figure 1 can readily be made to give a change of p around 40 Hz per millimetre of change in the position of the body, the frequency observed being that at which the tuned circuit resonates in each case.

Claims

p 1. An electromagnetic inductive transducer comprising a body of material which is electrically conductive or ferro-magnetic or which has both such properties, a coil and an electric circuit which comprises the coil and a source providing a cyclically varying supply to the coil, the body and the magnetic field produced by the coil being relatively movable in a range of movement in which the body is within the field and the length of the body in the direction of relative movement being relatively small compared with the range, the magnetic field being non-uniform in the path of relative movement so that inductance in the circuit varies in a predetermined relationship with the relative position of the body and the field and the circuit produces an output dependent on the inductance.
2. A transducer according to claim 1, said predetermined relationship being a linear relationship.
3. A transducer according to claim 2, the coil comprising turns in a stepped or in a tapered arrangement.
4. A transducer according to claim 3, the turns being in superimposed wraps of progressively decreasing length.
5. A transducer according to claim 3 or claim 4, the length of each p step of the arrangement being equal to or less than said length of the body.
6. A transducer according to any preceding claim, said coil being the first of two coils, the second coil of which is arranged about the first, said circuit containing both coils.
7. A transducer according to claim 6, said second coil being arranged oppositely to the first coil so that as the inductance in one coil increases it decreases in the other.
8. A transducer according to claim 7, said circuit comprising a bridge arrangement two limbs of which are formed by said first and second coils arranged so that the output of the bridge arrangement is greater than it would be using only one coil and the output is also linearised.
9. A transducer according to any preceding claim arranged to give an output dependent on the rate of flow of fluid through a tube having an internal tapered form in which tube said body is movable in dependence on fluid flow through the clearance of variable area between the body and the tube.
10. A transducer according to claim 9, said coil being arranged around said tube.
11. A transducer according to claim 9, said coil being arranged p alongside and parallel to said tube to allow the position of the body in the tube to be observed.
12. A transducer according to claim 1 or claim 2, at least one member of ferromagnetic material being arranged in relation to the coil so that the magnetic field traverses the member which is structured so as to make or help to make the field non-uniform.
13. A transducer according to claim 12 arranged to give an output dependent on the rate of flow of fluid through a tube having an internal tapered form in which tube said body is movable in dependence on fluid flow through the clearance of variable area between the body and the tube, said member or each said member being arranged alongside the tube to allow the position of the body in the tube to be observed.
14. A transducer according to claim 1 substantially as herein described having a coil substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.
15. A transducer according to claim 1 substantially as herein described having a coil substantially as herein described with reference to Figure 3 of the accompanying drawings.
16. A transducer according to claim 1 substantially as herein described having a coil substantially as herein described with reference to Figure 4 of the accompanying drawings.
17. A transducer according to claim 1 substantially as herein p described having coils substantially as herein described with reference to Figure 9 ofVthe accompanying drawings.
18. A transducer according to claim 1 arranged as part of a rotameter apparatus substantially as herein described with reference to Figure 1 of the accompanying drawings.
19. A transducer according to claim 18 having a coil made substantially as herein described with reference to Figure 12 of the accompanying drawings.
20. A transducer according to claim 18 but having instead of the coil shown in Figure 1 a coil substantially as herein described with reference to Figure 2; or with reference to Figure 3; or with reference to Figure 4; or with reference to Figure 9 of the accompanying drawings.