GB2049939A - Inductive displacement transducers - Google Patents

Abstract

Apparatus for the measurement or detecting of the position or change of position within a predetermined range of displacement of a constructional element (3) of non-magnetisable material which contains a ferromagnetic core (4), includes an exciting coil (5) and a detecting coil (7), arranged so that the coupling factor between the coils changes on displacement of the element (3). In order to increase the accuracy of the indication and to influence the characteristic coupling (transfer) curve, the coils (5, 7) are divided into sections. The series-connected sections of the exciting coil (5) rise in turns number in a mathematically monotonous series, as do the equally series-connected sections of the detecting coil (7) and the sections of the two coils are mutually alternatingly disposed so that by a change of position of the ferromagnetic core (4) not only the coupling factor but also the turns ratio is continuously changed. These coils are surrounded in addition by a similarly sectionally divided, short- circuited correction coil (6) dimensioned in accordance with the Biot-Savart law, the magnetic field of which influences the induction of the detecting coil and thus also the transfer curve. <IMAGE>

Description

SPECIFICATION Apparatus for accurately detecting and remotely indicating the position and/or change in position of axially displaceable constructional elements This invention concerns high accuracy apparatus for the electrical detection, particularly for the electrical indication and remote signalling, of the instantaneous position and/or a change in this instantaneous position of axially displaceable constructional elements or machine parts, which position or change of position depend on the instantaneous position of the machine or which depend on the instantaneous value of a physical parameter being measured in a measuring instrument.

In technological devices or machines there is frequently a need to indicate the position or a change in position within a predetermined displacement range of some displaceable constructional element or machine part, and in given cases to signal such position or change of position to a remote location and to register the direction and magnitude of the changes. In addition, many indicating and/or measuring instruments are known wherein the detected and/or measured physical parameter has an instantaneous value corresponding to the actual position of some component. Such a detecting or indicating instrument may for instance be a floattype liquid level detector, a rotameter (flowmeter) used for measuring the amount of flowing liquid, a diaphragm type pressure detector, etc.

These remote indication tasks mostly may be traced back to the position detecting of a constructional element which may, within a predetermined displacement range be displaced along an axis or straight line, and this task is generally solved electrically by measurement methods which are based on detecting a change in inductive coupling.

In such devices, the displaceable constructional element is generally made of a non-magnetisable material in which a ferromagnetic core is placed.

Along the range of displacement two or three coils are employed of which one, the exciting coil, is supplied with alternating current. The magnetic field of the exciting coil induces a voltage in the other coil or coils i.e. in the so-called detecting coil or coils, the magnitude of which is a function of the mutual inductance or inductive coupling coefficient which in turn changes with a change in the position of the ferromagnetic core. In solutions involving only two coils, the magnitude of the voltage induced in the detecting coil is measured whilst in the three coil solutions, the middle coil is excited while the two end detecting coils are connected in a bridge circuit or to some other phase-sensitive rectifier of known construction.

One of the most important disadvantages of such two-coil or three-coil position sensors is that the functional relationship between the displacement of the displaceable constructional element and the induced electric voltage or change of voltage is not linear.

A further complication of the above described situation is in the case of measuring instruments where even the original relation between the change in the characteristic physical parameter being measured and/or detected and the displacement of the displaceable constructional element is non-linear from the outset. In such a case, the electric voltage used for detecting or remote signalling and the actual detected physical parameter are related by an indirect function: y = f[g(x)] where both the functions y = f(u) and u = g(x) are respectively non-linear and thus the characteristic curve of the whole detecting and/or indicating and remote signalling apparatus can only be established by experiment.

In addition, this characteristic curve cannot be influenced or only slighly so, yet by virtue of its curvature, it decreases the accuracy of the detection and/or measurement and remote signalling.

The present invention seeks to overcome these disadvantages.

The present invention describes apparatus which, analogously to known constrictions, contains a displaceable constructional element maze of non-magnetisable material and containing a ferromagnetic core but which, in contrast however, is surrounded in its range of displacement by a coil system which ensures that a relatively small change in position of the constructional element corresponds to a relatively large change in the output voltage which is surrounded by a further coil system which enables the characteristic curve of transfer to be influenced.

In essence, the coil system employed contains two coils of equal length divided into coil sections of unequal numbers of turns in such a way that each section of the exciting coil is followed by a section of the detecting coil. The numbers of turns for both the exciting coil and the detecting coil section is established in such a way that the number of turns should form a mathematically definable monotonously increasing series.

However, the individual sections of the two coils are so disposed that the direction of increase of the number of turns of the individual sections of the exciting coil shuld be exactly opposite to the direction of increase of the number of turns of the individual sections of the detecting coil. This coil system is then surrounded by a third coil individual sections of which correspond to individual sections of both the exciting and the detecting coil. The different sections of the individual coils are connected together in series and the coil sections of the external third coil, the so-called correction coil, is short-circuited after this series connection.

The determination of the number of turns of the individual sections of the short-circuited correction coil is done on the basis of the per se known Biot-Savart law.

Two preferred embodiments of the invention shown in the drawings, wherein Figure la is a partially broken away cross-section, partly in elevation, of a device for detecting horizontal displacement, Figure 1 b is an end view of The, device of Figure 1 a, and Figure 2 is a device in longitudinal section for use as a remote indication of the output of a rota meter.

Two possible modes of construction of the apparatus according to the invention are described by way of non-limiting examples.

EXAMPLE 1 The apparatus shown in Figures 1 a and 1 b is constructed to detect horizontal displacement and has a connecting element 2 for transmitting the displacement to a constructional element 3 which can move along a path 1 and contains a ferromagnetic core 4, the constructional element 3 itself being made of a non-magnetisable material. The path 1 is surrounded by a coil system consisting of alternatingly disposed coil sections of an exciting coil 5 and a detecting coil 7. In this coil system, the coils consist of n sections which follow each other in such a way that the first section of the exciting coil 5 is followed by the nth section of the detecting coil 7, the second section of the exciting coil 5 is followed by the (n-1 )th section of the detecting coil 7 and so on.By way of example, the number of turns of the sections of the exciting coil 5 are according to the geometrical series a, = 10 and q = 1.5, thus 10; 15; 22.5; 33.7; 50, while the number of turns of the sections of the detecting coil are arranged according to the geometrical series in which a = 20 and q = 1.2 (but led in the opposite direction), that is the number of turns are 20; 24; 28.8; 34.5; 41.5; 50 etc.

The individual sections of the exciting coil 5 are connected with each other in series and am led out of the coil system by way of a terminal 8 and another non-illustrated terminal. The sections of the detecting coil 7 are also connected in series and similarly are taken out of the coil system via a terminal 9 and another non-illustrated terminal.

This coil system is surrounded by a compensating coil 6 which is also divided into sections corresponding to the coil sections 5/1; 7/n; 5/2; 7/n-1 etc. its sections are connected in series and are short-circuited via a terminal 10 and another non-illustrated contact terminal.

Assuming a five-section coil, i.e. n = 5, it can be seen that when the constructional element 3 containing a ferromagnetic core 4 is in its lower end position (left-hand end as seen in Figure 1 a), provides a coupling via its ferromagnetic core 4 between two coil sections, where the turns ratio between the exciting coil 5 and the detecting coil 7 is 10:41.5 while in its upper end position which in Figure 1a of the drawings is on the right, this ratio becomes 50:20. Thus the coupling ratio has changed by a factor of 10. This ratio can be increased by increasing the number of coil sections and by increasing the steepness (q) of the geometrical series.

By connecting the exciting coil 5 and the detecting coil 7 to some per se known phase sensitive rectifier or some other instrument such as is disclosed in Hungarian patent specification No. 164,298, the output voltage becomes poroportional to the distance from its end position of the constructional element 3 containing the ferromagnetic core 4, this end position being its bottom end position shown at the left of Figure 1 a.

The shape of-the characteristic coupling (transfer) curve may be widely varied by suitably selecting the number of turns of the sections of the exciting coil and of the detecting coil, by suitably selecting the, quotient of the geometrical series according to the Example and by suitably selecting the number of turns for the sections of the compensating coil 6, and in this way a linear relationship may with acceptable accuracy be approximated.

This is because under the effect of the lines of force terminating in the compensating coil 6, current is induced in the latter and the coil 6 also forms an opposing magnetic field and in this way the coil 6 also influences the magnitude of the induced voltage in the detecting coil 7 and thus may significantly affect the shape of the transfer curve.

EXAMPLE 2 The apparatus shown in Figure 2 is for the remote signalling of the value per unit time of the quantity of liquid passed by a rotameter (rotary flowmeter). In the rotameter tube 1' a float 3' is disposed. In the rotameter tube 1' there is a flow 3' made of a non-magnetisable material containing a ferromagnetic core 4'. The configuration and arrangement of the coils corresponds to that described above in Example 1 with a difference that here both terminals of each terminal pair 8, 9 and 10 have been shown. When no liquid is flowing in the rotameter in the direction indicated by the arrow 2', the float 3' will occupy its lowermost position and magnetically couples the coil sections 5/1 and 7/n.When fluid flow starts through the rotameter, the float 3' rises and, by virtue of inclined channels formed on its surface but not shown in the drawing, is set into rotation under the effect of the flowing medium. The extent of its rise is dependent on the conicity of the tube and is related by known function: h=(O) to the quantity of medium flowing through per unit time. Depending on the extent of the rise h, i.e. on the amount of liquid flowing through, the float 3' with the ferromagnetic core 4' disposed therein, couples the coil sections 5/2 and 7/n, then 5/2 and 7/n-1 , then 5/3 and 7/n-1 ,then 5/3 and 7/n-2 etc. which (as described above in Example 1), causes a rise in the coupling coefficient. Thus the output voltage of the phase sensitive rectifier connected to the terminals 8 and 9 becomes functionally related to the quantity of flow flowing through. The function: U=f(Q) will have a curve the shape of which can be widely influence or corrected practically to linearity on the one hand by suitably calculating and choosing the coil sections of the exciting coil 5 and the detecting coil 7 and on the other hand by suitably selecting the turns number of the sections of the compensating coil 6.

When the output signal of the phase-sensitive rectifier is not connected to an instrument, then the apparatus may be complemented by a registering or recording instrument which may be a digital counter or writer.

By using an integrating member in the transfer chain following the phase sensitive rectifier, the apparatus becomes suitable, e.g. in the case of a rotameter for the counting or measuring of the total liquid flowing through it or for the measuring and/or registering of the integral value of any other changing parameter during the whole time duration while by using a differentiating member it becomes usable for measuring and/or indicating changes.

In addition, the apparatus is suitable for the indicating and/or measuring of changes in the dimension due to thermal expansion, for the remote signalling of changes of shape under load and for other similar tasks where the essential problem is to detect displacement along an axis or straight line. The accuracy of the detecting is in practice settable to the desired value by suitable dimensioning.

Claims (5)

1. Apparatus for accurately detecting and remotely indicating signalling the position and/or change in position of constructional elements capable of displacement along an axis or straight line, which comprises a constructional element displaceable along a given path within a given range of displacement and which is made of a non-magnetisable material containing a ferromagnetic core, an exciting coil and a detecting coil surrounding the given path in the range of displacement, the said coils being divided into sections, and a similarly sectionally divided compensating coil surrounding the exciting and detecting coils, wherein the number of turns of the individual sections of the detecting coil vary according to a mathematically definable monotonous series and these coil sections are disposed between the coil sections of the exciting coil the number of turns of which latter also vary according to a mathematically definable monotonous series but in the opposite direction; while the sections of the compensating coil disposed around the coil sections of the exciting coil and the detecting coil in a like configuration.

2. Apparatus according to claim 1, wherein the sections of the exciting coil and the sections of the detecting coil are respectively connected in series while the sections of the compensating coil are connected in series and short-circuited.

3. Apparatus according to claim 1 or 2, wherein an alternating current supply is connected to the series-connected exciting coil while a voltage measuring instrument and/or a phase sensitive rectifier is connected to the detecting coil.

4. Apparatus according to any preceding claim, wherein the sections of the compensating coil are dimensioned according to the Biot-Savart law.

5. Apparatus according to claim 1 substantially as herein described with reference to and as shownin Figures 1 a and 1 b or Figure 2 of the accompanying drawings.