GB2080632A

GB2080632A - Differential transformers

Info

Publication number: GB2080632A
Application number: GB8121382A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-07-10
Filing date: 1981-07-10
Publication date: 1982-02-03
Also published as: GB2080632B; JPS5719613A; JPS6259762B2

Abstract

A differential transformer suitable for determining small displacements and hence for use in a vibrometer comprises a bobbin 11 having a hollow bore 21,there being a primary coil 12 wound in one or two layers for the entire length of the bobbin, from one end cheek to the other. A spacer 20 is fitted centrally over the primary coil, and between one end cheek and the spacer 20 is wound a secondary coil 14, whereas between the other end cheek and the spacer 20 is wound a second secondary coil 16. The two coils 14 and 16 are connected differentially, and a ferromagnetic core 18 is mounted on an operating rod 19 within the bore 21. The primary coil 12 is adapted to be energised at a relatively high frequency typically between 50kHz and 2MHz, and the output from the differentially- connected coils 14 and 16 is then relatively high but linear for small displacements of the core 18 from its central position. Modifications are disclosed in which the movable member is a non-magnetic conductive body, e.g Al, Ag, Cu or alloys thereof, disposed within, outside, or between the coils, and this may slide on a magnetic member. (Figures 9-23 not shown). <IMAGE>

Description

SPECIFICATION Differential transformers This invention relates to improvements in differential transformers, often employed for example as displacement detectors. The invention also relates to a displacement type vibrometer using a differential transformer arrangement of the invention.

A conventional design of differential transformer which has been in general use comprises a cylindrical bobbin having a primary coil wound round the middle portion thereof and two secondary coils wound one on each side respectively of the primary coil so that the secondary coils are separated by the primary coil.

The primary coil is arranged to be excited with an alternating current of several volts at a low frequency not exceeding 10 kHz, and the coil is formed from fine wire which is usually not more than 0.3 mm diameter. The current passing through the primary coil must be held below a safe level, which may be for example 10 mA to 20 mA for a 0.1 mm diameter wire. Accordingly, the primary coil requires a large number of turns, i.e.

several thousand turns even for a differential transformer adapted for measuring a displacement of + 10 mm and a high direct current resistance in order to provide an impedance of at least about 100 ohms, as derived from the high direct current resistance and inductive resistance, i.e. the reactance at the operating frequency.

Also, to obtain a large output voltage, it is necessary to have large voltages induced in the secondary coils.

This has lead to the requirement that for many cases each secondary coil has 2 to 5 times as many turns as the primary coil. Accordingly, the number of turns in each secondary coil has been very large and this has resulted in the differential transformer having a large size, requiring considerable quantities of materials and much assembly time to manufacture. There have however been recent developments in the manufacture of small differential transformers intended to be operated at a frequency not higher than 20 kHz, with casings and iron cores measuring about 5 mm and 1.6 mm respectively. These transformers are nevertheless suitable only for special purposes.

It is a principal object of the present invention to provide a differential transformer which at least reduces the above-stated drawbacks of the known conventional differential transformers, and which is intended to be energised at a frequency in the range of from 50 kHz to 2 MHz, the primary coil consequently having relatively few turns.

Accordingly, one aspect of this invention provides a differential transformer comprising a primary coil wound in at least one layer around a bobbin so as to extend over the entire length of the bobbin, and two secondary coils each wound directly over the primary coil with a prescribed number of turns, one secondary coil being wound from the central portion of the primary coil to one end thereof, and the other secondary coil being wound from the central portion of the primary coil to the other end thereof, and the primary coil being adapted to be energised at a frequency between 50 kHz and 2 MHz.

To be more specific, the inductive reactance of a coil generally is proportional to the frequency of the energising current. Therefore, energising a primary coil consisting of a relatively small number of turns wound in one or two layers over the entire length of a bobbin with a current at a much higher frequency than is used with conventional differential transformers raises the impedance of the primary coil, generally up to a value of at least 100 ohms. An adequate voltage applied to the primary coil at such a high frequency to ensure the safe current limit is not surpassed is able to induce a relatively high voltage in each of the secondary coils wound directly over the primary coil.

Results of experiments have shown that the number of the secondary coil turns need not be greater than 0.5 to 5 times and preferably 0.5 to 2 times, that of the primary coil. As is well known, the voltage which is induced by the primary coil in the secondary coil depends on the turns-ratio between the two coils. When the number of turns in the primary coil is reduced, therefore, an adequately high voltage can still be obtained in each secondary coil even if the number of turns in each secondary coil is also reduced. In view of this, this invention allows the manufacture of a small differential transformer, by decreasing the number of turns of the primary coil, thereby allowing the number of turns in the secondary coils also to be reduced, and hence the size of the entire transformer.

Generally, increasing the length of the bobbin of a differential transformer increases the measuring range or the range (generally called the stroke) within which the output voltage is appropriately proportional to the displacement of the iron core. However, an increase in the length of the bobbin results in an increase in the number of turns of the primary coil wound in one or two layers over the entire length of the bobbin and also increases both the d.c. resistance and the inductive reactance. As a result, the desired energising current becomes hardly obtainable unless a high voltage is applied. To keep the value of the impedance at much the same value for both long and short bobbins, the inductive reactance must be reduced with long bobbins by lowering the energising frequency, depending upon the length of the bobbin and the number of primary coil turns.

In other words, it is suitable to use a lower frequency for a differential transformer having a longer stroke.

Conversely, the use of a higher frequency is suitable for a differential transformer having a short stroke or when the coils have an especially small diameter.

According to the results of experiments, the use of a frequency of 100 kHz or thereabouts is suitable for a differential transformer prepared in accordance with the present invention, for a measuring range of +20 to t30 mm, and comprising a bobbin of 5 mm outside diameter, a primary coil wound in two layers over a winding width of 8 cm and an iron core of 4 to 6 cm in length. In this instance, the d.c. resistance of the primary coil is only 30 to 35 ohms, which is less than that for a conventional differential transformer.

However, the inductive reactance of the primary coil may be as high as 500 ohms. Furthermore, in the case of a differential transformer having a short measuring range, the use of a higher frequency will readily attain an impedance exceeding 100 ohms, and energising the primary coil with a voltage of several volts will not result in the current exceeding the safe level.

Generally, the inductance increases or decreases in proportion to the square of a coil diameter and the number of turns. Hence, the inductance can be lowered by using a bobbin of small diameter. To maintain the inductive reactance at an adequate value with a bobbin of small diameter, therefore, a high energising frequency is used. For example, where the bobbin is 4 mm in diameter and the stroke is +10 to t20 mm, a suitable frequency is between 300 and 500 kHz or thereabout. When the number of turns is fewer, i.e. the stroke is short (e.g. t2 mm), but with a bobbin of the same diameter (4 mm), a suitable energising frequency is about 800 kHz.

In cases where a bobbin of a very small diameter, say 2.2 mm, is used, a suitable energising frequency is between 800 and 1000 kHz, even if the stroke is t5 mm.

Results of various experiments which have been conducted for finding the practical limits concerning the number of coil turns, the coil diameter and the measuring range indicate that applying an alternating current of a high frequency exceeding 2 MHz to a primary coil causes a skin current (due to the capacity of adjacent wires) and this rapidly reduces the useful current for energising the coil. This makes it difficult to have a high voltage induced in each secondary coil. In view of this, the differential transformers according to the present invention should not be operated at a frequency much exceeding 2 MHz.

As for the lower limit of the usable frequencies, the limit found from the use of commercially available iron cores of conventional differential transformers may be considered appropriate. The conventional commercially-available iron cores range from 5 to 8 mm in diameter and have the advantage of mechanical strength. The use of these iron cores in differential transformers of this invention gives a distinct advantage of having a smaller outside diameter as compared to the conventional types while retaining their excellent mechanical strength. When the core diameter is between 5 and 8 mm, the outside diameter of the bobbin is about 10 mm.In the case of a bobbin of 9 mm diameter with a measuring range of +25 mm, for example, the frequency to be used is between 50 and 60 kHz, which is more than 5 times higher than the frequency used with a conventional differential transformer with the same core, while the outside diameter of the transformer is about one half that of the conventional one. Therefore, the lower limit of the frequency is set at 50 kHz.

The outside diameter of the differential transformer according to this invention can be made very small because of the small number of turns of the primary and secondary coils. Meanwhile, the recent advances in electronics has enabled the manufacture of small, stable amplifiers at low cost. This has obvied the need to increase the output voltage of a measuring device, and consequently, the energising voltage for the primary coil and the inductance thereof may be lowered. This, in turn, permits a further reduction in the outside diameter and the number of turns in the coils.

The differential transformer of this invention may be used in the production of a displacement type of vibrometer. Accordingly, a second aspect of this invention provides a vibrometer comprising a differential transformer of this invention, wherein the bobbin is formed as a tubular body in which a vibrating body is supported by spring means so as to be movable within the tubular body along the axis thereof, there being a fluid within the tubular body to attenuate the vibrations of the vibrating body and the primary and secondary coils being wound around the outer circumference of the tubular body.

Generally, a vibrometer of the displacement type and including a differential transformer is used to determine the vibrating conditions of a body of a certain given natural oscillation frequency, or the mass of a vibrating body supported by a spring arrangement within a fluid providing a suitable degree of attenuation, by measuring the displacement of the body when it vibrates.

The displacement is measured in terms of a d.c. output voltage from the differential transformer used in the vibrometer. However, in the case of a rapidly moving vibrating body, the vibrometer becomes incapable of measuring the displacement unless the response frequency of the output voltage is sufficiently high. With conventional vibrometers, the differential transformer has a primary coil energised at a frequency not exceeding 10 kHz, and because the response frequency of the output voltage is about one tenth of the energising frequency, it is not more than 1 kHz. With such a differential transformer, it has been difficult accurately to measure very rapid vibrations resulting from sudden accelerations, sudden decelerations, collisions and the like.

The vibrometer of the second aspect of this invention is based on the concept that the use of a differential transformer prepared according to the invention as described in the foregoing enables the primary coil to be energised with an alternating current at a high frequency; then, the response frequency of the d.c. output voltage of the secondary coil naturally can be increased in proportion to the energising frequency, so that a rapid vibrating state can accurately be measured.

In the vibrometer of this invention, a vibrating body is supported within a tubular body for movement in the axial direction thereof, and the primary coil is wound round the outer circumference of t:e tubular body in one or two layers, while the two secondary coils are directly wound over the primary coil with the required number of turns. In other words, the secondary coils are disposed close to the primary coils as to have a: voltage readily induced therein. Meanwhile, the primary coil is prevented from having an excessive impedance by winding it in one or two layers, so that an alternating current of a high frequency can be used to energise the primary coil.Accordingly, the response frequency of the d.c. output voltage also can be made rapid, for accurate measurement of a high speed vibrating condition, and in proportion thereto.

In accordance with the invention, the frequency of the current for energising the primary coil can be set at a value between 50 kHz and 2 MHz. The response frequency of the d.c. output voltage is obtained by rectifying and smoothing the difference voltage between the two secondary coils, giving a response frequency of about one tenth of the energising frequency. Thus, the response frequency is from 5 to 200 kHz, which is much higher than the response frequency of 1 kHz for the case of a conventional vibrometer. The vibrometer according to the invention is thus capable of accurately measuring a rapid vibrating condition.

The differential transformer of this invention as described above can be modified so as to be advantageous for the measurement of small displacements. In order to achieve this, a further aspect of this invention provides a differential transformer having a core or casing made from a conductive metal material of low permeability having a specific magnetic susceptibility of the order of from 10-3 and 10-6 and adapted to be connected to a movable object the movement of which is to be detected by the transformer, a primary coil disposed around the axis of movement of the core or casing and adapted to be energised with alternating current the frequency of which lies in the range of from 50 kHz to 2 MHz, and two secondary coils associated with the primary coil and arranged to produce a voltage difference proportional to the displacement of the core or casing.

The core used in a conventional differential transformer is made from a ferromagnetic metal material having a high permeability, such as iron the specific magnetic susceptibility of which is about 103 - 106 in electromagnetic units. This high permeability attracts many lines of magnetic force, which also are deflected as the core moves. This causes the voltages induced in the two secondary coils to vary accordingly, so that the mechanical displacement of the core appears as a differential output voltage. However, since the distribution of the lines of magnetic force produced by the primary coil varies according to the core movement, symmetry in the distribution of the magnetic force lines is lost. It is therefore difficult to ensure that the displacement of the core and the output voltage are kept in a correct proportional relation to each other.In an attempt to keep the two in a correct proportional relationship over a certain range, design efforts have been exerted on the frequency to be used, the number of coil turns, the width of the coil and the position and length thereof.

In the case of a differential transformer of a short stroke, however, the length of the coil hardly leaves any portion to have the magnetic force lines uniformly distributed thereover. Therefore, despite various attempts, it is difficult to ensure the displacement of the core and the output voltage are kept precisely proportional.

Furthermore, in cases where the stroke is short, a high frequency is used for energising the primary coil, because the coil width is short and the number of coil turns is small. The use of a high frequency, however, results in a considerable degree of eddy current losses arising in the ferromagnetic core. This lowers the advantageous effect attainable by having a high permeability, and eventually results in a lowered output voltage.

The differential transformer of this invention which has been described in the foregoing aims at reducing the size thereof as much as possible, and the primary coil is intended to be energised by an alternating current at a high frequency of 50 kHz to 2 MHz. However, the use of such a high frequency presents the problem that an eddy current loss takes place to a considerable degree in the iron core.

For general industrial measurements of deformations, use of a differential transformer having a short stroke often becomes necessary. Metal materials such as iron and steel that are widely in use have great rigidity and are not much deformed by external forces. Therefore measurements of the deformations of a structure, a machine, or the like which is made from such a rigid metal material necessitate the measurement of minute displacements. Furthermore, precision measurement such as the measurement of an error in the dimensions of a product, an assortment of ball bearings, measurement of the thickness of a thin plate, measurement of the deformation of a stress ring or the like is usually required to measure displacements of not more than 1 or 2 mm.Conventional differential transformers have been provided as detectors of such short strokes, but suffer from numerous shortcomings, as mentioned in the foregoing, and there is a demand for an improved product. This can at least in part be satisfied by the modified form of differential transformer of this invention, as described above.

As mentioned above, the modified differential transformer of this invention has a core or casing formed of a conductive metal of low permeability, while the primary coil is adapted to be energised at a high frequency.

The conductive metal of low permeability may be selected for example from aluminium, silver or copper, or alloys of these metals. The specific magnetic susceptibility of the conductive metal should be in the range of 10-3 to 10 B, in electromagnetic units, whereas the energising frequency should be within the range from 50 kHz to 2MHz.

In a conventional differential transformer with a core made of a ferromagnetic material, the core attracts many lines of magnetic force while the core is moving. This makes it difficult to have the displacement of the core and the output voltage strictly in proportion. This problem is solved by the modified differential transformer according to the invention by the use of a core made of a low permeability metal material, which does not spoil the magnetic force line distribution. This arrangement derives from the concept that, with such a core, the eddy current losses which arise in the core weaken the lines of magnetic force and thus serves to have the displacement of the core and the output voltage kept in a correct proportional relationship.

Generally, a conductive metal of low permeability does not strengthen nor weaken the lines of magnetic force, by virtue of the low permeability. Since such a metal material is not magnetised, there are no hysteresis losses due to the alternating field. There are only eddy current losses, the value of which is proportional to the square of the frequency. In case of a high frequency, therefore, the eddy current losses are very large and this causes a large magnetic reluctance, which weakens the lines of magnetic force.

Accordingly, with such a metal material used for the core of a differential transformer or detector arranged to be energised at a high frequency, the lines of magnetic force in the region of the core are weakened, due to the eddy current losses. Therefore, the voltages induced in the portions of the secondary coils located in that region become lower. When the core is central with respect to the two secondary coils, the output voltage is zero, but when the core moves, the lines of magnetic force are weakened to an extent proportional to the displacement of the core. Then, the induced voltage in one secondary coil located to one side becomes lower corresponding to the weakened lines of magnetic force, while the induced voltage in the other secondary coil to the other side increases, to an extent proportional to the displacement of the core.Accordingly, the-output voltage - which is the difference voltage between the two induced voltages, varies in proportion to the displacement of the core.

Since the core of the modified differential tranformertransformer of the invention is of low permeability, it does not attract the lines of magnetic force. Movement of the core, therefore, does not cause any change in the distribution of the lines of magnetic force in the coils so that symmetry can be retained. Unlike the conventional transformers using cores of high permeability, the displacement of the core and the output voltage of the differential transformer of the invention can be kept in a precisely proportional relationship.

This feature of the invention is particularly advantageous for a differential transformer or detector that has a short stroke and as a consequence short coils.

The eddy current loss We arising within unit volume can be expressed by the following formula: We = 1 ar2 2 Bm2t2[W1m3j 6p wherein p : Specific resistance sr: Ratio of circumference to diameter f: Frequency Bm: Max. magnetic flux density t: Thickness The eddy current loss is proportional to the square of the frequency and inversely proportional to the specific resistance. Accordingly, the eddy current loss increases as the specific resistance of the metal material decreases. Since silver and copper have smaller specific resistances than aluminium, the use of copper or silver selected from metals of low permeability is preferable for obtaining a larger output.

Furthermore, the specific resistance of iron is about four times that of aluminium. When an iron core is used, therefore, the eddy current loss is only about one quarter of that of an aluminium core, and this spoils to a great extent the effect of the permeability of iron.

As is apparent from the foregoing description, it is desirable to have as large as possible eddy current losses in the core, to obtain a higher output voltage. Such losses can be increased by impressing an energising current of high frequency on the primary coil. However, there is a limit on the frequency that is usable, having regard to the manufacture of the coil. Results of experiments indicate that for a differential transformer having a short stroke of +2 to +4 mm, use of a frequency between 200 kHz and 1 MHz of thereabout facilitates operations. For a longer stroke between 110 to +20 mm, which necessitates an increased number of turns in coil, a frequency in the range of not exceeding 200 kHz and not lower than 50 kHz is preferable. When the frequency exceeds 2 MHz, a skin current flows due to capacity between the strands of the coil.This tends to decrease the effective current for energising the coil. In view of this, the useful range of frequencies to be impressed on the primary coil is from 50 kHz to 2MHz, or thereabout.

The modified differential transformer of this invention as described above has been found still to have the shortcoming that the transformer fails to give a high output voltage when the core diameter is not more than 5 mm or thereabout. This is considered to be attributable to the following.

The lines of magnetic force are produced by the energising current flowing through the strands of the primary coil, and the extent of the magnetic field is small because of the high frequency of the energising current. When the coil diameter is large, most of the magnetic lines of force thus produced pass through the bore of the bobbin in which the core moves and therefore the value of the eddy current losscaused by the magnetic lines of force is great while the core is within the bobbin bore: this results in a large output voltage.

If however the diameter of the bore in the bobbin is smaller, because a small diameter coil is used,the magnetic lines of force produced by the primary coil are distributed throusturJparts other that e bobbin bore, so that the number of lines in t'ne bore are reduced. This lessens the eddy current losses in the core, and consequently the output voltage of the differential transformer is much reduced.

In an attempt to overcome this problem and to provide a differential transformer which displays good linearity (i.e. keeping the core displacement and output voltage in strict proportion over a given stroke), irrespective of the size of the core diameter, it is most advantageous for the transformer to be provided with an annular core, and there is a bar or tube of a ferromagnetic material having specific magnetic susceptibility of the order of from 1 10-3 to 1 of6 which bar or tube is mounted to serve as an auxiliary core for the transformer, the auxiliary core extending through the annular core.

An eddy current produced in a core generally flows along the outer circumferential surface of the core.

Therefore, a core made of a conductive metal material of low permeability does not have to be a solid metal bar but may be in the shape of a ring or annulus of thickness not exceeding one or two mm. In other words, a ring-shaped core not exceeding one or two mm in thickness and arranged to be movable in the axial direction of the differential transformer gives the same output characteristics as can be obtained with a core which is in the shape of a solid bar.

Through the central hole of the thin ring-shaped main core, an auxiliary core made of a ferromagnetic material of high permeability extends. This auxiliary core may be an iron rod or tube the outer diameter of which is as large as possible, and should be inserted within the bobbin bore to extend for at least the entire length of the primary coil. The positional relation between the main core and the auxiliary core may be either such that the main core is slidable on the auxiliary core, or that the auxiliary core extends through and is secured to the main annular core, to serve as a supporting rod therefor and to move in unison therewith.In the latter case, however, the auxiliary core must be longer than the entire length of the primary coil, so as to be able to extend for the whole length of the primary coil irrespective of the position of the main core within the range of its permissible displacement.

In the arrangement of an annular main core of conductive metal of low permeability and a ferromagnetic auxiliary core of high permeability extending therethrough for the entire length of the primary coil, almost all the lines of magnetic force produced by the primary coil are attracted by the auxiliary core and thus come to extend along the auxiliary core: this causes a decrease in the magnetic reluctance. Therefore, compared to a case where there is no auxiliary core, the magnetic lines of force produced by the primary coil are much strengthened by the presence of the auxiliary core. Besides, compared to a case without an auxiliary core, the provision of the auxiliary core ensures a homogeneous distribution of the lines of magnetic force within the bobbin bore.

The auxiliary core made of a ferromagnetic material, because it strengthens the lines of magnetic force, increases the voltages induced in the secondary coils. However, since the auxiliary core is symmetrically disposed to extend for the whole length of the primary coil, the voltages induced thereby in the two secondary coils are equal to each other. Therefore, the output (difference) voltage of the differential transformer is not changed by the presence of the auxiliary core.

With the auxiliary core of a ferromagnetic material arranged to extend for the whole length of the primary coil, the lines of magnetic force generated by the primary coil are strengthened, without causing a change in the output voltage of the differential transformer, but instead the magnetic force lines become homogeneously distributed within the bobbin bore. This not only increases the eddy current losses arising in the conductive metal main core of low permeability, in turn to increase the output voltage of the transformer, but also serves to improve the proportional relation of the core displacement to the output voltage - i.e. the linearity. Accordingly, the value of the residual zero voltage also becomes smaller.

Moreover, the advantageous effect of the ferromagnetic auxiliary core is obtained irrespective of whether the coil diameter is large or small. Those lines of magnetic force that would be distributed through parts other than the bobbin bore were no auxiliary core present are attracted by the auxiliary core and are thus forced to pass therethrough. Consequently, almost all the lines of magnetic force extend through the bore of the bobbin. The provision of the auxiliary core thus eliminates the likelihood of the use of a core of small diameter resulting in a lower output voltage.

The eddy current loss We is proportional to the square of frequency and inversely proportional to specific resistance, as mentioned in the foregoing. It is also proportional to the square of the maximum magnetic flux density. Accordingly, the eddy current losses increase with the maximum magnetic flux density. In the case of the differential transformer having an auxiliary core of a ferromagnetic material to increase the magnetic flux density, the magnetic reluctance is lowered, giving rise to greater eddy current losses. Therefore, the value of the output voltage of the differential transformer also increases. Furthermore, since the eddy current losses also increase as the specific resistance decreases, the use of silver or copper for the main core is more advantageous than the use of aluminium, so as to obtain a larger output. Alloys of silver or copper, or of other metals of low permeability, may however be used.

Though an energising current of high frequency is impressed on the primary coil in order to increase the eddy current losses, the manufacture of the coil inevitably imposes certain limitations on the range of usable frequency. However, as with the first-described modified form of differential transformer of this invention, results of experiments indicate that the useful frequency range is about 50 kHz to 2 MHz.

By way of example only, certain specific embodiments of this invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a sectional view of a known form of differential transformer; Figure 2 is a circuit diagram showing the connections of the differential transformer of Figure 1; Figure 3 is a sectional view of a first embodiment of differential transformer of the presentinvention; Figure 4 is a circuit diagram showing the connections for the transformer of Figure 3; Figure 5 is a graph showing the characteristics of the first embodiment; Figure 6 is a sectional view of a known vibrometer using a differential transformer; Figure 7 is a circuit diagram of the measuring circuit for use with the known vibrometer;; Figure 8 is a sectional view of a vibrometer using a differential transformer according to the present invention, as second embodiment thereof; Figure 9 is a sectional view of a third embodiment of the invention, being a differential transformer adapted to have a relatively short stroke; Figure 10 is a perspective view of a bobbin used in the third embodiment; Figure 11 is a circuit diagram showing the connections for the third embodiment; Figure 12 is a graph showing the relation between core displacement and output voltage, for the third embodiment; Figures 13, 14, 16 and 18 are sectional views of modified examples of the third embodiment shown in Figure 9; Figures 15 and 17 are circuit diagrams showing the connections for the modified examples of Figures 14 and 18 respectively;; Figure 19 is a graph showing the relation between core displacement and output voltage for the modified example shown in Figure 18; Figure 20 shows the fourth embodiment of the invention, being a modified form of differential transformer; Figure 21 is a perspective view of the bobbin used in the fourth embodiment; Figure 22 is a circuit diagram showing the connections of the fourth embodiment; and Figure 23 is a sectional view of a modified example of the fourth embodiment shown in Figure 20.

Referring to Figures 1 and 2, there is shown a typical known differential transformer which is in general use. A primary coil 2 is wound round the central portion of a bobbin 1 made of a non-magnetic material and is provided with lead wires 3. Secondary coils 4 and 6, which are respectively provided with lead wires 5 and 7, are wound round the respective outer portions of the bobbin 1 and are thus disposed one on each side respectively of the primary coil 2. The lead wires 5 and 7 of the coils 4 and 6 are differentially connected, as shown in Figure 2. There is provided an iron core 8 carried by a non-magnetic rod 9 within the bore of the bobbin 1. As is well known, when the primary coil 2 is energised by connecting the lead wires 3 to an a.c.

oscillator, a voltage proportional to the displacement of the iron core 8 is produced between the lead wires 5 and 7. Differential transformers of this construction have thus been widely used for the function of converting a mechanical displacement into a voltage.

However, the above-described conventional differential transformers have a relatively large outside diameter, because of the large number of turns in their coils. The casings usually range from 20 to 30 mm outside diameter, while the cores moving within the coils usually range from 5 to 8 mm diameter. Therefore, the transformers are suitable for measuring the displacement of large machines such as machine tools, production machines, vehicles, and so on, but not for measuring the displacement of small, light-weight devices such as a Bourdon tube used in a pressure gauge or a balance and so on.

First Embodiment Figures 3 to 5 illustrate a first embodiment of differential transformer of the invention. This embodiment comprises a primary coil 12 wound with one or two layers over the entire length of a bobbin 11 made of a non-magnetic material, the primary coil 12 having two lead wires 13. Two secondary coils 14 and 16 are wound directly over the primary coil 12, and the lead wires 15 and 17 of the secondary coils 14 and 16 are differentially connected as shown in Figure 4. The bobbin 11 has a central hollow bore 21 in which is provided an iron core 18 carried on a support rod 19 of non-magnetic material, arranged so that the core 18 can be slid axially.A split ring 20 made of a non-magnetic material, such as a plastics material, is snapped around the central portion of the primary coil 12 after the coil has been wound round the bobbin 11, to facilitate the winding of the two secondary coils 14 and 16.

A sufficiently high voltage can be induced in each of the secondary coils 14 and 16 by energising the primary coil 12 with an alternating current of several milliamperes, impressing several volts across the coil at such a high frequency as between 50 and 2000 kHz.

Referring to Figure 4, when the iron core 18 is midway between the secondary coils 14 and 16, equal voltages are induced in the secondary coils 14 and 16. Under this condition, therefore, the voltage between the differentially connected lead wires 15 and 17 is zero. Referring to Figure 3, if then the iron core moves to the left, the voltage induced in the secondary coil 14 increases while the voltage induced in the secondary coil 16 decreases, so that a voltage equal to the difference between the changed voltage appears between the lead wires 15 and 17. Conversely, if the iron core 18 moves to the right, the voltage induced in the secondary coil 14 decreases whereas that in the secondary coil 16 increases, so that a voltage equal to the difference therebetween appears between the lead wires.

Plotting the displacement of the iron core 18 as abscissa and voltage as ordinate, the voltagaappearing between the lead wires takes the form of a letter V, as shown in Figure 5, as with converlorral-dsntia1E transformers. Likewise, in order to secure the longest possible measuring range, or stroke, over which the displacement of the iron core 18 and output voltage are kept in linear proportion, a suitable length for the iron core 18 may be selected experimentally, according to the shape of the differential transformer and the frequency of excitation.

As described above, this embodiment of differential transformer has a primary coil energised with alternating current at a high frequency of between 50 and 2000 kHz, while the two secondary coils are wound directly over the primary coil. This arrangement permits a significant reduction in the number of turns required for both the primary and secondary coils. The reduced number of turns, in turn, permits a greatly reduced overall diameter and weight of the differential transformer, and thus facilitates the manufacture thereof.

In differential transformers arranged to be energised at low frequencies, the application of a voltage to the primary coil generally results in the generation of a magnetic attracting force which draws the iron core toward the centre of the coil. By contrast, in the differential transformer of this invention - which is arranged to be energised at a relatively high frequency - the magnetic attraction is so small that even a small, light iron core remains unattracted and this gives an operational advantage.

A well known manufacturing method for a differential transformer is to wind two wires at the same time but then the turns ratio between the primary and secondary coils becomes 1 : 0.5. Accordingly, the voltages induced in the secondary coils and hence the output voltages are small. Despite this disadvantage, if the differential transformer of this embodiment is made this way, improvements over the conventional products can be ensured.

The following Examples are given to illustrate specific details of this embodiment, and for each Example the outside and inside diameters of the bobbin, the numbers of turns and layers as well as the width of the primary coil, the numbers of turns and layers of the secondary coils, the energising frequency and the diameter and length of the iron core are given, and vary between the Examples. Each of these examples of differential transformers was manufactured as follows.

A primary coil was prepared by winding a 0.10 mm diametertransformerwire (except for Example 1 where 0.12 mm wire was used) to the specified number of turns in one or two layers and with a specified width over the entire length of a bobbin. Two secondary coils were prepared by winding polyurethane transformer wires of the same diameter directly over the primary coil, each secondary coil starting at the centre of the primary coil and extending one toward each end respectively of the bobbin, each secondary coil having the specified number of turns and layers. An iron core of the specified diameter and length was slidably mounted within the bobbin. With differential transformers manufactured in this manner, according to the invention, the primary coil of each transformer was energised with a 1 V sine wave at the specified frequency. It was then confirmed that the displacement of the iron core was substantially proportional to the output voltage within the specified proportioning range. The values given for each Example are approximate averages of the results obtained from experiments conducted on several transformers prepared in accordance with each Example.

The value of the output voltage was obtained without the differential transformer having a casing mounted thereon; that is, with the coils in a bare state. The output voltage may drop somewhat from the value shown if a casing is put on. However, if necessary, a higher voltage output can be obtained by the use of an amplifier, and this causes no problem since stable amplifiers are today available. The values bf the d.c.

resistance of the coils are very low, and it would appear that the values involved considerable measuring errors. However, errors in the d.c. resistance have little effect on the differential transformers of this invention, which are to be energised at a high frequency, since the impedance of the primary coil consists mostly of inductive reactance.

Example 1 A differential transformer of long stroke, which is easy to manufacture.

Plastic bobbin - Outside diameter: 5 mm; Inside diameter: 3.2 mm; Primary coil - Number of turns: 1100; Number of layers: 2; Winding width 8 cm; (d.c. resistance 34 ohms).

Secondary coils - Number of turns: 1000; Number of layers: 1 (d.c. resistance 31 ohms) Energising frequency - 100 kHz.

Primary coil impedance - 500 ohms.

Coil outside diameter - 8 mm or smaller.

(1) Iron core - Diameter: 3 mm; Length: 4cm; Proportioning range - +20 mm Maximum output -1.4V (2) Iron core - Diameter: 3 mm; Length: 6 cm; Proportioning range - +30 mm Maximum output -1.2 V Examples 2 to 6 show differential transformers of various strokes each using a bobbin of smaller diameter, measuring 4 mm.

Example 2 Plastic bobbin - Outside diameter: 4.0 mm; Inside diameter: 2.4 mm.

Primary coil -Numberofturns: 530; Number of layers: 1; Winding width: 64 mm; (d.c. resistance: 18 ohms).

Secondary coils - Number of turns: 430; Number of layers: 2 (d.c. resistance: 15 ohms).

Energising frequency -400 kHz.

Primary coil impedance - 300 ohms.

Iron core - Diameter: 2.2 mm Length: 5cm.

Proportioning range - +20 mm.

Maximum output - 0.8 - 1 V Example 3 Plastic bobbin - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 350 Number of layers: 1 Winding width: 43 mm (d.c. resistance: 14 ohms).

Secondary coils - Number of turns: 280 Number of layers: 2 (d.c. resistance: 12 ohms).

Energising frequency - 400 kHz Primary coil impedance - 200 ohms Iron core - Diameter: 2.2 mm Length: 32 mm Proportioning range - +10 mm Maximum output - 0.6 - 0.8 V Examples 4 and 5 show differential transformers wherein the ratio of the number of turns of the primary coil to that of each secondary coil was about the same. However, the primary coil was wound in one layer in the case of Example 4 and in two layers in the case of Example 5. A higher frequency was used in the former case where the number of turns was less than the latter. A frequency of not lower than 1 MHz proved favourable in Example 4 while that of about 500 kHz proved sufficient for Example 5.

Example 4 Plastic bobbin - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 180 Number of layers: 1 Winding width: 23 mm (d.c. resistance: 9 ohms).

Secondary coil - Number of turns: 130 Number of layers: 2 (d.c. resistance: 7 ohms) Energising frequency -1 MHz - 2 MHz Iron core - Diameter: 2.2 mm Length: 18 mum Proportioning range - +5 mm Maximum output - in the case of 1 MHz : 0.3 V he case of 1.5 MHz: 0.6 V In the case of 2MHz: 0.3 V Example 5 Plastic bobbin - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 370 Number of layers: 2 Winding width: 23 mm (d.c. resistance: 15 ohms) Secondary coils - Number of turns: 300 Number of layers: 4 (d.c. resistance: 13 ohms) Energising frequency - 400 kHz Primary coil impedance - 600 ohms Iron core - Diameter: 2.2 mm Length: 18 mm Proportioning range - +5 mm Maximum output - 0.5 V Example 6 A differential transformer of short stroke. In this case, the primary coil has a smaller number of turns and lower inductance. Accordingly, it is preferable to have the impedance of the primary coil raised by use of a higher energising frequency.

Plastic bobbin - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 200 Number of layers: 2 Winding width: 13 mm (d.c. resistance: 10 ohms) Secondary coils - Number of turns: 130 Number of layers: 4 (d.c. resistance: 8 ohms) Energising frequency - 800 kHz Primary coil impedance - 400 ohms Iron core - Diameter: 2.2 mm Length: 10 mm Proportioning range - +2 mm Maximum output - 0.3 V Xi; In Examples 7 and 8, the diameter of the bobbin on which the coils were wound was further reduced. The stroke was also shortened to +1.5 mm. The primary coil was wound in one layer in Example 7 and in two layers in Example 8. Because of the small number of turns, high frequencies were used.

Example 7 Plastic bobbin - Outside diameter: 3 mm Inside diameter: 2.2 mm Primary coil - Number of turns: 160 Number of layers: 2 Winding width: 11 mm (d.c. resistance: 8 ohms) Secondary coils - Number of turns: 140 Number of layers: 6 (d.c. resistance: 5 ohms) Energising frequency - 800 kHz Primary coil impedance - 280 ohms Iron core - Diameter: 2 mm Length 9 mm Proportioning range - +1.5 mm Maximum output -1.0 - 1.5 V Example 8 Plastic bobbin - Outside diameter: 3 mm Inside diameter: 2.2 mm Primary coil - Number of turns: 80 Number of layers: 1 Winding width: 11 mm (d.c. resistance: 8 ohms) Secondary coils - Number of turns: 110 Number of layers: 4 (d.c. resistance: about 6 ohms) Energising frequency -1.2 MHz Primary coil impedance - 400 ohms Iron core - Diameter: 2 mm Length: 9 mm Proportioning range - +1.5 mm Maximum output - 1.4 V Examples 9 and 10 show very small differential transformers provided with bobbins of especially small diameters. The stroke and the winding width of the primary coil were the same. However, the primary coil was wound in two layers in Example 9 and in one in Example 10.

Example 9 Aluminium bobbin - Outside diameter: 2.2 mm Inside diameter: 1.6 mm Primary coil -Numberofturns: 350 Number of layers: 2 Winding width: 22 mm (d.c. resistance: 8 ohms) Secondary coils - Number of turns: 270 Number of layers: 4 (d.c. resistance: 7 ohms) Energising frequency - 800 kHz Primary coil impedance - 220 ohms Iron core - Diameter: 1.4 mm Length: 18 mm Proportioning range - r5 mm Maximum output - 0.75 V Example 10 Aluminium bobbin - Outside diameter: 2.2 mm Inside diameter: 1.6 mm Primary coil - Number of turns: 180 Number of layers: 1 Winding width: 22 mm (d.c. resistance: 5 ohms) Secondary coils - Number of turns: 270 Number of layers: 4 (d.c. resistance: 7 ohms) Energising frequency -1 MHz Primary coil impedance - 40 ohms Iron core - Diameter: 1.4 mm Length: 18mm Proportioning range - 15 mm Maximum output - 0.8 -1 V Conventional differential transformers often have a core which is 5 to 8 mm in diameter and is arranged to move during a measuring operation, and also these transformers have a casing which is 20 to 40 mm external in diameter. They, therefore, have the disadvantage of being very heavy, but on the other hand, however, they have the advantage of great mechanical strength. Examples 11 and 12 show Examples of embodiments with smaller coil diameters than the conventional ones, while retaining the excellent mechanical strength, using large cores.

Since the increased coil diameter came close to 10 mm, the coil inductance increased and, accordingly, the value of energising frequency decreased. With a primary coil wound in two layers, a frequency of between 100 and 50 kHz or thereabout proved best.

Further, since the outside diameter of the coil was not larger than 11 mm, the outside diameter of the casing was held to 13 mm, or approximately one half of the outside diameter of the conventional differential transformer casings, even when a casing of 1 mm wall thickness was used.

Example 11 Plastic bobbin - Outside diameter: 9 mm Inside diameter: 7 mm Primary coil - Number of turns: 550 Number of layers: 2 Winding width: 35 mm (d.c. resistance: 38 ohms) Secondary coils - Number of turns: 450 Number of layers: 4 (d.c. resistance: 32 ohms) Energising frequency - 80 kHz Primary coil impedance - 600 ohms Iron core - Diameter: 6.5 mm Length: 25 mm Proportioning range - +10 mm Maximum output - 0.5 V Example 12 Plastic bobbin - Outside diameter: 9 mm Inside diameter: 7 mm Primary coil - Number of turns: 1020 Number of layers: 2 Winding width: 65 mm (d.c. resistance: 70 ohms) Secondary coils - Number of turns: 900 Number of layers: 4 (d.c. resistance: 65 ohms) Energising frequency - 60 - 50 kHz Iron core - Diameter: 6.5 mm Length: 50 mm Proportioning range - +25 mm Maximum output - 1.0 V at 60 kHz and 0.9 V at 50 kHzfrequency Primary coil impedance - 700 ohms at 60 kHz and 540 ohms at 50 kHz.

A typical vibrometer using a conventional type of differential transformer is arranged as shown in Figure 6 of the accompanying drawings. A casing 25 has its interior filled with a fluid 26 which normally would be silicon oil, and the casing is then tightly sealed. A differential transformer 27 comprises a non-magnetic tubular body 28 secured to the casing, and a vibrating body 29 which usually is made of iron and is carried by non-magnetic supporting rods 30 and 31, the rods being secured to the free ends of two plate springs 32 and 33 usually made of phosphor bronze. The other ends of the plate springs 32 and 33 are secured to the casing 25, so that the vibrating body 29 is movable within the tubular body 28 along the axis thereof.A primary coil 34 and secondary coils 35 and 36 are wound round the outer circumference of the tubular body 28, the primary coil 34 being wound round the middle portion of the tubular body 28, while the secondary coils 3.5 and 36 are wound respectively one to each side thereof. The primary coil is arranged to be energised with an alternating current. Vertical vibrations imparted to the casing 25 cause the plate springs to vibrate vertically, the vibrations being attenuated to a suitable degree by the fluid 26. The vibrations of the plate springs 32 and 33 are transmitted to the vibrating body 29, the displacement of which is then measured by a circuit arrangement as shown in Figure 7.

When an a.c. voltage is impressed on the primary coil 34 by an a.c. oscillator 37, an a.c. voltage corresponding to the position or displacement of the vibrating body 29 is induced in each of the secondary coils 35 and 36. The voltages thus obtained are rectified to produce a d.c. voltage by diodes 38 end 39, which voltages are smoothed by time-constant circuits constituted by capacitors 40 and 41, and resistors 42 and 43, as shown. These voltages are amplified by emitter-follower connected transistors 44 and 45, and the d.c.

output voltages appearing at the emitters of these transistors is extracted as the difference between the a.c.

voltages induced in the secondary coils 35 and 36. Therefore this voltage represents the displacement of the vibrating body 29.

As described above, in order to obtain a d.c. output voltage, the a.c. voltage produced in each secondary coil is rectified and smoothed to form a d.c. voltage with the ripple component reduced. In this instance, the smoothing action is performed by arranging the time constant determined by the respective resistor and capacitor pair to be generally of a value at least three times as great as the frequency of the a.c. induced in the associated secondary coil. Accordingly, the response frequency of the d.c. output voltage is generally about one tenth of the a.c. frequency induced in the secondary coil - i.e. the energising frequency applied to the primary coil.

As will be clearly understood from the above, the secondary coils of a differential transformer of a conventional vibrometer are spaced from the primary coil. When the primary coil is energised with an alternating current at a high frequency above 10 kHz, the lines of magnetic force do not sufficiently reach the secondary coils to induce a high voltage thereon. Therefore, the primary coil of the differential transformer in the conventional vibrometer has normally been energised at a relatively low frequency not exceeding 10 kHz. However, because the response frequency of the d.c. output voltage obtained from the secondary coils is about one tenth of the energising frequency applied to the primary coil, the use of a frequency not exceeding 10 kHz for energising the primary coil results in a response frequency not exceeding 1 kHz.A conventional vibrometer is therefore not entirely suitable for accurate measurements of rapid vibrations such as may result, for example, from a collision.

Second Embodiment: Figure 8 shows an embodiment of a vibrometer using a differential transformer according to the second aspect of the invention. In the vibrometer shown, a tubular body 46, made of a non-magnetic material such as a plastics material or a stainless steel, is completely filled with a fluid 47 such as silicone oil or air and the body is tightly sealed. A vibrating body 48 of a certain mass and made of a magnetic material such as iron is suspended by springs 50 and 54 within the tubular body 46. The vibrating body 48 is provided with a suitable number of holes 49 for attenuating the vibrations of the vibrating body 48 to an appropriate degree, which is adjustable by selecting the size and number of the holes. With the vibrating body 48 being thus carried by the springs 50 and 51, it is arranged to vibrate within the tubular body 46 in the axial direction thereof.The natural oscillation frequency of the vibrating body is set at a suitable value by selecting the ratio of the spring constants of the springs 50 and 51 to the mass of the vibrating body, in the same manner as with the conventional vibrometers.

The tubular body 46 has a primary coil 52 wound round its outer circumference in one or two layers, and is provided with lead wires 53. Two secondary coils 54 and'55 are wound directly over the primary coil 52 with a prescribed number of turns, the two secondary coils extending one from one end cheek to the middle of the primary coil, and the other from an opposed end cheek also to the middle of the primary coil. These secondary coils 54 and 55 are respectively provided with lead wires 56 and 57. The primary coil 52, the secondary coils 54 and 55 and the vibrating body 48 jointly constitute a differential transformer.

An alternating voltage at a high frequency of between 50 kHz and 2 MHz is impressed upon the primary coil 52 to energ ise it by causing an alternating current of several milliam peres to flow therethroug h, and a circuit arrangement similar to that shown in Figure 7 is required for measuring the displacement of the vibrating body 48. Voltages corresponding to the displacement of the vibrating body 48 are induced in the secondary coils 54 and 55, and the voltages thus obtained are rectified and smoothed by diodes 38 and 39, and capacitors 40 and 41. After that, the difference between the two voltages is extracted as a d.c. output signal.

When the tubular body 46 is caused to vibrate up and down (in Figure 8), the vibrating body 48 carried by the springs 50 and 51 naturally also vibrates up and down. Then, the displacement ofthe vibratfng body 48 caused by the vibration can be electrically measured to find the vertical vibrating conditions to which the tubular body 46 is subjected.

The embodiment described in the foregoing has a new structural arrangement, in that the tubular body is tightly sealed full of the fluid. However, it would of course be possible to have a tubular body and a casing separately arranged, in the same manner as in the conventional vibrometer as shown in Figur a; butWt;t the primary and secondary coils wound in accordance with the invention.

Generally, displacement type vibrometers are arranged to measure vibrational amplitudes not exceeding +10 mm, in most cases. In cases where they are used as a shock measuring device for measuring acceleration, the amplitude to be measured further decreases and generally does not exceed +2 mm. The size of a vibrometer thus is selected depending upon the purpose for which it is to be used. However, where the vibrometer has a large diameter and is intended for measuring large amplitudes, a primary coil wound round a tubular body will have a relatively large diameter and a relatively long coil. Since the number of turns in the coil is increased in such a case, the use of an excessively high frequency for energising the coil results in the primary coil having an excessively large inductive reactance.In such a case, therefore, the energising frequency is preferably arranged to be relatively low. Conversely, where the vibrometer is to be used for measuring short amplitudes, the use of a relatively high energising frequency is advantageous.

According to the results of experiments which were conducted on vibrometers having tubular bodies of 10 mm, 8 mm and 4 mm outside diameter (a detailed description will follow), the useful energising frequencies ranged from 50 to 200 kHz for a tubular body of 10 mm outside diameter, from 300 to 500 kHzfor a tubular body of 8 mm outside diameter and from 800 to 2000 kHz for a tubular body of 4 mm outside diameter.

According to the results of further experiments, use of energising frequency exceeding 2 MHz caused an increase in the skin current flowing over the surface of the coil, due to the capacity between adjacent turns of the coil. This in turn caused a decrease in the effective current for producing lines of magnetic force.

Therefore, the upper limit of the useful energising frequency is considered to be 2 MHz.

The second embodiment of the invention will more fully be understood from the following practical examples thereof, Examples 1 and 2 having the structural structural arrangement shown in Figure 7, whereas Examples 3 to 6 have the structural arrangement shown in Figure 6 but with the coils wound as defined in accordance with the present invention. These Examples represent experiments conducted by varying the outside and inside diameters of the tubular body; the number of turns, the number of winding layers, the width of the coils, and the energising frequency of the primary coil; the number of turns and the number of layers of the secondary coils; and the diameter and length of the vibrating body made of iron.In each example, a transformer wire of 0.10 mm diameter was wound round the tubular body to give a specified number of turns, to a specified winding width thereby to form the primary coil. The two secondary coils were prepared by winding transformer wires of the same diameter directly over the primary coil, starting from the centre of the primary coil and working toward one end for one of the secondary coils and toward the other end for the other secondary coil, with specified numbers of turns and layers. Within the tubular body, a vibrating body made of iron and of specified diameter and length was inserted.

With differential transformers formed into vibrometers and arranged as just-described, the primary coil of each vibrometer was energised by an impressed alternating voltage of 1 V, of sine-wave form at a specified frequency. Then, it was confirmed that the displacement of each vibrating body was substantially proportional to the output d.c. voltage within the specified proportioning range (a length to be used for measurement). The d.c. output voltage was obtained by extracting the difference between the two rectified and smoothed voltages of the two secondary coils. It was further confirmed that the response frequency of the output voltage was about one tenth of the energising frequency.

Examples 1 and 2 represent cases where the vibrating body had a large diameter (6.5 mm) thus resulting in a large diameter tubular body of 10 mm. The energising frequency was low for these cases. In Example 2, however, the energising frequency was higher than that for Example 1, because the primary coil in Example 2 was wound in one layer and thus had fewer turns.

Example 1 Plastic tubular body - Outside diameter: 10 mm Inside diameter: 7.5 mm Primary coil - Number of turns: 300 Number of layers: 2 Winding width: 20 mm (d.c. resistance: 27 ohms) Secondary coils - Number of turns: 310 Number of layers: 6 (d.c. resistance: 29 ohms) Vibrating body - Diameter: 6.5 mm Length: 15 mm Energising frequency - 50 kHz, 100 kHz Proportioning range - +8 mm Maximum output - 0.3.V (for both 50 and 100 kHz) Primary coil impedance - 260 ohms for 50 kHz and 400 ohms for 100 kHz Response frequency - 5 kHz when energising frequency was 50 kHz 100 kHz when energising frequency was 100kHz Example 2 Plastic tubular body - Outside diameter: 10 mm Inside diameter: 7.5 mm Primary coil - Number of turns: 160 Number of layers: 1 Winding width: 20 mm (d.c. resistance: 14 ohms) Secondary coils - Number of turns: 220 Number of layers: 4 (d.c. resistance: 20 ohms) Vibrating body - Diameter: 6.5 mm Length: 15mm Energising frequency - 200 kHz Proportioning range - +8 mm Maximum output - 0.5 V Primary coil impedance - 200 ohms Response frequency - 20 kHz In the following Examples 3 and 4, tubular bodies each measuring 8 mm in outside diameter were used. In Example 3, the winding width of the primary coil was 15 mm because a long proportioning range was required - i.e. the length to be used for measuring. By contrast, in Example 4 the winding width ofthe primary coil was short, measuring only 10 mm because a shorter proportioning range was required. A higher energising frequency was used in Example 4 where the number of turns in the coil was fewer than in Example 3.

Example 3 Plastic tubular body - Outside diameter: 8 mm Inside diameter: 6.5 Primary coil - Number of turns: 125 Number of layers: 1 Winding width: 15 mm (d.c. resistance: 13 ohms) Secondary coils - Number of turns: 180 Number of layers: 4 (d.c. resistance: 15 ohms) Vibrating body - Diameter: 5.5 mm Length: 12 mm Energising frequency - 300 kHz Proportioning range - 16 mm Maximum output - 0.35 V Primary coil impedance - 125 ohms Response frequency - 30 kHz Example 4 Plastic tubular body - Outside diameter: 8 mm Inside diameter: 6.5 mm Primary coil - Number of turns: 90 Number of layers: 1 Winding width: 10 mm (d.c. resistance: 12 mm) Secondary coils - Number of turns: 80 Number of layers: 4 (d.c. resistance: 10 ohms) Vibrating body - Diameter: 5.5 mm Length: 8 mm Energising frequency - 500 kHz Proportioning range - +3 mm Maximum output -0.18V Primary coil impedance - 135 ohms Response frequency - 50 kHz In the following Examples 5 and 6, tubular bodies of 4 mm in outside diameter were used. In each of these Examples, the coil diameter was small and the number of turns was also small. Therefore, a high energising frequency was used.

Example 5 Plastic tubular body - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 200 Number of layers: 2 Winding width: 13 mm (d.c. resistance: 10 ohms) Secondary coils - Number of turns: 130 Number of layers: 4 (d.c. resistance: 8 ohms) t Vibrating body - Diameter: 2.2 mm Length: 10 mm Energising frequency - 800 kHz Proportioning range - +2 mm Maximum output - 0.3 V Primary coil impedance - 400 ohms Response frequency - 80 kHz Example 6 Plastic tubular body - Outside diameter: 4 mm Inside diameter: 2.4 mm Primary coil - Number of turns: 180 Number of layers: 1 Winding width: 20 mm (d.c. resistance: 9 ohms) Secondary coils - Number of turns: 130 Number of layers: 2 (d.c. resistance: 7 ohms) Vibrating body - Diameter: 2.2 mm Length: 15 mm Energising frequency - 1 MHz and 2 MHz Proportioning range - l5 mm Maximum output - When energising frequency was 1 MHz: 0.3 V When energising frequency was 2 MHz: 0.3 V Primary coil impedance - 260 ohms for 1 MHz and 820 ohms for 2 MHz Response frequency - 100 kHz for 1 MHz and 200 kHz for 2 MHz Third Embodiment Figures 9 to 19 illustrate a third embodiment of the present invention. The embodiment shown in sectional view in Figure 9 is a differential transformer comprising a primary coil 60 and secondary coils 61 and 62 which are wound round a bobbin 63 and which are connected as shown in Figure 11.

The bobbin 63 is made of a non-magnetic plastics material and is provided with two annular recesses 64 and 65 of the same dimensions, in which the coils are wound. The bobbin 63 is also provided with an axial slit 66 through which the lead wires of the coils are passed. The primary coil 60 is wound in one or two layers at the bottoms of the two recesses 64 and 65 of the bobbin 60, in a continuous manner. In the recess 64, one secondary coil 61 is wound directly over the primary coil, with a prescribed number of turns. In the other recess 65, the other secondary coil 62 is also wound directly over the primary coil there, with a prescribed number of turns. The bobbin 63 has a hollow central bore 67 within which a core 68 made of a conductive metal of low permeability is mounted on a plastics supporting rod 69 to be movable along the axis of the bore 67.The primary coil 60 is provided with lead wires 70 and the secondary coils 61 and 62 with lead wires 71 and 72 respectively. The two ends of the lead wires 70 of the primary coil are arranged to have an alternating voltage at a high frequency impressed thereacross.

With the structural arrangement described above, voltages are induced by lines of magnetic force, produced by the primary coil 60, in the two secondary coils 61 and 62 which are wound directly over the primary coil. When the core 68 is midway between the two secondary coils 61 and 62, the voltages induced in these secondary coils are equal, and the voltage between the lead wires 71 and 72 is zero. When the core 68 moves to the left (as viewed in Figure 9), the voltage induced in the secondary coil 61 decreases while the voltage induced in the other secondary coil 62 increases, producing a voltage difference between the lead wires 71 and 72.Conversely, movement of the core to the right causes an increase in the voltage of the secondary coil 61 and a decrease in that of the secondary coil 62, resulting in a voltage representing the difference between the two induced voltages appearing between the lead wires.

Figure 12 shows the relation of the voltage produced between the lead wires 71 and 72 to the displacement of the core 68. The displacement of the core is plotted on the abscissa and the output voltage on the ordinate.

There obtains a relation in the shape of a letter V, as shown in Figure 12, in the same manner as with a conventional differential transformer.

A modification of the differential transformer described above in connection with Figure 9 is shown in Figure 13, in which the core 68 used in the case of Figure 9 is replaced by a hollow casing 73 which is arranged to be movable about the outer circumference of the bobbin 63, in the axial direction thereof. This casing 73 is made from a conductive metal of low permeability and is carried by a supporting tube 74 made of a plastics material. The operation of this modification is similar to that described in connection with Figures 9 to 12, and is suitable for use where the moving part must be located outside the coil assembly.

Further possible modifications are shown in Figures 14,16 and 18, all aiming at maintaining the output voltage precisely proportional to the displacement. When a member corresponding to the core is made from a ferromagnetic material of high permeability as is the case with a conventional transformer, it is difficult to manufacture the transformer so that the output voltage is precisely proportional to the displacement of the core. To solve this problem, each of the transformers shown in Figures 14, 16 and 18 uses a member corresponding to the core made of conductive metal of low permeability, and the primary coil is arranged to have an energising current of high frequency impressed thereon. This enables the differential transformer to keep the displacement and the output voltage precisely proportional.

In the case of Figure 14, two primary coil portions 75 and 76 are wound round two cylindrical tubular bobbins 77 and 78. After that, two secondary coils 79 and 80 are wound directly over the primary coil portions 75 and 76. The lead wires of the primary coil are connected in series while those of the secondary coils are differentially connected, as shown in Figure 15. A disc shaped core 81 is formed by a conductive metal material of low permeability, such as aluminium, and is carried by a supporting rod 82 made of a plastics material, so as to be movable to the left and right (Figure 14) in the axial direction of the supporting rod 82.

Moving the disc shaped core 81 toward and away from the end faces of the coils changes only those lines of magnetic force existing at the end faces of the coils. Since the voltages induced in the secondary coils are changed in this manner so as to obtain an output voltage, this transformer has the disadvantage that its output voltage is small. On the other hand, however, the transformer can advantageously be used where the required stroke is very short, because the permissible movement of the core is limited to a very small range.

Figure 16 shows a further modification of the differential transformer shown in Figure 14, and here the core is divided into two parts. The transformer operates on exactly the same principle as that shown in Figure 14, and movement of the divided core closer to and away from the end faces of the coils causes changes in the voltages induced in the two secondary coils. In this way, an output voltage can be obtained in proportion to the displacement of the divided core.

Atubular cylindrical bobbin 83 is provided with two recesses adjacent the ends thereof, and primary coil portions 84 and 85 are wound on the bottoms of the two recesses. The two secondary coils 86 and 87 are then wound directly over the two primary coil portions 84 and 85, respectively. As shown in Figure 17, the lead wires 88 of the two primary coil portions are connected in series, while the lead wires 89 and 90 of the secondary coils are differently connected. Disc-shaped cores 91 and 92 are made of a conductive metal material of low permeability such as aluminium and are carried by a supporting rod 93 made of a plastics material to lie one to each end of the bobbin 83.The disc-shaped cores 91 and 92 are arranged to be movable in the axial direction ofthe supporting rod 93.

The differential transformer of Figure 16 can be modified to form a detector of the non-contact measurement type by removing the core supporting rod 93 and by arranging one of the cores (94) immovably to abut the bobbin while allowing the other core 95 to move on its own. As shown in Figure 18, core 94 corresponds to the core 91 of Figure 16 and is arranged in close contact with the bobbin 83, whereas core 95 corresponds to core 92 and is for attachment to the object the displacement of which is to be measured. When the core 95 also comes in contact with the bobbin 83, the cores 94 and 95 equally affect the magnetic lines of force generated by the primary coil portions. Then, the voltages induced in the secondary coils are also equal to give a zero output voltage.However, when the core 95 moves away from the bobbin 83, there is a difference between the induced voltages in the two secondary coils in proportion to the extent to which the core 95 has moved away from the bobbin. This, therefore, enables the distance between the core 95 and the bobbin 83 to be detected. The relation of the displacement to the output voltage of the transformer or detector is as shown in Figure 19.

In the differential transformer of Figure 18, the cores 94 and 95 must have adequate and comparable eddy current losses. This requirement can be met by making these cores 94 and 95 of the same material. The cpre 95 does not have to be in the form of a flat disc, and could for example be constituted by a part of a large structure or machine, or may be a moving flat plate. In that case, the output voltage of the differential transformer represents a distance between the relevant member and the bobbin.If a product is allowed to pass below this differential transformer, any variation in the height of the product can be detected from the output voltage of the differential transformer, which shows the distance between the bobbin and the product Such non-contact measurement may be required in various cases such as where an object to be measured is moving at a high speed, where the object is at a high temperature, where the object is vibrating, or where the object is so soft that it cannot be touched without being deformed. The differential transformer of Figure 18 is highly advantageous for such applications because it allows accurate measurement 6f an object without contacting that object.

The transformers of this third embodiment have displacement and the output voltage adequately in proportion because of the use of a conductive metal for the core, which ensures the symmetry in the distribution of the magnetic lines of force is not ruined on the cores being displaced. A sufficiently high value of eddy current losses will be produced in the core, since the primary coil is arranged to be energised with a high frequency current. Therefore, a high output voltage can be obtained. Particularly, in the case of a differential transformer of short stroke and with small coil width, it is much easier to keep the core displacement and the output voltage in proportion using cores as specified, than in the case of a differential transformer using a core made of a ferromagnetic material.

The use of conventional differential transformers has suffered from the fear that their cores might be attracted by magnetic attraction produced by the energising current. In the case of this third embodiment of the invention, the use of a high frequency results in less magnetic attraction. Besides, the core of low permeability metal is not susceptible to the magnetic forces, eliminating the fear of attraction. Therefore, such differential transformers are highly advantageous for applications where precision measurement is to be accomplished, especially when using a light-weight core such as of aluminium. Furthermore, since the output voltage of the differential transformer is obtained in the form of a rectified and smoothed d.c. voltage, the response frequency is about one tenth of the energising frequency which itself is a relatively high frequency.Thus a sufficiently high response frequency can be obtained to permit measurement of high speed movements.

The following examples are given to illustrate specific details of differential transformers according to the third embodiment of the invention. Examples 1 to 5 are of differential transformers having the structural arrangement shown in Figures 9 or 13, Example 6 is of a transformer having the structural arrangement shown in Figure 14, and Example 7 is of a transformer having the structural arrangement shown in Figure 18.

In the Examples given below, there are specified the inside and outside diameters of the bobbin; the width of the coil recesses; bottom diameter of the recesses; the number of turns, number of layers and winding width of the primary coil; the number of turns and number of layers of each secondary coil; the energising frequency; and the material, outside diameter, inside diameter and length of the core or casing.

The differential transformers used in these Examples were manufactured as follows. The bobbins were made from a plastics material, to have a distance between the two recesses of 2 mm in Examples 1 to 5, 5 mm in Example 6 and about 20 mm in Example 7. In each example, the primary coil was prepared by winding a polyurethane wire of 0.10 mm diameter in the two recesses of the bobbin, in a continuous manner.

The secondary coils were prepared by separately winding polyurethane wires of the same diameter directly over the primary coil. The primary coil was energised with a 1 volt alternating current sine wave. It was then confirmed that the displacement of each core or casing was substantially in a correctly proportional relation to the output voltage.

Examples 1 and 2 are of high frequency type differential transformers with a coil diameter measuring 15 mm. A sufficiently high frequency was used to give high output voltages, which renders the transformers easy to use.

The examples include data obtained from transformers using cores or casings made of aluminium and brass. The use of iron cores or casings gave output voltages about one half or less than those obtained by' the use of aluminium or brass cores or casings of the same dimensions.

Example 1 Bobbin - Outside diameter: 18 mm Inside diameter: 13.2 mm Recess Width: 5 mm Bottom diameter of recess: 15 mm Primary coil - Number of turns: 76 Number of layers: 1 Winding width: 12 mm Secondary coils - Number of turns: 100 Number of layers: 3 Frequency used - 400 kHz and 600 kHz 1) Aluminium core - Outside diameter: 12.8 mm Inside diameter: 8 mm Length: 7 mm Proportioning range - +4 mm Maximum output - 1.1 V with frequency 400 kHz and 3.0 V with 600 kHz 2) Brass core - Outside diameter: 12.8 mm Inside diameter: 8 mm Length: 7 mm Proportioning range - +4 mm Maximum output - 0.9 V with frequency 400 kHz and 2.8 with 600 kHz 3) Aluminium casing - Outside diameter: 30 mm Inside diameter: 19 mm Length: 7 mm Proportioning range - +4 mm Maximum output - 0.5 V with frequency 400 kHz and 1.4 V with 600 kHz 4) Brass casing - Outside diameter: 30 mm Inside diameter: 19 mm Length: 7 mm Proportioning range - +4 mm Maximum output - 0.5 V with frequency 400 kHz and 1.4 V with 600 kHz Example 2 Bobbin - Outside diameter: 18 mm Inside diameter: 13.2 mm Recess width: 3 mm Bottom diameter of recess: 15 mm Primary coil - Number of turns: 40 Number of layers: 1 Winding width: 8 mm Secondary coils - Number of turns: 63 Number of layers: 4 Frequency used - 800 kHz and 1 MH 1) Aluminium core - Outside diameter: 12.8 mm Inside diameter: 8 mm Length: 5 mm Proportioning range - +2 mm Maximum output - 3.5 V with frequency 800 kHz and 0.5 V with 1MHz 2) Brass core - Outside diameter: 12.8 mm Inside diameter: 8 mm Length: 5 mm Proportioning range - +2 mm Maximum output - 3.5 V with frequency 800 kHz and 0.5 V with 1 MHz 3) Aluminium casing - Outside diameter: 30 mm Inside diameter: 19 mm Length: 5 mm Proportioning range - 12 mm Maximum output - 1.2 V with frequency 800 kHz 4) Brass casing - Outside diameter: 30 mm Inside diameter: 19 mm Length: 5 mm Proportioning range - +2 mm Maximum output - 1.1 V with frequency of 800 kHz In each of the following Examples 3,4 and 5, the differential transformer was prepared by winding the primary coil in one or two layers over the entire axial length of the recesses in a plastic bobbin of 10 mm outside diameter and 6.5 mm inside diameter, the bottom diameter of the recesses being 8 mm, The two secondary coils were wound directly over the primary coil with a specified number of turns, starting from the centre of the primary coil, one to the left and the other to the right. The coil diameter was reduced and the coil winding width was increased for a longer measuring range (or stroke). With the stroke arranged to be longer in this manner, the number of turns increases, and accordingly, the frequency used was lower.In addition to the data obtained from the use of aluminium and brass cores, data was obtained by using iron cores, for reference. The values of the maximum output voltages obtained with iron cores were lower than those obtained with aluminium and brass cores, even in the examples using low frequencies.

Example 3 Bobbin - Outside diameter: 10 mm Inside diameter: 6.5 mm Recess width: 7 mm Bottom diameter of recess: 8 mm Primary coil - Number of turns: 260 Number of layers: 2 Winding width: 16 mm Secondary coils - Number of turns: 190 Number of layers: 4 Frequency used - 100kHz 1) Aluminium core - Outside diameter: 6.3 mm Round bar Length: 13 mm Proportioning range - +5 mm Maximum output - 0.5 V 2) Brass core - Outside diameter: 6.3 mm Round bar Length: 13 mm Proportioning range - +5 mm Maximum output -0.4V 3) Iron core - Outside diameter: 6.3 mm Round bar Length: 13 mm Proportioning range - t5 mm Maximum output - 0.25 V Example 4 Bobbin - Outside diameter: 10 mm Inside diameter: 6.5 mm Recess width: 14 mm Bottom diameter of recess: 8 mm Primary coil - Number of turns: 500 Number of layers: 2 Winding width: 30 mm Secondary coils - Number of turns: 370 Number of layers: 4 Frequency used - 50 kHz and 100 kHz 1) Aluminium core - Outside diameter: 6.3 mm Round bar Length: 25 mm Proportioning range -+10 mm Maximum output - 0.4 with frequency 50 kHz and 0.6 with frequency 100 kHz 2) Brass core - Outside diameter: 6.3 mm Round bar Length: 25 mm Proportioning range - 10 mm Maximum output - 0.4 V with frequency 50 kHz and 0.6 with frequency 100 kHz 3) Iron core - Outside diameter: 6.3 mm Round bar Length: 25 mm Proportioning range - +10 mm Maximum output - 0.35 V with frequency 50 kHz and 0.5 V with frequency 100 kHz Example 5 The number of turns of the primary coil was the same as in Example 4.However, the number of turns per unit length was one half of that of Example 4 because the winding width was about twice as long as with the latter. Accordingly, a higher frequency was used.

Bobbin - Outside diameter: 10 mm Inside diameter: 6.5 mm Recess width: 29 mum Bottom diameter of recess: 8 mm Primary coil - Number of turns: 500 Number of layers: 1 Winding width: 60 mm Secondary coils - Number of turns: 400 Number of layers: 2 Frequency used - 200 kHz 1) Aluminium core - Outside diameter: 6.3 mm Round bar Length: 50 mm Proportioning range - +20 mm Maximum output - 1.0 V 2) Brass core - Outside diameter: 6.3 mm Round bar Length: 50 mm Proportioning range - t20 mm Maximum output - 0.9 V 3) Iron core - Outside diameter: 6.3 mm Round bar Length: 50 mm Proportioning range - t20 mm Maximum output - 0.6 V Example 6 A differential transformer arranged as shown in Figure 14. Two recesses of 3 mm width and 10 mm bottom diameter were provided in cylindrical tubular bobbins made of a plastics material. The primary coil was wound on the bottoms of the grooves. Then, secondary coils were wound directly over the primary coil portions. The transformer was arranged to measure the displacement of a disc-shaped core made of aluminium, with the measuring range set at +1 mm.

Primary coil - Wound with 35 turns in 2 layers on the bottom of each recess provided in left and right bobbins.

Secondary coils - Wound with 60 turns in 4 layers in the recesses of the left and right bobbins.

Frequency used - 900 kHz Aluminium core - Outside diameter: 20 mm Thickness: 4 mm Measuring range - +1 mm Maximum output - 1.2 V Example 7 A differential transformer arranged as shown in Figure 18. The primary coil was wound on the bottom4 of two recesses each of 3 mm width and 18 mm bottom diameter, provided in a cylindrical tubular bobbin made of a plastics material. The secondary coils were wound directly over the primary coil.

With one of the cores secured to the bobbin, in close contact therewith, data were obtained by allowing the other core to move, changing its position relative to the bobbin. The moving core does not have to be disc-like and may be replaced by a part of a machine or other structure, so long as such a part is made of the same kind of material measuring at least 3 mm in thickness, so as to obtain the same results.

Primary coil - Wound in two recesses, 30 turns in each recess, and in 2 layers Secondary coils - Wound with 60 turns in 4 layers, in each of the two recesses Frequency used - 800 kHz Aluminium core - 25 mm outside diameter, and to be displaced 4 mm thickness Measuring range - 2 mm Maximum output - 1 V Fourth Embodiment Figures 20 to 23 show differential transformers arranged in accordance with a fourth embodiment of the present invention. In the differential transformer shown in Figure 20, there is an auxiliary core secured to the main core, which auxiliary core extends through the ring-like main core and is arranged also to carry the main core. A modification of this arrangement is as shown in Figure 23, where the main core is arranged to be slidable on the auxiliary core. The former is easier to manufacture and will be more practicable on account of that.

Referring to Figures 20 and 21, a primary coil 100 and secondary coils 101 and 102 are wound on a bobbin 103 having a hollow central bore 104. A main core 105 is arranged to be movable axially within the bore 104, and the coil connections are as shown in Figure 22.

The bobbin 103 which is made of a plastics material is provided with two recesses 106 and 107 of the same dimensions, and an axial slit 108 through which the coil leads are taken out. The primary coil 100 is wound round the bottom portions of the recesses 106 and 107 in a continuous manner over their axial widths, in several layers. The secondary coils 101 and 102 are wound directly over the primary coil 100, onesecondary coil 8 in each of the recesses 106 and 107 respectively, with a specified number of turns in each coil.

The main core 105 is made to have a ring-like shape, out of a conductive metal of low permeability. An auxiliary core 109, made of ferromagnetic material with a bar-like shape, extends through and is rigidly secured to the core 105. The auxiliary core thus serves the combined purpose of carrying the cdre 105, so that the core 105 and the auxiliary core 109 are movable, in unison within the bore 104 of the bobbin, along the axis thereof. The auxiliary core 109 must of course be longer than the entire length of the primary coil 100. The primary coil 100 is provided with lead wires 110 and the secondary coils 101 and f02with lead wires 111 and 112. The primary coil 100 is arranged to have a alternating voltage at a high frequency impressed thereon.

With the differential transformer arranged as above described, lines of magnetic force produced by the primary coil 100 induce voltages in the two secondary coils 101 and 102 wound directly over the primary coil 100. When the core 105 is situated midway between the secondary coils 101 and 102, voltages of equal magnitudes are induced in the two secondary coils. This results in a zero voltage between the lead wires 11 f and 112. When the core 105 moves to the left as viewed in Figure 20, the voltage induced irithe secondary coil 101 is diminished while the voltage induced in the other secondary coil 102 is increased. Then, a voltage representative of the difference between the two induced voltages is produced between the lead wires 111 and 112.Conversely, when the core 105 moves to the right, the voltage induced in the secondary coil 101 is increased while the voltage induced in the secondary coil 102 is diminished, thus resulting in a voltage representative of the difference between the two induced voltages appearing between the twd lead wires.

With the displacement of the core 105 plotted on the abscissa and the voltage corresponding thereto plotted on the ordinate, the relation of the voltage to the displacement resembles a letter V, as shown in Figure 12, in much the same as with conventional differential transformers.

In the case of the differential transformer shown in Figure 23, the bobbin 103 has a blind central bore 113, and one end of an auxiliary core 114 is secured thereto. The auxiliary core 114 is made of a feirromagnetic material and extends part-way along the bore 113 from the closed end thereof. A core 115, made a conductive metal material of low permeability, is attached to an end of a supporting tube 11is an- 'ade,el'a plastics material, and is arranged to be slidable on the auxiliary core 114. Other parts of this differential transformer are identical with the corresponding parts of the transformer shown in Figure 20 and are indicated by the same reference numerals.With the exception that the core 1 is arranged to be slidable on the auxiliary core 114, the differential transformer of Figure 23 operates in the same manner as that shown in Figure 20.

As described above, the differential transformers according to the fourth embodiment of the invention have a ring-shaped core of a conductive metal material of low permeability, and an auxiliary core made of a ferromagnetic material of high permeability. The auxiliary core extends for the entire length of the primary coil, and passes through the central opening of the ring-shaped main core. This arrangement ensures that the symmetry of the distribution of magnetic force lines of the primary coil can be retained despite displacement of the main core, and that the distribution of magnetic force lines within the bore of the bobbin can be homogenised owing to the presence of the ferromagnetic auxiliary core. Therefore, the displacement of the main core and the output voltage can be kept in a correct proportional relation to each other.

Furthermore, since the magnetic flux density is enhanced by the ferromagnetic auxiliary core when the primary coil is being energised with high frequency current, the values of the eddy current losses in the main core become larger. Therefore, the output voltage which can be obtained is relatively large. It is a salient advantage, particularly for a differential transformer with coils of small width and diameter, that the output voltage can be prevented from becoming too low, in addition to maintaining adequate linearity.

To illustrate further the specific details of the fourth embodiment of this invention, examples are given below, together with some comparison examples for reference. In each of the examples, an auxiliary core made from an iron bar extended through and secured to a ring-shaped aluminium main core. In the comparison examples, an aluminium core having a ring shape and iron bar core were used.

In the following examples of differential transformers there are specified the inside diameter, outside diameter, width of the recesses and the recess bottom diameter for the bobbin; the number of turns, number of layers and winding width of the primary coil; the number of turns of the secondary coils; the energising frequency; and the material, outside diameter, inside diameter and length of the main core.

The bobbin was made of a plastics material. The distance between the two recesses of the bobbin was 2 mm. In each example, the primary coil was formed by winding a polyurethane wire of 0.10 mm in diameter into the two recesses of the bobbin, in a continuous manner throughout the two recesses. Then, the secondary coils were formed by separately winding polyurethane wires of the same diameter directly over the primary coil. The primary coil was energised with a sine wave alternating current of 1 volt. Furthermore, the frequency used was set at the value that gave the best linearity (about 1 - 0.5% or better).

It was then confirmed that the displacement of the core was substantially in a correct proportional relation to the output voltage.

In Example 1, the coil diameter was larger than in Example 2 and accordingly a lower frequency was used.

Also, in Example 1, the diameter of the core was relatively large, measuring 9.8 mm. Accordingly, in the comparison examples included in Example 1 below, the output voltage obtained with the use of an aluminium core by virtue of the eddy current losses was higher than the output voltage obtained with the use of an iron core, by virtue of the permeability. By contrast, in Example 2 where the core diameter was smaller (measuring 4 mm), the output voltage obtained with the use of an aluminium core was lower than the output voltage obtained with the use of an iron core. On the other hand, with an aluminium core combined with an auxiliary iron core in accordance with the fourth embodiment of the invention, the output voltage was larger and the residual zero voltage was smaller than with the comparison example transformers.

Example 1 Bobbin - Outside diameter: 15 mm Inside diameter: 10 mm - Recess width: 3 mm Recess bottom diameter: 11 mm Primary coil - Number of turns: 165 Number of layers: 4 Winding width: 8 mm Secondary coils Number of turns: 250 (two coils) Frequency used - 150 kHz 1) Comparison Aluminium core - Outside diameter: 9.8 mm Inside diameter: 5 mm Length: 5 mm Proportioning range - 12 mm Maximum output - 1.05 V Residual zero voltage - Not exceeding 30 mV 2) Comparison Iron core - Outside diameter: 9.8 mm Round bar Length: 5 mm Proportioning range - +2 mm Maximum output - 0.8 V Residual zero voltage - Not exceeding 50 mV 3) Core according to the fourth embodiment: Ring-shaped main aluminium core - Outside diameter: 9.8 mm Inside diameter: 6 mm Length: 5 mm Auxiliary iron core - Outside diameter: 6 mm Inside diameter: 3.2 mm Length: 50 mm Proportioning range - +2 mm Maximum output - 1.3 V Residual zero voltage - Not exceeding 5 mV Example 2 Bobbin - Outside diameter: 8 mm Inside diameter: 4.2 mm Recess width: 3 mm Recess bottom diameter: 5 mm Primary coil - Number of turns: 160 Number of layers: 4 Winding width: 8 mm Secondary coils - Number of turns: 224 (two coils) Frequency used - 300 kHz 1) Comparison aluminium core - Outside diameter: 4 mm Inside diameter: 3 mm Length: 5 mm Proportioning range - t2 mm Maximum output - 0.65 V Residual zero voltage - Not exceeding 35 mV 2) Comparison iron core - Outside diameter: 4 mm Round bar Length: 5 mm Proportioning range - ~2 mm Maximum output -iV Residual zero voltage - Not exceeding 50 mV 3) Core according to the fourth embodiment Ring-shaped main aluminium core - Outside diameter: 4 mm Inside diameter: 3 mm Length: 5 mm Auxiliary iron core - Outside diameter: 3 mm Round bar Length: 50 mm Proportioning range - +2 mm Maximum output - 1.4 V Residual zero voltage - Not exceeding 10 mV

Claims

1. A differential transformer comprising a primary coil wound in at least one layer around a bobbin so as to extend over tire entire length of the bobbin, and two secondary coils each wound directly over the primary coil with a prescribed number of turns, one secondary coil being wound from the central portion of the primary coil to one end thereof, and the other secondary coil being wound from the central portion of the primary coil to the other end thereof, and the primary coil being adapted to be energized at a frequency between 50 kHz and 2 MHz.

2. A differential transformer according to claim 1, wherein the number of turns in each secondary coil is from 0.5 to 5 times as many as the number of turns of the primary coil.

3. A differential transformer according to claim 2, wherein the number of turns in each secondary coil is from 0.5 to 2 times as many as the number of turns in the primary coil.

4. A differential transformer according to any of claims 1 to 3 wherein the bobbin has an axial bore extending therethrough, and there is a ferromagnetic core mounted on a non-magnetic rod within said bore, for movement along the axis of the bore.

5. A differential transformer assembly according to claim 4, wherein there is provided an oscillator arranged to supply an a.c. energizing voltage at a frequency of from 50 KHz to 2 MHz to the primary coil, and the two secondary coils are differentially connected thereby to yield an output voltage proportional to the displacement of the core from a central position thereof within the bobbin bore.

6. A vibrometer of the displacement type, comprising a differential transformer according to any of the preceding claims wherein the bobbin is formed as a tubular body in which a vibrating body is supported by spring means so as to be movable within the tubular body along the axis thereof, there being a fluid within the tubular body to attenuate the vibrations of the vibrating body dnd the primary and secondary coils being wound around the outer circumference of the tubular body.

7. A vibrometer according to claim 6, wherein the space within the tubular body is completelyfflled with fluid and the tubular body is tightly sealed to form a closed chamber containing the fluid and the vibrating body.

8. A vibrometer according to claim 7, wherein the vibrating body is supported between two springs each extending to one of the closed end faces respectively of the tubular body.

9. A vibrometer according to any of claims 6 to 8, wherein the primary and secondary windings are formed over a part only of the axial length of the tubular body.

10. A differential transformer having a core or casing made from a conductive metal mate-ria-sf low permeability having a specific magnetic susceptibility of the order of from 10-3 and 10-6 and adapted-to be connected to a movable object the movement of which is to be detected by the transformer, a primary coil disposed around the axis of movement of the core or casing and adapted to be energized wfth alternating current the frequency of which lies in the range of from 50 kHz to 2 MHz, and two secondary coils associatd with the primary coil and arranged to produce a voltage difference proportional to the displacement of the core or casing.

11. A differential transformer according to claim 10, wherein the core or casing is made from silver, copper, aluminium or alloys of silver, copper or alluminium.

12. A differential transformer according to claim 10 or 11, wherein the primary coil is wound with one or two layers on the bottoms of two annular recesses or grooves formed in a bobbin, and the two secondary coils are wound one in each recess or groove respectively directly over the primary coil therewithin.

13. A differential transformer according to any of claims 10 to 12, wherein the core is of disc-like shape, and the primary coil is divided into two parts which are wound on two bobbins disposed one to each side respectively of the disc-shaped core, the two parts of the primary coil being series-connected and there being two secondary coils wound directly one over each part of the primary coil respectively which secondary coils are differentially-connected.

14. A differential transformer according to any of claims 10 to 12, wherein the core is divided into two core-parts, and the two core parts are disposed one to each side respectively of the bobbin.

15. A differential transformer according to claim 14, wherein one of the two core-parts is secured to the bobbin in close contact therewith while the other core-part is arranged for movement by a moveable object the displacement of which is to be detected.

16. A differential transformer according to claim 15, wherein said other core-part is constituted by a part of the object the displacement of which is to be detected, whereby such displacement may be detected without contact between the object and the transformer.

17. A differential transformer according to any of the claims 10 to 12, wherein the transformer is provided with an annular core, and there is a bar or tube of a ferromagnetic material having specific magnetic susceptibility of the order of from 103 to 1 o6 which bar or tube is mounted to serve as an auxiliary core for the transformer, the auxiliary core extending through the annular core.

18. A differential transformer according to claim 17, wherein the auxiliary core is secured to the annular core to serve as supporting rod for that core, whereby the two cores may move in unison.

19. A differential transformer according to claim 17, wherein the annular core is arranged to slide on the auxiliary core.

20. A differential transformer assembly according to any of claims 10 to 19, wherein there is provided an oscillator arranged to supply an a.c. energizing voltage at a frequency of from 50 kHz to 2MHz to the prirnary coil, and the two secondary coils are differentially connected thereby to yi;;ld an output voltage proportional to the displacement of the core from a central position thereof within the bobbin bore:

21. A differential transformer according to claim land substantially as hereinbefore described with reference to Figures 3 to 6 of the accompanying drawings.

22. A vibrometer accordirig to claim 6 and substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

23. A differential transformer according to claim 10 and substantially as hereinbefore described with reference to any of Figures 9to 11,13, and t5,16 and 17,18, 20 and 21, 17, 18,20and21,an-d23oftNeaccornaryng drawings. - t