Displacement - Electricity Transducer

1. Field of the Invention

The present invention relates to a device for converting a mechanical displacement into an electric signal. More particularly, it relates to a displacement - electricity transducer which can simply detect a displacement at a very high sensitivity over a long term of use even where, as in the throttle valve of an automobile, a transport plane, or the like, the angular displacement is large and service temperature conditions are severe.

2. Description of the Prior Art

In recent years, with the growth of the automobile industry, the exhaust gas emitted from the automobile engine has caused a serious social problem as a source of environmental hazards. Therefore, a tendency has been seen to control the exhaust gas by any method such as a catalytic method. Regulation by legislation has also been considered in earnest. As a radical expedient, however, the development of a clean engine is necessary. It is believed the first essential to make the exhaust gas clean is to control the combustion within the engine.

To this end, a variety of engines are being developed and put into practical use as "clean" engines. These engines are not perfect, however. In order that the combustion explosion itself may be controlled so as to always keep the exhaust gas perfectly clean, it is necessary to further adopt a centralized control system based on an electronic circuit including a mini-computer.

That is, the prime aim of the future auto development will be to make it possible that all the sections of an automobile are electronically subject to centralized automatic control. In order to maintain the engine in the optimum state as a part of the means for achieving the aim, there has been a strong demand for an electronic detector which monitors and controls fuel injection and the air intake controller of the engine. In other words, there has been a strong demand for a device which precisely detects the displacement of the throttle valve for regulating the fuel and air quantities in the automobile engine.

Automobiles are used over extensive regions from cold to tropical climates. Further, the difference between temperatures at starting and at a steady running of the engine is large, and the service temperature range is as wide as from -40.degree. C. to +120.degree. C. The conditions of the service environment are naturally extremely severe irrespective of bad roads, dust, rain, snow etc. Accordingly, the displacement detector for automobile engines must be a structure which satisfactorily takes the thermal resistance, vibration resistance, moisture resistance corrosion resistance etc. into consideration and must simultaneously be a structure which is capable of a high sensitivity detection of the displacement. Moreover, the displacement detector must be structurally simple in order to be inexpensive in production and must be durable in order to repeatedly detect, for several million times, the displacements which vary frequently on account of use.

As a principal device of the displacement - electricity transducer fulfilling the above requisites, a variable inductor has been proposed which comprises a movable magnetic core and a fixed magnetic core oppositely arranged so as to have prescribed gaps in at least two places, both magnetic cores substantially forming two magnetic circuits each including one gap, coils being wound around parts of the respective magnetic circuits, so that when the movable magnetic core is relatively displaced, while being held at a predetermined spacing with respect to the fixed magnetic core, the substantially opposite area of the gap portion in one magnetic circuit increases, while the substantially opposite area of the gap portion in the other magnetic circuit decreases. (copending U.S. Pat. application Ser. No. 485,626, commonly assigned).

In the variable inductor, however, the magnetic reluctance of the magnetic core is not perfectly negligible in comparison with that of the gap, and the exciting coil is wound around a part not common to the two magnetic paths. For these reasons, a third magnetic path, other than the two magnetic paths, is formed, and mutual induction between the respective magnetic path arises, and, although slight is a problem.

An object of the present invention is to provide a displacement - electricity transducer which eliminates the problem of the prior art and which can convert a displacement into an electrical signal by a simple construction.

More concretely, an object is to provide a displacement - electricity transducer which can detect a displacement at a very high sensitivity over a long term of use under conditions of large angular displacements and a wide range of service temperatures, and which is free from the influence of the mutual induction between the respective magnetic paths constituting the detector.

The displacement - electricity transducer of the present invention for accomplishing this object is characterized by a movable magnetic core and a fixed core oppositely arranged so as to have predetermined gaps in at least two places. Both magnetic cores form substantially two magnetic circuits each including one gap, a first coil being wound around a part common to the two magnetic circuits, at least one second coil being wound around a part not common to the two magnetic circuits, so that when the movable magnetic core is displaced relative to the fixed magnetic core as the spacing of the gap portion is held constant, the substantial opposite area of the gap portion in one of the magnetic circuits increases, while the substantial opposite area of the gap portion in the other magnetic circuit decreases. An A.C. supply voltage is applied across both terminals of the first coil so as to derive the terminal voltage of the second coil.

The other objects and features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 is a side view of an embodiment of the present invention, and FIGS. 2 and 3 are characteristic curve diagrams of this embodiment;

FIG. 4 is a view showing a modification of a fixed magnetic core in the embodiment of FIG. 1, while FIG. 5 is a characteristic curve diagram in the case where the modified magnetic core is substituted for the fixed magnetic core in the embodiment of FIG. 1;

FIGS. 6, 7 and 8 are a sketch, a side view and a top view of another embodiment of the invention, respectively;

FIGS. 9 and 10 are a sketch and a side view of still another embodiment of the invention, respectively;

FIGS. 11 and 12 are sectional views each showing a modification of magnetic cores in the embodiment of FIGS. 9 and 10;

FIGS. 13 and 14 are a sketch and a top view of yet another embodiment of the invention, respectively;

FIGS. 15, 16 and 17 are a sketch, a top view and a magnetic core-sectional view of a further embodiment of the invention, respectively;

FIGS. 18 and 19 are a sectional view and a top view of a still further embodiment of the invention, respectively;

FIGS. 20, 21 and 22 are diagrams for explaining the output characteristics of a detector according to the present invention;

FIG. 23 is a sketch of another embodiment of the present invention, while FIG. 24 is a diagram of the output characteristics of the device in FIG. 23;

FIG. 25 is a diagram showing an example of a power source circuit in the detector according to the present invention;

FIG. 26 is a diagram of the output voltage-versus-temperature characteristics of a prior-art circuit and the circuit in FIG. 25; and

FIG. 27 is a diagram of the output voltage-versus-feedback resistance characteristic of the circuit in FIG. 25.

An embodiment of the present invention will be described in connection with FIGS. 1 and 2. In the figures, reference numerals 1 and 2 designate a fixed magnetic core and a movable magnetic core made of a material of high permeability, respectively. The movable magnetic core 2 is displaced in the direction of the arrow while maintaining the width of the gaps 9 and 10 constant. The primary coil 3 is wound around a central columnar part 6 of the fixed magnetic core 1, while the secondary coils 4 and 5 are wound around columnar parts 15 and 16 at both ends of the fixed magnetic core 1. Numerals 7 and 8 indicate magnetic paths. The voltage of an A.C. power source 11 is applied across the terminals of the primary coil 3.

Let l.sub.1 and l.sub.2 denote the effective magnetic path lengths of the magnetic paths 7 and 8, S.sub.l and .mu. denote the mean cross-sectional area and the permeability of the magnetic cores, .mu..sub.o and g denote the permeability and the width of the gaps, S.sub.g1 and S.sub.g2 denote the substantially opposite areas between the movable magnetic core 2 and the fixed magnetic core 1 with the gaps 9 and 10 therebetween, N denote the number of turns of the primary coil 3, and N.sub.1 and N.sub.2 denote the numbers of turns of the secondary coils 4 and 5. Assuming that the movable magnetic core 2 is displaced in the direction of the arrow in close adherence to the central columnar part 6 of the fixed magnetic core 1, the magnetic reluctances R.sub.G1 and R.sub.G2 of the respective magnetic paths 7 and 8 become: ##EQU1##

Using for the magnetic cores a high permeability material with which: ##EQU2## that is, ##EQU3## then, from Eqs. (1) and (2), ##EQU4## Further, both the magnetic cores are formed so that, even when the movable magnetic core 2 is displaced relative to the fixed magnetic core 1,

may always become constant. Then, the magnetic reluctance R.sub.G to which a magnetic flux interlinked with the primary coil 3 is subject is equal to the parallel connection consisting of the magnetic reluctances R.sub.G1 and R.sub.G2, and therefore becomes: ##EQU5## Accordingly, the inductance L of the primary coil 3 becomes constant as follows: ##EQU6## where I denotes a current flowing through the primary coil 3, and E.sub.G a magnetomotive force in the primary coil. Consequently, the input impedance of the primary coil 3 is not affected by the displacement of the movable magnetic core 2, and it is always constant. It is apparent from the above description that, in the device according to the present invention, the power source 11 does not suffer from any fluctuation by the displacement of the movable magnetic core 2.

Magnetic fluxes .PHI..sub.1 and .PHI..sub.2 passing through the respective magnetic paths 7 and 8 are: ##EQU7## Therefore, the output voltages V.sub.1 and V.sub.2 of the respective secondary coils 4 and 5 become: ##EQU8## where V denotes the applied voltage of the primary coil. It is thus apparent that the output voltages V.sub.1 and V.sub.2 of the secondary coils 4 and 5 are proportional to the opposite S.sub.g1 and S.sub.g2, respectively. It is also apparent that when N.sub.1 = N.sub.2, V.sub.1 + V.sub.2 = constant.

Since the opposing surfaces of the movable magnetic core 2 and the fixed magnetic core 1, which constitute the principal parts determining the displacement detection are arranged so as to hold the gaps 9 and 10 therebetween and not to be in contact with each other, the abrasion of the displacement detecting parts due to friction need not be considered. Assuming that the movable magnetic core 2 slides in close adherence to the central columnar part 6 of the fixed magnetic core 1, the abrasion due to the slide is unavoidable. However, when ferrite is used for the magnetic core, the abrasion is slight. In addition, even in the presence of the abrasion, if it is uniform, it will not affect the detection sensitivity in the first-order approximation as apparent from Equations (10) and (11). Furthermore, by making the contact area as large as is permissible in configuration, the abrasion of the contact part can be lessened. It is apparent from the foregoing that the gaps 9 and 10 at the displacement detecting parts are kept at substantially a constant width, so that the displacement - electricity transducer illustrated and described in this embodiment has a structure enduring repeated slides of several million times.

Although an explanation of the abrasion has been made in the case of sliding the movable magnetic core 2 where it is in close adherence to the central columnar part 6 of the fixed magnetic core 1, it is not always necessary to hold both the magnetic cores in close adherence during sliding. In some cases, it is possible to externally support both the magnetic cores 1 and 2 so as to define a slight spacing between the central columnar part 6 and the movable magnetic core 2, and to thus fully prevent the abrasion between both magnetic cores. In this case, the inductance of the primary coil 3 is held constant and that the output voltages of the secondary coils 4 and 5 are respectively proportional to the opposite areas S.sub.g1 and S.sub.g2.

Although the two secondary coils are provided in this embodiment, a single secondary coil may be provided.

FIGS. 2 and 3 are characteristic curve diagrams of the output voltages versus the displacement of the movable magnetic core 2 at the time when, in the embodiment of FIG. 1, the number of turns of the secondary coils is N.sub.1 > N.sub.2 and N.sub.1 = N.sub.2, respectively. It is understood from these diagrams that the output voltages vary linearly relative to the displacement of the movable magnetic core 2.

When the fixed magnetic core 1 in the embodiment of FIG. 1 is formed into the configuration shown in FIG. 4, the output voltage-versus-displacement characteristic curve diagram becomes that shown in FIG. 5.

When the columnar parts at both ends of the fixed magnetic core 1 in the embodiment of FIG. 1 are shaped into triangles congruent with each other, the output voltages vary parabolically relative to the displacement. In this manner, the present invention also has the remarkable merit that a displacement - electricity transducer having any desired output voltage-versus-displacement characteristic is acquired merely by designing the geometrical shape of the pole-faces holding the gas therebetween.

In the embodiment shown in FIG. 1, the device with the power source removed is a kind of transformer. The coupling coefficient between the primary coil and the secondary coil varies in response to the displacement of the movable magnetic core. The impedance as viewed from both terminals of the primary coil is fixed in the state in which the secondary coil is open. The device can, accordingly, be called a complementary, variable coupling coefficient transformer. For the sake of simplicity, it will be hereinafter termed the variable transformer.

In embodiments to be explained hereunder, the same reference numerals are affixed to the same constituent parts as in FIG. 1, and the description of such parts is omitted.

FIGS. 6, 7 and 8 are a sketch, a side view and a top view of another embodiment of the present invention, respectively. In this embodiment, parts common to the two magnetic paths are formed into circular cylindrical shafts 6a and 14a so that a fixed magnetic core 1a and a movable magnetic core 2a oppose each other. The opposing surfaces 12a and 13a of the fixed magnetic core 1a relative to the movable magnetic core 2a bordering the gap portions of the two magnetic paths have such a shape that, as shown in FIG. 8, each is surrounded by arcs of concentric circles of different radii and two straight lines passing through the center of the circles. The opposing surfaces are arranged at positions of symmetry of 180.degree. rotation about the center. On the other hand, the opposing surface of the movable magnetic core 2a relative to the fixed magnetic core 1a bordering the gap portions has such a shape that is surrounded by semicircles concentric with the circles of the opposing surfaces 12a and 13a. The fixed magnetic core 1a and the movable magnetic core 2a can turn about the center relative to each other. In this case, the shafts 6a and 14a forming the common magnetic paths are brought into adherence as close as possible. Therefore, the magnetic reluctances of the two magnetic paths are chiefly determined by the substantial opposite areas between the opposing surfaces 12a and 13a of the fixed magnetic core 1a and the opposing surface of the movable magnetic core 2a, and the influence of the magnetic reluctance at the contact part of the shafts can be eliminated. However, some spacing must be provided between the shafts 6a and 14a in order to prevent abrasion. In this embodiment, the operating temperatures range from -40.degree. C. to +120.degree. C., and hence, a ferrite whose Curie temperature is 130.degree. C. or higher is used as the material of the fixed magnetic core 1a and the movable magnetic core 2a. Of course, other materials of high permeability can be employed. As in the embodiment of FIG. 1, the characteristics of the terminal voltages of the secondary coils 4a and 5a versus the displacement of the movable magnetic core 2a become those shown in FIGS. 2 and 3.

FIGS. 9 and 10 are a sketch and a side view showing still another embodiment of the present invention, respectively. By replacing the movable magnetic core 2a in FIG. 6 with a sectorial flat plate, the same characteristics as in FIGS. 2 and 3 are obtained. As will be understood from FIG. 9, the center part of the sectorial flat plate 2b is constructed into a shape to completely cover the circular cylindrical part 6b at the central part of the fixed magnetic core 1b. The upper surface of the circular cylindrical part 6b protrudes above the opposing surfaces 12b and 13b to the movable magnetic core 2b. When the movable magnetic core 2b is brought into close adherence to the circular cylindrical part 6b, equal gaps or clearances 9b and 10b are defined.

FIGS. 11 and 12 are sectional views of modifications of the magnetic cores shown in FIG. 9. The magnetic cores illustrated in FIG. 11 are formed so that the opposing surfaces between the central circular cylindrical part 14c of the movable magnetic core 2c and the circular cylindrical part 6c of the fixed magnetic core 1c may be wide. The magnetic core structure, accordingly, has the merit that the magnetic reluctance of the portion of the opposing surfaces is low. A spacer 20c made of a material such as Teflon is inserted into an interstice which is defined among a supporter 21c secured to the fixed magnetic core 1c, the fixed magnetic core 1c itself, and the central cylindrical part 14c of the movable magnetic core 2c. Thus, both the magnetic cores can move relative to each other. On the other hand, the magnetic core structure illustrated in FIG. 12 is composed of the movable magnetic core 2d and the fixed magnetic core 1d which are shaped like a hand-drum, and a spacer 20d which is made of Teflon or the like and which is inserted between both the magnetic cores. The opposing surfaces of the movable magnetic core 2d and the fixed magnetic core 1d at the portion for inserting the spacer are wide. Accordingly, as in the magnetic core structure in FIG. 11, this structure has the merit that the magnetic reluctance of the portion of the opposing surfaces is low.

FIGS. 13 and 14 are a sketch and a bottom plan view showing yet another embodiment of the present invention, respectively. The opposing surfaces between the fixed magnetic core 1e and the movable magnetic core 2e are arranged on concentric cylinders having different radii. Thus, the characteristics in FIGS. 2 and 3 are obtained.

FIGS. 15, 16 and 17 are a sketch, a top view and a sectional side elevation of a further embodiment of the invention, respectively. The power source is omitted in FIG. 16, while the power source and the coils are omitted in FIG. 17. The displacement - electricity transducer of this embodiment has both the magnetic cores formed into such structure that, even in case where the transducer undergoes repeated strong vibrations as in an automobile and where a relative inclination consequently arises between the movable magnetic core and the fixed magnetic core, the impedance between the terminals of the primary coil is held constant and the output terminal voltage of the secondary coil does not fluctuate. In this embodiment, 1f and 2f indicate the fixed magnetic core and the movable magnetic core made of a high permeability material, respectively. Four magnetic circuits having gaps of a determined width are formed by both magnetic cores, and parts of the four magnetic paths are commonly formed at the central circular cylinder part 6f of the fixed magnetic core 1f. The movable magnetic core 2f turns in close adherence to the central circular cylinder part 6f of the fixed magnetic core 1f which is formed to be higher than the circumferential columnar parts 15f, 15'f and 16f, 16'f of the fixed magnetic core 1f. At this time, the gaps 9f and 10f between the movable magnetic core 2f and the circumferential columnar parts 15f and 16f, and the gaps 9'f and 10'f between the movable magnetic core 2f and the circumferential columnar parts 15'f and 16'f are held constant. The primary coil 3f is wound around the central circular cylinder part 6f, while the secondary coils 4f and 4'f are respectively wound around the circumferential columnar parts 15f and 15'f. In some cases, the secondary coils are also wound around the circumferential columnar parts 16f and 16'f in order to derive two outputs. The circumferential columnar parts 15f, 15'f and 16f, 16'f are congruent, and are arranged as illustrated in FIG. 16.

The gaps 9f, 9'f and 10f, 10'f are held at the determined value g.sub.0 in the absence of an external force attributed to vibration, whereas they generally fluctuate in the presence of vibration. Let g.sub.1, g'.sub.1, g.sub.2 and g'.sub.2 denote the widths or clearances of the respective gaps 9f, 9'f, 10f and 10'f; S.sub.g1 denote each of the substantial opposite areas between the circumferential columnar parts 15f and 15'f and the movable magnetic core 2f; S.sub.g2 denote each of the substantial opposite areas between the circumferential columnar parts 16f and 16'f and the movable magnetic core; N denote the number of turns of the primary coil 3f; N.sub.0 denote each of the number of turns of the secondary coils 4f and 4'f; and .mu..sub.0 denote the magnetic permeability of the gaps. Then, the magnetic reluctances R.sub.G1, R'.sub.G1, R.sub.G2 and R'.sub.G2 of the magnetic cicuits respectively including the gaps 9f, 9'f, 10f and 10'f become, similarly to Equations (4) and (5), as follows: ##EQU9## Assuming that an increase .DELTA.g.sub.1 takes place in the gap width on account of inclination due to the vibration,

The magnetic reluctances therefore become: ##EQU10## Let it be supposed that increments .DELTA.g.sub.0 and .DELTA.g.sub.1 in the gap width are sufficiently small in comparison with the gap width g.sub.0 at the time when no external force is exerted. Then, by the first-order approximation: ##EQU11## Both the magnetic cores are formed so that, even when the movable magnetic core 2f is displaced relative to the fixed magnetic core 1f,

may always become constant.

The magnetic reluctance R.sub.G to which a magnetic flux interlinked with the primary coil 3f is subject is given by: ##EQU12## Substituting Equations (15-1), (15-2), (15-3) and (15-4) into Equation (17), ##EQU13## Accordingly, similarly to Equation (7), the inductance L of the primary coil 3f becomes: ##EQU14## It is thus apparent that, even when a relative inclination occurs between the movable core 2f and the fixed magnetic core 1f on account of a vibration, the input impedance of the primary coil 3f is not affected by the first-order term of the increment .DELTA.g.sub.1 due to inclination.

Letting I denote a current flowing through the primary coil 3f, magnetic fluxes .PHI. and .PHI.' passing through the respective gaps 9f and 9'f become, from Equations (15-1) and (15-2), as follows: ##EQU15## where E.sub.g represents a magnetomotive force which is induced by the current I. The output voltages V.sub.1 and V'.sub.1 of the secondary coils 4f and 4'f having the number of turns N.sub.0 become: ##EQU16## A lead wire 17f is connected so that the voltages V.sub.1 and V.sub.2 may be added in series. An output voltage V at an output terminal 18f accordingly becomes: ##EQU17## It is thus understood that, even when a relative inclination occurs between the movable magnetic core 2f and the fixed magnetic core 1f due to vibration, the output voltage V at the output terminal 18f is not affected by the first-order term of the increment .DELTA.g.sub.1 due to inclination.

FIGS. 18 and 19 are a sectional side elevation and a top view of a still further embodiment, respectively. In the figures, the power source for applying the A.C. voltage to the terminals of the primary coil 3g is omitted. The movable core 2g and two fixed magnetic cores 1g and 1'g are made of a material of high permeability. The magnetic cores form two magnetic paths, which have respectively two gaps 9g and 9'g and 10g and 10'g with Teflon inserted therein. Shown at 19g is a member of a nonmagnetic material, which couples and fixes the fixed magnetic cores 1g and 1'g. The hand-drum-shaped movable magnetic core 2g, in which both sides of its cylinder part are formed to be semicircular, turns relative to the fixed magnetic cores 1g and 1'g while keeping the gap widths of the gaps 9g, 9'g and 10g, 10'g constant. The primary coil 3g is wound around the cylinder part of the hand-drum-shaped movable magnetic core 2g. The secondary coil 4g is wound around the fixed magnetic core 1g.

As in the embodiment shown in FIG. 15, this embodiment has the merit that even where the relative inclination appears between the movable magnetic core 2g and the fixed magnetic cores 1g and 1'g by reason of vibration, the impedance between the terminals of the primary coil 3g is held constant, the output terminal voltage of the secondary coil 4g being also held constant, and that even when a displacement in the vertical direction arises between both the magnetic cores, the impedance and the output terminal voltage are constant.

Where an external force due to the vibration is not exerted, the gaps 9g and 10g are held at a predetermined width g.sub.01, and the gaps 9'g and 10'g at a predetermined width g.sub.02. In general, however, the gap widths fluctuate in the presence of vibration.

Let g.sub.1, g'.sub.1, g.sub.2 and g'.sub.2 denote the widths of the respective gaps 9g, 9'g, 10g and 10'g; S.sub.g1 denote each of the substantial opposite areas of the gaps 9g and 9'g formed by the fixed magnetic core 1g and the movable magnetic core 2g; S.sub.g2 denote each of the substantial opposite areas of the gaps 10g and 10'g formed by the fixed magnetic core 1'g and the movable magnetic core 2g; N denote the number of turns of the primary coil 3g; N.sub.0 denote the number of turns of the secondary coil 4g; and .mu..sub.0 denote the magnetic permeability of the gaps. Then, similarly to the derivation of Equations (4) and (5), the magnetic reluctance R.sub.G1 of the magnetic circuit including the gaps 9g and 9'g becomes: ##EQU18## while the magnetic reluctance R.sub.G2 of the magnetic circuit including the gaps 10g and 10'g becomes: ##EQU19## Assuming that an increase .DELTA.g.sub.0 in the gap width takes place in the vertical direction on account of the vibration and that an increase .DELTA.g.sub.1 in the gap width further takes place on account of the inclination, and putting g.sub.01 + g.sub.02 = g.sub.0,

Therefore, the magnetic reluctances become: ##EQU20## The magnetic cores are so formed that, even when the movable magnetic core 2g is displaced relative to the fixed cores 1g and 1'g,

is always constant. In consequence, the magnetic reluctance R.sub.G, to which a magnetic flux interlinked with the primary coil 3g is subject, becomes (and it is not affected by .DELTA.g.sub.0 and .DELTA.g.sub.1): ##EQU21## It is, accordingly, apparent that the inductance of the primary coil 3g is not affected by vibration. Letting I denote a current flowing through the primary coil 3g, the output terminal voltage V of the secondary coil 4g becomes: ##EQU22## It is thus understood that the output terminal voltage V is free from the influence of vibrations.

As previously stated, teflon or the like material of low permeability is inserted into the gaps 9g, 9'g, 10g and 10'g. Accordingly, the magnetic cores 1g an 1'g and the magnetic core 2g are out of contact, and variations in the magnetic circuits, namely, variations in the inductance of the primary coil and the output voltage of the secondary coil attributed to the abrasion of the magnetic cores are preventable. Further, even when Teflon or the like material wears away, the inductance of the primary coil and the output voltage of the secondary coil are kept constant as apparent from the above explanation. Yet further, as is apparent from Equation (28), the output voltage is proportional to the opposite area S.sub.g1 .

In this embodiment, the magnetic cores are formed into such a structure that the movable magnetic core 2g is supported by the Teflon bonded onto the respectively two pole-faces of the fixed magnetic cores 1g and 1'g. However, they may also be formed into such a structure that Teflon is bonded onto the pole-faces on either the upper or lower side, while air gaps are defined on the other side, and that an external force is exerted so as to hold the Teflon between the movable magnetic core 2g and the fixed magnetic cores 1g and 1'g. Also, employable is such a structure that the gaps are air gaps and that the magnetic cores are externally supported.

The gaps in the foregoing embodiments may be air gaps or of any nonmagnetic material insofar as they are of a lower magnetic permeability than the magnetic cores. Accordingly, spacers of Teflon or the like may be inserted into the gaps in order to facilitate supporting the movable magnetic core by the fixed magnetic core. In some cases, very thin Teflon or the like is inserted into the contact part between the movable magnetic core and the fixed magnetic core in order to make small the coefficient or friction at the displacement of the movable magnetic core relative to the fixed magnetic core.

Description will now be made of another embodiment of the present invention constructed so that the angle .phi..sub.0 of the output 0 (zero) in the output-versus-rotational angle characteristic can be arbitrarily set.

In order to obtain with the device of FIG. 9 an output characteristic in which, as shown by solid lines in FIG. 20, the output voltage is V.sub.1 in an angular range of O-.phi..sub.1 and it varies rectilinearly from V.sub.1 to V.sub.2 in an angular range of .phi..sub.1 -.phi..sub.2, that is, the output characteristic given by: ##EQU23## The angular range .theta., in which the output characteristics of the respective secondary coils of the device are rectilinear, must be as follows: ##EQU24## Accordingly, .theta. must be changed in conformity with required specifications for V.sub.1, V.sub.2, .phi..sub.1 and .phi..sub.2. Moreover, since .theta. is usually limited to less than 180.degree., (.phi..sub.2 -.phi..sub.1) equal to or greater than 90.degree. cannot be realized.

The feature of this embodiment lies in utilizing the fact that, by arbitrarily selecting the turn ratio between the two secondary coils 4b and 5b of the device as shown in FIG. 9, the position of the output 0 can be arbitrarily set within the range of the rectilinear characteristic.

Now, let p denote the turn ratio between the coils 4b and 5b in the device illustrated in FIG. 9. Then, the outputs across both the terminals of the respective secondary coils 4b and 5b vary as shown in FIG. 21 with the angle of rotation of the rotor 2b, and an angular range .xi. in which the difference between the outputs of both the secondary coils begins to decrease rectilinearly and finally becomes 0 is given by: ##EQU25## where .alpha. is a constant which is determined by the geometrical shapes and sizes of the rotor and the fixed magnetic core and the magnetic properties of the material thereof. As is apparent from Equation (29), .xi. can be arbitrarily set between 0 and .theta. by appropriately selecting p.

Here, let it be supposed that the desired rectilinear variation part of the output characteristic ranges from .phi..sub.1 to .phi..sub.2 as shown in FIG. 22 and that the desired outputs at .phi..sub.1 and .phi..sub.2 are V.sub.1 and V.sub.2, respectively. Then, a point .phi..sub.0 at which the extension of the rectilinear part intersects the abscissa is given by: ##EQU26## Accordingly, by appropriately selecting the turn ratio p between the coil 4b and the coil 5b, it is possible to make:

It is apparent that outputs V.sub.1 ' and V.sub.2 ' at the respective angles .phi..sub.1 and .phi..sub.2 in the case of deciding the turn ratio in this way hold the following relation therebetween: ##EQU27## Consequently, when the input voltage to the primary coil 3b is decided, the desired output characteristic can be acquired by appropriately selecting the turn ratio of the primary coil to the secondary coil. Thus, according to the present invention, if (.phi..sub.2 -.phi..sub.1) is smaller than .xi., the desired rectilinear output characteristic can be attained by appropriately selecting the turn ratio between the two secondary coils and the turn ratios between the secondary coils and the primary coil and without specially changing the configurations of the rotor, the fixed magnetic core, etc.

FIG. 23 shows a concrete construction of this embodiment. The top 14b of the central protrusive part 6b and the tops 12b and 13b of the outer-peripheral protrusive parts of the fixed magnetic core 1b are flush, so that the rotor 2b can rotate in close adherence to the surfaces 14b, 12b and 13b. An angle .theta. by which each of the outer-peripheral protrusive parts of the fixed magnetic core 1b spreads when viewed from the center is 90.degree.. The angular range in which the coefficient of the mutual induction between the coils 4b and 5b wound around the outer-peripheral protrusive parts and the coil 3b wound around the central protrusive part 6b varies rectilinearly relative to the rotational angle of the rotor is about 90.degree.. The number of turns N.sub.1 of the primary coil 3b is 8 turns, and the number of turns N.sub.2 of one 4b of the secondary coils is 12 turns. The number of turns N.sub.2 ' of the other secondary coil 5b was changed to be 12, 10, 8, 6, 4, 2 and 0 turns. Further, the A.C. power source 11b is coupled to the coil 3b. The coils 4b and 5b are connected as in the figure so as to differentially operate. A voltage appearing across both the terminals of the secondary coils is rectified by a diode 20 and smoothed by a capacitor 21.

The output characteristics of the device are shown in FIG. 24. As is apparent from the figure, the angle .phi..sub.0 at which the output becomes 0 (zero) can be set substantially arbitrarily in a range of 45.degree.- 90.degree. by variously selecting the turn ratio between the two secondary coils. It is apparent that, in order to set the angle .phi..sub.0 in a range of 0.degree.- 45.degree., the ratios between the numbers of turns N.sub.2 and N.sub.2 ' may be selected to be converse to the foregoing. It is accordingly understood that the angle .phi..sub.0 of the output 0 can be arbitrarily set in the range of 0.degree.-90.degree. by the selection of the turn ratio. As previously stated, if the angle .phi..sub.0 of the output 0 is set at the desired value in this manner and the output at 0.degree. is set at the desired value by appropriately selecting the turn ratio between the primary coil and the secondary coil with respect to the supply voltage, the desired output characteristics rectilinearly varying between 0.degree. and .phi..sub.0 will be achieved.

Likewise, in the embodiments of FIG. 1, FIG. 6 and FIG. 13, the angle .phi..sub.0 at which the output becomes 0 can be arbitrarily chosen by appropriately selecting the turn ratio between the primary and secondary coils.

There will now be explained a power circuit in the displacement detector as set forth above.

In general, an oscillator constituting a power circuit is composed of active elements (transistor, complementary type insulated gate field-effect transistor, etc.) and passive elements (diode, capacitor, resistance). Therefore, where the oscillator is used in a place attended with a wide range of temperature changes, temperature compensation is usually provided.

As to conventional oscillators, it is known that where the amplification degree increases with the temperature rise, the output generally decreases. Accordingly, in order to stabilize the oscillation output against the temperature change, the oscillator may be operated in the direction in which the feedback ratio decreases with the temperature rise. A transistorized Colpitts oscillator is a known oscillation circuit. In order to effect the stable operation against the wide range of temperature changes, a feedback resistance has been contemplated which can easily alter the feedback ratio with the temperature rise. In the power circuit according to this embodiment, the oscillation amplitude is stabilized by the use of a negative temperature coefficient-resistance element whose resistance value decreases with a temperature increase. In addition, a dispersion in the coefficients of the negative temperature coefficient-resistance elements is compensated by combining auxiliary resistances.

FIG. 25 shows a concrete example of the circuit, in which TR denotes a transistor, R.sub.1 and R.sub.2 base bias resistances, R.sub.E an emitter resistance, R.sub.F a feedback resistance, C.sub.1 a D.C. eliminating capacitor, and C a capacitor for resonance.

The oscillation circuit includes a tank circuit of the capacitors C and the primary coil 3 of a displacement detector 32, and oscillates at a frequency given by: ##EQU28## where L denotes the inductance of the coil 3.

This embodiment is characterized in that fixed resistances R.sub..alpha. and R.sub..beta. and a temperature-dependent resistance, such as thermistor R.sub.T, are employed for the feedback resistance R.sub.F, so as to vary the feedback ratio in response to changes in ambient temperature.

As a result, whereas the output of the oscillator 31 varies at a curve a in FIG. 26 relative to the temperature change in the absence of the temperature compensation, it can invert the temperature coefficient as at a curve b in the presence of the temperature compensation as in the circuit of FIG. 25.

Further, by the way of the combination of the auxiliary resistances, the feedback resistance having an optimum temperature coefficient can be readily selected. An improvement of one or more orders over the above example is made very easily.

FIG. 27 shows the oscillation output level versus the feedback resistance. As seen from this figure, the D.C. output level can be easily altered by only the circuit part without adjusting the displacement detector portion 32.

While we have shown and described several embodiments in accordance with the present invention it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications known to a person skilled in the art and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.