Inductive transducer for linear displacements

Abstract

Inductive transducer for linear displacements 18, which comprises: - a measuring coil 1, which can be supplied with alternating current Is and which is clamped in a single layer and evenly onto a tubular support 2: - a measuring core 4 slidably arranged within the carrier 2, which is linearly displaceable relative to the measuring coil 1; - an evaluation circuit 20 for exciting the measuring coil 1 with an alternating current of constant amplitude Is, wherein the measuring coil 1 has a complex impedance Zs which changes depending on the position of the measuring core 4, characterized in that the complex impedance Zs of the measuring coil 1 of the inductive converter 18 is determined at a specific frequency fo of the alternating current Is through the measuring coil 1, and wherein the evaluation circuit 20 has means for evaluating inductive Xs and resistive Rs components of the complex impedance Zs of the measuring coil 1,

Description

[0001] The present invention relates to an inductive transducer for linear displacements, comprising a measuring coil energized with alternating current and a measuring core movable within the measuring coil. The arrangement of the measuring core inside the measuring coil results in an extremely compact transducer design.

[0002] Such inductive transducers are used to provide an electrical signal that varies with the linear displacement of a mechanical element. For example, the signal can vary depending on the position of a valve, a piston of a hydraulic cylinder, or a position-controlled electromagnet.

[0003] Inductive transducers for linear displacements are often used in industrial environments because they are also suitable for measurements under problematic environmental conditions, such as high temperatures and temperature gradients, the presence of strong magnetic fields and a relatively high degree of pollution.

[0004] Temperature fluctuations and temperature gradients along the measuring range affect the measurement results of an inductive transducer. Firstly, the complex impedance of the measuring coil changes. As the temperature changes, the electrical conductivity and magnetic permeability of the measuring core and the shielding of the measuring coil also change, which in turn affects the measuring coil and its complex impedance Zs via the feedback effect of the coupling.

[0005] At the same time, it is also important to achieve the largest possible linear measuring range in relation to the length of the transducer, whereby the transducer should have a simple structure.

[0006] As a measure to compensate for temperature influences on the measurement result, DE3225822A1 proposes using a temperature-dependent resistor as part of a resistor network, with the resistor mounted close to the measuring winding. To achieve a linear characteristic, the winding consists of several coils, and a slidably arranged core moves within the winding. A circuit is used to determine the change in inductance of the winding as a result of a core displacement. A disadvantage is the relatively complex design of the movement, and the temperature gradient cannot be fully compensated.

[0007] DE-A-2025734 discloses a transducer for linear displacements, in which a metallic body is displaceable relative to a measuring coil and in which the total impedance of the measuring coil is determined. The transducer is designed such that the resistive component of the total impedance of the coil is evaluated, with the inductive component of the impedance being insignificant compared to the resistive component.

[0008] US89301169.2 discloses a transducer for linear displacements, comprising a measuring winding with a magnetic core that is linearly displaceable relative to the measuring winding, with a device for exciting the measuring winding with an alternating current of a frequency sufficiently large that the inductive reactance of the magnetic core is not dominant over the resistive losses in the magnetic core. The losses in the magnetic core thus cause the voltage across the measuring winding, which is in phase with the current, to vary linearly relative to the linear displacement of the magnetic core. The device includes a synchronous demodulator for evaluating the voltage across the winding, which is in phase with the current. The transducer additionally includes a temperature sensing winding arranged adjacent to the measuring winding.

[0009] Publication WO 94 / 03778 discloses a method for compensating for temperature influences on the impedance of a measuring coil. In this method, the measuring coil can be simultaneously supplied with an alternating voltage and a direct voltage. The DC voltage drop across the ohmic resistance of the measuring coil is used for temperature compensation.

[0010] Some displacement sensors of the type LVDT (differential transformer) or DD (differential chokes) that are very common in practice are described, for example, in the book by W. Cassing, W. Stanek et al., "Electromagnetic Converters and Sensors". Expert Verlag, Volume 219.

[0011] It is common practice to use a measuring coil as a frequency-determined part of an oscillating circuit (US 3,891,918)

[0012] In section 4 of the publication “Archiv für Elektrotechnik 73 (1990)” a compensation of the temperature dependence of the coil quality is described, whereby a coil structure with two windings is provided, which could be enclosed in the same direction or in opposite directions.

[0013] Further transducers for linear displacements are also known from US 4284961, US 4115923, US 3891918, US 3654549, DE 3343885 A1, DE 35344680 A1.

[0014] The disadvantage of all the above-mentioned transducers for linear displacements is the relatively complex temperature compensation and the relatively complicated structural design.

[0015] Therefore, the invention is based on the object of designing and developing an inductive transducer for linear displacements of the type in question in such a way that reliable compensation of temperature influences on the measurement result is possible even with an inhomogeneous temperature distribution along the measuring range. The transducer should have a simple design and exhibit a linear characteristic. This object is achieved according to the invention by an inductive transducer for linear displacements having the features of claim 1.

[0016] According to the invention, a transducer contains a measuring coil capable of being energized with alternating current, which is wound in a single layer and evenly on a carrier. The carrier should be made as a tube made of non-magnetic, electrically conductive material. The carrier serves, on the one hand, to stabilize the measuring coil, since the measuring coil can practically be wound around the carrier, and, on the other hand, also as a guide for the movement of a measuring core, which slides within the carrier.

[0017] Advantageous materials for the core include materials known from practice. Non-ferromagnetic materials with high electrical conductivity are particularly suitable. It is important that the electrical conductivity of the core material is much greater than the electrical conductivity of the carrier material. For example, the measuring core could be made as a rod or tube made of brass. The carrier should be realized as a thin-walled tube made of stainless steel. Depending on the application, the transducer according to the invention can be provided with a housing that encapsulates the measuring coil and advantageously also shields it against dirt and interference, or additionally encloses the evaluation circuit. The housing material could be steel, with or without ferromagnetic properties. The housing could also be made of plastic, glass, or ceramic.

[0018] Preferably, an evaluation circuit for impedance measurement is provided on the measuring coil, wherein the evaluation circuit has means for determining the inductive component Xs and the resistive component Rs of the complex impedance Zs of the measuring coil.

[0019] In an advantageous embodiment of the evaluation circuit, the measuring coil is supplied with an alternating current at a specific frequency fo, wherein the Zs, Xs, and Rs should be determined and evaluated precisely at this frequency, and wherein the inductive component Xs is proportional to the position of the measuring core, and the resistive component Rs contains information about the ambient temperature T.

[0020] Furthermore, it would be recognized that at a frequency fo of the alternating current through the measuring coil, the resistive component Rs of the complex impedance Zs of the measuring coil is essentially independent of the relative position of the measuring core with respect to the measuring coil.

[0021] According to the invention, it was further recognized that the inductive component Xs of the complex impedance of the measuring coil Zs exhibits a relatively small, only additive change due to the influence of the ambient temperature T. Furthermore, it has been found that the inductive component Xs changes essentially linearly with respect to the displacement of the core. Therefore, a relatively simple temperature compensation of the converter could be achieved by a combination of inductive Xs and resistive Rs components of the complex impedance Zs of the measuring coil, whereby the impedance Zs should be determined at a certain frequency fo of the alternating current Is through the measuring coil. Depending on the electromagnetic properties, or electrical conductivity of the material of the core, the carrier around which the measuring coil is wound, and the magnetic permeability and electrical conductivity of the shielding, the frequency fo could be in the range of approximately 40 to 150 kHz.

[0022] There are various possibilities for advantageously embodying and developing the teachings of the present invention. Reference is made, on the one hand, to the claims subordinate to claim 1 and, on the other hand, to the following explanation of preferred embodiments of the invention with reference to the drawings.

[0023] The drawing shows: Fig. 1 a basic structure of an inductive converter according to the invention for linear displacements; Fig. 2 shows a schematic view of an embodiment of an application of the converter according to the invention using the example of valve lift measurement with a pressure tube; Fig. 3 in a schematic representation of an evaluation circuit an inductive converter for the linear displacements.

[0024] According to Fig. 1, an inductive transducer for linear displacements 18 contains a measuring coil 1 wound around a carrier 2. This is a long coil, namely a single-layer cylindrical coil with uniform windings. The measuring coil 1 is wound around a carrier 2 in the form of a thin-walled tube. A core 4, whose length corresponds to the measuring range of the transducer, is arranged slidingly inside the carrier 2.

[0025] Finally, the Fig. The linear displacement transducer 18 shown in Figure 1 also has a housing 3 that merely contains the measuring coil 1 with the carrier 2 and the core 4. The two connections of the measuring coil 1 to the evaluation circuit 20 are led out of the housing 3 via connectors 6. It is conceivable that the housing 3 also encloses the evaluation circuit 20 with all supply lines. The carrier 2 is made of an electrically conductive, non-ferromagnetic material with relatively low electrical conductivity. This could be a stainless steel with corresponding properties. The core 4 is made of a material with high electrical conductivity, which is much greater than the electrical conductivity of the material of the carrier 2.

[0026] It could be brass or other alloys. The core could be a rod or tube made of electrically conductive, non-magnetic material.

[0027] Housing 3 could be made as a tube made of magnetic or non-magnetic steel. Housing 3 could also be made of a plastic or glass tube.

[0028] The position of the core 4 relative to the measuring coil 1 can be determined using the evaluation circuit 20. Details of the measured value evaluation are described in connection with Fig. 3 is explained in more detail.

[0029] Fig. Figure 2 schematically shows a longitudinal section of an inductive transducer for linear displacements 19 with a pressure tube 8, wherein a measuring coil 9 is wound directly around the pressure tube 8. A core 7 slides in the inner region of the pressure tube 8. From the outside, the measuring coil 9 with the pressure tube 8 and the core 7 are enclosed by a housing 10. The pressure tube 8 could be made of non-magnetic stainless steel or titanium alloys with relatively low electrical conductivity. Brass or aluminum alloys could serve as the material for the core 7.

[0030] The Fig.The evaluation circuit 20 shown in Figure 3 for an inductive transducer for linear displacements 18 according to the invention comprises a voltage source, which here is shown in the form of an alternating voltage generator 11 with three outputs and a variable angular frequency. This alternating voltage generator 11 generates a sinusoidal voltage Us with a specific frequency fo at one output, wherein the measuring coil 1 is connected in feedback to an amplifier 12, and wherein the voltage Us is connected to the inverting input of the amplifier 12 via a resistor Ro. Therefore, an alternating current Is flows through the measuring coil 1 of the inductive transducer 18, the amplitude of which is adjusted by a resistor Ro. Two synchronous demodulators 13 and 14 are connected downstream of the output of the amplifier 10, wherein demodulator 14 is controlled with a square-wave voltage U1 and demodulator 13 with a square-wave voltage U2.Voltage U1 is in phase with the alternating voltage Us, while voltage U2 is shifted 90 degrees relative to U1. Low-pass filters 15 and 16 are connected downstream of the synchronous demodulators 13 and 14, which are designed to filter out high-frequency signal components that occur due to the circuit. The signals at the outputs of low-pass filters 15 and 16 are fed to an instrumentation amplifier 17 for differential calculation. The output signal of amplifier 17 is then determined using the following equation: Uout.=Ux−kUr, where Ux - the DC voltage which is proportional to the inductive component Xs of the complex impedances Zs of the measuring coil 1; Ur - the DC voltage which is proportional to the resistive component Rs of the complex impedances Zs of the measuring coil 1; k - a coefficient that depends on the design of the converter. For example, the coefficient k could be in the range of approximately 0.01 to 0.02. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 3225822A1

[0006] US 89301169.2

[0008] US 3,891,918 [0011, 0013] US 4284961

[0013] US 4115923

[0013] US 3654549

[0013] DE 3343885 A1

[0013] DE 35344680 A1

[0013] Cited non-patent literature

[0000] W. Cassing, W. Stanek et al. "Electromagnetic Transducers and Sensors". Expert Verlag, Volume 219

[0010] Archives for Electrical Engineering 73 (1990

[0012]

Claims

[1] Inductive transducer for linear displacements 18, which has: - a measuring coil 1, capable of being acted upon with alternating current Is, which is clamped in a single layer and uniformly onto a tubular support 2: - a measuring core 4 arranged slidably within the carrier 2, which is linearly displaceable relative to the measuring coil 1; - an evaluation circuit 20 to excite the measuring coil 1 with an alternating current of constant amplitude Is, wherein the measuring coil 1 has a complex impedance Zs which changes depending on the position of the measuring core 4, characterized by , that the complex impedance Zs of the measuring coil 1 of the inductive transducer 18 is determined at a certain frequency fo of the alternating current Is through the measuring coil 1, and wherein the evaluation circuit 20 has means for evaluating inductive Xs and resistive Rs components of the complex impedance Zs of the measuring coil 1, [2] Converter according to claim 1, characterized by, that the evaluation circuit 20 has an AC voltage generator 11 with three outputs Us, Ui and U2, wherein the sinusoidal output Us of the AC voltage generator 11 is connected via a resistor Ro to the inverting input of the amplifier 12, and wherein the measuring coil 1 is enclosed in the feedback of the amplifier 12. [3] Converter 18 according to claim 1, characterized by , that the measuring coil 1 is wound on a carrier 2 made of electrically conductive, non-ferromagnetic material, [4] Converter 18 according to claim 1, characterized by , that the core 4 is made of electrically conductive, non-ferromagnetic material, wherein the electrical conductivity of the material of the measuring core 4 is significantly greater than the electrical conductivity of the material of the carrier 2. [5] Converter 18 according to claims 1 and 3, characterized by that support 2 is formed as a thin-walled tube made of stainless steel. [6] Converter 19 according to claims 1 and 4, characterized by , that the measuring core 4 is formed as a rod or a tube made of brass and wherein the length of the core 4 corresponds to the length of the measuring coil 1. [7] Converter 19 according to claims 1 to 4, characterized by , that a pressure tube 8 serves as a support for the measuring coil 9, and wherein a core 7 is arranged slidably within the pressure tube 8. [8] Converters according to claims 1 to 7, characterized by , that the evaluation circuit 20 contains an instrument amplifier 17, wherein the output of the instrument amplifier 20 is the voltage Uout=Ux−kUr exhibits. [9] Converter 19 according to claims 1 to 8 characterized by , that the frequency fo of the alternating current Is via the measuring coil 1 lies in the range of 40 to 150 kHz.

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