Inductive Linear Displacement Sensors

Abstract

The invention relates to an inductive linear displacement sensor (1) comprising a primary coil (2) and two secondary coils (3, 3'), the secondary coils (3, 3') being inductively coupled to the primary coil (2). The linear displacement sensor (1) has a calibration coil (4) inductively coupled to the primary coil (2). The calibration coil (4) is arranged so that the signal generated by the calibration coil has only one zero crossing at the center of the linear displacement sensor (1).

Description

Technical Field

[0001] The present invention relates to an inductive linear displacement sensor comprising a primary coil and two secondary coils, the secondary coils being inductively coupled to the primary coil. The invention also relates to a system comprising a linear displacement sensor and a position sensor arranged movably relative to the linear displacement sensor, in particular in a linear direction relative to the linear displacement sensor, and a method for determining a linear displacement using the linear displacement sensor or a system comprising the linear displacement sensor and the position sensor arranged movably relative to the linear displacement sensor. Background Art

[0002] Such inductive linear displacement sensors are used, for example, in the automotive industry, large machinery, and automatic meter reading. Various concepts are known, such as the so-called linear differential transformer, in which the signals of the secondary coils—that is, the voltages induced by the primary coils—are subtracted from each other. Alternatively, the ratio of the signals can be used. This generates a linear characteristic curve that can be used to draw conclusions about the displacement.

[0003] Before using such an inductive linear displacement sensor, a calibration (so-called "teaching" of the sensor) must be performed, during which, among other things, the sensor's zero position must be determined. This is usually located in the center of the measuring range and, therefore, in the center of the characteristic curve. However, a disadvantage here is that various parameters have an influence on the sensor and, therefore, also on the characteristic curve, so that, in the worst case, zero position detection is no longer possible.

[0004] Against this background, the object therefore arises to improve inductive linear displacement sensors in such a way that an unambiguous zero position detection is possible without increasing the size of the proven sensor design. Summary of the Invention

[0005] This object is achieved by an inductive linear displacement sensor comprising a primary coil and two secondary coils, wherein the secondary coils are inductively coupled to the primary coil, wherein the linear displacement sensor has a calibration coil which is inductively coupled to the primary coil, wherein the calibration coil is arranged such that the signal generated by the calibration coil has only one zero crossing in the center of the linear displacement sensor. Preferably, the calibration coil is arranged in such a way that the signal generated by it assumes a different sign on either side of the center and particularly preferably a constant value.

[0006] With the linear displacement sensor according to the present invention, due to the well-defined zero position of the calibration coil's signal, calibration can advantageously be performed unambiguously with respect to the center of the linear displacement sensor or the center of its characteristic curve. Furthermore, this calibration is made possible in a particularly simple manner, meaning that complex control electronics for evaluating the calibration coil's signal can be omitted. Furthermore, the calibration coil provides an additional diagnostic option for the linear displacement sensor. This means that, if necessary, recalibration can be performed easily and reliably at any time during the operating life of the linear displacement sensor. In particular, this calibration option, based on evaluating the signal of the secondary coil, is advantageously independent of other methods.

[0007] According to an advantageous embodiment of the invention, the linear displacement sensor is designed to be planar, wherein, in particular, the coil is designed to be planar. The person skilled in the art will understand that in the present case, "coil" is to be understood as a primary coil, a secondary coil and a calibration coil. Furthermore, hereinafter, the term "sensor" is used synonymously for the linear displacement sensor. The fact that the sensor is designed to be planar means that, in particular, the coils are arranged in a single plane, i.e. the coils are designed to be flat. Preferably, the linear displacement sensor has, in particular, a substantially cuboid shape. The linear displacement sensor particularly preferably has a main extension plane which coincides with the plane in which the coils are arranged and a main extension direction which corresponds to the linear direction along which the displacement is determined. Consequently, the sensor advantageously requires less space and provides a flat surface along which the position sensor can be moved.

[0008] According to another advantageous embodiment of the present invention, the linear displacement sensor is configured as a circuit board, wherein the coil is designed as a single-layer or multi-layer conductive track. Such a circuit board is also known as a printed circuit board and can be easily and inexpensively manufactured using proven technology. By designing the coils as conductive tracks, the primary coil, secondary coil, and calibration coil can be arranged in a simple and reliable manner.

[0009] Preferably, the circuit board is arranged in multiple layers, with the coils arranged in one plane of the circuit board, in particular in the topmost plane, and at least one control electronics unit for operating the primary coil and / or for processing the signals generated by the secondary coil and / or the calibration coil, in particular arranged in a further plane below the topmost plane. In particular, the control electronics unit for processing the signals generated by the secondary coil is designed as an integrated circuit. Particularly preferably, the control electronics unit is designed at least partially as an application-specific integrated circuit (ASIC). Such a chip is a standard component, allowing for a cost-effective design of the control electronics unit. Those skilled in the art will appreciate that appropriate electrical connections must be provided between the various planes of the circuit board. Consequently, both the coils and the control electronics unit can advantageously be arranged on the circuit board in a space-saving manner.

[0010] An advantageous embodiment of the invention provides that the primary coil is operated with a particularly high-frequency alternating field. A person skilled in the art will understand that this produces an inductive oscillating circuit in which the secondary coil, and in particular also the calibration coil, is inductively coupled to the primary coil.

[0011] According to another advantageous embodiment of the present invention, secondary coils are provided that are designed with a sinusoidal geometry, in particular, offset by π / 2 relative to one another, wherein the primary coil is preferably designed with a rectangular geometry. A person skilled in the art will understand that this is particularly synonymous with one secondary coil being designed with a sinusoidal geometry and the other with a cosine geometry. In particular, the secondary coils are designed with only one measuring cycle. The corresponding geometry advantageously ensures that the secondary coils generate either a sine or a cosine signal. A person skilled in the art will understand that these signals can be combined via their quotient and an inverse tangent function to form at least a substantially linear signal. This means that the secondary coils generate a linear characteristic curve, from which a linear displacement can be determined.

[0012] According to a particularly advantageous embodiment, the calibration coil is designed such that it extends substantially parallel to the primary coil on either side of the center and switches sides at the center of the linear displacement sensor transversely to the main extension direction. In particular, this means that the signal generated by the calibration coil changes sign at the center of the linear displacement sensor, so that, for example, a positive voltage is measured to the left of the center and a negative voltage is measured to the right of the center. Consequently, the signal from the calibration coil has only one zero crossing, namely at the center of the linear displacement sensor. Preferably, the calibration coil is arranged flat and / or designed to be biaxially symmetrical, in particular about an axis extending in the main extension direction and passing through the center, and about an axis extending transversely to the main extension direction and passing through the center. Particularly preferably, the calibration coil has its windings at their maximum distance transversely to the main extension direction at the ends of the linear displacement sensor that are furthest apart from each other with respect to the main extension direction. According to the present invention, it is important that the calibration coil has exactly one zero crossing over the entire measuring range. Various forms are conceivable in this regard. For example, the substantially rectangular shape described above, a substantially triangular shape with two triangles touching at their vertices, or even a sinusoidal shape with only a single zero crossing. This provides a particularly advantageous definition of an unambiguous zero position, which can be used to calibrate the center of the linear displacement sensor in a simple manner.

[0013] Preferably, the linear displacement sensor is encapsulated, wherein the linear displacement sensor is surrounded by, in particular, a plastic material. This means, in particular, that the circuit board has no direct contact with the environment and is surrounded by a material that preferably has no influence on magnetic fields. This advantageously provides a linear displacement sensor that is protected from environmental influences such as dirt, heat, lubricants, and liquids.

[0014] According to a further advantageous embodiment, an analog circuit, in particular an operational amplifier, is provided which is arranged to detect and / or process the signal generated by the calibration coil. Commercially available ASIC chips usually have only two input terminals, which correspond to the two secondary coils. Advantageously, by using an analog circuit, such as an operational amplifier, there is no need to provide a proprietary control circuit system. In particular, due to the special design of the calibration coil, a particularly advantageous embodiment is possible in which only the amplitude of the calibration coil signal has to be output. This also means that, in an advantageous manner, the circuit of the calibration coil is independent of the circuit arrangement of the secondary coil. Thus, in a particularly advantageous manner, an additional diagnostic option is provided which is independent of the signal processing / availability at the sensor output or independent of the signal at the input of the ASIC chip, in particular in the case of a typical design of an inductive linear displacement sensor.

[0015] Another object of the present invention is a system comprising a linear displacement sensor according to the present invention and a position sensor arranged to be movable relative to the linear displacement sensor, particularly in a linear direction relative to the linear displacement sensor. Preferably, the position sensor is made of a magnetic material, particularly ferrite. Alternatively or additionally, the position sensor comprises a resonant circuit. Such a resonant circuit can, for example, be implemented as a circuit board with a coil of conductive tracks and, particularly preferably, a capacitor, similar to the linear displacement sensor.

[0016] Very particularly preferably, the position sensor is guided on the linear displacement sensor, for example, via a lateral groove extending over the length of the linear displacement sensor, into which a corresponding guide rail of the position sensor engages. A person skilled in the art will appreciate that other types of relative movable mounting between the linear displacement sensor and the position sensor are also possible. Alternatively, the position sensor can be freely moved relative to the linear displacement sensor. The inductive linear displacement sensor according to the invention is advantageously insensitive to movements of the position sensor transverse to the measuring direction, in particular transverse to the main extension direction of the linear displacement sensor.

[0017] The system according to the invention can achieve the same advantages as those described in connection with the linear displacement sensor according to the invention. The advantageous embodiments and features described in connection with this sensor can also be applied to the system individually or in combination.

[0018] Another object of the present invention is a method for determining a linear displacement using a linear displacement sensor or a system according to the present invention, wherein the primary coil is operated with an alternating field and wherein the characteristic curve of the linear displacement sensor is calibrated with respect to the center of the linear displacement sensor using a signal generated by a calibration coil. A person skilled in the art will appreciate that this is made possible by the special design of the calibration coil. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Further details and advantages of the present invention will be described below with reference to the exemplary embodiments shown in the accompanying drawings. In the drawings:

[0020] Figure 1 An inductive linear displacement sensor not according to the present invention is shown;

[0021] Figure 2 Shown according to Figure 1 The (useful) signal of the secondary coil of the linear displacement sensor;

[0022] Figure 3 An inductive linear displacement sensor according to an advantageous embodiment of the present invention is shown;

[0023] Figure 4 Shown according to Figure 3 the (useful) signals of the secondary coil and the calibration coil of the linear displacement sensor; and

[0024] Figure 5 Shown by Figure 3 The (useful) signal of the secondary coil generates the linear characteristic curve of the linear displacement sensor. DETAILED DESCRIPTION

[0025] exist Figure 1 , an inductive linear displacement sensor 1 not according to the present invention is shown. The linear displacement sensor 1 is designed here as a circuit board 5. For the sake of clarity, only one upper functional layer is shown here. Merely by way of example, a Cartesian coordinate system is shown in the following text, in which the x-direction corresponds to the main extension direction of the linear displacement sensor 1, and the plane formed by the x-axis and the y-axis corresponds to the main extension plane of the linear displacement sensor 1. In particular, further layers of the circuit board 5 are not shown here, which contain, among other things, further elements and control electronics required for the operation of the linear displacement sensor 1 and known to those skilled in the art.

[0026] For example, the various coils 2, 3, 3' are implemented in the form of conductive tracks in a circuit board 5. In the present case, a rectangular primary coil 2 and two secondary coils 3, 3' are provided within the rectangle spanned by the primary coil 2, the two secondary coils being sinusoidal and cosine, respectively, i.e. phase-shifted relative to one another by, in particular, a quarter period, in other words, π / 2.

[0027] Figure 2 The diagram in FIG shows the following diagram, in which the Figure 1 The (useful) signals of the secondary coils 3, 3' of linear displacement sensor 1 are plotted here over a linear displacement parallel to the x-axis. It can be seen that the signals have a sine or cosine curve, but they pass through less than a complete cycle. Based on the ratio of the signals of the secondary coils 3, 3', a linear characteristic curve can be obtained using an inverse tangent function or an arctan² function, from which the linear displacement can then be determined.

[0028] However, before using such an inductive linear displacement sensor 1 for the first time, the complete measuring path must be traversed and measured once in order to calibrate the sensor 1, in particular with respect to the center of the sensor 1. However, this calibration is sensitive to various influences and can therefore shift under certain circumstances, which is undesirable.

[0029] According to the present invention, by Figure 3The inductive linear displacement sensor 1 shown in the advantageous embodiment of FIG solves this problem. In this case, the structure of the coil geometry with respect to the primary coil 2 and the secondary coil 3, 3' corresponds essentially to that with Figure 1 Regarding the structure described. In addition, however, a calibration coil 4 is provided, which in this case extends to the left of the center parallel to the upper conductive track of the primary coil 2 and to the right of the center parallel to the lower conductive track of the primary coil 2. The calibration coil 4 passes through the circuit board exactly in the center. In other words, the calibration coil 4 is arranged in such a way that it has a zero crossing of its signal exactly in the center of the linear displacement sensor 1 and has a constant value of opposite sign on both sides of the center.

[0030] This clearly defines the center of the linear displacement sensor 1. The fact that the amplitude is substantially constant on both sides of the center also makes it advantageous to omit complex control electronics.

[0031] Figure 4 Shown according to Figure 3 Figure 4 shows the (useful) signals of the secondary coils 3, 3' and the calibration coil 4 of the linear displacement sensor 1. It can be seen how the signal from the calibration coil 4 has a clear zero crossing in the center and constant values ​​or zero crossings on both sides of the center, depending on the geometry of the calibration coil.

[0032] at last, Figure 5 Shown by Figure 3 The (useful) signals of the secondary coils 3, 3' generate the linear characteristic curve of the linear displacement sensor 1. With the aid of the signal from the calibration coil 4, the center can be accurately determined, thus allowing for simple calibration of the linear displacement sensor. The curve shown here is determined using the inverse tangent2 function (also known as arctan2 or atan2 function) based on the voltage induced and measured in the secondary coil.

Claims

1. An inductive linear displacement sensor (1) having a primary coil (2) and two secondary coils (3, 3'), wherein: The secondary coil (3, 3') is inductively coupled to the primary coil (2), characterized in that the linear displacement sensor (1) has a calibration coil (4) which is inductively coupled to the primary coil (2), wherein the calibration coil (4) is arranged so that the signal generated by the calibration coil has only one zero crossing at the center of the linear displacement sensor (1).

2. The linear displacement sensor (1) according to claim 1, characterized in that The linear displacement sensor (1) is designed to be planar.

3. The linear displacement sensor (1) according to claim 1, characterized in that The primary coil, the secondary coil and the calibration coil are designed to be planar.

4. The linear displacement sensor (1) according to claim 1, characterized in that The linear displacement sensor (1) is provided as a circuit board (5), wherein the primary coil, the secondary coil and the calibration coil are designed as single-layer or multi-layer conductive tracks.

5. The linear displacement sensor (1) according to claim 4, characterized in that The circuit board (5) is arranged in multiple layers, wherein the primary coil, the secondary coil and the calibration coil are arranged in one plane of the circuit board (5), and at least one control electronic unit for operating the primary coil (2) and / or for processing the signals generated by the secondary coil (3, 3') and / or the calibration coil is arranged as an integrated circuit.

6. The linear displacement sensor (1) according to claim 5, characterized in that The primary coil, the secondary coil and the calibration coil are arranged in the uppermost plane of the circuit board (5).

7. The linear displacement sensor (1) according to claim 5, characterized in that The primary coil, the secondary coil and the calibration coil are arranged in another plane below the uppermost plane of the circuit board (5).

8. The linear displacement sensor (1) according to claim 5, characterized in that The control electronics unit for processing the signals generated by the secondary coils (3, 3') is designed as an integrated circuit.

9. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that The linear displacement sensor (1) is packaged.

10. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that The linear displacement sensor (1) is surrounded by plastic material.

11. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that Analog circuitry is configured to detect and / or process the signal generated by the calibration coil (4).

12. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that An operational amplifier is arranged to detect and / or process the signal generated by the calibration coil (4).

13. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that The secondary coils (3, 3') are designed with a sinusoidal geometry that is shifted relative to one another.

14. The linear displacement sensor (1) according to claim 13, characterized in that The secondary coils (3, 3') are designed with a sinusoidal geometry that is shifted by π / 2 relative to each other.

15. The linear displacement sensor (1) according to claim 13, characterized in that The primary coil (2) is designed in a rectangular geometric shape.

16. The linear displacement sensor (1) according to any one of the preceding claims 1 to 8, characterized in that The primary coil (2) operates with a high-frequency alternating field.

17. A system comprising a linear displacement sensor (1) according to any one of the preceding claims 1 to 16 and a position sensor, the position sensor being movably arranged relative to the linear displacement sensor.

18. A system of a linear displacement sensor (1) and a position sensor according to claim 17, the position sensor being arranged movably in a linear direction relative to the linear displacement sensor.

19. A method for linear displacement determination using a linear displacement sensor (1) according to any one of claims 1 to 16 or a system according to claim 17 or 18, wherein: The primary coil (2) is operated with an alternating field, wherein the characteristic curve of the linear displacement sensor (1) is calibrated relative to the center of the linear displacement sensor (1) by means of the signal generated by the calibration coil (4).

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