DE102011115922B4 - Inductive sensor - Google Patents

Abstract

Inductive sensor (1) for detecting an electrically conductive object comprising:
- An LC resonant circuit (2) having a coil (5) and a capacitor (6), wherein the coil (5) comprises an inductor (3), a loss resistor (18) and a damping resistor (17) and a temperature coefficient of the loss resistance (18) is less than 3 * 10 ^-3 K ^-1 ,
- An excitation unit (16) for the displacement of the LC resonant circuit in an electromagnetic oscillation and
an evaluation unit (15) for detecting a change in at least one parameter of the LC resonant circuit (2) due to the electrically conductive object,
wherein the excitation unit (16) and the evaluation unit (15) with first and second electrical terminals (13, 14) are electrically connected to the LC resonant circuit (2),
wherein the LC resonant circuit (2) comprises an electrical amplifier (9) and a measuring resistor (8) connected in series with the coil (5), and
wherein with the amplifier (9) a voltage drop due to the loss resistance (18) on the coil (5) is at least partially compensated.

Description

The present invention relates to an inductive sensor according to claim 1 and to a method for operating an inductive sensor according to claim 9.

Inductive sensors are used in various technical applications for detecting electrically conductive objects, in particular metallic objects. The inductive sensor comprises a coil as an inductor and a capacitor as a capacitor, which form an LC resonant circuit. An excitation unit supplies electric power to the LC oscillation circuit, so that the LC oscillation circuit is set in an electromagnetic oscillation. The magnetic field generated by the coil induces eddy currents in the moving metallic object in the region of the coil, so that thereby the electromagnetic oscillation is changed or energy is withdrawn from the LC resonant circuit. This change can be detected with an evaluation unit and thus the metallic object in the area or in the vicinity of the coil can be detected or sensed.

The coils are generally made with a coil winding of a copper wire, because copper has a small electrical resistivity and thereby only a small voltage drop occurs at the coil. This leads to low losses in the LC resonant circuit. However, copper has a large temperature coefficient of 3.9 * 10 ^-3 K ^-1 , so that the specific electrical resistance depends strongly on the temperature. In a disadvantageous way, the resistance of the coil thus depends strongly on the temperature, so that the distance or switching distance between the metallic object and the coil within which the metallic object can be detected is subjected to large temperature-dependent fluctuations, ie the higher the temperature or higher temperature The higher the resistance of the coil, the smaller the distance within which the metallic object can be detected and vice versa.

The DE 201 08 904 U1 shows an inductive proximity switch for material-dependent detection of metallic objects with a sensor device in which at least one influenced by a metallic object resonant circuit is provided with at least one coil, for generating a distance-dependent sensor signal at a relative change in distance between metallic object and sensor device, and with an evaluation device, which is in operative connection with the sensor device, for evaluation of the sensor signal and for generating a switching distance. The coil winding of the coil of the resonant circuit consists of a resistance alloy. Resistor alloys have a small temperature coefficient, so that the inductive proximity switch is subject to only small temperature-dependent variations in the distance between the metallic object and the coil, within which the metallic object can be detected. However, resistance alloys have a high specific electrical resistance so that, in particular when using only one coil, metallic objects can only be detected with the inductive proximity switch at a very small distance from the coil, because a large voltage drop resp ., a large electrical loss occurs, ie the LC resonant circuit is strongly attenuated. The inductive proximity switch thus has a low sensitivity.

The DE 29 19 983 A1 describes a method for testing the electrical, magnetic or geometric properties of a body or its location by interaction with a coil or a capacitor, which perform self-excited electromagnetic oscillations in conjunction with an amplifier or for testing inductive or capacitive components, which themselves are part of the resonant circuit are. Here, an amplifier in cooperation with a coil and a capacitor forms a self-oscillating oscillator.

The object of the present invention is therefore to provide an inductive sensor and a method for operating an inductive sensor, which can detect a metallic object substantially independent of temperature and with high sensitivity, wherein the inductive sensor can be produced with low production costs.

This object is achieved with an inductive sensor for detecting an electrically conductive object, in particular a metallic object, with an LC resonant circuit having a coil and a capacitor, wherein the coil comprises an inductor, a loss resistor and a damping resistor and a temperature coefficient of the loss resistance smaller is 3 * 10 ^-3 K ^-1 , in particular less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 . Furthermore, an evaluation unit for detecting a change of at least one parameter of the LC resonant circuit is provided on the basis of the electrically conductive object, wherein the evaluation unit is electrically connected to the LC resonant circuit. The LC resonant circuit comprises an electrical amplifier and a measuring resistor connected in series with the coil, wherein the amplifier, a voltage drop due to the loss resistance at the coil is at least partially compensated.

The coil is represented in a model by a separate loss resistance and a separate damping resistance. In fact, the loss resistance and the damping resistance are formed by the coil itself.

The coil has a small temperature coefficient, so that thereby the sensitivity or the distance between the metallic object and the inductance, within which the metallic object can be detected, subject to only very small temperature-dependent fluctuations and also due to the amplification of the current by means of the In the LC resonant circuit of the sensor integrated amplifier realizes a high sensitivity and detection of the metallic object can be achieved at a great distance from the sensor.

In a preferred embodiment, the amplifier is connected in series with the inductor and the capacitor. The measuring resistor is connected in series with the inductance, wherein the measuring resistor is electrically connected from the inductance to the amplifier, in particular to an input terminal of the amplifier. The measuring resistor is required for the electrical circuit of the amplifier in the LC resonant circuit.

In another embodiment, the amplifier has an input terminal and an output terminal for amplifying the voltage applied to the input terminal with respect to the output terminal, and the input terminal is electrically connected at the node to a power line connecting the inductance and the measuring resistor and the output terminal is electrically connected to the capacitance connected.

In one variant, the amplification factor of the amplifier essentially corresponds to the sum of 1 and the ratio of the resistance of the loss resistance to the resistance of the measuring resistor. Thus, the voltage drop or the loss of the coil due to the loss resistance can be substantially compensated, so that even a large inductance resistance due to a large resistivity of a coil winding does not reduce the sensitivity of the inductive sensor.

In a supplementary embodiment, the inductance is formed as a coil with a coil winding, in particular of a wire and / or a stranded wire.

Suitably, the capacity is as a capacitor, for. As an SMD equippable ceramic capacitor formed.

In an additional variant, the specific electrical resistance at 20 ° C. of the coil winding, in particular of the wire and / or the strand, is greater than 2 * 10 ^-6 Ω cm, preferably greater than 5 * 10 ^-6 Ω cm or 10 ^-5 Ω cm, in particular greater than 2 * 10 ^-5 Ω cm or 10 ^-4 Ω cm.

Preferably, the temperature coefficient of the coil winding, in particular of the wire and / or the strand, is less than 3 * 10 ^-3 K ^-1 , preferably less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 , The sensitivity of the inductive sensor is thus dependent only to a very limited extent on the temperature.

In an additional embodiment, the coil winding, in particular the wire and / or the strand, at least partially, in particular completely, of a resistance alloy, preferably NiCr20AlSi and / or NiCr8020 and / or CuNi44 and / or CuMnl2Ni and / or CuMn7Sn. Resistor alloys advantageously have a small temperature coefficient and, disadvantageously, a high resistivity. The disadvantage of the large resistivity can be compensated with the aid of the amplifier.

In a further embodiment, the temperature coefficient of the measuring resistor is less than 3 * 10 ^-3 K ^-1 , preferably less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 . A small temperature coefficient of the measuring resistor prevents that the voltage given to the capacitance via the amplifier depends strongly on the temperature.

According to the invention, the sensor comprises an excitation unit for offsetting the LC resonant circuit into an electromagnetic oscillation and the excitation unit is connected to the first and second electrical Connection electrically connected to the LC resonant circuit. With the excitation unit, electrical energy is supplied to the LC resonant circuit and thereby the LC resonant circuit is put into an electromagnetic oscillation and / or kept in an electromagnetic oscillation. The excitation unit is preferably designed as a structural unit together with the evaluation unit.

In a supplementary embodiment, the evaluation unit comprises a switching unit, so that when a detected object in the region of the inductive sensor, the switch is open or closed and no detected object in the region of the inductive sensor, the switch is closed or opened.

In a further refinement, the inductive sensor is a switching sensor with a switch, the switch being open in particular in the case of at least one switch-on region of the at least one parameter and the switch being closed in the case of at least one switch-off region of the at least one parameter.

In a further embodiment, a method described in this patent application can be executed with the inductive sensor.

The method for operating an inductive sensor comprises the following method steps: placing an LC resonant circuit with a coil and a capacitor in an electromagnetic oscillation, wherein a current is conducted through an inductance of the coil, wherein the one coil winding of the coil has a temperature coefficient less than 3 * 10 ^-3 K ^-1 , preferably less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 , changing at least one electrical and / or magnetic parameter of the LC resonant circuit with an electrical conductive object, detecting and evaluating the change of the at least one electrical and / or magnetic parameter of the LC resonant circuit due to the electrically conductive object for local detection of the electrically conductive object to the LC resonant circuit, wherein the voltage drop across the measuring resistor of an amplifier electrically amplified and the amplified voltage supplied to the capacity w ill.

The at least one electrical and / or magnetic parameter comprises a frequency and / or amplitude and / or impedance of the LC resonant circuit.

In a further embodiment, the current and / or the voltage is amplified such that a voltage drop in the LC resonant circuit on the coil due to the leakage resistance is substantially balanced and / or the current through the inductor, in particular a coil with a coil winding with a specific electrical resistance at 20 ° C, which is greater than 2 * 10 ^-6 Ω cm, preferably greater than 5 * 10 ^-6 Q cm or 10 ^-5 Ω cm, in particular greater than 2 * 10 ^-5 Ω cm or 10 ^-4 Ω cm, is.

According to the invention, the stimulation of the electromagnetic oscillation of the LC resonant circuit is effected by an excitation unit.

• 1 ein Schaltbild eines induktiven Sensors und
• 2 ein Diagramm einer Resonanzüberhöhung des induktiven Sensors.

In the following, an embodiment of the invention will be described in more detail with reference to the accompanying drawings. It shows:

• 1 a circuit diagram of an inductive sensor and
• 2 a diagram of a resonance peak of the inductive sensor.

An inductive sensor 1 ( 1 ) is used in various technical applications for detecting a metallic object in the spatial area or in the vicinity of the inductive sensor 1 used. The sensor 1 includes an LC resonant circuit 2 and an evaluation unit 15 and an excitation unit 16 , The LC resonant circuit 2 is with a first electrical connection 13 and a second electrical connection 14 electrically with the evaluation and excitation unit 15 . 16 connected as a unit. With the excitation unit 16 becomes the LC resonant circuit 2 supplied electrical energy, so that the LC resonant circuit 2 is placed in an electromagnetic oscillation and thus by a coil 5 of the LC resonant circuit 2 an electromagnetic field is emitted. Located in the vicinity of the LC resonant circuit 2 or the coil 5 a metallic object or is a metallic object in the vicinity of the LC resonant circuit 2 or the coil 5 moves, induces that from the coil 5 emitted electromagnetic field in the metallic object an eddy current, so that thereby the located in the electromagnetic oscillation LC resonant circuit 2 Energy is withdrawn and thereby at least one parameter, for. As the frequency, the amplitude or the impedance of the LC resonant circuit 2 is changed. These Change of the at least one parameter can with the evaluation 15 recorded and thus also the metallic object are detected. Preferably, the evaluation unit comprises 15 also a switch, so that, for example, in a metallic object in the vicinity of the LC resonant circuit 2 the switch (not shown) is closed and no metallic object near the LC resonant circuit 2 the switch is open. Alternatively, it is of course also possible to generate an analog output signal via the evaluation of the at least one parameter.

The LC resonant circuit 2 includes a coil 5 with an inductance Ls as inductance 3 and a capacity C as capacity 6 , which with power lines 12 are connected. The sink 5 comprises a coil winding with a wire of a resistance alloy, so that the temperature coefficient of the wire of the coil winding is very small, that is, for example, about 10 ^-5 K ^-1 . Resistor alloys also have a high resistivity. The sink 5 of the LC resonant circuit 2 is in 1 represented with a coil model as an equivalent circuit diagram. The sink 5 has a loss resistance in the coil model 18 with a loss resistance R _s and a damping resistor 17 with a Dämppungswiderstand Rp on. In fact, the damping resistance 17 and the loss resistance 18 from the coil 5 educated. The loss resistance 18 is a consequence of the specific electrical resistance of the wire of the coil winding and the variable damping resistor 17 based on AC losses caused by the metallic object near the coil 5 and other effects such as skin and proximity effect. The size Rp of the damping resistance 17 during an electromagnetic oscillation of the LC resonant circuit is at several kQ, z. In the range of 5 to 50 kΩ.

In line with the coil 5 and the capacity 6 that with the power lines 12 connected to each other is an electrical amplifier 9 with an input connection 10 and an output terminal 11 arranged. The input connection 10 is electric with the power line 12 with the coil 5 connected and the output terminal 11 is with the capacity 6 connected. The power line 12 which the coil 5 with the input connector 10 or the amplifier 9 connects, points a node 19 on, on which a measuring resistor 8th to the power line 12 with connected. This power line 12 with the measuring resistor 8th is also connected to the second electrical connection 14 connected or forms the second electrical connection 14 , The electrical resistance R of the measuring resistor 8th and the electrical resistance Rs of the coil 5 and the wire of the coil winding are approximately equal and R and Rs are small in a range of approximately 2 to 30 Q. For example, R = R _s = 10 Ω. The amplifier 9 has a gain factor A on and the gain factor A is about $$\text{A}=1+{\text{R}}_{\text{s}}/\text{R}\text{,}$$

At R = R _s = 10Ω, the gain factor A = 2. Because of the loss resistance R _s the coil 5 occurs a voltage drop across the coil 5 during the electromagnetic oscillation of the LC resonant circuit 2 on.

This voltage drop due to the loss resistance R _s can thus with the amplifier 9 be substantially compensated by adjusting the gain and the correspondingly increased flow of capacity 6 be supplied. The wire of the coil winding of the coil 5 is made of a resistance alloy and has a high resistivity, so that the resulting loss resistance R _s compared to a wire made of copper is great. The loss resistance R _s leads to a damping of the electromagnetic oscillation of the LC resonant circuit 2 , This attenuation can be with the amplifier 9 be balanced, so that when an excitation of the LC resonant circuit 2 with the excitation unit 16 a larger resonance peak occurs than without amplification with the amplifier 9 , In 2 the abscissa represents the frequency in Hz and the ordinate the amplitude as the voltage in V. In the upper curve 20 in 2 is the resonance peak with gain and in the lower graph 21 the resonance peak is shown without amplification.

The inductive sensor 1 has a coil winding made of a resistance alloy. The resistance alloy has a small temperature coefficient, so that the inductive sensor 1 in its sensitivity, ie the degree of change of at least one parameter of the LC resonant circuit 2 with a metallic object near the coil 5 , is subject to very small temperature changes. The large specific electrical resistance of the resistance alloy causes a large loss resistance R _s and thus also a large attenuation of the electromagnetic oscillation of the LC resonant circuit 2 with the consequence that the sensitivity, ie the size of the spatial distance between the metallic object and the LC resonant circuit 2 within which a metallic object can be detected would be small because very small changes of the at least one parameter of the LC- resonant circuit 2 from the evaluation unit 15 below certain sizes can no longer be detected. With the help of the amplifier 9 can the damping due to the loss resistance R _s be substantially balanced, so that the LC resonant circuit 2 with the amplifier 9 has a high sensitivity, ie it can metallic objects in a large spatial distance to the LC resonant circuit 2 or the coil 5 be detected and this essentially independent of the temperature of the inductive sensor 1 ,

The impedance Z of the LC resonant circuit 2 with reinforcement is: $$\text{Z}\left(\text{s}\right)=\frac{R+Rs+s*Ls*\left(1+\frac{R+Rs}{Rp}\right)}{1+\frac{s*Ls}{Rp}+s*C*\left(R+Rs-A*R\right)+s*s*\text{Lp}*\text{C}*\left(1+\frac{\text{R}+\text{Rs}-\text{A}*\text{R}}{\text{rp}}\right)}$$

und die Impedanz Z beträgt bei Resonanz $$\text{Z}\left(\text{j}\mathrm{\omega }\right)=\frac{Lp\ast \left(R+Rs+Rp\right)}{Ls+C\ast \left(R-A\ast R+Rs\right)\ast Rp}$$

In the LC resonant circuit 2 is the resonance frequency ω $$\mathrm{\omega \; =}\frac{1}{\sqrt{Lp*C}}*\sqrt{\frac{\left(Ls+C*\left(\left(A-1\right)*R-Rs\right)*\left(R+Rs\right)\right)*Rp*Rp}{Ls*\left(\left(1-A\right)*R+Rs+Rp\right)*\left(R+Rs+Rp\right)}}$$

and the impedance Z is at resonance $$\text{Z}\left(\text{j}\mathrm{\omega }\right)=\frac{Lp*\left(R+Rs+Rp\right)}{Ls+C*\left(R-A*R+Rs\right)*Rp}$$

In the last equation, almost all variables fall out for R = R _s and A = 2, so that $$\text{Z}\left(\text{j}\mathrm{\omega }\right)=2*\text{R}+\text{rp}\text{,}$$

Rp is much larger than R or R _s (Rp >> R and R _s , respectively) because, for example, Rp is about 5 to 50 kQ and R = R _s = 10 Ω. If R and R _{s are} not substantially temperature-dependent or temperature-stable, the impedance essentially depends only on Rp, ie the AC losses and the attenuation by the target or the metallic object.

Overall, are considered with the inductive sensor according to the invention 1 significant benefits. The wire of the coil winding of the coil 5 consists of a resistance alloy, so that the loss resistance R _s only depends to a very small extent on the temperature or is substantially stable in temperature. As a result, the inductive sensor has 2 via a substantially non-temperature-dependent sensitivity or the switching distance of the inductive sensor 1 essentially does not depend on the temperature. With the electric amplifier 9 becomes the high loss resistance R _s the coil 5 balanced so that the LC resonant circuit 2 only has a low attenuation or a small impedance and thus has a high sensitivity and a large switching distance. The inductive sensor 1 is simply built and therefore inexpensive to manufacture in an advantageous manner.

LIST OF REFERENCE NUMBERS

1

Inductive sensor

2

LC resonant circuit

3

inductance

5

Kitchen sink

6

capacity

8th

measuring resistor

9

Electric amplifier

10

Input terminal of the amplifier

11

Output terminal of the amplifier

12

power line

13

First electrical connection

14

Second electrical connection

15

evaluation

16

excitation unit

17

damping resistor

18

loss resistance

19

junction

20

Upper curve

21

Lower curve

R _s

loss resistance

R _p

Damping resistance of the coil

R

Resistance of the measuring resistor

A

Amplification factor of the amplifier

L _s

Inductance of the coil

C

Capacitance of the capacitor

Z

impedance

Claims (12)

An inductive sensor (1) for detecting an electrically conductive object, comprising: - an LC resonant circuit (2) having a coil (5) and a capacitor (6), the coil (5) having an inductance (3), a loss resistor (18 ) and a damping resistor (17) and a temperature coefficient of the loss resistance (18) is less than 3 * 10 ^-3 K ^-1 , - an excitation unit (16) for displacing the LC resonant circuit into an electromagnetic oscillation and - an evaluation unit (15 ) for detecting a change of at least one parameter of the LC resonant circuit (2) due to the electrically conductive object, wherein the excitation unit (16) and the evaluation unit (15) with first and second electrical terminals (13, 14) with the LC resonant circuit ( 2) are electrically connected, wherein the LC resonant circuit (2) comprises an electrical amplifier (9) and a series-connected to the coil (5) measuring resistor (8) and wherein the amplifier (9) a Spannungsa bfall due to the loss resistance (18) on the coil (5) is at least partially compensated. Inductive sensor after Claim 1 , characterized in that the amplification factor of the amplifier (9) substantially corresponds to the sum of 1 and the ratio of the resistance of the loss resistor (18) to the resistance of the measuring resistor (8). Inductive sensor according to one of the preceding claims, characterized in that the amplifier (9) has an input terminal (10) and an output terminal (11) for amplifying the voltage applied to the input terminal (10) with respect to the output terminal (11) and the input terminal (11). 10) is electrically connected to the node (19) with a coil (5) and the measuring resistor (8) connecting power line (12) and the output terminal (11) is electrically connected to the capacitor (6). Inductive sensor according to one of the preceding claims, characterized in that the coil (5) as an inductance (3) with a coil winding, in particular of a wire and / or a strand is formed. Inductive sensor after Claim 4 , characterized in that the electrical resistivity of the coil winding at 20 ° C greater than 2 * 10 ^-6 Ω cm, preferably greater than 5 * 10 ^-6 Ω cm or 10 ^-5 Ω cm, in particular greater than 2 * 10 ^-5 Ω cm or 10 ^-4 Ω cm. Inductive sensor after Claim 4 or 5 , characterized in that the temperature coefficient of the coil winding is less than 3 * 10 ^-3 K ^-1 , preferably less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 . Inductive sensor after Claim 4 , characterized in that the coil winding, at least partially, in particular completely, of a resistance alloy, preferably NiCr20AISi and / or NiCr8020 and / or CuNi44 and / or CuMn12Ni and / or CuMn7Sn exists. Inductive sensor according to one of the preceding claims, characterized in that the temperature coefficient of the measuring resistor (8) is less than 3 * 10 ^-3 K ^-1 , preferably less than 10 ^-3 K ^-1 or 10 ^-4 K ^-1 or 10 ^-5 K ^-1 , is. Method for operating an inductive sensor (1), comprising the steps of: - placing an LC resonant circuit (2) with a coil (5) and a capacitor (6) in an electromagnetic oscillation with an excitation unit (16), wherein a current through an inductance (3) of the coil (5) is passed, wherein the one coil winding of the coil (5) has a temperature coefficient of less than 3 * 10 ^-3 K ^-1 , - changing at least one electrical and / or magnetic parameter of the LC resonant circuit (2) with an electrically conductive object, - detecting and evaluating the change of the at least one electrical and / or magnetic parameter of the LC resonant circuit (2) due to the electrically conductive object for the local detection of the electrically conductive object on the LC resonant circuit (2 ), wherein a voltage drop across a measuring resistor (8) is electrically amplified by an amplifier (9) and the amplified voltage is fed to the capacitor (6). Method according to Claim 9 , characterized in that the at least one electrical and / or magnetic parameter is a frequency and / or amplitude and / or impedance of the LC resonant circuit (2). Method according to one of Claims 9 or 10 , characterized in that the current and / or the voltage is amplified so that a voltage drop in the LC resonant circuit (2) on the coil (5) due to the loss resistance (18) is substantially balanced. Method according to Claim 11 , characterized in that the current through the coil (5) is conducted with a coil winding, wherein the coil winding has a specific electrical resistance at 20 ° C of greater than 2 * 10 ^-6 Ω cm, preferably greater than 5 * 10 ^-6 Ω cm or 10 ^-5 Ω cm, in particular greater than 5 * 10 ^-5 Ω cm or 10 ^-4 Ω cm, has.

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