DE9412765U1 - Inductive proximity sensor for material-independent distance measurement - Google Patents

Description

Title: Inductive proximity sensor for material-independent distance measurement Description

The invention relates to an inductive proximity sensor for measuring the distance of a measuring object, preferably made of metal. The output signal of a proximity switch around the adjusted switching point is almost only dependent on the measuring distance and independent of the material properties of the measuring object.

An inductive proximity switch for measuring distance usually works with an LC ParaHel oscillating circuit, because an oscillating circuit in resonance has a low auxiliary energy requirement. The alternating magnetic field of the coil of the LC oscillating circuit serves as a sensor and is influenced by an electrically conductive and magnetically conductive (i.e. soft magnetic or ferromagnetic due to its permeability) measuring object, usually made of metal. The influence of the sensor coil depends not only on the distance of the measuring object but also on the material properties of the various measuring objects, so that a material actuator for different materials must be introduced in practical application so that the measured distance or switching point is corrected accordingly. The material actuators for different metals are specified in the book by Andreas Schiff: "Inductive and capacitive sensors; The library of technology 24, Verlag moderne Industrie, 1989, page 28".
For sensor manufacturers, it is very time-consuming and costly to determine in advance the damping materials that a customer will use in large-scale production. Therefore, numerous circuits for inductive proximity switches are known that measure the distance without subsequent correction when measuring objects made of different metals approach.

All circuits have the disadvantage of a high level of construction effort in their implementation. The material-independent measurement is only implemented using additional coils and the voltage or current dividers formed by them, so

that the achievable measurement sensitivity is reduced.

The German patent 3919916 describes an inductive proximity switch for material-independent distance measurement of a measuring object. The sensor uses two spatially separated coils. By adjusting the magnetic coupling between the two coils, the material dependence of damping metals is reduced. However, this is difficult to achieve in practical production and the technical effort is high.

Another inductive proximity switch according to European patent 0537747 uses a resonator with a coil, two capacitors and a resistor instead of a conventional LC parallel resonant circuit. The frequency of the resonant circuit serves as the output signal. The capacitive voltage divider also significantly reduces the measurement sensitivity.

Another example is known from the German patent 3714433. In this inductive proximity switch there is an oscillator that delivers a fixed frequency and is loosely coupled to the sensor oscillating circuit. By adjusting the coupling, the material dependency is adjusted. Here, too, the measurement sensitivity is significantly reduced.

All known methods differ significantly from the conventional and most common measuring methods with an LC parallel oscillating circuit (a coil parallel to a capacitor). Implementing material-independent distance measurement on the basis of an LX>parallel oscillating circuit is therefore of great importance in industrial applications and has a significant advantage, because the previous sensor properties, e.g. the temperature behavior, are retained and the electronic circuit and mechanical structure used previously only need to be changed slightly or not at all.

The invention is based on the object of measuring the distance around its switching point using an inductive proximity switch, regardless of the material properties of the metal object, and of specifying a circuit which is constructed using a conventional LC oscillating circuit.

The invention solves this problem primarily according to claim 1 in that in the known measuring methods with an LC parallel or LC series resonant circuit, an additional measurement of the resonance frequency, a conversion of this frequency into a voltage dependent on this frequency and a mathematical linking of this frequency-dependent voltage with the voltage induced in the coil of the LC resonant circuit or with the current induced in the coil of the LC resonant circuit is carried out, so that the output signal around the switching point is almost only dependent on the distance.

In this invention, either an LC parallel resonant circuit or an LC series resonant circuit is used. The induced voltage or current on the resonant circuit is measured, rectified with a rectifier and output with an adjustable amplifier, preferably as a voltage output signal.

The second value measured is the resonance frequency of the oscillating circuit. This frequency is converted to a direct voltage using a frequency-voltage converter or converter and then amplified using an adjustable amplifier. The voltage output signal of the LC oscillating circuit is summed or subtracted with the voltage signal from the resonance frequency using an adder or subtractor, so that the output signal is the sum or difference of the two voltages.

The gains of the two adjustable amplifiers for the two voltage signals are set so that at the switching point the output signals are the same when a magnetically conductive (soft magnetic or hard magnetic or ferromagnetic) measuring object approaches and when an electrically conductive, non-ferromagnetic measuring object approaches. When a third measuring object made of a different ferromagnetic or non-ferromagnetic material approaches, the material dependency is significantly reduced.

Embodiments of the invention are explained in more detail below with reference to drawings:

Figure 1 shows an LC parallel resonant circuit consisting of a measuring coil and

a capacitor in the inductive proximity sensor, whereby the coil is damped with a metallic measuring object; Figure 2a shows the behavior of the induced voltage IL according to Fig. 1 as an output signal as a function of the distance a when different damping metals approach;

Figure 2b shows the behavior of the resonance frequency f for Fig. 2a; Figure 3a shows the basic behavior of the induced voltages for an aluminum and a steel plate, as well as the improved output voltages U _F and U F corrected by the invention at the switching point a».

U^ for Fig. 3a;
Figure 3b shows the basic behavior of the resonance frequency f for a

aluminum and a steel plate;
Figure 4 shows a block diagram for the realization of the material-independent

distance measurement;
Figure 5 shows a possible circuit for realizing the material-independent

distance measurement;
Figure 6 shows the behaviour of the output signal of the circuit according to Fig. 5 for the

Behavior according to Fig. 2a and 2b;
Figure 7 shows a block diagram for the realization of material-independent distance measurement when using digital components for digital signal processing.

According to Figure 1, the LC parallel oscillating circuit of an inductive proximity sensor, which is excited in resonance in an oscillator, consists of a coil 1 with a ferrite core and a capacitor 2 with the capacitance C connected in parallel to the coil, whereby the coil is damped with a metal object 3. The coil 1 is shown in Figure 1

represented by an equivalent resistance R and an equivalent inductance L. The admittance Y of the resonant circuit is:

-4-T (1)

R+jciL

In the case of resonance (angular frequency «q = 2π&iacgr;) the imaginary part of Y is zero. Therefore, <"> _n C = · (2)

By transformation one obtains

1-

A ² C

JlJT. \ L>

1 R^C

, where 1 > is assumed.

In the operating state of the inductive proximity sensor, the oscillating circuit is excited at its resonance frequency by feedback with active components both with metal damping 3 and without metal damping. The resonance frequency «« only depends on the value of the equivalent inductance L if the capacitor C is fixed. In this case, the complex induced voltage U decreases with the damping. According to equations (1) to (3), the effective value of U in the resonance case is

^R (4)

= / ( Jl. + R) CR

~ r L

CR

For an inductive proximity sensor as a switch, the switching distance a should be as large as possible, for example larger than 1/3 of the diameter of the coil. In such cases, L/CR > R. The approximation according to equation (4) can therefore be used. A typical curve of the voltage U when approaching different metal dampings as a function of the distance a around the frequency 120 kHz is shown in Figure 2a. The coil diameter was 25 mm. The subsequent signal evaluation in a conventional inductive proximity switch is carried out by rectifying the voltage U and then evaluating it with a Schmitt trigger and comparator to determine the switching point.

The other measurable quantity is the resonance frequency f. Its course as a function of the distance a is shown in Figure 2b.

For a simplified representation, the differences in the voltage U between an electrically conductive measuring object (here: an aluminum plate) and an electrically conductive and magnetically conductive (permeability) measuring object (here: steel plate) are shown in Figure 3a and the differences in the resonance frequency f in Figure 3b. At the switching point at a fixed distance a ₀ , the difference voltage AU(a _Q ) between the voltages U^a ₀ ) and U _Fe (a _Q ) is obtained (Figure 3a).

- U^a ₀ ) (5)

At the same time, a frequency difference Af(a _Q ) between the resonance frequencies of the oscillating circuit when using an aluminum plate fy(a ₀ ) ¹ ^ a steel plate

fp (a ₀ ) is present (Figure 3b).

fp (a ₀

In order to obtain the material independence of the switching point at the distance a ₀ , U and f are linked using the differential method by the following new function

where k is adjustable.

If a steel plate is located at a distance a _ß , the new output signal Ü&ldquor; (a ₀ ) is obtained for a measuring object made of steel.

If an aluminum plate is located at a distance a _fl , the new output signal U^a ₀ ) is obtained for a measuring object made of aluminum.

The two voltages become equal

if applies

At the operating point a _ß set in this way, the voltage ü(a ₀ ) no longer changes when using a measuring object made of aluminum or steel. The difference in the voltage ü (a ₀ ) when using an aluminum and a steel plate is

OJaJ - Ü _Fe (aJ

UJfiJ - IfJaJ - U _Fe (aJ

According to Figure 3a, an intersection point of the curves U^ia) and Up (a) is obtained at point a _ß . A material-independent measurement of the distance for steel and aluminum is thus realized at the operating point a _ß .

From Figures 3a and 3b it is clear that at any point a with a + a _Q the differences AU(a) and Af(a) change differently. This gives

= UJa) - U _Fe (ä) (13)

= /Ja) - f _Fe (fi) . (14)

Using equations (7) and (11) we obtain a new distance dependence u _pe (a) for a steel measuring object

and for a measuring object made of aluminium

At any point a, we now have

UJa) - Ü _Fe (a)

UJa) - kfjä) - U _Fe (a) +

= AU(a) -

UH)

and
AÜ(a) _

There exists therefore a neighborhood around point a _fl in which

\AÜ(ä)\ * \AU(a)\ .
From equation (18) we get

) _AU (a) . (20)

AU(a)j

Equation (19) is satisfied if in equation (20)

-1,1- ^IMEL , !. (21)

AAo ₀ ) &Dgr;&iacgr;&Zgr;(&agr;)

The environment is then determined by the condition

(O ₀ ) Δζ(α)

described.

Due to the monotony in the entire measuring range, for example according to Figure 2a and 2b,

^U Cu ^{> U} Al ^{> U} Fe ^And fcu > fjU ^> fpe ' ^Wem %Cu ^> %M ^And

The left inequality is automatically fulfilled, since sign[AU(a _Q )] = sign[AU(a)] and sign[Af(a ₀ )] = sign[Af(a)]. The fulfillment of the right inequality depends crucially on the choice of the point a _fl . With high resonance frequencies, for example here around 120 kHz, the right inequality is always fulfilled. The condition according to equation (22) is the prerequisite for the subsequent evaluation with a Schmitt trigger and comparator to work according to the invention. It is therefore possible to carry out a distance measurement at and around the switching point a _Q for steel and aluminum that is almost independent of the material.

When a third measuring object, for example a copper plate, is approached, the intersection point is not exactly the same as in Figure 3a with the aluminum plate, but the difference in the output signals between copper and steel is significantly reduced. The same applies to other ferromagnetic and non-ferromagnetic metals.

Figure 4 shows the block diagram of an embodiment of a material-independent distance measurement of an electrically conductive or magnetically conductive measuring object according to the invention. An LX>parallel oscillating circuit, consisting of the coil 1 and the capacitor 2, is excited to natural oscillations at its resonance frequency f by a known oscillator circuit 4, which is not shown in detail. The resonance frequency f of the oscillating circuit is measured by converting it into a frequency-proportional direct voltage signal U _f using a frequency-voltage converter 5. This direct voltage signal U _f is then amplified into the voltage U' _f using a known voltage amplifier 6, the gain factor k _f of which is adjustable. Parallel to 5 and 6, the alternating voltage signal U is measured at the oscillating circuit, in which the alternating voltage U is rectified using a known rectifier 7 and converted into the direct voltage U _z . This direct voltage signal IL· is then converted into the direct voltage U z using a known

&diams; · &diams;

#·
* ·· > &iacgr;&iacgr; ·· ·· . ·» * » #· ft * «

known voltage amplifier 8 into the voltage U ₂ ', whose amplification factor k _z is adjustable. The two direct voltage signals U ₂ ' and U' _f are subsequently subtracted from each other with an analog subtractor 9, so that the output signal

Ü{a) = & _z - u' _f (24)

represents the difference between the two voltage signals. The output voltage signal IJ (a) is evaluated with a comparator (not shown) known per se for setting the switching point, with a Schmitt trigger 10 known per se for setting any switching hysteresis that may be necessary, and is finally output to a subsequent switching amplifier 11 for controlling and switching higher power levels. The gain factors k. and k are set so that at the intended operating point or switching point a _fl the output signal ü(a) is the same when an electrically conductive, ferromagnetic metal plate, for example made of steel, approaches and when a non-ferromagnetic metal plate, for example made of aluminum, approaches. Each of the two voltage amplifiers 6 and 8 can, for example, be a differential amplifier known per se. The voltage U _f is applied to one input of the voltage amplifier 6 and an adjustable direct voltage source 12 with the constant voltage V^ is applied to the second input, where U _rf is subtracted from the voltage U- and is set so that the output voltage U' cannot reach the supply voltage of the subtractor 9. The voltage U ₂ is applied to one input of the voltage amplifier 8 and an adjustable direct voltage source 13 with the constant voltage U _r2 is applied to the second input, where U _rZ is subtracted from the voltage U ₂ and is set so that the output voltage U ₂ ' cannot reach the supply voltage of the subtractor 9. The embodiment according to Figure 4 implements a circuit according to equation (7) if the gain factor of the voltage amplifier 8 is set to k _z = 1, the voltage U _r2 = 0 and the gain factor of the voltage amplifier 6 is set to kj = k.

An inventive embodiment of the basic circuit according to Figure 4

with the two simplifications k = 1 and U _z = 0 is shown in Figure 5. Because U _z = 0 the direct voltage source 13 and the amplifier 8 according to Figure 4 are omitted. In the description only the essential circuit parts will be explained in more detail. The circuit modules used according to Figure 5 to implement the functional blocks described in Figure 4 are known per se. The IX parallel oscillating circuit with the measuring coil 1 and the capacitor 2 is excited at its resonance frequency by feedback (resistor Rl) with an operational amplifier IC1; the arrangement shown represents a possible embodiment for the oscillator 4. The subsequent operational amplifier IC2 works as a voltage follower or impedance converter with a gain of one, so that the oscillator 4 is not loaded by the subsequent circuit stages. The frequency-voltage converter 5 operates with a comparator IC3 and a known integrated phase-locked loop (PLL) circuit, for example the type 4046 from RCA. The comparator IC3 forms square-wave signals of constant amplitude from the sinusoidal input signals. The integrated PLL circuit, consisting of a phase comparator, a low-pass filter R6 and C4 and a voltage-controlled oscillator VCO, produces an output voltage U _f which is proportional to the frequency f of its input signal. The signal U _f is amplified by the amplifier 6. The amplifier 6 is designed as an instrument amplifier, consisting of the operational amplifiers IC8, IC9 and IC10, the gain k _f of which is set with the potentiometer r4 so that the output signal ü(a) at the switching point a _Q is the same when two different metal plates (for example steel and aluminum) approach each other. The second input of the instrument amplifier is the direct voltage V^ which is generated by the direct voltage source 12 in the manner shown with a Zener diode and an operational amplifier IC7. The amplitude of the direct voltage U- is adjusted with the potentiometer rl so that the output signal ü(a) cannot be as large as the supply voltage U _r of the operational amplifier ICH. The alternating voltage signal U of the oscillator 4 is rectified into the direct voltage signal -U _z with a known precision full-wave rectifier 7, consisting of the operational amplifiers IC4 and IC5 and the diodes Dl and D2.

the amplifier 8 is not present, the signal -U ₂ ' is equal to the signal -U ₂ . The direct voltage signals -U ₂ ' and U' _f are subtracted from one another using the adder 9, which acts as a subtractor and consists of the operational amplifier ICH and the resistors R15, R16 and R17, and then the output direct voltage signal ü(a) is formed, since the full-wave rectifier 7 inverts the signal U ₂ .

Ü(a) = -[ U' _f * (-Ufo = U' _z - U' _f (25)

This direct voltage signal ü (a) is evaluated with the known Schmitt trigger 10, so that a switching signal U is produced at the output of the Schmitt trigger 10. The subsequent switching amplifier 11 is not shown in Figure 5. The Schmitt trigger 10 consists of the two comparators IC12 and IC13 and an RS flip-flop IC14. The switching point a _fl and the switching hysteresis of the Schmitt trigger 10 are set with the two potentiometers r2 and r3.

Figure 6 shows the measurement results of the output signal U (a) corrected according to the invention as a function of the distance a between two metal objects (steel and aluminum) for an embodiment according to Figure 5 at an oscillator operating frequency f of about 120 kHz. The comparison with the oscillator amplitude U according to Figure 2a, which is evaluated in the previously known inductive proximity sensors, shows a significantly improved synchronization of the distance dependence of the output signal. According to Figures 2a and 2b, for a ferromagnetic metal object and for a large distance between the object, the distance dependence of the oscillator amplitude U is significantly greater than the distance dependence of the resonance frequency f. For a non-ferromagnetic metal object and for a large distance between the object, the behavior is reversed; i.e. the distance dependence of the oscillator amplitude U is in this case significantly less sensitive than the distance dependence of the resonance frequency f. This means that the measurement of the distance for a non-ferromagnetic metal object using the oscillator voltage amplitude U, i.e. the inductive voltage U of the measuring coil, is less suitable than using the resonance frequency f.

In a further embodiment of the invention, according to Fig. 7, the use of digital components for digital signal processing has proven to be effective in increasing accuracy and reducing the number of electronic circuits. For this purpose, the induced voltage U of the LC parallel resonant circuit or the induced current I of the LC series resonant circuit is converted by a rectifier 7 into the direct voltage signal U" and this is converted by a known analog/digital converter 14 into a digital signal X" proportional to it. Furthermore, the resonance frequency f of the resonant circuit is converted by a known digital frequency converter 15, preferably a frequency counter, into a frequency-proportional digital signal X. The subtraction of the two digital signals X _z and X _f is carried out by means of a digital subtractor 16 known per se, for example by writing one digital signal into a digital memory known per se and subtracting the second digital signal from this memory content, so that the remaining memory content represents the difference X between the two digital signals X _z and X _f . The quantizations of the analog/digital converter 14 and the frequency converter 15 are set so that the differences X for two different measurement objects made of different materials are the same at the operating point a _fl . The relationships explained in more detail above apply accordingly when the signals are digitized, so that further explanations can be omitted here. For a ferromagnetic metal object, the change in the oscillator voltage amplitude and thus the induced voltage U of the measuring coil is essentially recorded. The change in the amplitude of the induced voltage of the measuring coil is essentially a measure of the change in the quality Q of the measuring coil. In the case of an electrically conductive, non-ferromagnetic metal object, for example made of aluminum, the change in the resonance frequency f and thus essentially the change in the reciprocal of the pure coil inductance L or the change in the imaginary part of the complex coil conductance is recorded.

A proximity sensor operating according to the method according to the invention advantageously combines the known measurement of the quality of a measuring coil with the simultaneously measured coil inductance. A further advantage of the inventive

The aim of this method is to significantly increase the measurement sensitivity for non-ferromagnetic metal objects while maintaining the measurement sensitivity for ferromagnetic metal objects using the known quality measurement methods,
A further advantage of the method according to the invention is the use of resonance frequencies significantly above 10 kHz, so that the use of coils with small dimensions is possible and thus the condition according to equation (22) is fulfilled.

Claims (29)

tea* « » · · · &igr; Title: Inductive proximity sensor for material-independent distance measurement Claims 1. Inductive proximity sensor for material-independent measurement of the distance of an electrically conductive or a magnetically conductive, preferably soft magnetic or ferromagnetic, measuring object 3 using a parallel or series oscillating circuit, consisting of a coil 1 acting as an inductive proximity sensor and an oscillating circuit capacitor 2, wherein the oscillating circuit is excited to natural oscillations in an oscillator 4 at its resonance frequency f,
characterized, - that an electrical or electronic rectifier 7 is present in order to measure the induced alternating voltage U in the parallel resonant circuit or the induced current I in the series resonant circuit as a measure of the resonance resistance Z of the resonant circuit, so that the induced voltage U or the induced current I is converted into a signal U or X proportional to the resonance resistance, - that an electrical or electronic frequency converter 5 or frequency converter 15 is present to measure the resonance frequency f of the oscillating circuit, so that the resonance frequency f of the oscillating circuit is converted into a frequency-proportional signal U _f or X _f , and - that an electrical or electronic subtractor 9 or 16 is present in order to form the difference signal Ü or X from the signal U _z or &KHgr;_&khgr; and the signal U _f or X,. 2. Inductive proximity sensor according to claim 1,
characterized, - that a current/voltage converter converts the induced alternating current I of the series resonant circuit into an alternating voltage U and - that a rectifier 7 rectifies this alternating voltage U into the direct voltage U ₂ . 3. Inductive proximity sensor according to claim 2,
characterized, that the current/voltage converter is an ohmic resistor which is inserted into the series oscillating circuit, so that the induced alternating current I of the oscillating circuit flows through this ohmic resistor. 4. Inductive proximity sensor according to claim 2,
characterized, that the sensor coil 1 is the current/voltage converter, so that the alternating current I of the series resonant circuit generates the alternating voltage U at the sensor coil 1. 5. Inductive proximity sensor according to claim 2,
characterized, that the capacitor 2 is the current/voltage converter, so that the alternating current I of the series resonant circuit generates the alternating voltage U at the resonant circuit capacitor 2. 6. Inductive proximity sensor according to claim 2 or 3 or 4 or 5, characterized in that that the current/voltage converter has one of its two connections connected to the circuit ground, so that the alternating voltage TJ is generated against the circuit ground. 7. Inductive proximity sensor according to claim 1,
characterized, that a frequency/voltage converter 5 converts the resonance frequency f of the oscillating circuit into a frequency-proportional direct voltage U _f . 8. Inductive proximity sensor according to claims 1 to 6, characterized in that - that an electronic DC voltage amplifier 8 amplifies the DC voltage U _z into the DC voltage U' _z and - that electronic means are available to set and adjust the DC voltage gain k&rdquor; . 9. Inductive proximity sensor according to claim 1 or 7, characterized in - that an electronic DC voltage amplifier 6 amplifies the DC voltage U _f to the DC voltage U' _f and - that electronic means are available to set and adjust the DC voltage gain k _f . 10. Inductive proximity sensor according to claim 1 to 6 or 8, characterized in that - that the constant DC voltage source 13 generates the constant DC voltage U _z , which is subtracted from the DC voltage U _z by a subtractor in the DC voltage amplifier 8, and - that electronic means are available to set and adjust the constant direct voltage U _rZ . 11. Inductive proximity sensor according to claim 1 or 7 or 9, characterized in that - that the constant DC voltage source 12 generates the constant DC voltage U _ff , which is subtracted from the DC voltage LL by a subtractor in the DC voltage amplifier 6, and - that electronic means are available to set and adjust the constant direct voltage U _rf . 12. Inductive proximity sensor according to claim 8 or 9 or 10 or 11, characterized in that that either only the electronic DC voltage amplifier 6 with the constant DC voltage source 12 or only the electronic DC voltage amplifier 8 with the constant voltage source 13 or neither of the two DC voltage amplifiers and thus no constant voltage source is present. 13. Inductive proximity sensor according to claim 1 to 12, characterized in that that the subtractor 9 subtracts the direct voltage U' _f or IL from the direct voltage U' _z or U _z , so that the difference voltage Ü (a) arises when the direct voltage U' _z or U _z is greater than the direct voltage U' _f or U _f . 14. Inductive proximity sensor according to claims 1 to 12, characterized in that that the subtractor 9 subtracts the direct voltage U ' or U from the direct voltage U _f ' or U _f , so that the difference voltage Ü (a) arises when the direct voltage U _f ' or U _f is greater than the direct voltage Ly or U _z . 15. Inductive proximity sensor according to claims 1 or 8 to 14, characterized in that that electrical or electronic means are present for adjusting the direct voltage gains k _f or k of the direct voltage amplifiers 6 or 8 and electrical and electronic means are present for adjusting the constant voltage U _rf or U _rz of the constant voltage source 12 or 13, so that at a fixed operating point or switching point at the distance a of the measuring object 3 from the sensor coil 1, the differential voltage Ü.(a ) for a measuring object 3 made of material A is the same as the differential voltage Ü _ß (a ) for a measuring object 3 made of material B. 16. Inductive proximity sensor according to claim 1 to 6, characterized in that the converter 14 is an electronic analog/digital converter, so that the analog DC voltage U is converted into a proportional digital signal X. 17. Inductive proximity sensor according to claim 1 to 6, characterized in that that the converter 15 is an electronic frequency counter, so that the resonance frequency f of the oscillating circuit is converted into a frequency-proportional digital signal X _f . 19. Inductive proximity sensor according to claims 1 to 6 or 16 to 17, characterized in that that the subtractor 16 is a digital electronic subtractor, so that the digital difference signal X is formed from the subtraction of the digital signal X _z and the digital signal X _f . 19. Inductive proximity sensor according to claim 18,
characterized by
that the subtractor 16 is a digital semiconductor memory. 20. Inductive proximity sensor according to claims 16 to 19, characterized in that that electronic means for setting the quantization levels of the converter 14 and electronic means for setting the quantization levels of the converter 15 are present, so that in a fixed operating or switching point at the distance a _Q of the measuring object 3 from the sensor coil 1, the difference signal X _A (a _Q ) for a measuring object 3 made of material A is the same size as the difference signal X _ß (a _Q ) for a measuring object 3 made of material B. 21. Inductive proximity sensor according to claim 1 or 15 or 20, characterized in that that the material A or B of the measuring object 3 is an electrically conductive material, preferably metal. 22. Inductive proximity sensor according to claim 1 or 15 or 20, characterized in that that the material A or B of the measuring object 3 is a magnetically conductive material due to its permeability, for example a soft magnetic or ferromagnetic material. 23. Inductive proximity sensor according to claim 1 or 15 or 20, characterized in that that the material A or B of the measuring object 3 is an electrically conductive and due to its permeability magnetically conductive, preferably a soft magnetic or ferromagnetic material. 24. Inductive proximity sensor according to claim 21,
characterized, that the material A or B of the measuring object 3 is copper, brass, aluminium or bronze. 25. Inductive proximity sensor according to claim 22,
characterized, that the material A or B of the measuring object 3 is ferrite. 26. Inductive proximity sensor according to claim 23,
characterized, that the material A or B of the measuring object 3 is iron or steel. 27. Inductive proximity sensor according to claim 15 or 20,
characterized, that an electronic comparator 10 is present which compares the difference signal Ü (a) or X (a) with an adjustable constant comparison signal, so that the output signal of the comparator 10 indicates whether the difference signal Ü (a) or X (a) is greater or smaller than the comparison signal and so that the size of the comparison signal determines the operating or switching point at the distance a _Q. 28. Inductive proximity sensor according to claim 27,
characterized, that electrical or electronic means are available to set or adjust the size of the comparison signal, so that in this way the operating or switching point is set or adjusted at a distance a of the measuring object 3 from the sensor coil 1. 29. Inductive proximity sensor according to claim 27 or 28,
characterized, that the electronic comparator 10 contains a cut trigger so that a switching hysteresis is present around the operating or switching point, and
that electrical or electronic means are available to set and adjust the amplitude of the switching hysteresis.