Inductive presence or position sensor for detecting both ferrous and non-ferrous metals

An inductive sensor or detector includes as its sensitive element, preferably defining a front working plane of the sensor, a coil system forming an air-core transformer arrangement with a primary coil or winding (2) and a secondary coil or winding (3). The primary coil or winding of the system is associated with a capacitive component (4) in order to constitute a LC oscillating circuit whose oscillation is sustained by an adapted generator (5) in the form of an operational amplifier (6) and an associated resistance arrangement (R, R5, R13). The sensor also includes a signal processing circuit, for example signal adding (8), amplifying, converting (9) and/or evaluating circuits (10), fed by at least one signal provided by at least one component of the coil system. The inductive sensor comprises a direct or indirect feedback line (11) from the secondary coil or winding (3) to the input of the operational amplifier (6) of the generator (5).

BACKGROUND

The present application concerns the field of electromagnetic based detection and sensing, in particular in an industrial environment, and relates particularly to an inductive proximity sensor, detector or switch, which can work in a magnetic field and can detect both ferrous and non-ferrous metals.

More specifically, the present application concerns in particular an inductive sensor or detector of the type comprising:an inductive coil defining a front working plane of the sensor and associated with a covering plate or a plane part of a housing, said plate or part being disposed substantially perpendicular to the coil axis and parallel to its front working plane,means for supplying the coil or inductance repeatedly with current,means for processing signals which correspond to the voltages induced in said coil or inductance when fed, said induced voltages being influenced by the presence of objects or articles (targets) situated within a given detection area of said sensor.

Inductive proximity sensors using a coil as the sensitive element are already known. The working principals of this type of sensors are as follows.

When the coil with its associated flux field is placed close to the conductive target, the field establishes electric currents in the target. These currents are eddy currents, i.e. closed loops of induced current circulating (in a direction opposite to the current in the coil) in planes perpendicular to the magnetic flux, and generating their own magnetic field. Eddy currents normally run parallel to the coil windings and to the target surface. The eddy current flow is limited to the area in the target within the inducing magnetic field (seeFIG. 1).

The magnetic flux associated with the eddy currents opposes the coil's own magnetic flux. Decreasing the target-to-coil gap changes the inductance of the coil and thus the net flux of the system. The result is a change in the impedance of the coil and a voltage change across the coil. It is this interaction between the coil and the eddy current fields that is the  basis for determining target-to-coil position information with an eddy current position sensor.

The most common way of converting the impedance of the coil into electrical signal parameters is to make an LC generator with the inductance coil L as its sensing element. As the impedance of the inductance coil changes, parameters of the periodic signal at the output of the generator, such as amplitude and frequency, also change; thus making it possible, by providing a suitable electronic circuit, to detect a target as it approaches the sensing element of a detector. Similar designs were used in detectors described in the following patent and patent application documents: U.S. Pat. Nos. 4,942,372, 6,215,365, 6,664,781, DE-A-40 31 252, EP-A-0 304 272, 5,504,425, 6,335,619, 5,519,317, 5,952,822, EP-A-0 403 733, WO-A-00/76070.

As closest prior art, U.S. Pat. No. 5,027,066 discloses a distance detecting circuit that generates an electrical signal proportional to the linear displacement of an object. The functional diagram of the concerned device is shown inFIG. 4of said document and the concerned detector is actually a generator built around an operational amplifier67. Resistors R4and R5are used to set the required gain of the amplifier. The output of the amplifier is connected via resistor R6to an oscillating LC-circuit (elements71and65). Coil65is the primary winding of the transformer and its secondary windings63and64are connected to said primary winding via a moving core18. The linear movement of the core18changes the amount of induction factor between the transformer windings. Correspondingly, signal parameters change at the input of the detecting device24and the latter generates a voltage at its output that is proportional to the linear displacement of the core linked mechanically to the object.

The present application proposes an inductive position detector or sensor which shows at least some of the following improvements and additional features in comparison with the detectors known from the aforementioned documents, in particular from U.S. Pat. No. 5,027,066:

1. The detector or sensor should detect the presence of a target located at a certain distance in front of it whether this target is made of any ferromagnetic metal or of any non-ferrous/non-ferromagnetic metal.

2. The detector or sensor should be able to differentiate these two types of targets (ferromagnetic/non-ferrous).

3. The detector or sensor should remain operational when exposed to the effect of a constant or alternating magnetic field of industrial frequency.

4. The design of the detector or sensor should allow for its flush mounting with the frame of any material.

5. The sensor should be able to detect and evaluate an approaching article or object without any physical connection with the latter.

It is an aim of the present application to propose an inductive proximity (presence or position) sensor or detector showing at least some of the aforementioned benefits or improvements.

SUMMARY

To that end the present application concerns an inductive presence or position sensor or detector of the type comprising as its sensitive element, preferably defining a front working plane of the sensor, a coil system forming an air-core transformer arrangement with a primary coil or winding and a secondary coil or winding, said primary coil or winding of said system being associated with a capacity component in order to constitute a LC oscillating circuit whose oscillation is sustained by an adapted generator in the form of an operational amplifier and an associated resistance arrangement, the sensor also comprising signal processing means, for example signal adding, amplifying, converting and/or evaluating circuits, fed by at least one signal provided by at least one component of the coil system, inductive sensor characterized in that it comprises a direct or indirect feedback line from the secondary coil or winding to the input of the operational amplifier of the generator.

The present concepts will be better understood thanks to the following description and drawings of different embodiments of said invention given as non limiting examples thereof.

DETAILED DESCRIPTION

As shown onFIGS. 2 and 4to7, the concerned sensor or detector1is of the type comprising as its sensitive element, preferably defining a front working plane of the sensor, a coil system2,3forming an air-core transformer arrangement with a primary coil or winding2and a secondary coil or winding3. Said primary coil or winding2of said system2,3is associated with a capacitive component4in order to constitute a LC oscillating circuit whose oscillation is sustained by an adapted generator5in the form of an operational amplifier6and an associated resistance arrangement7. The sensor1also comprises signal processing means8,9,10, for example signal adding, amplifying, converting and/or evaluating circuits, fed by at least one signal provided by at least one coil component2or3of the coil system2,3.

In accordance with one aspect, said sensor or detector1comprises a direct or indirect feedback line11from the secondary coil or winding3to the input of the operational amplifier6of the generator5.

The general operating principle of the sensor1can for example be explained in relation withFIG. 2.

When the coil system2,3is approached by a target13(seeFIG. 10) of ferrous metal, the loss due to target eddy currents in the metal  causes the decline of the q-factor of the LC circuit2,4and of the amplitude of the sinusoidal oscillations of the U1 voltage. This, in turn, causes the decline of U2 voltage oscillation amplitude at the air transformer secondary winding output.

When the coil system2,3is approached by a non-ferrous target, the q-factor of the LC circuit2,4remains practically unchanged and the amplitude of the U1oscillations stays more or less constant. However, the degree of inductive coupling of the coils2and3decreases due to the diminishing value of the mutual induction factor M. Consequently, the oscillations at the secondary winding3output have smaller amplitude.

Preferably, the feedback line11comprises a low-stop filter12, in particular effective for frequencies below a few hundred Hertz, preferably below 60 Hz.

This high-pass or low-stop filter12is provided to ensure stability of circuit generation when exposed to external alternating magnetic fields of industrial frequency. Indeed, even if the circuit sensitive element (coil system2,3) has no core of ferromagnetic material, generation conditions can be affected by electromagnetic blast on windings2and3. Since industrial frequencies (around 50 or 60 Hz) differ by orders from the sensor generator5operating frequency (hundreds of KHz), the noise signal can be effectively suppressed by such a low-stop filter12, for example in the form of a double R-C circuit (seeFIG. 7).

According to a first embodiment, in connection withFIGS. 3 and 8, there is provided a basic embodiment of the invention wherein the sensor1delivers a single detection signal indicative of only a position information of a target object13with respect to the front working plane1′ of the sensor1, based on a single measurement signal representative of the voltage U2at the secondary coil or winding3.

In accordance with a second embodiment, in connection withFIGS. 2,5and6, there are provided more elaborate embodiments wherein the sensor1delivers a double detection signal indicative of a combined information of both position and constitutive material of a target object13approaching the front working plane1′ of the sensor1, said combined information signal being based on (a) measurement signal(s) provided to the processing means8,9,10and representative of the voltages U1and U2respectively at the primary and at the secondary coil or winding2,3.

In order to acquire interference free signals, it is preferred that the measurement signal representative of the voltage U2is picked up at the exit of a low-stop filter14, preferably the low-stop filter12incorporated in the feedback line11and that the measurement signal representative of the voltage U1is also picked up through a low-stop filter15(FIG. 7).

Low-stop or high-pass filters14and/or15have preferably a structure similar to the low-stop filter12, and serve the same purpose (suppression of noise generated by electromagnetic fields at industrial frequencies).

When the sensor or detector1has to provide a detection signal indicative of combined information (position and constituent material of the target object13), the processing means can be fed with two different signals, one representative of the U1 voltage and another representative of the U2 voltage. Said signals are combined by means of an adder8with preset ratios (for example an operational amplifier arrangement as inFIG. 7) which outputs a differential signal |U2−k U1| further processed by the following processing means9,10.

As an alternative, the measurement signal provided to the processing means8,9,10is representative of the differential voltage |U2−k U1|, said measurement signal being picked up at one end of the secondary coil or winding3, preferably through a low-stop filter12or15(having a similar structure), the other end of said latter being connected to a determined intermediate position IP of the primary coil or winding2, which defines the value of the coefficient k.

In this case, the adder can be replaced by a simple amplifier8(seeFIGS. 5 and 6).

As shown onFIGS. 2 and 4to7, the processing means preferably comprise, as components of a signal treatment chain, an adder with two inputs or a one way amplifier as a first chain component8, an AC/DC converter circuit as a second chain component9and a comparative circuit as a third chain component10, said latter issuing one or two logical output signal(s), depending of the number or the type of the input signal(s) at the first chain component8.

Converter9converts the sinusoidal signal applied to its input to a constant voltage level proportionate to the input amplitude.

The comparative circuit or decision box10initiates a logic signal at the first output (out1) with a target of ferrous metal approaching,  and at the second output (out2), with a target of non-ferrous metal approaching (in relation to embodiments ofFIGS. 2,5,6and7).

The evolutions of various signals with different types of targets approaching the front working plane1′ and the sensitive element (coil system2and3) are illustrated byFIG. 3:curve U1(Al) describes the amplitude variation of the voltage signal in point U1of the sensor circuit when a target of non-ferrous metal is approaching;curve U1(Fe) describes the amplitude variation of the voltage signal in point U1of the sensor circuit when a target of ferrous metal is approaching;curve U2(Al,Fe) describes the amplitude variation of the voltage signal in point U2of the sensor circuit when a target of any metal is approaching;curve U2*(Al,Fe) shows the amplitude of the linearly converted signal U2(U2*=kU2, where k=0.5 in the example shown on the diagram (when working the sensor1, the magnitude of coefficient k can be set by a choice of turns ratio in windings3(L2) and2(L1), namely, k=W2/W1where W2and W1are the number of turns in windings L2and L1, respectively);curves U3(Al) und U3(Fe) illustrate the signal amplitude variation at the output of adder8when a target13respectively made of non-ferrous metal U3(Al) and of ferrous metal U3(Fe) approaches the detector or sensor1.

As seen from the graphical diagrams ofFIG. 3, the approaching of a target13of non-ferrous metal (curve U3(Al)) results in an increasing amplitude of the signal at the output of adder4with respect to the initial level Uo, while the approach of a ferrous metal target (curve U3(Fe)) leads to its decrease. By comparing these output signals with thresholds Uo1and Uo2, an approach of a target13at a distance Do (from the working plane1′) can be detected, as well as the type of material this target13is made of.

A possible layout of the circuit components of sensor1, in connection with the constructive and functional embodiment ofFIG. 2, is shown onFIG. 4. The details disclosed by this drawing are self explanatory for a man skilled in the art, in particular when reading the present specification.

Nevertheless, one should notice that, when tuning the sensor circuit, resistors R3, R4are advantageously chosen so that despite any possible parameters variety of any other circuit components, no suppression of oscillation can possibly take place.

Thresholds Uo1and Uo2are preferably selected so that the comparators of the decision box (or comparing circuit)10operate when targets13reach a preset distance.

As indicated before, subtraction of signals with the necessary coefficients can be obtained directly in the coil system2,3.

To achieve this, winding3(L2) is connected to a top or intermediate position IP of winding2(L1) as shown inFIG. 5orFIG. 6. The windings2and3are thus opposite-connected and their signals are subtracted. Choosing the signal subtraction coefficients is done by selecting the place of the connecting point Ip from winding L1and by the number of turns in winding L2.

The circuit diagrams shown inFIG. 5andFIG. 6differ only by the location of the feedback connection to the generator5input (through low-stop filter12), whereas in both cases the differential amplifier forming an adder8(as inFIGS. 2 and 7) is replaced (in both circuits) by a common amplifier having an uncomplemented output (out) only.

If it is unnecessary to distinguish between the materials of the target13, the detector or sensor1can be embodied according to the diagram shown inFIG. 4. In this case a target (of any metal) approaching decision, regarding a preset distance Do, is made upon signal U2(Al,Fe) amplitude dropping to a value less than Uo3. The upper comparator U6and the first output out1shown inFIG. 7can be omitted from the decision-making circuit10and said latter comprises only one comparator as shown inFIG. 8.

A possible design of the system of coils2and3of the sensor1is shown inFIG. 9.

Preferably, the primary and secondary coils or windings2and3are mounted coaxially on a non-ferrous and a non-magnetic support17, preferably made of plastic material, the central axis X of said coils or windings2and3extending perpendicularly to the front working plane1′ of the sensor1and the secondary coil or winding3being situated proximate to said front working plane1′.

As can be seen fromFIG. 9, it is also preferred that the secondary coil or winding3has a flat structure with a large diameter compared to its thickness in its axial direction, the diameter of said secondary coil or winding3being at least slightly greater than the diameter of the primary coil or winding2.

Furthermore, the primary coil or winding2is situated at a distance D from the secondary coil or winding3in the direction opposite the front working plane1′, said distance D being adjusted in order for the sensor1to provide a uniform response signal for an approaching target object13, whether the latter is made of ferrous or of non-ferrous material.

Indeed, for the sensor1to have maximum sensitivity, coil3(L2) should be as flat as possible with the largest possible diameter. The slim construction of coil3enables the entire coil system2,3as a whole to be brought as close to the front working plane1′ and to the target13as possible, while the greater the diameter of this coil3, the greater the number of magnetic lines of force induced by eddy currents in the target which cross its turns.

To rule out the effect of the base material when flush mounting the sensor1, coil2(L1) should be constructed to have its diameter a few millimeters less than that of coil3(L2). This is decrease the density of magnetic lines of force crossing the detector body (housing+components) and the material of the support member into which the detector1is mounted or screwed, which decreases their effect on oscillating circuit parameters, correspondingly.

Preferably, the detector body or housing18(FIG. 10) should be of non-ferrous metal having low active resistance and, correspondingly, a thin skin layer. In this case the detector body shall act as a magnetic screen subduing the effect of the base material. Since currents flowing in coil3(L2) are insignificant as compared with those of coil2(L1), their interaction with the detector body18and with the support member can be neglected and the coil3can be made with the maximum diameter accommodated by design.

For the detector or sensor1to respond uniformly (i.e. produce the same signal amplitude variation at the decision box10input with a target approaching a selected distance), the primary coil or winding2is situated at a distance D from the secondary coil or winding3in the direction opposite the front working plane1′, said distance D being adjusted  in order for the sensor1to provide a uniform response signal for an approaching target object13, whether the latter is made of ferrous or of non-ferrous material.

To check the proper operation, a sensor model corresponding toFIG. 4was assembled, in which the coil system2and3was placed inside a body of brass18which was actually a diameter18pipe with a 1 mm thick wall, and tested (seeFIG. 10).

The coils2and3were wound on a plastic frame17having the following dimensions: d1=3.5 mm, d3=13 mm, D=1 mm. Coil2(L1) was wound with a 0.22 mm diameter wire, had 56 turns and a diameter d2=10 mm. Coil3(L2) had 28 turns of a 0.16 mm diameter wire and had a diameter d3=13 mm.

In the table below are listed operation ranges of the detector model vs. material of the target selected and material of the base into which the detector was embedded (experimentally obtained for the above design). A drawing is shown inFIG. 10that illustrates the principle of the experiment.

As seen from the tabulated data the operation range does not depend on the material of the support member or base into which the detector1is embedded and is only slightly dependent on the material of the target13.

An experiment was also performed that proved that the sensor1was still functional even when exposed to the effect of constant or alternating magnetic fields of 50 Hz of up to 200 millitesla.

The present invention is of course not limited to the preferred embodiments described and represented herein, changes can be made or equivalents used without departing from the scope of the invention.