DE102007058088A1 - Sensor for depth-selective detection of dielectric materials and method for operating such a sensor - Google Patents

Abstract

The invention relates to a sensor for locating dielectric objects (104), in particular a beam detector for searching dielectric objects (104) behind the surface (103) of building materials, comprising a plurality of electrodes (401, 402, 403, 404; 601, 602, 603, 604) constituting a transmitting electrode system (401, 402, 403; 602, 603, 604, 605) and a receiving electrode system (404, 601). According to the invention, it is proposed that the relative distance of the electrodes (401, 402, 403, 404; 601, 602, 603, 604, 605) in the direction orthogonal to the measuring direction of the sensor is dimensioned such that the capacitive coupling of the coupling paths between the transmitting electrode system and Receiving electrode system has different dependencies on the distance of an object to be detected (104) to the sensor. Furthermore, the invention relates to methods for operating such a sensor.

Description

State of the art

Disclosure of the invention

The The present invention relates to a sensor for locating dielectric objects according to the preamble of claim 1 and a method of operation such a sensor, in particular a method for evaluation the measurement signals of such a sensor.

sensors or detectors for locating, for example, behind Structural materials hidden, dielectric objects are currently working usually with capacitive methods. Such a capacitive one Sensor typically consists of several conductive ones Electrode surfaces and an evaluation circuit that allows a change in the effective between the electrodes To recognize capacity. This is exploited that the between two metallic electrodes forming capacity is changed, provided a dielectric in the range is spent in the the electric field between the electrodes intervenes.

such Capacitive sensors find their use, for example, as capacitive Proximity switch or as a so-called "bar searcher" (engl. Studfinder), which are used for the purpose, a substructure to be found behind a building material. A typical application for such a "bar searcher" is for example the task of a solid wooden beam behind a chipboard or to find a plasterboard. This is a much needed one Requirement, especially in the American economic area. If z. B. on a built in lightweight construction wall a harder It is important to hang this object to anchor in one of the massive support beams.

to Localization of these support beams become capacitive sensors used, whose electrodes are arranged such that the in the vicinity of the electrodes propagating electric field in engages the support beams.

Examples of known from the prior art capacitive sensors can be found in the US 3896425 A , of the US 6937951 A , of the US 4853617 A , of the US 5917314 A or in the EP 0428502 A1 ,

Various methods for controlling the electrodes are known from the prior art, or for evaluating the signals tapped at electrodes. For example, describes the US 6198271 A a method in which the change in the capacitance of an electrode relative to a common ground plane is detected by a time shift of a digital edge, which measurement capacitance is part of a passive RC network.

Alternatively, the beats US 3896425 A to evaluate the detuning of a capacitive voltage divider.

Also known from the US 4853617 or from the US5917314 , are sensors that use a plurality of sensor electrodes, with the aim of obtaining information about the lateral position of a wooden beam. For example, the US 5917314 A To insert three laterally offset electrodes to determine if the solid wooden beam is on the right, left, or center of a hand-held meter.

Object of the invention

Of the Invention is based on the object, starting from the detectors or sensors of the prior art, a capacitive sensor of the beginning specify type mentioned, configured the sensor characteristic is that the sensor is mainly limited to dielectrics in a limited Interval of distances relative to the sensor responds. D. H. that the sensor characteristic should be designed in such a way that signals that are mediated by dielectrics that are outside the limited local interval, that is, at much smaller or significantly larger distances to the Electrodes, at a significantly lower signal excursion lead, as signals, from within the distance interval located dielectrics are mediated.

Especially The task is to construct the sensor so that the Distribution of dielectrics in the vicinity of the sensor electrodes possible only a minor effect on the electrical properties of the sensor has, with the aim that this only noticeably responding to objects after a certain minimum distance.

About that It is also an object of the invention, such a sensor in particular cost-effective and with the lowest possible requirements to realize manufacturing tolerances and low installation costs.

The The problem underlying the invention is solved by a sensor for locating metallic objects with the features of Claim 1.

Advantages of the invention

The task of finding a wooden beam, which behind a continuous concealing Surface is found to be difficult for the following reasons.

In As a rule, the electrodes of the capacitive sensor are in Interior of a hand-held, in particular battery-powered locating device, which are moved over the wall surface can.

The Reaction by a dielectric object, d. H. in concrete Use case by the wooden beam, on the capacity An electrode is generally weaker, depending greater the distance of the object to the conductor surfaces of the electrodes. The electrodes are therefore typically immediately near the bottom of the handheld locator appropriate. For example, these may be as a copper structure be formed on a circuit board, in the, the wall facing Part of the housing of the locating device is mounted.

The electric field forming around the electrodes of the sensor accesses the measurement by the obscuring building material, eg. Legs Chipboard or plasterboard, through and into the area, There may be a wooden beam.

Becomes the beam detector at a constant distance to the surface Surface material moves, so there is a change the characteristics of the dielectric sensor as soon as the too finding wooden beams in the vicinity of the electrodes is located.

you takes advantage of the fact that the covering surface building material in terms of its thickness and its dielectric properties is approximately homogeneous. That is, in the absence of one Wooden beam is with respect to the electrical properties of the Sensors only a slight change observable if the sensor is at a constant distance over the surface is moved. Only when the capacitive sensor is in an area approaching, in which there is a wooden beam, one becomes noticeable modification of the electrical properties of the sensor observed.

in the practical use is therefore the operation of a bar searcher from the prior art from two separate steps. In one The first step is the sensor on an area of the wall put on, behind which no wooden beam is suspected. There will a so-called "calibration process" started in which the electrical properties of the sensor are determined. In one second step is then the sensor over the wall surface moved, and continuously the electrical Properties of the sensor determined. The presence of a wooden beam is detected by a change of the measuring signals, d. H. a change in the signals that are compared for calibration measurement and measurement after movement of the sensor results.

These Division into "Calibration" and "Measurement" is because of this, because the electrical properties of the sensor be much more influenced by the wall material, as being at a greater distance Wooden beams.

So Depending on the dielectric constant (or depending on the area Wall material or its moisture) and depending on the thickness of the wall material strong observed different electrical properties. As long as that Sensor signal dominated by the wall surface and the signal changes which are to be sought by the Object can be conveyed, far smaller, can on the calibration process not be waived.

The Division into "Calibration" and "Measurement" is insofar problematic as the risk of incorrect operation consists. Ie. the user could mistakenly use the device Calibrate above one of the invisible wooden beams and get it so that misleading measurement results or would have alternatively stop the measurement and with calibration in one place Repeat, a place where there is no wooden beam behind the paneling located.

Of the Calibration process itself is also problematic insofar as he only at approximately homogeneous wall surfaces works. Draws the wall surface, behind a wooden beam is sought, characterized in that thickness and / or Dielectric constant vary greatly depending on the location, so can a change in the electrical properties caused thereby of the sensor erroneously as presence or absence a wooden beam are interpreted.

at the operation of a bar search sensor according to the prior art Keep in mind that the exact position of the sensor Above the wall material a great influence on the Has measuring signals. If the meter is, for example, with a relatively large air gap above the to be examined Wandfläche proceed, so result from the larger Distance to the surface building material other measuring signals than one observes with an analog measurement with a smaller air gap would.

Around It is the air gap as constant as possible to keep it constant a common measure, the bottom of the locator to provide with felt gliders. These felt pads represent a defined, small distance of the housing of the locating device secure to the wall surface.

To solve the problem with variable air gap proposes the US 2005/0200368 to press the sensor head, in which the electrodes of the locating device are defined by means of a spring on the wall surface. An incorrect operation by the user, which z. B. results from the fact that the beam finder is tilted or lifted above the wall surface, can be counteracted so.

On a beam detector, the invention consists Task thus, the sensor as little as possible on an enlargement of a possible Air gap below the meter to react and suppress the influence that the presence of a flat wall material below the sensor causes. Much more should the influence of the flat wall material in favor the influence of a z. B. 2-3 cm depth Wooden object deliberately suppressed.

With the embodiment of the invention is for a capacitive bar search possible, aware of the influence the wall surface or below the device Air gap to be largely suppressed to the measurement signal. This allows, in particular, for complex mechanical devices, such as the known pressure device to dispense.

One immediate advantage of the present invention is that in this way a depth-selective detection characteristic can be realized.

This is achieved in that a sensor for locating dielectric Objects, in particular a bar detector for searching dielectric objects) behind the surface of building materials, a majority of electrodes comprising a transmitting electrode system and a Receive electrode system, the relative distance of the electrodes, in the direction orthogonal to the measuring direction of the sensor so dimensioned is that the capacitive coupling of the coupling paths between transmitting electrodes system and receiver selector have different dependencies from the distance of an object to be detected to the sensor.

In an advantageous embodiment of the invention Sensors are a plurality of transmitting electrodes as well as a single one Receiver electrode present.

there the distance of the transmitting electrodes, in particular the lateral Distance configured differently sized to the receiving electrode is. The lateral distance denotes a distance of the electrodes to each other in a plane that is substantially perpendicular to the Measuring direction of the sensor is.

About that In addition, the respective lateral extent of the transmitting electrodes be formed differently large. In particular it is advantageous if the lateral extent of the transmitting electrodes a function of the distance of the respective transmitting electrode to the Receiving electrode is and increases, for example, with this distance.

In In an advantageous embodiment, the lateral extent at least one transmitting electrode and / or their distance from the receiving electrode, in Essentially equal to the minimum distance between the electrodes and the surface of the medium to be measured.

Around specifically sensitive to the size of the Air gap to be between the meter and Forming the wall surface, both the width of the Electrode and the distance to the receiving electrode about the same chosen large, such as the distance between electrodes and flat wall medium. Both could, for example typically 1-4 mm, if the air gap is around 2 mm and remove the electrodes about 2 mm from the wall surface are arranged.

A at a large lateral distance from the receiving electrode located electrode is characterized by its capacitive Coupling to the receiving electrode both from the wall material, from the air gap between the wall and the sensor as well as from a low level Object is affected. For this purpose, your distance from the Receiving electrode of the same order of magnitude how their distance to the object to be found.

Farther is a user operation with the sensor according to the invention possible, which manages without a calibration process, since the Measurement signal is no longer dominantly influenced by the wall material, but the sensor primarily only one in larger Depth d. H. behind the surface responds.

Based on a capacitive proximity switch is the advantage in it, selectively objects only at a given distance to the sensor to detect and hide objects at a distance.

Advantageously, at least one of a pair of electrode elements is formed. The two electrode elements of the transmitting electrode can in particular be arranged symmetrically, and in this case preferably symmetrically with respect to the receiving electrode. This allows an axisymmetric reception characteristic. The axisymmetric Arrangement has the advantage that the resulting directional characteristic of the sensor is symmetrical to the axis. Advantageously, then tilting of the sensor around the axis of symmetry affect less on the display signals, as if an asymmetric system would be used.

In an advantageous embodiment of the invention Sensor is in addition to the receiving electrode system and at least one shielding electrode is provided to the transmitting electrode system.

Become the transmitting electrode system and receiving electrode system are planar trained and, for example, in a plane, in particular in a plane arranged a circuit board, so may advantageously at least a shield electrode can be arranged in a second plane of the printed circuit board.

Appropriately Way is the area electrode geometry together with the shielding surface on two different layers of a arranged conventional circuit board. In another part of the PCB can then advantageously, for example housed the components of a control and evaluation circuit become.

Farther is conceivable, for example, an additional, Provide passive electrode, with the example to external coupled AC potentials, eg. B. 50 Hz or 60 Hz, can be searched. So the bar searcher can as an additional function also recognize live power lines behind the wall.

About that In addition, it is also possible in an advantageous manner, the Sensor according to the invention in or on a machine tool to integrate a safe and secure working with one to allow such machine. For example, the sensor can be integrated in a drill or chisel tool or formed as a connectable with such a tool module be. As a possible installation location for the sensor according to the invention offers itself in an advantageous manner, a suction device for dust, which is connected to the machine tool, or connectable to it, and functionally nearby a wall to be processed is used.

The sensor according to the invention is advantageously operated in such a way that individual coupling paths between the electrodes of the transmitting electrode system and of the receiving electrode system are sequentially respectively applied with a drive signal IN _n . The output _signals U _xyz resulting from the drive signals IN _{n are} then provided with weighting factors F _xyz , in particular constant weighting factors, and combined by addition or subtraction to form a total signal.

It is advantageous if at least two weighting factors F _{xyz have} different signs.

With little effort, a method can be realized in which a linear weighting of the output _signals U _{xyz is} made.

In an alternative embodiment of the operating method according to the invention for the sensor, individual coupling paths between the electrodes of the transmitting electrode system and receiving electrode system are applied simultaneously, but with control signals IN _{n of} different amplitude and phase.

Further Advantages of the method or give it a working according to this method measuring device arising from the following drawing and the associated Description.

drawing

In The drawings are embodiments of the invention Sensors shown in the following description in more detail should be explained. The figures of the drawing, their description as well as the claims contain numerous features in combination. A person skilled in the art will also consider these features individually and summarize other, meaningful combinations.

It demonstrate:

1a the basic structure of a sensor geometry of a sensor for locating dielectric objects according to the prior art in a highly simplified, schematized sectional view,

1b an electronic equivalent circuit diagram for the structure according to 1a .

2a 1 is a sectional view through an alternative embodiment of a prior art electrode arrangement with a measuring electrode pair and a shielding electrode in a simplified, schematic representation,

2 B an electronic equivalent circuit diagram for the structure according to 2a .

3 a well-known example of the circuit-technical realization of a capacitive sensor, with an electrode geometry analogous to 2 .

4a An embodiment of an electrode arrangement according to the invention in a schematic sectional image representation,

4b an electronic equivalent circuit diagram for the structure according to 4a .

5 An embodiment of a drive and evaluation circuit according to the invention for a capacitive sensor with an electrode configuration analogous to 4 .

6 an alternative embodiment of a drive and evaluation circuit according to the invention for a capacitive sensor with an electrode configuration analogous to 4 .

7 an alternative embodiment of the electrode assembly according to the invention in a schematic plan view.

Description of the embodiments

to Illustrations of the invention are given below by way of example described the sensor of the invention. The Invention and in particular the use of multiple exciter electrodes However, it is in no way for use with bar searchers limited, but can also be used in other applications capacitive sensors are used.

In the following, the prior art will first be described by way of example ( 1 to 3 ), to then explain in detail in the next step how the above-described technical problem is solved with the aid of the invention. 1 and 2 show in a schematic sectional representation of the functional principle of a capacitive bar sensor. Two measuring electrodes ( 101 . 102 . 201 . 203 ) are above a laminar building material ( 103 ). In 1a and 2a located below the sensor electrodes ( 101 . 102 . 201 . 203 ) an enclosed object, for example a wooden block ( 104 ).

Practically important materials for the flat construction material ( 103 ) are, for example, Rigips, chipboard and hardboard whose thickness varies in practical use on construction sites between about 1 cm to about 3 cm. Typical relative dielectric constants of these materials are between 1.5 and 6. The typical relative dielectric constant of solid wood blocks is between 2 and 4, depending on moisture and wood type.

When a potential difference between the measuring electrodes is applied, a capacitive coupling or coupling of these electrodes occurs, so that an electric field is formed between them, which represents a coupling path. The size of the measuring electrode pairs ( 101 . 102 ) respectively. ( 201 . 203 ) is dimensioned such that the electrical field forming between them in the case of potential differences engages in the wall material and likewise in an optionally present wooden block. Thus, a coupling path for electrical signals arises, starting from the transmitting electrode into the half space lying in front of the electrode and thus possibly also into a medium to be examined, towards a receiving electrode. In the 1a . 2a and 4a the course of the electric field lines, which essentially marks this coupling path, is shown in a highly schematic manner by means of the circular arc segments. The exact course of the field lines and thus the respective coupling path depends strongly on the dielectric constant of the various dielectric media passed through the field. In particular, in practice, the field lines do not run exactly along circular arc segments, which have been used here only for illustration. Despite the simplistic representation, however, an essential property of capacitive sensors can be transferred from the schematic representation to the real field distribution:
The penetration depth of the field lines into a dielectric located below the electrodes depends directly on the relative distance between the two electrodes ( 101 . 102 ) 201 . 203 ) together. Widely spaced electrodes impart displacement currents along field lines that also extend at a greater distance to the electrode plane than closely spaced electrodes.

The two electrodes off 1a can in an electrical equivalent circuit, as in 1b is shown as a condenser, in which the capacitor plates are formed by the two flat electrodes.

The presence or absence of a dielectric wood block ( 104 ) affects by a change in capacity and can therefore be measured electrically.

A sensor with only two effective electrodes (example of 1 ) has compared to a sensor with a third electrode ( 202 ) (Example of 2 ) has the disadvantage that the electrically effective field is formed both above and below the electrode. This is disadvantageous when used in a hand-held locating device because z. B. the handle above the electrodes can be located and the position of the hand can have an impact on the measurement result. By inserting a shielding electrode ( 202 ), as in 2 can be counteracted, and the propagation of the electric field to the Un terseite of the locating device are limited. In the equivalent circuit diagram 2 B is then the additional coupling of the measuring electrode pair ( 201 . 203 ) to this shield electrode ( 202 ), as is also the case in the symbolic circuit diagram of the 2 B should be indicated ..

Like a control and evaluation circuit for a sensor analogous to 2 can look in practice is in 3 outlined.

To change the between the measuring electrodes ( 201 ) and ( 203 ) to measure effective capacity, by means of a voltage source, an AC signal ( 302 ) connected to one of the two electrodes. If, for example, a rectangular excitation signal ( 302 ), it may be advantageous to use a passive network ( 303 ) Smooth out harmonics before they are applied to the electrodes.

The use of a rectangular source signal ( 302 ) may be advantageous because it can be realized very frequency stable circuit technology with the help of inexpensive digital modules.

The capacitive network ( 306 ) corresponds to the three in 2a sketched electrodes, or the electronic equivalent circuit diagram in 2 B , wherein the shield electrode is grounded. Via the coupling path between the two measuring electrodes ( 201 ) and ( 203 ) flows when applying an alternating voltage, a displacement current, which at a high-impedance load resistor ( 305 ) causes a voltage drop in a measuring amplifier ( 301 ) is tapped and amplified with high input impedance.

A change in the effective capacitance between the two measuring electrodes can then be detected by changing the voltage at the output of the measuring amplifier. Ie. the presence or absence of a massive object ( 104 ) below the sensor is expressed in varying voltages at the output of the measuring amplifier ( 301 ) and can be recognized and displayed.

From the into the 1a and 2a In the case of highly schematic progressions of the field lines between the two measuring electrodes, it can be seen that most of the displacement current is mediated by field lines which are not guided by the object to be searched (FIG. 104 ), but rather by the flat wall material ( 103 ), which is located closer to the electrodes. A change in the thickness or the dielectric constant of this wall material ( 103 ) has a far greater influence on the voltage at the output of the measuring amplifier ( 301 ) as the presence of a more distant and possibly smaller object ( 104 ).

If present, enlargement or reduction of the air gap below the electrodes in practice also causes a significant voltage change at the output of the measuring amplifier (FIG. 301 ). This is practically significant because while looking for a wooden beam, ie while the sensor is above the wall surface ( 103 ), it may easily happen that the sensor is tilted or slightly raised. The signal change, which is due to the presence of the searched object ( 104 ) is then covered by the signal change by varying air gap.

These two major disruptive effects can be compared with the in 4 sketched advantageous electrode configuration of the invention counteracted. The original pair of sensor electrodes is here replaced by a plurality of measuring electrodes. A distinction can be made between two separate electrode systems:
On the one hand, a transmission system, which in the embodiment of 4 through the three electrodes ( 401 ) 402 ) and ( 403 ) is formed, and a receiving electrode system, which in the example of the 4 through a single electrode ( 404 ) is formed. Thus, starting from each transmitting electrode, there are different coupling paths towards the receiving electrode.

Looking at the electrodes ( 401 ) 402 ) and ( 403 ), they differ by their relative distance to the receiving electrode ( 404 ) and her size. The transmitting electrode ( 403 ) is close to the receiving electrode ( 404 ). In particular, for the capacitance forming between these two electrodes, the field profile within the air gap between the electrode surface and the surface of the wall or within the upper layers of the wall material ( 103 ) important. The capacitive coupling between these two directly adjacent electrodes is practically unaffected by the presence of the object ( 104 ), since the relative distance between the object ( 104 ) and these electrodes is too large. For the same reason, for the capacitive coupling of this electrode pair, the thickness of the flat wall material ( 103 ) not significant.

A Enlargement of the air gap between electrodes and wall material on the other hand has a large impact on the capacitive coupling, as well as a change the dielectric constant of the wall material.

In order to be particularly sensitive to the size of the air gap, which forms between the meter and the wall surface, both the width of the electrode ( 403 ) as well as the distance to the receiving electrode ( 404 ) is approximately the same size as the distance between the electrodes and the flat wall medium ( 103 ). For example, both could typically be 1-4 mm if the air gap is about 2 mm and the electrodes are located about 2 mm away from the wall surface.

At a large distance from the receiving electrode ( 404 ) located electrode ( 401 ) is distinguished by the fact that its capacitive coupling to the receiving electrode ( 404 ) both from the wall material ( 103 ), from the air gap as well as from a low-lying object ( 104 ) being affected. For this purpose, your distance from the receiving electrode ( 404 ) in the same order of magnitude as their distance to the object to be found ( 104 ). For example, the distance of the electrode centers from electrode ( 401 ) and ( 404 ) are about 2-6 cm, if objects in about 1-4 cm depth are to be found. For coupling to the electrode ( 404 ), the influence of the air gap is significantly lower than for the electrode ( 403 ), since the field is conducted to a considerable extent in the dielectric medium of the wall or possibly also in the air below the wall.

The electrode ( 402 ), which in a middle distance to the receiving electrode ( 404 ) is characterized in that its capacitive coupling to the electrode ( 404 ) practically only from the flat wall material ( 103 ) and the size of the air gap and not on the existence of a deep-seated object ( 104 ) is modified. Regarding the relative strength of the response to an increase or decrease in the air gap between the electrode and the wall surface, this electrode lies between the inner ( 403 ) and the outer ( 401 ) Electrode. The typical distance of this electrode ( 402 ) can be chosen with the aim of maximizing the properties of the wall material ( 103 ) should appeal. Too small a distance from the receiving electrode ( 404 ) is to be avoided, otherwise the influence of an enlargement of the air gap would be particularly strong. On the other hand, the distance to the receiving electrode ( 404 ) be sufficiently small that the electric field forming between them is only negligibly transformed into a deeper-lying wooden object ( 104 ) intervenes. At a typical thickness of the wall medium ( 103 ) of 1-3 cm, the size of the electrode ( 402 ) and their distance from the receiving electrode ( 404 ) also be about 1-4 cm.

The electrode ensemble off 4 is completed by a shield electrode ( 405 ), which is arranged above the transmitting and receiving electrode system. Neglecting the capacitive couplings within the transmitting and within the receiving electrode system, this results in the 4b sketched electrical equivalent circuit diagram, wherein the four lower capacitors, the coupling of the receiving and transmitting electrodes to the shielding ( 405 ) and the upper three capacitors describe the capacitive couplings of the individual transmitting electrodes to the receiving electrode ( 404 ).

It is essential for the invention that, due to the geometric design of the transmitting and receiving electrode system, the interfering signals and measuring signals have different effects on the capacitive couplings of the individual electrodes. This provides the basis for distinguishing between capacity changes such as B. caused by an increase in the air gap and capacity changes, as caused by the presence of an object to be searched ( 104 ) can be caused. This then makes it possible in an advantageous manner to have the sensor specifically addressed to objects at a greater depth.

5 schematically shows how a drive circuit for the electrode geometry 4 can look in practice. The end 3 Known a drive signal ( 302 ) is here by three independent control signals ( 503 . 504 . 505 ), which will be referred to below as IN ₁ , IN ₂ and IN ₃ .

For practical implementation, it is often favorable to use rectangular signal forms as drive signals, since these can be produced with inexpensive digital modules. These are then smoothed by means of a passive network and switched to the three exciter electrodes. The electrode arrangement ( 506 ) corresponds to the structure according to 4a and thus essentially the equivalent circuit diagram of 4b Accordingly, the electric field intervenes via different coupling paths into a medium to be examined. The sum of the high-impedance load resistor ( 502 ), which is mediated by the capacitive coupling between the receiving and excitation electrode system, causes a signal voltage at the input of a high-impedance measuring amplifier ( 501 ), which can then be evaluated digitally or analogously.

The technical idea of the invention lies in the existence of several, Independent coupling paths, each through distinguish a different depth characteristic. Leading this information suitable together so it is possible to produce a depth-selective overall signal.

One possibility for this merging is, in a sequential method, in each case one of the drive signals IN ₁ , IN ₂ and IN ₃ 5 active and the coupling of the associated excitation electrode ( 401 . 402 . 403 ) with the receiver electrode ( 404 ) separately, for example by the output signal of the amplifier ( 501 ) is digitized and stored. For the three consecutively activated input signals ( 503 . 504 . 505 ), sequentially measured output signals can then be evaluated with suitably selected, preferably constant, weight factors and summarized by addition or subtraction to form a total signal. The combined result signal then results from a, for example, linear superimposition of the three individual signals.

For example, with the aim of excluding the influence of the air gap, the weighting can be carried out in such a way that the outer ( 401 ) and inner ( 403 ) Are evaluated with a weighting factor of the same sign, and the middle excitation electrode ( 402 ) with a weighting factor of the other sign. The middle electrode has in principle a weaker dependence on the size of the air gap than the inner electrode ( 403 ), but a larger than the outer electrode ( 401 ). If the relationship which links the size of the air gap with the output voltage picked up at the measuring amplifier is linearized, or if this is approximated by a straight line, different slopes result for the different excitation electrodes.

If the weighting factors for the electrodes are chosen appropriately, a situation can be produced such that for the electrodes with the same sign ( 401 ) and ( 403 ) results in a sum equal to an influence of an increase in the air gap, as for the middle excitation electrode ( 402 ).

Specifically, this means that, for example, the magnification of the air gap, the amount of coupling and thus the measuring amplifier ( 501 ) tapped output signal for the excitation electrode ( 402 ) is reduced by a given voltage level dU ₄₀₂ .

Also reduces the to the electrodes ( 401 ) and ( 403 ) corresponding output voltage at the measuring amplifier dU ₄₀₁ and dU ₄₀₃ . The three voltages can now be so loaded with weighting factors F _xyz (F ₄₀₁ , F ₄₀₂ , F ₄₀₃ ) that the sum size .DELTA.S with ΔS = F 401 ·you 401 + F 402 ·you 402 + F 403 ·you 403 does not change or changes noticeably when the size of the air gap changes. This succeeds in particular if the weighting factors have different signs. It is particularly advantageous z. For example, if F ₄₀₂ on the one hand and F ₄₀₁ and F ₄₀₃ on the other hand have different signs.

For a given electrode geometry, the three signed weighting factors F ₄₀₁ , F ₄₀₂ and F _{403 are} in principle freely selectable. Apart from an absolute scaling of all three factors, two degrees of freedom therefore remain in the selection of F ₄₀₁ , F ₄₀₂ and F ₄₀₃ . These degrees of freedom can be used to compensate for up to two noise effects. For example, the factors may initially be chosen such that, as described above, the dependence of the sum signal S on the size of the air gap between the electrodes and the surface of a medium to be measured is minimized. However, one degree of freedom remains in the choice of weighting factors. This can be used to, for example, the influence of a change in the dielectric constant of the sheet wall material ( 103 ) to suppress the sum signal S.

A sum signal ΔS thus generated is excellent as an indicator of the presence of wood ( 104 ) at a greater depth behind the wall surface ( 103 ): it has only a small dependence on the size of the air gap between the electrodes and the surface and reacts only slightly to a change in the dielectric constant of the sheet-like wall material. The sum signal thus reacts more selectively to the presence of objects at greater depth, ie behind the surface material ( 103 ), as, for example, the isolated observation of the signal of the external excitation electrode ( 401 ).

So far, the evaluation of the individual electrode signals within a sequential method has been described in which in each case individual voltage levels were coupled in succession at several excitation electrodes and at a common receiver electrode, a measurement signal was tapped. It is naturally also possible to exchange receiver side and exciter side and z. B. the previous receiver electrode ( 404 ) Umzufunktionieren to the excitation electrode and to apply an AC signal. Measured variables would then be the capacitive coupling to the previous exciter electrodes ( 401 ) 402 ) and ( 403 ) resulting voltages. Disadvantage of the reversal of the transmitting and receiving side would be the necessity of a plurality of measuring amplifiers.

Since the reversal of the signal path has no influence on the advantages of the invention, in the following, when reference to receiver and Erre ger electrodes, the reversal of the signal path, as implicitly described with described.

The Existence of different capacitive coupling paths in the invention Sensor, in particular such coupling paths, each with different Depth characteristic, is in combination with the merge the respective individual signals of these coupling paths to a total signal .DELTA.S therefore a great advantage. This merge as outlined above by sequential measurement and digital, arithmetic operation. Especially advantageous is a linear combination of sequentially resulting Individual readings. (A more exact compensation generally becomes only a non-linear computational combination of the individual measured values However, in practice, can also be with less complex linear approximations produce good results.)

In such a linear combination of the measured variables of the respective individual electrodes, it may be advantageous to realize the sequential measurement, storage and then computational summary by a simultaneous measurement and linkage. This can be realized by the computational summation of the equation introduced above ΔS = F 401 ·you 401 + F 402 ·you 402 + F 403 ·you 403 is realized by an analog voltage summation. 6 shows a modification of 5 in which this is realized according to the invention.

For this purpose, the three excitation signals ( 503 ) IN ₁ , IN ₂ and IN ₃ according to 5 in the embodiment according to 6 by a single, for example digital excitation signal ( 703 ) with the frequency f replaced.

In the circuit, a synchronous second exciter signal which is phase-shifted by 180.degree. 704 ) generated. The network ( 710 ) corresponds to the network ( 506 ) the electrode system according to the invention ( 401 . 402 . 403 . 404 . 405 ) out 4 ,

The amplitude of the electrodes ( 401 ) to ( 403 ) can be achieved by suitable choice of capacitive or resistive voltage dividers ( 708 . 707 ). The at the receiver electrode ( 404 ) tapped voltage is formed when driving more than one exciter electrode by the sum of the individual over-coupling voltages. This corresponds to the summation of the three individual terms in the above equation.

The control of the different excitation electrodes with voltages of different magnitude is then equal to a weighting with a freely selectable weighting factor, analogous to the factors F ₄₀₁ , F ₄₀₂ and F ₄₀₃ from the above equation. A subtraction of electrode signals can be realized in that individual excitation electrodes with signals of the same frequency but shifted by 180 ° phase position ( 704 ).

The advantage of controlling the individual excitation electrodes with voltages of different amplitude and phase in each case is that only a single measuring amplifier ( 701 ) and the output signal of the measuring amplifier ( 701 ) immediately the summed measurement signal ΔS is available.

For the purpose of an internal calibration, it may be helpful to determine the amplification factor and any phase shift of the measurement amplifier ( 701 ) to determine. For this purpose it is possible, in addition to the main excitation signal ( 703 ) another independent exciter signal ( 713 ) via a reference path network ( 712 ) connect to the input of the measuring amplifier. The capacitive coupling of the reference signal ( 713 ) on the input of the measuring amplifier takes place in this case not via electrodes, but via conventional passive components ( 712 ).

When manufacturing the capacitive detector at the factory, the capacitive coupling of the reference signal ( 713 ) to the input of the measuring amplifier ( 701 ) are measured via the reference path and the measured value stored in a non-volatile memory. During this measurement, the exciter signal ( 703 ) shut off.

immediate before the use of the capacitive sensor, this measurement is again repeated. About a comparison of the original measurement when manufacturing the sensor in the factory with the shortly before the measuring operation repeated measurement of the reference signal it is possible Drift and aging of the amplifier reliable to recognize and possibly the output signals of the measuring amplifier computationally compensate. This is especially interesting because then within the measuring amplifier drift-prone and less expensive circuits can be used, without negatively affecting the measuring performance of the capacitive sensor.

Further shows 6 also another matching network ( 711 ), which also couples a signal to the input of the measuring amplifier by means of a coupling capacitor C_COMP, which can be realized, for example, as a normal passive component. Amplitude and phase This compensation signal is dimensioned so that, taking into account the signals coupling in via the electrode surfaces, the smallest possible voltage signal is formed at the input of the measuring amplifier when the capacitive sensor is used. This compensation network is to a certain extent provided for compensating the offset of the sensor and serves to prevent overmodulation in the measuring amplifier (FIG. 701 ) to avoid.

The 1 . 2 and 4 show possible arrangements of the electrodes in a schematic sectional view. 7 shows in contrast, a more concrete embodiment of an inventive electrode geometry in a plan view. The receiver electrode ( 601 ) corresponds to the electrode ( 404 ) according to the illustration of 4 , The exciter electrodes ( 602 ) 603 ) 604 ) are here semicircular at different distances to the receiver electrode ( 601 ) are arranged such that they each have a different lateral distance to the receiver diode. The electrodes ( 602 ) to ( 604 ) consist in each case of a pairwise right and left of an axis of symmetry ( 606 ) arranged electrode pair, which is electrically connected via lines. The transmitter or exciter electrodes ( 602 ) 603 ) 604 ) thus form an axis ( 606 ) mirror symmetric system.

Advantageously, the electrode geometry of the sensor according to the invention is mirror-symmetrical to a center axis ( 606 ) educated. This allows an axisymmetric reception characteristic. The axisymmetric arrangement according to 7 has the advantage that the resulting directional characteristic of the sensor is symmetrical to the axis. Advantageously, then tilting of the sensor around the axis of symmetry ( 606 ) less on the display signals than if an asymmetric system were used.

In the embodiment of 7 are the transmitting electrode system ( 602 . 603 . 604 . 605 ) and receiving selector system ( 601 ) are flat and lie in one plane ( 607 ). The plane is formed by a surface of a printed circuit board.

In addition to the embodiment according to 4 shows 7 a fourth exciter electrode ( 605 ), which can be used with the goal to deliberately hide objects at a greater distance, for example, at a depth of about 10 cm. For this purpose, this electrode is for example about 7-15 cm from the receiver electrode ( 601 ) and is suitably operated with the same phase position with which the middle electrode ( 402 ) out 4 is controlled.

On the side ( 607 ) of the detection area and thus the side of the electrodes facing away from the wall to be examined, ie, for example, on the underside of a corresponding board, there is a, in 7 not shown, planar shielding layer, which is at ground potential with respect to the electrodes of the capacitive sensor and analogous to electrode ( 405 ) is to be considered. Optionally, this electrode may also be dispensed with, with the disadvantage that then the sensitivity of the capacitive sensor is not limited to the area below the sensor.

Appropriately, the planar electrode geometry is arranged together with the shielding surface, not shown, on two different layers of a conventional circuit board. In the lower part of the circuit board off 7 For example, the components of a control and evaluation circuit could be accommodated.

It is also conceivable inside the semicircle formed by the electrodes 7 one more passive electrode ( 608 ) Provide, for example, with the externally coupled AC voltage potentials, eg. B. 50 Hz or 60 Hz, can be searched. Thus, the beam finder can also detect live power cables behind the wall as an additional function.

It can help with the function of the capacitive sensor be, this possibly added additional electrodes at ground potential to lay. For example, this is possible because of the measurement of the capacitive couplings of the invention Sensor takes place at a measurement frequency f, and the additional electrodes or also the shielding electrodes for this frequency f Ground potential are placed. This can be realized by that a passive LC series resonant circuit with resonant frequency f used is to electrically connect ground potential and auxiliary electrode. With respect to the measuring frequency f, this results in a niederimpedante Connection of additional electrode to earth while at the AC search frequency (eg, 50 or 60 Hz) a high-resistance connection results. Alternative to narrowband LC filters can also use simpler RC filters come with the disadvantage that these filters are also at AC voltage search frequencies on the additional electrode coupled signals to some extent be dampened.

In Advantageously, can also be a combination of capacitive sensor with a nearby metal detector realize.

If the capacitive sensor is to be combined with an inductive sensor, then it is advantageous for the electrode structures not to be planar, but much form more of many separate and as thin as possible individual conductors, for example by comb-like or meander-like structures that form a simply coherent surface in the mathematical sense. In this way, the formation of larger current loops within the capacitive electrodes can be effectively inhibited, thus avoiding that the sensitivity of an inductive sensor located in the vicinity of the capacitive sensor is impaired. The replacement of the contiguous surfaces by meander structures does not adversely affect the function of the capacitive sensor in any way.

One An essential feature of the invention is the use of a plurality of electrodes, which are characterized by their relative distance so is measured that their capacitive coupling each of a single Coupling path, which is formed by a pair of electrodes, a different dependence on the distance of one to having detecting object to the sensor. It is still essential that in a second step, the capacitive couplings of Electrodes are combined to form a total signal S, that forms a selective depth sensitivity.

The sensor according to the invention with its advantages set out above enables a reliable and inexpensive measuring device, in particular a detector for locating dielectric objects and a beam detector for use, for example, on construction sites, which can be simplified in terms of its handling and operation in particular, that he only those for the user relevant information (eg the presence of a wooden beam ( 104 )), irrelevant influencing variables for the user (such as, for example, the size of the air gap under the sensor or the dielectric constant of a wall surface material ( 103 )) but hides.

Of the Sensor is inexpensive to manufacture, in particular with low drift and stability requirements the measuring amplifier used. Furthermore it is possible in an advantageous manner, the inventive Integrate sensor in or on a machine tool to a Safe and safe working with such a machine too enable. For example, the sensor can be used in one Drill or chisel tool can be integrated or as a be formed with such a tool connectable module. As a possible installation for the inventive Sensor is also advantageously a suction device for dust, which is connected to the machine tool, or connectable to it, and functionally nearby a wall to be processed is used.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 3896425 A [0005, 0007]
• - US 6937951 A [0005]
• - US 4853617 A [0005]
• US 5917314 A [0005, 0008]
• EP 0428502 A1 [0005]
• - US 6198271 A [0006]
• - US 4853617 [0008]
• US 5917314 [0008]
• US 2005/0200368 [0026]

Claims (19)

Sensor for locating dielectric objects ( 104 ), in particular a bar detector for searching dielectric objects ( 104 ) behind the surface ( 103 ) of building materials, with a plurality of electrodes ( 401 . 402 . 403 . 404 ; 601 . 602 . 603 . 604 ), which is a transmitting electrode system ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) and a receiving electrode system ( 404 . 601 ), characterized in that the relative distance of the electrodes ( 401 . 402 . 403 . 404 ; 601 . 602 . 603 . 604 . 605 ) is dimensioned in the direction orthogonal to the measuring direction of the sensor such that the capacitive coupling of the coupling paths between the transmitting electrode system and the receiving selector system has different dependencies on the distance of an object to be detected ( 104 ) to the sensor. Sensor according to claim 1, characterized in that a plurality of transmitting electrodes ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) as well as a receiving electrode ( 404 ; 601 ) available. Sensor according to claim 1 or 2, characterized in that the spacing of the transmitting electrodes ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ), in particular the lateral distance, to the receiving electrode ( 404 . 601 ) is configured differently large. Sensor according to one of the preceding claims, characterized. in that the respective lateral extent of the transmitting electrodes ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) is different in size. Sensor according to one of the preceding claims, characterized in that the lateral extent of the transmitting electrodes ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) a function of the distance of the respective transmitting electrode ( 401 . 402 . 403 ; 602 . 603 . 604 ) to the receiving electrode ( 404 . 601 ) increases. Sensor according to one of the preceding claims, characterized in that the lateral extent of at least one transmitting electrode ( 401 . 402 . 403 ; 602 . 603 . 604 ) and / or their distance to the receiving electrode ( 404 . 601 ), is substantially equal to the minimum distance between the electrodes and the surface of the medium to be measured. Sensor according to one of the preceding claims, in particular according to claim 2, characterized in that at least one transmitting electrode ( 602 . 603 . 604 ) from a pair of electrode elements ( 602a . 602b ; 603a . 603b ; 604a . 604b ) is formed. Sensor according to claim 9, characterized in that the two electrode elements ( 602a . 602b ; 603a . 603b ; 604a . 604b ) of the transmitting electrode symmetrical, in particular symmetrical to the receiving electrode ( 601 ) are arranged. Sensor according to one of the preceding claims, characterized in that in addition to the receiving electrode system ( 404 . 601 ) and the transmitting electrode system ( 401 . 402 . 403 ; 602 . 603 . 604 ) at least one shielding electrode ( 405 ) is available. Sensor according to one of the preceding claims, characterized in that the transmitting electrode system ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) and receiving selector system ( 404 . 601 ) are formed flat. Sensor according to one of the preceding claims, characterized in that the transmitting electrode system and receiving selector system in a plane, in particular in a plane of a printed circuit board ( 607 ) are arranged. Sensor according to one of the preceding claims 9 to 11, characterized in that the at least one shielding electrode ( 405 ) in a second plane of the printed circuit board ( 607 ) is arranged. Sensor according to one of the preceding claims, characterized in that in addition to the receiving electrode system and the transmitting electrode system, a further sensor ( 608 ), in particular an inductive metal sensor is provided. Machine tool with a sensor after at least one of claims 1 to 13. Method for operating a sensor according to one of Claims 1 to 13, characterized in that individual coupling paths between the electrodes of the transmitting electrode system ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) and the receiving electrode system ( 404 . 601 ) are applied sequentially each with a drive signal IN _n . Method according to Claim 15, characterized in that the output _signals U _xyz resulting from the drive signals IN _{n are provided} with weighting factors F _xyz , in particular constant weighting factors, and are combined by addition or subtraction to form an overall signal. A method according to claim 16, characterized in that at least two weighting factors F _xyz , have different signs. A method according to claim 16 or 17, characterized in that a linear weighting the output _signals U _{xyz is} made. Method for operating a sensor according to one of Claims 1 to 13, characterized in that individual coupling paths between the electrodes of the transmitting electrode system ( 401 . 402 . 403 ; 602 . 603 . 604 . 605 ) and receiving electrode system ( 404 . 601 ) at the same time, but with control signals IN _n different amplitude and phase are applied.