WO2009062573A1

WO2009062573A1 - Apparatus and method for the capacitive force measurement

Info

Publication number: WO2009062573A1
Application number: PCT/EP2008/008582
Authority: WO
Inventors: Heinz KÜCK; Daniel Benz; Volker Mayer; Karl-Peter Fritz
Original assignee: Hahn-Schickard-Gesellschaft für angewandte Forschung e.V.
Priority date: 2007-11-13
Filing date: 2008-10-10
Publication date: 2009-05-22
Also published as: DE102007054027B4; DE102007054027A1

Abstract

In a force sensor for determining a force which acts on a component (1), a capacitor arrangement (3a, 3b) is mechanically coupled to the component (1) such that a change in the length of the component (1) effects a change in the capacitance of the capacitor arrangement (3a, 3b). An evaluation device (9) serves for determining the capacitance of the capacitor arrangement (3a, 3b) and for determining, on the basis of the determined capacitance, the force which acts on the component (1) in the force direction (12).

Description

Device and method for capacitive force measurement

The present invention is concerned with the measurement of forces exerted on a component and in particular with how these forces can be determined cost-effectively and with high precision by means of a capacitive measuring method.

Force measuring sensors which measure forces acting on mechanical components are used, for example, in monitoring the structure of construction machines or, in general, in monitoring mechanical structures. In some Wirkprinzipi.en while the deformation of the device itself, which is caused by the force acting on the component or component force, determined so as to close the forces acting on the component. The forces z. B. strain or compression forces, ie forces that cause a change in length on the component. In addition, such forces may be deforming forces, such as deflections, and such forces may be measured in an equivalent manner, as deflection also results in a force acting tangentially on the flexed component.

Thus, the forces acting on the mechanical structure or the component are often determined in-situ by means of deformations of the component itself.

To measure such deformations, strain gauges are used, for example, which are mechanically connected as rigidly as possible to the component to be monitored and in which the electrical resistance of the material forming the strain gauges changes depending on the strain state. Thus, an electrical resistance is determined which is compatible with the tensile stress acting on the strain gauge. load varies. However, the use of strain gauges is problematic because they usually do not have high mechanical stability due to the required special material properties. Therefore, they require special care in handling and processing and during operation, as the strain gauges can be mechanically easily destroyed. Furthermore, strain gauges are usually glued to the component to be monitored, the bond is subject to an aging process. The connection between the component and the DMS can therefore lead to a negative change in the sensor signal, for example due to drift and offset.

In view of the above-described problems of conventional force measuring sensors, there is a need to provide alternative force measuring methods or force measuring methods that have more robust and / or easier-to-use components, better long-term and drift behavior and thus can be used economically successful.

In some embodiments of the present invention, a force acting on a component is determined by measuring a variable capacitance, wherein a capacitor assembly is mechanically coupled to the component such that a change in length of the component causes a change in capacitance of the capacitor assembly. In this case, the arrangement of the electrodes is designed, for example, such that virtually no drift of the electrode geometry arises due to any connections between the component and the electrodes. The capacity is determined by means of an evaluation device, that is to say permanently measured, which can determine (calculate) the force acting on the component in the direction of force from a proven change in capacitance.

As the following detailed embodiments of the invention will show, it is by use of tion of capacitors possible, by means of simple geometrically suitable arrangement of capacitor surfaces or capacitances which are mechanically rigidly connected to the component to be monitored, to reliably and very precisely determine the force acting on the component force.

In some embodiments, two capacitor arrays are mechanically rigidly coupled to the component such that they experience capacitive changes with different signs as the component changes in length. If a capacitor arrangement obtains a higher capacitance, then the second capacitor arrangement will show a lower capacitance. If a difference between the two capacitances is formed by the evaluation device during the determination of the force, any undesired environmental influences which change both capacitances equivalently can not falsify the measurement result, since these eliminate each other. This can, for example, prevent moisture fluctuations from affecting the measurement result.

In the embodiments of the invention described below, a very simple structure consisting of few components is made possible due to the non-contact capacitive active principle. Likewise, the structure of the differential capacitance arrangement is very insensitive to aging or material fatigue processes. Furthermore, by using suitable materials and electrode arrangements, the thermal influence on the measurement signal can be kept very low.

Preferred embodiments of the present invention will be explained in more detail below with reference to the accompanying drawings. Show it:

Fig. Ia shows an embodiment of the present invention with 2 orthogonal capacitor arrangements; FIG. 1b shows an embodiment of the present invention with 2 orthogonal capacitor arrays; FIG.

Fig. Ic shows another embodiment of the present invention with 2 orthogonal capacitor arrangements.

Fig. 2 is an equivalent circuit diagram of the arrangements of Figs. Ia and Ib;

Fig. 3 shows an embodiment of the present invention with 2 orthogonal capacitor arrangements;

4 shows a further embodiment of the present invention;

Fig. 5 shows an embodiment of the present invention with 2 orthogonal capacitor arrangements;

Fig. 6 shows a further embodiment of the present invention with 2 capacitor arrangements;

FIG. 7 is an equivalent circuit diagram of that shown in FIG. 6

Arrangement;

Fig. 8 shows a further embodiment of the present invention

Invention with 2 capacitor arrangements;

Fig. 9 is an equivalent circuit diagram of the arrangement of Fig. 8; and

10 shows a further embodiment of the present invention with 2 capacitor arrangements.

FIGS. 1 a and 1 b show exemplary embodiments of the present invention, which differ in that, in FIG. 1 a, a surface of the component to be measured is shown mounted structure or the condenser assembly mounted there and the associated evaluation device in Fig. Ib in a cavity 16, that is arranged in a recess or bore in the component. Therefore, as far as the mode of operation of the individual components is concerned, first of all FIG. 1a is discussed, whereupon for FIG. 1b the differences to FIG. 1a are briefly illustrated.

In general, here as in the rest of the application text applies that functionally similar or functionally identical components are provided with the same reference numerals and that their description is mutually applicable to each other within the individual embodiments.

FIG. 1a shows a capacitor arrangement which is arranged on a component 1 or mechanically rigidly coupled thereto. Shown are four capacitor surfaces 3a, 3b, 5a and 5b, as well as two fastening elements 2a and 2b, on each of which one of the capacitor defining capacitor surfaces 3a and 3b are attached.

The capacitor surfaces 3a and 3b are arranged in the simplest case in principle on opposite sides of a gap parallel to each other, so that a plate capacitor with the plate spacing d (4a) is formed by the two capacitor surfaces.

Furthermore, FIG. 1 a shows two optional further capacitor surfaces 5 a and 5 b which are arranged on a carrier 5 which positions these parallel to one another at a predetermined distance between the capacitor surfaces 3 a and 3 b. The carrier 5 is in turn mounted on a support substrate 8 which is mounted on the surface of the component 1 such that changes in length of the component in a direction of force 12 do not transfer to the support substrate 8. This can be achieved, for example, in that the support substrate 8 only in a single location is rigidly connected to the device 1, so that the carrier substrate can slide on the surface thereof.

As can also be seen from the plan view and the sectional view in FIG. 1 a, the capacitor arrangement in FIG

1 a further further optional capacitor surfaces 3 c, 3 d,

7a and 7b, on the same scale the arrangement of

Condenser surfaces 3a, 3b, 5a and 5b repeated, the

Surface normals of the respective capacitor surfaces are aligned perpendicular to the direction of force 12.

In this case, a carrier 7, which holds the capacitor surfaces 7a and 7b mechanically, is also arranged on the support substrate 8, so that it also remains stationary (as well as carrier 5) when the component expands or contracts in the direction of force.

The fastening elements 2a and 2b are mechanically rigidly connected to the component 1, wherein the attachment points of the fastening elements 2a and 2b in the direction of force 12 have a predetermined distance 13 (1). In this case, the predetermined distance 1 defines the effective length, that is to say the length to whose change the capacitor arrangement is sensitive.

That is, a change in length of the distance 1 can be determined with the capacitor arrangement shown in FIG. 1a, as briefly described below. If a tensile force is exerted on the component 1 in the direction of force 12, this will stretch. Since the fastening elements 2a and 2b are mechanically rigidly connected to the component 1, the width of the gaps between the capacitor surfaces 3a and 5a or 3b and 5b is changed in a change in length of the distance 1 (13). During an expansion, ie when a tensile force is exerted, the gaps become wider, ie the capacitance between the capacitor surfaces 3a, 5a and 3b and 5b becomes smaller, since this is given by the following formula: ε _o ε _r A ε _o ε _r A c _mess = C ₁ + c ₂ =

Cl ₁ d.

_Cmess = C ₁ + C ₂ = ^ε ° ^{ε <A +>} + ^ε ° ^ε ' ^Ad *

^{1 2} Cl ₁ Ci ₂ Cl ₁ Ci ₂

C _mess (dχ + Cl _2).

The capacity is measured by the evaluation device 9 and can thus be used to determine the change in length on the route 1 or the change in length Δl of the route 13 (1) according to the above formula. With knowledge of Δl and the length 1, as well as the modulus of elasticity of the component 1 and its cross-sectional area A, the evaluation device can determine the force acting on the body 1 force. This can be done, for example, on the basis of the following relationship:

In principle, the carrier 5 and the capacitor surfaces 5a and 5b attached to the carrier 5 can also be omitted. The effective length 13 (1) in some embodiments is significantly greater than the distance between the measuring electrodes. This is due to the fact that an absolute change in length is determined by means of the capacitor arrangement. However, a fixed force causes, as the above formula can be seen, a relative change in length of the component 1. Therefore, in some embodiments, to achieve a precisely detectable absolute change in length, the attachment points of the fasteners have a certain, predetermined minimum distance 13 (1). -

The capacitor surfaces 3c, 3d, 7a and 7b, which are arranged perpendicularly to the direction of force 12, can contribute to further increasing the measurement accuracy of the arrangement discussed above, as will be briefly explained below. Due to the geometric arrangement, the partial capacitor arrangement between the capacitor surfaces 3c and 7a or 3d du 7b undergoes no change in the capacitance when the component 1 is stretched. By subtracting the measurement signals of both capacitor arrangements, however, a falsification of the measurement signal due to external influences, e.g. Humidity fluctuations, minimized, since the capacitance changes caused thereby act in the same direction on both sub-capacitor arrangements and are automatically compensated for in the subtraction. Thus, it can be ensured by the capacitor arrangement shown in FIG. 1a that the measurement signal remains largely free of external interference.

For this purpose, it is of course also necessary to connect the partial capacitor arrangement consisting of the capacitor surfaces 3c, 7a, 7b and 3d to the evaluation device so that it can carry out the optionally required subtraction.

In alternative embodiments, the capacitor surfaces 3a, 3b, 3c and 3d are formed by the carriers 2a and 2b themselves. For this, the surfaces of the fastening elements 2a and 2b facing the carriers 5 and 7 can be suitably processed (for example planarized), so that the carriers 2a and 2b themselves form the capacitor surfaces, provided that the carriers consist of electrically conductive material. Likewise, in alternative embodiments, the capacitor surfaces 5a and 5b and the capacitor surfaces 7a and 7b are formed by the carrier 5 and 7, if they are made of conductive material.

1a also shows an optional housing cover 10 which can serve to prevent the mechanical or electrical components of the force measuring sensor shown in FIG. nischer damage and negative environmental influences, such as humidity or harmful gases protect. Furthermore, the housing may advantageously form an electrically shielding Faraday cage.

Before discussing Fig. 1b briefly below, it should again be pointed out that the direction of force 12, as indicated in FIG. 1 a, is not to be construed restrictively in such a way that only expansion forces, that is to say tensile and compressive forces, act in Direction 12 act, can be determined by means of the force sensor. Rather, if the component 1 is bent, for example in a direction perpendicular to the direction of force 12, the force sensor also detect this bend, since then also a change in length in the direction of force 12 on the component is caused. This is of course to be set approximately equal to a force acting tangentially on the bending radius when the component is mechanically bent or deformed. Such a constellation can also be monitored or measured with the embodiments of the invention discussed here.

FIG. 1b shows a further embodiment in which the capacitor arrangement which has just been discussed does not rest on a surface of the component, but rather in a cavity 16, ie. H. is arranged in a recess or bore within the component 1.

In other words, FIG. 1b shows an alternative construction of a force measuring sensor. The sensor is constructed in a recessed cavity 16 of a structural mechanical element 1 (eg, a tensile bar).

Fig. Ic shows a further embodiment of the invention düng, which is based on the embodiments of Figures Ia and Ib, and is omitted in the carrier with the center electrodes. In this embodiment, the force on the component as well as in the preceding Be determined embodiments, which can be dispensed with additional mechanical components. This has the advantage of even lower implementation complexity.

Due to an attacking force F, the element with the length 1, the cross-sectional area A and the modulus E expands by the amount

Al = -, so F

E-A 1

The determination of the elongation takes place via a differential capacitance arrangement with the capacitances C _meSs ^{un <} d C _ref , which are defined in the equivalent circuit diagram of FIG. 2. For this purpose, two electrodes 3a and 3b are mounted on two carriers 2a and 2b on the side surfaces of the cavity, which ideally consist of the material of the structural mechanical element (of the component 1) and define a gap 4. The size of the gap 4 is changed due to the force acting by the amount .DELTA.l. Furthermore, the two carriers 2a and 2b may already consist of an electrically conductive material, and thus already contain the two electrodes 3a and 3b. In the gap 4, an electrode carrier or carrier 5 is positioned. The electrode carrier 5 can be designed, for example, as a multilayer printed circuit board with two electrode surfaces 5a and 5b. Likewise, a thin slice of a conductive material can be used directly as the electrode carrier. In the carrier 2a there is a recess 6 with two further electrodes or capacitor surfaces 3c and 3d, between which a further electrode carrier 7 is attached to the electrode surfaces 7a and 7b parallel to the direction of action of the force, so that the capacitor surfaces 3c, 3d, 7a and 7b form the reference capacitance C _ref . The electrode carriers 5 and 7 are mounted on a support substrate 8 and fixed in the cavity 16. The electrodes 3a and 3b form, with the electrodes 5a and 5b, the capacitance C _{meSs /} which, for example, example, in the wiring according to Fig. 2, the following capacities result:

r - r J - r - ^ε o ^ε r ^. ^ε o ^ε r ^ dl ^d 2

C = C + C _- ε ₀ ε _r ^r ° Ad ₂ ² + ε _o ε _r Ad _i d _x d ₂ Q ₁ Ci

^C mess = ~ 7 ~ 7 ~ ( ^d l + ^d 2) 'dl ^d 2

The evaluation of the differential capacitance between the measuring capacitance Cmess and the reference capacitance C _re f is done by means of an electrical circuit 9, which converts the differential capacitance into an analog or digital signal and is also applied to the substrate 8. The cavity 2 is closed by the cover 10.

2 shows an equivalent circuit diagram or a circuit diagram of how the individual capacitor surfaces 3a to 7b can be connected in order to determine the capacitances within the evaluation device according to one of the above formulas and thus also a change in length or a force acting on the component shut down.

In the case shown here, the measured capacitance C _{mess is the} capacitance formed by the capacitor surfaces 3a, 3b, 5a and 5b, while C _{ref is} the capacitance formed by capacitor surfaces whose surface normal points perpendicular to the direction of the force , However, the terminology is not to be construed as limiting, as the notation is merely arbitrary. In general, one could also define the measuring capacitance as the first capacitance and the reference capacitance as the second capacitance, since in some embodiments of the invention the capacitance C _{ref is influenced} by the action of force. kung also changes, as already described above. In particular, in some embodiments, the geometry is advantageously selected such that upon force, the first capacitance undergoes a change in one direction and the second capacitance undergoes a change in the opposite direction. Thus, by difference formation, on the one hand, an increased measurement signal can be obtained, on the other hand, interference influences which are impressed on the system from the outside, for example due to changes in humidity and temperature, can be reduced by subtraction.

The terminology C _re f is justified when, as in some embodiments of the invention, the second capacitor arrangement, ie the capacitor arrangement, which is formed from the capacitor surfaces 3 c, 7 a, 7 b and 3d, mechanically supported such or connected to the component 1 is that a change in length of the component causes no change in capacity. In this case, the second capacitance arrangement does not make a direct contribution to the measurement signal, but only serves to reduce impressions impressed on the system since they generally affect both capacitor arrangements equally, so that the error can be corrected by subtracting the two measured capacitance values ,

As can be seen from the equivalent circuit diagram of FIG. 2, short circuits may occur in the electrical circuit if, in the case of overload, the respective capacitor surfaces which form a capacitance are mechanically deflected so far that they are in direct electrical contact with one another devices. An embodiment of the invention in which this is prevented is shown in FIG. The exemplary embodiment shown in FIG. 3 is based essentially on the exemplary embodiment already explained in FIG. 1, so that a renewed description of the components already shown there is dispensed with at this point. In addition, in Fig. 3, the electrode assembly, ie some of the capacitor surfaces are provided with insulating pins 11, which prevent direct electrical contact of the electrodes 5a and 5b of the electrode carrier 5 with the electrodes 3a and / or 3b. The same applies to the electrodes or capacitor surfaces 7a and 7b equivalent.

The insulating pins 11 may be made of any suitable insulating material and moreover have any geometric shape, as far as they protrude so far beyond the capacitor surfaces in the direction of the surface normal of the capacitor surfaces that even taking into account a possible elasticity of the insulating pins an electrical short circuit opposite capacitor surfaces is reliably prevented. In particular, it should be noted here that in the present figures, the electrical pins 11 are not drawn to scale, which is required here for the sake of illustration, since in general the electrical pins 11 have a geometric extension which is so small that they are in the otherwise not visible in Fig. 3 case.

4 shows a further embodiment of the present invention in plan view, in which at least the support 5, which carries the insulated electrodes or capacitor surfaces 5a and 5b, is almost completely surrounded by the electrodes 3a and 3b, so that also in one Direction perpendicular to the direction of force 12 capacitor surfaces on the support 5 and the fastening elements 2a and 2b are arranged, which contribute to an effective capacitance change. However, since for these sub-areas the capacitance change is not caused by a change in the spacing, but rather by the effective areas A between the individual capacitor areas changing as the fasteners 2a and 2b are displaced relative to the carrier 5 in the direction of force 12, In the arrangement shown in Fig. 4, the capacitance changes are approximately linearly dependent on the strain in the direction of force 12. This is advantageous with regard to the evaluation of the measured capacitance signals and the calculation of the force acting on the component 1. Insulating pins 11, or spacer elements 11, which are arranged at the corners of the carrier 5, on the one hand can be used to prevent a short circuit. On the other hand, they can serve to fix their relative orientation during a relative movement between carrier 5 and fastening elements 2a and 2b, so that these spacer elements 11 can also be used as a guide for the mechanical components.

Generally speaking, it goes without saying that suitable materials are to be used for the production of the capacitor arrangements described in FIGS. 1 to 4 or in the following figures. For the formation of the capacitor surfaces, an electrically conductive material is therefore generally used which is electrically insulated from the respective carrier structures (the carriers 5 and 7 or the fastening elements 2a and 2b). For this purpose, the carriers or fastening elements can either be formed completely from electrically insulating material, or an electrically insulating layer can be applied between the fastening elements or carriers and the capacitor surfaces held by them.

Alternatively, it is also possible to make the carriers 5 or 7 or the holding structures 2 a and 2 b entirely of conductive material, so that the capacitance is formed directly by the carriers and holding structures, without it being necessary to apply the insulation for the capacitor surfaces to the Apply support structures or holders. In some preferred embodiments, at least the support substrate 8 is made as an insulating material or as a printed circuit board, so that the evaluation device 9, when mounted on the support substrate 8, is galvanically isolated from the rest of the arrangement to minimize any interference on the evaluation circuit.

In some embodiments, the capacitor assembly is completely surrounded by a housing shielding it from external controls to protect the assembly from mechanical and electrical interference.

5 shows another embodiment of the present invention in which the carrier 5 and the support substrate 8 holding the carrier 5 are not disposed on the surface of the component 1, but rather are fixed directly to the attachment members 2a and 2b. In this case, the support substrate 8 is fixed by way of example by means of a screw 17 only on one of the fasteners (in the case shown here fastener 2b), while it is slidably mounted on the other fastener (2a) or not connected to it, so that a longitudinal extent in Force direction 12 can not be hindered by a rigid connection of the two fasteners 2a and 2b. Also in the case shown in Fig. 5, the evaluation device 9 is arranged on the support substrate 8, so that it can be carried out galvanically separated from the rest of the arrangement, especially if only the capacitor surfaces 5a, 5b, 7a and 7b directly to the electronic readout with the Evaluation device 9 are connected.

Moreover, the arrangement of the supporting substrate 8 shown in FIG. 5 can minimize the susceptibility of the electric circuit to failure since stretching of the component 1 in this embodiment can not affect the supporting substrate 8, so that one mounted on the supporting substrate 8 Evaluation device 9 under no circumstances can experience the mechanical stress and the associated expansion or compression movements of the component 1. When using a thermal expansion of hardly subjected material for the Support substrate 8, this also applies to thermally induced tensions or expansions in the materials used. Furthermore, a more accurate geometric positioning of the capacitor surfaces relative to each other can be achieved by this arrangement, so that expensive adjustment steps can be omitted during commissioning of the force sensor.

6 shows a further exemplary embodiment of the present invention, in which the capacitor arrangements, which consist of the electrodes 3a, 3b, 5a and 5b on the one hand and of the electrodes 3c, 3d, 7a and 7b on the other hand, are arranged parallel to one another, see FIG the surface normals of the capacitor surfaces forming the capacitor arrangement are parallel to the direction of the force.

Due to the geometric design of the fasteners 2a and 2b is achieved, however, that a change in length of the component 1 in the first capacitor arrangement (consisting of the capacitor surfaces 3a, 3b, 5a and 5b) causes a change in capacitance, that of the second capacitor arrangement (consisting of the Capacitor surfaces 3c, 3d, 7a and 7b) is opposite. Due to the geometry of the fastening elements 2a and 2b, for example, when a force is exerted on the component 1 in the direction of force 12, so that it expands in the direction of force, the following change in capacitance results. The fastening elements 2a and 2b which are mechanically rigidly connected to the component 1 at the fastening points 14a and 14b move away from one another, since the effective length 1 between the fastening points 14a and 14b is also subjected to the expansion. Since the first capacitor surface 3a is attached to the fastening element 2b and the second capacitor surface 3b is fastened to the fastening element 2a, the capacitor surfaces also move away from each other, ie the gap dimensions di and d ₂ increase, so that overall the capacitance of the first capacitor arrangement is reduced. However, the capacitor face 3c is attached to the first attachment member 2a, and the capacitor face 3d is fixed to the second attachment member 2b, so that removal of the attachment members (a magnification of the length 1) causes the gap dimensions d ₃ and d _{4 to become} smaller, so that So the capacity of the second capacitor arrangement increases. The specially selected geometry of the fasteners 2a and 2b thus makes it possible to use parallel capacitance arrangements which undergo a differently directed capacitance change during the same movement.

Finally, it should be noted that the first fastening element 2a and the second fastening element 2b in Fig. 6 are connected to each other by means of a bending hinge 18, which the positioning accuracy of the carrier (the fastening elements 2a and 2b) with the capacitor surfaces 3a, 3b, 3c and 3d increased relative to the electrode carriers (the carriers 5 and 7). In this case, the fastening elements 2a and 2b can in particular be formed from a one-piece material, so that the decisive relative to the measurement accuracy relative positioning, or the distances 4a and 4b can be guaranteed with the highest precision.

In other words, FIG. 6 shows an embodiment of the present invention in which the capacitors between the electrodes 3a-5a, 3b-5b on the one hand and the electrodes 3c-7a, 3d-7b on the other hand undergo an opposite change (differential capacitance). Arrangement). In the case of the first-mentioned pair of capacitors, as the structure-mechanical element (component 1) expands, the gap increases and thus the capacitance is reduced, while the second-mentioned pair reduces the gap and thus increases the capacitance. In addition, the two carriers are made in one piece and the two halves via solid springs (bending hinges 18) positioned against each other. This increases the Positioning accuracy of the carrier with the electrodes 3a, 3b, 3c and 3d relative to the electrode carriers or the carriers 5 and 7.

Fig. 7 shows a possible wiring of the arrangement of Fig. 6, wherein between the electrodes 3a, 5a, 3b and 5b, a first capacitance Ci and between the electrodes 3c, 7a, 7b and 3d, a second capacitance C _{2 is} determined. Here, by direct contacting only those capacitor surfaces which are arranged on the carriers 5 and 7, with suitable positioning of the evaluation device 9, a galvanic separation of the evaluation device from the rest of the arrangement can be achieved.

Alternatively, it is also possible to connect the capacitor surfaces 3c, 3d, 3a or 3b directly to the evaluation device. Furthermore, it goes without saying that the evaluation device does not have to be arranged on the same substrate as the capacitor arrangement. Rather, this can also be mechanically and electrically separable connected to the capacitor arrangement. In this case, the capacitor arrangement or one of the fastening elements 2a and 2b would provide suitable contacts, so that an external evaluation device can be electrically conductively connected to the capacitor arrangement in order to measure the capacitances to be determined.

FIG. 8 is a plan view and a sectional view of another embodiment of the present invention based on the embodiment shown in FIG. 6. FIG. The sectional view shown in the right half is formed along a section line 15 to show the specific electrode arrangement of the embodiment shown in FIG.

In Fig. 8, the electrodes, which are arranged on the carriers 5 and 7, divided into two electrically separate partial electrodes. By this division For example, the capacitor surfaces 3a, 3b, 3c, and 3d may be used as floating center electrodes that need not be electrically contacted, as will become more apparent from the following consideration of FIG. 9. That is, in the case shown in FIG. 8, in an electrically advantageous manner of wiring the read-out or evaluation means 9, only part electrodes 5ai, 5a ₂ , 5bχ, 5b ₂ , 7ai, 7a ₂ , 7bi and 7b _{2 must be} connected to the evaluation device get connected. This has the great advantage that a galvanic separation of the circuit or the evaluation device 9 can be achieved if it is mounted on an insulating support substrate 8, on which, as discussed in detail in the preceding embodiments, the carrier 5 and 7th the electrodes 5ai, 5a ₂ , 5bi, 5b ₂ , 7ai, 7a ₂ , 7bχ and 7b ₂ are located. Galvanic separation means, for example, that the evaluation device 9 is not electrically connected to the electrodes 3a, 3b, 3c and 3d, so it is not connected via an ohmic connection. As can also be seen from the sectional view, in the exemplary embodiment of the invention shown in FIG. 8, the electrode shape is selected such that the sub-electrodes on the individual carriers are each formed by rectangular, electrically mutually insulated regions of the same electrode surface.

9 shows a circuit diagram of how the electrode arrangement of FIG. 8 can be electrically contacted, so that the galvanic isolation already mentioned above is achieved. As can be seen from the equivalent circuit in FIG. 9, only those electrodes are connected to the evaluation device 9 (which is located at the position of the capacitances Ci and C ₂ ), which are arranged on the electrically insulated carriers 5 and 7. These are the capacitor surfaces 5aχ, 5a ₂ , 5bi, 5b ₂ , 7ai, 7a ₂ , 7bχ and 7b _second A ground connection is required only for those capacitor surfaces mounted on the fasteners 2a and 2b. In particular, therefore, for These capacitor surfaces and the fastener itself can be used as a capacitor surface, if this is made of a conductive material. The galvanic isolation of the read-out circuit or of the evaluation device 9 proposed by FIG. 8 in connection with the circuit of FIG. 9 causes an increased reliability of the system and a higher accuracy of measurement by suppression of possible electrical interferences, otherwise via the fastening elements 2a and 2b of FIG the component to be measured 1 could be impressed into the system.

The individual capacitances to be measured with the floating center electrodes result in the case of a circuit, as shown in FIG. 9, from the geometric dimensions of the capacitor arrangements as follows:

C - ε ^• A ^{■ (dι + dz)}

2 - d _x and

C ₂ = ε ₀ • A (d ₃ + d ₄ )

2 • d ₃ • d ₄

For this purpose, it may be advantageous for the readout unit to be constructed and operated galvanically isolated with respect to the voltage supply (U _v ) and signal monitoring (U ₃ ) and / or connected to a suitable potential.

FIG. 10 shows a further embodiment of the invention in which the geometrical pitch of the part electrodes arranged on the carriers 5 and 7 differs from the arrangement shown in FIG. 8. In particular, an inner Teilkondensatorflächenbe- rich (5ai and 7a ₂₎ of an outer Teilkondensatorflä- chenbereich (7AI and 5a ₂₎ is shown in Fig. 10 partially enclosed, whereas in the embodiment shown in Fig. 8 arrangement, the capacitor surfaces from two electrically isolated from each other, juxtaposed, rectangular partial capacitor surfaces are formed.

As has been described with reference to the above-described embodiments, in some embodiments of the invention, a change in length or expansion or compression of a component on the change in capacitance of a mechanically rigidly connected to the component capacitance arrangement is determined, so that with knowledge of the modulus of elasticity of the component to a force applied to the component can be closed or recalculated. In this case, the exact nature of the attachment or the geometry of the fastening elements, which are the individual surfaces forming the capacitors of the capacitor arrangement or serve for their attachment, is irrelevant.

It is advantageous if a differential capacitance arrangement is created in which length changes in two capacitor arrangements affect a differently directed capacitance change, so that the measurement accuracy can be increased by increasing the measurement signal (better SNR). In addition, in two capacitor arrangements by subtraction external disturbances that have been impressed on the system, largely compensate.

The exact geometric shape of the fasteners is not crucial for a successful application of the inventive concept. Rather, the component or the component to be controlled may itself define some of the capacitor surfaces. This may be advantageous in particular if the embodiment described with reference to FIGS. 8 to 10 is used with floating side electrodes, since these are then readily formed by a slot milled in the component to be monitored or introduced in some other way Cavity, a hole or the like) can be formed.

Also, the evaluation device 9 does not have to be placed in the immediate vicinity of the capacitance arrangement, as is often the case in the preceding embodiments. Rather, this can also be performed as an external, electrically and mechanically separable from the capacitor assembly device. This has the advantage of considerably higher flexibility, but then the capacitor arrangement must have a contacting possibility by means of which the individual capacitor surfaces to be contacted for readout or evaluation of the capacitor arrangement can be electrically conductively connected to the evaluation device.

Finally, it remains to be noted that no requirements must be placed on the component 1 which is to be monitored by means of the force-measuring device, since the force-measuring device can be attached to any component or within any component in an extremely flexible manner. In particular, it is not necessary that the component 1 metallic, d. H. So consists of conductive materials. Rather, any type of materials, such as ceramics, wood, glass or similar substrates can be monitored or observed with the force measuring devices according to the invention.

Depending on the circumstances, the inventive method for determining a force acting on a component in a direction of force can be implemented in hardware or in software. The implementation can take place on a digital storage medium, in particular a diskette or CD with electronically readable control signals, which can cooperate with a programmable computer system such that the method according to the invention for determining a force acting on a component in a force action is carried out. Generally there is the Invention thus also in a computer program product with a program code stored on a machine-readable carrier for carrying out the method according to the invention, when the computer program product runs on a computer. In other words, the invention can thus be realized as a computer program with a program code for carrying out the method when the computer program runs on a computer.

Claims

claims

1. Force sensor for determining a force acting on a component (1) in a force direction (12), with the following features:

a capacitor arrangement (3a, 3b) having at least a first and a second capacitor surface, which are mechanically rigidly connected to the component (1) via separate fastening elements (2a, 2b), wherein a distance from attachment points at which the fastening elements (2a, 2b ) are connected to the component (1), in the direction of force (12) have a distance (13) which varies depending on the force and thus causes a change in a capacitance of the capacitor arrangement (3a, 3b); and

an evaluation device (9) for determining the capacitance of the capacitor arrangement (3a, 3b) and for determining the force acting in the direction of force (12) on the component (1) from the determined capacitance.

2. A force sensor according to claim 1, wherein the capacitor arrangement (3a, 3b) is mechanically connected to the component (1) in such a way that an elongation of the component leads to a reduction of the capacitance.

3. Force sensor according to claim 1 or 2, wherein a first capacitance defining the capacitor surface (3a) with a force direction (12) facing surface normal by a gap separated from a second, parallel capacitance defining capacitor surface (3b) is arranged.

4. Force sensor according to one of the preceding claims, wherein between the first (3a) and the second (3b) capacitor surface a third (5a), with the first capacitor surface (3a) a first partial capacitance bil- dend capacitor surface, and a fourth (5b) with the second capacitor surface (3b) a second partial capacitance capacitor surface are arranged on opposite sides of a support (5).

5. A force sensor according to claim 4, ^wherein the third (5a) and the fourth (5b) capacitor surface and its support on a support substrate (8) are arranged such that by a change in length of the component (1) no mechanical force on the mechanical distance between third (5a) and fourth (5b) capacitor surface determining carrier (5) is applied.

6. Force sensor according to claim 5, in which the support substrate (8) is mechanically rigidly connected only to the fastening element of the first (3a) or the second capacitor surface (3b).

7. A force sensor according to any one of claims 4 to 6, wherein the third (5a) and the fourth (5b) capacitor surface are formed of the material of the carrier.

8. Force sensor according to one of the preceding claims, wherein the evaluation device (9) with the first (3a) and the second (3b) capacitor surface is electrically connected to determine the capacitance of the capacitor assembly.

9. Force sensor according to one of claims 4 to 8, wherein the evaluation device (9) with the third (5a) and the fourth (5b) capacitor surface is electrically connected to determine the capacitance of the capacitor assembly.

10. A force sensor according to any one of claims 4 to 7, wherein the third capacitor surface (5a) of two electrically isolated from each other partial surfaces (5ai, 5a ₂ ) and the fourth capacitor surface (5b) consists of two electrically isolated from each other partial surfaces (5b _x , 5b ₂ ).

11. A force sensor according to claim 10, wherein the mutually isolated partial areas (5ai, Oa ₂ ; 5bi, 5b ₂ ) are electrically connected to the evaluation device (9) to determine the capacitance of the capacitor arrangement, wherein the evaluation of the first (3a ) and the second (3b) capacitor surface is galvanically isolated.

12. A force sensor according to one of the preceding claims, wherein the evaluation device (9) is adapted to a force on the component (1) with the modulus of elasticity E, the cross-sectional area A at a predetermined distance 1 (13) between the attachment points of fasteners 2a and 2b, which changes by Δl, using the following mathematical relationship:

13. Force sensor according to one of the preceding claims, wherein the evaluation device (9) is adapted to obtain a change in length for a capacitor arrangement of the capacitance C with capacitor surfaces A using the following relationship:

_ ε _o ε _R A

C

14. Force sensor according to one of the preceding claims, which additionally has a second capacitor arrangement (3c, 3d), wherein the evaluation device (9) is designed to determine the capacitance of the second capacitor arrangement (3c, 3d).

15. The force sensor according to claim 14, wherein the second capacitor arrangement (3c, 3d) has a fifth capacitance defining capacitor area (3c) and a sixth capacitance defining capacitor area (3d) which is separated by a gap from the fifth (3c ) Capacitor surface is arranged parallel to this.

16. A force sensor according to claim 14 or 15, wherein the second capacitor arrangement (3c, 3d) is mechanically coupled to the component (1) such that a change in length of the component (1) does not change the capacitance of the second capacitor arrangement (3c, 3d) causes.

17. A force sensor according to claim 16, wherein an area normal of the fifth (3c) and the sixth (3d) capacitor surface is perpendicular to the direction of force.

18. Force sensor according to claim 14 or 15, in which the second capacitor arrangement (3c, 3d) is mechanically coupled to the component (1) such that a change in length of the component (1) changes the capacitance of the second capacitor arrangement (3c, 3d) causes the change of the capacitance of the first capacitor arrangement (3a, 3b) is opposite.

19. A force sensor according to claim 18, wherein a surface normal of the fifth (3c) and the sixth (3d) capacitor surface is parallel to the direction of the force.

20. A force sensor according to claim 19, wherein the first (3a, 3b) and the second (3c, 3d) capacitor assembly are connected together via common fastening elements (2a, 2b) with the component (1).

21. A force sensor according to any one of claims 15 to 20, wherein between the fifth (3c) and the sixth (3d) capacitor surface, a seventh (7a), with the first capacitor surface (3c) forming a first partial capacitance capacitor surface, and an eighth (7b ), with the second capacitor surface (3d) a second partial capacitance capacitor surface are arranged on opposite sides of another carrier (7).

22. The force sensor according to claim 21, wherein the seventh (7a) and the eighth (7b) capacitor surface are formed from the material of the further carrier (7).

23. Force sensor according to one of claims 15 to 22, wherein the evaluation device (9) with the fifth (3c) and the sixth (3d) capacitor surface is electrically connected to determine the capacitance of the second capacitor arrangement.

24. Force sensor according to one of claims 15 to 23, wherein the evaluation device (9) with the seventh (7a) and the eighth (7b) capacitor surface is electrically connected to determine the capacitance of the second capacitor assembly.

25. Force sensor according to one of claims 20 or 21, wherein the seventh capacitor area (7a) consists of two electrically isolated from each other partial surfaces (7ai, 7a ₂ ) and the eighth capacitor surface (7b) consists of two electrically isolated from each other partial surfaces (7bi, 7b ₂ ) ,

26. A force sensor according to claim 25, wherein the mutually isolated partial areas (7a _if 7a ₂ , 7bi, 7b ₂ ) are electrically connected to the evaluation device (9) to determine the capacitance of the second capacitor arrangement, wherein the evaluation of the fifth (3c) and the sixth (3d) capacitor surface is galvanically isolated.

27. A force sensor according to one of claims 14 to 25, wherein the evaluation device is designed to use the difference of the capacitances of the first and the second capacitor arrangement in order to determine the force acting on the component (1) in the direction of force (12).

28. A force sensor according to any one of claims 14 to 27, wherein the fifth (3c) and the sixth (3d) capacitor surface of the material of the fastening elements (2a, 2b) are formed.

29. A force sensor according to one of the preceding claims, wherein the first (3a) and the second (3b) capacitor surface of the material of the fastening elements (2a, 2b) are formed.

30. A method for determining a force acting on a component in a force action, comprising the following steps:

Determining a capacitance of a capacitor arrangement, with at least a first and a second capacitor surface, which are separately mechanically rigidly connected via fasteners (2a, 2b) to the component (1), wherein a distance from attachment points at which the fastening elements (2a, 2b ) are connected to the component (1), in the direction of force (12) have a distance (13) which varies depending on the force and thus causes a change in a capacitance of the capacitor arrangement (3a, 3b); and Determining the force acting on the component using the determined capacitance of the capacitor assembly.

31. A computer program with a program code for carrying out the method according to claim 30, when the program runs on a computer.