DE3404262A1 - Capacitive sensor - Google Patents

Abstract

The invention relates to a capacitive sensor having a support member with two planar electrodes of corresponding size and with a third planar electrode of a diaphragm member, to which a pressure or a pressure difference can be applied. In order to obtain a sensor of this type which has high linearity in conjunction with improved measuremet accuracy, the diaphragm member (22) consists of a substrate (22a) to which a metal coating has been applied as third electrode (23), and there are present between the support member (21) and the diaphragm member (22) connecting means (30) which hold the support member (21) and diaphragm member (22) at a predetermined distance from one another. The connecting means (30) are arranged on the support member (21) outside the region of the first and the second electrode (24, 25) and between the first and the second electrode (24, 25) (Fig. 2). <IMAGE>

Description

Capacitive sensor

The invention relates to a capacitive sensor having a Support body on which a first and a second planar electrode with matching Size are applied, and with a third planar electrode of a membrane body, that of the first and the second electrode arranged opposite and with a Pressure or a pressure difference can be applied.

A known sensor of this type is shown in FIG. 1 as well as the figures 1a and 1b shown. The known sensor has (see FIG. 1, in which a section is represented by it) a housing 1 and planar electrodes 2 and 3. In addition, the known sensor has a membrane body 4 as a measuring membrane as well a support body 5. In addition, the known sensor is with connecting bores 6 and 9 provided in connecting pipes 7 and 8, to the electrical connections 11 and -12 are led; Another electrical connection 10 is with the membrane body 4 connected.

As Figure 1 also shows in detail, are on the support body 5 made of insulating material, the electrodes 2 and 3 arranged. The support body 5 is with the housing 1 made of metal, for example by metallization or on other way connected. The membrane body 4 is also connected to the housing 1, which consists of a metal measuring membrane 4, which by means of electron beam welding or is attached to the housing 1 by means of other welding processes. Like Figure 1a shows the electrodes 2 and 3 are concentric with each other arranged and have areas S1 and S2 which are coincident in size with each other and thus each have an area S.

The first (central) electrode 3 of the known sensor is with connected to the connecting pipe 7, in which a connecting bore 6 extends. This connecting hole 6 connects the interior of the sensor with the outside, so that over this Connection pipe a pressure to the membrane body 4 can be transmitted. The second too (Annular) electrode 2 is connected to a connecting pipe 8, which is also has a connecting bore 9. The first electrode 3 and the membrane body 4 form a capacitor with a capacitance C1, while the second electrode 2 and the membrane body 4 together have a further capacitor with the capacitance Represent C2. If a pressure is supplied through the connecting hole 6, then the membrane body 4 is deflected. This deflection is due in a known manner The following equations are determined on the assumption that the deflection is large in the mean Area of the membrane body 4, while the outer part of the membrane body 4 hardly is deflected. It is assumed that the capacitance C2 remains unchanged, as illustrated in the equivalent circuit diagram of Figure Ib.

The following relationships then result: The following relationship can be determined from equations (1) and (2): In these equations, £ o denotes the constant of electricity of the medium between the electrodes of the sensor and S denotes the area of electrodes 2 and 3, respectively; dg represents the distance between the electrodes 2 and 3 on the one hand and the membrane body 4 on the other side, while Ad denotes the main deflection of the membrane body 4.

In practice, however, this also occurs at the edge of the clamped area of the membrane body a deflection, whereby the capacitance C2 changes its value. This leads to a deterioration in linearity and a decrease in Measurement accuracy. In addition, the known sensor has the following disadvantages: a) Since the two electrodes 2 and 3 are arranged close to one another, a Develop stray capacitance between them.

b) The electrodes 2 and 3 are both arranged so that they touch the membrane body 4 opposite one another; therefore is the capacitance value of each capacitor formed small, so that the capacitance value easily interferes with the aforementioned stray capacitance can be influenced.

c) Since the known sensor has a structure made of metal and an insulating body owns, its production is generally quite expensive.

The invention is based on the object of a capacitive measuring sensor to create that avoids the disadvantages listed above, i.e. through a high linearity with improved measurement accuracy and low stray capacitances.

To solve this problem, there is a capacitive sensor of the type specified at the beginning, according to the invention, the membrane body consists of a substrate with applied metal coating as a third electrode, and between the support body and the membrane body there are connecting means that support body and holding membrane bodies at a predetermined distance from one another, wherein the connecting means on the support body outside the area of the first and second electrodes and are arranged between the first and second electrodes.

An advantage of the sensor according to the invention is that at him the measuring capacitor formed by means of the first electrode and by means of the Second electrode formed reference capacitor arranged spatially separated from one another so that a change in the capacitance of the reference capacitor does not occur. This improves the linearity and thus also the measurement accuracy. Further the sensor according to the invention has the advantage that it is by a comparatively small size. It should also be noted that as a result of the use a membrane body made of a substrate, a material is used that has considerably better elastic properties than metal.

Therefore, the deflection of the diaphragm is improved in terms of linearity, and hysteresis phenomena are also reduced.

In the sensor according to the invention, the substrate can be made of molten Quartz, rock crystal, glass, ceramic or sapphire consist. If necessary, can also the support body can be made from these materials.

The connecting means are advantageously in view on easy manufacturing of the entire sensor around a layer of a connecting material.

The first and second electrodes may be different in the probe Exhibit design. It is considered advantageous if the first and the second electrode have approximately the same shape. A circular shaped Formation of the two electrodes proved to be particularly advantageous.

One advantage of a probe designed in this way is that that the measuring capacitor formed by the first electrode and that of the second Electrode formed reference capacitor lie side by side and therefore these two capacitors can be arranged so close together that no stray capacitance can develop between them. Both capacitors advantageously have approximately the same temperature behavior. Another advantage of a probe is this Execution consists in having a matching electrode configuration has, whereby the influence of unequal stray capacities compared to the known Sensor is reduced, which has a beneficial effect on the linearity.

For manufacturing reasons, among other things, it can be advantageous be when the support body of the sensor according to the invention on its the membrane body facing side has two recesses, in which the first and the second Electrode is arranged; outside of the depressions there is a proportionate one thin layer of connecting material.

But it is also possible and if necessary also advantageous if the support body has a flat surface on its side facing the membrane body having on which the first and second electrodes lie, and if outside of the Area of these two electrodes is a relatively thick layer of connecting material is available. An advantage of this design can be seen in the fact that the support body can be made relatively easy.

If the sensor is - as is known per se - with a first circular and a second annular electrode comprising the first electrode, then the layer of connecting material is advantageously located in the ring area between the first and second electrodes.

In this embodiment too, the linearity is proportionate good because the layer of bonding material between the first and the second Electrode, causing a change in the capacitance of the second electrode formed reference capacitor is prevented. The layer in the ring area between the first and the second electrode also has the advantage that through them the two electrodes are spaced from each other, resulting in a reduction the stray capacitance leads. In addition, a sensor of this embodiment has the advantage that its manufacturing costs are relatively low, as in his No structure made of metal and insulating material is used.

In the case of the sensor with a circular and an annular electrode the support body advantageously has on its side facing the membrane body a recess in which the first electrode is arranged; owns on the verge the support body has an annular recess in which the second electrode is located; in the area outside the depression and the recess one is relative thin layer of connecting material present.

In particular with a view to easy producibility of the support body this advantageously has a plane on its side facing the membrane Surface on which the first and second electrodes lie; in the range between these electrodes is a relatively thick layer of bonding material available.

To the sensor according to the invention in the simplest possible way with To be able to connect external connections, advantageously have the first and the second electrode each have a lateral lead-out, the outer end of which each serves as a connection point.

The substrate of the membrane body of the sensor according to the invention can be measured in different ways; so it is considered beneficial if the substrate is relatively thick and on its facing the support body, the side carrying the metal covering is flat and on its other side has a recess, in the area of which the substrate acts as a measuring membrane; this Area is at least opposite the first electrode.

But it is also possible and under certain circumstances advantageous if that The substrate is thin like a membrane and has a flat surface on both sides.

To explain the invention, a section through a is shown in FIG Embodiment of the sensor according to the invention with two arranged side by side Electrodes and a plan view of the same embodiment shown in FIG. 3; in Figure 4 is a section through a further embodiment with side by side arranged electrodes, in Figure 5 another embodiment with also electrodes arranged next to one another, in FIG. 6 an additional exemplary embodiment with two electrodes arranged next to one another, in FIG. 7 an equivalent circuit diagram of the sensor according to Figures 4 to 6, in Figure 8 with an embodiment electrodes arranged concentrically to one another, in Figure 9 a Top view of the same exemplary embodiment with the membrane body removed, in the figure 10 only the support body of the exemplary embodiment according to FIG. 8, in FIG. 11 a further one Embodiment of a sensor with concentrically arranged electrodes, in FIG. 12 shows another exemplary embodiment with concentrically arranged electrodes, in Figure 13 a further embodiment with concentrically arranged electrodes, FIG. 14 shows a plan view of the first and second electrodes of the exemplary embodiment according to FIG. 13, in FIG. 15 an equivalent circuit diagram of the sensor according to the invention and in FIG. 16 a further equivalent circuit diagram is shown.

In the embodiment of Figure 2, a support body 21 is used, which consists of fused quartz, rock crystal, glass, ceramic or sapphire. A The membrane body 22 is made of a material similar to that of the support body 21.

The support body 21 has two depressions, which are essentially have the same size. A first electrode 24 and a second electrode 25 are housed in the wells It should be noted that the wells are circular are designed, as shown in FIG. 3 by means of the dashed lines 34 is indicated. The first electrode 24 and the second electrode 25 in the depressions are formed, for example, by vapor deposition or a similar process.

The membrane body 22 consists of a substrate 22a and a metal coating as a third electrode 23, for example by vapor deposition or the like Way is formed.

The substrate 22a has an area 26 due to a recess reduced Thickness present in the part of the substrate that is the first electrode 24 opposite. The substrate 22a serves in its area reduced in thickness 26 as a measuring membrane, because the metal coating 23 extends over the entire surface of the substrate 22a extends. In this way there is a changeable measuring capacitor with the capacitance value C1 by means of the first electrode 24 and the third electrode 23 and a reference capacitor with the capacitance value C2 through the second electrode 25 and the third electrode 23 formed.

The sensor shown has connecting pipes 28 and 29, the are each provided with a connecting bore 27, similar to this with reference to FIG Figure 1 has been described.

Via the connecting tube 28, the first electrode 24 is connected to an electrical Connection 32 and via the connection pipe 29 the second 25 with an electrical connection 33 connected, while the third electrode 23 is directly connected to an electrical connection 31 is connected, the connection being made by soldering or the like can.

Between the membrane body 22 and the support body 21 is located a layer 30 of a bonding material that is organic Lanyards, a low-melting glass or a glass at all can, with which the substrate 22a is connected to the support body 21.

If a sensor constructed in this way, a pressure or a pressure difference is supplied via the connecting bore 27, then the acting as a measuring membrane thinned area 26 of the substrate 22a deflected, causing a change in capacitance C1 of the measuring capacitor leads from the first electrode 24 and the third electrode 23 is formed. An influence on the capacitance of the reference capacitor from the second electrode 25 and the third electrode 23 results not, since the substrate 22a has a sufficient thickness.

Experiments have shown that the ratio of the thickness h1 of the thinned area 26 with respect to the thickness h2 of the substrate 22a in its remaining area should satisfy the condition that h2 / h1'-3. The capacitance value C2 can therefore be assumed to be unchangeable. Accordingly, the capacities C1 and C2 can then be expressed by the following relationships: The following equation can be derived from these relationships: Regarding these equations it should be noted that g is the dielectric constant of the medium between the electrodes, A is the effective area of electrodes 24 and 25, dg is the distance between electrodes 24 and 25 on the one hand and third electrode 23 on the other hand, and td is a size proportional to that applied Pressure describes. These equations correspond to relations (1) to (3) when the direction of pressure applied is the same. Therefore, a measured quantity corresponding to the pressure can be obtained by measuring the capacitances C1 and C2.

In the exemplary embodiment according to FIG. 4, both are one electrode 24 as well as the second electrode 25 applied to the surface of a support body 21, which is even. Here, too, the support body 21 and the substrate 22a are again over a layer 30 of connecting material connected to one another, the has a predetermined thickness. It is then not necessary to have recesses in the support body to be provided, whereby the production is simplified.

The exemplary embodiment according to FIG. 5 differs from that according to FIG. 4 in this respect from when the substrate 22 is thinned over its entire extent with him, so that in the production of the effort for the production of the recess is avoided. In this embodiment, however, both the capacitance value C1 and the Capacitance value C2 due to the thin design of the substrate 22a as a function change from pressure.

It may therefore be necessary, in addition, on the side of the Reference capacitor with the capacitance value C2 to provide means that have a feed prevent the pressure.

The exemplary embodiment according to FIG. 6 can be regarded as having been achieved thereby be that the arrangement of Figure 5 in two parts. is separated by a Arrangement according to Figure 5 is cut through in the middle. How this works The sensor is similar to that of Figure 5, with the advantage that stray capacitances are still can be further reduced.

The equivalent circuit diagram according to FIG. 7 applies to the exemplary embodiments discussed above according to Figures 2 to 6 and corresponds to that of Figure Ib. In the figure 7, C1 and C2 denote the capacitance values of the first electrode 24.

with the third electrode 23 formed measuring capacitor and of the reference capacitor formed by the second electrode 25 and the third electrode 23, while the connection designations correspond to those selected in FIGS. 2 to 6.

In the embodiment shown in FIGS of the sensor according to the invention, the support body 41 consists in turn of molten material Quartz, rock crystal, glass, Ceramic or sapphire. The supporting body 41 has two concentrically arranged electrodes, of which the first electrode 42 circular and the second electrode 44 circularly the first electrode 42 are comprehensively arranged.

These two electrodes 42 and 44 are in recesses on the support body 41 applied by vapor deposition or the like. The support body 41 has an annular shape Elevation 41 ', which can be seen particularly well in FIG. Furthermore, in the Carrying body 41 a connecting pipe 51 with a connecting bore 47 similar to that provided, as has already been mentioned in connection with FIG. The membrane body 46 is made thin like a membrane over its entire dimension and consists of a material similar to that used for the support body 41. The membrane body 46 has a metal covering over its entire surface facing the support body 41, which in turn is applied by vapor deposition or a similar process and the third electrode 43 forms.

The support body 41 and the membrane body 46 are connected to one another, so that their electrodes face each other; the connection is by means of a layer 45 of a bonding material such as low melting point glass, a any glass, organic compound or metal. It should be noted is in this context that a procedure is used in this connection can that makes use of a eutectic reaction, or a process by means of anodic bondings. Therefore, the third electrode 43 and the first electrode form 42 a plate capacitor with the capacitance C1, while the second electrode 44 forms a reference capacitor with the capacitance C2 with the third electrode 43. In the event of a pressure load, the capacitance C1 of the measuring capacitor changes during the capacitance C2 of the reference capacitor is unaffected by these pressures remain. The first electrode 42 is connected via the connecting tube 51 to a connection 50, the second electrode to an electrical terminal 48 and the third electrode brought up to an electrical connection 49, being the connection the connecting wires with the electrodes can be made by soldering or the like can.

In the capacitive measuring sensor shown in FIGS there is a different one in each case on both sides of the membrane body 40 Pressure as a result of the application of the connecting pipe 47, so that the membrane body 40 is acted upon with a pressure difference; according to this pressure difference the membrane body 40 is deflected. Therefore, the distance between the first electrode 42 and third electrode 43 and thus also the value of the capacitance C1, while the distance between the second electrode 44 and the third electrode 43 remains unchanged. In turn, one denotes the distance between the electrodes with dg, the dielectric constant of the medium between the electrodes with g, the area of the electrodes 42 and 44 with A and that caused by the pressure difference Change in distance between the first electrode 42 and the third electrode 43 with a d, then there is also a relationship between the capacitance values for this sensor and the distances or

Changes in distance, as already described in equation (6) is. Accordingly, a pressure difference can also be measured with this sensor the capacitances C1 and C2 can be determined.

In the embodiment of the invention shown in Figure 11 Sensor, the membrane body 40 has a comparatively large thickness and is provided with a recess 40 'to form the measuring membrane. The recess 40 ' is provided in the substrate 40a of the membrane body and introduced so far that the Substrate 40a is thinned like a membrane.

As has been proven experimentally, this results in a less effect of the hysteresis of the measuring membrane material than in the embodiment according to Figures 8 to 10. It has also been demonstrated that the ratio between the thickness hl of the thick part of the substrate 40a to the thickness h2 of the thinned part of the substrate 20 (<t = h1 / h2) should satisfy the condition g> -3.

In the embodiment shown in Figure 12 are in deviation of the exemplary embodiment according to FIGS. 8 to 10 and 11, the electrodes 42 and 44 applied to a support body 41 with a flat surface; the support body 41 therefore has no elevation. The electrodes 42 and 44 can in turn applied by vapor deposition or in a similar manner. By means of a layer 45 'made of a connecting material which is an organic binding agent, A low-melting glass or a glass at all can act, are the support body 41 and the membrane body 40 with the third electrode 43 are connected to one another. The layer 45 'is made of the connecting material between the first electrode 42 and the second electrode 44. This type of connection and structure of the probe facilitates its manufacture. The first electrode 42 is - like Figure 14 shows - provided with a lateral lead-out 42 ', which is left free by a Area of the second electrode 44 runs. In the embodiment according to FIG 12, the membrane body 40 can be replaced by a body that is a membrane-like has thinned area in its central part as shown in FIG is. The electrodes 42, 43 and 44 are in turn connected to electrical connections 50, 49 and 48 led.

Although concentric in the exemplary embodiments according to FIGS. 8 to 12 arranged electrodes 42 and 44 are only provided on the support body, Of course, a corresponding electrode arrangement can also be installed on the membrane body be present on the side facing the support body. A corresponding arrangement is shown in FIG. at this embodiment are both on the support body 41 electrodes 42 and 44 and on the membrane body 40 electrodes 62 and 64 are applied in an arrangement as shown in FIG. In this embodiment, too, the support body 41 and the membrane body are 40 connected to one another by means of a layer 45 'made of connecting material, see above that said electrodes are opposite one another. The electrodes are with electrical Connections 52, 52 'and 48, 48'. This embodiment is good for suitable for the cases in which individual capacity values are to be tapped.

FIGS. 15 and 16 show electrical equivalent circuit diagrams for the in FIGS. 8 to 14 sensor shown, FIG. 16 being the electrical Equivalent circuit diagram for a sensor according to the exemplary embodiments according to the figures 8 to 12 and FIG. 16 shows an equivalent circuit diagram for a sensor corresponding to Figure 13 shows. In FIGS. 15 and 16, the same reference numerals denote the corresponding connections in FIGS. 8 to 14.

16 figures 14 claims - blank page -

Claims (14)

Claims 3 capacitive sensor with a support body a first and a second planar electrode of the same size are applied, and with a third planar electrode of a membrane body, that of the first and the second electrode arranged opposite and with a Pressure or a pressure difference can be acted upon by d u r c h e k e n n z e i c h n e t that the membrane body (22) consists of a substrate (22a) with brought on There is a metal coating as a third electrode (23) and that between the support body (21) and connecting means (30) are present in the membrane body (22), the support bodies (21) and membrane body (22) hold at a predetermined distance from one another, wherein the connecting means (30) on the support body (21) outside the area of the first and the second electrode (24, 25) and between the first and second electrodes (24, 25) are arranged (Fig. 2). 2. Sensor according to claim 1, d a d u r c h g e k e n n -z e i c Note that the substrate (22a) is made of fused quartz, rock crystal, glass, ceramic or sapphire (Fig. 2) 3. measuring sensor according to claim 2, d a d u r c h g e k.e n n -z e i c h n e t that the support body (21) made of fused quartz, rock crystal, Glass, ceramic or sapphire (Fig. 2) 4. Sensor according to one of the preceding Claims that the connecting means consist of a layer (30) of a connecting material (Fig. 2). 5. Sensor according to one of the preceding claims d a d u r c h it is noted that the first and second electrodes (24, 25) are one have approximately the same shape (Fig. 2). 6. Sensor according to claim 5, d a d u r c h g e k e n n -z e i c h n e t that the first and the second electrode (24, 25) are circular are (Fig. 2). 7. Sensor according to claim 5 or 6, d a d u r c h g e k e n n z e i c h n e t that the support body (21) on its facing the membrane body (22) Side has two depressions in which the first and second electrodes (24, 25) is arranged, and that there is a relatively thin layer (30) of connecting material is located (Fig. 2). S. Measuring sensor according to claim 5 or 6, d u r c h g e k e n n z e i c h n e t that the support body (21) on its facing the membrane body (22) Side has a flat surface on which the first and second electrodes (24, 25) and that outside the range of these two electrodes (24, 25) a relatively thick layer (30) of connecting material is present (Fig. 4). 9. Sensor according to one of claims 1 to 5 with a first circular and a second annular electrode comprising the first electrode, d a d u r c h e k e n n n z e i c h n e t that the layer (45) is made of connecting material located in the ring area between the first and second electrodes (42, 44) (Fig. S). 10. Sensor according to claim 9, d a d u r c h g e k e n n -z e i c h n e t that the support body (41) on its facing the membrane body (40) Side a recess (41 ') has in which the first electrode (42) is arranged, and at the edge has a riny-shaped recess in which the second electrode (44) is located, and that in the area outside the recess and the recess a relatively thin layer (45) of connecting material lies (Fig. 8). 11. Sensor according to claim 9, d a d u r c h g e -k e n n z e i c h n e t that the support body (41) on its facing the membrane body (40) Side has a flat surface on which the first and second electrodes (42, 44) is, and that in the area between these electrodes (42, 44) is a relatively thick layer (45 ') of connecting material is present (Fig. 12). 12. Measurement error according to claim 8 or 11, d a d u r c h g e k e n n z e i c h n e t that the first and the second electrode (42, 44) each have a lateral Have lead-out (42 '), the outer end of which serves as a connection point (Fig. 12/14). 13. Sensor according to one of the preceding claims d a d u r c h it is noted that the substrate (22a) is relatively thick and on its side facing the support body (21) and carrying the metal coating (23) is flat and has a recess on its other side, in which Area the substrate acts as a measuring membrane (26), and that this area at least the first electrode (24) is opposite (Fig. 1). 14. Sensor according to one of claims 1 to 12 d a d u r c h g e it does not indicate that the substrate (22a) is membrane-like thin and on both Pages has a flat surface (Fig. 5).