DE102016124752A1 - Capacitive pressure sensor - Google Patents

Abstract

A pressure sensor (10) comprises an operation and evaluation circuit (170) and a pressure measuring cell (100) comprising: a measuring diaphragm (110); a mating body (120) connected thereto; a measuring chamber (140) formed therebetween; and a capacitive transducer (150); wherein a deflection of the measuring diaphragm (110) is pressure-dependent, wherein the capacitive transducer (150) has at least one edge region capacitor with a pressure-dependent edge region capacitance, the edge region capacitor an edge region electrode (152) on a surface of the counter body (120) and a membrane electrode (156) a surface of the measuring diaphragm (110), wherein the peripheral electrode (152) leaves free a central region of the surface of the counter body (120), wherein the pressure measuring cell (100) is dimensioned so that the measuring diaphragm (110) on the surface of the counter body ( 120) is applied when the pressure difference exceeds a first threshold, wherein the operation and evaluation circuit (170) is adapted to provide a pressure difference representing signal which depends exclusively on the edge area capacity when the pressure difference is more than 0.9 times of the threshold.

Description

The present invention relates to a capacitive pressure sensor, in particular a capacitive absolute pressure sensor. A generic pressure sensor comprises: an operating and evaluation circuit and a pressure measuring cell with a measuring diaphragm; at least one counter-body; a measuring chamber; and a capacitive transducer; wherein the measuring chamber is formed between the measuring diaphragm and the counter body, wherein the measuring diaphragm is connected to the counter body along a circumferential edge, wherein a deflection of the measuring diaphragm of a pressure difference between an on the outside of the measuring chamber remote from the outside of the measuring diaphragm applied media pressure and one in the measuring chamber the peripheral capacitance depends on the deflection of the measuring diaphragm, the peripheral capacitor being an edge region electrode on a surface of the counter body facing the measuring membrane and a membrane electrode on a surface of the measuring membrane facing the counter body wherein the peripheral edge electrode leaves free a central area of the surface of the counter body, wherein the pressure measuring cell is dimensioned so that the measuring diaphragm on the upper surface of the counter body abuts when the pressure difference exceeds a first threshold, and that is not applied to the surface of the counter body when the pressure difference is less than the first threshold. In capacitive pressure sensors according to the prior art, the capacitive transducer further comprises an inner condenser with an inner capacitance, wherein the inner capacitance depends on the deflection of the measuring diaphragm, wherein the inner condenser an inner electrode on the surface of the counter-body facing the measuring membrane and a membrane electrode on the opposite side facing the counter body Surface of the measuring membrane comprises; wherein the pressure measuring cell is dimensioned such that the measuring diaphragm in the region of the inner electrode rests against the counter body when the pressure difference exceeds the first threshold, and that it does not abut against the counter body in the region of the inner electrode when the pressure difference falls below the first threshold value; wherein the operation and evaluation circuit is arranged to provide a signal representing the pressure difference, which depends on the internal capacity and the edge area capacity, if the pressure difference is not more than K times the threshold, where K is necessarily less than one. A pressure sensor according to the prior art is disclosed, for example, in the published patent application DE 10 2011 078 557 A1 ,

If large pressure values are to be detected, the resolution is limited to low pressure readings, since the threshold value must be selected to be correspondingly high. Likewise, a desired sensitivity at low pressure values precludes measurement at high pressures. It is therefore the object of the present invention to provide a pressure sensor which mitigates this conflict of goals. The object is achieved by the pressure sensor according to independent claim 1.

The pressure sensor according to the invention comprises: an operating and evaluation circuit; and a pressure measuring cell comprising: a measuring diaphragm; at least one counter-body; a measuring chamber; and a capacitive transducer; wherein the measuring chamber is formed between the measuring diaphragm and the counter body, wherein the measuring diaphragm is connected to the counter body along a peripheral edge, wherein a deflection of the measuring diaphragm of a pressure difference between a remote on one of the measuring chamber outside of the measuring diaphragm applied pressure and one in the measuring chamber the peripheral capacitance depends on the deflection of the measuring diaphragm, the peripheral capacitor being an edge region electrode on a surface of the counter body facing the measuring membrane and a membrane electrode on a surface of the measuring membrane facing the counter body wherein the peripheral edge electrode leaves free a central area of the surface of the counter body, wherein the pressure measuring cell is dimensioned so that the measuring diaphragm on the upper surface of the counter body is applied when the pressure difference exceeds a first threshold, and that is not applied to the surface of the counter body when the pressure difference is less than the first threshold value; characterized in that the operating and evaluation circuit is adapted to provide a signal representing the pressure difference, which depends exclusively on the Randbereichskapazität and possibly a temperature readings when the pressure difference is more than K times the threshold, where K is a value of not more than 0.9, for example not more than 0.5 and in particular not more than 0.3.

In one development of the invention, the measuring chamber is evacuated, wherein the first threshold value is not more than 5 × 10 ⁴ Pa, for example not more than 2 × 10 ⁴ Pa, in particular not more than 1 × 10 ⁴ Pa.

In one development of the invention, the measuring diaphragm is not applied to the counter-body up to a pressure difference which does not exceed a second threshold value in the region of the edge-region electrode, the second threshold value being at least eight times, in particular not less than sixteen times, the first threshold value.

In one development of the invention, the measuring diaphragm is not up to a pressure difference, which does not exceed a second threshold, in the region of the edge area electrode on the counter body, wherein the second threshold is at least not less than 1bar, in particular not less than 2 bar.

In a further development of the invention, the operating and evaluation circuit is set up to provide a signal representing the pressure difference, which also depends exclusively on the edge area capacity and possibly a temperature measured values, if the pressure difference is not more than K times the threshold value.

In one development of the invention, the capacitive converter further has an inner capacitor with an inner capacitance, wherein the inner capacitance depends on the deflection of the measuring diaphragm, wherein the inner capacitor has an inner electrode on the surface of the counter body facing the measuring diaphragm and a membrane electrode on the surface of the counter body Measuring diaphragm comprises; wherein the pressure measuring cell is dimensioned such that the measuring diaphragm in the region of the inner electrode rests against the counter body when the pressure difference exceeds the first threshold, and that it does not abut against the counter body in the region of the inner electrode when the pressure difference falls below the first threshold value; wherein the operation and evaluation circuit is arranged to provide a signal representing the pressure difference, which depends on the inner capacity and the edge area capacity, when the pressure difference is not more than K times the threshold value.

In one development of the invention, the inner capacity at a pressure difference below the first threshold value, a greater pressure dependence than the edge area capacity.

gegeben ist, wobei C_i und C_r die Innenkapazität bzw. die Randbereichskapazität bezeichnen.In a further development of the invention, the operating and evaluation circuit is set up to determine the signal on the basis of a transfer function F at pressure differences below K times the first threshold value, which as $$\text{F}=\text{F}\left(\left({\text{C}}_{\text{i}}-{\text{C}}_{\text{r}}\right)/{\text{C}}_{\text{i}}\right)$$

where C _i and C _r denote the inner capacitance and the peripheral region capacitance, respectively.

In a further development of the invention, K has a value of not less than 0.1, in particular not less than 0.2.

wobei E der Elastizitätsmodul des keramischen Werkstoffs der Messmembran und µ dessen Poissonzahl ist, wobei y₀ der Abstand zwischen der Messmembran und dem Gegenkörper am Rand der Messkammer ist, wobei h die Materialstärke der Messmembran ist, wobei r_i der Radius der Messkammer, wobei der erste Schwellwert um nicht mehr als 10% der Designdruckdifferenz von der Designdruckdifferenz abweicht.In one development of the invention, the pressure measuring cell has a _design pressure difference dp _design , for which the following applies: $$d{p}_{DesiGn}:=\frac{16}{3\cdot \left(1-{\mathrm{\mu }}^2\right)}\cdot e\cdot {\left(\frac{H}{r_i}\right)}^4\cdot \frac{y_0}{H}.$$

where E is the modulus of elasticity of the ceramic material of the measuring membrane and μ its Poisson's number, where y _{0 is} the distance between the measuring membrane and the counter body at the edge of the measuring chamber, where h is the material thickness of the measuring diaphragm, where r _{i is} the radius of the measuring chamber, wherein the first threshold does not differ by more than 10% of the design pressure difference from the design pressure difference.

• 1: ein Ausführungsbeispiel eines erfindungsgemäßen Drucksensors.
• 2a: ein Diagramm von druckabhängigen Biegelinien der Messmembran eines Ausführungsbeispiels eines erfindungsgemäßen Drucksensors bei Druckdifferenzen kleiner 1 mbar;
• 2b: ein Diagramm von druckabhängigen Biegelinien der Messmembran des Ausführungsbeispiels des erfindungsgemäßen Drucksensors aus 2a bei Druckdifferenzen von 1 mbar oder mehr; und
• 3: ein Diagramm von Übertragungsfunktionen eines Ausführungsbeispiels eines erfindungsgemäßen Drucksensors.

The invention will now be explained in more detail with reference to the embodiments illustrated in the drawings. It shows:

• 1 : An embodiment of a pressure sensor according to the invention.
• 2a a diagram of pressure-dependent bending lines of the measuring membrane of an embodiment of a pressure sensor according to the invention at pressure differences less than 1 mbar;
• 2 B a diagram of pressure-dependent bending lines of the measuring membrane of the embodiment of the pressure sensor according to the invention from 2a at pressure differences of 1 mbar or more; and
• 3 : a diagram of transfer functions of an embodiment of a pressure sensor according to the invention.

This in 1 illustrated embodiment of a pressure sensor 10 has a pressure measuring cell 100 on. This comprises a circular disk-shaped, ceramic measuring diaphragm 110 and a circular disk-shaped ceramic counter body 120 of the same diameter.

The measuring diaphragm is along its circumference with the counter body along a circumferential joint 130 pressure-tight, being between the measuring diaphragm 110 and the opposing body 120 a measuring chamber 140 is formed. The joint has a thickness y _0, for example, 10 .mu.m to 30 .mu.m, whereby a distance between the measuring membrane 110 and the opposing body 120 is defined. The joint 130 has an inner radius r _i of, for example, 5 mm to 20 mm. The measuring membrane has, for example, a material thickness h of a few 10 .mu.m to a few 100 .mu.m. In particular, both have the measuring membrane 110 as well as the opposing body 120 Corundum, wherein the corundum, which is used for the measuring membrane, has a high purity in order to achieve a higher breaking stress. The material thickness of the counter body 120 is for example a few mm. She is chosen so that the opposite body 120 in comparison to the measuring membrane 110 is rigid, so practically has at most negligible deformations. The joint 130 has in particular a Zr-Ni-Ti alloy. Instead of the ceramic measuring membrane and a metallic measuring membrane can be used, in particular a measuring membrane of Ta, which has a thickness of several 10 microns, and is fixed by means of soldering to a counter body. The measuring chamber 140 is evacuated, ie its internal pressure is not more than 0.01 Pa, in particular not more than 0.001 Pa.

wobei E der Elastizitätsmodul des keramischen Werkstoffs der Messmembran und µ dessen Poissonzahl ist. Die Designdruckdifferenz ist jene Druckdifferenz, bei welcher die Auslenkung des Zentrums der Messmembram etwa der Stärke y₀ der Fügestelle 130 entspricht. Die Dimensionen der Druckmesszelle 1000 sind so aufeinander abgestimmt, dass die Designdruckdifferenz einen Wert von beispielsweise 1 hPa bis 100 hPa aufweist.The pressure measuring cell 100 has a design pressure difference dp _design for which: $$d{p}_{DesiGn}:=\frac{16}{3\cdot \left(1-{\mathrm{\mu }}^2\right)}\cdot e\cdot {\left(\frac{H}{r_i}\right)}^4\cdot \frac{y_0}{H}.$$

where E is the modulus of elasticity of the ceramic material of the measuring membrane and μ is its Poisson's number. The design pressure difference is the pressure difference at which the deflection of the center of the measuring diaphragm roughly equals the y ₀ strength of the joint 130 equivalent. The dimensions of the pressure cell 1000 are matched to one another such that the design pressure difference has a value of, for example, 1 hPa to 100 hPa.

For highly pure corundum E = 380 GPa and μ = 0.22 can be expected. With an inner radius r _{i of} the joint of 12 mm and a membrane thickness of h = 100 μm, a design pressure difference of approximately 20 hPa is achieved at a joint thickness of y ₀ = 20 μm.

The pressure measuring cell 100 also has a capacitive transducer 150 which is a peripheral electrode 152 on one of the measuring membrane 110 facing surface of the counter body 120 , an inner electrode 154 at the measuring diaphragm 110 facing surface of the counter body 120 and a full-surface membrane electrode 156 has on a surface of the measuring membrane facing the counter body. The electrodes include, for example, metals, in particular tantalum.

The edge area electrode 152 and the membrane electrode 156 together form a peripheral capacitor, while the inner electrode 154 and the membrane electrode 156 together form an inner condenser. The edge area condenser and the inner condenser have in one embodiment of the invention for a pressure difference of zero between the pressure in the measuring chamber and outside the measuring chamber substantially similar capacities and are preferably equal in capacitance. The capacities are similar if the larger does not deviate more than 5% of the value of the smaller one from the smaller one.

The pressure sensor 10 also includes an operating and evaluation circuit 170 which is set up to feed the capacitive converter and to determine its capacitances, to determine a pressure measurement value on the basis of the capacitances and to output the latter in particular via a two-wire line 180, for example a fieldbus, to a higher-order unit.

The operating and evaluation circuit 170 is via electrical feedthroughs 162 . 164 For example, Ta-pins, extending through the counter body 120 extend, with the inner electrode 154 and the peripheral region electrode 152 coupled to the capacitive transducer. The membrane electrode 156 is in galvanic contact with the joint 130 and is over a metallic coating 166 on the lateral surface of the counter body 120 or via an additional electrical feedthrough in the area of the joint to circuit ground.

For pressure differences below the design pressure difference, it is possible to determine the pressure-dependent deformation of the measuring diaphragm - and thus the pressure difference - by means of a transfer function which evaluates both capacities, for example dp = A * (Ci-Cr) / Ci. + B If the capacities at dp = 0 have the same capacity, B = 0. This transfer function is satisfactory for pressure differences up to half the design pressure difference. As the pressure differential approaches the design pressure differential, the deviations from the linearity become too large and the transfer function requires higher order terms or the operation and evaluation circuitry is overdriven. It has therefore hitherto been customary to specify for the nominal pressure range of a pressure sensor only pressure differences which are smaller than half the design pressure differential pressure.

When the pressure difference reaches the design pressure difference, the membrane electrode contacts the inner electrode, therefore, at pressure difference values greater than or equal to the design pressure difference, the capacity of the inner condenser can not be determined. The operating and evaluation circuit 170 is therefore adapted to determine a pressure-dependent signal by a transfer function depending on the capacity of the inner condenser and the capacity of the peripheral condenser up to a pressure difference corresponding to k times the design pressure difference and for pressure differences above k times the design pressure difference to output a pressure-dependent signal, which depends on the capacity of the Randbereichskondensators and does not depend on the capacity of the inner condenser.

The number k has, for example, a value of 0.3 to 0.6. The k-fold value of the design pressure difference corresponds to the conventional measuring span of the pressure sensor in which a pressure difference with a specified accuracy of 0.05% of the span can be measured. With a design pressure difference of the pressure measuring cell of 1 hPa and k = 0.5 results for the pressure sensor, a measurement accuracy of 2.5 · 10 ^-4 hPa.

In 2a and 2 B Bending lines y (p, r) of a measuring membrane of an absolute pressure measuring cell are shown with a design pressure difference of about 19 hPa and y ₀ = 20 microns, which were determined by means of an FEM method. 2a shows the bending lines for 0.1 hPa, 0.01 hPa and 0.001 hPa. Here, the usual bending lines can be seen, which in the first approximation with y (p, r) = p C ( 1 - (r / r _i ) ² ) ² . Where C is a constant and r _{i is} the inner radius of the joint 130 or the radius of the measuring chamber is. 2 B shows the bending lines y (p, r) for 1 hPa, 10 hPa, 100 hPa and 1000 hPa. For the first two pressure values again the usual bending lines occur, while the measuring diaphragm rests against the counter body at 100 hPa and 1000 hPa.

The deflection at 10 hPa corresponds approximately to the deflection at half the design pressure difference. Conventionally, this pressure measuring cell would be used as a pressure sensor with a nominal pressure difference of 10 mbar or 10 hPa. This is done, for example, by implementing a transfer function of the type F1 = (C _{i -Cr} ) / C _i or (C _p -C _r ) / C _p , wherein in the second, equivalent notation for the capacitance C _{i of} the inner capacitor, the usual Name C _p is used, whose greater pressure dependence indicates the index p. The radial regions of the inner electrode or pressure electrode on the one hand and the edge region electrode or reference electrode on the other hand are in 2a and 2 B each marked with C _p and C _r .

As based on 2 B can be seen, the measuring membrane is not even at a pressure of 1000 hPa at a hundred times the nominal pressure at the edge of the electrode. This allows the pressure above the nominal pressure to be determined by means of a transfer function which depends exclusively on the edge area capacity C _r . The transfer function can be, for example, of the form F2 = (C _r -C _r ( 0 )) / C _r ( 0 ) exhibit.

In 3 points of the two discussed transfer functions F1 and F2 are shown, which consist of the in 2a and 2 B shown bending lines result. The conventional transfer function F1 using both capacitances has sufficient dynamics to determine pressure readings even at pressures as low as a few hundredths Pa. At pressure values above 1000 Pa, this transfer function F1 is no longer meaningful to use. Instead, here offers the transfer function F2, which depends exclusively on the edge area capacity or reference capacity. However, the transfer function F2 reaches its limits with small pressure values, because unlike the conventional transfer function F1, which always works with current values of both capacities, F2 becomes a Difference between a current value and a stored zero point value formed whose validity is possibly subject to other factors, such as the temperature. In this respect, the suitability of this transfer function F2 decreases with decreasing pressure. However, depending on the accuracy requirements, a transfer function up to pressures below 1 Pa may be used which depends exclusively on the marginal area capacity. If the highest possible accuracy is desired over a wide measuring range, it is currently preferred to use the transfer function F1 for small pressures and to use the transfer function F2 for pressures above k times the design pressure difference, where k <1, in particular k <0 ,. 6 These transfer functions are in the operating and evaluation circuit 170 implemented.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102011078557 A1 [0001]

Claims (10)

A pressure sensor (10) comprising: an operation and evaluation circuit (170); and a pressure measuring cell (100) comprising: a measuring diaphragm (110); at least one counter-body (120); and a measuring chamber (140); and a capacitive transducer (150); wherein the measuring chamber (140) between the measuring diaphragm (110) and the counter body (120) is formed, wherein the measuring diaphragm (110) along a peripheral edge (130) with the counter body (120) is connected, wherein a deflection of the measuring diaphragm (110 ) depends on a pressure difference between a medium pressure applied to an outer side of the measuring diaphragm (110) remote from the measuring chamber (140) and a pressure prevailing in the measuring chamber (140), wherein the capacitive transducer (150) has at least one peripheral capacitor having a peripheral region capacitance the peripheral region capacitance depends on the deflection of the measuring membrane, wherein the peripheral region capacitor has an edge region electrode (152) on a surface of the counter body (120) facing the measuring membrane (110) and a membrane electrode (156) on a surface of the measuring membrane (110) facing the counter body (120) ), wherein the peripheral electrode (152) has a central portion of the surface of the surface s counter body (120) leaves free, wherein the pressure measuring cell (100) is dimensioned so that the measuring diaphragm (110) on the surface of the counter body (120) abuts when the pressure difference exceeds a first threshold, and that not on the surface of the counter body is present when the pressure difference falls below the first threshold; characterized in that the operation and evaluation circuit (170) is adapted to provide a signal representing the pressure difference, which depends exclusively on the Randbereichskapazität and possibly a temperature reading, when the pressure difference is more than K times the threshold, and K has a value of not more than 0.9, for example not more than 0.5 and in particular not more than 0.3. Pressure sensor after Claim 1 wherein the measuring chamber (140) is evacuated, and wherein the first threshold value is not more than 5 × 10 ⁴ Pa, for example not more than 2 × 10 ⁴ Pa, in particular not more than 1 × 10 ⁴ Pa, Pressure sensor according to one of the preceding claims, wherein the measuring diaphragm is not applied to the counter body up to a second threshold value of the pressure difference in the region of the edge area electrode, the second threshold value being at least eight times, in particular not less than sixteen times, the first threshold value. Pressure sensor according to one of the preceding claims, wherein the measuring diaphragm is not applied to the counter body in the region of the edge area electrode up to a pressure difference which does not exceed a second threshold value, the second threshold value being at least not less than 1 bar, in particular not less than 2 bar. Pressure sensor according to one of the preceding claims, wherein the operating and evaluation circuit is adapted to provide a signal representing the pressure difference, which also depends exclusively on the edge area capacity and possibly a temperature measured values, if the pressure difference is not more than K times the threshold value. Pressure sensor according to one of Claims 1 to 4 wherein the capacitive transducer further comprises an inner capacitor having an inner capacitance, wherein the inner capacitance depends on the deflection of the measuring diaphragm, wherein the inner capacitor comprises an inner electrode on the surface of the counter body facing the measuring membrane and a membrane electrode on the surface of the measuring diaphragm facing the counter body; wherein measuring diaphragm is dimensioned so that it rests in the region of the inner electrode on the counter body, when the pressure difference exceeds the first threshold, and that it does not abut against the counter body in the region of the inner electrode when the pressure difference falls below the first threshold value; wherein the operation and evaluation circuit is arranged to provide a signal representing the pressure difference, which depends on the inner capacity and the edge area capacity, when the pressure difference is not more than K times the threshold value. Pressure sensor after Claim 6 wherein the inner capacity at a pressure difference below the first threshold value has a greater pressure dependence than the marginal area capacity. Pressure sensor after Claim 6 or 7 , wherein the operating and evaluation circuit is adapted to determine the signal based on a transfer function F at pressure differences below K times the first threshold, which as $$\text{F}=\text{F}\left(\left({\text{C}}_{\text{i}}-{\text{C}}_{\text{r}}\right)/{\text{C}}_{\text{i}}\right)$$

where C _i and C _r denote the inner capacitance and the peripheral region capacitance, respectively. Pressure sensor according to one of the preceding claims, wherein K has a value of not less than 0.1, in particular not less than 0.2. Pressure sensor according to one of the preceding claims, wherein the pressure measuring cell has a _design pressure difference dp _design , for which applies: $$d{p}_{DesiGn}:=\frac{16}{3\cdot \left(1-{\mathrm{\mu }}^2\right)}\cdot e\cdot {\left(\frac{H}{r_i}\right)}^4\cdot \frac{y_0}{H}.$$

where E is the modulus of elasticity of the ceramic material of the measuring membrane and μ its Poisson's number, where y _{0 is} the distance between the measuring membrane and the counter body at the edge of the measuring chamber, where h is the material thickness of the measuring diaphragm, where r _{i is} the radius of the measuring chamber, wherein the first threshold does not differ by more than 10% of the design pressure difference from the design pressure difference.

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