Method for correcting a dual capacitance pressure sensor

Abstract

A method for correcting a dual-capacitance pressure sensor for measuring fluid pressure, comprising: at a first time, taking measurements of fluid pressure based on movements of a first membrane and a second membrane of the pressure sensor; at a second time, taking measurements of fluid pressure based on movements of the first membrane and the second membrane; determining a change in the measurement results based on movements of the first membrane between the first point in time and the second point in time; determining a change in the measurement results based on movements of the second membrane between the first point in time and the second point in time; Checking whether the changes in the measurements determined are based solely on a change in fluid pressure or whether the changes in the measurements determined are due to changes in the pressure sensor, and if the latter is the case, determining a correction for the measurements determined at the second point in time.

Claims

1. A method for correcting a pressure sensor for measuring a fluid pressure, the pressure sensor having a first membrane (3) and a second membrane (5), wherein the first membrane (3) and the second membrane (5) are connected to one another in such a way that they enclose a spatial volume in a hermetically sealed manner, and wherein the first membrane (3) and the second membrane (5) have different geometries, wherein a fluid can be supplied to the spatial volume enclosed by the membranes via a fluid supply element (29), and wherein each of the membranes (3, 5) is assigned to one or more reference electrodes (8, 9) which generate an electric field together with the assigned membrane (3, 5), wherein a change in the electric field caused by a movement of the first membrane (3) and the second membrane (5) is evaluated for measuring the fluid pressure, or each of the membranes (3, 5) is assigned at least two reference electrodes (8, 9) which generate an electric field, wherein a change in the electric field is evaluated for measuring the fluid pressure, the method comprising: taking measurements of the fluid pressure to determine measured values; comparing the determined measured values with reference values; determining, based on the comparing, if a thickness of the first membrane and the second membrane has changed; outputting a message if the measured values deviate from the reference values by more than a specified limit value; and determining a correction of the measured values if the measured values deviate from the reference values below or up to the specified limit value.

2. The method of claim 1, further comprising: at a first point in time, taking measurements of the fluid pressure based on movements of the first membrane and the second membrane to obtain the reference values; at a second point in time, taking measurements of the fluid pressure based on movements of the first membrane and the second membrane to obtain the measured values; determining a change in the measurement results based on movements of the first membrane between the first point in time and the second point in time; determining a change in the measurement results based on movements of the second membrane between the first point in time and the second point in time; checking whether the determined changes in the measurement results are based solely on a change in the fluid pressure or whether the determined changes in the measurement results are due to changes in the pressure sensor; and determining, in case the determined changes in the measurement results are due to changes in the pressure sensor, whether the deviation is above or below the specified limit value in order to determine whether the message is output or the measured values are corrected.

3. The method of claim 2, wherein it is checked whether the determined changes in the measurement results are due to a change in thickness of the first membrane and the second membrane by the same amount.

4. The method according to claim 2, wherein the fluid pressure in the volume of space is the same at the first point in time and at the second point in time.

5. The method according to claim 1, further comprising: reducing the pressure to a pressure range that is below a resolution capability of the second membrane, the second membrane having a better pressure resolution capability than the first membrane; determining a first output value for the electric field generated across the second membrane, the first output value representing the measured values; and comparing the first output value with a second output value, which was determined when the pressure range was previously lowered below the resolution capability, the second output value representing the reference values.

6. The method of claim 5, further comprising: setting the first output value as a zero point for measuring the fluid pressure with the second membrane; increasing the fluid pressure up to an abutment pressure at which the second membrane begins to abut its associated reference electrode, the abutment pressure representing the measured value; and comparing the abutment pressure determined in this way with a reference abutment pressure determined after the determination of the second output value, the reference abutment pressure representing the reference values.

7. The method of claim 5, further comprising: setting the first output value as a zero point for the measurement of the fluid pressure with the first membrane, the first membrane having a lower pressure resolution capability than the second membrane; increasing the fluid pressure to a measured pressure at which the first membrane begins to indicate a non-zero fluid pressure, the measured pressure representing the measured values; and comparing the measured pressure determined in this way with a reference measured pressure determined after the determination of the second output value, the reference measured pressure representing the reference values.

8. A pressure sensor for measuring a fluid pressure, comprising: at least one first and one second membrane (3, 5) which are connected to one another in such a way that they enclose a spatial volume in a hermetically sealed manner and which have different geometries, wherein the fluid can be supplied to the spatial volume enclosed by the membranes via at least one fluid supply element (29), and wherein each of the membranes (3, 5) is assigned to one or more reference electrodes (8, 9) which generate an electric field together with the assigned membrane (3, 5), wherein a change in the electric field caused by a movement of the first membrane (3) and the second membrane (5) is evaluated for measuring the fluid pressure, or each of the membranes (3, 5) is assigned at least two reference electrodes (8, 9) which generate an electric field, wherein a change in the electric field is evaluated for measuring the fluid pressure; and a control unit, wherein the control unit is configured to perform the method of claim 1.

9. The pressure sensor according to claim 8, wherein the first membrane (3) or the second membrane (5) is made of zirconium dioxide.

10. The pressure sensor according to claim 9, wherein the zirconium dioxide is doped with yttrium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 show a vertical cross section through a pressure sensor.

(2) FIG. 2 shows a horizontal cross section of the pressure sensor as in FIG. 1

(3) FIG. 3 show a vertical cross section through an alternative pressure sensor.

(4) FIG. 3a shows a horizontal cross section of an alternative pressure sensor.

(5) FIG. 3b shows a horizontal cross section of a further pressure sensor.

(6) FIG. 4 shows an exemplary pressure sensor with a small form factor.

(7) FIG. 5 show a vertical cross section through another pressure sensor.

(8) FIG. 5a is a cross sectional view A-A′ of FIG. 5.

(9) FIG. 5b is a cross sectional view B-B′ of FIG. 5.

(10) FIG. 6 show a vertical cross section through yet another pressure sensor.

(11) FIG. 6a is a cross sectional view A-A′ of FIG. 6.

(12) FIG. 6b is a cross sectional view B-B′ of FIG. 6.

(13) FIG. 7 show a vertical cross section through yet another pressure sensor.

(14) FIG. 7a is a cross sectional view A-A′ of FIG. 7.

(15) FIG. 7b is a cross sectional view B-B′ of FIG. 7.

(16) FIG. 8 show a vertical cross section through yet another pressure sensor.

(17) FIG. 8a is a cross sectional view A-A′ of FIG. 8.

(18) FIG. 8b is a cross sectional view B-B′ of FIG. 8.

(19) FIG. 9 is a flow diagram of a correction method for a pressure sensor.

(20) FIG. 10 schematically shows an example of the method of FIG. 9.

(21) FIG. 11 schematically shows a further example of the method of FIG. 9.

(22) FIG. 12 schematically shows another example of the method of FIG. 9.

DETAILED DESCRIPTION

(23) The invention is explained in more detail below with reference to the exemplary embodiments shown in FIGS. 1 to 12.

(24) FIG. 1 and FIG. 2 show cross sectional views offset by 90 degrees from one another through a pressure sensor. Via a vacuum flange 21 and a downstream opening of a TO base 22 (fluid supply element or fluid discharge element), the fluid to be sensed is connected to a small measuring or fluid chamber 12 arranged perpendicularly to the connection surface of the flange 21. The fluid chamber 12 itself is delimited by two membranes 3 and 5, with membrane edges 23, which are round in this case by way of example. The two membranes 3 and 5 preferably have a different thickness from one another.

(25) Roughly parallel to each of the membranes 3 and 5 is a planar, electrically insulating electrode support 19, preferably designed as a ceramic disc, which has at least two reference electrodes on its surface facing the respective membrane 3 and 5. By means of the electrodes 8 and 9 (electrode 1 or electrode 2), pressure-related changes in the distances between the electrically conductive membranes 3 and 5 and the electrode supports 19 can be measured capacitively.

(26) In their edge regions, the two membranes 3 and 5 and the two electrode supports 19 are fixed at a fixed distance from one another by means of spacer layers 6, the edges of the membranes 3 and 5 being hermetically sealed to one another.

(27) The structural unit formed from the electrode supports 19 and the membranes 3, 5 is surrounded by a housing with a housing wall 18, which delimits a reference space 13 in which a constantly desired reference pressure or a vacuum reference can be set.

(28) Compensation openings 31 are provided in the electrode supports 19 to produce a pressure equalization between the reference space 13 and the area enclosed between the electrode supports 19 and the membranes 3 and 5, respectively. If the measured pressure in the fluid supply is higher than the pressure in the reference chamber 13, the thin membrane 5 is initially deflected in the direction of the electrode carrier 19 associated with this membrane 5 when the pressure difference is low. If the measurement pressure continues to rise, the thick membrane 3 is also deflected in the direction of the electrode carrier 19 associated with this thick membrane 3. If the measurement pressure continues to rise, the thin membrane 5 can abut the electrode carrier 19 and be supported by it.

(29) In an overlapping measuring pressure range, both membranes can therefore be deflected without abutting an electrode carrier. The electrode carriers 19 with the spacer layers 6 can advantageously also be designed to be electromagnetically shielding.

(30) The reference pressure is set via a closable opening arranged in the housing 18 (closure 16 of the reference chamber). This opening can be provided, for example, in a second TO base integrated in the housing. Connecting wires, for example, can also be routed through this second TO base, which are used to supply the electrical supply to heating elements of a heater 17 that are attached to the electrode carriers 19.

(31) FIG. 3 shows advantageous developments and features of the exemplary embodiment of a pressure sensor shown in FIG. 1 and FIG. 2

(32) In a further development, a particle filter 24 can be connected upstream of the fluid inlet 29. Optionally, an evaluation electronics 25 arranged within the housing is connected to electrodes 8 and 9 via through-contacts in the ceramic electrode carriers 19. Their signals are routed via supply lines or connection wires 20 to hermetically sealed lead-through pins of connections 28 for a control electronics located outside the housing. Furthermore, a Pirani sensor 27 for monitoring the pressure in the reference space can optionally be connected via such lead-through pins. Further options are a getter holder 14 integrated into the housing wall 18 and the evacuation connection or closure 16 for the reference space 13, which is also set into the housing wall 18 in this exemplary embodiment.

(33) FIG. 4 shows an exemplary embodiment of a pressure sensor with a small form factor. Due to the fact that it is parallel to the flange surface, a very low overall height is achieved. Staggered contact surfaces on the spacer 32 facilitate assembly of the two membranes 3 and 5 with the required fixed distance from one another. In this case, the membrane 3 is designed as an annular membrane. The spacer 32 is firmly connected to the cup-shaped housing lower part, for example by welding, soldering or gluing. The ceramic 19 closest to the flange has an opening, preferably arranged centrally, for the passage of the fluid inlet 29 to one of the membranes 3, 5. The ceramic 19 serves as a carrier for the electrodes 8 and 9. The essential distances between the two ceramics 19 and the respective associated membrane 3 or 5 can be adjusted by one or more suitable contact surfaces provided on the spacer 32, the two ceramics 19 being connected by means of preferably resilient clamping elements 26 clamped between the wall of the housing 18 and said contact surfaces. All cavities located between the ceramics 19 and the insides of the housing are preferably connected to one another and form the reference space 13.

(34) An embodiment of the invention with a fluid inlet 29 and a fluid outlet 30 is shown in FIGS. 5, 5a and 5b. With such an embodiment, absolute pressure measurements as well as flow and differential pressure measurements are possible. The flat, approximately can-shaped housing shape is formed by two ceramic discs 19 or ceramic plates of any shape spaced at a distance from and parallel to each other, which are connected to each other hermetically sealed at their edges with suitable side walls 33. Two membranes 3 and 5, which are also spaced apart and parallel to one another and which are of different thicknesses, are also connected to one another in a hermetically sealed manner at their edges. Tubular elements for the fluid inlet 29 and the fluid outlet 30, which are tightly connected to the membrane structure, lead into two, for example opposite, edge regions of the membrane interior. These tubular elements are guided through one of the ceramic plates and tightly attached to this ceramic in such a way that the tubes also serve as spacers for setting the required distance between the membranes 3, 5 and the electrodes 8 and 9 arranged on the ceramics 19.

(35) In addition, heating elements of the heater 17 are integrated in the ceramics 19, which are designed in one or more parts.

(36) The fact that a pressure sensor can be formed solely by largely planar materials layered on top of one another is shown in FIGS. 6, 6a and 6b. Cross sectional FIG. 6a shows the structure of such a pressure sensor. Seen from bottom to top, a base plate 1, e.g. made of ceramic with attached electrodes 9, is followed by a spacer layer 2 and a thick membrane layer 3. The spacer layer 2 is cut out over a large area in the area of the membrane 3, e.g. in a circle, to form a lower reference space 13, into which the membrane 3 can move when the pressure increases.

(37) In order to form a suitable measuring space or fluid space 12, the second membrane 5 is arranged at a distance from the first membrane 3 by a further spacer layer 4.

(38) The connection of the fluid chamber 12 with the fluid to be measured takes place here by means of, for example, circular openings in the structural units 1, 2, 3 and 4. The second membrane 5 is placed on the spacer layer 4, which in turn is separated by a recessed spacer layer 6 from which the cover plate 7 closing the entire assembly final is separated. The intermediate space created by the cut-out forms an upper reference space 13. The cover plate 7 and the base plate 1 have electrodes 8 and 9, respectively, which each interact capacitively with the membrane electrode 10 applied to the associated membrane.

(39) As shown in FIGS. 6 and 6b, the upper and the lower part of the reference space 13 are connected to one another on two approximately opposite sides and are each accessible from the outside via an opening. One of the openings is optionally provided with getter 15 by means of a getter holder 14, the other can be hermetically sealed with a closure 16.

(40) Further alternative embodiments with a small form factor are shown in FIG. 7, FIG. 7a, FIG. 7b and FIG. 8, FIG. 8a, FIG. 8b, which differ in particular by the different, etchable layer materials. In addition, in the embodiment 7, all the connection openings are arranged on one side of the pressure sensor. In the embodiment shown in FIG. 8, these are on opposite surfaces of the pressure sensor.

(41) In all of the pressure sensors described above, at least one of the membranes 3, 5 can be made of zirconium dioxide. This membrane 3, 5 then has an extremely smooth surface, which makes it difficult for parts of the measurement fluid to accumulate on the membrane 3, 5. In addition, zirconium dioxide is chemically less vulnerable and therefore suitable for durable membranes. Doping the zirconium dioxide with yttrium can further enhance the effects mentioned above.

(42) In particular, zirconium dioxide has a smaller modulus of elasticity than aluminum oxide, for example, and has good chemical resistance and long-term stability. A membrane made of zirconium dioxide offers a higher sensitivity than a membrane made of aluminum oxide. In addition, a zirconium dioxide membrane has a smoother surface than an aluminum oxide membrane. This is partly due to the smaller grain size of zirconium dioxide.

(43) A correction method that can be carried out with each of the pressure sensors described above in the overlap measurement pressure range is shown schematically in FIG. 9.

(44) At S101, fluid pressure measurements are taken to determine measured values.

(45) At S102, the determined measured values are compared with reference values.

(46) If the measured values deviate from the reference values by more than a specified limit value, a corresponding message is output at S103.

(47) If the measured values deviate from the reference values below or up to the specified limit value, a correction of the measured values is determined in S104.

(48) In this way, maximum sensor operation can be combined with predictive maintenance.

(49) FIG. 10 schematically shows an example of the method of FIG. 9. At S201, measurements of the fluid pressure based on movements of the first membrane and the second membrane are taken out at a first point in time in order to obtain the reference values.

(50) At S202, fluid pressure measurements are performed based on movements of the first membrane and the second membrane to obtain measured values at a second point in time.

(51) At S203, a change in the measurement results based on movements of the first membrane between the first point in time and the second point in time is determined.

(52) At S204, a change in the measurement results based on movements of the second membrane between the first point in time and the second point in time is determined.

(53) At S205, it is checked whether the determined changes in the measurement results are based solely on a change in the fluid pressure or whether the determined changes in the measurement results are due to changes in the pressure sensor. If the latter is the case, it is determined whether the deviation is above or below the specified limit value in order to determine whether the message is output or the measured values are corrected.

(54) In this way, it can be determined in a simple and reliable manner whether the measured values of the pressure sensors described above need to be corrected.

(55) A further example for the method shown in FIG. 9 is shown in FIG. 11. At S301, the pressure is reduced to a pressure range that is below the resolution capability of the membrane with the better pressure resolution capability.

(56) At S302, a first output value is determined for the electric field generated by the membrane with better pressure resolution, the first output value representing the measured values.

(57) At S303, the first output value is compared with a second output value that was determined when the pressure range was previously lowered below the resolution, wherein the second output value represents the reference values.

(58) In this way, the zero point calibration can be used for checking the sensor and for predictive maintenance.

(59) Based on the method of FIG. 11, the method of FIG. 12 can also be used to implement the method of FIG. 9.

(60) At S401, the first output value is set as the zero point for measuring the fluid pressure with the membrane having the better pressure resolution capability.

(61) At S402, the fluid pressure is increased up to an abutment pressure at which the membrane with the better pressure resolution capability begins to abut its associated reference electrode, with the abutment pressure representing the measured values.

(62) At S403, the abutment pressure determined in this way is compared with a reference abutment pressure determined after the determination of the second output value, the reference abutment pressure representing the reference values.

(63) In this case, the abutment pressure can be used to check the integrity of the sensor. Likewise, the pressure at which the membrane with the poorer resolution capability begins to output non-zero measured values could also be used to compare measured values with reference values and thereby obtain information about the susceptibility of the sensor to faults or a necessary replacement/repair.

(64) While the present invention has been described with reference to exemplary embodiments, it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but, on the contrary, is intended to cover numerous other modifications, substitutions, variations and broad equivalent arrangements that are included within the spirit and scope of the following claims.

REFERENCE LIST

(65) 1 base plate 2 spacer layer 3 thick membrane layer 4 spacer layer 5 thin membrane layer 6 spacer layer 7 cover plate 8 reference electrode, electrode 1 9 reference electrode, electrode 2 10 membrane electrode (or full metal membrane) 11 metallization 12 fluid space 13 reference space, reference pressure chamber 14 getter mount 15 getter 16 closure of reference space or evacuation port 17 heating, heating elements 18 housing, housing wall 19 reference electrode carrier, ceramic 20 leads to heating, electronics or electrodes 21 vacuum flange 22 TO socket 23 membrane edge 24 particle filter (net) 25 evaluation electronics in the reference vacuum 26 clamping, direct or with a spring element 27 pirani sensor for reference pressure monitoring 28 connections to the control electronics 29 fluid delivery element, fluid inlet 30 fluid extraction element, fluid outlet 31 compensation opening 32 spacer 33 sidewall