Patent Application
20190064022
The invention relates to a capacitive vacuum measuring cell according to the preamble of claim 1 and to a method for capacitive pressure measurement according to the preamble of claim 13.
The capacitive readout of sensors is a common method used to measure path lengths or distances. “Capacitive Sensors: Design and Applications” by Larry K. Baxter (Wiley-IEEE Press August 1996, ISBN 978-0-7803-5351-0) exhaustively describes the principles and readout methods. It is characteristic of the known configurations that the capacitance to be measured is compared with a fixed standard capacitance. This is the reference element for such measurement; it can be designed as a fixed capacitor or integrated into the sensor.
The application of capacitive measurements or measuring cells for pressure measurement is known for example from US 323 21 14 and US 482 36 03. A vacuum measuring cell optimized especially for the measurement of low pressures is known from EP 1 070 239 B1, which describes the basic structure of a ceramic CDG (Capacitive Diaphragm Gauges).
If low pressures of about 0.1 mbar to 10−6 mbar are to be measured, conventional membrane production methods are unsuitable for this because of the resulting stresses in the materials. On the other hand, the vacuum measuring cell described in EP 1 070 239 B1 provides a clear improvement as a result of the structure and the manufacturing method described therein, but it is also not possible with this production method to clamp the membrane completely uniformly on all sides. The highest possible uniform clamping of the membrane and the complete switching off of intrinsic voltage conditions are for measurement methods and measuring cells a prerequisite for the most accurate measurement, since the membrane should deform in an ideally complete and rotationally symmetrical manner under the influence of pressure.
Thus, it is known that especially in the vicinity of the so-called 0-point—i.e. in the state of the membrane where no pressure acts on the membrane—the deviation of the membrane from the ideal deflection equation is largest, since the membrane approaches a relaxed state here. Thus, the measurement result is corrupted.
It is the object of the present invention to avoid the disadvantages of the prior art and to provide a measuring cell with which a more precise and reliable measurement is possible.
A vacuum measuring cell, which allows a more precise and/or reliable measurement of low pressures, is disclosed in claim 1 and comprises a first housing body having a membrane spaced therefrom and arranged in a sealed manner in the edge region, whereby an interposed reference vacuum space is formed.
The present invention is suitable both for a vacuum measuring cell with ceramic membrane and/or housing and for a vacuum measuring cell with metallic membrane and/or housing, or for example cells with ceramic housing and metallic membrane, or vice versa with metallic housing and ceramic membrane.
An edge region is generally understood here as a circumferential region of the membrane, e.g. between 0 mm and about 2 to 7 mm from the outer edge of a usually circular membrane, but which at least additionally includes the sealing surfaces. The opposite surfaces of the first housing body of the membrane lying at a small distance, for example from 2 to 50 μm, each comprise at least one electrically conductive layer. According to the invention, this also means embodiments in which in particular the membrane, but alternatively also the first housing body, or the housing as a whole or partially, for example by means of inserts or as a whole, is or are electrically conductive. An example of the material of a particularly suitable electrically conductive layer is gold, which is particularly suitable because of its high conductivity and chemical resistance. As an alternative with even higher chemical resistance, platinum can be used for example. If the membrane itself or a housing body made of conductive material is selected, this may, for example, also be stainless steel, which as required may additionally be coated with a more conductive material, for example as explained above. However, the use of aluminum or transparent, conductive oxides (TCO or “Transparent Conductive Oxide”) is also conceivable.
A second housing body is sealingly provided opposite the membrane in the edge region and forms therewith a measuring vacuum space into which connection means open for connection to a process space. This can be, for example, a vacuum chamber with the medium to be measured such as, for example, inert, reactive gas or a mixture thereof, which forms the process gas. The first housing body and the second housing body are sealingly connected in this case to the intermediate membrane in the edge region against an environment of the measuring cell and, as far as possible in the context of known production processes for example, symmetrically connected. The electrically conductive layer on the housing surface or/and the membrane surface comprises at least two housing electrodes (G1, G2, . . . Gn) which are electrically insulated from one another, or membrane electrodes (M1, M2, . . . Mn), which are arranged so that they form at least two measuring capacitances (C1, C2, . . . Cn) with at least one opposite electrode (G, G1, . . . Gn; M, M1, . . . Mn), so that the deflection of the membrane at a plurality of locations can be detected separately capacitively, wherein the electrodes are operatively connectible to a signal processing unit. The measuring capacitances are separately measurable, i.e. the measurement and evaluation can be done simultaneously by the signal processing unit, i.e. in parallel, for example, to detect very rapid pressure fluctuations, or alternatively sequentially, in particular especially periodically sequentially. The measurement sequence can be controlled in this case, for example, by the clock signal of the signal processing unit according to the required temporal signal resolution.
The electrodes are formed in a planar manner and can be formed, depending on requirements, in different geometries, e.g. circular, rectangular, annular or sector-shaped. In order to follow the deformation of the membrane particularly well and to achieve greater distinctive character of the measured values, the electrically conductive layer can comprise at least three, four or more electrically insulated housing electrodes (G1, G2, . . . Gn) and/or electrically insulated membrane electrodes (M1, M2, . . . Mn), wherein at least three, four or more in particular separately measurable measuring capacitances (C1, C2, . . . Cn) are formed. In this case, a first electrode (G1, M1) formed in the middle of the housing surface and/or the membrane surface can be surrounded by at least three or more further electrodes (G2, G3, . . . Gn; M2, M3, . . . Mn) which are symmetrically arranged, for example. Alternatively, the other electrodes can also be arranged asymmetrically or arbitrarily, in which case compensation can be achieved with a larger computational effort.
A part of the further electrodes (G2, G3, . . . Gn; M2, M3, . . . Mn) can be arranged on at least one circumference to the first electrode (G1, M1). In this case, at least four membrane electrodes (M1, M2, . . . Mn) and/or at least four housing electrodes (G1, G2, . . . Gn) can be arranged symmetrically in at least four different circular sections of the membrane circle, e.g. in at least four different circular ring pieces of at least one circular ring.
At least one of the opposing surfaces of the individual measuring capacitances, by the measurement of which a vector associated with the respective pressure can be formed, is small in this case in relation to the dimensions known from conventional measuring cells. In this case, the surface (AG, AM) of the housing electrodes (G1, G2, . . . Gn) or/and the membrane electrodes (M1, M2, . . . Mn) can in each case be less than 5000 mm2, in particular less than 200 mm2. However, the area should be in this case at least 0.1 mm2 for manufacturing reasons or for the reproducibility of the individual measurements.
In one embodiment, the electrical layer M thus comprises only one membrane electrode, which in this case can be equated with that of the electrical layer M, since any supply lines are not significant here. On the other hand, the membrane as a whole can form a membrane electrode M when the membrane is a metallic membrane. As a result of such an arrangement, only the surface of the first housing body which is opposite the membrane at a small distance has a plurality of electrodes, which is easier to realize from a production standpoint than to provide additional electrodes on the membrane which is very thin for vacuum measurements, which may in turn may still optionally change the deflection behavior of the membrane. In such a case, a plurality of housing electrodes are capacitively associated with an opposite, larger membrane electrode, i.e. a membrane electrode comprising at least the surfaces of the housing electrodes (G1, G2, . . . , Gn) in perpendicular projection to the surfaces. The measuring capacitances (C1 . . . Cn) can in each case, or even in the case of an arrangement provided with multiple electrodes on both sides, be a very small capacitance of, for example, Cn≤1 nF, in particular Cn≤50 . . . 60 pF, in particular Cn≤30 pF. In addition, the measuring cell may comprise a fixed standard capacitance Cs, which, for example, is designed as a fixed capacitor or integrated in the sensor.
The housing electrodes (G1, G2, . . . Gn) can be connected to the signal processing unit 16 and the membrane electrode M or the membrane electrodes (M1, M2, . . . Mn) to a supply, or alternatively conversely, the membrane electrodes (M1, M2, . . . Mn) to the signal processing unit and a single housing electrode G, which in this case can be equated with the electrically conductive layer G, or the housing electrodes (G1, G2, . . . Gn) to the supply. The measuring cell can in this case comprise the converter associated with the respective measuring capacitances (C1, C2, . . . Cn), in particular comprise the CDC (CDC stands for “capacitivity to digital converter”), which can operatively be connected to the signal processing unit. Alternatively, the converters can also be part of the signal processing unit.
To design the vacuum cell as compact as possible and to enable its installation ready for use without further measures, a signal processing unit can be integrated into the measuring cell, which includes an arithmetic unit with at least one memory and an output unit for outputting the calculated pressure value. In this case, the converter and, if necessary, an input unit, e.g. for entering external parameters such as ambient temperature and ambient pressure, and/or a standard capacitance Cs can be installed in the signal processing unit. Reference values can be stored in the memory of the signal processing unit in order to compare a measured actual value therewith.
Furthermore, an algorithm, in particular a best-fit algorithm for comparing the reference values with the measured actual values, can also be stored in the memory. This algorithm can also be supplied in a known manner from the outside, for example, via an external controller of the system control or be provided in a permanently wired manner in the microprocessor of the calculator.
The invention is also realized in a method for capacitive pressure measurement. For the method, a vacuum measuring cell is analogously used with a first housing body with a membrane which is spaced therefrom and sealingly arranged in the edge region, wherein between the membrane and the first housing body, a reference vacuum space is formed and the closely spaced opposite surfaces of the first housing body and the membrane are coated with an electrically conductive layer or are even fully or partially electrically conductive. In addition, the vacuum measuring cell comprises a second housing body which is likewise sealingly provided in the spatial area opposite the membrane, in order to form therewith a measuring vacuum space, into which connecting means open for connection to the medium to be measured. First and second housing body with the interposed membrane is thus sealingly connected, so that on the one hand reference and measuring vacuum space are separated from each other and on the other hand the seal to the outside is provided. This can be done in a known manner by elastic seals and/or in particular for high vacuum measurement by glass solders. For the method, capacitance measurements are thus carried out simultaneously in parallel or alternatively in temporal sequence on at least two, but better on at least three, in particular on four or more measuring capacitances (C1, C2, . . . Cn), which are each formed between housing electrode or housing electrodes (G, G1, G2, . . . Gn) and membrane electrode or membrane electrodes (M, M1, M2, . . . Mn). In this way, it is individually possible to compensate for intrinsic biases and/or manufacturing tolerances in the clamping symmetry of the respective membrane, since thus a reference vector (CR1, CR2, . . . CRn) can be generated for each pressure, e.g. within the scope of calibration of the measuring cell, for comparison with later measurements in practice. This can be realized particularly easily with measuring capacitances in which measurement is carried out between a plurality of housing electrodes (G1, G2, . . . Gn) and a large membrane electrode (M) comprising the surfaces of the individual housing electrodes. The conversion of the capacitance measurements takes place by means of converters, wherein CDC converters are particularly suitable since in this case the output values of the converters are already present in digitized form. For evaluation, measured values, for example in a signal processing unit connected to the measuring cell or integrated into the measuring cell, can be forwarded to the arithmetic unit of the unit and compared by means of an algorithm with the reference values stored in the memory, in order to calculate the output value therefrom which can be passed on, for example, via an output unit. For the comparison, a best-fit algorithm can be used.
In the following the invention will be explained with reference to figures and individual embodiments. It should be noted in this case that all features, albeit only mentioned in connection with individual embodiments or the description of individual figures, can be basically combined with other features or embodiments of the invention, unless from the general knowledge of the person skilled in the art a contradiction or incompatibility of the combination of such features results immediately. This also applies to all features and embodiments mentioned in the general part of the description, whereby a combination of such features is disclosed.
The drawings show as follows:
The measuring cell of the prior art shown in
For pressure measurements on media which are less critical, for example, with respect to their corrosive properties, it is possible, as is known, to also use a metallic membrane, which can form the membrane electrode as a whole due to its electrically conductive properties. If, instead of a ceramic material, a metal is also used for the first housing body 1, the housing electrode G and the electrically conductive bushings 6 must be formed in an insulated manner relative to the housing 1.
The principal mode of operation of such a vacuum measuring cell is shown in
Analogous to the vacuum measuring cells shown in
According to the invention, on each of the opposite surfaces 7, 8 of the housing 1 or the membrane 2, but in particular for the reasons mentioned above, a multi-electrode arrangement, as shown in
With a construction designed as described above and a method for operating a measuring cell as detailed above, the measuring pressure can be determined by comparing reference values of the capacitances measured individually for different pressures, e.g. as reference value vectors (CR22, CR22, . . . CR1n), (CR21, CR22, . . . CR2n), . . . (CRm1, CRm2, . . . CRmn), with the capacitance measured values (C1, C2, . . . Cn) measured at a measuring pressure, for example as a capacitance vector, as a result of which it is managed to individually consider and compensate individual differences, e.g. due to manufacturing tolerances, in the geometry of different measuring cells, in particular with respect to the geometry and pretension of the membrane. As a result, not only are more precise and reliable measurements possible, but also a particularly fine resolution Δp of the pressure to be measured can also be achieved for desired measuring ranges, for example particularly process-relevant ones, by depositing a greater number of reference values. Thus, the present invention offers the possibility of optimally designing vacuum measuring cells for very different pressures.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/053976 | 2/25/2016 | WO | 00 |
