CAPACITIVE VACUUM MEASURING CELL HAVING A MULTI-ELECTRODE

Abstract

The invention relates to a capacitive vacuum measuring cell having a first housing body (1) with a membrane (2) which is arranged at a distance therefrom so as to form a seal in the edge region (3) in such a way that a reference vacuum space (9) is formed therebetween, wherein opposite surfaces (7, 8) of the first housing body and of the membrane (2) comprise at least one electrode (G, G1, G2, . . . Gn, M1, M2, . . . Mn). A second housing body (4) is provided so as to form a seal with respect to the membrane (2) in the edge region and forms, with said membrane, a measuring vacuum space (10) in which connection means (5) are provided for connection to a process space.

Claims

1. Capacitive vacuum measuring cell having a first housing body (1) with a membrane (2) which is arranged at a distance therefrom so as to form a seal in the edge region (3) in such a way that a reference vacuum space (9) is formed therebetween, wherein opposite surfaces (7, 8) of the first housing body and of the membrane (2) comprise at least one electrode (G, G.sub.1, G.sub.2, . . . G.sub.n, M.sub.1, M.sub.2, . . . M.sub.n), wherein a second housing body (4) is provided so as to form a seal with respect to the membrane (2) in the edge region and forms, with said membrane, a measuring vacuum space (10) in which connection means (5) are provided for connection to a process space, characterized in that the electrodes (G, G.sub.1, G.sub.2, . . . G.sub.n; M.sub.1, M.sub.2, . . . M.sub.n) on the housing surface (7) and/or the membrane surface (8) comprise at least two, mutually electrically insulated housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) or/and membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n), which are arranged so that they form with at least one opposite electrode (G, G.sub.1, G.sub.2, . . . G.sub.n; M.sub.1, M.sub.2, . . . M.sub.n) at least two measuring capacitances (C.sub.1, C.sub.2, . . . C.sub.n), so that a deflection of the membrane can be detected capacitively at a plurality of locations, wherein the housing electrode (G) or the housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) and the membrane electrode (M) or the membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n) can be operatively connected to a signal processing unit.

2. Measuring cell according to claim 1, characterized in that the electrodes (G, G.sub.1, G.sub.2, . . . G.sub.n; M.sub.1, M.sub.2, . . . M.sub.n) comprise at least three or more electrically insulated housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) or/and mutually electrically insulated membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n), and at least three or more measuring capacitances (C.sub.1, C.sub.2, . . . C.sub.n) are formed.

3. Measuring cell according to claim 1, characterized in that a first electrode (G.sub.1, M.sub.1) formed in the middle of the housing surface (7) and/or the membrane surface (8) is surrounded by at least four further electrodes (G.sub.2, G.sub.3, . . . G.sub.n; M.sub.2, M.sub.3, . . . M.sub.n) arranged symmetrically thereto.

4. Measuring cell according to claim 1, characterized in that at least four membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n) and/or four housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) are symmetrically arranged in at least four different circular sections.

5. Measuring cell according to claim 1, characterized in that the surface (AG, AM) of the housing electrodes (G1, G2, . . . Gn) or/and membrane electrodes (M1, M2, . . . Mn) is less in each case than 5000 mm.sup.2, in particular less than 200 mm.sup.2, but in this case at least 0.1 mm.sup.2.

6. Measuring cell according to claim 1, characterized in that the membrane (2) comprises only one membrane electrode (M) or it is the membrane electrode.

7. Measuring cell according to claim 1, characterized in that the measuring capacitances (C.sub.1 . . . C.sub.n) each have a capacitance of C.sub.n100 pF, preferably C.sub.n50 . . . 60 pF, preferably C.sub.n30 pF.

8. Measuring cell according to claim 1, characterized in that the housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) are connected to the signal processing unit (16) and the membrane electrode (M) or the membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n) to a supply (14), or vice versa the membrane electrodes (M.sub.1, M.sub.2, . . . M.sub.n) to the signal processing unit (16) and the housing electrode (G) or the housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) to a supply (14).

9. Measuring cell according to claim 1, characterized in that the measuring cell comprises the converter associated with the respective measuring capacitances (C.sub.1, C.sub.2, . . . C.sub.n), which can be operatively connected to the signal processing unit.

10. Measuring cell according to claim 1, characterized in that the measuring cell is operatively connected to an integrated signal processing unit (13) which comprises an arithmetic unit (13), at least one memory (15) and an output unit.

11. Measuring cell according to claim 10, characterized in that reference values for comparing a measured actual value with the reference values are stored in the memory (15).

12. A method for capacitive pressure measurement with a vacuum measuring cell having a first housing body (1) with a membrane (2) which is arranged at a distance therefrom so as to form a seal in the edge region (3) in such a way that a reference vacuum space (9) is formed therebetween, wherein opposite surfaces of the first housing body (1) and of the membrane (3) are coated with an electrically conductive layer which is formed as an electrode (G, G.sub.1, G.sub.2, . . . G.sub.n, M.sub.1, M.sub.2, . . . M.sub.n), and a second housing body (4) is provided so as to form a seal with respect to the membrane (2) in the edge region in order to form, with said membrane, a measuring vacuum space (10) with connection means (5) for connection to a process space, characterized in that capacitance measurements are carried out on at least two, but in particular at least three measuring capacitances (C.sub.1, C.sub.2, . . . C.sub.n), which are each formed between the housing electrode (G) or housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) and membrane electrode (M) or membrane electrodes (M.sub.1, M.sub.2, M.sub.3 . . . M.sub.n), in such a way that the measurement results of the individual measuring capacitances (C.sub.1, C.sub.2, . . . C.sub.n) can be read out individually for each measured pressure p.sub.m.

13. Method according to claim 12, characterized in that capacitance measurements are carried out between at least three housing electrodes (G.sub.1, G.sub.2, . . . G.sub.n) and a membrane electrode (M).

14. Method according to claim 12, characterized in that the measured values are forwarded to an arithmetic unit (15) with at least one memory and compared by means of an algorithm with reference values stored in the memory to calculate and provide the output value therefrom.

Description

[0020] The drawings show as follows:

[0021] FIG. 1 shows a measuring cell of the prior art;

[0022] FIG. 2 shows the operation of a vacuum measuring cell;

[0023] FIG. 3 shows an embodiment of a vacuum measuring cell according to the invention;

[0024] FIG. 4 shows a numerical representation of a vector field formed from individual measured values;

[0025] FIGS. 5a and 5b show electrode arrangements according to the invention; and

[0026] FIG. 6 shows a circuit diagram of a vacuum measuring cell according to the invention.

[0027] The measuring cell of the prior art shown in FIG. 1 is shown in cross-section and has, at least concerning the adjacent surfaces 8, 9, a substantially rotationally symmetrical structure. In the present case, the first housing is made of an insulating material, for example a ceramic plate of aluminum oxide, which is sealingly connected, at a small distance from the ceramic membrane, to said membrane in the edge area and thereby forms a reference vacuum space 9. The distance between the two surfaces is usually set when mounting on the sealing material 11, which lies between the membrane edge 3 and the edge of the housing. In this way, a largely flat housing plate 1 can be used. In the same way, a measuring vacuum chamber 10 is formed in a second housing 4 on the opposite side of the membrane, which is connectable to a process space via a connecting piece 5 through an opening in the housing 4. The seal 3 on both sides of the membrane 2 may be formed, for example, from glass solder, which is easy to handle and can be applied, for example, by screen printing. In a typical measuring cell with an outside diameter of 38 mm and a free membrane inside diameter of 30 mm, the distance d.sub.0 between the capacitively effective surfaces is about 2 m to 50 m, preferably 12 m to 35 m. Here, for example, the first housing 1 is, for example, about 5 mm thick, the second housing 4 is, for example, about 3 to 6 mm, preferably 5 mm, thick. The second housing 4 may in this case be provided with a recess with a depth of about 0.5 mm in the inner region, as shown in FIG. 1, in order to increase the measuring vacuum chamber 10. Since in the present case both the housing 1 and the membrane 2 are made of an insulating ceramic, the housing 1 is coated on the reference vacuum side with a conductive layer which forms the housing electrode G and the membrane accordingly on the reference vacuum side with an electrically conductive layer which forms the membrane electrode M. The two layers are not electrically connected to each other. They can be painted, printed or sprayed, for example, or be applied with a vacuum method particularly suitable for the precise production of thin layers, e.g. vapor-depositing or sputtering. Furthermore, vacuum-tight, electrically conductive bushings 6 for connection to corresponding measuring means or measuring value converters such as CDG converters are provided for each electrode. In addition, getters (also not shown here) can be provided in order to maintain a long-term stable reference vacuum in the space 9. With regard to the advantageous embodiment and possible thinning of the electrode layers or the design of getters, reference is made to paragraph [0028] up to and including paragraph [0030] of EP 1 070 239 B1, which are hereby declared an integral part of the present description.

[0028] 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.

[0029] The principal mode of operation of such a vacuum measuring cell is shown in FIG. 2, which shows a membrane in rest position 2 and a pressurized membrane 2. The membrane 2, 2 with the thickness t is thereby deflected into the reference vacuum space 9 with the depth d.sub.0 by the amount w(p) as a function of the pressure and the clamping radius 2*R of the membrane 2, 2. Accordingly, a deflection takes place in the opposite direction when a vacuum is applied via the connection means 5.

[0030] Analogous to the vacuum measuring cells shown in FIGS. 1 and 2, a vacuum measuring cell according to the invention is shown in FIG. 3, which can be designed with respect to the choice of materials and geometry, with the exception of the electrode geometry and wiring, according to the examples of the prior art. In contrast to the measuring cell shown in FIG. 1, in the embodiment according to the invention three housing electrodes are provided here, which are arranged opposite a single membrane electrode. As a result, characteristic capacitance values (C.sub.j1, C.sub.j2, C.sub.j3) can be assigned to each specific pressure value p.sub.j for this measuring cell.

[0031] 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 FIGS. 5A and 5B, can be formed on the housing surface 7. The corresponding electrodes are, as shown, arranged symmetrically, for example, about a central housing electrode G.sub.1. As a result, for example, for a number m of measured pressure values p.sub.1, a vector containing n capacitance measurements corresponding to the number n of electrodes can be created and the sum m of the respectively obtained vectors can be represented as a vector field, as shown in FIG. 4. Depending on the desired accuracy of the measurement resolution, any number of reference measurement points defined by a respective n-dimensional vector can be recorded and saved for comparison purposes. This can also be done asymmetrically, for example in such a way that finer pressure steps p than in a less relevant pressure range are measured in a target pressure range and stored as reference vectors.

[0032] FIG. 6 shows the circuit diagram of a pressure sensor 12 according to the invention, which is constructed in this case from a membrane electrode and six housing electrodes, or vice versa, i.e. from a housing electrode and six membrane electrodes, and which is connected to a signal processing unit 13. Although the signal processing unit 13 may be provided externally, e.g. in a vacuum system controller, it is easily possible and advantageous, due to the current progressive miniaturization, even with vacuum measuring cells of small design, to integrate the signal processing unit in the measuring cell, since thus the sensor-related data are always connected to the right sensor and any possible confusion caused by incorrectly placed cables are excluded. The signal processing unit includes a supply 14 as a signal generator for the sensor and here also a voltage source for the unit, which can be battery-powered and/or connected to the mains. The central component is the arithmetic unit 15, which comprises a memory 17 with the saved reference values or has access to this memory. Furthermore, an algorithm is stored there or at another memory location, alternatively also hardwired or predetermined by the structure of the semiconductor, with the aid of which the arithmetic unit 15 compares the capacitance values or vectors which were sent via the converter 16 to the arithmetic unit or were already digitized with the reference vectors stored in the memory 17. This can be done for example with a best-fit method. In addition, the values for ambient temperature T.sub.amb or ambient pressure P.sub.amb, which were supplied in this case only by way of example via separate inputs, and the measurement of a standard capacitance C.sub.s (not shown here) can be used as required for correcting and further improving the accuracy of the measured values.

[0033] 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 (C.sub.R22, C.sub.R22, . . . C.sub.R1n), (C.sub.R21, C.sub.R22, . . . C.sub.R2n), . . . (C.sub.Rm1, C.sub.Rm2, . . . C.sub.Rmn), with the capacitance measured values (C.sub.1, C.sub.2, . . . C.sub.n) 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 p 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.