DE102009002662B4 - Capacitive pressure sensor as a combination sensor for recording other measured variables - Google Patents

Abstract

Capacitive pressure sensor as a combination sensor for detecting a further measured variable with a ceramic pressure measuring cell consisting essentially of a membrane (1) and a base body (2) and measuring electrodes (4,5,6,7) in the form of conductive surface areas, the predominant Part of the membrane surface is used solely for pressure measurement, characterized in that all measuring electrodes (4,5,6,7) are arranged in the cavity between the membrane (1) and the base body (2), the measurement of the different sizes takes place in different frequency ranges , whereby a calorimetric flow or point level measurement is carried out in addition to the pressure measurement.

Description

The invention relates to a capacitive pressure sensor as a combination sensor for detecting further measured variables according to the preamble of claim 1, and a ceramic measuring cell for a capacitive pressure sensor, which supplies a further measured value in addition to the actual measured pressure value.
Capacitive pressure sensors, which measure another variable in addition to the pressure, are known. The additional temperature measurement, as well as arrangements and methods for monitoring the functionality of the measuring cell, are widespread. For the latter, on the DE 10 2007 016 792 A1 referred to the applicant.

A pressure measuring cell with a temperature sensor is installed in DE 40 11 901 A1 described. Here it is proposed to arrange a temperature sensor as a resistance track made of a material with temperature-dependent resistance either on the thinner pane (membrane) or on the thicker pane (base body), but also in the area of the glass joint between the two panes.
The disadvantage of this arrangement on the base body is the thermal inertia, which does not allow the detection of comparatively rapid temperature changes, as is required for example for calorimetric flow measurement. In the case of the arrangement on the membrane, the unavoidable consumption of pressure measurement area or the influence on the dynamic behavior of the membrane and the associated loss of measurement accuracy is a disadvantage.

the DE 41 04 056 C1 discloses a method for automatically zeroing a capacitive ceramic pressure sensor. Here, at least one electrode of the pressure sensor is used at least temporarily as a test electrode that senses capacitively through the membrane to the metallic housing. If the capacitance value determined by the electrode reaches or falls below a capacitive threshold value corresponding to the membrane uncovered by the medium, an automatic zero point correction is carried out. The purpose of this measurement is the automatic compensation or correction of the long-term drift of the pressure sensor. To carry out the measurement, the various measurement capacitances are connected one after the other to a capacitance-to-frequency converter or a capacitance-to-time converter. Also disclosed is an additional electrode on the base body or on the inside of the membrane that does not belong either to the measuring capacitor or to the reference capacitor. In this case, the periodic switching of the ground electrode or the other electrodes of the pressure sensor is no longer necessary. A disadvantage is the use of the same evaluation circuit for such different measuring tasks as capacitive pressure measurement, which should not be influenced by the electrical properties such as conductivity and dielectric constant of the pressure medium, and capacitive limit level measurement, which uses precisely these properties to detect the medium present.

the DE 196 48 048 C2 relates to a capacitive pressure sensor with two membranes and a total of seven measuring capacitors. The capacitance of the sixth capacitor is temperature dependent and the seventh capacitor is responsive to changes in the dielectric constant of the print medium. For evaluation, all measuring capacitors are connected cyclically or alternatively to the same capacitance-frequency conversion circuit by a sensor selection unit.
The measuring capacitors are part of a relaxation oscillator that successively converts the capacitance of the capacitors into a pulse signal. Practice has shown that with the above capacitance-time converters or capacitance-frequency converters, especially with viscous or pasty media such. B. ketchup, problems in distinguishing between wetting and the actually pending medium occur.
Another disadvantage is the measuring cell, which has a very complicated structure and must be at least partially filled with the medium present, which, in addition to the high production costs, entails the risk of contamination or clogging. Of course, it initially makes sense to use the same electronics to evaluate the various measured variables such as pressure, temperature, conductivity and dielectric constant of the medium in question. However, as already explained above, it should be considered that the pressure measurement should not actually be influenced by the physical properties of the medium present, and a loss of measurement accuracy due to the adaptation of the common evaluation electronics to the fill level or limit level measurement is unacceptable. However, since disruptive buildup, which affects the temperature measurement, among other things, is also recognized as such, ie it is to be distinguished from the medium present, a sufficiently precise point level measurement must also be ensured.

In addition, also from the DE 698 18 762 T2 a pressure transducer is known which comprises a temperature sensor in the form of a temperature-sensitive resistance element.

The object of the invention is to at least partially overcome the disadvantages of the prior art, in particular the increased complexity or the loss of measurement accuracy in the pressure measurement wise to overcome.
For this purpose, the measuring principles used should be adapted to the respective measuring task. Furthermore, the sensors should be arranged on the membrane or combined with one another in such a way that the membrane surface is largely available for pressure measurement.
Measurands such as limit level, medium temperature or flow rate should be displayed as an additional benefit, but also used to correct the measurement results or to check the measuring cell. Furthermore, the integration in a measuring cell, in addition to saving material and assembly costs, should also bring the advantage that all measured values come from the same place; ie the various measured values actually correspond to one another.

This problem is solved according to the features mentioned in claims 1, 2 and 3. Advantageous configurations can be found in the dependent claims.

According to the invention, the membrane area required for temperature measurement is kept to a minimum and because of the resulting low thermal capacity, it can also be used for calorimetric flow measurement or even for limit level measurement.
In a further embodiment of the invention, the pressure measurement operating at a low frequency is combined with a capacitive limit level measurement operating at very high frequencies.
The pressure measurement is expediently carried out in the kilohertz range, while the capacitive limit level measurement works advantageously in the range between 100 and 200 MHz, but also into the GHz range. The temperature measurement, on the other hand, is carried out with direct current and the calorimetric flow measurement with pulsating direct current. Such different pulse shapes and frequencies require separate evaluation units with different measuring principles.

According to the invention, a ceramic pressure measuring cell is designed in such a way that it is also suitable for the above measurements. Furthermore, it has proven useful not only to use the results of the point level measurement to correct the zero point of the pressure measurement, but also to check that the measuring cell is functioning properly (cell health). In addition, it is advantageous to also display the available measured values. Under certain circumstances, this can be very helpful in identifying vacancies, detecting adhesions or even distinguishing between certain media.
For example, when cleaning a system for processing liquid or pasty foodstuffs, it makes sense to determine the buildup remaining on the pressure sensors or the cleaning agent that is still present.
The invention is explained in more detail below with reference to the exemplary embodiments illustrated in the figures.
the 1 until 4 show exemplary embodiments of the sensor circuit according to the invention. The different versions of the ceramic pressure measuring cell are shown in the 5 until 11 shown.
A first embodiment is described below with reference to 1 described:

The pressure is measured using the two capacitors 15 and 16. They consist of the electrodes 5 and 6 on the base body 2 and the common counter-electrode 4 on the membrane 1.
The capacitances 15 and 16 are influenced by the pressure exerted on the membrane 1. The measuring capacitance 16 located more in the middle of the membrane 1 is also increased more in the middle than the reference capacitance 15 located at the edge of the membrane 1 because of the greater deflection of the membrane 1.
The capacitors labeled 11 represent the parasitic capacitances. As the person skilled in the art can easily see, there are other parasitic capacitances which have not been shown for reasons of clarity.

The square-wave signal generated by generator 13 reaches the pressure transducer 19 directly, in this case the inverting input of a differential amplifier, and via the decoupling circuit 20 and the low-pass filter 14 to the reference capacitor 15, which essentially determines the time constant of the integrator 17.

The triangular signal produced at the output of the integrator 17 now reaches the differentiator 18 via the measuring capacitor 16, the low-pass filter 14 and the decoupling circuit 20, where the measuring capacitor 16 acts as a time-determining capacitance. The square-wave signal produced here reaches the non-inverting input of the operational amplifier belonging to the measured-value sensor 19 .
The circuit is dimensioned advantageously so that when unloaded
Membrane 1, and thus the measuring capacitance 16 in the initial state at the output of the differentiator 18, the original square-wave signal is produced again. In this case, the input signals of the evaluation circuit 19 cancel each other out, resulting in an output signal of almost zero.
The person skilled in the art is familiar with the zero adjustment with the aid of the existing capacitances and resistances, so that it is not discussed in any more detail here got to.
When the membrane 1 is loaded, the measuring capacitance 16 increases compared to the reference capacitance 15. The time constant of the differentiator 18 also increases compared to the time constant of the integrator 17. The signals at the two inputs of the measuring value recorder 19 now no longer completely compensate each other. The signal produced at the output of the measured-value recorder 19 is first fed to the sensor electronics 27 for further processing and finally to the microcontroller 29 . After the analog-to-digital conversion and standardization or scaling, the result can be supplied to a higher-level control unit via the I/O unit 28 and displayed by the display unit 30 .
The principle of pressure measurement is detailed in the writings DE 197 08 330 C1 and DE 198 51 506 C1 described by the applicant. (cf 1 out DE 197 08 330 C1 ) It is expressly pointed out that all of the configurations described in the above two publications by the applicant can be used.

Because of the low thermal capacity of the membrane and the good contact of the temperature sensor 23 with the medium, a calorimetric flow measurement is even possible in addition to the temperature measurement
The temperature sensor 23 is preferably a multi-layer PTC resistor, or else a semiconductor sensor, as described, for example, in WO 2003/100 846 A2.
Since the temperature measurement and the calorimetric flow measurement are practically carried out with direct current, the measuring lines can be decoupled with throttles 25.

For the calorimetric flow measurement, the sensor element, in this case the temperature sensor 23, is subjected cyclically to a heating current and, during a subsequent cooling phase, to a comparatively low measuring current. The length of time until a predetermined lower temperature threshold value is reached or the temperature reached after a predetermined time has elapsed is measured. In many cases, the medium temperature is measured with a second sensor. If you want to avoid this, you can also determine the flow velocity independently of the ambient temperature from the difference between two measurement phases with different heating capacities. The heat transfer function is obtained as the quotient of the difference in heat output. And associated temperature difference. Such a procedure is DE 38 41 637 C1 described and is not the subject of the invention.

To measure the limit level, the high-frequency generator 21 generates a sinusoidal signal of approx.
The medium 9 characterized by an equivalent circuit diagram of a capacitance, a resistance and an inductance is capacitively coupled to the limit level measuring electrode 12 located on the inside of the membrane 1 . The device housing serves as a ground connection.
The measurement frequency can be kept constant or varied over a frequency range of, for example, 120 to 170 MHz. The high-frequency alternating current flowing off to ground is measured by the two measurement sensors 22 both for the measuring electrode 12 and for the reference electrode 10 as impedance or admittance.

They are further processed by the sensor electronics 27 and finally recorded by the microcontroller 29. Detailed information about the medium present at the limit level measuring electrode 12 can be obtained on the basis of these measured values and reference values stored in the microcontroller 29 or learned during a teaching process.

Based on their frequency response, not only certain media can be identified, but adhesions can also be recognized and distinguished from a pending medium.
Details on the limit level measurement can be found in the documents also originating from the applicant DE 10 2007 059 702 A1 and DE 10 2007 059 709 A1
It should also be pointed out at this point that all of the configurations described in the two above-mentioned publications by the applicant can be used.
All measurements can be carried out one after the other, but also almost simultaneously. The limiting element is the processing speed of the microcontroller. Decoupling is effected by the low-pass filter 14, the band-pass filters 24 and the chokes 25. In addition, the signal paths can be interrupted by the switches in the decoupling circuit 20 and/or the assemblies can be separated from the power supply.
The decoupling circuit 20 is to be understood as a schematic diagram. In practice, the decoupling can be achieved additionally or exclusively by separating the assemblies from the operating voltage. Simultaneous operation of the two measuring arrangements is also possible and makes sense in the case of working frequency ranges that are so far apart and in particular because of the low-pass filter 14 and the band-pass filters 24 .
In the microcontroller 29, the measurement results subjected to a plausibility check. For example, if the system pressure is zero, it is determined whether it is just a drop in pressure or a real vacancy.
It is also checked whether the high-frequency alternating current flowing out via the measuring electrode during the limit level measurement is within a specified range. In particular, moisture penetrating through a tear or even a rupture of the membrane could lead to a significant exceedance. The temperature measurement is also extremely useful because it can be used to correct the other readings, especially the pressure readings.
The measured system pressure, the temperature or the flow rate and additional information about the type of medium present or error messages can be displayed by the display unit 30, but can also be sent via the input and output module 28 to a superordinate unit.

At the in the 2 shown embodiment, the temperature sensor 23 was combined with the limit level measuring electrode 12; ie the temperature sensor 23 is also supplied with the high-frequency signal for limit level measurement. In this way, the area provided for the limit level measuring electrode 7 on the membrane 1 or the base body 2 can be used twice, leaving more surface area for the most precise possible pressure measurement.
Due to the decoupling measures taken by the low-pass filter 14 and the band-pass filters 24, the measurements can preferably be carried out simultaneously, in particular the pressure measurement continuously. In this case, the decoupling circuit 20 could even be dispensed with, or its switches could remain closed all the time. In this way, the response time for the measurements can be significantly reduced. Depending on the measuring task, especially when there are high demands on the accuracy of the pressure measurement, it is also possible to carry out the limit level measurement at longer time intervals or only when requested by the operator or the higher-level control unit, but also only in the event of a sudden pressure change. The signal flow and the evaluation or display takes place as already described above.

In 3 was completely dispensed with the temperature measurement. The counter-electrode 4, which belongs both to the reference capacitance 15 and to the measuring capacitance 16, is subjected to the high-frequency signal in parallel or in succession. It thus also becomes the measuring impedance 12 for the limit level and thus the limit level measuring electrode 7.
In addition to its function as a counter-electrode 4 for pressure measurement, it is also used as a limit-level measuring electrode 7 .
This design represents the absolute minimum variant. It has the advantage that a "conventional" capacitive pressure measuring cell can be used

The signal processing and evaluation as well as the display takes place as already described above. The two measurements are controlled and evaluated by the microcontroller and, as already mentioned above, can take place simultaneously or at different times.

4 finally shows a variant without a temperature sensor. The pressure measurement and the level measurement are carried out with the same measuring cell, but with separate electrodes. The measurements can be carried out at different times, but also almost simultaneously.

The preferred arrangement of the temperature sensor in the measuring cell is given in 5 shown. It shows a ceramic pressure measuring cell according to the invention in an exploded view. The diaphragm 1 is held at a certain distance from the base body 2 by the glass solder layer 3 . On its side facing the membrane, the base body 2 has a recess 2a to accommodate the temperature measuring electrode 8 located in the center of the membrane, which in this case consists of the multilayer temperature sensor 23 . Furthermore, the reference electrode 5, which is also arranged on the base body and can be contacted from the outside, and the measuring electrode 6 are shown. These form the reference capacitance 15 and the measuring capacitance 16 for the capacitive pressure measurement with the counter-electrode 4 which is also located on the membrane and can also be contacted from the outside.
A separate limit level measuring electrode 7 is not shown. This can occupy the recessed sector in the ring-shaped reference electrode 5 . A corresponding evaluation circuit is in the 1 shown.
If the limit level measuring electrode 7 is dispensed with, the reference electrode 5 can usefully be supplemented to form an almost closed ring in the interest of the measuring accuracy of the pressure measurement. The figures are not to scale, but are to be understood as representations of principles.
The capacitive limit level measurement can also take place without a separate measuring electrode 7 by applying the high-frequency signal to the counter-electrode 4 or the temperature-measuring electrode 8 . In these cases, there must be no grounded metal layer between the high-frequency electrode and the medium. An associated sensor circuit can be found in 2 .

6 shows one of the surface of the counterelectr located on the inside of the membrane Rode 4 branched off separate limit level measuring electrode 7. The counter-electrode 4 was extended by a shielding tab 4a for shielding the pressure measuring electrode 6 and 4b for shielding the reference electrode 5. For reasons of clarity, the reference electrode 5 was not shown here completely as an almost closed ring which was only interrupted by the feed line to the pressure measuring electrode 6 . The arrangement of the limit level measuring electrode 7
Because of its high dielectric constant, there is good capacitive coupling with the pressure medium on the inside of the membrane.
In 7 was the off 6 known structure is extended by an additional temperature measurement area 8, which is also located in the plane of the counter-electrode 4, ie on the inside of the membrane. In a known manner, this can consist of an NTC-PTC or platinum resistance track. The arrangement on the membrane allows the medium temperature to be recorded quickly due to its good thermal coupling with the medium. In a specific embodiment, the
Limit level measuring electrode 7 and the temperature measuring electrode 8 are combined into a flat electrode. (see. 6 )

8th shows a ceramic pressure measuring cell with a separate one
Limit level measuring electrode 7. This is located on the base body in the plane of the pressure measuring electrode 6 and the reference electrode 5.
The wiring, especially for the high-frequency, becomes easier as a result. The counter-electrode 4 located on the membrane is recessed in the sector opposite the limit-level measuring electrode 7 to improve the radiation. Here, too, the reference electrode 5 could be supplemented to form an almost closed ring. In this case, the limit level measuring electrode 7 and the recess in the counter-electrode 4 must be moved in the direction of the contact point for the pressure-measuring electrode 6.

9 shows the limit level measuring electrode 7 in the plane of the pressure measuring electrode 6 and the reference electrode 5, i.e. on the base body 2. Of the shielding tabs 4a for the pressure measuring electrode 6 and 4b for the reference electrode 5 on the membrane 1, only 4a was shown. Here, too, the counter-electrode 4 is cut out to improve the emission of the limit-level measuring electrode 7 . If this recess were omitted, the counter-electrode 4 would have to be separated from ground during the limit level measurement, ie a sufficient transition resistance to ground potential would have to be ensured.

In 10 the arrangement was made 9 supplemented by the temperature measuring electrode 8. Otherwise, the above statements apply.

In 11 an embodiment with a measuring cell configured as a calorimetric flow sensor with two temperature measuring electrodes 8 is shown. The two temperature measuring electrodes 8 are located on the base body in the plane of the reference electrode 5. As already explained above, one of them is alternately subjected to a heating current and a significantly lower measuring current, while the other is used in a known manner to measure the medium temperature. Of course, a temperature measuring electrode arranged at the edge with the in 5 shown arrangement can be combined to form a calorimetric flow sensor.

reference list

1

Membrane,

2

body,

2a

Recess in the body

3

glass solder

3

counter electrode pressure measurement,

4a, 4b

Shielding tabs for measuring electrodes

5

reference electrode pressure

6

pressure measuring electrode

7

Point level measuring electrode

8th

Temperature measuring electrode (temperature measuring range)

9

Medium protection circuit (RLC)

10

Reference impedance limit level

11

parasitic capacities

12

Measuring impedance limit level, (transmitting electrode)

13

Square wave generator (pressure measurement)

14

low pass (pressure measurement)

15

Reference capacitance (pressure measurement)

16

Measuring capacity (pressure measurement)

17

Integrator (pressure measurement

18

differentiator (pressure measurement)

19

Pressure sensor

20

decoupling circuit

21

High-frequency generator limit level measurement

22

Limit value sensor

23

Temperature sensor, PTC resistor

24

Bandpass for high frequency

25

throttle(s)

26

control unit

27

sensor electronics

28

input/output unit

29

microcontroller

30

display unit

Claims (9)

Capacitive pressure sensor as a combination sensor for detecting a further measured variable with a ceramic pressure measuring cell consisting essentially of a membrane (1) and a base body (2) and measuring electrodes (4,5,6,7) in the form of conductive surface areas, the predominant Part of the membrane surface is used solely for pressure measurement, characterized in that all measuring electrodes (4,5,6,7) are arranged in the cavity between the membrane (1) and the base body (2), the measurement of the different variables takes place in different frequency ranges, In addition to the pressure measurement, a calorimetric flow or limit level measurement is carried out. Capacitive pressure sensor as a combination sensor for detecting a further measured variable with a ceramic pressure measuring cell consisting essentially of a membrane (1) and a base body (2) and measuring electrodes (4,5,6,7) in the form of conductive surface areas, the predominant Part of the membrane surface is used solely for pressure measurement, characterized in that all measuring electrodes (4,5,6,7) are arranged in the cavity between the membrane (1) and the base body (2), the measurement of the different variables takes place in different frequency ranges, suitable measuring electrodes (4,5,6,7) are provided for pressure measurement and for capacitive limit level measurement and the pressure measuring electrodes (4,5,6) are supplied with a low-frequency signal and the limit level measuring electrode (7) with a high-frequency signal and a limit level measurement based on the impedance or the admittance of the print medium. Capacitive pressure sensor as a combination sensor for detecting a further measured variable with a ceramic pressure measuring cell consisting essentially of a membrane (1) and a base body (2) and having measuring electrodes (4,5,6) in the form of conductive surface areas, the majority of the Membrane surface is used solely for pressure measurement, characterized in that all measuring electrodes (4,5,6) are arranged in the cavity between the membrane (1) and the base body (2), the measurement of the different variables takes place in different frequency ranges, suitable measuring electrodes (4,5,6) are provided for pressure measurement and for capacitive limit level measurement, which are at least temporarily supplied with a low-frequency signal and one of them also with a high-frequency signal, with a limit level measurement taking place on the basis of the impedance or the admittance of the pressure medium. Capacitive pressure sensor according to one of Claims 1 or 2 , wherein the measuring electrode (7) used for capacitive limit level measurement is also designed as a temperature sensor (23). Ceramic pressure measuring cell for a capacitive pressure sensor according to claims 2 until 4 , wherein the measuring electrode (4) acting as counter-electrode is arranged on the membrane (1) and has shielding tabs (4a and 4b) for the leads leading to the measuring electrode (6) and to the measuring electrode (5) acting as reference electrode. Method for operating a ceramic pressure sensor according to one of Claims 1 until 4 , wherein at least two measured variables are recorded essentially simultaneously. procedure after claim 6 , with at least two different measured variables being displayed or forwarded to a higher-level control unit. Procedure according to one of Claims 6 or 7 , whereby the various measured variables are used for a plausibility check. Procedure according to one of Claims 6 until 8th , whereby it is checked whether the high-frequency alternating current flowing via the measuring impedance (12), ie via the limit level measuring electrode (7) or the measuring electrode (4) is within a predetermined interval, and if a limit value is exceeded, the error message "measuring cell may be leaking" or a similar message is output.

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