Capacitive pressure sensor with temperature detection

Abstract

Capacitive pressure sensor with a pressure measuring cell (10) for detecting the pressure of a medium in a container and an evaluation circuit (30) for preparing and processing the measuring signals transmitted by the pressure measuring cell (10), wherein the pressure measuring cell (10) has a pressure-sensitive measuring capacitor (C M ) and a reference capacitor (C R ) and the pressure measuring cell (10) is connected to the evaluation circuit (30) via three lines (L1, L2, L3), wherein a first line (L1) is connected to a common electrode (COM) of measuring capacitor (C M ) and reference capacitor (C R ), a second line (L2) to the measuring capacitor (C M ) and a third line (L3) to the reference capacitor (C R ) leads, wherein the evaluation circuit (30) comprises a first operational amplifier (OP1), the low-impedance output of which is connected to the common electrode (COM) via the first line (L1), characterized by that the common electrode (COM) is used in addition to the temperature measurement of the medium applied to the pressure measuring cell (10) and the temperature measurement is carried out exclusively via the common electrode (COM), in that the common electrode (COM) is connected to a fourth line (L4) in addition to the third line (L3), by means of the third and fourth lines (L3, L4) the voltage drop across the common electrode (COM) is tapped and fed to an evaluation unit (AWE), and that at the output of the evaluation unit (AWE) a voltage signal U Temp corresponding to the temperature detected by the common electrode (COM), wherein the common electrode (COM) is designed as a meandering resistance track and the pressure sensor (1) comprises a constant current regulator (KSR), by means of which the common electrode (COM) is supplied with a constant current I via a fourth line (L4) konst is applied, whereby the constant current I konst from the output of the first operational amplifier (OP1), so that the course of the tapped voltage only depends on the medium temperature.

Description

[0001] The invention relates to a capacitive pressure sensor for detecting the pressure of a medium adjacent to the pressure sensor according to the preamble of claim 1.

[0002] Pressure gauges or pressure sensors are used in many industrial sectors for pressure measurement. They often have a pressure measuring cell, which acts as a transducer for the process pressure, and evaluation electronics for signal processing.

[0003] Typical capacitive measuring cells consist of a compact unit with a ceramic base body and a diaphragm. An annular spacer, such as a glass solder ring, is arranged between the base body and the diaphragm. The resulting cavity between the base body and the diaphragm allows the diaphragm to move longitudinally under pressure. Electrodes are provided on the underside of the diaphragm and on the opposite upper side of the base body, which together form a measuring capacitor. The application of pressure causes a deformation of the diaphragm, resulting in a change in the capacitance of the measuring capacitor.

[0004] Temperature measurement is often also required in conjunction with pressure measurement. For this purpose, it is known from DE 40 11 901 A1 to provide an annular resistance track arranged on the end face of the base body facing the diaphragm or on the end face of the diaphragm facing the base body.

[0005] However, due to space constraints, this known arrangement becomes problematic when the pressure measurement value is determined not only from the change in capacitance of a measuring capacitor, but – as known from DE 198 51 506 C1 – from the quotient of two capacitance values: a measuring capacitor and a reference capacitor. The quotient method is particularly advantageous because changes in the dielectric do not affect the measurement value determination. Therefore, the following assumes pressure sensors that operate according to the quotient method.

[0006] Since the presence of two concentrically arranged capacitors on the opposite sides of the diaphragm and base body results in correspondingly less space to accommodate a resistance track for temperature sensing, EP 1 174 696 B1 proposes integrating the resistance element into the glass solder ring. However, the available surface area for temperature sensing is correspondingly small. Furthermore, in the installed pressure measuring cell, a seal is typically located below the glass solder ring and thus in the area of ​​the resistance element on the medium side. This seal can influence temperature sensing due to its insulating effect.

[0007] Furthermore, WO 2014 / 154695 A1 discloses providing a multi-layer structure on the end face of the membrane facing the base body, consisting of the measuring electrode for pressure measurement, a resistance layer for temperature detection, and an intermediate insulation layer. However, this increases the manufacturing complexity of the pressure measuring cell.

[0008] Document EP 3 569 995 A1 discloses a combined capacitive pressure and temperature sensor. The central teaching is to design one of the two electrodes of the capacitive sensor (either the measuring electrode on the membrane or the counter electrode on the base body) as a structured, meander-shaped layer. This layer serves a dual purpose: It functions as a capacitor plate for pressure measurement and simultaneously as a resistor with temperature-dependent resistivity for temperature measurement. A specific embodiment describes the use of two intermeshing (combing) conductor tracks, which additionally allows for the detection of moisture.

[0009] Document DE 10 2010 062 622 A1 discloses a method and circuit for self-monitoring and fault diagnosis of a capacitive pressure sensor. The central teaching is the use of an additional, external capacitor (Cz) whose capacitance is independent of the process pressure. A control pressure reading is generated from this additional capacitor and one of the internal capacitors of the measuring cell. This control pressure reading is compared with the actual pressure reading (obtained from the internal measuring and reference capacitors). A significant deviation between the two values ​​indicates a fault, such as a diaphragm rupture, since such a fault (e.g., due to medium ingress) changes the capacitance of the internal capacitors but leaves the external additional capacitor unaffected.

[0010] The object of the invention is to provide a capacitive pressure sensor with a particularly simple and cost-effective possibility of temperature detection during pressure measurement.

[0011] The object is achieved according to the invention by a capacitive pressure sensor having the features of claim 1. Advantageous embodiments of the invention are specified in the subclaims.

[0012] The idea of ​​the invention is to use exclusively the membrane electrode COM, which is already present for pressure measurement and acts as a common electrode for the measuring and reference capacitor, is therefore only distanced from the medium to be measured according to the thickness of the membrane and has the maximum medium contact area, in addition to the pressure measurement also for the detection of the medium temperature, so that an additional resistance element for temperature detection can be dispensed with and the membrane only has a single-layer structure consisting of the membrane electrode COM.For this purpose, this common membrane electrode, which is formed in a meandering shape, for example, from metal with a measurable ohmic temperature coefficient, is supplied with a constant current via an additional, fourth line by means of a constant current regulator. The resulting voltage drop across the membrane electrode is fed to an evaluation unit, at the output of which a voltage signal is present that corresponds to the temperature detected by the membrane electrode. Although the charging currents for the measuring and reference capacitors superimpose the voltage drop across the membrane electrode in a time-resolved manner during the pressure evaluation cycle, their signed values ​​are identical and thus ineffective after averaging.

[0013] The design of the membrane electrode as a meandering resistance track is particularly advantageous because it allows for optimal use of the available surface area for the membrane electrode. On the one hand, the meandering layout allows for maximum length and thus the greatest possible ohmic resistance. On the other hand, a meander with the smallest possible spacing between the tracks achieves the largest possible electrode surface, which significantly determines the capacitance. The optimization of these two parameters—ohmic resistance and electrical capacitance—for a given surface area is advantageously achieved with a meandering resistance track.

[0014] The invention makes it possible to design the common membrane electrode with a low resistance, eliminating the need for highly costly sputtering technology during production. Instead, the common membrane electrode can be applied to the membrane using a screen-printing process, for example, as a meandering gold or platinum track, with minimal gaps to maximize the capacitively effective area and maximum length to maximize the effective ohmic resistance, with its reproducible change in magnitude over temperature according to its temperature coefficient.

[0015] A further advantage of the invention is the virtually non-existent temperature gradient, since the temperature measurement takes place in the direct vicinity of the medium.

[0016] The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

[0017] They show schematically: Fig. 1 a block diagram of a known capacitive pressure gauge, Fig. 2 a schematic sectional view of a known capacitive pressure measuring cell, Fig. 3 a known evaluation circuit for a capacitive pressure measuring cell according to Fig. 2, Fig. 4 an embodiment of the invention based on the known evaluation circuit according to Fig. 3.

[0018] In the following description of the preferred embodiments, like reference numerals designate like or comparable components.

[0019] In Fig. Figure 1 shows a block diagram of a typical capacitive pressure gauge used to measure a process pressure p (e.g., of oil, milk, water, etc.). The pressure gauge 1 is designed as a two-wire device and essentially consists of a pressure measuring cell 10 and an evaluation electronics unit 20. The evaluation electronics unit 20 has an analog evaluation circuit 30 and a microcontroller µC, in which the analog output signal of the evaluation circuit 20 is digitized and further processed. The microcontroller µC provides the evaluation result as a digital or analog output signal, e.g., to a PLC. The pressure gauge 1 is connected to a power supply line (12 - 36 V) for power supply.

[0020] Fig. Figure 2 shows a schematic representation of a typical capacitive pressure measuring cell 10, as used in a variety of capacitive pressure measuring devices. The pressure measuring cell 10 essentially consists of a base body 12 and a diaphragm 14, which are connected to each other via a glass solder ring 16. The base body 12 and the diaphragm 14 define a cavity 19, which – preferably only for low pressure ranges up to 50 bar – is connected to the rear of the pressure measuring cell 10 via a vent channel 18.

[0021] Both on the base body 12 and on the membrane 14, several electrodes are provided, which form a reference capacitor C R and a measuring capacitor C M The measuring capacitor C M is formed by the membrane electrode COM and the center electrode M, the reference capacitor C R through the ring electrode R and the membrane electrode COM. The measuring capacitor C M and the reference capacitor CR therefore have a common electrode COM arranged on the membrane.

[0022] The process pressure p acts on the membrane 14, which bends more or less depending on the pressure applied, whereby essentially the distance between the membrane electrode COM and the center electrode M changes. This leads to a corresponding change in the capacitance of the measuring capacitor C M . The influence on the reference capacitor C R is lower, since the distance between ring electrode R and membrane electrode COM changes less than the distance between membrane electrode COM and center electrode M. In principle, the reference capacitor C R not be sensitive to pressure.

[0023] In the following, no distinction is made between the capacitor's designation and its capacitance value. C M and C Rtherefore refer to both the measuring or reference capacitor itself and its respective capacitance.

[0024] In Fig. Figure 3 shows a known evaluation circuit 30 for the pressure measuring cell 10. A first line L1 leads to the common electrode COM of the measuring capacitor C M and reference capacitor C R , a second line L2 to the measuring capacitor C M and a third line L3 to the reference capacitor C R . The measuring capacitor C M is together with a resistor R1 in an integrating branch IZ and the reference capacitor C R together with a resistor R2 in a differentiating branch DZ. At the input of the integrating branch IZ there is a square-wave voltage U E0 which preferably alternates symmetrically around 0 volts (=ground) and has a period of approximately 300 µs. The input voltage U E0 is connected via the resistor R1 and the measuring capacitor CM Using an operational amplifier OP1, which acts as an integrator, the signal is converted into a linearly rising or falling voltage signal (depending on the polarity of the input voltage), which is output as a COM signal at the output of the integration branch IZ. The measuring point P1 is virtually grounded by the operational amplifier OP1.

[0025] The COM output is connected to a threshold comparator SG, which drives a square-wave generator RG. As soon as the voltage signal at the COM output exceeds or falls below a threshold, the comparator SG changes its output signal, whereupon the square-wave generator RG inverts its output voltage.

[0026] The differentiating branch DZ further consists of an operational amplifier OP2, a voltage divider with the two resistors R5 and R6, and a feedback resistor R7. The output of the operational amplifier OP2 is connected to a sample-and-hold circuit S&H. The measuring voltage U is present at the output of the sample-and-hold circuit S&H. Mess from which the process pressure p acting on the pressure measuring cell 10 is obtained.

[0027] The function of this measuring circuit is explained in more detail below. The operational amplifier OP1 ensures that the connection point P1 between the resistor R1 and the measuring capacitor C M is virtually grounded. This causes a constant current I1 to flow through the resistor R1, which charges the measuring capacitor C M charges until the square wave voltage U E0 its sign changes.

[0028] Out of Fig. 3 it can be seen that for the case R1= R2 and C M = CR The measuring point P2 in the differentiating branch DZ is at the same potential as the measuring point P1, i.e., at ground level, even if the connection between the measuring point P2 and the operational amplifier OP2 were not present. This applies not only in this special case, but always when the time constants R1 * C M and R2 * C R are equal to each other. During zero point adjustment, this condition is adjusted accordingly via the variable resistors R1 and R2. If the capacitance of the measuring capacitor C Mdue to pressure, the condition of equality of the time constants in the integrating branch IZ and the differentiating branch DZ is no longer met, and the potential at measuring point P2 would deviate from zero. However, this change is immediately counteracted by the operational amplifier OP2, since the operational amplifier OP2 continues to virtually ground the connection point P2. Therefore, a square-wave voltage U is present at the output of the operational amplifier OP2. R whose amplitude depends on the quotient of the two time constants and whose period corresponds to that of the input voltage U E0 , ie approximately 300 µs. It can easily be shown that the amplitude is directly proportional to the process pressure p - C R / C M - 1, with the dependence being essentially linear. The amplitude can be adjusted via the voltage divider formed by the two resistors R5 and R6.

[0029] The positive and negative amplitudes A+ and A- of the square wave are added together using a sample and hold circuit (S&H), and the magnitude A is used as the measuring voltage U Mess at the output of the operational amplifier OP3 and forwarded to the microcontroller µC (not shown). However, it could also be output directly as an analog value. The amplitude of the input voltage U E0 , which is present at the output of the square wave generator RG, is dependent on the measuring voltage U Mess to achieve better linearity. For this purpose, a voltage divider consisting of the resistors R 20 and R 10 This voltage divider is connected to a reference voltage VREF and is advantageously adjustable.

[0030] The positive operating voltage V+ is typically +2.5 V and the negative operating voltage V- is -2.5 V.

[0031] In Fig. 4 is a section of the known evaluation circuit according to Fig. 3 - in a modified representation - supplemented by the essential part of the invention in the form of the connected constant current regulator KSR and the evaluation / amplifier unit AWE. All reference numerals used correspond to those in Fig. 3. In particular, at the output of the first operational amplifier OP1, the Fig. 3 known circuit consisting of the threshold comparator SG and the square wave generator RG (not shown), whose output signal U E0 is fed back to the left side of the circuit. For illustration purposes, Fig. 4 only focuses on the elements essential to the invention.

[0032] The measuring capacitor C M and the reference capacitor C Rare each represented by a circular segment, with the common electrode COM in this case being designed as a low-resistance, meandering resistance track, which, for example, has a resistance value of 100 ohms and is preferably made of gold. The voltage drop across the common electrode COM is tapped, amplified, and evaluated by the evaluation / amplifier unit AWE. For this purpose, the evaluation / amplifier unit AWE is designed, for example, in the form of an instrumentation amplifier.

[0033] In order to be able to measure a voltage across the common electrode COM, a fourth line L4 is used by means of the constant current regulator KSR in addition to the capacitor C M , C R necessary charging current of approximately 1 µA, the common electrode COM is still charged with a constant current I konstof, for example, 100 µA. This means that the averaged voltage drop is only dependent on the temperature influence, and the evaluation of low-resistance metal meander structures is possible because the resulting voltage drop can be evaluated via differential amplifiers simultaneously with the pressure measurement. This current I konst from the output of the first operational amplifier OP1, so that the capacitive pressure measurement remains unadulterated.

[0034] Because the Fig. 3 shown evaluation circuit, the center and ring electrodes M, R for the measuring and reference capacitor C M , C R virtually grounded and the common electrode COM of measuring and reference capacitor C M , C Ris supplied with a triangular signal, the temperature evaluation and thus the inventive dual use of the common electrode COM is easily possible, since on the one hand parasitic capacitances still do not influence the pressure measurement and on the other hand the temperature-dependent voltage drop across the membrane superimposes the uniform triangular voltage in such a way that it can be evaluated in a simple manner.

[0035] At the output of the evaluation / amplifier unit AWE there is a voltage signal U Temp which corresponds to the temperature detected by the common electrode COM and follows the temperature curve linearly. List of reference symbols 1 pressure gauge 10 pressure measuring cell 12 basic bodies 14 Membran 16 Glass solder ring 18 Ventilation duct 19 Cavity 20 Evaluation electronics 30 Evaluation circuit C Mmeasuring capacitor C R Reference capacitor M center electrode R ring electrode COM membrane electrode IZ integration branch DZ differentiating branch SG threshold comparator RG square wave generator AWE evaluation / amplifier unit KSR constant current regulator L1 first line L2 second line L3 third line L4 fourth line

Claims

[1] Capacitive pressure sensor with a pressure measuring cell (10) for detecting the pressure of a medium in a container and an evaluation circuit (30) for preparing and processing the measuring signals transmitted by the pressure measuring cell (10), wherein the pressure measuring cell (10) has a pressure-sensitive measuring capacitor (C M ) and a reference capacitor (C R ) and the pressure measuring cell (10) is connected to the evaluation circuit (30) via three lines (L1, L2, L3), wherein a first line (L1) is connected to a common electrode (COM) of measuring capacitor (C M ) and reference capacitor (C R ), a second line (L2) to the measuring capacitor (C M ) and a third line (L3) to the reference capacitor (C R ) leads, wherein the evaluation circuit (30) comprises a first operational amplifier (OP1), the low-impedance output of which is connected to the common electrode (COM) via the first line (L1), characterized by , that the common electrode (COM) is used in addition to the temperature measurement of the medium applied to the pressure measuring cell (10) and the temperature measurement is carried out exclusively via the common electrode (COM), in that the common electrode (COM) is connected to a fourth line (L4) in addition to the third line (L3), by means of the third and fourth lines (L3, L4) the voltage drop across the common electrode (COM) is tapped and fed to an evaluation unit (AWE), and that at the output of the evaluation unit (AWE) a voltage signal U Temp corresponding to the temperature detected by the common electrode (COM), wherein the common electrode (COM) is designed as a meandering resistance track and the pressure sensor (1) comprises a constant current regulator (KSR), by means of which the common electrode (COM) is supplied with a constant current I via a fourth line (L4). konst is applied, whereby the constant current I konst from the output of the first operational amplifier (OP1), so that the course of the tapped voltage only depends on the medium temperature. [2] Capacitive pressure sensor according to claim 1, characterized by that the ohmic resistance value of the common electrode (COM) is in the range of 100 ohms. [3] Capacitive pressure sensor according to one of the preceding claims, characterized by that the common electrode (COM) is made of gold. [4] Capacitive pressure sensor according to one of the preceding claims, characterized bythat the common electrode (COM) is supplied with 100 µA by means of the constant current regulator (KSR).

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