Method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor
阅读说明：本技术 用于监测电容式压力传感器的压力测量单元的操作的方法 (Method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor ) 是由 海因茨·瓦尔特 曼弗雷德·毛鲁斯 于 2019-07-29 设计创作，主要内容包括：本发明涉及用于监测电容式压力传感器(1)的压力测量单元(10)的操作的方法,其中所述压力测量单元(10)包括压力相关的测量电容器(C-M)和压力相关的参考电容器(C-R),并且根据所述测量电容器(C-M)的电容值和所述参考电容器(C-R)的电容值来获取压力测量值(p)作为测量信号,其中将所述测量信号以交替的方波信号的形式供应至评估单元,所述信号的脉冲高度取决于所述参考电容器(C-R)的所述电容值与所述测量电容器(C-M)的所述电容值的商,并且通过所述电容值来确定信号的周期。(The invention relates to a method for monitoring the operation of a pressure measuring cell (10) of a capacitive pressure sensor (1), wherein the pressure measuring cell (10) comprises a pressure-dependent measuring capacitor (C) M ) Pressure dependent reference capacitor (C) R ) And according to the measuring capacitor (C) M ) And the reference capacitor (C) R ) As a measurement signal, wherein the measurement signal is supplied to an evaluation unit in the form of an alternating square-wave signal, the pulse height of which signal depends on the reference capacitor (C) R ) With the measuring capacitor (C) M ) And determining the period of the signal by the capacitance value.)
1. A method for monitoring the operation of a pressure measuring cell (10) of a capacitive pressure sensor (1),
wherein the pressure measuring unit (10) comprises a pressure-dependent measuring capacitor (C)M) Pressure dependent reference capacitor (C)R) And according to the measuring capacitor (C)M) And the reference capacitor (C)R) To obtain a pressure measurement value (p) as a measurement signal,
wherein the measuring signal is applied as an alternating square-wave signal URIs fed to the evaluation unit, the pulse height of the square wave signal being dependent on the reference capacitor (C)R) With the measuring capacitor (C)M) And by the quotient of the capacitance values of the measuring capacitor (C)M) Determines the period duration of the square wave signal such that there is a fixed relationship between the pulse height and the period duration within a nominal pressure range of the pressure sensor,
wherein, when a specific pressure value p is aimed at1，...，pnHas already been calibrated for a number of pulse height and period duration pairs (h)1，d1)，...，(hn，dn) Is stored as a target value and is,
wherein the actual value (h) of the pulse height and period duration pair is usedx-IST，dx-IST) To determine the target value (h) of the pulse height and period duration pairx-SOLL，dx-SOLL) For the currently measured pressure value pxAnd if the actual value (h) of said pulse height and period duration pairx-IST，dx-IST) And the pulse height and period are continuedTarget value (h) of time pairx-SOLL，dx-SOLL) With significant deviation therebetween, an error signal is generated.
2. Method according to claim 1, wherein the storage of the target value and the comparison with the actual value take place in a microcontroller (μ C).
3. The method of claim 2, wherein the first and second light sources are selected from the group consisting of,
wherein the microcontroller (μ C) comprises a timer (60), a memory (40) and a processing unit (50),
the method is characterized by comprising the following steps:
the timer (60) determines the alternating square wave signal URAnd outputting the alternating square wave signal as a value dx-ISTAnd applying said value dx-ISTTo the processing unit (50);
the alternating square wave signal UROf the currently measured pressure value pxAnd a trigger signal is fed to the memory (40), the trigger signal being dependent on the cycle duration d by the processing unit (50)x-ISTGenerated and the trigger signal precisely defines the alternate square wave signal U to be storedRSaid pulse height h ofx-ISTA point in time of the actual value of (a);
the processing unit (50) uses the actual value (h) of the pulse height and period duration pairx-IST，dx-IST) To determine a target value (h) for said pulse height and period duration pairx-SOLL，dx-SOLL) For the currently measured pressure value pxAnd comparing the two values of the pulse height and period duration pair.
4. A method according to claim 3, wherein the target value (h) of the pulse height and period duration pairx-SOLL，dx-SOLL) Is stored in a look-up table.
5. A method for monitoring the operation of a pressure measuring cell (10) of a capacitive pressure sensor (1),
wherein the pressure measuring unit (10) comprises a pressure-dependent measuring capacitor (C)M) Pressure dependent reference capacitor (C)R) According to said measuring capacitor (C)M) And the reference capacitor (C)R) To obtain a pressure measurement value (p) as a measurement signal,
wherein the measuring signal is applied as an alternating square-wave signal URIs fed to the evaluation unit, the pulse height of the square wave signal being dependent on the reference capacitor (C)R) With the measuring capacitor (C)M) And by the quotient of the capacitance values of the measuring capacitor (C)M) Determines the period duration of the square wave signal such that there is a fixed relationship between the pulse height and the period duration within a nominal pressure range of the pressure sensor,
wherein the pressure sensor (1) comprises a microcontroller (μ C) having a timer (60), a memory (40) and a processing unit (50),
the method is characterized by comprising the following steps:
defining the pulse height h in the form of a polynomialxAnd the period duration dxAnd storing said functional relationship in said memory (40);
the timer (60) determines the alternating square wave signal UROf said period duration dxAnd the alternating square wave signal U is appliedRAs a value dx-ISTTo the processing unit (50);
determining and having a pulse height h based on the polynomialx-ISTMeasured pressure value pxAssociated cycle duration dx-SOLL；
A period duration d to be determined based on the polynomialx-SOLLThe value of (d) and the actually measured period duration dx-ISTMaking a comparison and at dx-ISTAnd dx-SOLLWith significant deviation therebetween, an error signal is generated.
Technical Field
The present invention relates to a method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor.
Background
Capacitive pressure sensors or pressure measuring devices are used in many industrial fields for pressure measurement. They usually have a ceramic pressure measuring cell for the process pressure as a transducer and evaluation electronics for signal processing.
The capacitive pressure measuring unit consists of a ceramic substrate and a diaphragm, wherein a glass solder ring is arranged between the substrate and the diaphragm. The cavity created between the substrate and the diaphragm enables the diaphragm to move longitudinally as a result of the pressure effect. Therefore, this cavity is also referred to as a measurement cavity. On the bottom side of the diaphragm and on the opposite top side of the base body, there are provided corresponding electrodes which together form a measuring capacitor. The action of the pressure causes the diaphragm to deform, resulting in a change in the capacitance of the measuring capacitor.
By using the evaluation unit, the change in volume is detected and converted into a pressure measurement. Typically, these pressure sensors are used to monitor or control a process. Therefore, these pressure sensors are typically connected to a high level control unit (PLC).
A capacitive pressure sensor is known from DE19851506C1, in which a pressure measurement value is determined from the quotient of two capacitance values of a measurement capacitor and a reference capacitor. Although the pressure measuring cell is not specifically described in this patent specification, the illustrated circuit and the described method are suitable for capacitive pressure measuring cells. The pressure measuring device is particularly characterized in that, in order to evaluate the measuring signal at the output as a measure for the detected pressure measurement, this measurement is dependent only on the amplitude of the square-wave signal and not on the frequency of the square-wave signal.
A circuit arrangement for a capacitive pressure sensor is known from EP0569573B1, in which a pressure evaluation is also carried out using the quotient.
The business law usually assumes the following pressure dependence:
orOr
Wherein C isMIs a measure of the capacitance of a capacitor, CRIs the capacitance of the reference capacitor and p represents the process pressure to be determined. It is also conceivable to interchange C in the quotientMAnd CR. However, use of C in the denominatorMThe example given in favor of intrinsic linearization is the most common form. Therefore, unless otherwise specified, this form is assumed in the following examples.
The reliability of capacitive pressure sensors is increasingly important. The problem with capacitive pressure sensors operated according to the quotient method is that the entrance of a medium (which is caused by a diaphragm rupture or by a possible vent channel) cannot be detected due to the formation of the quotient, since the dielectric constant epsilonrWith corresponding changes in numerator and denominator. If entering the medium epsilonrEpsilon with airrSlightly different, this problem becomes worse. In particular, in the case where the medium to be measured is oil, the epsilon of the oilrUsually between 2 and 4, and e of airrIs 1.
For this purpose, EP2606330B1 suggests to use an additional capacitor whose capacity is independent of the diaphragm pressure to determine a control pressure measurement which is compared with the actual pressure measurement.
As other prior art regarding monitoring the operation of capacitive pressure sensors, the following documents are mentioned: DE102011083133A1, DE102010062622A1, US2014/0144206A1, US2006/0152380A1, DE102014201529A 1.
Disclosure of Invention
It is an object of the present invention to propose an alternative and cost-effective method for monitoring the operation of a pressure measuring cell of a capacitive pressure sensor.
This object is achieved by a method comprising the features of claim 1 and claim 5. Advantageous embodiments of the invention are specified in the dependent claims.
The basic idea of the invention is to recognize that within the nominal pressure range of the pressure sensor there is a fixed relationship between the pulse height or amplitude and the cycle duration or frequency and that when a medium enters the measurement chamber (said medium entering being caused by a diaphragm rupture or by an entry via a vent channel), the measurement capacitance C shown above is caused by the measurement capacitance CMAnd a reference capacitance CRThe square wave signal formed by the quotient of (a) varies significantly. Furthermore, due to the construction of the pressure measuring cell, in CMAnd CRThere is also a fixed relationship in the pressure correlation process of the two capacitance values, so that incipient cracks or breakages of the diaphragm, which influence the deformation behavior of the diaphragm, can be detected.
The medium, even if only a small amount, enters the measuring chamber because of epsilonrResulting in an increase in capacity depending on which electrodes and thus which dielectrics are affected.
If only the external reference electrode is affected, for example because the ventilation channel is usually arranged very close to the reference electrode, the pulse height becomes significantly smaller for the same cycle duration. If rupture of the membrane occurs in the outer zone and therefore CRRatio CMAnd more easily deformed, the same situation may occur.
In the case where it is rather unlikely that only the measuring electrode is affected or that an incipient crack or rupture of the membrane occurs in the central region, the exact opposite occurs, i.e. the pulse height becomes significantly larger for the same period duration.
In a more probable case, the capacitance C is if the medium is distributed uniformly in the measuring chamber, and thus on the reference electrode and the measuring electrode, due to a rupture of the membrane or an entry via the vent channelMAnd a capacitor CRTo the same extent. As mentioned above, in this case, the pulse heightRemain unchanged. However, the new finding is that in the case of a significant variation of the period duration, it is now possible to monitor the reliability of the pressure sensor by evaluating the frequency or the period duration of the square-wave signal, irrespective of the determination of the current pressure value.
The method according to the invention therefore involves an evaluation of the pulse height and the period duration relative to the actually measured pressure. For this purpose, the pressure value p is specified1，...，pnHas already been calibrated for a number of pulse height and period duration pairs (h)1，d1)，...，(hn，dn) Stored as a target value or as a function of the pulse height and the period duration in polynomial form. In the first case, the actual value (h) of the pulse height and period duration pair is usedx-IST，dx-IST) To determine a target value (h) for the pulse height and period duration pairx-SOLL，dx-SOLL) For the currently measured pressure value pxWherein an error signal is generated in case of a significant deviation between the actual value of the pulse height and period duration pair and the target value of the pulse height and period duration pair. In the second case, the stored polynomial describes the pulse height hxAnd a period duration dxFunctional relationship between them, such that for pulses having a pulse height hx-ISTMeasured pressure value pxThe period duration d can be determined by using a polynomialx-SOLLThe correlation value of (2). If the period duration d is determined according to a polynomialx-SollWith the actual measured period duration dx-ISTAnd if not, an error signal is sent out.
Thus, monitoring of the operation of the pressure measuring cell of the capacitive pressure sensor can be performed by a suitable signal evaluation with existing evaluation circuits and thus without any additional components, and capacitive error effects can be detected quickly and early.
The present invention is explained in detail based on exemplary embodiments with reference to the attached drawings.
Drawings
The figures show schematically:
FIG. 1 is a block diagram of a capacitive pressure measurement device;
FIG. 2 is a schematic cross-sectional view of a capacitive pressure measurement cell;
FIG. 3 is a known evaluation circuit for the capacitive pressure measuring cell according to FIG. 2; and
fig. 4 is the evaluation circuit of fig. 3 supplemented with a microcontroller for implementing the method according to the invention.
Detailed Description
In the following description of the preferred embodiments, the same reference numerals are used for the same or similar parts.
Fig. 1 shows a block diagram of a typical capacitive pressure measuring device for measuring a process pressure p (e.g. the pressure of oil, milk, water, etc.). The pressure measuring device 1 is designed as a two-line device and is substantially composed of a pressure measuring cell 10 and evaluation electronics 20. The evaluation electronics 20 comprise an analog evaluation circuit 30 and a microcontroller μ C, in which the analog output signal of the evaluation circuit 20 is digitized and further processed. The microcontroller μ C provides the evaluation result to the PLC, for example as a digital or analog output signal. The pressure measuring device 1 is connected to a voltage supply line (12V-36V) for energy supply.
Fig. 2 shows a schematic representation of a typical capacitive pressure measurement cell 10 as used in various capacitive pressure measurement devices. The pressure measuring cell 10 is substantially composed of a base body 12 and a diaphragm 14, the base body 12 and the diaphragm 14 being connected to one another by a glass solder ring 16. The base body 12 and the diaphragm 14 define a cavity 19, which cavity 19-preferably only in the low pressure range of at most 50 bar-is connected to the rear side of the pressure measuring cell 10 via a vent channel 18.
A plurality of electrodes are disposed on the substrate 12 and on the diaphragm 14, the electrodes forming a reference capacitor CRAnd a measuring capacitor CM. Measuring capacitor CMFormed by a diaphragm electrode ME and a central electrode M, a reference capacitor CRFormed by a ring electrode R and a membrane electrode ME.
The process pressure p acts on the diaphragm 14, the diaphragm 14 more or less bending depending on the applied pressure, wherein the distance between the diaphragm electrode ME and the center electrode M substantially varies. This results in a measuring capacitor CMA corresponding change in capacitance. Since the variation in the distance between the annular electrode R and the diaphragm electrode ME is smaller than the variation in the distance between the diaphragm electrode ME and the central electrode M, the reference capacitor C is thus formedRThe influence of (c) is small.
In the following, no distinction is made between the name (designation) of the capacitor and its capacitance value. Thus, CMAnd CRRepresenting the measurement or reference capacitor itself and their capacitance.
A known evaluation circuit 30 for a pressure measuring cell 10 is shown in more detail in fig. 3. Measuring capacitor CMAnd a resistor R1Arranged together in an integrating branch IZ, reference capacitor CRAnd a resistor R2Are arranged together in the differentiating branch DZ. Applying a square-wave voltage U to the input of the integrating branch IZE0The square wave voltage UE0Preferably symmetrically alternating around 0 volts. The input voltage U is obtained by using an operational amplifier OP1 serving as an integratorE0Via a resistor R1And a measuring capacitor CMIs converted into a linearly increasing or linearly decreasing (depending on the polarity of the input voltage) voltage signal, which is output at the output COM of the integrating branch IZ. Here, the measurement point P1 is actually in a grounded state due to the operational amplifier OP 1.
The output COM is connected to a threshold comparator SG which drives a square-wave generator RG. As soon as the voltage signal at the output COM is above or below the threshold value, the comparator SG changes its output signal, whereupon the square-wave generator RG inverts its output voltage, respectively.
In addition, the differential branch DZ is formed by an operational amplifier OP2, having two resistors R5And R6Voltage divider and feedback resistor R7And (4) forming. The output of the operational amplifier OP2 is connected to a sample and hold circuit S&H. In a sample and hold circuit S&Providing a measurement voltage U at the output of HMessFrom which an acting pressure is obtainedThe process pressure p on the force-measuring cell 10.
In the following, the function of the measurement circuit will be explained in more detail. The operational amplifier OP1 ensures the resistor R1And a measuring capacitor CMThe connection point P1 between is kept at actual ground. As a result, a constant current I1Through the resistor R1An electrical resistor R1For measuring capacitor CMCharging until square wave voltage UE0Changing its sign.
As can be seen from FIG. 3, for R1＝R2And CM＝CRIf the connection between the measurement point P2 and the operational amplifier OP2 were not to be present, the measurement point P2 in the differentiating branch DZ would even be at the same potential as the measurement point P1, i.e. at ground level. This applies not only to this special case, but also to the case where only the time constant R is used1*CMAnd R2*CRThe case of being equal to each other. During zero calibration, the voltage is respectively passed through the variable resistors R1And R2The state is set accordingly. If the capacitor C is measuredMThe condition that the time constants in the integrating branch IZ and the differentiating branch DZ are equal is no longer given, and the potential at the measuring point P2 will deviate from the value 0, due to the change in capacitance due to the effect of the pressure. However, this change is immediately cancelled by the operational amplifier OP2, because the operational amplifier OP2 continues to keep the connection point P2 effectively grounded. A square-wave voltage UR is thus provided at the output of the operational amplifier OP2, the amplitude of which depends on the quotient of the two time constants. It can easily be shown that the amplitude is proportional to the process pressure p-CR/CM-1, where the dependence is substantially linear. Can be via a voltage divider (which is formed by two resistors R)5And R6Form) to adjust the amplitude.
The positive amplitude A + and the negative amplitude A-of the square-wave signal pass through a sample-and-hold circuit S&H is added in quantities, wherein the quantity A is output as a measurement voltage U at the output of an operational amplifier OP3MessAnd forwarded to the microcontroller muc (not shown). However, the amount a may also be directly output as an analog value. According to the measured voltage UMessTo be arranged on square wave generatorInput voltage U present at the output of generator RGE0In order to obtain better linearity. For this purpose, a resistor R is provided20And a resistor R10Forming a voltage divider. The voltage divider is connected to a reference voltage VREF and can advantageously be regulated.
The positive operating voltage V + is typically 2.5 volts and the negative operating voltage V-is-2.5 volts.
Fig. 4 shows in principle an evaluation circuit known from fig. 3, which is supplemented with a microcontroller μ C. In this microcontroller, on the one hand, the comparator oscillator SG from fig. 3 is integrated, and on the other hand, it comprises the units necessary to implement the method according to the invention: a timer 60, a memory 40, and a CPU 50 as a processing unit. The elements arranged outside the microcontroller μ C are substantially identical and are therefore also denoted identically. To avoid repetition, only the elements essential to the invention will be discussed below.
On the one hand, the output signal of the threshold comparator SG is fed back to drive a square-wave generator RG, which is known from fig. 3. On the other hand, the signal is fed to the timer 60. In the timer 60, the time period behavior of the triangle signal is recorded, in particular the behavior with respect to reaching the set threshold value. Thereby deriving the cycle duration that is fed to the CPU 50. In this case, the period duration of the triangular signal is compared with the actual measurement signal URAre the same, it is particularly advantageous to detect the period duration in this way.
Furthermore, the microcontroller μ C comprises a memory 40, the memory 40 initially being provided with a voltage signal U known from fig. 3RForm of the currently measured pressure value px. Meanwhile, and not shown in detail for illustrative reasons, the pressure value pxIs also sent to the output of the microcontroller μ C, switch out or analog out, in order to output the measured pressure value as a switching or analog signal. Sample and hold circuit S known from fig. 3&H is also integrated into the microcontroller μ C as part of the evaluation circuit shown and is functionally identical to the simulation.
Furthermore, the trigger signal generated by the CPU 50 according to the cycle duration is also fed to the memory 40. The trigger signal accurately defines the voltage signal U to be storedRPulse height h ofx-ISTThe point in time of the actual value of. In terms of time, this point in time is advantageously exactly in the middle of the positive square wave pulse.
The values (h) of the multiple pulse height and period duration pairs are combined in a calibration process that occurs prior to application of the pressure sensor1，d1)，...，(hn，dn) Also stored as target values in the memory 40, in particular in a look-up table, for a specific pressure value p1，...，pn。
In the CPU 50, the actual value h of the pulse height and period duration pair is usedx-IST、dx-ISTTo determine a target value h of a pulse height and period duration pairx-SOLL、dx-SOLLFor the currently measured pressure value pxAnd if there is a significant deviation between the actual value of the pulse height and period duration pair and the target value of the pulse height and period duration pair, an error signal is generated which is output at the output terminal diag _ out.
List of reference numerals
1 pressure measuring device
10 pressure measuring cell
12 base body
14 diaphragm
16 glass solder ring
18 vent passage
19 cavity
20 evaluation electronics
30 evaluation circuit
40 memory
50 processing unit, CPU
60 timer
CMMeasuring capacitor
CRReference capacitor
M center electrode
R-ring electrode
ME diaphragm electrode
IZ integral branch
DZ differential branch
SG threshold comparator
RG square wave generator
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