DE102018118646B3 - Method for monitoring the function of a pressure measuring cell of a capacitive pressure sensor - Google Patents

Abstract

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) has a pressure-dependent measuring capacitor (C _M ) and a pressure-dependent reference capacitor (C _R ) and the pressure measured value (p) as the measuring signal the capacitance values of the measuring capacitor (C _M ) and the reference capacitor (C _R ) is obtained, wherein the measuring signal is supplied to an evaluation unit in the form of an alternating square wave U _R and its pulse height from the quotient of the capacitance values of reference capacitor (C _R ) and measuring capacitor (C _M ), wherein the pulse height is measured at least twice per period of the square wave signal U _R during at least one pulse width and the measured voltage values are stored in a memory (40), and wherein the at least two voltage values per pulse width are compared with each other in magnitude and at a significant Deviation to one another an error signal is generated.

Description

The invention relates to a method for monitoring the function of a pressure measuring cell of a capacitive pressure sensor.

Capacitive pressure sensors or pressure gauges are used in many industrial areas for pressure measurement. They often have a ceramic pressure measuring cell, as a transducer for the process pressure, and an evaluation for signal processing.

Capacitive pressure measuring cells consist of a ceramic base body and a membrane, wherein a glass solder ring is arranged between the base body and the membrane. The resulting cavity between the body and membrane allows the longitudinal mobility of the membrane due to a pressure influence. This cavity is therefore also referred to as a measuring chamber. On the underside of the membrane and on the opposite upper side of the main body electrodes are provided, which together form a measuring capacitor. Pressure causes a deformation of the membrane, which results in a capacitance change of the measuring capacitor.

With the aid of an evaluation unit, the capacitance change is detected and converted into a pressure measurement value. Typically, these pressure sensors are used to monitor or control processes. They are therefore often connected to higher-level control units (PLC).

From the DE 198 51 506 C1 a capacitive pressure sensor is known in which the pressure measured value is determined from the quotient of two capacitance values, a measuring capacitor and a reference capacitor. Although a pressure measuring cell is not specifically described in this specification, the circuit shown and the method described is suitable for capacitive pressure measuring cells. The special feature of this pressure gauge is that only the amplitude of the square wave signal is relevant for the evaluation of the measuring signal at the output, as a measure of the detected pressure measured value, regardless of its frequency.

From the EP 0 569 573 B1 a circuit arrangement for a capacitive pressure sensor is known, in which also a quotient method for pressure evaluation is used.

wobei C_M die Kapazität des Messkondensators, C_R die Kapazität des Referenzkondensators und p den zu bestimmenden Prozessdruck bezeichnet. Denkbar ist auch die Möglichkeit, C_M und C_R im Quotienten zu vertauschen. Das angegeben Beispiel mit C_M im Nenner stellt allerdings zugunsten der Eigenlinearisierung die gebräuchlichste Form dar. Im Folgenden wird daher von dieser Ausführungsform ausgegangen, sofern nicht anders angegeben.Quotient methods usually assume the following pressure dependencies: $$p~\frac{{\text{C}}_{\text{R}}}{{\text{C}}_{\text{M}}}\text{respectively}\text{,}p~\frac{{\text{C}}_{\text{R}}}{{\text{C}}_{\text{M}}}-1\text{or}p~\frac{{\text{C}}_{\text{M}}-{\text{C}}_{\text{R}}}{{\text{C}}_{\text{M}}+{\text{C}}_{\text{R}}}.$$

in which C _M the capacitance of the measuring capacitor, C _R the capacitance of the reference capacitor and p denotes the process pressure to be determined. It is also possible to C _M and C _R to exchange in the quotient. The given example with C _M however, in the denominator, the most common form is in favor of the self-linearization. Therefore, this embodiment is based on this embodiment, unless stated otherwise.

The reliability of capacitive pressure sensors is becoming increasingly important. Creep currents can be problematic on the measuring cell rear side facing away from the medium to be measured or in parts of the evaluation electronics which lead to signal drift. Cause of these leakage currents may be, for example, a condensing air humidity inside the meter.

As state of the art with respect to a function monitoring of capacitive pressure sensors, the DE 103 33 154 A1 and the DE 10 2014 201 529 A1 called.

The object of the invention is to propose a method for monitoring the function of a pressure measuring cell of a capacitive pressure sensor, by means of which a fault influence on the measurement result due to, in particular, moisture-induced leakage currents is made possible.

This object is achieved by a method having the features of claim 1.An advantageous embodiment of the invention is specified in dependent claim 2.

The core of the invention is the recognition that moisture on the measuring cell rear side facing away from the medium to be measured or in parts of the evaluation electronics and the resulting creeping currents entails a change of the measuring signal present in the form of an alternating square wave signal. Instead of uniform rectangular pulses with a constant amplitude or pulse height during the pulse width, the amplitude or pulse height is no longer constant in the faulty measuring signal, but is increasing or decreasing during the pulse duration.

The inventive method provides for detecting such erroneous measurement signals that at least twice the pulse height is measured per period of the square wave signal during at least one pulse width, ie either a positive or during a negative pulse or during both pulses, and the measured Voltage values are stored in a memory. Finally, the at least two voltage values per pulse width, for example, are compared with each other in absolute value by subtraction, and an error signal is generated in the event of a significant deviation from one another.

Thus, it is possible to carry out a functional monitoring of the pressure measuring cell of a capacitive pressure sensor with the existing evaluation circuit and thus without additional components by a clever signal evaluation and to detect resistive fault influences caused by leakage currents quickly and early.

The invention will be explained in more detail by means of embodiments with reference to the drawings.

• 1 ein Blockdiagramm eines kapazitiven Druckmessgeräts,
• 2 eine schematische Schnittdarstellung einer kapazitiven Druckmesszelle,
• 3 eine bekannte Auswerteschaltung für eine kapazitive Druckmesszelle gemäß 2,
• 4 eine Gegenüberstellung eines fehlerfreien Messsignals und eines durch Kriechströme beeinflussten Messsignals und
• 5 die Auswerteschaltung aus 3, ergänzt um einen Mikrocontroller zur Durchführung des erfindungsgemäßen Verfahrens.

They show schematically:

• 1 a block diagram of a capacitive pressure gauge,
• 2 a schematic sectional view of a capacitive pressure measuring cell,
• 3 a known evaluation circuit for a capacitive pressure cell according to 2 .
• 4 a comparison of an error-free measurement signal and a measurement signal influenced by leakage currents and
• 5 the evaluation circuit 3 , supplemented by a microcontroller for carrying out the method according to the invention.

In the following description of the preferred embodiments, like reference characters designate like or similar components.

In 1 is a block diagram of a typical capacitive pressure gauge 1 shown, which is used to measure a process pressure p (eg of oil, milk, water, etc.). The pressure gauge 1 is designed as a two-wire device and consists essentially of a pressure measuring cell 10 and an evaluation 20 , The evaluation electronics 20 has an analog evaluation circuit 30 and a microcontroller μC in which the analog output signal of the evaluation circuit 20 digitized and processed. The microcontroller μC provides the evaluation result as a digital or analog output signal z. B. a PLC available. For power supply is the pressure gauge 1 connected to a power supply line (12 - 36 V).

2 shows a typical capacitive pressure cell 10 , as it is widely used in capacitive pressure gauges, in a schematic representation. The pressure measuring cell 10 consists essentially of a basic body 12 and a membrane 14 over a glass solder ring 16 connected to each other. The main body 12 and the membrane 14 limit a cavity 19 , which - preferably only at low pressure ranges up to 50 bar - via a vent channel 18 with the back of the pressure cell 10 connected is.

Both on the body 12 as well as on the membrane 14 several electrodes are provided, which are a reference capacitor C _R and a measuring capacitor C _M form. The measuring capacitor C _M is through the membrane electrode ME and the center electrode M formed, the reference capacitor C _R through the ring electrode R and the membrane electrode ME ,

The process pressure p acts on the membrane 14 , which bends more or less in accordance with the pressurization, wherein substantially the distance of the membrane electrode ME to the center electrode M changes. This leads to a corresponding capacitance change of the measuring capacitor C _M , The influence on the reference capacitor C _R is lower, as is the distance between ring electrode R and membrane electrode ME less changed than the distance between the membrane electrode ME to the center electrode M ,

In the following, no distinction is made between the designation of the capacitor and its capacitance value. C _M and C _R Therefore, both denote the measuring or reference capacitor per se, as well as its respective capacity.

In 3 is a known evaluation circuit 30 for the pressure measuring cell 10 shown in more detail. The measuring capacitor C _M is together with a resistance R ₁ in an integrating branch IZ and the reference capacitor C _R together with a resistance R ₂ in a differentiation branch DZ arranged. At the input of the integrating branch IZ is a square-wave voltage U _E0 which preferably alternates symmetrically by 0 volts. The input voltage U _E0 is about the resistance R ₁ and the measuring capacitor C _M using an operational amplifier OP1 , which works as an integrator, converted into a linearly rising or falling voltage signal (depending on the polarity of the input voltage), that at the output COM of the integration branch IZ is issued. The measuring point P1 lies through the operational amplifier OP1 virtually on earth.

The exit COM is with a threshold comparator SG connected, which is a square wave generator RG controls. As soon as the voltage signal at the output COM exceeds or falls below a threshold value, the comparator changes SG be Output signal, whereupon the square wave generator RG its output voltage inverted in each case.

The differentiation branch DZ consists of an operational amplifier OP2 , a voltage divider with the two resistors R ₅ and R ₆ and a feedback resistor R ₇ , The output of the operational amplifier OP2 is connected to a S & H sample-and-hold circuit. At the output of the sample-and-hold circuit S & H is the measurement voltage U _Mess indicating the process pressure p which is on the pressure measuring cell 10 acts, is won.

The function of this measuring circuit is explained in more detail below. The operational amplifier OP1 ensures that the connection point P1 between the resistance R ₁ and the measuring capacitor C _M held virtually to ground. As a result, a constant current flows I ₁ , about the resistance R ₁ , the measuring capacitor C _M as long as it charges until the square-wave voltage U _E0 their sign changes.

Out 3 It can be seen that for the case R ₁ = R ₂ and C _M = C _R the measuring point P2 in differentiation branch DZ even then at the same potential as the measuring point P1 , that is at a mass level, lies when the connection between the measuring point P2 and the operational amplifier OP2 would not exist. This is true not only in this particular case, but whenever the time constants R ₁ * C _M and R ₂ * C _R are equal to each other. At zero point adjustment, this state is via the variable resistors R ₁ respectively. R ₂ adjusted accordingly. If the capacitance of the measuring capacitor C _M changes by pressure, the condition of the equality of the time constant in Integrierzweig IZ and in the differentiation branch DZ no longer given and the potential at the measuring point P2 would deviate from the value zero. This change, however, is made directly by the operational amplifier OP2 counteracted, since the operational amplifier OP2 the connection point P2 continues to hold virtual ground. At the output of the operational amplifier OP2 is therefore a square wave U _R whose amplitude depends on the quotient of the two time constants. It can easily be shown that the amplitude is directly proportional to the process pressure p ~ C _R / C _M -1, the dependence being essentially linear. The amplitude can be controlled by the voltage divider, which is controlled by the two resistors R ₅ and R ₆ is set.

By means of a sample and hold circuit S & H, the positive and negative amplitudes A + and A- of the square-wave signal are added together, the amount A being the measuring voltage U _Mess at the output of the operational amplifier OP3 output and forwarded to microcontroller μC (not shown). It could also be output directly as an analog value. The amplitude of the input voltage U _E0 at the output of the square wave generator RG is applied, depending on the measuring voltage U _Mess adjusted for better linearity. For this purpose, a voltage divider consisting of the resistors R ₂₀ and R ₁₀ intended. This voltage divider is connected to a reference voltage VREF and advantageously adjustable.

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

In 4 Ideally, they are compared with an error-free measuring signal (top) and a measuring signal (below) influenced by leakage currents. It can be clearly seen that the error-prone signal no longer has a constant amplitude or pulse height, but rises during a positive pulse and decreases during a negative pulse. To detect this situation, according to the invention, the pulse height of the measurement signal U _R per period during at least one pulse width duration, ie either a positive or during a negative pulse or during both pulses, measured at least twice and stored the measured voltage values in a memory. By a magnitude comparison of the at least two voltage values per pulse width, for example by a subtraction, such a waveform, as shown in the lower diagram, can be detected and the user can be made aware that a corresponding error influence exists, so that the measured pressure values possibly are drifting.

The illustrated situation arises, for example, in that, in the case of moisture on the measuring cell rear side remote from the medium to be measured, the low-impedance driven COM connection (cf. 3 and 5 ) separates the leakage current and the two high-impedance zero C _M - and C _R Connect the leakage current. Depending on where a flowable line has formed, the measuring capacitor experiences C _M or the reference capacitor C _R the larger leakage current effect. The waveform in 4 depicts the situation that the measuring capacitor C _M experiences the greater influence. Should the reference capacitor C _R experience the greater influence, would result during the pulse instead of a rising a falling waveform and instead of a falling a rising waveform.

5 basically shows that 3 known evaluation circuit, which, however, is supplemented by a microcontroller μC. In this microcontroller μC is on the one hand, the comparator oscillator SG out 3 integrated and on the other hand it contains the necessary for carrying out the method according to the invention units: a timer 60 , a store 40 and a CPU 50 as a processing unit. The elements located outside the microcontroller μC are essentially identical and therefore also identically designated. To avoid repetition, only the elements essential to the invention will be discussed below.

The output of the threshold comparator SG is returned to the one hand back to the square wave generator RG to control what is already off 3 is known. On the other hand, this signal is the timer 60 fed. In the timer 60 the temporal period behavior of the triangular signal is recorded, in particular with regard to the achievement of the set threshold values. From this, the period duration is derived, which the CPU 50 is supplied. The period of this triangular signal is identical in this case with that of the actual measurement signal U _R , so that the detection of the period in this way is particularly advantageous.

Furthermore, the microcontroller μC comprises a memory 40 , which first the currently measured pressure value p _x in the form of the 3 known voltage signal U _R is supplied. In parallel, and not shown for illustrative purposes, the pressure value p _x also to the output switch_out or analog out of the microcontroller .mu.C given to output the measured pressure values as a switching or analog signal. From 3 known sample & hold circuit S & H as part of the evaluation circuit shown there is then also integrated into the microcontroller microcontroller and there functionally identical replicated.

The memory 40 Furthermore, a trigger signal is supplied to the CPU 50 generated from the period. By this trigger signal is precisely defined, at which time or more precisely at which two times during the pulse duration, the pulse height of the voltage signal U _R should be saved. These times are advantageously at the beginning and at the end of the positive and / or negative square pulse.

In the CPU 50 the two voltage values detected during a rectangular pulse are compared in terms of absolute value and, in the event of significant deviation from a predetermined tolerance band, an error signal is generated which is output at the output diag_out.

LIST OF REFERENCE NUMBERS

1

pressure monitor

10

Pressure measuring cell

12

body

14

membrane

16

glass solder

18

vent channel

19

cavity

20

evaluation

30

evaluation

40

Storage

50

Processing unit, CPU

60

timer

C _M

measuring capacitor

C _R

reference capacitor

M

center electrode

R

ring electrode

ME

membrane electrode

IZ

Integrierzweig

DZ

Differentiating branch

SG

Threshold comparator

RG

square wave generator

Claims (2)

Method for monitoring the function of a pressure measuring cell (10) of a capacitive pressure sensor (1), wherein the pressure measuring cell (10) has a pressure-dependent measuring capacitor (C _M ) and a pressure-dependent reference capacitor (C _R ) and the pressure measurement (p) as a measuring signal from the capacitance values of the measuring capacitor (C _M ) and the reference capacitor (C _R ) is obtained, wherein the measurement signal to an evaluation unit in the form of an alternating square wave signal U _{R is} supplied, the pulse height of the quotient of the capacitance values of reference capacitor (C _R ) and measuring capacitor (C _M ) depends per period of the alternating square wave signal U _R during at least one pulse width of a positive and / or negative rectangular pulse of alternating square wave signal U _{R is} at least twice the pulse height is measured and the measured voltage values are stored in a memory (40), wherein the at least two voltage values per pulse width magnitude together be compared and an error signal is generated at a significant deviation from each other. Method according to Claim 1 in which the values for the pulse height and their comparison are stored in a microcontroller (μC).

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